NEW MATERIALS FOR OPTICAL SENSING OF EXPLOSIVES COPOLYMERS CONTAINING 2-VINYL-4,6-DIAMINO-1,3,5-TRIAZINE AND CO-CRYSTALS OF ELECTRON RICH AROMATIC MOLECULES AND 1,3-DINITROBENZENE by STEVEN KEITH MCNEIL DAVID E. NIKLES, COMMITTEE CHAIR MARTIN G. BAKKER CHRISTOPHER S. BRAZEL SHANLIN PAN SHANE C. STREET A DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in the Department of Chemistry in the Graduate School of The University of Alabama TUSCALOOSA, ALABAMA 2013
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CHAPTER 4: Polymer Thin Films Characterization by Variable Angle Spectroscopic Ellipsometry after Exposure to a Nitroaromatic Vapor .......................................127
4.8 Polystyrene Thin Films Containing 10-Methylphenothiazine .............................182
4.9 Polymer Thin Films Summary .............................................................................198
CHAPTER 5: Co-Crystals Containing Electron Rich Aromatic Molecules and Electron Poor Nitroaromatic Molecules .....................................................................................200
Table 1.1.2.1 Common explosives' vapor pressures ......................................................................5
Table 1.2.1 List of countries with estimated unexploded landmines ..........................................9
Table 1.3.2.1 Examples of the variety of AP mines based on their material, color, fuse, explosive charge, and weight .................................................................................12
Table 1.5.5.1 Explosive sensors limit of detection ranges ...........................................................39
Table 2.2.2.1 Experimental amounts and conditions for PS-co-PVDAT polymerizations .........45
Table 2.2.3.1 Experimental amounts for the PMMA-co-PVDAT copolymers and PMMA polymerizations ......................................................................................................47
Table 2.2.5.1 Experimental amounts for P2VP-co-PVDAT copolymers and P2VP polymerizations ......................................................................................................50
Table 2.2.6.1 Experimental amounts for PAM-co-PVDAT copolymers and PAM polymerizations ......................................................................................................52
Table 2.2.7.1 PVK-co-PVDAT copolymers and PVK experimental amounts for free radical polymerizations ......................................................................................................54
Table 2.2.8.1 Experimental amounts for the PS-co-PVK polymerizations .................................55
Table 2.2.9.1 Experimental amounts for PMMA-co-PVK copolymers polymerizations ............57
Table 2.3.1.1 Co-crystals experimental reagents, solvents, and descriptions of crystals ............64
Table 3.2.1 Polystyrene and PS-co-PVDAT 20 mol % VDAT copolymer 13C NMR peaks ...76
Table 3.2.2 Thermal decomposition temperatures, Td (10% weight loss for PS and PS-co- PVDAT copolymers) .............................................................................................79
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Table 3.2.3 Glass transition temperatures for PVDAT, PS-co-PVDAT copolymers, and PS ................................................................................................................................81
Table 3.2.4 PS-co-PVDAT copolymers GPC data ...................................................................82
Table 3.3.2 Thermal decomposition temperatures for PMMA and the copolymers of PMMA and PVDAT ...........................................................................................................88
Table 3.3.3 The glass transition temperatures for PMMA, PMMA-co-PVDAT copolymers, and PVDAT ...........................................................................................................90
Table 3.3.4 Molecular weights for the PMMA-co-PVDAT copolymers determined by GPC ................................................................................................................................90
Table 4.4.1 The Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a polystyrene film exposed to PNT vapors for ten seconds .................................................................................140
Table 4.4.2 The ellipsometry MSE, film thickness, refractive index, average change in refractive index (Δn), optical constants MSE, profilometer thickness, and spin coating parameters for a PS-co-PVDAT 20 mol % VDAT copolymer film produced from a 1% (w.t.) 1,4-dioxane solution .................................................142
Table 4.4.3 The ellipsometry Cauchy model MSE, thickness, refractive index, and optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 10 mol % VDAT copolymer film ............................................144
Table 4.4.4 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 5 mol % VDAT copolymer film ..............................................146
Table 4.4.5 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film ...........................................148
Table 4.4.6 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for five seconds .................................................................................................................151
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Table 4.4.7 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for twenty seconds .................................................................................................................153
Table 4.4.8 The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for forty seconds .................................................................................................................155
Table 4.5.1 The before and after Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a PMMA film exposed to 1,3-DNB for ten seconds ...................................................................160
Table 4.5.2 The ellipsometry Cauchy model parameters, profilometer thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co- PVDAT 1 mol % VDAT copolymer film exposed to 1,3-DNB for sixteen minutes ..............................................................................................................................162
Table 4.5.3 The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co- PVDAT 1 mol % VDAT copolymer film exposed to NB for twenty-five minutes ..............................................................................................................................164
Table 4.5.4 The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co- PVDAT 5 mol % VDAT copolymer film exposed to NB for twenty-five minutes ..............................................................................................................................166
Table 4.5.5 The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co- PVDAT 5 mol % VDAT copolymer film exposed to 1,3-DNB for twenty-five minutes .................................................................................................................168
Table 4.6.1 The Cauchy parameters, average change in refractive index, and spin coating parameters for a spin coated P2VP film exposed to PNT for five seconds. ........171
Table 4.7.1 The ellipsometry Cauchy model MSE, film thickness, average change in refractive index (Δn), optical constants MSE, profilometer measured thickness, and spin coating parameters for the P4VP film exposed to PNT for five seconds ..............................................................................................................................175 Table 4.7.2 The Cauchy model parameters and spin coating parameters for a PVI polymer film spin coated from a 3% (w.t.) EtOH solution exposed to PNT for 5 seconds .............................................................................................................................178
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Table 4.7.3 The before and after Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a PVI-co-PVA polymer film exposed to PNT for five seconds ..................................................181
Table 4.8.1 The before and after Cauchy model parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a polystyrene film containing 10-M exposed to 1,3-DNB for two hours ...............186
Table 4.8.2 The ellipsometry Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene solution containing 0.5% (w.t.) 10-M exposed to 1,3-DNB for three hours ....................................................................189
Table 4.8.3 The Cauchy model parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for three hours ............................................................................................................192
Table 4.8.4 The Cauchy parameters before and after exposure to 1,3-DNB, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene/toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for ten seconds ................................................195
Table 4.9.1 Polymer films average change in refractive index after a five-second exposure to a nitroaromatic vapor ..............................................................................................199
Table 5.2.1 1H NMR peak positions, splitting patterns, and integration values of the 1,3-DNB crystals, 9-EC co-crystals, and 9-EC crystals ......................................................208
Table 5.2.2 Comparison of NO2 asymmetric and symmetric stretching vibrations between 1, 3-DNB crystals and 9-EC + 1,3-DNB co-crystals ...............................................209
Table 5.2.3 Melting points of 1,3-DNB crystals, 9-EC crystals, and 9-EC co-crystals with 1,3- DNB .....................................................................................................................213
Table 5.3.1 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, 9-VC co-crystals, and 9-VC crystals .....................................................215
Table 5.3.2 NO2 asymmetric and symmetric stretching vibrations for 1,3-DNB crystals and 9- VC co-crystals ......................................................................................................217
Table 5.4.1 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, CBZ co-crystals, and CBZ crystals .......................................................220
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Table 5.4.2 NO2 asymmetric and symmetric stretching vibrations for 1,3-DNB crystals and CBZ co-crystals ...................................................................................................221
Table 5.5.1 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, PHZ co-crystals, and PHZ crystals ........................................................227
Table 5.5.2 NO2 asymmetric and symmetric stretching modes for the PHZ co-crystals and 1,3-DNB crystals..................................................................................................229
Table 5.5.3 Melting points of the 1,3-DNB crystals, PHZ crystals, and PHZ co-crystals .....233
Table 5.6.1 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, 10-M co-crystals, and 10-M crystals .....................................................235
Table 5.6.2 NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB crystals and 10-M co-crystals............................................................................................237
Table 5.6.3 Melting points of the 1,3-DNB crystals, 10-M crystals, and 10-M co-crystals ...241
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LIST OF FIGURES
Figure 1.3.3.1 Chemical structures of common explosives and TNT impurities found and used in landmines .........................................................................................................13
Figure 1.6.1 MZI consisting of a polymer waveguide with two optical paths ...........................40
Figure 3.1.1 The FTIR spectrum of PVDAT recorded in KBr ..................................................70
Figure 3.2.4 TGA curves for PS and PS-co-PVDAT 20 mol % VDAT copolymer ..................79
Figure 3.2.5 PVDAT, PS-co-PVDAT copolymers, and PS DSC curves ...................................80
Figure 3.3.1 FTIR spectra for the PMMA-co-PVDAT copolymers ..........................................83
Figure 3.3.2 1H NMR spectrum in DMSO-d6 for PMMA-co-PVDAT (20 mol %) ..................84
Figure 3.3.3 13C NMR spectrum in DMSO-d6 for the PMMA (80%)-co-PVDAT (20%) copolymer ..............................................................................................................86
Figure 3.3.4 TGA curves for PMMA and the copolymer containing 10 mol % VDAT ............88
Figure 3.3.5 PMMA and PMMA-co-PVDAT copolymers DSC curves ....................................89
Figure 3.4.1 FTIR spectrum for the PMA-co-PVDAT 20 mol % VDAT copolymer ...............91
Figure 3.4.2 1H NMR spectrum for the PMA-co-PVDAT 20 mol % VDAT copolymer ..........92
Figure 3.4.3 The 13C NMR spectrum (500 MHz, DMSO-d6) for the PMA-co-PVDAT 20 mol % VDAT copolymer ..............................................................................................93
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Figure 3.4.4 DSC curve for the PMA-co-PVDAT 20 mol % VDAT copolymer ......................94 Figure 3.5.1 FTIR spectra for P2VP and P2VP-co-PVDAT copolymers ..................................95
Figure 3.5.2 1H NMR spectrum for the P2VP-co-PVDAT 20 mol % VDAT copolymer in DMSO-d6 using the 360 MHz spectrometer ..........................................................97
Figure 3.5.3 The 13C NMR spectrum (500 MHz, DMSO-d6) for the P2VP-co-PVDAT 1 mol % VDAT copolymer ..............................................................................................98
Figure 3.5.4 P2VP and P2VP-co-PVDAT copolymers DSC curves .........................................99
Figure 3.6.1 PAM and PAM-co-PVDAT copolymers FTIR spectra .......................................101
Figure 3.6.2 The 1H NMR spectrum (360 MHz, D2O) for the PAM-co-PVDAT 20 mol % VDAT copolymer ................................................................................................102
Figure 3.6.3 The 13C NMR spectrum (500 MHz) recorded in D2O for the PAM-co-PVDAT 20 mol % VDAT copolymer .....................................................................................103
Figure 3.6.4 DSC curves for the PAM and PAM-co-PVDAT copolymers .............................104
Figure 3.7.1 The FTIR spectra for PVK and PVK-co-PVDAT copolymers recorded in KBr pellets ...................................................................................................................105
Figure 3.7.2 DSC curves for PVK and PVK-co-PVDAT copolymers ....................................107
Figure 3.8.1 The 1H NMR spectra for the PVK and PS-co-PVK copolymers recorded in CDCl3 (360 MHz) ............................................................................................................108
Figure 3.9.1 The FTIR spectra for KBr pellets containing PVK and PMMA-co-PVK copolymers ...........................................................................................................109
Figure 3.9.2 The 1H NMR spectrum for the PVK homopolymer recorded in CDCl3 using the 360 MHz spectrometer .........................................................................................110
Figure 3.9.3 The 1H NMR spectra for PMMA-co-PVK 50 and 20 mol % copolymers recorded in CDCl3 (360 MHz) ............................................................................................111
Figure 3.9.4 The 13C NMR spectrum for PVK recorded in CDCl3 (500 MHz) .......................113
Figure 3.9.5 The 13C NMR spectrum for the (50:50) PMMA-co-PVK copolymer ( 500 MHz, CDCl3)..................................................................................................................114
Figure 3.9.6 The DSC curves for the PMMA-co-PVK copolymers ........................................115
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Figure 3.10.1 The FTIR spectra for PVI homopolymer (black) and PVI-co-PVDAT 20 mol % VDAT copolymer (red) ........................................................................................116
Figure 3.10.2 The 1H NMR spectra for the PVI homopolymer and PVI-co-PVDAT copolymer ..............................................................................................................................117
Figure 3.10.3 The 13C NMR spectrum for the PVI-co-PVDAT copolymer recorded in DMSO-d6 (500 MHz) ............................................................................................................118
Figure 3.11.1 The FTIR spectra for the PVI homopolymer (red curve) and the PS-co-PVI 20 mol % copolymer (black curve) recorded in KBr pellets at room temperature ..............................................................................................................................119
Figure 3.11.2 The 1H NMR spectra for the PS-co-PVI copolymer and polystyrene homopolymer recorded in CDCl3 (360 MHz) ............................................................................121
Figure 3.11.3 The 13C NMR spectra for the PS-co-PVI copolymer and the homopolymer (PVI) ..............................................................................................................................122
Figure 3.12.1 The FTIR spectra for the PMMA-co-PVI copolymer and the PVI homopolymer recorded in KBr pellets ........................................................................................123
Figure 3.12.2 The 1H NMR spectra for the homopolymers, PVI and PMMA, and the PMMA- co-PVI copolymer containing 20 mol % vinylimidazole ....................................124
Figure 3.12.3 The 13C NMR spectrum for the PMMA-co-PVI copolymer containing 20 mol % VI recorded in DMSO-d6 (500 MHz) ..................................................................125
Figure 3.12.4 The DSC curves shown from 80 °C to 160 °C for the PMMA-co-PVI 20 mol % VI copolymer and PMMA homopolymer ............................................................126
Figure 4.1.1 Two linearly polarized waves combined out of phase producing elliptically polarized light. Modified from http://www.jawoollam.com/tutorial_2.html (accessed Feb. 15, 2013) ......................................................................................128
Figure 4.1.2 Light reflecting and refracting at the interface between air and the surface of a material. Modified from http://www.jawoollam.com/tutorial_3.html (accessed Feb. 15, 2013) ......................................................................................................129
Figure 4.1.3 Schematic representation for a typical ellipsometry measurement showing a polarization state change when linearly polarized light is reflected from a sample's surface. Modified from http://www.jawoollam.com/tutorial_4.html (accessed Feb. 15, 2013) ..............................................................................................................131
Figure 4.1.4 Schematic representation of a wave propagating through a film, producing multiple reflections and transmissions .................................................................131
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Figure 4.4.1 The before and after refractive index curves for a polystyrene film spin coated from a 3% (w.t.) toluene solution exposed to PNT for ten seconds ....................139
Figure 4.4.2 The change in refractive index for a PS-co-PVDAT 20 mol % VDAT copolymer film spin coated from a 1% (w.t.) 1,4-dioxane solution exposed to NB for five seconds ................................................................................................................141
Figure 4.4.3 The change in refractive index for a PS-co-PVDAT 10 mol % VDAT copolymer film produced from a 1% (w.t.) MEK solution exposed to NB for five seconds .................................................................................................................143
Figure 4.4.4 The change in refractive index for a PS-co-PVDAT 5 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for sixty seconds ..............................................................................................................................145
Figure 4.4.5 The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to NB for five seconds ..............................................................................................................................147
Figure 4.4.6 The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for five seconds ..............................................................................................................................150
Figure 4.4.7 The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for twenty seconds .................................................................................................................152
Figure 4.4.8 The change in refractive index for the PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for forty seconds ..............................................................................................................................154
Figure 4.5.1 The before and after refractive index curves for a PMMA film spin coated from a 3% (w.t.) toluene solution exposed to 1,3-DNB for ten seconds .........................159
Figure 4.5.2 The refractive index curves for a PMMA-co-PVDAT 1 mol % VDAT copolymer thin film spin coated from a 3% (w.t.) toluene polymer solution exposed to 1,3- DNB for sixteen minutes .....................................................................................161
Figure 4.5.3 The refractive index curves for a PMMA-co-PVDAT 1 mol % VDAT copolymer thin film spin coated from a 3% toluene solution exposed to NB for twenty-five minutes .................................................................................................................163
Figure 4.5.4 The refractive index curves for a PMMA-co-PVDAT 5 mol % VDAT copolymer film spin coated from a 1% (w.t.) toluene solution before and after exposure to NB for twenty-five minutes .................................................................................165
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Figure 4.5.5 The ellipsometry curves for a PMMA-co-PVDAT 5 mol % VDAT copolymer film spin coated from a 1% (w.t.) toluene solution before and after exposure to 1,3-DNB for twenty-five minutes .......................................................................167
Figure 4.6.1 The before and after ellipsometry curves for a P2VP polymer film spin coated from a 3% (w.t.) toluene solution exposed to PNT for five seconds ..................170
Figure 4.7.1 The before and after refractive index curves for a P4VP film spin coated from a 3% (w.t.) ethanol solution exposed to a concentrated PNT vapor for five seconds ..............................................................................................................................174
Figure 4.7.2 The PVI thin film spectroscopic ellipsometry curves showing a change in refractive index after a five second exposure to PNT .........................................177
Figure 4.7.3 The before and after refractive index curves for a thin PVI-co-PVA polymer film exposed to PNT for five seconds .........................................................................180
Figure 4.8.1 The ellipsometry curves for a spin coated polystyrene/10-M film from a 3% (w.t.) polystyrene solution containing 1% (w.t.) 10-M exposed to 1,3-DNB for two hours ....................................................................................................................185
Figure 4.8.2 Refractive index curves for a polystyrene/10-M film spin coated from 1% (w.t.) polystyrene/toluene polymer solution containing 0.5% (w.t.) 10-M exposed to 1,3-DNB for three hours .....................................................................................188
Figure 4.8.3 The refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3- DNB for three hours ............................................................................................191
Figure 4.8.4 The refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene/toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3- DNB for ten seconds ...........................................................................................194
Figure 5.1.1 Image of 1,3-DNB crystals ..................................................................................201
Figure 5.1.3 The FTIR spectrum of the 1,3-dinitrobenzene crystals .......................................203
Figure 5.1.4 Electronic absorption spectrum of the 1,3-DNB crystals in acetonitrile .............204
Figure 5.1.5 Diffuse reflectance spectrum of the 1,3-DNB crystals ........................................205
Figure 5.2.1 9-EC + 1,3-DNB crystals after drying for two days, producing yellow-orange tint crystals .................................................................................................................206
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Figure 5.2.2 1H NMR (360 MHz, CDCl3) spectra for the 9-ethylcarbazole crystals (9-EC), the co-crystals (9-EC co-crystals with 1,3-DNB), and 1,3-dinitrobenzene crystals (1,3-DNB) ............................................................................................................207
Figure 5.2.3 FTIR spectra of KBr pellets containing either 9-EC crystals (black curve) or the co-crystals containing 9-EC and 1,3-DNB (red curve) ........................................209
Figure 5.2.4 Electronic absorption spectra of 1,3-DNB crystals (black), 9-EC crystals (red), and 9-EC co-crystals with 1,3-DNB (blue) in acetonitrile ..................................210
Figure 5.2.5 Electronic absorption spectra in acetonitrile for the 9-EC co-crystals (red) and the sum of the spectra for 9-EC and 1,3-DNB crystals (black) .................................211
Figure 5.2.6 Diffuse reflectance spectra for the 9-EC crystals (black) and the co-crystals of 9- EC and 1,3-DNB (red) .........................................................................................212
Figure 5.3.1 Co-crystals of 9-VC and 1,3-DNB.......................................................................214
Figure 5.3.2 1H NMR spectra of the 1,3-DNB crystals, 9-VC crystals, and 9-VC co-crystals with 1,3-DNB (360 MHz, CDCl3) .......................................................................215
Figure 5.3.3 FTIR spectra of KBr pellets containing either 9-VC crystals (red) or the co- crystals of 9-VC and 1,3-DNB (black) ................................................................216
Figure 5.4.1 Crystals of carbazole (A) and co-crystals of carbazole and 1,3-DNB (B) ..........218
Figure 5.4.2 1H NMR spectra of CBZ crystals, CBZ co-crystals with 1,3-DNB, and 1,3-DNB crystals (360 MHz, CDCl3) ..................................................................................219
Figure 5.4.3 FTIR spectra for KBr pellets containing CBZ crystals (black) and the CBZ co- crystals with 1,3-DNB (red) .................................................................................221
Figure 5.4.4 Electronic absorption spectra in acetonitrile for 1,3-DNB crystals (black), CBZ crystals (red), and CBZ co-crystals containing 1,3-DNB and CBZ (blue) ..........223
Figure 5.4.5 Comparison of the electronic absorption spectrum in acetonitrile for the co- crystals containing 1,3-DNB and CBZ (red) and the sum of the spectrum for 1,3- DNB crystals and the spectrum for the CBZ crystals (black) ..............................224
Figure 5.5.1 Images of PHZ co-crystals (A) and PHZ crystals (B) .........................................226
Figure 5.5.2 1H NMR spectra (360 MHz, CDCl3) of the 1,3-DNB crystals, PHZ crystals, and the co-crystals made from PHZ and 1,3-DNB .....................................................227
Figure 5.5.3 FTIR spectra for KBr pellets containing either PHZ (black) or the co-crystal of PHZ and 1,3-DNB (blue) .....................................................................................228
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Figure 5.5.4 Electronic absorption spectra recorded in acetonitrile for 1,3-DNB crystals (black), PHZ crystals (red), and the co-crystals containing PHZ and 1,3-DNB (blue) ....................................................................................................................230
Figure 5.5.5 Electronic absorption spectra in acetonitrile for the PHZ co-crystals (red) and the sum of the spectra for 1,3-DNB crystals and PHZ crystals (black) ...............231
Figure 5.5.6 Diffuse reflectance spectra for PHZ crystals (black) and co-crystals containing PHZ and 1,3-DNB (red) .......................................................................................232
Figure 5.6.1 Images of 10-M co-crystals (A) and 10-M co-crystals with 1,3-DNB (B) ..........234
Figure 5.6.2 1H NMR spectra of 1,3-DNB crystals, 10-M crystals, and co-crystals containing 10-M and 1,3-DNB (360 MHz, CDCl3) ...............................................................235
Figure 5.6.3 Infrared spectra of KBr pellets containing either 10-M crystals (black) or the co- crystals containing 10-M and 1,3-DNB (red) ......................................................236
Figure 5.6.4 Electronic absorption spectra for 1,3-DNB crystals (black), 10-M crystals (red), and the co-crystals (blue) .....................................................................................238
Figure 5.6.5 Electronic absorption spectra for the co-crystals (red) and the sum of the spectra for 1,3-DNB crystals and 10-M crystals (black) ..................................................239
Figure 5.6.6 Diffuse reflectance spectra for the 10-M crystals (black) and the co-crystals containing 10-M and 1,3-DNB (red) ...................................................................240
Figure 5.6.7 50% probability ellipsoid plot of the asymmetric unit of the co-crystal. The dashed lines indicate distances that were less than the sum of the van der Waals radii ......................................................................................................................242
Figure 5.6.8 Short contact environment around 1,3-DNB A (left) and B (right). The green lines indicate distances that were less than the sum of the van der Waals distance .....243
Figure 5.6.9 Short contact environment around 10-M A (left) and B (right). The green lines indicate distances that were less than the sum of the van der Waals contacts ....244
Figure 5.6.10 The 1,3-DNB-10-M H-bonded chain along b. The green lines indicate the distances that were less than the sum of the van der Waals contacts ..................244
Figure 5.6.11 View along b axis of π- π stacking interactions between A chains. The green lines indicate the distances that were less than the sum of the van der Waals contacts ..............................................................................................................................245
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Figure 5.6.12 Packing down b axis showing only A chains (left) and all atoms (right). A chains are colored blue in both pictures. B chains are colored red. Crystallographic axes are color coded as a = red, b = green, c = blue ....................................................246
Appendix Figure 15 The PS-co-PVDAT 1 mol % VDAT copolymer GPC data .....................273
Appendix Figure 16 The PS-co-PVDAT 5 mol % VDAT copolymer GPC data .....................273
Appendix Figure 17 The PS-co-PVDAT 10 mol % VDAT copolymer GPC data ...................274
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Appendix Figure 18 The PS-co-PVDAT 20 mol % VDAT copolymer GPC data ...................274
Appendix Figure 19 FTIR spectrum of the PMMA-co-PVDAT 20 mol % VDAT copolymer ..................................................................................................................275
Appendix Figure 20 FTIR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer .................................................................................................................276
Appendix Figure 21 FTIR spectrum of the PMMA-co-PVDAT 5 mol % VDAT copolymer ..................................................................................................................277
Appendix Figure 22 FTIR spectrum of the PMMA-co-PVDAT 1 mol % VDAT copolymer ..................................................................................................................278
Appendix Figure 23 The 1H NMR spectrum of PMMA in DMSO-d6 using the 500 MHz spectrometer ............................................................................................279
Appendix Figure 24 The 1H NMR spectrum for the PMMA-co-PVDAT 1 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer ..........................280
Appendix Figure 25 The 1H NMR spectrum for the PMMA-co-PVDAT 1 mol % VDAT copolymer with the spectrum intensity increased showing the vinyl protons for either MMA or VDAT (6.18, 5.48, and 5.45 ppm) suggesting unreacted monomer present within the polymer matrix ..........................281
Appendix Figure 26 The 1H NMR spectrum of the PMMA-co-PVDAT 5 mol % VDAT in CDCl3 using the 500 MHz spectrometer .................................................282
Appendix Figure 27 The 1H NMR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer ..........................283
Appendix Figure 28 The 13C NMR spectrum of PMMA in DMSO-d6 using the 500 MHz spectrometer .............................................................................................284
Appendix Figure 29 The 13C NMR spectrum of the PMMA-co-PVDAT 10 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer ...........................285
Appendix Figure 30 The 13C NMR spectrum of the PMMA-co-PVDAT 5 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer ...........................286
Appendix Figure 31 The 13C NMR spectrum of the PMMA-co-PVDAT 1 mol % VDAT copolymer in DMSO-d6 using the 500 MHz spectrometer ......................287
Appendix Figure 32 TGA curve for the PMMA-co-PVDAT 1 mol % VDAT copolymer ......288
xxxv
Appendix Figure 33 The TGA curve for the PMMA-co-PVDAT 5 mol % VDAT copolymer ..................................................................................................................289
Appendix Figure 34 The TGA curve for the PMMA-co-PVDAT 20 mol % VDAT copolymer ..................................................................................................................290
Appendix Figure 35 GPC curve and data for the PMMA-co-PVDAT 20 mol % VDAT copolymer ...............................................................................................291
Appendix Figure 36 The GPC curve and data for the PMMA-co-PVDAT 10 mol % VDAT copolymer ...............................................................................................292
Appendix Figure 37 The GPC curve and data for the PMMA-co-PVDAT 5 mol % VDAT copolymer ...............................................................................................293
Appendix Figure 53 The PAM-co-PVDAT 10 mol % VDAT copolymer 13C NMR spectrum (500 MHZ, D2O) from 190 - 150 ppm showing the PMA carbonyl carbon signal (C3) and the PVDAT carbon signal (C7) confirming the presence of PVDAT in the copolymer ........................................................................309
Appendix Figure 54 The 13C NMR spectrum for the PMMA-co-PVK 20 mol % vinylcarbazole recorded in CDCl3 (500 MHz) .................................................................310
Appendix Figure 55 The DSC curve for the PVI homopolymer ..............................................311
xxxvii
LIST OF SCHEMES
Scheme 1.1 Classification of explosives .........................................................................2
Scheme 2.2.1.1 Free radical polymerization of PVDAT .....................................................43
Scheme 2.2.2.1 Free radical polymerization for the synthesis of PS-co-PVDAT random copolymers .................................................................................................45
Scheme 2.2.3.1 Free radical polymerization for the synthesis of PMMA-co-PVDAT random copolymers ....................................................................................47
Scheme 2.2.4.1 Free radical polymerization for the synthesis of a PMA-co-PVDAT random copolymer .....................................................................................48
Scheme 2.2.5.1 Free radical polymerization for the synthesis of P2VP-co-PVDAT random copolymers .................................................................................................50
Scheme 2.2.6.1 Free radical polymerization for the synthesis of PAM-co-PVDAT random copolymers .................................................................................................52
Scheme 2.2.7.1. Free radical polymerization for the synthesis of PVK-co-PVDAT random copolymers .................................................................................................54
Scheme 2.2.8.1. Free radical polymerization for the synthesis of PS-co-PVK random copolymers .................................................................................................56
Scheme 2.2.9.1 Free radical polymerization for the synthesis of PMMA-co-PVK random copolymers .................................................................................................57
Scheme 2.2.10.1 Free radical polymerization for the synthesis of a PVI-co-PVDAT random copolymer ..................................................................................................58
Scheme 2.2.11.1 Free radical polymerization for the synthesis of a PS-co-PVI random copolymer ..................................................................................................60
Scheme 2.2.12.1 Free radical polymerization for the synthesis of a PMMA-co-PVI random copolymer ..................................................................................................61
1
Chapter 1
Introduction
1.1 Explosives
Explosives are defined as energetic materials which react to produce rapid oxidation of
products accompanied by the generation of heat, light, or gas.1 Explosives are classified by
structure and performance, and fall into two broad categories, low and high explosives, with high
explosives being further classified into additional categories. Scheme 1.1.1 shows the
classification for high and low explosives. Low explosives, which include propellants and
pyrotechnics, burn at relatively low rates (cm/s) and are capable of producing heat, light, smoke,
gas, or sound with propellants.1 High explosives are capable of detonating at high velocities (1 to
9 km/s) and produce vast amounts of energy (400 to 1,200 cal/g).1 High explosives are further
classified into primary and secondary explosives based on their ability to detonate. Primary
explosives are very susceptible to initiation and are used as a source for igniting secondary
explosives. Secondary explosives, which consist of nitroaromatics and nitro-amines, are
prevalent for military and industry uses, since they are designed to detonate under specific
conditions.
2
Scheme 1.1.1. Classification of explosives.2
Explosives
High Explosives Low Explosives
Pyrotechnics Propellants Primary Explosives
Secondary Explosives
Military Explosives
Industrial Explosives
3
1.1.2 Types and Properties of Explosives
There are several different types of manufactured explosive materials designed to
detonate, but the most common are organic based compounds. The organic based secondary
explosives fall into two categories based on their structure: aromatic and aliphatic. Aromatic
explosives contain a benzene ring, and the aliphatic explosives do not. A variety of aromatic
explosives exists due to the fact one or more molecular subgroups can be substituted for a
hydrogen atom on the benzene ring. Aliphatic explosive materials primarily consist of aliphatic
nitrate esters (─ONO2), aliphatic nitramines (─N─NO2), and nitro-aliphatics (─NO2). The most
common aliphatic explosives are the nitrate esters. Examples of aromatic and aliphatic
a 1.04 mmols (0.17 g) of AIBN was used as the free radical initiator b 0.05 mmols (0.0821 g) of AIBN was used as the free radical initiator c 2 mmols (0.33 g) of AIBN was used as the free radical initiator * All polymerizations were performed at approximately 80 °C
Scheme 2.2.5.1. Free radical polymerization for the synthesis of P2VP-co-PVDAT random copolymers.
PAM 0 0 0 100 0.1 7.11 149 10.60 ≈ 80 °C a Polymer was synthesized using 0.05 mmols (0.0821 g) of AIBN and 25 mL of DMSO Polymers were synthesized using 2 mmols (0.33 g) of AIBN and 100 mL of DMSO
Scheme 2.2.6.1. Free radical polymerization for the synthesis of PAM-co-PVDAT random copolymers.
PS-co-PVK copolymers were attempted to be synthesized according to the method of
Chen and Sun.49 Scheme 2.2.8.1 shows the free radical polymerization for the PS-co-PVK
copolymers. Styrene was purified by distillation under reduced pressure and N-vinylcarbazole
was used as received. DMF was purified by distillation under reduced pressure from CaH2. For
the PS-co-PVK free radical polymerizations, the N-vinylcarbazole mol % concentrations were
varied from 10, 15, and 20 with styrene. In the synthesis for the 20 mol % PS-co-PVK
copolymer, 0.08 mols (8.33 g) of styrene and 0.02 mols (3.87 g) of vinylcarbazole were added to
a 250 mL three neck round bottom flask. The flask was placed in an oil bath on a hot plate, fitted
a 1 mmols (0.165 g) of AIBN was used as the free radical initiator b 100 mL of DMF was used as the reaction solvent *Polymers were synthesized using 2 mmols (0.33 g) of AIBN and 100 mL of DMSO
55
with a condenser containing 100 mL of freshly distilled DMF under a nitrogen atmosphere, and
equipped with a stir bar and thermometer. Once the reaction temperature inside the round bottom
flask reached 60 °C, 2.0 mmols (0.33 g) of recrystallized AIBN was added to the solution to
initiate the free radical polymerization. The reaction temperature was increased to approximately
80 °C and was stirred for five hours. Once the monomers and initiator dissolved in DMF, the
reaction solution appeared colorless. During the reaction, the solution gradually changed from
colorless to a light yellow color, indicating the reaction had come to completion. The polymer
solution was allowed to cool to room temperature and then was transferred to a 600 mL beaker.
The copolymer was precipitated in excess MeOH (approximately 250 mL), producing a white
polymer, and was stirred for several minutes. The polymer was collected by filtration and
washed successively with copious amounts of MeOH and D.I. H2O. The copolymer was dried
under vacuum at 50 °C overnight. Table 2.2.8.1 lists the experimental amounts of styrene, N-
vinylcarbazole, AIBN, DMF, yields, and temperatures for the PVK-co-PVDAT copolymers and
PVK polymerizations.
Table 2.2.8.1. Experimental amounts for the PS-co-PVK polymerizations.
The PMMA-co-PVI copolymer was synthesized according to the method of Chen and
Sun.49 Scheme 2.2.12.1 shows the free radical polymerization for the PMMA-co-PVI copolymer.
MMA and VI were purified by distillation under reduced pressure. DMSO was purified by
distillation under reduced pressure from CaH2. For the PMMA-co-PVI free radical
polymerization, the VI mol % concentration was 20 with MMA. In the synthesis for the 20 mol
% PMMA-co-PVI copolymer, 0.08 mols (8.01 g) of MMA and 0.02 mols (1.88 g) of VI were
added to a 250 mL three neck round bottom flask. The flask was placed in an oil bath on a hot
plate, fitted with a condenser containing 100 mL of freshly distilled DMSO under a nitrogen
atmosphere, and equipped with a stir bar and thermometer. Once the reaction temperature inside
the round bottom flask reached 60 °C, 2.0 mmols (0.33 g) of recrystallized AIBN was added to
the solution to initiate the free radical polymerization. The reaction temperature was increased to
approximately 70 °C and was stirred for five hours. Once the monomers and initiator dissolved
in DMSO, the reaction solution appeared colorless. During the reaction, the solution gradually
changed from colorless to a light orange color, indicating the reaction had come to completion.
The polymer solution was allowed to cool to room temperature and transferred to a 600 mL
beaker. The copolymer was precipitated in excess MeOH (approximately 300 mL), producing a
brittle white-orange polymer and was stirred for several minutes. The polymer was collected by
CH2 CH2
+ AIBN
DMSO, 70 °C
m n
N
N
N
N
61
filtration and washed successively with copious amounts of MeOH. The copolymer was dried
under vacuum at 50 °C overnight. The copolymerization produced a 64% yield (6.57 g).
CH2 CH2
+ AIBN
DMSO, 70 °C
m n
N
N
C
N
N
O
O
CH3
O
CH3
O
Scheme 2.2.12.1. Free radical polymerization for the synthesis of a PMMA-co-PVI random copolymer.
2.3 Co-Crystals with Nitroaromatics
2.3.1 General Co-Crystal Procedure with Nitroaromatics
1.0 mmol of the electron donor and electron acceptor were dissolved in separate test
tubes in the appropriate solvent by sonication or wrist action shaking. After the reagents
completely dissolved in the separate test tubes, they were transferred to a crystallization dish to
allow the solvent to evaporate at room temperature. After the solvent completely evaporated, the
crystals were collected from the crystallization dish. Table 2.3.1.1 lists the electron donors,
electron acceptors, and solvents used for growing co-crystals.
2.3.2 2,4-Diamino-6-methyl-1,3,5-triazine (MDAT) Co-Crystals with Nitroaromatics
MDAT co-crystals were attempted by the method of Xiao.50 5.0 mmols of MDAT and
5.0 mmols of a nitroaromatic reagent (2-NT, 3-NT, or 1,3-DNB) were added to a 250 mL three
neck round bottom flask in an oil bath on a hot plate. The flask was fitted with a condenser
containing 100 mL of EtOH and equipped with a stir bar and thermometer. The reaction
temperature was increased to approximately 50 °C and was stirred for three hours. During the
62
reaction, the nitroaromatic reagent dissolved, but MDAT was partially soluble in EtOH. After
three hours, the reaction solution was allowed to cool to room temperature and was then filtered.
The filtrate appeared colorless, but included small white particles. The filtrate was set aside for
one week and obtained white crystals with a faint yellow tint.
2.3.3 2-Vinyl-4,6-diamino-1,3,5-triazine (VDAT) Co-Crystals with 1,3-Dinitrobenzene (1,3-DNB)
1.0 mmol of VDAT (0.14 g) was dissolved in 5 mL of DMSO by gently heating in a test
tube. 1.0 mmol (0.17 g) of 1,3-DNB was dissolved in 15 mL of EtOH in a test tube by wrist
action shaking. The solutions were transferred to crystallization dish, allowing the formation of
crystals over six days. Light orange, needle like crystals formed and were removed from the
remaining DMSO.
2.3.4 9-Vinylcarbazole (9-VC) Co-Crystals with 1,3-Dinitrobenzene (1,3-DNB)
Three different ratios between 9-VC and 1,3-DNB were used to grow co-crystals. For the
1:1 ratio, 1.0 mmol (0.19 g) of 9-VC was dissolved in 10 mL of EtOH in a test tube by
sonication, producing a colorless solution. 1.0 mmol (0.17 g) of 1,3-DNB was dissolved in 10
mL of EtOH in a test tube by sonication, producing a light yellow color solution. When the two
solutions were combined in a crystallization dish, a bright yellow color solution rapidly formed.
The EtOH was allowed to evaporate at room temperature for two weeks. Yellow-orange clumps
formed in the crystallization dish after the EtOH evaporated.
For the 1:2 ratio, 1.0 mmol (0.19 g) of 9-VC was dissolved in 10 mL of EtOH in a test
tube by sonication, producing a colorless solution. 2.0 mmols (0.34 g) of 1,3-DNB was dissolved
in 15 mL of EtOH in a test tube by sonication, producing a light yellow colored solution. When
the two solutions were combined in a crystallization dish, a bright yellow color solution rapidly
63
formed. The EtOH was allowed to evaporate at room temperature for 3 weeks. Yellow-orange
needle like crystals formed in the crystallization dish after the EtOH evaporated.
For the 2:1 ratio, 2.0 mmols (0.38 g) of 9-VC was dissolved in 15 mL of EtOH in a test
tube by sonication, producing a colorless solution. 1.0 mmol (0.17 g) of 1,3-DNB was dissolved
in 10 mL of EtOH in a test tube by sonication, producing a light yellow color solution. When the
two solutions were combined in a crystallization dish, a bright yellow color solution rapidly
formed. The EtOH was allowed to evaporate at room temperature for four weeks. Yellow-orange
clumps formed in the crystallization after the EtOH evaporated.
64
Table 2.3.1.1. Co-crystals experimental reagents, solvents, and descriptions of crystals.
Electron Donor Electron Acceptor Solvent Description
Color Complex
Interaction 9-EC 2-NT EtOH White NO 9-EC 3-NT EtOH White NO 9-EC PNT EtOH White-Yellow NO 9-EC 1,3-DNB EtOH/Toluene Yellow-Orange YES 9-VC 2-NT EtOH White NO 9-VC NB EtOH White NO 9-VC 1,3-DNB EtOH/Toluene Yellow YES
Carbazole 2-NT EtOH Brown NO Carbazole NB EtOH Brown NO Carbazole 1,3-DNB EtOH/Toluene Brown YES
10-M 2-NT EtOH White NO 10-M 3-NT EtOH White NO 10-M PNT EtOH White NO 10-M 1,3-DNB EtOH Red-Purple YES
Phenothiazine 2-NT EtOH Brown NO Phenothiazine 3-NT EtOH Brown NO Phenothiazine PNT EtOH Brown NO Phenothiazine 1,3-DNB EtOH Brown YES
VDAT 2-NT H2O/EtOH (50:50) 65 °C White NO VDAT 3-NT H2O/EtOH (50:50) 65 °C White NO VDAT PNT H2O/EtOH (50:50) 65 °C White NO VDAT 1,3-DNB H2O/EtOH (50:50) 65 °C White NO MDAT 2-NT EtOH White NO MDAT 3-NT EtOH White NO MDAT PNT EtOH White NO MDAT 1,3-DNB EtOH White NO
Benzoguanamine 2-NT EtOH White NO Benzoguanamine 3-NT EtOH White NO Benzoguanamine PNT EtOH White NO Benzoguanamine 1,3-DNB EtOH White NO
2-VP 1,3-DNB EtOH White NO Vinylimidazole 1,3-DNB EtOH White NO
Acrylamide 1,3-DNB EtOH White NO
65
2.4 Instrumentation
FTIR spectra were recorded using a Jasco FT-IR 410 with the following parameters: 32
scans, 4 cm-1 resolution, % T, and a single background. 1 to 5 mg of a sample combined with
approximately 100 mg of dry KBr was ground into a fine powder. A pellet press was used to
produce a KBr pellet of the sample.
1H and 13C NMR spectra were recorded in CDCl3 or DMSO-d6 using a Bruker 500 or 360
MHz spectrometers. For recording 13C NMR spectra, the following parameters were used: D1
(relaxation delay) was set to zero, TD (time domain) was set to 16,000 points, and NS (number
of scans) was set to 60,000 scans. For processing the 13C NMR spectra, a line broadening value
of 20 was used as a smoothing function.
The glass transition temperatures (Tg) of the copolymers were collected by using a TA Q-
200 DSC. A small polymer sample was ground into a fine powder. 5 to 10 mg of the polymer
sample was heated with an initial heating ramp rate of 20 °C min-1 to the appropriate temperature
for DSC use and was held for three minutes. The polymer was then cooled to -40 °C with a
cooling ramp rate of 10 °C min-1, and was re-heated with a ramp rate of 10 °C min-1. The Tg was
determined as the inflection point between the upper and lower points of the heat capacity
transition.
Thermogravimetric analysis was performed with a TA 2950 TGA. A small polymer
sample was ground into a fine powder. The heating ramp rate was 5 °C min-1, with the
temperature being held at 75 °C for one hour in order to remove any residual water. The
temperature was increased to 600 °C with a heating ramp rate of 5 °C min-1. The decomposition
temperature (Td) was determined when 10% weight loss occurred.
66
Size exclusion chromatography (SEC) was performed on a Tosoh EcoSEC system with a
refractive index detector and a TSKgel Super HZ4000 column. PS-co-PVDAT copolymers were
dissolved in HPLC grade tetrahydrofuran (THF) at a concentration of 1.0 mg mL-1 and a flow
rate of 0.35 mL min-1 was utilized for the system. The injection port, column, and RI detector
were all set at 40 ˚C and the system was calibrated with polystyrene standards of narrow
polydispersity.
Silicon wafers were etched to produce 1 in. x 1 in. wafers for spin coating. The wafers'
surfaces were cleaned with D.I. H2O by spin coating and a small amount of acetone was applied
to a kimwipe to remove any dust from the etching process. The wafers' surfaces were dried using
filtered nitrogen. The wafers were stored in petri dishes.
Thin polymer films were spin coated using a Laurell (Model WS-400B-6NPP/LITE). The
polymer solutions were spin coated by two techniques: static and dynamic. The static spin
coating technique involved flooding the surface of the silicon wafer before spin coating. The
dynamic technique required the polymer solution to be dispensed during the spin coating process
at low speeds. A silicon wafer (1 in. x 1 in.) surface was flooded with a polymer solution. The
rotation speeds and time were varied in order to control film thicknesses. After spin coating, the
films were dried in an oven at 60 °C for 2 hours to remove any residual solvent.
Refractive Index measurements were performed on a J.A. Woollam Co. variable angle
spectroscopic (VASE) ellipsometer. Each of the films ψ and Δ were determined from 300 to
1000 nm at angles of 60°, 65°, 70°, 75°, and 80°. The data was fit and constrained to a Cauchy
Model with a polymer layer on a silicon wafer. An estimated SiO2 thickness of 20 Å was used
for the Cauchy model. The optical constants (n and k) were fitted to the Cauchy model. The
67
refractive index (n) and extinction coefficient (k) of the polymer films were recorded before and
after exposure to a concentrated nitroaromatic vapor.
The polymer films were exposed to concentrated nitroaromatic vapors using a simple
exposure apparatus. A large jar approximately 4 in. x 3 in. contained a large amount of a
nitroaromatic compound. A smaller jar approximately 2 in. x 2 in. was placed inverted inside the
larger jar with the nitroaromatic compound, so the bottom of the smaller jar could be used as a
sample holder allowing the film to rest above the nitroaromatic compound. Once the film was
placed inside the exposure apparatus and the lid tighten, the exposure time would be recorded.
After the film was exposed to a nitroaromatic vapor for a determined amount of time, the film
would then be removed and the refractive index would be measured using ellipsometry. The
exposure experiments were performed at temperatures between 22 - 25 °C.
A DekTak II A profilometer was used to measure film thickness to confirm the film
thickness measurements of the ellipsometer. A 1 mm scan was performed using a slow scan
method to measure the thickness of the films from an etched made into the films and observe
surface roughness.
A Siemens CCD Smart (Area Detector) and Enraf-Nonius CAD-4 computer controlled
X-ray diffractometer was used to measure the crystal structure. Steven Kelley determined the
crystal structure.
The melting points of the co-crystals with nitroaromatics were determined using an
Electrothermal IA9100. Approximately 2 mg of the co-crystals were ground into a fine powder
and transferred to a capillary tube. The instrument was dried by increasing the temperature to
300 °C with the lens removed to allow water vapor to escape the instrument. The instrument was
allowed to cool and equilibrate at 30 °C for 24 hours. After equilibrating for 24 hours, three
68
samples were placed in the capillary tube holder and heated at a ramp rate of 1 °C/min. The
melting point was determined from the first sign of liquid formation until the formation of a
meniscus after the sample completely melted.
A Shimadzu UV-3600 (UV/Vis - NIR) spectrophotometer was used to record the diffuse
reflectance spectra of the nitroaromatic co-crystals. A crystal sample of 200 mg was ground into
a fine powder and transferred to the holder with two Teflon spacers. A microscope slide was
used to make the sample flush with the top of the holder. The scan range for diffuse reflectance
measurements were from 900 - 200 nm by 0.5 nm.
A Cary 5G UV-Vis-NIR spectrophotometer was used to record the electronic absorption
spectra of the nitroaromatic co-crystals. Scan Mode was used to record the electronic absorption
spectra with the following parameters: Ave. time (s) - 0.100, Data interval (nm) - 1.00, Scan rate
Figure 3.10.3. The 13C NMR spectrum for the PVI-co-PVDAT copolymer recorded in DMSO-d6 (500 MHz).
was not observed due to the DMSO-d6 solvent signal overlapping with the methylene carbon
signal. The PVDAT aromatic carbon signals were observed at 166.91 (C4) and 178.82 (C5). The
observed PVDAT peaks indicated the copolymer was synthesized.
DSC experiments characterized the values of Tg for the homopolymer and copolymer.
The DSC curve for PVI (Appendix Figure 55) showed a Tg at 159 °C. The reported Tg value for
the PVI homopolymer is 182 °C.70 The synthesized homopolymer's Tg was lower than the
reported literature Tg. This decrease in Tg was attributed to impurities within the polymer matrix.
The copolymer Tg was not observed below 250 °C. This result suggested that the copolymer has
5060708090100110120130140150160170180ppm
119
a high thermal stability temperature. Introducing VDAT into the copolymer increased the Tg
significantly, which was attributed to an increase in hydrogen bonding and dipole - dipole
interactions, limiting chain mobility and resulting in a higher Tg.
3.11 PS-co-PVI Copolymer Characterization
The FTIR spectra for the PVI homopolymer and the PS-co-PVI 20 mol % vinylimidazole
copolymer were recorded in KBr shown in Figure 3.11.1. The copolymer spectrum did not
contain any strong PVI vibrational modes, which suggested that the copolymer was not
synthesized. The polystyrene vibrational modes were observed in the copolymer spectrum. The
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
PVI
PS-co-PVI
Figure 3.11.1. The FTIR spectra for the PVI homopolymer (red curve) and the PS-co-PVI 20 mol % copolymer (black curve) recorded in KBr pellets at room temperature.
120
polystyrene C=C overtones were located between 1937 - 1637 cm-1. The polystyrene C=C
aromatic vibrational modes were observed at 1600 and 1492 cm-1.
To confirm synthesis of the copolymer, 1H and 13C NMR experiments were performed to
characterize the copolymer's structure. The 1H NMR spectrum for the PS-co-PVI 20 mol %
vinylimidazole copolymer is shown in Figure 3.11.2 and compared to the polystyrene 1H NMR
spectrum. The two spectra were identical; suggesting the copolymer between styrene and
vinylimidazole was not synthesized. The 13C NMR spectra (Figure 3.11.3) also confirmed that
the copolymer was not synthesized. The polystyrene aromatic carbon peaks were observed
between 145.00 - 115.00 ppm and the polymer backbone carbon peaks (methine and methylene
carbons) were observed between 30.00 - 20.00 ppm. None of the PVI carbon signals were
observed in the copolymer's 13C NMR spectrum. A possible explanation for the un-synthesized
copolymer may be attributed the N-vinyl monomer. The N-vinylimidazole monomer produces a
highly reactive radical due to a lack of resonance stabilization in the propagation step of the
polymerization.71 The reactive propagating radical increases the possibility for chain transfer and
chain termination events, resulting in polymers with low molecular weights. In the literature, N-
vinylimidazole copolymers with styrene were synthesized by a controlled free radical
polymerizations.70, 71
121
Figure 3.11.2. The 1H NMR spectra for the PS-co-PVI copolymer and polystyrene homopolymer recorded in CDCl3 (360 MHz).
PVI methine, the PVI methylene, the PMMA OCH3, the PMMA quaternary carbon, and the
PMMA methylene carbon. The carbon signal located at 51.52 ppm corresponded to the PMMA
OCH3, overlapping with the PVI methine and PMMA methylene carbon signals. The PMMA
quaternary carbon signal was observed at 44.46 - 43.87 ppm, overlapping with the PVI
methylene carbon signals. The PMMA α-CH3 carbon signals were observed between 20.56 -
16.11 ppm.
Figure 3.12.3. The 13C NMR spectrum for the PMMA-co-PVI copolymer containing 20 mol % VI recorded in DMSO-d6 (500 MHz).
102030405060708090110130150170ppm
126
The DSC curves from 80 °C to 160 °C for the PMMA-co-PVI copolymer and PMMA
homopolymer are shown in Figure 3.12.4. The observed glass transition temperatures for the
copolymer and homopolymer were 132 °C (PMMA-co-PVI) and 103 °C (PMMA). The
copolymer exhibited a sharp endotherm transition compared to the homopolymer's gradual
endotherm transition. Incorporating PVI into the copolymer increased the Tg, compared to the
PMMA's Tg. This increase in Tg was attributed to an increase in hydrogen bonding, dipole -
dipole interactions, or the bulky vinylimidazole units limiting the polymer's chains mobility.
80 90 100 110 120 130 140 150
Hea
t Flo
w (W
/g)
E
ndot
herm
Temperature (οC)
PMMA-co-PVI
PMMA
Figure 3.12.4. The DSC curves shown from 80 °C to 160 °C for the PMMA-co-PVI 20 mol % VI copolymer and PMMA homopolymer.
127
Chapter 4
Polymer Thin Films Characterization by Variable Angle Spectroscopic Ellipsometry after Exposure to a Nitroaromatic Vapor In this chapter, a brief introduction to the fundamentals of ellipsometry will be discussed,
and the thin films' optical constants measured by ellipsometry will be presented. The first section
will discuss how ellipsometry is used to characterize the thickness and optical properties of thin
films. The next section will discuss the Cauchy model and data analysis. Lastly, the extracted
optical constants and thicknesses of the thin films after exposure to a nitroaromatic vapor will be
shown, providing supporting evidence that these films have the potential to be applied in the
development of a waveguide explosive sensor.
The polymers synthesized in Chapter 2 were used to make polymer solutions for spin
coating. Some of the polymers were not suitable for spin coating due to either insolubility in an
appropriate spin coating solvent or produced heterogeneous films (haziness or phase separation).
The data presented in this chapter is from the polymers that were used in spin coating of
homogeneous films without defects.
4.1 Ellipsometry Overview
Ellipsometry is a non-destructive technique that measures the change in the state of
polarization as light is reflected from the surface of a material.73 Ellipsometry is primarily used
to determine a material's thickness and optical constants, but also offers the ability to extract
128
properties such as composition, crystallinity, surface roughness, and other factors that are
dependent on the material's optical properties.
Ellipsometry utilizes elliptical polarized light, hence the technique's name. Light is an
electromagnetic wave traveling through space, consisting of an electric field vector and a
magnetic field vector. These vectors are mutually perpendicular to each other and perpendicular
to the propagation direction, allowing the wave to be described by its x and y components
traveling along the z-axis. Ellipsometry is concerned with the electric field vector (polarized
light). Elliptically polarized light is produced when two linearly polarized waves with the same
frequency are combined out of phase.73 If viewed (end-on) of the z-axis, the tips of the arrows
would appear to be moving on an ellipse shown in Figure 4.1.1.
Figure 4.1.1. Two linearly polarized waves combined out of phase producing elliptically polarized light. Modified from http://www.jawoollam.com/tutorial_2.html (accessed Feb. 15, 2013).
When an electromagnetic wave arrives at the interface between the air and film, the wave
can begin to slow, change direction, or be transmitted into the material (Figure 4.1.2). Not all of
the light enters the material, but some is reflected at the interface back into the air. The reflected
light from the material's surface allows the optical properties (n and k) to be characterized by the
complex index of refraction (Eq. 1), which describes how the light interacts with the material.
129
Figure 4.1.2. Light reflecting and refracting at the interface between air and the surface of a material. Modified from http://www.jawoollam.com/tutorial_3.html (accessed Feb. 15, 2013).
The complex index of refraction can be described by a real and an imaginary number:
(Equation 1)
where n is the index of refraction, k is the extinction coefficient, and j is the √ 1. The index of
refraction (n) describes the inverse measure of the phase of velocity for light as it enters a
dielectric material compared to the speed of light expressed as:
(Equation 2)
where c is the speed of light and ʋ is the phase velocity. The extinction coefficient (k) related to
the absorption coefficient (α) describes the light's loss of intensity as it travels through the
material expressed as:
(Equation 3)
(Equation 4)
where d is the distance traveled into the material.
The incident light is reflected or transmitted at the air/material interface (Figure 4.1.2). It
is known that the angle between the incidence light and the material ( ) is equal to the angle of
130
reflection ( ). The refracted angle ( ) as light is transmitted into a dielectric material (k = 0)
can be described by Snell's law where all terms are real numbers:
(Equation 5)
As previously mentioned, ellipsometry measures the change in the state of polarization
as light is reflected or transmitted at the air/surface interface. The electric field vectors of linear
polarized light are projected in two orthogonal components shown in Figure 4.1.3. The electric
field vector parallel to the plane of incidence is referred to as Ep and the electric field vector that
is perpendicular to the plane of incidence as referred to as Es. Both the Ep and Es are independent
components and can interact differently when reflected from the material's surface. The Fresnel
reflection coefficients describe the ratio of the amplitude of the incidence wave compared to the
reflected wave denoted as rs (perpendicular wave to the plane of incidence) and rp (parallel wave
to the plane of incidence). The Fresnel reflection and transmission coefficients provide
information about the phase and amplitude ratio between the p-wave and s-wave given by:
(Equation 6.1)
(Equation 6.2)
(Equation 6.3)
(Equation 6.4)
131
Figure 4.1.3. Schematic representation for a typical ellipsometry measurement showing a polarization state change when linearly polarized light is reflected from a sample's surface. Modified from http://www.jawoollam.com/tutorial_4.html (accessed Feb. 15, 2013).
When a film is present on a substrate, the incident light will be reflected or transmitted at
the film/air interface. The resulting transmitted wave will propagate through the film, producing
multiple reflections and transmissions between the film/air interface and the film/substrate
interface, shown in Figure 4.1.4. The presence of multiple waves in the film introduces
interference, which is dependent on the amplitude and phase of the electric fields.
Figure 4.1.4. Schematic representation of a wave propagating through a film, producing multiple reflections and transmissions.74
132
From the total reflection coefficients comparable to the Fresnel reflection coefficients, the phase
change in the wave as it propagates from the top to bottom through the film can be determined.
The film thickness can be determined by , film phase thickness, expressed as:
(Equation 7)
The p-waves and s-waves are not always in phase; after a reflection, there is a possibility of a
phase shift being produced, which can be different for both waves. The phase difference between
the p-wave and s-wave before the reflection and after the reflection can be described by the
parameter Δ, given by:
(Equation 8)
where is the phase difference before the reflection and is the phase difference after the
reflection. Similar to the phase shift, after the reflection a reduction in amplitude can be induced
for the p-wave and s-wave and the change in amplitude may not be the same for both waves. The
total reflection coefficients (the ratio of the amplitude for the reflected wave to the incidence
wave) for the p-wave (RP) and s-wave (RS) contain the magnitudes of the amplitude changes.
The tan Ψ is defined as the ratio of the magnitudes of the total reflection coefficients and is a real
number given by:
(Equation 9)
where Ψ is the angle whose tangent is the ratio of the magnitudes of the total reflection
coefficients.73 The complex number is defined as the complex ratio of the total reflection
coefficients that describe the change in polarization between the p-wave and s-wave expressed
as:
(Equation 10)
133
Ellipsometry addresses the phase and amplitude ratio between the p-wave and s-wave, which are
independent of each other. Using the fundamental equation of ellipsometry allows the two
independent parameters to be determined:
· (Equation 11)
where and Δ are measured quantities which characterize polarization effects of the surface
after the incident light has undergone a phase and amplitude change for the p-wave and s-wave.
These measured quantities are dependent on the wavelength, angle of incidence, optical
constants, and film morphology.
4.2 Data Analysis
Ellipsometry is able to determine the optical constants and film thickness by directly
measuring Δ and Ψ. After measuring ∆ and Ψ, a model is constructed to describe the sample's
response. The model includes layers with each optical constants and thickness of each layer
defined. If the optical constants and thickness are not known, an approximate value is given to
allow preliminary data calculations. The model and the Fresnel's equations provide the predicted
calculated response for ∆ and Ψ. The predicted values of ∆ and Ψ are then compared to the
experimental values of ∆ and Ψ. The optical properties and thickness of the unknown layer are
varied until the generated values of ∆ and Ψ are close to the experimental values. A fitting step is
performed to find the best fit between the generated data and experimental data. The fit between
the generated data and experimental data is evaluated by the mean square error (MSE). The MSE
quantifies the difference between the data curves, allowing parameters for the unknown material
layer to be adjusted until a minimum MSE is reached. When fitting the experimental and
generated data, the best fit is the assessment with the lowest value of MSE.
134
4.3 Cauchy Model
For many materials, there are electronic absorptions deep in the UV region of the
spectrum. This would lend to large values of k in this region. In the near infrared and through the
visible into the near UV there are no electronic absorptions and the value of k is zero. Through
the region from the near infrared to the near UV, the refractive index increases as the wavelength
decreases. This is optical dispersion. As the wavelength decreases into the UV region and
approaches the region where there are strong electronic absorptions, the refractive index
increases greatly. This is called anomalous dispersion.75
The Cauchy Dispersion model is an empirical model that is capable of describing the
wavelength dependence index of refraction for dielectric materials with little or no optical
absorption.76 The Cauchy model describes the relationship between the index of refraction and
wavelength given by:
(Equation 12)
where is the index of refraction, is the wavelength, and A, B, and C are Cauchy
parameters. The three parameters describe the index of refraction over a range of wavelengths.
The Cauchy model typically shows a decrease in refractive indices (n) as the wavelength
increases.
4.4 PS-co-PVDAT Films
Before spin coating thin films of the different copolymers, the copolymers solubility in an
ideal spin coating solvent (boiling point (b.p.) ≈ 100 to 130 °C) was determined. The ideal spin
coating solvent should possess suitable substrate wetting abilities, a b.p. that allows film
formation and does not quench the film in place (producing phase separation or haziness), and
also allows the films to be dried, removing the solvent leaving only the film. The lower VDAT
135
mol % copolymers (1 and 5 mol % VDAT) were soluble in toluene (b.p. 110 °C54), similar to
polystyrene. The 10 and 20 mol % VDAT copolymers were insoluble in toluene, but were
soluble in other polar organic solvents such as MEK (80 °C54), THF (66 °C54), DMF (153 °C54),
DMSO (192 °C54), pyridine (115 °C54), and 1,4-dioxane (101 °C54). The difference in solubility
between the lower mol % VDAT copolymers and higher VDAT mol % copolymers was
attributed to an increase in non-covalent interactions (hydrogen bonding and dipole - dipole
interactions). As the VDAT concentration increased, the solvents required a higher polar
solubility parameter. Copolymers containing concentrations greater than 20 mol % VDAT would
not likely be soluble in an appropriate spin coating solvents due to the insolubility of PVDAT.
To lower the b.p. of the higher b.p. polar organic solvents, attempts were made to include
lower b.p. solvents, which were miscible with the polar organic solvents such as EtOH, MeOH,
toluene, H2O, THF, or isopropanol. The amounts of the lower b.p. solvents were adjusted to a
maximum concentration in the higher b.p. solvents so that the copolymers remained in the
solution phase and did not precipitate. To increase the b.p. of the lower b.p. solvents, a similar
approach was used to include higher b.p. solvents. The adjusted b.p. attempts resulted in
heterogeneous films, which were not ideal for ellipsometry characterization.
Attempts were made to spin coat films of the PS-co-PVDAT 20 mol % VDAT copolymer
using THF, DMF, and 1,4-dioxane. The films spin coated from THF and DMF were very thin (≤
10 nm) and exhibited extreme surface roughness. It appeared that the solvents did not produce
viscous solutions, therefore creating thin films. Viscosity is a critical factor for spin coating thick
films. In addition, it was assumed that it would not be possible to remove DMF from the polymer
films due to the solvent's high b.p. and hygroscopic nature. The presence of DMF in the polymer
films may be potentially beneficial, acting as a plasticizing agent, allowing the films to become
136
more porous and polymer chains to be more mobile. 1,4-dioxane with low heat was capable of
dissolving the 20 mol % copolymer. After twenty-four hours, the 20 mol % VDAT copolymer
began to precipitate out of the 1,4-dioxane solution, but introducing heat allowed the copolymer
to dissolve back into the solution. Over time, the copolymer would precipitate out of 1,4-dioxane
and would not dissolve in the solvent.
The PS-co-PVDAT 10 mol % VDAT copolymer was soluble in MEK, 1,4-dioxane, THF,
and DMF. Similar to the 20 mol % VDAT copolymer, THF and DMF produced thin films with
rough surfaces. MEK produced films with transparent centers and haziness near the edges of the
silicon wafer. 1,4-dioxane produced homogenous films with no defects. The copolymer
precipitated out of the solvent over time.
For the PS-co-PVDAT copolymers, different film thicknesses were spin coated by
adjusting spin coating speeds and polymer solution concentrations. 0.3%, 1%, and 3% (w.t.)
copolymer solutions were prepared for spin coating, allowing variances in film thickness by
concentration. The 3% (w.t.) solutions produced films with a blue tint from the reflecting light,
indicating thick polymer films. The 1% and 0.3% (w.t.) solutions produced transparent films,
indicating thin copolymer films. The copolymer solutions were spin coated by both the static and
dynamic techniques. The static technique produced homogenous films with minimal surface
roughness, compared to the dynamic technique that produced homogeneous films with rough
surfaces. After determining appropriate spin coating parameters and concentrations for
producing quality films, the copolymer films were characterized by ellipsometry to determine
film thickness and optical constants (n and k) before and after exposure to a concentrated
nitroaromatic vapor.
137
The expected refractive index for the PS-co-PVDAT films were assumed to be similar to
the refractive index of polystyrene (n = 1.55 - 1.5977). The refractive index of PVDAT is not
known, but was estimated to possess a refractive index similar or higher than polystyrene. The
films were exposed to the concentrated high refractive index nitroaromatic vapors of PNT (n =
1.538), NB (n = 1.55654), and 1,3-DNB (n = 1.612) for a determined amount of time. The change
in film refractive index was attributed to the nitroaromatic vapor molecules with a higher
refractive index interacting with the PVDAT, by hydrogen bonding or electro-static interactions.
Figures 4.4.1 - 4.4.5 show a plot of the refractive index (n) as a function of wavelength for
polystyrene and the PS-co-PVDAT copolymers exposed to a concentrated nitroaromatic vapor.
Tables 4.4.1 - 4.4.5 provide the copolymers' Cauchy parameters fitted for the experimental data.
The ellipsometry curves shown in Figures 4.4.1 - 4.4.5 display expected refractive indices
for the Cauchy dispersion model. A decrease in refractive indices was observed as the
wavelength increased from the UV, through the visible, and into the near infrared. All of the
curves showed reasonable refractive indices that would be expected for polystyrene before
exposure to the nitroaromatic vapor. The extinction coefficient (k), describing the films' optical
absorption property, showed very little optical absorption (primarily in the UV region of the
spectrum from 380 - 300 nm) and did not affect the fit between the experimental and generated
data. The features observed between 700 - 1,000 nm were not expected, since the curves
typically lay flat in this region of the spectrum, due to no optical absorption. It was hypothesized
that these features may be attributed to surface roughness or a film defect, scattering or reflecting
the polarized light in a different manner than that of the bulk of the film. There was an observed
difference between the thicknesses measured by the profilometer and thicknesses determined by
the Cauchy model. The films' thicknesses measured by the profilometer were between 3 - 6 nm
138
less than the film thicknesses determined by the Cauchy model. These differences in thickness
occurred because of the etching process. It was likely that the etch made in the films did not
penetrate through the entire film to the substrate's surface. This would explain the slight
differences between the two measurements. The profilometer measurements were used to
confirm the ellipsometer's film thicknesses measurements, which were accurate. The low MSE
values for the spectra represented an effective comparison between the generated and
experimental data. The changes in refractive index for these spectra may not appear significant,
but MZI have shown the capability of detecting small changes in refractive index (10-6).
The refractive index curves for the polystyrene film exposed to PNT did not exhibit a
significant change in refractive index after exposure to the nitroaromatic. The PS-co-PVDAT
copolymers refractive index curves showed changes in the refractive index curves after the films
were exposed to a nitroaromatic vapor. This change in refractive index described the copolymer
films' affinity toward the nitroaromatics. The addition of PVDAT allowed nitroaromatic
molecules to form molecular complexes with the electron rich VDAT aromatic structure or the
nitro groups to form hydrogen bonds with the PVDAT amine groups.
The films exposed to concentrated nitroaromatic vapors showed varied results producing
large, minimal, or no change in refractive index. These changes may be attributed to the ability
of the nitroaromatic vapor molecules to enter the porous films and interact with PVDAT.
139
Figure 4.4.1. The before and after refractive index curves for a polystyrene film spin coated from a 3% (w.t.) toluene solution exposed to PNT for ten seconds.
1.54
1.56
1.58
1.60
1.62
1.64
1.66
1.68
1.70
1.72
1.74
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
140
Table 4.4.1. The Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a polystyrene film exposed to PNT vapors for ten seconds.
Polymer: PS, 3% (w.t.) Toluene
Nitroaromatic: PNT 10 sec. exposure
Before After
MSE 5.233 5.327
Thickness (Å) 813.3 ± 0.7 813.3 ± 0.7
A 1.567 ± 2.31 E-3 1.566 ± 2.37 E-3
B 4.359 E-3 ± 8.23 E-4 4.33 E-3 ± 8.35 E-4
C 7.621 E-4 ± 7.61 E-5 7.741 E-4 ± 7.66 E-5
Δn 0.0006
Optical Constants MSE 4.586 4.725
Dektak (Å) 780
Spin Coating 5,000 rpm for 40 sec.
141
Figure 4.4.2. The change in refractive index for a PS-co-PVDAT 20 mol % VDAT copolymer film spin coated from a 1% (w.t.) 1,4-dioxane solution exposed to NB for five seconds.
1.5
1.52
1.54
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
142
Table 4.4.2. The ellipsometry MSE, film thickness, refractive index, average change in refractive index (Δn), optical constants MSE, profilometer thickness, and spin coating parameters for a PS-co-PVDAT 20 mol % VDAT copolymer film produced from a 1% (w.t.) 1,4-dioxane solution.
Figure 4.4.3. The change in refractive index for a PS-co-PVDAT 10 mol % VDAT copolymer film produced from a 1% (w.t.) MEK solution exposed to NB for five seconds.
1.54
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
1.72
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
144
Table 4.4.3. The ellipsometry Cauchy model MSE, thickness, refractive index, and optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 10 mol % VDAT copolymer film.
Polymer: PS-co-PVDAT 10 mol % VDAT, 1% (w.t.) MEK
Nitroaromatic: NB 5 sec. exposure
Before After
MSE 2.122 3.035
Thickness (Å) 508.7 ± 0.3 511.3 ± 0.3
A 1.555 ± 1.49 E-3 1.558 ± 2.09 E-3
Δn 0.003
Optical Constants MSE 2.106 2.962
Dektak (Å) 454
Spin Coating 3,000 rpm for 40 sec.
145
Figure 4.4.4. The change in refractive index for a PS-co-PVDAT 5 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for sixty seconds.
1.54
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
1.72
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
146
Table 4.4.4. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 5 mol % VDAT copolymer film.
Figure 4.4.5. The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to NB for five seconds.
1.54
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
1.72
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
148
Table 4.4.5. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film.
Time exposure experiments were performed to determine the maximum concentration of
a nitroaromatic vapor that could be absorbed by the VDAT copolymer films to produce a
significant change in refractive index. PS-co-PVDAT 1 mol % VDAT copolymer films were
spin coated and exposed to PNT for five, twenty, and forty seconds to determine the maximum
change in refractive index over an extended exposure time. The ellipsometry curves shown in
Figures 4.4.6 - 4.4.8 show the change in refractive indices as a function of wavelength as the
exposure time to PNT was increased. Tables 4.4.6 - 4.4.8 provide the Cauchy parameters
determined before and after exposure to the PNT vapor.
The spectra's ranges were reduced to the region between 400 - 1,000 nm in order to
eliminate any optical absorption, which may have occurred in the UV region of the spectrum.
The ellipsometry curves displayed expected Cauchy dispersion model curves showing decreases
in refractive indices as the wavelength increased through the visible to the near infrared. The
refractive index curves before exposure to PNT were consistent with the refractive index for
polystyrene. All of the spectra displayed features in the region between 700 - 1,000 nm,
consistent with the previously shown spectra. The PS-co-PVDAT 1 mol % VDAT copolymer
film exposed to PNT for five seconds produced an average change in refractive index (Δn) of
0.002. When the exposure time was increased to twenty seconds, the average change in
refractive index (Δn) increased to 0.012. After exposure to PNT for forty seconds, there was no
observed increase in the average change in refractive index, suggesting the copolymer was
saturated with PNT.
The differences in film thickness measurements between the Cauchy model and
profilometer varied from 6 - 8 nm. These results were consistent with previously results and
confirmed the approximate thicknesses determined by the ellipsometer. The spin coated
150
copolymer film exposed to PNT for five seconds was approximately 12 nm thinner than the other
two copolymer films. The difference in film thickness could not be justified, but the profilometer
did confirm the film's approximate thickness.
Figure 4.4.6. The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for five seconds.
1.55
1.56
1.57
1.58
1.59
1.6
1.61
1.62
1.63
400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
151
Table 4.4.6. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for five seconds.
Figure 4.4.7. The change in refractive index for a PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for twenty seconds.
1.55
1.56
1.57
1.58
1.59
1.6
1.61
1.62
400 500 600 700 800 900 1000
Ref
ract
ive
Inde
x (n
)
Wavelength (nm)
Before
After
153
Table 4.4.7. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for the PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for twenty seconds.
Figure 4.4.8. The change in refractive index for the PS-co-PVDAT 1 mol % VDAT copolymer film produced from a 3% (w.t.) toluene solution exposed to PNT for forty seconds.
1.55
1.56
1.57
1.58
1.59
1.6
1.61
1.62
400 500 600 700 800 900 1000
Ref
ract
ive
Inde
x (n
)
Wavelength (nm)
Before
After
155
Table 4.4.8. The ellipsometry Cauchy model MSE, thickness, refractive index, optical constants MSE, profilometer measured thickness, and spin coating parameters for a PS-co-PVDAT 1 mol % VDAT copolymer film exposed to PNT for forty seconds.
Similar to the PS-co-PVDAT copolymers, the solubility of the PMMA-co-PVDAT
copolymers in spin coating solvents was determined. The PMMA-co-PVDAT copolymers
exhibited the same trend as the PS-co-PVDAT copolymers. As the VDAT mol % increased in
the copolymers, the copolymers required more polar organic solvents. The lower mol % PMMA-
co-PVDAT copolymers were soluble in toluene and MEK, and produced homogeneous films
with no visible defects. Toluene was used as the spin coating due to its ideal b.p. The 10 and 20
mol % copolymers were soluble in polar organic solvents such as DMF, DMSO, and 1,4-
dioxane. 1,4-dioxane was used to prepare spin coated thin films. All of the polymer solutions
were filtered to remove any particles present in the polymer solutions. The polymer solutions'
concentrations and spin coating speeds were varied, allowing a variety of film thicknesses to be
prepared. After the films were spin coated, the polymer thin films were dried in an oven at 60 °C
for two hours to remove any residual solvent.
Initial attempts were performed to expose the PMMA-co-PVDAT copolymers thin films
to nitroaromatic vapors for short amounts of time (five, ten, twenty, and thirty seconds). The
films did not always produce changes in refractive indices after exposure to the nitroaromatic
vapors. In order to produce refractive index changes, the exposure times were increased
significantly to determine whether the films had an affinity for the nitroaromatic vapors.
Polymer films spin coated from the 20 and 10 mol % copolymers were exposed to PNT,
NB, and 1,3-DNB for two minutes. There was no observed change in the refractive indices after
exposure to the nitroaromatics. Significant surface roughness was observed for the 3% and 1%
(w.t.) polymer solutions confirmed by the profilometer. The surface roughness affected the
157
refractive index measurements, which produced curved features in the ellipsometry spectra not
representative of the polymer films' refractive indices.
Polymer films prepared from the 1 mol % and 5 mol % copolymers solutions allowed
spin coated films with less surface roughness that allowed refractive index measurements
representative of the copolymer thin films. Since no change in refractive index for the 10 and 20
mol % copolymer films were observed when exposed to a nitroaromatic for two minutes, the
exposure time was increased to determine whether a change in refractive index would occur.
Spin coated films from the 1 mol % and 5 mol % copolymers were exposed to NB, PNT, and
1,3-DNB for several minutes. Figures 4.5.1 - 4.5.4 show the ellipsometry curves plotted as the
wavelength dependence of the refractive index (n) for the copolymer films. Tables 4.5.1 - 4.5.4
list the Cauchy parameters, profilometer measured thicknesses, average change in refractive
index, exposure times, and spin coating parameters for the copolymer films.
The refractive indices observed for the ellipsometry curves for the PMMA, 1 mol %, and
5 mol % copolymer films were in agreement with the reported PMMA refractive index in the
literature (n = 1.491477). The ellipsometry curves displayed ideal refractive indices for the
Cauchy dispersion model (refractive index decreased as wavelength increased). The features
observed in the ellipsometry curves were attributed to the copolymer films' surface roughness,
reflecting light in a different manner compared to the bulk of the film. Surface roughness was not
accounted for during the refractive indices measurements. The PMMA film exposed to 1,3-DNB
for ten seconds showed no change in refractive index after exposure to the nitroaromatic vapor.
However, the PMMA-co-PVDAT copolymer films produced a change in refractive index after
exposure to a nitroaromatic vapor, similar to the PS-co-PVDAT. Again, the addition of PVDAT
in the copolymer films showed an affinity for nitroaromatics vapor molecules. It was observed
158
that the copolymer films exposed to NB produced larger refractive index changes compared to
the films exposed to 1,3-DNB. These results correlated to the vapor pressures of the
nitroaromatics. 1,3-DNB has a low vapor pressure (0.027 Pa at 20 °C) compared to the vapor
pressure of NB (24 Pa at 20 °C).78 The higher vapor pressure of NB allowed the vapor phase
molecules to enter the amorphous films more readily and interact with the PVDAT units, which
produced a large change in refractive index. There were observed differences for the film
thickness measurements between the Cauchy model and profilometer. These small differences
between the profilometer and Cauchy model were negligible, but did confirm the approximate
film thicknesses. The PMMA-co-PVDAT films demonstrated mixed results after being exposed
to a nitroaromatic vapor by producing small, large, or no change in refractive index. These mixed
results may be linked to the ability of the nitroaromatic vapor molecules to enter the amorphous
film and interact with the PVDAT moieties giving rise to a change in refractive index.
159
Figure 4.5.1. The before and after refractive index curves for a PMMA film spin coated from a 3% (w.t.) toluene solution exposed to 1,3-DNB for ten seconds.
1.46
1.47
1.48
1.49
1.5
1.51
1.52
1.53
1.54
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
160
Table 4.5.1. The before and after Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a PMMA film exposed to 1,3-DNB for ten seconds.
Polymer: PMMA, 3% (w.t.) Toluene
Nitroaromatic: 1,3-DNB 10 sec. exposure
Before After
MSE 3.143 3.098
Thickness (Å) 1,205.2 ± 0.4 1,205.5 ± 0.4
A 1.474 ± 7.60 E-4 1.474 ± 7.51 E-4
B 3.414 E-3 ± 3.2 E-4 3.13 E-3 ± 3.21 E-4
C 1.944 E-4 ± 3.12 E-5 2.212 E-4 ± 3.17 E-5
Δn 7.5 E-5
Optical Constants MSE 3.03 2.91
Dektak (Å) 1,192
Spin Coating 5,000 rpm for 40 sec.
161
Figure 4.5.2. The refractive index curves for a PMMA-co-PVDAT 1 mol % VDAT copolymer thin film spin coated from a 3% (w.t.) toluene polymer solution exposed to 1,3-DNB for sixteen minutes.
1.46
1.47
1.48
1.49
1.5
1.51
1.52
1.53
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
162
Table 4.5.2. The ellipsometry Cauchy model parameters, profilometer thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co-PVDAT 1 mol % VDAT copolymer film exposed to 1,3-DNB for sixteen minutes.
Figure 4.5.3. The refractive index curves for a PMMA-co-PVDAT 1 mol % VDAT copolymer thin film spin coated from a 3% (w.t.) toluene solution exposed to NB for twenty-five minutes.
1.46
1.465
1.47
1.475
1.48
1.485
1.49
1.495
1.5
1.505
500 550 600 650 700 750 800 850 900 950 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
164
Table 4.5.3. The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co-PVDAT 1 mol % VDAT copolymer film exposed to NB for twenty-five minutes.
Figure 4.5.4. The refractive index curves for a PMMA-co-PVDAT 5 mol % VDAT copolymer film spin coated from a 1% (w.t.) toluene solution before and after exposure to NB for twenty-five minutes.
1.4
1.42
1.44
1.46
1.48
1.5
1.52
1.54
1.56
1.58
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
166
Table 4.5.4. The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co-PVDAT 5 mol % VDAT copolymer film exposed to NB for twenty-five minutes.
Figure 4.5.5. The ellipsometry curves for a PMMA-co-PVDAT 5 mol % VDAT copolymer film spin coated from a 1% (w.t.) toluene solution before and after exposure to 1,3-DNB for twenty-five minutes.
1.4
1.41
1.42
1.43
1.44
1.45
1.46
1.47
1.48
400 500 600 700 800 900 1000
Ref
ract
ive I
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(n)
Wavelength (nm)
Before
After
168
Table 4.5.5. The ellipsometry Cauchy parameters, profilometer measured thickness, average change in refractive index (Δn), and spin coating parameters for a PMMA-co-PVDAT 5 mol % VDAT copolymer film exposed to 1,3-DNB for twenty-five minutes.
The P2VP-co-PVDAT copolymers did not produce homogeneous films suitable for
ellipsometry characterization. However, the P2VP homopolymer did produce a homogeneous
film that was characterized by the spectroscopic ellipsometer to determine the film's optical
constants before and after exposure to a nitroaromatic vapor (Figure 4.6.1). Table 4.6.1 lists the
Cauchy parameters, average change in refractive index, and spin coating parameters. A 3% (w.t.)
toluene P2VP polymer solution was spin coated on a silicon wafer and exposed to PNT for five
seconds. The refractive index for the P2VP film before exposure was observed to be A ≈ 1.337
with a film thickness of ≈15 nm.* After the five-second exposure to PNT, the refractive index
increased to A ≈ 1.353 with a film thickness of ≈15 nm. The MSE values for the Cauchy
parameters and optical constants indicated a reasonable fit between the experimental and
generated data for the P2VP film. Exposure to the PNT vapor produced an average change in
refractive index (Δn) of 0.019 for the ellipsometry curves. The change in refractive index for the
P2VP film occurred due to the 2-vinylpyridine moieties hydrogen bonded with PNT nitro group.
Saloni et al. performed theoretical calculations for monomers incorporated in imprinted
polymers which described their ability to imprint with TNT in different solvents.79 2-
vinylpyridine theoretically was able to hydrogen bond to the TNT nitro functional groups for an
imprinted polymer. The features observed in the ellipsometry curves indicated a polymer film
with surface roughness. The film thickness was not measured by the profilometer to confirm the
ellipsometer's measured thickness. The change in refractive index for the P2VP film exposed to
* Since the refractive index for each film varied as a function of wavelength, it was decided to use the Cauchy parameter (A). The Cauchy parameter (A) is the refractive index at very long wavelengths when the refractive index does not change with wavelength. By using the parameter (A), we could compare the values for different films without the concern of the effect of dispersion.
170
PNT was greater than the required (Δn) of 0.003 to detect a change in refractive index for the
proposed MZI sensor.
Figure 4.6.1. The before and after ellipsometry curves for a P2VP polymer film spin coated from a 3% (w.t.) toluene solution exposed to PNT for five seconds.
171
Table 4.6.1. The Cauchy parameters, average change in refractive index, and spin coating parameters for a spin coated P2VP film exposed to PNT for five seconds.
Polymer: P2VP, 3% (w.t.) Toluene
Nitroaromatic: PNT 5 sec. exposure
Before After
MSE 3.507 3.235
Thickness (Å) 150 ± 2 146 ± 2
A 1.337 ± 9.52 E-3 1.353 ± 9.14 E-3
B 4.786 E-3 ± 1.77 E-3 5.817 E-3 ± 1.77 E-3
C 1.038 E-3 ± 1.66 E-4 9.918 E-4 ± 1.65 E-4
Δn 0.019
Optical Constants MSE 3.365 3.12
Dektak (Å)
Spin Coating 6,000 rpm for 1 min.
172
4.7 Commercial Polymers Films
PVI, PVI-co-PVA, and P4VP (commercial polymers) were purchased to examine their
capabilities of producing changes in refractive index after being exposed to a concentrated
nitroaromatic vapor. These polymers were chosen due to the known ability of amine bases to
form complexes with nitroaromatic species in solution. These polymers were also found to be
soluble in EtOH. Since the polymers were soluble in EtOH (b.p. ≈ 78 °C54), slower spin coating
speeds were employed for producing homogeneous films.
A 3% (w.t.) solution of P4VP was prepared in EtOH by gently heating and placing the
vial containing the polymer solution in the wrist action shaker until the polymer completely
dissolved. Before spin coating, the polymer solution was filtered twice using 0.45 μm PTFE
filters to remove any undissolved particles in the solution. The polymer solution was spin coated
by the dynamic technique. Approximately 1 mL of the polymer solution was dropped constantly
(≈ 1 drop per sec.) at 1,500 rpm for thirty seconds. After thirty seconds, the film formation was
allowed to proceed and dry at 3,000 rpm for forty-five seconds. After spin coating the polymer
film, the film was placed in an oven at ≈ 60 °C for two hours. After drying, the film appeared a
yellow-blue tint without any visible defects. The P4VP polymer film was then characterized by
ellipsometry to determine the film's thickness and optical constants. The P4VP film was exposed
to a concentrated vapor of PNT.
The refractive index of the P4VP film was A ≈ 1.580, which was similar to the reported
literature refractive index (n = 1.57280). The refractive indices as a function of wavelength
determined by spectroscopic ellipsometry are shown in Figure 4.7.1 and Table 4.7.1 provides the
Cauchy parameters, profilometer measured thickness, and spin coating parameters. The
ellipsometry curves displayed reasonable refractive indices expected for the Cauchy dispersion
173
model (refractive indices decreased toward the visible spectrum). The features observed in the
ellipsometry curves were attributed to surface roughness, similar to previously presented spectra.
When fitting the data to the Cauchy model, surface roughness was not accounted for. The small
MSE values obtained from the fitted data indicated a reasonable fit between the experimental and
generated data. The thickness measured by the profilometer differed by 4.0 nm when compared
to the thickness measured by the ellipsometer. This result was consistent with the previous
results, confirming the approximate thickness of the polymer film. Exposure to the PNT vapor
produced an average change in refractive index (Δn) of 0.014. This change in refractive index
was attributed to two types of interactions, π- π stacking or charge transfer complexes, as P4VP
does not undergo hydrogen bonding with nitroaromatics. This result was consistent with the
results Tenhaeff et al. observed with the ability of P4VP to detect a nitroaromatic vapor (TNT).80
The observed ability of P4VP to detect PNT by producing a significant change in refractive
index proposed the polymer to be an ideal material to incorporate in the MZI.
174
Figure 4.7.1. The before and after refractive index curves for a P4VP film spin coated from a 3% (w.t.) ethanol solution exposed to a concentrated PNT vapor for five seconds.
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
1.72
1.74
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
175
Table 4.7.1. The ellipsometry Cauchy model MSE, film thickness, average change in refractive index (Δn), optical constants MSE, profilometer measured thickness, and spin coating parameters for the P4VP film exposed to PNT for five seconds.
Polymer: Poly(4-VP), 3% (w.t.) EtOH
Nitroaromatic: PNT 5 sec. exposure
Before After
MSE 1.628 1.835
Thickness (Å) 166.5 ± 0.4 161.3 ± 0.4
A 1.580 ± 5.32 E-3 1.596 ± 6.42 E-3
B 2.795 E-3 ± 1.43 E-3 2.446 E-3 ± 1.74 E-3
C 9.30 E-4 ± 1.25 E-5 8.835 E-4 ± 1.50 E-5
Δn 0.014
Optical Constants MSE 2.31 3.16
Dektak (Å) 121
Spin Coating 1,500 rpm for 30 sec. and 3,000 rpm for 45 sec.
176
Similar to P4VP, the PVI homopolymer and PVI-co-PVA copolymer were soluble in
EtOH. Polymer solutions were prepared by dissolving the polymers in EtOH using the wrist
action shaker until the polymer completely dissolved. After the polymers completely dissolved,
the polymer solutions were filtered removing any undissolved polymer particles. The PVI and
PVI-co-PVA solutions were spin coated by the dynamic technique, producing thin polymer
films. After spin coating, the films were then placed in an oven at 60 °C for two hours to remove
any residual EtOH. After drying, the polymer thin films were characterized by spectroscopic
ellipsometry to determine the films' thicknesses and optical constants.
The PVI polymer film was exposed to a PNT concentrated vapor for five seconds. The
PVI refractive index curves were plotted as a function of wavelength shown in Figure 4.7.2 and
Table 4.7.2 provides the Cauchy parameters, average change in refractive index, and spin coating
parameters. The observed PVI refractive indices were consistent for the Cauchy model. Similar
to the P4VP films, the features observed in the curves were attributed to surface roughness. The
refractive index determined by the Cauchy model before exposure to PNT was approximately A
≈ 1.575. After exposure to the PNT vapor, the polymer film's refractive index increased to
approximately A ≈ 1.599. The five-second exposure to PNT produced an average change in
refractive index (Δn) of 0.014. The change in refractive index occurred due to two possible
electrostatic, hydrogen bonding, or van der Waals forces) or covalent interactions (Meisenheimer
complex). Kong et al. reported a possible Meisenheimer complex formed between a templated
cross-linked polymer containing imidazole units with TNT confirmed by 1H NMR titrations with
TNT.81 The MSE values suggested a good fit for the Cauchy model between the experimental
data and generated data. The thicknesses determined by the Cauchy model before and after
177
exposure to PNT were identical. The profilometer thickness measurement was not performed to
confirm the approximate film thickness determined by the ellipsometer.
Figure 4.7.2. The PVI thin film spectroscopic ellipsometry curves showing a change in refractive index after a five second exposure to PNT.
1.56
1.58
1.6
1.62
1.64
1.66
1.68
1.7
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
178
Table 4.7.2. The Cauchy model parameters and spin coating parameters for a PVI polymer film spin coated from a 3% (w.t.) EtOH solution exposed to PNT for five seconds.
Polymer: PVI, 3% (w.t.) EtOH
Nitroaromatic: PNT 5 sec. exposure
Before After
MSE 1.951 1.377
Thickness (Å) 215.8 ± 0.5 215.4 ± 0.3
A 1.575 ± 4.85 E-3 1.599 ± 1.01 E-3
B 1.281 E-3 ± 1.38 E-3 -2.412 E-3 ± 1.01 E-3
C 8.313 E-4 ± 1.21 E-4 9.621 E-4 ± 8.78 E-5
Δn 0.014
Optical Constants MSE 3.26 4.61
Dektak
Spin Coating 1,500 rpm for 30 sec. and 3,000 rpm for 45 sec.
179
A very thin PVI-co-PVA copolymer film was characterized by spectroscopic
ellipsometry before and after exposure to PNT for five seconds (Figure 4.7.3). Table 4.7.3 lists
the Cauchy parameters, average change in refractive index, profilometer measured thickness, and
spin coating parameters. The initial refractive index measured by the ellipsometer before
exposure to the nitroaromatic vapor was observed at A ≈ 1.268. After the five-second exposure
to PNT, the film’s refractive index increased to A ≈ 1.275, which produced an average change in
refractive index (Δn) of 0.007. The features observed in the before and after ellipsometry curves
were attributed to the film's surface roughness. The MSE values suggested a good fit between the
experimental and generated data from the Cauchy model. The 0.3% (w.t) polymer solution
produced an expected very thin polymer film (≈ 16 nm). There was a 2 nm difference in film
thickness observed between the profilometer and Cauchy model due to the etching process, but
this small difference approximately confirmed the Cauchy model's predicted film thickness. The
refractive index measured for the copolymer was a low value for two monomers with higher
refractive indices. This observed low refractive index will be addressed later in this chapter.
180
Figure 4.7.3. The before and after refractive index curves for a thin PVI-co-PVA polymer film exposed to PNT for five seconds.
1.26
1.28
1.3
1.32
1.34
1.36
1.38
1.4
1.42
300 400 500 600 700 800 900 1000
Ref
ract
ive I
ndex
(n)
Wavelength (nm)
Before
After
181
Table 4.7.3. The before and after Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a PVI-co-PVA polymer film exposed to PNT for five seconds.
Polymer: PVI-co-PVA, 0.3% (w.t.) EtOH
Nitroaromatic: PNT 5 sec. exposure
Before After
MSE 3.932 3.734
Thickness (Å) 167 ± 3 166 ± 3
A 1.268 ± 9.30 E-3 1.275 ± 8.91 E-3
B 5.961 E-3 ± 1.50 E-3 5.80 E-3 ± 1.45 E-3
C 4.78 E-4 ± 1.41 E-4 5.201 E-4 ± 3.16 E-4
Δn 0.007
Optical Constants MSE 3.586 3.444
Dektak (Å) 143
Spin Coating 1,500 rpm for 30 sec. and 3,000 rpm for 45 sec.
182
4.8 Polystyrene Thin Films Containing 10-Methylphenothiazine
In Chapter 5, the growth of co-crystals between 10-methylphenothiazine (10-M) and 1,3-
DNB is discussed. The co-crystals appeared red-purple in color, suggesting a strong interaction
between the electron donor and the electron deficient nitroaromatic. This result prompted the
investigations of 10-M included in polystyrene thin films to determine if a charge transfer
complex would form and cause a change in refractive index when exposed to 1,3-DNB vapors.
Polystyrene was synthesized according to the Chen procedure.49 Polystyrene was purified
by washing the polymer in EtOH in a one-neck round bottom. After purifying the polymer, the
polymer was dried at 50 °C under vacuum overnight.
3% and 1% (w.t.) stock polystyrene solutions were prepared by dissolving the polymer in
toluene. The polymer solutions were then placed in a wrist action shaker until the polymer
completely dissolved. After the polymer dissolved, the polymer solutions were filtered once to
remove any particles present in the solution.
The first experiments performed were to determine the maximum 10-M concentration
that could be included in polystyrene solutions to allow homogeneous spin coated polymer films.
(w.t.) 10-M were prepared by dissolving 10-M in the polystyrene/toluene solutions using the
wrist action shaker. After the 10-M dissolved in the polystyrene solutions, the solutions were
filtered again to remove any particles present in the solutions. Films were spin coated by the
static technique. The films were allowed to dry for twenty-four to forty-eight hours at room
temperature in a dark area, since 10-M was sensitive to the light. After drying, the polymer films
were inspected for any defects. The films spin coated from the polystyrene solutions containing
0.1% - 1% 10-M (w.t.) produced blue films with a slight purple tint. The film spin coated from
183
the 3% (w.t.) polystyrene solution containing 3% (w.t.) 10-M produced a blue-purple tint film
with a few white, needle-like crystals protruding from the film. The spin coated film from the
polystyrene solution containing 6% (w.t.) 10-M produced a blue-purple tint film with large areas
covered with white, needle-like crystals. The 10% (w.t.) 10-M spin coated film surprisingly did
not produce white, needle-like crystals, but rather produced a blue-purple tint film with a slight
green tint with cracks throughout, creating a mosaic pattern. The same experimental procedure
was performed for 1% (w.t.) polystyrene solutions which produced similar results with 1% (w.t.)
10-M being the maximum concentration included in polystyrene films that allowed
homogeneous spin coated films. Next, the polystyrene/10-M films' optical constants were
characterized by spectroscopic ellipsometry before and after exposure to 1,3-DNB vapors from
seconds to hours. Refractive index changes were unsuccessful for short exposure times. The
exposure times were increased until a change in refractive index was observed.
A polymer film spin coated from a 3% (w.t.) polystyrene solution containing 1% (w.t.)
10-M was exposed to 1,3-DNB for two hours (Figure 4.8.1). Table 4.8.1 lists the Cauchy
parameters, average change in refractive index, profilometer measured thickness, and spin
coating parameters. The refractive index curves displayed the trend observed for a Cauchy
dispersion model. The spectrum was fitted using the Cauchy model from 400 - 1,000 nm,
excluding any absorption, which may have occurred in the UV region. The initial refractive
index of the polystyrene/10-M film was A ≈ 1.567 and increased with decreasing wavelengths.
The 10-M/polystyrene film was exposed to 1,3-DNB for two hours. After the film was exposed
to 1,3-DNB, the film's refractive index increased to A ≈ 1.582. The average change in refractive
index from 400 - 1,000 nm was (Δn) ≈ 0.005. The MSE values for the Cauchy parameters and
optical constants suggested a reasonable fit between the experimental and generated data. The
184
profilometer’s measured thickness for the film was ≈ 95.9 nm, which was 7 nm thicker than the
thickness determined by the Cauchy model (≈ 89 nm). This difference in film thickness may be
due to the etching process, where the etch penetrated the silicon wafer's surface and created a
thicker film measurement. Even though there was a small difference between the film thickness
measurements, the profilometer measurement did approximately confirm the ellipsometer's
determined film thickness. The features observed in the ellipsometry curves follow the trend
observed for a film with some surface roughness.
185
400 500 600 700 800 900 1000
1.58
1.59
1.60
1.61
1.62
1.63
1.64
1.65
1.66
Ref
ract
ive
Inde
x (n
)
Wavelength (nm)
Before After
Figure 4.8.1. The ellipsometry curves for a spin coated polystyrene/10-M film from a 3% (w.t.) polystyrene solution containing 1% (w.t.) 10-M exposed to 1,3-DNB for two hours.
186
Table 4.8.1. The before and after Cauchy model parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a polystyrene film containing 10-M exposed to 1,3-DNB for two hours.
Polymer: 3% (w.t.) PS, 1% (w.t.) 10-M
Nitroaromatic: 1,3-DNB 2 hrs. exposure
Before After
MSE 5.053 5.86
Thickness (Å) 888.3 ± 0.4 889.6 ± 0.4
A 1.567 ± 1.10 E-3 1.582 ± 2.99 E-3
B 1.651 E-2 ± 6.21 E-3 1.200 E-2 ± 1.52 E-3
C -7.346 E-4 ± 9.08 E-5 -2.279 E-4 ± 1.94 E-5
Δn 0.005
Optical Constants MSE 4.281 5.474
Dektak (Å) 959
Spin Coating 5,000 rpm for 40 sec.
187
A polystyrene/10-M film was spin coated from a 1% (w.t.) polystyrene solution
containing 0.5% (w.t.) 10-M. The polystyrene/10-M film thickness and optical constants were
characterized by ellipsometry before and after exposure to 1,3-DNB for three hours (Figure
4.8.2). Table 4.8.2 lists the Cauchy parameters, average change in refractive index, profilometer
measured thickness, and spin coating parameters for the film. The refractive index before
exposure to 1,3-DNB was A ≈ 1.470. After the film was exposed to 1,3-DNB for three hours, the
film's refractive index increased to A ≈ 1.480. The increase in refractive index was due to the 10-
M incorporated in the polymer film that hydrogen bonded with the nitro groups of 1,3-DNB. The
features observed in the refractive index curves were due to the presence of surface roughness,
which was not accounted for in the model. After exposure to the nitroaromatic, the film did not
change in color compared to the observed color change for the 10-M co-crystal with 1,3-DNB.
The before and after Cauchy MSE values described a good fit between the experimental and
generated data. The Cauchy model determined a film thickness of ≈ 27 nm, but the profilometer
measured a film thickness of ≈ 31 nm. The difference in film thickness was consistent with the
previous result where the profilometer’s measured thickness was slightly larger than the
thickness determined by the ellipsometer. Again, this difference resulted from the etching the
process. The minimal difference between both measurements did confirm the approximate film
thickness. This result led to the investigation to determine if a smaller concentration of 10-M in a
polystyrene film could produce a change in refractive index over an extended exposure period.
188
400 500 600 700 800 900 10001.46
1.48
1.50
1.52
1.54
1.56
1.58
1.60
Ref
ract
ive
Inde
x (n
)
Wavelength (nm)
Before After
Figure 4.8.2. Refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene/toluene polymer solution containing 0.5% (w.t.) 10-M exposed to 1,3-DNB for three hours.
189
Table 4.8.2. The ellipsometry Cauchy parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene solution containing 0.5% (w.t.) 10-M exposed to 1,3-DNB for three hours.
Polymer: 1% (w.t.) PS, 0.5% (w.t.) 10-M
Nitroaromatic: 1,3-DNB 3 hrs. exposure
Before After
MSE 2.41 2.283
Thickness (Å) 267.3 ± 0.8 270.1 ± 0.7
A 1.470 ± 5.22 E-3 1.480 ± 2.99 E-3
B 1.112 E-2 ± 2.50 E-3 1.133 E-2 ± 2.42 E-3
C 8.341 E-4 ± 3.34 E-4 8.565 E-4 ± 3.23 E-5
Δn 0.01
Optical Constants MSE 2.553 2.413
Dektak (Å) 307
Spin Coating 3,000 rpm for 40 sec.
190
A polystyrene/10-M film was spin coated on a silicon wafer from a 1% (w.t.)
polystyrene/toluene solution containing 0.1% (w.t) 10-M. The polystyrene/10-M solution
produced a blue colored film with a slight purple tint, which was characterized by ellipsometry
before and after exposure to 1,3-DNB to determine the film’s thickness and optical constants
(Figure. 4.8.3). Table 4.8.3 lists the Cauchy parameters, average change in refractive index,
profilometer measured thickness, and spin coating parameters for the polymer film. The
refractive index determined by the Cauchy model for the film before exposure to 1,3-DNB was
A ≈ 1.464. After the film was exposed to 1,3-DNB, the refractive index increased to A ≈ 1.469.
The small concentration of 10-M included in the polystyrene film showed the ability to produce
a small change in refractive index of Δn = 0.004 due to hydrogen bonding with the 1,3-DNB
nitro groups. The MSE values described a good fit between the experimental and generated data
for the Cauchy model. There was a minute difference in film thickness observed between the
Cauchy model's determined film thickness and the profilometer's measured film thickness. This
result was similar to previous film thickness measurements where the profilometer measured a
film thickness greater than the Cauchy model's determined film thickness. This minute difference
(≤ 1 nm) confirmed the approximate film thickness determined by the Cauchy model. The
features observed in the refractive index curves between 600 - 1,000 nm indicated surface
roughness for the film, since typically the refractive index curves lie flat in the near-infrared
region.
The inclusion of small concentrations of 10-M in polystyrene films exposed to 1,3-DNB
for an extended period of time showed the ability to produce changes in refractive index by non-
covalent interactions. These results provided evidence that a polymer containing 10-M moieties
in greater concentration might have a strong affinity for 1,3-DNB giving rise to a significant
191
change in refractive index with a shorter exposure time. These results suggested 10-M would be
an ideal material to be applied in the MZI sensor for detecting nitroaromatics.
400 500 600 700 800 900 1000
1.46
1.48
1.50
1.52
1.54
1.56
1.58
Ref
ract
ive
Inde
x (n
)
Wavelength (nm)
Before After
Figure 4.8.3. The refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for three hours.
192
Table 4.8.3. The Cauchy model parameters, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for three hours.
Polymer: 1% (w.t.) PS, 0.1% (w.t.) 10-M
Nitroaromatic: 1,3-DNB 3 hrs. exposure
Before After
MSE 2.215 2.119
Thickness (Å) 257.4 ± 0.8 257.7 ± 0.8
A 1.464 ± 4.86 E-3 1.469 ± 4.61 E-3
B 9.162 E-3 ± 2.26 E-3 9.333 E-3 ± 2.14 E-3
C 9.846 E-4 ± 3.03 E-4 9.830 E-4 ± 2.87 E-4
Δn 0.004
Optical Constants MSE 2.312 2.205
Dektak (Å) 262
Spin Coating 3,000 rpm for 40 sec.
193
During one experiment, a film spin coated from a 1% (w.t.) polystyrene/toluene solution
containing 0.1% (w.t.) 10-M produced a change in refractive index after a ten second exposure to
1,3-DNB (Figure 4.8.4). The refractive index before exposure to the 1,3-DNB vapors was A ≈
1.494. After the ten-second exposure, the refractive increased to A ≈ 1.499, which produced a
0.005 average refractive index change (Table 4.8.4). The MSE values for the Cauchy model
described an ideal fit between the experimental and generated data. The film thickness
determined by the Cauchy model and the profilometer measured thickness were in agreement,
which confirmed the thickness determined by the ellipsometer. Features appeared in the near-
infrared region of the refractive index curves because of the presence of surface roughness. This
result was consistent with the previous result for a film spin coated from a 1% (w.t.)
polystyrene/toluene solution containing 0.1% 10-M exposed to 1,3-DNB for three hours. This
result showed that in one instance longer exposure times were not required to produce a
detectable change in refractive index.
194
400 500 600 700 800 900 10001.48
1.50
1.52
1.54
1.56
1.58
1.60
1.62
1.64
Ref
ract
ive
Inde
x (n
)
Wavelength (nm)
Before After
Figure 4.8.4. The refractive index curves for a polystyrene/10-M film spin coated from a 1% (w.t.) polystyrene/toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for ten seconds.
195
Table 4.8.4. The Cauchy parameters before and after exposure to 1,3-DNB, average change in refractive index, profilometer measured thickness, and spin coating parameters for a film spin coated from a 1% (w.t.) polystyrene/toluene solution containing 0.1% (w.t.) 10-M exposed to 1,3-DNB for ten seconds.
Polymer: 1% (w.t.) PS, 0.1% (w.t.) 10-M
Nitroaromatic: 1,3-DNB 10 sec. exposure
Before After
MSE 2.698 3.421
Thickness (Å) 273.4 ± 0.8 271.6 ± 0.7
A 1.494 ± 5.54 E-3 1.499 ± 5.06 E-3
Δn 0.005
Optical Constants MSE 2.86 2.577
Dektak (Å) 272
Spin Coating 3,000 rpm for 40 sec.
196
Most of the refractive indices for the polymers reported in this chapter were in agreement
with the reported literature refractive indices. There were some instances where the observed
refractive indices for thin polymer films (≤ 100 nm) exhibited lower refractive indices values
than the reported literature values. The lower indices of refraction were due to a radially
symmetric segmental orientation of specific groups on the polymer chains acting as an optical
retarder, produced during the spin coating process. Schwab et al. measured the optical
birefringence of rubbed thin polystyrene films to investigate the relaxation processes of
molecules at the polymer/air interface that showed shifts in the index of refraction.82 Hu et al.
investigated anomalies of refractive index for spin coated thin polystyrene films.83 Hu found that
the index of refraction was a function of film thickness for polystyrene films less than 100 nm
and the bulk refractive index could be recovered by annealing the films above the Tg. Hu noted
that the observed change in refractive index was due to the symmetric segmental orientation
produced by the spin coating process, since the orientation induced during the spin coating
process is typically radially symmetric about the spin axis and uniformly either in or out of the
plane of the spin coated film.83
The polymer films in this chapter represented possible materials that could be
incorporated in the MZI sensor to detect nitroaromatic explosives. The PS-co-PVDAT
copolymers, P2VP, PVI, PVI-co-PVA, and polystyrene/10-M films exhibited changes in the
index of refraction after exposure to a concentrated nitroaromatic vapor, suggesting that these
materials should be considered as possible MZI sensing materials. The copolymers containing
low concentrations of the VDAT monomer produced homogeneous films with minimal surface
roughness, allowing the optical constant to be fully characterized by spectroscopic ellipsometry.
VDAT appeared to be a promising monomer to synthesize electron rich copolymers, but as the
197
concentration was increased in the copolymers, problems with solubility in an ideal spin coating
solvent and the ability to spin coat homogeneous films with minute surface roughness excluded
these polymers from possibly being used as a sensing material. These were the main
disadvantages associated with utilizing VDAT, which led to the investigations of other polymers'
abilities to sense nitroaromatics by refractive index changes. P2VP, P4VP, PVI, and PVI-co-
PVA all showed the ability to interact with the nitro groups of the nitroaromatics, which
produced changes in the refractive index curves. Most of the polymers were not soluble in an
ideal spin coating solvent, but using the static spin coating technique allowed homogeneous films
to be casted with some surface roughness that were characterized by ellipsometry.
The polystyrene films containing low concentrations of 10-M showed the unexpected
ability to interact with the 1,3-DNB vapors over long exposure periods. These initial results
provided evidence of further work needed to synthesize a polymer rich in 10-M moieties. A
polymer containing larger concentrations of 10-M may have the potential to produce significant
changes in the index of refraction after short exposure periods to 1,3-DNB.
It should be noted that some of the polymers synthesized in Chapter 3 did not show
changes in refractive index when exposed to a concentrated nitroaromatic vapor. When the
nitroaromatic (1,3-DNB) was added to the polymers, PVK and PMMA-co-PVK copolymers, in
the solution phase and dissolved, changes in color (colorless to yellow) were observed for the
polymer/nitroaromatic solutions, suggesting a strong interaction between the electron rich
polymers and 1,3-DNB. These polymers could still be used in the MZI sensor to assist in the
detection of the nitroaromatic (1,3-DNB) in the solution phase, as opposed to the vapor phase.
Lastly, a concerning problem observed for the polymers in this chapter was the variant
changes in the index of refraction after exposure to the concentrated nitroaromatic vapors.
198
Minimal, large, or no change in refractive indices after exposure to the nitroaromatic were
observed, limiting the reliability of these sensing materials. Still, there was enough data to
support the possibility of using these polymer films as nitroaromatic sensing materials for a MZI
sensor.
4.9 Polymer Thin Films Summary
Polymer thin films were investigated to determine their affinity for nitroaromatics by
measuring the change in refractive index after exposure to a nitroaromatic vapor using
ellipsometry. To demonstrate the polymer films affinities for nitroaromatics, Table 4.9.1 lists the
polymer films' average change in refractive index after a five-second exposure to a nitroaromatic
vapor. From the PVDAT copolymers, the copolymers consisting of polystyrene and PVDAT
showed the most promise as the sensing material for the MZI. The PS-co-PVDAT copolymers
allowed films to be casted from an ideal spin coating solvent with minimal surface roughness
and did exhibit an affinity for nitroaromatics. The PMMA-co-PVDAT copolymers appeared to
have a greater affinity for nitroaromatics compared to the PS-co-PVDAT copolymers, but
extreme surface roughness was observed for the PMMA-co-PVDAT copolymer films and a
change in refractive index was not observed for some of the PMMA-co-PVDAT copolymer
films after the five-second exposure to a nitroaromatic vapor. From the commercial polymers,
PVI and P4VP showed the most promise for detecting nitroaromatics due to the change in
refractive index after exposure to a nitroaromatic vapor. A change in refractive index was
observed for the PVI-co-PVA polymer film after being exposed to PNT for five seconds, but
extreme surface roughness was observed for the polymer film.
199
Table 4.9.1. Polymer films average change in refractive index after a five-second exposure to a nitroaromatic vapor.
Polymer Nitroaromatic Exposure Time (sec.) ∆n
PVI-co-PVA PNT 5 0.010
P4VP PNT 5 0.014
PVI PNT 5 0.014
PS-co-PVDAT 1 mol % PNT 5 0.002
PS-co-PVDAT 1 mol % NB 5 0.013
PS-co-PVDAT 1 mol % 1,3-DNB 5 0.010
PS-co-PVDAT 5 mol % PNT 5 0.002
PS-co-PVDAT 5 mol % NB 5 0.005
PS-co-PVDAT 10 mol % NB 5 0.003
PS-co-PVDAT 10 mol % PNT 5 0.001
PS-co-PVDAT 10 mol % 1,3-DNB 5 0.003
PS-co-PVDAT 20 mol % NB 5 0.009
PS-co-PVDAT 20 mol % PNT 5 0.002
PMMA-co-PVDAT 5 mol % PNT 5 0.017
PMMA-co-PVDAT 20 mol % PNT 5 0.073
P2VP PNT 5 0.019
200
Chapter 5
Co-crystals Containing Electron Rich Aromatic Molecules and Electron Poor Nitroaromatic Molecules Nitroaromatic molecules will form molecular complexes with electron-rich aromatic
molecules, producing intense color changes. 1,3-dinitrobenzene and 2,4-dinitrotoluene both
formed jet-black molecular complexes with benzidine (4,4’-diaminobiphenyl).84 Chloroform
solutions containing aniline and either 1,3-dinitrobenzene, 1,4-dinitrobenzene or 1,3,5-
trinitrobenzene exhibited new absorptions extending from the UV region into the visible region
with extinction coefficients of 102 to 103 M-1 cm-1.85 Mixtures of picryl chloride with
hexamethylbenzene or picric acid with naphthalene formed highly colored solutions in
chloroform.86, 87
The crystal structure packing observed in complexes between an electron rich donor and
nitroaromatic exhibit a general trend with alternating stacking in charge transfer complexes. The
crystal structure of a 1:1 molecular complex between 1,4,-dinitrobenzene and phenazine showed
alternating stacks of phenazine and 1,4-dinitrobenzene molecules with a 361 pm distance
between the center of the phenazine molecule and the center of the 1,4-dinitrobenzene
molecule.88 A 1:1 molecular complex of 2-aminobenzimidazole and 1,3,5-trinitrobenzene also
showed alternating stacks.89 In each case, the electron-poor nitro aromatic rings and the electron-
rich aromatic rings were stacked face-to-face. This pattern of alternating stacking of donor and
acceptor aromatic molecules face-to-face was also observed in the crystal structure of the
molecular complex of tetrathiafulvalene and 1,3-dinitrobenzene and in the molecular complex of
201
4-iodotetrathiafulvalene and 1,4-nitrobenzene.90, 91 These intense color changes observed when
an electron donor forms a molecular complex with various nitroaromatics provide evidence of
the ability to detect nitroaromatics by using an electron rich polymer. The polymer could form a
molecular complex with the electron deficient nitroaromatics by hydrogen bonding or pi→pi*
interactions. Varieties of electron donor and acceptor combinations that were attempted are
shown in Chapter 2. This chapter will focus on the electron donors and acceptors which
produced co-crystals confirmed by 1H NMR, FTIR, UV/Vis, diffuse reflectance, and X-ray
crystallography.
5.1 1,3-Dinitrobenzene Crystals (1,3-DNB)
1,3-DNB crystals were produced by dissolving 1,3-DNB in EtOH and allowing the EtOH
to evaporate at room temperature for two days. Needle-like crystals formed with a faint white-
yellow color shown in Figure 5.1.1.
Figure 5.1.1. Image of 1,3-DNB crystals.
202
The 1H NMR (360 MHz, CDCl3) spectrum of the 1,3-DNB crystals was recorded shown
in Figure 5.1.2. The 1,3-DNB proton signals were characterized by 1H NMR. The reported 1,3-
DNB proton signals were in agreement with reported literature values. The positions of the
proton signals were used as a reference to determine the presence of 1,3-DNB in the co-crystals.
The FTIR spectrum for the 1,3-DNB crystals is shown Figure 5.1.3. The peaks located at
1540 and 1347 cm-1 were assigned to the NO2 asymmetric and symmetric stretching vibrations.
The C-H stretching vibrations appeared at 2873, 3049, and 3108 cm-1. The benzene ring
stretching vibrations were observed at 1614 and 1602 cm-1 with the benzene ring overtones
appearing at 2873, 3049, and 3108 cm-1. The NO2 asymmetric and symmetric stretching
vibrations were used as a reference to determine if a complex formed between the electron
donor and electron acceptor, producing a shift in the vibrational bands.
4000 3500 3000 2500 2000 1500 1000 500
20
30
40
50
60
70
80
90
100
% T
rans
mitt
ance
Wavenumbers (cm-1)
1540NO2 vas
1347NO2 vsym
CH stretchingvibrations
Benzene RingOvertones
Figure 5.1.3. The FTIR spectrum of the 1,3-dinitrobenzene crystals.
204
The electronic absorption spectrum for the 1,3-DNB crystals (Figure 5.1.4) was recorded
in acetonitrile at a concentration of 2.0 x 10-5 M. Acetonitrile was chosen as the applicable
solvent due to its UV/Vis solvent cut-off (≈ 190 nm for a 1 cm cuvette). The 1,3-DNB crystals'
electronic absorption spectrum displayed a λmax at 237 nm (ε = 1.80 x 104 M-1cm-1).
200 300 400 500 600 700 8000
5000
10000
15000
20000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB crystals
Figure 5.1.4. Electronic absorption spectrum of 1,3-DNB crystals in acetonitrile.
205
The diffuse reflectance spectrum for the 1,3-DNB crystals (Figure 5.1.5) was recorded at
room temperature. The reflectance was more than 50% in the visible region from 450 - 800 nm.
The reflectance decreased to approximately 5% from 450 - 400 nm, which gave rise to the
yellow tint for the 1,3-DNB crystals.
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10
20
30
40
50
60
% R
efle
ctan
ce
Wavelength (nm)
Figure 5.1.5. Diffuse reflectance spectrum of the 1,3-DNB crystals.
The melting point for the 1,3-DNB crystals was measured for comparison with the
melting point for co-crystals containing 1,3-DNB and electron-rich aromatic molecules. The
melting range was defined at the temperature that liquid formation was visible until the crystals
completely melted and formed a meniscus. The observed melting range was 90.9 - 91.3 °C (lit.
206
89 °C54). The narrow melting range provided evidence of crystals with few inhomogeneities or
individual components.
5.2 9-Ethylcarbazole (9-EC) Co-Crystals with Nitroaromatics
Attempts to prepare 1:1 co-crystals with 9-EC and either 2-NT, 3-NT, and PNT were not
successful. The solutions evaporated, producing a mixture of crystals of the pure compounds.
However, when the EtOH solutions of 9-EC and 1,3-DNB were mixed, a yellow-orange color
rapidly appeared. The EtOH was allowed to evaporate for two days, producing yellow-orange
needle-like crystals shown in Figure 5.2.1. 9-EC crystals were prepared by the same procedure
producing, white-brown, needle-like crystals.
Figure 5.2.1. 9-EC + 1,3-DNB crystals after drying for two days, producing yellow-orange tint crystals.
The 1H NMR spectra for the 9-EC co-crystals, 9-EC crystals, and 1,3-DNB crystals
(Figure 5.2.2) were recorded in CDCl3 to determine the ratio between the electron donor and
acceptor. Table 5.2.1 lists the peak positions, splitting patterns, and integration values for the 1,3-
DNB crystals, 9-EC crystals, and 9-EC co-crystals. The peaks observed in the 9-EC co-crystals
207
1H NMR spectrum at 9.06, 8.56, and 7.79 ppm were assigned to the 1,3-DNB incorporated in the
co-crystals. The 9-EC proton signals were observed at 8.09, 7.45-7.39, 7.23-7.19, 4.36, and 1.42
ppm. Neither the 9-EC nor 1,3-DNB peaks exhibited a chemical shift in the spectrum. The
integration value for the 9-EC proton signal at 7.22-7.20 ppm in the 9-EC and 9-EC co-crystals
spectra was not an accurate integration due to the CDCl3 solvent peak overlapping. The
integration of the spectrum revealed an approximate 1:1 ratio between 9-EC and 1,3-DNB.
Figure 5.2.2. 1H NMR (360 MHz, CDCl3) spectra for the 9-ethylcarbazole crystals (9-EC), the co-crystals (9-EC co-crystals with 1,3-DNB), and 1,3-dinitrobenzene crystals (1,3-DNB).
The FTIR spectra for the 9-EC crystals and co-crystals of 9-EC with 1,3-DNB are shown
in Figure 5.2.3. The symmetric and asymmetric stretching modes for the NO2 groups in the co-
crystal were shifted to lower energy, compared to those for the 1,3-DNB crystals. This shift
indicated intermolecular interactions between 9-EC and 1,3-DNB in the co-crystals. Table 5.2.2
shows the comparison between the NO2 asymmetric and symmetric stretching modes before and
after incorporation into the co-crystal.
209
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
3420
31183104
2981 1601
1536
1452
1345
30472978
2931
1594
14541377
1326
Free OH
CH stretching vibrations
9-EC
9-EC + 1,3-DNB
Figure 5.2.3. FTIR spectra of KBr pellets containing either 9-EC crystals (black curve) or the co-crystals containing 9-EC and 1,3-DNB (red curve).
Table 5.2.2. Comparison of NO2 asymmetric and symmetric stretching vibrations between 1, 3-DNB crystals and 9-EC + 1, 3-DNB co-crystals.
Crystals Color NO2 vas ( cm-1 ) NO2 vs ( cm-1 )
1,3-DNB white-yellow 1540 1347
9-EC white-brown N/A N/A
9-EC + 1,3-DNB yellow-orange 1536 1345
210
The electronic absorption spectrum of the 9-EC co-crystals with 1,3-DNB was recorded
in acetonitrile to determine if a charge complex formed in the dilute solutions. Figure 5.2.4
displays the electronic absorption spectra for the 1,3-DNB crystals, 9-EC crystals, and 9-EC co-
crystals with 1,3-DNB. There was no new absorption band that would be expected if a charge
transfer complex formed. There were no significant chemical shifts observed in the spectra
between 9-EC crystals and 9-EC co-crystals.
200 300 400 500 600 700 8000
10000
20000
30000
40000
50000
60000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB crystals 9-EC crystals 9-EC co-crystals
Figure 5.2.4. Electronic absorption spectra of 1,3-DNB crystals (black), 9-EC crystals (red), and 9-EC co-crystals with 1,3-DNB (blue) in acetonitrile.
211
To determine if the spectrum for the 9-EC co-crystals was the result of a charge transfer
complex or just the combination of free 9-EC and 1,3-DNB molecules in solution, the spectra of
the 1,3-DNB crystals and 9-EC crystals were combined and compared against the 9-EC co-
crystals spectrum (Figure 5.2.5). The similarities between the 9-EC co-crystals electronic
absorption spectrum and the combined 1,3-DNB and 9-EC crystals electronic absorption
spectrum was the result of the independent 1,3-DNB and 9-EC molecules in solution rather than
the formation of a charge transfer complex. Clearly, 1,3-DNB and 9-EC did not form a charge
transfer complex in dilute ~10-5 M acetonitrile solutions.
200 300 400 500 600 700 8000
10000
20000
30000
40000
50000
60000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB + 9-EC 9-EC co-crystals
Figure 5.2.5. Electronic absorption spectra in acetonitrile for the 9-EC co-crystals (red) and the sum of the spectra for 9-EC and 1,3-DNB crystals (black).
212
The diffuse reflectance spectra were measured for the 9-EC crystals and 9-EC co-crystals
with 1,3-DNB (Figure 5.2.6). The reflectance for the 9-EC crystals was greater than 70%
throughout the visible region. Below 400 nm, the reflectance decreased to less than 40% as the
UV light was absorbed by the crystals. The reflectance spectrum for the co-crystals had a
reflectance of less than 40% in the near infrared and red region of the spectrum. The reflectance
decreased to 10% at wavelengths below 500 nm. The difference in absorption was expected,
since the 9-EC crystals were white-brown, compared to the co-crystals which were yellow-
orange in color.
200 300 400 500 600 700 8000
20
40
60
80
100
% R
efle
ctan
ce
Wavelength (nm)
9-EC crystals 9-EC co-crystals
Figure 5.2.6. Diffuse reflectance spectra for 9-EC crystals (black) and the co-crystals of 9-EC and 1,3-DNB (red).
213
The melting points of the 9-EC crystals and 9-EC co-crystals were measured for
comparison. Table 5.2.3 lists the melting points for the 1,3-DNB crystals, 9-EC crystals, and 9-
EC co-crystals. The co-crystals with 1,3-DNB had a much lower melting range (48.4 - 50.1 °C)
compared to the 9-EC crystals (68 - 70 °C) and 1,3-DNB crystals (89 °C) melting ranges. This
lower, narrow melting point range indicated that there were few inhomogeneities or individual
components present. During the melt, the co-crystals produced a color change from a yellow-
orange to a red-orange color.
Table 5.2.3. Melting points of 1,3-DNB crystals, 9-EC crystals, and 9-EC co-crystals with 1,3-DNB.
Crystals Melting Point (°C) Lit. Value (°C)
1,3-DNB 90.9 - 91.3 89 54
9-EC 71.0 - 71.8 68 - 70 92
9-EC co-crystal 48.4 - 50.1
The 9-EC co-crystal structure was analyzed by X-ray diffraction. A survey scan of a co-
crystal revealed that the crystal structure was 1,3-DNB. Ito et. al. reported similar results with
carbazole derivative co-crystals with 1,3-DNB.92 Ito made reference that the crystal adducts
might be too small for X-ray diffraction characterization.
5.3 9-Vinylcarbazole (9-VC) Co-Crystals with 1,3-DNB
9-VC did not form co-crystals with 2-NT or NB. It did form a co-crystal with 1,3-DNB,
which was confirmed by an intense color change. When the two solutions of 9-VC and 1,3-DNB
were combined in a crystallization dish, a bright yellow color rapidly appeared. The EtOH
evaporated at room temperature for two weeks, producing yellow crystals with spots as shown in
214
Figure 5.3.1. 9-VC crystals were prepared by the same procedure producing white needle-like
crystals.
Figure 5.3.1. Co-crystals of 9-VC and 1,3-DNB.
The 1H NMR spectra of the 9-VC co-crystals, 9-VC crystals, and 1,3-DNB crystals
(Figure 5.3.2) were recorded in CDCl3 to determine the ratio between the electron donor and
acceptor. The peak positions, multiplicities, and integrations are listed in Table 5.3.1. The 1H
NMR spectrum for the 9-VC co-crystals with 1,3-DNB showed peaks located at 9.07, 8.57, and
7.79 ppm assigned to 1,3-DNB incorporated in the co-crystal. The 9-VC proton signals were
observed at 8.06, 7.65, 7.46, 7.32-7.25, 5.54, and 5.15 ppm. Neither the 9-VC nor 1,3-DNB
peaks exhibited a chemical shift in the spectrum. The integration values for the 9-VC proton
signals located between 7.32-7.25 ppm in the 9-VC and 9-VC co-crystal spectra were not an
accurate integration due to the overlapping CDCl3 solvent peak. The integration of the spectrum
revealed an approximate 1:2 ratio between 9-VC and 1,3-DNB.
215
Figure 5.3.2. 1H NMR spectra of the 1,3-DNB crystals, 9-VC crystals, and 9-VC co-crystals with 1,3-DNB (360 MHz, CDCl3).
Table 5.3.1. 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, 9-VC co-crystals, and 9-VC crystals.
The infrared spectra of the CBZ crystals and the co-crystals are shown in Figure 5.4.3
and Table 5.4.2 lists the NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB
crystals and CBZ co-crystals. The spectrum for the CBZ co-crystals showed that the NO2
asymmetric stretching mode (1537 cm-1) was red-shifted (3 nm) to lower energy, which indicated
that a weak complex occurred during the formation of the co-crystals. The NO2 symmetric
stretching mode (1347 cm-1) was at the same position as the symmetric stretch in the 1,3-DNB
crystals. Only one of the NO2 stretching vibrations exhibited a shift, suggesting a weak
intermolecular interaction between CBZ and 1,3-DNB.
221
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
Free OH
3419NH
1537
1347
CBZ co-crystals
CBZ crystals
Figure 5.4.3. FTIR spectra for KBr pellets containing CBZ crystals (black) and the CBZ co-crystals with 1,3-DNB (red).
Table 5.4.2. NO2 asymmetric and symmetric stretching vibrations for 1,3-DNB crystals and CBZ co-crystals.
Crystals Color NO2 vas ( cm-1 ) NO2 vs ( cm-1 )
1,3-DNB White-yellow 1540 1347
CBZ Light brown N/A N/A
CBZ + 1,3-DNB Light brown 1537 1347
222
The electronic absorption spectrum of the CBZ co-crystals with 1,3-DNB was recorded
in acetonitrile to determine if a charge transfer complex formed within the co-crystals. Figure
5.4.4 shows the electronic absorption spectra for the 1,3-DNB crystals, CBZ crystals, and CBZ
co-crystals with 1,3-DNB. The expected weak broad absorption band for a charge transfer
complex was not observed in the spectrum, indicating that a charge transfer complex did not
form. None of the CBZ peaks in the co-crystal spectrum exhibited any significant chemical
shifts. To determine if the CBZ co-crystal spectrum was the result of a charge transfer complex
or just the interaction between CBZ and 1,3-DNB molecules in dilute solutions, the sum of the
1,3-DNB and CBZ electronic absorption spectra were compared with the CBZ co-crystals
spectrum (Figure 5.4.5). The combined spectra closely matched the spectrum for the CBZ co-
crystals. From this result, it was assumed that the spectrum for the co-crystals with a
concentration of 10-5 M was simply the sum of the spectra for free CBZ and free 1,3-DNB
molecules. There was no charge transfer complex formed at that concentration.
223
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15000
20000
25000
30000
35000
40000
45000
50000
55000
60000
65000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB crystals CBZ crystals CBZ co-crystals
Figure 5.4.4. Electronic absorption spectra in acetonitrile for 1,3-DNB crystals (black), CBZ crystals (red), and CBZ co-crystals containing 1,3-DNB and CBZ (blue).
224
200 300 400 500 600 700 8000
10000
20000
30000
40000
50000
60000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB + CBZ CBZ co-crystals
Figure 5.4.5. Comparison of the electronic absorption spectrum in acetonitrile for the co-crystals containing 1,3-DNB and CBZ (red) and the sum of the spectrum for the 1,3-DNB crystals and the spectrum for the CBZ crystals (black).
225
The melting point ranges were measured for the CBZ crystals and the CBZ co-crystals
(Table 5.4.3). The co-crystals have a broad melting point range, compared with that of the 1,3-
DNB crystals and the CBZ crystals. The observed co-crystals' liquid formation temperature (82.4
°C) was lower than the liquid formation temperatures for the 1,3-DNB crystals (90.9 °C) and the
CBZ crystals (245.5 °C). The observed temperature when the co-crystals completely melted
forming a meniscus (209.3 °C) was higher compared to the 1,3-DNB crystals (91.3 °C), but
lower than the CBZ crystals (249.1°C). This broad melting range indicated the presence of
inhomogeneities or individual components within the co-crystals. During the melt, the CBZ co-
crystals changed from light brown to yellow in color before the first signs of the melt. Above 150
°C, the co-crystals changed color again from yellow to orange.
Figure 5.5.3 shows the infrared spectra for the PHZ crystals and co-crystals made from
PHZ and 1,3-DNB. Table 5.5.2 lists the NO2 asymmetric and symmetric stretching modes for the
1,3-DNB crystals and the PHZ co-crystals. The NO2 asymmetric stretching mode (1539 cm-1)
was red shifted (1 nm) to lower energy in the co-crystal spectrum, which suggests a weak
intermolecular interaction between the electron donor and acceptor. The NO2 symmetric
stretching mode (1347 cm-1) was located at the same position for the 1,3-DNB crystals
symmetric stretching mode.
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
PHZ PHZ co-crystal
1539
13473340 NH
Figure 5.5.3. FTIR spectra of KBr pellets containing either PHZ (black) or the co-crystal of PHZ and 1,3-DNB (blue).
229
Table 5.5.2. NO2 asymmetric and symmetric stretching modes for the PHZ co-crystals and 1,3-DNB crystals.
To determine if a charge complex formed within the co-crystals, the electronic absorption
spectrum was recorded in acetonitrile. Figure 5.5.4 shows the electronic absorption spectra for
the 1,3-DNB crystals, PHZ crystals, and the co-crystals. There were no shifts in the peak
positions for the co-crystal spectrum, compared to the spectrum for the PHZ crystals. There was
no observable charge transfer band present in the PHZ co-crystals spectrum. To determine if a
charge complex formed within the co-crystals, the sum of the PHZ crystals and 1,3-DNB crystals
electronic absorption spectra were compared to the co-crystals electronic absorption spectrum
shown in Figure 5.5.5. The two spectra were identical, with no observable differences. The
spectra had similar results as the CBZ co-crystals, which suggested that the PHZ co-crystals
electronic absorption spectrum was primarily the result of free PHZ molecules and 1,3-DNB
molecules in dilute solutions.
Crystals Color NO2 vas ( cm-1 ) NO2 vs ( cm-1 )
1,3-DNB White-yellow 1540 1347
PHZ White brown N/A N/A
PHZ co-crystal Light brown 1539 1347
230
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10000
20000
30000
40000
50000
60000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB crystals PHZ crystals PHZ co-crystals
Figure 5.5.4. Electronic absorption spectra recorded in acetonitrile for 1,3-DNB crystals (black), PHZ crystals (red), and the co-crystals containing PHZ and 1,3-DNB (blue).
231
200 300 400 500 600 700 8000
10000
20000
30000
40000
50000
60000
ε(M
-1 c
m-1)
Wavelength (nm)
Sum of 1,3-DNB and PHZ PHZ co-crystals
Figure 5.5.5. Electronic absorption spectra in acetonitrile for the PHZ co-crystals (red) and the sum of the spectra for 1,3-DNB crystals and PHZ crystals (black).
232
The diffuse reflectance spectra for the PHZ crystals and the PHZ co-crystals containing
1,3-DNB are shown in Figure 5.5.6. The spectrum for the PHZ crystals showed a diffuse
reflectance of approximately 80% from the NIR (800 nm) to 500 nm. Below 500 nm, the
reflectance decreased to less than 10% below 400 nm. This was consistent with the light brown
color of the PHZ crystals. The diffuse reflectance for the co-crystals showed a decrease in the
reflectance from the NIR to 400 nm. This difference cannot be explained as simply due to the
innocent presence of 1,3-DNB. The diffuse reflectance spectrum for 1,3-DNB (Figure 5.1.5)
showed a high reflectance greater than 50% through 800 - 500 nm region. Here a new feature
was observed suggesting a strong intermolecular interaction between PHZ and 1,3-DNB.
200 300 400 500 600 700 8000
20
40
60
80
100
% R
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ctan
ce
Wavelength (nm)
Phenothiazine crystals Phenothiazine co-crystals
Figure 5.5.6. Diffuse reflectance spectra for PHZ crystals (black) and co-crystals containing PHZ and 1,3-DNB (red).
233
Table 5.5.3 shows the melting ranges for the 1,3-DNB crystals, PHZ crystals, and PHZ
co-crystals. The co-crystals had a broad melting point range, compared to those for the 1,3-DNB
and PHZ crystals. The co-crystals' observed liquid formation temperature (71.4 °C) was lower
than those for the 1,3-DNB crystals (90.9 °C) and the PHZ crystals (186.9 °C). The temperature
at which the co-crystals completely melted and formed a meniscus (145.7 °C) was higher than
the 1,3-DNB crystals' melting temperature (91.3 °C), but lower than the PHZ crystals' melting
temperature (189.4 °C). This broad melting range indicated the presence of inhomogeneities or
individual components within the co-crystals. During the melt, the PHZ co-crystals began
shrinking and changing color from light brown to a dark-red before the first signs of liquid
formation.
Table 5.5.3. Melting points of the 1,3-DNB crystals, PHZ crystals, and PHZ co-crystals.
Crystals Melting Point (°C) Lit. Values (°C)
1,3-DNB 90.9 - 91.3 89 54
PHZ 186.9 - 189.4 184.9 54
PHZ co-crystal 71.4 - 145.7
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5.6 10-Methylphenothiazine (10-M) Co-Crystals with 1,3-DNB
As with other electron rich aromatic molecules, attempts to prepare co-crystals with 10-
M and 2-NT, 3-NT, and PNT were not successful. However, when the two solutions of 10-M and
1,3-DNB were combined in the crystallization dish, a dark red color rapidly appeared. The EtOH
evaporated for two days, producing reddish-purple crystals as shown in Figure 5.6.1 (B). 10-M
crystals were prepared by the same experimental procedure, which produced white, needle-like
crystals as shown in Figure 5.6.1(A).
Figure 5.6.1. Images of 10-M crystals (A) and 10-M co-crystals with 1,3-DNB (B).
The 1H NMR spectra of the 10-M crystals and co-crystals containing 10-M and 1,3-DNB
were recorded in CDCl3 in order to determine the ratio between the electron donor and acceptor
(Figure 5.6.2). The peak positions, peak multiplicities, and peak integrations are listed in Table
5.6.1. The 1,3-DNB peaks were observed at 9.06, 8.56, and 7.79 ppm. The 10-M peaks were
located at 7.17-7.11, 6.91, 6.80, and 3.36 ppm. The co-crystal spectrum integration revealed an
approximate 1.0:1.1 ratio between 1,3-DNB and 10-M. Neither the 1,3-DNB nor the 10-M
proton signals in the co-crystals spectrum displayed a shift in peak positions.
(A) (B)
235
Figure 5.6.2. 1H NMR spectra of 1,3-DNB crystals, 10-M crystals, and co-crystals containing 10-M and 1,3-DNB (360 MHz, CDCl3).
Table 5.6.1. 1H NMR peak positions, splitting patterns, and integration values of 1,3-DNB crystals, 10-M co-crystals, and 10-M crystals.
Figure 5.6.3 shows the FTIR spectra for 10-M and the co-crystals. Table 5.6.2 lists the
NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB crystals and the PHZ co-
crystals. The NO2 asymmetric and symmetric stretching modes were shifted 4 nm and 5 nm to
lower energy in the co-crystals, indicating a strong intermolecular interaction between the
electron donor and acceptor.
4000 3500 3000 2500 2000 1500 1000 500
% T
rans
mitt
ance
(A.U
.)
Wavenumbers (cm-1)
1536 1342
10-M co-crystal
10-M
Figure 5.6.3. Infrared spectra of KBr pellets containing either 10-M crystals (black) or the co-crystals containing 10-M and 1,3-DNB (red).
237
Table 5.6.2. NO2 asymmetric and symmetric stretching vibrations for the 1,3-DNB crystals and 10-M co-crystals.
Crystals Color NO2 vas ( cm-1 ) NO2 vs ( cm-1 )
1,3-DNB white-yellow 1540 1347
10-M White N/A N/A
10-M co-crystals red-purple 1536 1342
The electronic absorption spectra for the 1,3-DNB crystals, 10-M crystals, and the co-
crystals were recorded in acetonitrile (Figure 5.6.4). 10-M showed two peaks, an intense
absorption observed at 254 nm (ε=3.77 x 104 M-1 cm-1) and a weak absorption at 308 nm (ε=5.00
x 104 M-1 cm-1). 1,3-DNB had a single broad absorption at 237 nm (ε=1.80 x 104 M-1 cm-1). The
spectrum for the co-crystals made from 1,3-DNB and 10-M was simply the sum of the spectra
for 1,3-DNB and 10-M (Figure 5.6.5). This indicated that there was no intermolecular interaction
between 1,3-DNB and 10-M in the acetonitrile solution at 2.0 x 10-5 M concentration.
238
200 300 400 500 600 700 8000
5000
10000
15000
20000
25000
30000
35000
40000
45000
50000
55000
ε (M
-1 c
m-1)
Wavelength (nm)
2.0 x 10-5 M 1,3-DNB crystals 10-M crystals 10-M co-crystals
Figure 5.6.4. Electronic absorption spectra for 1,3-DNB crystals (black), 10-M crystals (red), and the co-crystals (blue).
239
200 300 400 500 600 700 8000
10000
20000
30000
40000
50000
ε (M
-1 c
m-1)
Wavelength (nm)
Sum of 1,3-DNB and 10-M 10-M co-crystals
Figure 5.6.5. Electronic absorption spectra for the co-crystals (red) and the sum of the spectra for 1,3-DNB crystals and 10-M crystals (black).
240
The diffuse reflectance spectrum (Figure 5.6.6) for the co-crystals containing 10-M and
1,3-DNB provided strong evidence for intermolecular interactions in the solid state. The
reflectance for the 10-M crystals was greater than 70% from 800 nm to 400 nm. Below 400 nm,
the reflectance dropped to less than 10%. This was consistent with the white color of the 10-M
crystals. The diffuse reflectance spectrum for the co-crystals was dramatically different. The
reflectance was slightly above 40% in the region from 800 nm to 650 nm. Below 650 nm, the
reflectance dropped below 10%. This was consistent with the dark red color of the co-crystals.
200 300 400 500 600 700 8000
20
40
60
80
100
% R
efle
ctan
ce
Wavelength (nm)
10-M crystals
10-M co-crystals
Figure 5.6.6. Diffuse reflectance spectra for 10-M crystals (black) and the co-crystals containing 10-M and 1,3-DNB (red).
241
This also indicated a strong intermolecular interaction between the 10-M and 1,3-DNB in the
solid state.
Table 5.6.3 lists the melting ranges measured for the 10-M crystals and 10-M co-crystals
with 1,3-DNB. The melting range for the co-crystals was significantly lower than the melting
range for the 10-M crystals and the 1,3-DNB crystals. The narrow melting range indicated the
existence of a co-crystal with few inhomogeneities or individual components. During the melt,
the co-crystals did not exhibit any color change.
Table 5.6.3. Melting points of the 1,3-DNB crystals, 10-M crystals, and 10-M co-crystals.
Crystals Melting Point (°C) Lit. Values (°C)
1,3-DNB 90.9 - 91.3 89 54
10-M 101.7 - 104.1 99-100 93
10-M co-crystal 61.7 - 63.4
The co-crystals were suitable for single crystal X-ray diffraction in order to determine the
structure. Steven Kelley obtained and interpreted the X-ray diffraction data. The following is his
interpretation of the structure. The 1:1 co-crystal of 1,3-DNB and 10-M crystallized in the chiral,
orthorhombic space group P212121 with two symmetry-independent formula units (Z = 8). None
of the atoms or molecules reside on special positions. The 1,3-DNB molecules were planar,
except for the nitro groups, which are twisted slightly out-of-plane. There were no statistically
significant differences in bond lengths for the two 1,3-DNB molecules, and the nitro groups on
both molecules had approximately the same orientation relative to the ring. The 10-M molecules
had the typical geometry of phenothiazine and its derivatives, with both of the phenyl rings
joining at an acute angle. The corresponding bond distances and N- and S-centered bond angles
of both 10-M molecules were statistically equivalent to each other and very similar to those in
the reported crystal structure of 10-M.94
242
Figure 5.6.7. 50% probability ellipsoid plot of the asymmetric unit of the co-crystal. The dashed lines indicate distances that were less than the sum of the van der Waals radii.
The short contact environments around the symmetry-independent molecules were
different (Figure 5.6.7). Both 1,3-DNB molecules made short contacts to five 10-M molecules,
but no 1,3-DNB molecules. Both accepted hydrogen bonds through either nitrate group. This
interaction explained the decrease in the peak positions for the asymmetric and symmetric
vibrational modes for the nitro groups in the co-crystals. The major difference was that 1,3-DNB
molecule A formed π-π contacts with the end of a 10-M molecule, while molecule B formed
those contacts with the center.
243
Figure 5.6.8. Short contact environment around 1,3-DNB A (left) and B (right). The green lines indicate distances that were less than the sum of the van der Waals distance.
The 10-M molecules also had different short contact environments (Figures 5.6.8 and
5.6.9). 10-M (A) only interacted with 1,3-DNB molecules through hydrogen bonding to the nitro
groups or inter-ring π-π stacking. The nitrogen and sulfur atoms of 10-M (A) were not involved
in short contacts. Molecule (B) made short contacts to two 10-M molecules as well as four 1,3-
DNB molecules. The 10-M (B) molecules interacted with each other through herringbone-type
C-H---π interactions between the phenyl rings. 10-M (B) did not π-stack with any 1,3-DNB
molecules; instead, it donated hydrogen bonds to nitro groups on two 1,3-DNB molecules and
accepted hydrogen bonds from 1,3-DNB molecules at the N and S atoms.
244
Figure 5.6.9. Short contact environment around 10-M A (left) and B (right). The green lines indicate distances that were less than the sum of the van der Waals contacts.
Infinite hydrogen bonded chains along b, formed by one of the 10-M phenyl rings
donating hydrogen bonds to 1,3-DNB molecules on either side of it, was a major structural
feature (Figure 5.6.10). Each of these chains only involved hydrogen bonds between 1,3-DNB
(A) and 10-M (A) or 1,3-DNB (B) and 10-M (B). The A and B chains were interdigitated with
each other, which allowed for extra hydrogen bonding between the chains as shown in Figure
5.6.10. The molecular recognition, which allowed co-crystallization, may stem from this
hydrogen bonding, as neither 1,3-DNB nor 10-M can form these chains without the other.
Figure 5.6.10. The 1,3-DNB-10-M H-bonded chain along b. The green lines indicate the distances that were less than the sum of the van der Waals contacts.
245
Other supramolecular structures can be described in terms of how the “A” and “B”
chains, the hydrogen bonded chains formed between 1,3-DNB and 10-M (A) respectively,
interacted with each other. Two adjacent A chains formed a dimer through π-π stacking. These
chain dimers interacted with each other through π- π stacking as well forming a 3-D network.
The interactions between these dimers are shown in Figure 5.6.11. The B chains were
interwoven into the A chain network through π- π stacking between 1,3-DNB (B) molecules and
10-M (A) molecules as well as hydrogen bonding of 1,3-DNB (A) molecules to the sulfur atom
on 10-M (B) molecules. The B chains also formed a network with each other through the 10-M-
10-M C-H π contact and hydrogen bonding between 1,3-DNB (B) molecules and the nitrogen
atom on 10-M. Figure 5.6.12 shows the network of A chain dimers network and the packing
down the b axis.
Figure. 5.6.11. View along b axis of π- π stacking interactions between A chains. The green lines indicate the distances that were less than the sum of the van der Waals contacts.
246
Figure 5.6.12. Packing down b axis showing only A chains (left) and all atoms (right). A chains are colored blue in both pictures. B chains are colored red. Crystallographic axes are color coded as a = red, b = green, c = blue.
247
Chapter 6
Conclusions and Future Works
Random copolymers of styrene or methyl methacrylate and the VDAT monomer showed
the potential to sense nitroaromatics by changes produced in the index of refraction after
exposure to a concentrated nitroaromatic vapor. The electron rich structure of VDAT presented a
problem for the solubility of copolymers in a suitable spin coating solvent. This solubility
dilemma limited the synthesis of copolymers with larger concentrations of VDAT due to the
insolubility of PVDAT. Copolymers rich in PVDAT moieties may be capable of producing
larger changes in the index of refraction after exposure to a nitroaromatic vapor, but a polar
solvent with an ideal boiling point with the ability to dissolve these electron rich copolymers
needs to be identified in order to allow the spin coating of homogeneous films. A disadvantage
of VDAT observed when synthesizing copolymers with other electron rich monomers was a
cross-linking effect, making the copolymers insoluble in solvents at or near room temperature.
VDAT appeared to have the ideal structure for sensing nitroaromatics due to its electron rich ring
containing amino functional groups, but these characteristics might be the monomer's downfall
as it limits the synthesis and solubility of polymers that would allow the production of thin films
for the MZI sensor.
The problems associated with the use of the PVDAT copolymers led to the investigations
of other polymers to determine their potential to detect nitroaromatics by changes in the
248
refractive index. Pyridine and imidazole based polymers (P4VP, PVI, and PVI-co-PVA) showed
the ability to sense nitroaromatics by changes in the refractive index after exposure to a
nitroaromatic vapor. These polymers were not soluble in an ideal spin coating solvent, but
homogeneous films were casted by adjusting the spin coating parameters for the dynamic
technique. There was surface roughness observed for these polymer films, which was expected
due to the low boiling point of EtOH. The change in the refractive index after exposure to a
nitroaromatic results attracted interest to synthesize copolymers with pyridine or imidazole
monomers with VDAT. These investigations resulted in brittle hard polymers, which had limited
solubility in spin coating solvents. Due to limited solubility, full characterization of the optical
constants was not possible; however, if suitable solvents were found for casting films of these
copolymers, these films would potentially have an affinity for nitroaromatics based on previous
results.
The interactions between the electron rich polymers and electron deficient nitroaromatics
led to research and the production of co-crystals between electron rich reagents and a
nitroaromatic. Attempts to produce co-crystals between VDAT and nitroaromatics were
unsuccessful due to VDAT solubility in polar organic solvents with high boiling points or H2O at
elevated temperatures. These unsuccessful attempts led to studies using other electron rich
reagents. Co-crystals were produced between 1,3-DNB with 9-VC, 9-EC, and 10-M. The color
changes associated with the formation of the complexes with 1,3-DNB suggested a strong
interaction between the reagents, which was confirmed by FTIR. The nitro groups' asymmetric
and symmetric stretching modes were red shifted to lower energy. The electronic absorption
spectra did not confirm a charge transfer complex in dilute solutions, but rather showed the
interaction between the electron donor molecules and the nitroaromatic molecules in the solution
249
phase. Only the 10-M co-crystal with 1,3-DNB was able to characterized by X-ray diffraction
allowing a crystal structure to be determined. The 10-M molecules interacted with the 1,3-DNB
molecules through hydrogen bonding and π-π contacts. At this time, no crystal structures have
been determined between the carbazole derivatives with 1,3-DNB. This obstacle may be due to
co-crystals' size produced during the slow evaporation process. The strong affinity the electron
donors had for 1,3-DNB suggested they should be considered as sensing materials.
The results for the 10-M co-crystals with 1,3-DNB led to the development of polystyrene
films containing small concentrations of 10-M to determine if the 10-M would interact with the
1,3-DNB vapors to produce a change in the index of refraction for the films. After the maximum
concentration of 10-M that could be included in a polystyrene film was determined, the optical
constants of the polystyrene/10-M film were characterized after long exposure times to 1,3-DNB.
Unexpectedly, the low concentrations of 10-M were capable of producing a change in refractive
index. This result confirmed that 10-M would be an ideal sensing material.
Future investigations should still focus on synthesizing electron rich copolymers with an
affinity for nitroaromatics. Polymers containing carbazole or phenothiazine derivatives should be
considered, since previous results showed the strong affinity these reagents had for 1,3-DNB.
One would expect these polymers to show a significant change in the index of refraction after
exposure to 1,3-DNB if the electron donating properties of reagent are not altered significantly.
These types of polymers should also be investigated as a solution phase nitroaromatic sensor due
to the color changes observed during the growing of the co-crystals.
Another important part of this project that must be considered is the development of
imprinted polymers from the homopolymers and copolymers that exhibited an affinity for
nitroaromatics. The polymer would be imprinted with a specific nitroaromatic, creating a specific
250
cavity for the targeted analyte in the polymer. After the targeted analyte is removed from the
polymer, a specific imprint site will be left where only the targeted analyte can enter the site and
interact at the recognition site. The development of imprinted polymer films would allow the
detection of a specific nitroaromatic and eliminate any false positives. A foreseeable problem
that must be considered is the solubility of VDAT in polar organic solvents. Removing the
majority of the solvent could be problematic for producing a thin imprinted polymer film.
For the last part of this research project, the copolymer films should be applied to a MZI
to determine the sensor's sensitivity. From experimental results in the literature, this will be an
exciting part of the project to see how sensitive the MZI is to changes in the index of refraction
after exposure to a concentrated nitroaromatic vapor. To determine the MZI sensitivity and limit
of detection, a vapor generator will need to be constructed to control the amount of a
nitroaromatic vapor required to determine a certain change in refractive index.
An additional side note for this project might be to evaluate the thermal properties of the
copolymers for heat resistant materials. The TGA characterization of the PS-co-PVDAT
copolymers and PMMA-co-PVDAT copolymers showed a significant increase in the
decomposition temperatures for the copolymers. Even though some of the copolymers were not
characterized by TGA to determine their decomposition temperatures, they may possess high
decomposition temperatures similar to liquid crystal polymers.
251
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Appendix Figure 25. The 1H NMR spectrum for the PMMA-co-PVDAT 1 mol % VDAT copolymer with the spectrum intensity increased showing the vinyl protons for either MMA or VDAT (6.18, 5.48, and 5.45 ppm) suggesting unreacted monomer present within the polymer matrix.
4.44.85.25.66.06.46.8ppm
0.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
5.5
6.0
6.5
7.0
7.5
282
Appendix Figure 26. The 1H NMR spectrum of the PMMA-co-PVDAT 5 mol % VDAT copolymer in CDCl3 using the 500 MHz spectrometer.
Appendix Figure 53. The PAM-co-PVDAT 10 mol % VDAT copolymer 13C NMR spectrum (500 MHZ, D2O) from 190 - 150 ppm showing the PMA carbonyl carbon signal (C3) and the PVDAT carbon signal (C7) confirming the presence of PVDAT in the copolymer.
160166172178184190ppm
C3
C7
310
Appendix Figure 54. The 13C NMR spectrum for the PMMA-co-PVK 20 mol % vinylcarbazole recorded in CDCl3 (500 MHz).
102030405060708090100110120130140150160170180ppm
20 mol % PVK
C=O1a 8a
7,25
4,6,3
5a 4a
8 1
(V+M)
10
9
CH (V)
C(M)
311
100 120 140 160 180-2.8
-2.6
-2.4
-2.2
-2.0
-1.8
-1.6
-1.4
Hea
t Flo
w (W
/g)
E
ndot
herm
Temperature (οC)
Appendix Figure 55. The DSC curve for the PVI homopolymer.