Pyrolysis-GC/MS/IR Analysis of Kraton 1107 Application Note 228-100 Authors Matthew S. Kl ee and Imogene Chang Hewlett-Packard Co. Avondale Divis ion Abstract The pyrolysis gas -chromato- graphic characterization of Kraton 1107 (isoprene -sty- rene elastomer) is desc ribed. Detection of capillary-col- umn effluent and ident ifi- cation of pyrolysates was accomplished with an MSD and IRD connected in paral- lel. Cryogenic cooling allowed separation of low boiling pyrolysates and focused peaks of higher boiling pyrolysat es. Pro- grammable column pressure control allowed fast analysis while maintaining chromato- graphic efficiency. Introduction Pyrolysis gas chromatogTaphy is a u seful tool for characteri zing polymers. Pol ymers which are not vol atile enough for standard g-as chromatogTaphic analysis can be thermally cleaved into small, volatile fragm ent s which are. Th e resul ting chr omatograms yield characteristic fingerprints r elating to the composition of the polymer, structural informa- tion relati ng to b ran ch ing and defects, and information on additives. Sin ce many fragments are pro- duced during the pyrolysis process, the ability to provide high - resolution separation of these fragments is very impor- tant. Capillary column chroma- tograp hy with low dead-volume connections and cryogenic cooling of both the col umn and inj ector ensme maximum reso lu- tion of pyrolysates. Since the flow r ate through a capillary column decreases with increas ing o ven temp er ature (the gas is more viscous, causing decreas ed fl ow with fixed head pressure), standard pressure-controlled systems suf fer both losses in efficiency and increases in analysis time . The HP 5890 SERIES II used in th is s tudy had a progTammable pressure controller which maintained column fl ow rate during the run by adjusting the co lumn head press ure. In ad dition, after pyrol ysates h ad been trapped, the injector was rapidly tempera- rf/n- HEWLETT PACKARD Gas Chroma tograp hy July 1989 ture programmed to ensure sharp injection profiles of solute bands. Selective detection of pyrolysa te peaks is very useful for accurate identification of the peaks rela t- ing to the starting polyme r, the mecha nisms of thermal degrada- t ion, and identification of add itives and residual solvents. The combination of mas s spec- tral and infrared detectors can provide complimentary informa- tion on mole cu- Jar structure and orientat ion and is, therefo re, extremely effective for polymer pyrolysis studies. Kraton 1107 is a co polymer of isoprene (2-m ethyl-1,3-butadi- ene) and styr ene and is a rubber- like elastomer. Krat on 1107 was the subject of an ASTM experiment aimed at developing a "molecular thermomete r" based on the ratio of isoprene to dipentene fragments (1), and its pyrolysis characteristics are well understood . Therefo r e, the pyrolysis-GC analysis of Kra ton 1107 was used in this study to demonstrate the effec- tive comb ination of the H P 5890 S ERIES II with MSD and IRD fo r t his type of analysis .
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Pyrolysis-GC/MS/IR Analysis of Kraton 1107
Application Note 228-100
Authors Matthew S. Klee and Imogene Chang Hewlett-Packard Co. Avondale Division
Abstract The pyrolysis gas-chromatographic characterization of Kraton 1107 (isoprene-styrene elastomer) is described. Detection of capillary-column effluent and identification of pyrolysates was accomplished with an MSD and IRD connect ed in parallel. Cryogenic cooling allowed separation of low boiling pyrolysates and focused peaks of higher boiling pyrolysates. Programmable column pressure control allowed fast analysis while maintaining chromatographic efficiency.
Introduction Pyrolysis gas chromatogTaphy is a useful tool for characterizing polymers. Polymers which are not volatile enough for standard g-as chromatogTaphic analysis can be thermally cleaved into small, volatile fragments which are.
The r esulting chromatograms yield characteristic fingerprints relating to the composition of the polymer, structural information relating to branch ing and defects, and information on addi tives.
Since many fragments are produced during the pyrolysis process, the ability to provide high-resolution separation of these fragments is very important. Capillary column chromatography with low dead-volume connections and cryogenic cooling of both the column and injector ensme maximum resolution of pyr olysates. Since the flow rate through a capillary column decreases with increasing oven temperature (the gas is more viscous, causing decreased flow with fixed head pressure), standard pressure-controlled systems suffer both losses in efficiency and increases in analysis time. The HP 5890 SERIES II used in this study had a progTammable pressure controller which maintained column flow rate during the run by adjusting the column head pressure. In addi tion, after pyrolysates had been trapped, the injector was rapidly tempera-
rf/n- HEWLETT ~a PACKARD
Gas Chromatography
July 1989
ture programmed to ensure sharp injection profiles of solute bands.
Selective detection of pyrolysate peaks is very useful for accurate identification of the peaks rela ting to the starting polymer, the mechanisms of thermal degradation, and identification of additives and residual solvents. The combination of mass spectral and infrared detectors can provide complimentary information on molecu- Jar structure and orientation and is, therefore, extremely effective for polymer pyrolysis studies.
Kraton 1107 is a copolymer of isoprene (2-methyl-1,3-butadiene) and styrene and is a rubberlike elastomer. Kraton 1107 was the subject of an ASTM experiment aimed at developing a "molecular thermometer" based on the ratio of isoprene to dipentene fragments (1), and its pyrolysis characteristics are well understood . Therefore, the pyrolysis-GC analysis of Kraton 1107 was used in this study to demonstrate the effective combination of the HP 5890 SERIES II with MSD and IRD for this type of analysis.
Experimental The instrumentation and conditions used for this study are listed in table 1.
Kraton 1107 was acquired from Chemical Data Systems, Oxford, PA. Pyrolyses were carried out at 700°C and 1000°C in quartz tubes using a platinum coil pyrolysis probe. Sample amounts are stated in the caption of figure 1 and were less than 1 mg.
Pyrolyses were carried out in a heated interface assembly (CDS, Oxford, PA) which is supplied for use with HP 5890 GCs. The standard interface needle assembly was too large to fit int o the on-column injector and was replaced with a 28-gauge needle via a 118 in.-1/16 in. Swagelok union and graphite!Vespel ferrule. The pyrolyzer-GC interface was used without the standard quartz liner, and was fed with an auxiliary supply of helium carrier gas. This auxiliary helium was connected to an event-controlled solenoid (in the Purge B position) such that it could be turned off and on within a time progTam. As long as the auxiliary pressure supplied to the pyrolyzer-GC interface exceeds that of the progTammable pressure controller CPPC), it overrides the PPC and supplies all of chromatogTaphic system's carrier gas flow through the interface. As soon as the solenoid turns off th e gas flow through the interface, the PPC resumes control of the system head pressure and no more He flows through the pyrolysis-GC interface.
Pyrolysates were separated by an HP-5 (5% phenyl methyl silicone) capillary column and detected by both MSD and IRD connected in parallel. The inlet end of the column was connected to the cool on-column injector. The outlet of the column was connected to one side of a 1/16 in. zero deadvolume Swagelok union. To the
other end of the union, two transfer lin es were connected via a two-holed gTaphite!Vespel ferrule: a short piece of 0.32 mm HP-5 coated capillary column was used for connection to the IRD, and a 2m x 0.10 mm id blank column was used for connection to the MSD.
Chemical Data Systems (Oxford, PA) Model1 22 275 Coil Off (Ballistic) 10 sec 700 and 1000°C
10-12 psi 0.5 min
Results and Discussion Pyrograms of Kraton 1107 at 700 and 1000°C are presented in figures 1 and 2. Figure 1 compares the t otal ion chromatogTams (mass spectrometry), and figure 2 compares the total reconstructed infrared chr omatograms. Chromatographic efficiency was maintained throug·h both detectors: peaks are symmetrical and peak widths correlate well between detectors. This is partly due to the low dead-volume connector between the analytical column and the transfer lines, and partly due to the constant-flow operation of the progTammable pressure controller. The column head pr essure was automatically increased (constant fl ow mode) during the run from the starting pressure of 5 psi at 0°C, to about 7.8 psi at the ending temperature of 275°C.
In general, the pyrogTams for the sample pyrolyzed at 1000°C contain the same fragments as for the 700°C pyrogTam. The sig·nificant differ ence between the hig·h- and low-temperature pyrolyses is that there are more molecular rearrangements and smaller molecular fragments at higher pyrolysis temperatures.
The identities of the major pyrolysis fragm ents and several of the minor fragments ar e g·iven in table 2. The identification of peak 11 (about 9.5 min) is not definitive by mass spectrometry, as can be seen in the library search results in figure 3. The library search yielded several possibilities with the the same empirical formula but different structures.
3 KRRTON 1107 :·.;.
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Figure 1. Total ion c hromatogram (mass spectral) for the pyrolysis of Kraton 1107. (A) 0.43 mg pyrolyzed at 700°C, (B) 0.51 mg pyrolyzed at 1000°C.
Figure 2. Total reconstructed infrare d chroma togra m s for the same analyses as in figure 1, (A) 700°C, (B) 1000°C.
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Figure 6. Infrared spectra of peaks 14, (A), and 12, (B), of the pyrogram in figure 2(A).
Conclusion The combination of a cryogenically cooled chromatographic system and parallel detection with an MSD and IRD has been shown to be very effective for the pyrolysis-GC analysis of Kraton 1107.
References 1. E. J. Levy and J . Q. Walker, J Chrom Sci, 22 (1984) 49- 55.
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4
Investigation of the infrared spectrum of peak 11 allowed unequivocal identification as styrene. Further confirmation was obtained by comparing the infrared spectrum of peak 11 with the styrene peak obtained by pyrolysis of polystyrene, as shown in figure 4.
The pyrogram of Kraton 1107 shows a separation of two fragments, peaks 12 and 14, which have the same molecular weig·ht and similar mass spectra (figure 5), indicating that they are structural isomers. The infrared absorption information shown in figure 6 confirms that they have the same general structure. The shift in absorption wavelengths from 1789 to 1829 em-~ and from 893 to 913 cm- 1 for peaks 14 and 12, respectively, indicate that the terminal vinyl group is disubstituted ( > C = CH2 )
for peak 14, and is monosubstituted (-CH = CH2 ) for peak 12. Peak 14 is 1-methyl-4-isopropenyl cyclo-hexene (dipentene), and peak 12 is probably 1,4-dimethyl-4-vinyl cyclohexene.
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F igure 4. Infrared spectra of (A) styrene resulting from the pyrolysis of polystyrene, and (B) peak 11 from the pyrogram 2(A).