ORIGINAL PAPER On Interaction Characteristics of Polyhedral Oligomeric Silsesquioxane Containing Polymer Nanohybrids Sang-Kyun Lim 1 • Jae Yun Lee 1 • Hyoung Jin Choi 1 • In-Joo Chin 1 Received: 1 November 2014 / Revised: 20 April 2015 / Accepted: 24 May 2015 / Published online: 3 June 2015 Ó Springer-Verlag Berlin Heidelberg 2015 Abstracts A generalized functional group of polyhedral oligomeric silsesquiox- ane (POSS) suitable for various polymer systems, e.g., polyolefins, polyesters and polyamides, is presented using theoretical and experimental approaches to examine thermodynamic interaction between a polymer and POSS. Both Flory–Huggins interaction parameter and maximum difference of the solubility parameter are uti- lized to study theoretically specific interaction between polymers and POSS nanoparticles. Flory–Huggins interaction parameter was estimated by the melting point depression method determined by DSC, while maximum difference of the solubility parameter was predicted using the method of Hoftyzer and van Krevelen. The interaction characteristics of the polymer/POSS nanohybrids are further tested by measuring the activation energy with the Kissinger method, in which the acti- vation energy was calculated using the temperature at the maximum degradation rate observed TGA. Viscoelastic, dynamic mechanical, thermal and mechanical properties of the polymer/POSS nanohybrids were also examined to correlate the theoretical and experimental results, finding that the isobutyl group was the most suitable functional group of POSS for polyethylene, poly(ethylene terephthalate), and Nylon 6. Keywords Polyhedral oligomeric silsesquioxane Thermodynamic interaction Interaction parameter Solubility parameter Activation energy Electronic supplementary material The online version of this article (doi:10.1007/s00289-015-1405-5) contains supplementary material, which is available to authorized users. & Hyoung Jin Choi [email protected]1 Department of Polymer Science and Engineering, Inha University, Incheon 402-751, Korea 123 Polym. Bull. (2015) 72:2331–2352 DOI 10.1007/s00289-015-1405-5
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ORIGINAL PAPER
On Interaction Characteristics of PolyhedralOligomeric Silsesquioxane Containing PolymerNanohybrids
Sang-Kyun Lim1• Jae Yun Lee1 • Hyoung Jin Choi1 •
In-Joo Chin1
Received: 1 November 2014 / Revised: 20 April 2015 /Accepted: 24 May 2015 /
Published online: 3 June 2015
� Springer-Verlag Berlin Heidelberg 2015
Abstracts A generalized functional group of polyhedral oligomeric silsesquiox-
ane (POSS) suitable for various polymer systems, e.g., polyolefins, polyesters and
polyamides, is presented using theoretical and experimental approaches to examine
thermodynamic interaction between a polymer and POSS. Both Flory–Huggins
interaction parameter and maximum difference of the solubility parameter are uti-
lized to study theoretically specific interaction between polymers and POSS
nanoparticles. Flory–Huggins interaction parameter was estimated by the melting
point depression method determined by DSC, while maximum difference of the
solubility parameter was predicted using the method of Hoftyzer and van Krevelen.
The interaction characteristics of the polymer/POSS nanohybrids are further tested
by measuring the activation energy with the Kissinger method, in which the acti-
vation energy was calculated using the temperature at the maximum degradation
rate observed TGA. Viscoelastic, dynamic mechanical, thermal and mechanical
properties of the polymer/POSS nanohybrids were also examined to correlate the
theoretical and experimental results, finding that the isobutyl group was the most
suitable functional group of POSS for polyethylene, poly(ethylene terephthalate),
aminopropylisobutyl-POSS (AB-POSS, C31H71NO12Si8, Fw = 874.58), and amino-
propylisooctyl-POSS (AO-POSS, C59H127NO12Si8, Fw = 1267.32). All the POSS
nanoparticles were purchased from Hybrid Plastics Inc.
Nanohybrids of the polymer and POSS nanoparticles were prepared by the melt
mixing method. Initially, the polymer was introduced into a torque rheometer
(Plastograph EC, Brabender, Germany) and melted at its melting point for 10 min
with a rotary speed of 60 rpm. POSS was then added to the melted polymer and
compounded for 15 min to concentrations of 0.5, 1 and 2 wt%. OM-POSS, OB-
POSS and OP-POSS were used for the preparation of PE and PET nanohybrids. In
the case of the Nylon 6 nanohybrids, AB-POSS, AO-POSS and AP-POSS were used
as the POSS nanoparticles.
Characterization
The nanostructures of the dispersed POSS nanoparticles in the polymer matrix were
examined by field-emission transmission electron microscopy (FE-TEM, JEM
2100F, JEOL, Japan) at an accelerated voltage of 100 kV. All ultrathin sections
(less than 3 lm) were microtomed using an ultramicrotome (RMC-MTX, USA)
with a diamond knife and observed by FE-TEM without staining. To measure the
mechanical properties, the nanohybrid films were subjected to uniaxial elongation
using a universal test machine (UTM, Houndfield Test Equipment, UK), with a
typical sample dimension of 10 mm width 9 50 mm length 9 0.1 mm thickness.
Mechanical tests were performed at room temperature and a crosshead speed of
5 mm/min. The rheological characteristics were also measured using a rotational
rheometer (HCR 300, Physica, Germany) equipped with a parallel plate geometry
(25 mm diameter) and TEK 350 temperature controller. All samples were measured
in the melt state with a gap distance of 1 mm.
The thermal stability of the samples was measured with thermogravimetric
analysis (TGA, Q50, TA Instruments, USA) by heating them from room
Polym. Bull. (2015) 72:2331–2352 2335
123
temperature up to 750 �C at a heating rate of 10 �C min-1 with air flowing. The
samples were also heated to 750 �C at heating rates of 20 �C min-1 and 40 �Cmin-1 with air flowing to calculate the activation energy. Differential scanning
calorimetry (DSC, Pyris Diamond DSC, Perkin-Elmer, USA) was employed to
determine the Flory–Huggins interaction parameter between the polymer and POSS
by monitoring the change in the melting point. To determine the equilibrium
melting temperature (Tm0 ), samples were heated 10 �C above their melting
temperatures (Tm) and maintained for 5 min for the complete melting of crystals.
Subsequently samples were cooled down to their crystallization temperatures (Tc)
and kept 30 min before heating to Tm ? 10 �C with a rate of 20 �C min21.
Results and discussion
Flory–Huggins interaction and solubility parameters
The Flory–Huggins interaction parameter is an important measure that can
determine the solubility of polymers in solvents or the compatibility between pairs
of chemical species such as polymer/polymer and polymer/nanoparticle combina-
tions, and can be obtained experimentally using several methods such as the melting
inverse-phase gas chromatography [81, 82], small-angle neutron scattering [83, 84],
and small-angle X-ray scattering [85, 86].
Nishi and Wang [76], in their analysis of the reduction of the melting temperature
of a crystalline polymer in the presence of an amorphous one, derived a simple
equation which related the melting point depression directly to the Flory–Huggins
interaction parameter. The relation between the melting point depression and the
Flory–Huggins interaction parameter of the mixture can be described by the
following equation.
T0m � T0
mix ¼ �BV
DHT0mð1� /iÞ2 ð1Þ
where Tm0 and Tmix
0 are the equilibrium melting temperature of the crystalline
polymer and the mixture, respectively, DH/V is the heat of fusion of the crystalline
polymer per unit volume, /i is the weight fraction of the crystalline polymer, and
B is the Flory–Huggins interaction parameter between the two components. The
Flory–Huggins interaction parameter B can be obtained from the slope of the plot of
(Tm0 - Tmix
0 ) vs (1 - /i)2. A negative value of B for the binary system indicates that
the two chemical species form a thermodynamically stable and compatible mixture.
As shown in Fig. 1, the equilibrium melting temperatures of the pure PE and the PE/
POSS nanohybrids were obtained using Hoffman–Weeks plots.
Figure 2 represents the melting point depression of PE in the PE/POSS
nanohybrids as a function of the PE content. According to Eq. (1), the B values
for the PE/POSS nanohybrids were determined from the slope of the straight line of
Fig. 2. When values of 30.9 cm3 mol-1 and 970.1 cal mol-1 were used for Viu and
DHiu, respectively, the estimated Flory–Huggins interaction parameters were
2336 Polym. Bull. (2015) 72:2331–2352
123
-1.95 cal cm-3, -3.39 cal cm-3 and -0.93 cal cm-3 for BPE/OM-POSS, BPE/OB-POSSand BPE/OP-POSS, respectively, indicating that the PE with OB-POSS is thermodynam-
ically most favorable hybrid.
Fig. 1 Hoffman–Weeks plotsof a PE, b PE/OM-POSS, c PE/OB-POSS, and d PE/OP-POSSnanohybrids
Polym. Bull. (2015) 72:2331–2352 2337
123
By following the same experimental procedures, the Flory–Huggins interaction
parameter between PET and OB-POSS was determined to be -1.11 cal cm-3,
which was smaller than that for PET/OM-POSS (-0.29 cal cm-3) and PET/OP-
POSS (-0.10 cal cm-3) (Fig. S1 and S2 in the Supporting Information). Likewise,
the Flory–Huggins interaction parameters of Nylon 6/AB-POSS, Nylon 6/AO-POSS
and Nylon 6/AP-POSS were -0.42 cal cm-3, -0.35 cal cm-3 and
-0.23 cal cm-3, respectively. Therefore, the interaction of the PET/OB-POSS
and Nylon 6/AB-POSS nanohybrids were found to be thermodynamically more
favorable than those of the other combinations.
The solubility parameter, d, which is the square root of the cohesive energy
density (the energy of vaporization per unit volume), was also used to predict the
thermodynamic interaction between the polymer and POSS nanoparticles. Since it is
not possible to obtain the molar vaporization energies for polymers, calculations
Fig. 2 Plots of the equilibriummelting points of PE in the a PE/OM-POSS, b PE/OB-POSS, andc PE/OP-POSS nanohybrids
2338 Polym. Bull. (2015) 72:2331–2352
123
based on the group contributions are used to determine the solubility parameters of
the polymers. The Small and Hoy method is generally used to compute the
solubility parameter, due to its simplicity. However, no specific forces, such as the
dispersion force, polar force and hydrogen bonding, are assumed to be active
between the structural units of the substances involved. Therefore, the Small and
Hoy method is considered to be unsuitable for crystalline polymers. In this study,
the solubility parameters of the polymers and POSS nanoparticles were calculated
according to the Hoftyzer and van Krevelen method using the following equations
[78].
dd ¼P
Fdi
V; dp ¼
ffiffiffiffiffiffiffiffiffiffiffiffiPF2pi
q
V; dh ¼
ffiffiffiffiffiffiffiffiffiffiffiffiPEhi
V
r
ð2Þ
d2t ¼ d2d þ d2p þ d2h ð3Þ
where dd, dp and dh are the dispersion, polar and hydrogen bonding components of
the solubility parameter, respectively. Fdi and Fpi are the dispersion and polar
portions of the molar attraction constant, respectively. The F-method is not appli-
cable to the calculation of dh. Hansen previously stated that the hydrogen bonding
energy, Ehi, per structural group is relatively constant, which led to Eq. (2). For
molecules with several planes of symmetry, dh = 0. A assumption was made to
calculate the solubility parameters of the polymers and POSS using the Hoftyzer
and van Krevelen method.
POSS is inherently an organic/inorganic hybrid, and the inorganic part of POSS,
mainly –Si–O–Si–, does not react completely in an organic material. The distance
between Si and O of the –Si–O–Si– is 1.64 A, and it is shorter than the sum of the
covalent radii of POSS, 1.76 A, meaning that there exists a partial double bond
character of –Si–O– [87]. Nonetheless, the barrier of rotation around the –Si–O–
axis, ca. 2.5 kJ mol-1 as well as the barrier of linearization of the –Si–O–Si– angle,
ca. 1.3 kJ mol-1, is very low. Thus, the siloxane chain is rigid, so much that the
–Si–O–Si– angle, 140�–180�, is much wider than the tetrahedral angle, the silicon
atom has a relatively large size and the substituents appear only at every second
atom in the chain [87]. These features also account for the relatively high steric
hindrance effect, which indicates the siloxane group is highly stable. Therefore, it
was assumed that the functional groups in the outer surface of POSS dominate the
solubility parameter, and the inorganic part with the siloxane bonding of POSS was
excluded from the calculation of the solubility parameter.
Table 1 lists the calculated solubility parameters for all the polymers and POSS
nanoparticles. Themaximum difference in the solubility parameter showed the lowest
value in the POSS functionalized with the isobutyl group. Themaximum difference of
the solubility parameter between PE andOB-POSSwas calculated to be 0.09 J1/2 cm-3/2,
which was significantly smaller than those for PE/OM-POSS (2.13 J1/2 cm-3/2)
and PE/OP-POSS (1.89 J1/2 cm-3/2). The maximum difference in the solubility
parameters of PET/OM-POSS, PET/OB-POSS and PET/OP-POSS were calculated
to be 1.58 , 0.46 and 1.34 J1/2 cm-3/2, respectively. Also, the maximum difference
in the solubility parameter of Nylon 6/AB-POSS (0.56 J1/2 cm-3/2) was the lowest
Polym. Bull. (2015) 72:2331–2352 2339
123
among the Nylon 6/POSS nanohybrids. The variation of the maximum difference in
the solubility parameter can be explained by Eq. (4), which interrelates the
thermodynamic terms [77, 78].
vAB¼Vr
RTðdA � dBÞ2 ð4Þ
where vAB is the Flory–Huggins interaction parameter of polymer A and POSS B,
R and T are the gas constant and temperature, respectively, and Vr is a reference
volume which is the molar volume of the smallest repeat unit. Therefore, it was
expected that the interactions between the three polymers, viz., PE, PET and Nylon
6, and POSS functionalized with the isobutyl group, would be more thermody-
namically favorable than those of the others. These results correspond well with the
result obtained based on the melting point depression method.
Activation energy
The activation energy of the polymer/POSS nanohybrids can be determined by
employing the Kissinger’s equation shown below [88].
lnb
T2max
¼ lnAR
Eþ ln nð1� amaxÞn�1
h i� �
� E
RTmax
ð5Þ
where b is the heating rate (K min-1), Tmax is the temperature at the maximum
degradation rate, A is the pre-exponential factor, amax is the maximum conversion,
and n is the reaction order. Tmax was determined from the differential TGA curves.
Figure 3 shows the plots of ln(bTmax-2 ) versus Tmax
-1 according to the Kissinger’s
method for the PE/POSS and PET/POSS nanohybrids containing 0.5 wt% of POSS.
Tables 2 and 3 list the values of ln(bTmax-2 ), Tmax
-1 and calculated activation energy for
all of the neat polymers and polymer/POSS nanohybrids. The activation energies of
the PE, PE/OM-POSS, PE/OB-POSS and PE/OP-POSS nanohybrids, which were
Table 1 Solubility parameters of polymers and POSS derivatives
Fda Fp
a Eh
(J mol-1)
V
(cm3 mol-1)
dd (J1/2
cm-3/2)
dp (J1/2
cm-3/2)
dh (J1/2
cm-3/2)
d (J1/2
cm-3/2)
Polymers
–CH2– 270 0 0 15.55 17.36 0 0 17.36
–COO– 390 490 7000 23.70 16.46 20.68 17.19 17.91
–CONH– 450 980 5100 28.30 15.90 34.63 13.42 18.01
POSS derivatives
–CH3 420 0 0 21.55 19.49 0 0 19.49
–C(CH3)3 1190 0 0 68.21 17.45 0 0 17.45
–C(CH3)7 2870 0 0 154.41 18.59 0 0 18.59
–Phenyl 1430 110 0 74.52 19.19 1.48 0 19.25
a J1/2 cm-3/2 mol-1
2340 Polym. Bull. (2015) 72:2331–2352
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calculated from the slope of the straight line in Fig. 3a, were 17.3, 27.1, 33.3 and
24.4 kJ mol-1, respectively. In the case of the PET/POSS nanohybrids, the acti-
vation energies of PET, PET/OM-POSS, PET/OB-POSS and PET/OP-POSS were
9.53, 11.43, 24.54 and 7.12, respectively (Fig. 3b). Likewise, the highest energy was
obtained for the Nylon 6/AB-POSS nanohybrids. The polymer nanohybrids with the
POSS functionalized with isobutyl group showed the highest energy, which means
that isobutyl-POSS was dispersed most uniformly throughout the polymer matrix
due to the increased interaction between the polymer and isobutyl-POSS.
Fig. 3 Determination of theactivation energies for the a PE/POSS and b PET/POSSnanohybrids. The POSS contentwas fixed at 0.5 wt%
Table 2 Activation energy data for the neat PE and PE nanohybrids
PE PE/OM-POSS
b (K min-1) 283.13 293.13 313.13 283.13 293.13 313.13
oscillation within the linear region of 1 % strain amplitude. The storage and loss
moduli increased with increasing POSS content compared to those of the neat PE
throughout the applied frequency range. The slope of the change in the storage
modulus decreased with increasing POSS content, exhibiting a monotonic increase
of G0 for all frequencies, as shown in Fig. 5a. The fact that G0 became less
dependent on the frequency may be due to the formation of a network structure of
the POSS, indicating that the PE/POSS nanohybrids possessed solid-like charac-
teristics [88, 89]. In fact, the PE/OB-POSS nanohybrid shows more solid-like
behavior than the PE/OM-POSS and PE/OP-POSS nanohybrids. Moreover, the PE/
OB-POSS nanohybrids exhibited a greater increase of their storage modulus than
Fig. 4 FE-TEM images ofa PE/OM-POSS, b PE/OB-POSS, and c PE/OP-POSSnanohybrids. The POSS contentwas fixed at 0.5 wt%
Polym. Bull. (2015) 72:2331–2352 2343
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the PE/OM-POSS and PE/OP-POSS nanohybrids throughout the frequency range,
which means that the interaction between PE and OB-POSS was more favorable.
Solid-like behavior can also be observed in the complex viscosity curve in Fig. 5c.
As shown in Fig. 5c, the complex viscosity of the PE/POSS nanohybrids was higher
than that of the neat PE in the whole frequency range. The PE/OB-POSS
nanohybrid, in particular, showed more rapid shear thinning behavior, suggesting
that the interaction between PE and OB-POSS, which exhibited solid-like
characteristics, was higher than that of the other samples. These results
corresponded well with those obtained based on the Flory–Huggins interaction
parameter and the solubility parameter.
The storage modulus describes the stiffness of a material. The storage moduli for
the PE/POSS nanohybrids, measured at 1 Hz using DMA, are shown in Fig. 6 as a
function of temperature. Below Tg, the G0 is high because the polymer is in the
glassy state. However, above Tg, the G0 decreases because the polymer chains
Fig. 5 Rheological propertiesof the PE/POSS nanohybrids:a storage modulus, b lossmodulus, and c complexviscosity. The POSS content wasfixed at 0.5 wt%
2344 Polym. Bull. (2015) 72:2331–2352
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become mobile and the polymer is in the rubbery state. The G0 of the PE/OB-POSSnanohybrids increases with increasing OB-POSS content below and above their Tg(Fig. 6a). The same trend is observed for the PE/OP-POSS nanohybrids, as shown in
Fig. 6b. It is because POSS particles are rigid and, thus, the addition of POSS would
increase the rigidity of the nanohybrid system [90]. In Fig. 6a, the a-transitiontemperature decreases monotonically for all the PE nanohybrids, suggesting that the
free volume in the PE increases upon the addition of the POSS nanoparticles. The b-transition temperature also shifts to a lower temperature, and the b-peak lowers and
broadens when POSS is added. Therefore, POSS has a plasticizing effect on the b-transition as well as on the a-transition due to the free volume on the local motions
[63–66]. The viscoelastic and dynamic mechanical behavior of the PET/POSS
nanohybrids were very similar to those of the PE/POSS nanohybrids (Fig. S5 and S6
in the Supporting Information).
Fig. 6 Dynamic mechanicalproperties of the PE/POSSnanohybrids
Polym. Bull. (2015) 72:2331–2352 2345
123
However, the Nylon 6/POSS nanohybrids exhibit a significantly higher Tg than
pure Nylon 6, as shown in Fig. 7. The reason for the increase of Tg upon the
incorporation of POSS could be twofold. First, the incorporation of POSS increases
the cross-linking density of the resulting nanohybrids. The increase in the cross-
linking density leads to a higher Tg, broader tan d peak and higher storage modulus.
Second, POSS is a rigid body, that is, the addition of POSS would increase the
rigidity of the nanohybrid system [91, 92]. Although the incorporation of POSS
increases the free volume in the Nylon 6/POSS nanohybrids, this effect can be
compensated by the increase in the cross-linking density.
Thermal and mechanical properties
POSS has been proved to be effective in improving the thermal stability of polymers
[2, 26, 28, 29, 57, 60, 61]. The decomposition temperatures (Td) based on the 5 wt%
loss of the neat polymers and polymer/POSS nanohybrids are listed in Table 4.
Table 4 indicates that as the chain length of the functional group of POSS is
increased, there is a distinct shift in the onset of weight loss to a higher temperature.
Interestingly, the Td recorded for the PE/OB-POSS nanohybrid (99.5/0.5 by weight)
shows that the onset of degradation is higher by about 30 �C. The Tds of the PET/
OB-POSS (99.5/0.5) and Nylon 6/AB-POSS (99.5/0.5) nanohybrids are about 10 �Chigher than those of the neat PET and Nylon 6, respectively. This may be due to the
fact that the oxidation of the alkyl-substituted POSS in air takes place on the organic
chains and leads to the cross-linking of the cage, producing a ceramic silica-like
phase [93, 94]. On the other hand, the long alkyl-substituted POSS, that is, AO-
Fig. 7 Tan d of a Nylon 6/AB-POSS and b Nylon 6/AP-POSSnanohybrids
2346 Polym. Bull. (2015) 72:2331–2352
123
POSS (composed of eight hydrocarbon chains), induces a reduction in the thermal
stability in comparison with AB-POSS.
The highest values of the tensile strength and elongation at break were observed
for the PE/OB-POSS nanohybrid (99.5/0.5), which were 92 and 36 % above those
of the neat PE, respectively. However, the mechanical properties decrease as the
amount of the POSS nanoparticles increases to 1 and 2 wt%. With respect to the
corresponding value of the neat PET, the addition of 0.5 wt% OB-POSS to PET
causes an increase in the tensile strength of about 30 %, while a more pronounced
increase in the elongation at break (300 %) is observed for the PET/OB-POSS
nanohybrid (99.5/0.5). In addition, the maximum tensile strength and elongation at
break are obtained in the Nylon 6/AB-POSS nanohybrid (95.5/0.5). These results
suggest that the interaction between the polymer and isobutyl-substituted POSS is
more favorable than that of the other samples. The thermal and mechanical results
are also in good agreement with the theoretical considerations based on the Flory–
Huggins interaction parameter, solubility parameter, and activation energy.
Conclusions
The Flory–Huggins interaction parameters between the polymers and POSS
nanoparticles were determined using the melting point depression method. In the
case of the PE nanohybrids, PE with OB-POSS was thermodynamically most
favorable. The thermodynamic interactions of both the PET/OB-POSS and Nylon
6/AB-POSS pairs were also found to be highly favorable according to the Flory–
Huggins interaction parameters. The maximum difference of the solubility
parameter between PE and OB-POSS was much smaller than those for PE/OM-
POSS and PE/OP-POSS, indicating that the interaction between PE and OB-POSS
was more favorable than that of the others. In the PET nanohybrids, the maximum
solubility parameter difference of PET/OB-POSS pairs was less than the others,
suggesting that the thermodynamic interaction of the PET/OB-POSS pair is most
Table 4 TGA results for the neat polymers and polymer nanohybrids