Gc. Rutledge, THERMOMECHANICAL PROPERTIES OF THE CRYSTAL PHASE OF POLY(ETHYLENE-TEREPHTHALATE) BY MOLECULAR MODELING, Macromolecules, 30(9), 1997, pp. 2785-2791
Theoretical values for the thermomechanical properties of poly(ethylen
e terephthalate) (PET) are determined self-consistently using the pcff
force field to compute the potential energy and quasiharmonic lattice
dynamics to determine the vibrational free energy. Complete sets of l
attice constants, thermal expansion coefficients, elastic properties,
and Gruneisen coefficients are reported between 0 and 400 K for the tr
iclinic PET unit cell. Mean square displacement matrices for the const
ituent atoms of PET were determined, from which a theoretical B-factor
for X-ray scattering of 4.0 Angstrom(2) at 300 K is estimated. The 50
% probability ellipsoids for thermal vibration of all atoms in the asy
mmetric unit are computed. Calculated lattice parameters at 300 K agre
e with experimental data, to within the accuracy of the method. Calcul
ated elastic constants for a transverse isotropic composite agree with
data from X-ray and ultrasonic velocity measurements on highly orient
ed samples. The tensile elastic stiffness constants are temperature-de
pendent, while the shear stiffnesses are roughly constant in the range
0-400 K. Thermal contraction along the chain direction is observed in
PET, consistent with results for other polymer crystals possessing ch
ains in fully extended conformations. The driving force for contractio
n is entropic in origin, arising from negative gamma(3) and gamma(6) G
runeisen coefficients.