SIMULATION OF THE TEMPERATURE-DEPENDENCE OF MECHANICAL-PROPERTIES OF POLYETHYLENE

Simulations of crystalline polyethylene at finite temperature are carr ied out using a molecular mechanics force field for the interatomic po tential and quasi-harmonic lattice dynamics for the vibrational free e nergy. The thermal expansion coefficient, determined by direct minimiz ation of the free energy, is in excellent agreement with experimental results for temperatures up to 250 K and remains in reasonable agreeme nt with experiment throughout the range of temperatures for which expe rimental results exist. The axial Young modulus at 300 K is found to b e 280 GPa, which agrees with the results of Raman scattering experimen ts with corrections for the effects of interlamellar coupling. The nu( ac) and nu(bc) Poisson ratios are found to increase substantially with temperature, whereas the nu(ab) and nu(bc) Poisson ratios are relativ ely temperature independent. The Gruneisen parameter is calculated as a function of temperature and is in agreement with the experimental va lues. The results of calculations using classical mechanics and quantu m mechanics for the vibrational energy are compared to assess the impo rtance of quantum effects as a function of temperature. Quantum effect s are largest for the c-axis thermal expansion coefficient, which is i n error by a factor of 2 at room temperature when classical mechanics is used.