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

Simulations of crystalline polyethylene at finite temperature are carried out using a molecular mechanics force field for the interatomic potential and quasi-harmonic lattice dynamics for the vibrational free energy. The thermal expansion coefficient, determined by direct minimization of the free energy, is in excellent agreement with experimental results for temperatures up to 250 K and remains in reasonable agreement with experiment throughout the range of temperatures for which experimental results exist. The axial Young modulus at 300 K is found to be 280 GPa, which agrees with the results of Raman scattering experiments 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 relatively temperature independent. The Gruneisen parameter is calculated as a function of temperature and is in agreement with the experimental values. The results of calculations using classical mechanics and quantum mechanics for the vibrational energy are compared to assess the importance of quantum effects as a function of temperature. Quantum effects are largest for the c-axis thermal expansion coefficient, which is in error by a factor of 2 at room temperature when classical mechanics is used.