MOLECULAR-BASIS FOR THE ANISOTROPIC TRANSVERSE THERMAL-EXPANSION OF SYNDIOTACTIC POLYPROPYLENE

The thermal expansion of crystalline syndiotactic polypropylene is sim ulated to elucidate the origins of the transverse thermal expansion an isotropy observed in these materials. The calculations are carried out using a lattice dynamics method and empirical force fields. For the t ransverse thermal expansion coefficients, the results obtained for dif ferent crystal structures and force fields bracket the experimental re sults. Thermal expansion is calculated to be greater along the b axis than along the a axis, in agreement with experiment. This transverse t hermal expansion anisotropy is found to arise from larger thermal stre sses, lower stiffness, and less Poisson coupling along the b axis.