TEMPERATURE-DEPENDENCE OF THE THERMAL EXPANSIVITY OF POLYMER CRYSTALS - LINEAR-CHAIN MODEL

A linear chain model is used to calculate the thermal expansivities of polymer crystals from room temperature down to liquid-helium temperature. Because of restraint by strong covalent forces, vibrations along the chains give a negligible contribution. Vibrations transverse to the chain cause the length projected along the chain direction to decrease. The expansivity along the chain direction is therefore negative, and has a temperature dependence characterized by the low Debye temperature of transverse vibrations. These vibrations, through a cubic term in the interchain van der Waals potential, also cause a positive expansivity, perpendicular to the chain direction. The theoretical predictions for the two expansivity parameters agree quite well with experimental data over the wide temperature range under consideration.