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 chains cause the length projected along the chain direction to decrease. The expansivity along the chain direction, α c ∥ is therefore negative, and has 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, α c ⟂ perpendicular to the chain direction. The theoretical predictions for α c ∥ and α c ⟂ agree quite well with experimental data over the wide temperature range under consideration.