Negative thermal expansivity of polymer crystals: Planar zig‐zag chain model

The thermal expansivity of polymer crystals in the direction parallel to the chain axis α ∥ is studied in a planar zig‐zag chain model. Using values of force constants for polyethylene, it was found that α ∥ = −1.3 × 10 −5 K −1 , in good agreement with experimental results. Torsional modes are found to contribute slightly more than half α ∥ . The expansivity is independent of the mass of the atomic groups on the chain and only weakly dependent on the interchain van der Waals interaction.