Theory of melting in simple polymers. II. Contribution from phonon anharmonicity

The effect of phonons is introduced in a previously described formalism for calculating the Helmholtz energy of a polymer crystal and its melt. The phonon frequencies are functionals of the distribution of torsional angles in the system, but this distribution must in turn be self-consistently determined by minimization of the total free energy, including phonon contributions. The theory was applied with the use of intrachain potentials calculated by molecular orbital techniques and interchain potentials found by molecular mechanics. Inclusion of phonons leads to improved agreement with the experimentally observed melting temperature in polymethylene, but aggravates a discrepancy in the case of polytetrafluoroethylene.