Macromolecular conformational volume and thermodynamics of crystal melting

Abstract

Conformational parts of the thermodynamical function change on melting, in particular the conformational volume ΔV conf, the conformational entropy ΔS conf and the pressure variation of the latter are analysed for an ideal polymer crystal. These quantities can be calculated from the data for low molecular compounds combined with the three-state rotational isomer model (with adjacent gauche states of opposite sign forbidden). The assumption of intramolecular dominance in the Gibbs energy difference between conformers in a polymethylene chain results in a negative value for ΔV conf and a positive value for the quotient ( ∂ΔS conf ∂P ) T which contradicts the available experimental data for polyethylene crystal fusion. The disagreement might be reconciled by the postulation of intermolecular contribution to the Gibbs energy difference between conformers favouring the existence of trans sequences of bonds in the chain and the correlation of molecular orientations in neighbouring chains.