The p‐phenylene linkage in linear high polymers: Some structure–property relationships

Abstract

AbstractThe high crystallite melting point of polyethylene terephthalate (Terylene) compared with the melting points of the aliphatic polyesters emphasizes the significance of the p‐phenylene linkage as an important factor determining polymer properties. A consideration of other polymer structures containing the p‐phenylene linkage shows that its influence is independent of any polar groups which may be present. X‐ray examination of the crystallite structure of polyethylene terephthalate and related substances has revealed no evidence of strong intermolecular forces.From a study of co‐polyesters based upon polyethylene terephthalate with aliphatic dibasic acids it is shown that the melting point of compositions containing more than 40 mole per cent of ethylene terephthalate is essentially independent of the aliphatic di‐basic acid employed. The Flory relationship between crystallite melting point and mole fraction of the units forming the crystallites shows fair agreement with the results obtained. The calculated latent heat of fusion of polyethylene terephthalate approaches closely to that of the corresponding aliphatic polyesters, and suggests that the high melting point induced by the p‐phenylene linkage is due to chain rigidity rather than to strong interchain attraction.This rigidity is consistent with the second order transition temperatures of such polyesters. It is shown that second order transition temperature is a linear function of the number of p‐phenylene linkages per unit chain length. The p‐phenylene linkage reduces the possibilities of change of shape available to a given length of chain, so accounting for the low entropy of crystallization and high melting point of polyethylene terephthalate.Evidence is advanced for the isomorphism of adipyl and terephthalyl linkages in linear polyamides. The absence of isomorphism in polyethylene adipate/terephthalate copolymers is ascribed to the weakness of the interchain forces.