On the Existence of Local Segmental Order in Undiluted Bulk Polymers

Abstract

The mean‐square end‐to‐end distance and its temperature coefficient, the change in the intramolecular conformational entropy on melting, and the optical anisotropy, as manifested by the strain birefringence, and its temperature coefficient are calculated in the rotational isomeric‐state approximation for unperturbed linear polyethylene chains containing short stretches of skeletal bonds exclusively in the all trans or planar zigzag conformation. The results are compared with the corresponding quantities calculated for a polyethylene chain free of such conformational constraints and with experimental data taken from the literature which were obtained on undiluted bulk polyethylene. This comparison shows that the optical properties of polyethylene are more sensitive to the local segmental order, as defined in this study, than are its dimensions and their temperature coefficient or the change in intramolecular conformational entropy on melting. An estimate of the upper limit of local segmental order in bulk polyethylene, which is consistent with experimental and calculated results, is presented, and the conclusion that little, if any, segmental order exists in bulk polymers is drawn. The effect of diluents on the strain birefringence of polymer networks is commented upon in this connection.