Orientation of amorphous polymers

Abstract

AbstractOrientation of amorphous polymers stretched at a temperature above their glass‐transition temperature, is involved in thermoforming processing.The molecular processes controlling the orientation and chain relaxation of polymers have been investigated by infrared dichroism in a large series of materials: polystyrene, polymethylmethacrylate of various tacticity and its copolymers with styrene and acrylonitrile.Polystyrene with hydrogenated and deuterated blocks leads to information on the behavior of each block (central part, chain ends) and allows a quantitative comparison with the Doi‐Edwards model for chain relaxation. In order to analyse the effect of polydispersity, blends of hydrogenated and deuerated polystyrene chains with various molecular weights have been studied. Short chains with molecular weights smaller than the molecular weight between entanglements, enhance the relaxation of long chains. Furthermore an anisotropic orientational coupling effect exists between a chain segment and its oriented surrounding.By comparing the orientation of polymers with different chemical structures, it results that they behave differently under temperature conditions where T ‐ Tg = const, but they undergo identical relaxations when the experiments are performed at temperatures chosen in such a way that the monomer friction coefficients are identical.In copolymers of styrene and methylmethacrylate, the two monomer units have different orientations due to local conformational constraints. This effect also accounts for the difference observed between an alternated and a random copolymer.