Molecular motions in glassy polymers with aromatic groups either in the side chain or in the backbone

Abstract

Dynamic mechanical properties (sound velocity v and damping factor Q −1) have been determined, below the glass transition, over a wide range of temperature at acoustic frequencies ( ⋍ 10 kHz), in polymers containing aromatic groups either in the side chain (polystyrenes, poly(vinyl benzoates), polyvinylpyridines, poly(vinyl nicotinates)) or in the backbone (poly(phenylene oxides)). The ortho and meta ring-substituted polystyrenes do not show any secondary mechanical relaxation effect in the glassy state; in all the other substituted polystyrenes a low-temperature damping peak (delta-process) is found at 90–100°K, the height and location on the temperature scale of which depend on the nature and position of the substituents. Para substitutions in the ring or α-substitutions in the main chain shift the delta-peak of the unsubstituted polystyrenes towards higher temperatures and increase apparent activation energies. Substitutions in β-position do not affect the delta-peak. Analogous results are found in poly(vinyl benzoates), which show also an additional relaxation effect (beta-process). The delta peak is connected with “wagging” motions of the ring and the beta-peak is associated with carbonyl group movements. A delta-process is also found in polyvinylpyridines and poly (vinyl nicotinates), which however is shifted towards higher temperatures as compared with the analogous phenomenon in polystyrene. In polymers with aromatic rings in the skeleton no damping peak is found at temperatures below room temperatures, but a relaxation effect is found, which covers a very wide temperature range and gives place to a flattened loss maximum at 400–450°K. Such a phenomenon is tentatively attributed to torsional oscillation of phenylene rings.