Relaxation in polymers

Abstract

Various molecular motions known to occur in high polymers are described and illustrated in connection with new magnetic resonance relaxation results. The motions include side-group reorientations, main-chain reorientation and translation and glass-transition phenomena. Classical hindered rotation and tunnelling are contrasted in connection with methyl group reorientation in polyvinyl acetate. A number of other methyl rotation situations are described. Polar side-group reorientation is discussed and the site-model parameters are evaluated for the well-characterized ester-group motion in polymethyl methacrylate. Main-chain reorientations are illustrated for several systems. In some cases all chains rotate and in others only a fraction of the chains are sufficiently free. This motion can become apparent abruptly at a first-order transition, as in linear polytetrafluoroethylene and polybutene-1, or gradually increase in frequency as the temperature is raised, as in hexafluoropropylene-tetrafluoroethylene copolymers and polychlorotrifluoroethylene. Translations parallel to the main chain axes are indicated for some fluorinated polymers. Fibre studies for tetrafluoroethylene polymers reveal these two types of main chain motions clearly. Glass-transition phenomena are described in connection with measurements on the extensively-studied orthoterphenyl. It is proposed that the rival configurational entropy and free-volume theories are about equally empirical.