Temperature dependence of orientation birefringence of polymers in the glassy and rubbery states

Abstract

AbstractThe temperature dependence of orientation birefringence has been measured for several different polymers, by producing samples with a fixed degree of orientation by mechanical stretching, and varying the temperature. The following three polymers were studied: polystyrene, poly (methyl acrylate), and poly(methyl methacrylate)—the last in both atactic and stereospecific forms. These all have negative orientation birefringence at room temperature. Measurements were carried out in a range of temperatures below the glass transition temperature, except for poly(methyl acrylate) which was crosslinked. The curves obtained for the different polymers showed considerable differences in form; they were reversible in regard to temperature cycling, and showed no appreciable time effects. Polystyrene showed little change in its orientation birefringence with temperature. The other polymers showed appreciable changes with temperature, and an actual change of sign of the birefringence from negative to positive with increasing temperature. The crossover temperature was near the glass transition temperature for the various stereo forms of PMMA, and measurement of this crossover temperature might therefore provide a fairly accurate method for determining the stereo composition of a PMMA sample. However, the crossover temperature is considerably above Tg in the case of PMA. We believe that these curves reflect thermal motions of the molecular groups with polarizability anisotropy (such as the phenyl and eater side groups) whose orientation gives rise to the birefringence. The small change is the case of polystyrene is undoubtedly a consequence of the rigidity of the phenyl ring and the steric hindrance of its immediate environment. These results may also reflect chain backbone motions, and other effects, in an indirect way. The theory and experimental data for a complete interpretation of these results is not yet available. However, some of the evident consequences of the present findings are discussed.