Studies on Birefringence Effect and its Sign for Some Polymers

The birefringence of polymers caused by stress has to be recognized not statically but rheologically in the course of creep. On the basis of the following definitions birefringence effect can be reasonably understood. ”The birefringence effect”, termed generically for the birefringence caused by stress, is classified into ”the-photo-elastic-effect” for the birefringence observed at the moment of stressing and ”the retarded-photo-elastic-effect” for the component of the birefringence corresponding to retarded elasticity under continues stress. The phenomena of the photo-elastic- and retarded-photo-elasitic-effect and these sign for some polymers in the course of creep are studied in the temperature range from -100℃ to 150℃. For polyester copolymers the photo-elastic-effect (α_e) in the glassy region shows small positive value and the maximum point of α_e is observed near the glass transition region. α_e decreases remarkably in the glass transition region and its sign is reversed to negative. The absolute value of negative α_e in the rubbery region is several hundred times as large as that in the glassy region. The phenomena of the retarded-photo-elastic-effect (α_r) show the similar trend a α_e with temperature. It can be considered that the birefringence behavior of polyester copolymers at each temperature is decided in the quantitative interrelation of positive and negative effects, corresponding to glassy and rubbery elasticities respectively. For epoxypolysulfide copolymers the sign of α_e is positive over all temperature range, but as for α_r the reversal of the sign is observed. Namely the sigh of α_r is positive at temperatures below the glass transition region but negative in the rubbery region.

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