Birefringence of amorphous polymers. V. Dynamic measurements on poly(α‐methyl styrene) and polycarbonate

The complex Young’s modulus, E*(ω), and the complex strain‐optical coefficient, O*(ω), of poly(α‐methyl styrene), PMS, and Bisphenol A polycarbonate, PC, were measured over the frequency range of 1–130 Hz around the glass‐to‐rubber transition point. The real part of O*(ω), O′, of PMS is negative over the entire temperature range considered and the imaginary part, O″, changes its sign from negative to positive with decreasing temperature. Both O′ and O″ of PC are positive over the entire temperature range used. These results are qualitatively different from those for polystyrene. The data were analyzed with a modified stress‐optical rule and the complex modulus was separated into two components (denoted by R and G). The G component, which is located in the glassy zone, is related to the high glass modulus, and the shapes of the G components of PMS, PC, and PS are very similar to each other. The R component, located at the long time end of the glass‐to‐rubber transition zone of PC, is quite different from those of PS and PMS. The difference is mainly due to differences in the plateau modulus, which for PC is about ten times higher than that of PS. The stress‐optical coefficient of the R component, C_R, varies much more with the molecular structure, but the coefficient of the G component, C_G, is positive and almost constant for these polymers.