Published data of dynamic birefringence and viscoelasticity of amorphous polymers were compared with the molecular expression of stress proposed by Gao and Weiner. The theory states that the stress is composed of contributions from the chain orientation (orientation term), the monomer orientation around the chain axis (rotation term), and the fluctuation of the local stress tensor (fluctuation term); the birefringence is composed of only two terms corresponding to the first two of the stress. The experimental data indicate that in the glassy zone and the high-frequency region of the glass-to-rubber transition zone the stress is attributable to the rotation and the fluctuation terms and the degrees of contribution vary with polymer species. For polymers with flat units (like polycarbonate), the fluctuation term is negligible and the relaxation spectrum in the glassy zone is low. For polymers with thin axisymmetric units (like polyisobutylene) or bulky irregular units (like poly(2-vinylnaphthalene)), the relaxation spectrum in the glassy zone is enhanced by the fluctuation term. It is also argued that, for polymers with thin axisymmetric units, the relaxation spectrum for the rotation term resembles that of a dilute solution; the role of rotational barrier along the chain is relatively enhanced since the rotational barrier from the surrounding is low because of the symmetry of the unit and because of the high fluctuation of the local stress. Some experiments are proposed to verify the statements. IntroductionThe strain-induced birefringence of polymers is related to the stress. In the rubbery and terminal flow zones of the stress relaxation or dynamic mechanical measurements, the deviatoric component of the refractive index tensor, An, is proportional to that of the stress tensor, u.

No abstract

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, CR, varies much more with the molecular structure, but the coefficient of the G component, CG, is positive and almost constant for these polymers.