Effects of wavelength on strain-induced birefringence of polymers

Strain-induced birefringence, Delta n, was measured at wavelengths 400 nm <lambda< 800 nm for uniaxially elongated films of polystyrene (atactic and syndiotactic), bisphenol A polycarbonate, poly(ethylene terephthalate), and poly(ethylene naphthalate). The magnitude of Delta n was always higher at lower lambda: the reduced birefringence, k(lambda)=Delta n(lambda)/Delta n(633 nm), was a decreasing function of lambda. The strain-induced crystallization in poly(ethylene terephthalate) (PET) and syndiotactic polystyrene (sPS) did not significantly affect k(lambda.). For polycarbonate (PC) and PET I-dependence was much stronger when the strain was applied in the melt state (corresponding to the stress-optical coefficient, SOC) than in the glassy state (corresponding to the photoelastic coefficient, PEG); the difference was very small for atactic polystyrene (aPS). These results may be explained with a model theory in which the SOC is associated solely with the chain orientation while the PEC is affected also by the rotation of repealing units around the chain axis induced by deformation of the material glassy state.