Structural effect on stretch-induced birefringence in polyimide films

Abstract

Seven polyimides (Pls) of low glass transition temperature were synthesized and their films were uniaxially stretched to observe the variation in their in-plane birefringence (An). The intrinsic birefringences (triangle n degrees) of their polymer chains were also estimated using the experimental average refractive indices and computationally calculated polarizability tensors. The observed stretch-induced An was markedly affected by the PI structure. A model that shows a good fit to experimental data is proposed to predict the orientation degree of PI chains in different PI films. The model, when combined with the analysis of the estimated An's, is useful in explaining the behavior of stretch-induced triangle ns in PI films.