Effect of backbone on the biaxial retardation of polyimide films in uniaxial stretch

A biaxial-retardation film prepared from the uniaxial-stretched polyimide (PI) film was developed for compensating the viewing angle of liquid-crystal displays. Good uniformity of in-plane birefringence in well-stretched PI films was observed visually with two crossed polarizers. The prism coupling method was used to measure the nx, ny, nz, and thickness of the stretched PI films. The birefringence variations of nx − ny (difference of refractive indices between x- and y-axes) and nx − nz during the stretching process were highly affected by PI structure. The polarizability tensors and intrinsic birefringences of the PI repeat units were estimated by Gaussian 98W. It was found that the stretch-induced birefringence is more related to the average polarizability per volume at the low elongation period.