MOLECULAR-WEIGHT AND FILM THICKNESS EFFECTS ON LINEAR OPTICAL ANISOTROPY OF 6FDA-PFMB POLYIMIDES

Different molecular weight samples of a polyimide, 6FDA-PFMB, were syn thesized from 2,2'-bis(3,4-dicarboxyphenyl) hexafluoropropane (6FDA) a nd 2,2'-bis(trifluoromethyl)-4,4'-diaminobiphenyl (PFMB) in refluxing m-cresol at high temperatures through a one-step polymerization route. Films of 6FDA-PFMB ranging from 1 to 100 mu m thickness were prepared via solution-casting and investigated using optical prism coupler wav e-guide experiments. It has been found that these films exhibit a line ar optical anisotropy in the refractive indices. Specifically, they ha ve a large index along the in-plane direction and a smaller one in the out-of-plane direction. Films with this linear optical anisotropy hav e been recognized to possess uniaxial negative birefringence. For film s with a fixed molecular weight, the refractive indices are constant i n the him thickness below 15 mu m, and the birefringence gradually dec reases with further increasing the film thickness. On the other hand, the refractive index along the out-of-plane direction decreases while the in-plane refractive index increases when the polyimide molecular w eight increases. Such films can be used as negative birefringent compe nsators in twisted and super-twisted nematic liquid crystal displays t o improve display viewing angles. (C) 1997 Elsevier Science Ltd.