In-plane birefringence and elongation behavior of uniaxially drawn aromatic polyimide films

Six kinds of aromatic poly(amic acid) films, the precursor of polyimides, w ere uniaxially drawn by constant loads during thermal curing with varying t he final curing temperature and the load. The in-plane birefringence (Delta n) generated by uniaxial orientation of po molecules in the Fim plane was i nvestigated in with their molecular structures and the elongation behavior of polyimide films. The polyimides derived from pyromellitic dianhydride (P MDA) showed much larger values of Deltan than the polyimides derived from d ianhydrides having rotatable diphenyl linkages. The Deltan former polyimide s increases with increasing the final curing temperature and the elongation between the most shrunken and the most elongated states during curing. In particular, the polyimides having rod-like structures show the largest valu e of Deltan. In contrast, the latter polyimides take maximal values of Delt an at much lower temperatures. The introduction of hexafluroisopropiridene (-C(CF3)(2)-)group into the main chain effectively lowers the values of Del tan. This result can be related to the inherent low polarizability anisotro py and the decreased molecular packing constant that are characteristic of highly fluorinated groups. These experimental results are qualitatively exp lained by the calculations of polarizability anisotropies of model compound s using FPT-MNDO-PM3 theory. Copyright (C) 2001 John Wiley & Sons, Ltd.