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

<jats:title>Abstract</jats:title><jats:p>Six kinds of aromatic poly(amic acid) films, the precursor of polyimides, were uniaxially drawn by constant loads during thermal curing with varying the final curing temperature and the load. The in‐plane birefringence (Δn) generated by uniaxial orientation of polyimide molecules in the film plane was investigated in relation with their molecular structures and the elongation behavior of polyimide films. The polyimides derived from pyromellitic dianhydride (PMDA) showed much larger values of Δn than the polyimides derived from dianhydrides having rotatable diphenyl linkages. The Δn of the former polyimides 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 value of Δn. In contrast, the latter polyimides take maximal values of Δn at much lower temperatures. The introduction of hexafluroisopropiridene (C(CF<jats:sub>3</jats:sub>)<jats:sub>2</jats:sub>) group into the main chain effectively lowers the values of Δn. This result can be related to the inherent low polarizability anisotropy and the decreased molecular packing constant that are characteristic of highly fluorinated groups. These experimental results are qualitatively explained by the calculations of polarizability anisotropies of model compounds using FPT‐MNDO‐PM3 theory. Copyright © 2001 John Wiley & Sons, Ltd.</jats:p>