| Abstract: | Anisotropy in a polyimide film was investigated in a quantitative manner by a nonlinear regression of the thermal shrinkage data obtained from thermomechanical analysis. The thermally induced shrinkage of this pyromellitic dianhydride–oxydianiline polyimide film at 573 K was directionally anisotropic in the film plane by as much as about 0.4%. The direction of maximum thermal shrinkage was inclined by about π/6 rad from the machine direction of the film. The thermal shrinkage behavior of the polyimide films in the vicinity of the glass‐transition temperature (Tg) showed an unusual anisotropic response. On the basis of a correlation between the anisotropy in the thermal shrinkage of the films and the molecular orientation of the polyimide, this characteristic thermal shrinkage behavior around Tg is suggested to be due to a recovery of the free volume lost by a physical aging process. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3222–3229, 2000 |
