Anisotropy in the thermal shrinkage of polyimide film

Anisotropy in a polyimide film was investigated in a quantitative manner by a nonlinear regression of the thermal shrinkage data obtained from thermom echanical analysis. The thermally induced shrinkage of this pyromellitic di anhydride-oxydianiline polyimide film at 573 K was directionally anisotropi c in time film plane by as much as about 0.4%. The direction of maximum the rmal shrinkage was inclined by about pi /6 rad from the machine direction o f the film. The thermal shrinkage behavior of the polyimide films in the vi cinity of the glass-transition temperature (T-g) showed an unusual anisotro pic response. On the basis of a correlation between the anisotropy in the t hermal shrinkage of the films and the molecular orientation of the polyimid e, this characteristic thermal shrinkage behavior around T-g is suggested t o be due to a recovery of the free volume lost by a physical aging process. (C) 2000 John Wiley gr Sons, Inc.