COMPATIBILITY EFFECT ON MOISTURE DIFFUSION IN POLYIMIDE BLENDS

Blends of rod-like PMDA-B (pyromellitic dianhydride-benzidine) and semi-flexible 6FDA-PDA (6F-dianhydride-phenylendiamine) polyimides with several different compositions were prepared. According to the results from X-ray diffractometry and FTi.r. spectrometry, these two polyimides are incompatible when mixed at room temperature for 20 min. When mixed at 50-degrees-C for 40 h, the polymers, thermodynamically incompatible, become compatible owing to exchange reactions. Bending beam diffusion experiments show that the diffusion of moisture in these films belongs to case I. The diffusion constant D is 0.10 x 10(-9) cm2 s-1 in PMDA-B and 1.65 x 10(-9) cm2 s-1 in 6FDA-PDA. In the blends, D increases with increasing content of 6FDA-PDA. The slow diffusion in PMDA-B can be attributed to its highly crystalline structure and relatively small interchain spacing. The diffusion of moisture is faster in the compatible films. Regardless of compatibility, diffusion in all the blends is much slower than in pure 6FDA-PDA. This can be attributed to the comparatively small average interchain spacings of the blends.