COMPATIBILITY EFFECT ON MOISTURE DIFFUSION IN POLYIMIDE BLENDS

Blends of rod-like PMDA-B (pyromellitic dianhydride-benzidine) and sem i-flexible 6FDA-PDA (6F-dianhydride-phenylendiamine) polyimides with s everal different compositions were prepared. According to the results from X-ray diffractometry and FTi.r. spectrometry, these two polyimide s are incompatible when mixed at room temperature for 20 min. When mix ed at 50-degrees-C for 40 h, the polymers, thermodynamically incompati ble, become compatible owing to exchange reactions. Bending beam diffu sion experiments show that the diffusion of moisture in these films be longs 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 increas es with increasing content of 6FDA-PDA. The slow diffusion in PMDA-B c an be attributed to its highly crystalline structure and relatively sm all interchain spacing. The diffusion of moisture is faster in the com patible films. Regardless of compatibility, diffusion in all the blend s is much slower than in pure 6FDA-PDA. This can be attributed to the comparatively small average interchain spacings of the blends.