Water sorption in thin films of high-temperature polyimides: the effect of imidization history

摘要: PMDA-ODA and BPDA-PDA polyimides in 10-50 Atrn thick films were prepared from the respective poly(amic acid) precursor by thermal imidization for 1-10 h at various temperatures over 230-400C. Their water sorption behaviours were studied in detail by gravimetry. Water sorption in both PMDA-ODA and BPDA-PDA followed a nearly Fickian process in spite of the morphological heterogeneity due to the ordered and less ordered phases. Water clustering or plasticization was very weakly evident on the water uptake versus humidity plots. Water sorption in both polyimides was highly dependent upon imidization history. Both diffusion coefficient and water uptake are decreasing functions of imidization temperature and time. This resulted mainly from the enhancement of overall crystallinity by high-temperature imidization because of their high-rT nature. The diffusion coefficient and water uptake were (1.0-3.0) x 10-9 cm2 s-1 and 0.4-4.5 wt% for PMDA-ODA and (0.9-3.0)x 10-cm2s-' and 0. I2-2.40wt%, depending on humidity,/iihidization history and film thickness. The differences between the water sorptions of the p'olyimides are interpreted by considering the differences in their morphologies and chemical affinities to water.