Abstract
By the usage of a bending beam technique, it is found that the diffusion of NMP (N-methylpyrrolidone) solvent in PMDA-ODA (pyromellitic dianhydride-4,4′-oxydianiline) imide films belongs to case I diffusion, while that in PMDA-PDA (pyromellitic dianhydride-p-phenylenediamine) belongs to case II. The difference may presumably be attributed to the fact that PMDA-ODA is relatively flexible and can form an amorphous and large void-rich film structure so that NMP molecules can penetrate more freely, while PMDA-PDA is rigid and can form a highly crystalline and large void-free film structure so that NMP molecules need to relax the imide chain in order for diffusion to proceed. The diffusion mechanisms do not vary with thickness ranging from 0.5 to 10 μm or curing temperature from 200 to 400°C, while the diffusion processes apparently become faster with the increase of thickness and decrease of curing temperature. For PMDA-ODA cured at 250°C, the calculated diffusion coefficient, D, increases from 1.2 to 3.7 × 10 -11 cm 2 /s for thickness increasing from 2.8 to 6.7 μm. For curing temperatures of 200, 250, 300, 350, and 400°C, D = 4.8, 3.3, 1.8, 0.60, and 0.46 × 10 -11 cm 2 /s, respectively. For PMDA-PDA cured at 250 °C, the calculated diffusion front velocity, υ, increases from 2.6 to 14 × 10 -8 cm/s for thickness increasing from 0.5 to 10.2 μm. For curing temperatures of 200, 250, 300, 350, and 400 °C, v = 1.0, 0.70, 0.34, 0.23, and 0.099 × 10 -7 cm 2 /s, respectively. The increases in the diffusion rate with respect to increasing thickness and decreasing curing temperature may be attributed to the decrease in film ordering and/or orientation. The reversibility in the diffusion process upon repeated diffusion/drying cycles indicates that the films studied must be cross-linked. The degree of cross-linking seems to increase with the increase of film thickness.