Abstract
A general formula correlating the bending curvature variation ratio of a layered structure caused by solvent‐induced swelling in its polymer overcoat with diffusion time under case II diffusion has been presented. In the event of case II diffusion, the diffusion front velocity, v, can be calculated by using this formula and measured by a bending‐beam apparatus. At room temperature, the diffusion of n‐methyl‐pyrrolidone (NMP) solvent in the film of pyromellitic phenylene diamine (PMDA‐PDA) is case II. While in PMDA‐B (‐benzidine) and benzophenone tetracarboxylic dianhydride (BPDA‐PDA), no diffusion progress can be observed. But, the diffusion in 6F‐dianhydride‐ (6FDA‐PDA) is case I with D = 0.85 × 10 −9 cm 2 /s. It becomes anomalous when mixing with 25% PMDA‐B, but becomes case II diffusion with more PMDA‐B. The preabsorbed moisture in the films does not affect the v value. In PMDA‐PDA, v = 7.3 × 10 −8 cm/s. In the 25/75 and 50/50 6FDA‐PDA/PMDA‐B blends, v = 6.3 and 11.3 × 10 −8 cm/s. Copyright © 1992 John Wiley & Sons, Inc.