A study of solvent diffusion in thin polyimide films using laser interferometry

Abstract

Using laser interferometry, we have determined in situ the thickness increase with time of thin supported polyimide films (4–8 μm in initial thickness) immersed in n‐methyl‐2‐pyrrolidone (NMP) as a function of NMP temperature (22–120°C). Similar experiments were also performed in dimethyl sulfoxide (DMSO) at 22°C with polyimide films of 4.1 μm in thickness. For NMP, the equilibrium fractional thickness increase (about 20%) is independent of initial polyimide thickness and temperature. The time scale for reaching equilibrium sharply decreases with temperature from 2–3 days at 22°C to 30–60 min at 120°C. Compared with NMP, the rate of DMSO sorption is considerably faster, reaching equilibrium swelling of about 28% in about 5 h at 22°C. To describe the transport process, we applied a phenomenological model proposed by Astarita and Sarti1 but reformulated in polymer fixed frame to enable straightforward comparison with the thickness data. Our analysis indicated that the transport of NMP is best described as anomalous, that is, intermediate between diffusion controlled and case II transport. The effective diffusion coefficient Deff and the front velocity U0 at 22°C were found to be 3–6 × 10−12 cm2/s and 8 × 10−9 cm/s, respectively. Our front velocity is in good accord with the value of 6 × 10−9 cm/s obtained for a similar polyimide based on gravimetric measurements.2 Both Deff and U0 show an activation energy of ∼56 kJ/mol. For DMSO, however, the transport is clearly case II. The front velocity at 22°C was found to be about 6 × 10−8 cm/s, which is about four times that obtained by Rutherford back‐scattering spectrometry.3 Copyright © 1989 John Wiley & Sons, Inc.