Poly(vinylidene fluoride) (PVDF) and ethylene-chlorotrifluoroethylene (ECTFE) exhibited non-Fickian diffusion behaviors in the transient uptake of aromatic solvents. The diffusional exponents at the final stage of uptake exhibited asymptotic values ranging from 0.9-1.0 for the sigmoidal sorption of PVDF and 1.2-2.0 for the drastic accelerating sorption of ECTFE. The asymptotic diffusional exponent decreased with rising temperature. PVDF film exhibited a linear increase in both machine and transverse directions with fractional uptake, whereas ECTFE film increased linearly in the transverse direction and exponentially in the machine direction. A rate-type viscoelastic [Camera-Roda and Sarti, 1990] model was applied to reproduce the anomalous sorption of fluoropolymer-solvent systems. The fitted Deborah numbers ranged from 0.2-0.3 for PVDF, and 2-5 for ECTFE. The diffusivity ratio, D_eq/D_o, of PVDFsolvent system was fitted as tenth order of magnitude lower than that of ECTFE-solvent system. The fraction of initial surface concentration, S_o, was fitted in the range of 0.6-0.8 for both ECTFE and PVDF. It is plausible to assume that diffusion coupled with comparable structural relaxation has the asymptotic diffusional exponents that are strongly dependent on accelerating curvature, and total sorption is contributed by relaxation-induced viscoelastic volume change along with concentration-driven Fickian diffusion.

Poly(vinylidene Fluoride) Ethylene-Chlorotrifluoroethylene Transient Sorption Relaxation Kinetics

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