- A diffusion Deborah number, DEB , is defined as the ratio of a characteristic relaxation time to a characteristic diffusion time. When DEB >> 1, elastic Fickian diffusion should occur. When DEB << 1, viscous Fickian diffusion should occur. When DEB = 1, viscoelastic diffusion should be anticipated.
In this study, step-change and oscillatory sorption experiments were conducted to verify anticipated behavior. For Fickian diffusion, the experimentally measured diffusion coefficient should be independent of the thickness of samples used in the experiment. The diffusion coefficient should also be independent of the frequency of penetrant vapor pressure oscillation in oscillatory experiments. The diffusion of methanol in polystyrene at 70(DEGREES)C with a concentration of less than 0.5 wt% methanol was found to be elastic Fickian. The diffusion of toluene in polyvinyl acetate at 90(DEGREES)C with a concentration around 17 wt% toluene was found to be viscous Fickian. For viscoelastic diffusion, anomalous phenomena would be observed. The apparent diffusion coefficient of water in polyvinyl acetate at the temperature range 35 to 45(DEGREES)C was found to increase with increasing sample thickness before levelling off at a thickness of 0.049 cm in step-change experiments. This is probably caused by the slow approach to equilibrium concentration at the surface of the polymer film when the vapor pressure is subject to a step-change. For the oscillatory experiment, the apparent diffusion coefficient increases when the frequency of vapor pressure variation is increased. The PVAc-water system at 90(DEGREES)C with less than 1% water showed that the apparent diffusion coefficient would decrease from a constant value, which represents the pseudo-elastic limit at high frequency, to another constant value, which is the viscous limit at low frequency, as the frequency is decreased. A structural relaxation process coupled with the diffusional transport was believed to be the cause of this anomalous behavior.
- Dissertation Note:
- Ph.D. The Pennsylvania State University 1985.
- Source: Dissertation Abstracts International, Volume: 47-01, Section: B, page: 2980.
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