SYNOPSISThe equilibrium and dynamic swelling behavior of glassy polymers immersed in solvents can be modified by controlling the history of the polymer sample, which includes prior swelling and the drying method, or by copolymerization with other monomers. In this paper, the swelling kinetics in water of ionic hydrogels of 2-hydroxyethyl methacrylate copolymerized with potassium 3-sulfopropylmethacrylate and/or ethylene glycol dimethacrylate have been studied at 23°C. The dimensional changes of a swelling polymer sheet can be controlled through incorporation of anisotropic stresses in the initially dry, glassy polymer. These anisotropic stresses do not affect the swelling kinetics as long as the sample is partially glassy. However, differences in the initial stresses cause sharply different swelling kinetics once the polymer becomes entirely rubbery, due to differences in dimensional changes. Increasing the percentage of ionic comonomer in the polymer increases the equilibrium degree of swelling and the water sorption rate without changing the time for equilibration or the swelling transport mechanism. In contrast, increasing the percentage of cross-linker in the polymer not only reduces the degree of swelling and water sorption rate, but also increases the equilibration time and shifts the water transport mechanism from Fickian diffusion to anomalous transport. I NTRO DU CTlO NThe equilibrium and dynamic swelling behavior of poly ( 2-hydroxyethyl methacrylate) (PHEMA) hydrogels has been studied by many researchers because of PHEMA's importance in biomedical applications.'-6 Control of these properties is particularly important for polymer-mediated drug delivery devices and hydrogel absorbents. The kinetic response depends both upon the history of a given gel sample and its chemical composition. Although the properties of glassy polymers are well known to be history-dependent, the effect of history on swelling kinetics has been little studied. Franson and Peppas found that repeated cycles of swelling and drying caused the degree of swelling to increase with each cycle without affecting the swelling rate or transport mechanism.2 Modification of the swelling behavior

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The surfaee diffusion of He, Ne, H,, Ar, 0,, N, and CO, through Vycor microporous glass was analyzed theoretically and experimentally in the low pressure range as a function of temperature. A statistical-mechanical treatment was carried out in terms of partition functions to derive a general equation of surface flow. It was possible to separate the surface diffusion from the total flow using a simplified working equation. The conventional method which assumes 82