Dimensionless analysis of swelling of hydrophilic glassy polymers with subsequent drug release from relaxing structures

Two dimensionless parameters, the diffusional Deborah number, De, and the swelling interface number, Sw, were used as indicators of solvent and solute transport behavior in swellable hydrogel systems. Polymer relaxation and concentration-dependent diffusion led to dynamically swelling polymers which displayed Fickian, Case II, or anomalous transport behavior. Experimental systems studied included crosslinked samples of poly(vinyl alcohol), designated PVA, and poly(2-hydroxyethyl methacrylate-co-methyl methacrylate), designated P(HEMA-co-MMA). Model solutes with molecular weights ranging from 200 to 17 000 were used to investigate release properties from these networks. Characteristic polymer relaxation times and swelling front velocities were determined experimentally. To gain an understanding of how polymer morphology and solute properties affected transport behavior, the calculated values of De and Sw were correlated to the diffusional exponent, n, used commonly to indicate the time dependence of sorption and release. The swelling interface number was found to correlate particularly with the transport kinetics, indicating anomalous and Case II solute transport when the Sw values were near 1.0.