Properties controlling the diffusion and release of water-soluble solutes from poly(ethylene oxide) hydrogels. 1. Polymer composition.

This study examines the state of water-association with poly(ethylene oxide), as evidenced by diffusivity, in a series of crosslinked polyurethanes made from poly(ethylene glycols) of a range of molecular weights. As a subsidiary underpinning exercise the correlation of diffusivity with water content at relatively high levels of swelling (> 45%) using a variety of semi-empirical equations was analyzed. Three water-soluble compounds with similar molecular weights and which exhibit minimal interaction with the polymer, as shown by their partition coefficients, were chosen for this part of the research programme. These were proxyphylline, morphine hydrochloride and caffeine. The best statistical correlations of the data were obtained for plots of: (a) diffusivity against weight percent water; and (b) log diffusivity against the reciprocal of the weight percent of water in the hydrogels. Proxyphylline results for the high levels of swelling compositions were augmented with data from lower swelling compositions and a clear break in the slope of diffusivity against percentage of water in the swollen hydrogel was obtained. This indicated a change in the nature of the diffusion at this point. The probability of this transition point corresponding to a change for diffusion through water bound as trihydrate to diffusion in free water is discussed.