Solute and penetrant diffusion in swellable polymers. II. Verification of theoretical models

Abstract

A mathematical model developed to describe diffusion of a penetrant and a solute in a swellable polymer slab was applied to the case of a hydrophilic polymer loaded with a soluble drug in which the penetrant (water) is sorbed and solute (drug) is desorbed. An exponential dependence of the penetrant and solute diffusion coefficients on penetrant concentration was chosen and shown to be adequate for description of the systems studied. Experimental verification of the model was conducted by using copolymers of 2‐hydroxyethyl methacrylate (HEMA) and N‐vinyl‐2‐pyrrolidone (NVP). The monomers were bulk polymerized with benzoyl peroxide initiator and cut into thin disks. Monomer mole fractions of HEMA in the copolymers were 0.707, 0.446, and 0.211. Swelling behavior of the samples was observed in water at 37 and 0°C. Solute‐containing samples were prepared and solute release from these samples into water was followed by monitoring the UV absorption of the release medium. The concentration dependence of the diffusivity of water and two model solutes, sodium trifluoroacetate and sodium heptafluorobutyrate, in the gels was studied by using the pulsed‐gradient spin‐echo NMR technique. The diffusivities measured by this technique followed the concentration dependence predicted by the free‐volume theory. The simple exponential dependence used in the model was an adequate approximation of this behavior in the case of a transient diffusion experiment.