Swelling mechanisms diffusion coefficients

When a slab of the PE0/PU hydrogel 2.8mm thick was swollen to equilibrium in an aqueous solution of morphine hydrochloride at 37°C, briefly rinsed, then transferred to water at 37°C, the rate of release of morphine was, as predicted by theory, proportional to time t O and 50° 0f the morphine content was released in the first 45 minutes. The release/time plot is shown in Figure 1. Results from experiments like this one can be used to calculate diffusion coefficients (8) in fully swollen hydrogels and help interpret the complex mechanism of diffusion in swelling hydrogels. 

To test for either adsorptive or electrostatic interactions, the SEC separation is performed at a variety of temperatures. If the separation occurs by size alone, the retention coefficient R(= V0/Ve) is independent of temperature very small variations may be observed as a result of gel swelling or microstructural changes to the gel. The presence of a significant dependence of R on T indicates the presence of a mechanism other than size exclusion. While R should not vary with 7 diffusion coefficients increase with T and so zone broadening occurs, leading to decreased resolution with increasing separation temperatures. 

Let us agree that s = 0 corresponds to a constant diffusion coefficient k(u,l) = 1). The analysis of this problem allows us to qualitatively explain many mechanisms of diffusion kinetics of elastomer swelling. 

Crosslinked poly(2-hydroxyethylmethacrylate), PHEMA, has been prepared by radical polymerization in presence of different amounts of water and water-diacetine solutions. While homogeneous samples can be prepared at all water-diacetine content, amounts of water higher than 40% causes the formation of macroporous opaque sponges. Mechanical properties of water swollen samples have been measured and related to the crosslink formation during the polymerization process. From water diffusion kinetics in sorption and desorption experiments at 37 C the diffusion coefficients have been calculated. Measurements of the swelling at equilibrium in water-diacetine mix-res indicate the presence of "cosolvency" phenomenon for the samples studied. 

Knowledge of the swelling characteristics of a polymer is of utmost importance in biomedical and pharmaceutical applications since the equilibrium degree of swelling influences the solute diffusion coefficient through these hydrogels [1], the surface properties and surface mobility [1,2,8], the optical properties, especially in relation to contact lens applications, and the mechanical properties. 

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