Study of Cryostructuring of Polymer System. 66. Properties and Microstructure of Poly(vinyl alcohol) Cryogels Formed in Frozen Dimethyl Sulfoxide with Additives of Urea and Then Hydrated by Replacing Organic Medium with Water

Abstract

“Primary” poly(vinyl alcohol) (PVA) cryogels have been obtained by cryogenic processing (freezing at –21.6°C for 12 h followed by defrosting via heating to 20°C at a rate of 0.03°C/min) of a 100 g/L solution of PVA in dimethyl sulfoxide (DMSO) in the absence and presence of urea (2 or 4 mol/L), which exhibits kosmotropic properties in such a medium. Subsequent hydration of the cryogels by replacing DMSO with water causes a decrease in the volume and weight of the samples, as well as leads to a significant increase in the elasticity modulus of resulting “secondary” cryogels. The absolute magnitude of such effects depends both on the concentration of urea in an initial PVA solution and on the volume ratio between gel samples and an aqueous extractant during their hydration. Using optical microscopy, it has been found that the presence of urea in the initial DMSO polymer solution in a concentration close to the limit of its solubility in such a medium induces the formation of a gel matrix with a wide-pore morphology. Since high-modulus secondary PVA cryogels are of great interest as materials for biomedical applications, the possibility of their functioning as carriers of drug delivery systems has been assessed in the work. Ibuprofen sodium salt has been used as a model drug. The analysis of the release kinetics of this substance within the framework of the Weibull function has been employed to show that the dynamic hydrogen bonding of its carboxylate groups with the hydroxyl groups of PVA decelerates the release of the drug from the polymer carrier, i.e., prolongs the release process. At the same time, the rate of the process depends on the urea content in the initial polymer solution most likely due to microstructural differences between the polymer phases of the macropore walls in the cryogel matrix.