1H NMR Self-diffusion Study of PVA Cryogels Containing Ethylene Glycol and Its Oligomers.

The multicomponent self-diffusion of the PVA cryogels prepared by a freezing-thawing treatment of the water/oligooxyethylene glycol solutions of PVA has been studied with the NMR FT-PGSE method at various temperatures and compared with the diffusion of such systems without glycols. The evaluated apparent activation energies, E(a), of the self-diffusion for the PVA chains showed that entrapment of ethylene glycol, EG, and diethylene glycol, DEG, into cryogels made them more friable. These glycols acted as cryoprotectants. On the contrary, triethylene glycol, TEG, and, especially, PEG-400 made cryogels firmer. The E(a) values for the self-diffusion of oligooxyethylene glycols exceeded those for the water molecules in the same cryogel and increased along with the lengths of oligomers. These values are also above those obtained for the PVA chains in cryogels with EG, DEG, and TEG as well as without additives. This fact is in conformity with the mechanism of the phase separation of water and co-solvent by freezing the multicomponent system. It is expected that the presence of TEG and PEG-400 in cryogels leads to a decrease in dimensions of the solvent-filled pores and the PVA filaments like that which was found out earlier for DMSO containing cryogels. Such a compartmentation within cryogels depends mainly on the dimensions of ice and polymer microcrystallites formed by freezing the solution. Copyright 1999 Academic Press.