Compartmentation in the poly(vinyl alcohol) cryogels. H-1 NMR self-diffusion study

The self-diffusion of the poly(vinyl alcohol) (PVA) cryogels prepared by fr eezing-thawing the aqueous and water/DMSO solutions of PVA was studied with the NMR FT-PGSE method. The temperature dependencies of the self-diffusion coefficients, D-s, for the PVA chains have a maximum at 45 degrees C due t o the syneresis of cryogels. They are quite different from the monotonous i ncrease of D-s for the aqueous solutions of EVA. Evaluated apparent activat ion energies, E-a, of the self-diffusion for the PVA chains in the PVA solu tions and cryogels in D2O are practically the same and equal 22-24 kJ mol(- 1) below the crucial point. The proton spin-lattice relaxation times, T-1, of the PVA chain also coincide with one another for solutions and cryogels. It means that molecular packing in cryogels depends mainly on the dimensio ns of the ice and polymer microcrystallites formed by freezing the solution . Above the crucial point polymer compartments become finner, and the chain mobility somewhat reduces. The strength of cryogels also increases along w ith growing the DMSO contents and decreases by the BSA addition. For an est imation of the cryogel morphology, effects of the restricted diffusion of b oth the water and PVA in a q-space have been taken into account. By introdu cing DMSO to cryogels, the solvent filled pores become smaller, and channel s become much shorter. The diameter of the PVA filaments is similar to thos e for all the cryogels, but the length of filaments with D2O is twice those for cryogels with a mixed solvent. Entrapment of BSA in the cryogel matrix by preparation leads to the increase of an average diameter of the water f illed pores, and destroys molecular packing the cryogel. (C) 2000 Elsevier Science B.V. All rights reserved.