H-1 NMR self-diffusion study of morphology and structure of polyvinyl alcohol cryogels

The multicomponent self-diffusion of the polyvinyl alcohol (PVA) cryogels p repared by a freezing-thawing treatment of aqueous and water-DMSO solutions of PVA has been studied with the NMR FT-PGSE method. The temperature depen dencies of the self-diffusion coefficients, D-s, for the PVA chains have a maximum at 45 degrees C due to the syneresis of cryogels. They are quite di fferent from the monotonous increase of D-s for the aqueous solutions of PV A. Evaluated apparent activation energies, E-a, of the self-diffusion for t he PVA chains in the PVA solutions and cryogels in D2O are practically the same and equal 22-24 kJ/mol below the crucial point. The proton spin-lattic e relaxation times, T-1, of the PVA chain also coincide with one another fo r solutions and cryogels. This means that molecular packing in cryogels dep ends mainly on the dimensions of the ice and polymer microcrystallites form ed by freezing the solution. Above the crucial point polymer compartments b ecome firmer, and the chain mobility somewhat reduces. The strength of cryo gels also increases along with growing the DMSO contents and decreases by t he BSA addition. For estimation of the cryogel morphology, effects of the r estricted diffusion of both the water and PVA in a q-space have been taken into account. By the introduction of DMSO to cryogels the solvent filled po res become smaller, and channels become much shorter. The diameter of the P VA filaments is similar to those for all the cryogels, but the length of fi laments with D2O is twice that for cryogels with a mixed solvent. Entrapmen t of BSA in the cryogel matrix by preparation leads to the increase of an a verage diameter of the water filled pores and destroys molecular packing th e cryogel. (C) 1999 Academic Press.