1H NMR Self-Diffusion Study of Morphology and Structure of Polyvinyl Alcohol Cryogels

The multicomponent self-diffusion of the polyvinyl alcohol (PVA) cryogels prepared by a freezing–thawing treatment of aqueous and water–DMSO solutions of PVA has been studied with the NMR FT-PGSE method. The temperature dependencies of the self-diffusion coefficients,Ds, for the PVA chains have a maximum at 45°C due to the syneresis of cryogels. They are quite different from the monotonous increase ofDsfor the aqueous solutions of PVA. Evaluated apparent activation energies,Ea, of the self-diffusion for the 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–lattice relaxation times,T1, of the PVA chain also coincide with one another for solutions and cryogels. This means that molecular packing in cryogels depends mainly on the dimensions of the ice and polymer microcrystallites formed by freezing the solution. Above the crucial point polymer compartments become firmer, and the chain mobility somewhat reduces. The strength of cryogels also increases along with growing the DMSO contents and decreases by the BSA addition. For estimation of the cryogel morphology, effects of the restricted diffusion of both the water and PVA in aq-space have been taken into account. By the introduction of DMSO to cryogels the solvent filled pores become smaller, and channels become much shorter. The diameter of the PVA filaments is similar to those for all the cryogels, but the length of filaments with D2O is twice that 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 filled pores and destroys molecular packing the cryogel.