Cryostructuring of polymeric systems. 58. Influence of the H2N-(CH2)(n)-COOH-type amino acid additives on formation, properties, microstructure and drug release behaviour of poly(vinyl alcohol) cryogels

The poly(vinyl alcohol) (PVA) physical cryogels that contained the additives of the H2N-(CH2)(n)-COOH-type amino acids (n = 1-5) have been prepared, and their physico-chemical characteristics, macroporous morphology and the solute release dynamics have been evaluated. These low-molecular solutes with respect of the PVA cryotropic gelation exhibited the kosmotropic influence, i.e. commencing from their concentration of about 0.5 mol/L the additives facilitated the growth in the rigidity and the heat endurance of the resultant cryogels. The efficiency of such influence was reduced with an increase in the length of oligomethylene spacer between the amino and carboxyl groups in the amino acid molecules. The average size of macropores also depended on the additive type and its concentration in the initial PVA solution; the cross-section of the pores decreased with increasing the solutes concentration. Upon the cryogel-entrapped amino acids release from the macroporous gel matrix a gradual transition from the release mechanism by a free diffusion for the hydrophilic glycine to the release according to the space-limited Fickian diffusion for the more hydrophobic e-aminocaproic acid was found. This effect was stipulated by the hydrophobic interactions between the carbochain core of the PVA macromolecules and the aliphatic moieties of the respective amino acid.