Vitamin B12 diffusion and binding in crosslinked poly(acrylic acid)s and poly(acrylic acid-co-N-vinyl pyrrolidinone)s

The diffusion mechanism of vitamin B12 in two types of crosslinked hydrogels, poly(acrylic acid) (cPAA) and copolymers of acrylic acid and N-vinyl pyrrolidinone (cP(AA-NVP)) was studied. The PAA and P(AA-NVP) synthesized by three different degrees of crosslinking have limited water absorption capabilities ranging from 3% to 18%. In the copolymers permeability of B12 is controlled by both intramolecular and intermolecular hydrogen-bonding between the pyrrolidinone and carboxylic acid side chains. The diffusion kinetic data in two types of polymers were best described by Peppas models instead of Higuchi models. Permeation from both crosslinked PAA and P(AA-co-NVP) copolymers followed a Super Case II transport mechanism, most likely driven by macromolecular chain relaxation and swelling of hydrophilic polymers. A special FTIR spectroscopic method for drug binding study, FTIR difference spectroscopy, is used to probe the strong interactions between vitamin B12 and the side chains of the hydrogels. The FTIR differential spectra of B12 in PAA hydrogels revealed dramatic changes of the spectral marker bands of B12 after binding in the crosslinked gels, indicating significant interactions occurring in the amide and phosphate moieties of B12. Such interactions retard the diffusion of vitamin B12.