MAGNETIZATION-TRANSFER NMR ANALYSIS OF AQUEOUS POLY(VINYL ALCOHOL) GELS - EFFECT OF HYDROLYSIS AND STORAGE-TEMPERATURE ON NETWORK FORMATION

The technique of magnetization-transfer nuclear magnetic resonance (MT -NMR) was used to probe the effects of concentration, degree of hydrol ysis, and storage temperature on the formation of a network in aqueous solutions and gels of atactic poly(vinyl alcohol) (PVA). The average degree of polymerization of the samples was 1250-1300, and the degrees of hydrolysis of the polymers were 99.5, 98.5, 97.4, and 87.6 mol %. The area of MT profiles and hence the extent of network formation incr eases nonlinearly with increasing concentration for all PVA samples ev aluated, with more extensive network associated with higher polymer co ncentrations. Network formation is minimal at all concentrations for P VA that is 87.6% hydrolyzed (12.4% residual acetate); the critical con centration of residual acetate groups necessary to disrupt the polymer -polymer hydrogen bonding responsible for forming the network is in th e range of 3%. The development of the network proceeds over a period o f as long as 18 weeks and is highly sensitive to storage temperature. An analysis of the Gaussian and Lorentzian components of MT profiles i ndicates that a sample whose profile area is greater than 50% Gaussian may be considered a gel from the point of view of NMR.