PULSED NMR-STUDY OF THE STRUCTURE OF POLY(VINYL ALCOHOL)-POLY(SODIUM ACRYLATE) COMPOSITE HYDROGEL

The heterogeneous structure of poly (vinyl alcohol) -poly (sodium acry late) composite hydrogels (PVA-PAA gels), prepared by repeatedly freez ing and thawing mixed aqueous solutions of PVA and PAA, has been studi ed using pulsed NMR. The free induction decay (FID) signals of the tra nsverse magnetization for the PVA-PAA gels were measured by both Carr- Purcell-Meiboom-Gill and solid-echo pulse sequences. The FID signal wa s decomposed into two or three decaying signals with various spin-spin relaxation times, based upon differences in the molecular mobility th at reflected structural heterogeneity. The structural heterogeneity of the PVA-PAA gel is quite different from that of PVA gel free of PAA. We have made observations on ''crosslinked'' domains in the PVA-PAA ge ls. They are formed by PVA solid chains, probably phase-separated in t he freezing-thawing process. The formation of crosslinked domains is i nfluenced by the number of freezing-thawing cycles. (C) 1994 John Wile y & Sons, Inc.