Water properties in the super-salt-resistive gel probed by NMR and DSC

The so-called super-salt-resistive gel, or poly (4-vinylphenol) (P4VPh) hydrogel, of different water contents (H 97-51%) was prepared by cross-linking with different amounts of ethylene glycol diglycidyl ether. H-1 NMR spectroscopy was used to investigate the dynamic properties of water in the get samples in terms of the spin-spin relaxation. The T-2 values in those hydrogels were analyzed by assuming a two-component system, namely, T-2(long) and T-2(short), and their fractions were obtained. In the higher water content region (75% <= H <= 97%), T-2(long) for P4VPh gel was almost constant or even slightly increased with decreasing temperature. On the other hand, T-2(long) for poly(vinyl alcohol) (PVA) gel (80% <= H <= 96%) significantly decreased with decreasing temperature, showing a natural behavior for water mobility in common hydrogels. Water in P4VPh gels of lower water contents (H = 70% and 51%) also showed intriguing behaviors: the T-2 values are much larger than those of gels with higher water contents and decreased with decreasing temperature only in the lower temperature range (< 10 degrees C). The fraction of T-2(long) values of P4VPh gel showed another contrast to those of PVA gel; the latter decreased with deceasing water content (normal behavior), while in the former gel the highest fraction (ca. 60% at 20 degrees C) was observed for a sample with the lowest water content (H = 51%). On the other hand, the results of DSC measurements for P4VPh gel were less specific than those of T-2 and comparable to those of common hydrogels such as PVA; with decreasing water content, the total amounts of free water and freezable bound water per polymer mass (g/g) decreased, while the amount of nonfreezing water per polymer also decreased.