Effect of neutralization of poly(acrylic acid) on the structure of water examined by Raman spectroscopy

The structure and property of water in aqueous solutions and cross-linked gels of poly(acrylic acid) (PAA) were investigated by using Raman spectroscopy. Relative intensity of the polarized O-H stretching band (C) due to an in-phase collective stretching of O-H oscillators within hydrogen-bonded water clusters was used to estimate degree of water-water hydrogen bonding. The C values vs degree of neutralization (alpha) plots showed a maximum at around alpha = 0.3 in the solutions of FAA with relatively high molecular weight (M-W = 50 000, 500 000), whereas the C value of PAA with relatively low molecular weight (M-W = 2000, 5000) decreased linearly with an increase in alpha. Defects of water-water hydrogen bonds within the interstitial water surrounded by the polymer chains were suggested in the solutions of PAA with high molecular weight and low degree of neutralization. The network of polymer chain supported by entanglement and hydrogen bonds between carboxyl groups on the polymer chain was destroyed, and concomitant release of interstitial water to bulk induced an increase in the C value with an increase of alpha at alpha < 0.3. Further neutralization brought about the perturbation of water structure because of increase in the concentrations of carboxylate ion and Na+.