Phase behaviour and sol-gel transition of poly(vinyl alcohol)-borate complex in aqueous solution

Phase behaviour and sol-gel transition of poly(vinyl alcohol) (PVA) aqueous solutions in the presence of borate ions were investigated. PVA molecules are known to form charged complexes with borate ions in aqueous solutions containing borax (Na2B4O7 · 10H2O) or a mixture of boric acid and sodium hydroxide. These complexes show the following viscosity and phase behaviour as a function of the concentrations of boric acid, sodium hydroxide, and salt. 1. At a given combination of PVA concentration and ionic strength, the system underwent a clear-opaque-clear transition with increasing boric acid concentration. 2. The intrinsic viscosity [η] decreased first and then increased with increasing boric acid concentration in alkaline solution. 3. The opaque region grew to some extent with time and/or by adding salt (i.e., NaCl). 4. When the polymer concentration increased, a thermoreversible sol-gel transition took place without phase demixing. The gelation concentration was close to the chain overlap concentration estimated from [η]. All the above phenomena can be explained by a balance of the excluded volume effect due to intra- and inter-chain crosslinks and electrostatic potentials.