Viscometric and light scattering studies on microgel formation by gamma-ray irradiation to aqueous oxygen-free solutions of poly(vinyl alcohol)

The gelation process of aqueous oxygen-free solutions containing poly(vinyl alcohol) (PVA) by gamma-ray irradiation was studied as a function of polymer concentration. Dynamic and static light-scattering techniques as well as the usual viscometric method were employed. PVA solutions turned into a ''bulk'' gel upon irradiation at high concentrations around 20 g/L, which corresponds to about 10 times that termed the critical concentration. Increases in the intrinsic viscosity ([eta]) and hydrodynamic radius (R-h) were observed before the gelation. The bulk gel did not form from diluted PVA solutions (1.6-2.0 g/L) but the solution became turbid due to the formation of microgel particles. In this process, both [eta] and R-h rapidly increased when increasing the irradiation dose up to a certain level ica. 0.3 Mrad) at which solution turbidity appeared. When the irradiation was continued over this dose, [eta] decreased and R-h levelled off. We thus examined the changes in the molecular weight (M-w) and radius of gyration (R-h) and found that R-g decreased while M-w increased almost linearly. Thus, a decrease in R-g/R-h from 1.5 to 0.98 was observed in doses >0.3 Mrad. On the basis of the results obtained, we discussed in detail the mechanism of microgel formation from PVA in its diluted solution by gamma-ray irradiation.