Rheo-SANS study on gelation of poly(vinyl alcohol)

Abstract

We have studied shear flow effects on physical gelation processes of poly(vinyl alcohol) solutions in a mixture of deuterated dimethyl sulfoxide and heavy water using time-resolved 2D-SANS technique with a rheological setup. Steady shear flow was applied on the sample solution after quenching from 100 to 25 °C, and the isothermal measurements were performed. The observed two-dimensional (2D) scattering patterns under the shear flow were isotropic while the growth rate of the scattering intensity was affected. It has been revealed in a previous report that the gelation of the solution was caused through crystallization between polymer chains to form the network. Hence, focusing on the scattering intensity from the crystallites, we have evaluated the crystallization rate. It was found that the crystallization rate was accelerated in a low shear rate region (shear rate, γ ˙ <5 s −1) and decreased with increasing the shear rate in a high rate region ( γ ˙ >5 s −1) after a maximum at γ ˙ =5 s −1. The results suggest that the local orientation of flexible polymer chains due to the shear flow assist crystal nucleation in the low shear rate region while the once-formed crystallites are destroyed by the shear in the high shear rate region.