Reentrant behaviour in polyvinyl alcohol-borax hydrogels

Abstract

Polyvinyl alcohol (PVA) hydrogels, cross-linked with varying concentrations of borax, were studied with Small Angle Neutron Scattering (SANS), X-ray Diffraction (XRD) and Differential Thermal Analysis (DTA). The SANS data satisfy the Ornstein-Zernike approximation. The hydrogels are modeled as PVA chains bound by borate cross-links. Water occupies the spaces within the three-dimensional hydrogel network. The mesh size ξ indicates reentrant behaviour i.e. at first, ξ increases and later decreases as a function of borax concentration. The behaviour is explained on the basis of the balance between the charged di-diol cross-links and the shielding by free ions in the solvent. XRD and DTA show the molecular size of water in the solvent and the glass transition temperature commensurate with reentrant behaviour.