Small-angle neutron scattering from poly(vinyl alcohol)-borate gels

The structure of poly(vinyl alcohol) (PVA) aqueous gels containing borate ions (i.e. PVA-borate gels) was investigated with small-angle neutron scattering. The correlation length ξ, a measure of the spatial length of concentration fluctuation, was estimated from a generalized Zimm plot for fractals having the fractal dimension of D. The variation of ξ with temperature showed a gel-to-sol transition. The correlation length remained constant in the gel state and decreased steeply with increasing temperature in the sol state. Even at the highest temperature of observation (95°C) ξ was a few times greater than the radius of gyration of the PVA molecules, which indicates presence of PVA clusters in the sol state. The scattered intensity functions for the gels at different temperatures could be superimposed by employing reduced variables, I(q)cξ3 and ξq, where c and q are the monomer concentration and the magnitude of the scattering vector, respectively. This indicates that the system is composed of domains of PVA clusters filling the space. The fractal dimension of PVA inside the domain is 2.6 to 2.8. This suggests that PVA chains in the domain are packed more densely than percolation clusters. In the sol state, the system is described as an ensemble of polydisperse clusters of D ⋍ 2.2.