Influence of observation temperature on light scattering of poly-N-isopropylacrylamide hydrogels

Abstract

Static and dynamic light scattering methods were applied to characterize the microstructure of gels synthesized by cross-linking copolymerization of N-isopropylacrylamide, with particular emphasis on the influence of measurement temperature. The total scattering intensity was divided into two parts: thermal scattering due to Brownian motion and static scattering due to topological or spatial inhomogeneity. Different methods of data evaluation, i.e. the non-ergodic and the partial heterodyne approaches, were carefully compared. We obtained consistent results clearly demonstrating that both parts of the scattering rise markedly upon increasing the observation temperature from 10 to 27.5 °C thus approaching the lower critical solution temperature. While the temperature dependence of the thermal scattering component is well understood, we attribute the rise of the static component to the establishment of local swelling equilibrium in a gel whose cross-link density features some inhomogeneity. This means that with rising temperature the more densely cross-linked regions deswell at the expense of the less densely cross-linked ones. As a result, the scattering contrast is enhanced thus leading to a larger static scattering intensity.