Determining hydrogel porosity through dielectric relaxation intensity ratios between water and hydrogel

Abstract

We outline a structural model of hydrogel that conceptualizes it as a combination of two components: solid and soft. The solid component of the system is the swollen polymer network with bound water. The solid component forms a backbone, whose pores contain the soft component: free water. We also present a method, based on this model, which can be used to determine the porosity of a gelatine- or agarose-based hydrogel. The porosity within the model is calculated as the relative volume of the system occupied by free water. The method is based on the theory of orientational defects, according to which the volume of free water in a solid state is proportional to the concentration of orientational defects. The numerical value of such concentration also correlates with the intensity of dielectric relaxation in a system. The porosity of the hydrogel can be determined as the ratio of the hydrogel's relaxation intensity to water's relaxation intensity. The intensity of the relaxation is the height of the peak of the imaginary component of the complex dielectric permittivity. Consequently, the porosity of a hydrogel can be deduced from the dielectric spectroscopy experiment. We report that the porosity of gelatine hydrogel is 0.48 and 0.35 for the mass concentration of polymer of 12% and 35%, respectively; the porosity of agarose is 0.83 and 0.69 for the mass concentration of the polymer of 1.5% and 2%, respectively.