Friction Coefficient of Well-Defined Hydrogel Networks

The friction coefficient between Tetronic gel and water is measured as a function of the polymer concentration of the gel and the temperature by a simply designed apparatus. Tetronic gel was prepared by cross-linking Tetronic macro-monomers through activated ester chemistry. The gel is expected to have homogeneous network structure. The polymer concentration C-p dependence of the friction coefficient f is well expressed by a power law relationship f similar to C-p(v) with the exponent of v = 1.5, which is in a good agreement with the prediction of the scaling theory. The friction coefficient normalized by viscosity of water, f(T)/eta(T), increases with temperature. When the network is homogeneous, the mesh size is given by the average distance between the nearest polymer-polymer contacts. Therefore, the increase in the ratio f(T)/eta(T) with temperature attributed to the decrease in the average mesh size of the network due to the dehydration of the chains at higher temperature. The friction coefficient for randomly cross-linked Tetronic gel prepared by enzyme-mediated cross-linking reaction of tyrosin-modified Tetronic is compared with that of the homogeneous Tetronic gel. The friction coefficient for the randomly cross-linked gel is about an order of magnitude smaller than that for homogeneous gel. It suggests that the friction coefficient is mainly governed by the spatial inhomogeneity frozen in the gel rather than the average cross-linking density of the gel.