The friction coefficient between the polymer network of poly(acrylamide) gel and water is measured as a function of the temperature, the polymer concentration of the gel, and the concentration of the crosslinker by a newly designed apparatus. The friction decreases in proportion to the viscosity of water as the temperature is increased, but the ratio of the friction coefficient and the fluid viscosity is constant in the temperature range from 0 to 60 °C. The concentration dependence of the friction coefficient is well described by a power law relationship f∼φv with the exponent of v = 1.5, which coincides with the prediction of the scaling theory. The friction decreases as the crosslinker concentration increases. The decrease, contradictory to our intuition, is presumably a result of the spatial structural inhomogeneity induced by the critical density fluctuations of the polymer network due to the small solubility of the crosslinker in water. In such an inhomogeneous network water passes through the regions with less network density avoiding the denser regions. This indicates that the effective pore size of a network is determined by the correlation length of the network inhomogeneities rather than by the distance between the neighboring two chemical crosslinkings.
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