Temperature dependence of probe diffusion in gels

Abstract

We present a dynamic light scattering study of the temperature dependence of the dynamics of probe diffusion in polyacrylamide gels and pregel solutions as a function of crosslink content and scattering angle. Our results show that the spinodal transition in polyacrylamide gels affects the movement of the probe particles. The dynamical behaviour ranges from overdamped vibration of particles controlled by the gel elasticity (relaxational behaviour) at low temperatures to the free diffusion of particles controlled by the sol viscosity at higher temperature. The decay rate of the relaxational mode decreases upon approaching the spinodal temperature, and this decrease is even more pronounced than that of gels without probe particles. Our results suggest that the friction felt by the 50 nm probe particles as they move in the gel appears to diverge at the spinodal temperature, scaling as ≈( T− T s) −1.5. This behaviour is observed over the entire temperature range for probe diffusion in a well gelled sample, whereas in a sample closer to the gel threshold the strong temperature dependence is observed only at low temperatures, the higher temperature behaviour being the same as that in pre-gel solutions. We propose an interpretation of the experimental data in terms of a simple theory of Brownian particles harmonically bound to the gel network.