Physical aging and structural recovery in a colloidal glass subjected to volume-fraction jump conditions.

Abstract

Three important kinetic phenomena have been cataloged by Kovacs in the investigation of molecular glasses during structural recovery or physical aging. These are responses to temperature-jump histories referred to as intrinsic isotherms, asymmetry of approach, and memory effect. Here we use a thermosensitive polystyrene-poly (N-isopropylacrylamide)-poly (acrylic acid) core-shell particle-based dispersion as a colloidal model and by working at a constant number concentration of particles we use temperature changes to create volume-fraction changes. This imposes conditions similar to those defined by Kovacs on the colloidal system. We use creep experiments to probe the physical aging and structural recovery behavior of colloidal glasses in the Kovacs-type histories and compare the results with those seen in molecular glasses. We find that there are similarities in aging dynamics between molecular glasses and colloidal glasses, but differences also persist. For the intrinsic isotherms, the times t_{eq} needed for relaxing or evolving into the equilibrium (or stationary) state are relatively insensitive to the volume fraction and the values of t_{eq} are longer than the α-relaxation time τ_{α} at the same volume fraction. On the other hand, both of these times grow at least exponentially with decreasing temperature in molecular glasses. For the asymmetry of approach, similar nonlinear behavior is observed for both colloidal and molecular glasses. However, the equilibration time t_{eq} is the same for both volume-fraction up-jump and down-jump experiments, different from the finding in molecular glasses that it takes longer for the structure to evolve into equilibrium for the temperature up-jump condition than for the temperature down-jump condition. For the two-step volume-fraction jumps, a memory response is observed that is different from observations of structural recovery in two-step temperature histories in molecular glasses. The concentration dependence of the dynamics of the colloidal dispersions is also examined in the equilibrium state and we find that the dynamic fragility index m is sensitive to the degree of softness of the soft colloidal dispersion, indicating that soft colloids make stronger glasses. Finally, we compare the present results with prior findings for similar thermoresponsive systems obtained with diffusing wave spectroscopy and discuss similarities and differences.