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.

Dynamical behavior of microgels of interpenetrated polymer networks

“…The relaxation time of "fragile" liquids increases very rapidly with decreasing temperature, while "strong" liquids show a much slower temperature dependence. For colloidal suspensions fragility must be defined replacing the inverse of temperature with concentration [15,18,19]. Hard-sphere colloidal suspensions are fragile and the absence of a wider range of fragility limits their versatility as a model system of the glass transition.…”

Introduction to the Colloidal Glass Transition

“…This method is termed "shear-rejuvenation." Alternatively, colloidal particles can be used for which their size is temperature- controllable, and thus temperature can be used to induce the particles to pack into a glassy state [54][55][56][57]. This is more analogous to the traditional temperature quench of a polymer glass.…”

Exploring the validity of time-concentration superposition in glassy colloids: Experiments and simulations

“…However, it is very difficult to determine the thermodynamic volume fraction of soft colloidal dispersions due to the interpenetration and compression of the hairy structure of the PNIPAM shell [30,76]. In the present work the effective volume fraction is used and was calculated at different temperatures based on the hydrodynamic size obtained from dynamic light scattering, and is given by φ eff (T) = φ eff(collapsed) [D H (T)/D collapsed ] 3 [77,78], where φ eff(collapsed) /φ ∞ is 0.291 at 37 °C and D collapsed is 92.3 nm. The effective volume fraction of the soft particle dispersion in the collapsed (particle) state was calculated based on the density and mass fraction [77,78], which may underestimate the effective volume fraction due to a small amount of water in the collapsed shell [79,80] even at high temperature.…”

“…In the present work the effective volume fraction is used and was calculated at different temperatures based on the hydrodynamic size obtained from dynamic light scattering, and is given by φ eff (T) = φ eff(collapsed) [D H (T)/D collapsed ] 3 [77,78], where φ eff(collapsed) /φ ∞ is 0.291 at 37 °C and D collapsed is 92.3 nm. The effective volume fraction of the soft particle dispersion in the collapsed (particle) state was calculated based on the density and mass fraction [77,78], which may underestimate the effective volume fraction due to a small amount of water in the collapsed shell [79,80] even at high temperature. We further remark that the effective volume fraction of the soft colloidal dispersion was also determined from relative viscosity measurements at very low concentrations using the Einstein-Bachelor equation [22,76].…”