Linear viscoelastic properties of aging suspensions

We have examined the linear viscoelastic behavior of poly-N-isopropylacrylamide (PNIPAM) microgel suspensions in order to obtain insight in the aging processes in these densely packed suspensions at various temperatures below the volume transition temperature. The system is found to display a strong aging behavior. The viscoelastic moduli are compared to the predictions of the soft glassy rheology model. The model predicts quantitatively the loss modulus $\ensuremath G^{\prime \prime }(\omega ,t)$, the elastic modulus $\ensuremath G^{\prime }(\omega ,t)$ and their ratio $\ensuremath G^{\prime \prime }(\omega ,t)/G^{\prime }(\omega ,t)$. A relative noise temperature of $\ensuremath (\mathcal{X}/\mathcal{X}_{g})\approx 0.62$ is obtained. Despite the fact that the microgel particles are more compressed and hence less mobile at a lower thermodynamic temperature T, $\ensuremath (\mathcal{X}/\mathcal{X}_{g})$ is found to be essentially independent of T within the range of $5\,^{\circ}$C to $10\,^{\circ}$C below the volume transition temperature.