249609 Aging in Colloidal Glasses: The Kovacs Signatures in Concentration Jump Conditions
The analogy between colloidal dynamics and the dynamics of molecular glasses remains an important area of study. Of particular interest to our team is the aging responses of the two systems. We have been investigating the dynamics of colloidal systems composed of thermosensitive particles that change diameter upon change of temperature and comparing the behavior to what is expected in molecular glass-formers. In particular, we have found that concentration jumps in these systems mimic three important behaviors of molecular glasses that were catalogued by Kovacs in the 1960s: the intrinsic isotherm (down-jump in temperature or increase in concentration), the asymmetry of approach (comparison of down-jump and up-jump experiments to demonstrate non-linearity of response), and memory ( two-step temperature (concentration) history in which the second step shows non-monotonic response). In our early work  we were able to show, using multispeckle diffusing wave spectroscopy, that although the three signatures are observed in the concentration jump conditions, they are not identical to the observations in molecular glasses. In the present work we expand our investigations to the rheological domain and following the aging behavior of the dynamic moduli. Such experiments were developed by Struik for molecular glasses and he found that the viscoelastic response was well described by free volume ideas, which are directly translatable to ideas of the fictive temperature. The concept of free volume is not applicable to experiments in colloids in which the response evolves at constant concentration. Hence, we are examining the idea of a fictive concentration (derived from the fictive temperature idea) and its applicability to complex concentration jump histories. The systems to are modifications of PNIPAAM as a corona grafted onto polystyrene cores. A series of different particle PNIPAAM fractions (different thermal sensitivity) is being investigated. Current results suggest that we see the aging suggested by the Kovacs signatures and quantitative comparisons between molecular and colloidal glasses will be made.
 A.J. Kovacs, Fortschritte der Hochpolymeren-Forschung, 3, 394(1963).
 X.J. Di, K.Z. Win, G.B. McKenna, T. Narita, F. Lequeux, S.R. Pullela, Z. Cheng, Phys. Rev. Lett., 106,
 L.C.E. Struik, Physical Aging in Amorphous Polymers and Other Materials, Elsevier, Amsterdam, 1978.
This work is supported by NSF CBET-1133279