(450g) Time-Concentration Superposition in Colloids Near to the Putative Glass Transition

Authors: 
Peng, X., University of Connecticut
McKenna, G. B., Texas Tech University
Wang, J., Stanford University
Li, Q., Texas Tech University
Chen, D., Texas Tech University
Zia, R. N., Stanford University
Similar to time-temperature superposition in polymers and small molecule glass-formers[1,2], the possibility that one can use time-concentration superposition to describe the behavior of colloidal systems has become of considerable interest. It is also known that polymeric glass-formers that have a strong sub-glass relaxation (β-relaxation) will not follow time-temperature superposition. In the case of colloidal systems, there is also a strong β-relaxation at high frequencies and this suggests the possibility that time-concentration superposition (TCS) may break down in these materials. Here we examine TCS in these materials using both experiment (rheological measurements) and simulation (Brownian dynamics) in which the colloidal dispersions are forced into their final concentration by making rapid changes in the particle diameter at constant particle number density. This is done in the simulations of near-hard sphere particles while the experiments are performed using soft sphere, thermosensitive particles (polystyrene core/poly(N-isopropylacrylamide)). In the simulations, the mean square displacement of the system that has been "aged" into an intransient (metastable equilibrium) state after the particle size-jump induced concentration change is transformed into the dynamic moduli using the generalized Stokes-Einstein approach. The dynamic moduli G'(ω) and G''(ω) are measured directly for systems that have achieved the intransient state through aging. We find that TCS does not hold for the full range of dynamics measured in either the simulations or the experiments because the glassy (long time or α-relaxation) and the high frequency β-relaxation show different concentration dependences. A further analysis of the results within the framework of the Baumgaertel-Schausberger-Winter (BSW) [3] relaxation function shows that the individual mechanisms themselves follow TCS with the α-relaxation showing typical glass-like strong dependence on the concentration while the β-relaxation shows a very weak dependence on the concentration. The significance of the results will be discussed.

[1] J.D. Ferry, Viscoelastic Properties of Polymers, 3rd. ed., Wiley, New York, 1980.

[2] M. L. Cerrada and G. B. McKenna,"Physical aging of amorphous PEN: Isothermal, isochronal and isostructural results," Macromolecules 33, 3065 (2000).

[3] M. Baumgaertel, A. Schausberger, and H. H. Winter, "The relaxation of polymers with linear flexible chains of uniform length," Rheol. Acta 29, 400 (1990).