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AGING OF SUPERCOOLED LIQUIDS :

  Supercooled liquids below glass transition temperature, Tg, fail to equilibrate on typical experimental times. One of the most striking feature of such glass forming systems is the slow approach towards equilibrium, called aging, that their response functions show. This behavior appears in a broad variety of disordered materials including structural glasses, spin glasses, disordered ferroelectrics and gels.

  I am conducting dielectric spectroscopy measurements of aging in a series of glass forming materials. By characterizing this aging on sorbitol for a variety of thermal histories, I have identified the remarkable property of "memory" in which the glass appears to become encoded with its thermal history. Currently, I am focusing on aging studies of the Johari-Goldstein relaxation on different glassy sytems to test various theories associated with the origins of this relaxation.

H. Yardimci, R. L. Leheny, “Memory in an Aging Molecular Glass”, Europhys. Lett. 62, 203 (2003).

 

AGING OF CLAY SUSPENSIONS:

 While the glass transition and aging in supercooled liquids introduces a number of interesting problems, the significant amount of research done in this area proves their difficulty. However, the general properties of slow dynamics and out-of-equilibrium behavior appear in many disordered systems. Thus, it can be informative to study a system whose microscopic behavior may be more accessible.

  Laponite is a synthetic clay consisting of charged disks, 25 nm diameter and 1 nm thick. Aqeous solutions of laponite, even at very small concentrations, undergo a transition from a liquid-like solution to solid-like gel. This transformation is very similar to a glass transition if time is considered to be the physical control parameter. Once the gelation is complete the laponite displays aging. I have made small angle and ultra-small angle scattering neutron scattering measurements at NIST Center for Neutron Research on laponite suspensions to provide information about how the structure evolves both as a function of time and and pH during gelation. I have also conducted dielectric measurements on laponite to probe the time dependence of the frequency dependent ionic conductivity.

R. Bandyopadhyay, D. Liang, H. Yardimci, D. A. Sessoms, M. A. Borthwick, S. G. J. Mochrie, J. L. Harden, R. L. Leheny, “Evolution of particle-scale dynamics in an aging clay suspension”, Phys. Rev. Lett. 93, 228302 (2004).

 

STRUCTURE AND DYNAMICS OF SELF ASSEMBLING POLYMER MICELLES:

  We investigate the phase behavior and local dynamics of aqueous solutions of the triblock copolymer polyethylene oxide (PEO) - polypropylene oxide (PPO) - polyethylene oxide (Pluronic F108 and F68) by neutron scattering. In solution the polymer self-associates into spherical micelles with PPO cores and corona of solvated PEO. For sufficiently high concentration the Pluronic evolves on heating from fully solvated polymer to a micellar liquid phase then to a micellar crystal phase. The temperature range of the micellar liquid region increases with decreasing chain length, a feature we attribute to fluctuation effects as predicted by recent theory. The local micellar dynamics probed in neutron spin echo display fast and slow modes that depend systematically on concentration and temperature as the liquid-crystal phase boundary is traversed. However, these dynamics are surprisingly insensitive to phase and macroscopic rheology. Contrast matching the PEO corona to the solvent reveals that the slow mode corresponds to Rouse modes of the PPO segments in the concentrated micelle cores. The fast mode is identified with longitudinal diffusive modes in the PEO corona characteristic of a polymer brush.

H. Yardimci, B. Chung, J. L. Harden, R. L. Leheny, “Phase Behavior and Local Dynamics of Concentrated Triblock Copolymer Micelles”, to be published in J. Chem Phys.

 

  ONGOING PROJECTS:

  • Aging of the Johari-Goldstein Relaxation

  • Neutron Spin-Echo Studies of Corona Dynamics in Polystyrene – Poly(acrylic acid) Copolymer Micelles