Dynamics and Thermodynamics of the Glass Transition 玻璃化转变的动力学与热力学.ppt

Dynamics and Thermodynamics of the Glass TransitionJ.S. Langer Workshop on Mechanical Behavior of Glassy Materials, UBC, July 21, 2007 What molecular mechanism is responsible for super-Arrhenius relaxation near the glass transition? What is the connection between the nonequilibrium dynamics of slow relaxation and the equilibrium thermodynamics of glassy materials? JSL Assumptions and Opinions Small molecules with short-ranged, frustrated interactions Basic problem: Compute transition rates between microstates (“inherent states”). Why do these rates become anomalously slow near the glass transition? These transitions are thermally activated molecular rearrangements. Assumptions and Opinions, cont’d. The RFOT theories are inconsistent with this molecular picture. They use Gibbsian statistical mechanics in a mean-field approximation to compute properties of an entropically favored phase, and use a droplet of this phase as a transition state in computing rates. But Gibbsian ergodicity is valid only when the transitions between microstates are much faster than the rates being computed. What, then, is the transition state? Why aren’t simple Arrhenius, activated processes effective? Effective Disorder Temperature Basic Idea: During irreversible plastic deformation of an amorphous solid, molecular rearrangements drive the slow configurational degrees of freedom (inherent states) out of equilibrium with the heat bath. Because those degrees of freedom maximize an entropy, their state of disorder should be characterized by something like a temperature. The effective temperature has emerged as an essential ingredient in STZ theories of large-scale plastic deformation. New Results from T. Haxton and A. Liu(cond-mat 0706.0235) MD simulations of a glass in steady-state shear flow over a wide range of strain rates, and bath temperatures ranging from well below to well above T0 Direct measurements of Teff in all these steady states Quantitative analysis b