V. Ginzburg
VVG Physics Consulting,
United States
Summary:
Philip W. Anderson, a Nobel Prize-winning theoretical physicist, wrote in 1995: “The deepest and most interesting unsolved problem in solid state theory is probably the theory of the nature of glass and the glass transition.” This challenge is especially intriguing and practically important for polymers. Many processes (mechanical recycling, lithography, application of paints to surfaces, extrusion of amorphous plastics, etc.) are designed based on our ability to understand the glassy dynamics, where the material viscosity can increase by several orders of magnitude as the material is cooled by only a few degrees. Today, the accepted paradigm is that as a polymer is cooled below its glass transition temperature, its “free volume” or, alternatively, “configurational entropy” decrease to zero at some finite temperature T0; at that temperature, the material becomes completely “frozen” as its relaxation time and/or viscosity diverge. This is described mathematically using Vogel–Fulcher–Tammann (VFT) or Williams–Landel–Ferry (WLF) equations. Recently, however, many researchers (G. McKenna, J. Dyre, H. Winter, and others) demonstrated that in many systems, the VFT formalism breaks down, and the low-temperature glass is better understood as “just another fluid”, albeit with extremely slow dynamics (with molecules moving over geological or astronomical timescales). In my talk, I will describe a new mean-field model aimed at capturing this behavior. The model – labeled “two states, two (time)scales” or TS2 – has been shown to successfully describe dielectric - and -relaxation in several amorphous polymers, such as PS and PMMA; it can also be extended to miscible polymer blends and random copolymers. By combining TS2 with the Sanchez-Lacombe lattice theory and adding the Tool-Narayanaswamy-Moynihan (TNM) relaxation equations, we are able to also describe the pressure-volume-temperature (PVT) data both above and below the glass transition. The theory can also capture the thickness dependence of the glass transition temperature (Tg) of thin films. Crucially, the theory can offer insights into the nature of yield and fracture of amorphous polymers near their glass transition. I will conclude by discussing future steps in theory development, including its practical applications (nanocomposites, recycling, membranes, lithography, etc.)