Original entry: Ian Burgess, Fall 2009
The glass transition is a phase transition that occurs in certain materials between the liquid and the glass phases. The transition to a glass is marked by a solidification of the material without the addition of any long range order to the molecular packing. Unlike crystallization, there is also no discontinuous change in any thermodynamic property. Figure 1 illustrates the difference between a crystalline and glassy transition. Kinetics hold the key to this behavior: when a liquid is cooled faster than the timescale required for nucleation and crystallization, an amorphous solid results. The resulting material behaves mechanically like a solid on laboratory timescales, but experiences continuous, slow rearrangement moving toward its equilibrium state. The structure of some materials prohibit crystallization and hence naturally transition to a glassy state when cooled. A crystalline material can also be brought through a glass transition when the liquid state is supercooled. The precise mechanisms for this transition to solid-like behavior is not well understood. The glass transition occurs at the glass transition temperature, <math>T_g</math>, which varies with the material and cooling rate. It should be noted that manipulating pressure, independent of temperature, can produce a glass transition.
In polymers above the glass transition temperature, chains have sufficient mobility to slide past each other and reconfigure under an applied stress. This mobility is substantially reduced below the glass transition. However, at temperatures above the glass transition, but below the melting point, polymers still have a finite stiffness.
 Debenedetti and F. H. Stillinger. "Supercooled liquids and the glass transition". Nature, Vol 410, 8 March 2001.
 Z. Fakhraai and J. A. Forrest, "Measuring the Surface Dynamics of Glassy Polymers" Science 319, 600 (2008).
 Kingery, W,D., Bowen, H.K., and Uhlmann, D.R., Introduction to Ceramics, 2nd Edn. (John Wiley & Sons, New York, 2006).