Glassforming liquids with microscopic to macroscopic two-state complexity

We review briefly the Adam-Gibbs-Goldstein approach to understanding the phenomenology of glassforming liquids and note three ways of scaling viscosity or relaxation time data which emphasize different aspects of the relationship between fragile and strong behavior in supercooling liquids. In all these cases, evolution from high temperature mobile to low temperature glassy states is smooth, the α-relaxation process being unimodal after the initial separation from the microscopic rattling mode. We then consider a series of cases where additional complexity enters in the form of new structural degrees of freedom which may be excited separately from the background structure modes and which are more slowly relaxing than the background modes. In first approximation, these may be considered as two-state systems superimposed on the normal glassformer background, and they may vary from (i) microscopic intramolecular degrees of freedom (such as a gauche-transisomerism) and microscopic intermolecular (such as unilateral hydrogen bonding to form molecular pairs) through (ii) mesoscopic cases in which large molecules can fold into tertiary structures (and the molecules themselves - proteins, RNAs - are mesoscopic glassforming systems), to (iii) macroscopic cases in which the glassforming liquid actually changes liquid phase through a first order transition.