Unifying the temperature dependent dynamics of glass formers

Strong changes in bulk properties, such as modulus and viscosity, are observed near the glass transition temperature, T-g, of amorphous materials. For more than a century, intense efforts have been made to define a microscopic origin for these macroscopic changes in properties. Using transition state theory (TST), we delve into the atomic/molecular level picture of how microscopic localized unit relaxations, or "cage rattles," evolve to macroscopic structural relaxations above T-g. Unit motion is broken down into two populations: (1) simultaneous rearrangement occurs among a critical number of units, n(alpha), which ranges from 1 to 4, allowing a systematic classification of glass formers, GFs, that is compared to fragility; and (2) near T-g, adjacent units provide additional free volume for rearrangement, not simultaneously, but within the "primitive" lifetime, tau(1), of one unit rattling in its cage. Relaxation maps illustrate how Johari-Goldstein beta-relaxations stem from the rattle of n(alpha) units. We analyzed a wide variety of glassy materials and materials with a glassy response using literature data. Our four-parameter equation fits "strong" and "weak" GFs over the entire range of temperatures and also extends to other glassy systems, such as ion-transporting polymers and ferroelectric relaxors. The role of activation entropy in boosting preexponential factors to high "unphysical" apparent frequencies is discussed. Enthalpy-entropy compensation is clearly illustrated using the TST approach.