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Strong changes in bulk properties, such as modulus and viscosity, are observed near the glass transition temperature, T₆, 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, we delve into the atomic/molecular level picture of how microscopic localized relaxations, or "cage rattles, " translate to macroscopic structural relaxations above T₆. Unit motion is broken down into two populations: (1) simultaneous rearrangement occurs among a critical number of units, n_, which ranges from 1 to 4, allowing a systematic classification of glasses that is compared to fragility; (2) near T₆, adjacent units provide additional free volume for rearrangement, not simultaneously, but within the "primitive" lifetime, ₁, of one unit rattling in its cage. Relaxation maps illustrate how Johari-Goldstein \beta relaxations stem from the rattle of n_ units. We analyzed a wide variety of glassy materials, and materials with glassy response, using literature data. Our four-parameter equation fits "strong" and "weak" glasses 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.
Schlenoff et al. (2024) studied this question.