Beyond the free volume theory: Introduction of the concept of cooperativity to the chain dynamics of biopolymers during vitrification

It is accepted in the literature that the viscoelastic properties of amorphous synthetic polymers in the rubber-to-glass dispersion relate to different modes in the time/temperature domain. The protocol of thermorheological simplicity as manifest through the combined WLF/free volume theory constitutes an early attempt to understand the nature of vitrification in spite of its obvious drawback of considering that all molecular retardation and relaxation processes have the same temperature dependence. Increasingly, thermorheological complexity is being recorded in the superposition of mechanical spectra in the viscoelastic master or composite curve, which is an observation that cannot be readily addressed. The newly proposed coupling theory aims to emphasize the distinct temperature dependence of molecular processes within the rubber-to-glass softening dispersion, thus identifying the local segmental motions as the main contributor to the viscoelastic spectrum at the vicinity of the glass transition temperature. Predictions of the coupling theory have explained the “anomalous” experimental facts in the viscoelasticity of amorphous synthetic materials, and the recently established relaxation time—intensity of intermolecular interaction correlations in the vitrification of the gelatin/co-solute mixture.