When the perturbation frequency imposed ona rubber falls within the glass transition zone of its viscoelastic spectrum, energy absorption is maximized. This phenomenon is the operative mechanism for various applications of elastomers requiring large energy dissipation. Nevertheless, a fundamental understanding of the glass transition is lacking. The diversity of properties that depend both on chemical structure and thermodynamic conditions makes modeling difficult and a first principles theory perhaps unachievable; indeed, the number of models for the glass transition seems to be inversely proportional to their ability to accurately describe the myriad behaviors. The progress made at quantifying the role of the thermodynamic variables temperature, T, and density, ρ, on the dynamics is described. An important aspect of the work was the discovery that relaxation times and viscosities of molecular liquids and polymers superpose when plotted against the scaling variable T/ρ γ, with the scaling exponent γ a material constant sensibly related to the nature of the intermolecular repulsive potential; thus, dynamic spectroscopy measurements can be used to quantify the forces between molecules. Other properties derive from the scaling behavior, including the Boyer-Spencer rule and the correlation of fluctuations in the potential energy with fluctuations in the virial pressure.
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