Entropy Theory of Polymer Glass-Formation Revisited. I. Characteristic Temperatures, Fragility, and Relaxation Times
J Dudowicz, Karl Freed, Jack F. Douglas
Considerable physical evidence supports the idea of Gibbs and DiMarzio (GD) that glass-formation arises when the configurational entropy of a liquid becomes critically small and the subsequent arguments by Adam and Gibbs (AG) that quantitatively relate the configurational entropy Sc to the relaxation time tau for structural relaxation. This classical entropy theory of glass-formation is revisited based on the lattice cluster theory (LCT) that incorporates monomer structure and the different rigidities of the polymer chain backbone and the side groups into the thermodynamic description of compressible semiflexible polymer melts. Recent claims of a breakdown of the entropy theory of glass-formation at elevated temperatures are resolved by identifying Sc of the AG model with Sc entropy per site calculated from the LCT. While this reinterpretation of Sc has a negligible effect near the glass-transition, it completely revises the predictions of the entropy theory at more elevated T.The Vogel-Fulcher-Tammann-Hesse (VFTH) temperature dependence of tau is recovered at low temperatures upon use of the AG relation between Sc and tau. At higher temperatures, however, Sc exhibits an unexpected quadratic dependence on T with a maximum at thew arrhenius temperature, TA. We introduce a Lindemann criterion to estimate a critical value of the specific volume that serves to define the kinetic glass transition temperature Tg. The LCT is used to investigate the origin of the observed dependence of fragility on molecular rigidity and structure. Numerous LCT-AG model calculations provide valuable insights into general trends of glass-formation, fragility, and experimental relaxation data. A comparison of the new entropy theory with other theories of glass-formation is also briefly discussed.
, Freed, K.
and Douglas, J.
Entropy Theory of Polymer Glass-Formation Revisited. I. Characteristic Temperatures, Fragility, and Relaxation Times, Journal of Chemical Physics, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=852488
(Accessed May 28, 2023)