The lattice model for linear polymeric glasses proposed by Gibbs and Di Marzio several decades ago is used to calculate the average relaxation time for the structure of the model as a function of temperature. Using the fact that the model exhibits the existence of a temperature $T_0$ for which the configurational entropy of the system vanishes, we identify this temperature with the threshold of infinitely long relaxation times. Using the analytical parameter-free expression for the specific heat for temperatures above $T_0$, we show that (i) there are substances for which a Vogel-Fulcher-Tammann-like (VFT) equation is accurately obeyed in the range T>$T_g$, where $T_g$ is the experimental glass transition temperature. On the other hand, in the range $T_0$$T_g$ a modified VFT equation is suggested.

• Received 20 March 1989

DOI:https://doi.org/10.1103/PhysRevB.40.7040