A thermodynamic approach to the fragility of glass-forming polymers

TL;DR

This paper establishes a thermodynamic framework linking the dynamic fragility of glass-forming polymers to their excess entropy and heat capacity, using the Adam-Gibbs relation, and clarifies the role of relaxation processes.

Contribution

It introduces a thermodynamic approach to understanding polymer fragility, accounting for additional relaxation processes affecting entropy correlations.

Findings

Clear correlation between fragility and excess entropy for some polymers

Relaxation processes unrelated to the structural relaxation influence thermodynamic properties

Removing temperature-independent relaxation contributions restores the entropy-fragility correlation

Abstract

We have connected the dynamic fragility, namely the rapidity of the relaxation time increase upon temperature reduction, to the excess entropy and heat capacity of a large number of glass-forming polymers. The connection was obtained in a natural way from the Adam-Gibbs equation, relating the structural relaxation time to the configurational entropy. We find a clear correlation for a group of polymers. For another group of polymers, for which this correlation does not work, we emphasise the role of relaxation processes unrelated to the  process in affecting macroscopic thermodynamic properties. Once an essentially temperature independent contribution of these processes is removed from the total excess entropy, the correlation between dynamic fragility and thermodynamic properties is re-established.