The consequence of excess configurational entropy on fragility: the case of a polymer/oligomer blend

TL;DR

This study demonstrates that increasing configurational entropy through polymer/oligomer blending can significantly alter fragility without changing fundamental glass properties, highlighting the role of entropy in glass dynamics.

Contribution

It introduces a method to modify fragility by controlling configurational entropy via blend composition, without affecting other glass properties.

Findings

Blend and pure 17-mer have same heat capacity in glass and liquid.

Fragility differs significantly between blend and pure 17-mer.

Extra configurational entropy from mixing influences fragility independently of fast dynamics.

Abstract

By taking advantage of the molecular weight dependence of the glass transition of polymers and their ability to form perfectly miscible blends, we propose a way to modify the fragility of a system, from fragile to strong, keeping the same glass properties, i.e. vibrational density of states, mean-square displacement and local structure. Both slow and fast dynamics are investigated by calorimetry and neutron scattering in an athermal polystyrene/oligomer blend, and compared to those of a pure 17-mer polystyrene considered to be a reference, of same Tg. Whereas the blend and the pure 17-mer have the same heat capacity in the glass and in the liquid, their fragilities differ strongly. This difference in fragility is related to an extra configurational entropy created by the mixing process and acting at a scale much larger than the interchain distance, without affecting the fast dynamics and the structure of the glass.