Towards the ideal glass transition by pinning in a dimer-polymer mixture

TL;DR

This study investigates how pinning a fraction of a polymer-dimer mixture affects glass transition dynamics, revealing significant slowing of motion but only minor changes in fragility, thus providing insights into the nature of glass formation.

Contribution

It introduces a novel approach using polymer-dimer mixtures to control and study glass transition dynamics through pinning effects.

Findings

Pinning significantly slows system dynamics.

Fragility shows only minor variation despite slowed dynamics.

Point-to-set length is strongly influenced by pinning degree.

Abstract

We use a mixture of a polymer and its dimer to control dynamics in a manner inspired by \emph{pinning} a fraction of the system. In our system of $\alpha$-methyl styrene, where the polymer has a glass transition at higher temperature than the dimer, at intermediate temperatures, the polymer acts to "pin" the dimer. Within this temperature range, we use differential scanning calorimetry to infer a point-to-set length which we find to be profoundly influenced by the degree of pinning. We determine the dynamics of the system with dielectric spectroscopy and find that while the dynamics are very substantially slowed by the "pinning", the fragility exhibits only a small change relative to the precision of our measurements. This may indicate that in the approach we have used, fragility has a relatively weak dependence on quantities such as the point--to--set length. % than one might expect, However, an alternative explanation is that the dimer may act to \emph{plasticize} the polymer and thus open routes to relaxation that may be inaccessible to fully pinned systems.