Molecular rotors to probe the local viscosity of a polymer glass

TL;DR

This study uses fluorescent molecular rotors to measure local viscosity and free volume in a polymer glass near its glass transition, revealing heterogeneous flow and shear banding under stress.

Contribution

It introduces a method to probe local viscosity in polymer glasses using environment-sensitive fluorescent rotors, providing spatially resolved insights into glass transition dynamics.

Findings

Heterogeneous shear banding observed in polymer films.

Local viscosity varies spatially during glass transition.

Fluorescent rotors effectively measure free volume changes.

Abstract

We investigate the local viscosity of a polymer glass around its glass transition temperature using environment-sensitive fluorescent molecular rotors embedded in the polymer matrix. The rotors' fluorescence depends on the local viscosity, and measuring the fluorescence intensity and lifetime of the probe therefore allows to measure the local free volume in the polymer glass when going through the glass transition. This also allows us to study the local viscosity and free volume when the polymer film is put under an external stress. We find that the film does not flow homogeneously, but undergoes shear banding that is visible as a spatially varying free volume and viscosity.