Molecular Dynamics of Azobenzene Polymer with Photoreversible Glass Transition

Abstract

Listen

Translate article

The azobenzene polymer with photoswitchable glass transition temperatures and reversible solid-to-liquid transitions has attracted much attention in recent years. The different glass transition temperatures of the azopolymer in the trans-state and the cis-state introduce evident dynamical heterogeneity during cis–trans interconversion. The correlation between molecular dynamics and rheology in azopolymers remains elusive. In this work, we studied the molecular dynamics of an unentangled amorphous azopolymer without a liquid crystal phase. We found that the Lodge–Mcleish self-concentration (SC) model can quantitatively describe the composition dependence of the glass transition temperature, revealing the local environment of the cis (or trans) segments. The terminal relaxation behavior can be well predicted by the Rouse model with the homogeneous bead friction coefficient and the heterogeneous bead–spring (HBS) model with varying friction coefficients for cis or trans subchains. The difference between two models lies in the relaxation time at small length scales and its distribution. The HBS model predicts broadening the relaxation time spectrum, whose width varies with temperature and the cis content. This prediction is consistent with the broadening of the glass transition temperature in azopolymers. Then, the models were used to study the kinetics of thermal cis-to-trans isomerization of the azopolymer in the melt, proving to be a first-order kinetic process and further justified by the ultraviolet absorption spectrum.