Isomer geometry controls local mobility in azopolymers: coarse-grained simulation insights.

Abstract

We use coarse-grained molecular dynamics to isolate how azobenzene isomer identity (<i>cis vs. trans</i>) modulates polymer dynamics in a guest-host setting without covalent attachment and without explicit photoisomerization. Segmental relaxation is quantified from the incoherent intermediate scattering function <i>F</i>_s(<i>k</i>,<i>t</i>), with relaxation times <i>τ</i>(<i>T</i>) extracted from the <i>F</i>_s(<i>k</i>,<i>τ</i>) = <i>e</i>^-1 criterion, fitted by Vogel-Fulcher-Tammann, and a glass-transition temperature <i>T</i>_g defined by a standard operational threshold. Across compositions, global structure (density and pair correlations) is nearly isomer-invariant. In contrast, within our model, <i>cis</i> systems exhibit systematically shorter <i>τ</i> and lower <i>T</i>_g than <i>trans</i>-differences consistent with a localized dynamic facilitation near chromophores. Voronoi analysis shows that the average monomer free volume around azobenzene is essentially insensitive to isomer identity, whereas <i>cis</i> chromophores occupy larger Voronoi cells at low <i>T</i>. Isoconfigurational ensembles (propensity analysis) reveal that monomers in the first-neighbor shell of <i>cis</i> are more mobile than near <i>trans</i>, and that immobilizing the chromophores suppresses this contrast. Overall, in this fixed-isomer equilibrium setting, our results cannot support a purely homogeneous free-volume softening between isomers (and, by construction, do not test illumination-induced macroscopic stress gradients); instead they point to a local, cooperative, mobility-dependent pathway that provides a geometry-only baseline for the still-debated microscopic origin of light-driven mass transport in azobenzene materials.