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

Abstract

We use coarse-grained molecular dynamics to isolate how azobenzene isomer identity (cis vs. trans) 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 F_s(k,t), with relaxation times τ(T) extracted from the F_s(k,τ) = e⁻¹ criterion, fitted by Vogel–Fulcher–Tammann, and a glass-transition temperature T_g defined by a standard operational threshold. Across compositions, global structure (density and pair correlations) is nearly isomer-invariant. In contrast, within our model, cis systems exhibit systematically shorter τ and lower T_g than trans—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 cis chromophores occupy larger Voronoi cells at low T. Isoconfigurational ensembles (propensity analysis) reveal that monomers in the first-neighbor shell of cis are more mobile than near trans, 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.