Isomer geometry controls local mobility in Azopolymers: Coarse-Grained simulation insights

Abstract

We use coarse-grained molecular dynamics to isolate how azobenzene \emph{isomer identity} (\textit{cis} vs.\ \textit{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 $\tau(T)$ extracted from the $F_s(k,\tau)=e^{-1}$ 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 \textit{cis} systems exhibit systematically shorter $\tau$ and lower $T_g$ than \textit{trans}$\,$—differences consistent with a \emph{localized dynamic facilitation} near chromophores. Voronoi analysis shows that the \emph{average} monomer free volume around azobenzene is essentially insensitive to isomer identity, whereas \textit{cis} chromophores occupy larger Voronoi cells at low $T$. Isoconfigurational ensembles (propensity analysis) reveal that monomers in the first-neighbor shell of \textit{cis} are more mobile than near \textit{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.