A local annealing strategy for equilibrium-like solute insertion into glassy polymer matrices

The insertion of small molecules into glassy polymer matrices is strongly influenced by the non-equilibrium dynamical state of the polymer. Using atomistic molecular dynamics simulations, we investigate sorbitol insertion into dry and hydrated poly(vinyl acetate) (PVAc) slabs and analyse how limited polymer mobility in the glass affects the potential of mean force (PMF). In the dry slab, the polymer chains are kinetically trapped below the glass transition temperature and cannot relax on accessible simulation timescales. As a result, direct PMF calculations yield an artificially large, non-equilibrium free-energy barrier dominated by steric constraints and kinetic trapping. In contrast, water-induced plasticization enhances local mobility, allowing relaxation around the solute and producing a substantially lower, more physically meaningful insertion barrier. To overcome the non-ergodicity of the dry glass, we introduce a local annealing strategy. Here, the solute is heated, promoting local relaxation of polymer chains in the immediate vicinity of the solute, prior to re-equilibration into a glassy state. This method reduces the insertion barrier by more than 70% and yields an equilibrium-like free-energy profile without disrupting, melting or softening the entire polymer film. The approach provides a practical and general route for modelling solute insertion in glassy polymers, where full equilibration is not computationally achievable on the timescales of atomistic simulation.