A micromechanically informed model under finite deformation explains structural evolution in glassy polymers

Structural evolution is an intrinsic feature of plastic deformation in glassy polymers, yet its microscopic origin and continuum representation remain elusive. Using molecular dynamics (MD) simulations, this work investigates deformation-induced structural evolution of microscopic plastic processes. Plastic deformation is found to initiate through spatially isolated non-affine rearrangements and to evolve into correlated, spatially extended structures after the onset of yielding. Non-affine displacement, bond rotation, and local volumetric fluctuation are strongly co-localized, indicating that they represent different manifestations of the same plastic events. Dilative and contractive volumetric strains coexist at the monomer scale, giving rise to distinct atomic-scale volume gradients. The evolution of spatial correlations reveals a progressive spreading of volumetric disturbance during yielding and post-yield flow, governed by competing generation and relaxation processes with distinct characteristic times. Based on these observations, a scalar volumetric disturbance variable is introduced to describe deformation-induced structural evolution. Its stochastic evolution is formulated in terms of plasticity-driven generation, thermally activated relaxation, and gradient-driven spatial redistribution. Upon ensemble averaging, this description yields a continuum-scale internal variable with a diffusion-type contribution. Incorporating this physically derived evolution equation, a thermodynamically consistent constitutive framework is established that captures strain softening and its evolution under deformation. At the representative volume element scale, the formulation recovers a mean-field description consistent with the observed macroscopic response. At the spatially resolved finite-element scale, the gradient term, with a coefficient estimated from the MD second-moment measure, regularizes the spatial heterogeneity of the structural variable while leaving its averaged evolution essentially unchanged. These results establish deformation-induced structural evolution in glassy polymers as an intrinsic physical process that can be coherently represented at the continuum scale through microscopic configurational dynamics.