Crack-tip deformation transitions and fracture mechanisms in glassy polymers revealed by particle-continuum coupling simulations

• Deformation stages are characterized by microscopic structural evolution, marking clear transitions from local yielding to necking to fibrillar breakdown. • The yielding-to-necking transition is driven by the coalescence of discrete plastic events, with the form of backbone bond rotations. • Necking is sustained by chain stretching within a connected plastic zone, giving rise to a linear relation between macroscopic stress and bond stretch. • Fibrillar instability initiates when spatial fluctuations in plastic activity increase, governed by a few disentanglement events. We investigate crack-tip deformation and fracture mechanisms of glassy polymers using a particle-continuum coupling method that embeds a molecular dynamics region within a finite element domain, with a coarse-grained atactic polystyrene glass as a model system. This approach enables molecular resolutions in the crack-tip region under strongly nonuniform deformation from the surrounding bulk at continuum scales, a regime that cannot be accessed by stand-alone molecular or continuum simulations. Across a broad range of temperatures, chain lengths, bond breakage criteria, and different geometric constraints, the deformation consistently organizes into three distinct stages governed by microscopic mechanisms at different scales: an early plastic regime, stable necking, and fibrillar instability, which are characterized by systematic changes in the microscopic deformation structure. The early plastic regime begins with local atomic dilatation and spatially distributed nonaffine rearrangements. Yielding is marked by the onset of coalescence of these localized plastic sites. Necking begins once this coalescence process is completed and a stable localized neck is established. In this regime, the load is carried by stretched entangled segments, leading to an approximately linear relation between the macroscopic stress and the average bond stretch that remains nearly insensitive to temperature. Continued loading destabilizes this state and leads to fibrillar breakdown, triggered by highly localized disentanglement events of polymer chains. Despite variations in temperature, molecular architecture and failure criteria, the neck and fibril stages exhibit consistent behavior: Stage transitions are driven by a small fraction of highly active regions, whereas the stabilized stages are governed by the collective response of the system.