Layer-Resolved Frictional Mechanisms of Engineering Polymers: Contrasting Spatial Modes of Dissipation in Polyoxymethylene and Polytetrafluoroethylene.

A fundamental molecular understanding of polymer friction remains elusive despite its critical engineering importance. Although all-atom molecular dynamics (AAMD) simulations can provide molecular-level insights, many studies have focused on individual polymers and/or sliding rates near or beyond the upper end of experimental conditions, often without the depth-resolved perspective needed to distinguish material-specific responses. Here, we use large-scale AAMD to identify two spatially distinct modes of frictional energy dissipation in polyoxymethylene (POM) and polytetrafluoroethylene (PTFE). After a brief interfacial transient, each system reaches a persistent quasi-steady response. POM exhibits a distributed mode that extends into the slab interior: deviations from the ideal Couette response and elevated orientational order persist several nanometers into the slab, and dissipation is dominated by bonded conformational relaxation through bond-angle and dihedral motions. By contrast, PTFE exhibits a mode localized near the interface: shear accommodation and the associated nonbonded energy dissipation─driven by van der Waals and electrostatic interactions─are concentrated within a narrow near-interface zone. In PTFE, this localized mode is accompanied by a pronounced subsurface temperature maximum. Notably, this thermal maximum is spatially offset from a deeper densification band, indicating distinct length scales for heat generation and structural packing. Together, these results establish spatial dissipation pathways and subsurface transport bottlenecks as key descriptors of polymer friction. This framework provides a molecular basis for tuning how and where shear work is absorbed in polymer tribomaterials.