Chain-Rigidity-Governed Mechanical Stiffening of Polymer Ultrathin Films

Increasing the miniaturization of functional materials and devices demands polymer films with thicknesses approaching molecular length scales. Under such extreme confinement, polymer mechanics deviate sharply from bulk behavior. Of particular importance is understanding the microscopic origin of such size effects to build design principles at the molecular level. Here, we uncover the microscopic basis of mechanical stiffening in ultrathin polymer films by integrating temperature-controlled atomic force microscopy-based deflection testing, single-molecule force spectroscopy, and molecular dynamics simulations. We demonstrate that confinement alters chain-level deformation pathways, shifting load transfer from weak intermolecular cohesion to direct intrachain stretching as the thickness decreases. Comparative studies of polycarbonate and poly(methyl methacrylate) films further confirm that chain rigidity─rather than chain alignment or entanglement density─dominates the extent of reinforcement. We unify these findings in a sliding–stretching competition mechanism, which reconciles discrepancies across diverse experimental techniques. This framework establishes molecular-level design rules for creating mechanically robust polymer nanofilms and layered architectures with tailored performance for next-generation flexible electronics and protective coatings.