Confinement-Enhanced Interfacial Anchoring beyond the Rule of Mixtures in Polymer-Graphene Heterostructures

Nanoconfinement is known to alter polymer mechanics, yet how it couples with interfacial interactions to govern stiffness and fracture in polymer-graphene layered nanocomposites remains fundamentally elusive. This work provides a mechanistic decoupling of these effects by investigating well-defined polycarbonate-graphene heterostructures across a range of polymer thicknesses (h_p). Using in situ MEMS tensile testing integrated with Raman spectroscopy, we demonstrate that geometric confinement significantly strengthens the interfacial anchoring. As the polymer thickness approaches molecular length scales (h_p < 2R_g), the PC chains undergo a critical conformational transition from random coils to flattened, interface-parallel configurations. This molecular ordering drives a shift in the deformation mechanism from bulk-like chain slippage to interface-governed bond stretching. Consequently, we observe an anomalous modulus stiffening that significantly exceeds the theoretical upper bounds of classical mixture rules, accompanied by a concurrent ductile-to-brittle fracture transition. Enhanced Raman strain sensitivity and spatially distributed graphene lattice degradation provide direct evidence of highly efficient stress transfer under strong confinement. Our findings reveal that the synergistic coupling of geometric and interfacial constraints dictates the mechanical landscape of polymer-graphene heterostructures, offering essential guidelines for engineering high-performance layered nanocomposites.