Visualizing interfacial thermal resistance in two-dimensional material-polymer composites from measurement to mechanistic understanding.

Two-dimensional (2D) material-based polymer composites are promising for advanced thermal management because of their mechanical robustness, electrical insulation, and structural tunability. However, their effective thermal conductivity remains far below the intrinsic potential of 2D fillers, primarily because of pronounced interfacial thermal resistance (ITR) at filler-polymer interfaces. This disparity indicates that heat transport is governed not only by filler properties but also by interfacial phonon transfer, local coupling, and structural heterogeneity across multiple length scales. Multiscale models, molecular dynamics simulations, and first-principles calculations have identified phonon spectral mismatch, interfacial disorder, weak bonding, and nonequilibrium energy transfer as key limitations on cross-interface heat flow. However, experiments often rely on spatially averaged thermal parameters, obscuring localized heat flow pathways and structure-dependent transport behavior. Visualization-based methods, including infrared thermography, luminescent thermal probes, scanning thermal microscopy, and advanced electron microscopy, bridge macroscopic thermal measurements with microscopic interfacial mechanisms. These approaches recast ITR from an indirectly inferred parameter into a spatially resolved, temporally trackable, and structurally interpretable transport process. By integrating visualization with multiscale thermal transport theory, this review defines the applicable length scales, mechanistic capabilities, and limitations of current methods and establishes a process-level framework for understanding ITR in 2D material-polymer composites. ITR is thus treated as a heterogeneous and structurally tunable phenomenon rather than a fixed scalar quantity. Future progress will depend on correlative, operando, and model-coupled visualization strategies that quantify dynamic interfacial heat transport under realistic conditions and guide rational composite design.