A New Insight into the Mechanical Reinforcement of Polymer Nanocomposites: Effect of Gradient Particle–Matrix Interfaces

The particle-matrix interface (PMI) plays a significant role in the macroscopic mechanical properties of polymer nanocomposites. However, the construction and morphology of this interfacial layer and its impact on the mechanical reinforcement of the composites are inadequately understood. In this study, we directly examine the PMIs of typical polymer composites with particle sizes ranging from 100 nm to 10 μm by probing the nanoscale elastic modulus distributions using atomic force microscopy (AFM) modulus mapping. Our results show a ubiquitous gradient distribution of the elastic modulus within the PMIs in all of the tested composites. More importantly, the ratio of the thickness of the gradient PMI to the particle size increases as the particle size decreases from the microscale to the nanoscale. Further analyses through theoretical calculations and finite element modeling reveal a more significant strengthening effect on the composite stiffness with a higher PMI ratio. We ascribe this reinforcement to strain relaxation within the PMIs and reduction of stress concentrations. We therefore propose this PMI ratio as a new metric parameter that governs the mechanical reinforcement of polymer nanocomposites, providing insights into the reinforcing mechanisms and guiding the design and optimization of polymer composites for further enhancement of mechanical properties.