Tailored structure and mechanical properties of polymer nanocomposites by introducing Janus polymer-grafted nanoparticles

Polymer nanocomposites (PNCs) filled with homogeneous polymer-grafted nanoparticles (HNPs) or Janus polymer-grafted nanoparticles (JNPs) have been widely studied, yet how polymer grafting and anisotropic Janus surface interactions jointly regulate self-assembled morphology and tensile response remains poorly understood. Here, we use coarse-grained molecular dynamics simulations to examine the equilibrium structure and uniaxial tensile mechanics of PNCs containing HNPs, JNPs, and their mixtures. The JNP fraction and graft-chain length were systematically varied at fixed numbers of GNPs and matrix polymer chains, while the matrix-chain length was kept constant. At equilibrium, HNPs form partially connected network-like aggregates through multivalent contacts among strongly adsorbing grafts, whereas JNPs assemble into more discrete clusters separated by pronounced voids owing to directional, orientation-dependent interactions. Mixed systems develop interlocking HNP-rich and JNP-rich regions, resulting in mesoscopically heterogeneous morphologies. Under uniaxial extension, increasing either the JNP fraction or graft-chain length reduces the small-strain modulus and overall stress level. Analysis of chain and nanoparticle orientations, together with the number of neighboring nanoparticles, shows that the modulus correlates with the early-stage alignment of strongly adsorbing grafts. Longer grafts alter the evolution of nanoparticle connectivity and suppress this early graft alignment along the tensile direction, thereby reducing the modulus. These results clarify how Janus surface patterning and graft architecture control load-transfer pathways in polymer nanocomposites.