Polymer nanocomposites (PNCs) are important functional materials with various applications because of their superior nanoparticle-reinforced properties. Among these enhanced macroscopic properties, mechanical reinforcement in PNCs is the most intriguing and among the first to be considered in applications from transportation, packaging to gas separation. However, understanding the mechanical reinforcement of PNCs remains a challenging task, especially in the large deformation regime where nonlinear effects emerge.This dissertation focuses on PNCs with well-dispersed spherical nanoparticles in the dilute and semi-dilute limit to investigate the mechanical reinforcement under large deformation at various Weissenberg number, ??=?̇?? with ?̇ being the Hencky strain rate and ?? the relaxation time of the polymer. At ??≪1, the nanoscale motion of nanoparticles first follows the macroscopic deformation. Beyond a critical elongation ratio defined by the interparticle spacing, the hydrodynamic interaction among nanoparticles leads to a strong deviation of the local spatial rearrangement of nanoparticles from the macroscopic deformation field. Further deformation leads to a deformation-induced nanoparticle network. More importantly, the elastic deformation of the network provides a strong enhancement to the mechanical strength of PNCs at large deformation.As ?? increases, strong microstructure rearrangement of nanoparticles is observed. Remarkably, the nanoparticle rearrangement does not affect the entanglement dynamics in the leading order and does not correlate with the macroscopic stress of the PNCs. These observations indicate that the deformation of matrix polymer plays a dominant role in the macroscopic stress of PNCs. To decouple the stress contributions from the matrix polymer and the nanoparticles, we further perform small-angle neutron scattering experiments that capture only the structure and dynamics of polymer matrices. Interestingly, the neutron experiments show that the magnitudes of polymer anisotropy in the PNC and the neat polymer are identical under the same deformation. Moreover, the stress relaxation of PNCs follows the time evolution of the structural anisotropy of the deformed matrix polymer. Similar phenomena are also observed for PNCs with nanoparticle aggregates and high nanoparticle loadings. These observations point to the absence of strain amplification or molecular overstraining in deformed PNCs and suggest the hydrodynamic effect as the leading molecular origin of the high mechanical strength of PNCs.To further quantify the molecular origin associated with the high polymer matrix contribution to the mechanical reinforcement, we carry out nonlinear rheology measurements for PNCs with different polymer molecular weights and nanoparticle loadings. The nonlinear rheological stress-strain curves of all these PNCs, if normalized by a constant dependent on PNC composition, are found to overlap with each other up to the stress overshoot point. This constant is directly correlated with the bulk polymer relaxation, instead of the interfacial polymers. These observations point to that the mechanical reinforcement of PNCs is controlled by the slowing down of the chain relaxation dynamics.
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