<p>Polymer nanocomposites (PNCs) benefit from the enhanced properties originating from the synergistic effects of nanoparticles (NPs) and a polymer matrix. However, the incorporation of NPs slows PNC relaxation, and particularly at high-particle-loading, the PNC melts become non-flowing because the interfacially adsorbed polymers bridge the particles, generating a kinetically quasi-permanent particle network. Herein, by introduction of bound polymer loops on the NP surfaces, we molecularly design a relaxation-enhanced PNC, where interfacial polymers adhering to the NP surfaces freely relaxed, enabling formation of a dynamic, loose particle network that facilitates flow of the PNC melts. The resultant molten high-NP-loading composites maintain fluid-like and low-viscosity dynamics, while the corresponding glassy materials possess enhanced toughness and strength. Thus, the demonstrated principle of preparing relaxation-enhanced PNCs by optimizing the way by which polymers are attached to the filler surface represents the state-of-the-art strategy for overcoming the tradeoff between the processability and mechanical performance of PNCs.</p>