Thermal Stiffening in Polymer Nanocomposites with Dynamically Heterogenous Interfaces

There is extensive research on polymer nanocomposite systems showing that fillers can improve the mechanical properties of polymer melts through hydrodynamic interactions between nanoparticles and polymer chains, distortion of shear fields, decreased matrix chain mobility, nanoparticle jamming, and percolation of a transient immobile high-density polymer network assisted by nanofillers. In the current study, dynamically asymmetric composite blends and polymer-grafted-nanoparticle polymer composites (PGNPCs) with molecular weights above the entanglement length were studied via Molecular Dynamics simulations. Both blends and PGNPCs were made up of two polymer chains having a large glass transition temperature (T_g) difference − grafted chains having the higher T_g. Simulation results showed vastly different temperature responses among the neat (Low-T_g polymer), blends, and PGNPCs. The entangled PGNPCs showed enhanced reinforcement, when compared to the neat polymer matrix at graft concentrations far lower than that of the blends. Additionally, a thermomechanical analysis showed a unique thermal stiffening behavior in PGNPCs (even at high temperatures) that was not observed in other systems. Two mechanisms were identified for the observed reinforcement in PGNPCs: (i) dynamic coupling of High-T_g grafts and Low-T_g matrix chains at high temperatures, (ii) increased number of graft/matrix entanglements with increasing interfacial thickness.