Molecular dynamics simulations towards understanding thermal stiffening in polymer nanocomposites with dynamically heterogenous interfaces

Abstract

There is extensive research on polymer nanocomposite systems showing that fillers can improve the mechanical properties of polymer melts. This large body of work has given rise to several postulated mechanisms to explain the mechanical reinforcement effect observed in polymer melts. These mechanisms nanoparticle and polymer jamming, free volume effects, shear field interactions, nanoparticle bridging, thermo-reversible cross-linking, and dynamically asymmetric bound layers.In the first two studies we examine the viscoelastic and dynamic properties of heterogeneously grafted nanocomposites (PGNs) as a function of grafting density and then brush height via Molecular Dynamics simulations. The nanocomposite is comprised of a nanoparticle with High-Tg polymer graft chains within a Low-Tg polymer matrix. The viscoelastic and dynamic properties were studied at a temperature below the Tg of the graft chains as a function of graft chain density. PGNs with the highest graft densities showed the greatest increase in shear storage modulus over bare nanoparticle system. In addition, shear storage moduli increased with increasing average brush height. Analysis of the simulation results revealed that the reinforcement was observed when matrix chain mobility decreased as a result of graft chains acting as immobile obstacles. In the next part of this work, we studied dynamically asymmetric composite blends and PGNs with Mw above the entanglement length for both the Low and High–Tg chains via Molecular Dynamics simulations. The PGNs were made up of two chains having a large glass transition temperature (Tg) difference, where the grafted chains have the higher Tg. In this study both the graft and matrix chains have high-Mw’s, above the entanglement length of the chains. Simulation results showed vastly different temperature responses between the neat, blended, and PGN systems. The entangled PGNs showed enhanced reinforcement, when compared to the neat matrix,at High–Tg concentrations far lower than that of the blended systems. Additionally, a thermomechanical analysis showed unique thermal stiffening behavior, for the high-Mw matrix PGN systems, a stiffening mechanism not observed in the other systems. Analysis of the simulation results revealed that the reinforcement mechanism for blended systems was proportional to the weight fraction of the High–Tg fillers and the slow-down of the dynamics of matrix polymer chains, which was reduced at elevated temperatures. However, the high-Mw matrix PGNs showed increased reinforcement at elevated temperatures as well as increased matrix and chain mobility. The following mechanisms were identified that were responsible for this PGN reinforcement: (i) At elevated temperature Low–Tg matrix chains dynamically coupled with High–Tg graft chains transmitting the applied stress through bulk of the polymer to the nanoparticle (ii) With increasing interfacial thickness, there were an increased number of graft/matrix entanglements the which drives the level of thermal reinforcement observed.