Viscoelastic and dynamic properties of polymer grafted nanocomposites with high glass transition temperature graft chains

Abstract

The viscoelastic and dynamic properties of dynamically asymmetric polymer-grafted nanocomposites (PGNs) are studied via molecular dynamics simulations. The model PGN is made up of two chains having a large glass transition temperature (Tg) difference, where the grafted chains have the higher Tg. The viscoelastic and dynamic properties were studied at temperatures between the Tgs of the graft and matrix polymers as a function of the average brush height. Simulation results showed that the static and dynamic properties of the glassy brush played an important role in reinforcing the overall nanocomposite. Although the bare nanofiller containing nanocomposite showed increased shear storage moduli compared to the neat low-Tg polymer, PGNs presented the greatest increases in the shear storage modulus. In addition, the shear storage modulus increased with increasing average brush height, reaching a maximum value at the brush height limit. Analysis of the simulation results revealed that the reinforcement of the shear storage modulus was mainly related to the slowing down of the dynamics of matrix polymer chains. The following mechanisms were identified that were responsible for this effect: (i) High-Tg grafted chains act as obstacles for matrix polymer chains. (ii) With increasing average brush height, grafted and matrix chains form a well-mixed morphology at the nanofiller interface, which leads to further slowing down of the matrix chain dynamics. (iii) Finally, at the brush height limit, grafted chains form a stiff and immobile percolated network, which leads to the observed maximum in the shear storage modulus.