Characterization of Morphology Evolution in a Polymer-Clay Nanocomposite using Multiscale Simulations

Molecular simulations provide an effective route for investigating morphology evolution and structure-property relationship in polymer-clay nanocomposites (PCNCs) incorporating layered silicates like montmorillonite (MMT), an important class of materials that show a significant enhancement over the constituent polymer for several properties. However, long relaxation times and large system size requirements limit the application to systems of practical interest. In this work, we have developed a coarse-grained (CG) model of organically modified MMT (oMMT) compatible with the MARTINI force field, a chemically-specific interaction model with high transferability. The dispersive and polar components of cleavage energy, basal spacing, and mechanical properties of MMT with tetramethylammonium (TMA) as intergallery ions were used to obtain a rational estimate for clay particle MARTINI bead types in accordance with the polarity of the functional group. The CG model provided accurate concurrent estimates for the structural, thermodynamic, and dynamical properties of PE in a PE/TMA-MMT PCNC, with less than 4% deviation from all atom (AA) simulations. The slow clay-induced redistribution of the PE-b-PEG block copolymer in the PCNCs was investigated using the developed CG model, with conformational changes occurring over a microsecond timescale. The preferential interaction coefficient and cluster analysis of individual blocks of PE-b-PEG were used to study the effect of clay arrangement (exfoliated versus tactoid) on copolymer reorientation and assembly at the clay surface. We find that the oMMT coated with PE-b-PEG acts as a neutral surface (small difference in polymer-polymer and polymer-oMMT+PE-b-PEG enthalpic interactions) and the primary influence of the nanofiller is a result of confinement and steric effect of the clay sheets on the PE chains.