Molecular dynamics simulations of organoclays and polymer nanocomposites

Understanding the interfacial interactions and structure is important to better design and manufacturing of nanoparticle-filled polymer nanocomposites. This paper presents our recent molecular dynamic studies on organically modified clays and polymer nanocomposites, including the swelling of clay minerals, molecular structure and dynamics in clay gallery, and interfacial interactions of polyurethane nanocomposites. The simulated results are in good agreement with the experimental measurements and observations. Quantitative analyses are made in atom density distribution, molecular tilt angle, order parameter, conformation, and mean squared displacement. Various layering structures (e.g., monolayer, bilayer, pseudo-trilayer and pseudo-quadrilayer) are observed in the gallery of organoclays, depending on the chain length of alkyl ammoniums and cationic exchangeable capacity of clays. In particular, the long alkyl chains do not lie flat within a single layer but interlace, and likely jump to the next layers. In polyurethane nanocomposite, the molecular interplays among clay surface, alkyl ammoniums and polyurethane chains lead to the absence of phase-separation of polyurethane, commonly observed in bulk polyurethane systems.