Structure, Conformations, and Diffusion in PDMS/Silica Nanocomposites via Atomistic MD Simulations

Poly-(dimethylsiloxane) (PDMS)-silica nanocomposites have attracted increasing attention due to their outstanding inherent properties, such as mechanical strength, self-healing, and superhydrophobicity. In this work, we explore the structure, conformations, and diffusion of neutral (nonionic) and ionic PDMS melts confined between nanosilica surfaces, using atomistic molecular dynamics, to provide a nanoscale insight into the interface and interphase, which play a crucial role in the design of novel nanocomposites. We investigate the effect of hydrogen bonding and ionic interactions, together with temperature, chain charge density, electrostatic strength, and charge localization, on the structure and dynamics of the PDMS chains. The chain charge density altered the structure of PDMS chains near the ionic functionalized nanosilica surface. In particular, it is observed that functionalized ionic chain-end PDMS obtains the largest dimensions, while a chain charge density of 10% lead to the contraction of PDMS chains in comparison with neutral PDMS chains. In addition, the ionic functionalization of the PDMS and nanosilica surface decrease the chain dynamics compared with the van der Waals dispersion and hydrogen bonding interactions. Hydrogen bonds between silanols and oxygen of PDMS are affected by the molecular weight of the neutral PDMS chains. Neutral short PDMS chains appear to have faster diffusion and interfacial dynamics, and the neutral long or ionic PDMS chains show a subdiffusive behavior. Charge localization or electrostatic strength has a substantial effect on ionic PDMS chains' structure, conformations, dynamics, and adhesion, while temperature has a negligible effect for neutral PDMS chains.