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

## TL;DR

This study uses simulations to explore how PDMS chains behave near silica surfaces, showing how factors like charge and temperature affect structure and movement.

## Contribution

The novel contribution is the atomistic MD investigation of PDMS conformations and diffusion in nanocomposites with ionic and neutral interactions.

## Key findings

- Functionalized ionic PDMS chains show larger dimensions compared to neutral chains.
- Chain charge density of 10% causes contraction in PDMS chains near silica surfaces.
- Hydrogen bonding and chain length influence diffusion and subdiffusive behavior in PDMS.

## Abstract

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.