Mechanical Property Modeling of Graphene Filled Elastomeric Composites

Abstract

Accessing improved elastomeric composites filled with functionalized graphene sheets (FGSs) requires an understanding of how the FGSs aggregate and how the position of FGSs affects the mechanical properties of the final composite material. In this thesis, I study both effects by devising models for 2-D particles in the 10s of microns scale and comparing my results with experiments. These models enable an understanding of the effect of the particles in a level that is hard to be studied experimentally or by molecular models.

In the first part, I present a model for aggregation of 2-D particles and apply it to study the aggregation of FGS in water with varying concentrations of sodium dodecyl sulfate (SDS). The model produces clusters of similar sizes and structures as a function of SDS concentration in agreement with experiments and predicts the existence of a critical surfactant concentration beyond which thermodynamically stable FGS suspensions form. Around the critical surfactant concentration, particles form dense clusters and rapidly

sediment. At surfactant concentrations lower than the critical concentration, a contiguous ramified network of FGS gel forms which also densifies, but at a lower rate, and sediments with time. This densification leads to graphite-like structures.

In the second part, I present a model for the prediction of the mechanical properties of elastomers filled with 2-D particles. I apply this model to the Poly-dimethylsiloxane (PDMS)-FGS system. For a perfect polymer matrix and when inter-particle forces are ignored the strength of the composite can be increased with the addition of particles but elongation at failure decreases relative to neat PDMS. Maximum load transfer to the particles is achieved when particles are covalently linked to span the whole polymer matrix. Minimum drop in elongation at failure can be achieved by maximizing the distance between the covalently linked particles. When the assumption of a perfect polymer matrix is relaxed, it can be shown that there is a certain particle concentration range for which elongation at failure can be increased as the particles can protect the polymer by redistributing high stresses created by inherent polymer defects that would lead to early failure.