(383b) Mechanical Properties of Graphene­–Polymer Nanocomposites

The exceptional mechanical, electronic, and thermal properties of graphene render it a highly promising filler for polymer­matrix nanocomposites. While preliminary experiments indeed show significant enhancement in the thermomechanical properties of graphene­polymer nanocomposites compared to those of the polymer matrix even at relatively low loading of filler, our fundamental understanding of structureâ??property relationships that govern such enhanced material response remains very limited.

Toward this end, in this presentation, we report results of molecular­dynamics simulation studies on the mechanical behavior of polymer (high-density polyethylene) nanocomposites reinforced by graphene and fullerenes with the aim of elucidating the underlying mechanisms that govern the mechanical response of these composite materials. Using a united-atom-based model of the glassy polymer matrix, we show systematic trends in the enhancement of the mechanical stiffness of the polymer composite as a function of filler concentration, filler size, and matrixâ??filler interfacial interaction strength that is governed by dispersive forces. For comparison of different filler reinforcement effects in such polymer­matrix nanocomposites, we also present systematic mechanical behavior studies of fullereneâ??polymer composites and show that, for fullerene fillers, the response is only weakly dependent on filler size with relatively high loadings required for a considerable improvement in the compositeâ??s stiffness. In contrast, graphene-polymer nanocomposites, with graphene circular flakes used as fillers, show an appreciable improvement in the stiffness reinforcement with increasing filler size. We explain the differences in the elastic response of (0D) fullerene­ and (2D) graphene ­reinforced polymer­ matrix composites through detailed atomic ­scale characterization of the nanocomposites in conjunction with the predictions of an analytical continuum-mechanics model for understanding the filler­size dependent response of the polymer nanocomposites. We also derive scaling relations for the dependence of the composite modulus for polymer-graphene nanocomposites as a function of the flake size at given filler concentration. Moreover, we report results of our continuum mechanical analysis for polymer-matrix reinforcement on the basis of shear-lag modeling and show a parametric dependence of the axial and interfacial shear stress at the graphene-polymer interface on the graphene flake size. Based on our atomistic simulations and this shear-lag analysis, we identify the range of filler length scale over which classical shear-lag theory becomes most effective for describing the polymer-matrix reinforcement.