Influence of Graphene Platelet Aspect Ratio on the Mechanical Properties of HDPE Nanocomposites: Microscopic Observation and Micromechanical Modeling.

A series of high-density polyethylene nanocomposites filled with different diameter sizes (5, 15, and 25 μm) of graphene nanoplatelets at various amounts (0.5-5 wt.%) are prepared by the melt-mixing method. The effect of diameter size and filler content on the mechanical properties is reported, and the results are discussed in terms of morphology and the state of dispersion within the polymer matrix. The measured stiffness and strength of the nanocomposites were found to be mainly influenced by the filler aspect ratio and the filler-matrix adhesion. Fractography was utilized to study the embrittleness of the nanocomposites, and the observations revealed that a ductile to brittle transition is caused by a micro-deformation mechanism change in the nanocomposites. Several micromechanical models for the prediction of mechanical properties of nanocomposites, taking into consideration filler aspect ratio, percolation effect, and interphase regions, are considered. The three-phase model proposed by Ji accurately predicts the stiffness of graphene nanoplatelets with a higher diameter size, while Takayanagi modified model II was found to show good agreement with the experimental results of smaller ones at low filler content. This study demonstrates that the diameter size of the filler plays a central role in determining the mechanical properties.