Effect of Processing and Particle Size on the Properties and Morphology of MWCNT-Polymer and SiC-Polymer Composites

Abstract

The objective of this study is to determine the effect of processing on the electrical properties and microstructure of MWCNTs/PMMA nanocomposites and the effect of particle size on SiC/PMMA composites. For the MWCNT composites, the mixing methods used were mechanical, solution, and melt mixing. Impedance spectroscopy, SEM, and optical microscopy were used to characterize the samples. Results show that the mechanically mixed composites have the lowest percolation threshold of 0.05 phr (0.05 wt % MWCNT). Melt mixed composites have the highest threshold of 4 phr (3.85 wt % MWCNT). Solution mixed composites have a percolation threshold of 2 phr (1.96 wt % MWCNT). Percolation theory was used to determine the percolation threshold and shows values very close to the experimental estimations. The microstructures are segregated, agglomerated, and randomly distributed for mechanically, solution, and melt mixed, respectively. These results indicate that the segregated microstructure allows for the CNTs to form a percolated network through the composite more easily. The equivalent circuits show that above percolation the CNT dominates the electrical properties and may be represented by a RL series circuit. Below or at percolation, the presence of PMMA plays a stronger role in the circuit. Silicon carbide/PMMA composites were fabricated using a mechanical mixing method using nano-, micro-, and whisker SiC. The results show clearly that the nanosized SiC reached percolation first at 2 phr (1.96 wt % SiC) and had a very defined microstructure. SiC whiskers were the opposite, with the least defined microstructure and a percolation at 10 phr (9.09 wt % SiC). The micron-sized SiC reached percolation at 7.5 phr (6.98 wt % SiC).