Influence of concentration, length and orientation of multiwall carbon nanotubes on the electromechanical response of polymer nanocomposites

Multiwall carbon nanotube (MWCNT)/polymethyl methacrylate (PMMA) composites were fabricated by solution casting with a range of MWCNT concentrations, two groups of MWCNT lengths and with or without the application of electric field. As-received (CNTs) and fragmented (FCNTs) MWCNTs were used to fabricate composites with MWCNT concentrations between 0.05 wt% and 0.7 wt%. A well-controlled procedure to systematically fragment the MWCNTs based on the application of high-density ultrasonic energy was implemented, and MWCNT length distributions were obtained by a formal statistical analysis. The relationship between the sparseness of the MWCNT network (macroscale morphology), electrical conductivity and piezoresistive sensitivity was evaluated. In order to evaluate the effect of the MWCNT alignment on the composite properties, specific MWCNT concentrations were selected for composites with CNTs and FCNTs under the application of an electric field. Composites with FCNTs exhibited a sparser network, higher transparency, lower electrical conductivity, higher percolation threshold and higher piezoresistive sensitivity than their counterparts with CNTs. On the other hand, composites with electric field-aligned MWCNT bundles presented higher electrical conductivity and piezoresistive sensitivity than their non-aligned counterparts, for both fragmented and non-fragmented cases. The evaluation of the combined effect of carbon nanotube length and alignment on the electromechanical properties of the composites represents an important step on the understanding of the structure-property relation of polymer nanocomposites.