Use of tailored blend morphologies to obtain electrically conductive composites

Abstract

This thesis aimed at designing electrically conductive polymer-based nanocomposites with an exceptionally low percolation threshold concentration through control of their morphology, and to evaluate how this morphology, hence the electrical properties, would be stable during post-processing. The relationships between morphological, electrical, and rheological properties of polypropylene/polystyrene filled with multiwall carbon nanotubes (PP/PS/MWCNT) composites prepared by conventional melt-mixing process were investigated. Composites with a percolation threshold concentration (PTC) of 0.3 wt.% MWCNT were obtained owing to the double percolation effect achieved by using co-continuous morphology of PP/PS matrix. The PTC was further decreased to 0.06 wt.% MWCNT upon using a thermal treatment based on the volume exclusion effect of PP crystals. An electrical conductivity of 10-5 S.m-1 was reached, for composites with 0.06 wt.% MWCNT, representing an increase of over 10 orders of magnitude compared to that of the PP/PS matrix alone. The co-continuous morphology of the PP/PS blend was not affected by the thermal treatment, indicating that other properties of the composite were not impacted. Furthermore, a model to characterize the co-continuous morphology of PP/PS/MWCNT composites using their rheological characterization was developed. The quantification of the morphology carried on by the model corroborated the one observed by scanning electron microscopy. The model was used to assess the morphology of composites subjected to thermal annealing, employed to enhance the electrical properties. The model was also used to probe the morphology evolution of PP/PS/MWCNT composites. The results showed a slight decrease in the characteristic domain size for composites with different MWCNT concentration, leading to a more refined co-continuous morphology, in line with the measured electrical properties. Specifically, the PTC was reduced from 0.28 wt.% to 0.06 wt.% MWCNT. Finally, the electrical and morphological properties of co-continuous PP/PS/MWCNT composites under deformation flow were evaluated. A delicate balance between MWCNT concentration, shear strain, and shear rates (the parameters of the deformation flow), which affected the morphology and electrical properties of the composites, was observed. The stabilization of morphology and electrical conductivity was achieved at critical levels of MWCNT concentration, and recovery was possible. The PTC and electrical conductivity were reduced during deformation but recovered after recovery step, even increasing by one order of magnitude.

Date	24 May 2023
Original language	American English
Awarding Institution	École de technologie supérieure
Supervisor	Nicole R. Demarquette (Supervisor) & Éric David (Co-supervisor)