Development of microcellular conductive foams based on polyetherimide with graphene nanoplatelets

Resumen

The aim of this dissertation was to develop and investigate novel cellular foams based on polyetherimide (PEI) and carbonbased nanoparticles such as, graphene nanoplatelets (GnP) and carbon nanotubes (CNT), using two main foaming methods; One being water vapour induced phase separation (WVIPS) method and the other being one-step foaming through dissolution of carbon dioxide (CO2) at its supercritical state. WVIPS method consists of initial preparation of polymer-solvent solution and nucleation of cells through phase separation due to water vapour absorption. The dissolution of supercritical CO2 (scCO2) in a high-pressure vessel has been used in order to consequently force a sudden expansion of the nanocomposite precursor in a one-step pressure drop which results in formation of cells. In WVIPS method, the concentration of the polymer in solvent during mixture showed to have a great impact on the morphology of the foams. Additionally, the composition seems to have various effect on the cellular structure. When a blend of PEI with polyamide-imide (PAI) was prepared using this method, the cellular formation evolved drastically depending on the amounts of PAI added to the mixture. Moreover, the incorporation of GnP and CNT seem to have affected the cellular structure and morphology with various levels of impact depending on whether the ultrasonication of the nanoparticles was applied. Using the one-step scCO2 dissolution foaming method, foams were obtained with homogenous closed-cell structure. The incorporation of GnP did not seem to affect the cellular structure of the PEI foams. However, the application of ultrasonication, melt mixing and one-step foaming seem to have induced a proper level of particle dispersion which was confirmed by X-ray diffraction analysis. The studies of mechanical properties of foams prepared via WVIPS method suggested that the densities of the foams alter their viscoelastic behaviour in a direct manner. Additionally, the mechanical behaviour followed a similar increasing trend by incrementing the amount of graphene content. Surprisingly, this behaviour changed when using CNT as reinforcement; A clear decreasing trend was observed in specific storage modulus of the foams by increasing the amount of CNT. Moreover, considering the case of polymeric blends, the mechanical behaviour seem to have been affected vastly by the structural changes. The PEI/PAI polymer composition played a key role in determination of the cellular structure and therefore, the eventual mechanical behaviour of these foams. The foams prepared through scCO2 dissolution showed an increasing trend of normalized modulus (Enorm) with increasing the density and a rise in specific modulus (Espec) with increasing the GnP volume fraction. In an overall view, the nanoparticles provided a general delay in degradation temperatures of the nanocomposite foams which could provide the possibility of their application in high temperature environments. Significant enhancements were achieved regarding the electrical conductivity of the foams. The ultrasonication of the nanoparticles have provided the possibility to increase the value of electrical conductivity by six orders of magnitude from 1.8 × 10E-7 S/m to 1.7 × 10E-1 S/m for foams containing 10 wt% of GnP. The foams with 1/1 ratio of GnP and CNT reached an electrical conductivity value of 8.8 × 10E-3 S/m containing only a total amount of 2 wt% in nanoparticles.