Electric field manipulation of polymer nanocomposites: processing and investigation of their physical characteristics

摘要

Research in nanoparticle-reinforced composites is predicated by the promise forexceptional properties. However, to date the performance of nanocomposites has notreached its potential due to processing challenges such as inadequate dispersion andpatterning of nanoparticles, and poor bonding and weak interfaces. The main objectiveof this dissertation is to improve the physical properties of polymer nanocomposites atlow nanoparticle loading. The first step towards improving the physical properties is toachieve a good homogenous dispersion of carbon nanofibers (CNFs) and single wallcarbon nanotubes (SWNTs) in the polymer matrix; the second step is to manipulate thewell-dispersed CNFs and SWNTs in polymers by using an AC electric field.Different techniques are explored to achieve homogenous dispersion of CNFs andSWNTs in three polymer matrices (epoxy, polyimide and acrylate) without detrimentallyaffecting the nanoparticle morphology. The three main factors that influence CNF andSWNT dispersion are: use of solvent, sonication time, and type of mixing. Once a dispersion procedure is optimized for each polymer system, the study moves to the nextstep. Low concentrations of well dispersed CNFs and SWNTs are successfullymanipulated by means of an AC electric field in acrylate and epoxy polymer solutions.To monitor the change in microstructure, alignment is observed under an opticalmicroscope, which identifies a two-step process: rotation of CNFs and SWNTs in thedirection of electric field and chaining of CNFs and SWNTs. In the final step, thealigned microstructure is preserved by curing the polymer medium, either thermally(epoxy) or chemically (acrylate). The conductivity and dielectric constant in the paralleland perpendicular direction increased with increase in alignment frequency. The valuesin the parallel direction are greater than the values in the perpendicular direction andanisotropy in conductivity increased with increase in AC electric field frequency. Thereis an 11 orders magnitude increase in electrical conductivity of 0.1 wt% CNF-epoxynanocomposite that is aligned at 100 V/mm and 1 kHz frequency for 90 minutes.Electric field magnitude, frequency and time are tuned to improve and achieve desiredphysical properties at very low nanoparticle loadings.