Nanotube -polymer composites: Tailoring the interface for improved mechanical properties

• Main Author: Eitan, Amram (Ami)
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2004
• Subjects: Materials science
• ISBN: 0496702971; 9780496702978

The discovery of carbon nanotubes introduced exciting opportunities in many scientific and technological fields. In particular, their high elastic modulus and their extremely high aspect ratio make carbon nanotubes an optimal candidate as a reinforcing agent in composite materials. The goal of the present research is to prepare uniform multi-walled carbon nanotube (MWNT)-polycarbonate composites, to explore their mechanical properties, and to investigate the reinforcement mechanisms that govern the mechanical properties. Surface modification of the MWNT was first developed to control the interactions between the MWNT and the polymer. As received (AR) and surface modified (EP) MWNT were embedded in bisphenol A polycarbonate by a solution-based process. Tensile tests of the composites showed an increase of the Young's modulus of the composite by 70% due to 5 weight % AR-MWNT compared to pure polycarbonate, whereas EP-MWNT composites of the same loading showed an increase of 95%. Yielding was preserved in all samples. Viscoelastic studies of AR-MWNT composites conducted by dynamic mechanical analysis suggest the existence of an immobilized polymer region caused by the incorporation of the MWNT. This immobilized polymer was detected by the broadening of the loss modulus peak towards higher temperatures, and by the broadening of the relaxation spectra towards longer relaxation times. The existence of an immobilized polymer layer was confirmed by electron microscopic examination of the fracture surface of the composites. The viscoelastic behavior of the EP-MWNT composites showed broadening as well as shifting of the loss modulus peak towards higher temperatures, and shifting of the relaxation spectra, toward lower frequencies. These findings are interpreted by the better inter-mixing that occurs between the immobilized polymer and the bulk polymer in the surface-modified MWNT composites, which affects the behavior of the bulk polymer. This mechanism was reinforced by load transfer studies conducted by strain-sensitive Raman spectroscopy of the composites. Better load transfer efficiency was measured in the EP-MWNT composites as compared to AR-MWNT composites. Based on the present research, reinforcement mechanisms are suggested in nanotube-polymer composites: (i) An adsorbed polymer, which is stiffer than the bulk polymer, increases the actual volume fraction of the reinforcing agent; (ii) Entanglements between the adsorbed polymer layer and the bulk polymer chains make the load transfer between the polymer and the reinforcement more efficient. The differences between the reinforcement mechanisms of traditional fiber composites and nano-filled polymer composites are discussed.