Carbon Nanofiller Reinforced Epoxy Nanocomposites

Abstract

Nanocomposites are unique materials with unique physical, mechanical and chemical properties that can find use in a wide range of applications. Nanocomposites are made of at least one nanosize material with polymeric systems or other materials. The incorporation of carbon nanotubes (CNTs) and nanofibers (CNFs) has led to countless possibilities for structural polymer nanocomposites with superior specific modulus, strength, and toughness. These materials have gained enormous attention in fabricating next-generation advanced structural materials with added thermal, mechanical, optical and electrical advantages. However, questions concerning the filler dispersion, effect of the nanofillers on the interface and the final properties of nanocomposites remain partially addressed. During this work, CNTs and CNFs were modified with covalent functionalization. This was achieved by adding carboxyl, ester and epoxy groups as well as monofunctional polyhedral oligomeric silsesquioxanes (POSS) molecules. The effects of these functionalization methods, as well as concentration of nanofillers on the quality of dispersion in epoxy matrix were explained. It was observed that functionalization of carbon nanotube helps to provide a better dispersion of carbon nanofillers. It was also detected that carbon nanotubes create a network structure after a critical threshold concentration that is called as percolation threshold. By using acidified SWCNTs, it was found that the percolation threshold value was 0.47 wt%.

Grafting monofunctional POSS molecules onto SWCNTs creates a phase separation phenomenon in epoxy/diamine systems. Whether this phase separation process provides a better dispersion of SWCNTs was discussed through rheological, calorimetric and visual analysis. It was found that phase separation creates POSS-rich domains that can help nanotubes create some kind of ordered arrays. This system did not show an improvement in storage modulus of epoxy/diamine system; however, the glass transition temperature of the system kept its value after incorporation of POSS functionalized SWCNTs. Changes in the structure of interface by incorporation of unmodified and modified CNTs and CNFs and the effects of these fillers on the thermo-mechanical behavior of epoxy system were studied. The observed alterations in the structure of epoxy nanocomposites were explained with partitioning and phase separation phenomena. It was found that systems with acidified SWCNT, MWCNT and CNF showed two relaxation peaks in nanocomposites assigned to a partitioning of monomers at the carbon interface. This produced a significant decrease of the glass transition temperature. However, esterification led to a single relaxation peak close to the one for the neat epoxy, for the three types of nanofillers without any penalty in the glassy and rubbery elastic moduli. Finally, the damping properties of epoxy elastomers reinforced with CNTs were analyzed. Results showed that epoxy nanocomposites with SWCNTs and MWCNTs have high damping capacity at high temperatures. The aid of CNTs incorporation on damping level at elevated temperatures was explained by “stick-slip” mechanism.