Study On Carbon Nanotube/Carbon Fiber Multi-Scale Reinforcement And Its Composite Interfacial Properites

Carbon nanotubes (CNTs) with tubular structures and excellent mechanical properties offer a kind of nanosized reinforcement for composite materials. Chemical method was employed to graft CNTs onto the carbon fiber surface to prepare a carbon nanotube/carbon fiber multi-scale reinforcement, and the interfacial properties of its composite were investigated by experiments and molecular simulation.Firstly, amino functional technics of CNTs were studied. Different acid systems and treatment conditions were used in the modification of CNTs and their functional efficiencies were evaluated. Results shows that sufficient carboxyl could be generated on the surfaces of CNTs without any damage to their properties in the system of nitric and sulphuric acid with a volume ratio of 3:1 in water bath (100℃) for 8 h. The carboxyl could be transformed into carbonyl chloride groups when multi-walled carbon nanotubes (MWCNTs) were put into the solution of DMF and excessive of thionyl chloride at 70℃with magnetic stirring for 72 h. Then hexamethylene diamine were grafted onto the end caps of CNTs. XPS and IR results indicate that there is 6.60% of C-NHx in the surface of CNTs, which suggests that preferable amino functional effect was obtained.Secondly, acylation functionalization treatment was carried out on carbon fiber surfaces. Surface reactivity of three kinds of carbon fiber, T300, T800 and home-made carbon fiber, were examined by acid treatment followed by acylation reaction with SOCl2. XPS was employed to analyze the surface chemical properties of specimens of acid treatment and acylation reaction. Results indicates that the content of–COOH group, 14.50%, after acid treatment on T300 surface is the maximal, as well as the content of Cl element which is up to 3.88% after acylation reaction, and Cl elements link to the fiber surface by acyl chlorine covalent bond. The crystallite size on treated carbon fiber surfaces decreased and unsaturated carbon atoms of carbon fiber edges and borders increased, leading to more reactive carbon fiber surfaces.In succession, to prepare a carbon nanotube/carbon fiber multi-scale reinforcement (MSR), MWCNTs functionalized at the end caps with hexamethylene diamine (HMD) are grafted onto the surfaces of carbon fibers treated with acylchloride. It was found that the grafting increases the weight of carbon fiber by 1.2%. SEM shows that MWCNTs were grafted onto carbon fiber surface by two means, one was sticking to the carbon fiber surface at different angles, and the other was that the amino groups at MWCNTs end caps reacted with the acyl chlorides of two carbon fibers respectively and joined them up.Single fiber polymer matrix droplet composite interfacial evaluation test indicate that the interfacial shearing strength (IFSS) was improved by 150% that that of the as received T300 composite. The mechanisms of the improved IFSS were discussed. It was found that the improved IFSS primarily relies in interfacial chemical bonding, vanderwaals effect, better soakage and mechanical joggling. Finally, interfacial structure models of carbon nanotube/carbon fiber multi-scale reinforcement (Model 1) and carbon fiber composites (Model 2) were established and analyzed respectively by molecular simulation, from which the mechanism of the improved IFSS was examined in view of the interfacial bonding energy. Simulation results indicate that the interfacial bonding energy of model 1 was improved by 118.28% than that of model 2, which was mainly contributed by chemical bond energy. The non-bond interaction of model 1was increased by 5.76% than that of the model 2. It is to say that the primary factor which affects the interfacial bonding strength of carbon fiber/epoxy composite is vanderwaals when there are no chemical bonds exist, and the other is the interfacial static interaction. Structure analyses show that the contact layer thickness of bigger that that of the model 2 (2.5 ?,1.0 ? respectively). The structure of first three layers of the carbon fiber changed in different degree, while the fourth layer remains unchanged. In model 2, only the first and the second layer structures changed at the interfacial interaction. In model 1, the maximum concentration of epoxy presented closer at contact layer and the concentration was higher in the 9.172 ? areas along Z direction than that in model 2. It can be concluded from the concentration distribution of epoxy that the interfacial bonding energy is higher that model 2.