Study On Carbon Nanotube/Carbon Fiber Multi- Scale Structure And It's Interfacial Reinforcing Effect

Carbon fiber reinforced polymer composites (CFRP) have been widely used in many fields such as aerospace industry, automotive industry, steamship industry, petroleum industryand chemical industry. The cohesive force between carbon fibers (CFs) and the matrix is weak because the CFs have small active specific surface area, low surface energy, and lipophobic surface. It much limites the applications of carbonfiber reinforced composites. To improve the adhesion behavior of the CFs/matrix interface, grafting carbon nanotubes (CNTs) onto the CFs to design a CNT/CF multi-scale structure has become a hot spot in recent years and attracted more and more attention.Based upon dendritic structure found in nature, we develope a new set of chemical grafting process to synthesize a hierarchical CNT/CF fibrillar structure by use of polyamide-amine (PAMAM) which has low viscosity, good film-forming and a large number of functional groups. In this method, many amino groups at the periphery of the PAMAM provide more active sites, which can react as a"bridge"to connect carboxyl functionalized CNTs and oxidated CFs together. By grafting CNT arrays that are dominated by dendrimers, the interfacial properities of composites are significantly enhanced in comparison with original composite. The giant enhancement is mainly attributed to the increase of specific surface area by CNTs grafting and the increase of functional group number by introduction of PAMAM dendrimers layer. This method avoids complex processing condition, catalyst contamination and high temperature damage in the chemical vapor deposition method. Forthermore, comparing with previous grafting method, this method is expected to provide us an optimum condition for grafting CNTs on CF with shorter reaction time (35h), no toxic chlorination contamination, perfect grafting morphology and readily controllability. It is readily scalable for industrial applications and can be further explored for the development of advanced structural composites.So far, the CNTs can either be grown directly onto a CF surface by chemical vapor deposition (CVD) or be anchored onto the functionalized CF surface by a chemical method. Whereas, the improvements in interfacial strength always came out at the expense of the fiber tensile strength and surface functional groups. In order to avoid the problems of high fiber strength loss and low interaction activity in these methods, oxidation treatment technics of CNTs and CFs are studied firstly. Different acid systems and treatment conditions are used in the modification of CNTs and CF samples.The functional efficiencies are evaluated to choose the optimum oxidation treatment condition. Results show that sufficient carboxyl can be generated on the surfaces of CNTs in the system of nitric and sulphuric acid with a volume ratio of 1:3 in water bath (100℃) for 8~12h. After oxidation treatment, there are many hydroxyl groups and carboxyl groups on the surface of the CNTs, which can provide active sites for further grafting reaction. And the dispersion of modified CNTs is improved significantly. The CFs should be oxidized in nitric acid at 100℃for 3 h. XPS and IR results indicate that there is 10.01% of -COOH content on the oxidized CF surface. Results of single fiber tensile test and Raman spectrum indicate that this oxidation treatment results in no obvious decrease in carbon fiber tensile strength.With low viscosity, good film forming ability and a grate number of functional groups, a kind of dendrimer, poly-(amidoamine) (PAMAM) is introduced for CF surface amino modification. Then, the carboxyl groups can be transformed into amino groups when CFs are put into the methanol solution of PAMAM at 80℃for 12 h. As a result, a large amount of amino groups covers the CF surface by chemical interaction and physical adsorption. After amination, the N element content on the CF surface is up to 5~10%, and the surface free energy of amino modified carbon fiber is up to 69.47 mJ/m2. CNT/CF multi-scale structure is finalized when amino-functionalized CF is dipped into the CNT suspension at the same interaction condition. The properties of the multi-scale structure are examined by SEM, FTIR, XPS, Raman, AFM and dynamic contact analyzer et al. The results show that CNTs are grafted onto the CF surface homogeneously and firmly. The network of CNTs surrounding the CF is almost unperturbed even after ultrasonic treatment in acetone for 15min. With an 18% content of CNTs on the CF surface, large amount of CNTs have been grafted onto CF surfaces, which results in about 2.6 times increase of the surface roughness and 40% increase of the surface free energy. The tensile strength of the multiscale structure only decreases 6% from the as-received values.Finally, we prepare the single fiber model composite and study the interfacial strength of the composite. The interfacial reinforcing mechanisms of the composite are also discussed. Single fiber composite fragmentation test and the single fiber-microdroplet tensile test are both conducted to quantify the adhesion property between CFs and resin matrix. The results of two methods both indicate that the IFSS of composite reinforced by CNT/CF MSR is improved by 160% than that of the as received T300 composite. This significant enhancement on interface strength is mainly attributed to two aspects: (i) hierarchical fibrillar structure by CNTs grafting, which can improve the physical bonding of the interface; (ii) large number of active sites thanks to the PAMAM dendrimer layer, which can not only provide large amount of chemical and physical bonds to connect CNTs and the CFs together, but also react with the epoxy groups of the matrix. Interfacial chemical bonding, Van der Waals effect and mechanical joggling at the composite interface between CF and epoxy matrix are significantly strengthened.