18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS PREPARATION AND CHARACTERIZATION OF CARBON NANOTUBE/CARBON FIBER MULTI-SCALE REINFORCEMENT C. Wang 1 , X. D. He 1 *, L. Y. Tong 2 , Y. B. Li 1 , Q. Y. Peng 1 , L. Mei 1 , R. G. Wang 1 1 Centre for Composite Materials and Structures, Harbin Institute of Technology, Harbin 150080, China, 2 School of Aerospace, Mechanical and Mechatronic Engineering, The University of Sydney, NSW 2006, Sydney, Australia * Corresponding author( xdhe@hit.edu.cn ) Keywords : carbon nanotubes; carbon fiber; dendrimers; multi-scale reinforcement; pullout model 1. Introduction simulations have been widely used in studying the interfacial mechanical properties of nanocomposites The interfacial shear strength between a CF and [12-13]. Liao et al. [14] studied the interfacial shear epoxy resin matrix plays an important role in stress of CNT/polystyrene system, in which the determining the mechanical properties of composite. interfacial shear stress was assumed to be constant The fiber-matrix interface in a composite can be along the axial direction of CNT. Gou et al. [15] improved via various surface treatments, such as carried out the pullout simulation of CNT from oxidation and coating etc; however, the epoxy matrix, and the interfacial shear strength improvement may be limited. Recently, some between CNT and epoxy matrix was estimated as researchers successfully grew CNTs onto CF or 75 MPa . Zheng et al. [16] studied the effect of fabrics by Chemical Vapor Deposition (CVD) [1-3] chemisorption on the interfacial properties between and showed that the insertion of CNTs improves the CNT and polymer by simulating the pullout of CNT, interfacial strength and subsequently the and the results show that by chemically delamination toughness [4-5]. However, despite of functionalizing CNTs the interfacial shear strength the enhancement, the catalyzer and the high can be improved. The above statement has indicated temperature involved in the CVD process can also that the simulation of MM and MD is an effective degrade the strength of carbon fiber and the method to investigate the mechanical properties of interfacial shear strength [6]. Recently, our research nano-reinforcement reinforced composites. group focused on using the chemical grafting In this study, firstly, the MM and MD were methods to assemble the CNTs onto CFs [7-8]. The employed to simulate the pullout of CNT from SEM photograph of CNTs/CF multi-scale epoxy resin matrix and the interfacial bonding reinforcement is shown in Figure 1. The mechanical characteristics of CNT and epoxy resin matrix were testing results shows that there is an excellent investigated. Then a simple micromechanical model interfacial shear strength between the CNTs/CF and matrix [9]. for evaluating the interfacial shear strength of CNTs/CF reinforced composites was established by In order to estimate the interfacial enhancement of combining the numerical results from MM and MD. CNTs/CF reinforced composites, it is important to determine the interfacial shear strength between 2. Computational model CNT and matrix. Wanger et al. [10] performed the 2.1 Molecular Model of CNT In this study, a double-walled CNT was fragmentation experiment ， and the results show that constructed by using Materials Studio with the there is strong stress transfer ability and the length of 59.03Å and diameter of 13.56Å, interfacial shear stress of CNT/polyurethane system respectively, as shown in Fig. 2. The unsaturated can reach 500 MPa . Cooper et al. [11] first detached boundary effect is removed by adding the hydrogen single CNT from matrix and showed that the atoms at the two ends of the CNT. interfacial shear strength of CNT/epoxy system 2.2 Molecular Model of Cured Epoxy Resin Matrix ranges from 35 to 376 MPa . Although the evidence The epoxy resin matrix comprises of DGEBA of experiments indicated that there is a strong resin and triethylenetetramine curing agent, the interfacial bonding between CNT and matrix, it is molecular structures of which are shown in Fig. 3. still a challenging issue to manipulate carbon For the epoxy resin matrix, the degree of nanotubes in experiments. The MM and MD
interfacial bonding energy γ scaled by the contact polymerization was set at 1. The single molecule models of the agent and resin were constructed first, area A [18] and then the energy minimization was performed to Δ E γ = (2) optimize the molecule structures. One hydrogen 2 A atom of amine groups on agent molecule can react In order to determine the interfacial shear strength with the epoxide group of one end of epoxy resin of CNT and matrix, the pullout simulation of CNT molecule first, and then the other hydrogen atoms of from matrix was performed. The interfacial shear amine groups on the agent molecule can further strength can be evaluated by the pullout energy of react with epoxide groups, the epoxide group of CNT from matrix, which is defined as the energy another end of epoxy resin molecular can react with difference between the fully embedded CNT and other agent molecule, as the reaction lasted, the entire pullout configuration [15-16]. The pullout agent molecules and epoxy resin molecules can energy can be divided into three terms which include generate cross-links [15]. Although the above cross- the energy change of CNT, matrix, and their linked mechanism can be used to generate a cured interaction energy after and before CNT pullout epoxy system, it is very complex to be applied in simulation as follows [15-16] larger molecular model system. In this study, we = − E E E employed a method of representative cross-linked 2 1 pullout total total ( ) (3) unit to simulate the cured epoxy system, and it has = Δ − Δ + − ( ) E E E E 2 1 2 1 matrix matrix been testified to be a valid method to obtain the ( ) + − E E requested epoxy system [17]. According to this 2 1 CNT CNT where and are the energy of method, the cured epoxy system is composed of the E E 2 1 total total simple representative cross-linked unit, which is CNT/epoxy resin system after and before CNT composed of four epoxy resin molecular and one pullout from matrix, respectively. and E E matrix CNT curing agent molecular as shown in Fig. 4. Δ are the energy of matrix and CNT. is the E 2.2.3 Molecular Model of CNT/Epoxy Resin System interaction energy between CNT and matrix. There Packing the representative cross-linked unit exists the following relation between the pullout molecular of epoxy resin and CNT into a supercell τ [15-16] energy and the interfacial shear stress in the range of50.00Å × 50.00Å × 65.00Å, and the i initial configuration was made to ensure that the ( ) ∫ l = π − δ τ δ 0 2 (4) E r l d CNT can be surrounded by the cured epoxy resin pullout i with a density of 1.00g/cm 3 . Then the molecular E pullout τ = (5) system of composite was equilibrated for 20ps while π 2 i rl keeping the CNT rigid. A further equilibrium of where r and l are the radius and length of CNT, 30ps was performed with non-rigid CNT. Lastly a respectively. δ is the pullout displacement of CNT. sufficient energy optimization was carried out to The three different pullout phases are shown in Fig. achieve the strongest interfacial bonding between 5. During the pullout of CNT from matrix, the CNT and matrix as shown in Fig. 4 [15]. changes of interfacial interaction energy and 3. Results and Discussion bonding energy with pullout displacement are shown 3.1 Interfacial Bonding of CNT and Epoxy Resin in Fig. 6 and 7, respectively. It can be seen that the Matrix absolute value of the interaction energy gradually The bonding strength of CNT and epoxy matrix is decreases duo to the reduction of the contact area related to the interfacial interaction energy. The between CNT and matrix, and the interfacial interfacial interaction energy can be calculated by bonding energy varies in a range from 0.15 to 0.19 the difference between the potential energy of kcal/molÅ 2 , and this indicates that there is a steady composite molecular system and the potential interface adhesion during pullout of CNT. From the energy of the matrix molecular and the embedded relation of the pullout energy and displacement as CNT as follows [16]: ( ) shown in Fig. 8, it is noted that the pullout energy Δ = − − (1) E E E E total matrix CNT almost linearly increases with the pullout Δ where E is the potential energy of the composite displacement. This is because the pullout load needs molecular system, is the potential energy of E to overcome the interfacial bonding energy, the matrix deformation energy of CNT and matrix. Finally we matrix, and is the potential energy of CNT. E CNT can obtain that the interfacial shear strength between Δ The interfacial interaction energy, E , is twice the
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