Molecular Dynamics Simulation Of Interfacial Self-Assembly Structure Of The Carbon Fiber Composites

In this work, the molecular self-assembled technique was applied on the interface modification of carbon fiber reinforced composites, thus the controllable interphase could be obtained. The interfacial functionary mechanism of the composites was studied by molecular dynamics simulation at molecular level. This research has great significance, both in theory and practices, for developing the surface modification methods of carbon fiber and clarifying the mechanisms of interfacial interaction.For epoxy resin (EP), the composite interphase was modified by thiols/Au self-assembled systems. First, the influence of the chain length and end functional groups on the interfacial properties between the SAMs/Au(111) and epoxy resin were studied by molecular simulations. Then, the CF surface was Au-plated, and the simulation results were verified by experiments.In the part of molecular simulations, the structures of methanethiol (MT) and 2-mercaptoethanol (ME) adsorbed on Au(111) surface were studied firstly by density functional theory (DFT) based on the first principle. The results indicated that the most stable adsorption site of the thiols was bri-fcc site. Based on the DFT study, the initial models of S(CH₂)_nX(X =-OH,-NH₂,-COOH,n=1¹7) on Au(111) surface atΘ=1 were built. The packing structures of the SAMs on Au(111) were studied by molecular dynamics (MD) method. And the influence of the chain length and end functional groups on the structure of SAMs was discussed. The simulation results indicate that the change trend of the SAMs with different end groups is almost the same. And the stability of the three types of SAMs is:S(CH₂)_nCOOH>S(CH₂)_nNH₂>S(CH₂)_nOH.The MD simulation results of the S(CH₂)_nX/Au(111) were used as the initial surface models, and then the interfacial models of SAMs/Au(111) and epoxy resin were prepared. The interface structures were calculated by the simulated annealing methods. The simulation results indicate that the chain length, n, is 10, 11 when the interface structures are stable. The stability of different system is: S(CH₂)_nNH₂/EP>S(CH₂)_nCOOH/EP>S(CH₂)_nOH/EP.The surface of the CF was metallized by the electroless Au plating. Then different types of thiols[S(CH₂)_nOH(n=2,6,11),S(CH₂)2NH₂,S(CH₂)2COOH] were self-assembled on the Au-plated CF surface, and the effects of the thiols on interfacial shear strengths (IFSS) of CF/EP composites were investigated. The results indicate that the IFSS of S(CH₂)11OH/EP comopsites is higher than those of S(CH₂)₂OH/EP and S(CH₂)6OH/EP composites. And the IFSS decreases firstly and then increases with the increase of the chain length. The order of the IFSS of the S(CH₂)₂X/EP composites is: S(CH₂)₂NH₂ > S(CH₂)₂COOH > S(CH₂)₂OH. The experiment results are in good accordance with the simulation results.For polyarylacetylene (PAA) resin, composite interphase was modified by hydroxylated surface/organic silane derivatives self-assembled systems. The effect of chain length on the interfacial properties of the composites was studied by MD simulation. The simulation results indicate that the interface energy decreases firstly and then increases gradually with the increase of the chain length. The interface energy is the lowest and the interface structure is most stable when n=100. The interface energy is determined by the entanglement interaction between the molecular chains of coupling agents and the PAA resin, and the coverage of the molecular chains of coupling agents on the CF surface.The surface of CF was modified by silane coupling agent using ozonization methods, and the interlaminar shear strengths (ILSS) and IFSS of the CF/PAA composites were tested. The results indicate that the properties of the composites are improved with the increase of the chain length. The analysis of the ILSS fracture morphology also indicates that the interfacial bond behavior is improved with the increase of the chain length. For the CF/PAA composites, the improvement of the interfacial properties is due to the interactions between the molecular chains of the coupling agents and PAA resin, and the interactions increase with the increase of the chain length. The experiment results prove that the MD simulation methods in this paper are available.