Interaction Mechanisms Between Carbon Nanotubes And Epoxy Resin And Its Effect On Glass Transition Temperature And Impact Toughness

For polymer materials,the improvement of temperature resistance and toughness is usually not possible to achieve simultaneously.On one hand,the polymer chains with more rigid groups are supposed to be more difficult to move,which corresponded to more rigid polymer molecules.Higher glass transition temperature(T_g)of the cross-linked polymer chain networks will be presented with shorter cross linking distances and higher cross-linking densities.On the other hand,flexible groups and linear chain segments,which have an excellent capacity to distortion and absorbing strain energy,are required to be grafted into the polymer molecules to improve their toughness.Polymer blending,copolymerization and molecular design are efficient in improving the temperature resistance and toughness of polymers.However,the two goals can't be always achieved simultaneously.Due to the special nano-scale,high aspect ratio,large specific surface area and excellent mechanical properties,carbon nanotubes(CNTs)are used to reinforce the polymer matrices with a goal to improve both T_g and toughness of polymer matrices in this paper.CNTs with particular aspect ratios can help to absorb more energy through various mechanisms,such as crack-expanding arresting,crack deflexion,interface debonding,crack bridging and CNTs pull out.These energy absorbing mechanisms are beneficial for the improvement of toughness of polymer.Meanwhile,because of the comparable dimensions of CNTs and polymer chain segments,motions of the polymer chain segments in the CNT/polymer composites are also supposed to be restricted,restrained,hindered or limited by the embedded CNTs through interfacial interactions.Accordingly,response of the segmental mobility to momentary temperature will be interfered,inducing a enhanced T_g of the composites.However,previously reported results indicated that T_gs of the polymer matrices were not always increased by incorporation of CNTs.In some cases,T_gs of the polymer decreased with the addition of CNTs,while totally contrary results were obtained by other researchers.The main purpose of this paper is to achieve a comprehensive and systematic understanding of the influence mechanisms of CNTs on the T_gs of the nano-phased polymer composites,and both experimental and molecular dynamic(MD)simulation methods were conducted.Taking epoxy resin for example,effects of CNT functionalization,contents and sizes on T_gs and impact toughness of the CNT/epoxy composites were investigated.Based on the above results,the influence rules of CNT configurations on T_gs and toughness of the epoxy resin were explored.The results indicated that the reinforcing effects were mainly depended on the interfacial interactions and bondings between the CNTs and the epoxy resin matrix,as well as the dispersion state and real aspect ratios of the CNTs in the composites.By optimizing the results,a content of 0.25 wt%of amino groups grafted CNTs with an aspect ratio of 10 is the best in reinforcing the epoxy resin.In order to characterize the interfacial interactions and bondings,surface energy analysis,atomic force microscopy and thermal analysis were conducted.The surface characteristics of the typically functionalized CNTs,the interaction as well as the reactive behavior between the functional groups grafted on CNTs and the epoxy groups were investigated.The experimental results indicated that the amino groups grafted CNTs had a more uniform distribution of surface energy,low surface polarity,low cohesive energy and better compatibility with epoxy resin.The amino groups and the epoxy groups possessed strong physical interactions and covalent bonds were also believed to be formed during the cross-linking reaction,which induced strong interphase bondings between the CNTs and the epoxy resin matrix.MD simulations were conducted to further investigate the influence mechanisms of CNTs on glass transition behavior of epoxy resin.The composite models reinforced by different types of CNTs were constructed.The densities,energies,Connolly free volume factions(CVF)and mean square distances of the epoxy molecules for each model were calculated.The simulation results indicated that the effects of CNTs on the T_gs of the composites should be attributed to the interphase changes,i.e.,stronger interphase interactions and lower CVF were both of advantage to the elevation of T_gs.Furthermore,effects of the CNTs on T_gs and impact toughness of the carbon fiber(C_f)/epoxy composites were investigated.The T_g of the C_f/CNT/epoxy composites increased with the embedding of CNTs.However,it showed a decreased impact toughness of the C_f/CNT/epoxy composites due to the CNT aggregations.Finally,the conclusions and prospect of future work were made.