| The epoxy microporous conductive composite was prepared by introducing microporous structure and conductive filler into the epoxy matrix.Since epoxy-based microporous conductive composites are lightweight,corrosion-resistant and have the advantages of high strength and easy molding process,it has received more and more domestic scholars’extensive attention.Study has shown that the introduction of micropores is beneficial to lower the over-permeability threshold of conductive fillers,triggering multiple reflections and absorption of electromagnetic waves.Furthermore,carbon-based nanoparticles,while acting as conductive functional fillers,can also improve the quality of the vesicles by acting as heterogeneous nucleating agents,thus improving the impact resistance of the composites and imparting electromagnetic shielding to the materials.However,the mechanism of the transformation process of the blister structure of epoxy resin/carbon nanocomposites on the rearrangement and orientation of nanofillers and their conformational relationships have not been systematically investigated.Based on this,the EP/MWCNTs composite microporous material was firstly prepared by a two-step foaming method of"Limited-Free".The effects of ball milling time,MWCNT content and type on the cell structure,expansion behavior,compression performance,electrical conductivity and electromagnetic shielding properties were investigated,as well as the structural evolution of MWCNT conductive network during the two-step foaming process.Secondly,the relationship between the cell structure and properties of EP/MWCNTs composites with apparent density under three foaming processes was investigated,and the differences in the anisotropic properties of the materials under the three processes were compared to further reveal the motion rearrangement law of MWCNT during the transformation of cell structure and analyze the intrinsic mechanism.Finally,carbon nanoconductive fillers with different dimensions were selected to form a compounded system with MWCNT,to investigate the transformation pattern of the complex conductive network in the two-step foaming process,and to establish the relationship between the microstructure and macroscopic properties of the material.The research results lay the foundation for an in-depth understanding of the mechanism of epoxy/carbon nanocomposite cell structure transformation on nano-filler rearrangement and orientation and the construction of high-performance functionalized epoxy resin-based composite foaming materials.The following results were obtained in the thesis:(1)Increasing the ball milling time improves the dispersion of MWCNT in the matrix,but decreases the viscoelasticity of the epoxy prepolymer and the length of MWCNT,thus weakening the anisotropy of the Limiteded foam,and the ball milling time for this transformation is about 16 min.The decrease of compression anisotropy is mainly related to the decrease of cell shape anisotropy ratio,the decrease of electromagnetic shielding anisotropy is mainly related to the decrease of oriented MWCNT conductive chain,while the decrease of anisotropic expansion behavior is mainly related to the decrease of MWCNT length.At the ball milling time of 16 min,increasing the MWCNT content or increasing the length-to-diameter ratio of MWCNT was beneficial to enhance the anisotropic expansion behavior and electromagnetic shielding anisotropy,but the effects on compression and conductive anisotropy were not obvious.(2)The EP/MWCNTs composite microporous materials were prepared by"Limited foaming","Limited-Free foaming"and"Limited-Limited foaming",all three foams showed a gradual decrease in cell diameter with increasing density,and the distribution of cell diameter became uniform.With the increase of density,the anisotropy ratio decreases of the Limited foam,and the MWCNT orientation weakens,which makes the material compression and electromagnetic shielding shift from anisotropy to isotropy,and the transition densities of both are 0.6g/cm~3and 0.8g/cm~3,respectively.The Limited-Free foaming further reduces the compression and electromagnetic shielding anisotropy,while the Limited-Limited foaming enhances the compression and electromagnetic shielding anisotropy,and the second foaming causes the reversal of the conductive anisotropy.The reason for the reversal at different densities is mainly related to the degree of destruction of the conductive network in the vertical direction,in which the Limited-Free foaming mainly destroys the vertical orientation of MWCNT,while the Limited-Limited foaming causes the transition from vertical to horizontal orientation of MWCNT.(3)Epoxy resin-based composite foams were prepared by selecting carbon nanoconductive fillers of different dimensions and MWCNT to form a compound system.Compared with zero-dimensional CB and two-dimensional GNP,the compound system of one-dimensional CNF and MWCNT improves more conductive and electromagnetic screen properties of the Limited foam and induces more significant anisotropic expansion behavior and anisotropic electromagnetic shielding properties,which is attributed to the synergistic orientation of CNF and MWCNT.The second foaming step reduces the compressive,conductive and electromagnetic shielding properties of all composites and weakens the compressive anisotropy,but does not necessarily reduce the electromagnetic shielding anisotropy ratio of the compounded system,which indicates the complexity of the effect of the cell hole regrowth process on the transformation of the multiple conductive network structure. |
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