Study on Preparation Methods and Electromagnetic Shielding Property of Microcellular PMMA/MWCNTs Composites
Author:Li Zuo Zuo
Supervisor:zhao guo qun
Electromagnetic radiation pollution has become increasingly serious and harmful with the extensive applications of electronic devices,telecommunication technologies and modern detection technologies.In this context,developing high-performance electromagnetic interference(EMI)shielding materials is the key to realize the protection from electromagnetic pollution.Conductive Polymer Composites(CPCs)have broad application prospects in the field of EMI shielding because of their attractive characteristics,such as light weight,good processability,chemical stability,low cost,broad absorption bandwidth and tunable electrical conductivity.Among the various conductive fillers used for preparing CPCs,carbon nanotubes,graphene and carbon nanofibers have become the most popular fillers due to their high aspect ratio,excellent electrical and mechanical properties.However,there are still some issues in the preparation and properties of the CPCs filled with carbon nanofillers.Firstly,carbon nanofillers easily tend to agglomerate in the matrix due to their structural characteristics of high aspect ratio and large specific surface area,which obviously limits their advantages in structure and performance.Moreover,the increasingly complex application environment puts forward new requirements on the performance of EMI shielding materials,such as higher shielding efficiency,lighter weight,and other versatility like thermal insulation.Supercritical fluid-assisted microcellular foaming technology provides an effective way for further weight reduction of CPCs.Meanwhile,the introduction of microcells can endow the CPCs with some new functional characteristics.However,there are still several fundamental scientific and technical issues in aspects of the effects of foaming parameters on cellular structure,the relationships between functionality and cell structures,the effects of foaming behavior on distribution of conductive fillers and performance of CPCs,the potential synergistic effect of hybrid fillers with different dimensions on cell structures and performance of foamed composites.Finally,traditional CPCs with random dispersion structure usually require quite high filler loadings to obtain the satisfactory electrical conductivity for EMI shielding purpose,leading to deteriorated processability and increased production costs.This is adverse to the commercial application of CPCs.Therefore,it is very urgent and necessary to fabricate high conductive and high-performance EMI shielding materials with as low as possible filler loadings by improving the dispersion of carbon nanofiller and designing the structure of composites.Among the various polymer matrices for preparing CPCs,polymethyl methacrylate(PMMA)with high strength,strong light stability and good biocompatibility is one of the most commonly used matrices.PMMA has good dielectric properties and strong absorption to infrared light,thus it is a promising candidate used as the wave-absorbing or thermal insulation materials.In addition,carbonyl groups have a strong adsorption capacity for carbon dioxide and PMMA also has a board foaming temperature range in the case of supercritical CO2 microcellular foaming process.Therefore,PMMA-based CPCs are more suitable for preparing lightweight EMI shielding materials.In this paper,amorphous PMMA and multi-walled carbon nanotubes(MWCNTs)were chosen as the polymer matrix and prime conductive fillers,respectively.Lightweight PMMA/MWCNTs foams with good mechanical properties,high electrical conductivity and EMI shielding performance were developed by using the supercritical CO2 microcellular foaming technology.The main research contents are arranged as follows:(1)PMMA/MWCNTs nanocomposites were prepared via four different methods,namely,melt blending cum injection molding(MI),melt blending cum compression molding(MC),solution blending cum compression molding(SC)and solution blending cum melt compounding(SM).The effects of preparation methods on the dispersion,orientation aspect ratio of MWCNTs and the mechanical,electrical properties and EMI shielding properties of PMMA/MWCNTs nanocomposites were investigated.The results show that the blending ways mainly affect the dispersion of MWCNTs on the nanoscale and the molding ways mainly affect the orientation and macroscopic distribution of MWCNTs on the micronscale.The solution-based methods can disperse MWCNTs more uniformly than the melt-based methods.The injection molding leads to significant alignment of MWCNTs,while the compression molding allows MWCNTs randomly distributed.Although the MWCNTs become shorter,the SM samples have the most uniform dispersion of MWCNTs and the best conductive network,thus they exhibit the lowest percolation threshold,the highest modulus,electrical conductivity and EMI shielding effectiveness(EMI SE)among all the samples.(2)A series of PMMA/MWCNTs nanocomposite foams with various cell structures were developed by regulating the foaming temperature,saturation pressure and MWCNTs contents in a supercritical CO2 microcellular foaming process.Bying characterizing the cell structures,electrical conductivity and EMI SE of the prepared foams,the foaming conditions-microstructure-electrical percolation-EMI shielding relationships in PMMA/MWCNTs foams were systematically studied.Foaming not only can reduce the density of nanocomposites,but also can promote the formation of continuous conductive network by redistributing and redispersing the MWCNTs,leading to a significant reduction to percolation threshold of foams.There is an optimal expansion ratio at which the composite shows the highest electrical conductivity,and the optimal expansion ratio increases with the MWCNTs content.Moreover,the cell structures can reflect and scatter the EM waves for many times in the interior of composite foams,which increases the absorption contribution to EMI SE.Therefore,the synergistic effects of conductivity improvement and microcellular structure enhance the EMI shielding performance of composite foams.(3)Highly-expanded PMMA/MWCNTs nanocomposite foams were fabricated by supercritical CO2 foaming.The cell structures,electrical and mechanical properties of nanocomposite foams were investigated.The effects of cellular structure and MWCNTs content on thermally-insulating property of foams were clarified by calculating the gas conduction,solid conduction and thermal radiation.The results show that the MWCNTs can endow nanocomposite foams with enhanced thermal insulation performance by blocking thermal radiation.The thermal conductivity of composite foams was significantly lower than that of pure PMMA foams,and decreased with the MWCNTs content.There is an optimal expansion ratio for minimizing the thermal conductivity of polymer foams,and the optimal expansion ratio can be up to more than 30-fold as the MWCNTs content increases.In addition,the mechanical properties of foams are improved by adding MWCNTs due to the reduced cell size and the enhanced cell wall of PMMA foams.(4)In this part,a new type of lightweight PMMA/MWCNTs nanocomposite foams with segregated conductive network and gradient multilevel cells were designed and fabricated via combining solution coating,hot compression molding and supercritical fluid-assisted foaming processes.The percolation behavior,mechanical and EMI shielding properties of the composites and composite foams were studied.Compared with the traditional PMMA/MWCNTs nanocomposites,the segregated nanocomposites have a lower percolation threshold and exhibit higher electrical conductivity and EMI SE values at lower MWCNTs loading,which fully confirm the superiority of constructing segregated structure in polymers.After foaming,gradient trimodal microcells were generated around each foamed PMMA grain because solution coating process makes the MWCNTs concentration vary gradiently along the radial direction of the PMMA microspheres.The gradient cell structure allows more incident waves to enter the interior of foams and provides numerous interfaces to reflect and attenuate the EM waves entering the interior of foams,so the EMI shielding mechanism of the composite foams is dominated by absorption.The synergy between segregated structure and gradient cell structure significantly improves the specific EMI SE,making the PMMA/MWCNTs composite foams more suitable for use as a lightweight wave-absorber.(5)1D MWCNTs and 2D graphene nanoplates(GNPs)were used as hybrid fillers to explore their potential synergistic effects on the final mechanical,electrical and EMI shielding properties of PMMA-based composites and composite foams.The optimal ratio of MWCNTs and GNPs was determined and the synergistic mechanism of hybrid fillers was revealed.The results show that,in unformed nanocomposites,all MWCNTs-GNPs hybrid fillers display obvious synergistic effect on the mechanical property while only the MWCNTs:GNPs(3:1)hybrid fillers show synergistic effect in improving the electrical and EMI shielding performances.Supercritical CO2 foaming process makes the GNPs in-situ exfoliate into thinner ones and causes the high-level orientation and redistribution of nanofiller.Meanwhile,the MWCNTs bridgings between thinner graphenes provides numerous pathways for electron transport.At an identical filler loading,the ternary bimodal composite foams exhibit higher electrical conductivity and EMI SE than the homogeneous foams filled with either single-MWCNTs or single-GNPs.This indicates that the MWCNTs and GNPs hybrid fillers have synergistic effects on the electrical and EMI shielding properties of the composite foam system.