Structural Control And Electromagnetic Property Of PMMA-based Physical Constraint Nanocomposite Foams

Electromagnetic wave absorption materials can absorb electromagnetic wave efficiently,are widely used in industrial electronic packaging,military radar communications,military camouflage and human body electromagnetic protection fields.The nanocomposite foam which using polymer as matrix and nanoparticles as phases additions is widely researched since it has low weight,high specific strength and well chemical stability.The researches about electromagnetic property of the nanocomposite foams are less.The influence of the cell structure of the nanocomposite foam on its electromagnetic property has not been studied yet.In this paper,the PMMA was selected as polymer matrix.The PMMA-based nanocomposite foams were prepared by physical restriction supercritical carbon dioxide?SCCO₂?foaming method.The macro dimension and microstructure of the foams were well controlled.Then the electromagnetic property of the PMMA-based nanocomposite foams was researched.Firstly,the SCCO₂ foaming of PMMA-based nanocomposite was studied.The diffusion of the SCCO₂ in polymer matrix was unsteady diffusion.It correspounded to Fick's second low.The PMMA foams with graded structure were prepared by unsteady diffusion with limited diffusion time.The foams with uniform cell structure were prepared by increasing the diffusion time.By adding nanoparticles in the polymer matrix,after foaming,the cell density of the foams increased 1² orders of magnitude since the heterogeneous nucleation effect during SCCO₂ foaming.The cell density increased from 10⁸ cells/cm³ to 10¹00 cells/cm³.The heterogeneous nucleation effect was influenced by the size of the nanoparticles.The heterogeneous nucleation can occur when the size of the nanoparticles was higher than the critical heterogeneous size.The 2 nm nanoparticles can be used as heterogeneous nucleation agent to increase the cell density of the foams at 20 MPa.The physical restriction SCCO₂ foaming method was proposed.The foams with flat surface and controlled dimension were prepared by physical restriction mold.The microstructure of the foams was not influenced by the physical restriction mold when the foaming temperature was lower than 110 ^o C.Secondly,The CNTs/PMMA nanocomposite foams with cell size of 2¹2?m?cell density of 10⁹ cells/cm³ and volume density of 0.33⁰.72 g/cm³ were prepared by physical restriction SCCO₂ foaming method.The cell structure of the nanocomposite foams was influenced by the foaming temperature and CNTs content.The best foaming temperature was 80 ^o C in this research.The foam foaming at this temperature had small and uniform cell size.The cell density is 10¹00 cells/cm³.The cell size decreased and cell density increased as the CNTs content increased.The the melt strength of the polymer matrix and adsorbing capacity of CO₂ were influenced by foaming temperature.The melt strength of the polymer matrix was high at low temperature?50^oC?which was difficult for the matrix to foam.While the melt strength of the polymer matrix was low,and the adsorbing capacity of CO₂ in the polymer matrix was small at high temperature?110 ^oC?.Then the cell size increased and cell density decreased.The cell size of the foam decreased and the cell density increased with the CNTs content increased.The best CNTs content was 6⁸ wt.%.The cell size was 3?m while the cell density was 10¹00 cells/cm³ with this CNTs content.The heterogeneous nucleation sites increased as the CNTs content increased.And the CNTs can form a net structure to restrain the growth of the cells.So the foams had smaller cell size and higher cell density with high CNTs content?6⁸wt.%?.Further more,the electromagnetic property of the CNTs/PMMA physical restriction nanocomposite foams at X-band?8.2¹2.4 GHz?was researched in this paper.The dielectric constant of the CNTs/PMMA physical restriction nanocomposite foams was 2.2²2 while the dielectric loss was 0.1¹0.The dielectric constant of the foams was significantly reduced by the introduction of the cells which can increase the impedance matching of the foams.The dielectric loss was fine by controlling the cell structure of the CNTs/PMMA physical restriction nanocomposite foams.As a result,the foams had well electromagnetic wave absorption property.The absorption band width of the foam was 2.7 GHz?9.7¹2.4GHz?,the lowest reflection coefficient was-27 dB,the absorbtion efficiency was99.8%.The electromagnetic wave absorption property of the foams was influenced by the thickness of the material,CNTs content and cell structure of the foam.The impedance matching of the foams was influenced by the thickness of the material.The electromagnetic wave cannot incidence to the material when the depth of the surface was smaller than the thickness of the foam.The conductivity of the foams was influenced by the CNTs content.The foams cannot loss the incidence wave as the CNTs content was low?lower than 4 wt.%?.While the wave will reflect by the surface of the foam as the CNTs content is high?higher than 8 wt.%?.The both cases would reduce the electromagnetic wave absorption property of the nanocomposite foam.The nanocomposite foam had well electromagnetic wave absorption property when the CNTs content was near the percolation threshold?1.6².2 vol%?.The CNTs can well-dispersed in the cell walls when the foams have uniform cell structure and high cell density.Thus the foams had better electromagnetic wave absorption property.When the density of the foam was 0.37 g/cm³,the specific absorption efficiency of the nanocomposite foam was all absorption in X-band.The smallest specific reflection coefficient was-50 dB.cm³/g.The CNTs distribution was influenced by the cell structure.And the density of the foams was decreased as the cell structure changed which would increase the specific absorption efficiency.Finally,the CNTs/PMMA physical restriction multilayer graded nanocomposite foam was designed by finite element calculation.And the electromagnetic wave absorption property of the nanocomposite foam was improved.According to the electromagnetic wave asborbtion theory,the surface of the multilayer graded nanocomposite foam had low was dielectric contant since the CNTs content was low?4⁶ wt.%?.The electromagnetic wave incident was then improved.The innerior of the multilayer graded nanocomposite foam had high dielectric loss since the CNTs content was high?8 wt.%?.Then the electromagnetic wave loss increased.Ultimately,the electromagnetic wave absorption property of the nanocomposite foam improved.According to the calculation result,the CNTs/PMMA multilayer graded nanocomposite foam was prepared by hot-pressing and physical restriction SCCO₂ foaming.The CNTs content of the upper layer was 6 wt.%and the thickness was 1.2 mm.The CNTs content of the under layer was 8 wt.%and the thickness was0.9 mm.The whole thickness was 2.1 mm.The surface was flat and the interface of the graded foam was tightness and smooth.The electromagnetic wave absorption band width of the multilayer graded nanocomposite foam was better than the monolayer foam with the same thickness and 6 wt.%CNTs content.The absorption band width of the graded foam was 3.5 GHz?8.9¹2.4 GHz?,the absorbtion efficiency was 99%.