The Research Of Multi-interface Structure Design, Integrated Preparation And Electromagnetic Property Of CNTs/PMMA Microporous Nanocomposite Foams

Electromagnetic wave absorbing material is a kind of functional material which can transform electromagnetic wave radiation energy into heat energy.Polymer based EMA composites with absorbing agents have been widely studied because of their characteristics of light weight,corrosion resistance and easy processing.However,the current polymer based EMA composites have the problems of low electrical conductivity and low electromagnetic wave absorbing performance.Reasonable structural design can solve the problem of high content,easy agglomeration and difficult dispersion of absorbent in the matrix to realize the unification of low absorbent content,high conductivity,low reflectance and high absorbability.However,the current polymer based EMA composites have the problems of low electrical conductivity and low electromagnetic wave absorbing performance.Based on Schelkunoff plane electromagnetic wave transmission theory and previous research work on EMA structure,three kinds of multi-interface structures including directional arrangement structure,gradient structure and alternating structure were designed and introduced into microporous nanocomposites foam.In this paper,the poly（methyl methacrylate）（PMMA）as polymer matrix and carbon nanotubes（CNTs）as wave absorbent,microporous nanocomposites foam with directional arrangement structure,gradient structure and alternate structure were one-step synthesized by combination of laminated melting hot pressing and physical constrained supercritical carbon dioxide（SCCO₂）foaming method.The relationship between the microporous nanocomposites foam with multi-interface structure and its electromagnetic properties in X-band is mainly studied.PMMA microporous foams with directional arrangement structure were one-step synthesized by combination of melting hot pressing and SCCO₂ foaming method.Multilayer interfaces were introduced into the PMMA matrix to induce directional growth and arrangement of gas nuclear at the interface.The interlayer spacing is controlled to be smaller than the critical nucleation size,the uniform and ordered directional structure was obtained.Under the foaming condition of 16 MPa and 80 ^oC,the average cell size was reduced from 17μm to7μm since directional arrangement structure,the cell density was increased from 4.74×10⁸cells/cm³ to19.6×10⁸cells/cm³.Furthermore,the CNTs/PMMA nanocomposites were prepared by mixing CNTs and PMMA via a totally enclosed mixing mill.On the basis of the preparation of PMMA microporous nanocomposites with directional arrangement structure,the CNTs/PMMA microporous nanocomposites foame with directional arrangement structure were also prepared.The control mechanism of its directional arrangement structure is consistent with that of the directional arrangement structure of PMMA microporous foam.In the X-band（8.2-12.4 GHz）,CNTs content was 8 wt.%,CNTs/PMMA microporous nanocomposites foam with directional arrangement structure,whose thickness was 2mm had good EMA properties,its minimum reflection loss was-24.8 d B and absorbing bandwidth was 2.5 GHz（9.1-11.6 GHz）,which was wider than 1.7 GHz of the normal microporous nanocomposites foam.This is due to the directional arrangement structure with a large number of parallel interfaces,promoting the multiple reflection and absorption of electromagnetic waves among the interfaces.PMMA-based microporous nanocomposites foam with thickness gradient structure was one-step synthesized by combination of melting hot pressing and SCCO₂foaming method.The thickness gradient structure is that the thickness of each layer decreases first and then increases from the bottom layer to the top layer.It scribes to difference of rheological properties of each layer of polymeric melts in the pressure-dirven flow.With the increase of the number of layers,the gradient structure becomes more complete and obvious.Since the microporous nanocomposites foam with gradient structure contains a PMMA foam layer and an 8CNTs foam layer,their cell structures were significantly different,so the prepared gradient-structured microporous nanocomposites foams were all bimodal cellular structure.With the layer increases,the cell density was increased from 2.4×10⁸ cells/cm³ to 9.7×10⁸ cells/cm³.The frequency dependency of the reflectivity of the PMMA-based microporous nanocomposites foam with gradient structure was analyzed from both theoretical calculation and experimental measurement,and the two are in good agreement.In X-band,CNTs content was 8 wt.%,PMMA-based microporous nanocomposites foam with gradient structure,whose thickness was 3.2 mm had nice EMA properties,its minimum reflection loss was-24.0 d B and absorbing bandwidth was 3.2 GHz（8.8-12.0 GHz）.This is mainly because the gradient structure improves the overall impedance matching,promotes the electromagnetic wave incidence,and the enhanced interlayer polarization improves the electromagnetic wave attenuation.The PMMA-based microporous nanocomposites foam was one-step prepared by combination of melting hot pressing and SCCO₂ foaming method.The PMMA foam layer and 8CNTs foam layer were distributed alternately,the cells showed bimodal porous,and the large and small cell size were randomly distributed.With the layer number increasing,the cell density was increased from 2.6×10⁸ cells/cm³ to 2.1×10¹⁰cells/cm³.The large and small size cells which was alternating distributed,was directional arrangement,it is mainly due to the layer spacing was controlled to be smaller than the critical cell size.The EMA properties of PMMA-based microporous nanocomposites foam with alternating structure were analyzed from both theoretical calculation and experimental measurement.It was found that the frequency dependency of the reflectivity of theoretical calculation and experimental measurement were in good agreement.In X-band,CNTs content was 8 wt.%,PMMA-based microporous nanocomposites foam with alternating structure,whose thickness was 3mm had excellent EMA properties,its minimum reflection loss was-20.75 d B and absorbing bandwidth was 3.53 GHz（8.49-12.02 GHz）.This is due to the fact that the alternating structure improves the impedance matching of the microporous nanocomposites foam,more electromagnetic waves are incident into the materials,the reflection of electromagnetic waves on the surface of the material is reduced,and the alternating structure microporous nanocomposites foam has strong internal loss characteristics.At the same time,the microscopic circuit of resistance-capacitance-inductance is formed in the microporous nanocomposites foam with alternating structure,which provides a channel for charge movement and attenuates its energy intensity by interacting with the incident electromagnetic wave.The thickness and absorbing agent content were normalized by normalizing the absorbing bandwidth and minimum reflection loss of the microporous nanocomposites foam with directional arrangement structure,gradient structure and alternating structure,respectively.The comparative analysis results show that the microporous nanocomposites foam with alternating structures has the largest normalized specific absorption bandwidth and the smallest normalized minimum reflection loss,indicating that it has the best EMA performance,followed by microporous nanocomposites foam with gradient structure and microporous nanocomposites foam with directional arrangement structure has relatively poor EMA properties.The main reason is that a large number of micro-parallel plate capacitors combine to form a complete microporous nanocomposite foam with alternating structure,so that the probability of electrons penetrating the micro-dielectric layer is very high,and the channels for mobile carriers grow,resulting in polarization and conductivity loss are much higher than that of microporous nanocomposites with directional arrangement and gradient structures,so the microporous nanocomposites foam with alternating structures has the best electromagnetic wave absorption properties.