High Performance Polyimide Honeycomb Structure Via Gel-ink Direct Writing And Its Wave Absorption Properties

Polyimide has attracted extensive attention of researchers due to its excellent mechanical properties and heat resistance.With the continuous development of aviation and aerospace fields,there is an urgent application demand for lightweight honeycomb structures with high specific strength and low density.Therefore,it is particularly crucial to expand molding methods for complex polyimide structures and functional properties,such as wave absorption.However,due to the insufficient melt fluidity of conventional polyimide and difficult dissolution in organic solvents,the molding for three-dimensional complex structures faces relatively obvious obstacles.The preparation for light-weight structures with high specific strength and high glass transition temperature is still an unattainable goal.As a flexible,fast and highly designable molding technology,3D printing technology has been applied in various fields,which provides new opportunities for the molding/processing and expanding applications of polyimide materials.At present,the printing processing for polymer materials is mostly accomplished by melt-deposition molding process.However,for the high temperature-resistant polyimide structures,due to their insoluble and non-melt material properties,it is usually difficult to process them by melt printing,which severely limits the fabrication for complex structures.In recent years,the strategy based on“secondary reaction”has realized 3D printing process for a variety of difficult-to-process inorganic non-metallic materials.The specific method is to mold and print the material at a lower temperature and after the high-temperature heating,the cross-linking or curing reaction occurred transforms the material's structure into a high-temperature resistant and high-strength structure.Some researchers introduced the concept of“secondary reaction”into the 3D printing for polyimide and studied the photosensitive polyimide 3D printing system based on the photocuring molding,but the added small-molecule additives can affect the heat resistance and mechanical properties of the final structure.In this paper,a simple direct-writing 3D printing method with hydrogel ink was developed.The hydrogel precursors of polyamic acid salt containing dynamic reversible non-covalent bonds were used as the printing ink and their rheological behaviors were adjusted based on intermolecular interactions to meet the printing requirements for direct ink writing.The printed honeycomb structure manifests excellent thermal and mechanical properties after high-temperature post-curing.In addition,in order to explore the structure-function integration possibility of 3D-printed polyimide,inorganic wave-absorbing fillers were introduced into the system to construct an organic-inorganic composite polyimide honeycomb and the wave-absorbing properties and mechanism were discussed.First,the regulation mechanism for gel precursor of polyamic acid salt was explored and the matrix preparation that could meet the direct writing printing of gel ink was realized.Based on the one-step method for salting and water-phase polymerization,3,3?,4,4?-biphenyltetracarboxylic dianhydride/p-phenylenediamine,3,3?,4,4?-biphenyltetracarboxylic dianhydride/3,4?-diaminodiphenyl ether,4,4'-(hexafluoroisopropylidene)diphthalic anhydride/3,4'-diaminodiphenyl ether were used to design and synthesize three hydrogels of polyamic acid salt with structures of different co-planar twist degrees.A method to characterize the intermolecular forces of polyamic acid salt precursor hydrogels was established by combining infrared spectroscopy and fluorescence emission spectroscopy.Based on the method established,the promotion or hindrance of different twist degrees on the intermolecular interactions such as hydrogen bonding and?-?stacking were studied and the aggregation state of the polyamic acid salt was analyzed.All three hydrogels of polyamic acid salt exhibit good shear-thinning behavior and shear modulus,which ensures that the smooth extrusion of gels and the self-supporting property after extrusion under shear force or high temperature.All the imidized polyimides have excellent heat resistance.Secondly,the preparation for high-performance polyimide honeycombs with all-aromatic ring structure was realized based on the direct writing of gel ink and high-temperature thermal curing printing strategy and their properties were studied.Based on the research results in Chapter 1,the hydrogel of 3,3?,4,4?-biphenyltetracarboxylic dianhydride/p-phenylenediamine structure with good thixotropy and temperature sensitivity was used as the printing ink for direct writing.The effects of printing process parameters(such as diameter of printing nozzle,printing temperature and printing speed)on the state and width of lines printed were studied and determined,and then the3D printing parameters for polyimide were adjusted.Combined with the high-temperature post-curing for“secondary reaction”,polyimide printing with low shrinkage and high interfacial fusion was successfully achieved.Based on the above-mentioned studies,various configurations,especially the polyimide honeycombs with triangular,square and hexagonal topologies were designed and fabricated by increasing number of printed layers and structural complexity.The interlayer melting situation and internal mass after the imidization process,as well as the room-temperature and high-temperature mechanical properties of honeycombs with different densities and shapes were systematically studied.The imidized polyimide honeycombs show a correlation between density and compressive strength,so the compressive strength could be regulated by varying the structure and density of the honeycombs.Using the gel ink printing strategy,the obtained polyimide honeycomb density has a density of 150-400kg/m³ and a compressive strength range of 12.6-43.5 MPa.In addition,the printed honeycomb had retention rates for high specific strength and high-temperature strength.The specific strength is up to 0.127 MPa/(kg/m³)and the strength retention rate for high temperature(300?)is close to two-thirds.Finally,based on the optimized high-performance polyimide honeycomb structure,multi-scale carbon materials were introduced to expand the wave-absorbing functional properties of 3D-printed honeycombs.Three inorganic nanomaterials,graphene,carbon nanotubes and nickel-coated carbon nanotubes were selected.With consideration of the significant effects of nanofillers'dispersion in matrix on the printing process,mechanical properties and wave-absorbing properties,the three nanomaterials were composited with polyamic acid salt hydrogel in the form of aqueous dispersion.The rheological behavior of the organic-inorganic composite gel printing ink was studied and the printing parameters were adjusted and optimized.Organic-inorganic microwave absorbing honeycombs with square topology were printed by the“secondary reaction”method combining direct writing with gel ink and thermal curing.The morphology,structure and mechanical properties of the composite honeycombs were discussed in detail.The focus was on the analysis of the wave-absorbing properties of organic-inorganic composite honeycombs.The electromagnetic parameters,impedance matching and attenuation constants of the wave-absorbing honeycombs could be varied by adjusting the type or added amount of wave-absorbing fillers,thus endowing the 3D-printed polyimide composite honeycombs tunable wave-absorbing properties.Among them,when the thickness of nickel-plated carbon nanotube honeycomb is 2.6 mm and the filler content is 5 wt%,the RL_min value is-56.56 d B,exhibiting the strongest electromagnetic wave-absorbing performance.When the thickness of graphene/PI honeycomb is 3.4 mm,the absorption is able to cover the entire X band(8.2-12.4 GHz),manifesting the widest effective absorption bandwidth.Finally,the absorption loss mechanisms of the three wave-absorbing honeycombs,graphene/PI,carbon nanotube/PI and nickel-coated carbon nanotube/PI were analyzed and discussed.In conclusion,based on the 3D printing strategy for polyamic acid salt hydrogel combining ink direct writing and high temperature thermal curing,this paper successfully prepared the polyimide honeycombs with high compressive strength,and endowed the polyimide honeycombs with good wave-absorbing functional properties by compositing them with inorganic nanofillers.The composite honeycombs have the potential to integrate structure and function so they are expected to be used in aviation and aerospace fields,providing new ideas for the further development of polyimide.