The Structure-property Relationship Of Polyimide Aerogels Prepared By Freeze-drying Technique

Benefitting from the chemical structural attributes of polyimide(PI)materials of resisting to high/low temperature,UV irradiation,and chemical corrosion,and meanwhile taking advantage of the physical structural characteristics of aerogel materials including light weight and highly porous,PI aerogels enjoy a promising prospect in cutting-edge and practical applications,such as the heat insulation materials in aerospace industry and the adsorbents in environmental governance,which makes them one of the most heated topic for PI or aerogel research.In the meantime,freeze-drying method is increasingly used in the preparation of aerogel materials owing to its high safety coefficient,low energy consumption,and facile operation.However,existing reports suggest that researchers have paid much attention to avoiding the high shrinkage rate and cracking of as-dried aerogels,whereas studies on the chemical structure of PI aerogel materials,especially on how the stiffness and flexibility of molecular chains as well as freeze-drying process would affect the comprehensive performance of aerogel products,have rarely been published.In this context and with the considerations on stiffness and flexibility of PI molecular chains,a series of PI aerogel materials holding different chemical structures were fabricated by using regular PI monomers and applying the freeze-drying technology.Specifically,water-soluble polyamide acid(PAA)precipitated fibers were prepared from the salt-formation reaction between polyamide acid(PAA)and organic weak base,followed by transformation into aqueous solutions of different solid contents.Then,PI aerogel materials with various chemical compositions and densities were obtained from freeze-drying and thermal imidization.The effects of PI chemical structure,solid content of salt solution,and freezing rate on the microstructure,molding shrinkage,along with the mechanical,thermal,and adsorption properties of the PI aerogels were investigated by means of SEM,TGA,universal material testing,and adsorption experiments.Results showed that chains with a higher rigidity would endow PI aerogels with smaller forming shrinkage,larger pore size,lower density,and thereby smaller compression strength under identical strain conditions.Moreover,the BPDA-ODA system exhibited a lower energy loss during cyclic compression.On such basis,two kinds of carbonaceous composites based on the flexible PI aerogels,carbon nanotube/PI composite aerogel and graphene/PI composite aerogel,were fabricated via freeze-drying technique with 4,4'-oxydianiline(ODA)and 3,3',4,4'-biphenyl tetracarboxylic acid dianhydride(BPDA)as the monomers and carbon nanotubes or graphene as the nano fillers.The microstructure,shrinkage,mechanical properties,thermal behaviors,electrical properties,and adsorption performance were characterized and analyzed by SEM,TGA,universal material testing,and universal electricity tests,so as to evaluate how the composition ratio of carbon material to PI could influence the molding process and comprehensive properties of as-prepared composite aerogels.It was found that the freeze-dried products would easily break down when the mass ratio of carbon material to PAAs was above 5:1,whilst composite aerogel materials with good flexibility and resilience could be obtained at this ratio equal to 2:1.