Preparation And Properties Of Polyimide And Its Modified Aerogel

Polyimide?PI?is a kind of polymer materials with excellent comprehensive properties,especially in high thermal stability,excellent mechanical properties and high insulation.Due to its unique rigid aromatic heterocycle structure,it has a wide range of applications both as structural materials and functional materials.It has been widely focused by numerous researchers as well as it has been called one of the most promising polymer materials in the 21 st century.Aerogels are the materials with unique porous structure which have ultra-low density,high porosity,low thermal conductivity and dielectric constant.Aerogels are widely used in the fields of catalysis,adsorption filtration,thermal insulation,sound absorption and noise reduction,photoelectricity and biological research.The earliest aerogels were the silica aerogels which classified as inorganic aerogels.Because they were fragile and have poor mechanical properties,more and more people pay attention to the study of polymer aerogels,such as polyurethane aerogels which have excellent mechanical properties.Unfortunately they are faced with another problem of low heat resistance,by contrast,polyimide aerogels have solved the problem of which organic aerogels have poor heat resistance.The traditional polyimide aerogels were prepared by sol-gel combined with supercritical drying.The linear aerogels had high shrinkage and low mechanical properties after supercritical drying in the early years.A number of researchers have focused their efforts on developing multifunctional diamines and dianhydrides monomers by cross-linking to reduce shrinkage.However,the preparation conditions of these monomers are harsh,the process is tedious,expensive and time-consuming,which is not suitable for mass production.On the other hand,supercritical drying is also faced with the disadvantages of low economy and high risk factor under high pressure.Therefore,this paper focuses on other methods to solve the problem.The costs and risks are significantly reduced by replacing supercritical drying with freezedrying.Water was used as diluent and pore-forming agent avoided a lot of organic solvents were used in the preparation of aerogel.When water was used as the solvent,it is difficult to adopt traditional cross-linked multifunctional group monomers to reduce shrinkage.This paper used other ways to solve the problem: first,the different linear polyimides were prepared by different dianhydrides?BTDA,BPADA,PMDA,BPDA?and diamine.Esting and analyzing a series of physical and chemical properties in order to select the optimal dianhydride-diamine aerogels as the modified materials in the next step.Secondly,the BTDA aerogels with better comprehensive properties in the four systems was selected for composite modification.Graphene oxide?GO?with a large number of oxygen-containing groups on the surface and nano-sized zirconium dioxide?ZrO₂?were selected with unique optical effects.Finally,the BPADA aerogels which have fine mechanical properties whereas high shrinkage rate were selected for the next modification.A certain amount of GO and nano-sized ZrO₂ were added separately and cooperatively for modification.Based on the green environmental protection,the simple and economic way was adopted to synthesis of the polyimide aerogels and its modification aerogels by preparation of linear system of polyimide and choosing the appropriate PI to combine the inorganic nano-sized materials.The results show that modified materials have significantly lower density,higher porosity and lower shrinkage,better mechanical properties and heat resistance.In this paper,a series of aerogels with high porosity??95%?,low density??70mg/cm3?,low shrinkage??20%?,favourable compressive resistance?T_d ? 460 ??,high thermal stability and excellent thermal insulation properties???0.027W/?m×K??were prepared,which are expected to enrich the species of thermal insulation materials and broaden the application field of polyimide materials.