Porous Polyimide Materials Based On Hydrogel Method: Preparation And Properties Manipulation

Polyimide(PI)material has been commercialized for decades.Compared with many other engineering plastics,its excellent mechanical strength,high temperature resistance,chemical corrosion resistance and low thermal expansion coefficient,make it play a great role in the fields of aerospace,automobile and ship,microelectronics science and photoresist.And polyimide porous material(foam,aerogel,porous membrane,etc.)endowed PI based materials with high porosity,high specific surface area,low density,and many other new features.Traditional polyimide materials are prepared by one-step or two-steps process.During the synthesis process,a large amount of organic solvents are directly discharged into the atmosphere,causing environmental pollution.Secondly,in the current process of preparing polyimide aerogels,the solvent is often removed by supercritical drying,and thus the cost of mass production is high.In addition,fabricating PI/inorganic hybrid films in organic solvents usually leads to a sharp decline in the flexibility of PI films,affecting their practical applications.In this work,polyamide acid(PAA),based on biphenyl tetracarboxylic anhydride(BPDA)and 4,4'-diamino diphenyl ether(ODA),was synthesized and partially salinized with trimethylamine(TEA)which turned it into water-soluble PAA carboxylate(PAAC).Through a simple sol-gel route,PAAC hydrogel was obtained and further lyophilized or naturally-dried for preparing PI aerogels or porous membranes.In this paper,a series of PI-based porous materials are all prepared by hydrogel method which is a safe and pollution-free green method and greatly reduces environmental pollution.The specific contents are listed as follows:(1)Polyamide acid(PAA)was prepared with ODA and BPDA and turned into water-soluble PAAC after being partially salinized with TEA.PI aerogel was prepared by freeze-drying and thermal imidization of PAAC hydrogel.The influence of concentration,aging time and freezing speed of hydrogels on the performance of PI aerogels were studied in detail.The results show that:1)The increase of solid contents can accelerate the formation of PAAC hydrogel and increase the gel molding temperature;2)Pore sizes of PI aerogels were mainly controlled by freezing speed of the PAAC hydrogel;The faster the freezing speed,the smaller the pores;3)The appropriate concentration for preparing PAAC hydrogels are limitted in a solid content range of 4-8%.The densities of PI aerogels decreased with the decrease of solid contents;4)Before the freeze drying of PAAC hydrogels,aging process can significantly increase mechanical performance of PI aerogels;5)The specific modulus(E/?)of PI aerogel after aging for 8 days is higher than that high-strength aerogels reported from literatures,confirming that the aging process of hydrogel state is the key factor to enhance the mechanical properties of PI aerogels.(2)PI aerogels prepared from hydrogel route are limited by the use of monomer,and the modification of PI aerogels can be realized before formation of hydrogels by coorperating nano materials in water solution.A high yield(>70%)functionalized water-soluble carbon quantum dots(C-dots)was prepared by a modified hydrothermal method to improve the performance of PI aerogels.PI/C-dots composite aerogels were prepared by direct mixing the C-dots with PAAC in water,with subsequent cooling,freeze-drying,and thermal imidization.Studies have shown that C-dots has a large number of functional groups,such as carboxyl group,hydroxyl group and chlorine element.As a cross-linking agent,C-dots can be uniformly dispersed among the PI molecular chains to enhance the intermolecular force of PI.Therefore,when the C-dots dosage reached 3 wt.%,the compression strength of PI/C-dots composite aerogels increased from 0.81 to 0.98 MPa(increased by 21.0%).In addition,the chlorine element contained in C-dots can improve the hydrophobicity of aerogel,and the contact angles between PI/C-dots and water can be increased from110.5°to more than 140°which significantly improves the anti-moisture ability of PI aerogels.(3)Polyimide/reduced graphene oxide(PI/rGO)composite aerogels were prepared from freeze drying and thermal imidization of PAAC/GO hydrogels.The nearly spherical cavity morphology formed in the frostgels during freezing process accounted for the resilience of PI or PI/rGO aerogels.PI/rGO composite aerogels with stable compression recovery performance were obtained by eliminating the adverse effects through pre-compression treatment.When the added amount of GO was 4wt.%,the energy loss coefficient(21.8%)of the composite aerogel reached the lowest level.When the deformation range of aerogel prepared by freeze-drying method is less than 20%,the energy loss coefficient showed no obvious change.In addition,PI/rGO composite aerogels have linear response to the magnitude and velocity of external forces.It is proved that PI based composite aerogels prepared by freeze-drying method can be used as a mechanical sensor with stable cyclic performance and wide stress induction range.(4)Composite hydrogel film of PAAC/GO was prepared by mixing PAAC and GO in water by simply tuning the temperature of the mixture.The formation mechanism of internal multilayer structures of PI/rGO films was proposed.The hydrogel process also avoids the aggregation of GO during solvent volatilization process,which is an excellent way to prepare homogeneous PI/inorganic nanocomposite films.In addition,the composite films prepared by hydrogel method have good malleability,due to the network structure of molecular chains and orderly stacking,which demonstrated an average elongation at break more than 35%.The hydrogel method for preparing PI/inorganic hybrid films avoid the poor toughness commonly faced in PI/inorganic hybrid films which were prepared using organic solvent.In addition,the porous films prepared by hydrogel method showed low dielectric frequency dependence.It also provides a feasible and effective method for preparing many other PI/inorganic hybrid materials with porous structure.(5)PAAC/GO/PEG composite hydrogel was successfully prepared by mixing PAAC,GO and PEG in aqueous solution with subsequently temperature regulation.Homogeneous PAAC/GO/PEG composite xerogel was obtained through natural drying of the hydrogel.Porous PI/rGO films were prepared by one-step in situ pyrolysis of PEG through a heating process.During the heating process,PEG was pyrolized at high temperature which produced large amount of gases.At the same time,PI/rGO composite films were reshaped above their glass transition temperature to obtain porous structure.An appropriate amount(2.0 wt%)of GO nanosheet was used as the reinforcement material to prevent the collapse of pore wall during cooling process.The PI/rGO porous composite film with ultra-low dielectric constant(?=1.93,PEG content at 40 wt.%)was obtained by simply adjusting the amount of PEG.At high frequency of 1.0 MHz,the dielectric loss of PI/rGO composite films decreased to0.001,which has a potential application prospect in the field of high-frequency communication.The porous PI/rGO films prepared by hydrogel route demonstrated very high toughness,with average elongation at break exceed 38%,high hydrophobic properties(CA>91.2°)and low densities(p<1.0 g/cm~3).It has important application value in the fields of high frequency communication and electronic devices.