Preparation And Performance Regulation Of Polymethacryloimide Aerogels And Their Composite Aerogels

Aerogel is a nano-scale,ultra-light solid material consisting of colloidal particles or polymer molecular chains that aggregate with each other and have a three-dimensional porous structure.Its pore is filled with gaseous media(usually air),which is called"air gel".This material has excellent properties such as low density,low thermal conductivity and high porosity,and is widely used in thermal insulation,filter element,aerospace and other fields.Polymethylacryloimide(PMI)is a class of polymers containing imide rings(-CO-N-CO-)in the main chain.Due to the presence of imine rings,PMI materials have excellent thermal stability,high strength and high rigidity.PMI foam is a closed-cell rigid foam material with higher specific strength,modulus and creep resistance than rigid foams such as polyvinyl chloride(PVC)and polyurethane(PUR).PMI foam performs well in high temperature and high load environment and is widely used in many fields such as aerospace,building exterior wall,road traffic,etc.However,up to now,the application of PMI products in the market is only closed-cell PMI foam,and the research on PMI open-cell materials(aerogels)is still in its infancy.In this paper,the preparation and properties of PMI aerogels will be studied,and the application of high-performance PMI materials in the field of opening will be explored.MAN/MAA copolymer was prepared by free radical solution polymerization with methacrylonitrile(MAN)and methacrylic acid(MAA)as monomers,acrylamide(AM)as cross-linking agent and azodiisobutyronitrile(AIBN)as initiator.As the copolymer is insoluble in water,triethylamine(TEA)was introduced into the system to treat it with water,and the water-soluble copolymer quaternary ammonium salt was obtained by washing and drying.Then the quaternary ammonium saline solution was prepared to prepare PMI aerogel by vacuum freeze-drying and thermoimidization.On this basis,the co-solvent modified PMI aerogel was successfully prepared by changing the composition of the freeze-dryer.PMI aerogels of low cost AN/MAA system were prepared by replacing expensive MAN with inexpensive acrylonitrile(AN).Based on the optimized formulation,PMI composite aerogels with two different lamellar components were obtained by introducing nanolamellar sodium montmorillonite and graphite into AN/MAA system copolymer.The pore structure of aerogel was characterized by scanning electron microscopy(SEM).The compression,thermal and flame-retardant properties of aerogel samples were analyzed by uniaxial compression test,vertical flame-retardant test,thermogravimetric analysis test(TG).The results show that:(1)The shrinkage of aerogels mainly occurs in the heat treatment stage.The modified aerogels with tert-butanol(TBA)/aqueous co-solvent have lower shrinkage(11.12%)and density(0.0711 g/cm~3),and effectively avoid the defects of collapse and stratification of aerogels.During the freeze-drying process,TBA and water form eutectic structure,which changes the crystal form of water and forms needle-like crystallization.At the eutectic point(TBA content 20%),the voids are the smallest and the diameter is only 9μm.The micro-morphology control of PMI aerogels in MAN/MAA system has been successfully realized.Compression properties of aerogels depend on their density,shrinkage and morphology of microstructures.The compression properties of PMI aerogels are reduced by using TBA/water co-solvent as freeze-dryer.(2)The pore wall thickness of PMI aerogels in AN/MAA system increases with the increase of the solid content of quaternary ammonium salts,but the pore size changes slightly with the diameters around 15μm.By changing the proportion of t-butanol in the freeze-dryer,PMI aerogels with different pore structures and sizes were obtained.When the content of t-butanol was at the eutectic point,the pore diameter was the smallest,only 8μm.The compressibility of PMI aerogels increases with increasing aerogel density.The addition of t-butanol has an adverse effect on the compression performance of aerogels,but the compression performance of modified aerogels in AN/MAA system is better than that in MAN/MAA system.(3)The sodium modified montmorillonite has good suspension and uniform dispersion in water.Quaternary ammonium saline solution has good thickening effect on graphite.The addition of graphite can improve the stratification of aerogels.The pore size of the aerogels modified by the two nano-lamellar materials increases significantly,and the compressive strength of the aerogels increases with the increase of Na-MMT content.Compared with lamellar graphite,the addition of lamellar Na-MMT can improve the flame retardancy of aerogels,mainly due to the catalytic carbon formation,heat absorption and oxygen flow retardation of Na-MMT.