Preparation Of Polyimide Boron Nitride Composites And Their Application In Water Purification

Boron nitride is a layered material that resembles graphite and is called white graphite.It is alternately occupied by B and N atoms in plane and connected by strong B-N covalent bond to form 2D lamellar,and 2D lamellar is connected by weak van der Waals force to form 2D stacking network.Boron nitride has many advantages,such as large specific surface area,more structural defects,high mechanical strength,high thermal conductivity,high thermal stability,and corrosion resistance,making it an ideal material for water treatment applications.In this paper,polyimide/dopamine-modified boron nitride/carbon nanotubes composite aerogel(PI/PDA@BN/CNT)and polyimide/Dopamine-modified boron nitride/carbon nanotubes flexible composite membrane(PBC)were prepared using polyimide as matrix combined with dopamine-modified boron nitride and carbon nanotube,and were applied to solar water evaporation and oil-water emulsion separation respectively.The main research contents include:(1)Preparation of PI/PDA@BN/CNT composite aerogel and its application in solar-driven evaporator.PI/PDA@BN/CNT composite aerogel with low bending tubular structure was prepared by four-directional freeze drying combined with imidization.Compared with polyimide aerogel(PI)with hydrophobic properties,the contact angles of polyimide/dopamine-modified boron nitride aerogel and PI/PDA@BN/CNT composite aerogel with dopamine-modified boron nitride(PDA@BN)decreased from 128°to 67°and 64°.The contact angle of polyimide/dopamine-modified boron nitride/carbon nanotubes composite aerogel(PI/PDA@BN/CNT-R)prepared by conventional freezing is 66°,which is much lower than 128°of PI aerogel.This caused by the large number of amino and hydroxyl hydrophilic groups on the surface of PDA@BN,which improves the hydrophilicity of the material,while the addition of CNT further improves the hydrophilicity of the PI/PDA@BN/CNT composite aerogel.The microstructure of the aerogel was observed by SEM.Compared with the internal three-dimensional pore structure of PI/PDA@BN/CNT-R,the PI/PDA@BN/CNT aerogel prepared by the four-directional freezing process has a tubular structure with low curvature,which is conducive to the transportation of water in the Solar-driven evaporation process.when the residual content was 95%,the thermal decomposition temperature of PI/PDA@BN and PI/PDA@BN/CNT composite aerogel increased from 490℃to 503℃and 510℃,compared with PI aerogel.PI/PDA@BN/CNT composite aerogel also has excellent recycling ability,chemical stability and efficient sewage purification capacity.(2)Preparation of PBC flexible composite membrane and its application in oil-in-water emulsion separation.By adjusting the p H of Tris,the boron nitride nanosheets were modified by dopamine hydrochloride,and the positive charge and hydrophilic properties of boron nitride were successfully endowed.The mixed solution of hydroxylated carbon nanotubes with negative charge,dopamine-modified boron nitride and polyamide acid were pumped and filtered into membranes by vacuum assisted extraction and filtration process.Through electrostatic action,the connection between PDA@BN layer and CNT layer becomes stronger.The addition of polyimide makes the composite membrane has good flexibility.PBC-0.75 flexible composite membrane has excellent underwater superhydrophobic properties,and its underwater oil contact angles with liquid paraffin(167°),cyclohexane(160°),n-heptane(162°)and petroleum ether(162°)are all over 150°.PBC flexible composite membrane has good separation ability for oil-in-water emulsion.The separation efficiency of PBC-0.75 for liquid paraffin,cyclohexane,n-heptane and petroleum ether oil-in-water emulsions reached99.83%,99.64%,99.32%and 99.17%.The separation flux of the flexible composite membrane for cyclohexane oil-in-water emulsions was the largest,reaching 389L/m~2·h·bar.The cyclohexane oil-in-water emulsion was cycled by PBC-0.75 for 8times.The second,fourth,sixth and eighth separation efficiencies were 98.79%,98.51%,97.39%and 97.38%,and the separation fluxes were 412,646,737 and 733L/m~2·h·bar,respectively.The surface PDA@BN of the composite membrane gradually subsists and tends to be stable around the sixth time with the increase of separation times.In addition,PBC-0.75 flexible composite membrane also has great chemical stability.The underwater contact angles between PBC-0.75 flexible composite membrane and cyclohexane still remain at 165°,160°and 163°after soaking in 1mol/L HCl,Na OH and Na Cl.