Synthesis Of Novel Hyperbranched Polyimides And Study On Properties Of Polyimide-ZnO Hybrid Film

Polyimides have plenty of outstanding properties.As a result,they are widely used as materials for adhesives,aviation,separation membranes,advanced composites,photoresists,microelectronic equipment and so on.However,the common polyimides are indissolvable and refractory,resulting in difficult processing.Thence,the improvement of solubility and molecular design of PI have become one of the hot topics of research and development of high performance PI.Because of hyperbranched polyimide having a large number of branched structures,it is difficult to crystallize and leads to a lack of chain entanglement.So,hyperbranched polyimide has a relatively good solubility,which can be substantially soluble in ordinary organic solvents such as DMF?DMAC?NMP.Solubility property has been significantly improved compared to linear polyimides.In recent years,water pollution caused by organic and toxic pollutants has aroused great attention.Photocatalysis is a green technique to decompose toxic chemicals in water and it draws continuing interest.A variety of nanostructured semiconductor metal oxides,such as Fe2O3,TiO2 and ZnO,have been developed as nanophotocatalysts for their ability to produce radicals and decompose most organic pollutants under UV irradiation or sunlight.Among them,ZnO is more affordable and effective in photocatalytic degradation for some organic dyestuffs and it has been widely studied and applied recently.ZnO nanoparticles range from lnm to 100nm in the size.When the size of particle is as same as lightwave or even less,size effect results in increased spacing between conduction band and valence band,with optical absorption significantly enhanced.So,it is frequently applied as a photocatalytic material.Besides,compared with conventional zinc oxide,nano-zinc oxide is a porous powder material.And there are numerous highly polar hydroxyl groups on its surface.More moisture can be absorbed,while nano-zinc oxide shows some hydrophilic properties.With the augment of surface-to-volume ratios,their small dimension increases the tendency of nanoparticle aggregation and leads to difficulties in the separation from water.Thereby catalytic efficiency is reduced.As a matrix,polyimide is hybridized with ZnO nanoparticles,which are uniformly adhered to the surface of the substrate by the method of ion exchange.This is a valid and low-cost way.Hybrid film also has photocatalytic and hydrophilic properties.In this study,4-hydroxyace-tophenone first had a nucleophilic substitution reaction with p-chloronitrobenzene to produce intermediate product 4-(4-nitrophenoxy)acetophenone(p,p-NPAP).Then,trimolecular p,p-NPAP had a condensation reaction to give the trinitro-compound 1,3,5-tris[4-(4-nitrophenoxy)phenyl]benzene(TNPOPB).Eventually,the trinitro-compound TNPOPB was reduced to the triamine monomer 1,3,5-tris[4-(4-amidophenoxy)phenyl]benzene(TAPOPB).The products of a three-step reaction were respectively characterized by FT-IR and 1H NMR.And the structures of the compounds were demonstrated.Then the obtained triamine has polycondensation reaction with three difficult commercial aromatic dianhydrides(BPADA?ODPA?BTDA)to generate six different end-capped hyperbranched polyimides,respectively.The structure and performance of the obtained hyperbranched polyimides were characterized by FTIR,XRD,TGA,DMA,mechanical measure,lysis tests,UV,GPC,contact angle and so on.The results indicated that they showed good solubility,thermal stability with glass transition temperatures(Tg)between 218? and 320? and temperature at 10%weight loss of 502.1-561.7? in nitrogen atmosphere.Meanwhile,they had decent mechanical properties whose tensile strength and modulus were higher than 72.37 MPa and 1.039GPa,respectively.Water uptake of less than 0.96%was obtained.The truncation wavelengths of the films were all greater than 400 nm with a good application prospect in UV protective coating.Most contact angles were more than 90°,promising to be used as hydrophobic material.Finally,the polyamic acid called the precursor of polyimide was synthesized through a copolymerization of three monomers including ODPA,TAPOPB and ODA.Drying solvent to give the films of PAA at 60 ? in the vacuum oven.Then the film was immersed into ZnC12 solution.After a succedent heat treatment,imidization of PAA and formation of ZnO nanoparticles can be simultaneously achieved in one step to obtain HBPI/ZnO composite film.SEM images manifested that ZnO nanoparticles are successfully immobilized on the surface of the membranes of hyperbranched polyimides,which dispersed homogeneously without obvious aggregation.Mechanical measure indicated that mechanical properties of hybrid materials have been some improvement compared with the pure film.The TG curves showed that the thermal performance of composite film has a slight lifting.In the meantime,after addition of ZnO nanoparticles,HBPI/ZnO nanocomposites exhibited decreased contact angle(CA)value from 101.5° to the 79.7°,which means that HBPI film turns from hydrophobicity(CA>90°)to hydrophilicity(CA<90°).Photocatalytic experiments showed the solutions of methylene blue faded gradually and the ultraviolet absorption peaks of them were also decreased.It showed composite film really accelerated the photocatalysis of methylene blue.