Preparation Of Porous Polyimide Microspheres By Volatilization Or Thermolysis Of Porogen

porous materials in recent decades has attracted considerable attention as they are largely adopted in a very wide range of extensive technical applications,including catalytic supports,separation and adsorption media,biomaterials tissue materials,microelectronic fabrication etc.because of its relatively low density,large specific surface area and chemical modification of the surface.The most active reserch on microspheres is the study of poly(methyl methacrylate)(PMMA)and polystyrene(PS),but the performance of these polymers on mecha-nics,heat resistance,solvent resistance,radiation resistance was far behind that of polyimide microspheres.The introduction of porous structure into the polyimide matrix will potentially be applied in catalytic supports,low dielectric materials,high temperature resistance in absorbing stealth materials to achieve the integration of material insulation,thermal insulation,noise reduction.In our previous study on the preparation of PI microspheres,a novel method was defined as non-aqueous emulsion system in which Liquid paraffin(LP)was the continue phase and N,N-dimethylformamide(DMF)was the dispersed phase using Span85 as a surfactant.These emulsions avoid the presence of not only water but also any protic solvent and offer the possibility to carry out water-sensitive reactions in emulsions.on the basis of non-aqueous reveser emulsion system,Although PI microspheres in micrometer size with high monomer concentration has been prepared through non-aqueous emulsion polymerization,it is still a tremendous challenge for incorporating pores into each PI microspheres.The basic idea associated with the design of porous polymers underlies the introduction of a porogenic agent(i.e.solvent,gas,small molecule or macromolecule)within polymer structures,followed by its selective removal.Combined with the characteristics of preparation of PI microspheres by non-aqueous emulsion,this paper will explore the mechanism of the pore-forming process,and provide a theoretical basis for the preparation of porous PI microspheres..Relared works are as follows:(1)forming pores by volatilization of porogenOrganic solvent(porogent)with low boiling point was added into a typical non-aqueous emulsion system.The introduction of a porogenic agent was complexed through hydrogen bonding to poly(amic acid)structure,followed by its selective removal through volatilization,which resulted from the heat generated by the cyclization reaction during chemical imidization process.The results of IR,TGA and SEM measurement indicated that hydrogen bonding interaction existing between polyamic acid and ethyl ethe played a critical role in the pore formation process and maintaining the consistency of Curing rate and the evaporation rate had an major effect on pore morphology.The best porogenic conditions:the temperature of system was 20℃,the monomer concentration was 10%,drop speed of Pyridine/acetic anhydride was 0.5s/drop.The specific surface of the porous microspheres was increased with increasing the ratio of ethyl ether/Liquid paraffin.The average particle diameter was about 20～30μm.(2)forming pores by thermolysis of mono-amino polyetherBlock copolymerization of mono-amino polyether oligomers and poly(amic-acid)(PAA)derived from Pyromellitic dianhydride(PMDA)and 4,4-Dia-minodiphenyl ether(ODA)was carried out in a typical non-aqueous emulsion system.Microspheres of triblock copolymers consisting of thermally stable PI backbone and thermally labile block polyether were precipitated through the chemical immidization using acetic anhydride/pyridine mixture and then heated to effect solvent removal and thermal imidization,resulting in microphase separation of the two dissimilar blocks.The results of IR,TGA and SEM measurement showed that the optimal thermolysis temperature of polyether block is 325 ℃,upon a thermal treatment the polyether block undergoes oxidative decomposition,leaving nanopores in PI microspheres,with the content of the porogen increases,the morphology of dispersed phase translated gradually from spherical into wormlike.The specific surface of the porous microspheres was increased with increasing the ratio of porogent.The average particle pore diameter was about 2～6nm.