Preparation And Thermal Properties Of Porous Poiyiniide Microspheres

The porous polyimide(PI) materials, combining both unique propertiessuch as good thermal, excellent mechanical properties of polyimide and theadvantages of porous structure such as low density, large specific surface, andlow-k, have significant business value and been widely used in aerospaceindustry, electronics industry, gas separation, insulating material, and so on.Currently, porous polyimide films have been fascinated many researchers, butthere are only a few studies concern on synthesis porous polyimidemicrospheres. Compared with porous polyimide films, porous polyimidemicrospheres possess larger specific surface and lower density and thereforehave greater load capacity, more wear-resistant, faster diffusion velocity, specialsize and interfacial effect. Therefore, the developments of porous polyimidemicrospheres as catalyst carriers, adsorption and separation materials and GCpacking columns, are very promising in application.Polyimide is generally formed by chemical or thermal cyclization ofpolyamic acid (PAA), which is obtained by the polymerization of dianhydrideand diamine monomers in polar aprotic solvent. Since the polymerizationprocess must be isolated from the aqueous phase, PI microsphere cannot beprepared by conventional emulsion polymerization and suspensionpolymerization. Until now, the study reports of porous PI is limited. the mainmethods to prepare PI microspheres include precipitation, reprecipitation anddispersion polymerization, and porous PI microspheres are mainly obtained byreprecipitation method in which oligomers or small molecules are used as porogens. Unfortunately, the methods above usually lead to the adhesion of PImicrospheres, induce pores being present at the surface of the microspheres, andit seems difficult to regulate the particle sizes and distributions. Therefore, it isof great importance to prepare porous PI microsphere with high solid content,controllable particle size and pore structure.Therefore, we propose a novel non-aqueous emulsion system, in which PImicrospheres are prepared by the thermal imidization of PAA, and the PAA isprepared through the polycondensation of aromatic diamine and dianhydridemonomers. According to this method, the presence of aqueous phase can beavoided, the particle size and distribution can be controlled. The adhesion of PImicrospheres can be solved since the dispersed PAA has been imidized duringthe microsphere formation. The porous PI microspheres with high thermalstability were then obtained using physical method (small molecules asporogens), chemical method (grafted oligomer thermal decomposition) andphysical-chemical method (nano-silica as template), sperately.The main conclusions are summaried as follows:(1) The optimization of recipe for non-aqueous emulsions. Thenon-aqueous emulsions were prepared using N’N-dimethylformamide as thedispersed phase, liquid paraffin as the continuous phase, and nonionicsurfactants Span85and Tween80as the stabilizing agents. The non-aqueousemulsion is most stable when the mass ratio of Span85and Tween80is4:1, thesurfactant amount is no less than10wt%and the ratio of DMF/LP is1:4.(2) Preparation of porous PI in the optimized non-aqueous emulsion withstabilizing agents as Span85/Tween80/DMF/LP. PI microspheres with30%solidcontent were prepared by the thermal imidization of PAA, which is preparedthrough the polycondensation of aromatic diamine and dianhydride monomers.The obtained PI microspheres have fine dispersion and controlled size. Thedegree of chemical imidization can be increased up to60%when the molar ratioof acetic anhydride and dianhydride is5:1. The thermal decomposition temperature T5%of PI microspheres is521℃, and the glass transitiontemperature Tgis320℃, indicating that the obtained PI microspheres have highthermal stability.(3) The size and morphology of PI microspheres can be modified by varingthe amount of surfactant, the structure of diamine monomer, solid content andimidization process. The morphology of microspheres is much better by usingdiamine (ODA) with a flexible segment as monomer. With increasing solidcontents, the particle diameter of the microspheres increase. When the solidcontent increases to40%, the stick-linking of microspheres formed. Themicrosphere size decreases gradually to about20μm with increasing surfactantcontents. During the transformation process from PAA to PI, completeimidization needs chemical and thermal imidization. The chemical imidizationreached the greated degree when the molar ratio of acetic anhydride/dianhydride(PMDA) is5:1. As the temperature reaches300℃, imidization is substantiallycomplete.(4) The porous PI microspheres were prepared using small molecules likeCS2, CH3OH, C3H6O, and LiCl as porogens. It was found that porous PImicrospheres with fine microporous structure and morphology can be obtainedonly by using CH3OH as porogen, due to the formation of hydrogen bonds withPAA. Although the porogens as LiCl et al. the obtained porous PI microsphereswere mostly irregularly shaped fragments because of the broken equilibrium ofnon-aqueous emulsion.(5) When methanol is used as the porogen, the temperature of the emulsion,the monomer concentration and pyridine/acetic anhydride dropping rate willinfluence the pore structures. It is beneficial to the formation of pores when theevaporation rate of methanol is consistent with the curing rate of microspheres.The final condition suitable for the formation of pores is as follows: temperature:40℃; monomer concentration:10%; dropping rate of Pyridine/acetic anhydride: 0.5s/drop. The obtained PI microspheres have high thermal stability with thethermal decomposition temperature T5%of517℃. Since the hydrogen bondbetween methanol and polyamic acid molecule is weak, most of the pores areclose resulting in the relatively low specific surface area, thus it can be makesused as low dielectric materials.(6) In the non-aqueous emulsion, porous PI microspheres can also beprepared by using grafted oligomer thermal decomposition. Compared withsmall molecule porogens, oligomer is chemically bonded with PAA, inducingthe obtained PI microspheres having cylindrical and communicating mesoporousstructures. In addition, the T5%of PI microspheres can reach about450℃,indicating the obtained PI microspheres can be used as catalyst supports atrelatively high temperature.(7) The microporous structures and morphologies of porous PImicrospheres can be impacted by the molecular weight of grafted oligomers aswell as thermal decomposition temperature and pressure. The pore structure ofPI microspheres is fractured and uneven using PPG1000as porogen, whilecylindrical pores dispersed uniformly inside and outside the PI microsphereswere obtained using PPG2500as porogen. Cylindrical pores with uniform sizedistribution of25~40nm is favorable to be formed when the PPG content isincreased. When the thermal decomposition temperature is slightly above theglass transition temperature, pores can be effectively formed.(8) The porous PI microspheres can be obtained throughsitu-polymerization using nano-silica (SiO2) sol as template in non-aqueousemulsion. When SiO2is used as porogen, the dispersion of SiO2in the PI matrixand the compatibility between the two phases are enhanced. Macroporousmicrospheres can be obtained since the silica matrix and PI are physically andchemically combined together. The obtained PI microspheres can be used asseparation and adsorption media due to their high stability. (9) The microporous structures can be affected by the size and content ofSiO2. Porous PI microspheres with fine microporous structure and morphologycan be obtained when the size of SiO2is100nm. The pores with sizes of about70~120nm were observed to be connected with each other. When the particlediameter of SiO2is20nm, the pores mostly exist on the surface with closed-cellstructure. The numbers and sizes of pores are increased with the increasing theamount of SiO2. In addition, the presence of water is beneficial to the formationof large and open pores.