Operation Optimization Of Polyimide Microspheres' Preparation And Pore-making Of Small Molecules

Macromolecule porous microsphere has a rapid development in preparative technique and pore forming mechanism owing to its excellent properties such as low density, easy functionalization etc., and has been applied to cosmetics, drug release, coating and so forth. So far, polyimide (PI) is one of the highest thermal stability polymers and has outstanding combination of properties such as good low temperature resistance, higher mechanical strength, and excellent dielectric properties. Therefore, more and more attention has been attracted to prepare porous PI microspheres with high thermal stability.In our previous study on the preparation of PI microspheres, a novel method was put forward named non-aqueous emulsion technique, in which polymer concentration can be largely increased and the product had a good morphology. Furthermore, this approach was convenient, easy to control and had a better reproducibility in the milder conditions. On this basis, two parts in this paper are as following:(1) operation optimization of emulsion system and polymer curing, further increase of polymer concentrationPI microspheres with higher polymer concentration about 35% can be prepared using non-aqueous emulsion system. Surfactant Span85 makes not only non-aqueous reverse emulsion stabilized but also PAA concentration increased. When concentration of surfactant Span85 was changed, the morphology, average diameter and particle size distribution of product varied. With the increase of Span85 concentration, the average diameter diminished and the size distribution became narrow. The suitable concentration of Span85 was obtained about 15%.In polymer curing process, the chemical imidization degree of poly (amic acid) (PAA) was up to the maximum about 60%, and the curing of particles' surface was complete when the ratio between acetic anhydride and dianhydride was 5:1 in mole. In order to make sure the complete conversion of PAA to PI, the further thermal imidization was made piecewise under 350℃with 1h dwell-time.(2) preparation of porous PI microspheres with liquid small molecules and inorganic agents as porogensTaking advantage of the good compatibility between solvent and polymer, several liquid small molecular porogens, with content below 30%, were added in polymerization process to get better results. While, ethanol and methanol with content below 25%, utilizing hydrogen bond, were added after polymerization reaction for 30min to get better results. When the ratio between triethylamine and dianhydride was 2:1 in mole, triethylamine was added after polymerization for 75min to get more pores.Solid inorganic agents as porogens were poured into emulsion system in solution form. With content below 25%, AlCl3 was added for 45min to get porous microspheres; with content below 30%, LiCl和LiBr was added for 30min to obtain fine microspheres, acquiring more pores.