Preparation Of Porous Microspheres Of Polyimides By Nonaqueous Emulsions

Porous polymer microspheres have been widely used in many areas such as filling in chromatography, biomedical, cosmetics and other fields, because of their special size effect and interface effect which is determined by their controlled pore structure. But researches on porous polymer microspheres are mainly about macromolecules which were prepared by the radical polymerization, and their low thermal stability limits the applications at high temperature, for example as catalyst support used in long-term high temperature conditions. Aromatic polyimide is by far one of the the highest thermal stability polymers, and it possess good low temperature resistance, high mechanical strength and excellent dielectric properties etc. Porous polyimide microsphere has the advantages both of polyimide and porous polymer microspheres, so it have broad application prospects. There are few reports in the literature on preparation of porous polyimide, and the existing methods for preparation of porous polyimide have kinds of problems such as poor reproducibility, uncontrollable of particle size and pore shape and so on. Emulsion polymerization is the most common technique for preparation of polymer microsphere. However, the water present results in hydrolysis of the components and additionally shifts the equilibrium of the esterification toward the diacid and dihydroxy compound, so high molecular weight polymer unable to be obtained. In this paper, we introduce the means of emulsion polymerization to the reaction of polycondensation by novel non-aqueous emulsion method, to avoid adhesion of microspheres at higher solid content and obtain a good dispersion of single polyimide microspheres. The main contents are as follows:(1) Novel non-aqueous emulsions were formulated by using N, N-dimethylformamide as the disperse phase and liquid paraffin as the continuous phase, and three kinds of non-ionic surfactant was selected as the emulsifier. The emulsion of poly(amic acid)s was prepared as condensation reaction was carried out by adding two monomers into the nonaqueous system, and polyimide particles with different morphologies were fabricated after imidization. When the Span85 was selected as emulsifier polyimide microspheres were obtained, and the process of mircosphere formation was studied. (2) The polyimide particle size was controlled by changing the volume ratio of liquid paraffin and N, N-dimethylformamide or changing the amount of Span85.(3) Certain proportion of Tween85 or block copolymer Pluronic (F127) were added into the nonaqueous emulsion to cooperated with Span85 to improve the stability of non-water emulsion. And the stability of emulision could be improved by adding F127 to the nonaqueous system.(4) Inorganic matter or oligomer were added ino the poly(amic acid)s as porogen. And the porous polyimide microspheres were fabricated though separating of the porogen from polyimide microspheres by washing or heating. Mechanium for formation of porous polyimide particles was briefly discussed.