Preparation of submicrometre polyimide particles by precipitation from solution

Abstract

The formation of submicrometre particles of high temperature polyimides by precipitation from a homogeneous solution in a mixed solvent of water and N-methyl pyrrolidone (H 2O/NMP) was investigated. The two polyimides consisted of those based on benzophenone tetracarboxylic dianhydride/2,2′-bis(4-aminophenyl)propane (BTDA-Bis A) and pyromellitic dianhydride/1,1-bis(4-aminophenyl)-1-phenyl-2,2,2-trifluoroethane (PMDA-3F diamine). Phase diagrams of the BTDA-Bis A polyimide/NMP/H 2O ternary system were obtained for two solvent compositions through cloud point measurements. The observed mixed solvent composition dependence of the cloud point behaviour suggests that the water weight fraction plays a fundamental role in the solubility characteristics of polyimides. Preliminary precipitation studies indicate that the size distribution of the polyimide particles is governed by the cooling rate below the cloud point curve. At an average cooling rate of 23°C min −1, narrow size distribution particles of 0.2 and 0.5 μm average diameter were obtained for the PMDA-3F diamine and the BDTA-Bis A polyimides, respectively. For slower cooling rates of 10 or 2°C min −1, the particle size distributions were notably broader, probably as a result of nucleation and growth over a larger temperature range.