Fabrication of Narrowly Dispersed Polyimide Microspheres via Poly(tetraalkylammonium amic acidate) Microspheres

Polyimide (PI) microspheres have excellent thermal and mechanical performances and find intensive applications in many fields. However, fabrication of PI microspheres with regular morphology and a narrow particle size distribution is still a big challenge. In this study, a facile method to fabricate narrowly dispersed PI microspheres is developed and investigated. Poly(amic acid) (PAA) is first synthesized from 4,4′-oxidianiline (ODA) and benzophenone-3,3′,4,4′-tetracarboxylic dianhydride (BTDA). Poly(tetraalkylammonium amic acidate) (PAAs), which is synthesized via reaction between tetraalkylammonium hydroxide and PAA, microspheres with regular morphology and narrow particle size distribution are fabricated by reverse phase induced precipitation of PAAs in water. Finally, narrowly dispersed PI microspheres are obtained by the thermal imidization of PAAs microspheres. PAAs and PI microspheres are analyzed and characterized by scanning electron microscope, dynamic light scattering laser particle size meter, Fourier transform infrared, differential scanning calorimetry, and thermogravimetric analysis. The results show that the water solubility of PAA can be enhanced by organic amines, which is conducive to the fabrication of narrowly dispersed PAA microspheres with regular morphology. Narrowly dispersed poly(tetraethylammonium amic acidate) (PAAs-TEAOH) microspheres with an average particle size of 569.9 nm and a PDI of 0.096 are obtained when 15 mol % carboxyl groups in PAA are neutralized by tetraethylammonium hydroxide (TEAOH). The organic amine can lower the thermal imidization temperatures of the PAAs microspheres. PI microspheres with a particle size of 420.0 nm are obtained through thermal imidization of PAAs-TEAOH microspheres. PI microspheres fabricated from PAAs microspheres show regular morphology, uniform particle size distribution, and excellent thermal properties, which have glass transition temperature higher than 275 °C. This study demonstrates that fabrication of narrowly dispersed PI microspheres via thermal imidization of PAAs microspheres is a simple and effective method, which will facilitate the application of PI microspheres in many fields.