Shape-Anisotropic Polyimide Particles by Solid-State Polycondensation of Monomer Salt Single Crystals

Abstract

Shape-anisotropic particles are of broad interest, e.g., for colloidal crystals or applications at interfaces such as particle-stabilized emulsions. Despite the wealth of accessible shapes of inorganic particles, anisotropic homopolymer particles are to date mostly limited to objects derived from spheres (e.g., ellipsoidal or disk-shaped particles). Here, we report the synthesis of shape-anisotropic, angular polyimide particles by thermal solid-state polycondensation (SSP) of monomer salts. We prepare monomer salt single crystals of relatively narrow size and shape distribution by growth inside hydrogels, and solve their crystal structure. Polyimide particles are obtained by simple heating and retain the shape of the initial salt crystals. Using high-temperature X-ray diffraction, thermal analyses and microscopy techniques, we investigate the mechanism of the transformation. The obtained polyimide particles are temperature-stable up to 640 °C and virtually insoluble in any solvent. This work sheds more light on the mechanism of SSP of monomer salts and reports a new methodology for accessing nonspherical homopolymer particles, which are due to their outstanding stability potentially of interest for applications under extreme conditions.