Modeling of Morphological Development in Polyimide Microspheres Obtained from Powdered Precursors

The synthesis of high performance polyimide foams by the solid-state powder foaming of poly(amic acid) powders, containing hydrogen bonded blowing agent (THF), is described. During the thermal inflation of poly(amic acid) particles into polyimide microspheres, their fusion has been shown to produce the desired cellular architecture. In the present work, the solid state powder foaming process is modeled by studying the concurrent phenomena such as transport processes and polymerization reactions in a single particle of spherical geometry and imposing the appropriate initial and boundary conditions. The simulation of the inflation process spans from poly(amic acid) particles to polyimide microstructures and the rheological and diffusional properties of the evolving polymer are related to its molecular changes (increase in molecular weight and cyclization) and loss of dissolved blowing agent. Morphological aspects of the initial precursor particles, as well as, plastization effects by the reaction solvents are found to be determinant parameters in the inflation process and determine the ultimate size of the resulting polyimide microspheres.