Fabrication of kinetically stable micropolymofoam particles and the spontaneous induction of morphological transformation

We report fabrication of low-density polymeric microparticles (micropolymofoam particles [μPFPs]) using the piezoelectric inkjet method. After ejecting a polymer solution prepared by dissolving polymers in a volatile organic solvent, microdroplets are periodically generated through an inkjet nozzle in the air. Rapid evaporation of the solvent as it falls through the air leads to a highly porous surface and internal morphology. Optical microscope observation confirms that the inside of the μPFPs is filled with air. Notably, the use of more than two different polymers results in kinetically stable particles, in which the heterogeneous polymer chains are randomly entangled. Shape transformation of the kinetically stable μPFPs occurs upon adding a swelling solvent, spontaneously changing their geometry from porous and rugged-shaped to a Janus-like structure with a clear phase boundary. This transformation is directly monitored using an optical microscope. The surface free energy calculation reveals that particles are transformed to adopt a thermodynamically stable structure; this is achieved by imparting fluidity to the randomly entangled polymer chains, which is energetically unfavorable. In addition, porosity control can be possible by selectively removing one polymer phase of kinetically stable particles comprising two different polymers. Furthermore, the developed inkjet method can be applied for producing composite particles encapsulated with functional nanoparticles and coating or patterning them directly onto a substrate.