Transforming micron-sized cross-link PS microspheres from isotropy to anisotropy with the assistance of supercritical carbon dioxide and sonication

Using “two-stage” dispersion polymerization method polystyrene (PS) microspheres with “hard shell and soft core” are synthesized. Through treating these cross-link particles with supercritical carbon dioxide (SC CO2), these microspheres were welded together. After ultrasonic treatment, these welded microspheres were successfully separated and the submicron-sized pits were engraved on the surfaces of these particles, which finally realized the transformation of these particles from isotropy to anisotropy. The transformation degree of these microspheres (size and the depth of the pits on the microspheres) could be adjusted by changing the temperature and pressure of SC CO2. The SEM results illustrated that the anisotropic degree of the microspheres enhanced with increasing the pressure of SC CO2 in certain range and then decreased. Elevating the temperature at the same pressure could greatly enhance the anisotropic degree of these microspheres. Slight changing the cross-link density of the shell could also have obvious effect on the transformation degree of the PS particles.