Formation of TiO2–polymer composite microparticles by rapid expansion of CO2 saturated polymer suspensions with high shear mixing

A method is reported for the preparation of highly loaded TiO2–polymer composite particles by rapid expansion of carbon dioxide (CO2) saturated polymer suspensions with high shear mixing. Suspensions of CO2 saturated polymer solution and TiO2 nanoparticles were agitated in high-pressure vessel equipped with a column agitator and mechanical seals. The rotation speed and peripheral velocity were set at 5000 rpm and 40.8 m/s, respectively. The polymers were poly(lactic acid) (PLA; M.W. = 10,000) and poly(styrene)-b-(poly(methyl methacrylate)-co-poly(glycidyl methacrylate)) copolymer (PS-b-PMMA-co-PGMA, M.W. = 5000). After agitation and rapid expansion of the CO2 saturated polymer suspensions to an aqueous phase, polymer microspheres were obtained and very small on the order of only micrometers. The structure and morphology of the microspheres were investigated by SEM, TEM, XRD and turbidity measurement. The polymer microspheres obtained were highly loaded TiO2–polymer composites. According to TEM photograph of the cross section of polymer particle, loading ratio of TiO2 in polymer particles is 52%. The TiO2 content in the polymer particles was increased by an increase in the rotational speed of the agitator, and consequently the shear stress in the high-pressure vessel. The shear stress between the column agitator and the inside wall accelerates the dispersion of TiO2 nanoparticles in CO2 saturated polymer suspensions. On the other hand, agglomerated TiO2 nanoparticles were observed on the surface of polymer microparticles produced by rapid expansion of mixtures with conventional mixing, and the loading ratio of TiO2 in the polymer matrix was very low. The loading ratio of TiO2 in the polymer particle was very low and less than 1%.