PARTICLE FORMATION AND GROWTH FOR SMALL MOLECULES AND POLYMERS BY NANOPRECIPITATION
The nanoprecipitation method is a simple and convenient way to produce nanoparticles from small molecules and polymers in solution. The underlying mechanism, which plays a pivotal role in the control of nanoparticle size and morphology in application, is not fully understood but is of great interest to researchers. My research is focused on organic nanoparticles in an aqueous medium prepared by nanoprecipitation, but the particle formation and growth for calcium carbonate inorganic nanoparticles is also studied. I presented studies of the formation of calcium carbonate prenucleation clusters in undersaturated solutions, which confirms the existence of initial clusters before the solubility limit is reached and suggests an alternative pathway for particle formation. In addition, I investigated the kinetics of nanoprecipitation by studying the mixing process with an imaging system, as well as the conditions for achieving a rapid and complete mixing for both small molecules (anthracene) and polymers (polystyrene) using an automated syringe pump system. The results suggest that the mixing process is critical in nanoprecipitation and can lead to distinct results in the particle formation process. I also studied the particle formation and growth at early stages immediately after mixing for both anthracene and polystyrene, using optical imaging system and static light scattering (SLS) as the characterization methods. Furthermore, I investigated the thermodynamics during phase transitions in nanoprecipitation by studying the phase diagrams and the effect of different initial conditions on the particle size and phase separation, such as the initial solute concentration, solvent/non-solvent ratio and the molecular weight of the samples for both small molecules and polymers. The results shed light on the possible mechanisms of particle formation and phase separation in nanoprecipitation, which can be nucleation and growth or spinodal decomposition, depending on the state that the system reaches after quenching. By comparing the result for anthracene and polystyrene, the difference in mechanisms for particle formation between small molecules and polymers is clarified. In contrast to small molecules, the glass transition of polymers is found to be a key factor that contributes to the phase separation and the resulting particle size and morphology of polymer nanoparticles in nanoprecipitation.
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