Precipitation polymerization of divinylbenzene to monodisperse microspheres: An investigation of the particle formation mechanism

Abstract

<p>This thesis addresses how the interactions between the reaction solvent and the forming polymer affects the morphology of the polymer formed in crosslinking polymerizations. Specifically, the mechanisms of nucleation, stabilization, growth and desolvation have been studied in the precipitation polymerization of divinylbenzene to monodisperse microspheres. These polymer beads have diameters in the micrometer range and may prove usefully for applications such as the separation of chemical mixtures using porous microspheres. The nucleation mechanism was found to proceed through transient microgels that are produced both by intermolecular reactions between oligomers and by further propagation. These microgels are transient and disappear from the reaction mixture at the same time as microspheres appear. This suggests that the microgels are the nuclei that grow to become the final microspheres. Internal curing reactions throughout the course of the polymerization transform the soft and deformable microspheres observed early in the reaction, into the rigid microspheres observed at the end of the reaction. As a result of both the nucleation and growth processes and the solvency of the polymerization medium, the microspheres have a gel surface that is formed in situ. When swollen with good solvents this surface gel serves as a steric stabilizer and when collapsed the microspheres flocculate together. Using a seeded polymerization method, the microspheres were shown to grow by a reactive growth mechanism. Oligomers were captured continuously from solution by radical reactions with existing microspheres. In contrast, microspheres having inert alkyl groups on their surface did not grow. Instead secondary or continuous nucleation was observed to produce new, smaller microspheres with broad size distributions. The microspheres observed in precipitation polymerization were then related to the morphologies more commonly observed in crosslinking polymerizations: microgels, macrogels, and coagulum. The volume occupied by the polymer was observed to decrease both with decreasing solvency of the continuous phase and with increasing crosslinking monomer concentration. Crosslinking in near theta-solvents was determined to be responsible for contraction of the polymer network into dense microspheres. The contraction of the polymer network is likely progressive, supporting the presence of a lightly crosslinked corona around the microspheres that acts as a steric stabilizer.</p>