(381c) Understanding Dispersion Polymerizations In Open and Confined Reaction Spaces towards New Applications

Emdadi, L., U.S. Army Research Laboratory
Lee, S. Y., University of Maryland

Dispersion polymerization is a well-known technique to produce monodisperse polymer particles of 1-15 microns. It consists of preparing a solution of a monomer, a solvent, a steric stabilizer and a chemical initiator. Since the polymer is poorly soluble in the solvent-enriched solution, growing polymer chains precipitate in the form of unstable primary particles which agglomerate to produce larger but stable polymer particles.

In general, dispersion polymerizations are carried out in open reaction spaces in the sense that no geometrical confinement is imposed for the polymer to precipitate. As a result, monodisperse polymer particles of several microns in size are generally produced. By varying the reaction conditions, however, other morphologies such as porous polymeric structures can also be developed after inducing the system phase inversion.

The use of confined reaction spaces to carry out dispersion polymerization is interesting because of the possibility of creating micro-particles with complex internal structures. Recently, our research group has developed a micro-dispersive suspension polymerization (MDSP) technique, where the dispersion polymerization takes place in the confined reaction space of a suspended monomer droplet of 10-150 microns in size. Although MDSPs share some characteristics with regular dispersion polymerizations in open spaces, some important difference imposed by the confinement has been identified: due to the presence of water in the surroundings of the confined reaction space, a compositional gradient is induced within the suspended droplets. By electrostatic attractions, the more polar species are forced to preferentially accumulate at the droplet/water interface region, whereas the nonpolar species accumulate at the core region of the droplet. For instance, using a slightly polar monomer such as methyl methacrylate (MMA) and a nonpolar solvent such as n-hexane, a pseudo-homogeneous polymerization is promoted at the droplet surface region to produce a dense polymeric structure or shell. In the core region, the solvent concentration is very high thus inducing the precipitation of polymer chains under “starved” conditions. Smaller polymer sub-particles of 300-500 nm in diameter are densely packed within a polymer capsule resulting in the pomegranate-like particle. Similarly to regular dispersion polymerizations, an intra-droplet phase inversion can also occur, thus generating multi-hollow micro-particles.

The applications of the polymer micro-particles from this study are still under investigation. Our research group has developed an in situ micro-encapsulation technique that uses pomegranate-like particles as a sacrifice template to produce ultra-porous silica structures characterized by a huge specific surface area, a wide-open and interconnected internal structure with a unique pore size distribution. More details will be presented at the meeting.