(143b) Morphology and Molecular Weight Control of Core-Shell Polymer Nanoparticles

Colloidally stable core-shell polymer nanoparticles can be simply synthesized using microemulsion polymerization through the semi-continuous addition of a second monomer to form the shell around seed polymer particles. However, there are two main limitations of traditional semi-continuous microemulsion polymerization. The first disadvantage is the lack of control over the molecular weight and polydispersity. While the second limitation is the challenge in maintaining the location of the shell forming polymer, since the morphology of the core-shell polymer is determined by equilibrium thermodynamics that is dictated by the polymer chemistry and composition. For instance, if the added polymer thermodynamically favors to be in the core more than the first seed polymer, phase inversion occurs between the core and shell polymers. In this work, we aim to produce core-shell polymer nanoparticles by employing a controlled polymerization technique, i.e. reversible addition-fragmentation chain transfer (RAFT), in semi-continuous microemulsion polymerization which enables us to obtain predefined molecular weight polymers and architectures that are inaccessible by current synthetic techniques.

RAFT is a successfully implemented method for controlling homogenous polymerization for producing polymers with low polydispersity and predefined molecular weight. The RAFT technique relies on a chain transfer agent (CTA) which controls the propagation of the polymer chains. Performing RAFT in heterogeneous microemulsion polymerization has been proven to facilitate the synthesis of latex nanoparticles with predetermined molecular weight and low polydispersity. In recent work, we have investigated enhancing the polymerization control in RAFT microemulsion polymerization by fixing the location of the CTA at the surface of the polymer particle through attaching a surface-active moiety to the CTA structure. Here, we extend our work to investigate the use of surface-active CTA for the synthesis of core-shell polymer nanoparticles. There are several advantages for using surface-active CTA in RAFT semi-continuous microemulsion polymerization:

(1)   Block copolymer shell formation:The polymer propagation is constrained to the corona of the particle, which promotes the formation of a block co-polymer shell with further second monomer addition.

(2)   Prevention of phase inversion:The covalent attachment of the surface-active CTA to the end of the polymer introduces an energetic penalty for transferring the shell polymer to the core of the particle, and thus diminishes the driving force for phase inversion with the right concentration of surface-active CTA per particle.

(3)   Surface functionality:The surface-active CTA permits further surface functionalization by simple chemical modification of the CTA.

 In this work we investigate the core-shell synthesis of n- butyl acrylate (BA) and styrene (St) using RAFT microemulsion with surface-active chain transfer agent, 2-((11-(((benzylthio)carbonothioyl)thio)undecanoyl)oxy)-N,N,N-trimethylammonium iodide (B11T). In the literature, it has been shown experimentally and theoretically that at 50:50 St to BA ratio, St favors to form the shell while BA favors to be in the core, irrespective of monomer addition sequence. We hypothesize that, with enough B11T chain transfer agents, phase inversion can be prevented during St/BA semi-continuous microemulsion polymerization.

The number average molecular weight, polydispersity, and rate of polymerization are analyzed during the semi-continuous microemulsion polymerization to assess polymerization control and kinetics, while the final morphology of the polymer particles will be directly visualized by transmission electron microscopy. The results will be compared against core-shell synthesis using traditional semi-continuous microemulsion polymerization, and shall provide an understanding of the requirement to prevent phase inversion.