(163a) RAFT Polymerization of Emulsified Microemulsions

Internally-structured polymer nanoparticles are desirable for many applications including nanoreactors, bioelectrodes, sensors, and drug delivery vehicles because of their high interfacial area, their ability to encapsulate both hydrophilic and hydrophobic components, and the possibility of functionalizing the particle surface.  Larsson et al. were the first to propose dispersing the viscous hexagonal and cubic lyotropic liquid crystal microstructures formed by lipids in aqueous solutions to maintain the advantages provided by the high interfacial area of these microstructures while improving the processibility.[1-4]  Subsequently, Glatter et al. used small angle x-ray scattering (SAXS) and cryogenic transmission electron microscopy (cryoTEM) to investigate the internal microstructure and thermodynamic stability of dispersed liquid crystals and emulsified microemulsions.[5]  These studies showed that the microstructures within the dispersed drops correspond to the microstructures formed in bulk at the same temperature and composition, and that the microstructural transitions were thermally reversible.

Our previous studies of the polymerization of emulsified inverse lauryl acrylate (LA)/bis(2-ethylhexyl) sulfosuccinate sodium salt (AOT)/water microemulsions stabilized by Pluronic F127 have shown that the polymerization kinetics and polymer properties depend on the concentration of the inverse microemulsion in the aqueous dispersion, and the composition of the inverse microemulsion.  Our current studies aim to control not only the internal structure of the polymer particles, but the molecular weight and polydispersity of the polymers in the particles.  Molecular weight control has been implemented by using the controlled polymerization method of reversible addition-fragmentation chain transfer (RAFT).  The polymerization kinetics and particle size have been measured by reaction calorimetry, and dynamic light scattering, respectively.  RAFT chain transfer agent (CTA) concentrations ranging from 0.002 to 0.090 mol chain transfer agent/mol monomer have been investigated.  All of the polymerizations reach approximately 70% monomer conversion, however the necessary polymerization time increases significantly from 20 minutes with 0.002 mol CTA/mol monomer to 60 minutes with 0.090 mol CTA/mol monomer.  The uncontrolled emulsified microemulsion polymerizations produced particles approximately 35 nm in diameter.  In contrast, these RAFT emulsified microemulsion polymerizations produced polydisperse polymer particles with an average diameter of approximately 200 nm.  The gel permeation chromatography data showed that the number average molecular weight corresponded to predicted values at CTA concentrations above 0.04 mol CTA/mol LA.  The polydispersity of the particle diameter did not demonstrate a clear trend. This can be attributed to the initial distribution of CTA in the microemulsion drops and the diffusion of CTA between propagating particles and uninitiated drops.

1.   Larsson, K. Cubic lipid-water phases - structures and biomembrane aspects, Journal of Physical Chemistry 1989, 93, (21), 7304-7314.

2.   Larsson, K. Colloidal dispersions of ordered lipid-water phases, Journal of Dispersion Science and Technology 1999, 20, (1-2), 27-34.

3.   Larsson, K. Aqueous dispersions of cubic lipid-water phases, Current Opinion in Colloid & Interface Science 2000, 5, (1-2), 64-69.

4.   Larsson, K. In Bicontinuous cubic lipid-water particles and cubosomal dispersions, Meeting on Mesoporous Crystals and Related Nano-Structures Materials, Stockholm, SWEDEN, Jun 01-05, 2004; Terasaki, O., Ed. Stockholm, SWEDEN, 2004; pp 41-51.

5.   Yaghmur, A.; de Campo, L.; Sagalowicz, L.; Leser, M. E.; Glatter, O. Emulsified microemulsions and oil-containing liquid crystalline phases, Langmuir 2005, 21, (2), 569-577.