(743d) 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 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 depend on the concentration of the inverse microemulsion in the aqueous dispersion and the composition of the inverse microemulsion. The compositions of the inverse microemulsions studied are α = 82 wt% and 91 wt%, and γ = 10 wt%, where α = mass_LA/(mass_LA + mass_water) and γ = mass_AOT/(mass_AOT + mass_LA + mass_water), and the polymerization temperature is 55 C. These inverse microemulsions were dispersed in a solution of water and Pluronic F127 for a final dispersion composition of 1 wt% Pluronic F127, and 5, 10 or 20 wt% LA/AOT/water microemulsion, with the remainder of the dispersion being water. Mixing was maintained throughout the polymerization at 500 rpm. All of the polymerizations reach at least 70% monomer conversion. Increasing the concentration of the inverse microemulsion from 5 wt% to 10 wt% increases the rate of polymerization. Further increasing the concentration of the inverse microemulsion to 20 wt% does not change the initial rate of polymerization, however a high rate of polymerization is maintained to a greater monomer conversion than the polymerizations with 10 wt% inverse microemulsion. Despite the observed kinetic differences, the particle diameter measured by dynamic light scattering is approximately 35 nm for polymerizations at all microemulsion concentrations. Transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM) provide direct visual evidence of the production of poly(lauryl acrylate) particles that are approximately 40 nm in diameter that contain numerous 5 nm diameter clusters of the sodium ions associated with the AOT used to stabilize the inverse microemulsion, suggesting that the internal inverse microemulsion structure may be maintained during the polymerization.

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.