Block co-polymer micelles offer a way to solubilize hydrophobic components such as diagnostic agents or drug molecules for applications in aqueous environments. By varying the type of polymer, relative block lengths, or preparation conditions, a variety of morphologies can be accessed. Non-spherical micelles, such as worm-like cylindrical micelles, have attracted the attention of researchers because of their ability to encapsulate a greater amount of hydrophobic cargo per micelle as a result of their aspect ratio. Here, we have explored the interfacial instability process proposed by the Discher and Hayward (1, 2) groups to synthesize non-spherical micelles, including worms and higher order structures. In the interfacial instability process, an emulsion is generated by combining polymers and nanoparticles dissolved in a water-immiscible solvent with an aqueous emulsifier. The emulsion droplets experience transiently negative surface tension promoting progressive fission yielding smaller droplets. Simultaneously, solvent is evaporating. Micelles form when the critical micelle concentration is reached.
The effect of process conditions on the final structure of the wormlike micelles and their nanoparticle solubilizing capacity was examined. The resultant micelles exhibited a range of morphologies from fused spheres, smooth cylinders, branched cylinders, and cross-linked networks depending on the completion time for the interfacial instability process. Rate of emulsion evaporation was found to be the critical parameter, which can be tuned to obtain different structures at the same polymer concentration. The intensity of mixing during the emulsion generation step was found to be another important parameter in determining the predominant polymer phase in the aqueous solution. Future efforts will be directed towards elucidating the mechanistic pathways at play in some of these different structures.
Further, nanoparticles with different surface ligands were introduced to evaluate the encapsulation behaviour as a function of micelle formation rate. Nanoparticles clustered in the end-caps and high curvature regions along the cylinder length in some cases, whereas in others, they dispersed inside the wormlike micelle uniformly. This behaviour was dependent primarily on nanoparticle capping ligand. Future work will explore the role of nanoparticle composition: surface chemistry, size, or shape on dispersion characteristics inside the wormlike micelles.
Finally, we studied the growth of filamentous structures in aqueous micellar systems when stored at high final concentrations of the polymer. This highlighted the possibility of self-assembly to continue in solution even after the interfacial instability process is complete.
The interfacial instability process is a tuneable micellization technique which can effectively be applied to both crew cut as well as star-like micelle forming polymers. By examining the effect of different process variables and operating conditions, we hope to better understand the underlying self-assembly process and scale-up potential.
- Geng, Y. and Discher, D.E. (2005), “Hydrolytic degradation of poly(ethylene oxide)-block-polycaprolactone worm micelles”, J. AM. CHEM. SOC., September, pp. 12780–12781.
- Zhu,J. and Hayward, R. (2008), “Spontaneous Generation of Amphiphilic Block Copolymer Micelles with Multiple Morphologies through Interfacial Instabilities”, J. AM. CHEM. SOC., February, pp. 7496-7502.