Structural characterization of amphiphilic homopolymer micelles using light scattering, SANS, and Cryo-TEM

Patterson, Joseph P. and Kelley, Elizabeth G. and Murphy, Ryan P. and Moughton, Adam O. and Robin, Mathew P. and Lu, Annhelen and Colombani, Olivier and Chassenieux, Christophe and Cheung, David and Sullivan, Millicent O. and Epps, Thomas H. and O’Reilly, Rachel K. (2013) Structural characterization of amphiphilic homopolymer micelles using light scattering, SANS, and Cryo-TEM. Macromolecules, 46 (15). 6319–6325. ISSN 0024-9297

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Abstract

We report the aqueous solution self-assembly of a series of poly(N-isopropylacrylamide) (PNIPAM) polymers end-functionalized with a hydrophobic sulfur–carbon–sulfur (SCS) pincer ligand. Although the hydrophobic ligand accounted for <5 wt % of the overall homopolymer mass, the polymers self-assembled into well-defined spherical micelles in aqueous solution, and these micelles are potential precursors to solution-assembled nanoreactors for small molecule catalysis applications. The micelle structural details were investigated using light scattering, cryogenic transmission electron microscopy (cryo-TEM), and small angle neutron scattering (SANS). Radial density profiles extracted from the cryo-TEM micrographs suggested that the PNIPAM chains formed a diffuse corona with a radially decreasing corona density profile and provided valuable a priori information about the micelle structure for SANS data modeling. SANS analysis indicated a similar profile in which the corona surrounded a small hydrophobic core containing the pincer ligand. The similarity between the SANS and cryo-TEM results demonstrated that detailed information about the micelle density profile can be obtained directly from cryo-TEM and highlighted the complementary use of scattering and cryo-TEM in the structural characterization of solution assemblies, such as the SCS pincer-functionalized homopolymers described here.