Small-Angle Neutron Scattering Characterization of Micelles Formed by Poly(dimethylsiloxane)-graft-polyether Copolymers in Mixed Polar Solvents

Abstract

The self-assembled structure of an amphiphilic poly(dimethylsiloxane)-graft-polyether copolymer has been investigated in binary solvents consisting of water and one of the following polar organic solvents: 2-propanol, ethanol, formamide, or glycerol. Micelle structural parameters were obtained by fitting the core−corona form factor and hard-sphere interaction structure factor to small-angle neutron scattering (SANS) data obtained at contrast conditions where the solvents were fully deuterated and the copolymer was not. The siloxane copolymer forms in water at 20 °C micelles that consist of a “dry” core with a radius of about 6 nm and a solvated (about 40% polymer) corona with a thickness of 2 nm; the relatively thin corona may be due to the comb-type architecture of the siloxane copolymer. The micelle radii, micelle core radii, micelle association numbers, and polymer volume fractions in the micelle core and corona decreased with increasing cosolvent/water ratio in the mixed solvents containing 2-propanol, ethanol, or formamide. On the contrary, the addition of glycerol led to an increase of the micelle association number. The above findings are discussed in terms of the relative location of the polar organic solvents in the interior or exterior of the siloxane copolymer micelles. In particular, the micelle association numbers observed for different water−cosolvent mixtures can be correlated to the octanol−water partition coefficients of the respective cosolvents. A transition from spherical to ellipsoid micelles was observed upon an increase in the temperature: this is attributed to the desolvation of the polyether chains. This structural transition correlates with the solvent quality in the mixed solvents: ethanol and 2-propanol, which solvate the interior of the micelles, tending to keep the micelles spherical, while increased glycerol content in the mixed solvent leads to the formation of ellipsoids at a fixed temperature.