Photophysical Studies Provide Thermodynamic Insights into Block-Copolymer Micelle Formation in a Selective Solvent

Abstract

A large number of theoretical [1–4] and experimental efforts [5–16] have been directed toward describing the micellization of block copolymers in solvents selective for one of the blocks. For appropriate conditions of concentration, temperature, and pressure, spherical micelles maybe formed from aggregates of approximately 100 block copolymer molecules in which the shell or corona is composed of the block having favorable enthalpic interactions with the solvent, and the core is composed of the block having unfavorable solvent interactions. In the present study, we are interested in two aspects of the micellization phenomenon: (i) the effect of solvent size on the equilibrium between the individual block copolymer chain (the unimer) and the micelle, as interpreted through the closed-association model, and (ii) the extent to which homopolymer chains of the core block material may be solubilized in the micelle core. Both of these issues involve thermodynamic questions that can be addressed by use of photophysical experimental techniques, the first taking advantage of intrinsic excimer fluorescence from the core block and the second incorporating a fluorescently labeled, free probe molecule that partitions between the micelle core and the surrounding solvent.