Block copolymer micelles

Typical block copolymers are linear macromolecules that are composed from two joint dissimilar blocks. When dissolved in solvents that are good for one block and poor for the other, they self-assemble into uniform spherical micelles where the insoluble blocks are segregated in the core and the soluble blocks form a shell that keeps the micelles in solution. These micelles are similar to the well-known detergent micelles but are much larger and much more stable. We are especially interested in copolymers having one hydrophobic and one hydrophilic block that form micelles in aqueous media. Using living anionic polymerization we have prepared block copolymers of styrene and methacrylic acid. The shell of the micelles they form in buffered solutions is a polyelectrolyte; that makes them very stable. We are studying both the fundamental aspects of these fascinating materials and the ways they can be utilized for practical purposes.

In the more basic studies we are using numerous techniques of polymer physical chemistry for characterizing the micelles: static and quasi-elastic light scattering, sedimentation in the ultracentrifuge, viscometry, gel permeation chromatography. We have developed techniques for following the rate of exchange of individual copolymer molecules among different micelles. Practically, our micelles are a promising vehicle for controlled release of drugs and other active agents, for scavenging pollutants, for formulation of coatings, etc. We are loading the micelles with fluorescent probes, measuring the rate of release, and comparing the results with data obtained by computer simulation of the diffusion within the micelles under various conditions.

Thermodynamic interactions in mixtures

For the study of thermodynamic interactions in mixtures we have developed a powerful technique: inverse gas chromatography (IGC). In IGC a polymer is used as a stationary phase in an usual GC experiment and individual solvent probes are injected and their elution profiles analyzed. This analysis yields polymer-solvent interaction coefficients with a high accuracy. We have obtained thousands of these coefficient and used them for a development of a general model of intermolecular interaction: the surface of every molecular species is ascribed characteristic values for dispersive, polar, electron donor, and electron acceptor interaction. These values (components of solubility parameters) were obtained by regression of highly redundant experimental data. Interaction between two substances is then obtained by combining their solubility parameters. We are extending this evaluation to cover a broad range of temperatures and also for pairs of substances, mixtures of which were not measured experimentally, for example, for mixtures of two solvents or mixtures of two polymers. The next goal is the interaction on surfaces; IGC is also eminently suitable for this purpose.