Micelle formation of randomly grafted copolymers in slightly selective solvents

Amphiphilic surfactants, molecules with chemical moieties that interact differently with the solvating medium, are important for technological applications and ubiquitous in biology. Understanding how to control surfactant properties is, therefore, of wide-ranging importance. Using a combination of light scattering experiments and field theory, we demonstrate that the behavior of polymeric surfactants can be controlled sensitively by manipulating molecular architecture. We find that branched polymeric amphiphiles can be much better surfactants than traditional linear analogs. This is indicated by micelle formation in solvents that are very slightly selective for the backbone of the branched molecule. Our experimental and theoretical findings also suggest that, for a given chemistry and architectural class, surfactant properties of polymeric amphiphiles are very sensitive to subtle changes in architectural features. Specifically, we find that choosing a particular branching density optimizes the propensity for micelle formation. The sensitivity of macromolecular surfactant properties to molecular architecture can perhaps be profitably exploited in applications wherein only certain chemical moieties are allowed. The physical origin of this sensitivity is the importance of conformational entropy penalties associated with the pertinent self-assembly process. This is in contrast to self-assembly of small molecule systems where conformational entropy is not of such significance.