Structure and composition of mixed micelles of polymerized and monomeric surfactants.

Micelle structure and composition has been determined by small-angle neutron scattering for mixed micellar solutions of in situ polymerized ω-methacryloyloxyundecyl-trimethylammonium bromide (MUTAB) in equilibrium with its monomeric form at various concentrations, as well as in mixtures with a fluorinated cationic surfactant, heptadecafluorodecylpyridinium chloride (HFDePC), and the non-ionic surfactant, C12E7. Whereas polymerized MUTAB is immiscible with HFDePC and forms two populations of distinct spheroidal micelles, it mixes with C12E7 in all proportions and forms a single average micelle structure depending on composition. These results allow us to explain the origin of the previously reported formation of mixed worm-like micelles of polymerized and monomeric MUTAB that coexist with globular monomeric MUTAB micelles as a consequence of the unfavourable electrostatic interactions that accompany the uncoiling of polymerized MUTAB chains in unimer micelles when swollen by monomeric cationic surfactants.

Micellization of monomeric and poly-ω-methacryloyloxyundecyltrimethylammonium surfactants.

We have used small-angle neutron scattering to study how micelle morphology of the tail-polymerizable surfactants MUTAB and MUTAC (ω-methacryloyloxyundecyltrimethylammonium bromide and chloride) is affected by classic self-assembly modifiers such as temperature changes, salt addition, and counterion exchange, as a function of their conversion from monomer into polymer amphiphile in aqueous solution. Contrary to common assumptions about polymerized surfactants, these systems remain in dynamic equilibrium under all conditions examined and at all conversions (except for a small amount of high-molecular-weight precipitation by MUTAC). Counterintuitively, the polymerized methacrylate backbone has little influence on aggregate morphology, except for the formation of rod-like mixed micelles of polymerized and unpolymerized surfactant at intermediate conversions. The addition of salt produces a transition to rod-like micelles at all conversions except in the unpolymerized surfactant, which has some characteristics of an asymmetric bolaform surfactant and retains its spheroidal geometry under almost all conditions.

Polymerizable cationic micelles form cylinders at intermediate conversions.

The structural evolution of micelles of the polymerizable surfactant omega-methacryloyloxyundecyltrimethylammonium bromide (MUTAB) during UV-initiated polymerization in aqueous micellar solution has been followed by small-angle neutron scattering. Although the micelles are short spheroids both before and after polymerization, a significant, distinct population of rodlike micelles develops during the reaction, which accounts for as much as 40 vol % of the micellized surfactant and coexists with the spheroids and dissolved monomer. These coexisting micelle populations are shown to remain in dynamic equilibrium throughout the reaction and can be understood by treating it as a ternary mixture of surfactant, amphiphilic polyelectrolyte, and water.

Structure changes in micelles and adsorbed layers during surfactant polymerization.

We have studied the self-assembled structures formed by the cationic surfactant 11-(methacryloyloxy)undecyltrimethylammonium bromide (MUTAB) using small angle neutron scattering as it undergoes UV-initiated polymerization in bulk solution, and the subsequent adsorbed structures at the mica/solution interface using atomic force microscopy. MUTAB forms spheroidal aggregates in aqueous solution with an axial ratio of 2-3 both before and after polymerization, as previously reported in numerous studies. However, at intermediate conversions the micelles surprisingly form elongated structures up to 200 A long. Except for the unpolymerized MUTAB, which forms a featureless adsorbed layer, the micelle structures are largely retained after adsorption: structures are elongated at 50% conversion and globular at 100% conversion. These results demonstrate that the structure of the unpolymerized micelle is not simply kinetically-trapped during polymerization, but undergoes extensive reorganization as the reaction proceeds. This has implications for attempts to capture micellar structure by polymerization and use it to template nanostructured materials or for encapsulation.