Dynamics of Molecular Exchange In Solutions of Block Copolymer Micelles

Block copolymers offer certain advantages over traditional low molecular weight surfactants in a variety of engineering applications in both aqueous and organic media. Increasing the molecular weight of an amphiphilic molecule enhances interfacial activity but drastically reduces the critical micelle concentration, which leads to dramatically slower dynamics for molecular exchange resulting in non-equilibrium micelle configurations. Model poly(styrene-b-ethylene-alt-propylene) (PS-PEP) diblock copolymers have been prepared in normal and partially deuterated forms by anionic polymerization and catalytic hydrogenation and mixed in squalane (C₃₀), a model saturated oil, at concentrations between 0.5 and 15% by weight. Linear dynamic mechanical spectroscopy and small-angle x-ray scattering were employed to ascertain the state of order or disorder, size, size distribution and other structure features associated with the resulting spherical micelles, which contain a PS core surrounded by a solvated PEP corona. Time resolved small-angle neutron scattering measurements afforded access to the rate of block copolymer exchange at 1% and 15% loadings. These results demonstrate a remarkable and non-intuitive sensitivity of the molecular exchange process to relatively minor core block polydispersity and the state of micelle ordering.