Relaxation phenomena and development of structure in a physically crosslinked nonionic microemulsion studied by photon correlation spectroscopy and small angle x-ray scattering

We performed small angle x-ray scattering (SAXS), photon correlation spectroscopy (PCS), and oscillatory frequency sweep experiments to investigate the static and dynamic properties of a transient network. The system under investigation is an oil in water microemulsion containing the nonionic surfactant C12E5 (penta ethylene glycol mono dodecyl ether). The microemulsion consists of micelles (spherical oil droplets covered by the surfactant) with radius R(m)approximate to 10 nm, which are dispersed in water. Adding a triblock copolymer with a hydrophilic midblock and hydrophobic end blocks (stearate-poly ethylene oxide-stearate) to the microemulsion leads to the interconnection of the micelles and thus to the formation of a transient network. We describe the structural behavior under addition of the triblock copolymer with a hard sphere model based on the Percus-Yevick pair correlation function. In the dynamic light scattering experiment we observe three relaxational processes. The fast and the slow relaxation time vary with scattering vector q as tau(i)=(D(i)q(2))(-1) whereas the intermediate relaxation process is independent of the scattering vector q. The relaxation time of this local process compares well with the terminal relaxation time measured in an oscillatory shear experiment. The nature of this intermediate process is therefore attributed to the characteristic lifetime of a network junction point. (C) 2000 American Institute of Physics. [S0021-9606(00)50514-5]