Structure and ordering kinetics of micelles in triblock copolymer solutions in selective solvents

Abstract

AbstractWe have used small‐angle x‐ray scattering (SAXS), and small‐angle neutron scattering (SANS) to study the micelle structure of a polystyrene‐block‐poly(ethene‐co‐butene)‐block‐polystyrene triblock copolymer in dilute ‐ semidilute solutions in solvents selective for either the outer styrene block (dioxane) or for the middle block (heptane or tetradecane). Measurements of equilibrium structure factors showed that micelles were formed in both types of selective solvents. In the case of dioxane, the micelles are isolated whereas in the case of heptane or tetradecane, a bridged micellar structure may be formed at higher copolymer concentrations. In both cases we observed an ordered cubic structure of insoluble domains (micellar cores) at high concentrations (> 8 %). The micellar scattering function was fit to the Percus‐Yevick interacting hard‐sphere model. The temperature dependence of the core radius, the hard‐sphere interaction radius and the volume fraction of hard spheres were obtained. We also used synchrotron‐based time‐resolved SAXS to examine the kinetics of ordering of the micelles on a cubic lattice for many different temperature jumps into the ordered cubic phase starting from the disordered micellar fluid phase in different solvents at different concentrations. The time evolution of the structure changes was determined by fitting the data with Gaussians to describe the structure factor of the ordered Bragg peaks and the Percus‐Yevick structure factor was used to describe the micellar fluid. The time dependence of the peak intensities and widths as well as of the micellar parameters will be presented. The results showing the kinetics of the transformation from the fluid to the ordered phase were analyzed using the Mehl‐Johnson‐Avrami theory of nucleation.