Coronal structure of star-like block ionomer micelles: An investigation by small-angle neutron scattering

The coronal structure of starlike block ionomer micelles has been probed directly by small-angle neutron scattering (SANS), using specific contrast labeling of different regions of the corona. Solutions of PS-b-dPS-b-PS-b-PACs starlike block ionomer micelles in toluene are studied; contrast is provided by perdeuterated polystyrene labels situated at different distances from the ionic core. For deuterated labels situated at 390 PS units from the core, the scattered intensity is found to scale as I similar to q(-1.7) at intermediate q, suggesting a semidilute environment for the labels and local swollen coil behavior. For labels 180 PS units from the core, steric perturbations are more extensive than those found under semidilute conditions, such that single chain scaling is not observed at any length scale; scattered intensity is found to scale as I similar to q(-1.3) at intermediate q. The stiffness of coronal chains next to the core is even greater, as indicated by I similar to q(-1.0) scaling for dPS labels attached directly to the ionic core; these probes exhibit rigid-rod behavior at length scales less than ca. 76 Angstrom. In Kratky representations of the three samples, a maximum characteristic of polymer stars becomes more pronounced as the distance of the dPS labels from the core decreases, indicating greater contributions from interchain scattering. Increases in chain stiffness and interchain scattering suggest a significant increase in local segment densities with decreasing distance from the ionic core. Interparticle scattering as a function of polymer concentration is also investigated. The position of the Bragg peak scales with concentration as c(1/3), suggesting liquid ordering of the solution. Radii of gyration determined from Guinier analysis agree well with light scattering results, with the exception of micelles in which dPS probes are attached directly to the core. In this case, the neutrons "see" only the inner part of the spherical brush, and R-g from SANS data is less than R-g for the whole micelle.