Studies on Morphology and Proton Conductivity of Sulfonated Block Copolymer Micelles

Abstract

We present an electric-field triggered sphere-to-cylinder transition of negatively charged block copolymer micelles in with radically low electric-field of 30V/cm. The system investigated is dilute solutions of strong polyelectrolyte containing ionic block copolymers, i.e., poly(styrenesulfonate-b-methylbutylene). We have carried out in-situ small angle x-ray scattering experiments equipped with DC power supply, combined with electron microscopy and atomic force microscopy. The application of small electrical fields across the solutions of spherical micelles results in the transient morphology of interconnected spheres, which are eventually transformed into cylindrical shape with time. The E-field induced cylindrical micelles reverted to spherical micelles when the E-field is switched off. Second, an in-depth analysis of proton mobilities in model ionic block copolymers has been carried out. The system of interests is a series of sulfonated poly(styrene-b-methylbutylene) (PSS-b-PMB) copolymers. Dilute solutions of PSS-b-PMB copolymers in methanol were examined where the PSS domains have an ability to conduct protons by offering sufficiently protonated conditions. A nearly monodisperse molecular weight distribution of PSS-b-PMB copolymers yields highly uniform spherical ionic micelles. In particular, on virtue of the self-assembly nature of block copolymers, the system revealed well-defined ionic PSS domains with different thickness ranging from 3.0 to 7.8 nm. The proton transport in PSS-PMB copolymers was found to be facilitated by the decrease in the ionic domain sizes with proton mobilities () ranging from 1.96 x 10-4 to 8.48 x 10-4 cm2/V s. Notably, unique scaling relationship between the values and the micelle radii (RH),   RH-1.67, was described, which was rationalized by the different proximity of acid groups at the surfaces of ionic domains. The validity of the scaling behavior is verified by examining body-centered cubic forming concentrated solutions. Interestingly, when the same analysis is applied to the hydrated samples possessing different domain geometry, i.e., cylindrical ionic domains, it also reveals the scaling behavior, although an obtained exponent is significantly low as -0.35.