(641f) Structure and Dynamics of Strongly Microphase-Separated Block Copolymers with High Contrast

Strongly microphase-separated block copolymers (BCPs) form sharp interfaces and typically large domains on the order of 100 nm. This is achieved with high molecular weight, that also results in mechanical robustness due to entanglements. Thus, high contrast BCPs with a glassy and a rubbery block are used as thermoplastic elastomers and are also of interest as battery electrolytes and separation membranes. This work studied structural dynamics of such block copolymers as a function of grain size and block volume fraction. The block copolymers studied were cylindrical and lamellar polystyrene-b-poly(ethylene oxide) (PS-b-PEO). The grain size was controlled via the rate of solvent evaporation during solution casting. Morphologies and grain sizes were evaluated using small angle x-ray scattering and atomic force microscopy. The ability to control grain size was found to depend on which homopolymer formed the matrix majority. Grain size was only found to increase with slow evaporation in the BCP comprising a rubbery PEO majority. Structural dynamics of the BCPs was measured with X-ray photon correlation spectroscopy (XPCS). XPCS is a powerful technique that tracks changes in scattering contrast, i.e. structure, by correlating time-resolved scattering intensity. The most significant correlation was found to be with PS volume fraction (not grain size as expected). This correlation occurred at temperatures near PS T_g. Other details, such as effects of temperature and domain size, will also be presented to rather definitively demonstrate that the anomalous dynamics that have previously been reported in BCPs with XPCS is rooted in the soft glassy nature inherent in the glassy block rather than a collective structural behavior.