November 12, 2014

Speaker: Robert Wickham, Associate Professor, Department of Physics, University of Guelph

Title: Simulation of structure and dynamics in the disordered micelle regime of a diblock copolymer melt

Abstract: Diblock copolymers are long, chain-like molecules composed of two sub-chains, or blocks, of different chemical species. In a dense melt of diblock copolymers, there is competition between an effective repulsion that tends to separate unlike blocks and entropy, which tends to coil the chain. At low temperatures, a compromise between these two tendencies is reached and this results in microphase separation of the blocks into ordered domain structures at nanometer scales. This process is known as molecular self-assembly.

There has been a tremendous focus on the low-temperature ordered phases: for example, the body-centred cubic (bcc) arrangement of spherical polymeric aggregates called micelles. However, experiments reveal that the high-temperature disordered phase has a surprising amount of structure itself, with indications of localized, strongly segregated domains – a disordered liquid of micelles – existing as part of the disordered phase. This disordered micelle regime is challenging to study theoretically because one needs to go beyond the mean-field theories typically employed, which predict that the disordered phase is featureless, and include composition fluctuations in the model for a proper description of the structure.

In this talk, I will discuss large-scale, fluctuating, dynamical field-theoretic simulations we use to comprehensively characterize the structure and dynamics of the disordered micelle regime. Our simulations provide insight into how and when the disordered micelle liquid is formed; the nature of the micelles as polydisperse hard spheres; and the sluggish dynamics of the micelle liquid as it is cooled, which has implications for the kinetics of formation of the bcc ordered phase.