Physics Colloquium: Creating Useful Nanostructured Soft Materials (and single crystals) by Directed Self-Assembly

Prof. Chinedum Osuji, University of Pennsylvania

Abstract: Controlling structure and thereby manipulating the use properties of condensed matter is a central concern in materials science. A striking disparity exists in our ability to enact such structural control in self-assembled soft materials, relative to hard condensed matter systems. This talk examines strategies for directing self-assembly in nanostructured soft matter to create useful materials, including single crystals and materials with bespoke textures. Our focus is on self-assembly of block copolymers and small molecule mesogens. The ability to precisely control their chemical functionality combined with the readily tunable characteristic length scales (~1-100 nm) of their self-assembled mesophases position these systems as a versatile and attractive class of materials for compelling applications ranging from membranes for size and chemo-selective transport, to optics, and lithography. As a result there is intense interest in elucidating the physical processes relevant for directing their self-assembly to create application relevant structures, with a goal of exploiting such fundamental understanding to create useful materials or devices. This presentation discusses such directed self-assembly of soft nanostructured materials and emerging methods for generating highly ordered and heterostructured systems. In particular, we focus on recent advances in the creation of highly ordered nanostructured membranes for water purification, and small length scale structures for lithography in microelectronics. While much of the progress to date has involved the use of a single ‘field’ to direct self-assembly (e.g. shear, magnetic, electric fields; surface forces/confinement), the use of more than one stimulus to control structural order has the potential to produce materials with true single crystal texture. We show that such single-crystal textures can be produced over unprecedented length scales in a small molecule columnar mesophase. Texture control is accomplished by judicious application of magnetic fields and physical confinement, or by sequential applications of dynamic and static magnetic fields across pertinent phase transitions. We discuss the physical origin of the unique biaxial texture control as revealed by in situ X-ray scattering experiments.