Chemical Engineering Seminar

Molecular self-assembly of block copolymers and small molecule surfactants gives rise to a rich phase behavior as a function of temperature, composition, and other variables.  The ability to precisely control their chemical functionality combined with the readily tunable characteristic length scales (~1–100 nm) of their self-assembled mesophases identifies these systems as a versatile and attractive class of materials for compelling applications ranging from selective transport to lithography.  A longstanding problem in this area is our inability to reliably and rapidly generate well-ordered structures with specified orientations in, and over, application-relevant geometries, and dimensions, respectively, i.e. to direct their self-assembly in useful ways.

   We consider the directed self-assembly of such soft mesophases using magnetic fields, principally through the use of in situ x-ray scattering studies.  Field alignment is predicated on a sufficiently large product of magnetic anisotropy and grain size to produce magnetostatic interactions which are substantial relative to thermal forces.  We examine the role of field strength on the phase behavior and alignment dynamics of a series of soft mesophases.  Directed self-assembly in the block copolymers considered proceeds by nucleation of randomly aligned grains which thereafter rotate into registry with the field, rather than by selective nucleation of aligned grains.  This is consistent with estimates which show that magnetic fields as large as 10 T have little discernable impact on the phase behavior of systems considered, with shifts in order-disorder transition temperatures of 5 mK or smaller.  We highlight the tradeoff between decreasing mobility and increasing anisotropic field interaction that dictates alignment kinetics while transiting from a high temperature disordered state to an ordered system at lower temperatures.  The ability to produce highly ordered functional materials over macroscopic length scales is demonstrated and we explore the role of alignment and connectivity in controlling anisotropic ionic transport in nanostructured systems.