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Squishy Physics Seminar
Prof. Chinedum Osuji
Department of Chemical and Environmental Engineering
Yale University, New Haven CT 06511
Directed Assembly of Nanostructured Soft Materials by Magnetic Field
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 the 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. In this presentation I will discuss recent
advances in scalable approaches for directing the assembly of soft
nanostructured materials, and novel routes for generating highly ordered soft
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.
Wednesday, May 11, 2016
2nd floor Maxwell Dworkin Lounge
Pizzas start at 5:20 p.m.