Featured in Science magazine, Editors' Choice Thermoreversible, Epitaxial fcc ↔ bcc Transitions in Block Copolymer Solutions. Uncharged block copolymer micelles display thermoreversible transitions between close-packed and bcc lattices for a range of concentration, solvent selectivity, and copolymer composition. Using small-angle x-ray scattering on shear-oriented solutions, University of Minnesota and Northwestern University investigators discovered that highly aligned fcc crystals can transform epitaxially to bcc crystals, with fcc/bcc orientational relationships that are well established in martensitic transformations in metals. The transition is driven by decreasing solvent selectivity with increasing temperature, inducing solvent penetration of the micellar core. This provides access to a highly organized, "single crystal-like" organization in a polymer with a structure (in this case, bcc) that ordinarily is difficult to achieve, through shearing a structure (fcc) that is highly susceptible to flow organization. This discovery represents a unique approach to manipulating the internal structure of complex polymers, which is vital to realizing the potential of self-assembled nanomaterials in advanced technologies. [Joona Bang, Kristin L. Brinker, Wesley R. Burghardt, Timothy P. Lodge, and Xiaohui Wang, Phys. Rev. Lett, 89, 215505 (2002)] The American Physical Society
 .jpg)
FCC in Vorticity Direction (left) and BCC in Vorticity Direction (right)
|
Last
modified on April 3, 2008