W0313

Nano-Structured Materials from Self-Assembly of Block Copolymers in the Condensed State and Fluid Media. Wignall George, Solid State Division, Oak Ridge National Laboratory, Mail Stop 6393, Oak Ridge, TN 37831 USA.

Block copolymers are molecules containing two or more chemically distinct polymers and when the components are incompatible, segregation occurs. The chemical bond between them constrains the separation to length scales of the order of the block radius of gyration, thus producing complex nano-structures on length scales of 1-100 nm by self-assembly in the condensed state. Similarly, block copolymers self-assemble when they are dispersed in a solvent selective for one of the blocks and form micellar aggregates, consisting of a core formed by the insoluble blocks, surrounded by a shell of solvated blocks extending into the continuous phase. Recently, it has been demonstrated that micelles can also form in supercritical solvents such as CO₂ by using blockcopolymer stabilizers, consisting of both CO₂-phobic (e.g. polystyrene) and CO₂-philic (e.g. fluoro-polymer) blocks. These micelles have been shown to be capable of solubilizing CO₂-phobic materials (which dissolve in the core), similar to the way in which hydrophobic oil may be solubilized in water by coating the oil droplets with the hydrophilic (water-soluble) component of a detergent. As the CO₂-density is increased, the solvation of the block CO₂-philic block becomes greater, which should result in a decrease in aggregation and eventually unimers (i.e. single molecules). Thus, we anticipate the existence of a critical micelle density (CMD), which is analogous to the familiar critical micelle concentration observed in aqueous media, and recent experiments [Langmuir, 16(2), 416 (2000); J. Appl. Cryst., 33, 641-644 (2000)] demonstrate this phenomenon, which is distinctive of highly compressible fluids. The size and shape of the micelles may be determined by small angle neutron scattering (SANS), which shows that the self-assembly can be reversibly controlled, as the pressure is tuned to control the solvent quality. This paper will review the use of small angle scattering techniques to characterize self-assembly in both the condensed and solution states.