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We are pursuing a molecular-level understanding of polymer
dynamics -- how individual macromolecules diffuse, flow, and relax
stress. Of particular interest are experiments designed to test, and
discriminate among, contemporary theoretical treatments of polymer
liquids. Ultimately, such information will also impact the synthesis,
characterization, processing, and end-use of polymeric materials. Our
primary approach is experimental, but synthesis, computer simulation,
and analytical calculations are also employed when appropriate.
Currently, we are most interested in multicomponent systems --
copolymers, homo-polymer blends, and their mixtures -- in solution and
in the bulk (melt) state. Such materials are of great commercial
interest, due primarily to the potential flexibility for tailoring
superior combinations of physical properties. The overall scientific
challenge is to understand how the thermodynamic interactions among the
components control both structure and dynamics. For example, the (net)
repulsive interactions between blocks of a block copolymer can lead to
spontaneous self-assembly into a variety of microstructures, each with
a periodicity set by the molecular size, i.e., in the tens of
nanometers. The same interactions may cause a blend of the
corresponding homopolymers to undergo macroscopic phase separation.
However, this separation is often quite slow, and may be arrested
(e.g., by vitrification, crystallization, or added copolymer
surfactant) to produce interesting morphologies with characteristic
dimensions on the micron scale. In these, and other situations under
study, the already distinctive dynamic properties of polymers may
couple in unexpected ways to structural features.
A host of experimental techniques are employed, including structural
probes, such as scattering of light, x-ray, and neutrons, and
microscopy. Measurements of collective dynamics, by dynamic light
scattering, rheology, and flow birefringence, are also routinely
pursued. However, our current primary focus is on chain diffusion.
Here, we employ the transient optical grating technique of forced
Rayleigh scattering, the ion beam analysis approach of forward recoil
spectrometry, and pulsed-field gradient NMR.
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