APS Today beta

Semidilute solutions are characterized by the overlapping of linear polymer chains with each other. Dynamics of semidilute solutions has been well described by the Brownian motion of monomer units (“blob”) between two neighboring entanglement points and the reptation of the entire chain in a “tube” made of other surrounding chains. In the past, many dynamic laser light-scattering (LLS) measurements showed that besides the fast relaxation related to the “blobs”, there exists an additional slow relaxation mode. In the earlier time, such a slow mode was wrongly identified as the reptation. LLS cannot distinguish one chain from another in a semidilute solution so that it cannot observe the chain reptation. Later, this slow mode was attributed to possible problems in the sample preparation, such as dust particles or a concentration gradient. Whether this slow mode is real has remained a challenging problem since 80’s. To avoid these potential problems, we used different un-conventional ways, instead of a simple increase of polymer concentration, to induce an in situ dilute-semidilute transition. These methods include the temperature-induced coil-to-globule transition of long polystyrene chains, in situ high-vacuum anionic polymerization of styrene in cyclohexane, and in situ RAFT living bulk polymerization of methyl methacrylate. Our results confirm that this slow mode is real with no ambiguity and showed that it appears whenever the solvent quality becomes less good. Our study leads to a fundamental question whether semidilute solutions are “homogeneous” as stated in textbooks and previous theories. By considering that the segments or monomer units near the entanglement points are different from those in the middle between two neighboring entanglement points; namely, semidilute solutions are “inhomogeneous”, we can satisfactorily explain the existence of the two modes.
[ Hide ]

This two-day workshop is organized within the COMPRES infrastructure development initiative, which is aimed at creating state-of-the-art high-resolution inelastic x-ray scattering (IXS) techniques for characterizing the properties of materials under the high-P-T conditions of planetary interiors.
[ Hide ]

Understanding material behavior under extreme conditions is central to modern materials research, as highlighted in a recent DOE workshop on "Basic Research Needs for Materials under Extreme Environments" (June, 2007, Washington D.C., http://www.sc.doe.gov/bes/reports/list.html.) Over the past decade or two, a large number of new materials and novel phenomena have been discovered and predicted at extreme conditions, paralleling advancements in high-pressure technologies, the nation's large-scale experimental user facilities, condensed matter theories, and high-performance computers. Yet, because of the single event and destructive nature of the experiments, gaining microscopic insights into dynamic materials response has been a significant scientific challenge, despite unprecedented scientific opportunities for mechanistic understanding of material phenomena in short time and length scales.
[ Hide ]

The objective of the Small-Angle Scattering Short Course 2008 is to raise the capabilities of the small-angle scattering (SAS) community by providing an intermediate-level course for those in need of a better understanding of SAS theory, and techniques utilized at the APS. The SAS short course offers an overview of SAS theory, capabilities, and data reduction and analysis tools to enable the community to submit highly effective beam-time proposals and to facilitate better utilization of the resources at the APS.
[ Hide ]