Rheo-X-Ray Investigation of Structure Development in Polymer Melts Under Uniaxial Extensional Flow

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ARRA STATEMENT:This award is funded under the American Recovery and Reinvestment Act of 2009 (Public Law 111-5).TECHNICAL SUMMARY The ability of flow fields to strongly perturb molecular-scale structure in polymers has profound scientific and technological consequences. The nonlinear rheology of polymers is directly attributed to stretching and alignment of polymer molecules under flow. Materials with more complex internal structure?polymer blends, ordered block copolymers, polymer nanocomposites, for example?exhibit even more dramatic modes of flow-structure coupling. Further, polymer technology frequently hinges on the ability of flow fields during processing to impart molecular or microstructural anisotropy in the resulting product. X-ray scattering is a powerful tool for in situ structural studies during both flow and processing. To date, however, x-ray scattering methods have never been applied to well defined, homogenous extensional flows. Since extensional flows involve exponential stretching of fluid elements, they are dramatically more effective than shear flows at inducing molecular or microstructural alignment. These same characteristics make it notoriously difficult to produce homogenous extensional flows for laboratory testing. In this project, researchers at Northwestern will develop unique instrumentation that will allow, for the first time, in situ synchrotron-based studies of polymer structure development in well-defined uniaxial extensional flow. This experiment will be built around a proven, commercially available test fixture designed to facilitate extensional rheometry using conventional rotational rheometers. For synchrotron studies, this fixture will be integrated into a testing platform with computerized motion control and a custom-fabricated convection oven designed to facilitate x-ray access for both small- and wide-angle x-ray scattering. This instrumentation will open new vistas of scientific inquiry in virtually every class of polymer material that has been the object of flow-induced structure studies. Within this project, three specific application areas will be explored: (i) alignment of ordered block copolymers; (ii) particle orientation in polymer nanocomposites; and (iii) flow-induced crystallization of semi-crystalline polymers.NON-TECHNICAL SUMMARYMuch of the vast plastic materials industry hinges upon changes in molecular structure induced in processes such as fiber spinning, extrusion and molding, that are essential to achieve the desired performance in the end product. Using powerful x-rays at the Advanced Photon Source at Argonne National Laboratory), researchers from Northwestern University will develop a unique new instrument that will enable these molecular-scale structural changes to be studied in ?extensional flow?? flows in which molten plastics are stretched much in the same way that taffy is pulled. Extremely bright x-ray beams will facilitate ?real-time? measurements of nanometer-scale structural dynamics in a wide range of high-performance materials. Further, the instrumentation developed under this proposal will be made available to researchers across the country, thus making a permanent contribution to US scientific infrastructure.