(138g) In Situ Nanostructure Characterization of Complex Fluids Under Arbitrary Processing Flows

In situ small angle neutron scattering under flow (flow-SANS) has become a critical tool for measuring and formulating processing-structure-property relationships of polymeric fluids. However, sample environments and associated measurement methods for flow-SANS/SAXS have largely limited these measurements to steady state flows and simple rheometric deformations (pure shearing or elongation) that fail to capture the complex nonlinear and time-varying deformations encountered during polymer processing. Recently, significant advances in neutron detection as well as the design of new fluidic devices have opened up new capabilities for probing complex, time-varying deformations that more reliably emulate real processing flows. Here, we will summarize the key advances leading to these capabilities, and illustrate their usefulness with two examples involving the flow-induced structuring in polymer nanocomposites. In the first example, we use time-resolved rheo-SANS, involving simultaneous flow-SANS and rheological measurement, to explore the kinetics and mechanics of shear-induced aggregation of nanoparticle suspensions in associative polymers during startup and cessation. The results show that, over a wide range of conditions, clustering is dominated by competition of hydrodynamic interactions and Brownian motion of the dispersed nanoparticles, rather than by polymer normal stresses as originally proposed. In the second, we use a newly developed fluidic four-roll mill (FFoRM) in order to probe how the flow-induced alignment of rodlike nanoparticles in polymer solutions depends on the type of applied deformation.