Structural response of an ordered block copolymer melt to uniaxial extensional flow

We report in situ small-angle X-ray scattering (SAXS) studies of a cylindrically ordered styrene-ethylene butylene-styrene (SEBS) triblock copolymer melt subjected to uniaxial extensional flow. The flow is applied by stretching strips of polymer melt using a counter-rotating drum extensional flow fixture housed in a custom oven designed to provide X-ray access. SAXS patterns show two distinct modes of structural response during extensional flow: deformation of the microscopic structure, and re-orientation of PS microdomains towards the flow direction. The d-spacings of the hexagonally ordered domains measured parallel and perpendicular to the flow direction deform affinely until Hencky strains of similar to 0.2. Departures in extensional viscosity from linear viscoelastic predictions are observed at similar strain. The azimuthal dependence of the primary diffraction peak reveals a complex re-orientation process whereby PS microdomains rotate toward the stretching direction. At intermediate strains, a '4-point' diffraction pattern indicates the presence of two discrete populations of microdomain orientation, attributed to a buckling instability of microdomains initially oriented perpendicular to the stretching direction. Flow-induced deformation and orientation both relax upon cessation of flow, albeit at very different rates, suggesting that these two modes of structural response are largely decoupled.