title: Structural response of an ordered block copolymer melt to uniaxial extensional flow.
abstract: 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 ∼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.
authors.affiliation: Department of Chemical and Biological Engineering, Northwestern University, Evanston, IL 60208, USA. w-burghardt@northwestern.edu.
authors.forename: Ruinan
authors.initials: R
authors.lastname: Mao
authors.forename: Erica M
authors.initials: EM
authors.lastname: McCready
authors.forename: Wesley R
authors.initials: WR
authors.lastname: Burghardt