(342i) SAXS Studies of the Structure of a BCC-Ordered Block Copolymer Melt Subjected to Uniaxial Extensional Flow

We report in situ small-angle x-ray scattering (SAXS) investigations of a spherically-ordered block copolymer melt subjected to uniaxial extensional flow.  The sample studied is a styrene-ethylene butylene-styrene (SEBS) triblock copolymer with a low styrene content (13 %) which results in spherical polystyrene microdomains ordered in BCC lattice.  Samples are prepared by extended annealing in the melt after clearing above the order-disorder transition, resulting in ordered samples that are macroscopically random in terms of the orientation distribution of the BCC 'grains'.  Samples are subjected to uniaxial extensional flow in a wind-up counter-rotating drum extensional flow fixture housed in a custom built oven designed for x-ray access.  The combination of high intensity synchrotron radiation and fast CCD detectors allows for time-resolved study of the structural response during flow.  Upon application of flow, initially isotropic diffraction rings in SAXS patterns become deformed, reflecting flow-induced distortion of the BCC lattice. The peak intensity also concentrates azimuthally, indicating macroscopic alignment of the BCC lattice. There is evidence that extensional flow leads to progressive disordering of the BCC structure, with loss of higher order peaks and the emergence of a more diffuse 'halo' of scattering.  So long as the primary diffraction peak is visible in directions parallel and perpendicular to the stretching direction, the deformation of the lattice d-spacing follows affine deformation.  Indications of ordering persist to higher strains in samples stretched at higher extension rates, and evidence of affine lattice deformation persists to high strains (Hencky strains of 1 - 2) under these high deformation rate conditions.  The azimuthal redistribution of diffraction peak intensity provides evidence that the BCC lattice orients with the 100 direction along the flow direction, although the lattice deformation and simultaneous degradation of long range ordering complicates interpretation of the orientation state.  Vestiges of oriented BCC diffraction persist throughout the flow, specifically diffraction peaks attributed to 110 planes oriented with normals at 45˚ with respect to the stretching direction.  We speculate that these features are the most long-lived because lattice planes with this orientation are not subjected to either extreme compression or dilation by the applied extensional flow.