Persistence of surface-induced alignment in block copolymers upon large-amplitude oscillatory shear processing

Subjecting lamellae-forming poly(styrene-b-isoprene) (SI) diblock copolymer s to large-amplitude oscillatory shear (LAOS) offers a route to microstruct ural alignment. This study examines the spatial uniformity of both isotropi c (unaligned) and LAGS-aligned states of an SI diblock copolymer and its bl end with homopolystyrene. Unlike typical studies in this vein, we do not re strict our examination to the bulk but characterize the state of alignment near the shearing surfaces. Transmission electron microscopy (TEM) and smal l-angle X-ray scattering (SAXS) results reveal that lamellae near the surfa ces adopt a parallel orientation, regardless of bulk orientation. This near -surface parallel region is uniform and extends ca. 2 mu m into the bulk. I n the case of perpendicular bulk alignment, the mutually orthogonal bulk an d surface lamellae are separated by a boundary region containing defects si milar to +1/2 and -1/2 disclinations. Our results indicate that the near-su rface parallel region is created upon quenching through the order-disorder transition temperature. Furthermore, the near-surface parallel region persi sts, relatively unchanged, even when the bulk of the specimen becomes perpe ndicular.