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

Subjecting lamellae-forming poly(styrene-b-isoprene) (SI) diblock copolymers to large-amplitude oscillatory shear (LAOS) offers a route to microstructural alignment. This study examines the spatial uniformity of both isotropic (unaligned) and LAGS-aligned states of an SI diblock copolymer and its blend with homopolystyrene. Unlike typical studies in this vein, we do not restrict our examination to the bulk but characterize the state of alignment near the shearing surfaces. Transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) results reveal that lamellae near the surfaces adopt a parallel orientation, regardless of bulk orientation. This near-surface parallel region is uniform and extends ca. 2 mu m into the bulk. In the case of perpendicular bulk alignment, the mutually orthogonal bulk and surface lamellae are separated by a boundary region containing defects similar to +1/2 and -1/2 disclinations. Our results indicate that the near-surface parallel region is created upon quenching through the order-disorder transition temperature. Furthermore, the near-surface parallel region persists, relatively unchanged, even when the bulk of the specimen becomes perpendicular.