INTERDEPENDENCE OF SHEAR DEFORMATIONS AND BLOCK COPOLYMER MORPHOLOGY

A lamellar poly(styrene-b-isoprene) diblock copolymer is well-aligned, as observed by TEM and SAXS, by continous, nonlinear oscillatory shearing, at a surprisingly low strain amplitude, specifically 5 %, and a temperature 20-degrees-C above the glass transition temperature of the PS block. This aligned state exhibits lamellae parallel to the sample plane, that is, with the normal of the lamellae parallel to the shear gradient. The onset of nonlinear viscoelastic effects occurs at small strains, approximately 1 %. Oscillatory shear within the linear regime does not effectively align the block copolymer morphology. Steady shear disrupts well-aligned samples by introducing defects, namely wall and focal conic defects. We distinguish between intermediate and global length scales of alignment using TEM and SAXS, respectively. Although significant morphological differences exist between the various samples, the dynamic mechanical responses of these copolymer samples are indistinguishable over the frequency range examined. Our experimental results are inconsistent with the proposed grain rotation mechanism as the sole mechanism of alignment of block copolymer; a destruction/reformation mechanism is necessary to explain the transformation of the initial morphology to having global alignment.