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 shea ring, 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 regim e does not effectively align the block copolymer morphology. Steady sh ear disrupts well-aligned samples by introducing defects, namely wall and focal conic defects. We distinguish between intermediate and globa l length scales of alignment using TEM and SAXS, respectively. Althoug h significant morphological differences exist between the various samp les, the dynamic mechanical responses of these copolymer samples are i ndistinguishable over the frequency range examined. Our experimental r esults are inconsistent with the proposed grain rotation mechanism as the sole mechanism of alignment of block copolymer; a destruction/refo rmation mechanism is necessary to explain the transformation of the in itial morphology to having global alignment.