Unit cell deformation under elongational stress of oriented SIS three‐block copolymers

Abstract A 50, 000 molecular weight SIS three‐block copolymer, containing 30 percent by weight of polystyrene and, presenting the cylindrical morphology, has been oriented by a novel shearing process at high temperature. The orientation has been studied by low‐angle X‐ray diffraction; the intensity of the (100) reflexion of the hexagonal array has been measured as a function of the angular position of the sample with respect to the direct beam. The samples have been found to be very similar to single crystals; not only the axes of the cylinders lie parallel to the shearing direction, but also the (100) lattice‐planes keep the same orientation in space throughout the whole volume of the specimen. Special probes for the mechanical tests have then been cut in the oriented material, parallel and perpendicular to the axis of the cylinders. The deformation under elongational stress of the external shape of the probes has been studied with a traveling microscope; when the cylinders are oriented perpendicular to the stress, the sample deforms elastically, the, size of the probe corresponding to the length of the cylinders remaining constant up to 200 percent deformation. Low‐angle X‐ray diffraction has been used to study the structural deformation under the same elongational stress; with the stress perpendicular to the cylinders, the unit cell seems to deform in an.affine way with respect to the macroscopic aspect; with the stress parallel to the cylinders, the structure is destroyed rapidly.