Perpendicular deformation of a near-single-crystal triblock copolymer witha cylindrical morphology. 1. Synchrotron SAXS

A poly(styrene-block-isoprene-block-styrene) (SIS) triblock copolymer with a polystyrene (PS) cylinder morphology was processed via roll-casting to pr oduce a near single-crystal texture. Deformation experiments normal to the cylinder axis were carried out using synchrotron small-angle X-ray scatteri ng (SAXS) with the beam both parallel and perpendicular to the cylinder axi s. In situ measurement of load and displacement enabled morphological infor mation to be mapped to the stress-strain curve. Results indicate that the d eformation proceeds in two stages. Deformation to strains of approximately 100-130% is nearly affine. The rubber matrix extends along the stretching d irection (SD) while contraction occurs almost exclusively along the neutral direction due to the constraint imposed by the aligned PS cylinders. A mea sured Poisson's ratio of 0.9 compares favorably with a value of 1 expected for a perfect composite. At deformations beyond 130% an X-pattern is observ ed at perpendicular incidence. The angle between the arms of the X increase s asymptotically, while the intercylinder spacing remains fairly constant. The initial hexagonal pattern undergoes a continuous distortion and provide s further evidence for the deformation discontinuity in the rate of lateral contraction. The morphological observations in reciprocal space are interp reted in terms of a kinking instability in which the plane strain state imp osed by the oriented cylinders is relieved by their kinking into a chevron pattern. The kinking transition strain is expected to be dependent on the d egree of cylinder misorientation, the shear resistance of the rubber matrix , and the bending resistance of the PS cylinders.