Deformation behavior of an S‐B‐S copolymer

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Abstract
In accordance with previous studies, ‘single crystal’ tyr samples were prepared from the polystyrene‐polybutadien‐ polystyrene (S‐B‐S) three block copolymer Kraton 102 consist ing of a hexagonally packed parallel array of glassy S cylinder within a rubbery B matrix. Following earlier investigations o the pronounced anisotropy of the modulus, the actual deformation mechanism is examined in terms of the microstructures on straining perpendicular and parallel to the cylinder direction up to and beyond yielding. On perpendicular straining, a one to‐one relation is observed between ‘lattice’ and sample stray up to about 10 percent, coupled with complete recovery. For higher strains, the deformation becomes increasingly irreversible dependent on the time of stressing, a behavior which is found to be caused by cracks forming along crys tallographically defined directions of the hexagonal macrolai tice. On parallel straining, the stress‐strain curves display yield behavior at ∼3 percent strain, the yielded material be coming more compliant even at small strains on repeated test ing. However, on storage, the sample rehardens, a proces accelerated by heat treatment. This behavior is explained in terms of a breaking up and reformation of the cylinders and is supported by the subsequent experiments. These includ measurement of birefringence, which in the reversible small strain region corresponds to expectations from uniform strain in both S and B phases, but which beyond the yielded region displays behavior such as expected from deformation of the B phase alone. The postulated break‐up of cylinders on yielding is directly confirmed by electron microscopy on Iongitudinal sections thin enough to contain a single layer of cylinders. The yield behavior itself is treated theoretically using the composite mechanics approach. Two treatments are applied, the trad itional shear lag theory and a new random rod breaking theory In the latter case, in particular, very good agreement with experiment is found in that both the observed stress to yield and the electronmicroscopically seen fractured rod length in the yielded product are predicted.