Styrene‐diene triblock copolymers: Orientation conditions and mechanical properties of the oriented materials

Abstract

SYNOPSISThe organization of the hexagonal structure of some block8copolymerslhas beenobtained directly from the melt. We have pointed out the temperature and shear conditions which give a well-organized and oriented structure. In order to represent the viscous and elastic behavior of the melt, we have drawn flow curves and have measured the die swell ratio. A flow mechanism, corresponding to grain structure, is proposed. The results obtained enabled us to build a molding device. Fracture tests under tension on ABA poly(styrene-b-isoprene) samples exhibiting hexagonal structure showed a highly anisotropic behavior: If the crack propagates in a plane perpendicular to the isoprene cylinder axes or parallel with it, the fracture is either brittle or ductile accordingly. In the latter case, branching also occurs on both sides of the main crack. By using optical and scanning electron microscopy, the corresponding fracture surface morphologies have been studied. The brittle fracture surface shows a pattern of alternating rough and smooth bands which is characteristic of pure polystyrene. On the other hand, the ductile fracture surface is composed of an array of elongated "chips." They are disposed practically parallel to the fracture propagation direction. Moreover, indentations made with a spherical steel penetrator led to residual ellipsoidal impressions. By measuring their dimensions, the anisotropy of the mechanical properties was displayed.