The mechanical properties of a `single crystal' of SBS copolymer - a novel composite material

Abstract

The mechanical properties of a novel fibre-reinforced material formed from the segregated microphases of a three-block copolymer are described. Such a material was prepared by extruding a commercial polystyrene-polybutadiene-polystyrene block copolymer, followed by an annealing process. Low-angle x ray diffraction and electron microscopy revealed that the dispersed polystyrene phase was in the form of cylinders which form a hexagonal lattice with the cylinder axes parallel to the original extrusion direction.As expected, the resulting mechanical anisotropy was found to be very pronounced. In particular, the dependence of Young's modulus on the angle between the stress direction and the original extrusion direction was compared with the predictions of some current theories of fibre reinforcement based on hexagonal symmetry. Complete agreement with these theories can only be obtained if the Poisson's ratio of the polybutadiene matrix is taken as 0·37 compared with the bulk value of 0·50. The morphological consequences of this result are also discussed.Torsional modulus measurements were also conducted but led to unexpectedly large values compared with those derived from the Young's modulus data alone. This is undoubtedly associated with the extreme anisotropy exhibited by these samples and requires further detailed examination.