A Shear-Induced Martensitic-like Transformation in a Block Copolymer Melt

The coexistence of two cylindrical microstructures of different symmetries has been observed in a sheared and quenched poly(styrene-d8/polybutadiene/poly(styrene-d8) (SBS) block copolymer (23 wt % styrene-d8) by small-angle neutron scattering and transmission electron microscopy. Near the order-disorder transition (ODT) temperature, the equilibrium cylindrical microstructure with d(100) = 21 ± 2 nm and cylinder diameter of about 12 nm orients in the shear field as expected based on previous reports. Above a critical shear rate and at an appropriate strain, a new shear-induced cylindrical microstructure forms. The shear-induced structure consists of grains of cylinders with a smaller diameter (8-9 nm) and a d(110) spacing of 12 nm. The transformation which occurs, from the original p31m space group with a unit cell dimension a = 25 nm to a p3ml space group of lower symmetry, also with a = 25 nm, is analogous to martensitic transformations which occur in metals during deformation processes. This transformation may be a result of the preferred orientation of the original cylindrical microstructure relative to the shear field and the imposition of a nonsymmetric (simple shear) deformation.