Shear-induced changes in the order-disorder transition temperature and the morphology of a triblock copolymer

A summary of our work on a triblock copolymer under steady shear is presented. The experiments were conducted using small-angle neutron scattering (SANS), as a function of shear rate and temperature, and transmission electron microscopy (TEM) on quenched specimens. The triblock copolymer is composed of polystyrene-d 8/polybutadiene/polystyrene-d 8, and the ordered microstructure normally consists of hexagonally packed cylinders. Two temperature-dependent, characteristic shear rates, -γCI and γC2, are identified from the scattering results. The first characteristic shear rate identifies the shear rate required to go from a disordered state to an ordered state, while the second characteristic shear rate is interpreted as the shear rate necessary to produce a different ordered morphology. This latter transition was previously identified as being analogous to a martensitic transition in metal alloys. The supporting SANS and TEM evidence for the new ordered phase is presented. Dynamical aspects of the structural transition between different ordered triblock morphologies are discussed using the modei of a martensitic-like transformation.