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 pr esented. The experiments were conducted using small-angle neutron scat tering (SANS), as a function of shear rate and temperature, and transm ission electron microscopy (TEM) on quenched specimens. The triblock c opolymer is composed of polystyrene-d(8)/polybutadiene/polystyrene-d(8 ), and the ordered microstructure normally consists of hexagonally pac ked cylinders. Two temperature-dependent, characteristic shear rates, gamma(C1) and gamma(C2), are identified from the scattering results. T he first characteristic shear rate identifies the shear rate required to go from a disordered state to an ordered state, while the second ch aracteristic shear rate is interpreted as the shear rate necessary to produce a different ordered morphology. This latter transition was pre viously identified as being analogous to a martensitic transition in m etal alloys. The supporting SANS and TEM evidence for the new ordered phase is presented. Dynamical aspects of the structural transition bet ween different ordered triblock morphologies are discussed using the m odel of a martensitic-like transformation.