Viscoelastic characterization of an order-order transition in a mixture ofdi- and triblock copolymers

The phase behavior and linear viscoelasticity of a mixture of a matched dib lock and triblock copolymer of styrene and ethylene-butene-1 are reported. The mixture undergoes reversible thermotropic transitions from a cylindrica lly ordered state at low temperatures to a spherically ordered state at int ermediate temperatures to a disordered state at high temperatures. The morp hological assignments were based on TEM and SANS measurements, which also r evealed a decrease in the domain size in the spherically ordered state as c ompared to the cylindrically ordered state. The kinetics of the order-order transition are extremely slow, with the sample exhibiting rheological char acteristics similar to a disordered material immediately after heating from the cylindrical to the spherical state. The transition exhibits a long ind uction period wherein the sample continues to behave like a disordered mate rial followed by a rapid increase in the storage modulus and a final platea u value. The kinetics and rheological characteristics of a material quenche d ti om the disordered state to the spherical state are remarkably similar to those obtained after heating from the cylindrical to the spherical state , suggesting that the transition from cylindrically ordered to spherically ordered microdomains proceeds via a mesoscopically disordered state.