Shear-induced long-range alignment of BCC-ordered block copolymers

Abstract

Effect of large shear on an asymmetric block copolymer with nanospherical domains has been studied using rheology and small angle X-ray scattering. The material investigated was a triblock copolymer poly (styrene-b-(ethylene-co-butylene)-b-styrene) swollen in a midblock-selective solvent. When cooled below the order- disorder transition temperature (TODT), the system forms a locally ordered structure of grains with body-centered cubic (BCC) lattice. Isothermal shearing, either at constant rate or with large amplitude oscillatory shear (LAOS) at low frequencies and strain amplitude greater than or equal to 2.0, leads to the destruction of the BCC lattice (isothermal "shear melting"). Upon cessation of the shear, the BCC structure recovers with kinetics similar to the one after thermal quench from above TODT. Under certain experi- mental conditions, LAOS leads to alignment of the BCC lattice. The lattice orientation depends primarily on shear- ing frequency. At low frequencies, there exists an upper and lower bound on strain amplitude where monodomain tex- tures can be obtained. Upon alignment, the modulus drops by about 30% of that of the polycrystalline structure. Mea- surement of rheological properties offers an indirect method for distinguishing between polycrystalline structure (grains) and monodomain texture.