Direct Imaging of the Orientational Dynamics of Block Copolymer Lamellar Phase Subjected to Shear Flow

Abstract

An imaging torsional parallel-plate shear cell, producing a well-defined range of shear rates, was used to probe the dynamics of shear-induced structural transitions in complex fluids. The photonic properties of high molecular weight poly(styrene-b-isoprene) symmetric diblock copolymer solutions were exploited to visualize the parallel-to-perpendicular shear-induced orientation transition of the lamellar phase. The online imaging demonstrated that the transition is purely shear rate controlled. Exploiting different shear pulse profiles demonstrates that below a critical value shear flow aligns the lamellar phase in the parallel orientation and in the perpendicular orientation at shear rates above this critical value. The reverse perpendicular-to-parallel shear-induced orientation transition of the lamellar phase was also detected. However, experimental observations suggest that the mechanisms of the forward and the reverse orientation transition are different. It was also found that at low shear rates, when the perpendicular alignment is unstable, the change in lamellae orientation from perpendicular back to parallel is strain controlled. There is a good correlation between results obtained from the imaging technique and by other techniques such as rheology (both steady shear and dynamic measurements), ex situ polarized light imaging, and small-angle X-ray scattering.