Ordering Kinetics and Alignment of Block Copolymer Lamellae under Shear Flow

Abstract

The effect of large-amplitude oscillatory shear flow on a concentrated block copolymer solution with lamellar order was studied by in-situ small-angle neutron scattering. Microstructural changes were studied as a function of temperature, frequency of the oscillatory flow field, and thermal history prior to turning on the shear field. We find that the alignment path depends mainly on thermal history prior to turning on the shear field and is independent of frequency and temperature. At long times, the lamellae were aligned parallel to the shearing plates, regardless of frequency, temperature, and thermal history. We refer to this as the parallel orientation. Monotonic changes from the unaligned to the aligned state were found when the shear field was turned on after the sample was completely ordered. The alignment kinetics, in this case, occurs in two stages. The first stage consists of a rapid rotation of the grains so that the lamellar normals lie in the velocity gradient−vorticity plane. This is followed by a slower process wherein the lamellar normals get increasingly localized in the velocity gradient direction. We also studied ordering kinetics under shear, by turning on the shear field before significant ordering had taken place. In this case, the first stage of ordering resulted in the formation of lamellae aligned perpendicular to the shearing plates in addition to the parallel lamellae, regardless of temperature and frequency. Eventually the perpendicular lamellae were transformed to parallel lamellae via an undulation instability.