The initiation, structure, and dynamics of defects control both the structure and physical properties of materials. In this work, kink band defects are systematically introduced into a lamellar poly(styrene-b-ethylene propylene) diblock copolymer by applying various rates and total strains of steady shear. However, kink bands are only produced when the shear strain exceeds a critical value. The mere existence of kink bands implies the presence of a preferential slip plane parallel to the lamellae, which we estimate exists within the polystyrene microdomains. Furthermore, our results regarding the dependence of kink band geometry on shear rate and strain suggest that these defects are formed by the rotation of lamellae. Based on a rotation mechanism, the characteristic size of the kink bands and their spatial distribution, it appears that preexisting defects initiate kink bands. Insights gained from steady-shear-induced kink bands are extended to oscillatory shear alignment of this high molecular weight diblock copolymer, allowing us to circumvent a previously reported shear-stabilized parallel-transverse biaxial texture.

Recent studies have induced a biaxial texture in lamellae-forming poly(styrene-b-ethylene propylene) diblock copolymers by applying large amplitude oscillatory shear. According to small-angle X-ray scattering, this biaxial texture consists of “parallel” lamellae (normal to lamellae aligned perpendicular to the shearing surfaces) and “transverse” lamellae (normal to lamellae aligned parallel to the shearing direction). This study determines the arrangement of these two populations of lamellae by using field emission scanning electron microscopy to examine the microstructure and superstructure. The transverse lamellae are separated from the parallel lamellae by sets of parallel wall defects which have two characteristic orientations. This parallel−transverse biaxial morphology shows an astonishing resemblance to that of kink bands. The formation of kink bands suggests that the transverse lamellae could be the result of buckling in the parallel lamellae. Relaxation of the kink band superstructure during quiescent annealing occurs primarily by lamellae tilting, rather than twisting, and produces a variety of defect structures.

In situ small-angle X-ray scattering (SAXS) rheology is used to study the dynamic process of shear alignment in a lamellar poly(styrene-b-ethylene propylene) diblock copolymer, at temperatures far below the microphase separation transition temperature. We have focused on the alignment dynamics at time scales shorter than 1 cycle of deformation. To extract this valuable information, we use prealigned specimens and follow subtle changes in the orientation during a steady shear deformation. Two notable changes in the azimuthal SAXS intensity are observed:  the maximum of the main peak induced during prealignment shifts to lower azimuthal angles and a secondary peak develops and shifts to higher angles. We effectively modeled the shift of the main peak maximum by assuming that lamellae rotate with the vorticity component of shear. This provides conclusive evidence for lamellar rotation in block copolymers, as opposed to a discontinuous transformation such as lamellar dissolution and reformation. Lamellar rotation also supports our previously proposed mechanism for kink band formation.