Non-equilibrium structures in block copolymers: Kink bands, lamellar contraction, and solvent -induced morphologies

This dissertation has been focused on three aspects in the processing-induced non-equilibrium structures in block copolymers, kink band defects and lamellar contraction during shear alignment, as well as morphological transitions by solvent processing methods. We examined kink band evolution under shear in a lamellar diblock copolymer. Using ex situ transmission electron microscopy (TEM) and in situ small angle x-ray scattering (SAXS)-steady shear technique, our study provided conclusive evidence for lamellar rotation within kink bands and boundary transformations during kink band evolution. By comparing the SAXS data with a theoretical model, we were able to quantitatively characterize the dynamics of kink bands under shear. We also examined lamellar contraction in the parallel lamellae using the same in situ SAXS-steady shear method. Specifically, we observed a molecular weight and shear rate dependence of lamellar contraction. These results indicated correlation between lamellar contraction, chain conformation distortion and applied shear stress. Solvent processing was used in this dissertation to examine effects of morphology and chain architecture on mechanical properties of triblock copolymers of polystyrene and polyisoprene. Specifically, solvents with varying selectivity were used in the casting process to achieve morphological transitions in a given triblock copolymer, that is, without varying the composition or molecular weight. In both SIS and ISI triblocks, we found that increasing the solubility parameter of the casting solvent increases the effective PS volume fraction, therefore inducing morphological transitions relative to the equilibrium states. The elastic moduli of these as-cast morphologies were measured using microtensile tests. While the moduli increases with increasing PS composition, as expected, we also found that at fixed composition the moduli increased with increasing connectivity of the glassy microdomains. At fixed molecular weight and composition, the average modulus increases with increasing glassy domain connectivity, in the order of PS cylinders, lamellae, and double gyroid structure. Furthermore, with the same total molecular weight, composition and microdomain morphology, the SIS triblocks exhibit higher moduli than the ISI, because the rubbery PI block in SIS is doubly tethered by glassy PS domains