Deformation behavior of cylindrical block copolymer bicrystals : pathway to understanding block copolymer grain boundaries

摘要

Model bicrystals made by adhering pieces of near-single-crystal styrene-isoprene-styrene (SIS) cylindrical block copolymer (BCP), produced by a roll-casting process; yield various types of pure tilt grain boundaries. The study of the deformation of the bicrystals, each containing one grain boundary, enables a deeper understanding of the influences grain boundaries and the incompatibilities between them have on mechanical behavior. Mechanical properties and deformation of near-single-crystal systems provide a reference base for the expected bicrystal behavior. We consider various aspects of incompatibility that can arise from joining two highly anisotropic grains together (i.e. Young's modulus, Poisson's ratio, deformation mode(s)). Experimentally, the structure of grain boundaries was characterized using atomic force microscopy (AFM). In deformation experiments, optical microscopy was employed to examine the deformation gradient in the specimen and in situ small angle x-ray scattering (SAXS) was used to monitor the microdomain structural evolution. Finally, finite element simulations illustrated the state of strains of the bicrystal. The symmetric (45-45) bicrystal turns out to be the most complex system, despite of the simplest geometry.