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

Abstract

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.

(cont.) Due to the opposite orientation of the grains, the deformation of the symmetric bicrystal results in a rigid translation of the grain boundary. A triangular shaped influence region indicates that the influence distance varies along the grain boundary length. A portion of the influence region has limited expansion and is slightly sheared along the grain boundary. Another portion of the influence region experiences high tension. For the asymmetric (90-45) bicrystal, the deformation is mostly influenced by the difference in deformation modes: dilation vs. shearing. The distortion due to deformation in the diagonal grain induces rotation and advances the deformation in the perpendicular grain near the grain boundary. For the asymmetric (90-0) bicrystal, the most influential factors are the Young's modulus and the Poisson's ratio. The much softer perpendicular grain assumes most of the deformation by extensive dilation and lateral contraction. Near the grain boundary, the perpendicular grain is constrained by the rigid parallel grain such that its deformation is impeded. The influence region in the perpendicular grain is narrow and invariant along the grain boundary length.