Epitaxial growth and shearing of the body centered cubic phase in diblock copolymer melts

Abstract

Two poly(ethylenepropylene)–poly(ethylethylene) (PEP-PEE) diblock copolymer melts, containing 25% and 83% by volume PEP, were investigated using small-angle neutron scattering (SANS) and rheological measurements. The SANS measurements were performed with the aid of an in situ shearing device operated directly in the neutron beam. Each sample was observed to possess three equilibrium phases: two ordered phases at low temperature and a disordered phase at elevated temperatures. The low and high temperature ordered phases have been evaluated to be hexagonally packed (hex) cylinders and body centered cubic (bcc) spheres, respectively. Application of a large amplitude dynamic shear deformation to the hex phase leads to well-aligned cylinders, with a specific crystallographic orientation relative to the shear plane. Upon heating through the cylinder-to-sphere transition, the bcc phase grows epitaxially, with the [111] direction coincident with the original cylinder axis, leading to a well-defined twinned microstructure. SANS measurements performed while the bcc specimens were dynamically sheared revealed a rich compliment of microstructural rearrangements, with the twinned state appearing at low and high shear rates, and two-dimensional disordering at intermediate shear rates.