EPITAXIAL-GROWTH AND SHEARING OF THE BODY-CENTERED-CUBIC PHASE IN DIBLOCK COPOLYMER MELTS

Two poly(ethylenepropylene)-poly(ethylethylene) (PEP-PEE) diblock copo lymer melts, containing 25% and 83% by volume PEP, were investigated u sing small-angle neutron scattering (SANS) and rheological measurement s. The SANS measurements were performed with the aid of an in situ she aring device operated directly in the neutron beam. Each sample was ob served to possess three equilibrium phases: two ordered phases at low temperature and a disordered phase at elevated temperatures. The low a nd high temperature ordered phases have been evaluated to be hexagonal ly packed (hex) cylinders and body centered cubic (bcc) spheres, respe ctively. Application of a large amplitude dynamic shear deformation to the hex phase leads to well-aligned cylinders, with a specific crysta llographic orientation relative to the shear plane. Upon heating throu gh 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 pe rformed while the bcc specimens were dynamically sheared revealed a ri ch compliment of microstructural rearrangements, with the twinned stat e appearing at low and high shear rates, and two-dimensional disorderi ng at intermediate shear rates.