Crystal Orientation Changes in Two-Dimensionally Confined Nanocylinders in a Poly(ethylene oxide)-b-polystyrene/Polystyrene Blend

Abstract

A lamella-forming poly(ethylene oxide)- b -polystyrene (PEO- b -PS) diblock copolymer has been blended with a low molecular weight polystyrene (PS) homopolymer to form a miscible polymer blend. The PEO volume fraction is 0.32, and the order−disorder transition temperature ( T ODT ) of this blend is 175 °C. Therefore, the PEO blocks form nanocylinders surrounded by a PS matrix below the T ODT . Since the glass transition temperature of the PS is 64 °C and the PEO crystal melting occurs at ∼50 °C, the PEO-block crystallization takes place in a two-dimensionally confined glassy environment. The cylinder diameter is determined to be 13.7 nm, based on small-angle X-ray scattering (SAXS) and transmission electron microscopy results. Using simultaneous two-dimensional SAXS and wide-angle X-ray scattering techniques, the crystal orientation (the c -axes of the PEO crystals) within the nanocylinders is found to change simply depending upon the crystallization temperature ( T c ). At very low T c (<−30 °C), PEO crystals are randomly oriented within the confined cylinders. Starting at T c = −30 °C, the crystal orientation changes to be inclined with respect to the cylinder axis, â . The tilt angle from â continuously increases with increasing T c, and finally it becomes 90° when T c ≥ 2 °C. Crystallographic analysis indicates that the crystal c -axis orientation at each T c corresponds to a uniform crystal orientation.