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

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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 (TODT) of this blend is 175 °C. Therefore, the PEO blocks form nanocylinders surrounded by a PS matrix below the TODT. 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 (Tc). At very low Tc (<−30 °C), PEO crystals are randomly oriented within the confined cylinders. Starting at Tc = −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 Tc, and finally it becomes 90° when Tc ≥ 2 °C. Crystallographic analysis indicates that the crystal c-axis orientation at each Tc corresponds to a uniform crystal orientation.

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