Poly(ethylene oxide) crystal orientation changes in an inverse hexagonal cylindrical phase morphology constructed by a poly(ethylene oxide)-block- polystyrene diblock copolymer

A polyethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymer with number-average molecular weights of 7.7k g/mol for the PS block and 21.4k g/mol for the PEO block was used to study the PEO crystal orientation changes at different crystallization temperatures (T_x) via small- and wide-angle X-ray scattering techniques. For this diblock copolymer, an inverse hexagonal cylinder (IHC) phase morphology was identified with PS cylinders hexagonally packed within the PEO matrix. In this IHC morphology, the PEO blocks were tethered on the convex interfaces of the PS domains, and the crystallization of PEO blocks was outside of the cylinders. The crystal orientation of the PEO blocks (the c-axis of the PEO crystals) after crystallization among the PS cylinders was, for the first time, found to change with respect to the long cylinder axis, â, depending solely on T_x. At very low T _x's, when the samples were quenched into liquid nitrogen, the crystals possessed a random orientation. When -30°C ≤ T_x ≤ 5°C, PEO crystals had an orientation with their c-axis parallel to â. Within the temperature region of 10°C ≤ T_x, ≤ 20°C, the c-axis crystal orientation changed to be tilted with respect to â (the tilting angle was defined to be between the c-axis of the PEO crystals and â). This tilting angle increased with increasing T_x. Finally, a major crystal orientation with the c-axes of PEO crystals perpendicular to â was observed when T_x reached 30°C. Furthermore, it was particularly interesting that the PEO crystals in the IHC phase were oriented in two dimensions when T_x = 30°C. Namely, the PEO crystal growth was specifically grown along the {101̄0} planes of the hexagonal PS cylinders. The crystallite sizes were estimated by the Scherrer equation. The PEO crystal sizes, at least along one dimension, were on the scale of the sizes limited by the distance between the neighboring glassy PS cylinders in the hexagonal lattice.