Nanotailored crystalline morphology in hexagonally perforated layers of a self-assembled PS-b-PEO diblock copolymer

Nanoscale tailored polymer crystalline morphology is studied in a polystyrene-b-poly(ethylene oxide) (PS-b-PEO) diblock copolymer with number-average molecular weights for the PS and PEO blocks being 17 000 and 11 O00 g/mol, respectively. The PEO volume fraction is 0.39. After large amplitude planar reciprocating shear, a hexagonally perforated layer (HPL) structure is obtained. Since the glass transition temperature of the PS blocks (72 °C) is higher than the PEO crystal melting temperature (∼51 °C when the crystallization temperature, T_c, is lower than 40 °C), the PEO block crystallization is confined within the HPL structure. The PEO crystal (the c axis) orientation within this complex confined environment is investigated using simultaneous synchrotron two-dimensional smallangle and wide-angle X-ray scattering (SAXS and WAXS) methods. The PEO crystal orientations with respect to the layer plane (or the {000l}) of the HPL structure have been found to be dependent upon T_c. At very low T_cs (below -50 °C), the PEO crystals have a random orientation. Between -50 °C ≤ T_c ≤ -10 °C, the PEO crystal c axes preferentially orient parallel to the layer plane. Above T_c = 0 °C, the crystal c axes orient inclined to the layer plane of the HPL structure, and the tilt angle with respect to the layer plane increases with T_c. Contrary to the confined polymer crystallization in nanolamellar structure, however, the c axis orientation perpendicular to the layer plane is not found at least up to T_c = 40 °C in this HPL confined environment. Meanwhile, the ribbonlike PEO crystal growth is specifically tailored along the {101̄0} planes of the hexagonal PS perforations. Apparent crystallite size analyses using the Scherrer equation confirm the one-dimensional crystal growth at high T_cs. Using time-resolved WAXS experiments, the crystal orientation is observed to occur in the early stage of crystallization with a crystallinity of ∼7 wt %. Based on the results of specifically designed self-seeding experiment, the crystal orientation is determined by the crystal growth (surface nucleation) in the confined HPL phase rather than the preorientation of primary nuclei.