Initial-stage growth controlled crystal orientations in nanoconfined lamellae of a self-assembled crystalline-amorphous diblock copolymer

Crystallization temperature (T_c)-dependent crystal orientations within nanoconfined lamellae have been studied in a self-assembled poly(ethylene oxide)-block-polystyrene (PEO-b-PS) diblock copolymer. The copolymer possesses number-average molecular weights of M_n^PEO = 8.7 K and M_n^PS = 9.2 K, and it forms a microphase-separated lamellar structure in the melt. It has been found that the PEO crystal (the c axis) orientations are determined by only varying the T_c. Based on real-time simultaneous two-dimensional (2D) small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) experiments, the formation of crystal orientation is found in an early stage of PEO crystallization confined between the PS lamellae. To understand whether the crystal orientation is determined during the primary nucleation or crystal growth step, specifically designed self-seeded crystallization experiments are carried out using 2D SAXS and WAXS techniques. Experimental results suggest that primary nuclei (self-seeds) do not possess specific orientation with respect to the PS lamellar surface normal, disregarding the history of crystal orientation in the samples generated before the self-seeding process. It is found that the initial stage of crystal growth determines the final crystal orientation in the nanoconfined lamellae. Studies of the correlation lengths (apparent crystallite sizes) along both [120] directions of the PEO crystals with various crystal orientations indicate that the PEO crystals formed in nanoconfined lamellae undergo a change from a one-dimensional to a two-dimensional growth with increasing T_c.