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 nan oconfined lamellae have been studied in a self-assembled poly(ethylene oxid e)-block-polystyrene (PEO-b-PS) diblock copolymer. The copolymer possesses number-average molecular weights of (M) over bar (PEO)(n) = 8.7K and (M) ov er bar (PS)(n) = 9.2K, and it forms a microphase-separated lamellar structu re in the melt. It has been found that the PEO crystal (the c axis) orienta tions are determined by only varying the Tc.(1) Based on real-time simultan eous two-dimensional (2D) small-angle X-ray scattering (SAXS) and wide-angl e 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 durin g 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) d o not possess specific orientation with respect to the PS lamellar surface normal, disregarding the history of crystal orientation in the samples gene rated before the self-seeding process. It is found that the initial stage o f crystal growth determines the final crystal orientation in the nanoconfin ed lamellae. Studies of the correlation lengths (apparent crystallite sizes ) along both [120] directions of the PEO crystals with various crystal orie ntations indicate that the PEO crystals formed in nanoconfined lamellae und ergo a change from a one-dimensional to a two-dimensional growth with incre asing T-c.