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The orientation of the crystal stems in diblock copolymers, where crystallization occurs within the cylindrical microdomains present in the melt mesophase, is investigated via X-ray scattering on flow-aligned specimens. A series of ethylene-b-(3-methyl-1-butene) diblocks (E/MB) is compared with an ethylene-b-(vinylcyclohexane) diblock (E/VCH), where MB is rubbery (Tg < Troom) and VCH is vitreous (Tg = 134 degreesC) at the freezing point of the E block. All of the diblocks contain 26-28 wt % E, leading to a morphology of hexagonally-packed E cylinders. Crystallization in all of these materials can be confined to the cylindrical microdomains. Not only do the crystals align preferentially within the semicrystalline cylinders, but this orientation varies depending on the ability of chains to diffuse during the crystallization process. When chain diffusion is most rapid, alignment is observed with the chain axis in the crystals perpendicular to the cylinder axis and the b axis (fast growth axis) coincident with the cylinder axis. However, when the chain mobility is limited, the crystal stems tilt with respect to a plane which is normal to the cylinder axis, allowing better accommodation of amorphous material at the crystallite surface.
Quiram et al. (Wed,) studied this question.