CHAIN ORIENTATION IN BLOCK-COPOLYMERS EXHIBITING CYLINDRICALLY CONFINED CRYSTALLIZATION

The orientation of the crystal stems in diblock copolymers, where crys tallization 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), whe re MB is rubbery (T-g < T-room) and VCH is vitreous (T-g = 134 degrees C) 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 cylinder s. Crystallization in all of these materials can be confined to the cy lindrical microdomains. Not only do the crystals align preferentially within the semicrystalline cylinders, but this orientation varies depe nding on the ability of chains to diffuse during the crystallization p rocess. 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. Howe ver, when the chain mobility is limited, the crystal stems tilt with r espect to a plane which is normal to the cylinder axis, allowing bette r accommodation of amorphous material at the crystallite surface.