Imaging block copolymer crystallization in real time with the atomic force microscope

The crystallization of two diblock copolymers, one forming a cylindrical mesophase and the other a spherical mesophase in which the crystallizable unit, polyethylene, is the minority component, has been followed in situ using high-temperature atomic force microscopy. Contrast is obtained between the crystallizable and noncrystallizable microdomains in the melt as well as between the melt and the crystalline material, allowing the influence of the melt structure on crystallization to be observed in real time. In both cases the mesophase structure is destroyed by crystallization, but its influence over the growth dynamics is followed. In the cylinder former, transport of material to the crystal growth front is found to occur primarily by diffusion, rather than by flow, leaving the melt structure unperturbed. Crystal growth rates vary considerably between individual crystallites, with more rapid growth along existing ethylene-rich domains. Melting occurs randomly along the crystallites, leading to their breakup into small blocks. In the sphere former crystallization leads to a densely branched seaweed structure, the morphology controlled by the directionally varying diffusion rate between the ethylene-rich spherical domains. On increasing supercooling the extent to which the spherical melt structure controls the crystal morphology increases, with crystal growth directions dominated by sphere-sphere nearest-neighbor angles.