Controlled polymer crystallization through block copolymer self-assembly

The crystallization behavior of a series of polyethylene-containing semicrystalline diblock copolymers was examined via transmission electron microscopy (TEM) and small-angle x-ray scattering (SAXS). In semicrystalline-glassy diblocks, where the short block is crystallizable, crystallization was always confined within the nanoscale domains created by microphase separation, due to the presence of an already-vitreous matrix. Thus, the melt morphology was exactly reproduced in the solid state. A specially devised staining protocol for TEM even revealed the individual crystallites that form within the microdomains. In particular, TEM conducted on a pre-aligned cylinder-forming sample revealed exactly one ribbon-like crystal per microdomain, running along the cylinder axis. As for semicrystalline-rubbery diblocks, the fluid matrix could just as effectively restrict crystallization to within block copolymer microdomains provided that the interblock segregation strength during crystallization was sufficiently high. Weak interblock segregation during crystallization resulted in "breakout", where the melt structure was completely destroyed, yielding lamellar crystallites. The dynamics of crystallization under confinement were probed by time-resolved small- and wide-angle x-ray scattering (SAXS/WAXS) coupled with differential scanning calorimetry. Systems with confined crystallization exhibited first-order crystallization kinetics, in stark contrast to the sigmoidal kinetics typically observed in quiescently crystallized bulk polymers. Such first-order kinetics indicate that crystallization is effectively isolated within individual microdomains. Additionally, the temperature dependence of the nucleation rate indicates that nanoscale confined crystallization is homogeneously nucleated. This is also drastically different from bulk homopolymers where crystallization is predominantly nucleated by active impurities within the samples. In addition to understanding spatially confined polymer crystallization behavior, we examined two semicrystalline block copolymers where polyethylene comprises the matrix. Simultaneous SAXS/WAXS experiments conducted on fully crystallized samples reveal that the presence of amorphous nanoscale cylinders, whether glassy or rubbery, can induce an unexpected two-dimensional preferential orientation of the crystals that form within the matrix. These ribbon-like crystals tend to grow to large extents along the cylinder axis, as when crystals are confined inside cylinders, but the crystals are also preferentially oriented in the plane of the cylinder radii due to the similarity in length scales between the intercrystallite and intercylinder spacings.