Dynamics of Structure Formation in Crystallizable Block Copolymers

The dynamics of crystallization-driven microphase separation in semicrystalline block copolymers are investigated using diblocks of ethylene-(ethylene-alt-propylene) (E/EP) and ethylene-ethylethylene (E/EE). Simultaneous, time-resolved small-angle and wide-angle X-ray scattering (SAXS and WAXS) on heating and cooling are used to study the formation of the microphase-separated lamellar structure and the crystallization of the ethylene blocks, respectively. Sharp first- and higher-order SAXS peaks (as many as four reflections) evolve rapidly and simultaneously on cooling from the homogeneous melt, characteristic of a highly-ordered domain structure with relatively sharp interfaces. These semicrystalline diblocks thus form microdomains which appear strongly segregated, even though the small undercoolings used would put amorphous block copolymers in the weak segregation regime. Crystallization of the E blocks exactly tracks the development of the microdomain structure. During the initial rapid structure development, the SAXS peak positions are constant, suggesting that the ordered structures nucleate and simply grow to fill the sample, without any accompanying internal rearrangement. At longer times, a small decrease of the microdomain spacing is observed, attributed to the relaxation of a structure initially out of equilibrium.