Formation of H* Phase in Chiral Block Copolymers: Morphology Evolution As Revealed by Time-Resolved X-ray Scattering

The ordering of a chiral block copolymer (BCP*) polystyrene-b-poly(d-lactide) during the solvent-casting process was studied in real time using in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) simultaneously. Three different solvents, namely benzene, dimethylformamide, and 1,3-dioxolane, were used. It was found that, according to the solution phase behavior of these BCPs*, there were three different stages of morphology development during solvent removal. Initially, the development of the morphology during solvent removal was the same for all solvents studied. However, for strongly selective solvents, crystallization dominated and no H* phase formed. For moderately selective solvents, crystallization was initially suppressed, a microphase separated H* phase formed, and then crystallization occurred, leading ultimately to a crystalline H* phase. The results clearly showed that crystalline ordering is not required for the H* phase to form in mutually good solvents. Consequently, the preferred handedness of molecular packing of the chiral block in the amorphous state necessarily underpins the transfer of chirality from the segmental level to the microphase-separated morphology level.