New aspects for the hierarchical cooperative motions in photoalignment process of liquid crystalline block copolymer films

The realignment process of azobenzene-containing liquid crystalline (LC) block copolymer films by irradiation with linearly polarized light (LPL) was investigated in detail by time-resolved grazing incidence X-ray scattering measurements using a synchrotron beam. By using a newly synthesized diblock copolymer possessing a poly(hexyl methacrylate (PHMA) block with a lowered glass transition temperature (T_g = -5 °C), intense X-ray scattering signals were obtained, which favorably provided precise and detailed time course profiles and information on the interplay between the hierarchical structures involved along the realignment process. The LC layers were softened and fluctuated at an early stage by LPL irradiation which induced shrinkage of the cylinder to cylinder distance of microphase separation patterns before the alignment change. Interestingly, the ordering process of the MPS cylinder array of larger hierarchical structure proceeded faster than that of the azobenzene LC layer of smaller feature size in the realigned state. Decisive evidence was obtained to support the subdomain rotation mechanism in the realignment process.