Pathways toward Photoinduced Alignment Switching in Liquid Crystalline Block Copolymer Films

Abstract

The pathways toward linearly polarized light (LPL)-induced alignment switching in a diblock copolymer film composed of liquid crystalline (LC) azobenzene (Az) and amorphous poly(butyl methacrylate) (PBMA) blocks were studied in detail using polarized UV−vis absorption spectroscopy, grazing incidence small-angle X-ray scattering measurements, and polarized optical microscopy and transmission electron microscopy observations. The hierarchical structures of microphaseseparated cylinders of PBMA in a smectic LC Az layer matrix were prealigned by LPL and then irradiated by orthogonal LPL, which resulted in alignment switching to the orthogonal direction. In this process, the large prealigned domains were divided into substantially smaller domains at the submicrometer level, and then the structures were realigned in the orthogonal direction in a strongly cooperative manner, most likely through the domain rotation mechanism. The alignment change consisted of three stages: (i) fluctuations in the smectic layer of LC Az side chains in the initial state and breaking up of smaller grains to the submicrometer level before the orientation change (induction period), (ii) actual rotation of the divided domains driven by the photoinduced reorientation of Az mesogens (action period), and (iii) slower fusion and growth of smaller domains in the orthogonally realigned direction (postgrowth period). New aspects of dynamic self-assembly behavior in which different hierarchical structures are involved are proposed.

Exploitation of Self‐Assembly Phenomena in Liquid‐Crystalline Polymer Phases for Obtaining Multifunctional Materials

“…Finally, in order to assess the suitability of the system for the optical devices, pure polarization holographic gratings with large birefringence, which showed a periodic molecularly oriented structure, were fabricated using a 325 nm He-Cd laser in various polarization modes. In a recent work, Sano et al used linearly polarized light (LPL) for inducing alignment in a diblock copolymer film consisting of liquid-crystalline azobenzene and amorphous poly(butyl methacrylate) (PBMA) blocks; such structural and analytical techniques as polarized UV-Vis absorption spectroscopy, grazing incidence small-angle X-ray scattering, polarized optical microscopy, and transmission electron microscopy were exploited for characterizing the obtained materials [35]. More specifically, LPL was used for pre-aligning the hierarchical structures of microphase-separated cylinders of PBMA in the smectic liquid-crystalline azobenzene layer; then, the samples were irradiated by orthogonal LPL, thus promoting an alignment switching to the orthogonal direction.…”

Dynamic photoinduced realignment processes in photoresponsive block copolymer films: effects of the chain length and block copolymer architecture

“…[18][19][20] Typical 2D GI-SAXS image and 1D in-plane intensity profiles are displayed in Fig. The real time scattering changes were monitored for both the LC smectic layer (wider regions) and cylindrical MPS structure (smaller regions) in the block copolymer systems.…”

“…[7][8][9][10] In the photoprocess, the pre-aligned state by initial linearly polarized light (LPL) can be ''realigned'' by successive LPL with the electric vector E in a different direction. 18,19 This approach revealed a strongly cooperative interplay between the different hierarchical structures of the MPS cylinder pattern and the smectic LC layer. [14][15][16][17] Regarding the mechanism of photoinduced alternations of MPS structure, we have attempted to make in situ observations of the dynamic alignment changes in thin films of a diblock copolymer consisting of poly(butyl methachrylate)-block-azobenzene (Az)-containing LC polymer (PBMA-b-PAz).…”

“…[14][15][16][17] Regarding the mechanism of photoinduced alternations of MPS structure, we have attempted to make in situ observations of the dynamic alignment changes in thin films of a diblock copolymer consisting of poly(butyl methachrylate)-block-azobenzene (Az)-containing LC polymer (PBMA-b-PAz). 19 Additional efforts using a homologous diblock copolymer possessing poly(hexyl methacrylate) (PHMA-b-PAz) provided precise and in-depth understanding for this dynamic process. 18 By capturing the intermediate state of the realignment process by transmission electron microscopy (TEM) and polarized optical microscopy (POM), and in situ synchrotron grazing angle X-ray scattering (GI-SAXS) measurements, the realignment was found to take place via the sub-domain rotation mechanism retaining the hierarchical structure.…”

Photoalignment of Liquid Crystalline Polymer Films Containing Mesostructures and a Free Surface Command Layer

“…Chemical structure of PBMA-b-PAz and a schematic drawing of the alternative changes of LC orientation and MPS cylinders of PBMA-b-PAz by LPL irradiation.Very recently, Sano et al[24] succeeded in the observations of the intermediate state during the photo-realignment process. Evaluations by dichroic ratio of the Az mesogens, GI-SAXS, polarized optical microscopy, transmission electron microscopy (TEM) provided a more detailed view of the dynamic realignment process as follows (Figure 2).…”