Hierarchical Self-Assembly in Liquid-Crystalline Block Copolymers Enabled by Chirality Transfer

Photoreversible Order-Disorder Transition in an Ionic Liquid Solvated Block Polymer

The structure of a solution of poly(methyl methacrylate)-block-poly(benzyl methacrylate-stat-(4-phenylazophenyl methacrylate)) in the ionic liquid 1-ethyl-3-methyl imidazolium bis(trifluoromethanesulfonylimide) can be controlled with light. We explored the phase behavior of this block polymer at a concentration of 30 wt % as a function of temperature. Remarkably, this solution shows ordering into hexagonally closed packed spheres (HCP) upon heating under visible light, as confirmed by small-angle X-ray scattering. Small amplitude oscillatory shear rheology was used to demonstrate that the system could readily be converted between HCP and disorder by switching between visible and UV light wavelengths at 100 °C. This switching was cycled six times, without any noticeable change in the ordered or disordered state. The dependence on illuminating wavelength arises from the changes in polarity of the azobenzene moiety, which modulates its solubility in this ionic liquid to a significant extent. This is the first example of reversible light-triggered ordering of a "coil-coil" block polymer, a phenomenon that has been previously observed only in liquid crystalline systems.

Synthesis and Photoinduced Anisotropy of Polymers Containing Nunchaku-Like Unit with an Azobenzene and a Mesogen

A series of polymers containing nunchaku-like unit with an azo chromophore and a mesogen group was successfully prepared and photoinduced anisotropy of the obtained polymers was minutely investigated. Firstly, monomers containing nunchaku-like unit with an azo chromophore and a mesogen group linked by flexible group were synthesized. The structure of the monomers was confirmed via NMR COSY spectra. Subsequently, the obtained monomers were polymerized into corresponding polymers through RAFT polymerization. The prepared polymer samples were characterized through NMR, FTIR, gel permeation chromatography (GPC), and UV-vis testing while the thermal properties of the samples were investigated through differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) measurements. The photoinduced isomerization of the polymers, which was researched in situ via measuring UV-vis spectra of the polymer solutions and spin-coated films under irradiation with 450 nm light or putting in darkness, demonstrated the rapid trans-cis-trans isomerization of the polymers. When irradiated with a linearly polarized light, significant photoinduced birefringence and dichroism were observed, suggesting photoinduced isomerization of azobenzene can drive orientation of mesogen in the system. This study blazes a way to design the optical materials with light-controllable birefringence and dichroism.

Synthesis of Polyazobenzenes Exhibiting Photoisomerization and Liquid Crystallinity

While only a few studies have investigated the synthesis of main chain-type polyazobenzenes, they continue to draw an increasing amount of attention owing to their industrial applications in holography, dyes, and functional adhesives. In this study, dibromoazobenzene was prepared as a monomer for constructing azo-based π-conjugated polymers. Miyaura⁻Suzuki cross-coupling polymerization was conducted to develop copolymers containing an azobenzene unit as a photoisomerization block and a pyrimidine-based liquid crystal generator block. The prepared polymers exhibited thermotropic liquid crystallinity and underwent cis and trans photoisomerization upon irradiation with ultraviolet and visible light. Furthermore, the photoisomerization behavior was examined using optical absorption spectroscopy and synchrotron X-ray diffraction spectrometry.

Photoresponsive polymers with multi-azobenzene groups

Photoresponsive polymers with multi-azobenzene groups are reviewed and their potential applications in photoactuation, photo-patterning, and photoinduced birefringence are introduced.