Photomobile polymer materials with crosslinked liquid-crystalline structures: molecular design, fabrication, and functions.

Crosslinked liquid-crystalline polymer materials that macroscopically deform when irradiated with light have been extensively studied in the past decade because of their potential in various applications, such as microactuators and microfluidic devices. The basic motions of these materials are contraction-expansion and bending-unbending, which are observed mainly in polysiloxanes and polyacrylates that contain photochromic moieties. Other sophisticated motions such as twisting, oscillation, rotation, and translational motion have also been achieved. In recent years, efforts have been made to improve the photoresponsive and mechanical properties of this novel class of materials through the modification of molecular structures, development of new fabrication methods, and construction of composite structures. Herein, we review structures, functions, and working mechanisms of photomobile materials and recent advances in this field.

Sunlight-Driven Smart Windows with a Wide Temperature Range of Optical Switching Based on Chiral Nematic Liquid Crystals.

Photoresponsive liquid crystals are promising materials for sunlight-driven smart windows, which can automatically change their optical states in response to sunlight and control energy flow between the inside and outside of a building. Herein, liquid-crystalline systems are developed that show a transparent-scattering transition upon irradiation with sunlight in a wide temperature range. Push-pull azobenzenes with axial chirality have been newly developed as photochromic chiral dopants to allow changes in mesostructures of liquid crystals in response to sunlight. To realize optical switching, photochromic and photoinert chiral compounds with opposite handedness of helical twisting are doped in liquid crystals. This liquid crystalline sample with a compensated nematic phase is transparent in its initial state. Upon irradiation with sunlight, this sample transforms to a scattering state due to the formation of helical mesostructures along with photoisomerization of azobenzene moieties and the change in the helical twisting power. After the cease of irradiation, the sample reverts to the transparent state through thermal back isomerization of azobenzene moieties. This system significantly improves the operating temperature range of sunlight-driven smart windows based on liquid crystals: the transparent-scattering transition is observed at 4-42 °C. The present mechanism allows development of autonomous and wireless smart windows adaptable to various environments.

Inducing Motions of Polymers in Liquid Nitrogen with Light.

Polymer materials that show macroscopic deformation in response to external stimuli are feasible for novel soft actuators including microactuators. Incorporation of photochromic moieties, such as azobenzenes, into polymer networks enables macroscopic deformation under irradiation with light through photoisomerization. Under cryogenic conditions, however, it has been difficult to induce macroscopic deformation as polymers lose their soft nature due to the severe restrictions of molecular motions. Here, activation of molecular motions and macroscopic deformation in liquid nitrogen only with light for polymers containing photochromic moieties is reported. Photoinduced bending of polymer networks with normal azobenzenes in liquid nitrogen is enabled by preliminary UV irradiation at room temperature to produce cis-isomers. To realize photoinduced deformation directly in liquid nitrogen, polymer networks are functionalized with bridged azobenzenes, which exist as cis-isomers in thermodynamic equilibrium. The films with bridged azobenzenes exhibit reversible photoisomerization and bending upon irradiation with light in liquid nitrogen without the need of preliminary irradiation, implying that the change in conformation of polymer chains can be isothermally induced even under cryogenic conditions. Achievement of flexible motions under cryogenic conditions through isothermal processes will greatly expand the operating temperature range of soft actuators.

Conformation of single polymer chain in rubbed thin film observed by fluorescence imaging.

The conformation of poly(methyl methacrylate) (PMMA) chains in a thin film after the rubbing process was investigated through the direct observation of the single chains by scanning near-field optical microscopy (SNOM) and excitation polarization modulation microscopy (EPMM). The rubbing at room temperature hardly changed the dimension on the whole chain scale in spite of the increase in orientational order on the segmental scale. The increase in the chain dimension along the rubbing direction was observed in the film rubbed at the higher temperature, which showed a surface morphology with fine groove. The extension ratio of the whole chain in the rubbed film was much smaller than that in the uniaxially stretched film. This indicates that the rubbing process mainly induces the conformational change on the length scale of the monomer unit rather than for the whole chain.

Photomobile Liquid-Crystalline Elastomers with Rearrangeable Networks.

Incorporation of dynamic covalent bonds into photomobile liquid-crystalline elastomers with a polysiloxane backbone enables the alignment of mesogens and macroscopic shapes to be controlled through the rearrangement of the network topology, even after the formation of chemically crosslinked networks. The reshaped samples show various sophisticated 3D motions upon irradiation with UV and visible light, depending on their initial shapes.