Anisotropic Bending and Unbending Behavior of Azobenzene Liquid‐Crystalline Gels by Light Exposure

Abstract

crosslinker are controlled, it is technically possible to prepare a gel membrane reflecting the specific color at a certain temperature. This method to prepare the interconnecting porous gels will give us the potential to study appropriate smart gels that may have interesting applications, such as in tunable optical filters, actuators, and sensors. Studies to confirm this are underway. ExperimentalSynthesis of Colloidal Crystals as Templates: To prepare the closest-packed colloidal crystals, a 15 wt.-% aqueous solution of silica spheres, having a diameter of 291 nm as determined from SEM measurements, was used. The growth of the crystal by the gravity sedimentation method was conducted in a flat Petri dish at 20 C. The stable dried crystals were obtained within 1 week as the water evaporated, and then completely dried in vacuo at 60 C. It is generally accepted that the crystalline arrays produced by the gravity sedimentation method have a cubic close-packed structure containing polycrystalline domains, similar to that of a natural opal. The thickness of the crystal can be easily controlled by the colloidal concentration and the iterative treatment. On the other hand, fine ordered crystals were created by the solvent evaporation method as follows: the colloidal suspension (ca. 15 wt.-%) was dropped onto a clean microscope slide and was placed in a thermostatic chamber at 90 C, in which the solvent gradually evaporated. A high quality crystal can be prepared with a thickness of up to 1 mm.Synthesis of Porous Hydrogels and Cylindrical Hydrogels: The thermosensitive gels were prepared by free-radical polymerization as follows. First, N-isopropylacrylamide (NIPA, 11.3 g), N, N¢-methylene bis(acrylamide) (0.513 g) as a crosslinker, and benzoylperoxide (0.048 g), the initiator, were dissolved in degassed and nitrogen-saturated 1,4-dioxane to a final volume of 50 mL. The solution was then infiltrated into the colloidal crystals in a Petri dish, and the polymerization was conducted at 60 C for 40 h. Afterwards, the samples were immersed in a 5 wt.-% HF aqueous solution to remove the SiO 2 . The gels for a swelling measurement were prepared in micropipettes of 100 lm diameter. The resulting porous gels and the cylindrical gels were washed carefully with distilled water for 1 week.Measurements: The swelling measurement was carried out by monitoring the diameter of the cylindrical gel in water. The temperature was controlled by using a temperature control system with circulating water. The reflection spectra were obtained by an Ocean Optics USB2000 fiber optic spectrometer [13] We did not use a macroporous gel to determine the swelling behavior.Therefore, a small difference in swelling size between the porous gel and cylindrical gel seems to be present, because the quantitatively different values were obtained in the wavelength of the peak of the reflection spectra from a porous gel as a function of temperature in Fig. 4b. Nevertheless, the experimental results qualitatively prove our description.

Photomechanical Effects in Azo-Polymers Studied by Neutron Reflectometry

“…In fact, a large number of the unusual photomotions observed in azo systems are probably related to this photomechanical effect. For instance, the macroscopic bending and unbending of liquid crystalline azo films 8,9 may be a manifestation of the photocontraction effects described here. The free surface of these films contracts to a greater extent than the bulk (since the light intensity is higher at the surface, owing to sample absorption), and this gradient in contraction leads to macroscopic deformation.…”

“…Although extensively studied, this surface mass transport phenomenon is not yet fully understood. 6 The azos have also been shown to generate macroscopic photomechanical effects, such as the expansion of thin films floating on a water surface, 7 the bending of freestanding polymer films irradiated with polarized light, 8,9 and even macroscopic motion of floating films. 10 Recently we investigated the photomechanical effect in azobenzene thin films by measuring the light-induced expansion of these materials using ellipsometry, 11 an optical technique that determines thickness and refractive index by analyzing the polarization change induced in a probe laser beam upon reflection.…”

Photoswitchable Nanomaterials Based on Hierarchically Organized Siloxane Oligomers

“…In these materials, contraction along the LC director occurs with isomerization of the planar trans-azobenzenes to the bent cis-azobenzenes. By limiting the depth of photoactivity, generally through utilizing the inherently high absorption coefficient of the azobenzenes, this contraction can give rise to anisotropic bending in poly mer films, [7,8] gels, [9] fibers, [10][11][12] and even crystals. [13] Different modes of motion arise with more complex molecular arrangements.…”

Optical and Physical Applications of Photocontrollable Materials: Azobenzene-Containing and Liquid Crystalline Polymers

“…The film homogeneously aligned by rubbing bends toward the UV irradiation because of shrinkage of the surface of the film [17] (Figure 1(b)). On the other hand, in the homeotropically aligned liquid crystalline azobenzene thick film, photodriven isotropic expansion occurs only near the film surface at the light incident side upon irradiation with UV light.…”