Photoinduced changes in the upper critical solution temperature (UCST) were investigated for polymer solutions in an azobenzene-doped liquid crystal solvent. The UCST of poly(methyl methacrylate) (PMMA) and polystyrene (PS) solutions dropped upon irradiation with UV light, which induces trans-cis photoisomerization of the doped azo dye. In the case of PMMA solutions, the photoinduced drop in UCST was significantly larger than that expected from previous studies using azo-based polymers and common solvents. Moreover, the UCST of PS solutions was also decreased under the photoirradiation, in a direction opposite to that expected from the contribution of polarity. X-ray diffraction data of the solvent suggest that decreased intermolecular interaction in the solvent (i.e. larger distance between the solvent molecules) is responsible for the photoresponsive behavior of UCST. The proposed mechanism is consistent with the Flory-Huggins theory. Using such photoresponses in UCST, isothermal transition between 2-phase and 1-phase states by photoirradiation was demonstrated.
(Phys. Chem. Chem. Phys., 20, 5850-5855 (2018))
Colloidal composites consisting of polymer-brush-afforded silica particles (P-SiPs) and a nematic liquid crystal (LC) exhibited three gel states with distinct viscoelastic and/or optical properties depending on temperature: (1) opaque hard gel, (2) translucent hard gel, and (3) translucent soft gel. We demonstrated that the transitions of the optical property and the hardness of the gels were due to the phase transition of the LC matrix and the glass transition of the grafted polymers of P-SiPs, respectively. We then revealed that the gelation (the formation of the translucent soft gel) was caused by the phase separation of P-SiPs and LC matrix in an isotropic phase based on spinodal decomposition. In addition, the particle concentration and molecular weight of the grafted polymer of P-SiPs were observed to significantly affect the elastic moduli and thermal stability of the composite gels. By the addition of an azobenzene derivative into an LC matrix, we achieved photochemical switching of the transparency of the composites based on the photoinduced phase transition of LCs, while keeping self-supporting ability of the composite gel.
(ACS Appl. Mater. Interfaces, 8, 29649-29657 (2016))
Photo-control of a bilayer-to-nonbilayer phase transition (the liquid-crystalline Lalpha phase to the inverted hexagonal HII phase) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) by photoisomerization of incorporated stilbene molecules was examined utilizing differential scanning calorimetry (DSC), small-angle X-ray diffraction (SAXD), ultraviolet (UV)/visible absorption, and attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopies. cis-Stilbene lowered the transition temperature, Th, to a greater extent than trans-stilbene, and the difference was at most ca. 10 degree C. At temperatures higher than Th of POPE/cis-stilbene but lower than that of POPE/trans-stilbene, photo-isomerization from the trans to the cis form of the stilbene molecules by irradiation with UV light caused a Lalpha-HII phase transition. The UV irradiation partially induced the HII phase at a constant temperature due to the incomplete photoisomerization of stilbene (ca. 60%). The reduction of Th by the incorporation of stilbenes was caused mainly by reducing the spontaneous radius of curvature of the lipid monolayer, R0. The greater bulkiness of cis-stilbene as compared to the trans form resulted in a more effective reduction of R0 and stabilization of the HII phase.
(Langmuir, 32, 7647-7653 (2016))
1,2-Dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayer with stilbene was examined using phase-contrast and fluorescence microscopies, differential scanning calorimetry, and Fourier transform infrared and UV/visible absorption spectroscopies. The results indicate that the temperature of gel/liquid-crystalline phase transition of DPPC bilayer with cis-stilbene is much lower than that with trans-isomer. Photoirradiation on trans-stilbene in DPPC bilayer suppresses the pretransition (Lbeta' to Pbeta'), while the gel/liquid-crystalline phase transition (Pbeta' to Lalpha) changes little due to the poor photoisomerization rate in both gel and liquid-crystalline phases. It is shown that photoisomerization of incorporated stilbene and the phase behavior of the lipid bilayer depend on each other.
(Chem. Lett., 43, 1352-1354 (2014))