Yanlei Yu

Photomechanics: Directed bending of a polymer film by light

Polymer solutions and solids that contain light-sensitive molecules can undergo photo-contraction, whereby light energy is converted into mechanical energy. Here we show that a single film of a liquid-crystal network containing an azobenzene chromophore can be repeatedly and precisely bent along any chosen direction by using linearly polarized light. This striking photomechanical effect results from a photoselective volume contraction and may be useful in the development of high-speed actuators for microscale or nanoscale applications, for example in microrobots in medicine or optical microtweezers.

Nucleation mechanism and crystallization behavior of poly(ethylene terephthalate) containing ionomers

By means of differential scanning calorimetry, the influences of AClyn® on the crystallization of poly(ethylene terephthalate) (PET) were investigated and compared with Surlyn®. AClyn initiates heterogeneous nucleation and substantially increases the crystallization rate of PET while Surlyn dissolves in molten PET and reacts with PET chains, producing ionic chain ends. They subsequently associate into ionic clusters which, eventually, act as the true nucleation species. AClyn is more effective to accelerate nucleation than Surlyn. Crystallization kinetics analysis by the Avrami equation indicates that an athermal nucleation seems to occur in the case of both AClyn and Surlyn.

Photoresponsive Behavior of Azobenzene Liquid-Crystalline Gels

The azobenzene liquid-crystalline gel (LCG) films with macroscopic uniaxial alignment were prepared to investigate photoresponsive behavior. By polarizing microscopy, fiber-like structures aligned in one direction were observed. Furthermore, with a confocal laser scanning microscope (CLSM), it was confirmed that the fiber-like structures were formed even in the bulk of the LCGs. When the LCG films were irradiated with UV light below Tg, changes in the microscopic surface morphology were observed, while no appreciable macroscopic change was observed. On the other hand, when the LCGs were plasticized by suspending in solvents or heating above Tg, the LCG films showed macroscopic changes (e.g. bending) upon UV light irradiation. The bending and unbending could be repeated just by changing the wavelength of the irradiation light. It was suggested that the bending is induced by an absorption gradient which produces a volume difference between the front surface area and the bulk of the gel films.

Precisely Direction-Controllable Bending of Cross-Linked Liquid-Crystalline Polymer Films by Light

Cross-linked liquid-crystalline polymer films were prepared by polymerization of mixtures containing azobenzene monomers and cross-linkers with azobenzene moieties. Two types of the cross-linked liquid-crystalline polymer films were prepared: monodomain films and polydomains films. Photoirradiation of the monodomain films with UV linearly polarized light (LPL) gave rise to bending of the films toward actinic light along the polarization direction, when the light polarization was parallel to the rubbing direction. When the two directions were perpendicular to each other, the bending was isotropic without a preferential direction. On the other hand, exposure of the polydomain films to UV LPL led to bending of the films always along the polarization direction of the actinic light. This means that one can induce bending of the film along any direction precisely by choosing the polarization direction of the actinic light.

Photoinduced Deformation Behavior of Crosslinked Azobenzene Liquid-Crystalline Polymer Films with Unimorph and Bimorph Structure

The use of a crosslinked azobenzene liquid-crystalline polymer (CALP) system for micro-integrated actuators becomes especially attractive. CALPs can deform along the alignment direction of mesogens upon irradiation of UV/visible light owing to photochemical phase transition. In this study, to improve the bending speed, CALP films with a homogenous alignment on one surface and a homeotropic alignment on the opposite surface (hybrid alignment) were prepared and their bending behavior was investigated. The films showed photoinduced bending whose direction was determined by the surface alignment treatment, and the bending speed was greatly enhanced upon irradiation from both surfaces of the film at the same time.

Preparation and Characterization of Crosslinked Azobenzene Liquid-Crystalline Polymer Fibers

We report the synthesis and physical properties of crosslinked liquid-crystalline polymer (CLCP) fibers consisting of a liquid-crystalline monomer and an acrylate monomer with a hydroxyl group that acts as a reactive site for crosslinking. By differential scanning calorimetry, the CLCP fibers showed a glass transition temperature around 60°C. Polarizing optical microscopy revealed that we obtained fibers with high orientation of mesogens along the fiber axis. The CLCP fibers showed deformations induced by heated the fiber to an isotropic phase.

Preparation and Photoresponsive Behavior of Plastic Films Coated with Azobenzene Liquid-Crystalline Polymer Layers

Plastic films coated with crosslinked liquid-crystalline polymer (CLCP) layers containing azobenzene moieties (plastic/CLCP bilayer films) were prepared by thermal polymerization of mixtures of an azobenzene monomer and a crosslinker placed between a plastic film and a glass substrate. When the glass substrate was untreated, the obtained bilayer film bent slightly away from the UV light source when irradiated on the CLCP layer side. This bending direction was the same as that of a hybridly aligned CLCP film. When the glass substrate was coated with a rubbed polyimide to strengthen the homogeneous alignment of the azobenzene moieties of the CLCP surface layer, the obtained bilayer film bent toward the irradiation direction of the incident UV light. It was the same as a homogeneously aligned CLCP film and the degree of bending was larger than that of the untreated sample. This means that the photoinduced bending behavior of the plastic/CLCP bilayer films strongly depends on the alignment of the azobenzene moieties of the CLCP surface layer facing the incident light.

Preparation and Characterization of Azobenzene Liquid-Crystalline Elastomer Films with Homeotropic Alignment

ABSTRACT Azobenzene liquid-crystalline elastomer (LCE) films with homeotropic alignment were prepared by in-situ photopolymerization of mixtures of an LC monomer and a cross linker, both of which contain azobenzene chromophores. Thermodynamic and mesomorphic properties of the monomers and the LCE films were determined by differential scanning calorimertry and polarizing optical microscopy. X-ray diffraction patterns revealed that the LCE films show a smectic A phase. Polarized UV spectra and conoscopic observation with a polarizing optical microscope demonstrated that the azobenzene mesogens are aligned homeotropically in the films.

The Influence of Aliphatic Low Molar Mass Polyesters on Crystallization of Poly(ethylene terephthalate)

Several poly(1,4-butylene sebacate) (SBT) with low molar mass were synthesized, and their number-average molar masses were determined by vapor phase osmometer (VPO). Thermogravimetric analysis (TGA) indicated that they display a good thermal stability. The measurement of solution viscosity of poly(ethylene terephthalate) (PET) certified that the addition of SBT leads to an only small decrease in molar mass of PET in the case of higher molar mass SBT. The compatibility of PET with SBT was studied by scanning electron microscope (SEM); higher molar mass SBT shows a larger tendency towards phase separation. The influence on crystallization of PET was investigated using differential scanning calorimetry (DSC). It was revealed that SBT significantly accelerates the crystallization rate of PET at low temperature and also enhances the crystallization rate of PET at high temperature.

Liquid-crystalline elastomers with photomechanical properties

Photoinduced bending and unbending behavior of liquid-crystalline elastomers is discussed. Various modes of bending have been achieved with various alignments of the photoactive mesogens in the elastomers: oriented Liquid-crystalline elastomer films were found to undergo anisotropic bending and unbending behavior only along the rubbing direction, when exposed to alternate irradiation of unpolarized UV and visible light; in the case of polydomain Liquid-crystalline elastomer films, a single film could be bent repeatedly and precisely along any chosen direction by using linearly polarized light.