Synthesis of a Photoresponsive Liquid‐Crystalline Polymer Containing Azobenzene

The synthesis of an oriented liquid‐crystalline photoresponsive polymer, prepared by polymerization of mono‐ and di‐acrylates, both of which contain azobenzene chromophores, is reported. The prepared free‐standing polymer film shows strong reversible photoinduced deformation upon exposure to unpolarized UV light at 366 nm, as a result of an optically induced isomeric change of the azobenzene moieties in the polymer network. The synthesis process is relatively simple and more efficient compared to conventional ones, and can be used to synthesize other liquid‐crystalline photoresponsive polymers. The use of this photoresponsive polymer film as an optical high‐pass/low‐pass switch under UV or natural light irradiation for a laser beam is demonstrated. This photoresponsive polymer may have applications in robotic systems, artificial muscles, and actuators in microelectromechanical systems (MEMS) and labs on chips.

     

Photoswitchable Smectic Liquid‐Crystalline Elastomers

Summary: We succeeded in the synthesis of azo side chain containing polysiloxanes with broad smectic C* and A phases. In these polymers the phase transition temperatures can be shifted reversibly by up to 17 °C by irradiation with UV (cis) or VIS (trans) light. Thin films of these polymers in the smectic phase (both on substrates and as free‐standing films) orient perfectly in a homeotropic manner. As a consequence, the azo chromophores do no longer absorb during a perpendicular illumination with light (dichroism). It is thus possible to crosslink these films photochemically to prepare “photoswitchable smectic LC elastomers”.

The trans‐cis isomerization in homeotropically oriented LC elastomers.     

Reversible Photoinduced Switching of Permeability in a Cast Non‐Porous Film Comprising Azobenzene Liquid Crystalline Polymer

Permeation characteristics of an azobenzene‐containing liquid crystalline (LC) non‐porous film are investigated using a metallic corrosion method. Thin films (300 nm) are fabricated by the solution casting of an azobenzene side‐chain LC polymer on freshly polished carbon steel coupons. Coated coupons are treated under the following conditions: a) gradual annealing at a cooling rate lower than 1 °C · min⁻¹ from 150 °C (above its T_g) to room temperature, and b) irradiation at 465 nm (20 mW · cm⁻²) with either circularly polarized light (CPL) or non‐polarized light (NPL). The morphology of these films is characterized using X‐ray diffraction, polarized optical microscopy, and transmission measurements. The results suggest that the annealing treatment resulted in the formation of a polydomain structure consisting of locally ordered small smectic domains that lack mutual orientation. Ordered micro domains are surrounded by disordered phases. CPL and NPL irradiation generates a monodomain orientated structure and an isotropic liquid crystal glass, respectively. The permeability of these non‐porous films treated by CPL, NPL, and annealing are found to be 6.14 × 10⁻⁴, 1.92 × 10⁻², and 1.56 × 10⁻³ cm³ · m⁻² · d⁻¹. An orientation‐dependent structure model is constructed to explain the permeation phenomenon, considering the ordered phase is impermeable, only the disordered phase is accessible to penetrating molecules. Fast switching of gas permeation is demonstrated by alternative irradiation of the film with CPL and NPL, which results in an approximately 30‐fold difference in the permeability of the non‐porous film.

     

Thermotropic Liquid‐Crystalline Polymer Derived from Natural Cinnamoyl Biomonomers

Summary: The compound 4‐hydroxycinnamic acid (4HCA), a natural biomonomer, is polymerized by melt polycondensation to yield a liquid‐crystalline biopolymer (P4HCA) with UV reactivity. L929 cells were successfully incubated on P4HCA films at 37 °C.

Structure of poly(4‐hydroxycinnamic acid) (P4HCA) and its crossed‐polarizing optical micrograph in the nematic state. Inset image: optical micrograph of L929 mouse fibroblasts adhered on P4HCA film after 24 h incubation at 37 °C.     

On the Uniaxial–Biaxial Nematic Phase Transition in Liquid‐Crystalline Polymers and Elastomers

We study how the uniaxial–biaxial nematic phase transition changes its nature when going from a low‐molecular‐weight liquid crystal to a liquid‐crystalline elastomer or polymer (the latter above the Maxwell frequency) and find a qualitative change due to the presence of a coupling to the strain field in these materials. While this phase transition can be of second‐order in low‐molecular‐weight materials, as is also experimentally observed, we show here that the order of this phase transition is changed generically to no phase transition at all or to a first‐order phase transition in mean‐field approximation. We analyze the influence of an external mechanical stress field above the uniaxial–biaxial nematic phase transition and find that either biaxial nematic order is induced, which is linear or quadratic in the stress intensity, or no response to an external stress results at all, depending on the relative orientation of the applied shear with respect to the director of the uniaxial nematic phase.     

New Architecture of Liquid‐Crystalline Polymers from Fluorinated Vinylcyclopropanes

A new class of fluorinated polymers was prepared by radical ring‐opening homopolymerization of vinylcyclopropane monomers with a perfluorinated (CF₂)_nF chain (n = 6, 8, or 10). The polymers were in fact copolymers composed of 1,5‐linear and cyclobutane isomer units, the relative content of which depended on n. Surprisingly, they formed liquid‐crystalline mesophases (SmB_d and/or SmA_d), which was attributed to phase separation of the incompatible fluorocarbon and hydrocarbon components of the repeat unit.     

Branched Azobenzene Side‐Chain Liquid‐Crystalline Copolymers Obtained by Self‐Condensing ATR Copolymerization

Summary: The one step synthesis of a series of branched azobenzene side‐chain liquid‐crystalline copolymers by the self‐condensing vinyl copolymerization (SCVCP) of a methyl acrylic AB* inimer, 2‐(2‐bromoisobutyryloxy)ethyl methacrylate (BIEM), with the monomer 6‐(4‐methoxy‐azobenzene‐4′‐oxy)hexyl methacrylate (M), by atom transfer radical polymerization (ATRP) in the presence of CuBr/N,N,N′,N′,N″‐pentamethyldiethylenetriamine as a catalyst system, and in chlorobenzene solvent, is reported. The degree of branching (DB), and the molecular weights and polydispersities of the resultant polymers were determined by NMR spectroscopy and size exclusion chromatography, respectively. The phase behaviors of the branched copolymers were characterized by differential scanning calorimetry (DSC) and thermal polarized optical microscopy (POM). The degree of branching of the branched copolymers could be controlled by the comonomer ratio in the feed and influenced their liquid‐crystal properties. Liquid‐crystal properties were not exhibited when the comonomer ratio was low. Comonomer ratios greater than 8 gave polymers with a lower number of branches, which exhibited both a smectic and a nematic phase.

A polarized optical micrograph of the smectic phase texture of a polymer synthesized here with a higher comonomer feed ratio (magnification × 400).     

Effect of Molecular Weight on Photoinduced Birefringence in a Chiral Liquid Crystalline Azodye Polymer

Summary: The evolution of the photoinduced birefringence in thin films of narrow polymer fractions is studied and compared with the behavior of the non‐fractionated polymer. The Δn_ind value decreases by increasing the degree of polymerization ( ) within the oligomeric range but becomes independent of molecular weight starting from a of ≈70. Thermal pretreatment of the films results in higher photoinduced birefringence. The films show good stability of the photorecording.

Birefringence induced after 10 min, Δn_ind(600) and its growth rate at the same moment versus molecular weight.     

Thermal and Photochemical Control of Molecular Orientation of Azo‐Functionalized Polymer Liquid Crystals and Application for Photo‐Rewritable Paper

A photo‐responsive multi‐bilayered film consisting of azobenzene polymer liquid crystals (PA6Az1) and poly(vinyl alcohol) (PVA) has been prepared on a glass substrate by alternate spin coating of the polymer solutions. The reflectivity of the multi‐bilayered film disappears by annealing at 80 °C. The disappearance of the reflection by the annealing is related to the thermal out‐of‐plane molecular orientation of PA6Az1 even in the multi‐bilayered film, which leads to a very small difference in refractive indices between PA6Az1 and PVA. The reflectance of the multi‐bilayered film is increased again by UV irradiation because of the transformation from the out‐of‐plane orientation to an in‐plane random orientation. In this way, on–off switching of the reflection is achieved by combination of the thermally spontaneous out‐of‐plane molecular orientation and following photoisomerization of PA6Az1 comprising the multi‐bilayered film.

     

Dynamic Temperature Distribution in Photo‐induced Alignment of Azobenzene Liquid Crystalline Polymer

A theoretical model has been established to describe the dynamic temperature distribution during the alignment of an azobenzene liquid crystalline polymer irradiated by a linearly polarized laser beam. The dynamic heat diffusion equations are used, and the relationship between heat source item and time is introduced, based on experimental results. With the model, the contours of the temperature distribution at different time have been worked out. It can be found from the theoretical model that there is a maximum temperature rise during the photo‐induced alignment, which is coincident with the analysis of experimental observations. The existence of a minimum laser power and an offset photo‐alignment temperature T_off required to carry on photo‐induce alignment are explained based on the theoretical model.

Temperature distribution after two minutes.     

Precise Control of Photoinduced Birefringence in Azobenzene‐Containing Liquid‐Crystalline Polymers by Post Functionalization

A series of functionalized liquid‐crystalline polymer materials with different degrees of functionality was synthesized by a post Sonogashira cross‐coupling reaction of a polymer precursor. The post‐functionalization was carried out under mild conditions and showed a high yield. Although a highly birefringent azotolane group was introduced into the polymer precursor, the photoresponse of the functionalized liquid‐crystalline materials was not obviously changed. By adjusting the content of azotolane groups, precise control of the photoinduced birefringence was successfully obtained after thermal enhancement upon annealing. The present method to gain precise control of photoinduced birefringence might enable one to finely photocontrol the optical performances of materials, and may have a potential application as an advanced process for photonic materials.

     

Studies on Lyotropic Liquid‐Crystalline N‐Alkyl Chitosans in Formic Acid

A series of derivatives of chitosan – N‐alkyl(methyl, ethyl, propyl and butyl) chitosans – were synthesized from completely deacetylated chitosan. The degree of substitution (from 0.15 to 0.81) of the N‐ethyl chitosan were obtained by controlling the molar ratio of the reactants. All the products showed lyotropic liquid‐crystalline properties regardless of the length of the side chains and the degree of substitution. The critical concentration (C*) of the samples were measured by both microscopy and refractometry. C* seemed not to vary with the degree of substitution (ds) in the case of a given subsitituent chain, but rose dramatically depending on the length of the substituent group as this was varied from methyl to butyl. The results were explained according to Flory's classical theory as well as experimental of X‐ray diffraction measurements.     

Photoinduced Bending Behavior of Cross‐linked Azobenzene Liquid‐Crystalline Polymer Films with a Poly(oxyethylene) Backbone

Cross‐linked azobenzene liquid‐crystalline polymer films with a poly(oxyethylene) backbone are synthesized by photoinitiated cationic copolymerization. Azobenzene moieties in the film surface toward the light source are simultaneously photoaligned during photopolymerization with unpolarized 436 nm light and thus form a splayed alignment in the whole film. The prepared films show reversible photoinduced bending behavior with opposite bending directions when different surfaces of one film face to ultraviolet light irradiation.

     

A New Chemo‐Enzymatic Route to Side‐Chain Liquid‐Crystalline Polymers: The Synthesis and Polymerization of 6‐(4‐Methoxybiphenyl‐4′‐oxy)hexyl Vinyl Hexanedioate

A novel synthetic method combining chemo and enzymatic synthesis strategies was employed to prepare a vinyl acetate type monomer, 6‐(4‐methoxybiphenyl‐4′‐oxy)hexyl vinyl hexanedioate (VA‐LC). Homo‐ and copolymers of VA‐LC with maleic anhydride (MAn) were prepared by conventional free radical polymerization using 2,2′‐azobisisobutyronitrile (AIBN) and 1,1′‐azobis (cyclohexane carbonitrile) (AHCN) as an initiator at 95 and 60 °C, respectively. The thermal properties of the generated polymeric material were investigated by differential scanning calorimetry (DSC), and the optical texture was inspected by polarizing optical microscopy (POM). While the monomer VA‐LC does not exhibit liquid‐crystalline properties, poly(VA‐LC), and the alternating copolymer of VA‐LC with maleic anhydride both displayed such properties.

     

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.     

A Main‐Chain Liquid‐Crystalline Oligomer Prepared by in situ Photopolymerization of an LC Monomer Having Cinnamate Moieties

Summary: A liquid‐crystalline (LC) compound, having a cinnamate moiety on each end of the molecule, was synthesized and irradiated with UV light in its LC phase in the presence of a triplet sensitizer. Various measurements of the irradiated sample revealed that the linearly structured LC oligomers were formed by [2+2] cycloaddition of the cinnamate moieties, and that the resultant cyclobutane units dominantly assumed an anti head‐to‐head configuration.

Schematic structures of the LC oligomer obtained by photopolymerization.     

Phase Separation and Organisation of Colloidal Spheres Suspended in Sheared Lyotropic Liquid‐Crystalline Polymers

Summary: Organisation behaviours of spherical particles suspended in sheared, lyotropic, liquid‐crystalline polymer solutions have been investigated using polarizing optical microscopy. We find that in a nematic phase the particles phase separate and adopt anisotropic chain‐like structures along the director. An earring defect is observed around a single particle whereas a cross or strings defect between neighbouring particles is found to serve as a repulsive barrier to prevent the particles from contacting each other. A theoretical analysis is presented to explain this new phenomenon.

An optical micrograph of 0.01 wt.‐% glass spheres suspended in a nematic solution of 40 wt.‐% ethyl cellulose in chloroform under an external shear force.     

Enthalpy Relaxation of Polymers: On the Possible Role of Liquid‐Crystalline Order

Isothermal physical ageing experiments were performed by differential scanning calorimetry to probe the enthalpy relaxation in a methacrylate copolymer carrying azobenzene mesogenic side groups. Further evidence of the ability of the configurational entropy model developed by Gomez Ribelles in describing the structural relaxation mechanism of polymers is provided. The trend of the equilibrium structural relaxation time was also determined as a function of the reduced temperature T_g/T. The comparison of the aging dynamics of the copolymer with those of previous analogous copolymers containing different amounts of azobenzene counits allowed us to highlight effects of the liquid‐crystalline nematic order on the properties of structural relaxation.

     

Liquid‐crystalline thermosets by the curing of dimeric liquid‐crystalline epoxyimine monomers with 2,4‐toluene diisocyanate

We studied the curing processes of several series of dimeric liquid‐crystalline epoxyimine monomers with 2,4‐toluene diisocyanate (TDI) alone or with added catalytic proportions of 4‐(N,N‐dimethylamino)pyridine. We obtained isotropic materials or liquid‐crystalline thermosets with different degrees of order, which depended on the structures of the monomers. To fix ordered networks, we had to do the curing in two steps when TDI was used alone as the curing agent. However, when a tertiary amine was added in catalytic proportions, the ordered networks were fixed in just one step. In this way, we were able to fix both nematic and smectic mesophases. The significance of the polarization of the mesogen for obtaining liquid‐crystalline thermosets was demonstrated. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2521–2530, 2003     

Liquid‐crystalline thermosets from liquid‐crystalline epoxy resins containing bisazomethinebiphenylene mesogens in the central core: Copolymerization with a nonmesomorphic epoxy resin

A new series of liquid‐crystalline epoxy resins was synthesized, and their mesomorphic behavior was investigated with differential scanning calorimetry, polarized optical microscopy, and wide‐angle X‐ray scattering. These glycidylic compounds had central aromatic imine mesogens derived from benzidine and aliphatic spacers of up to 10 methylene units that linked the mesogens to the glycidylic groups. Crosslinking these monomers with primary aromatic diamines led to nematic networks, some of which contained crystal inclusions. However, through curing with tertiary amines as catalytic agents or through copolymerization with different proportions of the nonmesomorphic epoxy monomer and primary amines as crosslinking agents, smectic C organized thermosets were prepared when the spacers had at least four methylene carbons. When they had fewer than four, the networks were nematic. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 3631–3643, 2004