2‐Naphthoxymethyl‐Substituted Polystyrenes for Homeotropic Liquid‐Crystal Alignment Layers

A series of 2‐naphthoxymethyl‐substituted polystyrenes (P2NAP) were prepared by the reaction of poly(chloromethylstyrene) with different amounts of 2‐naphthol to investigate the liquid‐crystal (LC) alignment properties of these polymer films. P2NAP with only 2‐naphthoxymethyl side groups (100% conversion of chloromethyl to 2‐naphthoxymethyl) was obtained when 180 mol‐% of naphthol was used in the reaction at 80 °C for 12 h. LC cells fabricated with unrubbed P2NAP films exhibited a homeotropic LC alignment behavior with a pretilt angle of approximately 90°. The homeotropic LC alignment behavior of the unrubbed LC cell of the P2NAP film was ascribed to the head‐on type incorporation of a nonpolar bulky 2‐naphthoxymethyl group into the phenyl side group of the polystyrene.

     

Photo‐Orientation Phenomena in Photochromic Liquid Crystalline Azobenzene‐Containing Polymethacrylates with Different Spacer Length

Synthesis and investigation of the phase behavior and photo‐optical properties of two chiral liquid‐crystalline polymers with azobenzene‐containing side groups having different spacer lengths, 6 and 10 methylene groups ( PMAzo‐6 and PMAzo‐10 , respectively) are described. Formation of different smectic phases and high‐temperature cholesteric phase is studied by polarizing optical microscope, differential scanning calorimetry, and X‐ray investigations. It is shown that UV‐irradiation induces effective reversible E –Z photoisomerization in polymer solutions and films. Atomic force microscopy (AFM) study reveals substantial changes in the surface topography of the polymer PMAzo‐6 film after UV‐irradiation, whereas PMAzo‐10 surface remains the same. Irradiation by polarized light (457 nm) results in photo‐orientation process in polymer films consequential in significant alignment of the chromophores in direction perpendicular to the polarization plane of the incident light. A significant difference is found in thermal stability of the photoinduced alignment; an annealing of PMAzo‐10 irradiated samples results in a slight decrease of dichroism values (down to 0.57); whereas the dichroism increases up to very high values (0.91) for PMAzo‐6 .

     

Photo‐Crosslinked Side‐Chain Liquid‐Crystalline Elastomers for Microsystems

Photo‐crosslinkable side‐chain liquid‐crystalline polymers (LCPs) containing photoreactive benzophenone cores are synthesized in order to obtain their corresponding side‐chain liquid‐crystalline elastomers (LCEs). This strategic synthesis allows thin elastomeric films and their integration into microsystems for actuators and micromachines to be obtained. As an example of this principle, a gripper was developed. The position of its arms can be changed by applying voltages from 1.5 to 3.5 V at different rates. Small changes in the liquid‐crystalline elastomer film cause strains of up to 150% in the microdevice and the capacity to move up to 400 times its own mass due to the nematic‐to‐isotropic transformation.

     

Reconstruction of Achiral Polymer for Structural Chirality Using Cholesteric Liquid Crystal Medium and Light‐Driven Chiroptic Modulation

A series of optically inactive polythiophene derivatives bearing the azobenzene moiety are synthesized in an isotropic toluene solution. The polymers are then dissolved in a cholesteric liquid crystal (Ch*LC) medium within the liquid crystal (LC) temperature range. The dissolution process in Ch*LC produces intermolecular π‐stacking with structural chirality. Although the polymers thus prepared have no chiral center or axial chirality in the primary structure, they do exhibit consistent chiroptical activity derived from three‐dimensional chiral aggregations. Furthermore, photochemical cis–trans isomerization of the substituents allows light‐driven chiroptic modulation of the polymer.

     

Reversible Photo‐Mechanical Switching Behavior of Azobenzene‐Containing Semi‐Interpenetrating Network under UV and Visible Light Irradiation

Summary: Light‐induced reversible changes in elasticity of semi‐interpenetrating network (semi‐IPN) films bearing azobenzene moieties were achieved under both ultraviolet (UV) and visible light irradiation. The semi‐IPN film was prepared by a cationic copolymerization of azobenzene‐containing vinyl ethers in a linear polycarbonate (PC) film as a matrix. When the irradiation was switched on and off, the semi‐IPN film showed rapid reversible deformation with the same behavior occurring over a range of wavelengths, including both the UV and visible regions. The observed reversible deformation of the film was attributed to the decrease in the film's elasticity, which was assumed to be caused by the frequent trans‐cis cycling isomerization of azobenzene moieties taking place during the UV and visible light irradiation. This cycling makes it difficult for the azobenzene groups to aggregate, thus hindering their ability to function as pseudo‐crosslinking points.

     

A Triple‐Shape Memory Material via Thermal Responsive Behavior of Liquid Crystalline Network Incorporating Main‐Chain/Side‐Chain LC Units

In the last several years, multiple‐shape memory liquid crystalline networks (LCNs) have received more and more attention due to the basic theoretical research on them and their wide potential applications. In this article, a novel main‐chain/side‐chain liquid crystalline monomer and its corresponding polymer networks based on the thiol‐ene click reaction are reported. Properties of the synthesized liquid crystalline monomer are well studied with nuclear magnetic resonance (NMR), Fourier transform infrared (FTIR) spectra, differential scanning calorimetry (DSC), and polarized optical microscopy (POM). The as‐prepared free‐standing LCN films are investigated well by FTIR, DSC, POM, and X‐ray diffraction (XRD), which show them having good liquid crystalline properties. Tensile test and dynamical mechanical analysis (DMA) results indicate the LCN films have excellent thermal mechanical properties. By adjusting the crosslinking densities, LCN films exhibit two thermal transition temperatures (T_g and T_NI) that can be utilized to trigger the triple‐shape memory behaviors. The cyclic thermal mechanical analysis conducted by DMA reveals that LCN films exhibit good triple‐shape memory properties with high‐shape fixity ratio (R_f (S1→S2) is 99.2% and R_f (S2→S3) is 99.3%) and shape recovery ratio (R_r (S3→S2) is 92.4% and R_r (S2→S1) is 98.5%).     

Polymer‐Dispersed Liquid Crystals Based on Polystyrene and EBBA: Analysis of Phase Diagrams and Morphologies Generated

As an example of thermoplastic/liquid crystal blends that do not exhibit a liquid‐liquid immiscibility region in their phase diagrams, a polystyrene (PS)/N‐4‐ethoxybenzylidene‐4′‐butylaniline (EBBA) blend was analyzed. The complete phase diagram was built up using thermal transitions determined by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). The boundary of the nematic + isotropic region was fitted with the Flory‐Huggins‐Maier‐Saupe model, extended to consider the polydispersity of PS. Main factors controlling the morphologies generated by thermal‐induced phase separation (TIPS) were the initial EBBA concentration in the blend and the cooling rate. Cooling at a fast rate led to small nematic domains with a narrow size distribution. Slow cooling rates led to the coexistence of very large and small dispersed domains. This was because of the large extent of coalescence of the droplets first generated associated with the continuation of the nucleation/growth process in a medium of increasing viscosity. The use of fast cooling rates might be important for the generation of a narrow size‐distribution of nematic droplets by TIPS in polymer‐dispersed liquid crystals (PDLC) used in electrooptical devices.

Micrograph obtained by transmission optical microscopy without crossed polarizers, showing nematic domains dispersed in a glassy matrix arising by a fast cooling of the blend from 130 °C; composition of blend: 70 wt.‐% EBBA.     

Preparation and Self‐Repairing of Highly Oriented Structures of Ultrathin Polymer Films

Thin and ultrathin polymer films play an important role in a variety of applications, such as electronics, liquid crystal alignment, adhesion, and so on. It is well established that the properties of semicrystalline polymeric materials depend remarkably on their multiscale structures; for example, an increase of more than a factor of 100 has been reported for the electrical conductivity of doped and aligned conjugated macromolecules compared with their nonoriented counterparts. Consequently, sophisticated methods have been developed for preparing highly oriented polymer ultrathin films. It should be pointed out that the highly oriented crystalline structure of polymers can be maintained only at temperatures below their melting points. Otherwise, structure rearrangement will result in the loss of orientation and therefore the properties. Therefore, self‐repairing of highly oriented polymer structures is of great significance for the long‐term application of polymer thin films in some specific fields. In this review paper, the preparation and self‐repair of the highly oriented structure of polymer thin films is described in the hopes that this will afford useful information for further development of polymeric materials for advanced applications.     

Thermotropic Liquid Crystals of Main‐Chain Polyesters having a Mesogenic 4,4′‐Biphenyldicarboxylate Unit, 14

Summary: Anisotropy in molecular motions of a glassy smectic CA (SmCA) liquid crystal of a main‐chain BB‐5(3‐Me) polyester was examined by dynamic mechanical analysis (DMA) for fibrous monodomain samples with two distinct orientations of smectic layers perpendicular and parallel to the fiber axis. The α‐process attributed to micro‐Brownian motion of the polymer on the glass transition shows clear anisotropy explainable by the nature of the smectic layer structure. Time‐temperature superposition is applicable to the α‐process, so that the frequency of the micro‐Brownian motion in the SmCA phase can be estimated despite the narrow frequency range in DMA. The results suggest that the decoupling of the molecular motions in two characteristic directions are parallel and perpendicular to the layer. The amplitude of the micro‐Brownian motion in the layer normal direction is 3 times smaller than that in the layer direction, and the frequency in the layer normal direction is 2.5 times higher than that in the layer direction.

Illustration of molecular packing structures: fiber A (left side) and fiber B (right side) with the orientations of polymers parallel and perpendicular to the fiber, respectively.     

Liquid Crystals Embedded in Polymeric Electrolytes for Quasi‐Solid State Dye‐Sensitized Solar Cell Applications

A new series of liquid crystal embedded in polymeric electrolytes was developed for obtaining high efficiency in quasi‐solid state dye‐sensitized solar cells (DSSCs). The polymeric electrolytes were composed of iodide and tri‐iodide redox species in polyacrylonitrile (PAN) as a polymer matrix and liquid crystals (E7 or ML‐0249) for increasing the order parameter of electrolyte components with easy transport of redox species. The highest efficiency (6.21 and 6.29% at 1 sun) was obtained for the quasi‐solid state DSSCs using E7 to PAN and ML‐0249 to PAN, respectively, under AM 1.5 G illumination and an aperture mask condition. The high efficiencies of the quasi‐solid state DSSCs are due to the effective formation of pathways through liquid crystal orientation for the transport of redox species.

     

Size‐Dependent Isotropic/Nematic Phase Transition Behavior of Liquid Crystalline Peptide Nanowires

We present size‐tunable assemblies of peptide nanowires and study their liquid crystalline phase behavior. An aromatic peptide molecule of diphenylalanine was rapidly assembled into one‐dimensional nanowires, whose sizes were tunable according to the initial concentration of diphenylalanine in the preparing solution. The stable dispersion of peptide nanowires in an organic solvent exhibited colloidal nematic liquid crystalline phases. The liquid crystalline phase transition was governed by the repulsive electrostatic interaction among peptide nanowires as well as their shape anisotropy and polydispersity. Our work provides an interesting model system to investigate the liquid crystalline phase behaviors of a biomolecular assembly in terms of its assembled morphology and intermolecular interactions.

     

Bulk Polymerization Photo‐Initiator ZnO: Increasing of the Benzoyl Formic Acid Concentration and LED Illumination

Polymerization in volume/bulk is one of the most important processes used in science and in industry to form solid polymers used in multiple different applications. The migration and transport of molecular polymerization initiators are problematic for commercially used photo‐polymerization procedures. ZnO not only is a well‐known photo‐semiconductor but has the potential to be used as an enlarged size photo‐initiation system. A high reactivity modification, firstly systematically investigated using novel light‐emitting diode illumination and two benchmarked resins is presented here. The curing potential of layers up to 1.4 mm is proven and first indications of differences in oxygen dependencies of such initiating systems are discovered. Additionally, the optimized synthesis procedure (0.45 mol L^?1 Zn²⁺ at room temperature) is presented which allows to perform systematic variation of the ZnO surface. Starting from one approach/batch, the content of the modification is systematically increased.     

Reactive Coatings in Glass Capillaries: Preparation of Temperature‐ and Light‐Responsive Surfaces and Accurate Determination of Wettability Switching

The inner walls of thin glass capillaries are coated with a reactive precursor coating that can be converted into different temperature‐ and light‐responsive coatings. The switching range of wettability can be determined by measuring the meniscus height of water inside these capillaries at different temperatures and upon UV‐light irradiation. In comparison with the sessile drop measurement, very accurate equilibrium contact angles are obtained by this method.

     

Control of In‐Plane and Out‐Of‐Plane Reorientation in Photo‐Cross‐Linkable Copolymer Liquid‐Crystal Films by Irradiation with Linearly Polarized Ultraviolet Light and Annealing

This paper describes thermally enhanced in‐plane and out‐of‐plane reorientation of mesogenic groups in copolymer liquid‐crystal (CPLC) films that have a photo‐cross‐linkable 4‐(4′‐methoxycinnamoyl)biphenyl (MCB) side group and a 4‐methoxybiphenyl (4MB) side group. The side groups are cross‐linked by irradiation with linearly polarized ultraviolet (LPUV) light and subsequent annealing. Due to axis‐selective photo‐cross‐linking, a small negative optical anisotropy is observed for all CPLCs after exposure to LPUV light. When the composition of the 4MB comonomer is less than 45 mol‐%, thermal treatment in the liquid‐crystalline temperature range causes in‐plane reorientation of the film and results in an in‐plane order parameter of 0.71. In contrast, when the composition of the 4MB is increased, out‐of‐plane reorientation is generated. Three‐dimensional molecular reorientation and inclined out‐of‐plane reorientation are achieved by adjusting the exposure angle.

Angular dependence of UV absorption spectrum of film 3 . The inset shows a conoscope observation of the film. The film was annealed at 130 °C for 10 min.     

Studies on Preparation and Fluorescent Properties of a Novel Photo‐Sensitive Nanoparticle Composed of Europium Ion and Cinnamic Acid Derivative

A novel fluorescent and photo‐sensitive nanoparticle was self‐assembled from an europium‐based random copolymer, europium coordinated poly(methylacrylic acid)‐co‐poly(cinnamyl acrylate) (PMCFA‐Eu³⁺), which was synthesized from methylacrylic acid (MAA), and Eu³⁺‐cinnamyl acrylate derivative (CFA) by radical polymerization. DLS and TEM results indicated the formation of spherical nanoparticles with 120 nm in diameter. The PMCFA‐Eu³⁺ complex showed stronger fluorescence than Eu³⁺, indicating the effective energy transferred from the ligand to Eu³⁺. Moreover, the photo‐crosslinking of the cinnamate groups induced a decrease in the diameter and an increase in the fluorescent properties of the PMCFA‐Eu³⁺ nanoparticles. This functional nanoparticle might be useful as a carrier and a fluorescence probe in biomedical and fluorescent fields.

     

Photoluminescent Color and Polarized Light Emission Tuning of Fluorene Derivatives Using a Photoreactive H‐Bonded Liquid Crystalline Polymer

Three fluorene (FL) derivatives with pyridine end groups were synthesized to control the photoluminescence (PL) wavelength and polarized light emission using a photoreactive H‐bonded polymer containing cinnamic acid side groups (P6CAM). The FL derivatives, which possessed pyridine end groups, formed H‐bonds with cinnamic acid, and changed the PL behavior of the derivatives. The controllability λ_max of the PL depended on the position of the N‐atom at the pyridine end group and the degree of the photoreaction of the P6CAM. For F1, λ_max of the PL was tuned from 470 to 518 nm. Employing P6CAM as the photoalignment layer to reorient the cinnamic acid side groups realized polarized PL up to PL_||/PL_? = 3.9. Furthermore, photopatterning of the PL color and the direction of the polarized light emission using patterned P6CAM films were demonstrated.

     

Photocontrol of Birefringence and In‐Plane Molecular Orientation in Copolymer Liquid Crystal Films with 4‐Methoxyazobenzene and Photo‐Cross‐Linkable Side Groups

Control of the in‐plane orientation direction in a polymethacrylate copolymer liquid crystal film substituted with 4‐methoxyazobenzene (4MAz) and 4‐methoxycinnamoyloxybiphenyl (4MCB) side groups, and control of the reversible birefringence of the resultant oriented film are described. High‐order in‐plane molecular orientation was generated by irradiating with linearly polarized (LP) 365 nm light and subsequent annealing. By adjusting the exposure dose, the reorientation direction was controlled parallel and perpendicular to the electric vector ( E ) of the LP light. Cooperative reversion of the orientation direction for both the 4MAz and 4MCB groups was observed, and the orientational order, S, varied from ?0.79 to +0.75, without the side groups aggregating. The degree of the photoreaction and the distribution of the photoproducts from the 4MAz and 4MCB side groups through the axis‐selective photoreaction explain the reversion of the orientation direction. Optical recording, erasing, and birefringent control of the resultant oriented films were demonstrated by employing the photoisomerization of the 4MAz side groups using nonpolarised (NP) 405 nm light.

Order parameters of a P1 film after irradiating with LP 365 nm light (circles) and that following subsequent annealing (squares), as a function of exposure energy.     

Star‐Like Poly(N‐isopropylacrylamide) and Poly(ethylene glycol) Copolymers Self‐Arranged in Newfound Single Crystals and Associated Novel Class of Polymer Brush Regimes with V‐Type Tethers

Linear poly(ethylene glycol) (PEG)‐block‐poly(N‐isopropylacrylamide) (PNIPAM) and star‐like three‐arm PEG‐star‐(PNIPAM)₂ copolymers having one PEG and two PNIPAM blocks are synthesized by atom transfer radical polymerization (ATRP). Single crystals of these block copolymers are grown from amyl acetate and toluene dilute solutions. To recognize PNIPAM and PEG thicknesses, small angle X‐ray scattering (SAXS) is applied. V‐type brushes behave differently from linear brushes because doubly grafted PNIPAM blocks from a common point onto PEG substrate exert a higher osmotic pressure, leading to a thinner crystal. In addition to three ordinary regimes, a fourth or extremely stretched regime is detected for V‐type PNIPAM brushes. Although in PEG₅₀₀₀‐star‐(PNIPAM)₂ single crystals with overall PNIPAM molecular weight of 37 000 g/mol, each PNIPAM arm is shorter than PNIPAM grafts in linear PEG₅₀₀₀‐block‐PNIPAM₂₆₀₀₀ single crystals, their switching point from second to third regime is significantly lower (22 vs 33 °C). The V‐type configuration of PNIPAM brushes cause them to be entered into the extremely stretched or fourth regime, which has not been previously detected for coily brushes. The boundary between third and fourth regimes for PEG₅₀₀₀‐star‐(PNIPAM)₂ single crystals is verified at 31.5 and 23.5 °C in amyl acetate and toluene, respectively.     

Effect of Molecular Structure and Thermal Treatment on Photo‐optical Properties of Photochromic Azobenzene‐containing Polymer Films

Three novel photochromic azobenzene‐containing comb‐shaped polyacrylates are synthesized and their phase behavior and photo‐optical properties are studied. The influence of the side photochromic group structure, thermal treatment, and light irradiation on aggregation of azobenzene chromophores in thin spin‐coated films of the polymers is investigated in detail. Special attention is paid to studying the photo‐orientation processes in polymers films induced by polarized blue light (473 nm). The relationship between the photochromic group architecture, phase behavior, thermal treatment of films, and kinetics of chromophore photo‐orientation is established and discussed. It is found that the position of the N?N bond in chromophores plays an important role in the kinetics of the process, but does not affect the maximum value of dichroism.