Reactive Liquid Crystal Research Articles

Investigation of Surface Orientation on UV Reactive Mesogen-Doped Liquid Crystals

Investigation of Surface Orientation on UV Reactive Mesogen-Doped Liquid Crystals

Birefringence-modulated total internal reflection in liquid crystal shells

The combination of anisotropic boundary conditions and topological constraints acting on a spherical shell of nematic liquid crystal confined between aqueous phases gives rise to peculiar but well-defined configurations of the director field, and thus of the optic axis that defines the impact of the nematic birefringence. While the resulting optics of nematic shells has been extensively investigated in transmission, studies of the reflection behavior are scarce. Here we show that nematic shells exhibit specific light guiding paths mediated by birefringence-modulated total internal reflection (TIR) within the shell. With stabilizers promoting tangential boundary conditions, shells show immobile antipodal spots revealing the locations of maximum effective refractive index, but their intensity is modulated by the polarization of the illuminating light. With normal-aligning stabilizers, shells instead show bright arcs separated by dark spots, and these follow the rotation of the polarization of the illuminating light. Reflection polarizing microscopy thus offers a valuable complement to the more common characterization in transmission, adding data that can be helpful for accurately mapping out director fields in shells of any liquid crystal phase. Moreover, the TIR-mediated light guiding paths may offer interesting handles to localize photopolymerization of reactive liquid crystal shells or to dynamically modulate the response of light-triggered liquid crystal elastomer shell actuators.

Photonic calcium and humidity array sensor prepared with reactive cholesteric liquid crystal mesogens

Ion and humidity sensor array films have been developed using an interpenetrating polymeric network (IPN) of photonic networked cholesteric liquid crystals (CLCs) intertwined with network-structured poly(acrylic acid). Selective curing of a photonic CLC network film infiltrated with an acrylic acid/tri(propylene glycol) diacrylate mixture produced an array film of dots with photonic IPN structures on a simple photonic CLC background. Water isolation by individual hydrophilic dots against a hydrophobic background allowed testing of small sample amounts. Subsequent acetone/KOH treatments of the IPN film (IPNacetone/KOH film) prevented hydrogen bonding between carboxylic groups, maximized swelling of the film with water, and increased the speed of photonic color change. A full range of reflected colors, i.e., blue (dry) to red (humid), allows this film to act as an optical humidity sensor. The replacement of K+ with divalent ions that bridge the carboxylic groups prevented film swelling. Given that the photonic band gap wavelength is inversely proportional to the CaCl2 concentration, the amount of Ca2+ can be determined from the reflected color by the naked eye. This cost-effective, convenient IPNacetone/KOH film has been successfully applied as a biosensor for detecting Ca2+ in human blood serum and saliva and as an anti-counterfeiting label that reveals a colorful image under humid conditions.

Liquid crystal thermosets. A new class of high-performance materials

ABSTRACT The evolution of the liquid crystalline order during the cure of a reactive thermotropic liquid crystal (LC) is studied using two phenylethynyl-terminated biphenol and naphthalenediol model compounds. The nematic order in the melt of both model compounds at elevated temperature is monitored with X-ray Diffraction (XRD) and C-13 Nuclear Magnetic Resonance (13C NMR). XRD shows a cybotactic nematic phase; the global nematic order decreases with curing and eventually the resulting cross-linked liquid crystal thermoset (LCT) exhibits an isotropic morphology. The NMR method tracks the phenylethynyl end-group and yields data about the cure kinetics of this class of reactive LC mesogens. The observations may be used to make inferences about the cure behaviour and morphology of phenylethynyl-terminated random copolyester macromonomers.

Tunable nonpolarizing one-dimensional crossed resonant gratings incorporating electrically driving liquid crystal

A tunable nonpolarizing filter composed of one-dimensional crossed resonant gratings and an electrically driving liquid crystal (LC) cell is designed, fabricated and demonstrated. The filter is designed by using different grating periods to obtain nonpolarizing spectra for oblique incidence. The filter sample is fabricated through thin-film deposition, ultraviolet lithographic patterning, deep reactive ion etching and LC injection. Finally, the resonance spectra of the fabricated sample are measured by a spectrometer and the measured results are consistent with the theory. The experimental results show that the nonpolarizing resonance spectra can be tuned by electrically driving the LC cell of the proposed device.

Synthesis of new chiral mono- and diacrylates for ferro- and antiferroelectric liquid crystals

The synthesis and characterization of new, chiral and reactive single liquid crystals based on biphenyl benzoate core are described. Compounds are terminated with partially perfluorinated alkoxy chain (monoacrylates) or acryloyloxy-alkoxy chain (diacrylates) on the non-chiral side of molecule, while the opposite terminal side is chiral acryloyloxy unit. New families of smectic LCs present alternative molecular approach to the monomer design. Compounds are characterized by mass spectrometry (electron ionization) analysis, proton and carbon nuclear magnetic resonance. They show liquid crystal behavior in broad temperature ranges, confirmed by a polarizing thermomicroscopy and differential scanning calorimetry. Detailed synthetic procedures are attached. Synthesized compounds stand as promising candidates for polymer stabilization of chiral liquid crystalline materials.

Well-Adhering, Easily Producible Photonic Reflective Coatings for Plastic Substrates.

The development of well-adhering, easily producible photonic reflective coatings is still a challenge. Here, an easy-to-produce, industrial viable process is reported that uses a primer layer of the so-called type II photoinitiator to obtain an excellent adhesion between a plastic substrate and one-dimensional (1D) photonic liquid crystalline coatings. Furthermore, a good alignment of the reactive cholesteric liquid crystal mixture is obtained using a bar-coating process, without alignment layers or surfactants. After photopolymerization, cross-hatch tape tests show a good adhesion of the photonic coating having a reflection band of 50% transmission with almost no scattering. Additionally, we demonstrate the ability to create well-adhering ∼100% reflective coatings by coating double layers and the ability to create single-layered cholesteric broadband reflectors using solely a reactivity gradient created by the primer layer. Our new interfacial method gives new opportunities to use reflecting 1D photonic coatings in industrial processes and applications and allows the bonding of almost any polymer to a plastic substrate.

Biosensor Array of Interpenetrating Polymer Network with Photonic Film Templated from Reactive Cholesteric Liquid Crystal and Enzyme‐Immobilized Hydrogel Polymer

AbstractA biosensor array is fabricated using an interpenetrating polymer network consisting of photonic film templated from reactive cholesteric liquid crystal (CLC) and enzyme‐immobilized polyacrylic acid (PAA). The solid‐state photonic film on the glass substrate is successfully templated by ultraviolet (UV) curing of the reactive CLC mixture of a reactive mesogen mixture of RMM 727 (from Merck) and a nonreactive chiral dopant of (S)‐4‐cyano‐4′‐(2‐methylbutyl)biphenyl following the extraction of the chiral dopant. The acrylic acid monomer mixed with a cross‐linker of tri(propylene glycol) diacrylate is infiltrated into the extracted space of the photonic film, and UV‐cured with a photomask to obtain a patterned array‐dot film. The interpenetrated cholesteric liquid crystal/hydrogel polymer network (CLC‐hydrogel‐IPN) array is further functionalized in the individual dots with urease, for a model study of biosensor array applications. The dots of the CLC‐hydrogel‐IPN array respond independently to the urea by a color change with high sensitivity and stability. Thus, the patterned CLC‐hydrogel‐IPN can be used as a new biosensor array for cost‐effective and easy visual detection without any sophisticated instruments.

Acetylene-containing highly birefringent rod-type reactive liquid crystals based on 2-methylhydroquinone

ABSTRACTNew highly birefringent reactive liquid crystal materials based on the 2-methylhydroquinone core were designed and synthesised. Rod-type liquid crystal compounds bearing photo-crosslinkable reactive group of acryloyl, methacryloyl, cinnamoyl, furylacryloyl group were synthesised by introducing acetylene groups via Sonogashira coupling to obtain high birefringence, and lateral groups such as fluoro and methyl to adjust the temperature of the liquid crystal phase. The synthesised compounds were characterised using nuclear magnetic resonance spectroscopy, mass spectrometry and elemental analysis. In addition, their thermal behaviour was investigated using differential scanning calorimetry and polarised optical microscopy. After aligning the synthesised compounds, liquid crystal films were prepared by photo-irradiation. Photo-elastic modulator results showed that the obtained liquid crystal films had high birefringence (Δn) values of 0.32–0.40.

Smart shell membrane prepared by microfluidics with reactive nematic liquid crystal mixture

Smart, uniform-sized, solid-state nematic liquid crystal (NLC) (NLCsolid) shell membranes were successfully fabricated from a reactive mesogen mixture (RMM727) and 4-cyano-4′-pentylbiphenyl (5CB) using a microfluidic method that combines flow-focusing and co-flow glass capillary geometries after UV curing and 5CB extraction. The NLC shells having planar anchoring with poly(vinyl alcohol) (NLCp shells) showed unavoidable defects as per the Poincaré theorem, with a total of +2 defect strengths on the top of the shell (often the weakest point of the shell), and were usually ripped during UV curing. The NLC shells having homeotropic anchoring with sodium dodecyl sulfate/polysorbate 80 (1/1, w/w) (NLCh shells) had a defect-free structure and could be formed into NLCsolid shells without ripping when the density of the RMM727/5CB mixture matched that of the aqueous medium in the microfluidic channel (controlled using a glycerol/water mixture). The thin part of the NLCh shell was easily ripped during UV curing without density matching. The thus-produced NLCsolid shells exhibited good swelling/shrinkage properties depending on the solvent quality and temperature, which could be further utilized for encapsulating/releasing Rhodamine 6G in/from the core of the NLCsolid shell. The pores in the NLCsolid shell (the size of which was controlled by the 5CB content of the RMM727/5CB mixture) were completely closed in a poor solvent and open in a good solvent. The encapsulation/release properties were studied based on the fluorescence intensity of encapsulated Rhodamine 6G. This study provides a method for preparing uniform-sized and robust NLCsolid shell membranes that can be used for several applications, such as in smart actuators, sensors, and parts of microelectromechanical systems.

Photonic Spring‐Like Shell Templated from Cholesteric Liquid Crystal Prepared by Microfluidics

Solid‐state photonic shells are prepared with a reactive cholesteric liquid crystal (CLC) mixture that consists of a reactive mesogen mixture and the nonreactive chiral dopant. The shells are prepared using a microfluidic glass‐capillary chip, followed by UV‐curing and extraction of the chiral dopant. CLC shells with relatively uniform thickness are produced by density matching and annealing. The produced solid‐state CLC (CLCsolid) shells show a good reversible swelling/shrinkage behavior that is dependent on the solvent quality and temperature. The swelling or shrinkage simultaneously induces a color change in the reflection point at the center and in the cross‐communication dots. Encapsulation and release from the CLCsolid‐shell cores are successfully tested with Rhodamine 6G by controlling the solvent quality and temperature. Nanosized Fe3O4 particles encapsulated in the core give the CLCsolid shells mobility, and are assembled at the desired location using a magnet. The color changes caused by external stimuli can be applied to new types of small hollow sensors that will respond to changes in temperature, and solvent quality. The shells can be used, for example, as a chemical container that can be triggered by temperature to release for a delayed reaction, or in a security patch controlled by tuneable multicolored patterns.

Visible Light Responsive Liquid Crystal Polymers Containing Reactive Moieties with Good Processability.

Two types of novel reactive linear liquid crystal polymers (LLCPs) with different azotolene concentrations have been synthesized and processed into films and fibers by solution and melting processing methods. Then, the LLCPs in the obtained monodomain fiber and polydomain film were easily cross-linked with difunctional primary amines. The resulted cross-linked liquid crystal polymers (CLCPs) underwent reversible photoinduced bending and unbending behaviors in response to 445 and 530 nm visible light at room temperature, respectively. The post-cross-linking method provides a facile way to prepare the CLCP films and fibers with different shapes from LLCPs, which can be processed by traditional melting and solution methods.

High birefringent reactive discotic liquid crystals based on asymmetrical triphenylene with phenyl-acetylene moieties

ABSTRACTTriphenylene (TP)-based liquid crystals, a category of discotic liquid crystals (DLCs), are easy to synthesise, thermally stable and can undergo self-assembly. A new DLC compound, 2,7,10-tris[4-(6-acryloyloxyhexyloxy)benzoate]-3,6,11-tris[(4-hexylphenyl)ethynyl]-triphenylene, was obtained with a 49% yield using catechol as a starting material and features acetylene groups on its TP core. This material can form films with high birefringence through its cross-linkable acrylate ends. Here, we synthesised this new compound and characterised it using nuclear magnetic resonance spectroscopy, mass spectrometry and elemental analysis. The thermal behaviour was also investigated using differential scanning calorimetry and polarised optical microscopy. This new asymmetric TP-based DLC compound exhibited a nematic liquid-crystalline phase between −20 and 170°C and formed an optical anisotropic film with a high birefringence (Δn = 0.21–0.25).

Electrospinning of reactive mesogens

The reinforcement potential of reactive liquid crystals or reactive mesogens (RMs) in electrospun fibers was investigated through the blending of two types of RMs (RM257 and RM82) with two types of thermoplastics; polyamide 6 (PA6) and poly(methyl methacrylate) (PMMA). Polymer/RM blends were electrospun from solution into nanofibers followed by UV-radiation induced photo-polymerization in order to cross-link the RMs. The phase behaviour of the polymer/RM blends was investigated using optical microscopy and differential scanning calorimetry (DSC). Mechanical tests of the electrospun nanofibers showed the biggest improvement in mechanical properties for the PA6/RM257 system containing 33wt% RMs, with a 20% increase in tensile strength and 150% increase in Young’s modulus compared to pure PA6 nanofibers. The work potentially provides a new route towards designed nanofibers with oriented microstructures, ultimately leading to a methodology for creating high performance electrospun nanofibers.

Structure-performance relation of liquid crystal photoalignment with in-situ formation of protection layers

We demonstrate a stabilized liquid crystal photoalignment using a surface-localized polymer method. A protection layer is developed on a photoalignment layer (PAL) through phase separation of a mixture composed of reactive monomers (RMs) and liquid crystals (LCs). The RM is polymerized on the PAL which enhances its stability against heat and light with short wavelength. The effects of the concentration and molecular structure of RMs on the electro-optical response and surface anchoring of photoaligned LC device are studied. The concentration of RMs affects the effective cell gap of the LC device. The rigid core length of the RM structure modulates the surface anchoring strength of the alignment layer. Both effective cell gap and surface anchoring strength are key elements for the enhanced dynamic response of LC devices.

Photoresponsive Fiber Array: Toward Mimicking the Collective Motion of Cilia for Transport Applications

Remote transport of material is an utmost useful, but challenging, property expanding the design possibilities of many applications such as microfluidics or robotics where species can be carried without interfering with its environment. Nature has solved the problem of transport in e.g., the respiratory system by a concerted motion of cilia. This study addresses a new method to fabricate an array of small parallel fibers acting as cilia placed side by side on a substrate. The fibers consist of a crosslinked liquid crystal main chain polymer functionalized with coreactant azobenzene molecules. The fibers bend toward a light source in a concerted manner. When placed in a liquid, the cooperative bending motion of the fibers creates a flow able to efficiently carry objects. The proposed fabrication process of the fibers is scalable to large area and requires an optimized rheology which is achieved by converting low molecular weight reactive liquid crystal acrylate monomers to oligomers using a multiplication of the monomeric units by the Michael addition reaction with dithiol. The oligomer properties and the elasticity of the fibers are adjusted by changing the thiol spacer leading to optimized manufacturing and maximized optical response.

Selective Adsorption: Responsive Nanoporous Smectic Liquid Crystal Polymer Networks as Efficient and Selective Adsorbents (Adv. Funct. Mater. 32/2014)

A nanoporous polymer network based on reactive smectic liquid crystals is used as an efficient and selective adsorbent. The inside cover image shows the selective adsorption of a blue dye from a mixture of dyes, described in detail on page 5045 by D. J. Broer, A. P. H. J. Schenning, and co-workers. The dye can be released and the adsorbent can be re-used. Cover design by ICMS Animation Studio TU/e.

A highly effective reactive liquid crystal for the improved β‐nucleation of isotactic polypropylene

We report here the synthesis and characterization of a reactive liquid crystal (RLC) as a novel polymeric nucleating agent for the promotion of the nucleation efficiency of isotactic polypropylene (iPP). The RLC was synthesized by an in‐situ photo‐polymerization and was then grafted onto the molecular chain of iPP by the reactive blending. The phase transition and crystalline morphologies of RLC‐iPP in the β‐nucleation were studied. It is found that the nucleation efficiency of β‐crystals of iPP can be increased to 42% with a very small amount of RLC grafting, which is much higher than the reported nucleation efficiency of polymeric nucleating agents up to now (23%). In addition, we found that the nucleation efficiency of β‐crystals is strongly related to the concentration of RLC for the reactive blending. The nucleation efficiency was decreased from 42% to about 17% with the increase of RLC concentration from 0.5% to 4%. We propose a possible nucleation mechanism for this interesting phenomenon. It is expected that this new β‐nucleation RLC will have potential industrial applications in the future. POLYM. ENG. SCI., 54:2112–2120, 2014. © 2013 Society of Plastics Engineers

Synthesis and characterisation of reactive liquid crystals containing an azo group

A new series of liquid crystal (LC) monomers – not only contain a double bond but also contain an azo group – were designed and synthesised. The length of side groups in the LC monomers containing azobenzene ester varied from 1 to 2 methylene units, and the length of the substituted groups in the main chain varied from 1 to 3 methylene units. The molecular structures of the intermediates and target compounds were confirmed by Fourier transform infrared, ultraviolet and visible spectrum and nuclear magnetic resonance (NMR) spectroscopy. The thermal phase behaviour of the LC monomers was investigated by polar optical microscopy coupled with hot stage and differential scanning calorimetry (DSC). In this paper, the effect on the LC zone with the substituents were investigated, and with the increased methylene of main chain, the melting point and the phase transition temperature of the substance will be lowered and LC regions will be narrower.

Paper No P21: Normal‐ and Reverse‐Mode Light‐Scattering Properties in Polymer‐Stabilized Nematic Liquid Crystals

AbstractA normal mode property of which a light‐scattering state electrically changes to a transparent state and a reverse mode with the opposite switching property are demonstrated in an oriented reactive mesogen and liquid crystal composite cell. The light‐scattering domain is produced in the transparent cell by partly irradiated with a UV light under a suitable voltage application. Scattering and transparent patterns simultaneously electrically turn to the transparent and scattering patterns without electrode partitions.