Mesogen Content Research Articles

Controlling the Optical Properties of Transparent Auxetic Liquid Crystal Elastomers.

Determining the tunability of the optical coefficients, order parameter, and transition temperatures in optically transparent auxetic liquid crystal elastomers (LCEs) is vital for applications, including impact-resistant glass laminates. Here, we report measurements of the refractive indices, order parameters, and transition temperatures in a family of acrylate-based LCEs in which the mesogenic content varies from ∼50 to ∼85%. Modifications in the precursor mixture allow the order parameter, ⟨P2⟩, of the LCE to be adjusted from 0.46 to 0.73. The extraordinary refractive index changes most significantly with composition, from ∼1.66 to ∼1.69, in moving from a low to high mesogenic content. We demonstrate that all LCE refractive indices decrease with increasing temperature, with temperature coefficients of ∼10-4 K-1, comparable to optical plastics. In these LCEs, the average refractive index and the refractive index anisotropy are tunable via both chemical composition and order parameter control; we report design rules for both.

Liquid crystallinity and magnetorheological performance of polymeric liquid crystal magnetic composites

ABSTRACT Magnetorheological fluids (MRF) are regarded as a novel ‘intelligent’ material because they show the ability to respond to external stimuli. However, with the extension of working time, the agglomeration and sedimentation of magnetic particles become a serious problem, which has a great impact on the magnetorheological performance. Many methods have been proposed to overcome the particle settling problem, including magnetic particle modification and carrier liquid modification, but they all show negative effects on the magnetorheological performance of MRF. A new method to prepare the magnetic liquid crystal (LC) composites [PEA-COOH]SO3H@Fe3O4 series, with Fe3O4 nanoparticles as core and side-chain supramolecular LC polymers with different mesogen contents as coating, were reported. The chemical structure, magnetic property and LC properties were investigated by a variety of techniques. In order to study the effects of the mesogen content and LC phase state of magnetic LC composites on magnetorheological performance and sedimentation stability, MRF based on silicone oil carrier liquid and [PEA-COOH]SO3H@Fe3O4 dispersed phase were prepared. It was found that dynamic yield stress, plastic viscosity and the sedimentation ratio increased slightly with increasing mesogen content in the magnetic LC composites. What’s more, MRF prepared by magnetic LC composites annealed in LC state exhibited better magnetorheological performance and sedimentation stability than those in glassy state and isotropic state. These results indicated that the synergetic effects of LC orientation and magnetic field-induced arrangement of Fe3O4 magnetic particles can improve the magnetorheological performance and sedimentation stability of MRF.

Mechanotropic Elastomers

AbstractLiquid crystal elastomers (LCEs) are anisotropic polymeric materials. When subjected to an applied stress, liquid crystalline (LC) mesogens within the elastomeric polymer network (re)orient to the loading direction. The (re)orientation during deformation results in nonlinear stress‐strain dependence (referred to as soft elasticity). Here, we uniquely explore mechanotropic phase transitions in elastomers with appreciable mesogenic content and compare these responses to LCEs in the polydomain orientation. The isotropic (amorphous) elastomers undergo significant directional orientation upon loading, evident in strong birefringence and x‐ray diffraction. Functionally, the mechanotropic displacement of the elastomers to load is also nonlinear. However, unlike the analogous polydomain LCE compositions examined here, the isotropic elastomers rapidly recover after deformation. The mechanotropic orientation of the mesogens in these materials increase the toughness of these thiol‐ene photopolymers by nearly 1300 % relative to a chemically similar elastomer prepared from wholly isotropic precursors.

Mechanotropic Elastomers.

Liquid crystal elastomers (LCEs) are anisotropic polymeric materials. When subjected to an applied stress, liquid crystalline (LC) mesogens within the elastomeric polymer network (re)orient to the loading direction. The (re)orientation during deformation results in nonlinear stress-strain dependence (referred to as soft elasticity). Here, we uniquely explore mechanotropic phase transitions in elastomers with appreciable mesogenic content and compare these responses to LCEs in the polydomain orientation. The isotropic (amorphous) elastomers undergo significant directional orientation upon loading, evident in strong birefringence and x-ray diffraction. Functionally, the mechanotropic displacement of the elastomers to load is also nonlinear. However, unlike the analogous polydomain LCE compositions examined here, the isotropic elastomers rapidly recover after deformation. The mechanotropic orientation of the mesogens in these materials increase the toughness of these thiol-ene photopolymers by nearly 1300 % relative to a chemically similar elastomer prepared from wholly isotropic precursors.

Thermoresponsive liquid crystalline polymer membranes that undergo phase transition at body temperature

Biological membranes regulate their properties and functions by drastic changes in their organized structure and mobility in response to changes in environmental factors such as pH, temperature, and biomolecule concentration. Liquid crystalline polymers (LCPs) are widely used as functional materials in a variety of fields because, similarly to biological membranes, they undergo abrupt changes in their organized structures and mobilities at their phase transition temperatures. In order to design smart membranes that regulate drug permeability as a function of temperature, we focused on well-organized and dynamic LCPs that undergo liquid crystalline-isotropic phase transition around body temperature. In this study, side-chain LCPs composed of a flexible siloxane main chain and pendant mesogens were synthesized and LCP membranes were prepared by casting the LCPs/xylene on porous support membranes. The nematic-isotropic phase transition temperature (TNI) of the resulting LCPs strongly depended on the mesogen content. Although the permeation of drugs through the LCP membranes was hindered at temperatures below TNI, their drug permeability was drastically enhanced over TNI. Thermoresponsive regulation of drug permeation through the LCP membranes is attributed to the drastic changes in their well-organized and dynamic structures that are induced by the nematic-isotropic phase transition.

(Meth)acrylate liquid crystalline polymers for membrane applications

ABSTRACTSynthesis and characterization of a series of linear copolymers with liquid crystalline side chains is reported. Methacrylate monomers containing cyanobiphenyl as a mesogenic group were prepared. These monomers were then copolymerized with 2‐ethylhexyl acrylate in varying molar ratios. The structure and composition of the monomers and corresponding polymers were determined by 1H‐NMR, elemental analysis and size‐exclusion chromatography. Thermal properties of the polymers were studied using differential scanning calorimetry, polarized optical microscopy, and X‐ray scattering techniques. Increasing mesogen content resulted in an increase of the glass transition temperature of the copolymers. In addition, above a threshold mesogen content the copolymers exhibited smectic mesophases. Using a solution casting technique, a membrane was fabricated to study single gas transport behaviors. Permeabilities, were in the order of CO2 > propylene > O2 > N2 > CH4 > propane. Diffusion coefficients correlated well to Lenard–Jones Diameter. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015, 132, 42694.

Biodegradable Poly(ester-urethane)s Containing Poly(butylene succinate) and Mesogen Segments

A series of biodegradable segmented poly (ester-urethane) s were prepared by solution polymerization of poly (butylene succinate) (PBS), mesogenic diol (MD), and hexamethylene diisocyanate (HDI). The MD content was varied from 0 to 40 mol% so that the effects of the mesogen content on the thermal and physical properties, and enzymatic degradation were examined respectively. It was found that introducing mesogen segments could increase the thermal stability and the elastic properties, while reduced the phase transition temperatures, the degree of relative crystallinity (Xc) and the enzymatic degradation rate.

Structural and photoresponsive behavior of a series of (phenylene)azobenzene-containing liquid-crystalline methacrylic copolymers

A series of side-chain liquid-crystalline methacrylic copolymers, having various amounts of 4′-(4-decyloxyphenyl)-4-(hexyloxy)-azobenzene as the mesogenic group, were synthesized and characterized by differential scanning calorimetry, polarizing optical microscopy and X-ray diffraction to determine their mesomorphic properties, and by UV−visible spectrometry to elucidate their optical absorption characteristics. Also, birefringence and surface relief gratings were photoinduced in the films using a laser beam of 413 nm. Copolymers with high mesogen contents display a polymorphic behavior of the smectic type. The smectic order tends to disappear as the content of mesogens in the copolymer decreases, going from extended lamellar domains of chromophores packed in a parallel position, typical for smectic liquid crystals, to small aggregates of chromophores dispersed in an amorphous matrix. Important hypsochromic shifts (Δλmax ≈ 20 nm), associated with π interactions between neighboring aromatic groups, confirm the aggregation of chromophores in lamellar structures. The light-induced birefringence (0.005 − 0.06) is not as high as expected but it corroborates that there exists a tight interplay between photoinduced orientation and intrinsic molecular order. Surface relief gratings were recorded in copolymers with low mesogen content (15 − 30 mol%), showing by atomic force microscopy sinusoidal profiles with an average depth of around 150 nm. © 2013 Society of Chemical Industry

Synthesis and Properties of Biodegradable Liquid Crystalline Poly(ester-Urethane)s Based on Poly(L-Lactic Acid)

A series of biodegradable segmented liquid crystalline poly(ester-urethane)s were prepared by solution polymerization of poly(L-lactic acid) (PLLA), mesogenic diol prepolymer poly(butylene terephthaloyldioxy dibenzoates) (MD), and hexamethylene diisocyanate (HDI). The MD content was varied from 0 to 40 mol% so that the effects of the mesogen content on the thermal and physical properties, and hydrolytic degradation were examined respectively. It was found that introducing mesogens units could increase the thermal stability and the elastic properties, while reduced the phase transition temperatures and the hydrolytic degradation rate.

Synthesis and Characterization of Biodegradable Segmented Liquid Crystalline Poly(<i>Ester-Urethane</i>)s

A series of biodegradable segmented liquid crystalline poly(ester-urethane)s were prepared by solution polymerization of poly(butylene succinate) (PBS), mesogenic diol prepolymer poly(butylene terephthaloyldioxy dibenzoates) (MD), and hexamethylene diisocyanate (HDI). The MD content was varied from 0 to 40 mol% so that the effects of the mesogen content on the thermal and physical properties, hydrolytic degradation and morphological textures were examined respectively. It was found that introducing mesogens units could increase the thermal stability and the elastic properties, while reduced the phase transition temperatures and the hydrolytic degradation rate. The mesogenic monomer and mesogenic diol could exhibit a schlieren texture characteristic of nematics, and some of the poly(ester-urethane)s could display a grainy texture.

Kinetics of the nematic phase growth across the isotropic-nematic phase transition in polymer-dispersed liquid crystals

Polymer-dispersed liquid crystals (PDLCs) composed of poly(dimethyl siloxane), cured poly(dimethyl siloxane) and polysulfone (as matrices), and an azomethine compound (as an embedded mesogen varying in weight from 5 to 80%) were prepared via solvent-induced phase separation. After preparation, they were heated to the melt and then cooled; phase transitions upon both heating and cooling were detected with a differential scanning calorimeter and a polarising optical microscope (POM). The nematic droplets appearing in the POM images across the isotropic-nematic phase transition were treated statistically and described with principles of irreversible thermodynamics. Furthermore, kinetics of the nematic phase growth across this phase transition was studied and described analytically with the universal law for cluster growth. Both flexibility of the polymer matrix and the mesogen content in PDLCs were shown to influence the processes studied.

Biodegradable liquid crystalline aromatic/aliphatic copolyesters. Part I: Synthesis, characterization, and hydrolytic degradation of poly(butylene succinate- co-butylene terephthaloyldioxy dibenzoates)

To increase the thermal and mechanical properties of the aliphatic polyester poly(butylene succinate) (PBS), a series of potentially biodegradable liquid crystalline aromatic/aliphatic random copolyesters were prepared by melt polycondensation of new mesogenic monomers dimethyl 4,4′-(terephthaloyldioxy) dibenzoate (MTB), dimethyl succinate, and 1,4-butanediol. The synthesized copolyesters were characterized by means of proton nuclear magnetic resonance spectroscopy ( 1H NMR), gel permeation chromatography (GPC), viscosity measurements, differential scanning calorimetry (DSC), thermogravimetry (TG), X-ray diffraction (XRD), polarizing light microscopy (PLM) and mechanical property measurements. The MTB content was varied so that the effects of the mesogen content on the thermal and mechanical properties, degradable behaviours and mesophase were examined. It was found that introducing the rigid rod mesogens could increase the thermal stability and the mechanical properties, while it reduced the melting temperature ( T m), the crystallization temperature ( T c), the degree of relative crystallinity ( X c) and the hydrolytic degradation rate. Only the homopolyester poly(butylenes terephthaloyldioxy dibenzoates) was able to show the schlieren texture characteristic of nematics.

Thermotropic behavior of lithocholic acid derivative linked hydroxyethyl cellulose

AbstractVarious mesogenic lithocholic acid (LA) derivatives [3‐acetyl LA (LAAC); 3‐propionyl LA (LAP), 3‐cinnamoyl LA (LACin); 3‐benzoyl LA (LAB), 3‐(4‐nitrobenzoyl) LA (LANB); and methyl‐3‐(3‐carboxypropionyl) LA, i.e., LA methyl ester monosuccinate (LAMeMS)] were reacted with hydroxyethyl cellulose (HEC) to obtain a series of LA derivatives of HEC. These polymers were characterized by the mesogen content, IR, differential scanning calorimetry, and hot‐stage coupled polarizing optical microscopy. The degree of substitution (DS) ranged from 0.27 to 1.11. Polymers HEC‐LAB (DS = 1.11), HEC‐LANB (DS = 0.80), HEC‐LACin (DS = 0.76), and HEC‐LAP (DS = 0.27) exhibited a mesophase on heating whereas HEC‐LAB also showed a mesophase on cooling. The other polymers HEC‐LAAC (DS = 0.89) and HEC‐LAMeMS (DS = 0.36) did not reveal mesophase formation with either heating or cooling. It is observed that the formation of a mesophase is affected by the type of mesogen that is used rather than the mesogen content of the polymer. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 1995–2001, 2006

Liquid crystalline properties of unsegmented and segmented polyurethanes synthesised from high aspect ratio mesogenic diols

AbstractA series of novel tetrad high aspect ratio mesogenic diol monomers 4‐{[4‐(n‐hydroxyalkoxy)‐phenylimino]‐methyl}‐benzoic acid 4‐{[4‐(n‐hydroxyalkoxy)‐phenylimino]‐methyl}‐phenyl ester were prepared with varying alkoxy spacer length (n=2,4,6,8,10) by reacting 4‐formylbenzoic acid 4‐formylphenyl ester and 4‐(n‐hydroxyalkoxy) anilines. Two series of thermotropic main chain liquid crystalline unsegmented polyurethanes (PUs) were obtained by the polyaddition of the mesogenic diols with hexamethylene diisocyanate (HMDI) and methylene bis(cyclohexylisocyanate) (H12MDI) in dimethylformamide respectively. The effect of the incorporation of a third component namely polyol on the liquid crystalline properties of the polyurethanes was also studied. Linear segmented PUs were synthesised by a two‐step block copolymerisation method. The PUs synthesised were based on six spacer mesogenic diol chain extender, soft segments poly(tetramethylene oxide)glycol (PTMG) (Mn= 650,1000,2000) and polycaprolactone diol (PCL) (Mn=530,1250,2000) of varying molecular weights and different diisocyanates including HMDI, H12MDI and methylene bis(phenylene isocyanate) (MDI). Structural elucidation was carried out by elemental analysis, fourier transform infra red (FT‐IR), nuclear magnetic resonance (1H NMR and 13C NMR) spectroscopy. Inherent viscosity of the unsegmented polymers measured in methanesulphonic acid at 26°C was in the range of 0.13 ‐ 0.65 dL/g while the molecular weights and molecular weight distribution of the segmented polyruethanes was determined using gel permeation chromatography (GPC). Mesomorphic properties were studied by differential scanning calorimetry (DSC) and hot stage polarising optical microscopy and the thermal stability was determined by thermogravimetric(TG)analysis. The monomeric diols and the polyurethanes exhibited nematic texture and good mesophase stability. It was observed that the partial replacement of the mesogenic diol by the polyol of varying molecular weights influenced the phase transitions and the occurrence of mesophase textures. The phase transition temperatures of the investigated polyurethanes showed dependence on the chain length of the soft segment and on the content of the mesogen moiety. A higher content of mesogenic moiety was needed to obtain liquid crystalline property when the soft segment length was increased as observed in the case of PTMG. Grained and threaded textures were observed depending on the molecular weight of the soft segment, the mesogen content and the diisocyanate. The stress‐strain analyses showed that the polymers bused on high molecular weight PTMG soft segment have elastomeric property while the PCL based PUs displayed no elastomeric property.

Synthesis and properties of segmented main‐chain liquid‐crystalline polyurethanes with a high aspect ratio mesogenic diol as a chain extender

AbstractMain‐chain liquid‐crystalline polyurethanes were synthesized based on a high aspect ratio mesogenic diol (4‐{[4‐(6‐hydroxyhexyloxy)‐phenylimino]‐methyl}‐benzoic acid 4‐{[4‐(6‐hydroxyhexyloxy)‐phenylimino]‐methyl}‐phenyl ester) as a chain extender; polycaprolactone (PCL) diol soft segments of different number‐average molecular weights (530, 1250, or 2000); and different diisocyanates, including 1,4‐hexamethylene diisocyanate (HMDI), 4,4′‐methylene bis(cyclohexyl isocyanate) (H12MDI), and 4,4′‐methylene bis(phenyl isocyanate) (MDI). The structure of the polymers was confirmed with Fourier transform infrared spectroscopy, and differential scanning calorimetry and polarizing microscopy measurements were carried out to examine the liquid‐crystalline and thermal properties of the polyurethanes, respectively. The mesogenic diol was partially replaced with 20–50 mol % PCL. A 20 mol % mesogen content was sufficient to impart a liquid crystalline property to all the polymers. The partial replacement of the mesogenic diol with PCL of various molecular weights, as well as the various diisocyanates, influenced the phase transitions and the occurrence of mesophase textures. Characteristic liquid‐crystalline textures were observed when a sufficient content of the mesogenic diol was present. Depending on the flexible spacer length and the mesogenic content, grained and threadlike textures were obtained for the HMDI and H12MDI series polymers, whereas the polyurethanes prepared from MDI showed only grained textures for all the compositions. The polymers formed brittle films and could not be subjected to tensile tests. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 1527–1538, 2002

Studies on Thermotropic Main Chain Liquid Crystalline Segmented Polyurethanes. II. Synthesis and Properties of H 12 MDI- and MDI-Based Polyurethanes from High Aspect Ratio Mesogenic Diol as Chain Extender

Previous studies on thermotropic main chain liquid crystalline polyurethanes (TLCPUs) prepared from 4-{[4-(6-hydroxyhexyloxy)-phenylimino]-methyl}-benzoic acid 4-{[4-(6-hydroxyhexyloxy)-phenylimino-methyl}-phenyl ester (mesogenic diol) as chain extender were continued. In this article, the main chain liquid crystalline polyurethanes are based on the mesogenic diol as chain extender, poly(tetramethylene oxide)glycol (PTMG) soft segment of different molecular weight PTMGs (M n =650, 1000, 2000), different diisocyanates including 4,4′-methylene bis(cyclohexyl isocyanate) (H 12 MDI) and 4,4′-methylene bis(phenyl isocyanate) (MDI). The polyurethanes were investigated by FTIR spectroscopy for structural confirmation and DSC and hot stage optical polarizing microscope to study the mesomorphic properties. The molecular weight distribution was determined by GPC. The effect of partial replacement of the mesogenic diol by 20-50 mol% PTMG and the effect of the diisocyanates on the liquid crystalline properties were studied. It was observed that the partial replacement of the mesogenic diol by PTMG of varying molecular weights and also the diisocyanates influenced the phase transitions and the occurrence of mesophase textures. Characteristic liquid-crystalline textures were observed when sufficient content of the mesogenic diol was present. If the flexible spacer was longer, a higher content of mesogenic agent was needed to obtain LC properties. Depending on the flexible spacer length and the mesogenic content, grained and threadlike textures were obtained for the H 12 MDI series polymers while the PUs prepared from MDI showed only grained texture for all the compositions. Elastomeric behavior was observed in the polymers from PTMG 2000 and H 12 MDI with mesogen content 20-49 mol%.

A Series of Novel Liquid Crystalline Polymers Showing a Nematic Discotic and/or a Nematic Columnar Phase

A series of novel liquid crystalline side chain polyacrylates was prepared, bearing a disk-shaped pentakis(4-methylphenylethynyl)benzene mesogen via an undecanoxy spacer. By using the concept of the polymer analogous reaction, a variety of copolymers with different alkyl acrylate “co-monomers” and increasing mesogen contents were synthesized. The thermal properties of the polymers were investigated by optical polarization microscopy and differential scanning calorimetry. The results indicated that the liquid crystalline characteristics of the polymers are not determined by nature of the co-monomers, but mainly by the weight fraction of rigid mesogen present in the material. All polymers exhibit a nematic columnar phase at lower temperatures. Polymers with a high mesogen content (>61%-wt) exhibit an additional nematic discotic phase at high temperatures, which is unique for polymeric materials.

Behavior of ER Fluids based on Three Variants of a Liquid Crystalline Polysiloxane

ER fluids of three types, composed of liquid crystalline polymethylsiloxanes (LCSs) differing in the range of polysiloxane chain length and mesogenic group content diluted by polydimethylsiloxanes (DMSs) with or without phenyl groups, were prepared and the ER effect of each was investigated in terms of its temperature dependence and rise time. Type A fluid exhibited an ER effect with a short rise time but a strong temperature dependence. The ER effect of the Type B fluid showed little temperature dependence but a comparatively long rise time. The Type C fluid exhibited a short rise time, comparable to that of Type A, and a temperature independence similar to that of Type B. For all three types, the decay time of the ER effect was long. Investigation of the decay time for the Type A fluid showed that it was affected particularly by shear rate and DMS dilution ratio and also by temperature and DMS viscosity, and indicated that deformation by shear stress was necessary for a return of the fluid to its original non viscous structure.

Thermal and mechanical properties of liquid crystalline epoxy resins as a function of mesogen concentration

Liquid crystalline thermosets are known to exhibit a number of improved characteristics in comparison with traditional plastics. We now describe quantitatively how the thermal and mechanical properties of a number of liquid crystalline epoxy resins vary with polymer mesogen concentration, using biphenyl- and biphenol-based diepoxides with varying spacer lengths and a range of aromatic diamine curing agents. Although macroscopically isotropic, the degree of order at the microdomain level can be related to the network physical properties, and thus increasing mesogen content leads to increased intermolecular interactions between the polymer chains and hence reduced micro-Brownian motion and reduced free volume. These properties are manifested as increased elastic moduli at high temperatures, reduced thermal coefficients of expansion, increased decomposition onset temperatures and reduced solvent absorption.

Relation between molecular structure and electrorheological effects in liquid crystalline polymers

The influence of the molecular structure of homogeneous fluids comprised of liquid crystalline polysiloxanes (LCSs) diluted in polydimethylsiloxane (DMS), on the increase in their viscosity upon application of an electric field (electrorheological effect, or the ER effect) was investigated, particularly in relation to the degree of polysiloxane polymerization, the composition of their mesogenic side chains, and the composition of alkyl spacers inserted between these two components. A large ER effect was observed in LCSs having spacer length of 3 carbons (C3) and diluted with DMS, even though little such effect was observed with no DMS dilution. The ER effect generally increased with increasing DMS dilution and with increasing mesogenic group content, main chain length, and spacer length. At the same time, however, the miscibility of the DMS solvent with the LC polymer decreased, resulting in instability and difficulty in measuring the shear stress at temperatures up to 40°C or higher. When spacers containing an ether bond were employed, on the other hand, little or no polymer-solvent phase separation occurred, and the ER effect was strong and stable down to temperatures of 30°C or lower. With all of the LCSs, the ER effect was observed at temperatures above their isotropic phase transition temperature as measured by DSC.