Synthesis and structure of polysiloxane liquid crystalline elastomers with a mesogenic crosslinking agent

A new mesogenic crosslinking agent M‐1 was synthesized to minimize the perturbations of a nonmesogenic crosslinking agent for liquid crystalline elastomers. The synthesis of new side‐chain liquid crystalline elastomers containing a rigid mesogenic crosslinking agent M‐1 and a nematic monomer M‐2 is described by a one‐step hydrosilylation reaction. The chemical structures of the obtained monomers and elastomers were confirmed by ¹H NMR and FTIR spectroscopy. The mesomorphic properties and phase behavior were investigated by differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction measurements. The influence of the crosslinking units on the phase behavior is discussed. The elastomers containing less than 15 mol % of the crosslinking units showed elasticity, reversible phase transition, and nematic‐threaded texture. However, when the crosslinking density reached 21.6 mol %, the mesophase of polymer P‐8 disappears. The adoption of a mesogenic crosslinking agent diminishes the perturbation of a nonmesogenic crosslinking agent on mesophase of liquid crystalline elastomers, and isotropic temperature and a mesomorphic temperature range slightly decreased with increasing content of the crosslinking agent. In addition, X‐ray analysis shows nematic polydomain network polymers can transform into smectic monodomain by stress induction, leading to the orientation formation macroscopically. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1736–1742, 2004     

Synthesis and mesomorphic properties of a new side‐chain,chiral smectic,liquid‐crystalline elastomer

The synthesis of the new chiral monomer 4‐(10‐undecylen‐1‐yloxy)biphenyl‐4′‐[(S)‐2‐methyl‐1‐bu‐ toxy]benzoate (M₁), the nematic crosslinking agent biphenyl 4,4′‐bis(10‐undecylen‐1‐yloxybenzoate) (M₂), and the corresponding liquid‐crystalline elastomer is described. The chemical structures of the chiral monomer and crosslinking agent have been characterized with Fourier transform infrared, elemental analyses, and proton and carbon‐13 nuclear magnetic resonance spectra. The mesomorphic properties have been investigated with differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction. Monomer M₁ shows different smectic phases (smectic A, chiral smectic C, and smectic B) and a cholesteric phase, and M₂ exhibits a nematic phase. The liquid‐crystalline elastomer shows smectic A and chiral smectic C phases. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 4234–4239, 2006     

Synthesis and characterization of a series of smectic liquid crystalline elastomers

A series of new smectic and cholesteric liquid crystalline elastomers was prepared by graft polymerization of mesogenic monomer with the chiral and nonmesogenic crosslinking agent using polymethylhydrosiloxane as backbone. The chemical structures of the monomers and polymers obtained were confirmed by Fourier transform infrared (FTIR), proton nuclear magnetic resonance spectra (¹H‐NMR). The mesomorphic properties were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X‐ray diffraction measurements (XRD). M₁ showed smectic (S_B, S_C, S_A) and nematic phases during the heating and the cooling cycles. Polymer P₀ and elastomer P₁ exhibited smectic B phase, elastomers P₂–P₅ showed smectic A phase, P₆ and P₇ showed cholesteric phase, and P₈ displayed stress‐induced birefringence. The elastomers containing less than 15 mol % M₂ displayed elasticity and reversible phase transition with wide mesophase temperature ranges. Experimental results demonstrated that the glass transition temperatures decreased first and then increased; melting temperatures and the isotropization temperatures and the mesophase temperature ranges decreased with increasing content of crosslinking unit. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 97: 498–506, 2005     

Study on the Optical and Mesomorphic Properties of Cholesteric Side-Chain Liquid Crystalline Elastomers Containing a Nematic Cross-Linking Agent

To study the effect of the content of the nematic cross-linking units on the mesophase behaviors, the optical and mesomorphic properties of liquid crystalline elastomers (LCEs), aseries of LCEs have been synthesized by hydrosilylation reaction with poly(methylhydrogeno)siloxane, a cholesteric liquid crystalline monomer, and a nematic cross-linking agent. The chemical structures and properties of the synthesized LCEs have been investigated by use of various techniques. Homopolymer P₀ bearing only cholesteric component displays a smectic A phase, but elastomers P₁-P₅ containing different content of nematic cross-linking units show a cholesteric mesophase. The reflection wavelengths of theLCEs show a weak temperature dependence at lower temperatures but a strong temperature dependence at higher temperatures. Furthermore, the maximum reflection wavelengths of the LCEs can be stabilized over a wide temperature range when the LCEs are heated, suggesting that the helical structure and pitch of the cholesteric phase can be stabilized with a polymer network.     

First liquid single crystal elastomer containing lactic acid derivative as chiral co-monomer: Synthesis and properties

For the first time new type of liquid single crystal elastomers forming the chiral smectic A^* (SmA^*) phase has been prepared using chiral lactic acid derivative as a co-monomer. The synthesis and the basic characterization of the ferroelectric liquid crystalline co-monomer based on differential scanning calorimetry and small/wide-angle X-ray scattering are reported and discussed. The monomer possesses the paraelectric SmA^* and the ferroelectric SmC^* phases over a broad temperature range. The preparation of new smectic liquid crystalline elastomers together with characterization of their mesomorphic and structural properties is also reported. New elastomers possess the orthogonal paraelectric SmA^* phase over 50 K broad temperature range.     

Synthesis and phase behavior of nematic liquid‐crystalline elastomers derived from smectic crosslinking agent

A mesogenic crosslinking agent M‐1 was synthesized to minimize the perturbations of nonmesogenic crosslinking agent for liquid‐crystalline elastomers. The synthesis of side‐chain liquid‐crystalline elastomers containing a rigid mesogenic crosslinking agent M‐1 and a nematic monomer M‐2 was described by a one‐step hydrosilylation reaction. The chemical structures of the obtained monomers and network polymers were confirmed by Fourier transform infrared and ¹H‐NMR spectroscopy. The mesomorphic properties and phase behavior were investigated by differential scanning calorimetry, polarizing optical microscopy, and X‐ray diffraction measurements. The influence of the crosslinking units on the phase behavior was discussed. The liquid‐crystalline elastomers containing less than 15 mol % of the crosslinking units showed elasticity, reversible phase transition, and threaded texture. The experimental results demonstrated that isotropic temperature and liquid‐crystalline range of polymers P‐1–P‐7 decreased a little as the concentration of crosslinking agent M‐1 increased, and the use of mesomorphic crosslinking agent M‐1 promotes the arrangement of liquid‐crystalline units from P‐1 to P‐5. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 1712–1719, 2005     

Effect of the different rigidity of the chiral crosslinker on phase behaviors of side‐chain chiral liquid crystalline elastomers

In the present work, the phase behaviors of two series of side‐chain liquid crystalline elastomers ( PI and PII series) derived from the same nematic liquid crystalline monomer and the different rigidity of chiral bisolefinic crosslinking units have been compared and studied extensively, and the effect of the different rigidity of crosslinker on the phase behavior of elastomers has been discussed. The chemical structure of the monomers and polymers obtained were confirmed by FTIR and ¹H NMR spectroscopy. The phase behaviors were investigated by differential scanning calorimetry, polarizing optical microscopy measurement, thermogravimetric analyses, and X‐ray diffraction measurement. The two series of elastomers showed smectic or cholesteric phases. When the amount of different crosslinking units was less than 15 mol %, both of the elastomers displayed elasticity, reversible phase transition with wide mesophase temperature ranges, and high thermal stability. It is shown that the isotropization temperature values of PII series are higher than those of PI series, and the glass transition temperature values of PII series varied smoothly and that of PI series changed smoothly first and then abruptly with increasing the contents of crosslinkers with different rigidity. In addition, PI series showed an interesting change in LC texture near clearing point, but PII series did not. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Synthesis and characterization of a novel liquid crystalline star‐shaped polymer based on α‐CD core via ATRP

Well‐defined side‐chain liquid crystalline star‐shaped polymers were synthesized with a combination of the “core‐first” method and atom transfer radical polymerization (ATRP). Firstly, the functionalized macroinitiator based on the α‐Cyclodextrins (α‐CD) bearing functional bromide groups was synthesized, confirmed by ¹H‐NMR, MALDI‐TOF, and FTIR analysis. Secondly, the side‐chain liquid crystalline arms poly[6‐(4‐methoxy‐4‐oxy‐azobenzene) hexyl methacrylate] (PMMAzo) were prepared by ATRP. The characterization of the star polymers were performed with ¹H‐NMR, gel permeation chromatography (GPC), differential scanning calorimetry (DSC) and thermal polarized optical microscopy (POM). It was found that the liquid crystalline behavior of the star polymer α‐CD‐PMMAzo_n was similar to that of the linear homopolymer. The phase‐transition temperatures from the smectic to nematic phase and from the nematic to isotropic phase increased as the molecular weight increased for most of these samples. All star‐shaped polymers show photoresponsive isomerization under the irradiation with Ultraviolet light. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Main‐chain liquid‐crystalline ionomers bearing potassium sulfonate groups

A series of thermotropic main‐chain liquid‐crystalline (LC) ionomers were prepared, which contained potassium sulfonate groups pendent to the chains. The polymers were prepared in an esterifying reaction with potassium ion contents ranging between 0 and 3.9 wt %. The content of potassium ion was characterized by spectrophotometric analysis with sodium tetraphenylboron as the titrant. Chemical structures were determined by various experimental techniques including Fourier transform IR spectroscopy and ¹H‐NMR. LC properties were characterized by differential scanning calorimetry, polarizing optical microscopy, and X‐rays. All of the polymers displayed nematic or smectic mesophases. With increasing potassium sulfonate ionic concentration in the polymers, the melting temperatures and isotropic transition temperatures changed little, whereas the temperature of the smectic A–nematic phase transition increased. The ionic aggregation was tangled with the rigid mesogenic groups of LC segments to form multiple block domains, leading the soft main chains to fold and form a lamellar structure due to their electrostatic interactions. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 2021–2026, 2005     

Bistable light scattering effects in a (side chain liquid crystalline polymer)/(low molecular weight liquid crystal) composite and in a ferroelectric liquid crystalline polymer

The electro-optical effects and aggregation states of liquid crystalline polymer (LCP)/ low molecular weight liquid crystal (LC) composite and ferroelectric liquid crystalline copolymer (FLCP) have been investigated. The nematic LCP was observed to be miscible with the nematic LC over wide ranges of concentration and temperature. The binary mixture showed an induced smectic phase in the range of 80/20-20/80 mol%. The electro-optical effects of the LCP/LC composite in an induced smectic phase could be classified into the turbid (light- scattering) and the transparent states upon application of AC and DC electric fields, respectively. The transient scattering mode was obtained by repeated voltage polarity reversal in the chiral smectic C phase of FLCP. The reversible transparent-opaque (light scattering) change was observed in the chiral smectic C state upon application of DC and AC electric fields, respectively. Both transparent and light-scattering states of the LCP/LC composite and the FLCP could be maintained, even after the electric field had been turned off (memory effect). The bistable effects of LCP/LC composite and FLCP are opposite under the same conditions. A novel type of electro-optical effect on light scattering was obtained for liquid crystalline polymer in the smectic states.     

Preparation and properties of azobenzene-containing amphiphilic miktoarm star polymers

Summary A series of novel miktoarm star polymers composed of a poly(ethylene oxide) (PEO) and two side-chain liquid crystalline azobenzene-containing polymethacrylate (PEO-(PMMAZO)₂) were prepared using atom transfer radical polymerization (ATRP). Bifunctional macroinitiator PEO-Br₂ was synthesized by condensation reaction in two steps and characterized by ¹H NMR, ¹³C NMR and IR. Kinetic study showed that it was a first order reaction referred to the monomer MMAZO, namely, 6-(4-methoxy-4’-oxy-azobenzene)hexyl methacrylate. The liquid crystalline behaviors of the miktoarm star polymers were studied by differential scanning calorimetry (DSC) and polarized optical microscope (POM). They exhibited smectic and nematic mesophases when Mn was beyond 9.4×10³ g/mol. The phase transition temperatures of the smectic and nematic phases increased while the melting temperature of PEO decreased with increasing molecular weight of the LC block. Compared with diblock polymer PEO-PMMAZO, the melting temperature of PEO in miktoarm star polymer decreased more rapidly.     

New side‐chain nematic liquid crystalline elastomers containing smectic crosslinking units

A series of siloxane‐based liquid crystalline elastomers containing the smectic crosslinking agent M‐1 and nematic monomer M‐2 were synthesized by a one‐step hydrosilication reaction. The chemical structures of the monomers and polymers obtained were confirmed by FTIR spectroscopy. The mesomorphic properties and phase behavior were investigated by differential scanning calorimetry (DSC), polarizing optical microscopy (POM), and X‐ray diffraction (XRD) measurements. The influence of the crosslinking unit on the phase behavior was discussed. The experimental results demonstrated that the glass transition temperatures of elastomers had no remarkable change, and isotropization temperatures decrease with increasing the content of the crosslinking agent M‐1. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 3755–3760, 2004     

Synthesis,Characterization and Texture Observations of Calamitic Liquid Crystalline Compounds

Several divinylic mesogenic monomers were synthesized based on coupling the monomer 4-(4-pentenyloxy)benzoic acid with chlorohydroquinone, 2,5-dihydroxy- acetophenone, methylhydroquinone or 2-methoxyhydroquinone. This resulted in novel mesogens of phenylene esters with different lateral substituent groups. The effect of the lateral substituent group on the thermotropic phase behavior for these liquid crystalline compounds was investigated using DSC and optical polarized microscopy. All the mesogens proved to have a wide nematic liquid crystalline range. Only the phenylene ester, which has a methoxy lateral substituent, exhibited both nematic and smectic phases. Structural confirmation of all new derivatives was accomplished by ¹H- and ¹³C-NMR spectroscopic analysis, along with CH elemental analysis.     

Phase identifications and monotropic transition behaviors in a thermotropic main-chain liquid crystalline polyether

A thermotropic liquid crystalline (LC) polyether was synthesized by condensation polymerization of 1-(4-hydroxyphenyl-2-(2-methyl-4-hydroxypheyl)ethane) and α,ω-dibromononane, and abbreviated as MBPE-9. Multiple phase transitions were found via differential scanning calorimetry (DSC) upon heating and cooling. Based on wide-angle X-ray diffraction (WAXD) and polarized light microscopy (PLM) experiments, two monotropic LC nematic (N) and smectic phases and three crystalline (orthorhombic K₁, triclinic K₂ and triclinic K₃) phases were identified. A study on kinetics showed that only when the N phase formation rate was faster than the crystallization rate of K₁ phase which directly grew from the melt, the presence of the N phase could accelerate both the overall crystallization and the linear crystal growth rates of K₁ phase. In a narrow temperature region, two distinct crystal growth rates could be observed: one was directly from the isotropic melt, and another was from the N phase.     

Thermal and structural properties of tri-O-substituted cellulose ethers

Tri-O-substituted cellulose ethers prepared by the use of SO₂–diethylamine (DEA)–dimethylsulfoxide (DMSO) and powdered sodium hydroxide were characterized by measurement of melting points and degradation points and by differential scanning calorimetric (DSC) and X-ray analyses. Their thermal and structural characteristics were ascertained to be dependent on the kinds of substituents. Among the ethers examined, four kinds of arylmethylcelluloses showed the characteristics of thermotropic liquid crystals (smectic). Tri-O-α-naphthylmethyl-cellulose showed the three transformations due to a solid–smectic phase, a smectic–nematic phase, and a nematic–isotropic phase transformation. This thermotropic liquid crystal was enantiotropic below 200°C.     

Synthesis,characterization, and mechanical properties of a novel terphenyl liquid crystalline epoxy resin

A novel terphenyl liquid crystalline (LC) epoxy resin was synthesized and characterized by ¹H‐NMR, Fourier transform infrared spectroscopy, differential scanning calorimetry (DSC), and polarizing optical microscopy. Depending on the curing temperature, the synthesized resin formed both smectic and nematic LC phases. A time‐temperature‐transformation diagram was constructed to optimize the curing process, which helped in the preparation of LC and isotropic system. The terphenyl epoxy resin obtained exhibited higher acid resistance than a comparable Schiff‐base epoxy resin, and also displayed excellent fracture toughness. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41296.     

Rheology of low nematic transition temperature thermotropic liquid crystalline copolyester HBA/HQ/SA

The rheology of a liquid crystalline copolyester of hydroxybenzoic acid, hydroquinone, and sebacic acid (HBA/HQ/SA copolyester) was studied on both a rotational and a capillary rheometer. DSC studies show that the copolyester has a crystalmesophasic and a broad mesophasic-isotropic transition at 170°C and 220°C. Optical texture observations show the mesophase is characterized by line defect textures, which are characteristic of a nematic structure. At 220°C, both isotropic and nematic phases coexist with the latter being the major. As temperature reaches 250°C, a clear dominance of isotropic phase is observed. At this temperature, the nematic phase of irregular shapes randomly disperses within the isotropic matrix. Subsequent rheological studies were thus conducted in crystal/nematic biphase, single nematic phase, nematic/isotropic biphase, and the near single isotropic phase. Dynamic strain sweep measurements show that a linear viscoelastic region exists at all temperatures tested. The maximum strain amplitude for the linear viscoelastic region is found to be highly structure dependent; it is > 100% in the nematic phase, ∼20% in the biphases, and only about 5% in the isotropic phase. The concurrence of curves obtained at different temperatures in a Cole-Cole plot of G′ vs. G″ indicates similar structures in the nematic phase and biphases. Measurements of steady shear viscosity using a rotational rheometer and a roundhole capillary rheometer show that in the nematic phase the copolyester behaves as a shear thinning fluid for a wide shear rate range of 1 ∼ 10,000 s⁻¹, in which the power law index is about 0.6 ∼ 0.8, and the viscosity is < 10 Pa.s at shear rates >1 s⁻¹.     

Effect of the length of the carbochain on the phase behavior of side‐chain cholesteric liquid‐crystalline elastomers

In this study, we prepared two series of new side‐chain cholesteric liquid‐crystalline elastomers (PI and PII) derived from the same chiral bisolefinic crosslinking units and different nematic liquid‐crystalline monomers. The chemical structures of the monomers and polymers obtained were confirmed by Fourier transform infrared and ¹H‐NMR spectroscopy. The phase behavior properties were investigated by differential scanning calorimetry, thermogravimetric analysis, and polarizing optical microscopy. The effect of the length of the carbochain on the phase behavior of the elastomers was investigated. The polymers of the PI and PII series showed similar properties. Polymer P₁ showed a nematic phase, P₂–P₇ showed a cholesteric phase, and P₄–P₇, with more than 6 mol % of the chiral crosslinking agent, exhibited a selective reflection of light. The elastomers containing less than 15 mol % of the crosslinking units displayed elasticity, a reversible phase transition with wide mesophase temperature ranges, and a high thermal stability. The experimental results demonstrate that the glass‐transition temperatures first decreased and then increased; the isotropization temperatures and the mesophase temperature ranges decreased with increasing content of crosslinking agent. However, because of the different lengths of the carbochain, the glass‐transition temperatures and phase‐transition temperatures of the PI series were higher than those of the PII series, and the PI and PII elastomers had their own special optical properties. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1204–1210, 2005     

Phase behavior in liquid crystallization for diblock copolymers consisting of rubbery amorphous and side-chain liquid crystalline components

Phase behavior in liquid crystallization was studied for a series of liquid crystalline (LC) diblock copolymers consisting of rubbery amorphous and side-chain liquid crystalline components, poly(n-butyl acrylate) (PBA) and poly[11-(4′-cyanophenyl-4″-phenoxy)undecyl acrylate] (PLC), respectively, using a time-resolved small-angle X-ray scattering (SAXS) techniques, DSC and polarized optical microscopy (POM). The block copolymers used had three kinds of copolymer compositions, 44, 20 and 15 wt% of PLC compositions (BLC44, BLC20 and BLC15, respectively). BLC44 showed a smectic liquid crystalline structure. In the process of liquid crystallization for BLC44, the SAXS peak due to the microphase separation structure existing before liquid crystallization was changed continuously to be at a smaller angular side, and at almost the same time, a new peak appeared at a further smaller angular side and developed. The former peak disappeared with the development of liquid crystallization. The behavior of these SAXS peaks suggests that the microphase separation structure was changed discretely at the transition from isotropic to smectic and that two phases coexist in the early stage of the liquid crystallization. The coexistence of two peaks in the early stage of the liquid crystallization corresponded to the POM observation. In the isotropization process, coexistence of two phases was not observed. For BLC20 exhibiting a cylindrical structure in both isotropic and liquid crystalline states, the liquid crystalline structure was not smectic but probably nematic, and the spacing was changed continuously in liquid crystallization. No liquid crystallization was observed in SAXS, POM and DSC for BLC15. The orientation of smectic layers within lamellar domains was investigated using 2D-SAXS images. The smectic layer was aligned perpendicularly to the lamellar interface.     

The glass transition and the glassy state in isotropic and liquid crystalline polymers

PVT-measurements on atactic polystyrene shows that the Ehrenfest equations, which describe a thermodynamic transformation of the second order, are not applicable for the glass transition. The introduction of one internal ordering parameter ζ in addition to the conventional variables T and P, is sufficient to describe the behaviour in case of atactic PS in the isotropic glassy state. The usually observed way dependences in the glassy state can be explained by the concept of ordering parameters. In recent years liquid crystalline side chain polymers were developed. The liquid crystalline phases of these polymers can be supercooled and frozen-in like isotropic liquids. As the nematic or smectic liquid crystalline structure freezes-in, and can be conserved by this process, glasses with anisotropic properties are obtained. These glasses show e.g. a high optical birefringence. The glass transition of the liquid crystalline polymers can only be described by the assumption of at least two parameters ζ_i. Therefore the Ehrenfest equations are not applicable for the glass transition of isotropic as well as anisotropic glasses.