Dan-Shu Yao

Preparation and properties of siloxane liquid crystalline elastomers with a mesogenic crosslinking agent

A mesogenic crosslinking agent M-1 was synthesized to minimize the perturbations of non-mesogenic crosslinking agents in liquid crystalline elastomers. The synthesis of new side chain liquid crystalline elastomers containing the rigid mesogenic crosslinking agent M-1 and nematic monomer M-2 by a one-step hydrosilylation reaction is described. The chemical structures of the monomers and network polymers obtained were confirmed by FTIR and 1H NMR spectroscopy. The mesomorphic properties and phase behaviour were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction. The influence of the crosslinking units on phase behaviour is discussed. Liquid crystalline elastomers containing less than 15 mol % of the crosslinking units showed elasticity, reversible phase transitions and a threaded texture. The experimental results demonstrated that the glass transition temperature of polymers P-1–7 increased with increasing concentration of crosslinking agent M-1; but the isotropic temperature and liquid crystalline range decreased slightly.

1,2-Propanediol-linked chiral symmetric and non-symmetric liquid crystal dimers containing trifluoromethyl

ABSTRACT Four symmetric and non-symmetric chiral liquid crystal dimers containing trifluoromethyl groups, termed as TFBA-PD-TFBA, UEBBA-PD-TFBA, UEBA-PD-TFBA and UEA-PD-TFBA, respectively, have been synthesised and characterised. UEA-PD-TFBA exhibited chiral nematic phase, whereas the other three dimers displayed chiral smectic A phase. X-ray diffraction (XRD) has revealed the structure of the smectic A phase for TFBA-PD-TFBA to be intercalated, whereas that for UEBBA-PD-TFBA and UEBA-PD-TFBA to be monolayer and interdigitated, respectively. In addition, the weaker peak corresponding to a shorter layer spacing was observed for UEBBA-PD-TFBA and UEBA-PD-TFBA. Considering the results of XRD measurements and computer simulations, the structural model corresponding to the shorter layer spacing is assigned as horseshoe-like shape. The absence of smectic behaviour for UEA-PD-TFBA reveals that the weaker aromatic–aromatic interactions cannot stabilise the smectic A phase. GRAPHICAL ABSTRACT

The effect of mesogenic and non-mesogenic crosslinking units on the phase behaviour of side-chain smectic and cholesteric elastomers

Chiral monomer (M1 ), mesogenic and non-mesogenic crosslinking agents (C1 and C2 ), and the corresponding liquid crystalline elastomers (P1 and P2 series), have been synthesised. Their chemical structures have been characterised by Fourier transform infrared or 1H nuclear magnetic resonance and their phase behaviour investigated by differential scanning calorimetry, polarising optical miscoscopy, thermo-gravimetric analysis (TGA) and X-ray diffraction. The effect of the crosslinking unit on the phase behaviour of the elastomers has been studied. M1 showed a cholesteric oily streak and focal conic texture. C2 exhibited a nematic enantiotropic thread-like and schlieren texture, and a monotropic fan-shaped texture in the SA phase. Due to the introduction of the mesogenic crosslinking unit, elastomers, P2-1 −P2-5 , exhibited a cholesteric phase, while elastomers, P1-1 −P1-4 , derived from a non-mesogenic crosslinking unit, exhibit a SA phase. As the content of the crosslinking unit increased, the T g of the P1 series initially decreased and then increased, and the T i of the series decreased. In the P2 series the T g increased, but the T i initially increased and then decreased. TGA confirmed that all the elastomers had improved thermal stability.

Effect of ionic aggregates of sulphonate groups on the liquid crystalline behaviours of liquid crystalline elastomers

A series of siloxane‐based liquid crystalline elastomers containing biphenyl benzoate mesogenic units and ionic Brilliant Yellow moieties was synthesized. The chemical structures and liquid crystalline properties of the samples were characterized by FTIR, 1H NMR, DSC, POM and XRD. The effective crosslink density of the ionic elastomers was determined by swelling experiments in organic/buffer mixtures. All the polymers displayed a smectic mesophase. It was shown that the glass transition and melting point temperatures of the polymers increased slightly with increasing content of ionic and mesogenic groups in the polymers, while the liquid crystal mesophase region decrease slightly.

Cholesteric liquid crystalline thermosets: synthesis, structure and properties of ChLCTs/precursor polymers

The synthesis of six liquid crystalline monomers M1–M6 and three series of side chain cholesteric liquid crystalline polymers P1–P3 is described. The chemical structures of the monomers were characterized by FTIR and 1H NMR spectroscopy. The structure–property relationships of M1–M6 and P1–P3 are discussed. Their phase behaviour and optical properties were investigated by differential scanning calorimetry and polarizing optical microscopy. The monomers M1–M3 exhibited cholesteric phases; M4–M6 showed nematic or smectic phases. The polymer series P1–P3 showed cholesteric phases. Experimental results demonstrated that the selective reflection of the cholesteric monomers and homopolymers shifted to the shorter wavelength region (blue shift) with increasing length of the flexible spacer. The selective reflection of the copolymers shifted to the longer wavelength region (red shift) with increasing content of nematic units.

Synthesis and properties of new non-symmetric liquid crystal dimers containing mandelic acid and cyano group

ABSTRACT Four non-symmetric dimers containing mandelic acid as the chiral core have been synthesised, termed as QBMA-B, QBMA-BCN, QBMA-BBCN and QBBMA-BBCN, respectively. Chemical structures and liquid crystal (LC) properties of the dimers were characterised by FTIR, 1H-NMR, differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and polarised optical microscopy (POM). The results indicated that the rigidity and conformation of the molecules of the dimers played important effects on their mesophase properties. QBMA-B did not display any mesophase, QBMA-BCN and QBMA-BBCN exhibited nematic (N) phases, while QBBMA-BBCN displayed cholesteric (ch) phase, which indicated that chiral mandelic acid could induce cholesteric phase, but some attention should be paid to the molecular conformation to obtain cholesteric phase. For the four dimers, melting temperature (Tm) increased first and then decreased, inferring that molecular conformation played a more important effect besides molecular weight and rigidity. Graphical Abstract

Mesomorphic properties of non-symmetric three-arm chenodeoxycholic acid-derived liquid crystals

ABSTRACT Six three-arm star-shaped liquid crystals (LCs) based on chenodeoxycholic acid (CDCA), termed as G-BH, G-YD, G-FD, G-DJ, G-DZ and G-BX, respectively, have been synthesised. CDCA was used as the chiral core and the nematic side arm, 6-(4-(ethylbenzoyloxy) phenoxy)-6-oxohexanoic acid, was chosen to be introduced into the two hydroxyl of CDCA to synthesise cholesteric LC (CDCA2EA) and different structures were introduced into the carboxyl group of CDCA to prepare the three-arm star-shaped LCs. Chemical structures and LC properties of the six three-arm LCs were characterised by FTIR, 1H-NMR, differential scanning calorimetry, thermogravimetric analysis, X-ray diffraction and polarised optical microscopy. G-BH and G-DJ displayed cholesteric phase with a long helical pitch; G-BX displayed nematic phase; and G-YD, G-FD and G-DZ displayed cholesteric phase on heating and on cooling. These results indicated that not only the chiral core CDCA but also the structures of the side arms played an important role in inducing the cholesteric phase of the CDCA-derived LCs. G-DZ displayed selective reflection, a wider ∆λ and red shift on cooling. Graphical Abstract

Non-symmetric chiral nematic liquid crystal dimers containing trifluoromethyl and 1,2-propanediol

ABSTRACT A series of non-symmetric liquid crystal (LC) dimers with the same chiral core 1,2-propanediol (PD) have been synthesised, termed as ABBA-PD-TFBA, PBBA-PD-TFBA, ABA-PD-TFBA, PBA-PD-TFBA and AA-PD-TFBA, respectively, in which one of the two mesogenic groups, the fluorinated mesogenic unit, was kept fix and the other arm was different. The intermediate compounds and LC dimers were characterised by FTIR, 1H NMR, differential scanning calorimetry, thermogravimetric analysis, polarised optical microscopy and X-ray diffractometer (XRD). The results of the measurements indicated that ABBA-PD-TFBA, PBBA-PD-TFBA and ABA-PD-TFBA displayed optical activity and enantiotropic chiral nematic phase, and PBA-PD-TFBA was an enantiotropic nematic LC while AA-PD-TFBA was a monotropic LC, displaying both nematic phase and smectic A phase on cooling. The results indicated that PD was able to induce the chiral nematic phase, nevertheless, the rigidity of the mesogenic arm, the flexibility of the terminal group and even the type of the terminal chemical bond played an important effect on the thermal properties of the dimers, and even on the formation of the chiral nematic phase. It is also worth noting that C=C at the terminal helped to stabilise the LC phase. Graphical Abstract

Multi-arm azobenzene liquid crystal based on cholic acid: synthesis and mesophase properties

ABSTRACT A new series of multi-arm liquid crystals (D1–D5) based on cholic acid as the chiral core and ω-[4-(p-substituted azobenzeneoxy carbonyl]valeric acid (B1–B5) as the mesogenic arms were designed and synthesised. All the mesogenic arms and the multi-arm liquid crystals were structurally well-characterised using elementary analysis (EA), 1H NMR and FT-IR. The photochemical properties were determined by UV spectra. The optical rotations were measured by Automatic Polarimeter. The phase behaviours were investigated by means of polarised optical microscopy, X-ray diffraction and differential scanning calorimetry, while the effect of the terminal groups of azobenzene on mesogenic and photochemical properties were discussed. The results demonstrated that the mesogenic arms exhibited nematic phase except B3. The multi-arm compounds showed chiral nematic except D3. Cholic acid played an important role in inducing chiral nematic of the multi-arm liquid crystals. Tm or Ti of B1–B5 and D1–D5 are influenced by the effect of electrical effects of the terminal groups and the molecular shapes. Furthermore, the multi-arm compounds exhibited well light sensitivity, owning the photoisomerisation of the tran-form azobenzene to the cis-form azobenzene. λmax of D1–D5 displayed blue shift first and then red shift with the different electrical effects of the terminal groups of mesogenic arms. GRAPHICAL ABSTRACT

Synthesis and mesomorphism of novel multi-arm liquid crystals with cholic acid as chiral centre linking Schiff base moieties as mesogens

ABSTRACT A new series of multi-arm chiral liquid crystals (LCs) D1–D3 were synthesised and characterised. Cholic acid was used as the core and ω-[4-(p-alkoxybenzoloxy)phenoxycarbonyl]valeric acid (B1–B3) was used as the mesogenic arms, containing different terminal substituent X (B1: X= -OCH3, B2:X= -CH3, B3: X= -Cl). Their structures and mesomorphic properties were investigated by Fourier-transform infrared spectroscopy, nuclear magnetic resonance hydrogen spectrometer, differential scanning calorimetry, polarised optical microscopy and X-ray diffraction, respectively. The mesogenic B1–B3 displayed smectic B phase. The multi-arm LC D1 displayed cholesteric, while D2 and D3 exhibited nematic phase. The formation of cholesteric phase of D1 was affected by both the chiral core – the bulky cholic acid and the polarity of the terminal substituent of the mesogenic arm. That D1 displayed cholesteric phase but D2–D3 did not indicated that the stronger polarity of the terminal group OCH3 of D1 played an important part in stabilising the cholesteric phase. The multi-arm LCs D1–D3 all showed ultraviolet activity. The wavelength of maximum absorption of D1–D3 was affected by the terminal substituent of the mesogenic arm. Graphical Abstract