Xiao-Zhi He

Synthesis, structure and characterization of side chain cholesteric liquid crystalline polysiloxanes

A series of liquid crystalline homopolysiloxanes and copolysiloxanes were synthesized. The chemical structures of the monomers M1 –M7 were confirmed by FTIR and 1H NMR spectroscopy. The structure–property relationships of the monomers and polymers are discussed; their phase behaviour and optical properties were investigated by differential scanning calorimetry, thermogravimetric analysis, and polarizing optical microscopy. All the monomers, except M2 and M7 showed smectic and nematic phases; the copolymers P8 –P15 displayed cholesteric phases. The homopolymers P1 –P7 exhibited smectic phases. The selective reflection of cholesteric monomers and copolymers shifted to longer wavelengths with increasing length of the rigid mesogenic core, with decreasing length of the flexible spacer, or with increasing content of nematic units. Experimental results demonstrated that a flexible polymer backbone, a rigid mesogenic core and a long flexible spacer tended to produce a lower glass transition temperature, higher thermal stability, and wider mesophase temperature range.

Synthesis and characterization of side chain liquid crystalline polymers exhibiting cholesteric and blue phases

A series of cyclosiloxane‐based cholesteric liquid crystalline (LC) polymers were synthesized from a cholesteric LC monomer cholest‐5‐en‐3‐yl(3β) 4‐(2‐propenyloxy)benzoate and a nematic LC monomer butyl 4‐[4‐(2‐propenyloxy)benzoxy]benzoate. All the polymers exhibit thermotropic LC properties and show cholesteric phases. Most of the polymers display four types of phase transition behaviour corresponding to glass transition, melting point, cholesteric phase–blue phase transition and clearing point. The mesophase temperature range of the blue phases are as broad as 20°C. The blue phase was confirmed by the apperance of planar textures and cubic packings. With an increase of non‐chiral component in the polymers, the clearing point decreases slightly, while the glass transition and melting temperatures change little. In the reflection spectra of the polymer series the reflected wavelength broadens and shifts to longer wavelength with increase of the non‐chiral component in the polymer systems, suggesting that the helical pitch P lengthens.

Cholesteric star-shaped liquid crystals induced by a maltose core: synthesis and characteristics

Three esters of maltose (A1–A3) with 4–(4–(alkoxybenzoyloxy)phenoxy) –6–oxohexanoic acid (b1–b3) side-arms have been synthesised. All the maltose derivatives were laevo-rotationary, unlike their parent cores. The side arm b1 did not display liquid crystalline (LC) properties, and b2 and b3 displayed thermotropic nematic LC properties. The star-shaped compound (SSC) A1 with b1 side-arms did not display a mesomorphic phase. Unlike the nematic schlieren texture provided by side-arms b2 and b3, the star-shaped liquid crystals (SSLC) A2 and A3 displayed Grandjean and oily texture in the cholesteric phase. The results suggest that the LC properties of the side-arms have an important influence on the formation of LC properties in an SSC, and that the maltose core is important in determining the mesomorphic phase type. In other words, the SSC displayed LC properties only when the side-arms were also LC, and the maltose core induced a cholesteric phase in the SSLC with nematic side-arms. The mesomorphic regions for A2 and A3 were 39.1 and 53.7°C during the heating cycle and 63.8 and 107.0°C during the cooling cycle, respectively. The longer terminal chain rendered the mesomorphic region broader.

Synthesis and characterization of side chain cholesteric liquid crystalline elastomers derived from chiral bisolefinic crosslinking units

In this work we prepared a nematic monomer (4′‐allyloxybiphenyl 4′‐ethoxybenzoate, M1 ), a chiral crosslinking agent (isosorbide 4‐allyloxybenzoyl bisate, M2 ) and a series of new side chain cholesteric liquid crystalline elastomers derived from M1 and M2 . The chemical structures of the monomers and polymers were confirmed by FTIR and 1H NMR spectroscopy. The mesomorphic properties were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy and X‐ray diffraction. The effect of the content of the crosslinking unit on phase behaviour of the elastomers is discussed. Polymer P1 showed a nematic phase, P2 –P7 showed a cholesteric phase; P6 formed a blue Grandjean texture over a broad temperature range 145–209.6°C, with no changed on the cooling. Polymers P4 –P7 , with more than 6 mol % of chiral crosslinking agent, gave rise to selective reflection. Elastomers containing less than 15 mol % of the crosslinking units displayed elasticity, reversible phase transition with wide mesophase temperature ranges, and high thermal stability. Experimental results demonstrated that, with increasing content of crosslinking agent, the glass transition temperatures first fell and then increased; the isotropization temperatures and mesophase temperature ranges decreased.

Synthesis and characterisation of chiral liquid crystalline elastomers containing four-arm crosslinking agent

A new series of side-chain chiral liquid crystalline elastomers derived from M1 (cholest-5-3-ol(3β)-4-(2-propen-yloxy)]benzoate) and MC(2,5-[3,5-bis(4-(3-(4-(allyloxy)phenyl)propanoyloxy)benzoyloxy)benzoic acid]isosorbide diester). The structures of monomers and elastomers measured by using Proton Nuclear Magnetic Resonance Spectra (1H-NMR) and Fourier transform infrared spectroscopy (FTIR) separately are consistent with our design. IIP~VIP all appeared blue Grandjean (GJ) texture on the heating cycle or cooling cycle. The glass sheets of IIP~VIP were made under 150°C and measured its ultraviolet–visible spectrophotometry by PerkinElmer Lambda 950 instrument (Shelton, CT, USA). IIP~VIP all have absorptions at about 481~483 and 561~562 nm. The optical activities were measured at different temperatures on heating and cooling cycles. And the blue selective reflection of IIP~VIP on the round glass sheet can be seen. The elastomers containing less than 6 mol% of the crosslinking units displayed elasticity, reversible phase transition and high thermal stability. The glass transition temperatures reduced first and then increased, the isotropisation temperatures and the mesophase temperature ranges increased first and then decreased with increasing content of crosslinking unit. The thermogravimetric analysis (TGA) results showed that the temperatures at which 5% weight loss occurred (Td) were greater than 310°C for all the polymers.

Synthesis and characterization of a series of side chain chiral smectic liquid crystalline elastomers

A series of new chiral smectic liquid crystalline elastomers was prepared by graft polymerization of a nematic monomer with a chiral and non‐mesogenic crosslinking agent, using polymethylhydrosiloxane as backbone. The chemical structures of the monomers and polymers obtained were confirmed by FTIR and 1H NMR. The mesomorphic properties were investigated by differential scanning calorimetry, thermogravimetric analysis, polarizing optical microscopy and X‐ray diffraction. Monomer M 1 showed a nematic phase during heating and cooling. Polymer P 0 exhibited a smectic B phase; elastomers P 1–P 3 showed the smectic A phase, P 4–P 6 showed a chiral smectic C(SmC*), and P 7 displayed stress‐induced birefringence. Elastomers containing less than 15 mol % M 2 displayed elasticity, reversible phase transitions with wide mesophase temperature ranges, and high thermal stability. With increasing content of the crosslinking unit, glass transition temperatures first increased, then fell, then increased again; isotropization temperatures and mesophase temperature ranges steadily decreased.

Cyclic versus linear siloxane liquid crystalline oligomers: phase behaviour

The phase behaviour of cyclic and linear liquid crystalline polysiloxanes containing two different cholesteric mesogens is reported. The thermal properties of the synthesized monomers and oligomers were investigated by polarizing optical microscopy and differential scanning calorimetry. The influence of the structure of cyclic siloxane on the mesomorphic properties of cyclic liquid crystalline polysiloxanes is discussed. Similar trends in the thermal transitions of cyclic and linear compounds containing the same mesogenic composition were observed, though the cyclic oligomers showed poorer mesomorphic properties as compared with their linear analogues.

The effect of different arms on the properties of chiral branched-arm liquid crystals based on isosorbide as the chiral core

A new series of two-arm ( MA1 , MA2 ), four-arm ( MA3 ) and six-arm ( MA4 ) chiral liquid crystalline compounds containing isosorbide as the chiral core were synthesised. Four precursors of branched-arm A1–A4 were first obtained, and then were esterised separately with isosorbide to obtain four chiral branched-arm liquid crystals ( MA1–MA4 ). The structure and properties have been characterised. The results show that MA1 was smectic–cholesteric phase with a fan-like and oily streak texture; MA2–MA4 showed a cholesteric phase with oily streak texture, or lined texture and finger print texture. Isosorbide successfully induced a cholesteric phase. The melting point and clearing point values for MA1–MA4 first increased and then decreased. The branched-arm and chiral core quantity affected the liquid crystalline properties. At the same time, the difference in side-arms resulted in different directions of rotation. MA1–MA2 showed right-handed rotation and had selective reflection; MA3–MA4 demonstrated left-handed rotation and did not have selective reflection.

Polymerised ionic liquid crystals bearing imidazolium and bipyridinium groups

ABSTRACT Novel kinds of polymerised ionic liquid crystals (PILCs) bearing imidazolium and bipyridinium groups were synthesised and characterised in this work. Some bromo-polyesters were synthesised in an esterification chain reaction using 2,3-dibromosuccinyl dichloride, isophthaloyl chloride and 4,4′-biphenol. The polyester imidazolium bromides (PIBs) and polyester dipyridinium bromides (PDBs) were obtained by a quaternisation reaction between the bromo-polyesters, N-methylimidazole and 4-4′-bipyridine, respectively. The polyester imidazolium tetrafluoroborates (PITs) and polyester dipyridinium tetrafluoroborates (PDTs) were synthesised using the corresponding PIBs and PDBs in an anion-exchange reaction. The chemical structures, liquid crystalline properties and molecular weights of these polymers were characterised by multiple experimental techniques. All the PILCs including PIBs, PDBs, PITs and PDTs display smectic A phase on heating and cooling cycles. The liquid-crystalline properties of bromo-polyesters are influenced by the length of flexible spacer and composition of polyester backbones, while those of PILCs are determined by the ionic groups as additional important influent factors. In comparison with those of the mother bromo-polyesters, the d-spacing of PILCs reduced slightly due to Im+–Br−, Dp+–Br−, Im+–BF4− and Dp+–BF4− ion pairs in the polymer systems. Monocationic imidazolium salts display weaker interionic and intermolecular interaction, higher mobility and lower viscosity than dicationic dipyridinium salts. GRAPHICAL ABSTRACT

Synthesis and characterization of a novel liquid crystal‐bearing ionic mesogen

A new ionic liquid crystal bearing a sulphonate group in the mesogenic core, potassium 2‐allyloxy‐5‐cholesteryloxycarbonylbenzenesulphonate, was synthesized. Its chemical structure was determined by various techniques including FTIR and 1H NMR. Its liquid crystalline properties were characterized by DSC, POM and SAXS; it exhibits a smectic C mesophase. In dielectric constant measurements a maximum Δε value of −43.0 D was obtained at 210°C. The morphology of this ionic liquid crystal indicated a slant array of ionic mesogenic units under a static electric field.