Side-chain Cholesteric Liquid Crystalline Elastomers Research Articles

Side-chain cholesteric liquid-crystalline elastomers containing azobenzene derivative as cross-linking agent – synthesis and characterisation

ABSTRACTThe cholesteric polysiloxanes (PLQ series) were obtained by reacting cholesteric monomer and phenolic hydroxyl monomer in different ratios with polysiloxanes. The chiral azo-containing polysiloxanes liquid-crystalline elastomers (LCEs) were synthesised by esterifying PLQ series with acyl acid of azo diacid catalysed by 4-dimethylaminopyridine (DMAP). The chemical structures and mesomorphic properties of monomers and polymers were confirmed by conventional spectroscopic methods. The elastomers displayed elastic properties, reversible phase transition and high thermal stability. The Tg values changed irregularly and Ti values decreased at the beginning and then increased afterwards. The chiral azo-LCEs showed colourful textures and a Grandjean texture could be observed, which exhibited a red shift with increasing content of azo moiety. PLQ2–PLQ6 exhibited selective reflection in the visible light region, but PLZ1–PLZ4 can only observe selective reflection when appropriate mechanical pressure was imposed on the polymers. The PLZ series was carefully investigated by ultra violet–visible spectroscopy.

Wide-band reflective films produced by side-chain cholesteric liquid-crystalline elastomers derived from a binaphthalene crosslinking agent

A series of novel side-chain cholesteric liquid-crystalline elastomers based on polysiloxane and cholesterol derivate monomer were prepared by adopting a crosslinking agent containing binaphthalene group. The chemical structures and mesomorphic properties of the monomers and elastomers were confirmed by FT-IR, 1H NMR, DSC and POM measurements. Worthily, the elastomers exhibited unusual temperature dependence of the helical twisting power (HTP) which is demonstrated resulting from coordination of the crosslinking agent and the mesogenic units. With increase in temperature, the HTP of elastomers containing small quantity of the crosslinking unit exhibited a turning point, while that of elastomers comprising much more crosslinking unit shifted straight. Furthermore, a single layer wide-band reflective film with non-uniform pitch distribution was prepared by utilizing the HTP variety of elastomers with change in temperature. From scanning electron microscopy (SEM) investigations, the mechanism of the broadband reflection was verified.

Side-chain cholesteric liquid crystalline elastomers containing isosorbide as chiral agent – synthesis and characterisation

In this work the new-style nematic monomer M1 , chiral crosslinking reagent MC and a series of new side-chain cholesteric liquid crystalline elastomers derived from M1 and MC were prepared. The effect of the content of the chiral crosslinking unit on phase behaviour of the elastomers has been discussed. Polymer P1 showed nematic phase, P2 –P7 showed cholesteric phase, P3 formed Grandjean texture in the heating cycle and turned out a blue Grandjean texture in the cooling cycle, P2 –P3 with less than 6 mol% of chiral crosslinking agent gave rise to selective reflection. The elastomers containing less than 15 mol% of the crosslinking units displayed elasticity, reversible phase transition and high thermal stability. Experimental results demonstrated that the glass transition temperatures reduced first and then increased, and the isotropisation temperatures and the mesophase temperature ranges decreased with increasing content of crosslinking unit.

Side-chain cholesteric liquid crystalline elastomers derived from a mesogenic crosslinking agent: I. Synthesis and mesomorphic properties

The synthesis of new side-chain cholesteric elastomers derived from a cholesteric monomer and mesogenic crosslinking agent is presented. The chemical structures of the monomers obtained were confirmed by elemental analyses, FT-IR, 1H NMR, and 13C NMR. The mesomorphic properties and thermal stability were investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), polarizing optical microscopy (POM), and X-ray diffraction (XRD) measurements. M 1 showed cholesteric phase, and M 2 displayed enantiotropic nematic phase and monotropic smectic phase. The elastomers containing less than 12 mol% of the crosslinking units revealed reversible mesomorphic phase transition, wide mesophase temperature ranges, and high thermal stability.

Synthesis and phase behavior of side-chain cholesteric liquid-crystalline elastomers derived from a chiral crosslinking agent

A series of new side-chain cholesteric liquid-crystalline elastomers derived from 4-(10-undecylen-1-yloxy)-4′-(4-ethoxybenzoyloxy)biphenyl and isosorbide di-(10-undecylen-1-ylenate) were synthesized. The chemical structures of the monomers obtained were confirmed by element analyses, FT-IR, 1H NMR and 13C NMR. The mesomorphic properties and thermal stability were investigated by polarizing optical microscopy, differential scanning calorimetry, thermogravimetric analysis, and X-ray diffraction measurements. The influence of the content of the crosslinking unit on the phase behavior of the elastomers was discussed. Monomer M 1 showed nematic and different smectic (S A, S C, S B) phase on cooling cycle. Elastomers containing less than 27 mol% of the crosslinking units revealed elasticity, reversible mesomorphic phase transition, wide mesophase temperature ranges, and high thermal stability. Experimental results demonstrated that the glass transition temperature, melting temperatures, clearing temperature and mesophase temperature ranges decreased with increasing the content of the crosslinking unit.

Structures and properties of cholesteric liquid crystalline elastomers

The synthesis of new side-chain cholesteric liquid crystalline elastomers containing mesomorphic crosslinking agent M-1 and cholesteric monomer M-2 was described by a one-step hydrosilylation reaction. The chemical structures of the obtained network polymers were confirmed by FT-IR spectroscopy. The mesogenic properties and phase behavior were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction measurements. The mesomorphic crosslinking agent was adopted to diminish the disturbance of the non-mesogenic crosslinking agent on liquid crystalline elastomers. The influence of the crosslinking units on the phase behavior was discussed. The liquid crystalline elastomers P-2–P-6 containing less than 10.8 mol% crosslinking agent showed elasticity, reversible phase transition, and cholesteric grandjean texture. The experimental results demonstrated that the glass transition temperatures and the isotropization temperatures of the networks polymers decreased a little with increasing concentration of crosslinking units but the cholesteric phase was not disturbed.

Synthesis and properties of cholesteric liquid crystalline elastomers

The synthesis of new side-chain cholesteric liquid crystalline elastomers, containing the flexible non-mesogenic crosslinking agent M-1 and the cholesteric monomer M-2, is described by a one-step hydrosilication reaction. The chemical structures of the monomers and network polymers obtained were confirmed by FT-IR spectroscopy. Their mesogenic 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 network polymers showed elasticity, reversible phase transitions, and cholesteric Grandjean texture. The experimental results demonstrated that the glass transition temperatures and isotropization temperatures of network elastomers decreased as the concentration of crosslinking units was increased, but the cholesteric phase was not disturbed.

Side-Chain Cholesteric Liquid Crystalline Elastomers Derived from a Mesogenic Cross-Linking Agent

The synthesis of the two monomers M1 and M2 and a series of new side-chain cholesteric liquid crystalline elastomers P2−P8 is presented. The chemical structures of the monomers and elastomers obtained were confirmed by FTIR or 1H NMR spectroscopy. The cross-link density of the elastomers was determined by swelling experiments. The mesomorphic properties were investigated by differential scanning calorimetry (DSC), thermogravimetric analyses (TGA), polarizing optical microscopy (POM), and X-ray diffraction (XRD) measurements. Monomer M1 showed a cholesteric phase, and M2 revealed smectic and nematic phases. The effect of the cross-link density on the phase behavior of P2−P8 is discussed. Elastomers P2−P6 containing less than 12 mol % of the cross-linking units displayed elasticity, reversible liquid crystalline phase transition, wide mesophase temperature ranges, and high thermal stability. Elastomer P7 displayed stress-induced birefringence, and P8 showed only elasticity with no other texture. Experimental results demonstrated that the glass transition temperatures increased, and the isotropization temperatures and the mesophase temperature ranges of P2−P6 decreased with increasing cross-link density.

Synthesis and Properties of Side-Chain Cholesteric Liquid Crystalline Elastomers Containing Tetra-Vinyl Crosslinking Units

The synthesis of side-chain cholesteric liquid crystalline elastomers containing crosslinking agent pentaerythriol tetra-p-allyloxybenzoate (M-1) and mesogenic monomer 4-cholesteryl-4’-undecenyloyloxybenzoate (M-2) is presented by hydrosilication reaction. The chemical structures of the monomers and liquid crystalline networks were confirmed by Fourier transform infrared (FT-IR) and proton nuclear magnetic resonance (1H NMR) spectroscopy. Mesomorphic properties and phase behavior were investigated by differential scanning calorimetry, polarizing optical microscopy, and X-ray diffraction measurements. The influence of crosslinking units on phase behavior was discussed. The networks containing less than 8 mol% of crosslinking units showed elasticity, reversible phase transition and cholesteric grandjean texture. The glass transition temperatures increased and clearing temperatures decreased with concentration of crosslinking units.