Zihui Cheng

Electrically controllable selective reflection of chiral nematic liquid crystal/chiral ionic liquid composites.

A chiral nematic liquid crystal/chiral ionic liquid composite with unique electro-optical characteristics is reported. The composite can be switched electrically between three different light states: transparent, scattering, and mirror reflecting (see images). Moreover, the reflection bandwidth can be controlled accurately and reversibly by adjusting the intensity of the electric field applied.

Magnetite nanoparticles/chiral nematic liquid crystal composites with magnetically addressable and magnetically erasable characteristics

A magnetite (Fe3O4) nanoparticle/chiral nematic liquid crystal (N*-LC) composite was prepared and filled into a planar treated cell. The Fe3O4 nanoparticles had been modified by oleic acid so that they could be better dispersed in the composite. When a magnetic field was scanned on the outer surface of the cell locally, Fe3O4 nanoparticles moved towards the inner surface of the cell correspondingly, and the black expected information was displayed. When the magnet was applied to the opposite outer surface, the information was erased. After polymer network walls were prepared in the composite, the resolution of the information displayed increased. Then, through the formation of hydrogen bonds between the nanoparticles and chiral pyridine compound (CPC) doped in the composite, the pitch length of the N*-LC could be adjusted by altering the intensity of the applied magnetic field. The composite doped with CPC could potentially be used as a material for a type of reflective colour paper with magnetically controllable characteristics.

Bandwidth-controllable reflective cholesteric gels from photo- and thermally-induced processes

A bandwidth-controllable reflective gel has been investigated from photo- and thermally-induced processes. Due to the pitch of cholesteric liquid crystal (CLC) composite increasing as the temperature rises, the CLC with short pitch is frozen by UV-curing the polymer network at low temperature and the CLC with long pitch is fixed by heat curing the polymer network at high temperature. A non-uniform pitch distribution of CLCs forms in the gel when temperature becomes low. It is demonstrated that the memory effect of the polymer network is an important mechanism for the resulting gel.

Liquid crystalline and thermo-optical properties of cyclic siloxane tetramers containing cholestryl-4-allyloxy-benzoate and biphenyl-4-yl 4-allyloxybenzoate

A series of cyclic siloxane tetramers covering the whole composition range were synthesised using cyclotetrasiloxane, cholestryl-4-allyloxy-benzoate and biphenyl-4-yl 4-allyloxybenzoate, and the effects of the cholesteryl-based mesogenic units on liquid crystalline and thermo-optical properties were studied. The presence of a cholesteric mesogen was observed to widen the mesogenic temperature range and tended to induce smectic A phases in the tetramers containing a higher mole fraction of cholestryl-4-allyloxy-benzoate (X chol). The selective reflection of cholesteric tetramers shifted to shorter wavelengths with increasing X chol and temperature. Blue phases can be easily observed for those tetramers containing more than 50.0 mol% cholesteric mesogen. The shorter pitch sample showed the wider blue phases range. The blue phases range width increased from 2.8°C to 18.5°C as X chol increased from 0.5 to 1.0.

Bandwidth-controllable reflective polarisers based on the temperature-dependent chiral conflict in binary chiral mixtures

In this study, we demonstrate a new method for obtaining a binary chiral mixture (BCM) which exhibits an unprecedented temperature-dependent helical twisting power (HTP) by utilising the chiral conflict between two chiral dopants with opposite handedness and HTP temperature dependence. Different from the common chiral dopant reported before, the addition of such a BCM could increase the clearing temperature of the host liquid crystal (LC). Due to the sufficiently large HTP value of the BCM, the three primary colours (blue, green and red) can be easily reflected by the nematic liquid crystal (N-LC)/BCM mixture at different temperatures. Moreover, the large HTP temperature dependence could be maintained and even enhanced in the longer wavelength region by adjusting the content of the BCM. Finally, based on the large HTP temperature dependence and the positive affect on the host LC of the BCM, a thermally bandwidth-controllable reflective polariser that could work in the visible region (400–750 nm) was prepared from the N-LC/BCM/polymer network composite.

Broadband reflection mechanism of polymer stabilised cholesteric liquid crystal (PSChLC) with pitch gradient

In order to reveal the nature of the formation of a broadband reflection property for polymer stabilised cholesteric liquid crystal, the spatially periodic helix structure of epoxy cholesteric liquid crystal is ‘frozen’ by cationic polymerisation following the formation of a polymer network induced by ultraviolet radical polymerisation of acrylate monomer. The fracture of the helix structure is observed by scanning electron microscopy, which reveals the presence of pitch gradient. Since the liquid crystal monomers have a UV absorption characteristic, a polymer network concentration gradient is formed after ultraviolet radical polymerisation. Using an attenuated total reflection Fourier transform infrared (ATR-FTIR) spectrum, the concentration gradient of the polymer network and that of the chiral dopant, with the opposite trends, are confirmed. The existence of both gradients, resulting in the formation of the pitch gradient, are considered as the natural mechanism for the formation of the broadband reflection.

Influence of linkage and terminal group on the liquid crystalline and helical twisting behaviours of cholesteryl esters

Three series of cholesteryl esters with different linkages and terminal groups have been synthesised and characterised. All the esters except cholesteryl 6-bromohexanoate can exhibit thermotropic liquid crystalline phases in certain temperature regions. The insertion of a 4-phenoxyl and a 4-biphenylyloxy group in the ester can increase the molecular length, broaden the whole mesophase width and heighten the clearing temperature. The increasing molecular length can enforce the layered arrangement in series A and B (resulting in a monolayer smectic A phase), while destroying the layer stacking in series C (resulting in a suppressed or absent interdigitated smectic A phase). Meanwhile, the molecular structure played an important role in the molar helical twisting power and its temperature dependence.

Chiral nematic liquid crystals with helix inversion from (R)-1,1′-binaphthyl and cholesteryl ester moieties

In this study, on the concept of intramolecular chiral conflict between the (R)-1,1′-binaphthyl and cholesteryl ester moieties, we have designed and synthesised a new liquid crystal (LC) (R)-dicholesteryl 6,6′-[1,1′-binaphthyl-2,2′-diylbis(oxy)]dihexanoate [(R)-DC]. A helix inversion could be observed for the chiral nematic liquid crystal (N*-LC) comprising the commercial nematic LC (N-LC) host SLC1717 and (R)-DC on heating. As a comparison, (S)-dicholesteryl 6,6′-[1,1′-binaphthyl-2,2′-diylbis(oxy)]dihexanoate [(S)-DC] was also prepared. Due to the intramolecular chiral superposition between the (S)-1,1′-binaphthyl and cholesteryl ester moieties, the N*-LC comprising SLC1717 and (S)-DC also exhibited excellent temperature sensitivity.

A helix inversion from the temperature-dependent intramolecular chiral conflict

A new liquid crystal, cholesteryl (S)-4′-(2-methylbutyl)biphenyl-4-carboxylate (CB15-COOChol*), has been designed, synthesised and characterised. CB15-COOChol* exhibits multiple liquid crystalline phases: smectic A (SmA), nematic (N) and chiral nematic (N*). When CB15-COOChol* is heated, two N* phases exhibiting opposite handedness (NR* and NL*) and opposite temperature dependence of the helical pitch separated by an N phase are observed above the SmA phase. The entire rare phase transition sequence can be described as crystalline Cr→SmA→NR*→N→NL*→I isotropic. The helix inversion behaviour occurs mainly due to the temperature-dependent intramolecular chiral conflict between the (S)-4′-2-methylbutyl and cholesteryl ester moieties with opposite handedness and opposite temperature dependence of the helical twisting power value.

Broadband reflective liquid crystalline films prepared from liquid crystals with negative dielectric anisotropy

When a certain range of frequencies of AC electric fields are applied to a chiral nematic (N*) liquid crystal (LC) with negative dielectric anisotropy ((N*-LCN), usually prepared from a nematic LC with negative dielectric anisotropy (N-LCN) doped with a chiral dopant (CD), the long axes of the LC molecules will be oriented perpendicular to the electric field, which will be helpful to improve the orientation degree compared with the orientation induced by rubbing on the inner surfaces of the substrates sandwiching the LC. After a photopolymerisable N*-LC monomer/N-LCN/CD/an ultraviolet (UV) absorber/a photoinitiator mixture with an appropriate composition is irradiated with UV light, while an electric field with an appropriate frequency and intensity is being applied to it during the UV curing, not only the planar orientation of a high degree but also a pitch gradient of the helical structure of the LC molecules are formed in the resulted polymer network/N*-LCN composite.