Xinghe Fan

Organic–inorganic hybrid bent-core liquid crystals with cubic silsesquioxane cores

Organic–inorganic hybrid bent-core liquid crystals (BCLCs) with different numbers of bent-core mesogens end-attached to inorganic POSS cores or a larger siloxane core were synthesized by hydrosilylation reactions. Their chemical structures were confirmed by NMR, IR, elemental analysis, and GPC. Antiferroelectric SmC phases are found for almost all samples. However, the layer spacing depends on the number of bent-core mesogens around the inorganic core and size of the siloxane core. Two bilayer SmCAPA phases can be observed when eight bent-core mesogens are end-attched to POSS. Monolayer SmCPA phases are obtained upon increasing the content of POSS or the size of the siloxane core.

Water soluble multi-walled carbon nanotubes prepared via nitroxide-mediated radical polymerization

Nitroxide-mediated radical polymerizations of 4-vinylpyridine and sodium 4-styrenesulfonate were carried out on the surfaces of MWNT to afford poly(4-vinylpyridine)-grafted MWNT (MWNT-P4VP) and poly(sodium 4-styrenesulfonate)-grafted MWNT (MWNT-PSS). The covalent bonding of polymers to the MWNT surface was confirmed by IR and TGA investigations. SEM, TEM and AFM investigations also showed a polymer-wrapped MWNT structure. Due to the presence of basic pyridine units in the structures, MWNT-P4VP exhibited good solubility in acidic aqueous solutions, but tended to aggregate in neutral or basic solutions. In comparison, MWNT-PSS could form stable dispersion solutions in aqueous solutions throughout the pH range from 1 to 14.

Synthesis, characterisation and liquid crystal properties of 2,5‐bis[5‐alkyl(alkoxy)phenyl‐1,3,4‐oxadiazole]bromobenzenes

A series of new thermal bilateral liquid crystal compounds with the phenylenebis‐1,3,4‐oxadiazole structure was synthesised. The molecular structures of the oxadiazole compounds were confirmed by FT‐IR and 1H NMR spectroscopy, elemental analysis and mass spectrometry. Thermogravimetric analysis indicates that the compounds in an atmosphere of nitrogen have good thermal stability. Measurements using differential scanning calorimetry, polarising optical microscopy and temperature‐dependent wide‐angle X‐ray diffraction indicated that the liquid crystalline properties are related to the length of the end groups. When the end group was tert‐butyl or alkoxy with four and six carbons, the compounds exhibit no liquid crystal phase. However, compounds with end groups containing more than eight carbons show significant bidirectional thermally‐induced liquid crystal properties and the structure of the liquid crystal phase is the lamellar smectic A phase. All the compounds exhibit blue fluorescence.

Synthesis and properties of highly birefringent liquid crystalline materials: 2,5-bis(5-alkyl-2-butadinylthiophene-yl) styrene monomers

In order to obtain liquid crystals with high birefringence (Δn), a series of new liquid crystals monomers incorporating styrene, a 2,5-disubstituted thiophene ring and diacetylenes, which were highly conjugated along the molecular long axis, were synthesized. Their physical properties were evaluated. As expected, they exhibited high Δn values of 0.4–0.7 and a low melting point and were of potential use for polymer-dispersed liquid crystal displays (PDLCDs), cholesteric displays and laser beam steering applications.

ABA type liquid crystalline triblock copolymers by combination of living cationic polymerizaition and ATRP: synthesis and self-assembly

Lamellar and hexagonal-coil-cylinder self-assembled structures of ABA type triblock copolymers containing mesogen-jacketed liquid crystalline polymer (MJLCP) as the rod block, and polyisobutylene (PIB) as the coil middle block were discovered. PIB was synthesized by living cationic polymerization of isobutylene initiated by 1,4-bis(2-chloro-2-propyl)benzene (p-DCC), and then a small amount of styrene was introduced at the end of the PIB chains to form the difunctional PIB macroinitiator with -CH2CH(C6H5)Cl end groups for further atom transfer radical polymerization (ATRP). 2,5-Bis[(4-methoxyphenyl)oxycarbonyl]styrene (MPCS) was block-copolymerized from the difunctional PIB macroinitiators at 110 °C. The molecular characterization of the triblock copolymers was performed with 1H NMR, 13C NMR, gel permeation chromatography (GPC), and thermogravimetric analysis (TGA). Their phase structures and transitions were investigated by differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS), and polarized optical microscopy experiments. It was demonstrated that the triblock copolymers formed lamellar structures at moderate rod fractions and hexagonal coil cylinders in the rod matrix at high rod fractions. The d-spacing of the microphase-separated structures was influenced by the liquid crystalline phase of rod-like PMPCS blocks.

Competition between liquid crystallinity and block copolymer self-assembly in core–shell rod–coil block copolymers

Core-shell type of architecture revealed the subtle competition between liquid-crystalline ordering and block copolymer (BCP) self-assembly in a rod-coil BCP system.

Synthesis and self-assembly of a linear coil-coil-rod ABC triblock copolymer

An interesting order-order transition between two different complex nanostructures was observed in a new liquid crystalline linear coil-coil-rod ABC triblock copolymer (triBCP). First, the ABC triBCP, poly(dimethylsiloxane)-bpolystyrene-b-poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PDMS-b-PS-b-PMPCS), was synthesized through sequential atom transfer radical polymerization. The degrees of polymerization of PDMS, PS, and PMPCS blocks are 58, 159, and 106, and the corresponding volume fractions of PDMS, PS, and PMPCS are 0.09, 0.29, and 0.62, respectively. The phase behaviors of the PDMS-b-PS diblock copolymer precursor and the final triblock copolymer were studied by smallangle X-ray scattering, one-dimensional wide-angle X-ray scattering, and transmission electron microscopy experiments. The PDMS-b-PS precursor self-assembles into hexagonally packed cylinders with a relatively small periodic size after thermal annealing. When the triblock copolymer is annealed at a relatively low temperature (120 °C) at which the PMPCS block is in the amorphous state, the triBCP forms core-shell hexagonally packed cylinders (CSH) with a relativly large periodic size. After the triBCP is annealed above 140 °C at which the PMPCS block transforms to the liquid crystalline (LC) phase, the nanophase-separated structure transforms to a three-phase four-layer lamellar structure (LAM-3P4L). Thus, accompanied with the transition of the PMPCS blocks from the amorphous state to the LC phase, the order-order transition from CSH to LAM-3P4L occurs in the PDMS-b-PS-b-PMPCS ABC triBCP.

Mesogen-jacketed liquid crystalline polymers: from molecular design to polymer light-emitting diode applications

This critical review covers the recent progress in the chemical structure design and light-emitting performance evaluation of electroluminescent mesogen-jacketed liquid crystalline polymers (MJLCPs). By applying the concept of MJLCPs, two main chromophoric building units, electron-transporting and hole-transporting units with large steric hindrance, are attached to the backbones solely or together. We highlight the structure–properties–performance relationships of electroluminescent MJLCPs. Device performances can be enhanced by the “jacketing effect”, which inhibits the aggregation of conjugated structures, fluorescence quenching, and excimer formation.

Preparation and properties of highly birefringent liquid crystalline materials: styrene monomers with acetylenes, naphthyl, and isothiocyanate groups

Two new series of styrene monomers with different alkyl chain length were successfully synthesised. The chemical structures of synthetic intermediates and monomers were confirmed by Fourier transform infrared spectroscopy, 1H nuclear magnetic resonance spectroscopy, mass spectrometry and elemental analysis. Their thermal stability, transition temperatures and phase sequences were investigated by polarised light microscopy, thermogravimetric analysis, differential scanning calorimetry and one-dimensional wide-angle X-ray diffraction. The birefringent and photoelectric properties, UV–vis and fluorescence spectroscopy were also measured using Abbe refractometer and fluorescent apparatus. The results indicated that all the monomers exhibited a liquid crystalline phase at higher temperature, high birefringence values between 0.4 and 0.7, and good photoluminescence properties. Optical anisotropy of homologous molecules was reduced with the increase of alkyl chain length due to the decrease of molecular polarisation. Moreover, a significant effect of the chemical structure on the photoluminescence properties was also found and discussed according to the length of π-conjugation.

Synthesis and self-assembly of a triarm star-shaped rod-rod block copolymer

We designed and synthesized a triarm star-shaped rod-rod block copolymer (BCP), (poly{2,5-bis[(4-methoxyphenyl)-oxycarbonyl]styrene}-block-poly(γ-benzyl-l-glutamate))3, (PMPCS-b-PBLG)3. The triarm core with three PMPCS-N3 segments was prepared by copper-mediated atom transfer radical polymerization of 2,5-bis[(4-methoxyphenyl)-oxycarbonyl]styrene initiated by a trifunctional initiator and a subsequent azide reaction. And the PBLG block with alkyne functionality was synthesized through ring-opening polymerization of γ-benzyl-l-glutamate N-carboxyanhydride initiated by propargylamine. Finally, Huisgen’s 1,3-dipolar cycloaddition was employed to combine the triarm (PMPCS-N3)3 and PBLG segments. The chemical structure of the BCP was confirmed by 1H-NMR spectroscopy, Fourier-transform infrared spectroscopy, and gel permeation chromatographic analysis. Results from differential scanning calorimetry, polarized light microscopy, one-dimensional and two-dimensional wide-angle X-ray diffraction, and transmission electron microscopy techniques demonstrate that the triarm star-shaped rod-rod BCP self-assembles into a hexagon-in-lamella morphology, with the PMPCS block in the columnar nematic phase and the PBLG block in the hexagonal columnar arrangement packed in bilayers due to the rigid nature of the two blocks and the covalent connections in the star-shaped BCP.