A hybrid ionic liquid crystal comprising imidazolium surfactant-graphene oxide

ABSTRACT To produce graphene oxide liquid crystal composites with better controllability, imidazolium surfactant with biphenyl mesogenic core (CBphCM) has been synthesised and used to modify the negatively charged surface of graphene oxide (GO) to obtain a hybrid ionic liquid crystal (CBphCM-GO). The process of GO and CBphCM recombination was monitored by various experimental techniques such as FT-IR, Uv-vis, Raman and XPS. The experimental results indicated that the C=O bonds on GO sheets are all reduced and C−O bonds are partially reduced due to the imidazolium units being electrically active and nitrogen-rich, and the presence of the N−O bonds indicates that a new hybrid is formed between CBphCM and GO. The recombination destroys the electrostatic repulsion balance among the GO lattices, and the CBphCM-GO is observed a fluffy appearance by TEM. The liquid crystal properties of CBphCM and CBphCM-GO were investigated by DSC, polarised optical microscopy (POM) and variable temperature X-ray diffraction, respectively. As an ionic liquid crystal, CBphCM has SmA as well as the rare SmC phases exhibits de Vries-like behaviour with a layer shrinkage. The liquid crystal properties of CBphCM-GO are derived from CBphCM, and the layer shrinkage behaviour from SmA to SmC phase has also been observed. GRAPHICAL ABSTRACT


Introduction
Design and synthesis of organic/inorganic hybrid liquid crystals (LCs) have been one of the most active research areas in both chemistry and material science over recent years because the rich physical properties of inorganic components could be properly introduced into LC systems [1][2][3].Ionic liquid-crystalline (ILCs) are a class of liquidcrystalline compounds that contain ions and possess the collective characteristics of ionic liquids as well as liquid crystals, showing high viscosity, self-assembly ability and dynamic molecular order, etc. [4][5][6].The properties of ILCs can be controlled by the judicious selection of the organic cation and its anionic counterpart and also by changing the mesogenic cores [7].The proper use of ILCs has been found to be a versatile strategy for the expansion of the components and the development of CONTACT Yunxia Jiang jiangyunxia@jlict.edu.cnSupplemental data for this article can be accessed online at https://doi.org/10.1080/02678292.2023.2261874.
Graphene oxide (GO) is a negatively charged material containing abundant carboxyl groups on graphene oxide sheets: primarily epoxy and hydroxyl groups on the basal plane and carboxylic acid, esters, phenols, and quinones at the sheet edges [16].The high oxidation degree favours GO dispersion in polar solvents and provides convenient sites for further chemical functionalization.The designed imidazolium surfactant can share structural and dynamic features with both thermotropic liquid crystals and phases of surfactant/water mixtures.This unique property allows imidazolium to be used as an efficient carrier for integrating moieties paired to external stimuli and other functional groups.Due to cation-anion Coulombic interactions and π-π interactions between imidazolium surfactant and GO, imidazolium surfactant can exfoliate of graphene from graphite [17][18][19], stabilise graphene [20,21], phase transfer GO sheets between the aqueous and organic phases [22,23] and even reduce GO sheets [17,23,24].
In this paper, we synthesised a novel imidazolium surfactant containing biphenyl mesogenic core (abbreviated as CBphCM).The CBphCM molecule basically consists of four different segments: ionic moiety, flexible spacer, mesogenic core and flexible alkyl tail (Scheme 1).By introducing an imidazolium head group, the amphiphilic nature of the CBphCM can dramatically enhance the nanophase separation within the molecule.And the π + -π + interactions of the imidazolium head groups can also increase the stability of the smectic phases as well as the stability of the SmC phase and the tendency towards de Vries-like behaviour with minimum SmA → SmC layer contraction [25][26][27].The π-conjugated backbone and negatively charged surface of GO provide a perfect environment for the incorporation of positively charged CBphCM to yield a hybrid ionic liquid crystal (CBphCM-GO).The process of GO and CBphCM recombination was monitored using FT-IR, Uv-vis, Raman and XPS spectra.The morphology of CBphCM-GO was observed by SEM and TEM.The liquid crystal properties of CBphCM and CBphCM-GO were investigated in detail by differential scanning calorimetry (DSC), polarised optical microscope (POM) and variable temperature X-ray diffraction, respectively.These investigations proposed an interesting relationship between LC behaviours, functionalising GO and selforganised structure.

Materials and methods
4, 4ʹ-biphenol and 1-bromododecane were purchased from Aldrich. 1, 10-dibromodecane and 1-methylimidazole were obtained from Fluka.Graphene oxide was purchased from Soochow Hengqiu Graphene Technology.All these starting compounds for synthesis were used without further purification.All the used solvents are analytical grade.Doubly distilled water was used in the experiments.H NMR spectrum was recorded on a Bruker Avance 500 instrument using (CD 3 ) 2 SO as solvent and TMS as internal reference (0.00 ppm).Elemental analysis (C, H, N) was performed on a Flash EA1112 from ThermoQuest Italia S.P.A. FT-IR measurement was carried out on a Bruker Vertex 80 V FT-IR spectrometer equipped with a DTGS detector (32 scans) with a resolution of 4 cm −1 on KBr pellets.Mass spectrum was recorded on a Matrixassisted laser time of flight mass spectrometry (Autoflex speed TOF).Scanning electron microscopy (SEM) observation was recorded on the conducting resin substrate with a ZEISS SUPRA 40 apparatus.Raman spectroscopy was carried out using a JY-T64000 (HORIBA JOBIN YVON) with incident laser light.Uv-vis absorption spectra were recorded on a 754 N spectrophotometer.X-ray photoelectron spectroscopy (XPS) was conducted using a Thermo Scientific ESCALAB 250Xi system with an A1 K α source, 15 kV tube voltages, and 10 mA current.TGA was conducted with a Perkin-Elmer TG/DTA-7 instrument and the heating rate was 10°C min −1 .Differential scanning calorimetry (DSC) measurements were performed on a Netzsch DSC 204.The samples were examined at a scanning rate of 5°C min −1 by applying several heating and cooling cycles from −20 to 240°C.Polarized optical microscopy (POM) observation was performed on a Leica DMLP (Germany) microscope equipped with a hot stage.Variable-temperature XRD experiments were performed on a Rigaku X-ray diffractometer (D8 Discover Gadds, using Cu Kα radiation of a wavelength of 1.54 Å) with a PTC-20A temperature controller.TEM images were taken using a JEOL 2100 system, and the sample was made with the microwaveassisted dispersion of the corresponding GO and CBphCM-GO in deionised water.SAXS measurements were conducted on a modified Xeuss 2.0 system (Xenocs, France).This setup was equipped with a multilayer focused Cu Kα X-ray source (GeniX 3D Cu ULD) and thus generating an X-ray beam with a wavelength of 0.154 nm.A custom-made HCS 412W (Instec, America) hot stage attached with liquid nitrogen pump.

Synthesis
Synthesis of CBphCM was carried out following the route as shown in Scheme 2.

Synthesis of CBphCM
C 12 BphC 10 Br was prepared according to the reported literature [28].A solution of C 12 BphC 10 Br (4.1 g, 7.15 mmol) and 1-methylimidazole (1.1 g, 13.41 mmol) in 80 mL of acetone was stirred for 24 h under refluxing.The solvent was evaporated, and the residue was washed with ethyl acetate three times, dried.The dried powder was added into anhydrous ethanol solution, heated and filtered.Then, the filtrate was evaporated, and the residue was washed acetone solution three times, dried to obtain a white powder (1.7 g).The yield is about 36.3%.

Synthesis of CBphCM-GO
CBphCM (100 mg) was dispersed into aqueous solution, and added dropwise to 20 mL aqueous dispersion of GO (0.05 mg/mL).This mixture was stirred for 2 h at 80°C, and the colour of solution changed from brown to black, stopped heating, cooled the mixed solution to room temperature.The solution was then transferred to dialysis membrane (MD 44) and dialysed against distilled water for 24 h.After evaporating solvent, the sample was further dried in vacuum until the weight remained constant.

Characterization of CBphCM and CBphCM-GO
Fourier transform infrared spectrometry (FT-IR) in Figure 1(a) gives the characteristic absorption of GO, CBphCM and CBphCM-GO.The GO shows the characteristic broad peak of O−H at 3400 cm −1 , and other stretching peaks at 1733 cm −1 , 1623 cm −1 , 1219 cm −1 and 1051 cm −1 , which represent C=O, C=C, C−OH, and C−O groups, respectively [29].The detailed assignments of characteristic FT-IR absorption bands of CBphCM are summarised in Table S1.FT-IR spectrum of CBphCM-GO is very similar to that of CBphCM, and the characteristic peak of GO disappears because CBphCM dominates and shields the characteristic peak of GO.Uv-vis spectra of GO, CBphCM and CBphCM-GO are shown in Figure 1(b).The GO displays an absorption maximum at 230 nm which is due to the π-π* transition of aromatic C=C bonds and a shoulder at 305 nm which corresponds to the n-π* transition of the C=O bond [30].The characteristic peaks of CBphCM at 262, 275 and 285 nm represent the π-π* transition of benzene ring [31].CBphCM-GO Scheme 2. Schematic synthetic route of CBphCM.exhibits peaks at the same wavelength with CBphCM due to the absence of any strong charge transfer composite formation between GO and CBphCM [32].The inset shows that the colour of the complex solution changes from brown to black during the process reaction, suggesting that the restoration of π-conjugated network within the GO occurres upon chemical reduction.
In order to confirm the effective recombination of CBphCM and GO, the characteristics of GO, CBphCM and CBphCM-GO are also analysed through Raman scattering because it is very sensitive towards the changes of carbon framework.In Figure 2, the characteristic peaks at 1300 cm −1 , 1550 cm −1 and 2700 cm −1 , which represent D band, G band and 2D band of GO, respectively [33].The detailed assignments of characteristic Raman absorption bands of CBphCM are summarised in Table S2.These characteristic peaks are consistent with the FT-IR spectra of CBphCM.When GO is incorporated into CBphCM, some characteristic peaks of CBphCM disappear or weaken.CBphCM-GO still retains the three characteristic bands of GO, but we cannot judge the reduction degree of GO by calculating the ratio of I D /I G because the characteristic peak at 1605 cm −1 of benzene ring of CBphCM overlaps with the G band of GO.The 2D band which is affected by the number of GO layers and the internal stress between them becomes more obvious in CBphCM-GO, which indicated that the decrease in the number of layers of GO sheets due to the more uniform dispersion of GO in CBphCM [33].
Using X-ray Photoelectron Spectrometer (XPS) experiment to monitor the change of chemical composition in the process of GO and CBphCM recombination.The survey scans for peak assignment are found in Figure 3(a).The GO used in this study contains about 34.53% oxygen, and the carbon-to-oxygen ratio (C/O) of GO closes to 1.9.In order to further confirm the interaction between the imidazolium surfactants and GO, the detailed information of different elemental signals in XPS is discussed.In the C (1s) XPS spectrum of GO, the signal can be decomposed into three main contributions, underling the presence of at least six types of carbon bonds: sp 2  At the beginning of the mixing of GO and CBphCM, the position of carbon functional groups shifts to the

Liquid crystalline properties
Thermal transition behaviours of CBphCM and CBphCM-GO are investigated by TGA and DSC (Figure 4).TGA curves show that CBphCM and CBphCM-GO have good thermal stability and remained stable up to 240°C.Table 1 summarises the phase transition temperatures, enthalpies, and assignments for CBphCM and CBphCM-GO.An exothermic peak appears at 215.12°C when CBphCM is cooled from the isotropic phase to SmA phase.Then, a weak broad peak at 190.67°C corresponds to an enthalpy of transformation of 0.97 J/g.We consider that the phase transition is still the SmA phase from the POM observation and XRD results.Another exothermic peak at 154.15°C is assigned from SmA phase to SmC phase.As the    further cooling, the focal conic texture begins to break at 154°C as shown in Figure 5(c), suggesting SmA transforms into the more ordered SmC phase.Figure 5(d) is completely broken focal-conic texture.During the cooling process, SmA phase and SmC phase are also observed in CBphCM-GO, and the black reduced GO is observed in liquid crystal textures in Figure 5(e-f).

Mesophase structure
In order to evaluate the molecular arrangements in their mesophases, temperature-dependent X-ray diffraction (XRD) is performed on CBphCM and CBphCM-GO during the cooling processes as shown Figure 6.
According to the XRD data of CBphCM and CBphCM-GO, d values in the mesophase are summarised in Table S3.The XRD pattern of CBphCM has three sharp diffraction peaks in the small-angle region at 200°C, and a broad diffraction peak at 19.88° in the wide-angle region indicates liquid-like arrangement of the molecules as shown in Figure 6(a).When the temperature decreases to 180°C, a new sharp peak 4 appears.As the temperature further decreases to 140°C, another a new diffraction peak 5 appears which suggests a highly ordered SmC phase.Many sharp reflections detect at 105°C from the small-angle to wide-angle region represent that a true Cr structure is formed.The XRD patterns of CBphCM-GO are shown in Figure 6(b).Three sharp diffraction peaks are also observed in the smallangle at 185°C.When further cooled to 130°C, there is peak 4. A new weak diffraction peak (2θ ≈ 22.83°) from reduced GO is observed in the wide-angle region when CBphCM-GO is in the crystalline state, which corresponds to a spacing of 4.06 Å [20].
One parameter for de Vries-like behaviour is the maximum layer contraction at the SmA-SmC phase The next necessary quantity to decide whether these materials are de Vries-like materials is the optical tilt angle of the SmC phase.The optical tilt angles Ɵ opt were measured by POM as function of reduced temperature T-T AC in the absence of an electrical field upon cooling.Figure 8 shows the optical tilt angles Ɵ opt of CBphCM.At T-T AC = 0, the optical tilt angles Ɵ opt of the SmC phase increased significantly and then steadily increased upon further cooling.CBphCM shows values of Ɵ opt up to 29°.Unfortunately, because the CBphCM-GO liquid crystal texture is covered by graphene oxide, we did not observe the optical tilt angle of CBphCM-GO by POM.
From the maximum layer contraction and the optical tilt angle Ɵ opt , the reduction factor R was calculated according to equation R = Ɵ Xray (T)/Ɵ opt (T) = cos −1 [d C (T)/d AC ]/Ɵ opt (T) for a given temperature [25,35].At T-T AC = −25 K, Ɵ opt = 28.5°, the R values of CBphCM are 0.482, which means that CBphCM exhibits some degree of de Vries-type behaviour.
Combining the results of the theoretical calculation and experimental observations, the possible molecular packing modes for CBphCM and CBphCM-GO in the smectic phase can be proposed.According to the results calculated by GaussView 5.0, the length of CBphCM is 40.1 Å along the longitudinal axis.The d AC /l ratios at the SmA-SmC phase transition is 1.8.This value means the CBphCM forms an interdigitated SmA phase, and the CBphCM can be considered to have structural features of the dimer [36].A double bilayer model with interdigitated alkyl chains, as shown in Figure 9(a).The neighbouring flexible alkyl tails overlap each other and the biphenyl groups are faceing each other in the lateral direction.The interaction of the biphenyl groups keeps the orientation order of liquid crystal molecules in the layer.The imidazolium parts associate to form a smectic layer and all the layers are further linked with the imidazolium salts through electrostatic interactions.When the tilted SmC phase is formed, the extent of interdigitation neighbouring flexible alkyl tails decreases in the bilayer structure to compensate the layer contraction as shown in Figure 9(b).The molecular packing mode of CBphCM-GO is shown in Figure 9(c).In case of the CBphCM-GO, CBphCM is linked to GO sheets through electrostatic interaction and weak π−π interaction between imidazole rings and GO sheets.

GO and CBphCM-GO morphology characterization
The morphology of GO and CBphCM-GO is characterised using SEM and TEM. Figure S3 shows the typical SEM images of the resulting GO and CBphCM-GO.It can be clearly observed that large-scale CBphCM-GO with so many crumpled silk veil waves, something like tremella, is successfully obtained in Figure S3(b), indicating the efficient reduction of GO [37].Individual GO sheets resemble crumpled silk veil waves in lowmagnification images as shown in Figure 10(a).The CBphCM-GO has a fluffy appearance due to the chemical functionalization and the re-aggregation of smaller flakes as shown in Figure 10(b).A lattice resolved highresolution TEM (HRTEM) image of the stacked  CBphCM-GO lattice is shown in the inset of Figure 10(b).An internal short-range ordered crystalline structure is observed.The average inter-planar distance is measured to be 2.10 nm.This result is approximately consistent with the d 3 value of the strongest diffraction peak observed by XRD patterns of CBphCM-GO.These results indicate that the chemical modification puts a strong impact on the domain structure of the graphene lattice.The functionalization of GO by the CBphCM destroys the electrostatic repulsion balance existed among the GO lattices [38].

Conclusion
In this article, a novel imidazolium surfactant (CBphCM) with biphenyl mesogenic cores has been designed and successfully synthesised.As an ionic liquid crystal, CBphCM exhibits the characteristic of typical de Vries-type behaviour, which undergoes a SmA to SmC phase transition with an unusual small layer contraction on cooling.During the recombination process of CBphCM and GO, the C=O bonds on GO sheets are all removed and C−O bonds are partially removed by the chemical reduction, and the presence of the N−O bonds indicates that a new hybrid (CBphCM-GO) is formed between CBphCM and GO.CBphCM-GO retains the liquid crystal properties of CBphCM, and also exhibits the layer shrinkage behaviour from SmA to SmC phases.In summary, we successfully introduce GO into ionic liquid crystal systems, and hope that orderliness and self-organised structure of liquid crystal offer valuable routes for the functionalization of graphite materials.
aromatic and sp 3 carbons (284.56 eV), C−O−C epoxy ring and C−OH (286.65 eV), C=O and O−C=O (288.4 eV) [34].The atomic percentage (atom %) for different carbon functional groups is calculated with respect to the total area of the C 1s peak.The majority consists of C−O bonds (44.44 atom %) and C=O (11.11 atom %).The O (1s) signal contains a main signal centred at 532.4 eV and divides into two contributions C−O−C epoxy ring/C −OH (532.54 eV) and C=O/O−C=O (531.8 eV).
transition.Therefore, the smectic layer spacing d of CBphCM and CBphCM-GO as a function of temperature was measured by small-angle X-ray scattering (SAXS).The layer distances d were calculated from the (001) reflex at different temperatures, where d AC denotes the layer distance at the SmA -SmC transition temperature T AC .The profiles of normalised layer spacings d/d AC vs. reduced temperature T-T AC for CBphCM and CBphCM-GO are summarised in Figure 7. Upon cooling an expansion in the SmA phase until the transition temperature was obserbed.At transition into the SmC phase, the layer spacing initially decreased, then increased upon further cooling.The maximum layer contractions of CBphCM and CBphCM-GO are 2.86% (T-T AC = −25 K) and 2.41% (T-T AC = −20 K), respectively.

Figure 7 .
Figure 7. Layer spacing d/d AC vs. reduced temperature T-T AC profiles of CBphCM (a) and CBphCM-GO (b).

Figure 8 .
Figure 8. (Colour online) Optical tilt angle Ɵ opt vs. reduced temperature T-T AC for CBphCM (a).The optical micrographs of CBphCM at 150°C in an ITO glass cell with rubbed Nylon alignment layers (b).

Figure 10 .
Figure 10.TEM images of (a) GO and (b) CBphCM-GO, inset in (b): a lattice resolved HRTEM image of the stacked CBphCM-GO.

Figure 9 .
Figure 9. (Colour online) Proposed molecular arrangement structures of CBphCM in SmA phase (a), SmC phase (b) and CBphCM-GO in SmA phase (c).