Incorporation of graphene oxide into C₁₂E₄/C₁₂mimBr hybrid lyotropic liquid crystal and its thermo-sensitive properties

Abstract

Graphene oxide (GO) was successfully incorporated into a hybrid lyotropic liquid crystal (LLC) matrix formed by two kinds of surfactants n-dodecyl tetraethylene monoether (C₁₂E₄) and 1-dodecyl-3-methylimidazolium bromide ionic liquid (C₁₂mimBr). By changing the ratios of C₁₂E₄ and C₁₂mimBr, two types of C₁₂E₄/C₁₂mimBr LLC matrixes (lamellar and hexagonal phase) were formed and the effects of the concentration of GO and temperature on the properties of GO/C₁₂E₄/C₁₂mimBr LLC composites were systematically investigated by polarized optical microscopy (POM) observations, small-angle X-ray scattering (SAXS) and rheological measurement. Both POM observations and SAXS results indicated that GO can be well-dispersed in the hybrid LLC matrixes at room temperature. Moreover, after the incorporation of GO, the temperature tolerance of GO/C₁₂E₄/C₁₂mimBr LLC composites were enhanced compared with pure C₁₂E₄/C₁₂mimBr hybrid LLC and aggregated GO was not observed in the C₁₂E₄/C₁₂mimBr LLC hybrid hexagonal matrix with the increase of temperature while it can be observed in the C₁₂E₄/C₁₂mimBr LLC hybrid lamellar matrix. The results of rheological measurements showed that the addition of GO were helpful for enhancing the mechanical properties of C₁₂E₄/C₁₂mimBr LLC. Thus, the success preparation of GO/hybrid LLC composites can highly improve the thermal stability of these materials and widen the applications of GO/LC materials in nanotechnology, electrochemical, drug delivery systems and bioengineering areas.

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Introduction

Lyotropic liquid crystal (LLC) is a special kind of the self-assembly of amphiphilic molecules within the scope of the long range ordered arrangement, and its unique functions cannot be replaced by other materials.^1–3 LLC can be used as a template to synthesize or assemble nonfunctional materials, which has received widespread attention.^4–8 As we all know, the structure of LLC matrix can be regulated by changing its composition such as the type of amphiphilic molecules, the concentration, the solvent conditions, etc. Except for traditional surfactants, long-chain alkyl ionic liquids are special kinds of amphiphilic surfactant molecules which can also form LC. Ionic liquids are often moderate to poor conductors of electricity, non-ionizing, highly viscous and frequently exhibit low vapor pressure.⁹ In addition the other properties are diverse due to their purely ionic character, such as low combustibility, excellent thermal stability, wide liquid regions, and favorable solvating properties.^10–12 Therefore, long-chain alkyl-imidazolium ionic liquids has become hotspots in research fields due to its rich phase behavior.^13–15 For example, Zhang et al. investigated the phase behavior of C₈mimCl/H₂O/alcohols (1-hexanol, 1-octanol, 1-decanol, and 1-dodecanol) at 25 °C, the lamellar phases were found in all systems, and the length of alkyl chain of alcohols on the impact of LLC structures were also discussed.¹⁶ Moreover, Zhang et al. also compared two ternary systems C₁₆mimBr/H₂O/C₁₀H₂₁OH and CTAB/H₂O/C₁₀H₂₁OH, and investigated the effect of hydrophilic group on the structure of LLC and finally got the formation mechanism of complex crystal.¹⁷

Long-chain alkyl ionic liquids/traditional surfactants mixed systems can enrich the variety of LLC matrix and offer a new way for their application.¹⁸ Moreover, it is also interesting to build orderly organic/inorganic hybrids by doping nanoparticles (especially with special property, such as optical, electrical, magnetic and biocompatibility) into LLC matrix. In addition to showing the nature of every component of these hybrids, new features were also exhibited duo to the interaction between those components.^19–21 As a special carbon material, graphene is the one with the most potential due to its outstanding physical, chemical and electronic properties.²² Graphene oxide (GO) is a graphene sheet with carboxylic groups at its edges and phenol, hydroxyl and epoxide groups on its basal plane which makes it can be dispersed in water by simple ultrasonication and became a material with good hydrophilic and biocompatible properties.^23,24 Those groups can be also combined with small organic molecules by hydrogen bonds and π–π bonds which induce GO–LLC composites has potential applications in drug release and dye adsorption.^25–27 For example, Sun et al. incorporated GO into PNIPAM hydrogel by one-step strategy and their PNIPAM-co-AA microgels showed dual thermal and pH response with good reversibility.²⁸ In our previous work, a detailed survey has been carried out on GO/C₁₂E₄-LLC composites.²⁰ Furthermore, we incorporated CNTs into LLC matrix formed by nonionic surfactant or ionic surfactant through spontaneous phase separation process, respectively, and the properties of CNTs-nonionic surfactant and CNTs-ionic surfactant were compared.^29,30

In this paper, GO was successfully incorporated into a hybrid LLC matrix formed by nonionic surfactant n-dodecyl tetraethylene monoether (C₁₂E₄) and long-chain alkylimidazolium ionic liquids 1-dodecyl-3-methylimidazolium bromide (C₁₂mimBr). The properties of GO/C₁₂E₄/C₁₂mimBr LLC composites were systematically investigated by polarized optical microscopy (POM) observations, small-angle X-ray scattering (SAXS) and rheological measurement. Compared with LC formed by single non-ionic surfactant, it is expected that the hybrid LC may be have a better property of thermal stability due to less influence of temperature changes of ionic liquids, so the effects of temperature on the properties of GO/C₁₂E₄/C₁₂mimBr LLC composites were investigated. Moreover, it can be concluded that the liquid crystal composite formed by ionic liquids C₁₂mimBr and the traditional surfactant C₁₂E₄ enriched the variety of liquid crystal template.

Experimental

Chemicals

Nonionic surfactant (C₁₂E₄) was purchased from Acros Organics (USA) and the purity was greater than 99%. Long-chain alkylimidazolium ionic liquids (C₁₂mimBr) was purchased from Lanzhou institute of chemical physics with the purity greater than 99%. GO (diameter: 0.5–5 μm, thickness: 0.8–1.2 nm) was obtained from Nanjing XFNANO Materials Tech Co., Ltd. All the above reagents were used without further purification. Water used in the experiments was triply distilled using a quartz water purification system.

Methods and characterization

GO (25 mg) is added into water (5 mL) directly by ultrasonication for 4 h, then stock GO dispersion was obtained and can stand for at least two weeks. Sample preparation: the total mass fraction of solute (C₁₂E₄ and C₁₂mimBr) was 70 wt%, the solvent is water or a dispersion of GO, samples were prepared by mixing the substances with a certain amount.

TEM observations were carried out on a JEOL JEM-100 CXII (Japan) at an accelerating voltage of 80 kV and about 5 μL of solution was placed on a TEM grid and the excess solution was wicked away with filter paper. Field-emission scanning electron microscopy (FE-SEM) observations were carried out on a JSM-6700F. For the sample preparation, a drop of GO dispersion was placed on a silica wafer to form a thin film. The wafers were freeze-dried in a vacuum extractor at −55 °C. Small-angle X-ray scattering (SAXS) observations were carried out on a HMBG-SAX X-ray small-angle scattering system (Austria) with a Ni-filtered Cu Kα radiation (0.154 nm) operating at 50 kV and 40 mA. The distance between the sample and detector was 27.8 cm. Polarized microscopy observations were carried out on AXIOSKOP 40/40 FL (ZEISS, Germany) microscope and Nikon Eclipse E400 microscope equipped with a LINKAM THMS 600 heating/cooling stage.

The rheological measurements were carried out on an Anton Paar Physica MCR302 rheometer with a plate–plate system (diameter, 25 mm; 0°). In oscillatory measurements, an amplitude sweep at a fixed frequency was performed prior to the following frequency sweep in order to ensure that the selected stress was in the linear viscoelastic region. The viscoelastic properties of the samples were determined by oscillatory measurements in the frequency range of 0.01–100 Hz. The samples were measured at 20.0 ± 0.1 °C with the help of a cyclic water bath.

Results and discussion

Dispersion states of graphene oxide

TEM and SEM images of the freeze-dried 5 mg mL⁻¹ GO dispersion were shown in Fig. 1. The TEM image of GO (Fig. 1A) shows that it contains several graphitic layers and some place of it fold to induce wrinkles. For the SEM image (Fig. 1B), a 3D film network structure was observed and GO sheets appeared in a loose pore structure which is due to the force balance between electrostatic repulsion and binding interactions (hydrogen bonding, π-stacking, hydrophobic effect, etc.).³¹ Then, the GO dispersion was diluted to the desired concentration for later use.

Fig. 1 TEM (A) and SEM (B) images of freeze-dried of 5 mg mL⁻¹ GO solution.

Phase behavior of hybrid C₁₂E₄/C₁₂mimBr LC composites

First, the phase behavior of C₁₂E₄/C₁₂mimBr mixed system was investigated by fixing the total concentration of C₁₂E₄ and C₁₂mimBr at 70 wt% and changing the ratio of C₁₂E₄ and C₁₂mimBr. The SAXS results are shown in Fig. 2. It can be seen that two peaks were detected for all the samples. The relative position ratio of these two peaks for the samples a–d was 1 : 2 when the concentration of C₁₂mimBr is below 30 wt% which is expected for the (001) and (002) reflections of the lamellar phase. The samples e was not an uniform phase by macroscopic observation, and the SAXS results also showed that it was not a lamellar or hexagonal phase, so we have not do further research about it. When the concentration of C₁₂mimBr exceeds 50 wt%, the relative position ratio of these two peaks for the samples f–h changed to which is expected for the (001) and (002) reflections of the hexagonal phase.^32–34 Thus, it can be concluded that the phase state of C₁₂E₄/C₁₂mimBr hybrid composites can be controlled by changing the ratio of C₁₂E₄ and C₁₂mimBr. On the basis of the above results, the sample b which represents the lamellar phase and the sample g which represents the hexagonal phase, respectively, were selected as the templates for the further study.

Fig. 2 SAXS results (A) and (B) the amplification part of 2.45–3.5 nm⁻¹. (a) 70 wt% C₁₂E₄, (b) 60 wt% C₁₂E₄/10 wt% C₁₂mimBr, (c) 50 wt% C₁₂E₄/20 wt% C₁₂mimBr, (d) 40 wt% C₁₂E₄/30 wt% C₁₂mimBr, (e) 30 wt% C₁₂E₄/40 wt% C₁₂mimBr, (f) 20 wt% C₁₂E₄/50 wt% C₁₂mimBr, (g) 10 wt% C₁₂E₄/60 wt% C₁₂mimBr and (h) 70 wt% C₁₂mimBr.

The effect of c_GO on the properties of GO/C₁₂E₄/C₁₂mimBr LLC composites

POM images of the above hybrid LLC composites were shown in Fig. 3. Fig. 3 (A–D) showed the POM results of pure C₁₂E₄/C₁₂mimBr LLC while Fig. 3 (a–d) showed the results of GO/C₁₂E₄/C₁₂mimBr LLC composites. Under POM observations, it can be seen that samples of (A, a, B, b) showed Maltese crosses which are characteristic of lamellar structures, and samples of (C, c, D, d) showed fan-like texture which are characteristic of hexagonal structures. Compared the POM images of C₁₂E₄/C₁₂mimBr LLC with and without GO, it can be found that the textures of these composites were similar, indicating that GO were well-incorporated into C₁₂E₄/C₁₂mimBr LLC matrix without destroy their structures.^35,36

Fig. 3 POM images of LLC matrix without GO. (A) 70 wt% C₁₂E₄, (B) 60 wt% C₁₂E₄/10 wt% C₁₂mimBr, (C) 10 wt% C₁₂E₄/60 wt% C₁₂mimBr and (D) 70 wt% C₁₂mimBr. POM image of composites with GO. (a) 70 wt% C₁₂E₄, (b) 60 wt% C₁₂E₄/10 wt% C₁₂mimBr, (c) 10 wt% C₁₂E₄/60 wt% C₁₂mimBr and (d) 70 wt% C₁₂mimBr.

To further obtain the details about the phase behaviors of GO/C₁₂E₄/C₁₂mimBr LLC composites, SAXS measurements were carried out. Fig. 4A showed the variation of SAXS results of 60 wt% C₁₂E₄/10 wt% C₁₂mimBr LLC composites as a function of the concentration of GO (c_GO), two peaks were detected with a relative position ratio of 1 : 2 which respected the lamellar phase. Moreover, the position of the peaks moved to left a bit with the increasing of c_GO. The d-spacing of the lamellar lattice was calculated by d = 2π/q₁ (q₁ was the value of the first peak). It was found that the d-spacing of the lamellar lattice were increased from 4.33 nm to 4.76 nm as c_GO increases. Fig. 4B showed the variation of SAXS results of 10 wt% C₁₂E₄/60 wt% C₁₂mimBr LLC composites as a function of c_GO, two peaks were detected with a relative position ratio of which respected the hexagonal phase. The d-spacing of hexagonal phase was calculated by d = 4π/(q₁) (q₁ was the value of the first peak). Interestingly, it can be seen that with the increase of c_GO, the d-spacing remained nearly the same (∼4.29 nm). It was clearly observed that the d-spacing of lamellar phase was larger than the hexagonal phase which may be because of the length of the imidazole ring shorter than EO chain (Fig. 4C).

Fig. 4 The results of SAXS, (A) lamellar liquid crystal (60 wt% C₁₂E₄/10 wt% C₁₂mimBr), (B) hexagonal liquid crystal (10 wt% C₁₂E₄/60 wt% C₁₂mimBr). The concentration of GO were (a) 0 mg mL⁻¹, (b) 0.3 mg mL⁻¹, (c) 0.6 mg mL⁻¹ and (d) 0.9 mg mL⁻¹. (C) The structure and the length of C₁₂E₄ and C₁₂mimBr. Color code for atoms: blue, nitrogen; red, oxygen; dark gray, carbon; light gray, hydrogen.

Rheological measurements can give macro properties of the system in real-time and then reflect the change of internal microstructure; thus, it is an important method to investigate LLC materials.^37,38 We compared the properties of GO/C₁₂E₄/C₁₂mimBr LLC composites between lamellar phase and hexagonal phase and the results are shown in Fig. 5. The stress sweep test was executed at a fixed frequency (Fig. 5A, a), a critical stress value (yield stress, τ*) was appeared. The shear modulus sharply decreased above the τ* and the system shown properties of Newtonian-like flow.³⁹ In addition, the addition of GO greatly increased the range of the linear viscoelastic region of both lamellar phase and hexagonal phase, and the shear modulus increased with the increasing of c_GO. For example, G′ of lamellar phase of 60 wt% C₁₂E₄/10 wt% C₁₂mimBr without GO was 2780 Pa, and the value of G′ increased progressively from 3216 Pa to 3826 Pa when c_GO increased from 0.3 mg mL⁻¹ to 0.9 mg mL⁻¹. Moreover, the same phenomenon occurred with the increase of c_GO for the hexagonal phase LLC. For example, G′ of hexagonal phase of 10 wt% C₁₂E₄/60 wt% C₁₂mimBr LLC without GO was 84 500 Pa, and the value of G′ increased progressively from 91 900 Pa to 95 200 Pa when c_GO increased from 0.3 mg mL⁻¹ to 0.9 mg mL⁻¹.

Fig. 5 Rheological properties of liquid crystal composites as a function of GO, (left) lamellar liquid crystal composites (60 wt% C₁₂E₄/10 wt% C₁₂mimBr) and (right) hexagonal liquid crystal composites (10 wt% C₁₂E₄/60 wt% C₁₂mimBr). (A and a) G′ and G′′ as a function of the applied stress (τ) at f = 1.0 Hz (lamellar liquid crystal) and f = 100 Hz (hexagonal liquid crystal), (B and b) G′ and G′′ and (C and c) η* as a function of f.

In the whole frequency measurement, the system of both lamellar phase and hexagonal phase showed a strong elastic response (Fig. 5B, b). Elastic modulus of GO/C₁₂E₄/C₁₂mimBr LLC composites also increased continuously with the increase of c_GO. For example, G′ of lamellar phase and hexagonal phase of C₁₂E₄/C₁₂mimBr LLC without GO were 2364 Pa and 163 217 Pa, respectively. When c_GO was 0.9 mg mL⁻¹, the value of G′ increased to 3364 Pa and 268 006 Pa, respectively. The η* of both systems decreased with the increase of frequency indicating a shear-thinning behavior (Fig. 5C, c). Moreover, the value of η* increased with the increase of c_GO which indicated that the addition of GO enhanced the mechanical property of C₁₂E₄/C₁₂mimBr LLC material.

From all of the above experimental results, it can be seen that for our systems, GO is not only used for improving mechanical properties but also C₁₂E₄/C₁₂mimBr LLC can tune the system's morphology from lamellar to hexagonal, where the lamellar can work as a ‘trigger’ for releasing back the GO, while hexagonal is more stable.

The effect of temperature on the properties of GO/C₁₂E₄/C₁₂mimBr LLC composites

Cloud point is characteristic of nonionic surfactants containing polyoxyethylene chains. Nonionic surfactants exhibit reverse solubility versus temperature behavior in water and therefore “cloud out” at some point as the temperature is raised.^29,40 So the LLC composites formed by nonionic surfactants are easy to destroy at high temperature. The variation of the SAXS results of 70 wt% C₁₂E₄ LLC as a function of temperature were shown in Fig. 6. It can be seen that two peaks were detected with a relative position ratio of 1 : 2 when the temperature is below 40 °C and the shape of the peaks gradually widened and disappeared when the temperature is above 40 °C. However, the peaks reappeared when the temperature returned to 20 °C.

Fig. 6 (A) The variation of the SAXS results of 70 wt% C₁₂E₄ LLC as a function of temperature. (B) The amplification part of (A) of 2.5–3.5 nm⁻¹.

In order to improve the thermal stability of the LLC materials, C₁₂mimBr and GO were incorporated into the system. For the composites, SAXS measurements were carried out as a function of temperature (Fig. 7). It can be clearly seen the addition of C₁₂mimBr greatly increased the thermal stability of C₁₂E₄ LLC. Compared 60 wt% C₁₂E₄/10 wt% C₁₂mimBr LLC with 70 wt% C₁₂E₄ LLC, the peaks of 60 wt% C₁₂E₄/10 wt% C₁₂mimBr LLC became wider and shorter after the temperature above 70 °C as shown in Fig. 7A, indicating the lamellar phase were destroyed. This temperature (70 °C) is much higher than that of 70 wt% C₁₂E₄ (40 °C). Besides, the thermal stability of the lamellar C₁₂E₄/C₁₂mimBr composites (80 °C) was slightly increased with the addition of 0.3 mg mL⁻¹ GO. In other words, the addition of GO into C₁₂E₄/C₁₂mimBr LLC was beneficial to the increase of their thermal stability. For the hexagonal phase (10 wt% C₁₂E₄/60 wt% C₁₂mimBr LLC and 0.3 mg mL⁻¹ GO/10 wt% C₁₂E₄/60 wt% C₁₂mimBr LLC), when the temperature increased from 20 °C to 80 °C, two peaks were detected with a relative position ratio of and it maintained no deformation of the shape of peaks even at high temperature (Fig. 7B, b), indicating the high thermal stability of the hexagonal C₁₂E₄/C₁₂mimBr LLC and GO/C₁₂E₄/C₁₂mimBr LLC composites. Hence, it can be concluded that the properties of the LLC materials can be controlled by adjusting the content of C₁₂mimBr and GO.

Fig. 7 The SAXS results of LLC composites: 60 wt% C₁₂E₄/10 wt% C₁₂mimBr, (A and a); 10 wt% C₁₂E₄/60 wt% C₁₂mimBr, (B and b). The concentration of GO were 0 mg mL⁻¹ (A and B) and 0.3 mg mL⁻¹ (a and b).

To gain further details about the state of GO/C₁₂E₄/C₁₂mimBr LLC composites, POM measurements were carried out at different temperature (Fig. 8). For 0.3 mg mL⁻¹ GO/60 wt% C₁₂E₄/10 wt% C₁₂mimBr LLC, Maltese crosses were appeared and the aggregated GO was not found at a reunited state, which indicating that GO was well-dispersed in the C₁₂E₄/C₁₂mimBr LLC matrix at 20 °C (Fig. 8A₁). When the temperature was increased to 80 °C, the texture disappeared and we observed large GO aggregates under the microscope. However, the Maltese crosses texture reappeared when the temperature dropped to 20 °C, but the aggregated GO was still maintained and can not recovered to the dispersed state, indicating that after the increase of the temperature, the aggregation process of GO was irreversible even the structure of C₁₂E₄/C₁₂mimBr LLC was recovered. For the hexagonal phase (0.3 mg mL⁻¹ GO/10 wt% C₁₂E₄/60 wt% C₁₂mimBr LLC), when the temperature increased from 20 °C to 80 °C, the texture had not changed and the aggregation of GO was not occurred during the whole process of heating, indicating that the thermal stability greatly increased with the increasing of the concentration of C₁₂mimBr which lead GO keep a good dispersion in LLC matrix due to the stable structure.

Fig. 8 POM images of liquid crystal composites as the change of temperature, c_GO = 0.3 mg mL⁻¹, (A) 60 wt% C₁₂E₄/10 wt% C₁₂mimBr and (B) 10 wt% C₁₂E₄/60 wt% C₁₂mimBr. The temperature were (A₁ and B₁) 20 °C, (A₂) 80 °C, (B₂) 90 °C and (A₃ and B₃) back to 20 °C from high temperature.

Nonionic surfactants became turbid and phase separation occurred when the temperature reached above its cloud point, they are sensitive to temperature while C₁₂mimBr was an ionic surfactant and they did not have cloud point and the influence of temperature on their phase transition wasn't obvious. As the temperature rises, the structure of GO/C₁₂E₄ or GO/C₁₂E₄/C₁₂mimBr lamellar LLC composites was destroyed easily at high temperature due to its major component was C₁₂E₄. Then the aggregation of GO was occurred when the structure of lamellar LLC damaged and the aggregated GO can not disperse again even when the temperature recovered to room temperature and the lamellar LLC of C₁₂E₄ or C₁₂E₄/C₁₂mimBr recovered.

However, the addition of large amounts of C₁₂mimBr greatly increased the thermal stability of GO/C₁₂E₄/C₁₂mimBr hexagonal LLC and the structure of GO/C₁₂E₄/C₁₂mimBr hexagonal composites had no obvious change during the heating or cooling process because the major component was C₁₂mimBr in the GO/C₁₂E₄/C₁₂mimBr hexagonal LLC composites. We concluded that, the addition of C₁₂mimBr and GO not only can change the phase behavior of LLC but also can improve its thermal stability.

Formation mechanism of GO/C₁₂E₄/C₁₂mimBr LLC

Through the above results, we discussed the mechanism of the system in this paper. The experiments showed that the d-spacing of C₁₂E₄/C₁₂mimBr LLC composites became shorter with the concentration of C₁₂mimBr increasing, and there are several reasons which are shown as follow. First, it can be seen that the length of C₁₂mimBr (∼1.9 nm) was shorter than that of C₁₂E₄ (∼3.1 nm) (Fig. 4C), this is because that the imidazole ring of C₁₂mimBr is shorter than EO chain of C₁₂E₄. Thus, it induced the molecule of C₁₂mimBr is shorter than the C₁₂E₄ when the alkyl chain are the same (C₁₂). Besides, because the imidazole ring of C₁₂mimBr is rigid and the EO chain of C₁₂E₄ is flexible, when a small amount of C₁₂mimBr was added to the system of C₁₂E₄ LLC, there are strong intermolecular hydrogen bonds interaction between EO chains and imidazole ring which induced the structure of C₁₂E₄/C₁₂mimBr LLC becoming looser compared with that of C₁₂E₄ LLC and the EO chains of C₁₂E₄ arranging around the imidazole ring of C₁₂mimBr and becoming bent.

Furthermore, when GO sheets were incorporated into the hydrophilic part of C₁₂E₄/C₁₂mimBr LLC matrix, because the presence of the oxygen-containing functional groups such as –COOH and –OH on the surfaces of GO, there are strong hydrogen bonds interaction between GO and EO chains or GO and imidazole ring which can enhance the mechanical property of C₁₂E₄/C₁₂mimBr LLC material. Moreover, because the d-spacing of C₁₂E₄/C₁₂mimBr lamellar composites is large and its structure is a little loose, thus, the addition of GO can further enlarge the hydrophilic region and increase the d-spacing of the GO/C₁₂E₄/C₁₂mimBr lamellar composites. But for C₁₂E₄/C₁₂mimBr hexagonal phase, because the C₁₂E₄/C₁₂mimBr hexagonal phase is a close-packed structure and the molecular arrangement is compact, therefore the GO sheets can be only well-distributed among the columns of the GO/C₁₂E₄/C₁₂mimBr hexagonal LLC composites and the d-spacing had no significant changes with the increase of the concentration of GO. The details of the schematic illustration of the formation of GO/C₁₂E₄/C₁₂mimBr lamellar and hexagonal LLC composites were shown in Scheme 1.

Scheme 1 Schematic illustration of the formation of GO/C₁₂E₄/C₁₂mimBr lamellar and hexagonal LLC composites.

Conclusions

To conclude, GO have been successfully incorporated into the lamellar and hexagonal LLC composites formed by C₁₂E₄ and C₁₂mimBr. From the experimental results, it can be observed that GO has been well-dispersed in the matrix and the additions of GO were helpful for mechanical properties of the system. Moreover, with the increase of the concentration of C₁₂mimBr, phase behavior of the lyotropic liquid crystals matrix changed from lamellar to hexagonal phase. It can be seen that the thermal stability of the LLC composites was greatly increased after the addition of C₁₂mimBr and GO into C₁₂E₄ LLC.

Acknowledgements

We gratefully acknowledge financial support from the National Natural Science Foundation of China (21173128, 21203109) and Ji'nan Youth Science and Technology Star Program (2013040).

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