The role of supramolecular chemistry in LCs and graphene based LCs can be viewed in different ways which firstly involves the supramolecular approach of formation of discotic liquid crystals, secondly aqueous and organic phase formation of liquid crystals in graphene system and lastly involves a wider range of the self‐assembly of graphene systems onto supramolecular matrix of various liquid crystals. Recent exploration by Kim et al. 54 on the chemical functionalization of highly purified graphene suspension shows colloidal LC phases and subsequently, the graphene based liquid crystals have emerged as an advance field in soft condensed matter. Graphene can be chemically modified via functionalization 55 and this functionality can produce different surface and Basel‐edge defect. Alignment of LCs can be achieved on functionalized graphene surface. Different temperature and pH dependent modification of graphene causes the extremely large Kerr effect, 56 which is useful in liquid crystal displays (LCD). On another hand water and organic phase induced liquid crystallinity, liquid crystal functionalization of graphene, LC assisted self‐assembly in graphene, photo‐responsive liquid crystallinity, self‐assembly of chemically modified graphene in nematic (N), ferroelectric, discotic LCs, LCs from polymer grafted graphene, self‐assembly of graphene nanoparticles in DLCs open a new way towards many applications like optoelectronics, energy storage, optical switching, highly conducting electrodes, large flexible wet‐spun fibers, supercapacitors, lasing, etc. This review focuses on recent advances in liquid crystal field of discotic colloidal liquid crystals of graphene oxide (GO) and reduced graphene oxide (rGO), nanocomposites of DLCs with GO/rGO, their physical properties and various applications.

The ideas, the concept and the importance of the nanoscience started by the statement “ There's plenty of room at the bottom ” by an American Physicist, Richard Feynman, in 1959. 39 Over the past two decades nanostructured materials such as, metallic nanoparticles (NPs), semiconducting nanomaterials like quantum dots (QDs), nano wires (NWs), nanorods (NRs), carbon nanotubes (CNTs), fullerenes and graphene have received tremendous interest in science and technology. Graphene, a one atom thick two dimensional (2D) honeycomb lattice of single atomic layer of sp 2 bonded carbon, which is the basic building block of all graphitic materials including 0‐D fullerenes, 1‐D carbon nanotubes (CNTs), and 3‐D graphite and also is the core of ever‐green ongoing research due to its side lined properties like mechanical, optoelectronical, thermal which leads to a long pathway towards the application of graphene based devices in common life. 40 It has emerged to hold potential applications in light emitting diodes, 41 solar cells, 42 , 43 touch displays, 44 biological labelling, 45 ultrafast lasers, 46 , 47 energy‐storage materials, 48 ‘paper‐like' materials, 49 , 50 polymer composites, 51 , 52 liquid crystal devices, 53 etc.

Increasing the size of aromatic core leads to an extension of π‐orbital's, which enhance π–π overlapping with neighboring molecules along the column and produces high charge carrier mobility and, thus enhance semiconductor properties of DLCs. So, here PAC are playing a crucial role in the construction of highly conjugated semiconducting and photoconductive materials for device applications. 22 , 23 Therefore, it is obvious that during the formation of mesomorphism the aromaticity passes from benzene to utmost graphene. In this contest, the recent discovery of “Nano‐graphene” synthesized from the bottom‐up approach of large polycyclic aromatic cores suggest the same. 24 - 26 Thus, graphene may be considered as the largest DLC core (Figure 1 ). High charge mobility leads to the superiority of these organic economic semiconductors towards the organic light emitting diodes (OLEDs), organic field‐effect transistors (OFET), photovoltaics and sensors. 27 , 28 Owing to their extraordinary electronic and self‐healing properties, these one‐dimensional organic semiconductor systems have been explored for both fundamental and technological importance's which have appeared in several articles. 22 - 38 A number of discotic molecules also form lyotropic LCs.

In DLCs, the rigid core provides crystalline character while the surrounding flexible aliphatic chains cause the liquid‐like nature of the mesophase. The spontaneous arrangement of rigid aromatic cores in one‐dimensional stacks of columns, owing to strong π–π interactions of polycyclic aromatic cores (PAC), exhibits one‐dimensional charge and energy transport. In the columnar structure, owing to strong intracolumnar interactions, the core‐core separation is quite low (∼0.35 nm) while the columns‐to‐columns separation is very high (2–4 nm). Therefore, the self‐assembled columnar structure of DLCs may be considered as quasi‐one‐dimensional molecular wire where charge and energy migration occur in an organized manner ( Figure 2 ). The bandwidth in these materials reaches close to 1.1 eV, similar to graphite, 16 and high charge carrier mobility (0.2–1.3 cm 2 V −1 s −1 ) is reported. 17 - 19 Moreover, simulated studies indicate that a defect‐free assembly of such materials may lead to mobilities in excess of 10 cm 2 V −1 s −1 . 20 Further, the exciton diffusion length in columnar DLCs exceeds to 70 nm, which is higher than known conjugated polycyclic aromatic cores (10–20 nm). 21 Because of these unique features in conjunction with easy processability, these materials may be extremely important in various optoelectronic applications.

It is observed that LC phases are formed either by putting an amphiphilic compounds in a solvent or by thermal treatment on organic substances. The first category is called “lyotropic liquid crystals where the solvent is often water and such liquid crystals are very important in biological systems. For instance, DNA and nano‐DNA form various LC phases when dissolved in water over a range of concentrations and temperatures. 6 , 7 Colloidal suspension of small discotic molecules in solvent is often termed as “chromonic LCs” which have been extensively studied as sensors, optical compensators, cosmetics and drugs. 8 When a compound or a mixture of compounds is heated or cooled to get LC properties, it is called thermotropic liquid crystal. A majority of thermotropic LCs (about 95%) are derived from rod‐like molecules (calamitic liquid crystals) which have found application in flat panel displays, gas sensors, thermal sensors, etc. 9 - 14 From the discovery in 1888 till 1977 these calamitic LCs ruled the field. However, in 1977 an experimental breakthrough came when Chandrasekhar and co‐workers reported the findings of mesomorphic properties of benzene‐hexa‐ n‐ alkanoates 15 and quoted that “…. what is probably the first observation of thermotropic mesomorphism in pure, single component systems of relatively simple plate‐like, or more appropriately disk‐like molecules ”. These are now commonly referred to as “discotic liquid crystals (DLCs)”. DLCs are generally composed of two contrasting parts; a rigid core surrounded by flexible aliphatic chains. Microphase segregation in these two non‐compatible parts leads to mesomorphism. Most DLCs are derived from aromatic hydrocarbons such as benzene, naphthalene, triphenylene, hexabenzocoronene, etc. ( Figure 1 ).

Liquid crystals (LCs) “ Nature's delicate phase of matter ” also referred to as mesophases, usually assert that these materials are intermediate states between crystalline solids and conventional liquids displaying both the order and mobility characteristics at supramolecular, molecular and macro‐nanoscopic levels. 3 LCs coupled with their supramolecular approach to self‐healing and self‐aligning properties have the strong impact on various daily life device applications such as display devices, sensors, etc. These delicate materials are not only having an essential role in material science but also in biological systems. 4 Biomolecules (deoxyribonucleic acid (DNA), enzymes, lipids and proteins etc.), which shows self‐assembly and self‐healing characteristic form various liquid crystalline phases under appropriate conditions. Over the past 125 years, since the discovery of calamitic liquid crystals in 1888 by Reinitzer, 5 who realized a specific unusual double melting behavior of cholesterol benzoate, there have been a growing interest in family of LCs.

Mother nature creates fascinating supramolecular architectures using the approach of spontaneous self‐assembly and supramolecular self‐aligning where multiple small prototypes spontaneously get together to form an accommodated hierarchical organization. This spontaneous encounter or “ get together ” process can be driven by weak interactions like van der Walls, ionic, hydrophobic, hydrogen bonds, coordination bonds and π–π interactions. “ Self‐assembly ” itself indicates its meaning, which is recognized over the past 3–4 decades in natural sciences. 1 , 2

Graphene oxide liquid crystals (GOLC) forms stable LC phase by dissolving in water or organic solvents at different concentrations but it is very important to optimize the concentration of graphene oxide, size of the graphene flakes, monodisperse and polydisperse of graphene nanosheets formation, and modified functional groups which changes the behavior of LC phase formation. Aqueous GO suspensions exhibit nematic LC phase at low concentrations. 54 But in GO the presence of salts and impurities hinder the LC phase formation. Increasing the ionic strength or decreasing the pH deteriorate the repulsive interaction resulting the coagulation of the graphene sheets. 54 It is very important for GOLC formation to reduce the surface repulsion. The GOLC formation depends upon size, shape and density of graphene nanosheets. 67 Polydisperse sheets causes broad biphasic region of GOLC. As the graphene concentration increases, the flakes undergo transitions from an isotropic (I) dispersion to a biphasic system and then to a discotic nematic liquid crystal. 68 Below the onset of percolation graphene flakes self‐assemble into nematic liquid crystals. 69 The large size poly dispersability induces the broad I‐N transition in LC phases, 70 this could be due to the higher aspect ratio of polydisperse graphene in dispersion. Increasing the size of graphene sheets changes the amphiphilicity leading to much lower needed concentration for the formation of the LC phase. 71

Since the first preparation method of graphene oxide in nineteenth century, 57 - 59 a large number of methods have been described for generating graphene and chemically modified graphene from graphite and derivatives of graphite. 60 - 62 Graphene oxide mainly successfully produced by oxidation of graphite by Brodie, 57 Staudenmaier 58 and Hummers 59 methods and these all methods involve chemically oxidation of graphite in presence of strong acids and oxidants. Hummer's method involves oxidation of graphite primarily using sodium nitrate, potassium permanganate, and sulfuric acid, but due to the presence of high amount of evolution of nitrous gas it was modified by Ruoff and co‐workers. 51 The functionality of graphene affect the formation of LC phase. Graphene can be chemically functionalized covalently and non‐covalently. 63 The functionalization of pristine graphene sheets with organic functional groups develops the dispersability of graphene in common organic solvents after attachment of certain organic groups. This organic covalent functionalization of graphene achieved via the formation of covalent bonds between free radicals or dienophiles and CC bonds of pristine graphene and the formation of covalent bonds between organic functional groups and the oxygen groups of GO. Addition of free radicals to sp 2 carbon atoms of graphene via functionalization of nitro benzyl group on graphene opens up new properties of graphene. Tour and co‐workers demonstrated that upon heating of a diazonium salt, a highly reactive free radical is produced, which attacks the sp 2 carbon atoms of graphene forming a covalent bond. 64 Hydroxylated aryl groups grafted covalently on graphene by the diazonium addition reaction which initiates polymerization of styrene on graphene. 65 An alternative free radical addition method includes the reaction of benzoyl peroxide with graphene sheets. 66 Graphite functionalized with randomly distributed aromatic regions (sp 2 carbon atoms) and oxygenated aliphatic regions (sp 3 carbon atoms) containing hydroxyl, epoxy, carbonyl, and carboxyl functional groups. The edge of layers is functionalized with carboxylic group while the epoxy and hydroxyl groups lie above and below each graphene layer. The presence of oxygen groups on the surface of GO introduces chemical reactivity and hydrophilicity.

Since the early 20 th century, the LC behavior of tobacco mosaic virus roads, V 2 O 5 rods based‐anisotropic colloidal systems have been studied extensively. 72 - 74 These anisotropic colloidal particle systems exhibit liquid crystallinity in presence of suitable solvents. 75 , 76 Under a specific temperature and concentration condition the LC phases of anisotropic molecules exhibit dynamic anisotropic molecular scale structures with switchable anisotropic material properties. 77 - 81 From lower to higher concentrations the overlap of excluded volumes causes the colloidal particles to be aligned into a nematic ordering. Anisotropic particles in solvents form colloids which show an isotropic phase at a low concentration, but upon changing the particle concentration it transit into a biphasic mixture with coexisting isotropic and nematic phases. Also the geometric shape of the molecules and particles is the major driving forces for the formation of various LC phases, such as nematic, smectic, cholesteric, and columnar phases. 82 These observations were also observed in graphene based colloidal liquid crystals. GO dispersions of 2D anisotropic colloid exhibit a rich range of arrested states of fluid, glassy and gel states coexisting with LC ordering. Such colloidal particle system forms LC systems, and the perquisite and properties of such colloidal LC systems were studied firstly by Onsager. 83 , 84 Graphene oxide forms anisotropic colloidal liquid crystal system in presence of different solvents such as water, organic solvents, where it serves as a particle in system. The Onsager theory demonstrated the LC phase dependency on critical concentration, size, and shape. Above the critical concentration for the formation of the LC phase, the nematic phase is stabilized. At higher concentrations, anisotropic particles restricted more with large excluded volumes which induces the entropic loss. Such phenomenon was observed in graphene based colloidal systems also, where graphene is an anisotropic particle in solvent.

4 Discovery of Graphene Oxide Liquid Crystals

Since the discovery of the synthesis of graphene oxide from graphite powder by Hummers et al. in 1958, there has been many reports to modify graphite chemically and use in numerous applications of this “Atomic thick chicken‐wire”.85 Several reports on macroscopically dispersion of large graphene flaks in water or polar organic solvents for the formation of graphene oxide liquid crystals (GOLCs) have been appeared.

4.1 LC Formation by Superacid Assisted Spontaneous Exfoliation of Graphene In 2010, Behabtu et al. firstly reported the spontaneously exfoliation of graphene and their liquid crystalline phase formation.86 It was a breakthrough which was firstly attempted to chemically obtain the colloidal liquid crystals of graphene. Pristine graphene can be easily dispersed up to 2 mg mL−1 concentration in chlorosulfonic superacid which exfoliates the graphene without induction of sonication, any covalent functionalization, and surfactant stabilization. To dissolve graphene at higher concentration a lowest amount of protonation is required. It is known that rigid anisotropic molecules show an isotropic‐liquid crystal transition behavior upon increasing the concentration in a common dispersant solvent.87 Similarly such graphene system showed formation of isotropic‐liquid crystalline behavior at high concentrations up to 20‐30 mg mL−1. These high concentration of graphene liquid crystal materials can be an alternatives to replace functionalized carbon thin film in various coatings. The thin films of isotropic solutions with 8 µm thickness showed higher conductivity up to 110 000 S m−1 , with sheet resistance of 1000 Ω sq−1 in an 80% transparent (at 550 nm) film.

4.2 Graphene Oxide Nematic Liquid Crystals In 2011, Kim et al. firstly demonstrated and studied the aqueous phase formation of graphene oxide liquid crystals by mild sonication of aqueous GO dispersion, which was a new equivalent addition to the class of the carbon based liquid crystals.54, 88, 89 As synthesized graphene oxide was dispersed in deionized water which exhibit an inhomogeneous chocolate‐like milky appearance to naked eyes. It was not a precipitation of graphene but was an indication of the formation of a nematic liquid crystal (Figure 3a). A low concentration (0.05‐0.6 wt%) aqueous dispersion of GO was immobilized for more than 3 weeks for the formation of LC phases in dispersion. In Figure 3a it is clearly seen that the upper part symbolizes the isotropic phase while the lower dense chocolate part showed formation of optical birefringence which confirms optically active nematic LC of biphasic behavior (where both isotropic and nematic phases coexist). The phase consists of both, dark and bright brushes in the bottom phase of the sample. Figure 3b is a typical nematic schlieren texture of graphene oxide dispersion of 0.3 wt% concentration which reflect the local orientation of graphene oxide platelets, where ±1/2 disclinations were determined by the rotation director of the brushes (Figure 3c). The local orientation of GOLC phases were explained in Figure 3d, is a Cryo‐SEM (scanning electron microscopy) image of GOLC with 0.5 wt% concentration. It reveals the typical disclinations morphology with continuous nematic phase. Furthermore they established that the concentration values were based on polydisperse hard‐disc modal, and the presence of the impurities in dispersion could lead to deformation of the LC phase. In addition the study also focused on the effect of the external stimuli on the macroscopic orientation of GOLC. When a magnetic field (H, 0.25 T) was applied to the device composed of GOLC, segregated liquid‐crystal orientational domains were initially separated by disclinations, gradually reoriented and then merged into a large domain.54 Figure 3 Open in figure viewer PowerPoint 54 f m 's of 5 × 10−4, 1 × 10−3, 3 × 10−3, 5 × 10−3, 8 × 10−3, and 1.0 × 10−2 (from 1 to 6). (f) GO aqueous dispersions in test tubes with f m 's of 1.0 × 10−4, 2.5 × 10−4, 5 × 10−4, 1.0 × 10−3, 5 × 10−3, 1.0 × 10−2, and 2.0 × 10−2 (from 1 to 7). (g) Schematic models for isotropic (left) and nematic (right) phases of GO aqueous dispersions. Reproduced with permission. 90 (a) (Left to right) 0.5 wt% GO dispersion exhibiting a milky appearance; phase‐separated 0.2 wt% dispersion three weeks after preparation; three phase‐separated dispersions (0.05, 0.2, 0.5 wt%); coagulated 0.01 wt% dispersion upon adding 50 mm NaCl. (b) Typical nematic schlieren texture of a 0.3 wt% dispersion with 1 = 2disclinations and a +1 disclination. (c) Disclination morphologies of GOLC upon successive rotations of crossed polarizers. (d) SEM image of a graphene oxide liquid crystal in a freeze‐dried sample (0.5 wt%). Blue and red symbols indicate +1/2 and ‐1/2 disclinations, respectively. Reproduced with permission.POM (polarising optical microscopy) images between crossed polarizers of (e) GO aqueous dispersions in planar cells with's of 5 × 10, 1 × 10, 3 × 10, 5 × 10, 8 × 10, and 1.0 × 10(from 1 to 6). (f) GO aqueous dispersions in test tubes with's of 1.0 × 10, 2.5 × 10, 5 × 10, 1.0 × 10, 5 × 10, 1.0 × 10, and 2.0 × 10(from 1 to 7). (g) Schematic models for isotropic (left) and nematic (right) phases of GO aqueous dispersions. Reproduced with permission.Copyright 2011, American Chemical Society. In another study, Xu and Gao established a study of aqueous phase nematic liquid crystals of graphene oxide dispersions and their self‐dependent phase behavior.90 They also presented the isotropic‐nematic solid phase diagram versus mass fraction (GO) and salt concentration (NaCl). The GO dispersion with sheet diameter of 2.1 µm and polydispersity index of 83% shows the transition from isotropic to nematic phase, the phase from at 0.025 wt% and reached to maximum stable phase concentration at 0.5 wt%. Figure 3e shows POM textures of GOLC, where the emergence of isolated birefringence can be seen and also the isotropic to nematic phase transition that starts at lower f m values (maximum mass fraction) at 2.5 × 10−4, while this f m value increased, the stable birefringence spreads the whole dispersion along with typical vivid‐schlieren texture of nematic LCs. As seen in Figure 3f, birefringence were absent at the dispersion with f m values at 1 × 10−4 (tube 1) but at f m values at 2.5 × 10−4, the birefringence starts emerging and thread‐like textures indicates the preliminary formation of a nematic phase (tube 2), which closeness to branches at higher concentration with more compactness to the optical textures (from tubes 3 to 7), and at the f m values at 5 × 10−3 , birefringence becomes colorful (tube 5) indicating the formation of a uniform nematic phase. The GOLC phases were further characterized by SAXS (small‐angle X‐ray scattering) to confirm the presence of LC phase, and it was confirmed that with concentrations >10 mg mL−1 higher order lamellar phases could be present in high concentration of the graphene sheets which ascribed to the Bragg's reflection. To calculate the interlayer spacing d = T/Φ (Φ‐ volume fraction, T‐ monoatomic thickness) were applied. The estimated d values (T = 0.8 nm) were around 41.6, 52.0, and 69.3 nm for their respective f m of 2.5 × 10−2, 2 × 10−2, and 1.5 × 10−2. The d values in experimentally obtained by the SAXS diffusive peak and are 45, 52, and 63 nm at f m 's of 2.5 × 10−2, 2 × 10−2, and 1.5 × 10−2, respectively. Which concludes that the d(100) value decreases with increasing f m of GO for the formation of lamellar phase. A schematic illustration also represents I and N phases (Figure 3g). In same contest, Aboutalebi et al. manifested a report on spontaneous formation of LC in ultra‐large graphene oxide dispersions in water at lower concentrations at 0.1 wt%.67 The pre exfoliation of large graphene sheets were without any ultra‐sonication to form large graphene sheets. The formation of liquid crystallinity in graphene occurred at low GO content with ultra‐high aspect ratio, (>30 000). “The reason behind this is the steric hindrance which arises from the overlapping of colloidal particles in concentrated dispersions which further results in an entropy‐driven arrangement, forcing the particles to shape into a long‐range order that resembles liquid‐crystal molecules.” Dan et al. demonstrated similar studies on the formation of lyotropic liquid crystals of giant aqueous flaks of graphene with aspect ratio above 10 000.68 It was intriguing that as the concentration of graphene increases, the large graphene flaks undergo in a transition from an isotropic phase to biphasic system where both isotropic and liquid crystal phase coexist and then to a liquid crystal phase of discotic nematic. This exhibits a gel‐like phase with uniform director along the micrometer scale of large graphene flaks. The higher ratio (D/h 104 and higher) of graphene flaks leads to formation of gel‐like LC behavior instead of previously reported fluidic nematic liquid crystals of graphene and GO.54, 77 The samples with concentrations of 0.03 wt% showed no significant polarization and birefringence under the polarized optical microscope (Figure 4a) but the birefringence at very lower concentrations (0.05 wt%) can be seen (Figure 4b), and above 0.1 wt% entire sample display an entire range of fully covered birefringence at various stages of rotation angles which further proved that the single‐LC phase is present in the system with further gel‐like formation at very high concentrations (Figure 4c,e). The appearance of uniform brightness in entire surface indicates that a well‐aligned director (n) scales is present, and also that the giant GO flakes are well directionally oriented perpendicular to the wide top and the bottom sides of the rectangular capillary. In such large domains Frank elastic constant is calculated and is 100 ‐fold higher compared to low‐molecular‐weight discotic nematics. Figure 4 Open in figure viewer PowerPoint 68 92 (a–c) Optical microscopy images obtained without analyzer (left) and between crossed‐polarizers (right) of GGO aqueous suspensions, demonstrating phase behavior vs concentration: (a) isotropic (0.03 vol %), (b) biphasic (0.07 vol%) and (c) nematic (0.14 vol%). The scale bar in (a–c) is 10 mm. (d) Phase behavior vs concentration. (e,f) POM of GGO nematic phase, inside a capillary, (g) schematic showing the alignment of flakes. Reproduced with permission,(h) Phase diagram depicting the glass, gel and LC arrested states in GO aqueous dispersions. (i)Variation of the state of GO dispersions with increasing salt concentration. Reproduced with permission.Copyright 2014, American Chemical Society. 87 71, 91 91 71 3) was ρ iso D3 = 2.7 and ρ nem D3 = 4.3 for isotropic and nematic transition concentrations, respectively. 78 −1, but POM investigations of 0.25 mg/mL concentration showed formation of nematic liquid crystals which is higher than theoretical calculated values which is due to the wrinkling behavior of the flexible GO sheets. They found that at concentration of 2.5 mg mL−1 and sheet size of >1.5 mm, the aqueous dispersion display complete nematic nature with notable yield. Such GOLC dispersions can be used for fabrication of wet‐spinning fibers. This LC phase can be introduced in smaller size of sheets, the mixture by adding small amount of ultra‐large GO sheets into isotropic dispersion (4 wt% large GO). At lower concentration of 1.3mg/mL compared to 2.5 mg mL−1, much lower dispersions (1.3 mg mL−1) display birefringent and also introduced partial spinnability. These results suggested that the small dispersion of GO sheets can control and induce liquid crystallinity and these GOLC used to fabricate the wet‐spinnable fibers. Similarly if the mass fraction of large GO to small GO sheets (from 4% wt to 10% wt) increased, spinnable LC GO dispersion can be achieved. The rheological behavior and applications of such graphene spinal fibers in textiles will be discussed later in application part. (1) It is very difficult to understand the self‐assembly process involved in formation of GOLC phases. Upon the formation of LC phase in graphene it was revealed that the structure and properties of GOLC is especially vulnerable on sheet concentration, temperature, size, shape and density of the graphene sheets. The fundamental process involved in the formation of the nematic liquid crystals is “entropy‐driven” as modal and theoretical prediction were provided by Onsanger.In order to understand the formation and processing of the graphene oxide liquid crystals and considering the entropy driven formation of nematic isotropic phase, Jalili et al. reported that GO are flexible where entropy is associated with configuration alsoand in the nematic phase the loss in configurational entropy is associated to GO sheets which are parallel to the director. To understand the graphene concentration and their processebility for liquid crystal formation in water with different sheet aspect ratios, they calculated a critical size for the dispersion of the graphene sheets responsible for the formation of isotropic‐nematic phase transition.Based on rheological and POM investigations, they proposed a rational relation between GO sheet size and polydispersity, concentration, liquid crystallinity, and spinnability.The Equation 1 is a model system for LC phases of charged colloidal platelets which gives the critical theoretical volume fraction (Φ). The polydispersity (σ) of the system was determined by dividing the standard deviation of the diameter distribution (23 µm) by the mean (37 µm). The experimentally calculated dimensionless number density (ρD) was ρ= 2.7 and ρ= 4.3 for isotropic and nematic transition concentrations, respectively.The equation gives a theoretical biphasic region between 0.05 and 0.09 mg mL, but POM investigations of 0.25 mg/mL concentration showed formation of nematic liquid crystals which is higher than theoretical calculated values which is due to the wrinkling behavior of the flexible GO sheets. They found that at concentration of 2.5 mg mLand sheet size of >1.5 mm, the aqueous dispersion display complete nematic nature with notable yield. Such GOLC dispersions can be used for fabrication of wet‐spinning fibers. This LC phase can be introduced in smaller size of sheets, the mixture by adding small amount of ultra‐large GO sheets into isotropic dispersion (4 wt% large GO). At lower concentration of 1.3mg/mL compared to 2.5 mg mL, much lower dispersions (1.3 mg mL) display birefringent and also introduced partial spinnability. These results suggested that the small dispersion of GO sheets can control and induce liquid crystallinity and these GOLC used to fabricate the wet‐spinnable fibers. Similarly if the mass fraction of large GO to small GO sheets (from 4% wt to 10% wt) increased, spinnable LC GO dispersion can be achieved. The rheological behavior and applications of such graphene spinal fibers in textiles will be discussed later in application part. In 2014, Konkena and Vasudevan92 presented a study clearing the fact that GO dispersions at various concentrations and with different salt concentrations can behave as a frustrated class of 2‐D anisotropic colloid with arrested states. They demonstrated that these arrested states could be easily accessed by changing the volume fraction and/or ionic strength of the salt. They also studied that by changing the concentration of the salt where the relative magnitude of the repulsive and attractive forces differ and plays a role in formation of these “specific arrested' states (Figure 4h). This modal proposed that at low salt concentrations the dominant force is long‐range electrostatic repulsion that transforms the fluid state in a repulsive “Wigner glass arrested state”, while at high concentrations the dispersion undergoes from a fluid to gel‐like formation and in this state the attractive forces dominate results to the formation of gels which displays a nematic to columnar liquid‐crystalline transition. (Figure 4i) In another study, Xu et al. studied the formation of liquid crystalline phase in monolayer, bilayer and multilayer of aqueous dispersion of GO.93 They also observed LC behavior in soild‐state of GO. Different thicknesses of GO were used; typically, for monolayer GO‐12 (1 nm), bilayer GO‐6 (1.6 nm) and multilayer GO‐1 (2 nm), respectively. Different concentrated samples were used in study and GOLC retained schlieren texture. The uniformity of the spacing between flakes was revealed in SAXS study and was at scattering peak at 0.09 nm−1. A drop‐casted thin film of aqueous dispersion of concentrated GOLC fabricated and showed optimized electrical conductivity of 10 S cm−1. Such composites can be sued in functional energy materials, electronic circuits etc.

4.3 Giant Graphene Oxide Liquid Crystals Various reports on dispersion of graphene oxide sheets with few layers for the formation of liquid crystals at various concentrations, salt concentrations have been reported but it was very interesting that large monolayers exfoliated sheets from graphite oxide (GtO) can also exhibit property of the formation of liquid crystallinity. To account this concept, in 2014 Tong and co‐workers established a study and reported that these giant GtO can also have a possibility to exhibit long range lamellar liquid crystallinity.94 GtO is a layered material which forms aqueous colloidal suspensions.95 They prepared a GtO with modification with the distribution of lateral width of 0.2 mm to 31.7 mm and thickness of 0.86 nm to 62.87 nm to perform the liquid crystallinity. They found that at high concentration upto 1.45 wt% aqueous dispersions of GtO, LC phase can be formed which can be clearly seen in Figure 5a of typical schlieren texture of nematic phase exhibiting both the dark and bright brushes. These LC phases were obtained directly by chemical oxidation of graphene and without any sonication. In Figure 5a a “vivid‐wave‐like pattern with typical fan‐like texture” for the nematic phase is clearly observed which is correspond to typical columnar mesophase or lamellar LC phase. This fanlike texture confirmed the high anisotropy suggesting a coherent long‐range ordering in GtO, which is further characterized in freeze‐fracture morphology Cryo‐SEM, TEM (transmission electron microscopy) and SAXS. Figure 5b is showing the parallel flakes which are perpendicular to the surface with disclination value of s = +1/2 which demonstrates that the GtO flaks were bent along the director orientation and this orientation forms the liquid crystalline anisotropy. On the basis of these result a modal represented in Figure 5c, which reveals that GtO layers assembled in a lamellar structures. Upon external electrical (50 V) field these system exhibits a different texture under POM, the optical textures sharply changes from green pattern to yellow pattern due to change in orientation. These excellent alignment properties of such materials makes them interesting candidate for promising optoelectronics applications. Figure 5 Open in figure viewer PowerPoint 94 (a) POM images of graphite oxide aqueous dispersions exhibiting the fan‐like LC textures. (b) SEM images with wave‐like textures formed by freeze‐dried GtO LC samples. c) Proposed lamellar structural model to explain the hierarchical self‐assembly of GtO LCs. Reproduced with permission.Copyright 2014, Royal Society of Chemistry.

4.4 GO Liquid Crystals in Organic Solvents It is clear that large graphene sheets and flakes form lyotropic nematic liquid crystalline phase upon dispersion in water. To extend this work, recently Wallace et al. manifested the formation of colloidal graphene oxide liquid crystalline phase amphiphilic self‐assembly in wide range of organic solvents like ethanol, acetone, tetrahydrofuran (THF), N,N‐dimethylformamide (DMF), N‐cyclohexyl‐2‐pyrrolidone (CHP) by dispersing ultra large graphene sheets.96 As these solvents are common for the processing of various polymers, the dispersion and formation of LC phase in large graphene sheets provide a new platform for the production of self‐assembled novel functionalized GOLC composites. The POM micrographs of these organic phase dispersion of graphene sheets are clearly revealing the formation of birefringence and formation of lyotropic LC phase (Figure 6a–f). For these organic solvents the isotropic to nematic phase transition was found in the range of 0.25‐0.50 mg mL−1 concentration of graphene sheets. These organic phase dispersions were stable for several months without any phase degradation. This is due to the more adaption of solvent in‐between graphene sheets for balancing the steric and repulsive force which further enables graphene to form supramolecular interactions like H‐bonding. This further compensate the loss of rotational entropy occurs in the self‐assembly process and also with confined solvent molecules and electrostatic repulsion among two neighboring graphene sheets. Furthermore, these GOLCs were used to introduce liquid crystallinity in single wall carbon nanotubes (SWCNT) dispersions. The maximum concentration of SWCNT dispersion in GOLC for the formation of birefringence and nematic LC property was 10 wt%. The water‐free nature of these GOLCs in organic solvents enables the SWCNT to assemble and the SWCNTs do not agglomerate and crash out of the dispersion. Such LC formulation in GOLC‐SWCNT composites can be used for the fabrication of layer‐by‐layer 3‐D assemblies. Figure 6 Open in figure viewer PowerPoint 96 ‐1) demonstrating lyotropic LC phases, and (o–q) micrographs of GO dispersions at lower concentrations (0.5–1.5 mg ml−1) showing isotropic and biphasic textures. Reproduced with permission. 97 (a–f) POM images of LCGO in various organic solvents at a GO concentration of 2.5 mg/mL. (a) Water, (b) DMF, (c) CHP, (d) THF, (e) acetone, and (f) ethanol. Reproduced with permission.Copyright 2013, American Chemical Society. (g–q) (g) Chemical transformations induced by the reaction of graphite oxide and NMP or other polar aprotic solvents, (h) Graphite oxide flakes (i) a gel‐like dispersion of GO formed upon addition of NMP, (j) 0.2 wt% GO dispersion in DMF showing anisotropic texture, (k–n) Optical micrographs of GO in organic solvents (2 mg ml) demonstrating lyotropic LC phases, and (o–q) micrographs of GO dispersions at lower concentrations (0.5–1.5 mg ml) showing isotropic and biphasic textures. Reproduced with permission.Copyright 2013, Elsevier. In another study, Sharif et al. encouraged the selection of organic solvent for the formation of monolayers sheets of graphene oxide via exfoliation.97 It is very critical for the formation of nematic lyotropic LC phase in very large graphene sheets with a high aspect ratio and also this process required an intense amount of the ultra‐sonication to exfoliate graphene in organic solvents. They reported the formation of lyotropic nematic liquid crystalline phase in 0.1 mg/mL dispersion of graphene in various organic solvents like DMF, NMP (N‐methyl‐2‐pyrrolidone) and suggested that these polar aprotic organic solvents induce chemical transformations, which involves the ionization of GO sheets without any sonication and mechanical agitation. (Figure 6g). In addition, when aprotic solvent was added to the GO, it becomes yellow‐dark‐brown viscous gel like structure (Figure 6h,i) at concentration higher than 0.5 wt%. The graphene flaks were easily dispersible in protic solvents like ethanol, methanol etc. by means of solvent exchange method without any further sonication process and results in the spontaneous formation of nematic lyotropic GOLC formation at very low concentrations (0.1 wt%) (Figure 6j‐n). Also the isotropic‐biphasic transition was found in these GO dispersions at concentration range of 0.025–0.3 wt% (Figure 6o‐q). The formation of LC phase was not hindered by increasing the pH strength of the solution in protic solvents. The application of such assemblies in conjunction with polymer matrix can be employed for the fabrication of 3‐D self‐assembled layered composites with good electrical conductivity up to 1050 S m−1.

4.5 Aqueous LCs of Reduced Graphene Oxide The dispersion of graphene oxide flaks in water can spontaneously form liquid crystals. This process makes graphene an excellent candidate for future supplication in optoelectronic but due to occurring of the oxidation process most of graphene electronic functionalities were lost during treatment. In order to minimize this loss and to see the formation of LC phases in aqueous dispersion of reduced graphene oxide, Poulin et al.98 realized that by stabilizing the post reduction of graphene in the presence of bile salts (BS) removes the aggregation of rGO sheets and stabilizes the rGO flakes against the aggregation and retain the water‐based liquid crystallinity. The liquid‐crystal phases are observed up to large dilution levels of about 1.0 to 1.5 wt % of rGO. This diluted boundary of the isotropic to liquid‐crystal transition is due to the large aspect ratio of the rGO flakes.99, 100 The LC structures were characterized by SAXS which confirms that the exfoliation of single‐layer graphene with thickness of 0.5 nm. The POM characterization confirms the formation of LC phases. Birefringence and typical nematic textures were observed for concentrated GO and rGO‐BS materials at 6.6 and 5.9 wt%, respectively. In the absence of bile salt, the rGO was unstable and found to form aggregates and does not show any liquid crystal transition.

4.6 pH Dependent Formation of GOLC The formation of LC phases by the dispersion of GO in solvents also depends on the pH of the solution. Majumdar and co‐workers reported that in graphene oxide dispersion isotropic to nematic phase transition is pH dependent.101 In biphasic regime, a highly organized perfect‐spherical droplet‐like organization of nematic phase of uniform size (20 ± 2.8 µm) was realized. It is known that charge functionalized groups like carboxylic and hydroxyl groups present on the edge and basal planes show protonation or deprotonation upon changing the pH. These functional planes and the present groups on these planes strongly dependend on the pH. The pKa of GO is 4 for the carboxylic groups and 9 for the phenolic groups.102 Nonetheless, at very lower pH the dominating forces like H‐bonding between H 2 O and functionalized –OH groups (oxidized) acts and stabilize the aqueous GO suspensions.103 Following this they have studied the I‐N phase transition by the salt concentration and influence of pH. Different samples by dispersing graphene in water in nine concentrations between 1 and 20 mg/ml and five pH values (1, 2, 6, 9 and 14) were made for studies. The results reveal that at pH 1 and 14 there was no formation of any LC phase which is due to the GO aggregation, while at pH 2, 6 and 9 complete phase separation was observed where bottom part exhibits nematic and top part exhibits isotropic phase. It was cleared that at pH 4, the pKa of ‐COOH groups grafted via functionalization on graphene oxide responsible for the depletion interaction in sheets. This interaction segregates the large GO (515 nm ± 76%) sheets into the nematic phase and smaller (330 nm ± 51%) ones into the isotropic phase. They also noticed that formation of GOLCs happened in small size of GO‐H 2 O droplets. It is a key factor that how to organize large graphene oxide sheets into highly oriented textures of liquid crystals at low concentrations. To encounter this phenomenon, very recently Shi and co‐workers established a method to induced liquid crystallinity in GO suspension at low concentration (3.2 mg/mL) by increasing the pH value using KOH.104 The aspect ratio of the GO sheets was 3500 which was sufficient large to construct the liquid crystalline phase. Different concentrations of GO were studied using POM to corroborate the liquid crystalline phase construction. At graphene oxide concentration of 0.5 mg mL−1 (0.132 m KOH) rod‐like nematic texture were noticed. But when the concentration of GO is increased to 0.25 mg mL−1 an isolation happened in birefringence domains which continuously associated with each other (Figure 7a). Also, at higher concentration of 3.5 mg mL−1 ordered lamellar liquid crystalline features which were highly aligned and gradual throughout the dispersion were realized (Figure 7b) compared to pure GO LC. The effect and comparison to clarify whether K+ or OH− ions induced the texture transformation, a 3.5 mg mL−1 GO suspension containing 0.132 M KCl characterized which showed more disordered microstructure (Figure 7c). In KCl‐GO suspension a large amount of flocculated particles precipitated because of neutralizing the electric double‐layer of colloidal GO sheets by K+ ions revealing that the induction of liquid crystallinity in graphene dispersion was dominant by OH− ions. Figure 7 Open in figure viewer PowerPoint 104 105 105 106 POM images of (a) GO, (b) GO + KOH, and (c) GO + KCl suspensions, respectively; Scale bar = 400 µm. Reproduced with permission.Copyright 2016, Wiley‐VCH. (d‐g) POM images of GO LCs with successively increased volume fractions (φs) of 0.23, 0.38, 0.60, 0.76%. (h) Top‐view SEM images show the fracture morphology of GO CLCs at volume fraction φ 0.98% confined in a circular cavity. (i) The screw dislocation of neighboring GO lamellar blocks with twist vectors and (j, k) POM images between crossed polarizers of GO CLCs in central (j) and lateral (k) domains. Reproduced with permission.Copyright 2011, Nature Publishing Group. Schematic illustrations for (l) one pitch of GO CLCs. The vectors (ns) of the lamellar blocks rotate anticlockwise along the helical axis. Reproduced with permission.Copyright 2011, Nature Publishing Group. (m) Helical lamellar phase for GO‐g‐PAN dispersions in DMF. The blue strings indicate grafted PAN chains, and GO sheets are simplified as yellow meshes. Reproduced with permission.Copyright 2013, American Chemical Society.