Footsteps of graphene filled polymer nanocomposites towards efficient membranes—Present and future

: Due to rising global environmental challenges, air/water pollution treatments technologies especially membrane techniques have been focused. In this context, air or purification membranes have been considered effective for environmental remediations. In the field of polymeric membranes, high performance polymer/graphene nanocomposite membranes have gained increasing research attention. The polymer/graphene nanomaterials exposed several potential benefits when processed as membranes. This review explains utilizations of polymer and graphene derived nanocomposites towards membranes formation and water or gas separation or decontamination properties. Here, different membrane designs have been developed depending upon the polymer types (poly(vinyl alcohol), poly(vinyl chloride), poly(dimethyl siloxane), polysulfone, poly(methyl methacrylate), etc.) and graphene functionalities. Including graphene in polymers influenced membrane microstructure, physical features, molecular permeability or selectivity, and separations. Polysulfone/graphene oxide nanocomposite membranes have been found most efficient with enhanced rejection rate of 90%–95%, high water flux >180 L/m 2 /h, and desirable water contact angle for water purification purposes. For gas separation membranes, efficient membranes have been reported as polysulfone/graphene oxide and poly(dimethyl siloxane)/graphene oxide nanocomposites. In these membranes, N 2 , CO 2 , and other gases permeability have been found higher than even >99.9%. Similarly, higher selectivity values for gases like CO 2 /CH 4 have been observed. Thus, high performance graphene-based nanocomposite membranes possess high potential to overcome the challenges related to water or gas molecular separations.


Introduction
Generally, membrane based technologies have been used to remove toxic nanomaterials from environment [1].Among membrane materials, polymeric based materials and nanomaterials have been adopted for the separation purposes [2,3].Owing to technical benefits of nanocarbons, graphene, fullerene, and carbon nanotube have been adopted as unique and valuable nanostructures [4,5].Graphene derived nanomaterials possess fine tendencies towards separation applications [6].Especially, graphene has been reinforced in polymers to form the high performance nanocomposite membranes [7].The polymer and graphene derived nanocomposite membranes have been efficiently used for the separation of hazardous molecules [8].For the fabrication of polymer/graphene membranes, facile processing approaches have been used [9].Solution processing, phase inversion method, infiltration technique, and other facile methods have been reported [10,11].The polymer/graphene membranes have been developed using polymers like polyamides, polysulfone, poly(dimethyl siloxane), poly(methyl methacrylate), and several others [12].The nanocomposite membranes own superior nanofiller dispersion, pore sizes, molecular permeation, and selectivity properties [13].Owing to effective characteristics, polymer/graphene nanocomposite membranes have been applied for technological sectors focusing water, gaseous, and chemical separations [14].The resulting membranes have been applied for the commercial scale water purification systems, gas sensing, and separation systems, fuel cells systems, and myriad of other technical areas [15,16].Most importantly, the polymer and graphene based nanocomposite membranes have been fabricated for the gas and water purification.Graphene mostly develop torturous pathways in the matrices to facilitate the gas or water based ionic or molecular diffusion processes [17].Homogeneous graphene dispersion in polymeric membranes has been used to enhance the separation of impurities and toxic molecules from air mixtures or contaminated water [18,19].In this concern, several membrane processes have been studied like nanofiltration, microfiltration, ultrafiltration, and reverse osmosis [20][21][22].Afterwards, graphene derived nanocomposite membranes have been efficiently used for removing pollutants [23].The polymer/graphene nanocomposite membranes own structural advantages relative to reported nanocomposite membrane designs in terms of facile processing and resulting performance benefits [24].Research progressions have led to the advancements of efficient air/water membranes [25].
For efficient graphene membrane fabrication, membranes, mechanisms of molecular transport need to be thoroughly understood.The self-standing nanocomposite membranes must be researched for new design novelties [26].In addition to graphene, graphene derivatives like graphene oxide, reduced graphene oxide, etc. may widen the potential of these membranes.The ultimate thinness of the membranes has been desirable to allow high flux [27].The narrow pore size distribution and surface chemistry have been identified as desirable factors to promote molecular sieving and diffusion through the membranes.According to literature, the interlayer spacing between graphene nanosheets can promote molecular transportation through the membrane [28].To better withstand the high temperature, pressure, and humidity conditions, membrane support materials must be used [29].Such efforts fill gaps between the membrane designs-to-large-scale productions-to-commercialization of the novel graphene nanocomposite membranes.
This review article discusses the developments of graphene nanocomposites towards efficient membrane applications.Inclusion of graphene in polymers has improved the membranes permeation and selectivity properties towards the water purification and gaseous molecular separation.In this article, design, structure, and properties of polymeric membranes filled with graphene or graphene oxide nanofillers have been scrutinized.Subsequently, the microstructure, durability, stability, permeability, and other membrane properties have been used.The competence of nanocomposite membranes has been studied for gas or water purification, especially for the separation of toxins, pollutants, and unwanted species.The polymer and graphene based nanomaterials have high surface area and exceptional structure for efficient membrane performance.Accordingly, formation of polymer/graphene membranes has extended the scope towards air purification and water management.
To the best of the knowledge, this article is ground-breaking presenting efficient graphene based membranes.The review outline, included literature, as well as relevant discussions are novel and based on recent research assumptions for graphene based membranes.Moreover, hardly any recent topical comprehensive review reports observed on the polymer/graphene nanocomposite membranes.Need of this review article also arises due to remarkably increased research reports on the graphene nanocomposite membranes in past two-three years.Hence, there is utmost need of a recent innovative review on polymer/graphene nanocomposite membranes.According to recent reports on polymer/graphene nanocomposite membranes, it can be stated that substantial progressions have been made in this field up till now.This article will be definitely beneficially needed for the field scientists/researchers to expand the research towards the future success of high performance industrial scale nanocomposite membranes.

Graphene
Graphene is a one atom thick nanosheet of sp 2 hybridized carbon atoms [30].Figure 1 shows the structure of graphene and related derivative forms.This remarkable nanocarbon was discovered in 2004 [31].Graphene has been developed using various techniques such as graphite exfoliation, plasma process, chemical vapor deposition, and chemical or organic synthetic strategies [32].It is a transparent carbon nanostructure [33] having high thermal conductivity of 3000-5000 W/mK [34], Young's modulus of ~1 TPa [35], and weak van der Waals forces [36].Graphene oxide is a graphene based nanocarbon, which is usually formed through oxidation and stripping of graphite.This graphene derivative has graphene nanosheet structure with carboxylic, hydroxyl, carbonyl, or other oxygen containing surface groups [37].Graphene and graphene oxide have been employed to form the nanocomposite materials [38].Graphene nanocomposites have been studied for high electron conductivity, thermal and chemical stability, and physical properties [39].In addition, high-tech potential of graphene has been developed for coatings, membranes, energy devices, and biomedical sectors [40].
Graphene is composed of a single carbon atom layer arranged in two-dimensional honeycomb lattice.Wide range of two dimensional nanomaterials like graphene, graphene, nanoclays, MXenes, silicane, hexagonal boron nitride, transition metal dichalcogenides, etc. have been reported.The two dimensional carbon nanostructure like graphene has probabilities to tailor and functionalize through surface defects, modified groups, number of layers, morphology, etc. Graphene have been explored for the doping, modification, strength, conductivity, and other physical characteristics, relative to other two dimensional nanomaterials.Consequently, graphene has been found stronger than other zero, one, or two dimensional materials due to their structural strength.Especially, compared with other carbon nanomaterials like carbon nanotube (one dimensional) and fullerene (zero dimensional), graphene nanosheets have revealed remarkable potential due to high surface area nanosheet nanostructure with better compatibility with polymer matrices.Research has realized the importance of graphene two dimensional monolayers in several fields with a special emphasis on the benefit to our society.

Polymer nanocomposite based membranes
Polymer based membranes have been produced using the range of polymer matrices and preparation approaches [41].Numerous carbonaceous nanoparticles and inorganic nanofillers were filled in the different polymeric matrices to develop the nanocomposite membranes [42].Combination of these nanoparticles with polymers resulted in the formation of membranes having significant physical characters [43].The resulting properties depend upon the nanoparticle type, nanoparticle amount, and polymer types as well [44].These nanocomposite membranes have fine microstructure and separation properties for different type of molecules [45,46].In other words, polymeric nanocomposite membranes act as molecular sieves for molecular separations [47].The molecular permeation mechanisms depend on the interactions between the polymer and nanoparticles and their mutual effects [48].In inorganic nanoparticle filled membranes, silica nanoparticles have been used as nanofillers [49,50].For silica filled membranes, permeation and selective separation of O2, CO2, and N2 gaseous molecules have been studied.Functional silica nanoparticles filled poly(vinylidene-fluoride-hexafluoropropylene) membranes were prepared through phase separation method [51].The molecular separation for CO2 molecules was studied.The nanocomposite membrane with 40 wt.% nanoparticles revealed CO2 uptake of 33.75 mg/g [52,53].Silica nanoparticles developed fine pathways for gas diffusion [54,55].Titania nanofiller [56] and zinc oxide nanoparticle [57] have also been used with the polymers.Such membranes may have high structural robustness and CO2/H2 selectivity of 2.77.
In addition, carbon nanoparticles have been reinforced in the polymeric membranes [58].Various nanocarbon nanoparticles have been used as efficient nanofillers with polymers like carbon nanotube, nanodiamond, fullerene, etc. to form the nanocomposite membranes.Graphene based nanomaterials own high surface area to volume ratio, light weight, facile processing, and structural flexibility [59].Inclusion of very minor amounts of graphene nanofiller in the nanocomposites has found to enhance the physical features due to interfacial properties [60].Interfacial bonding has been found to directly affect the mechanical and thermal properties of the nanocomposites.As compared to zero and one dimensional nanocarbon like fullerene and carbon nanotube, graphene nanostructure has advantageous two dimensional nanostructure with light and strong nanosheet nanostructure and intrinsic charge mobility and permeability features [61].Therefore, graphene nanocomposites reveal range of potential applications and remarkable properties from high performance nanocomposites to technical nanostructures like membranes.In nanocomposites membranes, graphene has better alignment, dispersion, porosity, and tortuous pathway formation than one dimensional nanostructures for better molecular permeability [62].The polymer-based nanocomposite membranes have been applied in the water purification and gas permeation purposes [63][64][65].

Efficiency of graphene nanocomposite membranes for water or gas separations
Owing to lack of inherent robustness, structural, and fouling drawbacks, polymeric membranes have been continuously replaced with the nanocomposite membranes for better performance [66,67].Consequently, nanocomposite membranes have been recognized for controlled and advantageous thermal stability, selectivity, and permeability features [68].For water remediation applications, solution processing, blading, phase separation, and related membrane fabrication strategies have been focused [69].The membrane matrices like poly(vinyl alcohol), polysulfones, nylons, and numerous others [70,71].Graphene and graphene oxide nanofillers have been used for the development of efficient membranes.Polystyrene, polysulfone, and polyethersulfone have been widely used as ultrafiltration membranes due to fine strength, durability, pH operating range, and chemical stability [72].Though, their uses in water treatment have been restricted due to hydrophobicity and related reduced permeability properties.Widely used ultrafiltration polymeric membrane materials have hydrophobic properties.Poly(vinyl fluoride), poly(vinyl chloride), and poly(methyl acrylic acid) have been adopted for these membranes.Membrane hydrophobicity has been found to decrease the water flux due to the organic compound accumulation on the membrane surface.In this regard, polymer modification has been suggested to induce membrane hydrophilicity to enhance the membrane antifouling properties for enhanced water filtration processes.Future of polymer based water treatment membranes relies on adoption of new modified polymer matrices as well as using nanoparticle nanofillers in the nanocomposite matrices.
Mosty membranes have been used to remove the soluble and non-soluble impurities through the processes of ultra-filtration, reverse osmosis, nanofiltration, microfiltration, etc. [73].Ultrafiltration membranes have pore sizes of 0.01-0.1 µm, which are smaller than microfiltration membranes.However, these pores are larger than the pores of nanofiltration (0.0001 µm) and reverse osmosis membranes.Nanofiltration has been used to remove small organic molecules like viruses.Ultrafiltration has been found to remove the bacteria, microbes, and suspended solids from the water.Reverse osmosis works like filter media which attracts contaminants.The efficiency of the membrane filtration processes depends on the polymer type, surface functional groups, and physical characteristics of the polymeric membranes.The polymer modification has been used to attain the efficient membrane separation processes and desired membrane performance.The modification may involve the incorporation of copolymers and nanoparticles into the polymeric membranes to form the blends or nanocomposites.
For the fabrication of polymer/graphene nanocomposite membranes, efficient techniques have been used [74,75].Solution casting technique follows the principle of Stokes' law [76].In this technique, polymer is dissolved in solvent.The nanoparticles are also dispersed in solvent.Both the polymer solution and nanoparticle solution are mixed to form a homogeneous phase.Later, the solution phase is evaporated to from a polymer film or membrane.Phase inversion technique has also been focused for polymer/graphene nanocomposite membranes [77].In this process, controlled transformation of polymer is performed from liquid to solid phase.Consequently, steps like precipitation, controlled evaporation, and immersion precipitation are involved in this method.Furthermore, the polymer/graphene nanocomposites have been formed by interfacial polymerization [78].Interfacial polymerization involves steps like oil phase, emulsification and solvent evaporation.All these technologies have been used to form the nanocomposite membranes with finely dispersed graphene and derived nanofillers.
Consequently, poly(vinyl alcohol) and poly(vinyl chloride) matrices have been considered important as important matrices for the graphene nanofillers [79][80][81].Production and properties of poly(vinyl chloride) and graphene oxide nanocomposite membranes have been produced through the phase inversion method [82].According to microscopic studies, these membranes revealed macro-void structure.The nanocomposite membranes were investigated to remove bovine serum albumin from water.The separation performance was observed due to hydrophilic nature of the membranes.Poly(vinyl alcohol) matrix has been filled with the graphene or graphene oxide nanoparticles [83].These nanomaterials based on poly(vinyl alcohol) and graphene oxide possess hydrogen and hydrophilic binding interactions.Moreover, the membrane design based on poly(vinyl alcohol) and graphene oxide was reported by Castro-Muñoz and researchers [84].The poly(vinyl alcohol)/graphene oxide based mixed matrix membranes were prepared using the dense-film casting and solvent evaporation methods.Inclusion of 1 wt.% graphene oxide in membrane resulted in permeate flux of 0.14 kg m −2 h −1 .Sun and co-workers [85] used the pressure assisted filtration process for the production of poly(vinyl alcohol)/graphene oxide system.Including 10 wt.% nanoparticles caused superior water flux and salt rejection of 98 kgm −2 h −1 and 99.9%, respectively.Thakur and co-researchers [86] utilized direct laser writing method to form laser induced graphene based three dimensional porous carbon nanomaterial.Three dimensional laser induced graphene had superior electron conductivity.Then, nanocomposite of poly(vinyl alcohol) and laser induced graphene have been prepared for the formation of membranes.The poly(vinyl alcohol)/laser induced graphene nanocomposites own fine mechanical, physical, and surface wettability characters.Figure 2 displays a scheme for development of poly(vinyl alcohol) and laser induced graphene nanocomposite derived nanocomposite based water treatment membranes for nt.Consequently, the ultrafiltration poly(vinyl alcohol)/laser induced graphene nanocomposite membranes had shown separations of solute particles and bacterial species.Initially, laser induced graphene was coated on a polyethersulfone substrate.Then, the laser induced graphene was coated with the poly(vinyl alcohol) to form the nanocomposite membrane.Scanning electron microscopic studies on laser induced graphene and poly(vinyl alcohol)/laser induced graphene nanocomposite membranes revealed development of porous three dimensional network with consistent pore size distributions.The rejection rate was found as 99.9%.
The polysulfone and graphene derived nanocomposites formed some advantageous membrane design combinations [87].Zinadini et al. [88] reported on the polysulfone and graphene oxide derived membranes.Addition of nanoparticles to the membrane systems resulted in unique microstructure and high water flux [89].The polysulfone/graphene oxide derived membranes revealed contact angle of 55°-65° and water flux of >20 kg/m 2 h.Hydrogen bonding interactions have also been observed between the polysulfone matrix and graphene oxide leading to the formation of efficient hydrophilic membranes.Here, wet phase inversion method has been preferred for the fabrication of polysulfone and graphene oxide derived membranes [90].Rezaee and co-workers [91] reported on the polysulfone/graphene oxide nanocomposite membranes using solution technique.Table 1 demonstrates influence of adding graphene oxide amounts on the pure water flux, porosity, and pore structure of the membranes.Enhancing the graphene oxide contents from 0.5 to 1 wt.% enhanced the pure water flux from 20 to 50 L/m 2 h.The membrane porosity was also enhanced from 78% to 87%.Table 1.Effect of GO content on pure water flux and pore structure parameters of the prepared membranes [91].GO = graphene oxide; PSF = polysulfone; PSF/GO = polysulfone/graphene oxide.Reproduced with permission from Springer (Creative Commons CC BY).Adding 1 wt.% graphene oxide amount caused higher pure water flux, porosity, and pore diameter properties, relative to the neat polymer and other nanofiller loaded membranes.The effect of change in pH on the rejection rate was studied for the membranes with different nanofiller contents (Figure 4).Better arsenate rejection performance was observed for 1 and 2 wt.% nanoparticle loading.Consequently, the nanofiller addition caused high separation efficiency due to homogeneous membrane structure, morphology, pore sizes, and optimum porosities [92].Hence, polymer/graphene and polymer/graphene oxide nanocomposites have been studied for the water remediation and filtrations [93].However, these membranes need further research efforts to resolve the challenges of low membrane stability and fouling effects.Tulugan et al. [94] formed polysulfone/graphene nanocomposite derived nanofiltration membranes.water flux of neat polysulfone membrane (33.2L/m 2 /h) was improved with the inclusion of graphene to 183.6 L/m 2 /h.Moreover, the nanofiltration membranes have high adsorption rate of 79.8%, relative to neat polymer membrane (26.7%).Alshahrani et al. [95] used interfacial polymerization method for polysulfone/reduced graphene oxide membrane fabrication.Including 0.015% nanofiller in polyamide led to water permeability of 48.9 L/m 2 h, higher than the neat polyamide membrane (25.0L/m 2 h).In addition, these membranes have high salt rejection of 80-95%.Yu et al. [96] developed polyamide-polysulfone membranes through interfacial polymerization.The water permeability of membranes was found as 48.90 L/m 2 h at 22 bar, which was superior than the neat polyamide membrane of 25.0 L/m 2 h.Salt rejection was observed in the range of 80%-95%.
In addition to water permeation or desalination membranes, the polymer/graphene membranes have been frequently investigated for gas molecule transportation [97].Koenig and workers [98] formed pristine graphene membranes on silicon substrate for the separation of H2 and CO2 gases.Structure and morphology of membranes have been reported.In addition, membranes have been studied for CO2/CH4, CO2/O2, and CO2/N2 permeation and separation processes [99].The performances were found to be related to the membrane pore sizes as well as affinity towards different molecular species [100].Subsequently, graphene designs have been investigated for fine gas separations [101].To improve the properties of graphene towards gas permeation, polymer and graphene based membranes have been reported [102].Li and researchers [103] fabricated the polymer/graphene nanocomposite membranes with pore size of 0.34 nm.The membranes were tested for high selectivity for H2/CO2 and H2/N2 gases.These membranes still need to be focused for better pore sizes towards CO2 sieving [104].For gas separation membranes, poly(dimethyl siloxane) has been considered [105].Ultrathin membranes of poly(dimethyl siloxane) have been designed focusing the carbon dioxide and other toxic gases removal [106].Nevertheless, pristine poly(dimethyl siloxane) membranes have certain drawbacks due to lack of structural robustness.In this regard, reports on poly(dimethyl siloxane) and graphene oxide derived nanocomposite membranes have been found in literature [107].Such nanocomposite membranes have been prepared using ultrasonication and solvent based methods.The poly(dimethyl siloxane)/graphene oxide membranes own fine CO2 permeability and CO2/CH4 separations characteristics.Poly(methyl methacrylate) is a thermoplastic polymer widely applied for the membrane applications [108].Baldanza and researchers [109] produced the poly(methyl methacrylate) and graphene based nanocomposite membranes through wet deposition technique.The 'lift off/float-on' method was used for the formation of these membranes [110].Figure 5 shows the formation of poly(methyl methacrylate) and twenty layer graphene based nanocomposite membrane.The membrane thickness and nominal volume fraction were observed around 550 nm and 0.06%, respectively.According to the scanning electron microscopy, regular lamination sequence was observed.Neat poly(methyl methacrylate) and poly(methyl methacrylate)/graphene membranes were studied for the permeability coefficients of humidified CO2 and O2 (Figure 6).The nanocomposite membrane had significantly low permeability properties.Adding graphene nanofiller reduced the CO2 and O2 permeability coefficient of the membrane to 1.30×10 −17 and 0.21×10 −17  2).The property was declined owing to the development of diffusion pathways in the membrane.These membranes had high permeability coefficients suitable for commercial scale uses of poly(methyl methacrylate)/graphene membranes [111].Polysulfone is also an important thermoplastic polymer for gas purification membrane systems [112].In this contest, the mixed matrix membranes of polysulfone have been reported [113].These membranes have been reported for the separation or selective separation of CO2 and other noxious gases [114].Sainath and co-workers [115] produced the mixed matrix polysulfone/graphene oxide membrane for selective separation of gases.Adding 0.25 wt.% graphene oxide caused 3-4 times higher CO2/CH4 selectivity, relative to pristine membrane.The results were obtained due to better nanofiller dispersion and formation of the diffusing routes in the nanocomposite membranes [116].Gas separation membranes of copolymers have also gained success towards gas separation applications such as poly(1-trimethylsilyl-1propyne)/graphene oxide [117][118][119] and poly(phenyl sulfonepyridine)/graphene oxide nanocomposites [120].Similarly, poly(2,6-dimethyl-1,4-phenylene oxide) has been adopted for gas separation membrane matrix [121][122][123].Rea and co-workers [124] fabricated the poly(2,6-dimethyl-1,4-phenylene oxide) and graphene based nanocomposite membrane.The 0.3 to 15 wt.% nanoparticle contents were filled in the membranes.Figure 7 shows the morphology of poly(2,6-dimethyl-1,4-phenylene oxide)/graphene nanocomposite membrane.Few layer graphene was observed in the polymer matrix showing layered morphology and fine dispersion.The gas permeability was found to decrease at 35 and 65 º C with the addition of graphene (Figure 8).The effect on the permeation properties was observed due to the variation in loading level and dispersion in the polymer matrix.Hence, numerous polymeric membranes have been prepared with the graphene or graphene oxide nanofillers for gas permeation [125].The gas permeability and selectivity properties rely on the nanoparticle alignment in the matrices, which may affect the membrane pore sizes and microstructures [126,127].

Important prospects
Numerous polymer/graphene nanocomposite membranes systems have been proposed for the superior water permeation, desalination, and gas separation and selective separation characteristics.Key points of this review article include the (i) fundamentals of graphene; (ii) fundamentals of polymer membranes; and (iii) efficiency of graphene nanocomposite membranes for water remediation or gas separations; (iv) important aspects of the nanocomposite membranes; (v) graphene amount and dispersion; (vi) graphene interactions with membrane matrix; (vii) membrane porosity, morphology, and surface properties; and (viii) membrane permeability and selectivity properties.Additionally, graphene based systems have advanced mechanical stability and thermal stability properties.Generally, fine graphene dispersion in matrices has been considered for fine molecular transportation characters such as barrier, permeation, and selective separation (Figure 9).These membrane properties can be limited due to poor nanoparticle dispersion, surface properties, and imperfect membrane pores formation.Another limiting factor is the fabrication of graphene based membranes on industrial or commercial levels.The large scale processing depends upon technique, polymer/nanofiller types and functionalization affecting the microstructure, durability, and water/gaseous molecular transportation.According to a literature comparison (Table 3), polysulfone nanocomposite membranes have been prepared with the carbon nanotubes [128], zeolites [129], and silicon dioxide [130].The polysulfone/graphene or graphene oxide based membranes revealed better nanoparticle dispersion, antifouling, water flux, and permeability properties.By comparing the utilization of graphene or graphene oxide nanofillers in the water purification membranes, most of the membranes have been prepared using the graphene oxide nanofiller.Inclusion of graphene oxide or reduced graphene oxide to membranes led to superior water flux, permeability, and rejection properties.Reason seems to be the functionalization of graphene nanosheets causing better interactions and dispersion with the polymers, relative to neat graphene nanofiller.
Solution casting, phase inversion, and ultrasonication.techniques have been frequently used include for gas separation polymer/graphene and polymer graphene oxide membranes.As compared to polysulfone/graphene and polysulfone/graphene oxide membranes [115,131], lower CO2 permeability and ideal CO2/CH4 selectivity 4.2% and 2.7%, respectively were observed for commercial polysulfone/zeolite membranes [132].For gas separation membranes, both graphene, graphene oxide, and modified graphene oxide have been applied.Number of studies have been reported on graphene and derivative based membranes [133].By comparing various studies on graphene and graphene derived membranes (Table 4), graphene oxide based membranes were found to have higher selectivity and permeability values than the graphene based systems.For example, the polysulfone [103] system revealed much higher gas selectivity than the corresponding graphene based membranes [115,131].The reason seems to be the nanostructure of graphene with impermeability towards molecular passage.However, the formation of graphene oxide or modified graphene nanostructures may form surface defects leading to better compatibility, interactions, and interface formation.Consequently, superior permeability and selectivity of graphene oxide based membranes have been observed.Thus, the polymer/graphene oxide membranes reveal better gas separation properties to overcome the tradeoffs between permeability and selectivity of the nanocomposite membranes.Nevertheless, these membranes are still in developmental stages and further studies have been desirable to understand the transport mechanisms as well as structural specifications.Here, research progress in the field of polymer/graphene nanocomposite membranes need to be analyzed according to the membrane design, type, and specific end application in order to access the crucial foremost difficulties in this field.Research progress in the field of polymer/graphene membranes can be primarily categorized as desalination or separation membranes for the removal of salts, biological, and organic pollutants by attaining optimally high water flux.Design and essential characteristics of polymer/graphene nanocomposite based water permeation membranes have been studied.
The nanocomposite membranes have been investigated for the morphological properties, permeability, flux, desalination, and toxin removal.The mechanical properties like flexibility, strength, toughness, and important properties of membranes have been deliberated.Graphene nanoparticle dispersion has been found important to enhance the matrix-nanofiller interactions to improve the final membrane characters.In this context, compatibility between the polymer and graphene nanoparticles may cause better nanoparticle dispersion and miscibility effects.The molecular diffusion and permeability properties rely on the pore size, shape, and nanoparticle dispersal in the polymeric matrices.All these properties not only affect the selectivity/permeability features but also the membrane strength and functional life time towards membrane applications.Major challenges identified in this sector has been found as complications owing to poor nanoparticle dispersion, phase separation, optimum fabrication parameters, and identification of perfect membrane designs towards commercial scale production of these membranes.In this way, desirable barrier effects can be achieved for selective molecular transportation through the membranes to separate the salts, toxic ions, biological species, and other toxins.Thus, not much research has been observed regarding the separation mechanisms and overcoming the challenges to fabricate well defined designs for commercial level use.Future research in the mentioned research directions will be beneficial towards the formation of efficient water separation membranes.
Secondly, an important application of polymer/graphene nanocomposite membranes have been observed for the gas separation.Here, matrices like polysulfone, poly(dimethyl siloxane), poly(methyl methacrylate), and other block copolymers have been used and filled with graphene nanofillers using facile solution, sonication, phase inversion, infiltration, and other techniques.For this application, efficient design combinations have been observed for separating the toxic or desired gas from gaseous mixtures.The resulting membrane must have optimum porosity, permeability, selectivity, other membrane features for the separation of molecular species.As discussed above, all the membrane characters have been found dependent upon the processing, nanoparticles alignment, functionality, and compatibility with the polymer phases.By controlling all these features, complex gas mixture can be separated using novel membrane designs.In addition, membrane thicknesses have also been found important to control the gas transport and flux characters, but also the membrane durability and cyclic performance.For gas separation, identification of perfect processing technique, membrane parameters, and ultrathin polymer/graphene nanocomposite membrane formation have been found as limiting factors or challenges.In this field, there is lack of research regarding the separation mechanisms, structureproperty relationship, as well as well-defined membrane designs towards the separation of specific gaseous pollutants.Hence, due to lack of targeted research in this area, desired permeability, selectivity, and working life have been found challenging.More focused research efforts have been definitely needed in these directions to form high performance membranes through facile processing with welldefined parameters.

Conclusions
Hence, this article presents gas separation performance of polymer/graphene nanocomposite membranes keeping in view the important literature reports.Graphene as well as modified graphene nanoparticles have been filled in the nanocomposite membranes.The resulting membrane systems have been analyzed for fine water and gas molecular separation as well as permeation properties.The polymer/graphene nanocomposite membranes have been examined for the nature of pores, microstructure, sturdiness, and gas or water molecular separation efficiencies.Various combinations of polymers and graphene or modified graphene have been developed for the formation of efficient membrane systems.Nevertheless, there are several challenges in the way of the formation and application of polymer/graphene nanocomposite membranes.The related challenges may be comprised of polymer type, nanoparticle modification, nanoparticle dispersion, and nanoparticle interaction with the polymer.Despite of the advantages, there are numerous problems limiting the rapid development of graphene based nanocomposite membranes.Even facile solution methods have been used, no perfect design with all the defined parameters, high surface area, and even thickness has been identified, so far, for large scale functioning.Facile methods have been found ineffective to produce membranes with all defined membrane parameters on large scale.Pore clogging and membrane fouling (biofouling, microfouling, macrofouling) due to the presence of organic/inorganic pollutants (dyes, metal particles, microbes, bacteria, etc.) have been found to prevent the rapid water purification.Hence, the development of evenly structured ultrathin, enduring, light weight, low price, antifouling, and extended life polymer/graphene membranes have been found difficult to attain for large scale commercial systems.Overcoming all these challenges may yield fine future opportunities towards the high-tech commercial grade graphene filled membranes.
Thus, the research progress on graphene nanocomposite membranes led to several progresses in types, design, and applications to overcome the challenges in this field.For increase in physical properties, nanoparticle dispersion has been found important for the matrix-nanofiller interactions, microstructure, mechanical features, and for advanced membrane characteristics.Consequently, the compatibility of matrix-graphene has been recommended to improve for better miscibility and reinforcing effects.The membrane performance also depends upon the pore shapes, sizes, and distribution in the matrices.The random nanofiller dispersion or pore distribution in membranes may influence the strength, durability, and life time of the membranes.For commercial scale membrane production, membrane design features must be analyzed.Hence, future research must resolve the challenging to identify the directions for high performance gas separation membranes.

Figure 3
specifies the construction of poly(vinyl alcohol)/laser induced graphene nanocomposite membranes.

Figure 2 .
Figure 2. Schematic of poly(vinyl alcohol) and laser induced graphene nanocomposite membranes for water remediation [86].Reproduced with permission from ACS.

Figure 5 .
Figure 5. (a) Schematic illustration of the iterative 'lift-off/float-on' process combined with wet depositions adopted to produce the Gr-PMMA nanolaminates; (b) Thickness evaluation of the single Gr-PMMA layer deposited on a Si wafer: representative cross-section of the scratch and atomic force microscopy image as inset; and (c) scanning electron microscopy image in the cross-section plane of the nanolaminate [109].Gr = graphene; Gr-PMMA = poly(methyl methacrylate/graphene nanocomposite); APS = ammonium peroxydisulfate.Reproduced with permission from MDPI.

Table 3 .
Specifications of polymer/graphene nanocomposite membranes for water separation.

Table 4 .
Specs of polymer/graphene nanocomposite membranes for gas separation.