Effect of graphene oxide in the formation of polymeric asymmetric membranes via phase inversion
Graphical abstract
Introduction
Phase inversion (or phase separation) is a well-known technique for the fabrication of asymmetric filtration membranes used in wastewater treatment, biochemical processing, and pharmaceutical production [1]. In this method, a homogeneous polymer solution is cast into a 2D film (in the case of flat sheet membranes) or a tube (in the case of hollow fiber membranes) and immersed in a nonsolvent liquid, where solidification of the film takes place. The polymer solution (referred to as the dope solution) typically comprises the polymer, a solvent, and an additive. Immersion precipitation is the most widely used method to produce asymmetric membranes, however, the solid membrane can be also precipitated from the solution by controlled evaporation of the solvent, contact with a nonsolvent vapor (vapor-induced precipitation), controlled cooling of the polymer solution (in the case of polymer melts - known as thermally-induced precipitation), or combinations of these. In all cases, the polymer phase ‘inverts’ from a solution into a solid but the different methods result in different membrane morphologies. In most ultrafiltration applications and for use as a supporting layer in thin-film composite membranes (for nanofiltration and reverse osmosis applications), an asymmetric membrane morphology is preferred. Membranes with this structure have a dense skin layer containing small pores and a more open sub-layer made up of larger or finger-like pores. When the casting solution is immersed in the nonsolvent (or coagulation) bath, the top layer solidifies spontaneously. Then, due to the inward diffusion of the nonsolvent into the solution, large pores are formed. Hence, the membrane has a dense skin layer and a porous sublayer. The presence of large macrovoids can threaten the mechanical integrity of the membrane in high pressure applications, however, they are commonly found in membranes for ultrafiltration processes [2]. The morphology of a membrane can broadly be defined by the shape and size of the membrane pores and pore walls. Determining the morphology of a membrane is of interest because it plays a vital role in its performance. It is generally well known that the morphology depends on both the rate of outer diffusion of the solvent into the water bath and the rate of inner diffusion of the water into the casting solution. Slow precipitation causes a sponge-like morphology whereas fast precipitation produces finger-like pores (Fig. 1). The demixing rate can be rationalized by the phase equilibrium (thermodynamic properties) and the diffusive behavior of solvent and nonsolvent (kinetic properties) [3,4].
A common approach to improving the performance of asymmetric membrane is the introduction of an additive. In general, additives act as nonsolvent materials and they are usually leached out from the casting solution, leading to an increased porosity. These additives are known as pore formers (or pore-forming agents). However, additives do not always perform as pore formers and the final morphology of the membrane depends on the thermodynamic and kinetic properties of the casting solution [5,6]. Moreover, additives such as graphene oxide (GO) often stay trapped in the final structure of the solidified film, which is referred to as a mixed matrix membrane (MMM), and GO is better defined as a filler rather than an additive.
Ternary phase diagrams, binary interaction parameters, mutual diffusion between solvent/nonsolvent and polymeric solution viscosity can be used to investigate the thermodynamic and kinetic features of a casting solution and hence help to predict the final morphology of the membrane [4,[7], [8], [9]]. In this way, Sadrzadeh and Bhattacharjee [5] were able to define two non-dimensional parameters to study the changes in thermodynamic and kinetic properties of polyethersulfone (PES) after adding pore formers such as polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG). It was concluded that there is a trade-off between the thermodynamic and the rheological properties of the casting solution. Lower demixing rates resulted in denser, sponge-like morphologies and higher demixing rates led to more open structures with larger sub-layer pores. With the same approach, Mohsenpour et al. [10,11] explored the effect of TiO2, PVP, and different types of polyurethanes on the morphology of polyvinylidene fluoride (PVDF) and PES membranes. It was reported that by increasing the additive concentration until a point where the thermodynamic parameter was greater than the kinetic parameter, the porosity and pure water flux (PWF) of the membranes increased. Mazinani et al. [4] employed a ternary phase diagram to study the effect of solvent and nonsolvent on the morphology of the membranes made from a new form of polyetherimide called ExtemⓇ. The membranes produced using n-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMAc) and dimethylformamide (DMF) as solvents and water as the nonsolvent showed a finger-like morphology, while the use of dimethyl sulfoxide (DMSO) as a solvent produced a sponge-like morphology. Barzin et al. [7,12] correlated the ternary phase diagram with the formation of macrovoids in PES membranes. They employed the Flory-Huggins theory and cloud point titrations to construct the ternary phase diagram for PES/DMAc/water and PES/NMP/water systems, theoretically and experimentally. It was found that despite better miscibility between DMAc and water (which favors the formation of finger-like pores), more sponge-like structures were observed for PES/DMAc/water compared to PES/NMP/water.
Graphene oxide (GO) is a hydrophilic additive/filler which has attracted great interest in different applications including water and gas purification, ultrafiltration, pervaporation, and energy storage [13,14]. GO can be directly used as a separating layer [15] or as a matrix filler to improve the performance of polymeric membranes [16]. In our previous work we prepared PES/GO and PES/APTS-GO MMMs and reported an increase of PWF from 2 to 13 L m−2 h−1 with a 0.5 wt% GO [17,18]. Furthermore, the rejection of bovine serum albumin (BSA) increased from 89.2 to 97%. Also, the anti-fouling properties of the membrane were improved as indicated by the flux recovery ratio and BSA adsorption. Reduced GO (rGO) with different degrees of reduction was used as a filler in PVDF membrane for membrane distillation and ultrafiltration applications [19,20]. The optimum performance of MD system was obtained for the membrane containing 0.5 wt% rGO with 58% degree of reduction. This was attributed to higher porosity and the larger pore size of this membrane [19]. While others have reported improved performance in terms of flux, rejection and antifouling properties when adding GO to dope solutions [21,22], little attention has been paid to the theoretical underpinning of these observations and as far as we know, no-one has yet constructed ternary phase diagrams comparing the effect of different GO additions.
In this work, the effect of GO and rGO on the morphology of polymeric membranes prepared by immersion precipitation, has been studied by measuring their thermodynamic and kinetic properties. In the phase inversion process, the choice of solvent has a significant effect on the morphology of the membranes. NMP, DMAc, DMSO, DMF and dihydrolevoglucosenone (Cyrene) have been selected to study binodal lines, viscosity and final morphology of PES membranes prepared with them. Nontoxic DMSO and Cyrene have been chosen as solvents for the fabrication of PES/GO MMMs. Moreover, Cyrene is a green bioderived solvent that shows the same polarity as the traditional organic solvents like DMF and NMP [23]. As far as we know this is the first time PES/GO membranes have been prepared using Cyrene as a solvent. Also, DMF has been used as a solvent of PVDF for ease of comparison with previous work [24]. The Flory-Huggins theory has been exploited for PES/GO and PVDF/GO systems to draw theoretical binodal and spinodal lines, which have been validated using cloud point experiments. In addition, the rheological properties of the casting solutions have been studied. Moreover, characterizations including scanning electron microscopy (SEM), contact angle, pore size and porosity determination, and PWF measurements have been carried out to assess the morphology of the prepared membranes.
Section snippets
Phase diagram
A graph of the mixing Gibbs free energy of a polymer solution (ΔGm) versus composition for a ternary system is shown in Fig. 2a. The binodal line (purple) is constructed from the points where the first derivative of ΔGm with respect to composition is zero (). Furthermore, the second derivative ΔGm versus composition makes the spinodal line (green) () [25]. The intersection point of the binodal and spinodal lines is called the critical point and the regions between these lines
Flory Huggins theory
An extended form of the Flory-Huggins theory [equation (5)] is used to calculate the Gibbs free energy of mixing for a quaternary system [37].Where, R, T, ni and are the gas constant, absolute temperature, number of moles, and volume fraction of component i, respectively. Subscripts 1, 2, 3, and 4 refer to nonsolvent, solvent, polymer, and additive, respectively. is the binary interaction parameter between
Materials
Commercial grade PES (ultrason E6020P, MW = 58 000 g mol−1 and glass transition temperature, Tg = 225 °C) and PVDF with different molecular weights (MW = 180 000, 275 000 and 534 000 g mol−1) were purchased from BASF Ludwigshafen, Germany, and Sigma Aldrich, respectively. They were employed as the base polymer for membrane fabrication. Solvents DMF, (99.9%), NMP, (99.5%), DMAc (99.8%) and Cyrene™ (99%) were provided by Sigma Aldrich and DMSO, (99.8+%) was provided by Acros Organics. Dextrans
Purely polymeric PES membranes
The cross-sectional SEM images of PES membranes prepared with different solvents are shown in Fig. 3. Three distinct morphologies can be observed: 1) PES membranes prepared with either NMP, DMSO or DMAc show extended finger-like pores with a thin sponge-like layer beneath it, 2) membranes prepared with DMF have smaller finger-like pores and a sponge-like structure extended to the top layer 3) and those prepared with Cyrene as solvent show cellular voids and very small finger-like pores near to
Conclusions
In the present investigation, the effect of different solvents on the morphology of neat PES membrane has been studied using thermodynamic and kinetic properties of polymer casting solutions which are key factors that have been found to influence the morphology of phase inversion membranes. It has been observed that PES/NMP, PES/DMAC, and PES/DMSO show extended finger-like pores, PES/DMF exhibit finger-like and sponge-like morphology and PES/Cyrene contain cellular voids. The appearance of
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
S. Mohsenpour and J. Shokri thank the University of Manchester for funding their Ph.D studies. P. Gorgojo acknowledges the Spanish Ministry of Economy and Competitiveness and the European Social Fund through the Ramon y Cajal programme (RYC2019-027060-I/AEI/10.13039/501100011033). The authors are also grateful to the EPSRC for funding under the grant number EP/S032258/1.
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