Polymeric Membranes. Alberto Figoli, Silvia Simone, and Enrico Drioli CONTENTS 1.1 INTRODUCTION

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1 1 Polymeric Membranes Alberto Figoli, Silvia Simone, and Enrico Drioli NTENTS 1.1 Introduction Membrane Preparation Techniques Sintering, Stretching, and Track-Etching Membrane Preparation by PI Membrane Preparation Techniques via PI Thermodynamic Principles of PI Phase Diagrams for TIPS and DIPS Solubility Parameters Trade-ff between Thermodynamic, Kinetic, and Membrane Morphology Peculiarities of Hollow-Fiber Membrane Preparation through PI Examples of Membrane Preparation for Pressure-Driven Separation Processes Microfiltration Ultrafiltration NF and Solvent-Resistant NF Reverse smosis Gas Separation Pervaporation Examples of Membrane Preparation for M Gas/Liquid ontactors Liquid/Liquid ontactors Membrane Distillation D and Membrane rystallizers onclusions utlook References INTRDUTIN A membrane can be defined as an interphase between two adjacent phases acting as a selective barrier, regulating the transport of substances between the two compartments ([1], p. 2217). Membrane technologies are widely recognized as advanced separation/ concentration processes, which are ideally placed to aid process intensification [2], thanks to the possibility of exploiting the synergy between different membrane operations in an integrated system [3]. Membrane processes are now 3

2 4 Membrane Fabrication widespread at the industrial level, a result of the advances in membrane performance connected to higher productivity, enhanced selectivity, and improved stability. Nowadays, membranes are prepared using a wide variety of techniques, mainly depending on the membrane material but also on the application. In this chapter, the most common membrane preparation methods are described, with peculiar focus on phase inversion (PI) or phase separation (PS), which is the foremost technique for preparing polymeric membranes. The thermodynamic principles of PI, the main factors affecting membrane morphology and properties as well as the peculiarities of hollow-fiber membranes preparation through PI, are examined. Furthermore, significant examples of membrane preparation for selected applications, spanning from classical pressure-driven processes, such as reverse osmosis (R), nanofiltration (NF), ultrafiltration (UF), and microfiltration (MF), to more recent emerging processes, such as membrane contactors (Ms), are reported. 1.2 MEMBRANE PREPARATIN TEHNIQUES Sintering, Stretching, and Track-Etching There are several techniques for preparing membranes; the selection of the appropriate method depends on the material and the final membrane application. Membrane properties can be modulated, to a certain extent, by properly choosing the preparation technique and acting on the key process conditions. In Table 1.1, the main membrane materials, preparation techniques, and applications are summarized. PI, the most used membrane preparation technique, is discussed in detail in Section 1.3. The other most used techniques, usually employed in membrane preparation, are introduced in this section. TABLE 1.1 Main Membrane Materials, Preparation Techniques, and Applications Materials Techniques Applications rganic polymers Inorganic materials α-al 2 3 α-al 2 3 /γ-al 2 3 γ-al 2 3 /Ti 2 /Zr 2 Stainless steel Palladium Sintering Stretching Track-etching Phase inversion Sintering Glass (polycarbonate) Track-etching MF MF MF/M MF MF/UF/NF/R MF/UF NF UF MF GS Source: A. Bottino et al.,. R. himie, 12, , 2009.

3 Polymeric Membranes 5 TABLE 1.2 Principle of Sintering Method, Membrane Materials and Properties Schematic of the Process Materials Used Powders of polymers Polyehtylene Polytetrafluoroethylene Polypropylene Membrane pore-size distribution μm Powders of metals Stainless steel Tungsten Porosity 10% 20% with polymers Powders of ceramics Aluminum or zirconium oxide 80% with metals Powders of graphite arbon Powders of glass Silicalite The sintering technique allows the preparation of symmetric membranes and is generally used to prepare ceramic or metallic membranes for application in UF and MF (Table 1.1). A powder consisting of particles (of the material) of a certain size is pressed and heated, at or just below the melting temperature [4]. The principle of the sintering method is shown in Table 1.2 (from Ref. 5). The pore size and porosity of the membranes obtained are generally affected by two main factors, namely, particle size and sintering profile, but also by temperature, heating/cooling rates, and dwelling time [5,6]. Membranes prepared by sintering can be produced as disks, cartridges, or fine-bore tubes. Hollow-fiber ceramic membranes can be prepared using a three-step process based on a combination of PI and sintering methods involving the (1) preparation of a spinning suspension, (2) spinning of ceramic hollow-fiber precursors, and (3) final sintering [7]. The stretching technique is also used for producing MF polymeric membranes. A homogeneous polymer of partial crystallinity, in the shape of a film or hollow fiber, is stretched perpendicularly to the axis of crystallite orientation [4]. Relatively uniform pores, with diameters of μm, are formed as a result of a partial fracture of the film (Figure 1.1) [5]. Polytetrafluoroethylene (PTFE), polypropylene (PP), and polyethylene membranes can be prepared by this technique. For instance, elgard PP membranes are obtained by monodirectional stretching, whereas Gore-Tex membranes are produced by bidirectional stretching [4]. The membranes produced generally show high permeability to vapor and gases, although, due to the intrinsic material hydrophobicity, they are quite impenetrable to aqueous streams. Therefore, they are interesting for application as water- repellent textiles and contactors [5]. Track-etching allows the preparation of membranes having uniform cylindrical pores. A thin dense polymer film is exposed to high-energy particle radiation, which damages the polymer matrix. The damaged polymeric material is then etched away

4 6 Membrane Fabrication (a) (b) (c) FIGURE 1.1 Membranes produced by (a) sintering, (b) stretching, and (c) track- etching. (Reprinted from H. Strathmann et al., Basic Aspects in Polymeric Membrane Preparation, In: E. Drioli and L. Giorno, eds., omprehensive Membrane Science and Engineering, vol. 1: Basic Aspects of Membrane Science and Engineering. Elsevier, Amsterdam, the Netherlands, 2010, pp , opyright 2010, with permission from Elsevier.) in an acid (or alkaline) bath [4,5]. Membrane porosity is generally around 10%, and is affected by residence time in the irradiator [5]. Pore dimensions are usually within the range μm. MF membranes, in silicon nitride, showing high porosity and narrow pore size distribution, coupled with very low flow resistance and minimal fouling tendency, were produced by laser interference lithography and silicon micromachining technology (Figure 1.2) [8]. Such membranes are often referred to as microsieves. The template leaching technique is suitable for preparing porous membranes from polymers, which do not dissolve in common organic solvents [9], or from glass, metal alloys, and ceramics [5]. FIGURE 1.2 Field emission scanning electron microscopy image of a membrane produced by laser interference lithography. (Data from S. Kuiper et al., Journal of Membrane Science, 150, 1 8, 1998.)

5 Polymeric Membranes MEMBRANE PREPARATIN BY PI Membrane Preparation Techniques via PI PI or PS is indeed the most common method for preparing polymeric membranes. It is based on the separation of an initially homogeneous system into two distinct phases, consisting of a polymer, a solvent, and, eventually, other additives. The solid phase, or polymer-rich phase, will give rise to the membrane matrix, whereas the solvent-rich liquid phase, or polymer-lean phase, will originate from the membrane pores. There are four techniques distinguished on the basis of the mechanism exploited to induce such separation, often called demixing or precipitation. These four techniques are evaporation-induced PS (EIPS), vapor-induced PS (VIPS), temperature-induced PS (TIPS), and nonsolvent-induced (or diffusion-induced) PS (NIPS or DIPS) [1,5]. In TIPS, precipitation is induced by lowering temperature. In DIPS, precipitation of the casting solution is obtained by immersion into a nonsolvent bath. In VIPS, the nonsolvent is adsorbed from a vapor phase, which can also contain other gases such as air or nitrogen. In EIPS, precipitation is induced by the evaporation of a volatile solvent from the casting solution. According to some authors, there are only two types of PI, namely, temperature-induced and diffusioninduced, whereas immersion precipitation, vapor adsorption, and solvent evaporation are considered as three types of DIPS [10]. Phase inversion is extremely versatile and allows the preparation of membranes from several different polymers, as long as the polymer is soluble in a solvent, and the system shows a miscibility gap over a defined concentration and temperature range. Membranes, which have morphology and properties suitable for an impressive variety of processes, can be obtained. 1.4 THERMDYNAMI PRINIPLES F PI All the recipes reported in the literature for membrane preparation are based on the same principles, that is, thermodynamic and kinetic, such as the relationship between the chemical potentials and diffusivities of the individual components and Gibb s free energy of mixing of the entire system. Their interplay during membrane formation produces the final membrane structure; therefore, a better understanding of all these parameters is the optimum way to achieve a deeper knowledge of the membrane formation mechanisms, and how to tailor and optimize membrane morphology and properties. From a thermodynamic point of view, the two main mechanisms of PS, thermally induced and nonsolvent induced, are described with the aid of binary and ternary phase diagrams (Figure 1.3). Phase diagrams represent a useful instrument to better understand the mechanism of membrane formation. This is often called phenomenological description of the phase separation process [5] Phase Diagrams for TIPS and DIPS TIPS is based on a latent solvent or diluent that behaves as a good solvent at temperatures close to the melting of the polymer, but that works as a nonsolvent at lower

6 8 Membrane Fabrication Temperature (a) T 1 T 2 Solvent (S) Liquid phase A B B B Solid phase Polymer (P) ritical point Binodal Spinodal A Liquid phase Polymer (P) B Spinodal temperatures. TIPS is often used for polymers that are not soluble at room temperature, such as polyolefins [1]. The TIPS technique consists of the following: Dissolving the polymer in the latent solvent asting the solution in the desired shape Phase separating as a result of solution cooling Extracting the latent solvent by means of a more volatile substance Final drying of the membrane The system must show a miscibility gap over a certain range of temperature and composition. In the binary phase diagram, the miscibility gap is surrounded by the spinodal curve, although the region in between the spinodal and the binodal curves is metastable. In Figure 1.3, it is seen that the temperature T 1 of an initially homogeneous system, located at the point A, decreases to reach T 2. The corresponding point B is located inside the miscibility gap and as a consequence the system will demix in two phases, which is indicated by B and B. The first one represents the polymerrich phase and forms the solid membrane structure. The other phase forms liquidfilled membrane pores. Depending on the polymer type, dope composition (polymer concentration and solvent type), and the cooling rate, PI can proceed both through solid liquid (S L) and liquid liquid (L L) demixing, giving rise to different membrane structures and properties. ther phenomena, such as gelation and vitrification, can also take place. L L PS takes places when the temperature reaches the binodal curve. Two mechanisms of membrane formation may occur in this case: spinodal decomposition (SD) and nucleation and growth (NG). Although the latter occurs only in the metastable region comprised between the binodal and spinodal lines, SD takes place in the unstable region under the spinodal line. S L PS takes place only if, during solution cooling, the crystallization temperature of the polymer is reached. If the polymer is amorphous, gelation through L L separation takes (b) Binodal ritical point Solvent (S) B Miscibility gap B Nonsolvent (NS) FIGURE 1.3 (a) Binary and (b) ternary phase diagrams describing TIPS and DIPS processes. (Reprinted from H. Strathmann et al., Basic Aspects in Polymeric Membrane Preparation, In: E. Drioli and L. Giorno, eds., omprehensive Membrane Science and Engineering, vol. 1: Basic Aspects of Membrane Science and Engineering, Elsevier, Amsterdam, the Netherlands, 2010, pp , opyright 2010, with permission from Elsevier.)

7 Polymeric Membranes 9 place, which is arrested by the vitrification of the polymer-rich phase at the glass transition temperature [10]. As reported in the literature [10], in the binary phase diagram of an amorphous polymer, the intersection between the binodal curve and the glass transition boundary is defined as Berghmans point. The L L demixing is interrupted by gelation, which leads to vitrification of the polymer-rich phase. The time between the beginning of the PS and the final vitrification is referred to as the gelation time. If the cooling rate is not infinitely slow, the final structure will be a porous glass. When the system PS starts within the metastable region, which is found between the spinodal and binodal curves, it is commonly referred to as nucleation and growth, or NG. In the metastable region, indicated as II in the binary phase diagram (Figure 1.4), it can be noticed that the polymer-rich phase gives rise to a continuous matrix, whereas the polymer-lean phase produces isolated pores, that is, NG of the solvent phase in the polymer-rich phase. n the contrary, a suspension of polymerrich phase in a continuous polymer-lean phase, NG of the polymer-rich phase in the solvent phase, can be obtained from region III of the phase diagram. When demixing starts in the unstable region, the mechanism is called SD, which is defined as a spontaneous process that does not need a nucleus [10]. As reported in the literature, the PS structures obtained by NG and SD will grow and coarsen during the gelation time. If this is infinite, two fully separate phases could be obtained. If the gelation time is short enough, the SD will give rise to a morphology with high interconnectivity, since the coalescence process is quickly Temperature T A T gel B Liquid phase I III IV M A II B B Unstable Metastable ritical point Binodal curve Spinodal curve rystallization curve V Liquid solid demixing 0 Polymer composition A solvent-rich phase A polymer-rich phase An interpenetrating three-dimensional network FIGURE 1.4 Schematic phase diagram of TIPS. (Data from A.G. Fane et al., Membrane technology for water: Microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. In: P. Wilderer, ed., Treatise on Water Science, Academic Press, xford, 2011, pp ) 1

8 10 Membrane Fabrication stopped by vitrification. As the gelation time increases, the coalescence phenomena may result in a closed cell structure. A good knowledge of the properties of the polymer/solvent system could help to adjust the gelation time, thus allowing control of the interconnectivity between pores and the final membrane morphology. Bicontinuous structures can be obtained through SD. However, the spinodal area can be directly reached during cooling only at the critical point, the point where the binodal and spinodal curves coincide. When composition is different, during cooling, the system must cross the metastable area first. In this case, in order to prevent demixing and improve pore interconnectivity, it can be useful to employ fast cooling [12]. Semicrystalline and crystalline polymers can also crystallize, giving rise to chain-folded lamellae and supramolecular architectures as axialites and spherulites [12]. NIPS consists in the preparation of a homogeneous polymer dope by dissolving the polymer in a suitable solvent. After casting in the desired shape, polymer precipitation is induced by immersion in a coagulation bath containing a nonsolvent. NIPS can be described by ternary phase diagrams. The system must exhibit a miscibility gap, for a defined range of polymer/solvent/nonsolvent (P/S/NS) compositions. Ternary phase diagrams always refer to a certain temperature. Similarly to what is described for TIPS phase diagrams, the metastable region is between the binodal and spinodal curves and the unstable region is delimited by the spinodal curve. onsider a point A as the initial system composition; this point is located in the stable region since only polymer and solvent are present. By adding a nonsolvent, the system composition will change, and point A will move toward point B. Going from A to B, the system composition changes due to S NS exchange. nce the miscibility gap is reached, PS takes place. The upper boundary of the miscibility gap, B, is the polymer-rich phase, and the lower boundary, B, is the polymer-lean phase. In Figure 1.5, four regions can be recognized: region I, one solution phase; region II, two liquid liquid phases; region III, two liquid solid phases; region IV, one solid phase. Starting from a generic point A in region I, the system can follow four different paths. If the system follows path 1, it reaches region IV of the phase diagram after a glass transition; a homogeneous glassy film is obtained (vitrification). When the system reaches the point S 1, L L PS takes place, resulting in phase S 2 (polymer-rich phase) and S 3 (polymer-lean phase). When S 1 is located in the metastable region at high polymer concentration (path 2-1), similar to what is described for TIPS, membrane formation proceeds through NG of the polymer-lean phase, resulting in noninterconnected pores. Bicontinuous structure can be obtained following path 2-2, which enters directly in the unstable region (SD). When following path 2-3, the system enters in the metastable region at low polymer concentration, giving rise to low-integrity powdery agglomerates [11]. The point where the binodal and spinodal curves coincide is also here and is called the critical point. It represents the maximum solvent concentration in the coagulation bath that still allows the fabrication of a solidified membrane [10]. Two further important definitions are the delay time and gelation time. Delay time is defined as the time interval between the immersion in the coagulation bath and the beginning of the liquid liquid demixing [10]. Looking at the phase diagram, it is easy to understand that, depending on the delay time, the system crosses

9 Polymeric Membranes 11 P (Polymer) Vitrification IV 1 S (Solvent) A I 1 S 2 K> K<1 2-2 S 0 K<1 S 1 S 3 S 1 II S 1 ritical point Binodal Spinodal Gelation boundary different regions; this affects membrane morphology. For instantaneous demixing, the system immediately reaches the unstable region; when the delay time is longer, the system passes through the metastable region. The gelation time is defined as the time interval between the onset of the demixing and the solidification of the polymer solution. This applies when the system enters region IV of the phase diagram, when the polymer-rich phase vitrifies after reaching the Berghmans point. This also influences membrane morphology since the system composition with solvent/nonsolvent exchange can proceed only before the vitrification. The formation of the final membrane structure is indeed a complicated process, in which all the described mechanisms are involved. NG, SD, and gelation exert a significant influence on the final membrane. When PS proceeds through NG, the increase of the gelation time will promote the formation of interconnected pores. The final membrane morphology also depends on the polymer type. For glassy polymers, such as cellulose acetate (A), polyamide (PA), and polyimide, PI is mostly controlled by liquid/liquid demixing, whereas for semicrystalline polymers, such as polyvinylidene fluoride (PVDF), solid liquid demixing and polymer crystallization can also take place during PI. Usually, L L demixing gives rise to membranes having cellular morphology and/or finger-like macrovoids, with pores generated by the polymer-lean phase, surrounded by a matrix created by the polymer-rich phase. S L demixing often results in particulate structure, which is made up of interlinked semicrystalline spherulites. III Path 2 2-1: 2-2: 2-3: N (Nonsolvent) FIGURE 1.5 Ternary phase diagram describing membrane formation through immersion precipitation. (Data from A.G. Fane et al., Membrane technology for water: Microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. In: P. Wilderer, ed., Treatise on Water Science, Academic Press, xford, 2011, pp )

10 12 Membrane Fabrication VIPS and EIPS are not treated in detail in this section. However, as reported in the literature, most of the concepts illustrated regarding TIPS and NIPS also apply to these two techniques Solubility Parameters Solubility parameters are indexes usually employed to evaluate the interactions between the polymer, solvent, and nonsolvent, such as the solvent s ability to dissolve a given polymer, the miscibility between solvent and nonsolvent, and the coagulation power of a nonsolvent toward a polymer of interest. These interactions will strongly affect the path followed by the P/S/NS system during PI, and hence the final membrane morphology. In general, the closer the solubility parameters of two chemical species, the more compatible they are. The Hildebrand solubility parameter for a pure liquid substance is defined as the square root of the cohesive energy density: where: ΔH v is the heat of vaporization V m the molar volume δ= ΔHv RT V m According to Hansen s theory [13], δ, the Hildebrand parameter, can be calculated using the three components: δ d, which represents the energy from dispersion bonds; δ h, which represents the energy from hydrogen bonds between molecules; and δ p, which represents the energy from dipolar intermolecular forces: ( δd 2 + δ 2 p + δ 2 h ) The interactions between polymer (P), solvent (S), and nonsolvent (NS) can be evaluated by calculating the difference between their solubility parameters using the following equations [14,15]: 1/ 2 ( ) + ( ) + ( ) P,S d,p d,s p, P p,s h,p h,s P S: δ = δ δ δ δ δ δ ( ) + ( ) + ( ) P NS d,p d,ns p,p p,ns h,p h,ns P NS: Δδ = δ δ δ δ δ δ ( ) + ( ) + ( ) S NS d,s d,ns p,s p,ns h,s h,ns S NS: Δδ = δ δ δ δ δ δ The mutual interaction between P, S, and NS strongly affects the mechanism of membrane formation. For instance, when the difference between the solubility parameters of P and S is small, S has a strong dissolving capacity. As a consequence,

11 Polymeric Membranes 13 the path followed during PI to reach the miscibility gap and finally, the membrane morphology, is affected, as discussed in Section If the difference between the P and NS parameters is large, NS will have a strong coagulant power. As a consequence, fast L L demixing could take place. Finally, the difference between the NS and S parameters influences the S NS exchange during coagulation. For example, Wang et al. [14] prepared PVDF MF membranes for wastewater treatment and studied the effects of different solvent compositions, in particular N,N dimethylformamide (DMF), N,N dimethylacetylamide (DMA), triethyl phosphate (TEP), dimethyl sulfoxide (DMS), and their mixtures (50/50) on the produced membranes features. These four solvents have affinity for PVDF that decreases in the following order: TEP > DMA > DMF > DMS. The properties of the produced membranes varied with the solvent type and, hence, with the difference of solubility parameters between P, S, and NS, which was water in all cases. When changing solvent type, S NS diffusivities reduce as follows: DMF > DMA > TEP > DMS. The morphology of the top-layer was found to be more dependent on the affinity between P and NS, whereas the S NS exchange rate was found to affect the pore structure in the sublayer. A larger difference between P and S parameter (PVDF DMS couple) will induce sudden PS after immersion in the coagulation bath. However, DMS had lower diffusion rate in the coagulation bath, which delayed solidification and caused the development of a sublayer with macrovoids. This also increased membrane thickness. In contrast, when using pure DMF, DMA, and TEP, the prepared membranes showed lower surface porosity and shorter finger-like pores in the sublayer. The mixture of DMF and TEP, which has the highest dissolving capacity for PVDF, delayed PS a lot, resulting in a nonporous top layer. The use of solvent mixtures could delay the S NS exchange and, in general, promote the growth of macrovoids in the sublayer. However, for the DMF TEP mixture, the growth of macrovoids is limited, due to the development of a skin layer that prevents the S NS exchange trade-ff between Thermodynamic, Kinetic, and Membrane Morphology The morphology of membranes produced through immersion precipitation and, in particular, the dichotomy sponge versus finger-like structure is a clear example of interplay or, as also reported in the literature, trade-off between thermodynamic and kinetic factors. Finger-like macrovoids, generally formed during membrane preparation, represents unwanted morphology, being connected to low mechanical strength. Several studies proposed different mechanisms to explain and/or avoid macrovoids formation. Early studies suggested interfacial hydrodynamic instabilities, caused by surface tension gradients, as a possible origin of macrovoid initiation [16 18]. According to Ray et al. [19], macrovoids formation is connected to concentration gradients at the interface between the polymer solution and the nonsolvent bath. Smolders et al. [20] connected macrovoids formation to the type of demixing i.e., delayed or instantaneous. According to the mechanism proposed in their work, macrovoids are produced under the skin layer from newly formed nuclei of the diluted phase if the solvent concentration exceeds a certain threshold value and if the composition in front of the nuclei remains stable for a suitable period.

12 14 Membrane Fabrication Macrovoids formation can be avoided by delayed demixing, and increasing polymer and/or nonsolvent concentration in the polymeric dope. ther studies suggested to introduce solvent into the coagulation bath [21 23], to increase solvent evaporation time [24], to work with a S NS pair with low miscibility [25] or to use organic additives such as polyvinylpyrrolidone (PVP) [26,27]. Regarding the preparation of hollow fiber, Simone et al. [28] reported that the effect of PVP on macrovoids formation depends on concentration. At low concentration, the presence of PVP increases the dope instability, thus promoting faster demixing and enhancing macrovoids development (thermodynamic effect). At high concentration, PVP increases the dope viscosity, thus delaying demixing and avoiding macrovoids formation (kinetic effect). These findings are in agreement to what is described in the early work by Lee et al. [29], who investigated the trade-off between thermodynamic enhancement and kinetic hindrance during PI. They prepared polysulfone (PSU) membranes and analyzed the system PSU/DMF/PVP. The increase of PVP concentration reduces the thermodynamic stability of dope solutions and should induce faster L L demixing. However, the increase of PVP concentration was able to induce the formation of macrovoids and also increase membrane permeability until a certain threshold value (7.5%). Further increase of PVP caused an increase of dope solution viscosity. This caused a rheological hindrance of the demixing. The overall diffusion between components was delayed due to kinetic factors. This study is a clear example of how, during membrane formation, the same factor (PVP concentration) could influence both the thermodynamic and kinetic properties of the system. Sadrzadeh and Bhattacharjee [30] discussed complex systems composed of polyethersulfone/1-methyl-2-pyrrolidone/additive; the additive was either polyethylene glycol (PEG) or PVP with different molecular weights. They demonstrated that two dimensionless parameters can be calculated, for each system, to quantify the thermodynamic enhancement and the kinetic hindrance to PI due to additives; these parameters could be used to predict membrane morphology. A simple model [31] was used to calculate diffusion rates of the solvent and nonsolvent in the coagulation bath. Another recent and interesting study [32] showed, by direct microscopic observation, the influence of solvent and nonsolvent type during PI. The polymer polysulfone (PSU), the solvents NMP and N,N-DMF, and the nonsolvents water and glycerol were studied. Although PSU/DMF/water system resulted in sponge-like morphology, the finger-like macrovoids developed when using PSU/NMP/water system. In both systems, at the polymer coagulation bath interface, there was fast S NS exchange, which caused the formation of a skin layer. However, the morphology of the membrane produced from PSU/DMF/water was mainly sponge-like, due to the slow nonsolvent influx, which was hindered by the formation of a skin layer. According to Hansen s solubility parameters, NMP is a better solvent for PSU than DMF, hence, the formation of the skin was slow. Void lengths were found to decrease exponentially with increasing polymer concentration. The thickness of the skin layer was reported to increase with PSU percentage. Macrovoids formation in the PSU/NMP/ water membrane was avoided by inducing the formation of a viscous gel layer, which caused similar effects to polymer precipitation by VIPS, which was also found to inhibit macrovoids formation in the PSU/NMP/water system [33]. Authors proposed that void growth takes place by convective nonsolvent flow, through the polymer

13 Polymeric Membranes 15 solution, driven by gradients in interfacial energy. An increase in viscosity might avoid voids formation by hindering the supply of nonsolvent. This confirmed the kinetic effect of viscosity on macrovoids formation. Macrovoids formation was, finally, hindered by using a poor nonsolvent, a mixture of water and glycerol or water and NMP. The addition of solvent in the coagulation media delays the S NS exchange and hence the nonsolvent influx, which is responsible for voids growth. The effect of glycerol is connected both to its lower nonsolvent power and to its viscosity, which further delayed the nonsolvent influx. 1.5 PEULIARITIES F HLLW-FIBER MEMBRANE PREPARATIN THRUGH PI Depending on their dimensions, it is possible to distinguish hollow-fiber membranes (diameter < 0.5 mm), capillary membranes (0.5 mm < diameter < 5 mm), and tubular membranes (diameter > 5 mm) [34]. The preparation of hollow-fiber membranes through PI is more complex, with respect to flat sheet, due to the higher number of parameters involved. However, hollow-fiber modules are usually preferred, because they ensure space savings, more productivity, and reduction of costs, which is also connected to maintenance, as these modules can be backflushed [35]. Hollow-fiber preparation requires a polymeric dope of suitable viscosity, usually a few thousand of centipoises. As a consequence, the polymer and additive concentrations in the dope are usually higher, thus affecting porosity and pore size. Furthermore, this could result in non-newtonian rheological behavior [36]. There are three main methods for preparing hollow fibers and capillaries, namely, wet spinning, melt spinning, and dry spinning [37]. There are several parameters that are known to affect fiber morphology, properties, and performance. These parameters can be divided into the following four categories: Parameters connected to the dope composition include polymer and additives type, concentration, viscosity, and temperature. Parameters connected to the spinning experiment include temperature, dope extrusion rate, spinneret type (double/triple), geometry and dimensions, airgap length, and atmosphere (moisture). Parameters connected to the coagulation include temperature, composition and injection rate of the bore fluid (BF), and temperature and composition of the coagulation bath. Parameters connected to the eventual post-treatments include take-up speed (stretching), chemical or thermal posttreatments (additive leaching, thermal annealing), and drying techniques (hexane, glycerol, etc.). The type of polymer determines some key membrane features, such as its hydrophilicity/hydrophobicity and fouling tendency, structure, mechanical properties, and chemical resistance [36]. Polymer concentration is a key parameter for the thermodynamics and kinetics of PI. Higher polymer concentration will reduce the

14 16 Membrane Fabrication solvent volume fraction, and as a result, less nonsolvent is required to achieve PS. Moreover, due to its effect on viscosity, it will affect the kinetics of S NS exchange. Higher polymer concentration could result in the formation of a thicker skin, which will delay coagulation of the inner layers. Tasselli et al. [38] observed that polymer concentration affected the thermodynamics of PI in the preparation of modified poly(ether ether ketone) (PEEK-W) hollow fibers; the binodal curve was found to shift toward the P S axis, indicating less nonsolvent tolerance, as the polymer concentration increased. Polymer concentration affected the kinetics as well, due to its effect on dope viscosity. Fiber morphology was found to be affected by increasing polymer concentration; finger-like voids at the outer surface were reported to reduce significantly. The effect of nonsolvent was more pronounced at higher polymer concentration, as expected from the shift of the binodal curve toward the P S axis. Polymer concentration was found to also affect fiber performance, with typical trade-off between flux and rejection. Sukitpaneenit and hung [39] observed an increase of dope viscoelastics properties with polymer concentration in the preparation of PVDF hollow fibers, which influenced fiber morphology and, in particular, macrovoids formation. This was attributed to the increased shear and elongation viscosities, due to greater degree of chain entanglement, which reduced nonsolvent penetration during coagulation. Pore-forming additives are known to affect the delicate balance between kinetics and thermodynamics. Additives with high molecular weight (Mw) are usually retained in the fiber structure, thus modifying hydrophilicity/hydrophobicity [30]. For instance, PVP is known to affect the thermodynamic and kinetics of the PI process due to its hydrophilicity (thermodynamic enhancement) and its effect on the dope viscosity (kinetic hindrance). Tasselli et al. [40] observed that macrovoids growth was suppressed in PEEK-W hollow fibers, until complete sponge-like structure was obtained by increasing the PVP concentration. The increase of PVP concentration from 0 to 20 wt% reduced porosity from 84% to 74%. Water permeability was found to decrease, whereas dextran rejection and fiber mechanical strength were found to increase. Temperature is a key parameter referring to both the dope and the coagulants [36]. Indeed, temperature affects dope viscosity. Peng et al. [41] observed that more macrovoids can be observed in the cross-sectional morphology of Torlon polyamideimide fibers when increasing the spinneret temperature due to the reduction of dope viscosity. The temperature of both inner and outer coagulant will affect the interdiffusion between solvent and nonsolvent at the fiber walls, thus affecting the kinetics of PI. hung and Kafchinski [42] observed that a more porous structure was formed in 6FDA/6FDAM polyimide fibers by increasing the external coagulant temperature due to delayed demixing (connected to increased solubility). Fiber coagulation is much more complicated, with respect to flat-sheet membrane, since it involves two surfaces. The thermodynamics and kinetics of PI are affected by both the coagulants. As discussed in Section 1.4, the main factors are the nonsolvent power, its mutual affinity with the dope solvent, the solubility parameter differences between P NS and S NS, the solvent and nonsolvent diffusivities connected to their molecular size, and, obviously, temperature. Tasselli and Drioli [43] showed that hollow-fiber morphology, transport, and mechanical properties can be tailored

15 Polymeric Membranes 17 by varying the composition of the BF. In particular, the effect of different R H BFs, with R=H; H 3 ; 2 H 5 ; n 3 H 7 or n 4 H 9, on the properties of PEEK-W hollow-fiber membranes was examined. Going from water to alcohols with progressively longer aliphatic chain, the binodal curve moved toward higher nonsolvent concentrations, indicating less nonsolvent power. Phase diagrams were found to be similar by adding low molecular weight PVP (Luviskol K-17, Mw 12 kda) to the dope solution, showing that the additive influenced mostly the kinetics, rather than the thermodynamics of PI. The different composition of BFs also influenced the kinetics of PI, mainly due to the increasing nonsolvent molecular dimensions, which further reduced diffusivity. Tasselli et al. [38] observed that increasing solvent (DMAc) percentages in the BF affected fiber dimensions, thus increasing the diameter and reducing the thickness. This was attributed to the effect of solvent percentage on the degree of fiber inflation, normally caused by BF injection. Higher solvent percentage induced a delayed onset of demixing, resulting in a softer skin at the inner surface, which was easier to inflate when compared to a rigid skin produced by sudden coagulation induced by water. Increase of solvent concentration in the BF was found to decrease rejection without affecting permeability due to a more open skin layer. Fiber properties can be modulated by acting on the atmosphere of the air gap. Tasselli and Drioli [40] found that the relative humidity percentage in the air-gap atmosphere strongly affected the morphology of the outer layer of PEEK-W hollow fibers. Although all membranes prepared under unsaturated conditions showed similar morphology and water permeability, the presence of supersaturated water vapor and microdroplets in the air gap induced the formation of a macroporous skin at the outer surface, which induced local PS at the outer surface of the fibers. The rheology of spinning experiments is complicated and involves both the shear stresses experienced by the dope within the spinneret and the elongational stresses in the air-gap region connected to gravitational force or additional stretching during the take-up of fibers. Tasselli et al. [38] observed that an increase of the air gap-induced higher stretching of the nascent fiber due to the gravitational force. This resulted in elongation, higher spinning rate, and hence reduction of fiber dimensions. Peng et al. [36] pointed out that the supplementary elongational stress may be caused by the take-up device and influence the properties of hollow fibers by inducing extra phase instability, thereby enhancing PS and promoting orientation and packing. The air gap could modulate the effect of the rheological die-swell and affect the morphology of fibers. Peng et al. [44] found that there was a critical air-gap distance, which varied with the dope composition, above which the effects of die swell and chain relaxation were suppressed, and as a consequence nonsolvent intrusion and macrovoids formation were hindered. The rheology, molecular orientation, and finally the fiber morphology are strongly affected by the spinneret architecture. The structure of a common spinneret, and the phenomena that take place during extrusion are depicted in Figure 1.6. Peng et al. [36] pointed out that if fibers are extruded using a small annular gap, the shear rates will be higher, thus inducing molecular orientation, chain packing, and confounding macrovoids formation. According to Widjojo and hung [45], shear stress within the spinneret could also help to eliminate the irregularities of the outer surface besides macrovoids formation.

16 18 Membrane Fabrication Bore fluid Dope Spinneret Air-gap region Die swell Stretching oagulant bath Dense layer Solvent exchange Microporous layer FIGURE 1.6 Schematic diagram of area near the spinneret and the formation of nascent hollow fiber during PI. (Data from N. Peng et al., Progress in Polymer Science, 37, , 2012.) In the literature, different modifications of the spinneret design were proposed as follows: (1) spinnerets with different flow angles, to tailor pore size distribution and control pure water permeability [46] and (2) spinnerets with microstructured annulus or needle, to increase the active surface area [47,48]. It is possible to produce dual-layer hollow fibers, for several applications, using a triple orifice spinnerets. The production of dual-layer fibers is more complex due to the presence of two dope solutions. ne of the most critical issues is delamination of the outer layer. A modified spinneret, with an indented middle tube, has been proposed to induce interdiffusion between the two fiber layers and prevent delamination [49]. Fiber posttreatments and drying procedures represent further important steps. Thermal treatments and cross-linking could be required to improve mechanical and chemical stabilities and/or selectivity [50 53]. After spinning, fibers are usually washed with deionized or milli-q water, to exchange the residual solvent. Then a suitable drying procedure should be applied. Some authors suggest to gradually exchange water with low-surface tension liquids, such as methanol, ethanol, or hexane, in a multistep process, involving fibers soaked in different mixtures or pure liquids. See, for instance, Abed et al. [54] (ethanol and hexane), Xu et al. [55] (50% ethanol, pure ethanol, ethanol hexane 1:1), Mansourizadeh et al. [56], and Wang et al. [57] (methanol and hexane). ther studies suggest the use of water/glycerol mixtures [28,38,40,43].

17 Polymeric Membranes EXAMPLES F MEMBRANE PREPARATIN FR PRESSURE-DRIVEN SEPARATIN PRESSES Membrane pressure-driven processes, namely, MF, UF, NF, and R are normally carried out in the liquid phase. Although water permeates through the membrane, other species are partially or completely rejected. According to Fane et al., The MF UF range can be considered as a continuum [11]; both processes involve porous membranes. MF is carried out using symmetric membranes, with pore size ranging from 0.05 to 10 μm. UF, instead, requires asymmetric membranes, with pore size from 1 to 100 nm. The NF and R spectrum is also considered as a continuum [11]. NF/R membranes are usually thin-film composite (TF) structures with nonporous skin. The most important features of pressure-driven membrane processes are resumed in Table 1.3. MF is used to remove suspended solids, algae, and bacteria. Membranes can be prepared from either polymeric or inorganic materials. The structure is usually symmetric. UF also removes viruses, colloids, and macromolecules. The separation ability of a UF membrane is usually expressed in terms of molecular weight cut off (MW). This is defined as the molecular weight of the solute that is 90% retained by the membrane. Typical MW values for UF membranes ranges from 1 to 300 kda [11]. Although for MF membranes permeability is usually affected by the entire membrane thickness, performance of the UF membranes mostly depends on the skin-layer properties. R membranes can remove even monovalent ions, as Na + and l ; the typical application is seawater desalination. The literature defines these membranes as dense TABLE 1.3 Typical Properties of Pressure-Driven Membrane Processes Microfiltration Ultrafiltration Nanofiltration Reverse smosis Pore size (nm) 50 10, ~2 <2 Water permeability > (lm 2 h 1 bar 1 ) perating pressure (bar) MW (Da) Not applicable ,000 >100 >10 Targeted contaminants in water Membrane materials Bacteria and algae, suspended solids Polymeric, inorganic Bacteria, virus, colloids, and macromolecules Polymeric, some inorganic Di- and multivalent ions, natural organic matter, and small organic molecules Thin-film composite polyamide, cellulose acetate, and other materials Dissolved ions and small molecules Thin-film composite polyamide and cellulose acetate

18 20 Membrane Fabrication or with subnanometer pores. Depending on their performances, expressed in terms of permeability and rejection to Nal, these membranes are usually divided into seawater R (SWR) membranes and brackish water R (BWR) membranes. NF can remove bi- and multivalent ions and small organic molecules, being located between the UF and R range. Besides water softening, another relevant application of NF membranes is solvent separation, which requires materials with high chemical stability or solvent-resistance nanofiltration (SRNF) membranes. R and NF membranes can be prepared by PI, to produce an integrally asymmetric structure, made up of one polymeric material, or by interfacial polymerization (IP), to obtain a TF structure, with a skin of cross-linked aromatic PA over a support layer such as PSU on a reinforcing fabric Microfiltration MF represents a viable alternative to conventional processes for the treatment and recycling of water and wastewater [58,59]. Its increasing application is mainly connected to the progressively more severe regulation both regarding environmental safety and drinking water quality [60]. MF is also widely applied in the agro- industrial sector [61,62] and in biotechnology [63]. ne of the main obstacles to the full application of MF is membrane fouling, which induces a rapid decline in productivity. Furthermore, the physical and chemical cleaning procedures, backflushing, and treatment with various cleaning agents are time-consuming and may reduce membrane lifetime, resulting in reduction of plant capacity and efficiency [64]. Apart from the adjustment of hydrodynamic conditions, most work has been devoted to the preparation of MF membrane with high productivity, desired selectivity, and low fouling, coupled with the enhancement of mechanical, chemical, and thermal stabilities. Among the different polymeric materials, the most widely used for producing MF membranes through the PI process are PVDF, PSU, and PES, together with polycarbonate, A, diacetate, triacetate, and their blends [11]. Hydrophilic surfaces are usually preferred for fouling reduction. However, although A is naturally hydrophilic, PVDF, PSU, and PES can be modified using different additives. Susanto et al. [65] prepared MF membrane in PES, using triethylene glycol (TEG) and Pluronic, a poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) triblock copolymer, as nonsolvent and wettability modification agent, respectively. Membranes were prepared by combining the immersion precipitation technique with exposure to humid air before coagulation (combination of VIPS and NIPS). The best performances were observed for membranes prepared with the dope having the following composition: PES/NMP/TEG/Plu = 10/30/55/5 (wt%) with 3 min exposure in humid air (50% 60% RH) before coagulation (Figure 1.7). Incorporation of nanoparticles (NPs) is reported as an effective method to improve the hydrophilicity, thus reducing the susceptibility to fouling of polymeric membranes. For instance, Hong and He [66] prepared PVDF MF membranes using PEG 600 as additive and Zn particles as nanofiller. Similarly, Dong et al. [67] prepared PVDF membranes using PEG 600 as an additive and Mg(H) 2 NPs as filler. PVDF MF membranes were prepared through TIPS using nano-ti 2 particles as filler and dimethyl phthalate (DMP) as diluent by Shi et al. [68]. The same authors obtained interesting

19 Polymeric Membranes 21 FIGURE 1.7 Scanning electron microscopy images of the surface morphology (left) and cross-sectional morphology (right) of a PES membrane prepared from dope PES/NMP/Plu/ TEG = 10/30/5/55 wt% (Pluronic PE6400) with 3 min of exposure to humid air. (Data from H. Susanto et al., Journal of Membrane Science, 342, , 2009.) results by modifying the nano-ti 2 particles using carboxyl-functional ionic liquid ([H2Hmim]l) [69]. Another strategy for improving the hydrophilicity of PVDF membranes is surface treatment. PVDF MF membranes with negative charge were produced by direct sulfonation with chlorosulfonic acid [70]. UV-photo-grafting modification on PVDF MF membranes was carried out using 2- dimethylaminoethyl methacrylate (qdmaem) and 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) and benzophenone as the photo-initiator by Hilal et al. [71] Ultrafiltration Membranes for UF can be prepared through the PI process using polymers such as PVDF, PSU, PES, A, but also polyacrylonitrile (PAN) and modified poly(ether ether ketone) [11,38]. The main difference with MF is the need for asymmetric membranes, with thin selective skin and support layer with reduced resistance. The main targets for preparing optimized membranes for UF are obtaining a sharp cut-off, improved mechanical and chemical resistances, and reduced susceptibility to fouling. For instance, research on PVDF membrane production focuses mainly on the reduction of its intrinsic hydrophobicity to reduce fouling, by methods such as the introduction of nanosized alumina [72]. Al 2 3 PVDF nanocomposite membranes have been successfully tested for the treatment of oily wastewater, showing better resistance to fouling [73]. rganic inorganic PVDF silica (Si 2 ) composite hollow fibers showed improved hydrophilicity and permeability and enhanced antifouling properties in UF experiments [74]. PVDF membranes were modified by PVP and were used to purify flavonoids from crude Ginkgo biloba extraction products [75]. oating with hydrophilic polymers, such as chitosan or poly(vinyl alcohol), has also been performed for fouling reduction [76,77]. In addition, other techniques, such as blending and grafting, have been tested to modify the surface properties of PVDF membranes [78 81].

20 22 Membrane Fabrication Recently, the use of Ti 2 NPs for preparing mixed matrix membranes (MMMs) was reported by several authors, due to Ti 2 NPs commercial availability, good stability, and excellent photocatalytic, antibacterial, antifouling, and UV-cleaning properties. Song et al. [82] studied the preparation of photo- catalytically active PVDF/ Ti 2 hybrid membranes, which were tested for natural organic matter removal in both dead-end and cross-flow UF experiments. PES is also widely used for the preparation of UF membranes [83,84]. Razmjou et al. [85] prepared PES UF membranes with Ti 2 NPs modified mechanically or both mechanically and chemically. PES MMMs were also prepared by coating Ti 2 NPs onto the membrane surface [86] NF and Solvent-Resistant NF The most commonly used polymeric materials for preparing NF membranes are A, polyimide and PA; other materials are polyvinyl alcohol (PVA), sulfonated PSU, and inorganic materials, such as some metal oxides. NF membranes can be integrally asymmetric or TF. A typical TF membrane is made up of a dense selective skin (PA) on the top of a microporous PSU or PES layers; a nonwoven fabric acts as mechanical support (Figure 1.8). The main advantages of TF are connected to the possibility of controlling and optimizing the properties of each layer, in order to attain the desired selectivity and permeability coupled with excellent mechanical resistance. There are different techniques for preparing composite membranes, such as lamination, coating, and plasma polymerization. However, the IP technique is the most applied technique for preparing TF membranes [5]. It is based on the reaction of monomers on the surface of the porous support film at the interface between two immiscible media. IP was first developed by adotte et al. [87,88], who found that composite membranes with high flux, high rejection to aqueous sulfate ions, coupled with low selectivity toward chloride ions, can be produced by interfacial cross-linking of piperazine (PIP) with trimesoyl chloride (TM)/isophthaloyl chloride mixture. The basic mechanism for polymerization of PA layers, based on PIP and TM, is shown in Figure 1.9. Fully aromatic PA can be obtained from TM and m-phenylenediamine (MPD), whereas semiaromatic PA can be obtained from TM and PIP (Figure 1.10). The research on TF membranes aims at enhancing membrane productivity and selectivity, improving resistance against chemicals as chlorine and solvents, and reducing Polyamide μm Polysulfone μm Backing layer 200 μm FIGURE 1.8 Typical structure of a TF membrane. (Data from A.G. Fane et al., Membrane technology for water: Microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. In: P. Wilderer, ed., Treatise on Water Science, Academic Press, xford, 2011, pp )

21 Polymeric Membranes 23 H H H N N N NH NH n H 1 n (a) N N N N (b) n H FIGURE 1.9 (a) Fully aromatic polyamide based on TM and MPD. (b) Semiaromatic polyamide based on TM and PIP. (Data from A.G. Fane et al., Membrane technology for water: Microfiltration, ultrafiltration, nanofiltration, and reverse osmosis. In: P. Wilderer, ed., Treatise on Water Science, Academic Press, xford, 2011, pp ) Porous support (a) HN (b) Piperazine NH Impregnation liquid 1 + reactant A l + l Trimesoyl chloride Immersion liquid 2 + reactant B l N N Polymerization N N N 1 n omposite membrane N + Hl FIGURE 1.10 (a) Basic mechanism for the preparation of TF membranes by IP. (b) Reaction between piperazine (PIP) and trimesoyl chloride (TM) for preparing semiaromatic polyamide layer. (Data from H. Strathmann et al., Basic Aspects in Polymeric Membrane Preparation, In: E. Drioli and L. Giorno, eds., omprehensive Membrane Science and Engineering, vol. 1: Basic Aspects of Membrane Science and Engineering. Elsevier, Amsterdam, the Netherlands, 2010, pp ) the susceptibility to fouling. A recent review by Lau et al. summarized the development of TF membrane technology over the last decade [89]. ther techniques for NF membrane preparation are photo- or thermal-grafting, dip-coating, electron beam irradiations, and plasma-initiated polymerization. ther strategies for improving the performance of NF membranes include blending and

22 24 Membrane Fabrication incorporation of nanofillers. Mansourpanah et al. [90] prepared NF membranes from a blend of PES/polyimide and modified them using ethylenediamine as a cross-linker and newly synthesized modifiers (PEG-triazine). Redox-initiated graft polymerization and sulfonation were compared as modifying methods for NF membranes by Van der Bruggen [91]. Ti 2 NPs were assembled on the surfaces of PES/polyimide NF membrane by Mansourpanah et al. [92]. PES/polyimide blend membranes were also H functionalized using different concentrations of diethanolamine. Solvent-resistant NF (SRNF) represents a fairly new and interesting application of NF in different industrial fields (e.g., food, chemical, and pharmaceutical) for purification, recovery, or recycling of oligomers, catalysts, and solvents. This process requires membranes to be able to withstand aggressive environments, with high chemical resistance, coupled with desired permeability and selectivity. Not only TF but also integrally skinned asymmetric membranes can be used for SRNF. The most widely used polymers for the preparation of SRNF membranes are polyimide, PAN, polyelectrolyte complex membranes (PEMs), and polydimethylsiloxane (PDMS). Polyelectrolyte complexes (PEs) are a variegate group of multicomponent polymeric materials that can be used for preparing membranes for different applications. The main aspects regarding physical background and the preparation and application of PEMs were recently reviewed by Zhao et al. [93]. PEs are normally insoluble in common organic solvents; therefore, they represent promising materials for producing SRNF membranes [93]. Vankelecom et al. [94] prepared multilayered PE SRNF membranes for filtration of polar aprotic solvents (DMF and tetrahydrofuran [THF]). Recently, polymers belonging to the sulfone family, such as PSU, have also been reported for the preparation of SRNF membranes. Holda et al. [95] prepared SRNF membranes from PSU using a mixture of NMP and THF (70/30) as solvent. Polyphenylsulfone (PPSU) (Figure 1.11) shows great potential for the preparation of SRNF membranes, since it has high chemical and mechanical resistance and capacity to operate at high temperatures. Darvishmanesh et al. [97] prepared PPSU hollow-fiber NF membranes and studied their isopropanol (IPA) permeability and rejection to Rose Bengal and Bromothymol blue, as well as their resistance to several solvents. Although the produced fibers were visually stable in most of the solvents except MEK, permeability tests showed that the membranes were not stable in acetone and toluene Reverse smosis R is the most relevant membrane-based technique for seawater desalination [98]. Similar to NF, R is carried out using asymmetric membranes with a nonporous skin layer. Membranes can be integrally skinned or TF. The most important technique for the preparation of such membranes is IP, which has been already described in Section devoted to NF membranes. As reported by Lee et al. [99], the studies about the preparation of polymeric membranes for R application, from 1950 to 1980, focused on the search for optimum membrane materials. Subsequently, the performance of R membranes was improved by controlling membrane formation reactions and using catalysts and additives.

23 Polymeric Membranes 25 Polysulfone T g = 190 S n Polyethersulfone T g = 220 S S n Polyphenylsulfone T g = 220 S FIGURE 1.11 hemical structure of the sulfone family. (Data from S. Darvishmanesh et al., Journal of Membrane Science, 384, 89 96, 2011.) As reported by Misdan et al. [100], the three main challenges of R TF membranes in the desalination industry are fouling propensity, boron rejection, and chlorination. The main strategies for the development of R membranes with reduced tendency to fouling were reviewed by Kang and ao [101]. These include (1) the development of new R materials or improvement of IP process; (2) the surface modification of existing R membranes by physical methods, adsorption and coating, or chemical methods, including grafting and plasma polymerization; and (3) the preparation of hybrid polymeric/inorganic membranes. For instance, Li et al. [102] reported on the preparation of TF membranes using two novel synthesized tri- and tetra-functional biphenyl acid chloride 3,4,5-biphenyl triacyl chloride 3,3,5,5 -biphenyl tetraacyl chloride and MPD. Lee et al. [99] proposed that zeolite membranes, thin-film nanocomposite membranes, carbon nanotube (NT) membranes, and biomimetic membranes could offer an attractive alternative for improving the performance of R membranes. Ti 2 NPs are an interesting candidate to solve the problem of fouling and biofouling. Kwak et al. [103] prepared a polymeric/hybrid R membrane composed of aromatic PA thin films underneath Ti 2 NPs. Kim et al. [104] prepared TF aromatic PA membrane, with Ti 2 self-assembled on the surface. Moreover, Madaeni and Ghaemi [105] prepared PVA R membranes with a coating of Ti 2 NPs. Buonomenna [106] highlighted that nanotechnology is one of the most promising strategies for producing the so-called nanoenhanced membranes (NEMs), which are R membranes with enhanced properties. NEMs can be produced by exploiting inorganic materials, such as zeolites or Ti 2, as an alternative. Bioinspired NEMs can be produced using NTs or aquaporins. n

24 26 Membrane Fabrication Gas Separation The application of polymeric membranes for selective separation of gas mixtures is becoming a viable alternative to traditional gas separation (GS) technologies, such as pressure swing adsorption and cryogenic separation. Membrane GS results in several advantages, as it does not require any phase transition and moving parts, and can, therefore, be also used in remote locations [107]. The development of new materials and membranes for GS is exhorted by the growing necessity for low emission plants and clean industrial processes. As pointed out by Nunes and Peinemann [108], inorganic membranes are usually preferred because many processes at the industrial level are carried out at high temperature. However, polymeric membranes can be used for H 2 /hydrocarbon separation in the platformer off gases from refineries and for 2 separation in coal plants. Polymeric membranes for GS can be symmetric or asymmetric, but should have a dense selective layer. Three types of membrane structures can be employed: (1) homogeneous dense membranes (symmetric); (2) integrally skinned asymmetric membranes; and (3) composite membranes. The choice of the membrane material is of crucial importance, since the transport mechanism is based on the affinity between the membrane matrix and the permeating species. Besides investigating the most suitable material available, studies on GS membranes aim at finding the most appropriate techniques for producing defect-free membranes, showing high performance in terms of flux and selectivity (limited by typical trade-off), coupled with good mechanical/thermal/chemical resistance. Brunetti et al. [107] produced integrally skinned asymmetric membranes from PEEK-W and studied the influence of different preparation parameters, such as the composition and temperature of the coagulation bath and casting knife gap set, on the membrane morphology and transport properties, permeance, and selectivity. Membranes were prepared by immersion precipitation, using THF as solvent. Three different coagulation baths were tested: a mixture of methanol/water 70/30, pure methanol (MeH), or pure IPA. Interesting advances in the field of GS membrane materials are polymers with high-free volumes, such as poly(1-trimethylsilyl-1-propyne), poly(4-methyl-2- pentyne), and polymers of intrinsic microporosity [108]. Regarding the emerging application of 2 capture, the copolymer class Pebax (Arkema) showed promising results. For instance, Bondar et al. [109] reported interesting values of 2 /N 2 and 2 /H 2 selectivity for different grades of Pebax membranes. The preparation of MMMs using, for instance, zeolite fillers, seems another promising strategy for improving GS membrane performance and overcoming the typical trade-off between productivity and selectivity. Although the preparation of defect-free zeolite membranes is very difficult, MMMs offer the possibility of combining their superior transport properties to the simplicity of processing polymer membranes [110]. Besides zeolites, different types of carbon-based fillers, such as carbon molecular sieves (MSs), fullerenes, and NTs, have been recently reported as promising materials for preparing high-performance MMMs for GS. Vu et al. [111]

25 Polymeric Membranes 27 reported the preparation of polyimide membranes filled with MS particles, which showed considerable improvement in permselectivity for 2 /H 4 and 2 /N 2 gas couples, and higher permeability to 2 and 2, with respect to the neat membrane Pervaporation Pervaporation (PV) is a membrane-based process used to separate the components of a liquid mixture. It requires dense membranes. The liquid feed is heated up and placed in contact with the active layer, whereas a vacuum or a sweep gas is applied downstream. The driving force is a chemical potential gradient through the membrane cross section. The separation phenomenon is explained according to the solution diffusion model. The selective separation depends on the different dissolution of feed molecules into the membrane matrix and their diffusivity. Based on the feed composition and the target of the separation, PV is generally classified into the following three categories: hydrophilic, hydrophobic, and organophilic. The first type is carried out to dehydrate organic compounds or to extract water from a mixture using hydrophilic membranes [112,113]. The second type is carried out using hydrophobic membranes and can be applied for recovering organic solvents, removing alcohol from alcoholic beverages, and recovering aroma compounds from fruit juices [114,115]. Finally, the third type is useful to separate organic/organic mixtures [116,117]. PV is now established as a good alternative to traditional separation processes, for instance, extraction or distillation, thanks to its ease of operation, lower costs, and reduced requirement of chemicals. An interesting application of PV is the separation of organic/ organic mixtures made up of close-boiling points liquids or forming an azeotrope, because distillation is less-efficient and more expensive particularly in such cases. Zereshki et al. [118] prepared poly(lactic acid) (PLA) homogenous dense membranes, which were used for selective separation of methanol/methyl tert-butyl ether (MeH/MTBE). This separation is of great interest since the mixture of MeH and MTBE forms a minimum boiling azeotrope. PLA/PVP membranes were applied for PV separation of ethanol/cyclohexane azeotropic mixture [119]. Ethanol/ cyclohexane mixture separation through PV was also carried out using membranes produced from a blend between PEEK-W and PVP [120]. Similar to what was observed for PLA/PVP membranes, although all the prepared membranes were ethanol selective, the addition of PVP improved membrane performance. Furthermore, PVP enhanced the selective adsorption of ethanol from the membrane, thus enhancing selectivity. PEEK-W dense membranes were also used to carry out the separation of MeH/ MTBE azeotropic mixture through PV [121]. The search for new membrane materials is of peculiar interest to carry out organic/ organic separations. When the feed contains particularly aggressive solvents, it is necessary to find membrane materials that are able to withstand such harsh environments. Simone et al. [122] prepared asymmetric dense membranes using a copolymer of ethylene chlorotrifluoroethylene (ETFE), commercialized by Solvay Advanced Polymers as Halar. ETFE represents an exceptionally promising material for

26 28 Membrane Fabrication preparing membranes for separations involving organic solvents, due to its exceptional resistance to a broad group of aggressive and corrosive chemicals, coupled to high temperature resistance and durability. The incorporation of nanofillers, for instance, zeolites, represents a promising strategy for improving membrane performance in PV. Dobrak et al. [123] prepared PDMS composite membranes and investigated the effect of two types of fillers, namely, commercial zeolite silicalite (BV 3002) and laboratory-made colloidal silicalite-1, on membrane performance in the removal of ethanol from ethanol/water mixtures through PV. Filler incorporation increased membrane stability by crosslinking. Furthermore, the PDMS membrane filled with commercial zeolites showed a significant increase of selectivity. Incorporation of BV 3002 fillers into a PDMS composite membrane was also found to enhance the performance in PV tests of toluene removal from water [124]. 1.7 EXAMPLES F MEMBRANE PREPARATIN FR M Among the new membrane operations, Ms have shown great potential for the development of environmentally sustainable technologies. An M generally uses a polymer matrix to create an interface for the mass transfer and/or the reaction between two phases. Ms and related operations were recently described in detail by Drioli and riscuoli [ ]. As further references, the books by R. Baker [128] and N. Li [129] can be consulted. Ms are an extension of the traditional concept of membrane operations. Ms can work with either hydrophobic or hydrophilic membranes, depending on the process and nature of the feed. Depending on the physical state of the two phases, different types of Ms can be built up (Figure 1.12). Typical examples of M include gas/liquid, liquid/liquid with immiscible phases, and liquid/liquid with miscible phases, including membrane distillation (MD), osmotic distillation (D), and membrane crystallizers. There are Membrane operations Traditional (MF, UF, NF, R, GS, PV) Membrane contactors Gas/liquid Liquid/liquid Liquid/vacuum Supported liquid membranes Membrane distillation Thermal-driven smotic FIGURE 1.12 Membrane contactor categories.