Poly(ether ether ketone) - Springer Link

ISSN 0965545X, Polymer Science, Ser. A, 2009, Vol. 51, Nos. 11–12, pp. 1355–1366. © Pleiades Publishing, Ltd., 2009.

Poly(ether ether ketone) Derivative Membranes—a Review of Their Preparation, Properties and Potential1 Johannes Carolus Jansen and Enrico Drioli Institute on Membrane Technology, ITMCNR, Via P. Bucci 17/C, 87030 Rende (CS), Italy email: [email protected]

Abstract—The present paper gives an overview of the properties and performance of membranes of a poly(ether ether ketone) derivative with a cardo group in the chain, known in the literature as PEEKWC or PEKC. This is one of the typical examples of a new polymer, emerged in the last two decades, with the potential to be applied as a membrane material in a wide range of application fields. Due to the presence of the cardo group in the backbone, the polymer is soluble in several common organic solvents, in contrast to the traditional poly(ether ketone) (PEK) and poly(ether ether ketone) (PEEK). It is therefore more versatile and its solubility allows the use of nonsolventinduced phase inversion techniques to prepare membranes with a wide range of different morphologies and transport characteristics. The present review will show the current state of the art and will testify that PEEKWC offers interesting perspectives in especially the fields of gas sep aration, biomedical applications and—in its sulfonated form–in fuel cells. Examples of successful applica tion in microfiltration, ultrafiltration, nanofiltration, pervaporation, membrane contactors, catalytic mem branes and some other applications, such as packaging and molecular imprinting will also be shown. DOI: 10.1134/S0965545X09110200 1

FOREWORD

From the early stage of membrane research activi ties, polymer scientists have been interested in explor ing the potentialities of their materials in developing new selective and permeable membranes, to be uti lised in a large variety of different operations and industrial applications. Prof. Nikolai Platé was one of those outstanding polymer scientists who anticipated the future of polymeric membranes in the biomedical fields, in gas separations and various other areas. As an expert in polymer chemistry and polymer physics, his intuitions permitted the creation of an important membrane research activity at his Institute, which spread out over various Russian Research Centres in the subsequent years. It is a pleasure and an honour to dedicate this manuscript to his name. INTRODUCTION In the last several decades, membranebased oper ations have grown in importance in an increasing number of separation processes because of several advantages over the traditional technologies, also in important fields like water treatment, gas separation, food and beverage processing etc. Since the develop ment of the first asymmetric cellulose acetate mem branes by Loeb and Sourirajan [1], which enabled the achievement of fluxes of practical interest also for commercial applications, the polymeric membrane 1 The article is published in the original.

market has been dominated by a relatively small num ber of either rubbery, glassy or semicrystalline poly mers. By far the most common rubber is poly(dime thyl siloxane) (PDMS), which is mainly used in gas and vapour separation and in pervaporation. Semi crystalline polymers such as polyamide (PA), polyeth ylene (PE), polypropylene (PP), poly(vinylidene fluo ride) (PVDF) and poly(tetrafluoro ethylene) (PTFE) are mostly used for porous micro and ultrafiltration (UF) membranes, because the crystalline domains would be impenetrable for diffusive transport in their dense membranes. Only amorphous glassy polymers find widespread application in the entire range from dense to porous membranes. Some of the most com mon glassy polymers used as membrane materials are commercial polymers like cellulose acetate (CA), polyimides (PI), poly(ether imide)s (PEI), polycar bonate (PC), polysulfone (PSf) and polyethersulfone (PES). The membrane market is dominated by the same polymers since years, without great changes. At the same time, thanks also to polymer scientists like Nikolai Platé, there has been a continuous search for new materials, leading for instance to the evalua tion of more sophisticated polymers such as glassy poly(vinyltrimethylsilane) [2] and amorphous poly acetylenes [3, 4] with a high free volume and conse quently high permeability, or glassy perfluoropolymers [5] for applications which require a particular chemi cal or physical stability. Also the booming interest in sulfonated polymers such as Nafion® for fuel cell applications, driven by the search for alternative

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JOHANNES CAROLUS JANSEN, ENRICO DRIOLI

O

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PEEKWC Fig. 1. Chemical structure of semicrystalline PEK and PEEK, and amorphous glassy PEEKWC or PEKC.

energy sources and conversion technologies, is a good example of how new polymeric membrane materials may be developed. Whereas for instance perfluoropolymer mem branes with their particular properties may open new markets, occasionally new polymers are presented that can simply compete with currently used commercial materials with similar properties. A typical example is the poly(ether ether ketone) with phthalidecardo group [6], obtained by solution polycondensation of phenolphthalein and dichlorobenzophenone, and in the literature usually abbreviated as PEEKWC or PEKC (Fig. 1). As yet the polymer is not commer cially available on the world market and its future applicability will depend on the necessity of EINECS/ELINCS registration and, for food or bio medical applications, on its eventual FDA approval. Nevertheless, it presents a series of favourable proper ties which makes it a truly interesting candidate as a new membrane material. An overview of properties and of potential application fields is described below. Unsubstituted Poly(ether ketone)s Versus PEEKWC The members of the polyetherketone family, such as poly(ether ketone) and poly(ether ether ketone), are generally semicrystalline and are characterized by an extremely high thermal stability while they are vir tually insoluble in common organic solvents and at low temperature. Generally, in organic solvents their solu bility is limited to those solvents with boiling points higher than their own melting point, mainly because of their insoluble crystalline fraction. Indeed, only few examples of polyetherketone membranes have been reported, using very harsh conditions for their prepa ration. For instance solutions of PEK can be prepared in concentrated sulfuric acid [7–9], which might slightly sulfonate the polymer and thus facilitate its dissolution. Indeed, substituted PEEKs have a higher

solubility in organic solvents than the parent polymer [10] because they are often amorphous. Also PEEKWC is an amorphous glassy polymer with quite similar properties as PES and PSf. It is miscible with the latter [11]. The Tg of PEEKWC was reported to be 228°C, and it also exhibits a secondary transition (βtransition) at about 70°C, which coincides with a small change in the activation energy for the perme ation of large penetrant molecules [12]. It may further present considerable enthalpy relaxation [13], and it can be foreseen that physical aging will cause time dependent changes in the free volume and the trans port properties. Applications of PEEKWC The amorphous character of PEEKWC, due to the presence of the bulky phenolphthalein moiety, which prevents regular packing of the polymer chains in a crystal lattice, confers it a good solubility in several common organic solvents, enabling the preparation of membranes by simple solution casting methods [14], while the polymer maintains the excellent chemical, mechanical and thermal properties of normal PEEK. It is part of a larger family of similar polymers with dif ferent pendant groups [15] and has been studied for various applications. Numerous papers on thin film guesthost systems study its potential for optical and electrooptical devices [e.g., 16–19]. The first reports on the use as membranes date back to 1987–1988 and were focused on ultrafiltration [20, 21], while its potential for gas separation has been recognized shortly after, in 1990, by two different groups [12, 22]. For instance Liu et al. found promising values for the CO2/CH4 selectivity and O2/N2 selectivity of 32 and 5.7, respectively at 25°C for a 50 micrometer thick dense membrane [12]. There has been a significant development in the preparation of PEEKWC gas sep aration membranes by phase inversion, recently

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Comparison of the transport properties of an asymmetric dense PEEKWC flat film membrane and an asymmetric PEK hollow fibre membrane PEEKWC flat film membrane after silicone coating [48] a

Selectivity

Permeance Gas (M) Nitrogen Oxygen Methane Helium Hydrogen CO2

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PEK Hollow fibre membrane [8]

h bar)

5.93 × 10–3 3.77 × 10–2 6.14 × 10–3 0.521 n.d.c 0.187

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2.19 13.9 2.27 193 – 69.2

1.00 6.4 1.03 88 – 32

0.97 6.14 1.00 85 – 30.5

Permeance GPU

Selectivity (M/CH4)

0.126

1.00

4.95 0.972

39.3 7.71

Note: a Effective skin thickness of 38 nm based on helium permeance. b 1 GPU = 1 barrer/µm = 10–6

3

m STP /(cm2 s cm Hg).

c

Not determined on the reported sample. However, the helium and hydrogen permeability are nearly identical [J.C. Jansen, unpub lished results].

resulting in asymmetric dense membranes with an ultra4hin dense skin of less than 40 nm, exhibiting a high permeance and high selectivity (table). PEEKWC and Its Derivatives—Chemical and Physical Properties Besides PEEKWC itself, synthesised by polycon densation of phenolphthalein and the diphenylke tonedihalide, various other chemical modifications of PEEK have been reported. For instance, sulfonated PEEK has recently received considerable attention in view of possible application as a proton exchange membrane for fuel cell applications [23]. Analogously, the sulfonated form of PEEKWC is known since the early years [24, 25] and it has been proposed for a vari ety of applications, such as gas drying [26], fuel cell membranes [27–30] or for pervaporation [31, 32]. Nitrated PEEKWC has been reported as well [33, 34]. Light scattering analysis of PEEKWC samples by Siddiq et al. [35] show a polydispersity close to the ideal value of 2 for condensation polymers, and a molar mass distribution ranging from about 10000 to 300000 g/mol. Samples with a higher molar mass tend to have a much wider distribution, and can be frac tionated by selective precipitation from N,Nyimeth ylformamide (DMF) with ethanol [36]. Dilute solu tions of these fractions in chloroform present clusters that can be removed by filtration, after which the poly dispersity drastically reduces. Jansen et al. used a poly mer with a high molar mass and a distribution ranging from about 1000 g/mol to 5000000 g/mol, having a polydispersity of 16 according to multidetector SEC analysis with light scattering and viscosity detection [37]. This polymer was fractionated into samples with a range of different molar masses and a narrower molar mass distribution, using N,Ndimethylacetamide POLYMER SCIENCE

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(DMA) as the solvent and water as the nonsolvent. The molar mass was found to have a notable influence on the solution properties and, consequently, on the membrane formation by the phase inversion process. In spite of the suppression of the crystallinity of the parent polymer by the cardo group, the polymer pre serves its excellent thermal stability, with an onset of weight loss of over 400°C in thermogravimetric analy sis (TGA) [38]. In combination with the polymer’s high Tg this guarantees sufficient stability under demanding conditions and during membrane cleaning operations, heat sterilization included. Depending on the conditions, the polymer may exhibit significant physical aging [39] and enthalpy relaxation phenomena [13]. Since this influences the free volume and thus the transport properties, it is rel evant for applications that use dense membranes, in particular for gas separation. Mechanical properties, such as fatigue [40] and frictionwear [41] have been studied as well, but such properties are probably more relevant for application as engineering plastic. Membranes are not usually subjected to wear or to periodic stresses and deforma tions. Nevertheless, such properties might gain some importance in for instance submerged modules, where fibres are in continuous motion to reduce fouling and polarization phenomena. For deformations near the ultimate tensile strength (UTS), failure occurs after several hundreds or thousands of cycles. These num bers increase to over 100000 cycles at lower stresses of 0.3 UTS. For much smaller stresses and deformations in submerged membranes fatigue is therefore not expected to be a problem. In other applications such as ultrafiltration, the stresses on the membrane due to the internal pressure are relevant. Fibres are generally not subjected to longitudinal tensile forces but the internal 2009

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100 for water/acetic acid mixtures with 10% water, and a flux of 800 g/m2 h and a separation factor of 48 at 70% water in acetic acid. The use of additives in the polymer slightly reduced the selectivity but sig nificantly improved the water flux [31]. Nanofiltration and Reverse Osmosis Reverse osmosis (RO) membranes, largely applied in desalination [69] are on the borderline between the dense membranes for gas separation or pervaporation

applications, and porous membranes for pressure driven filtration processes, while nanofiltration (NF) membranes usually consist of porous films. In some cases they can also be composed of dense polymers with an extremely high free volume [70] or of dense polymers which undergo so strong swelling in the sol vent, that they can almost be considered porous. Both RO and NF membranes often contain charged groups or strong permanent dipoles to combine the size selec tivity of the polymer matrix, with an additional elec trostatic interaction to reject solutes with the opposite charge. To the best of our knowledge, no reverse osmosis membranes of PEEKWC have been reported so far, while the first nanofiltration membranes stem from 2005 by Qiu et al. [71]. The first papers are all based on photoinduced grafting of hydrophilic monomers like acrylic acid [71, 72], 2hydroxyethyl methacrylate (HEMA) [73] or ionic monomers like sodium styrenesulfate (SSS) [74] and sodium allyl sul fonate (SAS) [75] on a porous ultrafiltration mem brane support. Thus the actual separation is performed by the layer of graft polymer on top of the PEEKWC membrane. These membranes may reach nearly 100% sodium sulfate retention. Li et al. reported another approach to obtain nanofiltration membranes by in situ interfacial copolymerization of poly(amidoam ine) (PAMAM) dendrimer and trimesoyl chloride (TMC) on an existing ultrafiltration membrane [76]. Recently Buonomenna et al. reported very high retention values of several water soluble organic dyes, using pure PEEKWC nanofiltration membranes pre pared by phase inversion methods, without any further treatment [77]. The very high retention of relatively small positively charged dyes such as methylene blue (320 g/mol) and neutral red (289 g/mol), and much lower values for neutral or negatively charged mole cules, suggest an important role of local charges or dipoles in the selective skin of the membrane. Intro duction of amino groups by plasma treatment enhanced the sensitivity towards the charge of the sol utes, causing complete retention of positively charged methylene blue and complete permeation of methyl orange, having a similar molar mass but a negative charge [78]. This confirms the importance of electro static repulsion in nanofiltration. Ultrafiltration and Microfiltration The first reports on PEEKWC for membrane appli cations were on its potential use in ultrafiltration [20, 21]. Since then numerous studies have been con ducted. Most of the previously mentioned nanofiltra tion membranes are based on flat ultrafiltration mem brane supports prepared under different conditions [71–76]. The UF membranes were cast from pure sol vents DMF [76] and Nmethylpyrrolidone (NMP) [73], or from solvent/pore former mixtures NMP/PVP [72, 75], DMF/Poly(ethylene glycol) (PEG) [76] or DMA/PEG [79].

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Fig. 3. Dextrane rejection (Mw =12 kg/mol) and pure water permeability of PEEKWC hollow fibre ultrafiltration membranes as a function of the PEEKWC concentration (left) and as a function of the PVP concentration in the dope solution at 16% PEEKWC (right) (Courtesy F. Tasselli).

Tasselli et al. presented a series of systematic stud ies on the preparation of capillary PEEKWC ultrafil tration membranes and found that the dextrane rejec tion of the membranes increases and the water perme ability decreases with increasing polymer content [51]. Nearly complete rejection can be realized by using PVP in the dope solution [42]. Trends are displayed in Fig. 3. In contrast, a high humidity in the air gap or the use of alcohols instead of water as the bore fluid improves the water permeability of the membranes, but at the expense of the dextrane rejection [52]. Some of these membranes were successfully applied in the ultrafiltration of kiwifruit juice [80] and in biomedical applications [81]. The high Tg and the good chemical stability allows thorough cleaning pro cedures to counteract the effect of fouling, as well as heat sterilization when aseptic operation is required. Catalytic Membranes Various studies have been conducted on the poten tial use of PEEKWC in catalytic membranes, either by supporting a catalytically active film like Nafion® [82] on a porous PEEKWC membrane or by dispersion of different types of catalytically active species in the dense or porous matrix. Cyclodextrin: Various derivatives of cyclodextrin have been dispersed in the PEEKWC matrix [83] and its catalytic activity in the hydrolysis of pnitropheny lacetate [84] and of phosphate esters [85] has been investigated. Modelling studies show that the cyclo dextrin ring becomes slightly distorted in the polymer matrix [86] and this may have influence on its catalytic activity. Oxidation catalysts. Photocatalytic membranes containing Decatungstate salts [87] were developed for the photooxidation of alcohols in water. However, under the conditions used for the photocatalytic reac tion, considerable degradation of the membranes took POLYMER SCIENCE

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place in the case of PEEKWC and polysulfone. These materials were therefore unusable for the given appli cation, in contrast to for instance PVDF and PDMS, which showed a much higher stability. A better performance of PVDF membranes com pared to those of PEEKWC was also observed in the case of membranes containing Ti(IV)/trialkanolamine complexes for the oxidation of dibenzylamine to nitrone [88]. Recently Buonomenna et al. developed PEEKWC microcapsules with an immobilized water soluble ammonium molybdate catalyst for the effi cient and selective oxidation of benzyl alcohols and other alcohols to the corresponding aldehydes [89]. Biomedical and Pharmaceutical Applications The most common medical applications of mem branes are for controlled release of pharmaceutical products, for instance by transdermal drug delivery [90, 91]. Recently Figoli et al. have developed micro capsules of PEEKWC for potential use as controlled release systems [92–94]. An example is shown in Fig. 4. De Bartolo et al. have studied biocompatibility of PEEKWC membranes through plasma protein affinity studies, in relation to the surface morphology and physicochemical properties. They demonstrated that PEEKWC has a lower affinity to plasma proteins than several commercial polymers [95–97]. Pure PEEKWC membranes [98] and their blends with poly urethane [99, 100] are also suitable for liver cell culture in biohybrid systems (Fig. 5). Cell adhesion and met abolic behaviour of rat and human liver hepatocytes were investigated, and especially on the blend mem brane with PU the cells were able to maintain their functions at high level over a long time span. Plasma treatment of the PEEKWC–PU blend membranes has been used as an approach to change the surface prop erties [101]. The immunomodulatory activity of 2009

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10 µm 500 µm

1 mm

Fig. 4. External view and cross section of a PEEKWC microcapsule prepared by nonsolvent induced phase separation. Inset: zoom of the outer surface (Courtesy A. Figoli).

herbal supplements were evaluated in a hollow fibre membrane bioreactor confirming the results of in vivo observations [102], demonstrating the usefulness of the bioreactor as the support system that reproduces physiological parameters such as a constant perfusion of medium, nutrients and oxygen, maintaining the in vitro integrity of lymphocyte viability and functions [81]. Rougher PEEKWC membranes are not equally successful for the growth of Hippocampus neuronal cells, which prefer smoother surfaces of fluorocarbon polymer [103]. Fuel Cells PEEKWC in its sulfonated form (S–PEEKWC, Fig. 6), offers interesting perspectives for application as fuel cell membranes [27–30]. Tresso et al. devel oped electrode devices with S–PEEKWC by employ ing siliconbased micro fabrication techniques [104]. The sulfonated polymer itself is obtained by direct sul fonation of the parent polymer, which is usually car

ried out in solution/dispersion, using either H2SO4 [24, 25] or chlorosulfonic acid [105]. The latter method has the advantage that it suffers less from chain degradation. The SO3H groups give a strongly hydrophilic character to the sulfonated PEEKWC. While this makes the polymer permeable for water vapour and suitable for gas drying [26] or pervapora tion [31], the combination of strong water sorption and the presence of strongly acidic groups, makes this polymer also an excellent proton conductor, suitable for application as the solid electrolyte in fuel cells. At 100% relative humidity the proton conductivity of PEEKWC is 1 to 3 orders of magnitude lower than that of Nafion® 117. Nevertheless, it increases more rap idly with increasing temperature and with the degree of sulfonation (DS) and reaches for instance 2.5 × 10 ⎯2 S cm–1 at 115°C and 100% relative humidity for DS = 0.82 against 2 × 10–1 S cm–1 for Nafion under the same conditions [27]. Therefore at higher temper ature the power density of sulfonated PEEKWC may exceed that of Nafion® membranes under the same conditions, as reported by Paturzo et al. [29, 106]. It thus offers interesting perspectives for high tempera ture fuel cell applications, desirable to reduce the problem of carbon monoxide poisoning of the catalyst. Another advantage of SPEEKWC is the much lower O O

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SO3H

SO3H

O

SPEEKWC 5 µm Fig. 5. SEM image of pig hepatocytes in adhesion on PEEKWC–PU membrane. Cells establish intercellular junctions in a threedimensional structure (Courtesy L. De Bartolo).

Fig. 6. Generalized chemical structure of S⎯PEEKWC. The position and the degree of sulfonation depend on the experimental conditions of the sulfonation reaction but the preferential position is the orthoposition with respect to phenolic ether of the phenolphthalein unit [24].

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methanol permeability than Nafion® 117 [27]. Although these measurements were done at low vapour activity by the time lag method under condi tions quite different from those actually occurring inside the fuel cell, the results suggest that S⎯PEEKWC will exhibit reduced methanol crossover in direct methanol fuel cells. No significant improve ment in performance is obtained upon blending of S⎯PEEKWC with zirconium phosphate sulfophe nylenphosphonate [107], which was reported to improve the performance of S–PEEK [108]. Also sev eral heteropolyacids like tungstophosphoric acid (H3PW12O40), silicotungstic acid (H4SiW12O40) and phosphomolybdic acid (H3PMo12O40) were studied to improve the ion exchange capacity and general perfor mance of SPEEKWC membranes in relation to pos sible application in fuel cells [109]. Miscellaneous Applications Packaging: The favourable combination of trans port parameters for different gases makes PEEKWC/poly(αpinene) blend membranes poten tially suitable for packaging of fruit and vegetables [110]. In this case, rather than a barrier material, the blend offers a controlled respiration rate which pro vides the proper atmosphere for the packaging of fresh foods. For instance, some of the studied films have combination of a water vapour transmission rate (WVTR) of 10–100 g m–2 day–1, an O2 permeability of about 1000 cm 3STP m–2 day–1 bar–1 and a CO2/O2 selec tivity of 4–6.7. This is in the proper range for several fresh fruits and vegetables. Addition of antimicrobic additives has been suggested to further increase the shelf life of the packaged products [111]. Textile: Porous PEEKWC/PU blend membranes were demonstrated to have a high water vapor trans mission rate, while they are impermeable to liquid water at low pressures. This offers perspectives for the use of these membranes as breathable waterproof tex tile materials [112]. Membrane contactors: Gugliuzza et al. hypothe sized that the honeycomb patterned membranes, first developed by Tian et al. [45] could be used for contac tor applications [46], where precise control of the average pore size, the pore size distribution and spatial arrangement of the pores are important. In membrane distillation tests under the same conditions a some what higher transmembrane flux was obtained than with a commercial membrane. Molecular recognition membranes: PEEKWC membranes with embedded βcyclodextrin derivatives have been developed for selective recognition of narin gin, the bitter component of grapefruits [113]. The presence of the cyclodextrin strongly enhanced the naringin absorption capacity of the membranes, which was nearly absent in a pure PEEKWC membrane, and POLYMER SCIENCE

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the capacity increased with increasing cyclodextrin concentration in the dope solution. Molecular Modelling Studies Atomistic molecular modelling has proven to be a useful tool for the investigation of transport properties of small gas molecules in polymer membrane matri ces. Several simulation studies have been reported on the properties of PEEKWC [61, 114] and its nitrated and sulfonated [61] and its alkylated derivatives [62]. It was found that the predicted gas transport properties of small gas molecules in the glassy polymer mem brane show often a large scatter in simulated gas diffu sion and solubility values, while the absolute values may differ significantly from the experimental results. The quality of the predictions of the transport proper ties based on molecular simulation depends princi pally on the quality of the membrane model. Numeri cal analysis of structural features of the membrane model should therefore be used for preselecting only the realistic ones for further simulations, using transi tionstate theory (TST) approach. This will reduce the scatter in predicted gas transport properties, as was demonstrated for gas solubility and diffusion in alky lated PEEKWC membranes [62]. CONCLUSIONS The present review has shown that amorphous glassy PEEKWC is a highly versatile polymer for use as a membrane material, with similar physicochemical properties as its semicrystalline counterparts PEK and PEEK, but with much better processability because of its higher solubility in organic solvents and the consequent ease of membrane preparation by phase inversion techniques. Lack of commercial avail ability, EINECS/ELINCS registration and FDA approval may hamper direct commercial introduction of such membranes, but a series of excellent properties makes this polymer and its sulfonated derivative nev ertheless particularly interesting. The combination of good chemical, thermal and physical properties confer PEEKWC membranes great potential in various fields, such as ultrafiltration, gas separation, biomedical applications and in its sulfonated form in fuel cells. This is especially due to the ease of formation of membranes of virtually any kind, from porous membranes for micro and ultrafil tration, to nearly dense or completely dense mem branes for nanofiltration and gas separation. In fact, since its first appearance in the membrane field less than two decades ago, tremendous progress has been booked in the PEEKWC membrane preparation and nowadays a careful control of the membrane porosity and of the morphology in general is possible. The good permselectivity of the bulk polymer, in combination with the possibility to form asymmetric membranes with an ultrathin dense skin of less than 2009

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40 nm, enables the production of gas separation mem branes with an even higher selectivity than the bulk polymer. Very similar preparation conditions yield membranes with a good performance in the nanofil tration of small organic dye molecules, especially after plasma treatment of the membranes for enhancement of their selectivity. The particular surface properties of PEEKWC grants this polymer a relatively low plasma protein affinity compared to several other polymers, which can be a major advantage in foulingfree cell culture systems. On the other hand, depending on the mem brane treatment, the surface properties can be tailored to improve cell adhesion. Finally, one of the main advantages of S–PEEKWC for fuel cell applications it its very low methanol per meability, an important factor to limit the methanol crossover in direct methanol fuel cells. Furthermore, the power density remains more stable at higher tem peratures than the commercial and widely used Nafion® membranes, opening perspectives for the use in medium temperature fuel cells. Summarizing we may conclude that PEEKWC is an extremely versatile polymeric membrane material, which offers particular advantages in series of specific applications and which justifies a strong and continu ing research effort. REFERENCES 1. S. Loeb and S. Sourirajan, Adv. Chem. 28, 117 (1963). 2. N. A. Platé, S. G. Durgarjan, V. S. Khotimskii, et al., J. Membr. Sci. 52, 289 (1990). 3. N. A. Platé, A. K. Bokarev, N. E. Kaliuzhnyi, et al., J. Membr. Sci. 60, 13 (1991). 4. K. Nagai, T. Masuda, T. Nakagawa, et al., Prog. Polym. Sci. 26, 721 (2001). 5. A. Yu. Alentiev, Yu. P. Yampolskii, V. P. Shantarovich, et al., J. Membr. Sci. 126, 123 (1997). 6. H. C. Zhang, T L. Chen, and Y. G. Yuan, CN Patent No. CN 85,108,751 (1987). 7. P. J. Brown, S. Ying, and J. Yang, AUTEX Res. J. 2, 101 (2002). 8. J.P. Yang and P. J. Brown, Chin. J. Polym. Sci. 26, 263 (2008). 9. J.P. Yang and P. J. Brown, ePolymers 076 (2007). 10. W. Risse and D. I. Sogah, Macromolecules 23, 4029 (1990). 11. Q. Guo, J. Huang, T. Chen, et al., Polym. Eng. Sci. 30, 44 (1990). 12. W. Liu, T. Chen, and J. Xu, J. Membr. Sci. 53, 203 (1990). 13. Z. Mingqiu, M. Kancheng, and Z. Hanmin, Polym. J. (Tokyo) 25, 541 (1993). 14. Y. P. Handa, J. Roovers, and P. Moulinié, J. Polym. Sci., Part B: Polym. Phys. 35, 2355 (1997). 15. Z. Wang, T. Chen, and J. Xu, J. Appl. Polym. Sci. 63, 1127 (1997).

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