ВЫСОКОМОЛЕКУЛЯРНЫЕ СОЕДИНЕНИЯ, 2009, том 51, № 11, с. 2101-2112
УДК 541.64:542.954
POLY(ETHER ETHER KETONE) DERIVATIVE MEMBRANES -A REVIEW OF THEIR PREPARATION, PROPERTIES AND POTENTIAL
© 2009 г. Johannes Carolus Jansen and Enrico Drioli
Institute on Membrane Technology, ITM-CNR, Via P. Bucci 17/C, 87030 Rende (CS), Italy
e-mail: e.drioli@itm.cnr.it.
Abstract—The present paper gives an overview of the properties and performance of membranes of a po-ly(ether ether ketone) derivative with a cardo group in the chain, known in the literature as PEEKWC or PEK-C. 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 much versatile and its solubility allows the use of nonsolvent-induced 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 separation, biomedical applications and — in its sulfonated form — in fuel cells. Examples of successful application in microfiltration, ultrafiltration, nanofiltration, pervaporation, membrane contactors, catalytic membranes and some other applications, such as packaging and molecular imprinting will also be shown.
FOREWORD
From the early stage of membrane research activities, polymer scientists have been interested in exploring the potentialities of their materials in developing new selective and permeable membranes, to be utilised in a large variety of different operations and industrial applications. Prof. Nikolai Plate 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, membrane-based operations 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 development of the first asymmetric cellulose acetate membranes by Loeb and Sourirajan [1], which enabled the achievement of fluxes of practical interest also for commercial applications, the polymeric membrane market has been dominated by a relatively small number of either rubbery, glassy or semi-crystalline polymers. By far the most common rubber is poly(dimeth-yl siloxane) (PDMS), which is mainly used in gas and vapour separation and in pervaporation. Semi-crystal-
line polymers such as polyamide (PA), polyethylene (PE), polypropylene (PP), poly(vinylidene fluoride) (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 common glassy polymers used as membrane materials are commercial polymers like cellulose acetate (CA), polyim-ides (PI), poly(ether imide)s (PEI), polycarbonate (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 evaluation of more sophisticated polymers such as glassy poly(vinyltrimethylsilane) [2] and amorphous poly-acetylenes [3, 4] with a high free volume and consequently high permeability, or glassy perfluoropolymers [5] for applications which require a particular chemical or physical stability. Also the booming interest in sulfonated polymers such as Nafion® for fuel cell applications, driven by the search for alternative energy sources and conversion technologies, is a good example of how new polymeric membrane materials may be developed.
Whereas for instance perfluoropolymer membranes 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
2101
O'
PEK
PEEK
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O
O
PEEKWC
n
Fig. 1. Chemical structure of semi-crystalline PEK and PEEK, and amorphous glassy PEEKWC or PEK-C.
example is the poly(ether ether ketone) with phtha-lide-cardo group [6], obtained by solution polycon-densation of phenolphthalein and dichloroben-zophenone, and in the literature usually abbreviated as PEEKWC or PEK-C (Fig. 1). As yet the polymer is not commercially available on the world market and its future applicability will depend on the necessity of EINECS/ELINCS registration and, for food or biomedical applications, on its eventual FDA approval. Nevertheless, it presents a series of favourable properties which makes it a truly interesting candidate as a new membrane material. An overview of properties and of potential application fields is described below.
Unsubstitutedpoly(ether ketone)s versus PEEKWC
The members of the polyetherketone family, such as poly( ether ketone) and poly( ether ether ketone), are generally semi-crystalline and are characterized by an extremely high thermal stability while they are virtually insoluble in common organic solvents and at low temperature. Generally, in organic solvents their solubility 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 preparation. 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 (P-transi-tion) at about 70°C, which coincides with a small
change in the activation energy for the permeation 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 transport 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 different pendant groups [15] and has been studied for various applications. Numerous papers on thin film guest-host systems study its potential for optical and electro-optical 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 separation membranes by phase inversion, recently resulting in asymmetric dense membranes with an ultra-thin dense skin of less than 40 nm, exhibiting a high permeance and high selectivity (table).
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 silicon coating [48]
Permeance3 Selectivity Permeance GPU Selectivity (M/CH4)
Gas (M) 3 2 mSTP /(m2 h bar) GPUb (M/N2) (M/CH4)
Nitrogen 5.93 x 10-3 2.19 1.00 0.97
Oxygen 3.77 x 10-2 13.9 6.4 6.14
Methane 6.14 x 10-3 2.27 1.03 1.00 0.126 1.00
Helium 0.521 193 88 85
Hydrogen n.d.c - - - 4.95 39.3
CO2 0.187 69.2 32 30.5 0.972 7.71
PEK Hollow fibre membrane [8]
a Effective skin thickness of 38 nm based on helium permeance. b 1 GPU = 1 barrer/^m = 10-6 mSTP /(cm2 s cm Hg).
c Not determined on the reported sample. However, the helium and hydrogen permeability are nearly identical [J.C. Jansen, unpublished results].
PEEKWC and its derivatives — chemical and physical properties
Besides PEEKWC itself, synthesised by polycon-densation of phenolphthalein and the diphenylke-tone-dihalide, 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 variety of applications, such as gas drying [26], fuel cell membranes [27—
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