Perspective pubs.acs.org/Macromolecules
Polymeric Gas Separation Membranes Yuri Yampolskii* A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 29 Leninsky Prospect, 119991, Moscow, Russia
ABSTRACT: This short Perspective conveys to the general reader of Macromolecules basic approaches of materials science of polymeric membranes for gas and vapor separation. The relations between the polymer structure and transport properties of rubbery and glassy membrane materials are considered. On the basis of acquired information, several methods for quantitative prediction of permeability were developed, and their comparative analysis is given in the Perspective. The past decade was marked by the appearance of a number of novel interesting membrane materials, which will be briefly described in the text. In conclusion, novel approaches for achieving highly permeable and permselective materials (e.g., mixed matrix membranes) will be considered as well as several relevant but not solved so far problems of membrane gas separation.
1. INTRODUCTION The principles of gas separation by means of nonporous polymeric membranes were formulated earlier than those for other membrane separation methods. As early as the 19th century, because of the works by J. K. Mitchell, A. Fick, and T. Graham,1 it became clear that the mass transfer in polymeric and liquid films is possible in the absence of through pores. While measuring the permeation rates of different gases through polymeric films, for example, films of natural rubber, T.Graham noted that there was no relation between the permeation rates and known gas diffusion coefficients. Therefore, great permeation rate was observed for carbon dioxide distinguished by the largest solubility in membrane materials among all gases tested. These observations led to the “solution−diffusion” mechanism of transport through polymeric membranes. According to this model, the characteristic of the permeation rate, permeability coefficient, P, can be presented as the product (1) P = DS where D is the diffusion coefficient and S is the solubility coefficient. The permeability coefficient characterizes the flux J (mol/m2 s) through a membrane under the pressure drop (driving force) Δp (Pa) and normalized to the unit thickness of the membrane L (m). The unit of P in the SI system is mol/s m Pa. However, a more widely used and accepted nonsystem unit for P is 1 Barrer = 10−10 cm3(STP) cm/cm2 s cm Hg. The permeability coefficients of various gases in polymers vary in a wide range from 10−4 to 104 Barrer. © 2012 American Chemical Society
Another key characteristic of gas-separating membranes is their selectivity. The ideal selectivity (or separation factors) can be defined as αAB = PA /P B
(2)
where PA and PB are the permeability coefficients of gases A and B, respectively. Commonly, the more permeable gas is taken as A, so that αAB > 1. Keeping in mind eqs 1 and 2, the ideal selectivity can be partitioned into diffusion and solubility selectivity αAB = (DA /DB)(SA /SB) = α D ABαS AB (3) These simple relationships hold for gases that weakly interact with each other and with polymer matrix during the separation process. This is true for permanent gases at relatively low pressure. For gases having high solubility (e.g., carbon dioxide or hydrocarbons), the ideal separation factors give a rather inaccurate measure of the “actual” selectivity of a membrane in the process of mixture separation. In such cases, the following equation α*AB = (yA /yB )/(xA /x B)
(4)
is more suitable. Here yA and yB are the mole fractions of the components in the permeate (the stream permeated through Received: January 30, 2012 Revised: March 6, 2012 Published: March 28, 2012 3298
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tives.14 These studies indicated that there exist straightforward correlations between chemical structure of polymer repeat unit and the observed gas permeation parameters. 2.1. Rubbers. The most detailed results were obtained for siloxane polymers with different side groups attached to Si atom. It was shown15 that when the size of the side groups increases, the chains become less flexible, as is shown by the increases in the glass transition temperature and the decreases in permeability (Table 2). In this respect, rubbery polymers
the membranes) while xA and xB are the corresponding mole fractions in the feed. Although polymeric membranes can be used today for separation of nearly all imaginable gas mixtures, large industrial applications have been realized only for the following processes:2−6 - air separation (obtaining of technical grade nitrogen or oxygen enriched air) - hydrogen separations (separation of the mixtures H2/N2, H2/CH4, and H2/CO in refineries and petrochemical industry) - separation of CO2/CH4 mixtures (natural gas “sweetening”) and CO2/N2 (treatment of flue gas, etc.) - vapor/gas separations (numerous systems) Table 1 presents some of the leading industrial companies that are active in membrane gas separation and the principal membrane materials used.
Table 2. Permeability Coefficients, P, of Siloxanes with Different Side Groups15 P, Barrer
Table 1. Principal Membrane Companies and Membrane Materials Used2,4 company Permea (Air Products) MEDAL (Air Liquid) Generon Separex (UOP) Aquila Ube MTR Helmholtz Centrum (formerly GKSS) Grasysa Kryogenmasha Air Liquid OPW Vaposaver a
R1
R2
Tg, K
CO2
CH4
C3H8
CH3 CH3 CH3 CH3
CH3 C3H7 C2H4CF3 C6H5
150 153 203 245
4570 1520 1214 226
1450 531 201 36
8580 2950 584 139
differ from glassy polymers, where the introduction of larger side groups often results in an increased permeability, as will be later discussed. The variations of the flexibility of the main chains, while the side chains are intact, is also a strong way to change the permeability, as Table 3 indicates for polydime-
principal membrane materials used polysulfone polyimides tetrabromopolycarbonate cellulose acetate poly(phenylene oxide) polyimide silicon rubber silicon rubber
Table 3. Effects of the Variation of the Structure of Main Chains in Rubbery Polymers12,16−18 P, Barrer
polyimide, polysulfone poly(vinyltrimethylsilane), tetrabromopolycarbonate ethyl cellulose poly(trimethylsilyl propyne)
Producer of membrane modules and installations only.
polymer
Tg, K
−Si(CH3)2O− −Si(CH3)2CH2− −C(CH3)2CH2−
150 181 208
H2 650 6.4
O2
N2
CH4
600 99 1.2
280 40 0.3
950 130
thylsiloxane, polydimethylsilmethylene, and polyisobutylene. The correlations between Tg values and permeability for structurally less related hydrocarbon rubbers shows that the effects of chemical structure of repeat units are exerted mainly via the flexibility of the main chains of rubbery polymers. It is interesting that flexible Si−O bonds that enhance the chain mobility and thus increase permeability can exert effects, even if they are located in the side chains. Therefore, polystyrenes substituted in para position with radicals containing several Si−O bonds (e.g., Si[CH3][OSi(CH3)2] or Si[CH3]2[OSi(CH3)3], etc.) reveal decreased glass-transition temperatures, increased gas permeability, and reduced separation factors as compared with unsubstituted polystyrene.19 Recently, similar effects were demonstrated for another type of polymers, substituted metathesis polynorbornenes.20 2.2. Polyolefins. Extensive studies were performed during the 1960s to 1980s on gas permeation properties of polyolefins (polyethylene of high and low density, polypropylene, poly(4methylpentene), etc.).8,21−24 However, the polymers of this important class are semicrystalline, so their transport properties are affected mainly by the degree of crystallinity and morphology of the heterophase structure.25 2.3. Glassy Polymers. The gas permeation properties of glassy polymers are much more sensitive to chemical structure of repeat units than those of rubbers. It is sufficient to point out that the permeability coefficients in the glassy state can be as low as P(CO2) = 0.0003 Barrer for polyacrylonitrile26 or as
Although hundreds of polymers have been investigated so far as potential membrane materials, only several ones, as can be seen from Table 1, found actual application in industrial gasseparating plants. It is a long way from a polymer that showed a good combination of permeability and selectivity to industrial application of membranes based on it. The requirements for production of robust gas separation membranes include sufficiently good mechanical and film-forming properties, absence of microdefects in thin layers of composite membranes, thermal and chemical stability under the conditions of the separation process, and absence of aging in thin films (reduction of permeability in time). Often, high cost of production of potential membrane materials is regarded as the main obstacle for using a polymer as the material of industrially produced gas-separation membranes.
2. “STRUCTURE−PROPERTIES” RELATIONS The history of membrane gas separation shows that every period had its favorite types of polymeric materials of separating membranes. In the early years of membrane studies, Thomas Graham had at his disposal only natural polymers − gutta percha, natural rubber, and gelatin. On the advent of polymer science in the middle of the 20th century, numerous synthetic polymers became available, and so diverse studies of gas permeation properties were carried out with polyolefins,7−9 vinylic polymers, like poly(vinyl acetate),10 polytetrafluoroethylene,11 siloxane,12 and other rubbers,13 cellulose deriva3299
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Table 4. Enhancement of Permeability Due to Introduction of Si(CH3)3 Groups
1
Low density PE (0.91 g/cm3).
large as P(CO2) = 27 000 Barrer in poly(trimethylsilyl propyne).27 The most spectacular demonstration of strong effects of polymer chemical structure on permeation rate was obtained with polymers of different classes containing bulky side groups: (Si(CH3)3, Ge(CH3)3, and CH(CH3)2). Table 4 summarizes some data on the effect of the introduction of Si(CH3)3 groups into different main chains. It is seen that independently of the structure of main chains the appearance of this group as the side chain results in a substantial increase in the permeability coefficients, sometimes accompanied by some growth of the separation factors. The most detailed information on such effects was obtained for polyacetylenes.28 Examination of numerous Si-substituted glassy polymers allows a formulation of the rules for stronger enhancement of permeability: (1) Symmetry of side groups. The strongest effects are observed for completely symmetrical substituents; for example, the effect of SiMe3 group is stronger than that of SiMe2Et.28 (2) A bulky group should be attached directly to the main chain and not via a spacer. (3) Long linear substituents (e.g., SiMe2-n-C6H13) show significant decreases in permeability. A compelling explanation of such behavior can be given on the basis of free volume theory and application of probe methods.29 In polymers with aromatic backbones (polyimides, polycarbonates, polysulfones), a marked increase in permeability can be achieved by symmetrical or asymmetrical substitution of phenylene rings.36 Different radicals were used in such
substitution, for example, CH3 and other linear and branched alkyls, CF3, Cl, and so on. Examples of the effects of such modification of diamine moyety of 6FDA polyimides are given in Table 5. The increases in permeabiluity correlate with the Table 5. Properties of 6FDA Polyimides Containing Various Substituted Diamine Fragments37 a
a
R1
R2
P(O2), Barrer
α(O2/N2)
substituent volume, Å3
H H Me Et i-Pr
H CF3 Me Et i-Pr
2.8 5.2 11.0 18.4 47.1
5.65 5.71 4.17 4.20 3.76
16.5 79.0 90.8 158.8 226.8
Structure of 6FDA dianhydride and diamine fragments
total size of side groups shown in the last column. These data indicate that substitution results in a more loose chain packing (growth of free volume, which is accompanied by decreases in permselectivity α(O2/N2)). Another element of the design of polymers with aromatic backbones is the structure of connector groups. These effects are also revealed in the transport properties of polycondensation materials.36 Bulkier connector groups can stiffen the chain backbones, inhibit their dense packing, and thus increase free 3300
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If the splitting of repeat units is appropriate and the number of the input parameters is sufficiently great, then it is possible to make relatively accurate predictions within the coefficient of 2 to 3. Such accuracy seems to be really good because the range of variation of the permeability coefficients in polymers amounts to 5 to 8 decimal orders (depending on the gas molecule). Better accuracy is hardly possible because it is mainly determined by “noise” in the input parameters: thus, several authors who studied polyimide Kapton reported the values of P(O2) in the range 0.2 to 0.8 Barrer, so such big variation should affect the group contributions parameters deduced from these data. It is interesting that the accuracy of the predictions of separation factors is usually better. Apparently, this is a due to similar influences of the aforementioned “noise” on the permeability coefficients of both gases, P(A) and P(B), so it affects the ratio P(A)/P(B) to a smaller extent. This is equally true for predictions using molecular dynamics, and the illustration of this will be given later on in this section. Meanwhile, it must be admitted that the group contribution and other similar approaches have their limitations. They are based on the input parameters, that is, on the increments obtained for already studied polymers with diverse chemical structure. Therefore, they do not provide means for strong breakthrough, for example, for the methods to overcome the upper bounds on the Robeson diagrams45,46 or predict transport properties of polymers that include entirely new components of chemical structure, because the corresponding increments are absent. Such weakness is not characteristic of computer modeling of nanostructure of membrane materials and calculation, on this basis, the S, D, and P values for different gases. These methods of computer simulation made great progress during the past decade.47−50 The method of molecular dynamics was used by most of the authors.51−53 Recently, different variants of the Monte Carlo (MC) methods also acquired some popularity.54,55 It is important that the MC methods allow us to take into account connectivity56 of membrane materials, an issue especially important for glassy polymers that reveal size sieving permeation. The accuracy of the prediction differs in different works. Some authors claim that it is easier to predict accurately the solubility coefficients. Often, the predicted S values coincide with the experimental ones within the range of 20−30%.51 The predictions are much less reliable in the case of the diffusion and, hence, permeability coefficients. It is commonly accepted that if the Dpred and Dexp values differ by a factor 2−5 then the prediction is considered to be as successful. According to the opinion of other research,50,57,58 the accuracy of the predictions of the S values is less accurate. Such predictions are sensitive to many factors, and the appropriate choice of the force field is maybe the most crucial. It can be assumed that realistic assessment of the accuracy of the S values for small penetrants based on contemporary computer simulations corresponds to the factor 2 as compared with the experimental values, which also show some scatter. As for the diffusion coefficients for such penetrants as O2, N2, CO2, the predicted values are within a factor of 1.5 to 2 from the experimental values. As has been mentioned, the predictions of the separation factors are often more successful than those of the permeability and diffusion coefficients. It is illustrated in Table 7 by a comparison of the experimental and predicted permeability coefficients and separation factors for two substituted
volume and permeability. This situation is illustrated for the family of polyimides in Table 6. Table 6. Effects38 of the Connector Groups in 6FDA Polyimides of General Formula:
connector group X
P(CO2), Barrer
α(CO2/CH4)
−O− −CH2− −C(CH3)2− −C(CF3)2
23 19.3 30 63.9
60.5 44.9 42.9 39.9
So far, only the effects of nonpolar groups were considered. Meanwhile, the introduction of the substituents that are capable to dipole−dipole interactions or can form hydrogen bonds can strongly influence the transport parameters due to increasing interchain interactions or interactions with some penetrants. For example, in carboxylated poly(vinyltrimethylsilane) when the content of −COOH groups amounted to 20 mass %, permeability coefficients of hydrocarbon gases decreased two-fold, whereas permeability coefficients of water vapors increased by a factor of 4.5.30 The introduction of the −COOH group in the m-plenylenediamine moyety of 6FDA polyimide results in a decrease in P(O2) from 3 Barrer (for nonsubstituted polymer) to 1 Barrer with a certain accompanying increase in separation factor α (O2/N2): 8 versus 6.7.39
3. PREDICTIONS AND MODELING As soon as a sufficient body of the data had accumulated on gas permeation parameters for polymers with different chemical structure, especially for polymers that belong to the same classes, it became tempting to proceed from qualitative analysis of the data to the development of quantitative models with predictive ability. A detailed discussion of different approaches for predictions was made by Alentiev and Yampolskii.40 The methods for predictions were conceived for facilitating directed search for advanced membrane materials − not studied so far or even synthesized. Several approaches for predictions were proposed. The most successful ones are based on the group contribution methods that are common in analyzing, for example, thermodynamic and kinetic properties of various compounds. A key question in such methods is the way of “splitting” polymer repeat units into fragments for which corresponding increments (group contributions) are being sought after. The smallest fragments can be atoms in different valence states (e.g., C(sp3) or C(sp2)).40 Robeson et al.41 proposed to split repeat units into larger fragments (e.g., phenylene rings). An important class of membrane materials, polyimides, can be treated as alternating copolymers of diamine and dianhydride fragments. The prediction of the transport parameters of polyimides is more successful if the increments characteristic for such groups are found, especially bearing in mind that the number of chemical structures of polyimides of different chemical structure is very big (more than 300).42 There were also attempts to use the graph theory approach43 and artificial neural networks44 for the prediction of the gas permeation parameters. 3301
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backbone. A feature of these materials is that they incorporate sites of contortion or spiro-center that force the polymer chains to form a loosely packed matrix. These polymers are distinguished by great inner porosity (∼700 m2/g), as measured by the BET method, as well as gas and vapor solubility coefficients larger than those reported for any studied polymers, including PTMSP, the great free volume as measured by positron annihilation lifetime spectroscopy and 129Xe/NMR, and relatively large permeability coefficients (Table 8). The data points of archetypal PIM, referred to as PIM-1, are located on some Robeson diagrams above the upper bound of 2008.67 Interestingly, PIM-1 also shows unprecedented sensitivity to the protocol of film formation: treatment of the films of this polymer by alcohols increases the observed permeability by a factor of three to four. Unusual properties of PIM-1 were also confirmed by the results obtained by other research groups.68 The works with PIM-1 and other PIMs showed that high permeability combined with good selectivity can be achieved not only by means of bulky Si(CH3)3 groups attached to rigid main chains. The interest in this polymer is confirmed by further recent publications69 on cross-linking and UV irradiation of the parent PIM-1 structure that resulted in polymers with attractive membrane properties. Despite the fact that the transport properties of amorphous polyacetylenes are extensively studied,28 recently it was shown that even within the class of disubstituted polyacetylenes another design can result in extremely high gas permeability. Hu et al.70 demonstrated that a group of indan-based polyacetylenes exhibits permeability coefficients larger than those known for PTMSP, which had been considered as the polymer having the highest gas permeability among known polymers. Permeability coefficients of indan-based polyacetylenes are combined, however, with very poor selectivity of separation of light gases (Table 9). It is an indication of the extremely opened pore structure of these polymers. The introduction of one or more Si(CH3)3 groups into other stiff backbone chains can also result in the formation of novel highly permeable polymers with unusual properties − Si-substituted additional type norbornene polymers, whose properties will be discussed in more detail in Section 6.3.20,34,71,72
Table 7. Experimental and Predicted Gas Permeation Parameters51 for p-Substituted Polystyrenes −(CH2−CHPhR)− method calc exp calc exp
P(O2), Barrer
P(N2), Barrer
R = SiMe3 323 102 55.8 16.1 R = SiMe2(CH2CH2CF3) 338 90.9 38 12
α(O2/N2) 3.2 3.5 3.7 3.2
polystyrenes. The predictions were made by molecular dynamics method. However, the progress of the programs of simulations (rates of computing) and the sizes of supercomputers used is so fast that it can be imagined that soon computer simulations will successfully compete with experimental procedures for the determination of D, P, and S values as well as sorption isotherms.
4. NOVEL MEMBRANE MATERIALS The efforts of membrane scientists are traditionally directed to the creation of more permeable and more permselective materials, which, in addition, must be sufficiently robust and resistant to plasticization effects that reduce the selectivity of mixture separation. Until the 1970s, it was commonly accepted that glassy polymers have low permeability, although sometimes high permselectivity. However, the discovery of, first, poly(vinyltrimethyl silane) (PVTMS)59,60 and, more importantly, poly(trimethylsilyl propyne) (PTMSP)61,62 disproved this viewpoint. The design of these polymers (presence of a bulky Si(CH3)3 substituent attached to the main chain) and the highest permeability coefficients found for PTMSP among all of the polymers known for a long time determined the directions of the search for advanced membrane materials with great permeability (mainly polyacetylenes). Meanwhile, some highly permeable polymers with entirely different design also showed rather high gas permeability. Thus, amorphous Teflon AF2400 or glassy copolymer containing 87 mol % 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole and 13 mol % tetrafluoroethylene has permeability coefficients for O2 and CO2 about 1000 and 2500 Barrer, respectively.63,64 The reason for high permeability of this and related polymers is the rigidity of the main chain (high-energy barriers for rotation of adjacent dioxole rings) and low tendency to interchain interactions that cause a loose packing of the chains.65 In addition, it was shown that all amorphous perfluorinated polymers exhibit unusual solubility behavior: reduced hydrocarbon gas and vapor solubility.66 This results in attractive positions at Robeson diagrams46 of the data points that correspond to perfluorinated polymers for such gas pairs as He/CH4, N2/CH4, and He/H2 and low tendency for plasticization of membranes in the presence of hydrocarbon vapors, common impurities in gas mixtures.66 These circumstances justify the growing interest in perfluorinated and partially fluorinated polymers as membrane materials. The stiffness of main chains was shown to be the governing factor that determines physicochemical and transport properties in another group of novel polymers synthesized by English researchers − polymers of intrinsic microporosity (PIM).67 These are ladder polymers without any single bond in the
5. NEW APPROACHES FOR CONTROL OF THE GAS PERMEATION PARAMETERS 5.1. Mixed Matrix Membranes. The experience accumulated in numerous studies of membrane materials having different structure indicates that there are limitations in achieving, using such an approach, materials with very advanced membrane properties. Since the 1990s, it has become clear that there exists another way to influence the gas permeation properties of membrane materials. This approach, very popular currently, is known as “mixed matrix membranes” (MMM). It implies the introduction of inorganic or organic−inorganic fillers with small particle size (desirably in the range of 10− 100 nm) into polymeric matrices. In many cases, such a strategy resulted in enhancing either permeability or permselectivity (or often both) and some data points on Robeson diagrams46 that are located above the upper bound of 1991 or even 2008 are based namely on MMMs. Preparation of MMMs, investigation of their transport and mechanical properties, and study of their nanostructure have attracted the attention of the membrane community during the past decade; hundreds of articles have been published, including some reviews.73−75 This direction of 3302
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Table 8. Permeability Coefficients (Barrer) and Separation Factors of PIM-1 and PIM-Polyimides67
Table 9. Permeability Coefficients (Barrer) and Separation Factors of Indan-Based Polyacetylenes70
with the prediction of Maxwell’s equation for nonpermeable particles of the dispersed phase, which holds for common filled polymers and which predicts a decrease in permeability and diffusivity for larger loadings of fillers.76,77 It is important that strong effects on permeability are observed only for larger concentrations of nanoparticles (>20− 30%), whereas at small loading the P values do not depend on the concentration of nanoparticles or sometimes slightly decrease. It can be assumed that marked changes of P are observed above the percolation threshold. This is in line with the qualitative model proposed by Takahashi and Paul,88 who assumed that at larger loading of the nanoparticles additional microcavities on the boundary nanoparticle−polymer matrix form infinite cluster. In some cases, the transport properties observed for MMM are entirely different as compared with those of pure polymers. For example, pure amorphous Teflon AF2400 is characterized by size-sieving patterns: the permeability coefficients decrease when the size of penetrants (gaseous hydrocarbons) increases.64 However, MMMs based on this polymer combined with zeolites and fumed silica become solubility-controlled permeation materials: permeability coefficients for butane are larger than those of methane.79,80
research is seemingly a further development of numerous studies of filled polymers. (See, e.g., refs 76 and 77.) However, new phenomena became clear when the size of the fillers approached nanoscale. In filled polymers, the introduction of nonporous inorganic fillers into polymers resulted, as a rule, in a decrease in permeability with no changes of permselectivity. In MMMs, the opposite behavior is observed: permeability increases upon the introduction of nanofillers, and these changes are caused, in the most cases, by increasing the diffusion coefficients. It is important that these changes are different for different penetrants; hence, selectivity can be changed as compared with pure polymer matrices. Numerous polymers have been studied as continuous matrices in MMMs: high permeability polyacetylenes,78 amorphous Teflons AF,79,80 and also conventional glassy polymers81,82 and even some rubbers.83 Nanoadditives of different structures have been introduced into these polymers, for example, fumed silica,78,79 carbon nanotubes and carbon molecular sieves,84,85 zeolites,80,81 TiO2,86,87 and so forth. A common feature of most MMMs studied is an increase in permeability with respect to various gases when the volume fraction of nanoparticles in the membrane increases significantly. (See, e.g., refs 78, 79, 82, and 87.) This is in contrast 3303
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Certain combinations of polymer matrices and nanoadditives show common trade-off between Pi and αij (e.g., systems polysulfon/fumed silica82 or polyimide Matrimid/MOF89); however, in many cases, a more attractive behavior has been reported: joint increases in both Pi and αij. (See, e.g., systems amorphous Teflon AF2400/fumed silica79 or polybenzimidazole/metal organic frameworks (MOFs).90) Some of the studied nanoparticles (zeolites, or MOF) can be considered as the fillers with inner porosity, whereas other ones do not have empty inner cavities (fumed silica, TiO2, MgO). Hence, qualitatively different effects can be expected for these two types of nanofillers. Meanwhile, numerous studies showed that different types of nanoparticles (those with inner porosity and its absence) reveal similar trends for permeability and permselctivity for various gases. This seeming contradiction requires elucidation. An extremely important problem is a substantiated search for optimal combinations of polymer matrices and nanoparticles. There are examples when the introduction of different nanoparticles into the same polymer led to quite different results. Ahn et al.91 added silica nanoparticles into the polymer of intrinsic microporosity PIM-1 and observed a typical tradeoff between P(O2) and α(O2/N2), so the data points on Robeson diagram did not overcome the upper bound of 2008. The introduction of a kind of MOF, zeolitic imidazolate framework ZIF-8, into the same polymer resulted in a more permeable and more permselective systems, so the data points for various content of nanoadditive are above these upper bounds (Figure 1).92 Despite obvious advantages that are provided by MMMs and a wide field of further studies, there are many problems that are to be solved. 1. Preparation of membranes. There exists no well-defined procedure for preparation of dense films or composite membranes based on MMMs. It is not clear how to prepare membranes with uniform (and maybe nonuniform) distribution of nanoparticles and how to avoid agglomeration of nanoparticles. 2. Problems of functionalization. A common wisdom is that without appropriate functionalization it is difficult to achieve a good compatibility of nanoadditives and polymer matrix. However, the formation of additive free volume at the boundary between nanoparticles and the matrix implies that complete compatibility might be detrimental for achieving greater increases in permeability and diffusivity. There are examples that prove that nonfunctionalized additives give good results and opposite examples that demonstrate that under the condition of good miscibility produced membranes show only weak increases in permeability or even decreases. No approaches have been proposed for the calculation of the surface energy of the introduced nanoparticles in advance. 3. Aging. It is known that polymers with extra high gas permeability are prone to physical aging, that is, a reduction of permeability in time.28 Because the introduction of nanoparticles into such polymers as polyacetylenes, amorphous Teflons AF, or PIM-1 results in a further increases in their permeability (already high in pure polymers), the problem of aging becomes very relevant.
4. Mechanical properties. The introduction of some nanoparticles (e.g., carbon nanotubes) leads to reinforcing of mechanical properties of polymers, which can make MMMs more robust. However, it is true only for small concentrations of the fillers. At larger concentration (20−40%), an opposite effect is often observed: the membranes become brittle. So this problem desperately requires a solution. 5. Modeling. Unfortunately, relatively little has been made regarding modeling MMMs.93 Good contemporary models require simulation of rather big unit volumes of composite system (cubes with the edge at least 500 nm). Such scale is beyond now the possibility of molecular dynamics, so new approaches are desperately needed. 6. It is really a challenge to transfer the achievements reached with MMMs based on experiments with dense films (thickness of 30 μm or more) and nanoparticles having the size in the range 20−200 nm to industrially used composite membranes (often in the form of hollow fibers) with the thickness of thin selective layers of 100− 300 nm. It means that much smaller nanoparticles must be introduced into polymer matrices. Again, any clustering that typically occurs in the process of the preparation of MMMs would be detrimental to the resulting properties of the membranes. These technological problems still wait for their solution. 7. A selection of combination of polymer matrix and nanoparticles. A logical requirement to polymers used as matrices in prepared MMMs is that they must have sufficiently good combinations of permeability and permselectivity, even in pure state. Therefore, the introduction of nanoparticles will only further improve these characteristics. However, overcoming the upper bounds on Robeson diagrams is only a part of the story. Practice requires certain values of the permeability coefficients and separation factors. So a search for optimal combinations of polymers and nanoparticles should not be made at random. The demands of further industrial application must be taken into account. 5.2. Desililation. It was demonstrated recently that there are alternatives to the principle of control of free volume in polymers via the introduction of bulky substituents and, hence, obtaining highly permeable polymers. Diphenylacetylene can easily polymerize; however, the obtained poly(diphenylacetylene) is insoluble in any organic solvents, so it is impossible to prepare and test films of this polymer. Meanwhile, its close structural analog, poly[1-phenyl-2-p(trimethylsilyl) phenylacetylene)], is soluble in common organic solvents, like toluene or chloroform. Teraguchi and Masuda proposed94 carrying out solid-state desililation of the films produced from this polymer and thus obtained the film of poly(diphenylacetylene). Trifluoroacetic acid was used as the desiliation agent. Interestingly, it was found that poly(diphenylacetylene) obtained in this manner has high permeability: its permeability coefficient P(O2) = 6000 Barrer, much higher than that of the parent Si-containing polymer (1550 Barrer). It can be assumed that this effect is caused by large free volume elements that form in solid film in the sites, where originally presented Si(CH3)3 groups were removed. This also means that these large free volume elements are retained in the matrix due to highly stiff polymer structure. Although subsequent studies showed that permeability of the 3304
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Figure 1. Correlation of permeability and permselectivity for the O2/N2 (a) and H2/CH4 (b) gas pairs.92 The solid line is the upper bound of 2008.46 Triangles indicate PIM-1/ZIF-8 films after ethanol treatment, squares indicate PIM-1/ZIF-8 films as cast.
latter polymer size distribution of free volume elements is shifted to smaller radii (average cavity size of 5 Å), whereas in the Si-containing polymer the average cavity size is larger (7.5 Å). All this means that further studies of this interesting method for production of highly permeable polymers are quite desirable. 5.3. Thermal Rearrangement of Membrane Polymers. Thermal transformation of polymers can result in entirely new structures, and some of them show very interesting membrane properties. In particular, pyrolysis of polyimides at 500−700 °C leads to carbon membranes with an excellent combination of
desililated polyacetylene depends on some not completely clarified factors and can be not so great (P(O2) = 910−1100 Barrer),95 for other Si-containing side groups, even greater effects were noted:95,96 thus, removal of the much larger side group, Si(CH3)2C8H17, resulted in an increase P(O2) from 28 to 3300 Barrer.95 According to Wang et al.,97 desililation leads to some reduction of the permeability coefficients, and the reason for this is a decrease in the diffusion coefficients. Computer simulation of nanostructure of free volume in poly[1-phenyl-2-p(trimethylsilyl) phenylacetylene)] and poly(diphenylacetylene)97 also did not clarify the situation: it was shown that in the 3305
dx.doi.org/10.1021/ma300213b | Macromolecules 2012, 45, 3298−3311
Macromolecules
Perspective
permeability and selectivity.98 Therefore, it is possible to speak about another “structure−properties” relationship: structure of the pyrolyzed polyimides and resulting properties of carbon membranes. However, if thermal rearrangement process is carried out at lower temperatures (e.g., at 350−450 °C), it is possible to obtain polymeric products with the transport properties significantly changed as compared with those of the parent polyimides. It was suggested99,100 that mild thermal treatment of polyimides with −OH and −SH side groups in the ortho position results in thermal rearrangement with the formation of insoluble polybenzoxazoles or polybenzothiazoles, respectively, that revealed good combination of P(CO2) and α(CO2/CH4): the corresponding data points are above the upper bound line on Robeson diagram. Insolubility of the obtained membranes makes them less sensitive to the impurities of liquid hydrocarbons in the separated natural gas. Although there are some doubts regarding the proposed mechanism of thermal transformations,101,102 the quality of the obtained membranes makes quite desirable further investigations of this new method for preparation of gas-separating membranes.
by direct fluorination of polyimide Matrimid.110 Zimmerman and Koros111 also reported very permselective membranes based on fluorine containing polypyrrolone. 6.2. Separation of CO2 from Gas Mixtures. These processes are also among the oldest in membrane technology. Membrane removal of carbon dioxide from natural gases (“sweetening” of natural gases) before they can be passed to the pipeline has obtained wide distribution since the early 1980s.2,6 Capture of CO2 acquired especially great importance because of the problems of environmental protection (related to global warming). Therefore, a search for membrane materials with high permeability P(CO2) and separation factors α(CO2/N2) became very relevant. For the treatment of synthesis gas, another and even more difficult separation problem appears: treatment of H2/CO2 mixtures. The challenge of this task is caused by large and similar permeability coefficients of the both components in most polymers. Two types of membrane materials were proven to be the most attractive in this regard. They will be briefly discussed below. Pebax Copolymers. It was shown that multiblock copolymers having the general formula HO-[C(O)-PA-C(O))-PEO]n-H, where PA is a hard block of nylon-6 or nylon-12 and PE-O is flexible block of polyethylene oxide, are suitable for the removal of carbon dioxide from gaseous mixtures.112 Subsequent studies113−115 showed that high permeability P(CO2) and separation factors α(CO2/N2) and α(CO2/CH4) are caused by exceptionally large solubility coefficients of carbon dioxide due to interactions with the PEO component of the material. Cross-linking of poly(ethylene oxide) suppresses the crystallinity normally found in high-molecular-weight, linear poly(ethylene oxide) and makes the membrane even more permeable.116,117 Some of the cross-linked Pebax membranes showed very impressive transport parameters: CO2 permeability coefficient of ∼500 Barrer and a mixed gas CO2/H2 selectivity of 30 at −20 °C.116,117 Further investigations of Pebax copolymers and related structures also gave interesting results: P(CO2) = 110−130 Barrer and α(CO2/N2) = 37− 49118 or P(CO2)=98 Barrer and α(CO2/N2) = 48.119 Composite Membranes Containing Ionic Liquids. Ionic liquids are organic salts having melting points below ambient temperature or at least
