Gas permeation parameters and other physicochemical properties of a polymer of intrinsic microporosity: Polybenzodioxane PIM-1

Journal of Membrane Science 325 (2008) 851–860

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Gas permeation parameters and other physicochemical properties of a polymer of intrinsic microporosity: Polybenzodioxane PIM-1 Peter M. Budd a , Neil B. McKeown b , Bader S. Ghanem b , Kadhum J. Msayib b , Detlev Fritsch c , Ludmila Starannikova d , Nikolai Belov d , Olga Sanfirova d , Yuri Yampolskii d,∗ , Victor Shantarovich e a

Organic Materials Innovation Centre, School of Chemistry, University of Manchester M13 9PL, UK School of Chemistry, Cardiff University, Cardiff CF10 3AT, UK Institute of Polymer Research, GKSS Research Centre Geesthacht GmbH, Max-Planck-Strasse 1, D-21502 Geesthacht, Germany d A.V. Topchiev Institute of Petrochemical Synthesis, Russian Academy of Sciences, 29 Leninsky Prospect, 119991, Moscow, Russia e N.N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 4 Kosygin Street, 119991 Moscow, Russia b c

a r t i c l e

i n f o

Article history: Received 20 March 2008 Received in revised form 3 September 2008 Accepted 8 September 2008 Available online 16 September 2008 Keywords: Gas permeation Selectivity Gas sorption Polybenzodioxane Free volume

a b s t r a c t A detailed study of gas permeation, thermodynamic properties and free volume was performed for a novel polymer of intrinsic microporosity (PIM-1). Gas permeability was measured using both gas chromatographic and barometric methods. Sorption of vapors was studied by means of inverse gas chromatography (IGC). In addition, positron annihilation lifetime spectroscopy (PALS) was employed for investigation of free volume in this polymer. An unusual property of PIM-1 is a very strong sensitivity of gas permeability and free volume to the film casting protocol. Contact with water in the process of film preparation resulted in relatively low gas permeability (P(O2 ) = 120 Barrer), while soaking with methanol led to a strong increase in gas permeability (P(O2 ) = 1600 Barrer) with virtually no evidence of fast aging (decrease in permeability) that is typical for highly permeable polymers. For various gas pairs (O2 /N2 , CO2 /CH4 , CO2 /N2 ) the data points on the Robeson diagrams are located above the upper bound lines. Hence, a very attractive combination of permeability and selectivity is observed. IGC indicated that this polymer is distinguished by the largest solubility coefficients among all the polymers so far studied. Free volume of PIM-1 includes relatively large microcavities (R = 5 Å), and the results of the PALS and IGC methods are in reasonable agreement. © 2008 Elsevier B.V. All rights reserved.

1. Introduction During the last two decades, a number of novel polymers have been prepared and described that reveal unusually high gas permeability and diffusivity, large solubility coefficients and great free volume [1]. Most of them belong to the class of amorphous glassy polyacetylenes and are, in a sense, analogs of poly(trimethylsilyl propyne), the most permeable of all the polymers known, discovered in 1983 by Masuda and Higashimura [2]. Analysis of structure–property relationships of this and other polymers indicates that the prerequisites for high permeability and large free volume are the presence of a bulky group attached directly to the main chain (that is, without spacers) and the stiffness of the main chain. So a relevant question arises as to whether this is the only design of the repeat units of polymers that enables the realization of extreme values of gas permeability of a polymer. In spite of

∗ Corresponding author. Tel.: +7 495 9554210; fax: +7 495 6338520. E-mail address: [email protected] (Y. Yampolskii). 0376-7388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2008.09.010

some exceptions (e.g. amorphous glassy perfluorinated polymers [3]), very little evidence can be found in the literature that suggests otherwise. Recently, a novel type of polymers with relatively high gas permeability was described: polybenzodioxanes. The ladder-type new polymers present different design of repeat unit. The most interesting member of this class is the product of condensation of 5,5 ,6,6 -tetrahydroxy-3,3,3 ,3 -tetramethyl-1,1 -spirobisindane and tetrafluoroterephtalonitrile (PIM-1). Its structure is shown below.

These polymers – termed polymers of intrinsic microporosity or PIMs [4,5] – show relatively high gas permeability, extremely large

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inner surface area as measured by the BET method, large free volume as estimated via positron annihilation lifetime spectroscopy (PALS), and great sorption capacity. All these features are characteristic to many polyacetylenes, however PIMs have entirely different structure and this circumstance makes further investigation of the properties of these novel membrane materials quite relevant. In this work we extend the studies of these interesting membrane materials. The work includes the results of the investigation of the transport and other physicochemical properties of PIM-1. Gas permeation parameters (permeability, diffusion and solubility coefficients) were measured at different temperatures for films prepared in various ways. Three “states” of PIM-1 films are discussed, corresponding to films that have contacted with water during preparation (state 1), films prepared by solvent casting without contact with water (state 2) and films post-treated by soaking in methanol (state 3). In addition, thermodynamic properties of the polymer were studied by means of inverse gas chromatography. Free volume of this polymer, as estimated using PALS is also discussed.

Fig. 1. Molar mass distribution from gel permeation chromatography (polystyrene calibration) for PIM-1.

720 ± 6 m2 g−1 ). TGA measurements of different PIM-1 samples were carried out using an Iris TG 209 F1 instrument (NETZSCH) at the linear rate of heating of 20 K/min.

2. Experimental 2.1. Polymerization Polymerization was carried out in a Radleys LARA reactor equipped with an anchor-type stirrer. The glass reactor body was dried overnight at 100 ◦ C before use. 5,5 ,6,6 -Tetrahydroxy3,3,3 ,3 -tetramethyl-1,1 -spirobisindane (44.950 g, 0.1320 mol) and tetrafluoroterephthalonitrile (26.418 g, 0.1320 mol) were added to a 1-L vessel and the reactor flushed with nitrogen. Anhydrous dimethylformamide (DMF, 846 cm3 ) was added by double-ended needle under nitrogen. A stream of nitrogen was bubbled through the pale brown solution as it was stirred (300 rpm) at room temperature for 1 h, then the contents of the reactor were heated to 65 ◦ C over 30 min, under a nitrogen purge. The stirring rate was increased to 500 rpm and anhydrous K2 CO3 (155 g, 1.123 mol, 8.51 mol equiv.) was added. An exotherm was noticed immediately after the addition (to 73 ◦ C), and an immediate colour change to yellow was apparent. After 2 h, the stirring rate was reduced to 300 rpm, and the reaction mixture was then stirred at 65 ◦ C under a nitrogen purge for 68 h. The reaction mixture was cooled to 20 ◦ C then filtered, and the filter cake washed with DMF (300 cm3 ), acetone (300 cm3 ) and water (1 L). The yellow solid was stirred with water (4 L) in a 5-L vessel for 30 min. The solid was filtered off, and washed with water (2 L) and acetone (700 cm3 ). The filter cake was then washed with 1,4-dioxane (2 L in 100 cm3 portions), with frequent stirring. The solid was rinsed with acetone (700 cm3 ), and 10% water in acetone (semi-pure yield 57.77 g, 95.0%). To purify the batch further, the PIM-1 was washed on a sinter with water (4 L), acetone (600 cm3 ), 1,4-dioxane (4 L in 200 cm3 portions), acetone (600 cm3 ), water (1 L) and acetone (400 cm3 ), then dried overnight at 95 ◦ C (final yield 55.53 g, 91.4%). 2.2. Characterization Multi-detector gel permeation chromatography (GPC) analysis was carried out in chloroform at a flow rate of 1 cm3 min−1 using a Viscotek VE2001 GPC solvent/sample module with two Viscotek GMHHRM columns and a Viscotek TDA302 triple detector array (light scattering, viscosity and refractive index detectors). Gel permeation chromatogram is shown in Fig. 1. The absolute weight–average molar mass was determined as 370,000 g/mol. N2 adsorption analysis of a powdered sample at 77 K was carried out using a Coulter SA3100 instrument (BET surface area

2.3. Gas permeation measurements Two techniques were used in the determination of the permeability of PIM-1. One method (Moscow) was based on use of a gas chromatographic setup with differential thermostated cell with a penetrant pressure drop of 1 atm. The films were cast on horizontal table, so uniform thickness was achieved. A precise micrometer was used in determination of film thickness averaged over the whole area of the film (error ±2%). Measurements of permeation of several gases (He, H2 , O2 , N2 , CO2 , CH4 ) was performed in the temperature range 22–55 ◦ C. Pure penetrant gas with a pressure of 1 atm was passed through the upstream part of the cell, while the gas carrier (He in the most cases and argon in the runs with He and H2 as penetrants) was passed though the downstream part of the cell. After attaining steady-state conditions, as checked by a catharometer, a sample of the permeate was taken for GC analysis. Simultaneously, the flow of the permeate was measured using a soap bubble flowmeter. In order to check possible effects of the measurement protocol (effects of heating during investigation of the Arrhenius dependence, influence of highly sorbed gases like CO2 ) the determination of oxygen permeability at room temperature was carried out before and after completing of the series of experiments. The error in the determination of permeability coefficients was ±5%. Gas permeation parameters were also measured (Geesthacht) in the temperature range 30–55 ◦ C with pure gases, using a pressure increase time-lag apparatus operated at low feed pressure (typically 200–300 mbar), starting with an oil-free vacuum (<10−4 mbar). Permeate pressure increase with time was recorded by two MKS Baratron pressure sensors (10 mbar max (permeate), 1 bar max (feed)) that were connected directly to a computer [6]. Software developed in the Labview environment ensures automated measurements. Typically, the total time of measurement is set to four time-lags with an automatically adapting data sampling rate to yield 200 data points. For H2 and He, usually only 200 data points, measured at a speed of 20 data points per second, are necessary to describe time-lag and steady-state gas transport completely. Time-lags below 1 s can be detected and reproduced precisely. Feed pressure was varied from 0.1 to 1 bar. Permeate pressure was recorded up to 0.05–9 mbar, depending on the feed gas. Fig. 2 is given as an example of the experimental curve using He as the penetrant. It indicates that the technique permits reliable mea-

P.M. Budd et al. / Journal of Membrane Science 325 (2008) 851–860

853

the net retention volume VN can be found: VN = (tr − ra )Fp 0 ,T col j32 ,

(1) cm3 /s

where Fp 0 ,T col is the flux of gas carrier, reduced to the atmospheric pressure and the column temperature, j32 is the correction for pressure drop in the column, and Tcol is the temperature of the experiment. After accounting for the mass of the polymer in the column, the specific retention volume Vg can be calculated: Vg =

Fig. 2. An example of experimental time-lag curve.

2.4. Inverse gas chromatography A large pore Inerton AW support with low surface area (about 0.5 m2 /g) was chosen as a solid carrier for the polymer phase. The PIM-1 samples were deposited on Inerton AW from 3 to 5 mass% solutions in tetrahydrofuran. The polymer concentrations estimated by back-extracting in a Soxhlet apparatus were 8.3 ± 0.3 mass%. Dry solid carrier coated with the polymer phase was packed into a stainless steel column 1.5-m long and with inner diameter 3 mm. The measurements were performed in a LKhM8MD chromatograph with thermal conductivity detector in the range 45–250 ◦ C. Helium served as a gas carrier and an air peak was used for estimation of ta (see below). The inlet pressure was checked using a high-sensitivity manometer to introduce the corrections into Eqs. (1) and (4) below, while the outlet pressure was taken as atmospheric. The inverse gas chromatography (IGC) method is based on measurement of the retention times of sorbed (tr ) and “non-sorbed” (ta ) components [9]. The value ta that corresponds to virtually weakly sorbed component (e.g. air or methane) is needed to account for the dead volume of the chromatograph. Using the values tr and ta ,

(2)

where wL (g) is the mass of polymeric phase in the column. The IGC method can be used for determination of the infinite dilution solubility coefficient [10]: S=

surements of very short time-lags. A more detailed description of this experimental setup is given elsewhere [7]. Permeability coefficient, P, was calculated from the slope in the steady-state region and apparent diffusion coefficient, D, from the time-lag, , using the Daynes–Barrer equation D = l2 /6, where l is the membrane thickness. The solubility coefficients were estimated as the ratio S = P/D. Average experimental errors of this method are as follows: P, ±7%; D, ±7% (somewhat higher in the case of “fast” gases—He and H2 ); S, 10%. An unusual peculiarity of this work is that the same property, gas permeability, was measured using two independent methods. In order to exclude systematic errors of both methods we selected a standard reference polymer, poly(vinyltrimethylsilane) (PVTMS), which has been tested many times using different techniques and which does not reveal a big sensitivity to the protocol of film casting. The following permeability coefficients of this polymer at room temperature (in Barrer) were obtained using the barometric method: H2 215, He 178, O2 45, N2 10.9. The P values measured using the GC method and reported earlier [8] are as follows: H2 220, He 160, O2 44, N2 11 (in Barrer). This excellent agreement indicates that the differences in the P values observed using the two methods cannot be ascribed to any systematic errors. The PIM-1 films were cast from 2% solutions in THF over the surface of stretched cellophane bottom of hollow cylindrical vessels or over hydrophobic glass or Teflon surface. The details of post-treatment of the samples are considered later in the paper.

VN , wL

1 Vg exp p0

 (2B − V ) 11 1 RT

p0 J34



(4)

Here p0 = 1 atm is the standard value, the second virial coefficient B11 (cm3 /mol) and molar volume V1 (cm3 /mol) of solutes can be calculated as recommended by Reid and Sherwood [11], p0 (Pa) is the inlet pressure in the column, and J34 is the correction for pressure drop in the column. The temperature dependence of S allows a determination of enthalpy of sorption Hs :

 H  s

S = S0 exp −

RT

(5)

and the partial molar enthalpy of mixing Hm : Hm = Hs − Hc ,

(6)

where Hc is the enthalpy of condensation of the solute [12]. 2.5. Positron annihilation lifetime spectroscopy The positron annihilation lifetime decay curves were measured at room temperature in ambient and an inert atmosphere using an EG@G Ortec “fast-fast” lifetime spectrometer. A nickel-foilsupported [22 Na] sodium chloride radioactive positron source was used. Two stacks of film samples, each with an overall thickness of about 1 mm, were placed on either side of the source. Measurements were performed in ambient atmosphere (in contact with air) and in an inert (nitrogen) atmosphere: the reason for this was to eliminate the contribution of additional o-positronium decay due to the reaction with sorbed oxygen, which is important for high free volume materials. The time resolution was 230 ps (full width at the half maximum (FWHM) of the prompt coincidence curve). The contribution from annihilation in the source material, a background, and instrumental resolution were taken into account in the PATFIT program for treating the experimental lifetime data. The integral statistics for each spectrum was equal to (1.5–2.0) × 106 coincidences. Resulting data were determined as an average value from the several spectra collected for the same sample. 3. Results and discussion 3.1. Gas permeation parameters The transport properties of this polymer have been reported earlier [5]. In the present work a more detailed study was undertaken. The permeability coefficients P were measured by barometric and gas chromatographic methods described in Section 2. Barometric, time-lag methods allowed also a determination of the diffusion coefficients D, and, hence, the solubility coefficient could be deduced from the observed P and D values. The gas chromatographic technique resulted only in permeability coefficients. In the

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P.M. Budd et al. / Journal of Membrane Science 325 (2008) 851–860 Table 2 Gas permeation parameters of PIM-1 film (state 2). Gas

He H2 O2 N2 CH4 CO2

Fig. 3. Arrhenius dependence of the permeability coefficients P (Barrer) of PIM-1 (sample GC1, GC measurements).

two methods different boundary conditions were realized. However, for light gases, which served as the objects of the investigation, in the majority of cases these differences were insignificant, so the results of the two methods could be compared. The most unusual feature of PIM-1 as a membrane material is an extraordinarily great sensitivity of the observed permeability coefficients on the protocol of film forming. As will be seen from the subsequent text, the observed permeability coefficients vary by one order: e.g. for oxygen the P values varied from 120 Barrer to 1600–1800 Barrer at room temperature. Hence, it is worthwhile to focus on the details of film casting and treatment and one may speak of several states of the polymer under investigation. Some insight into the state of the films that revealed different permeability was provided by TGA tests in the range 25–140 ◦ C. Three films were tested: (a) a film cast from chloroform solution (state 2), (b) a similar film that contacted water while peeling from the bottom of the casting dish (state 1) and (c) a film that was kept in methanol overnight (state 3). All the samples were kept in a vacuum chamber at room temperature until constant weight was achieved. The films (a) and (b) showed very similar TGA behavior. The removal of low molecular mass components started at 80–100 ◦ C and the final mass loss was 2.2 and 2.3%, respectively. On the other hand, sample (c) showed a much smaller content (0.5%) of residual low molecular mass compound (methanol). 3.1.1. Gas chromatographic determination of permeability The “as-received” sample of PIM-1 was dissolved in THF and a film with a thickness of 199 ␮m was obtained by casting over the surface of cellophane bottom in the casting cup (sample GC1). The peeling of the film from the cellophane was achieved by applying a weak stream of water. The film was subjected to vacuum drying to constant weight. Investigation of gas permeability was carried out in the temperature range 21–58 ◦ C and the pressure drop of a penetrant 1 atm. Arrhenius dependences of the permeability coefficients are shown in Fig. 3. The permeability coefficients of O2 , N2 ,

Gas chromatographic method (sample GC2)

Barometric method (sample B2)

P (25 ◦ C), Barrer

P (30 ◦ C), Barrer

EP (kJ/mol)

P (30 ◦ C), Barrer

EP (kJ/mol)

760 1630 580 180 310 4390

786 1670 590 190 330 4350

5.0 3.6 1.1 7.5 10.9 −1.5

740 1600 530 155 240 3700

5.4 3.2 3.3 10.5 10.9 −4.5

CO2 and CH4 (at 25 and 30 ◦ C), as well as the activation energies of permeation, are presented in Table 1. Special experiments indicated that the order of measurement (that is, the effects of highly sorbed gases like CO2 ) does not affect permeation rates of lighter gases in subsequent runs. It should be noted that the observed values for sample GC1 were substantially lower than those reported earlier for this polymer. Thus, according to Ref. [5] the permeability coefficient of oxygen is 370 Barrer and of nitrogen is 92 Barrer at 30 ◦ C. In order to eliminate possible effects of strongly sorbed water or impurities (oligomers) in the sample, reprecipitation of the sample was performed. A solution of 1.25 g PIM-1 in 50 ml THF was slowly added (with vigorous mixing) to the mixture of 200 ml of acetone and 50 ml THF. After completion of this procedure the mixing was resumed for ∼30 min. The polymer was filtered out, several times washed with acetone, methanol and then dried in ambient conditions. From the sample obtained in this manner a film was cast from THF solution (sample GC2). The thickness of the film was 107 ␮m. The film was subjected to additional drying in vacuum at room temperature. This film was removed from cellophane bottom without contact with water. The results of gas permeation measurements with this sample are given in Table 2 and the corresponding Arrhenius dependence is shown in Fig. 4. First, Table 2 indicates that a strong increase in the P values for all the gases is observed after reprecipitation of the PIM-1 sample. The observed P values are significantly higher than those reported in [5]. Marked changes are also observed in the activation energies of permeation (Fig. 3); the dependences became even weaker in most cases, while for carbon dioxide the more pronounced negative activation energy is observed. Experiments at different temperatures in the range 25–55 ◦ C during 10 days did not result in a noticeable change in permeability: P(O2 ) decreased not more than by ∼3%, P(N2 ) by ∼7%. Storing high permeability, large free volume polymers (e.g. PTMSP) in methanol prior to measurement is a frequently used procedure to delay or reverse the effects of physical aging (see

Table 1 Gas permeation parameters of the PIM-1 film (state 1). Gas

He H2 O2 N2 CH4 CO2

Gas chromatographic method (sample GC1)

Barometric method (sample B1)

P (25 ◦ C), Barrer

P (30 ◦ C), Barrer

EP (kJ/mol)

P (30 ◦ C), Barrer

– – 150 45 114 1550

– – 157 49 122 1540

– – 6.4 11.9 9.1 −0.37

155 270 128 51 – 950

Fig. 4. Arrhenius dependence of the permeability coefficients P (Barrer) of PIM-1 (sample GC2, GC measurements).

P.M. Budd et al. / Journal of Membrane Science 325 (2008) 851–860 Table 4 Permselectivity of PIM-1 in different states.

Table 3 Gas permeation parameters of the PIM-1 film (state 3). Gas

He H2 O2 N2 CH4 CO2

Gas chromatographic method (sample GC3) P (23 ◦ C), Barrer – – 1,610 500 740 12,600

855

Barometric method (sample B3) P (30 ◦ C), Barrer 1,320 3,300 1,530 610 1,160 11,200

for example [13]). Bearing this in mind, the PIM-1 film GC2 was immersed in methanol for 5 days then its gas permeation parameters measured (sample GC3). GC test indicated that after 1 day contact with ambient atmosphere no measurable quantity of MeOH was present in the film. The results are presented in Table 3. It is seen that a treatment in methanol leads to a significant, nearly threefold, increase in gas permeability. The values of P(O2 ) and P(CO2 ) of PIM-1 treated in such manner allow one to consider this polymer among the most permeable membrane materials, such as substituted polyacetylenes and amorphous Teflon AF (see Ref. [1], p. 241, 253). Aging, i.e. a reduction of permeability in time due to various possible mechanisms, is a peculiarity of many high free volume polymers (see e.g. Refs. [14–17]). Since aging is more characteristic for highly permeable polymers, it was of interest to study the time dependence of the permeability coefficients in the PIM-1 sample after treatment with methanol (sample 3). The film was kept for different periods at ambient temperature and in contact with air. The measurements of permeability were performed periodically using the gas chromatographic method. It was shown (Fig. 5) that a certain decrease in the P(O2 ) and P(N2 ) values is observed, however, the rate of these changes is relatively small: oxygen permeability was reduced by about 23% and nitrogen permeability by 40% after 45 days of observation, so this “aging” resulted in some increase in permselectivity P(O2 )/P(N2 ) of the sample, from 3.3 to 4.2. It is of interest to compare the rates of aging for PIM-1 and PTMSP, the polymer for which these phenomena are best documented. Since the rate of aging can depend on the conditions (temperature, selection of the test gas, etc.), so the results obtained for room temperature and oxygen being used as a test gas were selected from extensive literature on this subject. In many cases, PTMSP reveals much greater rate of aging: for example, in the conditions of individual gas permeation Nakagawa et al. [16] reported a reduction of the P values of O2 by a factor of 5 during 30 days. Consolati et al. observed a decrease in oxygen permeability by a factor of about 16 for 40 days [18]. Similar rate of aging (as sensed by O2 permeability) was reported by Nagai et al. [17]. On

Fig. 5. Time dependence of the permeability coefficients of oxygen and nitrogen through the PIM-1 film treated by methanol (lines are drawn to guide eye).

State

P(O2 ) (Barrer)

A O2 /N2

CO2 /CH4

CO2 /N2

1 (wet) 2 (reprecipitated) 3 (MeOH treated)

150 584 1610

3.3 3.2 3.3

13.6 14.2 17.0

34.4 24.2 25.5

the other hand, mixed gas permeability in separation of the mixture of 2%C4 H10 /98%H2 showed much smaller rate of aging: after 40 days mixture was only 75–85% of the original value. The influences of the protocol of film forming (choice of a solvent, heat treatment, etc.) is a common phenomenon in gas permeation studies, but very seldom if anywhere one meets with so strong effects as in the case of PIM-1. This behavior appears to be linked to strongly adsorbed residual solvent (see discussion of TGA above) and work is in progress to elucidate further the structural peculiarities of PIM-1 treated in different ways. Further information about free volume in different states of this polymer is obtained using the PALS method (see later). A typical trade-off behavior is well known for gas permeability (Pi ) and permselectivity (Pi /Pj ) in polymer membranes. It is of interest to examine how the permselectivity varies at so great changes of the permeability coefficients. Results are given in Table 4. A conclusion can be made that strong variations of the P values are not accompanied by significant changes in the separation factors ˛ij = (Pi /Pj ). Further analysis of the relationships between permeability and permselectivity of different PIM-1 samples is provided by Fig. 6, where Robeson diagrams are presented for different gas pairs. For the pairs O2 /N2 and CO2 /CH4 the upper bound lines were taken from the original work by Robeson [19]. As this article did not present the diagram for the pair CO2 /N2 , the corresponding upper bound was obtained using the Data base [20]. It should be noted that the Robeson upper bounds correspond to the data collected until 1991, whereas the latter upper bound is based on the data reported approximately until 2004. It can be seen from Fig. 5 that for all three gas pairs considered, most of the data points are located in these diagrams above the upper bounds drawn by Robeson. In the case of the gas pair CO2 /N2 the data points are significantly above even the upper bound drawn through most recent gas permeation parameters. All this confirms the conclusions made earlier [5] that PIM-1 can be considered as a promising material for gas separation membranes, especially for separation of the mixtures containing carbon dioxide. 3.1.1.1. Barometric determination of permeability. In the standard procedure, freshly cast films were subjected to 1 day removal of the residual solvent in a closed chamber under argon stream at room temperature either on a glass or Teflon® surface. When the films were prepared on a glass surface, they were peeled from the surface by immersion in water and wet films (sample B1) were obtained. Their prolonged heating in the range 30–55 ◦ C under high vacuum (below 10−4 mbar) resulted in films (sample B2) having a state similar to those obtained on a Teflon surface using the standard procedure. Finally, overnight storage of the films in liquid methanol, with subsequent heating in vacuum at 120 ◦ C for 16 h, resulted in another more permeable state of the film (sample B3). It is important to note, as will be shown later, these states of the polymer films correspond well to the “states” obtained in the gas chromatographic experiments, however, in the latter case they were obtained by different procedures. Table 1 presents the permeability coefficients of a PIM-1 sample contacted with water and measured using the barometric method (sample B1).

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Since barometric measurements resulted in determination of the D and S values for various gases, it is possible to consider which of the components of permeability is responsible for the high permeability of PIM-1. Table 5 summarizes these data for various “states” of PIM-1 and compares these values with those reported for PTMSP, whose permeability, diffusion and solubility coefficients are considered to be the largest among all the polymers. Several observations can be made after examining this table. First, it is obvious that different treatment of the PIM-1 films results also in some changes in the D and S values. “Wet” samples reveal smaller diffusion and solubility coefficients than the sample after heat treatment that likely leads to removal of sorbed water. Hence, both thermodynamic and kinetic components of permeability contribute to the increases in the P values. Second, the observed D and S parameters of PIM-1 differ significantly from those of PTMSP. On the one hand, the diffusion coefficients of PIM-1 are smaller than those of PTMSP by one or two orders (depending on the gas and the sample). On the other hand, solubility coefficients of gases in PIM-1 in many cases are markedly larger than those in PTMSP. This is a quite unexpected result that indicates that we deal in the case of PIM-1 with the material having the greatest gas solubility coefficients observed. Later on in this paper it will be shown that the solubility coefficients of gases determined as the ratio P/D and of vapors estimated via the IGC method are in good agreement and both confirm the conclusion of unusually high S values in this material. Note that high values of the solubility coefficients of light gases have been discussed in our previous work [5]. The reasons for this should be sought for in the structure of this polymer, and can be attributed in particular to the polar cyano-groups. In addition, cyano-groups will interact strongly with water and may explain this behavior in permeation of films treated by water. Combined action of smaller D and larger S values explains lower values of the permeability coefficients in this polymer as compared with PTMSP. Sorption thermodynamics: The work included the determination of the specific retention volumes Vg and various physicochemical parameters derived from them at different temperatures for the following solutes: n-alkanes C3 –C10 , benzene and toluene, perfluorobenzene and perfluorotoluene, 4-fluorotoluene, 2,3,4,5,6pentafluorotoluene, normal alcohols C1 –C4 , acetone, tetrachlorocarbon, chloroform, tetrahydrofuran. Fig. 7 shows the retention diagrams, that is, vant’Hoff plots for Vg of several solutes as an illustration. The temperature dependences for these and other solutes are linear, indicating constant enthalpies of sorption.

Using Eq. (4) the solubility coefficients of various solutes in PIM-1 were found. It is common to compare the S values at some standard temperature, for which 35 ◦ C is usually taken. In Fig. 8, the solubility coefficients of the n-alkane series are compared for PIM-1 and other polymers. Information on solubility coefficients is available for many polymers, but we have taken for comparison those which reveal the largest solubility coefficients: PTMSP [13,22], PVTMS [23], and addition type poly(trimethylsilyl norbornene) [24]. For PIM-1 the solubility coefficients are based both on IGC measurements and found via the formula S = P/D (P and D were obtained in barometric measurements). The correlation of S versus Tc2 , where Tc is the critical temperature of the solute, holds for glassy and rubbery polymers and is observed for solutes having a wide range of properties (see Ref. [1], p. 7). It can be seen in Fig. 8 that PIM-1 exhibits the largest solubility coefficients, irrespective of solute size. The difference between the S values of PIM-1 and PTMSP increase as the size of the solute increases: for n-heptane and higher alkanes the difference is as great as one order of magnitude. The IGC-based values of S and those determined using the P and D values fall on the same correlation line. These results show that PIM-1 has the highest solubility coefficients of all the polymers studied so far. This may be attributed to the inner microporosity and the availability of highly polar cyano- and ether-groups, as has been briefly discussed earlier [5]. The slopes of the lines in the retention diagrams allow one to estimate the enthalpies of sorption, Hs , which are given in Table 6. The values of Hs are negative for all the solutes, which is typical for vapor sorption. An unusual observation here is the absolute values of Hs in PIM-1: thus, the enthalpy of sorption of n-hexane, e.g. in metathesis poly[bis(trimethylsilyl)norbornene] is −10 kJ/mol [25], whereas the same value in PIM-1 is −63.8 kJ/mol. Since Hs includes the contributions of enthalpy of condensation and partial molar enthalpy of mixing (Eq. (6)), and the former parameter is a characteristic of the solute, strongly negative Hs values should be a manifestation of a very exothermic process of mixing (negative Hm values). Indeed, these parameters, also shown in Table 6, are much more negative than those observed in other glassy polymers. It indicates that the process of dissolution in PIM-1 is not accompanied by overcoming interchain interactions and involves insertion of solute molecules into large pre-existing microcavities of the nano-structure of PIM-1. Mixing in this polymer is also accompanied by great restrictions of inner degrees of freedom, as is manifested in large negative Sm values (Table 6). Free volume: The estimation of free volume in this work was performed using the PALS and IGC methods. Some insight on free

Table 5 Diffusion and solubility coefficients of PIM-1 and, for comparison, PTMSP. Polymer D (×107 cm2 /s) PIM-1

PTMSP [16] [21]

State 1 (wet)a 2 (after heating at 45 and 55 ◦ C)b 3 (after MeOH treatment)

He 190 360 680

100 290 500

1100

1800 2600

PTMSP [16] [21] a

O2

N2

CO2

CH4

9.3 15 39

5.7 4.3 16

3.5 4.5 16

– 1.3 7.1

–

S (×103 cm3 (STP)/cm3 cm Hg) 1 (wet) PIM-1 2 (after heating at 45 and 55 ◦ C) 3 (after MeOH treatment

b

H2

Contacted to water and dried at 120 ◦ C for 16 h at about 5 mbar. Dried additionally for several days with pressure below 10−4 mbar.

220 520

150 440

0.82 2.6 1.9

2.7 6.9 6.6

14 40 39

9.1 35 37

2.0 –

2.9 5.7

14 17.2

12 14.7

250 330

160 360

270 770 700

– 160 163

76 82.2

27 47.4

P.M. Budd et al. / Journal of Membrane Science 325 (2008) 851–860

857

Fig. 7. Retention diagrams in PIM-1.

[29]. It should also be mentioned that in rubbery polymers, where the dissolution process proceeds according to the Flory–Huggins model, the Hm values do not depend on solute size. Fig. 9 presents this dependence for PIM-1 and for two other glassy polymers. It obviously shows very negative values of Hm in PIM-1. The curve of Hm versus Vc reveals some tendency to approaching a minimum, but measurements were not possible for alkanes larger than n-decane, because the large solubility in PIM-1 would require experiments to be performed at too high a temperature. It can be concluded that the size of free volume elements in PIM-1 according to the IGC method is at least larger than the size of C9 H20 –C10 H22 , which, assuming a spherical microcavity, corresponds to a radius of 6.0–6.2 Å. A rough correlation that exists between gas permeability of several glassy polymers and the radius R of microcavity as determined by the IGC method indicates that this radius in PIM-1 should be close to 6.0 Å. Fig. 6. (a) Robeson diagram for the pair O2 /N2 : the points are from the Data base of TIPS [20], dashed line shows Robeson’s upper bound of 1991; (b) Robeson diagram for the pair CO2 /CH4 : the points are from the Data base of TIPS [20], dashed line shows Robeson’s upper bound of 1991; (c) Robeson diagram for the pair CO2 /N2 : the points are from the Data base of TIPS [20], solid line dashed line is the upper bound drawn manually for all the data points except those of PIM-1 (1 state 1; 2 state 2; 3 state 3).

volume in PIM-1 was previously provided by nitrogen adsorption measurements [5]. Also, data on free volume in PIM-1 based on PALS measurements have been reported [26,27]. In this work, the PALS method was applied to check the variations of free volume in different states of PIM-1. Inverse gas chromatography: The dependences of Hm values on solute size have been extensively used for estimation of the size of free volume elements in glassy polymers [28]. It has been demonstrated for many examples that the dependences of the negative Hm values on the solute’s critical volume Vc , which serves as a measure of their molecular size, in glassy polymers passes through a minimum at certain Vc(min) values. These parameters correlate with gas diffusivity and permeability of glassy polymers. The mean free volume size deduced from them is in agreement with the results of the determination of free volume in glassy polymers using positron annihilation lifetime spectroscopy and 129 Xe NMR

Fig. 8. Solubility coefficients in different glassy polymers—1: PIM-I, 2: addition type poly(trimethylsilyl norbornene) [24], 3: PTMSP [13], 4: PVTMS [23].

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Table 6 Enthalpies of sorption and partial molar enthalpies and entropies in PIM-1.

Table 7 Parameters of PAL spectra of PIM-1.

Solutes

Hs (kJ/mol)

∞ Hm (kJ/mol)

∞ Sm (J/mol K)

Atmosphere

Propane n-Butane n-Pentane n-Hexane n-Heptane n-Octane n-Nonane n-Decane Cyclohexane Benzene Toluene Perfluoropbenzene Perlfuorotoluene 4-Fluorotoluene 2,3,4,5,6-Pentafluorotoluene Methanol Ethanol n-Propanol n-Butanol Tetrachlorocarbon Chloroform Tetrahydrofuran Acetone

−42.8 −51.3 −61.0 −63.8 −68.7 −80.0 −87.1 −88.1 −59.5 −56.8 −71.0 −62.1 −78.2 −72.5 −77.1 −42.8 −51 −59 −64 −60 −60 −60 −58

−24 −21 −26 −28 −30 −38 −40 −40 −27 −25 −33 −25 −33 −33 −33 −7 −16 −21 −24 −26 −25 −27 −27

−56 −51 −58 −61 −62 −77 −77 −72 −56 −48 −66 −46 −67 −64 −61 −32 −44 −54 −56 −50 −50 −56 −60

Standard (state 2) Air 1.15 ± 0.16 2.13 ± 0.30 N2

Positron annihilation lifetime spectroscopy: Measurements of positron annihilation lifetimes were performed in contact with air (in presence of oxygen) and in a nitrogen atmosphere. Previous experiments have shown that experiments with polymers in vacuum and in nitrogen atmosphere gave only slightly different lifetimes, though the second way was safer for the source and more reliable for the estimation of free volume size. Experiments in air were useful for revealing the chemical interactions of orthopositronium (o-Ps) with oxygen in free volume. It is known that in polymers oxygen is able to quench o-Ps in the pores of a polymer, thus reducing the lifetimes [30,18]. This effect is more pronounced for high free volume polymers but virtually disappears for the polymers having smaller sizes of free volume elements [31]. The experiments under both conditions (air and nitrogen) and PATFIT analyses of the lifetime distributions showed that the PAL spectra are characterized by the two relatively long o-Ps lifetimes  3 and 4.

 3 (ns)

 4 (ns)

I4 (%)

9.17 ± 1.75 6.14 ± 0.61

4.30 ± 0.08 5.81 ± 0.19

10.25 ± 0.42 17.79 ± 0.87

Methanol (state 3) Air 1.84 ± 0.13 1.80 ± 0.36 N2 Air again 1.72 ± 0.20

9.62 ± 0.43 5.77 ± 0.71 10.9 ± 0.7

4.48 ± 0.11 7.09 ± 0.19 4.42 ± 0.19

9.2 ± 0.65 17.25 ± 0.65 9.2 ± 1.10

Water (state 1) Air N2

8.91 ± 0.73 7.38 ± 0.99

4.53 ± 0.09 5.58 ± 0.08

10.25 ± 0.46 15.38 ± 0.37

1.43 ± 0.10 1.35 ± 0.17

I3 (%)

Table 7 indicates that the replacement of air with nitrogen results in a noticeable increase in the observed lifetimes  4 . The  4 values obtained in the runs in contact with air do not differ significantly for the three states of PIM-1. However, the quenching process of ortho-positronium conversion (o-Ps) ↔ (p-Ps) on oxygen unpaired electrons or o-Ps annihilation in the bound state (Ps-O2 ) provide an additional route for the disappearance of o-Ps, so these values do not allow lifetimes to be related realistically to the size of free volume elements. They will be used only to illustrate the oxygen penetration into the pores. Pore size estimations were made based on the data obtained in the absence of oxygen. The lifetimes  4 obtained in the atmosphere of N2 are sensitive to the state of PIM-1 samples, as manifested in their gas permeation parameters. The shortest time is observed for the sample that had contact with water during film preparation (state 1 with the smallest P values) and the longest lifetimes are characteristic for the sample post-treated with methanol (state 3, which showed the higher P values). Therefore, the film forming protocol affects the sizes of free volume elements in PIM-1. Interestingly, the changes in the intensity I4 , that can serve as a rather rough measure of the concentration of free volume elements were hardly noticeable for all samples in the air or for those in absence of oxygen, respectively. However, the intensities are essentially increasing on going from air to nitrogen and decreasing again when the pores were refilled with air (see “state 3” as an example). These results show that oxygen can be an inhibitor of positronium formation too. Using the well accepted model of Tao and Eldrup [32,33] we calculated the radii of free volume elements in this polymer, which are presented in Table 8. It is seen that the samples, which showed greater permeability include larger free volume elements. Interestingly, the values obtained for the radii of free volume elements correlate with the results of TGA tests. The sample soaked with methanol contained very little of “residual solvent” which was manifested in larger R values. The samples that included larger quantity of “residual solvents” (water and chloroform) are characterized by smaller radii of free volume elements. It is of interest to compare the results of the estimation of free volume radius in PIM-1 using different methods. The results are shown in Table 9 where the data obtained via IGC are compared with various PALS studies as well as results of N2 adsorption analysis and molecular dynamics simulation. Such a comparison should be made with caution because different approaches have certain peculiarities and complications. Thus, the IGC method gives the Table 8 Radii of free volume elements of PIM-1 in different states at ambient temperature.

Fig. 9. Dependence of the partial molar enthalpy of mixing of n-alkane series in PIM-1 and for comparison in other glassy polymers. 1: PIM-1, 2: addition type poly(trimethylsilyl norbornene) [24], 3: PVTMS [23].

State

R3 (Å)

R4 (Å)

Standard (2) Methanol (3) Water (1)

3.00 2.69 2.17

5.22 5.75 5.11

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859

Table 9 Radii of free volume elements (Å) as estimated by different methods. Polymer

PIM-1 PTMSP a b c d

PALS

IGC

N2 adsorption

MD

R4

R4

R3

Rmin

Pore size distribution

Radius distribution

5.2 (this work)a 6.8 [31]a

4.85 [26]a 7.5 [18]a

4.8 [27]b –

6.0 (this work)c –

6–8 (this work)d 6.5–14 [5]d

1–6 [34] 2–10 [35]

Four components lifetime distribution. Three components lifetime distribution, measured in vacuum. Temperature range of 45–250 ◦ C. At 77 K.

values averaged over a wide temperature range (above room temperature) and there are some doubts about the density of sorbed phase and, hence, the choice of critical volume as a scaling parameter. The PALS data can be obtained for different fits (three or four components treatment) of the original experimental data. Bearing in mind these limitations and complications, one can conclude by analyzing Table 9 that fairly good agreement is reached between the results of different probe methods. It can be added that the PALS results reported in Ref. [27] also revealed somewhat higher lifetimes in the PIM-1 sample exposed to methanol than in the standard sample. The nitrogen adsorption isotherm at 77 K analyzed by the Horvath–Kawazoe method using a slit-pore model gave a wide pore width distribution with a maximum at 6.5 Å [5]. If one assumes a spherical free volume element, this corresponds to a radius R of about 3.3 Å at 77 K. The PALS method gave a higher radius R of 4.2 Å at low temperature, which is, however, within the size distribution obtained by nitrogen adsorption. Molecular dynamics simulation [34] also gave a broad size distribution of free volume (the radii in the range 2–10 or 1–6 Å depending on the model used), and the results of the probe methods (Table 9) are within this distribution. Therefore, several independent methods for the determination of free volume in PIM-1 indicated that this polymer includes fairly large free volume elements. Since the transport parameters of PIM-1 and PTMSP were compared in detail in this work, it was worthwhile to compare also free volume in these membrane materials. It is made in Table 9. The data presented show that PTMSP is characterized by somewhat larger sizes of free volume elements according to PALS, low temperature N2 adsorption and MD simulation in agreement with higher permeability and diffusivity of this polymer. Another interesting feature characteristic for both polymers is virtual independence of positron annihilation lifetimes (free volume radii) on temperature [18,26]. This observation can be related to the idea of the presence of a more or less continuous “hole phase” in both polymers additional to the free volume organized in isolated holes [34]. Apparently, due to opened porosity and unusual large size of free volume elements in these polymers the effect of temperature on small scale mobility of atomic groups in “the walls” of free volume elements is less pronounced than in conventional glassy polymers. 4. Conclusions This study revealed a strong sensitivity of the gas permeation parameters of PIM-1 to film forming protocol, stronger than that of the majority of membrane materials investigated. Contact with water during film preparation results in a decrease in permeability, whereas post-treatment with methanol leads to a large increase in permeability (P(O2 ) values reach 1600 Barrer). These changes in gas permeability correlate with free volume sizes in the samples as determined by the PALS method. In the Robeson diagrams for various gas pairs, the data points are located above the upper bound, thus making PIM-1 an attractive material for gas separation membranes. It was found that PIM-1 behaves in many aspects as a typical high free volume polymer, exhibiting, in particular, in

low activation energies of permeation. An unusual feature of PIM1 is that it has the highest solubility coefficients of all polymers studied. This observation is consistent with a very large inner surface area, as was noted for this polymer earlier. Different methods for evaluation of free volume in this polymer (PALS, IGC, nitrogen low temperature adsorption, molecular dynamics simulation) indicated rather large size (or size distribution) of free volume elements. The results of different methods are in fairly good agreement.

Acknowledgement This work was supported by INTAS program (project #051000008-7862).

Nomenclature List of symbols B11 the second virial coefficient (m3 /mol) D diffusion coefficient (cm2 /s) EP activation energy of permeation (kJ/mol) Fp 0 ,T col flux of gas carrier (cm3 /s) Hc enthalpy of condensation (kJ/mol) Hm partial enthalpy of mixing (kJ/mol) Hs enthalpy of sorption (kJ/mol) I intensity in positron annihilation lifetime spectroscopy (%) J23 , J34 corrections of the pressure drop in the column P permeability coefficient (Barrer) R universal gas constant (J/mol K) R3 , R4 radii of free volume elements (Å) S solubility coefficient (cm3 (STP)/cm3 atm) partial entropy of mixing (J/mol K) Sm ta retention time of “non-sorbed” component (s) tr retention time of sorbed component (s) critical temperaure (K) Tc Tcol temperature of the experiment (K) Vc critical volume of solute (cm3 /mol) Vf size of microcavity (Å3 ) specific retention volume (cm3 /g) Vg VN net retention volume (cm3 ) V1 molar volume of solute (cm3 /mol) wL mass of the polymer in the column (g) Greek symbols ˛ permselecitvity  polymer density (g/cm3 )  positron annihilation lifetimes (ns)

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