Stabilization of gas transport properties of PTMSP with porous aromatic framework: Effect of annealing

Journal of Membrane Science 517 (2016) 80–90

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Stabilization of gas transport properties of PTMSP with porous aromatic framework: Effect of annealing A.V. Volkov a,n, D.S. Bakhtin a,b, L.A. Kulikov c, M.V. Terenina c, G.S. Golubev a, G.N. Bondarenko a, S.A. Legkov a, G.A. Shandryuk a, V.V. Volkov a,b, V.S. Khotimskiy a, A.A. Belogorlov a,b, A.L. Maksimov a,c, E.A. Karakhanov c a

A.V. Topchiev Institute of Petrochemical Synthesis RAS, Moscow, Russian Federation National Research Nuclear University MEPhI, Moscow, Russian Federation c Moscow State University, Moscow, Russian Federation b

art ic l e i nf o

a b s t r a c t

Article history: Received 12 May 2016 Received in revised form 18 June 2016 Accepted 23 June 2016 Available online 23 June 2016

This work was focused on the study of physical aging of PTSMP and preventing this effect via incorporation of porous aromatic frameworks (PAF-11). The dense PTMSP membranes with PAF-11 content of 0, 1, 5 and 10 wt% were stepwise annealed in air at 100 °C for up to 510 h with constant monitoring of O2, N2 and CO2 permeability (measured at 30 °C). As-cast and aged PTMSP samples were characterized by means of helium pycnometry, dynamic mechanical analysis and IR spectroscopy. Gas transport characteristics of the PTMSP sample containing 10 wt% of PAF-11 became stable upon annealing at 100°С within the short time interval (100–200 h). However, for all other samples containing 5 wt% of PAF-11 or less, gas permeability gradually decreased with time. According to the IR analysis, upon high-temperature treatment, PAF-11 nanoparticles tended to migrate from the sub-layer region into the membrane matrix bulk. According to the helium pycnometry, the introduction of PAF-11 to the PTMSP matrix provided a loosened packaging of polymer chains, and the structure became more stabilized since the porous filler served as a “physical” cross-linker. & 2016 Elsevier B.V. All rights reserved.

Keywords: PTMSP PAF-11 Aging Annealing Gas separation

1. Introduction Hydrophobic high free volume glassy polymers (or polymers of intrinsic microporosity) such as disubstituted polyacetylenes, polybenzodioxane PIM-1, and polynorbornenes are considered as promising membrane and sorbent materials for different gas and liquid-based applications [1–11]. The excess of free volume composed of interconnected free volume elements is naturally formed during the membrane casting due to rigid nature of polymer chains providing superior membrane permeability and solubility selectivity as compared with the performance of conventional glassy or rubbery polymers [1–3,9]. Drawbacks of highly permeable glassy polymers are related to the tendency for relaxation of their non-equilibrium free volume and concomitant deterioration in membrane permeability with time [6,12–20]. Noteworthy is that physical aging phenomenon is known to be more pronounced when membranes are used in their dry rather than the swollen state. For example, in the case of the 1 mm thick film of poly[1(trimethylsilyl)-1-propyne] (PTMSP), nitrogen or helium n

Corresponding author. E-mail address: [email protected] (A.V. Volkov).

http://dx.doi.org/10.1016/j.memsci.2016.06.033 0376-7388/& 2016 Elsevier B.V. All rights reserved.

permeability decreases by a factor of 4 after their testing for 200 h [21], whereas no noticeable reduction in ethanol permeability during 230 h of operation was observed for the thin-film composite membrane with the 0.9 mm thick PTMSP layer [22]. The effect of physical aging on gas permeability for different membrane materials is being actively studied in the literature, both from a theoretical and an empirical point of view. One of the most efficient methods to recover the gas transport characteristics of aged PTMSP is concerned with the soaking of membranes in “poor” solvents (for example, methanol or ethanol) when the fractional free volume is restored upon polymer swelling and desorption of solvent molecules [16,19,20,23]. However, the recent breakthrough research in this area [24–27] involves the use of a new type of fillers, porous aromatic frameworks (PAF) [28]. The introduction of PAF into highly permeable glassy polymers improves their gas permeability characteristics and markedly suppresses the physical aging due to reduced macrochains mobility as a result of the partial intrusion of polymer segments into porous fillers. In particular, it was demonstrated that dense films based on PTMSP, PMP or PIM-1 contained 10 wt% of PAF revealed only 5–7% of the reduction in CO2 permeability over about 240 days, while non-filled PTMSP, PMP, and PIM-1 exhibited a more pronounced decrease in the gas transport for 38–62% [24]. Another advantage

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of PAF fillers is concerned with their porous structure that boosts gas permeability of the mixed matrix membranes. For example, after one year of storage PTMSP with 10 wt% of PAF-1-Li6C60 demonstrated CO2 permeability of 50,600 Barrer, whereas unfilled PTMSP films showed the drop in permeability from 29,800 down to 13,600 Barrer during the same period [25]. The effect of PAF-1 fillers on the physical aging of membrane materials was studied by measuring gas permeability coefficients for 100–150 mm thick, dense films upon their storage at ambient conditions for 240–365 days [24–27]. In this work, stabilization of gas transport properties of PTMSP was studied by using 30–40 mm dense films loaded with PAF-11 fillers. Taking into account good chemical stability of different highly permeable glassy polymers confirmed by the FTIR analysis [14,29,30], the duration of aging experiments was substantially shortened by annealing of PTMSP membranes at 100 °C.

2. Experimental part 2.1. Synthesis of PAF-11 All initial materials were purchased from commercial suppliers and used as received. Tetrakis (4-bromophenyl)methane was synthesized according to the technique described elsewhere [31]. In the round bottomed flask equipped with the stirrer and placed in the silicone bath, tetrakis(4-bromophenyl)methane (1060 mg, 1.6677 mmol, 1 eq.) and 4,4′-biphenyldiboronic acid (807 mg, 3.34 mmol, 2 eq.) were added to the mixture of 50 mL N,N′-dimethylformamide and 7 mL aqueous K2CO3 (2 mol/L). After 3 freeze-pump-thaw degassing cycles Pd(OAc)2 (38 mg, 0.167 mmol, 0.1 eq.) and PPh3 (240 mg, 0.915 mmol, 0.55 eq.) were quickly added to the solution and repeatedly degassed by three freeze-thaw-pump cycles. Then, the reaction mixture was heated at 140 °C and stirred for 24 h under argon atmosphere. The synthesis of PAF-11 with tetrahedral geometry is presented in Fig. 1. The mixture was cooled down to room temperature, the residue was filtered, washed with THF (4
50 mL), CHCl3 (4
50 mL), EtOH (4
50 mL), water (4
50 mL), and dried in vacuum that allowed to obtain PAF-11 as an off-white powder (720 mg, yield 94%). 2.2. Synthesis of PTMSP PTMSP was synthesized in the toluene solution using catalytic system: TaCl5 with cocatalyst triisobutylaluminum (TIBA) [32]. Polymerization reactions were carried out under the following conditions: [Monomer]/[Catalyst] ¼50, [Cocatalyst]/[Catalyst]¼ 0.3, [Monomer]0 ¼0.75 mol/l, T ¼25 °C (Mw ¼900,000, Mw/ Mn ¼1.9, [η]25 toluene ¼6.0 dl/g). 2.3. Preparation of PTMSP/PAF-11 membranes The PTMSP/PAF-11 casting solutions with different filler content (0, 1, 5 and 10 wt%) were prepared by mixing 0.5 wt% PTMSP

Br

Br

and PAF solutions in chloroform. Before mixing with polymer solutions, the PAF-11 solution was placed in the ultrasonic bath for 15 min. Before membrane casting, the PTMSP/PAF solutions were stirred using the magnetic bar for 15 min and placed in the ultrasonic bath for another 15 min. The PTMSP/PAF-11 solutions were cast onto a cellophane support and blanketed by a glass plate for slow evaporation of solvents at ambient conditions (about 200 h) until the constant weight of the samples was attained. The resulting thickness of the PTMSP/PAF-11 membranes was varied in the range of 30–40 mm. 2.4. PAF-11 characterization PAF fillers were characterized by the Fourier transform infrared spectroscopy (FTIR), magic-angle spinning nuclear magnetic resonance (MAS NMR), nitrogen adsorption measurements, and transmission electron microscopy (TEM). The FTIR spectra were taken with a Nicolet IR2000 instrument (Thermo Scientific) using multiple distortions of the total internal reflection method with Multi-reflection HATR accessories equipped with a ZnSe crystal 458 at different wavelengths with a resolution of 4 cm  1 in the range from 4000 to 400 cm  1. All FTIR spectra were taken by averaging 100 scans. The solid-state NMR spectra were collected using the Varian NMR Systems with a resonance frequency of 500 MHz for 1H MAS NMR spectra and 125 MHz for 13C cross polarization (CP) MAS NMR spectra in an impulse mode at a spinning speed of 10 kHz. The measurements of specific surface area and porosity were performed using nitrogen adsorption-desorption isotherms at 77 K on a Micromeritics Gemini VII 2390 V 1.02t apparatus. Before the analysis, the 300-mg-sample was dried at 100 °C under high vacuum for 8 h. The surface area was calculated using the BET model; average pore size and pore volume were calculated according to the BJH method. The samples were examined using the method of transmission electron microscopy (TEM) on LEO912 AB OMEGA microscope with an electron tube voltage of 100 kV. 2.5. Membrane characterization The ATR-FTIR spectra of virgin and PAF-loaded PTMSP films were collected on an IR HYPERION-200 microscope coupled with FTIR IFS-66 v/s vacuum Bruker spectrometer in the range of 600– 4000 cm  1 (30 scans, the resolution was 2 cm  1). The IR spectra at high temperatures were in situ recorded in the transmission mode using a high-temperature Bruker adapter. Mechanical properties of dense membranes were measured by the dynamic mechanical analysis (DMA/SDTA861e Mettler Toledo) under the following experimental conditions: polymer strip (length 10.5 mm, width 2.5 mm, thickness 60–70 mm), stretch mode at 1 Hz, load amplitude was 0.1 H, strain amplitude was 10 mm, the temperature range 20–200 °C, heating rate 2 °C/min, argon atmosphere. The helium pycnometer Micro-Ultrapyc 1200e (Quantachrome Instruments, USA) was used to obtain the skeletal density of samples.

B(OH) 2

Br

+ Br

81

Pd(OAc)2, PPh3 DMF/H2O, K2CO3 140 î Ñ, 24h

2 B(OH) 2

Fig. 1. Synthesis of the polyaromatic framework (PAF-11).

2

PAF-11

82

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Fig. 2. FTIR spectra of 4,4′-biphenyldiboronic acid, tetrakis(4-bromophenyl)methane and PAF-11.

Fig. 3. Solid-state

13

C CP/MAS NMR spectra of PAF-11.

2.6. Gas transport parameters Single gas permeation measurements were carried out at a temperature of 30.070.1 °C and a feed pressure of 0.05–0.8 bar using a constant volume/variable pressure experimental setup (GKSS Time-Lag Machine) [33]. Then, the permeability coefficient obtained from the measurements was plotted against the feed pressure, and true value of the permeability coefficient was determined from the region where it had constant values at different feed pressure. The measurements were performed for the as-cast PTMSP/PAF membranes and the same membranes after their annealing in air at 100 °C for 50, 100, 200, and 510 h. Permeability coefficient P expressed in Barrer was estimated by the linear extrapolation of experimental data to zero trans-membrane pressure. Diffusion coefficient D was

estimated using the time-lag method as D¼l2/6θ, where l is the membrane thickness, θ is the experimental time lag before the attainment of the steady state permeation regime. Solubility coefficient S was evaluated in terms of the solution-diffusion permeation model as S¼P/D. Ideal selectivity of the pair of gases was calculated as the ratio of permeabilities of individual gases.

3. Results and discussion 3.1. Synthesis and characterization of PAF-11 In this study, porous aromatic framework containing 4 benzene rings at edges was synthesized from 4,4′-biphenyldiboronic acid

A.V. Volkov et al. / Journal of Membrane Science 517 (2016) 80–90

Fig. 4. The TEM images of PAF-11.

Fig. 5. N2 adsorption isotherms of PAF-11 at 77 K.

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and tetrakis(4-bromophenyl)methane. The completion of the cross-coupling reaction was controlled by the FTIR measurements. As follows from Fig. 2, the final product (PAF-11) shows the strong δoop C–H band at 808 cm  1 and less intensive ν C ¼Cp-Ar and δip Csp3-CAr bands at 1485 and 1005 cm  1. At the same time, the spectra of PAF-11 contained no B-OH bands at 3370 cm  1 and C-Br bands at 1076, 532 and 512 cm  1 attributed to tetrakis(4-bromophenyl)methane and 4,4′-biphenyldiboronic acid that proved an almost complete quantitative conversion of monomers to the final product of the coupling reaction. The structure of PAF-11 was also studied using the solid-state 13 C CP/MAS NMR spectroscopy, and the corresponding spectrum is presented in Fig. 3. The most intensive signal at 129 ppm corresponded to unsubstituted carbon atoms in benzene rings which were spaced from the central sp3-C atom. The signal at 66 ppm can be attributed to the quaternary carbon atom. Relative intensities of signals were the function of the number of similar C atoms and their screening; hence, the signal at 141 ppm may be attributed to the substituted phenyl carbon atoms linked to each other, while the signals at 132 and 147 ppm – to unsubstituted and substituted carbon atoms in the nearest position to the central sp3-C atom of the benzene ring. These findings correlate with are with the work of Yuan et al. [34], who observed the similar spectrum for PAF-11. The completion of the reaction can be also verified by the absence of the signals typical for halogen or boronic acid in 13C CP/MAS NMR spectra of the final product.

Fig. 6. Dynamic mechanical analysis: Young's modulus as a function of temperature.

The TEM images showed that the polyaromatic frameworks synthesized in this work consist of 150–250-nm-sized spheres and their agglomerates (see Fig. 4). The porous structure of PAF-11 was also characterized by N2 adsorption at 77 K. As shown in Fig. 5, adsorption and desorption isotherms showed pronounced

Fig. 8. PTMSP film stored for 17 years at ambient conditions.

Fig. 9. The skeletal density of PTMSP-based films estimated by helium pycnometer plotted against PAF-11 content (the trend line is plotted only for the as-cast PTMSP/ PAF films).

Fig. 7. PTMSP, PTMSP/PAF-11 (1%), PTMSP/PAF-11 (5%) and PTMSP/PAF-11 (10%) films after 510 h of annealing at 100 °C.

A.V. Volkov et al. / Journal of Membrane Science 517 (2016) 80–90

As-cast PTMSP (a)

85

As-cast PTMSP/PAF (b)

PAF

PAF PAF

Aged PTMSP (c)

Aged PTMSP/PAF (d)

PAF

PAF PAF

Fig. 10. Schematic visualization of possible macrochains packaging in virgin and loaded PTMSP material before and after thermal annealing.

Fig. 11. ATR-FTIR spectra: 1 – as-cast PTMSP, 2 – annealed PTMSP, 3 – as-cast PTMSP/PAF-11 (10%), 4 – annealed PTMSP/PAF-11 (10%).

0,8

1557

1 2 3

Absorbance

0,6

1365

0,4

1433

1483

0,2

0,0 1600

1550

1500

1450

1400

1350

1300

Fig. 12. Transmission IR spectra of PTMSP/PAF-11 (10%): 1 – as-cast at 25 °C, 2 – at 170 °C, 3 – after cooling at 25 °C.

Fig. 13. ATR-FTIR spectra in the region of 750 cm  1: 1 – as-cast PTMSP, 2 – annealed PTMSP, 3 – as-cast PTMSP/PAF (10%), 4 – annealed PTMSP/PAF (10%).

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Fig. 14. ATR FTIR spectra of the films at 1250 cm  1: (a) 1 – as-cast PTMSP, 2 – annealed PTMSP, 3 – as-cast PTMSP/PAF-11 (10%), 4 – annealed PTMSP/PAF-11 (10%), (b) 1 – PTMSP, 2 – PTMSP after swelling in ethanol for 30 min, 3 – PTMSP after swelling in ethanol for 60 min.

hysteresis effect as reported for PAF-11 in [34]. Using the BET equation, the following parameters of PAF-11 were estimated: BET apparent surface area – 240 m2/g, pore volume – 0.35 cm3/g, the average pore size was estimated at the level of 5.8 nm by the BET method and 7.5 nm by the BJH method. Lower surface area in contrast to the PAF-11 reported in [34] (704 m2/g) indicated that PAF-11 possessed more pronounced mesoporous structure. 3.2. Physicochemical characteristics and gas transport properties of PTMSP/PAF-11 3.2.1. Macroscopic and mechanical properties The primary visual inspection of dense membranes revealed that incorporation of PAF-11 fillers resulted in a partial loss of transparency of the films and slight changing of their color. The study of mechanical properties of as-cast PTMSP-based samples was performed by dynamic mechanical analysis (DMA) in the stretch mode within the temperature range of 20–200 °C (heating rate is 2 °C/min). As shown in Fig. 6, there is a certain improvement of Young's modulus of mixed matrix materials with the addition of PAF-11. However, the difference in mechanical properties of dense films with varied PAF-11 loading became less pronounced at a higher temperature. PTMSP membranes with different PAF-11 loading were annealed on the air at 100 °C to facilitate structural rearrangements of macrochains and, thus, to accelerate physical aging as used before [15–17,35]. The samples were cooled down to ambient

Fig. 15. N2, O2 and CO2 permeability coefficients for PTMSP and PTMSP/PAF-11 membranes containing 1%, 5% and 10% of PAF plotted against annealing time at 100 °C. Dashed lines show the level of gas permeability coefficient after annealing for 200 (PTMSP) and 510 h (PTMSP/PAF-11).

temperature after 50, 100, 200 and 510 h of heat treatment for measurements of gas permeance. Fig. 7 presents the general view of corresponded PTMSP samples with different PAF-11 loading after 510 h of annealing. There was interesting observation that

A.V. Volkov et al. / Journal of Membrane Science 517 (2016) 80–90

unfilled PTMSP could not withstand long-term exposure to the high temperature at air and was broken into big fragments. At the same time, virgin PTMSP possessed long-term mechanical stability during the storage at ambient conditions (see Fig. 8). To that end, it can be concluded that addition of porous fillers provided better mechanical resistance of polymeric membranes. The change in the polymeric chains packaging was evaluated by helium pycnometry that provides the skeletal density of the material excluding the volume of microvoids accessible for helium molecules. Fig. 9 represents the skeletal density of fresh and annealed (510 h) PTMSP samples. The skeletal density of as-cast PTMSP (0.992 g/cm3) was in good agreement with the literature data ranged within 0.990–0.997 [18,24]. As expected, the aging of unfilled PTMSP by annealing (510 h at 100°) was accompanied by the increase of the density up to 1.11 g/cm3. It should be noticed that no significant change of the experimental densities measured by helium pycnometry was reported for dense PTMSP films (100 mm) aged by short-term annealing at 100 °C [18]. In this study, PTMSP films of 30–40 mm were experienced cyclic steps of heating, cooling followed by gas permeation measurements, and this difference in pre-history of PTMSP samples might result in the obtained results. The opposite situation was found for loaded PTMSP, when freshly casted samples demonstrated linear increase of the skeletal density with increasing of PAF-11 loading, while the thermal annealing resulted in the drop of the corresponding density down to nearly the same level of 1.01–1.03 g/cm3 regardless of PAF-11 concentration (see Fig. 9). It cannot be explained only by additive effect of the skeletal density of as-cast PTMSP (0.992 g/cm3), annealed PTMSP (1.11 g/cm3) and PAF-11 (1.631 g/cm3). The results of skeletal density can be interpreted as follows: incorporated hydrophobic porous fillers have certain compatibility with the PTMSP material which assists sorption and/or intrusion of polymer chain fragments into PAF-11 interior. Hence, this disturb packing of macromolecules, thus providing a loosened structure of the polymeric matrix and an enhanced access of helium molecules into the interchain space. Hence, increased skeletal density of the as-cast PTMSP/PAF-11 membranes can be explained by the formation of loosened regions of PTMSP, whereas reduced skeletal density upon thermal treatment can attest the appearance of more closely packed regions of the polymer with their limited access for helium molecules. Incorporated hydrophobic porous fillers can be considered as a “physical cross-linker” due to sorption and/or intrusion of polymer chain fragments into aromatic porous framework interior as previously reported for PTMSP/PAF-1 system [24– 27]. Hence, this disturbs packing of macromolecules providing a loosened structure of the polymeric matrix (see schematic representation on Fig. 10a and b) and an enhanced access of helium molecules into the interchain space resulted in higher density values for loaded samples. Thermal annealing accelerates the physical aging and the glassy polymer shows the tendency to the densification of macrochains packing (see Fig. 10a and c), and it is typically accompanied by reduction of smaller and bigger size pores in PTMSP [16,19,24]. In loaded membranes, rearrangements of macrochains in the loosened PTMSP regions physically cross-linked by PAF-11 particles can lead to the development of denser fragments and regions with increased interchain distances. This speculation can be supported by the PALS data reported for PTMSP/PAF-1 membranes [24]: aging of loaded PTMSP was attributed by the increase of the average size of bigger free volume elements (from 15.9 to 16.5 Å). To this end, increased skeletal density of the as-cast PTMSP/PAF-11 membranes can be explained by the formation of loosened regions of PTMSP, whereas reduced skeletal density upon thermal treatment can attest the appearance of more closely packed regions of the polymer with their limited access for helium molecules.

87

3.2.2. IR-spectroscopy Fig. 11 shows typical ATR-FTIR spectra for virgin PTMSP and PTMSP films with the filler content of 10% before and after annealing (510 h at 100 °C). In the ATR-IR spectra collected using the Ge crystal, the maximum depth did not exceed 0.65 mm. However, IR spectra in the transmission mode (the whole membrane was analyzed) did not reveal any change in the intensity of PAF-11 peaks (e.g. see a peak at 1483 cm  1 on Fig. 12) after high-temperature treatment confirming the same concentration of PAF-11 nanoparticles in PTMSP as before. By this means, such observation can be explained by the migration of bulky PAF-11 fillers from the sub-surface area into the bulk of the membrane matrix via rearrangements of polymer chains at high temperatures. Comparing the IR spectra of the initial porous filler and the PAF-loaded membranes, no shift in the spectral bands of PAF-11 was observed, and this fact proves the absence of any specific interactions between PAF-11 and PTMSP macrochains. At the same time, in the presence of PAF-11, some spectral bands of PTMSP are seen to be distorted, in particular, two splitted peaks with a wavelength of 751 or 1243 cm  1 became wider, and their intensity ratio was changed. As was shown in [36], these two peaks corresponding to the deformation vibrations in the ¼ С–Si(CH3)3 group are sensitive to conformational changes in PTMSP and polymer microstructure. Fig. 13 illustrates that upon the addition of 10 wt% of PAF-11, the absorption band at 748 cm  1 with a well-pronounced shoulder at 755 cm  1 in the spectra for virgin PTMSP is transformed into one broad and symmetric peak with a maximum at 750 cm  1. As a result of the heat treatment, the less pronounced shoulder at 748 cm  1 was recovered; however, as compared with unfilled PTMSP, the peak remains broader. Interestingly, PTMSP swollen in ethanol demonstrated the similar changes in IR spectra, which were corresponded to the deformation vibration of H–C–H in methyl groups of –Si(CH3)3 (1240– 1250 cm  1) [23]. Moreover, as compared with the initial polymer, the swollen PTMSP experienced some conformational changes related to the transformation of the vibration of the ¼ С–Si(CH3)3 segments: the bands at 1557 cm  1 (C ¼ C bond) and at 1250 cm  1 (deformation vibrations of CH3-groups at Si-atom) and 750 cm  1 (¼C–Si) [36]. PAF-11 filled PTMSP showed similar changes in the corresponding IR spectra: 750 cm  1 (see Fig. 13), 1250 cm  1 (see Fig. 14) and 1557 cm  1 (see Fig. 11). In the case of the PTMSPethanol system, the intensity of the 1250 cm  1 band decreased in the proportion of the exposure time of PTMSP in alcohol (for PTMSP, the equilibrium degree of swelling in ethanol is 55%). Therefore, taking into account changes in the intensity of the 1250 cm  1 band for PTMSP/PAF-11, one can assume that the ascast membranes experienced maximum conformational changes, Table 1 Changes in relative gas permeability coefficients and ideal selectivity after annealing at 100 °C for 200 (PTMSP) and 510 h (PTMSP/PAF).

PTMSP PTMSP/ PAF (1%) PTMSP/ PAF (5%) PTMSP/ PAF (10%)

Decrease in gas permeability coefficient, %

Selectivity

N2

O2

CO2

Before After Before After Before After

41 57

38 51

35 49

1.48 1.47

1.56 1.68

5.60 5.89

6.14 6.95

3.79 4.01

3.94 4.14

57

51

49

1.47

1.66

5.80

6.77

3.94

4.08

38

33

30

1.47

1.58

5.64

6.30

3.84

3.99

O2/N2

CO2/N2

CO2/O2

88

A.V. Volkov et al. / Journal of Membrane Science 517 (2016) 80–90

Fig. 16. Diffusion coefficients D and solubility coefficients S¼ P/D of N2, O2 and CO2 in PTMSP/PAF-11 membranes containing 0, 1%, 5% and 10% of PAF-11 plotted against the time of annealing at 100 °C.

and, upon annealing, partial relaxation took place. 3.2.3. Effect of PAF-11 filler on gas transport properties of the PAFloaded PTMSP membranes Fig. 15 presents gas permeability coefficients plotted against the duration of annealing (at 100 °C). As can be seen, due to the incorporation of the filler, gas transport characteristics of the PAFloaded samples tended to increase as compared with those of the virgin PTMSP. For PTMSP/PAF-11 (10%), with increasing the duration of annealing up to 200 h, the gas permeability coefficient

approached its stable value, and further thermal treatment above 200 h had no effect on the transport characteristics. When the content of PAF-11 in PTMSP was below 10%, gas permeability coefficients gradually decreased, and annealing at 100 °C even for 510 h appeared to be insufficient to provide the stable permeability behavior. Moreover, gas permeability of membranes was improved due to the incorporation of PAF-11 porous filler into PTMSP, for example, initial CO2 permeability coefficients of different PTMSP/PAF-11 membranes increase by 10–28% as compared with the virgin PTMSP sample. This fact correlates with the

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literature when incorporation of PAF-1 filler into PTMSP films provided nearly 25% improvement in CO2 permeability coefficient [24,25]. As it was discussed earlier, the unfilled PTMSP was broken after 510 h of annealing (see Fig. 7). Therefore, in Fig. 15, the data on gas permeability of the PTMSP membrane after annealing for 510 h are missing. Thus, incorporation of PAF-11 filler into the PTMSP matrix provided not only improvement in gas transport characteristics of the resultant membranes but also ensured the high mechanical stability of the PTMSP samples at elevated temperatures. Changes in relative gas permeability coefficients (Pt/P0, where t is the annealing time) and ideal selectivity upon annealing are summarized in Table 1. It seems that low content of PAF-11 (at least 5% or lower) allowed to obtain the mixed matrix membranes with improved gas permeability (see Fig. 15) and with a slight enhancement in ideal gas selectivity. However, upon annealing, gas permeability coefficients for the PTMSP/PAF-11 (1% and 5%) films markedly decreased by 49–57% from the original values. After annealing for 200 h, gas permeability coefficients of the PTMSP samples dropped by 35–41%. On the other hand, the lowest drop in the gas permeability coefficients (30–38%) was found for PTMSP/PAF-11 (10%), and the sample preserved its stable performance and maximum gas permeability coefficient after annealing for 510 h: P(N2)¼3700 Barrer, P(O2) ¼5900 Barrer and P(CO2)¼ 23,500 Barrer. As follows from Fig. 15 and Table 1, PTMSP/PAF-11 (1%) and PTMSP/PAF-11 (5%) samples showed the similar tendency for physical aging and similar performance (in terms of gas transport characteristics and ideal selectivity). Therefore, it can be concluded that there is no need to increase the level of PAF loading above 10 wt% to achieve PTMSP stable performance, and the optimum concentration of PAF filler in the membrane can be found between 5 and 10 wt%. Besides, 200 h of annealing at 100 °C was sufficient to observe the stabilization of gas transport properties of PTMSP/PAF-11 (10%), while it took about 90 days of storage at room temperature to reach the stable performance of PTMSP loaded with 10% of PAF-1 [24]. 3.2.4. Diffusion and sorption Fig. 16 shows the diffusion coefficient D and solubility coefficient SQP/D of N2, O2 and CO2 in PTMSP with different PAF-11 content plotted against the annealing time. As can be seen, the incorporation of porous fillers resulted in improvement of diffusion coefficient, while calculated S values were higher for virgin PTMSP. Mesoporous structure of PAF-11 is characterized by quite high internal surface area and by regular pores, which are attractive for intercalation of fragments of PTMSP chain. Therefore, the maximum gas diffusivity of PTMSP/PAF (10%) can be explained by a higher loading of porous fillers which facilitates gas transport through membranes. Besides, PAF-11 serves as a “physical” crosslinker due to the partial penetration of the PTMSP chain fragments into the pores of PAF fillers. As the content of PAF-11 decreased, characteristics of loaded membranes approached those of virgin PTMSP – higher values of gas diffusivity and lower values of gas solubility (see Fig. 16). Meanwhile, all membranes showed the similar behavior upon prolonged thermal annealing – diffusion coefficients of N2, O2 and CO2 were markedly reduced whereas the S values remained virtually unchanged.

4. Conclusions In this work, physical aging of PTSMP and its prevention via incorporation of porous aromatic frameworks was studied. Gas transport characteristics of dense PTMSP membranes with a thickness of 30–40 mm containing 0, 1%, 5% and 10% wt of PAF-11 were studied upon annealing at 100 °C for 50, 100, 200 and 510 h.

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For the as-cast PTMSP membranes containing 1 and 5 wt% of PAF11, permeability of N2, O2 and CO2 increased by 21–28% as compared with that of unfilled PTMSP; as the PAF-11 content increased up to 10 wt%, gas permeability increased by 9–10%. The incorporation of PAF-11 fillers was shown to improve mechanical stability of the PTSMSP-based mixed matrix materials, whereas the unfilled PTMSP was stable only up to 200 h of annealing at 100°С and it failed to survive thermal treatment in air at 100°С for 510 h and experienced its breakdown into big-sized fragments. Gas transport characteristics of the PTMSP sample containing 10 wt% of PAF-11 became stable upon annealing within the short time interval (100–200 h at 100 °C). However, for all other samples containing 5 wt% of PAF-11 or less, gas permeability gradually decreases with time. According to the IR analysis, upon high-temperature treatment, PAF-11 particles with a size of 150–200 nm tended to migrate from the sub-layer to the bulk region of PTMSP. Moreover, the introduction of PAF-11 to the PTMSP matrix led to conformational changes of PTMSP chains and provided a loosened packaging of polymer chains; as a result, the structure became more stabilized since the porous filler served as a “physical” cross-linker. In the bottom-line, it can be concluded that high-temperature annealing offers a promising route to speed up the discovery of new type of additives that could stabilize gas transport properties of polymers of intrinsic microporosity. The best results on stabilization of gas transport characteristics of PTMSP were obtained for the samples containing 10 wt% of PAF-11.

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