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Highly Gas Permeable Polyamides Based on Substituted Triphenylamine Debaditya Bera,† Venkat Padmanabhan,‡ and Susanta Banerjee*,† †

Materials Science Centre and ‡Chemical Engineering Department, Indian Institute of Technology, Kharagpur 721302, India S Supporting Information *

ABSTRACT: We present synthesis, characterization, and gas transport properties of a series of new aromatic polyamides (PAs) containing tri-tert-butylphenol-substituted triphenylamine moiety (TPA). The PAs showed a good combination of thermal and mechanical properties (Tg up to 284 °C, Td10 up to 400 and 460 °C in air and nitrogen, respectively, and tensile strength up to 68 MPa). Gas transport properties of these membranes were investigated for different gases at 35 °C. The PAs showed high gas permeability with moderate gas selectivity [PCO2 = 119.0, PO2 = 29.0 barrer and PCO2/PCH4 = 37.19, PO2/PN2 = 9.67]. The effect of the substituted TPA unit on the gas transport properties of these membranes has been investigated and analyzed using molecular dynamics simulations. A thorough structure/property relationship for this class of materials has been established that may serve as a guide to future design of PA-based gas separation membranes.

1. INTRODUCTION Membrane-based gas separation is an active area of research due to its high demands in industrial applications, such as preparation of nitrogen or oxygen-enriched air, removal of CO2 for natural gas “sweeting”, and mitigation of carbon dioxide from greenhouse gas-producing sources.1,2 Membrane-based gas separation offers many advantages over the other traditional ones like, pressure swing adsorption, cryogenic distillation due to its mechanical simplicity, cost-effectiveness, high-energy efficiency, and environment-friendly techniques.2 However, large material cost for high output applications, difficulty in attaining high product purity, and limited thermal and chemical stability are some of the major challenges that limit the full development of polymer membrane-based gas separation.3 Moreover, there exists a trade-off between permeability and selectivity for these membranes as indicated by the Robenson upper bound relationship.4 Matrimid, which is being currently used in many commercial gas separation applications, generally demonstrate high selectivity but low permeability, thus limiting its application to small and medium scale gas separation operations.5 Thus, the key paradigm of membrane science regarding materials for gas separation is to search for polymers with high permeability and good selectivity, required for costeffective large-scale separation applications.2,5 Though, there are no strict “design rules” for polymers to obtain optimal gas separation,6 it has been observed that chains with a rigid backbone induce size-based selectivity for small gas molecules like CO2 and O2 at the upper bounds.4 Also, the structural modifications which lead to low polymer chain packing increase the polymer fractional free volume (FFV), resulting in high gas permeability through the membrane. Another important criterion for practical application of these © XXXX American Chemical Society

polymeric materials is that they should be structurally robust and thermally stable. So, a careful selection of these properties that increase the polymer gas interactions and control the FFV and segmental mobility is crucial to obtain effective gas separation. Several classes of glassy polymers such as modified cellulose,7 poly(arylene ether)s,8 polyimides,5,9,10 and polyamides11−16 have been investigated for gas separation applications. Although polyamides have shown some promising gas separation behavior, their insolubility and processing difficulty, due to strong interchain hydrogen bonding that results in high cohesive energy density and dense chain packing, limit their usage in commercial applications.15 Several approaches including introduction of bulky pendent groups, kink structures in the polymer backbone, etc., have been adopted to hinder the polymer chain packing and to improve its solubility.11,12,14,15 Such modifications have shown to increase the polymer backbone rigidity and FFV in these materials that are required for superior gas permeability and selectivity.4 In this context, polymer containing triphenylamine (TPA) can be advantageous due to the rigid three-dimensional propeller-shaped structure of the TPA that has been shown to reduce the polymer chain packing and improve the solubility.17−22 Although polymers containing TPA and its derivatives have been investigated primarily as an organic electrochromic material, they have gained importance recently due to their unique gas separation behavior.18−22 The rigid propeller structure of TPA that imparts stiffness to the polymer Received: May 15, 2015 Revised: June 11, 2015

A

DOI: 10.1021/acs.macromol.5b01044 Macromolecules XXXX, XXX, XXX−XXX

Article

Macromolecules Scheme 1. Synthesis of the Triphenylamine Monomer 6

comparison has been made with the state-of-the-art data using Robeson diagrams. We also performed a detailed analysis of the structural orientation of polyamides and their effect on gas permeability using molecular dynamics simulation. Finally, we established a structure−property relationship of these PAs for further development and broader adoption of polymer containing TPA to address the profound industrial challenges of membrane-based gas separations.

backbone, which in turn increases the interchain separation, helps in achieving both high permeability and selectivity.11 Yen et al. synthesized a series of PIs derived from Me3TPA-based dianhydride monomer and reported that introduction of Me3TPA groups effectively improved the gas permeability with a slight decrease in permselectivity.18 Chang et al. studied the gas permeation properties of polyimides containing different methoxy-substituted TPA units and reported that there was an improvement in gas permeability with little decrease in gas selectivity.19 Li et al. also investigated the gas permeation properties of polyimides containing p-tert-butylsubstituted TPA and found an improvement in the O2 and CO2 permeability with a little decrease in selectivity compared to their unsubstituted analogues.20 In case of trimethyl-substituted TPA-containing polyimides, Yen et al. reported higher permeability and selectivity for CO2/CH4 gas pair.21 Hu et al. investigated the gas transport properties of polyimides based on TPA-containing pendant anthraquinone moiety and observed high CO2 permeability.22 In our previous work, we reported gas transport properties of PAs (8a-8e) containing adamantane substituted TPA11 that displayed high gas selectivity but moderate gas permeability. In addition to TPA, the tert-butyl group has also been considered as a structural moiety for improving polymer solubility and gas permeability in different polymer systems.15,23,24 Espeso et al. synthesized aromatic polyamides containing tert-butyl group and reported improved gas permeability with selectivity comparable to many commercially available polymers.23 Calle et al. observed more than 3 times increase in gas permeability for tert-butyl-substituted polyimides in comparison to its unsubstituted analogue.24 Considering the above facts, in continuation of our quest for the development of new PA membranes with improved gas permeability, in this work, we synthesized a series of PAs containing tri-tert-butylphenol-substituted TPA and studied their gas separation properties. In addition, to understand the efficiency of these PAs in the field of gas separation, a

2. EXPERIMENTAL SECTION 2.1. Materials. 2,4,6-Tri-tert-butylphenol, palladium on activated carbon (1 wt %), triphenyl phosphite (TPP), 4,4′-(hexafluoroisopropylidene)bis(benzoic acid), 5-tert-butylisophthalic acid, isophthalic acid, terephthalic acid, 2,6-naphthalenedicarboxylic acid, and 1-fluoro4-nitrobenzene were purchased from Sigma-Aldrich, USA, and used as received. Cesium fluoride (CsF), also purchased from Sigma-Aldrich, was dried for 6 h at 160 °C before use. N,N-Dimethylacetamide (DMAc) and tetrahydrofuran (THF) were purchased from Merck, India, and used as received. Dimethyl sulfoxide (DMSO) (Merck) was dried using calcium hydride. 1-Methyl-2-pyrrolidone (NMP) (Merck) was purified by stirring with NaOH and distilled from P2O5 prior to use. Pyridine (Merck) was purified by stirring with NaOH and distilled under reduced pressure. CaCl2 and anhydrous K2CO3 (Merck) were dried for 12 h at 140 °C before use. Methanol (Rankem, India) was used for precipitation of polymers. The synthesis of the new TPA monomer (6) is depicted in Scheme 1. 2.2. Equipment. Carbon, hydrogen, and nitrogen contents of the compounds were determined using a Vario EL (Elementar, Germany) elemental analyzer. FTIR spectra of the monomers (using KBr pellets) and polymers (membrane) were recorded from a NEXUS 870 FTIR (Thermo Nicolet) spectrophotometer at room temperature. 1H NMR spectra were obtained using a Bruker 200, 400, and 600 MHz instrument (Switzerland) using DMSO-d6 and pyridine-d5 as solvents. Gel permeation chromatography (GPC) was performed with a Waters GPC instrument (Waters 2414). THF was used as eluent, and polystyrene was used as standard. RI detector was used to record the signal. Differential scanning calorimetric (DSC) analysis were performed on a NETZSCH DSC 200PC differential scanning calorimeter with 7 ± 1 mg samples, at a heating rate of 20 °C min−1 for determining the glass transition temperature (Tg). Tgs were B


Article

Macromolecules Table 1. Physical Properties of the Polyamides polymer PA PA PA PA PA

I II III IV V

Mn (g/mol)

PDIa

densityb

2θ (deg)

d-spacing (Å)

Vw (cm3/mol)

FFVEXP c

FFVSIM d

69 100 78 500 63 000 81 000 79 000

2.1 3.1 2.8 2.4 2.6

1.07 1.15 1.15 1.13 1.16

15.27 14.38 16.16 16.06 16.56

5.80 6.15 5.48 5.51 5.35

435.89 483.77 397.43 397.68 423.64

0.160 0.188 0.105 0.122 0.089

0.228 0.250 0.194 0.202 0.166

Polydispersity index. bDensity (g/cm3) measured at 25 °C. cFFVEXP = the FFV values of the PAs determined experimentally. dFFVSIM = the FFV values of the PAs determined from atomistic molecular dynamics (MD) simulations.

a

The corresponding permeability coefficient (P) and diffusion coefficient (D) values were used to determine the solubility coefficient S of each gas for all the polymers using eq 3:

estimated at the midpoint of change in slope of the baseline on the second DSC heat scan. Thermal decomposition behavior of these polymers was investigated using a NETZSCH TG 209 F1 instrument at a heating rate of 10 °C min−1 under synthetic air (N2:O2 is 80:20) and nitrogen. The densities of the membranes were measured using a Wallace high precision densimeter-X22B (UK) (isooctane displacement) at 30 °C. The density data (ρ) were used to determine the fractional free volume (FFV) of polymer by using the equation

S = P /D

The ideal permselectivity toward a gas “A” relative to another gas “B” was calculated from the individual gas permeability using eq 4: αP(A/B) = PA /PB

FFV = (V − 1.3Vw)/V

(1)

where T0 and p0 are the standard temperature and pressure (T0 = 273.15 K, p0 = 1.013 bar), T is the temperature of the measurement, d is the thickness of the film, and (dp/dt)s was obtained from the slope of the increments of downstream pressure vs time plot. The reproducibility of the measurements was checked from four independent measurements using the same membrane, and it was better than ±5% depending on the nature of gas molecules. In the case of high permeable gases like CO2 and O2 the error limit was within 1− 3%, whereas for low permeable gases like N2 and CH4 it was within 2− 5%. The gas permeability values were taken as an average of four independent experiments. The effective diffusion coefficient D is calculated from the time lag “θ” according to eq 2:

D = d 2/6θ

(4)

2.3. Molecular Dynamics (MD) Simulations. Atomistic molecular dynamics (MD) simulations of PA polymers were performed using LAMMPS molecular dynamics simulations package.26 For each PA system, an amorphous cell, containing 20 chains with 10 repeat units each, was first built using the Avogadro molecular editing package followed by energy minimization using the conjugate gradient method, with maximum distance of line search chosen to be 0.1 A. OPLA-AA force field was used for all types of PA polymers modeled with explicit atoms. The structures were then equilibrated under isobaric−isothermal thermodynamic conditions (NPT ensemble) for 5 ns, with a time step of 1 fs, until the conformational distribution, the box side length, and the potential energy fluctuated around constant values. Subsequently, all the systems were compressed using the method of shrinking boxes to achieve system densities equal to the experimental values, as given in Table 1. The systems were then dynamically equilibrated in a canonical (NVT) ensemble for 10 ns at a temperature of 300 °C, which is above the glass transition temperature of all the PA polymers. The temperature was controlled using a direct velocity rescaling procedure with a temperature window of 10 K. Under these conditions, greater temperature fluctuations are allowed, but the disturbance in the trajectory is insignificant. After equilibration, the temperature was lowered down to 35 °C followed by another equilibration run at these conditions in an NVT ensemble for 10 ns before the final productions runs were performed to collect data. The production runs were carried out for another 1 ns with coordinates data sampled every 1 ps. The free-volume regions within the simulation box were determined by the general method of grid scanning. A grid with a spacing of 0.3 Å was first established, and the location of each grid point was optimized to maximize its distance from all the atoms of the polymer chains. If the distance for all surrounding atoms is greater than their atomic radius, the coordinates of the grid were saved, together with the shortest distance to an atom, according to the algorithm described by Voorintholt et al.27 The saved information about the shortest distance allows determining the accessible positions for probes with different radii, with the shortest distance being the maximum radius of the probe molecule. The total free volume within the simulation box was calculated by setting the probe radius to 0. The fractional free volume (FFV) was then calculated by taking the ratio of the total free volume to the total volume of the simulation box. The fractional free volume data for all systems were obtained as average values calculated over 1000 samples collected from production runs to prevent redundant correlations. The fractional accessible volume (FAC) was estimated by considering various sizes of the (spherical) probe molecule. We also analyzed the free volume distribution (FVD) of the polymer to get a better insight into the distribution of free volume element which affected the gas diffusivity. To estimate the diffusivities of various gas molecules through the different kinds of PA membranes, five molecules of each gas were

where V is the specific volume (V = 1/ρ). The van der Waals volume (Vw) was estimated by the Hyperchem computer program, version 8.0.15,25 The dielectric constants (ε) of polymer films were measured using the parallel plate capacitor method with a HIOKI 3532-50 LCR Hi Tester from 100 kHz to 1 MHz at a temperature of 25 °C. The mechanical properties of PA membranes were measured with TINIUS OLSEN H5KS. Samples with dimension of 10 mm × 25 mm and a thickness of around 0.06−0.07 mm were used for the measurement. Tests were done using a crosshead speed of 5% min−1 of the specimen length. Wide-angle X-ray diffractograms were obtained from Panalytical, X’Pert PRO X-ray diffractometer using a Cu Kα (λ = 0.154 nm) source operated at 40 kV and 30 mA. Permeabilities of CO2, O2, N2, and CH4 were measured through the polymer membranes (thickness around 0.07−0.08 mm) using an automated diffusion permeameter (DP-100-A) manufactured by Porous Materials, Inc., USA, under 3.5 bar of applied gas pressure at 35 °C. Prior to the experiments all the membranes were degassed for 12 h under vacuum using a turbomolecular pump at the operating temperature within the permeation cell. The effective permeation area (A) with in the permeation cell was 5.069 cm2. To the upstream side of the film, 3.5 bar (pi) pressure was applied instantaneously, and in the downstream side a reservoir of constant volume (119 cm3) was connected with a pressure transducer to monitor the total amount of gas passed through the film. Ultrahigh pure XL grade gases, Linde India, were used for all the measurements. The time lag method was used to measure the gas transport property. In this technique the mean permeability coefficient P was determined from the steady state gas pressure increment (dp/dt)s in the calibrated volume V of the product side of the cell. The permeability coefficients were calculated from eq 1 and expressed in barrer.

P = [VdT0/App T ](dp /dt )s i 0

(3)

(2) C


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Macromolecules

Figure 1. 1H NMR spectrum of the diamine (6) in DMSO-d6 (*signals at 3.38 and 2.48 ppm are for H2O and DMSO-d6, respectively). stretching), 3013 (aromatic C−H stretching), 2964, 2866 (aliphatic C−H stretching), 1620, 1424 (aromatic CC ring stretching), 1507 (N−H bending), 1228 (C−N stretching for aromatic−NH2), 1268, 1106 (C−O−C stretching). 1H NMR (DMSO-d6, 200 MHz, δ ppm): 7.30 (s, 2H), 6.75 (bs, 1H), 6.53 (bs, 1H), 6.32 (bs, 1H), 5.55 (bs, 1H), 4.57 (s, 2H), 1.29 (s, 9H), 1.19 (s, 18H). 2.4.3. 4-(2′,4′,6′-Tri-tert-butylphenoxy)-4′,4″-dinitrotriphenylamine (5). To a solution of compound 4 (7 g, 19.79 mmol) and 10.52 g (69.29 mmol) of CsF in 70 mL of dried DMSO, 1-fluoro-4nitrobenzene (2) (6.30 mL, 59.39 mmol) was added slowly under a nitrogen atmosphere. The reaction mixture was heated at 140 °C for 36 h and then slowly poured into ice water. The yellow precipitated compound was then collected by filtration and washed thoroughly with water. Yield: 9.32 g (∼79%). Anal. Calcd for C36H41N3O5 (595.73 g mol−1): C, 72.58%; H, 6.94%; N, 7.05%. Found: C, 71.87%; H, 6.59%; N, 6.87%. FTIR (KBr, cm−1): 3083 (aromatic C−H stretching), 2957, 2863 (aliphatic C−H stretching), 1602, 1498 (aromatic CC ring stretching), 1580, 1338 (NO stretching), 1222, 1106 (C−O−C stretching), 840 (C−N stretching for aromatic− NO2). 1H NMR (DMSO-d6, 400 MHz, δ ppm): 8.17 (d, J = 9.0 Hz, 4H), 7.33−7.19 (m, 5H), 7.11 (d, J = 9.2 Hz, 4H), 5.97 (bs, 1H), 1.30 (s, 9H), 1.24 (s, 18H). 2.4.4. 4-(2′,4′,6′-Tri-tert-butylphenoxy)-4′,4″-diaminotriphenylamine (6). The dinitro compound (5) (9 g, 15.10 mmol), Pd−C (Pd content 1%, 0.46 g), and 200 mL of ethanol were taken in a three-neck round-bottom flask. Hydrazine monohydrate (70 mL) was added slowly to the reaction mixture at 50 °C. The reaction mixture was heated to reflux for another 36 h. After completion of the reaction as monitored by TLC, the mixture was filtered to remove Pd−C. The filtrate was concentrated to remove excess hydrazine and ethanol. The remaining filtrate was then precipitated out in excess water, filtered, and washed several times with distilled water. The product was dried under vacuum at 60 °C. Purification of the product was done by silica gel column chromatography using ethyl acetate/hexane (2:8) as eluent to obtain the reddish yellow compound (6). Yield: 5.5 g (68%). Anal. Calcd for C36H45N3O (535.76 g mol−1): C, 80.7%; H, 8.47%; N, 7.84%. Found: C, 80.34%; H, 7.97%; N, 7.67%. FTIR (KBr, cm−1): 3411, 3337 (N−H stretching), 3007 (aromatic C−H stretching), 2955, 2863 (aliphatic C−H stretching), 1605, 1424 (aromatic CC ring stretching), 1502 (N−H bending), 1228 (C−N stretching for aromatic−NH2), 1259, 1103 (C−O−C stretching). 1H NMR (DMSO-d6, 600 MHz, δ ppm): 7.31 (s, 1H), 6.85 (bs, 1H), 6.72 (s, 1H), 6.66 (d, J = 6.0 Hz, 4H), 6.47−6.43 (m, 5H), 5.63 (bs, 1H), 4.83 (s, 4H), 1.28 (s, 9H), 1.21 (s, 18H) (Figure 1).

randomly inserted in the simulation box such that they do not overlap with the existing atoms of the polymer chains. All gas molecules were modeled with explicit atoms. Simulations, in NVT ensemble maintained at a temperature of 35 °C, were then performed for 100 ns to monitor the mobility of gas molecules within these systems. The mean-squared displacement (MSD) of the center of mass was plotted for each gas molecule in all the PA membranes as a function of time. The diffusion coefficient of all the gases in each type of PA membrane was then calculated from the MSD using Einstein’s relation ⟨MSD⟩ = 6Dt The angle brackets indicate ensemble average. The diffusion coefficients were obtained as the average of all the five molecules. 2.4. Monomer Synthesis (Scheme 1). 2.4.1. 4-(2′,4′,6′-Tri-tertbutylphenoxy)nitrobenzene (3). In a three-neck round-bottom flask 2,4,6-tri-tert-butylphenol (1) (10 g, 38.10 mmol), dry potassium carbonate (7.90 g, 57.15 mmol), and 57 mL of dry DMSO were taken under a nitrogen atmosphere. 1-Fluoro-4-nitrobenzene (2) (4.45 mL, 41.91 mmol) was added slowly to the reaction mixture with constant stirring. The reaction was continued for 24 h at 145 °C. After completion of the reaction (as was checked by TLC), the pale yellow compound was precipitated out in water and recrystallized using ethyl alcohol. Yield: 11.40 g (∼78%). Anal. Calcd for C24H33NO3 (383.52 g mol−1): C, 75.16%; H, 8.67%; N, 3.65%. Found: C, 75.08%; H, 8.35%; N, 3.42%. FTIR (KBr, cm−1): 3081 (aromatic C−H stretching), 2970, 2869 (aliphatic C−H stretching), 1608, 1427 (aromatic CC ring stretching), 1583, 1341 (NO stretching), 1240, 1100 (C−O−C stretching), 846 (C−N stretching for aromatic−NO2). 1H NMR (CDCl3, 200 MHz, δ ppm): 8.26 (dd, J = 9.0 Hz, 1H), 8.03 (dd, J = 9.2 Hz, 1H), 7.39 (s, 2H), 7.18 (dd, J = 9.9 Hz, 1H), 6.10 (dd, J = 9.2 Hz, 2H), 1.36 (s, 9H), 1.23 (s, 18H). 2.4.2. 4-(2′,4′,6′-Tri-tert-butylphenoxy)phenylamine (4). A threeneck round-bottom flask was charged with the compound 3 (10 g, 26.07 mmol), Pd−C (Pd content 1%, 0.4 g), and ethanol (150 mL). Hydrazine monohydrate (40 mL) was added dropwise to the reaction mixture at 50 °C. The reaction mixture was refluxed for another 24 h. After completion of the reaction as monitored by TLC, Pd−C was filtered off from the reaction mixture. Excess hydrazine and ethanol were removed and the remaining mixture was precipitated out in excess water. The product was dried under vacuum at 60 °C. Purification of the product was done by silica gel column chromatography using ethyl acetate/hexane (1:9) as eluent to obtain the light pink compound (4). Yield: 7.92 g (∼86%). Anal. Calcd for C24H35NO (353.54 g mol−1): C, 81.53%; H, 9.98%; N, 3.96%. Found: C, 81.32%; H, 9.35%; N, 3.45%. FTIR (KBr, cm−1): 3438, 3362 (N−H D


Article

Macromolecules Scheme 2. Synthesis of the Polyamides (PA I−V)

Table 2. Thermal, Mechanical, and Dielectric Constant Values of the Polyamides Tdb (°C) polymer PA PA PA PA PA a b

I II III IV V

Tga (°C)

air

N2

TSc (MPa)

modulusd (GPa)

elongation at break (%)

dielectric constant (1 MHz)

263 271 258 281 284

415 404 403 411 411

445 460 447 453 451

45 49 54 64 68

1.52 1.71 1.81 1.80 1.80

4 7 5 11 9

2.5 2.3 2.8 2.6 3.0

From DSC measurements, heating rate at 20 °C min−1 (Tg was taken as the midpoint of the change in slope in the second heating DSC curves). 10% weight loss temperature in air and N2 heating rate at 10 °C min−1. cTensile strength. dYoung’s modulus.

2.5. Polymerization and Membrane Preparation. Polymerization was carried out by reacting the synthesized diamine, 4-(2′,4′,6′tri-tert-butylphenoxy)-4′,4″-diaminotriphenylamine (6), with five structurally different aromatic diacids in NMP in the presence of TPP and pyridine (Scheme 2). A representative procedure for the polyamide preparation (PA I) is given below. The diamine (6) (0.707 g, 1.32 mmol), 5-tert-butylisophthalic acid (0.293 g, 1.32 mmol), calcium chloride (0.36 g), NMP (6 mL), pyridine (1.37 g, 17.34 mmol), and TPP (1.66 g, 5.35 mmol) were taken in a 100 mL round-bottom flask equipped with a stirrer. The reaction was carried out under a blanket of nitrogen. The mixture was heated to 100 °C while constantly stirring for 6 h. The highly viscous polymer solution was then cooled and slowly poured into a large amount of methanol (300 mL) with constant stirring. The fibrous precipitates of the polymer were filtrated and washed thoroughly with methanol followed by distilled water for removal of adsorbed solvent and CaCl2. The polymer was dried overnight in an oven maintained at 65 °C under vacuum. The other PAs were also prepared by the aforementioned procedure. Detailed structural characterizations of these polymers are described in the Supporting Information. The PA membranes were prepared via casting from their respective homogeneous DMAc solutions [≈10% (w/v)] onto the clean glass Petri dishes. The Petri dishes were placed in an oven at 80 °C for overnight, followed by slow heating up to 150 °C and allowed it to

heat for another 6 h. Finally, the membranes were kept under vacuum at 160 °C for 48 h to remove any residual solvent. Then, the polymer membranes were removed by immersing the Petri dishes in boiling water. Finally, the membranes were dried again under vacuum for 4 h at 160 °C for the removal of any absorbed moisture. Finally, membranes with a thickness of 0.07−0.08 mm were obtained and used for characterization and testing in the following studies.

3. RESULT AND DISCUSSION 3.1. Monomer Synthesis. As depicted in Scheme 1, the diamine monomer, 4-(2′,4′,6′-tri-tert-butylphenoxy)-4′,4″diaminotriphenylamine (6) was prepared starting from 2,4,6tri-tert-butylphenol (1) and 1-fluoro-4-nitrobenzene (2). The mononitro compound (3), which was synthesized by the first aromatic nucleophilic substitution (SNAr) reaction between compound 2 and potassium phenolate of 1, was reduced to prepare the aniline derivative (4). The second SNAr reaction between compounds 4 and 2 produced the intermediate dinitro compound (5), which was finally reduced to obtain the diamine monomer (6). The crude diamine (6) was purified by column chromatography and used directly for the synthesis of PAs. The most relevant change that occurred during the conversion of E


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Macromolecules

Figure 2. Representative 1H NMR spectrum of PA I in pyridine-d5 (*signals at 8.74, 7.59, and 7.22 ppm are for pyridine-d5 and 5.1 ppm for H2O).

transparent PA membranes were obtained from their respective DMAc solution. All PAs showed comparatively same density values except PA I (Table 1). The lowest density of PA I is due to the presence of the extra tert-butyl group (acid component) that further disrupts the chain packing.12 The highest FFV exhibited by PA II, albeit its relatively high density, is attributed to the presence of hexafluoroisopropylidene [⟩C(CF3)2] group in the polymer backbone.15 All the PAs showed one broad amorphous halo in their X-ray diffractograms (Figure 3) around 2θ ≈ 15° and an

dinitro compound (5) to diamine monomer (6) was the appearance of strong absorption bands of −NH2 at 3411, 3337 cm−1 (N−H stretching). The formation of the diamine monomer (6) was also further confirmed by 1H NMR of this compound (Figure 1). In Figure 1. the signals in the 1H NMR at δ = 5.63 ppm assigned to the amine proton and the two signals in the range of δ = 1.28−1.21 ppm assigned to methyl protons suggested the formation of compound 6. Also, the signals in the range of δ = 7.4−6.4 ppm are due to the aromatic protons present in the diamine monomer. 3.2. Polymer Synthesis and Their Properties. 3.2.1. Polymer Synthesis and Characterization. The PAs were prepared by the condensation reaction between diamine monomer (6) and five different aromatic diacids (1:1 mole ratio) in NMP solvent (Scheme 2) using TPP and pyridine as the condensing agents. The physical properties of all PA membranes are summarized in Tables 1 and 2. The characteristic absorption band for −NH in −CONH appeared at 3291−3261 cm−1 in the FTIR spectra, whereas no signals associated with the free amine (above 3400 cm−1 ) were observed. Additional absorption bands at about 2900 cm−1 that is typical of the aliphatic C−H stretching corresponding to the tert-butyl groups were also observed. The elemental analysis of all the synthesized PAs were in good agreement with their proposed structures, indicating a high purity. The repeat unit structures of the novel PAs were also well confirmed by the 1H NMR spectra. A representative 1H NMR spectrum of PA I in pyridine-d5 is shown in Figure 2. The singlet above 11.00 ppm in Figure 2 corresponds to the proton of the amide group in all the PAs. There were no observable peaks corresponding to free amine or acid protons, indicating a high conversion to polyamides with high molecular weight (M̅ n = 45 000−94 000 and PDI = 2.0−2.9, Table 1). 3.2.2. Polymer Properties. 3.2.2.1. Solubility and Crystallinity. All PAs were soluble [10% (w/v)] in polar solvents like, DMAc, DMF, NMP, and pyridine at room temperature but were insoluble in DMSO. Similar findings were also reported in our earlier publication.11 The propeller-shaped TPA units containing the bulky tert-butyl groups were responsible for disrupting the polymer chain packing, resulting in their enhanced solubility in THF where the analogue PAs (8a−8e) containing adamantane-substituted TPA were insoluble.11 The

Figure 3. Wide-angle X-ray diffraction plots of the polyamide membranes.

additional shoulder around 2θ ≈ 28°. The π−π stacking of amide and phenyl rings in ordered domains could be responsible for the appearance of this shoulder.10,11 The amorphous nature of the PAs can be attributed to the substituted TPA units that disrupt the polymer chain packing.17,19 The maximum in the broad bands and the calculated d-spacing (intersegmental distance) values are tabulated in Table 1. The position of the maximum gradually shifted from 2θ = 14.38°, for PA II, to higher values of 2θ in the order PA I < IV < III < V, which could be assigned to their symmetrical closely packed polymer structures. PA II has the highest d-spacing in the series due to the incorporation of the ⟩C(CF3)2 linkage, which disrupts the interchain packing, thus F


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Macromolecules reducing the H-bonding.28 The intersegmental distance (dspacing) between the polymer chains obtained from X-ray diffractograms is a measure of openness in the polymer and influenced their FFVs.10 The order of FFV for this series of PAs is PA II > I > IV > III > V, which is same as the order of their dspacing (Table 1). 3.2.2.2. Thermal Properties. The thermal behavior of this series of PAs was evaluated by DSC and TGA. These PAs did not show any peaks corresponding to melting or crystallization on the DSC thermograms (Figure 4), indicating the amorphous

TPA moiety which resulted in enhanced polymer chain separation by disrupting the close packing.10 The increase in chain separation of this series of PAs compared to their analogues (PA 8a−8e) can be observed from their higher FFV and d-spacing values.11 The TGA (Figure 5) revealed that all the PAs have high thermal stability with 10% decomposition temperatures in the range of 403−415 and 445−460 °C under air and nitrogen, respectively. The values are reasonably high considering the presence of TPA with the tert-butyl groups in the PAs. It is interesting to find that PA I showed higher Td10 value in air than the other PAs in this series, which can be attributed to the weight gain of PA I during oxidation of extra tert-butyl group present in the diacid component.17 All these PAs showed a high char yield of more than 47% (under nitrogen) irrespective of the introduction of pendant tert-butyl groups, indicating a high aromatic content. 3.2.2.3. Mechanical Properties. Polymer membranes with sufficiently good mechanical properties are required for the production of robust gas separation membranes.1 Table 2 summarizes the mechanical properties of these new classes of PA membranes. All membranes exhibited tensile strength up to 68 MPa and Young’s modulus up to 1.81 GPa. The elongation at break for these PA membranes ranges between 4 and 11% depending on the repeat unit structure. In this series, PA I containing the extra tert-butyl group in the diacids component showed the lowest tensile strength and modulus that can be attributed to the reduction in polymer interchain interactions caused by the bulky tert-butyl group.30 Although PA II has the highest FFV in this series, it showed higher mechanical property than PA I due to the increased intermolecular forces caused by the presence of strongly interacting −CF3 groups in the former.31 Thus, the combination of thermal and mechanical properties exhibited by these PAs makes them suitable for membranebased gas separation processes. 3.3. Gas Transport Properties. 3.3.1. Effect of Chemical Structures on Gas Transport Properties. The mean gas permeability (P) of four different gases (CO2, O2, N2, and CH4) and the ideal permselectivity (α) values for important gas pairs CO2/CH4 and O2/N2 are summarized in Table 3. The diffusion coefficients (D) and solubility coefficient (S) along with their solubility selectivity and diffusivity selectivity values are presented in Table 4.

Figure 4. DSC curves of the polyamides (heating rate: 20 °C min−1).

nature of these materials, which was also evident from the XRD diffractograms. The high Tg values of these PAs (Table 2) were due to the presence of rigid TPA cores containing tert-butyl groups.17 The Tg values of these PAs increased with increasing stiffness of the polymer backbone, mostly depending on the rigidity of diacids used as a starting material. The increasing order of Tg for this series of PAs is PA III < I < II < IV < V. The higher Tg for PA II is the consequence of hindered inter segmental mobility due to the presence of the ⟩C(CF3)2 linkage on its backbone.29 This series of PAs (PA I−V) showed a lower Tg compared to the analogue PAs (PA 8a−8e, Tg 299−330 °C) containing the adamantane-substituted TPA moiety.11 The low Tg of this series of polyamides can be attributed to the bulkiness of three tert-butyl groups present as a pendant group in the

Figure 5. TGA thermograms of the polyamide membranes in air (a) and nitrogen (b) (heating rate: 10 °C min−1). G


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Table 3. Gas Permeability Coefficients (P) in barrer and Permselectivities (α) of the Polyamides and Their Comparison with Other Commercially Available Polymers and Some Polyamides Reported Earlier polymer

P(CO2)

P(O2)

P(N2)

P(CH4)

α(CO2/CH4)

α(O2/N2)

ref

PA I PA II PA III PA IV PA V Matrimid cellulose acetate TBI-SO2b HFA-TERTc APA3d APA4d APA5d PA 8ae PA 8be PA 8ce PA 8de PA 8ee

86.0 119.0 51.0 69.7 41.5 8.7 6.56 8.60 22.0 43.0 27.0 36.0 53.5 61.5 14.5 23.5 12.6

20.2 29.0 12.8 17.0 10.8 1.90 1.46 2.00 5.88 9.38 6.47 7.88 13.0 14.2 3.8 5.0 3.5

2.5 3.0 1.7 2.0 1.4 0.27 0.23 0.35 1.06 1.73 1.18 1.51 1.3 1.4 0.4 0.6 0.3

2.9 3.2 2.1 2.3 1.7 0.24 0.20 0.28 0.72 2.24 1.21 1.40 1.1 1.2 0.3 0.5 0.2

29.65 37.19 24.28 30.30 24.41 36.00 32.80 31.80 30.31 19.2 22.3 25.7 48.64 51.25 48.33 47.00 63.00

8.08 9.67 7.53 8.5 7.71 7.00 6.40 5.40 5.06 5.5 5.5 5.2 10.0 10.14 9.50 8.34 11.67

a a a a a 43 44 17 45 16 16 16 11 11 11 11 11

From this study 1 barrer = 10−10 cm3 (STP) cm/(cm2 s cmHg). bGas permeability coefficient (P) measured at 35 °C and 10 atm of applied pressure. cP(CO2), P(N2), and P(CH4) measured at 10 atm and P(O2) measured at 2 atm of applied pressure at 35 °C. dGas permeability coefficient (P) measured at 3 bar of applied gas pressure and at 30 °C. eGas permeability coefficient (P) measured at 3.5 bar of applied gas pressure and at 35 °C. a

Table 4. Gas Diffusion Coefficients, D (10−8 cm2/s), Solubility Coefficients (S) in 10−2 cm3 (STP)/(cm3 cmHg), Diffusivity Selectivity (αD), and Solubility Selectivity (αS) Values of the Polyamides at 35 °C and 3.5 bar CO2 polymer PA PA PA PA PA

I II III IV V

O2

N2

CH4

CO2/CH4

O2/N2

D

S

D

S

D

S

D

S

αD

αS

αD

αS

7.8 10.6 4.6 5.0 3.0

11.02 11.23 11.08 13.94 13.83

9.8 13.6 5.5 7.7 4.0

2.06 2.13 2.33 2.21 2.7

3.0 3.5 1.9 2.1 1.3

0.83 0.86 0.89 0.95 1.08

2.7 3.1 1.5 1.9 1.2

1.07 1.03 1.49 1.21 1.42

2.89 3.42 3.07 2.63 2.5

10.30 10.9 7.44 11.52 9.74

3.23 3.88 2.89 3.67 3.07

2.48 2.47 2.61 2.32 2.5

diameter of 3.94 Å. Thus, CO2 molecule may have a larger effective size than O2, and as a result D(O2) > D(CO2).33 This fact can also be explained batter by considering the effective molecular diameters (def) provided by the Teplyakov− Meares.34 They have provided that the def is higher for CO2 (3.02 Å) over O2 (2.89 Å) and which could be other reason behind the lower diffusivity of CO2 over O2. The solubility coefficients of CO2 for this series of PAs are very high compared to the other three gases due to the strong quadrupole of CO2 molecule which induces an electrostatic dipole− induced dipole interaction between CO2 and the ⟩CO group of the amide linkage.16 Also, the presence of extra nitrogen atom in the TPA moiety increases the favorable acid base interaction with CO 2 molecule, resulting in high CO 2 solubility.13,35 The kinetic diameter or the molecular size controls the diffusion coefficients of gases in polymers; similarly, the gas condensability which depends on its critical temperature is a governing factor for the gas solubility in polymer membranes. The high critical temperature of CO2 (31.04 °C) over O2 (−118.6 °C), N2 (−146.9 °C), and CH4 (−82.3 °C) is responsible for its high inherent condensability which favors its higher solubility,35 consequently resulting in higher permeability of CO2 through these PAs membranes. Also, the higher critical temperature of CH4 molecule over N2 molecule helps to enhance the solubility of the CH4 molecule (Table 4), which

All the PAs in this series showed high gas permeability with moderate selectivity depending on their backbone structure. This is attributed to the cumulative effect of the propellershaped TPA moiety and the bulky tert-butyl substituents. The gas permeability coefficients for all the PAs followed the order P(CO2) > P(O2) > P(CH4) > P(N2), which is the reverse order of their kinetic diameter, except for CH4 and N2 (Å): CO2 (3.3) < O2 (3.46) < N2 (3.64) < CH4 (3.8). The high permeability of CH4 over N2 is due to the affinity of CH4 with the bulky tert-butyl groups (bulky alkyl substituents) of the PAs.32 The higher solubility coefficient of CH4 over N2 for this series of PAs also supported the above fact (Table 4). This permeability behavior of CH4 and N2 is typical for high permeable polymers and can be observed in the α(N2/CH4) vs PN2 Robeson plot.24 However, the diffusivity values of these four gases through these membranes did not follow the same order. The observed diffusivity order was D(O2) > D(CO2) > D(N2) > D(CH4). Though, CO2 was expected to diffuse faster than O2 considering its lower kinetic diameter, the favorable interaction between CO2 with the polar amide group present in the polyamide backbone causes the diffusivity of CO2 to be lower. The high polarizability of CO2 and its strong quadrupole is behind this favorable interaction. There is no such type of interaction between O2, N2, and CH4 with the PAs. Further CO2 is less spherical compared to other diatomic molecules like O2 and has a “kinetic diameter” of 3.3 Å and a collision H


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Macromolecules also finally effected to obtain the P(CH4) > P(N2) for this series of polyamides. Generally, with increase in FFV, the gas diffusivity through the polymer membranes also increases, which ultimately results in high gas permeability. The relation between gas permeability and FFV is given by the equation P = A exp( −B/FFV)

where A and B are constants that depend on temperature and type of gas.11,36 The FFV and gas permeability maintain a linear relationship; i.e., with increase in FFV, the permeability also increases due to improvement in gas diffusivity.10 Generally for polymer systems, the logarithm of permeability decreases roughly linearly with increasing reciprocal of FFV.16 This trend is nicely followed by this series of PAs (Figure 6). The increasing order of gas permeability coefficients of these PAs for all the gases was V < III < IV < I < II, which is also the order of gas diffusivities.

Figure 7. Dependence of gas permeability vs dielectric constant of the polyamide membranes for CO2, O2, N2, and CH4 gases.

In this series of PAs, the solubility selectivity values (Table 4) played a major role in determining the permselectivity for CO2/O2, CO2/N2, and CO2/CH4 gas pairs. This is due to the relatively small diffusion selectivity compared to solubility selectivity for more soluble and more condensable gases (like CO2).12 However, for determining the permselectivity of the comparatively smaller molecules like O2 and N2, the diffusivity selectivity plays a major role.41 The order of gas selectivity for CO2/CH4 and O2/N2 is II > IV > I > V > III. The permselectivity values, which depend on the polymer repeat unit structures, varied between 24 and 37 and 7.5 and 9.67 for CO2/CH4 and O2/N2 gas pairs, respectively. The high permselectivity of PA II is not only due to its packing disruptive ⟩C(CF3)2 linkage, which stiffens the polymer backbones by restricting the torsional motion of phenyl rings around it, but also due to the increased interchain interactions caused by the polar ⟩C(CF3)2 linkage.15,31 These interactions give rise to additional intermolecular forces which increases the energy barriers for diffusion jumps of gas molecules, resulting in high gas selectivity.31 PA IV showed high permselectivity compared to PA I and III, which is in accordance with their Tg, as with increase in Tg, permselectivity also increased.9 In the case of PA I and III, the tert-butyl substitution in the acid moiety (PA I) enhanced the stiffness of the polymer backbone (high Tg = 263 °C), which resulted in high gas selectivity.31 3.3.2. Molecular Dynamics Simulation of the Polymer Structure vs Gas Transport Properties and Correlation with the Experimental Results. Molecular dynamics simulation was performed on the PA structures to get a better insight of the structural orientation of the polymer chains and their effect on the gas transport properties. The FFV values of the PAs were determined from both atomistic molecular dynamics (MD) simulations (FFVSIM) and experiments (FFVEXP). The equilibrium structures of all the PAs from MD simulations (optimized polymer boxes) are shown in the Supporting Information (Figure S1). It can be observed from Table 1 that the FFVSIM values are comparatively higher than the FFVEXP values but follow the same trend. A similar type of observation was also reported by Maya et al.10 Simulations allowed us to analyze and correlate the distribution of free volume elements (FVD) in these PAs with gas permeability and selectivity. It is already established that FVD and sizes of the free volume

Figure 6. Dependence of gas permeability vs reciprocal of fractional free volume of polyamides for CO2, O2, N2, and CH4 gases.

In the case of PA I, incorporation of the tert-butyl group improves the permeability of all four gases when compared to the unsubstituted structural analogue, PA III. This is due to considerable increase in gas diffusivity (≈2 times higher for PA I) as a result of enhanced FFV due to the presence of the tertbutyl substitution.36 In the case of PA III and IV, the high permeability of the latter is due to its high gas diffusivity, which is due to the free rotation of phenylene rings present in the para position compared to the restricted rotation of the meta-linked phenylene rings of the PA III.37,38 Also, the high d-spacing and high FFV of the PA IV compared to the PA III supported the above fact.38 The dielectric constant (ε) values for this series of PA membranes are tabulated in Table 2. The order of ε was II < I < IV < III < V. PA II containing ⟩C(CF3)2 showed lowest ε in this series due to its highest fluorine content. The low polarizability of the C−F bonds and disruption of polymer chain packing by the bulky −CF3 groups caused the lowering in ε.39 Matsumoto et al.40 used ε to predict the gas permeability through the polymer membrane as it is a function of molar volume (related to FFV) and molar polarizability. They observed a linear relationship between CO2 and CH4 gas permeability with ε in several polymers.40 A similar linear relationship was also observed between gas permeability and ε for this series of PAs (Figure 7). I


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Figure 8. Free volume distributions (FVD) of the polyamides.

success of our MD simulation for predicting the polymer properties.1 As stated earlier, the FVDs (Figure 8) for all the five PAs helped us to better understand the effect of FFV on gas diffusivities in this series of polymers. The transport properties of small- and medium-sized penetrant molecules are greatly affected by the free volume distribution in amorphous polymers.31 The FVDs for all the PAs in this series were similar and consistent with the increasing bulkiness of the diacid moiety. The presence of higher free volume elements bigger than 60 Å3 in the case of PA I and II is due to the presence of the extra tert-butyl group and the hexafluoroisopropylidene moiety, respectively. Whereas for PAs III, IV, and V the bigger volume elements are smaller in size (≈40 Å3). The diffusion coefficients for all the gases were in the order PA II > I > IV > III >V, which is in good agreement with the presence of bigger free volume elements in the PAs, i.e., with the trend of FVDs in the PAs. The presence of free volume elements bigger than 70 Å3 in PA II is responsible for its higher gas diffusivity over PA I. These facts are also supported by the fractional accessible volume for different gas molecules (Figure 10) in this series of PAs. It can be observed that the order of fractional accessible volume is PA II > I > IV > III >V, which is also according to the order of gas diffusivities. Hence, it can be concluded large accessible volume in the polymer matrix corresponds to enhanced gas diffusivity, which finally improves

elements (Figure 8) influenced the gas permeability, diffusivity, and permselectivity of the polymer membrane.11,31 The trends of diffusivity for this series of PAs for all these four gases were also determined using simulations, and it was observed that it followed the same trend (Figure 9) with close proximity to the experimental values. This close proximity between the experimental and simulated values signifies the

Figure 9. Diffusivity of the four gases determined by molecular dynamic simulation for this series of polyamides. J


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The high gas separation efficiency (permselectivity) of these membranes can be supported and explained from their distribution of free volume elements. The presence of large number of smaller free volume elements (size 10 Å). These “hourglass-shaped” (Figure 13) interconnected channels of free volume elements are responsible for the selective transport of gas molecules with enhanced permeability.42 3.3.3. Comparison with Other Commercially Available Polymer and Previously Reported Polyamide Membranes. We compared the gas permeability values of these PA membranes with a few commercially available polymers (e.g., Matrimid43 and cellulose acetate44). The gas permeability data of polyamides containing the tert-butyl moiety were also considered for further comparison.15,16,45 All these data are reported in Table 3. The permselectivity for CO2/CH4 gas pair against permeability coefficient of CO2 and permselectivity for O2/N2 gas pair against permeability coefficient of O2 for this series of PAs and different other polymers have been plotted in Robeson plots in Figures 14 and 15, respectively. From these plots, it can be observed that PA II showed the highest CO2 and O2 permeability with high selectivity over other aromatic glassy polymers (Table 3). This improvement is due to an increase in free volume, brought about by the presence of bulky tert-butyl groups, while retaining a significant degree of chain rigidity to maintain the gas selectivity. In comparison to our previous series of PAs containing adamantane-substituted TPA (PA 8a−8e),11 the present series of PAs showed higher permeability but relatively lower selectivity. This could be attributed to the presence of three tert-butyl groups in the substituted TPA moiety which increased the gas permeability by enhancing the gas diffusivity (higher FFV) but relatively lowered the molecular size sieving ability. Overall, it can be stated that the PAs in this investigation showed a good combination of high permeability and moderate selectivity for gas permeation applications.

Figure 10. Fractional accessible volume of the different gas molecules for these series of PAs.

the gas permeability. From Figure 10, it can observe that the fractional accessible volume is significantly higher in the case of PA II and III when compared to other polymers in the series, which is in good agreement with their FVD (Figure 8). Another important fact which comes out from Figure 10 is that for all the polymer system in this series the percent of accessible volume decreased sharply after the probe radius of 1 Å; i.e., the change in accessible volume is much more in the case of the gas molecule with larger radius (N2 and CH4). This affects the overall diffusivity of the gas molecules and can be observed from the difference in diffusivity for CO2 and O2 compared to N2 and CH4 (Figure 9). We also analyzed the trajectories of different gas molecules in these polymer systems (Figure 11) and found that they follow an order that is in agreement with the gas diffusivity values. We note that O2 moves larger distances within the system (shown by blue line) compared to other gas molecules as discussed earlier. For an in-depth analysis of gas diffusivities in the PAs, we plot the mean-squared displacement (MSD) of the gas molecules as a function of time (Figure 12). We observed that the MSD is highest for O2 and follows a trend similar to that of diffusivity for all the PAs studied in this work. We also note that the MSD follows the same trend as FFD for this series of PAs and is highest for PA II, which also has the highest FFV.

Figure 11. Gas molecules trajectory in the polyamide systems (O2 = blue, CO2 = red, N2 = brown, CH4 = green). K


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Figure 12. Gas molecules mean-squared displacement through these polyamide systems.

Figure 13. Size and shape of the free volume elements of this series of PAs (representative one for the PA II). The black one signifies the free volume elements in the simulated polymer box.

4. CONCLUSION A series of novel PAs with high molecular weight and a set of desirable physical properties for gas separation applications were successfully synthesized from a new tri-tert-butylphenolsubstituted TPA-containing diamine monomer. The presence of the pendant tri-tert-butylphenol along with the packing disruptive TPA core acts favorably to reduce the polymer chain packing and simultaneously increase the chain rigidity. These structural features favor an enhanced solubility of these PAs in

various organic solvents. A good combination of thermal and mechanical properties allowed using these PAs for gas permeation investigation. The PAs showed high gas permeability for CO2 and O2 with moderate gas separation efficiency for CO2/CH4 and O2/N2 gas pairs. In fact, for the O2/N2 gas pair, most of the PAs even surpass the present Robeson upper bound. The introduction of tri-tert-butylphenol-substituted propeller-shaped TPA core in these PAs boost the gas permeability while maintaining good selectivity. Gas diffusivity L


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ACKNOWLEDGMENTS D.B. acknowledges IIT Kharagpur for providing him a senior research fellowship. The financial support from Department of Science and Technology (DST), India, in the form of a sponsored project (Grant SR/S3/ME/0008/2010) is gratefully acknowledged.

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Figure 14. Permeability/selectivity trade-off map for CO2/CH4 separation. Values are for this series of polyamides and also taken from some other polymers reported earlier.12,16,17,43−45

Figure 15. Permeability/selectivity trade-off map for O 2 /N 2 separation. Values are for this series of polyamides and also taken from some other polymers reported earlier.12,16,17,43−45

and permeability through these PA membranes were in accordance with the increasing FFV and dielectric constants of these PAs. Overall, it can be concluded that the approach of using TPA diamines containing bulky tert-butyl groups is a good route to develop PA membranes with a combination of good permeability and selectivity, suitable for gas permeation applications.

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ASSOCIATED CONTENT

S Supporting Information *

Detailed characterization of the polymers; equilibrium structures of all the PAs (PA I−V) from MD simulations. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01044.

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*(S.B.) E-mail [email protected]; Tel +913222283972; Fax +91-3222255303. Notes

The authors declare no competing financial interest. M


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