Highly permeable polyimide membranes with a structural pyrene containing tert-butyl groups: Synthesis, characterization and gas transport

Journal of Membrane Science 563 (2018) 134–141

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Highly permeable polyimide membranes with a structural pyrene containing tert-butyl groups: Synthesis, characterization and gas transport

T

⁎

R. Sulub-Suluba, M.I. Loría-Bastarracheaa, H. Vázquez-Torresb, J.L. Santiago-Garcíaa, , ⁎ M. Aguilar-Vegaa, a

Unidad de Materiales, Centro de Investigación Científica de Yucatán, A. C., Calle 43 No. 130, 32 y 34, Chuburná de Hidalgo, C.P. 97205 Mérida, Yucatán, México Departamento de Física, Universidad Autónoma Metropolitana Iztapalapa, Av. San Rafael Atlixco 186, Col. Vicentina, Apdo. Postal 55-534, C.P 09340 Ciudad de México, México b

A R T I C LE I N FO

A B S T R A C T

Keywords: Polyimides Aromatic dianhydride High gas permeation membrane Fractional free volume Thermal resistance

Three new polyimides with high gas permeability based on a new dianhydride 3,8-di(4-tertbutylphenyl)pyrene1,2,6,7-tetracarboxylic DPt, containing tert-butyl moieties are reported. Chemical structures of the resulting dianhydride monomer and polyimides were confirmed by FTIR and 1H-NMR. All polyimides show high thermal stability with onset decomposition temperatures above 490 °C, and glass transition temperatures above 340 °C. The incorporation of a pendant tert-butyl group into the polymer provided high gas permeability with moderated selectivity. The polyimide formed from DPt and 2,4,6-trimethyl-m-phenylenediamine, DPt-TMPD, showed 2035 Barrer CO2 permeability which could be attributed to its inefficient chain packing from the incorporation of tertbutyl groups that increased the polymer FFV. The obtained gas permeability coefficients and gas selectivities are similar to those reported for PIM-PI polymers.

1. Introduction Polymer membranes have great potential for gas separation applications due to their separation efficiency, low operating costs and ease of operation [1–3]. To achieve a balanced efficiency for gas separation, membranes should possess both high gas permeability and high selectivity. However, there exists a trade-off between gas permeability and selectivity, that is, as gas permeability increases the ideal gas separation factor decreases and vice versa, as indicated by the Robeson upper bound relationship [4,5]. Aromatic polyimides are among the most attractive materials for gas separation membranes due to their excellent balance of mechanical, thermal, chemical and electrical properties [6–8]. The main obstacle for the application at large scale of polyimides is their low permeability, although they possess high selectivity for most gas pairs of industrial interest, thus limiting its application to small and medium scale gas separation operations [9–11]. There is an interest to develop polyimide membranes with increasing permeability without impairing their selectivity. Modification of polyimide backbone by incorporating bulky groups for gas separation applications have been reported to improve gas permeability and selectivity in polyimides [12–15]. These bulky pendant groups make the structure rigid and simultaneously help to disrupt chain packing reducing local segmental mobility. They increase fractional free volume

⁎

(FFV), which results in improved gas permeability [16]. In this regard, different types of pendant bulky groups such as phenyl [17,18], triptycene [19,20], adamantane [21–23], triphenyl [24] and tert-butyl [12,25–27] have been introduced. In particular, it has been proved that polyimides containing bulky tert-butyl groups had an improved solubility, high permeability, and moderated selectivity with good thermal stability and mechanical properties [28,29]. Qiu et al., reported improvement in gas permeability for polyimides containing tert-butyl (PCO2 = 86 and PCO2/PCH4 = 27) [30]. Kim et al., synthesized polyimides containing tert-butyl groups and reported high O2 permeability and improvement in the O2/N2 selectivity (PO2 = 52 and PO2/PN2 = 4.2) [31]. Taking into consideration these results we have aimed to synthesize a new dianhydride monomer 3,8-di(4-tert-butylphenyl)pyrene-1,2,6,7tetracarboxylic dianhydride, containing symmetrically placed tert-butyl groups. We also describe the synthesis and characterization of three aromatic polyimides prepared from two ortho methyl-substituted diamines, and one unsubstituted flexible diamine. The presence of the tertbutyl bulky groups in the dianhydride moiety is expected to improve solubility and increase gas permeability coefficients in membranes from these polyimides. The polyimides synthesized should also present outstanding thermal stability and high mechanical properties due to the large number of conjugated benzene rings in their final structures.

Corresponding author. E-mail addresses: [email protected] (J.L. Santiago-García), [email protected] (M. Aguilar-Vega).

https://doi.org/10.1016/j.memsci.2018.05.054 Received 17 January 2018; Received in revised form 5 April 2018; Accepted 25 May 2018

Available online 26 May 2018 0376-7388/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 1. Reaction scheme to obtain DPt dianhydride.

2. Experimental

stirrer and reflux condenser. The reaction mixture was boiled at 100 °C to remove water. After, the mixture was refluxed for 3 h at 200 °C. Then, the mixture was cooled down to 100 °C and 1.6 g of NaOH were added slowly; the mixture was again refluxed for 3 h. When the solution was cooled, a clear brown precipitate (2) was formed, which was filtered off and dried, to give a 95% yield. The product (2) was recrystallized from a solvent mixture ethanol/CHCl3 (2:1). m.p: 180 °C. Elemental analysis, calculated for C32H36 (420.64): C, 91.37%; H, 8.63%; measured: C, 91.66%; H 8.34%. 1H-NMR (600 MHz, CDCl3) δ 7.88 (d, J: 9 Hz, 2H), 7.32 (t, J: 15 Hz, 2H), 7.20 (m, 6H), 7.05 (d, J: 8.4 Hz, 4H) 4.35 (s, 4H) 1.22 (s, 18H).

2.1. Materials Naphthalene (99%, Sigma Aldrich), 4-tert-butylbenzoyl chloride (98% Sigma Aldrich), aluminum chloride (AlCl3, 99% Sigma Aldrich), hydrazine monohydrated (98%, Sigma Aldrich), diethylene glycol (99%, Sigma Aldrich), sodium hydroxide (NaOH, 98.3% J.T. Baker), nitrobenzene, NB, (99.5%, Sigma Aldrich), maleic anhydride (99%, Fluka), iodine (99%, Sigma Aldrich), acetic acid (100%, J.T. Baker), toluene (98% Sigma Aldrich), hexane (98.5%, TEDIA), hydrochloric acid (37%, J.T. Baker), methanol (99%, J.T. Baker), ethanol (99%, J.T. Baker), N,N-dimethylformamide, DMF, (99%, Sigma Aldrich), N,N-dimethylacetamide, DMA, (99.%, Sigma Aldrich), tetrahydrofuran, THF, (99%, Sigma Aldrich), chloroform (CHCl3, 99.98%, J.T. Baker), 1-methyl-2-pyrrolidone, NMP, (99.5%, Sigma Aldrich), pyridine (99.8%, Sigma Aldrich), benzoic acid (99.5%, Sigma Aldrich), 4,4´-methylenebis(2,6-dimethyl-aniline), MBDAM, (99%, Sigma Aldrich), were used as received. 2,4,6-trimethyl-m-phenylenediamine, TMPD, (96%, Sigma Aldrich) was purified by sublimation and 2,2-bis[4-(4-aminophenoxyphenyl)] hexafluoropropane, BAPHF, (96%, Sigma Aldrich) was purified by crystallization.

3.3. 3,8-di(4-tert-butylphenyl)pyrene-1,2,6,7-tetracarboxylic dianhydride (DPt) Into a 125 mL round bottom flask, 25 g of maleic anhydride were dissolved with 25 mL of nitrobenzene and the solution was boiled at 100 °C for water removal. 5 g of 1, 5-di(4-tertbutyl)benzyl naphtalene (2) and a few iodine crystals were added and the mixture refluxed for 4 h. At this point, 8 mL of solvent were distilled from the mixture under vacuum. After cooling, 25 mL of acetic acid were added to clean the product. The acetic acid solution was filtered to give 86% yield of a brown solid (3). DPt (3) was recrystallized from nitrobenzene. m.p: 405 °C. C40H30O6 (606.67): C, 79.19%; H, 4.98%; O, 15.82%; measured: C, 79.73%; H 4.20%; O, 16.07%.

3. Monomer synthesis 3.1. 1,5-di(4-tertbutyl)benzoyl naphthalene (1)

3.4. Polymer synthesis In a three neck flask 80 mmol of aluminum chloride were added slowly to a solution of 40 mmol of naphthalene and 50 mmol of 4-tertbutylbenzoyl chloride. The mixture was heated at 110 °C for 4 h. After this time, 50 mmol of 4-tert-butylbenzoyl chloride and 50 mmol of aluminum chloride were added again. The mixture was stirred at 110 °C overnight. At this point the mixture was cooled down until it solidified and it was then separated by the addition of ice and hydrochloric acid (HCl) to obtain a red solid. The resultant solid was thoroughly washed with water and filtered. Toluene was added to the filtrate forming a solution and the organic phase washed with hot water. The washed toluene (organic phase) solution was concentrated by solvent evaporation and then dissolved in hexane and recrystallized with ethanol twice to obtain white crystals (1). m.p.: 220 °C. Elemental analysis, calculated for C32H32O2 (448.61): C, 85.68%; H, 7.10%; O, 7.13%; measured: C, 85.54%; H, 7.06%; O, 7.4%. 1H-NMR (600 MHz, CDCl3) δ 8.14 (d, J: 9 Hz, 2H), 7.76 (d, J: 9 Hz, 4H), 7.53 (d, J: 7.2 Hz, 2H), 7.46–7.41 (m, 6H), 1.29 (s, 18H).

Three polyimides were prepared using DPt by a one-step hightemperature polycondensation reaction as described in Fig. 2. For a typical reaction, 1 mmol of diamine 2,4,6-trimethyl-m-phenylenediamine, TMPD, was dissolved in 6 mL of nitrobenzene in a 50 mL threeneck flask. 1 mmol of dianhydride (DPt) was added to this reaction mixture and heated to 80 °C with vigorous stirring under nitrogen atmosphere. At this temperature 0.3 mL of pyridine (2 mmol) were also added, and the mixture was maintained at that temperature for 1 h. In the next step, the temperature was increased to 120 °C and then 488.5 mg of benzoic acid (2 mmol) were added. The reaction mixture was heated and maintained at 200 °C for 48 h under continuous stirring. At the end of the reaction, the polymer solution was poured slowly into 600 mL of methanol under strong agitation to obtain a yellow fibrous precipitate. The polymer was filtered off, washed with ethanol and dried under vacuum at 200 °C for 24 h. The product was obtained with a 93% yield of DPt-TMPD. The other two polyimides, DPt-MBDAM and DPt-BAPHF, were synthesized following the same procedure.

3.2. 1, 5-di(4-tertbutyl)benzylnaphtalene (2) 3.5. Film preparation 4 g of 1 were added to 40 mL of diethylene glycol and 2.5 mL of hydrazine in a 125 mL round bottom flask equipped with magnetic

Dense films from the polyimides were prepared by a solvent 135

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Fig. 2. Reaction scheme for DPt based polyimides synthesis.

measured in a density gradient column (Techne Corporation) based on aqueous calcium nitrate solutions, between 1.10 and 1.23 (g/cm3) at 23 °C. Mechanical properties were measured with a Shimadzu AGS-X universal testing machine with a 100 N load cell at a crosshead speed of 1 mm/min. Measurements were performed using films with dimensions of 0.5 cm long by 2 cm wide and thicknesses of 100 µm (DPt-TMPD and DPt-MBDAM) and 40 µm (DPt-BAPHF) were used. An average of at least three individual determinations was performed for each polyimide.

evaporation casting method. The polyimide was dissolved in nitrobenzene, 4% (wt/v), and then was filtered through a 0.2-mm Teflon syringe filter. The filtered solution was deposited onto a glass plate within an aluminum ring and the solvent was slowly evaporated at 80 °C for 12 h. The solid polyimide films were washed with methanol and finally, dried under vacuum at 230 °C and kept at this temperature for 48 h. The complete removal of residual solvent was confirmed by TGA. 3.6. Monomer and polyimides characterization

3.8. Gas transport properties Elemental analyses were carried out using a Thermo Scientific Flash 2000 elemental analyzer. FTIR spectra were obtained with a Nicolet 8700 Thermo Scientific FTIR spectrometer using KBr pellets for monomer reaction intermediates and DPt dianhydride, Attenuated total reflectance (ATR-FTIR) was used to obtain the spectrum of polymer films. 1H-NMR spectra were recorded on a Varian, 600 MHz VNMRS spectrometer with CDCl3 as solvent and tetramethylsilane as reference. Solubility was determined by dissolving 5 mg of each polyimide in 1 mL of CHCl3, THF, NMP, DMF, DMAc and NB, respectively at room temperature. The inherent viscosities (ηinh) were determined using an Ubbelohde viscometer No. 50 with a polyimide concentration of 0.5 g/ dL in DMF for DPt-TMPD and nitrobenzene for DPt-MBDAM at 30 °C.

Gas permeability coefficient, P, for six pure gases (He, H2, O2, N2, CH4 and CO2) were determined using a constant volume permeation cell of the type described elsewhere [26], according to the following equation:

PA =

273 VL dp 76 ATp0 dt

(1)

where P is the permeability coefficient of gas A through the membrane expressed in Barrer [1 Barrer = 10−10 [cm3 (STP) cm cm−2 s−1 cmHg−1], V is the constant volume of the permeation cell, A and L are the exposed area and the thickness of the membrane, respectively. T is the temperature at which the measurement is carried out (298.15 °K), p0 is the pressure of the feed gas in the upstream of the membrane, and dp/dt is the pressure increase with time under steady state conditions measured in the permeation cell. The gas diffusion coefficient, D, was obtained by the time lag method from the intercept with the time axis using the relation: D= l2/(6θ), where l is the film thickness and θ is the time lag. The gas solubility coefficient, S, was obtained using the relation: S = P / D .

3.7. Thermal and physical characterization Thermal stability analyses were performed via thermogravimetric analysis (TGA) using a TGA-7 Perkin Elmer thermobalance at a heating rate of 10 °C/min between 50 and 800 °C under nitrogen atmosphere. Differential scanning calorimetry (DSC) measurements were carried out using a Mettler-Toledo DSC 1 Star System on 4–5 mg samples at a heating rate of 10 °C/min (from 40 to 500 °C) under a 100 cc/min nitrogen flux. X-ray diffraction (XRD) was carried out in a Bruker D8 Advance diffractometer using Kα (Cu) radiation with a wavelength (λCu = 1.542 Å), in the range of 2 θ from 5° to 60°. Polyimide densities were

4. Results and discussion The dianhydride DPt was synthesized according to Fig. 1. In this synthesis, a Friedel-Craft's acylation reaction between naphthalene and 136

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Fig. 3. FTIR spectrum of DPt monomer.

benzoyl chloride was used to obtain the 1,5-di(4-tert-butyl) benzoylnaphthalene (1), which was reduced through a Wolff-Kishner's reduction to obtain 1,5-di-(4-tert-butyl) benzylnaphthalene (2). Finally, a Diels-Alder´s reaction between (2) and maleic anhydride gives to the product 3,8-di(4-tert-butylphenyl)pyrene-1,2,6,7-tetracarboxylic dianhydride (3) (DPt). DPt was purified twice by recrystallization in NB before use. The FTIR spectrum of DPt shows the characteristic absorption bands in 2960 cm−1 and 1365 cm−1(C-H and C-CH3 stretching of the tert-butyl group), respectively. At 1840 and 1770 cm−1 carbonyl bands (C˭O, symmetric and asymmetrical stretching) are also found, see Fig. 3. Three polyimides were synthetized by the one-step high-temperature polycondensation reaction in nitrobenzene, as solvent, in the presence of pyridine and benzoic acid, as described in the scheme of Fig. 2 [32]. The structures of the polyimides were confirmed by FTIR and 1HNMR spectroscopy (Figs. 4 and 5). FTIR spectra of the rigid polyimides based on DPt are shown in Fig. 4. These spectra show that all polyimides exhibited the characteristic imide absorption bands around 1770 cm−1 and 1710 cm−1 (C˭O asymmetrical and symmetrical stretching), 1370 cm−1 (C-N stretching), and they also show bands around 2860–2960 cm−1 (C—H stretching of alkyl groups). The loss of the amide and carboxyl bands confirmed that the polymers were fully imidized. Fig. 5 shows DPt-TMPD 1H-NMR spectrum as an example, in which all the signals have been assigned to the corresponding protons in the polyimide. Polyimides solubility was determined qualitatively in several organic solvents and the results are reported in Table 1. The polyimide, DPt-TMPD, was soluble in both polar and non-polar solvents. While, DPt-MBDAM showed lower solubility, since it is only soluble in THF, CHCl3 and NB; finally, DPt-BAPHF showed the lowest solubility since it is partially soluble in nitrobenzene. These differences in solubility can be attributed to the effect of the ortho symmetric methyl substitution present in the polyimides prepared from diamines TMPD and MBDAM; this is in contrasts with DPt- BAPHF since the latter structures have methyl substitutions at ortho position in the phenyl ring, which restrict rotation avoiding packing and thus improving solubility, as has been reported elsewhere [17]. Physical properties of DPt polyimides are listed in Table 2. Inherent viscosity, ηinh, values are related to moderate molecular weights, as has been reported in other similar polyimide structures [33]. However, the three polyimides synthesized have the ability to form flexible membranes. ηinh was 0.36 and 0.63 dl/g for DPt-TMPD and DPt-MBDAM, while for DPt-BAPHF it was not determined owing to its low solubility even in NB which precluded its measurement due to slow precipitation during the measurements. It is known that the fractional free volume (FFV) affects gas

Fig. 4. ATR-FTIR spectrum of DPt based polyimides.

Fig. 5. 1H-NMR spectrum of DPt-TMPD polyimide.

transport properties of polymeric membranes. Densities and fractional free volume of the membranes are also listed in Table 2. The FFV was calculated using the experimental polyimides density data and the following equation:

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Table 1 Solubility of DPt based polyimides. Polymer

DPt-TMPD DPt-MBDAM DPt-BAPHF

Table 4 Thermal properties of the DPt based polyimides.

Solubility CHCl3

DMAc

DMF

THF

NMP

DMSO

NB

++ ++ –

++ – –

++ – –

++ ++ –

++ – –

– – –

++ + +-

Polyimide

Td (°C)

T5 (°C)

T10 (°C)

Char yield (%)a

Tg (°C)

DPt-TMPD DPt-MBDAM DPt-BAPFH

552 528 547

556 532 554

590 578 575

65 66 62

397 370 340

Td: onset degradation temperature, T5 y T10: degradation temperature at 5 y 10% weight loss, respectively. a Measured at 800 °C under N2 atmosphere.

++: soluble at room temperature; +: soluble by heating; +-: partially soluble; -:insoluble. Table 2 Inherent viscosity, density and fractional free volume (FFV) of DPt based polyimides. Polyimide

ηinh (dL/g)

Density (g/cm3)

Vdw (cm3 /mol)

FFV

DPt-TMPDa DPt-MBDAMb DPt-BAPHF

0.36 0.63 –

1.10 1.11 1.23

408.3 467.3 566.4

0.201 0.183 0.168

a b

ηinh Measured in DMF at 30 °C. ηinh Measured in nitrobenzene at 50 °C.

FFV =

V0−1.3 Vw V0

(2)

where V0 is the specific volume of the polyimide (cm3 g−1) determined from the measured density, and Vw is the Vander Waals volume (cm3 g−1) which was calculated by the group contribution method of Zhao et al. [34]. The densities of these DPt based polyimides are in the range of 1.10–1.23 g cm−3. The FFV values were in the range from 0.160 to 0.201 which are significantly higher even than those reported for other high FFV polyimides [25]. This result can be attributed to the incorporation of the symmetric tert-butyl group moiety in the dianhydride structure (DPt) that contributes to an increasing rigidity and a higher fractional free volume presented by the as synthesized polyimides. The diamine used for polyimide preparation exhibited the following FFV order DPt-TMPD > DPt-MBDAM > DPt-BAPHF. This result is attributed to the presence of the methyl group into the ortho position of the diamine moiety, which disturbs chain packing and results in higher FFV; consequently, DPt-TMPD and DPt-MBDAM show higher FFV than DPt-BAPHF. Table 3 shows tensile strength, elongation at break and Young´s modulus values for DPt based polyimides. Tensile strength was found to be in the range of 52–54 MPa for DPt polymide films. Elongation at break was between 6% and 7.5% and Young's modulus values lay between 0.98 and 1.17 GPa. DPt-TMPD exhibited the highest tensile strength and tensile modulus and lowest elongations at break as compared to DPt-MBDAM and DPt-BAPHF which can be attributed to a more rigid structure. In general, the three polyimides showed quite similar mechanical properties where there is little difference within the uncertainty of the measurements. Young's modulus around 1 GPa and tensile strength around 53 MPa lead to highly rigid polyimides. The modulus values observed in these polyimides were comparable to those reported for other rigid polyimides [35,36].

Fig. 6. Thermogravimetric analysis curves of DPt based polyimides.

4.1. Thermal properties DPt based polyimides thermal properties were evaluated by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results are illustrated in Table 4. The onset of decomposition temperature (Td), as well as 5 wt% (T5) and 10 wt% (T10) weight loss were determined from the TGA thermograms shown in Fig. 6. The three polyimides showed high thermal stability with the Td above 500 °C. T5 ranges between 532 and 556 °C while T10 values are between 575 and 590 °C. The polyimides showed excellent thermal stability and char yield, above 62 wt% at 800 °C. This high char yield and thermal resistance can be explained by the presence of the conjugated, pyrene type, phenyl structures of these polyimides. The differential TGA curves of the polyimides are also illustrated in the inset of Fig. 6 and exhibit clearly the two degradation steps. The first step in all polyimides occurred with a minimum around 580 °C, and has been associated with the loss of the tert-butyl groups. The second step occurred with a minimum around 650 °C and it was associated with the main chain polymer degradation. Glass transition temperatures, Tg , for DPt based polyimides were determined by DSC. As it is reported in Table 4, the DPt based polyimides exhibited Tg , values above 340 °C, which decreased in the following order: DPt-TMPD > DPt-MBDAM > DPt-BAPHF. The DPtTMPD exhibited the highest Tg , value of these polyimide series with 397 °C. DPt-BAPHF showed the lowest Tg, value which is due to a larger degree of conformational freedom for this DPt based polyimide attributed to the presence of the ether linkages in the main polymer chain. Fig. 7 shows X-ray diffraction patterns of the three DPt based polyimides that are amorphous patterns. DPt-TMPD and DPt-MBDAM showed maxima peaks located at 14.82° and 14.90° 2θ, respectively. On the other hand, DPt-BAPHF showed two overlapped maxima one at 15.02° and the other at 17.16° 2θ. These maxima were used to calculate the average d-spacing between polymer chains using Bragg's law (d = nλ /2sinθ) . The calculated d values correspond to d-spacing of 6.02

Table 3 Mechanical properties of DPt based polyimide membranes. Polymer

Young's Modulus (GPa)

Tensile Strength (MPa)

Elongation at break (%)

DPt-TMPD DPt-MBDAM DPt-BAPHF

1.17 ± 0.07 0.98 ± 0.06 1.02 ± 0.1

54 ± 4 52 ± 6 52 ± 5

6.6 ± 0.9 7 ± 0.6 7.5 ± 0.6

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permeability coefficient (PCO2 = 2035) with an ideal CO2/CH4 selectivity of 9.9 at 2 atm and 35 °C. DPt-TMPD has 2 times the gas permeability coefficients of DPt-MBDAM, with similar selectivity. This is attributed to MBDAM structure, which contains two connected phenyl rings by a methylene moiety that allows a larger number of conformations, which in turn decrease the permeability coefficient, while the lack of flexible joints into TMPD increase chain rigidity in fixes the structure which in turn increases permeability [17,45]. Also, DPt-TMPD has 6–9 times and DPt-MBDAM has 2–3 times DPt-BAPHF gas permeability coefficients for the five gases tested (H2, N2, O2, CH4, CO2). This behavior is attributed to the ortho-methyl substitution of the TMPD and MBDAM diamines use for DPt-TMPD and DPt-MBDAM polyimides preparation which hinders the chain rotation, increases FFV and gas permeability coefficients, which is consistent with the reports in the literature [46]. The DPt based polyimide membranes show gas permeability coefficients that are two orders of magnitude above those of Matrimid®. DPt-TMPD shows the most attractive gas transport properties where its CO2 permeability is ~300 times higher than Matrimid®. Furthermore, the CO2 permeability coefficient of DPt-TMPD is 27% higher than that reported before for DPPT-TMPD, which has a similar base structure without the presence of the pendant tert-butyl groups [17]. Thus a large enhancement on gas permeability coefficients was achieved with the presence of the tert-butyl moiety. Table 5 also shows that the gas permeability of these DPt based polyimides was similar to the one reported for some polyimides with intrinsic microporosity (PIM-PI) [43,44,47,48]. Recently, PIM-polyimides have been developed showing both high permeability and good selectivity. This class of polymers has attracted interest because they offer additional structures that could be cast into films [8]. In particular, the polyimide DPt-TMPD, synthesized in here, shows similar permeability and selectivity than PIM-PI-9 and PIM-PI-11 [43] and CO2 permeability similar for KAUST-PI but with lower selectivity [44]. It is important to emphasize that the PIM-PI's are synthetized using more expensive diamines with a more sophisticated structure. The diffusion coefficients in DPt-TMPD, DPt-MBDAM and DPtBAPHF were determined by the time lag method. The apparent pure gas diffusion coefficients, D, are presented in Table 6. Apparent gas diffusion coefficients in the three based DPt polyimides DPt-TMPD > DPtMBDAM > DPt-BAPHF follow the same tendency observed in gas permeability coefficients. The decreasing order of apparent diffusion coefficients through these polyimides was DO2 > DCO2 > D N2 > DCH4 . Generally, gas diffusion coefficients decrease as penetrant size increases. However, CO2 can interact with the polymer matrix that slows the migration of the penetrant through the polymer. This behavior has been reported in glassy polymers and particularly for other rigid polyimides [48,49]. Table 6 also shows apparent gas solubility coefficients, S, for DPt based polyimide membranes determined at 2 atm and 35 °C. These data

Fig. 7. X-ray diffraction patterns of DPt based polyimides.

Å and 5.98 Å for DPt-TMPD and DPt-MBDAM, respectively, and 5.94 Å and 5.21 Å for DPt-BAPHF. The maximum at ∼6 Å is attributed to loosely packed polymer chains and it is usually found in polymers which maintaining their conformation forming microporous structures [37], while the maximum corresponding to a d-spacing around 5 Å is assigned to chain distances for polymeric chains that are more efficiently packed [17,38]. The d-spacing values follow the order DPtTMPD > DPt-MBDAM > DPt-BAPHF. The larger d-spacing value for the DPt-TMPD and DPt-MBDAM was attributed to the ortho substitution in the phenyl ring of the base diamine used for polyimides synthesis which would inhibit chain packing. FFV values also follow the order of d-spacing maxima since an increasing d-spacing corresponds to a higher FFV, see Table 2. 4.2. Gas transport properties Gas transport properties for DPt based polyimides were measured for pure gases (CO2, H2, He, O2, CH4 and N2) at 2 atm and 35 °C. Gas permeability coefficients and ideal selectivity of the three polyimide membranes are presented in Table 5. The measured gas permeability coefficients decrease in the following order P(CO2) > P(H2) > P (He) > P(O2) > P(CH4) > P(N2). These polyimides do not follow the order of the reported gas kinetic diameters He (2.6 Å), H2 (2.8 Å), CO2 (3.30 Å), O2 (3.46 Å), N2 (3.64 Å), CH4 (3.80 Å). Furthermore, they show higher permeability for CH4 than for N2. They also show higher permeability for CO2, than for He and H2. This behavior is observed usually in membranes prepared from polymers of intrinsic microporosity [39,40] and some highly rigid polyimides [12,41]. This result is usually attributed to an increase in FFV that favors the permeability of CH4 and CO2, due to the higher solubility coefficient of these gases [41]. As show in Table 5, DPt-TMPD membrane shows higher CO2

Table 5 Gas permeability coefficients and Ideal selectivity at 2 atm and 35 °C for DPt based polyimides. Polymer

DPt-TMPD DPt-MBDAM DPt-BAPFH DPPT-TMPDa[17] Matrimid®[42] PIM-PI-9 [43] PIM-PI-11[43] KAUST-PI-1[44] KAUST-PI-2[44]

Permeability (Barrer)

Selectivity (αA / B )

PHe

P H2

PO2

P N2

PCH4

PCO2

αO2 / N2

αCO2 / CH4

αC02 / N2

490 285 177 390 – 400 332 1771 1026

1007 542 237 – 23.7 840 624 3983 2368

348 158 56 280 1.7 295 208 627 490

111 52 16 73 0.25 94 65 107 98

205 86 21 108 0.19 170 129 105 101

2035 932 320 1600 6.5 2180 1523 2389 2071

3.1 3.0 3.6 3.8 6.8 3.1 3.2 5.9 5

9.9 10.8 15 14.8 34.2 12.8 11.8 23 21

18.2 17.9 20 21.9 26 23.2 23.4 33 21

1 Barrer= 10−10 cm3(STP) cm cm−2 s−1 cmHg−1. a Measured at 3 atm and 30 °C. 139

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lead to permeability-selectivity performance moving in a parallel to the Robeson upper bound following the trade-off. Also is can be observed that the 2 polyimides DPt-TMPD and DPt-MBDAM are quite permeable and present gas selectivity values that are quite similar to the ones reported for first generation PIM-PI's. A combination of high gas permeability coefficients and moderate selectivity for the DPt based polyimides is due to the effect of the tert-butyl groups present in these polyimides.

Table 6 Apparent gas diffusion and solubility coefficients for DPt based polyimides. Polyimide

DPt-TMPD DPt-MBDAM DPt-BAPFH

Diffusion

Solubility

DO2

D N2

DCH4

DCO2

SO2

S N2

SCH4

SCO2

73.3 38.5 15.1

27.2 13.2 4.7

13.2 5.5 1.6

59.6 29.6 11.1

4.7 4.1 3.7

4.1 4.0 3.4

15.6 15.7 13.3

34.1 31.5 28.8

Diffusion coefficients (D × 10−8) cm2 s−1. Solubility coefficients (S × 10−2) cm3 (STP) cm3 cm Hg.

5. Conclusion A new dianhydride monomer 3,8-di(4-tertbutylphenyl)pyrene1,2,6,7-tetracarboxylic dianhydride containing symmetrical tert-butyl groups was synthesized with high purity. The dianhydride monomer was used to prepare three different polyimides. Membranes prepared from these polymers showed high mechanical strength, thermal stability and glass transition temperature. All polyimides exhibited large gas permeability coefficients, which are attributed to the presence of a large FFV due to an inhibited chain packing produced by the presence of the rigid pendant bulky tert-butyl groups. On the other hand, the significant increase in gas permeability found in the rigid polyimides reported here presents the usual trade-off in gas selectivity following closely the Robeson´s upper bond. DPt-TMPD twice the gas permeability of DPtMBDAM, and six to nine times the one found for DPt-BAPHF for five gases tested (H2, N2, O2, CH4, CO2). This trend was attributed to the ortho-methyl substitution of the TMPD and MBDAM diamines which hinders the chain rotation and increases FFV. Acknowledgments

Fig. 8. CO2 permeability versus selectivity for CO2/CH4.

Rita Sulub-Sulub gratefully acknowledges a scholarship from CONACYT (Mexico's National Council for Science and Technology) under grant 389245. Partial funding by CONACYT-SENER LENERSE II grant 254667. The authors acknowledge the help of Dr. Griselda Castruita for XRD analysis. We also grateful to Dr. Patricia Quintana Owen and Dr. Emmanuel Hernández for 1H-NMR analysis form The National Laboratory of Nano and Biomaterials (LANNBIO). Partial funding from grants FOMIX-Yucatán 2008-108160, CONACYT LAB2009-01-123913 is acknowledged. References [1] S.K. Sen, S. Banerjee, Spiro-biindane containing fluorinated poly(ether imide)s: synthesis, characterization and gas separation properties, J. Membr. Sci. 365 (2010) 329–340. [2] L. Wang, Y. Cao, M. Zhou, S.J. Zhou, Q. Yuan, Novel copolyimide membranes for gas separation, J. Membr. Sci. 305 (2007) 338–346. [3] S. Luo, Q. Liu, B. Zhang, J.R. Wiegand, B.D. Freeman, R. Guo, Pentiptycene-based polyimides with hierarchically controlled molecular cavity architecture for efficient membrane gas separation, J. Membr. Sci. 480 (2015) 20–30. [4] L.M. Robeson, Correlation of separation factor versus permeability for polymeric membranes, J. Membr. Sci. 62 (1991) 165–185. [5] L.M. Robeson, The upper bound revisited, J. Membr. Sci. 320 (2008) 390–400. [6] Y. Liu, Y. Zhang, Q. Lan, S. Liu, Z. Qin, L. Chen, C. Zhao, Z. Chi, J. Xu, J. Economy, High-performance functional polyimides containing rigid nonplanar conjugated triphenylethylene moieties, Chem. Mater. 24 (2012) 1212–1222. [7] D.-J. Liaw, F.-C. Chang, M.-K. Leung, M.-Y. Chou, K. Muellen, High thermal stability and rigid rod of novel organosoluble polyimides and polyamides based on bulky and noncoplanar naphthalene−biphenyldiamine, Macromolecules 38 (2005) 4024–4029. [8] D.F. Sanders, Z.P. Smith, R. Guo, L.M. Robeson, J.E. McGrath, D.R. Paul, B.D. Freeman, Energy-efficient polymeric gas separation membranes for a sustainable future: a review, Polymer 54 (2013) 4729–4761. [9] D. Bera, V. Padmanabhan, S. Banerjee, Highly gas permeable polyamides based on substituted triphenylamine, Macromolecules 48 (2015) 4541–4554. [10] L.S. White, T.A. Blinka, H.A. Kloczewski, If Wang, Properties of a polyimide gas separation membrane in natural gas streams, J. Membr. Sci. 103 (1995) 73–82. [11] C. Zhang, P. Li, B. Cao, Effects of the side groups of the spirobichroman-based diamines on the chain packing and gas separation properties of the polyimides, J. Membr. Sci. 530 (2017) 176–184. [12] M. Calle, A.E. Lozano, J. de Abajo, J.G. de la Campa, C. Álvarez, Design of gas separation membranes derived of rigid aromatic polyimides. 1. Polymers from diamines containing di-tert-butyl side groups, J. Membr. Sci. 365 (2010) 145–153.

Fig. 9. O2 permeability versus O2/N2 selectivity.

show that the apparent solubility coefficient decreases in the following order: SCO2 > SCH4 > SO2 > S N2 . It can be observed that the solubility coefficients for these polymers are much higher than those usually found in glassy polymers reported in the literature [43,48,50]. Budd et al. assigned this behavior, in part, to a microporous character of these type of polymers which provides a higher gas uptake capacity [50]. For visualizing the performance of the synthesized polyimides, a comparison was carried out with Matrimid® [42] and PIM-PI's that present high gas permeability coefficients [43,44,47]. Figs. 8 and 9, show the correlation between permeability and selectivity for the CO2/ CH4 and O2/N2 gas pairs in a Robeson upper bound plot [5]. From these plots it can be observed that for the pairs CO2/CH4 and O2/N2, the polyimides based on the DPt dianhydride and the different diamines 140

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