Journal of Membrane Science

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1 Journal of Membrane Science 365 (20) Contents lists available at ScienceDirect Journal of Membrane Science journal homepage: Design of gas separation membranes derived of rigid aromatic polyimides. 1. Polymers from diamines containing di-tert-butyl side groups Mariola Calle, Angel E. Lozano, Javier de Abajo, José G. de la Campa, Cristina Álvarez Instituto de Ciencia y Tecnología de Polímeros, Consejo Superior de Investigaciones Científicas (CSIC), Juan de la Cierva, 3, Madrid, Spain article info abstract Article history: Received May 20 Received in revised form 26 August 20 Accepted 28 August 20 Keywords: Polyimides Fractional free volume Rigidity Gas permeability X-ray diffraction A series of polyimides has been obtained from the experimental diamine 1,4-bis(4-aminophenoxy)2,5- di-tert-butylbenzene (TBAPB) and three commercial dianhydrides, i.e. pyromellitic dianhydride (), 3,3,4,4 -byphenyltetracarboxylic dyanhidride (BPDA) and 4,4 -hexafluoroisopropyliden diphthalic anhydride (6FDA), following classical polyimidation methods. Analogous polyimides with diamines 2,2-bis(4-aminophenyl)hexafluoropropane (6FpDA) and 1,4-bis(4-aminophenoxy)benzene (APB) have been also prepared for comparative purposes. All polyimides showed high thermal stability, with decomposition temperatures above 490 C, and glass transition temperatures higher than 270 C. The rigid, rod-like structure of, combined with bulky diamines TBAPB and 6FpDA, yielded polyimides with high fractional free volume (FFV), close to that of 6FDA 6FpDA. The high FFV of TBAPB contrasts with its structure that has fairly large degree of short range order as evidenced by X-ray diffraction. The gas permeation properties were greatly dependent on the dianhydride moiety and, as a rule, they compared well with those of polyimides used for gas separation. The polymers from, TBAPB and 6FpDA, showed the best gas productivity values. By means of molecular modelling calculations, it has been observed that monomer TBAPB preferably adopts a contorted, rotation-restricted conformation, what can explain the special characteristics observed in the reported polyimides. 20 Elsevier B.V. All rights reserved. 1. Introduction ver the past decades, investigation on polymer membranes for gas separation has focused on aromatic, glassy polymers, and particularly on rigid, high glass transition temperature (T g ) polymers, such as polyimides [1,2]. Aromatic polyimides have achieved growing importance as high performance materials, thanks to their excellent balance of mechanical and thermal properties; however, despite their undeniable favourable characteristics they did not deserve special attention as membrane-forming materials until the boost of thermoplastic and soluble polyimides, which took place in the eighties of the last century [3,4]. In this regard, commercial and experimental dianhydrides and diamines of the most varied chemical structures have been used as condensation monomers to attain novel soluble aromatic polyimides [5 8]. In this time, many soluble, processable polyimides have been reported and evaluated as materials in applications related with advanced technologies, but most of them have remained as experimental polymers, and only a very few have achieved technical significance [4,9,]. Corresponding author. Tel.: address: cristina.alvarez@ictp.csic.es (C. Álvarez). Regarding gas permeability, penetration of small molecules is severely restricted through polymer matrixes with strong interchain attraction forces and high degree of molecular packing, as it is the case of wholly aromatic classical polyimides. This is especially true for commercial polyimides as Kapton and Upilex. The permeability of these materials to gases is rather low in comparison with other glassy polymers such as polycarbonates or polysulfones [11 13]. Much more favourable for this application are technical, soluble polyimides, such as Matrimid and Ultem, particularly Matrimid, which offers permeability to oxygen about 2 barrers, and 2 /N 2 selectivity above 6 [14,15]. Thus, much of the research work is being addressed to investigate new polyimides that exhibit higher permeability to specific gases without greatly impairing their inherent good selectivity. The challenge at this respect is to manipulate their chemical structure to achieve the most favourable balance of transport properties. The majority of the chemical modifications attempted have been directed to enhance diffusivity through an increment of the fractional free volume (FFV). In this regard, fluorine-containing polyimides, mainly those having the group hexafluoroisopropylidene, have reached special importance as they provide a favourable balance of permeability and selectivity [16,17]. The most orthodox way to improve the gas separation properties of a polymer material is to increase FFV by introducing /$ see front matter 20 Elsevier B.V. All rights reserved. doi:.16/j.memsci

2 146 M. Calle et al. / Journal of Membrane Science 365 (20) ,4-Bis(4-aminophenoxy)2,5-di-tert-butylbenzene (TBAPB) was synthesized in two steps, according to the previously reported method [22], from 2,5-di-tert-butylhydroquinone (Aldrich, 99%) and p-chloronitrobenzene (Aldrich, 99%) via nucleophilic aromatic substitution in the presence of potassium carbonate (K 2 C 3 ) (Panreac) and DMF (Aldrich, 99.5%) as solvent, followed by catalytic reduction with hydrazine hydrate and Pd/C as catalyst. Anhydrous N-methyl-2-pyrrolidinone (NMP) and N,Ndimethylformamide (DMF) were purchased from Aldrich and used as received Polymer synthesis and membrane preparation Fig. 1. Molecular modelling of 1,4-bis(4-aminophenoxy)2,5-di-tert-butylbenzene TBAPB. bulky groups and to increase at the same time the rigidity of the main chain, because the use of structures with high rigidity results in a strong size sieving ability. Thus, the approach followed in this work has consisted of using diamines with bulky side groups conveniently placed to produce an increase in both FFV and rigidity, improving the gas permeation properties. According to this premise, the diamine 1,4-bis(4-aminophenoxy) 2,5-di-tertbutylbenzene (TBAPB) should be a good candidate to develop membranes with both high permeability and selectivity. The presence of tert-butyl groups in ortho positions in the central ring should bring about both effects. The bulky and contorted conformation of diamine TBAPB is shown in Fig. 1. This work should provide new relationships to design and create new structures with tailor-made properties, decreasing the gap between classical materials and new ones, as are, for instance, the novel polymers with intrinsic microporosity (PIMs), ladder-like polymers with spiranic moieties and excellent gas separation properties [18 21]. Taking into consideration these antecedents, TBAPB has been made to react with various commercially available aromatic dianhydrides to yield a series of polyimides having improved gas permeability and correct permselectivity. The election of dianhydride monomers has been done to achieve this goal at a maximal extent, without loss of the positive characteristics of polyimides, particularly without significantly lowering transition temperatures and mechanical properties. These polyimides have been reported by Liaw and Liaw [22] and Yang and Hsiao [23], but they showed only the effect of the diamine on mechanical and optical properties. Herein we examine the effect of both tert-butyl groups and dianhydride on the gas permeation, diffusion and separation performance. A comparison is also made of analogous polyimides derived from diamine 2,2-bis(4-aminophenyl)hexafluoropropane (6FpDA). Furthermore, a theoretical study has been carried out by molecular modelling to calculate properties directly related to permeability, such as torsional mobility and rotational energy barriers. 2. Experimental 2.1. Materials Pyromellitic dianhydride (, from Merck), 3,3,4,4 - byphenyltetracarboxylic dyanhidride (BPDA, from Chriskev), 4,4 -hexafluoroisopropyliden diphthalic anhydride (6FDA, from Chriskev) and 2,2-bis(4-aminophenyl)hexafluoropropane (6FpDA, from Chriskev) were sublimated just before use. 1,4-Bis(4- aminophenoxy)benzene (APB, from Criskev) was purified by recrystallization from ethanol. A series of polyimides was synthesized from TBAPB diamine and three dianhydrides via a conventional two-step procedure, which involves the preparation of a soluble polyamic acid precursor in a first step and the obtaining of polyimide by an imidization reaction through a heat-promoted or chemically promoted cyclodehydration process in the second step. For comparative purposes, two analogous polyimides derived from dianhydride and 6FpDA or APB diamines and a polyimide based on 6FDA dianhydride and 6FpDA diamine were also prepared by the same procedure. The chemical structure of the polyimides studied is given in Fig. 2. The following general route was used for the synthesis of poly(amic acid)s: a three-necked flask, equipped with a mechanical stirrer under nitrogen atmosphere, was charged with 3.0 mmol of diamine and 3.0 ml of NMP. The mixture was stirred at room temperature until all solid was dissolved. Then, the solution was cooled to 0 C and the dianhydride (3.0 mmol) was added followed by 3.0 ml of NMP. The reaction mixture was stirred for 15 min at 0 C and then the temperature was raised up to room temperature and left overnight. The poly(amic acid) solution obtained was diluted with NMP and filtered through a 5.0 m PTFE Whatman filter, spread on a glass plate and left at 80 C overnight to remove most of the solvent. The polyamic acid film was imidized by sequential heating under N 2 atmosphere at 0, 150, 200 and 250 C, and subsequently under vacuum at 300 C, for 1 h each, respectively. Films were m thick. Chemical cyclodehydration of 6FDA TBAPB and 6FDA 6FpDA poly(amic acid)s was also carried out. The imidization was accomplished by adding acetic anhydride (25 mmol) and pyridine (25 mmol) to the aforementioned poly(amic acid) solution and stirring at room temperature for 6 h, followed by heating for a further hour at 60 C. When cooled to room temperature, the polymer solution was poured dropwise into 500 ml of water, washed several times with water and extracted in a Soxhlet with ethanol to remove traces of solvent and oligomers. The polymer was dried overnight under vacuum at 120 C. The casting of the polyimide was done from a 7% (w/v) filtered solution in NMP, onto a glass plate, and the sequential following thermal treatment was applied to get polyimide film: 80 C/overnight/air; 180 C/24 h/n 2 ; heating to 300 C under vacuum Thermal and physical characterization Fourier transform infrared (FTIR) measurements were performed with a PerkinElmer RX-1 spectrometer. The scan range was from 4000 to 600 cm 1. A TA Instruments Q500 Thermogravimetric Analyzer employing the Hi-Res TM option [24] was used to determine the onset temperature of weight loss, T d, and the char yield at 900 C. Hi-Res scans were run at 50 Cmin 1 with resolution and sensitivity parameters of 4.0 and 1.0, respectively, in the temperature range of C. The purge gas was nitrogen (60 ml/min) and the sample mass was about mg.

3 M. Calle et al. / Journal of Membrane Science 365 (20) R N Ar N F 3 C CF 3 Ar: BPDA 6FDA C(CF 3 ) 2 R: 6FpDA APB TBAPB Fig. 2. Chemical structure of dianhydrides and diamines. Differential scanning calorimetry (DSC) measurements were carried out using a TA instruments DSC Q2000 system. The experiments were conducted at a constant heating rate of 20 Cmin 1 under nitrogen atmosphere (50 ml min 1 ). Weight-average molecular weight (M w ), number-average molecular weight (M n ) and polydispersity index (M w /M n ) were determined by size exclusion chromatography (SEC) with PerkinElmer Series 200 pump, oven and UV vis detector. Wavelength was 271 nm, oven temperature was 70 C, and solvent flow was 0.3 ml/min. Polyethyleneglycol standards were used as references. The samples were prepared by dissolving 1 mg of polymer in 1 ml of N,N-dimethylformamide. The software TotalChrom from PerkinElmer was used for data acquisition and analysis. Two Polymer Labs Resipore columns (3 m, 300 mm 4.6 mm, molecular weights range from 200 to 400,000) were used. Eluent was N,Ndimethylformamide containing 0.1% (w/v) LiBr. Polymer density () was measured on films by a buoyancy method in a solution of calcium nitrate in water at 25 ± 1 C (Table 1). The density of a water solution was increased by adding saturated calcium nitrate solution until it equalled that of the sample and then it was measured by pycnometry. The fractional free volume (FFV) was calculated from the density data using the following relation [25]: FFV = V 1.3V w V (1) where V (=1/) is the polymer specific volume and V w is the van der Waals volume, that was estimated using the Hyperchem computer program, version 7.0 [26]. This program considers the dependence of van der Waals volume on chemical environment and it can estimate V w for the structural unit of a polymer chain. Wide-angle X-ray diffraction (WAXS) patterns were recorded in the reflection mode at room temperature by using a Bruker D8 Advance diffractometer provided with a Goebel mirror and a PSD Vantec detector. Cu K (wavelength = 1.54 Å) radiation was used. A step-scanning mode was employed for the detector, with a 2 step of and 0.5 s per step. The average d-spacing was determined based on the Bragg s law: n = 2d sin (2) where d is the d-spacing, the scattering angle and n is an integer number (1, 2, 3,...) related to the Bragg order Permeability measurements A barometric permeation method was used to determine steady state pure gas permeability at 30 C. The downstream pressure was kept below 2 mbar, while the upstream pressure was maintained, for all gases, at 3 bar. For the permeation experiments, oxygen, nitrogen, methane and carbon dioxide were used. The purity of the gases was greater than 99.5% for CH 4 and 2 and Table 1 Thermal and physical properties. Abbreviation T g a ( C) Char yield (%) T d ( C) (g cm 3 ) V w (cm 3 mol 1 ) FFV APB b FpDA b TBAPB b BPDA TBAPB FDA TBAPB FDA TBAPB (C) FDA 6FpDA (C) a Middle point of the endothermic step during the second scan of DSC measurements. b Polyimides derived from dianhydride were annealed at a temperature C above T g, after imidization protocol.

4 148 M. Calle et al. / Journal of Membrane Science 365 (20) Transmittance (a.u.) -APB -6FpDA -TBAPB BPDA-TBAPB 6FDA-TBAPB 6FDA-TBAPB (C) 6FDA-6FpDA (C) Wavenumber (cm -1 ) Fig. 3. FTIR spectra of the polyimide films. greater than 99.99% for the other ones. The absence of pinholes was checked by helium permeation at three upstream pressures. Under conditions of steady state of permeation, the permeability coefficient, P, was determined from the slope of the downstream pressure versus time plot (dp(t)/dt) according to the following equation: P = Vl dp(t) ATp 0 dt where A and l are respectively the effective area and the thickness of the film. T is the temperature of the measurement in K and p 0 is the pressure of the feed gas in the upstream chamber. P is usually expressed in barrers [1 Barrer = (cm 3 (STP) cm)/(cm 2 s cm Hg)]. The diffusion coefficient, D, was obtained from the intercept with the time axis using the relation: D = l 2 /(6), where l is the film thickness and is the lag time. The ideal permselectivity, A/B, and diffusion selectivity of the membrane for a pair of gases A and B, are defined as the ratio of the individual gas permeability coefficients and diffusion coefficients, respectively. 3. Results and discussion As mentioned above, all the polyimides derived from diamines APB and TBAPB and also polyimide 6FpDA, were synthesized by a conventional two-stage procedure involving the formation of poly(amic acid) followed by thermal imidization. FTIR measurements were carried out to check the progress of the imidization. Fig. 3 shows the FTIR spectra of the polyimide films. The presence of characteristic imide group absorptions in the regions 1778 and 1720 (C asym. and sym. stretching, respectively), (C N stretching) and cm 1 (imide ring deformation) and the absence of the C band of poly(amic acid) around 1680 cm 1 indicated that polyimides were well imidized. n the other hand, as 6FDA TBAPB was soluble, it was possible to obtain also the polyimide by chemical imidization in order to examine the effect of the imidization method on gas permeabilities. Additionally, for a comparative purpose, 6FDA 6FpDA was also prepared by chemical imidization of its poly(amic acid). The polymers chemically imidized are marked with (C) in Table Thermal and physical properties Some relevant thermal and physical properties of the membranes are summarized in Table 1. All polymers showed high thermal stability in inert atmosphere, with degradation temperature above 490 C. Polyimides containing the TBAPB diamine presented a slightly lower thermal stability and (3) Heat flow (W g -1 ) 200 Endotherm -APB -6FpDA -TBAPB BPDA-TBAPB 6FDA-TBAPB 6FDA-TBAPB (C) 6FDA-6FpDA (C) T (ºC) 0.05 w g w g Fig. 4. DSC curves of the polyimide films, at 20 C min 1, corresponding to the second heating scan. lower char yield than those derived from other diamines. This fact can be related to the loss of tert-butyl moieties that starts weight loss before the generalized degradation of polymer backbone. Glass transition temperature, T g, was strongly affected by chemical structure, as shown in Fig. 4. For TBAPB diamine, the following trend of T g s was found: > BPDA > 6FDA. The planar and rigid structure of dianhydride yielded polyimides with the highest T g, whilst the non-coplanar and semi flexible 6FDA dianhydride gave polyimides with the lowest T g. n the other hand, the polyimides containing 6FpDA showed higher T g than those derived from TBAPB diamine. Despite these observations, it was difficult to recognize trends as the chain rigidity is not the only factor affecting T g. In fact, when comparing the T g s of 6FDA TBAPB obtained from both imidization methods, a higher T g was observed for the chemically imidized polyimide by about 12 C. By GPC measurements, it was also verified that chemical and thermal imidization yielded polyimides with very different molecular weight. The weightaverage molecular weight and polydispersity index found were 15,500 g mol 1 and 3.0 for polyimide prepared by thermal imidization and 55,500 g mol 1 and 2.2 for the chemically imidized. Thus, it appears that during the thermal cyclization a decrease of Mw happens. An analogous effect of the imidization method on molecular weight has been reported by Matsumoto and Xu [27] for a similar polyimide. Densities and fractional free volume (FFV) values of the membranes are also listed in Table 1. The FFV fall within the experimentally reported values for polyimides, which are usually in the range [28], with the exception of APB due to its semicrystalline structure [29]. For polyimides containing the diamine TBAPB, the bulky [C(CF 3 ) 2 ] group in the dianhydride 6FDA makes the chain packing less efficient, and consequently, the 6FDAbased polyimides obtained showed the highest FFV. Surprisingly, the two-dimensional flat structure of dianhydride yielded polyimides with higher FFV than BPDA based polyimides and closer to that of 6FDA polymers. To our knowledge, only very few based polyimides have similar FFV [28].

5 M. Calle et al. / Journal of Membrane Science 365 (20) Dianhydride Intensity 6FDA Θ Fig. 5. WAXS diffractograms (vertically shifted), as a function of 2, of the polyimides containing 6FpDA diamine. The trend in FFV is indicated by the arrow Structural characterization Figs. 5 and 6 compare the X-ray diffraction profiles, at room temperature, of structurally related polyimides. A broad amorphous halo was observed for the polyimides containing 6FpDA diamine (Fig. 5). The amorphous nature of these polyimides can be attributed in part to the presence of the bulky [C(CF 3 ) 2 ] group in the diamine, which leads to a loosely packed structure. It is known that the position of the halo maximum can be considered as an indicator Intensity -APB Θ Dianhydride 6FDA (C) 6FDA BPDA Fig. 6. WAXS diffractograms (vertically shifted), as a function of 2, of the polyimides containing diamines TBAPB and APB. The trend in FFV is indicated by the arrow FFV - FFV Fig. 7. Molecular modelling of the structure of polymer TBAPB. of the most probable intersegmental distance between the chains, as calculated from Bragg equation (Eq. (2)). From Fig. 5, it can be seen that the dianhydride structure scarcely changes this distance ( 6 Å), even if the patterns differ in both shape and width of the halo. n the one hand, the WAXS pattern of 6FpDA shows a clear shoulder on the high-angle side. The position of this shoulder corresponds to distances near 3.8 Å and it could be assigned to stacking of imide and phenyl rings in ordered domains [30]. n the other hand, the marked asymmetry on the low-angle side of the 6FDA 6FpDA halo indicates a considerable contribution of larger intersegmental distances to the global scattering pattern. Because d-spacing can be used as index of openness of the polymer matrix, both facts could be related to the higher FFV value found for 6FDA 6FpDA. Fig. 6 shows the X-ray diffraction patterns for all polyimides containing the TBAPB diamine, including 6FDA TBAPB (C), as well as that of APB, which shows a semicrystalline pattern, as reported elsewhere [29]. The low FFV of this polyimide is in agreement with the value of crystallinity of 37%. The percentage of crystallinity of APB was estimated using the X-ray diffraction profile of an amorphous polyimide made from and 4,4 -oxydianiline (DA) in our laboratory. TBAPB polyimides were found to be amorphous polymers, probably because the tert-butyl groups in the diamine prevented efficient packing or crystallization of the polymer chains. The position of the amorphous halo maximum at 14.7 (2) for 6FDA TBAPB corresponds to a preferential intersegmental distance of 6.0 Å, similar to the distance observed in the analogous polyimide derived from diamine 6FpDA. Both polyimides have the highest FFV, as seen in Table 1. Polyimide BPDA TBAPB shows two maxima at 17.6 and 12.5 (2) that correspond to 5.0 and 7.1 Å, respectively, indicating some regularity in the packing, while the pattern of TBAPB presents some weak and broad crystalline reflections on the amorphous halo. Therefore, it seems clear that the introduction of tert-butyl groups in the diamine markedly reduced the crystallinity as compared with APB. Surprisingly, polyimide TBAPB, with a fairly large degree of short-range order, has higher FFV than BPDA TBAPB and similar to that of amorphous 6FpDA. Moreover, the pattern of TBAPB shows an additional low angle peak at 5.1 (2) corresponding to a d-spacing of 17.3 Å, which has to be due to a repeat distance along the molecular chain. This distance correlates quite well with the length of the rigid moiety containing pyromellitic unit in the simulated structure of the model compound depicted in Fig. 7 (17.7 Å). The length of the moiety was calculated by the program Materials Studio 5.0 [31], using the CMPASS force field [32,33]. Therefore, X-ray data indicate that TBAPB has a high molecular order along the chain molecular axis, as exemplified in Fig. 7, but there is no order in the lateral packing of the chains.

6 150 M. Calle et al. / Journal of Membrane Science 365 (20) n comparing the patterns of 6FDA TBAPB, obtained from different imidization methods, it could be observed that the chemically imidized polyimide showed an amorphous halo at 14.8 (2), similar to that of the thermally imidized one, and an additional peak at 4.8 (2) that corresponds to a distance of 18.4 Å. This distance, by similarity with that found in TBAPB, could be also assigned to an intramolecular distance, indicating that the chains take up more extended conformations under chemical imidization conditions. Moreover, it has been already reported by Ree et al. [34] that the orientation of the chains is strongly dependent on the imidization method and on other parameters such as imidization temperature, heating rate, substrate and film thickness. For highly aromatic polymers, like the polyimides studied here, phenylene motions tend to be the most significant contributors to the molecular chain mobility at a temperature far below T g [35].In diamine APB, the ether group acts as a hinge group, allowing for the rotation of phenyl rings. Thus, one might expect that the large steric effects of the two tert-butyl groups in diamine TBAPB should strongly restrain the rotations of phenyl rings. To quantitatively evaluate this effect, a theoretical study of the energy involved in the aromatic ring rotations for APB and TBAPB was performed [26]. The rotation barriers around the bond joining the central aromatic ring to the ether group for the two model compounds of APB and TBAPB are shown in Fig. 8. It can be seen that TBAPB has a maximum of about 18 kcal/mol when the dihedral angle (defined by atoms 1, 2, 3, and 4) is close to 180, while APB can rotate around the same bond from 0 to 360 much more easily, with maximum barrier energy of only 0.6 kcal/mol around 120. Therefore, the rotation of phenyl rings is practically free for APB while the bulky tert-butyl groups in TBAPB restrict the number of conformations to be adopted by the molecule. The broad and flat energy well in TBAPB indicates that there are still a large number of probable conformations. From the results discussed so far, it can be concluded that the presence of bulky tert-butyl groups in diamine TBAPB not only separates the polymer chains, which is consistent with the decrease of crystallinity, but also contributes to the local rigidity of the chain in the glassy state. As to the polyimides derived from, the morphology of the PI films depends mainly on the chemical structure of diamine moiety, in such a way that polyimides can be obtained of very varied order degree, from semicrystalline ( APB) to amorphous ( 6FpDA). In the case of TBAPB, the rod-like structure favours that chains preferentially adopt extended conformations, similar to those shown in Fig. 7, leading to a fairly higher FFV than would be expected for a structure with certain degree of short-range order. This result is not surprising since we had observed earlier the ability of diamine TBAPB to give aromatic polyamides with a significant order degree [36] Gas transport properties Tables 2 and 3 list the permeability coefficient and ideal permselectivity values, along with diffusion coefficients and diffusion ΔH (kcal mol -1 ) N R: tert-butyl 60 1 R R Dihedral angle (degrees) N R: H Fig. 8. Energy of rotational barriers versus dihedral angle of model compounds of APB and TBAPB, as determined by semiempirical methods. selectivities, for polyimide films measured at 3 bar and 30 C. Several interesting features can be observed in these tables. The effect of the imidization process on the behaviour of the membranes can be found if thermally and chemically imidized 6FDA TBAPB polymers are compared. Although the same thermal treatment up to 300 C was applied in both cases, permeability was significantly higher for the chemically imidized polymer, around 1.6 times higher for the more permeable gases, 2 and C 2, and 1.8 times higher for the less permeable ones, N 2 and CH 4, while the selectivity decreased. This effect, which has been previously reported for similar polymers [37], might be attributed to the formation of a less dense packed structure by chemical imidization [38]. The permeability has usually been related to FFV and, generally, high permeability and low selectivity result from high FFV. However, in this case, a mere difference of 1.5% in FFV between both polymers gives way to an increase of permeability above 55% for the chemically imidized whilst a decrease of a % in selectivity for 2 /N 2 and a 40% for C 2 /CH 4 is observed. The effect of the diamine can be seen if permeation data of APB, TBAPB and 6FpDA are compared. In the first two cases, where only the presence of two tert-butyl groups distinguishes the diamines, an extraordinary difference in perme- Table 2 Permeability coefficients a and permselectivities at 3 bar and 30 C. Abbreviation P He P 2 P N2 P C2 P CH4 2 /N 2 C2 /CH 4 APB b FpDA b TBAPB b BPDA TBAPB FDA TBAPB FDA TBAPB (C) FDA 6FpDA (C) a Permeabilities in barrers, where 1 barrer = cm 3 (STP) cm/cm 2 scmhg. b Polyimides derived from dianhydride were annealed at a temperature C above T g, after imidization protocol.

7 M. Calle et al. / Journal of Membrane Science 365 (20) Table 3 Diffusion coefficients a and diffusion selectivities at 3 bar and 30 C. Abbreviation D( 2) D(N 2) D(C 2) D(CH 4) D 2 /D N2 D C2 /D CH4 APB b FpDA b TBAPB b BPDA TBAPB FDA TBAPB FDA TBAPB (C) FDA 6FpDA (C) a D has units of 8 cm 2 s 1. b Polyimides derived from dianhydride were annealed at a temperature C above T g after imidization protocol. ability between both polyimides can be observed. It is caused by a significant increase in FFV, which rises from for APB to for TBAPB. The permeability ratios P (TBAPB) /P (APB) depend on the kinetic diameter of the gases, going from 16 for the smallest gas (He) to 59 for the biggest one (CH 4 ). However, the ratio for C 2 does not follow the order of kinetic diameters (He, 2.60 Å; C 2, 3.30 Å; 2, 3.46 Å; N 2, 3.64 Å; CH 4, 3.80 Å), as it is higher than that corresponding to 2 and very similar to that of N 2. This result cannot be solely explained by the diffusivity term and thus the effect of gas solubility has to be considered for C 2 and CH 4 [39]. A similar effect can be seen when diamines 6FpDA and TBAPB are compared. Although 6FpDA showed relatively high gas permeability, the values are still higher for TBAPB. In this case, the increase in permeability ratio P (TBAPB) /P (6FpDA), which ranged from 1.0 for He to 3.0 for CH 4, is not caused by an increase in FFV, which is practically the same in both polymers. Also in this case, the augment in permeability is not directly related to the increase in diffusion coefficient. In fact, as it can be seen when Tables 2 and 3 are compared, going from 6FpDA to TBAPB causes an improvement in D (TBAPB) /D (6FpDA), which goes from 2.16 for 2 to 3.83 for CH 4. However, the increase in the ratio P (TBAPB) /P (6FpDA) only goes from 1.38 for 2 to 3.01 for CH 4. Consequently, it seems that the solubility contribution is significantly higher in 6FpDA and counteracts in some way the increase in diffusion coefficient. The effect of the solubility coefficient is even more remarkable when the same diamines with dianhydride 6FDA are compared. In this case [polymers 6FDA TBAPB (C) and 6FDA 6FpDA (C)], diffusion coefficients are almost double for TBAPB (between 1.9 times higher for 2 and 2.3 for methane), in spite of its lower FFV. However, the permeability values do not follow the same tendency, and, except for the biggest gas (CH 4 ), they are higher for polymer 6FDA 6FpDA. That means that in this case, the solubility contribution is even able to reverse the order determined by the diffusion coefficients. The dianhydride effect on the permeability could be also established on comparing polyimides based on TBAPB diamine, which were all prepared by thermal imidization. The trend of permeability was > 6FDA > > BPDA, irrespective of the analyzed gas. Again, this trend for dianhydrides and 6FDA does not correlate with that displayed by their FFV in Table 1. Thus, the short rigid dianhydride seems to lead to a packing of polymer chains, where the chain separation caused by tert-butyl groups of the diamine is preserved. In this way, as to diffusion contribution, a comparison between TBAPB and 6FDA TBAPB shows that, in spite of the significantly higher FFV of the latter, the diffusion coefficients are higher (between 1.85 and 3.60 times higher) for the former one. In general, permeability coefficients decreased in the following order: He > C 2 > 2 >N 2 >CH 4, which is also the order of increasing kinetic diameter of gases. However, the TBAPB membrane showed higher permeability to CH 4 than to N 2 and, more surprisingly, permeability was higher to C 2 than to He. The first behaviour is typical of polymers with very high permeability and can be observed in the Robeson representation of (N 2 /CH 4 ) versus P(N 2 ) [40], where an inversion in selectivity appears when permeability increases. In vitreous polymers, this fact seems to be related to an increase in FFV that favours the diffusivity of CH 4 and, because of the higher solubility of this gas, it gives way to a permeability value above that of N 2. The second behaviour, P(C 2 )>P(He), can also be observed in the Robeson paper [40], but it is much less frequent and appears mainly in polymers with intrinsic microporosity [18,20,41] and it has also occasionally been observed in some polyimides [42]. Also in this case, a relative increase of diffusion of C 2, along with the higher solubility of this gas, gives way to an increase in permeability, which overcomes the value of P(He). Consequently, it seems that the behaviour of TBAPB is similar to that of polymers with very high FFV, even if this is not the case (the FFV of 6FDA TBAPB and 6FDA 6FpDA are higher), and it should be attributed again to the different distribution of free volume caused by the combination of a bulky and contorted diamine with a very rigid dianhydride, which hinders the packing of chains. The performance of these membranes for gas pairs 2 /N 2 and C 2 /CH 4 is shown graphically in Figs. 9 and respectively. The dashed and solid lines in the figures represent the Robeson s upper bound of 1991 and 2008, respectively. For the pair 2 /N 2, it can be observed that the change of dianhydride moiety leads to polymer performance moving in a parallel direction to the Robeson s upper bound, except for the semicrystalline APB that showed comparatively low performance in terms of permeability/permselectivity trade-off. For the pair C 2 /CH 4, the polymers derived from and diamines 6FpDA or TBAPB lay by far closer to the Robeson upper bound than the polymer from APB. Moreover, both 6FpDA and TBAPB exhibited good overall values but they did not α 2 /N Matrimid Diamine APB TBAPB 6FpDA 1 BPDA 6FDA 6FDA(C) 6FDA(C) Robeson P 2 (barrer) Robeson Fig. 9. Selectivity versus permeability with the Robeson s trade off curve for 2/N 2 [40]. Commercial polyimide Matrimid has been included [14].

8 152 M. Calle et al. / Journal of Membrane Science 365 (20) α C 2 /CH Matrimid Diamine APB TBAPB 6FpDA BPDA 6FDA(C) 6FDA 6FDA(C) 0 Robeson 1991 PC 2 (barrer) 00 Robeson Fig.. Selectivity versus permeability with the Robeson s trade off curve for C 2/CH 4 [40]. Commercial polyimide Matrimid has been included [14]. overcome the C 2 /CH 4 permselectivity offered by the fluorine-rich 6FDA 6FpDA. For comparative purposes, a commercial polyimide like Matrimid has been included [14]. For the pair 2 /N 2, its behaviour is similar to that observed for the polymers of this work, all them lying in a line parallel to the upper bound. However, for the pair C 2 /CH 4 the performance of Matrimid is very poor, if compared to the other polymers (the distance to the upper bound is higher in this case). 4. Conclusions By introducing a bulky group conveniently placed in an aromatic diamine it has been possible to increase in a synergistic way the rigidity of the molecular chain and the fractional free volume of polyimides. The reaction of the bulky, contorted diamine TBAPB with rigid dianhydrides and 6FDA yielded polyimides with very high FFV, close to that of 6FDA 6FpDA. The presence of tert-butyl groups in diamine TBAPB prevented the efficient packing of the polymer chains. Moreover, in TBAPB, the rod-like structure of dianhydride could not alleviate the separation effect produced by the tert-butyl groups. In fact, the dianhydride favoured that chains preferentially adopt extended conformations as confirmed by X-ray diffraction and modelling calculations. Polyimides with dianhydride and diamines TBAPB and 6FpDA displayed gas permeabilities comparable to those of 6FDA 6FpDA, but slightly lower permselectivities. The reason is not entirely clear but it could be related to a less favourable combination of distribution and value of FFV. From these results, it can be concluded that the approach of using diamines with bulky side groups on well-chosen positions permits to develop membranes with both high permeability and selectivity. Moreover, the cheap dianhydride combined with these diamines appears to be a promising alternative to the classical fluorine-containing polyimides. Acknowledgements The Financial support provided by CSIC (Project PIE I066) and Spanish Ministerio de Ciencia e Innovación (Project MAT and program Consolider, project MULTICAT) is gratefully acknowledged. References [1] Polymer membranes for gas and vapor separation, in: B.D. Freeman, I. Pinnau (Eds.), ACS Symp. Series 733, Washington, [2] Y. Yampolskii, I. Pinnau, B.D. Freeman (Eds.), Materials Science of Membranes for Gas and Vapor Separation, Wiley, New York, [3] K.L. Mittal (Ed.), Polyimides. Synthesis, Characterization and Applications, Plenum Press, New York, [4] J. de Abajo, J.G. de la Campa, Processable aromatic polyimides Adv. Polym. Sci., vol. 140, Springer-Verlag, Berlin, 1999, pp [5] S.C.M. Kwan, C. Wu, F. Li, E.P. Savitski, F.W. Harris, S.Z.D. Cheng, Laser light scattering studies of soluble high performance fluorine-containing polyimides. 1. Polyimide synthesized from 2,2 -bis(3,4-dicarboxyphenyl)-hexafluoropropane dianhydride and 2,2 -(trifluoromethyl)-4,4 -biphenyldiamine, Macromol. Chem. Phys. 11 (1997) [6] M.K. Ghosh, K.L. Mittal (Eds.), Polyimides: Fundamentals and Applications, Marcel Dekker, New York, [7] Z.Y. Yang, Synthesis of new fluorinated polyimides, J. Fluorine Chem. 111 (2001) [8] H. Chou, D. Sahadeva, C.F. Shu, Synthesis and characterization of spirobifluorene-based polyimides, J. Polym. Sci. A: Polym. Chem. 40 (2002) [9] T. Takekoshi, Polyimides, Adv. Polym. Sci. 94 (1990) [] K.L. Mittal (Ed.), Polyimides and ther High temperature Polymers: Synthesis, Characterization and Applications, VSP, Utrech, [11] A.K. Fritzsche, Gas separation by polysulfone hollow fiber membranes, Polym. News 13 (1988) [12] M. Aguilar-Vega, D.R. Paul, Gas transport properties of polycarbonates and polysulfones with aromatic substitutions on the bisphenol connector group, J. Polym. Sci. B: Polym. Phys. 31 (1993) [13] K.J. Lee, J.Y. Jho, Y.S. Kang, J. Won, Y. Dai, G.P. Robertson, M.D. Guiver, Gas transport and dynamic mechanical behavior in modified polysulfones with trimethylsilyl groups: effect of degree of substitution, J. Membr. Sci. 223 (2003) 1. [14] L. Shao, L. Liua, S.-X. Cheng, Y.-D. Huanga, J. Mac, Comparison of diamino cross-linking in different polyimide solutions and membranes by precipitation observation and gas transport, J. Membr. Sci. 312 (2008) [15] S. Sridhar, R.S. Veerapur, M.B. Patil, K.B. Gudasi, T.M. Aminabhavi, Matrimid polyimide membranes for the separation of carbon dioxide from methane, J. Appl. Polym. Sci. 6 (2007) [16] S.A. Stern, Polymers for gas separations: the next decade, J. Membr. Sci. 94 (1994) [17] M.R. Coleman, W.J. Koros, The transport properties of polyimide isomers containing hexafluoroisopropylidene in the diamine residue, J. Polym. Sci. B: Polym. Phys. 32 (1994) [18] P.M. Budd, K.J. Msayib, C.E. Tattershall, B.S. Ghanem, K.J. Reynolds, N.B. McKeown, D. Fritsch, Gas separation membranes from polymers of intrinsic microporosity, J. Membr. Sci. 251 (2005) [19] N.B. McKeown, P.M. Budd, K.J. Msayib, B.S. Ghanem, H.J. Kingston, C.E. Tattershall, S. Makhseed, K.J. Reynolds, D. Fritsch, Polymers of intrinsic microporosity (PIMs): bridging the void between microporous and polymeric materials, Chem. Eur. J. 11 (2005) [20] S. Thomas, I. Pinnau, N. Du, M.D. Guiver, Pure- and mixed-gas permeation properties of a microporous spirobisindane-based ladder polymer (PIM-1), J. Membr. Sci. 333 (2009) [21] N. Du, G.P. Robertson, I. Pinnau, S. Thomas, M.D. Guiver, Copolymers of intrinsic microporosity based on 2,2,3,3 -tetrahydroxy-1,1 -dinaphthyl, Macromol. Rapid Commun. 30 (2009) [22] D.-J. Liaw, B.-Y. Liaw, Synthesis and properties of polyimides derived from 1,4- bis(4-aminophenoxy) 2,5-di-tert-butylbenzene, J. Polym. Sci. A: Polym. Chem. 35 (1997) [23] C.-P. Yang, F.-Z. Hshiao, Synthesis and properties of fluorinated polyimides based on 1,4-bis(4-amino-2-trifluoromethylphenoxy)-2,5-ditertbutylbenzene and various aromatic dianhydrides, J. Polym. Sci. A: Polym. Chem. 42 (2004) [24] P.S. Gill, S.R. Sauerbrunn, B.S. Crowe, High resolution thermogravimetry, J. Therm. Anal. Calorim. 38 (1992) [25] W.M. Lee, Selection of barrier materials from molecular structure, Polym. Eng. Sci. 20 (1980) [26] Hyperchem, Computational Chemistry, versión 7.0, Hypercube Inc., Gainesville, FL, [27] K. Matsumoto, P. Xu, Gas permeation properties of hexafluoro aromatic polyimides, J. Appl. Polym. Sci. 47 (1993) [28] J.Y. Park, D.R. Paul, Correlation and prediction of gas permeability in glassy polymer membrane materials via a modified free volume based group contribution method, J. Membr. Sci. 125 (1997) [29] J. Liu, D. Kim, F.W. Harris, S.Z.D. Cheng, Crystal structure, morphology and phase transitions in aromatic polyimide oligomers. 2. Poly(1,4-phenyleneoxy-1,4-phenylene-oxy-1,4-phenylene pyromellitimide), Polymer 35 (1994) [30] J. Wakita, S. Jin, T.J. Shin, M. Ree, S. Ando, Analysis of molecular aggregation structures of fully aromatic and semialiphatic polyimide films with synchrotron grazing incidence wide-angle X-ray scattering, Macromolecules 43 (20) [31] Materials studio v , Accelrys Software Inc.

9 M. Calle et al. / Journal of Membrane Science 365 (20) [32] H. Sun, CMPASS: an ab initio forcefield optimize for condensed-phase applications overview with details on alkane and benzene compounds, J. Phys. Chem. B 2 (1998) [33] H. Sun, P. Ren, J.R. Fried, The CMPASS force field: parameterization and validation for phosphazenes, Comput. Theor. Polym. Sci. 8 (1998) [34] M. Ree, K. Kim, S.H. Woo, H. Chang, Structure, chain orientation, and properties in thin films of aromatic polyimides with various chain rigidities, J Appl. Phys. 81 (1997) [35] M. Kochi, C. Chen, R. Yokota, M. Hasegawa, P. Hergenrother, Isomeric biphenyl polyimides. II. Glass transitions and secondary relaxation processes, High Perfor. Polym. 17 (2005) [36] J. Espeso, E. Ferrero, J.G. de la Campa, A.E. Lozano, J. de Abajo, Synthesis and characterization of new soluble aromatic polyamides derived from 1,4-bis(4- carboxyphenoxy)-2,5-di-tert-butylbenzene, J. Polym. Sci. A: Polym. Chem. 39 (2001) [37] L. Shao, T.-S. Chung, G. Wensley, S.H. Goh, K.P. Pramoda, Casting solvent effects on morphologies, gas transport properties of a novel 6FDA/-TMMDA copolyimide membrane and its derived carbon membranes, J. Membr. Sci. 244 (2004) [38] W. Jang, D. Shin, S. Choi, S. Park, H. Han, Effects of internal linkage groups of fluorinated diamine on the optical and dielectric of polyimide thin films, Polymer 48 (2007) [39] K. Ghosal, B.D. Freeman, Gas separation using polymer membranes: an overview, Polym. Adv. Tech. 5 (1994) [40] L.M. Robeson, The upper bound revisited, J. Membr. Sci. 320 (2008) [41] K. Nagai, T. Masuda, T. Nakagawa, B.D. Freeman, I. Pinnau, Poly(1- trimethylsilyl)-1-propyne and related polymers: synthesis, properties and functions, Prog. Polym. Sci. 26 (2001) [42] J. de Abajo, J.G. de la Campa, A.E. Lozano, Designing aromatic polyamides and polyimides for gas separation membranes, Macromol. Symp. 199 (2003)