Patent Publication Number: US-2022219127-A1

Title: Poly(imide-oxadiazole) membranes for gas separation applications

Description:
TECHNICAL FIELD 
     The present disclosure is directed to polymers for gas separation membranes. More specifically, the polymers include polymers based on poly(imide-oxadiazole) for use in gas purification technologies. 
     BACKGROUND 
     Natural gas supplies 22% of the energy used worldwide, and makes up nearly a quarter of electricity generation. Further, natural gas is an important feedstock for the petrochemicals industry. According to the International Energy Agency (IEA), the worldwide consumption of natural gas is projected to increase from 120 trillion cubic feet (Tcf) in the year 2012 to 203 Tcf by the year 2040. 
     Raw, or unprocessed, natural gas is formed primarily of methane (CH 4 ), however it may include significant amounts of other components, including acid gases (carbon dioxide (CO 2 ) and hydrogen sulfide (H 2 S)), nitrogen, helium, water, mercaptans, and heavy hydrocarbons (C 3+ ), among other components. These contaminants must be removed during gas processing in order to meet the standard pipeline specifications of sales gas. In particular, the removal of acid gases (CO 2  and H 2 S) has been a significant research topic due to the problematic effects of acid gases on natural gas heating value, pipeline transportability, and pipeline corrosion in the presence of water. 
     Currently, the majority of gas processing plants remove CO 2  and H 2 S from natural gas by absorption technology, such as amine adsorption. However, several drawbacks are associated with this technology, including energy usage, capital cost, maintenance requirements, and the like. 
     SUMMARY 
     An embodiment described in examples herein provides a gas separation membrane. The gas separation membrane including a poly(imide-oxadiazole) polymer including an oligomer having a structure including: 
     
       
         
         
             
             
         
       
     
     In this structure, Ar 1 , Ar 2 , and Ar 3  include aromatic moieties, and Ar 1 , Ar 2 , and Ar 3  are each independently selected. 
     Another embodiment described in examples herein provides a method for forming a gas separation membrane. The method includes, obtaining a diamine oxadiazole monomer, obtaining an imide monomer, and reacting the diamine oxadiazole monomer with the imide monomer to form a polymer. The polymer is dissolved in a solvent to form a polymer solution. A dense film is formed from the polymer solution. The dense film is tried to form the gas separation membrane. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIGS. 1A and 1B  are reactions scheme for the preparation of symmetric and asymmetric diamine oxadiazole monomers. 
         FIG. 2  is a reaction scheme for the preparation of poly(imide-oxadiazole) homopolymers. 
         FIG. 3  is a reaction scheme for the preparation of random or block poly(imide-oxadiazole) copolymers. 
         FIG. 4  is a reaction scheme for the preparation of the diamine oxadiazole monomer 4,4′-(1,3,4-oxadiazole-2,5-diyl)dianiline (termed BAO). 
         FIG. 5  is a reaction scheme for the preparation of the 6FDA-based poly(imide-oxadiazole) homopolymer, 6FDA-BAO. 
         FIG. 6  is a reaction scheme for the preparation of a 6FDA-based poly(imide-oxadiazole) random copolymer, 6FDA-Durene/BAO at a 1:1 ratio of 6FDA-Durene to BAO. 
         FIG. 7  is a  1 H NMR spectrum of 4,4′-(1,3,4-oxadiazole-2,5-diyl)dianiline (BAO) in DMSO-d 6 . 
         FIG. 8  is a Fourier transform infrared (FTIR) spectrum of 4,4′-(1,3,4-oxadiazole-2,5-diyl)dianiline (BAO). 
         FIG. 9  is a  1 H NMR spectrum of 6FDA-BAO homopolymer in DMSO-d 6 . 
         FIG. 10  is a  1 H NMR spectrum of 6FDA-Durene/BAO (1:1) random copolymer in DMSO-d 6 . 
         FIG. 11A  is a plot of thermogravimetric analysis (TGA) curves of the prepared polymers. 
         FIG. 11B  is a plot of the first derivatives of the DSC curves (DTG) of the prepared polymers. 
         FIG. 12  is a simplified process flow diagram of a permeation apparatus used for measuring single gas and mixed gas permeation properties. 
         FIG. 13  is a plot of the measured values for CO 2 /CH 4  compared to permeability-selectivity trade off curves. 
         FIG. 14A  is a change in a sweet mixed-gas permeability of 6FDA-Durene/BAO (1:1) copolymer at different feed pressures and 22° C. 
         FIG. 14B  is the change in the sweet mixed-gas selectivity coefficients of 6FDA-Durene/BAO (1:1) copolymer at different feed pressures and 22° C. 
         FIG. 15A  is a change in a sweet mixed-gas permeability of 6FDA-Durene/BAO (3:1) copolymer at different feed pressures and 22° C. 
         FIG. 15B  is the change in the sweet mixed-gas selectivity coefficients of 6FDA-Durene/BAO (3:1) copolymer at different feed pressures and 22° C. 
         FIG. 16  is a method for synthesizing and using a polymer for forming a membrane to separate a gas mixture 
     
    
    
     DETAILED DESCRIPTION 
     As separation technologies advance, the use of polymeric membrane-based technology for gas separation has been increasingly explored over the past years due to the potential for energy efficiency, small footprint, and low capital cost. Although current membrane technology does not outperform absorption systems, hybrid systems using absorption and membranes have proved to be a potentially attractive alternative. For membrane systems, it is desirable to have polymeric membranes with improved separation performance. 
     Polymeric membranes are thin semipermeable barriers that selectively separate some gas compounds from others. Generally, polymeric membranes do not operate as a filter, where small molecules are separated from larger ones through a medium with pores, rather it separates based on how well different compounds dissolve into the membrane and diffuse through it, for example, using a solution-diffusion model. 
     Numerous polymeric membranes for gas separation have been developed in the decades, but few are currently commercialized for use in sour gas separation applications. Examples of polymeric materials used to form gas separation membranes include cellulose acetate (CA), polyimides (PI), and perfluoropolymers, such as polytetrafluoroethylene (PTFE), perfluorocycloalkene (PFCA), and the like. These polymeric materials are generally semi-crystalline polymers having a T g  of greater than about 100° C. 
     One of the main characteristics used to select and modify polymeric materials is the chemical structures. Various classes of polymers were studied for this purpose. For example, polyimides and polyoxadiazoles have been studied for potential application in gas separation, such as sour mixed-gas separation. A successful candidate would be able to form membranes with high CO 2  and H 2 S permeability coefficients with high to moderate CO 2 /CH 4  and H 2 S/CH 4  selectivity coefficients, while withstanding the harsh chemical, physical, and thermal conditions encountered during the purification of natural gas. 
     Embodiments described in examples herein provide a new hybrid material that combines the advantages of polyimides and polyoxadiazoles in one material, which is called poly(imide-oxadiazole). This material could be under the form of a homopolymer, random copolymer, or block copolymer. 
     The chemical structure of poly(imide-oxadiazole) contains two different types of rings to link the constituting monomers an imide ring and oxadiazole ring. The preparation the poly(imide-oxadiazole) generally includes two main steps. To begin, an oxadiazole ring is formed within a diamine monomer, and then a polycondensation reaction between the oxadiazole diamine monomer with a dianhydride monomer is used to form the imide ring in a later stage. 
       FIGS. 1A and 1B  are reactions scheme for the preparation of symmetric and asymmetric diamine oxadiazole monomers. The synthetic methodology allows the preparation of a large variety of new polymers, including but not limited to, homopolymers, random copolymers, block copolymers, terpolymers, and so on. The change can be implemented at every stage of the preparation of the polymer. For example, during the preparation of the oxadiazole diamine monomer, a variety of aminobenzoic acid derivatives could be used, as shown in  FIGS. 1A and 1B . 
     As shown in  FIGS. 1A and 1B , to form an diamine oxadiazole monomer, an aromatic structure having a carboxylic acid and an amine group, such as para-aminobenzoic acid, among many others, is reacted with hydrazine sulfate, N 2 H 4 .H 2 SO 4  at elevated temperatures, using polyphosphoric acid (PPA) as solvent. Two different aromatic structures may be used to form an asymmetric diamine oxadiazole monomer. For example, amino benzoic acid structures that may be used to form the diamine oxadiazole monomer in embodiments include: 
     
       
         
         
             
             
         
       
     
     among others. 
     Symmetric diamine oxadiazole monomer structures that may be formed in embodiments include: 
     
       
         
         
             
             
         
       
     
     among others. 
     Asymmetric structures may include any combinations of the benzoic acid precursors above, among many others. Examples of asymmetric structures include: 
     
       
         
         
             
             
         
       
     
     among others. 
     The symmetric (Ar 1 ═Ar 2 ) or the asymmetric (Ar 1 ≠Ar 2 ) diamine oxadiazole monomers disclosed in here could be combined with a variety of dianhydride monomers to form poly(imide-oxadiazole) homopolymers as shown in the general schematic in  FIG. 2 . 
       FIG. 2  is a reaction scheme for the preparation of poly(imide-oxadiazole) homopolymers. The reaction may be carried out in n-methyl pyrrolidone (NMP) at an elevated temperature (180° C.-200° C.) using LiCl as a catalyst. Other solvents may be used, such as m-cresol and dimethylacetamide (DMAc), and alternative catalysts system could be used, such as CaCl 2 ), MgCl 2 , and ZnCl 2 . Polymers in various embodiments can be formed from any combinations of imide monomers, including, for example: 
     
       
         
         
             
             
         
       
     
     among others. 
     Any number of polymers can be formed from combinations of these monomers, including, for example: 
     
       
         
         
             
             
         
       
     
     among many others. 
     Other changes could be employed through using the symmetric (Ar 1 ═Ar 2 ) or the asymmetric (Ar 1 ≠Ar 2 ) diamine oxadiazole monomers disclosed in here with a large combination of diamine and dianhydride co-monomers widely commercially available or that could be prepared, to form random or block copolymers with different molar ratios of the various co-monomers forming the copolymers. 
       FIG. 3  is a reaction scheme for the preparation of random or block poly(imide-oxadiazole) copolymers. In this example, an aromatic diamine is combined with the monomers described above to provide two different oligomeric structures further control over the properties. The aromatic diamine may include any of the following structures: 
     
       
         
         
             
             
         
       
     
     among many others 
     As described herein, the synthetic methodology allows the preparation of a large variety of new polymers, including but not limited to, homopolymers, random copolymers, block copolymers, terpolymers, and so on. The change can be implemented at every stage of the preparation of the polymer. For example, during the preparation of the oxadiazole diamine monomer, a variety of aminobenzoic acid derivatives could be used. The polymers allow the development of polymer membranes with high acid gas permeability coefficients against methane, while maintaining a good selectivity coefficient, which makes this kind of materials attractive for potential use in natural gas purification. 
     EXAMPLES 
     Materials 
     All materials listed in this work were used as received. Polyphosphoric acid (PPA, ≥83% phosphate as P 2 O 5  basis), and 4-aminobenzoic acid (reagent plus, ≥99%) were purchased from Sigma-Aldrich, USA. 4,4′-(hexa-fluoroisopropylidene)diphthalic anhydride (6FDA, 99.0%), and hydrazine sulfate (ACS reagent, ≥99.0%) were acquired from Alfa Aesar. 1-methyl-2-pyrrolidinone (NMP, biotech grade, ≥99.7%) was purchased from Honeywell. Lithium chloride (reagent grade) was obtained from Matheson Coleman &amp; Bell Manufacturing Chemists. 2,3,5,6-tetramethylbenzene-1,4-diamine (Durene; purity 98.0%) was obtained from TCI America. Methanol (98.0%) was purchased from Fisher Scientific and sodium hydroxide (≥98.0%) from Fluka. 
     Membrane Fabrication 
     The polymeric membranes studied in this work were prepared using the solution casting method. Solutions with concentration of 3 wt. % polymer in N,N-dimethylformamide (DMF) were prepared. Then, 11 mL of the solution were filtered through a 0.45 μm PTFE filter to remove any possible solid impurities and poured into a leveled 5.5 cm diameter flat glass Petri dish. The casting dish was placed in a preheated oven at 90° C., under a gentle flow of nitrogen to allow a slow evaporation of the solvent. After 24 hours, the obtained membrane was further dried at 180° C. under vacuum for another 24 hours. If needed, the membrane was peeled off from the Petri dish using deionized water and dried at 100° C. under vacuum for 12 hours. The thickness of the prepared membranes was determined in the range of 60-120 μm; noting that for an individual membrane, the standard deviation of thickness uniformity was less than 3%. 
     Characterization Techniques 
     Proton Nuclear Magnetic Resonance Spectroscopy ( 1 H-NMR) 
     The  1 H-NMR spectra of the prepared polymers and monomers were recorded using a JEOL 500 MHz NMR spectrometer in deuterated chloroform (CHCl 3 -d) or dimethyl sulfoxide (DMSO-d 6 ) accordingly. 
     Fourier Transform Infra-Red Spectroscopy (FTIR) 
     The Fourier Transform Infrared (FTIR) spectra were recorded using a Thermo Scientific Nicolet iS50 spectrometer in transmission mode. Samples could be of the form of powder solid or membrane. 
     Thermogravimetric Analysis (TGA) and Glass Transition Temperature (T g ) 
     The thermogravimetric analysis (TGA) plots and the differential scanning calorimetry (DSC) traces were performed using a NETZSCH STA 449 F3 Jupiter®. The TGA plots were recorded at a temperature range from 30° C. to 650° C. with a heating rate of 10° C./min under a nitrogen atmosphere. The glass transition temperature (T g ) was determined from the DSC traces over two consecutive cycles. Each cycle consists of heating the sample at a temperature range between 30° C. and 450° C., using a heating rate of 10° C./min under a nitrogen flow. The first run is aimed to clear the thermal history of the sample, and the T g  values were determined after the second cycle. 
     Syntheses of Monomers and Polymers 
     Example 1: Preparation of 4,4′-(1,3,4-oxadiazole-2,5-diyl)dianiline (BAO) 
       FIG. 4  is a reaction scheme for the preparation of the diamine oxadiazole monomer 4,4′-(1,3,4-oxadiazole-2,5-diyl)dianiline (termed BAO). In a 250-ml three-neck round bottom flask equipped with a nitrogen inlet and a mechanical stirrer, polyphosphoric acid PPA (41.26 ml; 85 g; d=2.06 g/mL; weight ratio PPA/hydrazine sulfate=15) was introduced to the reaction vessel and heated to 100° C. with vigorous stirring for one hour to remove traces of moisture from the solvent. Hydrazine sulfate (5.69 g, 43.8 mmol) was then added to the reaction vessel and the mixture was heated to 160° C. until the solid was completely dissolved, then 4-aminobenzoic acid (5 g, 36.5 mmol) was added and the reaction mixture was stirred for additional 8 hours at the same temperature. The reaction mixture was precipitated in a 1M NaOH aqueous solution. The solid obtained through vacuum filtration using a fritted funnel, was then transferred to deionized water and the mixture was further stirred at 80° C. overnight. The solid was then collected through filtration and dried in a vacuum oven at 100° C. for 24 hours. The final monomer 4,4′-(1,3,4-oxadiazole-2,5-diyl)dianiline (4.00 g, 15.86 mmol, 36.2% yield) was then crystallized from 1,4-dioxane as yellow solid.  1 H NMR (500 MHz, DMSO-d 6 ) δ 7.71 (d, J=8.2 Hz, 4H), 6.68 (d, J=8.2 Hz, 4H), 5.87 (s, 4H) 
     Example 2: Preparation of 5-(2-(1,3-dioxo-2-(4-(5-(p-tolyl)-1,3,4-oxadiazol-2-yl)phenyl)isoindolin-5-yl)-1,1,1,3,3,3-hexafluoropropan-2-yl)-2-methylisoindoline-1,3-dione (6FDA-BAO) 
       FIG. 5  is a reaction scheme for the preparation of the 6FDA-based poly(imide-oxadiazole) homopolymer, 6FDA-BAO. In a 100-ml three-neck round bottom flask equipped with a nitrogen inlet and a mechanical stirrer, 4,4′-(1,3,4-oxadiazole-2,5-diyl)dianiline (1.000 g, 3.96 mmol) (BAO) and 5,5′-(perfluoropropane-2,2-diyl)bis(isobenzofuran-1,3-dione) (1.761 g, 3.96 mmol) (6FDA) were added to the reaction vessel followed by NMP (12.00 ml) and 5% LiCl (0.600 g). The mixture was heated at 200° C. for 8 hours. The heat was removed and the reaction mixture was allowed to cool down below 100° C., then the resulting highly viscous solution was poured into water in thin fibers. The fibrous polymer obtained by vacuum filtration using a fritted funnel was ground, rinsed with water, filtered and dried under reduced pressure for 24 h at 60° C. to produce the 6FDA-BAO (2.57 g, 3.73 mmol, 94% yield) as a white off powder.  1 H NMR (500 MHz, DMSO-d 6 ) δ 8.31 (d, J=7.9 Hz, 4H), 8.22 (d, J=7.3 Hz, 2H), 8.00 (d, J=7.4 Hz, 2H), 7.81 (s, 2H), 7.76 (d, J=6.9 Hz, 4H). 
     Example 3: Preparation of a Random Copolymer of 6FDA-Durene/BAO (1:1) 
       FIG. 6  is a reaction scheme for the preparation of a 6FDA-based poly(imide-oxadiazole) random copolymer, 6FDA-Durene/BAO at a 1:1 ratio of 6FDA-Durene to BAO. In a 100-ml three-neck round bottom flask equipped with a nitrogen inlet and a mechanical stirrer, 2,3,5,6-tetramethylbenzene-1,4-diamine (0.326 g, 1.982 mmol) (3,6-diaminodurene), 4,4′-(1,3,4-oxadiazole-2,5-diyl)dianiline (0.500 g, 1.982 mmol) (BAO) and 5,5′-(perfluoropropane-2,2-diyl)bis(isobenzofuran-1,3-dione) (1.761 g, 3.96 mmol) (6FDA) were added to the reaction vessel followed by NMP (12.00 ml) and 5% LiCl (0.600 g). The mixture was heated at 200° C. for 8 hours. The heat was removed and the reaction mixture was allowed to cool down below 100° C., then the resulting highly viscous solution was poured into water in thin fibers. The fibrous polymer obtained was ground, rinsed with water, filtered and dried under reduced pressure for 24 h at 60° C. to afford 6FDA-Durene/BAO (1:1) (2.378 g, 1.883 mmol, 95% yield) as an off-white powder.  1 H NMR (500 MHz, DMSO-d 6 ) δ 8.46-7.49 (m, 20H), 2.07 (s, 12H). 
     Example 4: Preparation of 6FDA-Based Poly(Imide-Oxadiazole) Random Copolymer: 6FDA-Durene/BAO (3:1) 
     The reaction scheme of  FIG. 6  was also used to prepare a random copolymer of 6FDA-Durene/BAO at a 3:1 ratio. 
     In a 100-ml three-neck round bottom flask equipped with a nitrogen inlet and a mechanical stirrer, 2,3,5,6-tetramethylbenzene-1,4-diamine (0.488 g, 2.97 mmol) (3,6-diaminodurene), 4,4′-(1,3,4-oxadiazole-2,5-diyl)dianiline (0.250 g, 0.991 mmol) (BAO) and 5,5′-(perfluoropropane-2,2-diyl)bis(isobenzofuran-1,3-dione) (1.761 g, 3.96 mmol) (6FDA) were added to the reaction vessel followed by NMP (12.00 ml) and 5% LiCl (0.600 g). The mixture was heated at 200° C. for 8 hours. The heat was removed and the reaction mixture was allowed to cool down below 100° C., then the resulting highly viscous solution was poured into water in thin fibers. The fibrous polymer obtained was ground, rinsed with water, filtered and dried under reduced pressure for 24 h at 60° C. to afford 6FDA-Durene/BAO (3:1) (2.378 g, 1.883 mmol, 95% yield) as a white off powder.  1 H NMR (500 MHz, Chloroform-d) δ 8.43-7.55 (m, 12H), 2.13 (s, 12H). 
     Chemical Characterization 
     The various chemical structures of the compounds prepared in this disclosure were confirmed using  1 H NMR in deuterated solvents. Examples of the spectra for these compounds are illustrated below. 
       FIG. 7  is a  1 H NMR spectrum of 4,4′-(1,3,4-oxadiazole-2,5-diyl)dianiline (BAO) in DMSO-d 6 . The chemical structure of the diamine oxadiazole monomer was confirmed by  1 H NMR in deuterated DMSO. The spectrum depicts the aromatic protons corresponding to the benzene rings as two duplets at 7.71 and 6.68 ppm, and the amine protons are illustrated in a singlet peak at 5.87 ppm. The spectrum shows the symmetry within the structure of the monomer, in addition to the high purity of the solid obtained. 
       FIG. 8  is a Fourier transform infrared (FTIR) spectrum of 4,4′-(1,3,4-oxadiazole-2,5-diyl)dianiline (BAO). The presence of the functional groups within the structure of the BAO monomer, such as free primary amine groups were confirmed using Fourier transform infrared (FTIR) spectroscopy. The primary amine stretching bands are illustrated between 3198 and 3468 cm −1 . The stretching bands for the C═N of the oxadiazole ring and C═C of the phenyl groups could be found at around 1600 cm −1 . The stretching band at 1177 cm −1  could be attributed to the ether (C—O—C) bond in the oxadiazole ring. The peaks between 700 and 900 cm −1  could be attributed to the aromatic C—H bonds. 
       FIG. 9  is a  1 H NMR spectrum of 6FDA-BAO homopolymer in DMSO-d 6 . The chemical structure of the 6FDA-BAO homopolymer was confirmed by  1 H NMR in deuterated DMSO. The spectrum illustrates the corresponding peals of 6FDA moiety as two duplet and one singlet at 8.21, 8.00 and 7.81 ppm, respectively, and the corresponding peaks for BAO as two duplets at 8.31 and 7.76 ppm, respectively. Moreover, the spectrum illustrates the high purity of the polymer prepared. 
       FIG. 10  is a  1 H NMR spectrum of 6FDA-Durene/BAO (1:1) random copolymer in DMSO-d 6 . The chemical structure and the desired molar ratio between the durene moiety and BAO monomer in the 6FDA-Durene/BAO (1:1) random copolymer were confirmed by  1 H NMR in deuterated DMSO. The integration of the peak areas confirms the ratio between the aromatic and aliphatic regions in the spectrum. For instance, the durene diamine monomer does not have aromatic protons. However, it possesses 12 aliphatic protons that correspond to its four-methyl groups. The aromatic region shows a total of 20 protons that could be distributed as follows: eight aromatic protons for BAO, six aromatic protons for 6FDA connected to BAO, and six aromatic protons for 6FDA connected to the durene moiety. Similarly, the molar ratio between the durene moiety and BAO monomers in 6FDA-Durene/BAO (3:1) random copolymer was confirmed using the corresponding  1 H NMR spectrum. 
     Thermal and Physical Properties 
     The thermal properties of the prepared polymers were measured using thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) and the results are illustrated in  FIGS. 11A and 11B . 
       FIG. 11A  is a plot of thermogravimetric analysis (TGA) curves of the prepared polymers.  FIG. 11B  is a plot of the first derivatives of the DSC curves (DTG) of the prepared polymers. The decomposition temperatures at 5% and 10% were determined (Table 1) to assess the thermal stability of the prepared polymers during the harsh conditions of gas separation tests. The T d5%  of the prepared polymers were found to be higher than 480° C., which is similar to high thermally stable membranes used in gas separation technology. The first derivatives of the TGA curves were calculated and the values are listed in Table 1. These values (&gt;538° C.) indicate the highest temperature at which the polymer degrades the fastest, are additional indication to the high thermal stability of the prepared polymers. 
     The glass transition temperatures (T g ) of the prepared polymers were calculated from their corresponding DSC traces and the values are listed in Table 1. These temperatures are indicative of the rigidity of the polymeric chains, which is somehow could be correlated to their performance during gas separation testing. The values obtained as similar to other glassy polymers used in gas separation technology. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Thermal properties of the prepared polymers. 
               
            
           
           
               
               
               
            
               
                   
                 DSC 
                 TGA 
               
            
           
           
               
               
               
               
               
            
               
                   
                 T g    
                 T d5%    
                 T d10%    
                 DTG 
               
               
                 Poly(imide-oxadiazole) sample 
                 (° C.) 
                 (° C.) 
                 (° C.) 
                 (° C.) 
               
               
                   
               
               
                 6FDA-BAO 
                 374 
                 500 
                 521 
                 551 
               
               
                 6FDA-Durene/BAO (1:1) 
                 417 
                 480 
                 510 
                 543 
               
               
                 6FDA-Durene/BAO (3:1) 
                 379 
                 481 
                 507 
                 538 
               
               
                   
               
            
           
         
       
     
     The fractional free volume (FFV) values of membranes prepared from the studied polymers were calculated using the following equation: 
     
       
         
           
             
               
                 F 
                 ⁢ 
                 F 
                 ⁢ 
                 V 
               
               = 
               
                 
                   V 
                   - 
                   
                     V 
                     0 
                   
                 
                 V 
               
             
             , 
           
         
       
     
     where V is the specific volume and V 0  is the occupied volume by the polymer. Note that V is the reciprocal of the polymer density and can be determined experimentally. The densities of the prepared polymers were measured using a Mettler Toledo XPE205 balance equipped with a density kit using cyclohexane (d=0.777 g/cm 3 ) as the buoyant liquid at 20° C. The density values reported in Table 2 are the average values of at least five different measurements, with error values (standard deviation) below 5%. The occupied volume (V 0 ) values were calculated from the van der Waals volumes (V w ) using Bondi&#39;s equation: 
         V   0 =1.3× V   w  
 
     The van der Waals volumes of copolymers were calculated from the individual V w  of the constituent homopolymers taking into consideration their different molar ratios in the copolymer backbone using the following equation: 
         V   w   =X   1   V   w     1     +X   2   V   w     2   , 
     where X 1  and X 2  are the molar ratios, and V w1  and V w2  are the van der Waals volumes of the constituent homopolymers. In our case, we have estimated the van der Waals volumes using a web simulation tool rather than the Bondi&#39;s group contribution method due to missing volume values within the functional groups reported. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Density and fractional free volume (FFV) values of the  
               
               
                 prepared poly(imide-oxadiazole) samples. 
               
            
           
           
               
               
               
               
               
            
               
                   
                 V 0   
                 V 
                 d 
                   
               
               
                 Poly(imide-oxadiazole) sample 
                 (cm 3 /g) 
                 (cm 3 /g) 
                 (g/cm 3 ) 
                 FFV 
               
               
                   
               
               
                 6FDA-Durene 
                 0.6123 
                 0.7735 
                 1.2929 
                 0.2083 
               
               
                 6FDA-BAO 
                 0.5854 
                 0.6909 
                 1.4473 
                 0.1528 
               
               
                 6FDA-Durene/BAO (1:1) 
                 0.5989 
                 0.7312 
                 1.3677 
                 0.1810 
               
               
                 6FDA-Durene/BAO (3:1) 
                 0.6056 
                 0.7483 
                 1.3363 
                 0.1907 
               
               
                   
               
            
           
         
       
     
     The results listed in Table 2 show a clear increase in the FFV for copolymer membranes when compared to that of 6FDA-BAO homopolymer due to the incorporation of the somewhat bulkier durene moiety. For example, the FFV value of the homopolymer 6FDA-BAO is 0.1528, and that of the 6FDA-Durene is 0.2083. The FFV of the copolymers prepared from these two monomers are between the FFV values of their constituent homopolymers. As indicated by the data listed in Table 2, the higher the molar ratio of the durene moiety in the copolymer backbone, the higher the FFV value. These results are correlated to the gas permeation results in the next section. 
     Dense Membrane Preparation 
     Dense membranes of the polymers with a thickness around 100 μm were prepared using the solution casting method in dimethyl formamide (DMF) as the solvent. A polymer solution of 3 wt. % concentration was prepared and 12 mL of this solution was filtered through a 0.45 μm Teflon filter to remove any solid particles impurities, and then transferred into a glass Petri dish of a 5 cm diameter. The Petri dish was placed on a leveled support in a vacuum oven preheated to 80° C. and the solvent was slowly evaporated under a gentle nitrogen flow. When the membrane formed (after ˜36 hours), the oven temperature was increased to 180° C. and vacuum was applied to remove any traces of residual solvent within the membrane matrix. The formed membrane was cut using a 4 cm diameter cutter, for a perfect fitting into the membrane cell of the gas permeation testing system. 
     Pure-Gas and Mixed-Gas Permeation Measurements 
     Pure-Gas Permeation Measurement 
     The pure-gas permeation properties of the prepared polymeric membranes were measured using an in-house built constant volume/variable pressure permeation system. The membrane was placed in the permeation cell and subjected to a selected gas feed (i.e., He, N 2 , CH 4  and CO 2 ) for a specific time, to reach a permeation steady-state at a constant feed pressure of 100 psi and a temperature of 22° C. The permeability coefficients (P) were calculated from the slope 
     
       
         
           
             
               ( 
               
                 
                   d 
                   ⁢ 
                   
                     p 
                     p 
                   
                 
                 
                   d 
                   ⁢ 
                   t 
                 
               
               ) 
             
             ss 
           
         
       
     
     of the steady state (ss) of the permeate pressure (p p ) versus time curve using the following equation: 
     
       
         
           
             
               
                 
                   P 
                   = 
                   
                     1 
                     ⁢ 
                     
                       0 
                       
                         1 
                         ⁢ 
                         0 
                       
                     
                     ⁢ 
                     
                       
                         
                           
                             V 
                             d 
                           
                           ⁢ 
                           l 
                         
                         
                           
                             p 
                             f 
                           
                           ⁢ 
                           A 
                           ⁢ 
                           R 
                           ⁢ 
                           T 
                         
                       
                       ⁡ 
                       
                         [ 
                         
                           
                             
                               ( 
                               
                                 
                                   d 
                                   ⁢ 
                                   
                                     p 
                                     p 
                                   
                                 
                                 
                                   d 
                                   ⁢ 
                                   t 
                                 
                               
                               ) 
                             
                             ss 
                           
                           - 
                           
                             
                               ( 
                               
                                 
                                   d 
                                   ⁢ 
                                   
                                     p 
                                     p 
                                   
                                 
                                 
                                   d 
                                   ⁢ 
                                   t 
                                 
                               
                               ) 
                             
                             leak 
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     where, V d  is the permeate tube volume (cm 3 ), l is the membrane thickness (cm), p f  is the gas feed pressure (cmHg), A is the membrane effective surface area (cm 2 ), R is the universal gas constant (R=0.278 cm 3 ·cmHg/cm 3 (STP)·K), Tis the operational temperature (K), 
     
       
         
           
             
               ( 
               
                 
                   d 
                   ⁢ 
                   
                     p 
                     p 
                   
                 
                 
                   d 
                   ⁢ 
                   t 
                 
               
               ) 
             
             ss 
           
         
       
     
     is the steady-state pressure variation in the permeate tube (cmHg), and 
     
       
         
           
             
               ( 
               
                 
                   d 
                   ⁢ 
                   
                     p 
                     p 
                   
                 
                 
                   d 
                   ⁢ 
                   t 
                 
               
               ) 
             
             leak 
           
         
       
     
     is the leak rate of the system, which is usually very small and may be neglected. The permeability coefficient is expressed in Barrer, where 1 Barrer=10 −10  cm 3 (STP)·cm/cm 2 ·s·cmHg. 
     The ideal selectivity coefficient for two selected gases A and B is calculated from the ratio of their corresponding permeabilities (PA and PB) using the following expression: 
     
       
         
           
             
               
                 
                   
                     ∝ 
                     
                       A 
                       / 
                       B 
                     
                   
                   ⁢ 
                   
                     = 
                     
                       
                         P 
                         A 
                       
                       
                         P 
                         B 
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     The diffusivity coefficients D (cm 2 /s) of the gas penetrants were calculated by the time-lag method using the following expression: 
     
       
         
           
             
               
                 
                   D 
                   = 
                   
                     
                       l 
                       2 
                     
                     
                       6 
                       ⁢ 
                       θ 
                     
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
     where θ(s) is the time-lag determined from the pure-gas permeability measurement, and l (cm) in the membrane thickness. The solubility coefficient S (cm 3 (STP)/cm 3 ·cmHg.) can be then calculated from the permeability and diffusivity coefficients using the following equation: 
     
       
         
           
             
               
                 
                   S 
                   = 
                   
                     P 
                     D 
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
     Mixed-Gas Permeation Measurement 
     The permeation of gases through glassy polyimide membranes is affected by the presence of other gases in the flow. It is known that a small partial pressure of a condensable species such as CO 2  in the feed gas can significantly reduce the permeability of a gas relative to its permeability individually. Therefore, the competition for Langmuir sorption sites for non-plasticized polymers will lead to a decrease of all penetrants permeabilities. Moreover, the selectivity coefficients may also decrease because of bulk flow and the change on the dynamic free volume (plasticization). Therefore, the permeability of a gas i in the mixed gas permeation process is given by: 
     
       
         
           
             
               
                 
                   
                     P 
                     i 
                   
                   = 
                   
                     
                       P 
                       total 
                     
                     ⁢ 
                     
                       
                         
                           y 
                           i 
                         
                         ⁡ 
                         
                           ( 
                           
                             
                               p 
                               p 
                             
                             - 
                             
                               p 
                               f 
                             
                           
                           ) 
                         
                       
                       
                         
                           
                             x 
                             i 
                           
                           ⁢ 
                           
                             p 
                             p 
                           
                         
                         - 
                         
                           
                             y 
                             i 
                           
                           ⁢ 
                           
                             p 
                             f 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
     P total  is derived from the permeability expression for mixed gas: 
     
       
         
           
             
               
                 
                   
                     P 
                     total 
                   
                   = 
                   
                     Jl 
                     
                       Δ 
                       ⁢ 
                       
                           
                       
                       ⁢ 
                       p 
                     
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
     The selectivity coefficient (a* ij ), which is the ability of a polymeric membrane to separate a binary feed gas mixture, is defined as follows: 
     
       
         
           
             
               
                 
                   
                     α 
                     ij 
                     * 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           y 
                           i 
                         
                         
                           y 
                           j 
                         
                       
                       ) 
                     
                     ⁢ 
                     
                       ( 
                       
                         
                           x 
                           j 
                         
                         
                           x 
                           i 
                         
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     where y i  and y j  are the mole fractions of gases i and j at the permeate side, and xi and x j  are the mole fractions of gases i and j at the feed side. 
     To reflect the real properties of the membrane in the case of a non-ideal gas mixture, the modified expression of the selectivity (α i/j   m,* ) is represented by, 
     
       
         
           
             
               
                 
                   
                     ∝ 
                     
                       i 
                       / 
                       j 
                     
                     
                       m 
                       , 
                       * 
                     
                   
                   ⁢ 
                   
                     = 
                     
                       
                         P 
                         i 
                         * 
                       
                       
                         P 
                         j 
                         * 
                       
                     
                   
                 
               
               
                 
                   ( 
                   8 
                   ) 
                 
               
             
           
         
       
     
     where P i * and P j * are the mixed gas permeability coefficients of components i and j determined by the fugacity driving force definition. 
       FIG. 12  is a simplified process flow diagram of a continuous-flow gas permeation device  1200  used for measuring single gas and mixed gas permeation properties. A test gas cylinder  1202  provides a single gas or gas mixture for testing. A pressure transducer  1203  measures the gas pressure from the test gas cylinder  1202 . An actuated valve  1204  allows the test gas to flow through or pressurize the interior of a sample membrane  1206  that is surrounded by a chamber  1208 . A second actuated valve  1210  allows retentate samples to be taken from the inside of the sample membrane  1206  as opposed to permeate samples from the chamber  1208 . A pressure transducer  1212  measures the pressure of the permeate in the chamber  1208 . 
     A mass flow meter  1214  measure the amount of permeate exiting the chamber  1208 . A sample collector  1216  can be used to collect samples of permeate or retentate for analysis. A pressure transducer  1218  is used to measure the pressure of gas in the sample collector  1216 . A carrier gas cylinder  1220  provides a carrier gas, such as helium, for a gas chromatograph  1222 . The flow rate of the carrier gas is set by a needle valve  1223 . After collection of a gas sample in the sample collector  1216 , other valves are closed, and an actuated valve  1224  is opened to sweep the sample to the gas chromatograph  1222 . A vacuum pump  1226  is used to pull a vacuum on the system before and between test runs. 
     The pure-gas permeation properties of membranes prepared from the studied polymers were determined using the permeation system  1200  in a constant-volume mode. For this study, four different pure gases were used: He, N 2 , CH 4 , and CO 2 . The polymeric membranes permeability and selectivity coefficients were calculated from the steady state of the pressure versus time curve, using a constant feed pressure of 100 psi and an operating temperature of 22° C. The obtained results are listed in Table 3. The permeability coefficients are listed in Barrer, where 1 Barrer=10 −10  cm 3  (STP)·cm/cm 2 ·s·cmHg. 
     
       
         
           
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                 Pure gas permeability and selectivity coefficients for the polymeric 
               
               
                 membranes measured at 100 psi feed pressure and at 22° C. 
               
            
           
           
               
               
               
            
               
                   
                 Permeability coefficients 
                 Selectivity  
               
               
                   
                 (Barrer) 
                 coefficients 
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 Polyimide 
                 He 
                 N 2   
                 CH 4   
                 CO 2   
                 He/CH 4   
                 N 2 /CH 4   
                 CO 2 /CH 4   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
            
               
                 6FDA-Durene 
                 451 
                 55.9 
                 46.1 
                 740 
                 9.77 
                 1.21 
                 16.0 
               
               
                 6FDA-BAO 
                 26.8 
                 0.609 
                 0.247 
                 25.6 
                 109 
                 2.47 
                 63.0 
               
               
                 6FDA-Durene/BAO (1:1) 
                 158 
                 6.96 
                 4.27 
                 130 
                 37.1 
                 1.60 
                 30.5 
               
               
                 6FDA-Durene/BAO (3:1) 
                 187 
                 17.0 
                 11.6 
                 261 
                 16.0 
                 1.50 
                 22.4 
               
               
                   
               
            
           
         
       
     
       FIG. 13  is a plot of the measured values for CO 2 /CH 4  compared to permeability-selectivity trade off curves. In the early 1990s, a research survey determined that polymers with a high selectivity have a low permeability and that the opposite is also true, that materials with a low selectivity have a high permeability. This provides the plot line labeled as “Upper Bound 1991.” The research survey was updated to reflect advancements in membrane technology in an article in 2008, providing the plot line labeled “Upper Bound 2008.” See L. M. Robeson, “The Upper Bound Revisited,” Journal of Membrane Science 320, 390-400 (2008). 
     The gas permeation properties of the 6FDA-BAO homopolymer membrane demonstrated that the membrane possesses relatively low CO 2  permeability coefficient (25.6 Barrer), while the CO 2 /CH 4  selectivity coefficient of 63.0 is considered relatively high. The copolymerization methodology, employed in this disclosure, with a permeability-enhancing monomer, such as a durene moiety (FFV=0.2083; Table 2), showed that the CO 2  permeability coefficient of the membranes prepared from Durene/BAO copolymers were improved by increasing the FFV within the membrane matrix, compared to that of the 6FDA-BAO homopolymer. In general, membranes prepared from glassy polymers suffer from permeability-selectivity trade off. This is also observed for our studied membranes. However, the gas permeation properties of the 6FDA-Durene/BAO (1:1) membrane afforded permeability and selectivity coefficients in the desired potential commercially favored range. 
     Sweet Mixed-Gas Permeation Measurements. 
       FIG. 14A  is a change in a sweet mixed-gas permeability of 6FDA-Durene/BAO (1:1) copolymer at different feed pressures and 22° C.  FIG. 14B  is the change in the sweet mixed-gas selectivity coefficients of 6FDA-Durene/BAO (1:1) copolymer at different feed pressures and 22° C. Since natural gas is a mixture of gases, it is important to study the mixed-gas separation performance of the polymeric membranes. For that, the best performing polymeric membrane 6FDA-Durene/BAO (1:1) was chosen as a potential candidate. Therefore, its membrane was subjected to a sweet mixed-gas containing 10, 60, 29 and 1 vol. % of CO 2 , CH 4 , N 2  and C 2 H 6 , respectively. The permeation measurements were recorded at different feed pressures (300-1000 psi) at a fixed temperature of 22° C. The obtained results are listed in Table 4. 
     
       
         
           
               
             
               
                 TABLE 4 
               
             
            
               
                   
               
               
                 Sweet mixed-gas permeability and selectivity coefficients of 6FDA- 
               
               
                 Durene/BAO (1:1) at various feed pressures and 22° C. 
               
            
           
           
               
               
               
               
            
               
                   
                 P 
                 Permeability coefficients (Barrer) 
                 Selectivity coefficients 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Polymer 
                 (psi) 
                 N 2   
                 CH 4   
                 C 2 H 6   
                 CO 2   
                 N 2 /CH 4   
                 C 2 H 6 /CH 4   
                 CO 2 /CH 4   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 6FDA-Durene/BAO 
                 300 
                 2.28 
                 2.11 
                 0.496 
                 84.5 
                 1.08 
                 0.235 
                 40.0 
               
               
                 (1:1) 
                 500 
                 2.08 
                 1.87 
                 0.485 
                 62.7 
                 1.11 
                 0.259 
                 33.5 
               
               
                   
                 700 
                 2.23 
                 2.05 
                 0.534 
                 59.4 
                 1.09 
                 0.260 
                 29.0 
               
               
                   
                 900 
                 2.43 
                 2.13 
                 0.480 
                 55.4 
                 1.14 
                 0.225 
                 26.0 
               
               
                   
                 1000 
                 2.70 
                 2.34 
                 1.16 
                 61.1 
                 1.15 
                 0.496 
                 26.1 
               
               
                   
               
            
           
         
       
     
     As can be seen from  FIG. 14A , the permeability coefficients of C 2 H 6 , N 2  and CH 4  were slightly increasing with the increase in feed pressure up to 1000 psi. However, for CO 2 , the permeability coefficient decreased by about 34% when the upstream pressure is increased from 300 to 900 psi. This change in mixed-gas CO 2  permeability coefficient is attributed to the competition on Langmuir sorption sites between CO 2  and the other existing gaseous molecules in the mixture. However, when the feed pressure was increased to 1000 psi, the permeability coefficients of all of the penetrants increased, indicating some plasticization within the membrane matrix. For example, this allowed the CO 2  permeability to increase to 61.1 Barrer. 
     Accordingly, the CO 2 /CH 4  selectivity coefficient decreased from 40.5 to 26.1 when the pressure increased from 300 to 1000 psi, because of the decrease of the mixed-gas CO 2  permeability and the insignificant change in CH 4  permeability. On the other hand, the C 2 H 6 /CH 4  and N 2 /CH 4  selectivity coefficients were essentially constant up to a feed pressure of 1000 psi, except for C 2 H 6 /CH 4 , where the selectivity doubled at 1000 psi. These results of permeability and selectivity at such elevated pressure and for such a multicomponent gas mixture make 6FDA-Durene/BAO (1:1) a very attractive potential material for industrial natural gas sweetening application. 
       FIG. 15A  is a change in a sweet mixed-gas permeability of 6FDA-Durene/BAO (3:1) copolymer at different feed pressures and 22° C.  FIG. 15B  is the change in the sweet mixed-gas selectivity coefficients of 6FDA-Durene/BAO (3:1) copolymer at different feed pressures and 22° C. Membranes prepared from 6FDA-Durene/BAO (3:1) were studied in a similar fashion to that of 6FDA-Durene/BAO (1:1) using the same gas mixture composition and same testing conditions of pressure and temperature. The obtained data are listed in Table 5. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Sweet mixed-gas permeability and selectivity coefficients of 6FDA- 
               
               
                 Durene/BAO (3:1) at various feed pressures and 22° C. 
               
            
           
           
               
               
               
               
            
               
                   
                 P 
                 Permeability coefficients (Barrer) 
                 Selectivity coefficients 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Polymer 
                 (psi) 
                 N 2   
                 CH 4   
                 C 2 H 6   
                 CO 2   
                 N 2 /CH 4   
                 C 2 H 6 /CH 4   
                 CO 2 /CH 4   
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 6FDA-Durene/BAO 
                 300 
                 6.74 
                 6.60 
                 1.47 
                 183 
                 1.02 
                 0.223 
                 27.7 
               
               
                 (3:1) 
                 500 
                 6.47 
                 6.37 
                 1.61 
                 154 
                 1.02 
                 0.253 
                 24.2 
               
               
                   
                 700 
                 6.32 
                 6.00 
                 1.36 
                 133 
                 1.05 
                 0.227 
                 22.2 
               
               
                   
                 900 
                 6.06 
                 6.03 
                 1.37 
                 110 
                 1.00 
                 0.227 
                 18.2 
               
               
                   
                 1000 
                 5.82 
                 5.66 
                 1.32 
                 109 
                 1.03 
                 0.233 
                 19.3 
               
               
                   
               
            
           
         
       
     
     The data from Table 5 show the benefit of increasing the Durene/BAO molar ratio form 1:1 to 3:1 in 6FDA-Durene/BAO (3:1), where the CO 2  permeability coefficient at 300 psi, for example, was found to be 183 Barrer versus 84.5 Barrer for 6FDA-Durene/BAO (1:1) at the same pressure. However, the permeability-selectivity trade-off was prominent, where the CO 2 /CH 4  selectivity coefficient was calculated to be 27.7 versus 40.0 for 6FDA-Durene/BAO (1:1) at the same pressure (300 psi). 
     Furthermore, when the feed pressure was increased from 300 psi to 1000 psi, the CO 2  permeability coefficient dropped by around 40% with a drop on the CO 2 /CH 4  selectivity coefficient by around 30%, with no signs of plasticization within the membrane matrix. These results are of great interest compared to other glassy polymers reported in the literature, especially at such elevated feed pressure of gas mixture (1000 psi). 
     The present invention discloses a new hybrid class of polymeric materials, poly(imide-oxadiazole), which shows to have great potential to be used in gas separation technology. This class of polymers, combines the advantages offered by two families of polymers, polyimides and polyoxadiazoles, into one material. The current invention can change the perspective of designing polymers with enhanced gas transport properties through a specific molecular engineering process. 
       FIG. 16  is a method  1600  for synthesizing and using a polymer for forming a membrane to separate a gas mixture. The method  1600  begins at block  1602  with the synthesis or purchase of the monomers. This may be performed by the techniques described herein. At block  1604 , the polymer used for the membrane is synthesized, for example, using the techniques described herein. At block  1606 , a membrane is formed from the polymer, for example, using the solvent evaporation techniques described herein. At block  1608 , the membrane is used to separate gas mixtures, for example, to sweeten natural gas by the separation of acid gases, such as H 2 S, CO 2 , or COS. 
     EMBODIMENTS 
     An embodiment described in examples herein provides a gas separation membrane. The gas separation membrane including a poly(imide-oxadiazole) polymer including an oligomer having a structure including: 
     
       
         
         
             
             
         
       
     
     In this structure, Ar 1 , Ar 2 , and Ar 3  include aromatic moieties, and Ar 1 , Ar 2 , and Ar 3  are each independently selected. 
     In an aspect, the gas separation membrane includes a homopolymer of the structure: 
     
       
         
         
             
             
         
       
     
     In an aspect, the gas separation membrane includes a copolymer of the structure: 
     
       
         
         
             
             
         
       
     
     In this structure, Ar 4  includes an aromatic moiety. 
     In an aspect, the gas separation membrane includes a random copolymer. In an aspect, the gas separation membrane includes a block copolymer. 
     In an aspect, the gas separation membrane includes a copolymer of the structure: 
     
       
         
         
             
             
         
       
     
     In an aspect, the gas separation membrane includes a copolymer that includes an oligomer of the structure: 
     
       
         
         
             
             
         
       
     
     In an aspect, the gas separation membrane includes a copolymer that includes an oligomer of the structure: 
     
       
         
         
             
             
         
       
     
     In an aspect, the gas separation membrane includes a copolymer that includes an oligomer of the structure: 
     
       
         
         
             
             
         
       
     
     In an aspect, the gas separation membrane includes a copolymer that includes an oligomer of the structure: 
     
       
         
         
             
             
         
       
     
     In an aspect, the gas separation membrane includes a copolymer that includes an oligomer of the structure: 
     
       
         
         
             
             
         
       
     
     In an aspect, the gas separation membrane includes a copolymer that includes an oligomer of the structure: 
     
       
         
         
             
             
         
       
     
     In an aspect, the gas separation membrane includes a monomer with the structure: 
     
       
         
         
             
             
         
       
     
     In an aspect, the gas separation membrane includes a monomer with the structure: 
     
       
         
         
             
             
         
       
     
     In an aspect, the gas separation membrane includes a monomer with the structure: 
     
       
         
         
             
             
         
       
     
     In an aspect, the gas separation membrane includes a monomer with the structure: 
     
       
         
         
             
             
         
       
     
     In an aspect, the gas separation membrane includes a monomer with the structure: 
     
       
         
         
             
             
         
       
     
     In an aspect, the gas separation membrane includes a monomer with the structure: 
     
       
         
         
             
             
         
       
     
     Another embodiment described in examples herein provides a method for forming a gas separation membrane. The method includes, obtaining a diamine oxadiazole monomer, obtaining an imide monomer, and reacting the diamine oxadiazole monomer with the imide monomer to form a polymer. The polymer is dissolved in a solvent to form a polymer solution. A dense film is formed from the polymer solution. The dense film is tried to form the gas separation membrane. 
     In an aspect, the diamine oxadiazole monomer is formed by heating an amino benzoic acid with hydrazine sulfate to form a symmetric diamine oxadiazole monomer. 
     In an aspect, the amino benzoic acid includes 4-amino-3-methylbenzoic acid, 4-amino-3,5-dimethylbenzoic acid, 4-aminobenzoic acid, 4-amino-3-(trifluoromethyl)benzoic acid, 3-amino-4-methylbenzoic acid, 3-aminobenzoic acid, 6-amino-2-naphthoic acid, 6-aminopicolinic acid, or 3,4-diaminobenzoic acid. 
     In an aspect, the diamine oxadiazole monomer is formed by heating at least two amino benzoic acids with hydrazine sulfate to form an asymmetric diamine oxadiazole monomer. 
     In an aspect, the at least two amino benzoic acids include 4-amino-3-methylbenzoic acid, 4-amino-3,5-dimethylbenzoic acid, 4-aminobenzoic acid, 4-amino-3-(trifluoromethyl)benzoic acid, 3-amino-4-methylbenzoic acid, 3-aminobenzoic acid, 6-amino-2-naphthoic acid, 6-aminopicolinic acid, or 3,4-diaminobenzoic acid, or any combinations thereof. 
     In an aspect, the polymer is formed by heating the diamine oxadiazole monomer with the imide monomer with a lithium chloride catalyst. 
     In an aspect, the polymer is formed by heating the diamine oxadiazole monomer, the imide monomer, and an aromatic diamine with a lithium chloride catalyst. 
     In an aspect, the aromatic diamine includes 
     
       
         
         
             
             
         
       
     
     or any combinations thereof. 
     Other implementations are also within the scope of the following claims.