Structural Determination of Extem XH 1015 and Its Gas Permeability

Apr 12, 2010 - NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore, 28 Medical Drive, Singapore 117456, ...
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Ind. Eng. Chem. Res. 2010, 49, 12014–12021

Structural Determination of Extem XH 1015 and Its Gas Permeability Comparison with Polysulfone and Ultem via Molecular Simulation Jianzhong Xia,†,‡ Songlin Liu,‡ Pramoda Kumari Pallathadka,§ Mei Lin Chng,‡ and Tai-Shung Chung*,‡ NUS Graduate School for IntegratiVe Sciences and Engineering, National UniVersity of Singapore, 28 Medical DriVe, Singapore 117456, Singapore, Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore, and Institute of Material Research and Engineering, 3 Research Link, Singapore 117602, Singapore

By employing high resolution 1H and 13C NMR spectroscopy combined with elemental analysis and FTIRATR, we have determined the basic chemical structure of Extem XH 1015, a new brand of polyetherimide with good thermal, mechanical properties, and processability. Bisphenol-A dianhydride (BPADA) and diamino diphenyl sulfone (DDS) are found to be the monomers for this newly developed polyetherimide. The gas permeability of this new polymer is reported for the first time in the literature. Polysulfone (PSU) and Ultem are employed as reference samples for the elucidation of permeability and selectivity differences among them because of their structural similarities. In addition to qualitative comparison of chain rigidity and packing with gas transport properties, computational simulations powered by Material Studio are performed at a molecular level to quantitatively investigate the relationship between the fractional accessible volume (FAV) and gas permeability. The FAV differences among these polymers increase with an increase in gas molecules diameters; thus these polymers have similar permeability for small gas molecules but diverse for large gas molecules. Their selectivity differences are also discussed in terms of FAV ratio. The FAV concept is proved to be more effective than fractional free volume to analyze and predict gas separation performance. 1. Introduction Throughout history, human beings have witnessed many evolutions involving materials. Now it is the world of new materials, especially after the emergence of engineering plastics. Polyimide is one of these high performance engineering plastics aiming at replacing metals as well as targeting other specialty applications such as electronics, automotives, and membranes. However, not all of the polyimide materials meet industrial specifications because of their poor processability rendered by high chain rigidity and high glass transition temperatures. One of the exceptions is PEI (polyetherimide), which is a particular class of polyimides that not only retains high temperature characteristics but also possesses good melt processability. It has balanced physicochemical properties between performance and practicability due to the introduction of ether groups. One of the commercialized PEIs was developed by GE (General Electric) Plastics in 1986 under the trademark Ultem. It was synthesized from bisphenol A dianhydride (BPADA) and metaphenylene diamine (MPD) through condensation polymerization.1 This polyetherimide has already been employed in many applications, such as healthcare applications, pharmaceuticals, and membrane separations. A gas separation membrane made from Ultem in Paul’s group2 has shown impressively high selectivity for many gas pairs, especially for the helium/methane gas pair. In recent years, Extem XH1015, a new brand of polyetherimide, was developed by GE and has been commercialized in Europe since 2007. This so-called amorphous thermoplastic * To whom correspondence should be addressed. E-mail: [email protected]. † NUS Graduate School for Integrative Sciences and Engineering, National University of Singapore. ‡ Department of Chemical and Biomolecular Engineering, National University of Singapore. § Institute of Material Research and Engineering.

polymer offers a higher Tg (267 °C), exceptional dimensional stability, high strength, stiffness, and creep resistance at elevated temperatures, outstanding flame, smoke, and low smoke-toxicity performance without additives. Even though the Extem XH resin is not the strongest material at room temperature, it outperforms many other high-performance amorphous and semicrystalline thermoplastics at temperatures exceeding 240 °C by retaining most of its tensile strength and creep resistance.3 Unlike other semicrystalline thermoplastic polyimides, this fully amorphous Extem XH does not need any postheat treatment to achieve its desired property. On the basis of our previous understanding on polyetherimide,2 this new amorphous material may have potential to be a membrane material due to its higher Tg as compared to Ultem. However, the dense Extem membrane does not show impressive gas separation performance. Further endeavor is hindered by the lack of information on its chemical structure to explain its structure relationship with gas permeability and to explore methodologies for chemical and physical modifications to enhance its application potentials. A bundle of patents4-8 filed by GE provided few clues on this new PEI, but the detailed structure under the trademark of Extem had never been released. Therefore, the first objective of this study is to investigate its chemical structure with the aid of NMR, along with FTIRATR and elemental analysis. Scheme 1 illustrates the structure we propose, as well as its synthesis route. Interestingly, Extem’s main chemical repeat unit is quite similar to another grade of commercial PEI, named Ultem XH6050, whose chemical structure also consists of sulfone groups.9 This so-called PEIS (polyetherimide sulfone) combines the desired properties of polyetherimide and polysulfone into a single resin, which include good processability, high Tg, and good thermal stability at high temperatures. The good melt processability may be derived from the unique characteristics of polyetherimde and polysulfone. Gallucci and Malinoski10 reported that the end group of PEIS

10.1021/ie901906p  2010 American Chemical Society Published on Web 04/12/2010

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Scheme 1. Synthetic Route of Extem XH1015

plays an important role on its Tg and thermal stability. The PEIS, which has the same structure with Ultem XH6050, exhibits a higher Tg (258 °C) by employing more anhydride as the end groups. This suggests that the same strategy may also be employed to improve the Tg of Extem XH1015. Their similar physical properties indicated in ref 3 indirectly support our hypothesis. Since polysulfone (PSU) and Ultem contain part of the repeat unit in Extem as illustrated in Figure 1, the second and the third objectives of this work are to correlate its gas permeation properties with its chemical structure via both experiments and molecular simulation and to further investigate its potential as a material for gas separation. 2. Background on Simulation for Gas Permeability Polymer scientists have made significant attempts to theoretically predict the intrinsic permeability of polymers according to monomer moieties, chain structures and rigidity, and functional groups.11 Salame12 developed a method named “Permachor” to forecast oxygen permeability in barrier-type polymers via the calculation of chemical group’s empirical factor which contributes to the total permeability. Bicerano13 proposed the use of packing density, cohesive energy, and rotational freedom of a polymer as important parameters to predict permeability. However, the method developed by Lee14 to correlate gas permeability with free volume has received great attention and high remarks. The free volume concept proposed by Lee was defined as (V - Vo), where V was the specific volume obtained experimentally, while Vo was 1.3 times of Vw which is the sum of van der Waals volume of the groups calculated following the Bondi’s method15 or Park and Paul’s method.16 In glassy polymers, the permeability coefficient (P) can be expressed in eq 1. Extensive experiments suggest that P is much more dependent on diffusion coefficient (D) rather than on solubility coefficient (S).17 P ) DS

(1)

Though many parameters affect the diffusion coefficient, the free volume plays a much more crucial role than others. Thus, a relationship between the permeability coefficient and the free

Figure 1. Chemical structures of PSU, Extem, and Ultem.

volume has been established through experiments18,19 despite its oversimplifications: P ) A exp(-B/FFV)

(2)

where A and B are constants for a particular gas, while FFV is the abbreviation of fractional free volume defined as following. FFV ) (V - Vo)/V

(3)

The FFV value can be theoretically estimated from the molecular structure with the aid of the compass force field theory as described in Chang et al.’s work.20 In Material Studio, the van der Waals volumes could be estimated accurately by employing a probe radius of 0 nm based on the Connolly task21 after the polymer periodic amorphous cell is constructed and optimized by molecular dynamics. The total volume of the polymer amorphous cell could also be calculated from the Connolly task. Thus, the FFV which describes the ratio of vacant volume to the total volume of the polymer could be obtained. However, the FFV value alone could not indicate the effective space for a specific gas penetrant. In Hofmann et al.’s work,22 the size distribution of free volume elements was applied to overcome the drawback of FFV. Wang et al.23 also have calculated the cavity size distribution which showed a good correlation with the PALS (positron annihilation lifetime study) experimental investigation. Some recent reports by Chang et al.20,24 on simulation of aromatic polyimide membranes for gas separation have already pointed out the limitation of FFV and employed fractional accessible volume (FAV) and relative FAV for replacement. The concept of FAV which was first employed by Tung et al.25 was used to calculate the accurate information for a particular gas passing through the membrane. The FAV value can be defined as following. FAV ) (V - Vo)/V The form of this equation is similar to that of FFV. However, both V and Vo are estimated from the Connolly task simulation by using different probe diameters. The mechanism of this Connolly task is to employ a hard sphere with a particular diameter as a probe to detect the available vacancy inside an optimized amorphous polymer cell. The FFV concept developed from the Bondi’s method is universally applicable to all gases, while the Connolly approach calculates FAV for each individual penetrant. Generally, the FAV would increase when the dimension of detection probe decreases. Significant experimental and theoretical works have validated eq 2 to correlate gas permeability of polymers with similar structures of relatively larger molecules, like methane, with their FAV values. Therefore, we intend to estimate Extem’s permeability and correlate it with its structure via Material Studio and to explore the fundamentals of molecular design of polymeric materials for gas separation.

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3.3. Molecular Simulation. Molecular dynamic simulations were conduct using Material Studio 4.4 from Accelrys. Polymer chains consisting of 10 repeat units of PSU, Ultem, or Extem were constructed via polymer build function, followed by energy minimization separately. Initiator and terminator were unknown because of limited publications or information on these commercial polymers. The isotactic configuration with random torsion and head-to-tail orientation was assumed prior to the amorphous cell construction. Besides, polymer chains consisting of 5 repeat units were also constructed in order to discuss the chain packing morphology. Two polymer chains of PSU, Extem, and Ultem were put together and minimized in order to discuss structural effect on chain packing. The initial density of the polymer periodic cell was set as 0.1 g/cm3 in order to eliminate aromatic ring catenation and chain scission during amorphous cell construction as suggested by Heuchel and Hofmann.28 Four polymer chains were employed for amorphous cell construction based on compass force field calculations by the Amorphous Cell module. Conformational optimization and fine convergence with a maximum iteration of 10 000 was performed before molecular dynamics simulations to proceed. The detailed information for cell models built was listed in Table 1. The equilibrium stage temperature was set to 308 K when MD (molecular dynamics) by Discover module was launched. This is because all the pure gas permeability data were collected at 35 °C. A set of MD under isothermal-isobaric (NPT) mode were performed to compress amorphous cell density from 0.1 g/cm3 to the experimental density. The simulated densities are listed in Table 2. The total MD time for each polymer was 500 ps in order to allow the amorphous cell to reach equilibrium. The fractional accessible volume (FAV) was calculated from the free volume and occupied volume simulated by the Connolly task. The FAV distribution of each polymer was plotted by using different diameter probes to detect available volume in the amorphous cell. For one particular diameter, there is one unique FAV, which means only particles smaller than the probe could go through or occupy the volume under this definition. Thus, this so-called Connolly task corresponds to the kinetic diameters of gases employed. In this study, we choose probe diameters ranging from 2.0 to 5.0 Å, not only because even the diameter of smallest helium molecules is bigger than 2.0 Å, but also the number of cavities larger than 5.0 Å is very small and can be ignored, compared to that of the smaller cavities.

Table 1. Atom Numbers and Cell Dimensions for PSU, Extem, and Ultem Amorphous Cells dimension (Å) atom numbers

a

b

c

2168 3288 2768

28.9 33.3 31.3

28.9 33.3 31.3

28.9 33.3 31.3

PSU Extem Ultem

3. Experimental Section 3.1. NMR Characterization, Elemental Analysis, and FTIR-ATR. NMR analyses were conducted using a Bruker 400 FT-NMR spectrometer at a resonance frequency of 400.132 MHz for 1H and 100.621 MHz for 13C, while 1H NMR spectra (1D, homonuclear decoupling) were obtained from an Extem XH 1015 pellet sample dissolved in DMSO-d6 using a 5 mm BBO (broadband observe) probe. Other spectra were performed in CDCl3 and chemical shifts are given in parts per million units. The elemental analysis was carried out using a Flash EA 1112 Elemental Analyzer manufactured by Thermo Electron. The analyzer was able to determine CHNS (carbon, hydrogen, nitrogen, and sulfur) on dried samples. Oxygen content was obtained by the subtracting the ash percent plus the total for carbon, hydrogen, nitrogen, and sulfur. The FTIR-ATR measurement was performed using a PerkinElmer FT-IR Spectrometer Spectrum 2000 with scanning time 32. The Extem thin film used in FTIR-ATR was made by ring casting. 3.2. Membrane Preparation and Gas Permeability Measurements. A commercial Extem XH 1015 polymer was obtained from the SABIC Innovative Plastics (formerly GE Plastics), MA, and was dried at 120 °C overnight under vacuum before use. Dicholoromethane (DCM) was purchased from Merck and utilized without further purification. Flat dense Extem membranes were fabricated using a solution casting method described elsewhere.26 In short, a solution was prepared by dissolving 2 wt % of the polymer sample in a solvent of DCM. The solution was then filtered through a 1 µm filter (Whatman) and cast onto a Si wafer with a metal ring on top. The whole plate was leveled horizontally and covered with a glass plate. A small gap was left to allow slow solvent evaporation to form a dense membrane. The nascent membrane was further dried at 250 °C under vacuum for 48 h to remove residual solvents. Pure gas permeability measurements were performed using a variable pressure constant-volume method described elsewhere.27 Pure gas permeability of He, H2, O2, N2, CH4, and CO2 was tested in order and replicated three times. All the measurements were conducted at 35 °C and 10 atm except H2 at 3.5 atm. The ideal selectivity of membranes for pure gases A and B is defined as follows: RA,B )

PA PB

4. Results and Discussion 4.1. Extem Structure Determination. The 1H NMR and 13C NMR spectra of Extem XH 1015 are illustrated in Figures 2 and 3, respectively. The 2D-NMR spectra used to assign each carbon and proton in the aromatic region are given in Figures 4 and 5. These spectra are in complete agreement with the proposed polymer structure. In addition to NMR spectra and FTIR-ATR shown in Figure 6, the elemental analysis results of this polyetherimide sulfone shown in Table 3 also generally agree with the theoretical values for the proposed structure. Very

(4)

where P is the gas permeability.

Table 2. Gas Permeation Performance of Flat Dense PSU, Extem, and Ultem Membranes permeability (barrera)

PSU33 Extem Ultem34

ideal selectivity

He

CO2

O2

N2

CH4

He/CH4

CO2/CH4

O2/N2

density (g/cm3)

FFVc

FFVd

13 11.1 9.4

5.6 3.28 1.33

1.4 0.81 0.41

0.25 0.13 0.051

0.25 0.13 0.036

56 85 261

22.4 25.2 36.9

5.6 6.2 8.0

1.24 (1.22b) 1.31 (1.32b) 1.27 (1.28b)

0.143 0.131 0.137

0.186 0.171 0.158

1 barrer ) 1 × 10-10 cm3(STP) cm/(cm2 s cmHg) ) 7.5005 × 10-18 m2/(s Pa). b Density simulated from Material Studio. c FFV calculated from the Bondi-Park and Paul’s method. d FFV calculated from Material Studio. a

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Figure 2. H NMR spectrum of Extem XH1015.

Figure 5. HETCOR spectrum of Extem XH1015.

Figure 3. C NMR spectrum of Extem XH1015.

Figure 6. FTIR-ATR spectrum of Extem XH1015 thin film. Table 3. Experimental and Theoretical Elemental Analysis element

C

H

N

S

experimental theoretical

69.35 70.49

3.95 3.83

3.79 3.83

4.10 4.37

(110-170 ppm). However, the 1H-13C HETCOR spectrum does not show any proton connected to this carbon. The above evidence all strongly support a 2,2-substituted propane structure in the Extem’s backbone. Although the area integration curve of all proton peaks is not shown in Figure 2, the peak area ratio of a-h protons could be determined roughly to be 3:2:1:2:1:2:1:2, which is wellcoincident with the theoretical protons ratio based on the chemical structure of Scheme 1. Fortunately, aromatic protons, Figure 4. COSY spectrum of Extem XH1015.

minor differences are found due to experimental error and/or different element compositions of the polymer end group. The aliphatic parts of the 1H and 13C NMR spectra of Extem can be completely interpreted with the aid of the 1H-13C HETCOR spectrum (not shown here because its position is too far away from the aromatic region). The adsorption of the C-12 carbon at 30.9 ppm of the 13C NMR spectrum correlates well with a very sharp singlet of H-a protons at 1.64 ppm of the 1H NMR spectrum, revealing the existence of methyl group. C-13 at 42.5 ppm is also in the aliphatic region due to a relative low chemical shift compared to these carbons in the aromatic region

Figure 7. Chain morphologies of PSU, Extem, and Ultem with five repeat units.

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Figure 8. Morphology of two polymer chains with five repeat units for PSU, Extem, and Ultem.

Figure 9. Fractional accessible volumes and relative FAV values of PSU, Extem, and Ultem, probed with different diameters.

except H-g, all belong to the AX two-spin system.29 Each peak of these protons is observed as a small doublet that is split by the proton nearby. The 1H-1H correlation (COSY) performed in CDCl3 is shown in Figure 4, revealing that H-b (7.04-7.06 ppm) is split by H-d (7.28-7.30 ppm) through a 3-bond coupling, as do the H-h (8.13-8.15 ppm) and H-f (7.72-7.74 ppm) pair, as well as the H-e (7.63-7.65 ppm) and H-c (7.15-7.17 ppm) pair. The H-g at 7.18 ppm is assigned as an isolated proton because of its absence from the COSY spectrum, although it is overlapped by H-f. Figure 3 shows the signal assignment for a 13C NMR spectrum with 1H noise decoupled during acquisition. These carbon atoms at the high frequency are mainly attached to heteroatoms, making them valuable in the determination of monomers, especially diamino monomer. Carbon atom C-1 (165.9 ppm) is assigned to the imide carbonyl group. C-5 (164.2 ppm) and C-8 (152.7 ppm) constituting the BPADA ether segment are assigned due to the direct connection to the oxygen atom. A similar carbon chemical shift is also found for C-17

(147.5 ppm) in the polysulfone 13C NMR spectrum.30 The assignment of each peak marked in Figure 3 is assisted by the correlations shown in the 1H-1H COSY (Figure 4) and 1H-13C HETCOR (Figure 5) spectra to fit the structure of polyetherimide sulfone shown in Scheme 1. In the case of FTIR-ATR spectra as shown in Figure 6, bands at around 1781 cm-1 (attributed to CdO asymmetric stretch of imide groups), 1717 cm-1 (attributed to CdO symmetric stretch of imide groups), and 1360 (attributed to C-N stretch of imide groups) are characteristic imide peaks as indicated in the work of Shao et al.31 Although parts of the band at 1320 cm-1 (attributed to -SO2- asymmetric stretch) are overlapped by strong adsorption of the imide group at 1360 cm-1, the existence of sulfone group can still be proved by the strong peak at 1150 cm-1 (attributed to a -SO2- symmetric stretch).32 4.2. Chain Morphology Comparison. Figure 7 shows the single chain morphologies of PSU, Extem, and Ultem containing five repeat units at the completely extended state. Although the

Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010 Table 4. Kinetic Diameters of Various Gases gas

He

H2

CO2

O2

N2

CH4

diameter (Å)

2.60

2.89

3.30

3.46

3.64

3.80

morphologies are far from real conformation in our membranes, it is still valuable for the qualitative discussion to correlate membrane gas permeation performance with their chemical structures. As we can see in this figure, Ultem, being the most rigid polymer, takes a much extended chain conformation; while PSU, the softest one, exhibits a zigzaglike morphology. Usually, an increase in chain stiffness accompanies with a decrease in segmental mobility. As a result, from PSU, Extem to Ultem, the thermal motions generating transient gaps for penetrants to diffuse decrease. These calculated chain morphologies correlate well with their gas selectivity as shown in Table 2. The sequence of chain rigidity in these polymers can also be elucidated from their monomers. PSU and Extem both have bisphenol A and diamino diphenyl sulfone (DDS) moieties. The additional heterocyclic imide group in Extem induces a planar and relatively larger ring in the backbone, which increases the chain rigidity. The introduction of heterocyclic imide group also enhances the interchain attractions. As shown in Figure 8, where two Extem polymer chains are constructed at fully extended state, two heterocyclic imide rings are partially aligned parallel to each other to minimize the total energy. This interchain

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attraction effect increases the chain packing efficiency, thus leading to a lower permeability. Ultem also has the same phenomenon, while PSU polymer chains could not be aligned parallel, which correlate well with its high permeability. Although the discussion is only valid in the ideal case, the polymer chain packing will still follow the principle of minimum total potential energy in the real materials. Comparing Extem with Ultem, the only difference is that the meta-phenylene diamine (MPD) moiety in Ultem is replaced by DDS in Extem. Since the DDS structure can reduce the effective chain packing due to tetrahedral configuration of the -SO2- group, Extem has a higher permeability and a lower selectivity than Ultem. 4.3. Molecular Simulation. The FFV values calculated by both the Bondi-Park and Paul method16 and Material studio were listed in Table 2 in order to compare their gas separation performance quantitatively. PSU has the largest FFV, which correlates well with experimental data of the highest permeability. However there is a contradiction when it comes to Extem and Ultem. The FFV value of Extem calculated by the Bondi-Park and Paul method is smaller than that of Ultem, while the permeability of Extem is much larger than that of Ultem. Obviously, the FFV concept alone cannot interpret this discrepancy because FFV just gives a sum of the total free volume with no differentiation for different penetrants with different dimensions. In order to explain this contradiction, the fractional accessible volume (FAV) distributions of PSU, Extem, and Ultem, as well as their relative FAV values raised by Chang et al.20 in terms of PSU/Ultem and Extem/Ultem were calculated and listed in Figure 9. In our case, the relative FAV values were defined, respectively, as follows: FAV(PSU) - FAV(Ultem) × 100% FAV(Ultem)

(5)

FAV(Extem) - FAV(Ultem) × 100% FAV(Ultem)

(6)

and

Figure 10. Correlation between gas permeability and 1/FAV.

Figure 11. FAV ratios of Extem/PSU and Ultem/PSU probed by different diameters.

The selection of horizontal axis value is based on the kinetic diameter of the probe gas molecule as shown in Table 4 for easy interpretation with permeation data. For one particular gas, three FAV values were calculated from these three polymers. It can be seen that the FAV of PSU is always the largest, which correlates well with its highest permeability of all gases as presented in Table 2. Meanwhile, the FAV values of Ultem are always the smallest, correlating well with its lowest gas permeability. Furthermore, for all the three polymers, gas permeability and the reciprocal of FAV follow linear relationships for relatively larger gas molecules, especially for methane and nitrogen as shown in Figure 10, even though their polymer structures are different. In Table 2, we also notice that when the gas molecule diameter is small, the difference in their gas permeability is small, while when the gas diameter increases, this difference expands. For example, the helium permeability coefficients of PSU, Extem, and Ultem are 13, 11.1, and 9.4 barrers with a ratio of 1.38: 1.18:1. Meanwhile, the methane permeability coefficients become 0.25, 0.13, and 0.036 barrers with a ratio of 6.94: 3.61:1. These phenomena could be well explained using the relative FAV concept. As plotted in Figure 9, the relative FAV values of PSU/Ultem and Extem/Ultem increase when the probe diameter increases. In other words, the FAV difference between PSU and Ultem becomes larger when the probe diameter increases. This indicates that the fractional

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accessible volume is a more precise tool in analyzing the gas permeability than the total free volume. The gas selectivity of polymer dense membranes shows a good correlation with the FAV ratio, which was defined, respectively, as follows: FAV(Extem) × 100% FAV(PSU)

(7)

FAV(Ultem) × 100% FAV(PSU)

(8)

139 0033 (A-Star), as well as the Singapore National Research Foundation (NRF) for the support on the project entitled “Molecular Engineering of Membrane Materials: Research and Technology for Energy Development of Hydrogen, Natural Gas and Syngas” with grant number of R-279-000-261-281. The authors also thank Dr. S. K. Yen and Dr. Y. C. Xiao for their valuable discussion and Mr. C. H. Lau for his help on NMR experiments.

and

As shown in Figure 11, the FAV ratio of Ultem/PSU is about 76% for helium, while it is only 63% for methane. This means, using PSU as a reference, the fractional accessible volume for both helium and methane transport in Ultem is less than in PSU. However, the decrease of FAV for methane is more severe than that for helium. Thus, the helium/methane selectivity increases from 56 for PSU to 261 for Ultem. The same approach could be used to explain the helium/methane selectivity increase in the Extem membrane. Another method is to investigate the slope of a straight line connecting any two data points of interest on the FAV curve. For any gas pair in one particular polymer, if the slope of the line between the selected two data points is negative, the selectivity of this gas pair for this particular polymer is higher than the reference polymer. For example, if helium and methane are chosen in the Ultem/PSU FAV ratio curve, the slope of line between these two points is definitely negative. Therefore, the selectivity of helium/methane is better in Ultem than in PSU. When comparing gas selectivity of more than two polymers, this approach is still validate by just choosing one reference polymer. For example, if there are two straight lines linking helium and methane in the Ultem/PSU and Extem/ PSU FAV ratio curves, the absolute value of the slope for Ultem/ PSU is obviously larger than that for Extem/PSU. This demonstrates that the helium/methane selectivity difference between PSU (56) and Ultem (261) is larger than that between PSU (56) and Extem (85). This method could explain other gas pairs as well, like O2/N2. The absolute value of the slope between O2 and N2 in the Extem/PSU FAV ratio curve is smaller than that for Ultem/PSU. This correlates well with their gas selectivity difference between PSU (5.6)/Extem (6.2) and PSU (5.6)/Ultem (8.0), respectively. 5. Conclusions The chemical structure of Extem XH1015 has been determined by NMR spectra, FTIR-ATR, and elemental analysis with the aid of published patent information. Bisphenol-A dianhydride (BPADA) and diamino diphenyl sulfone (DDS) are confirmed to be the monomers of this newly developed high temperature polyetherimide. Gas permeability of Extem dense membranes is reported for the first time. Due to the structural similarities, PSU and Ultem are used for comparison of fractional free volume (FFV), fractional accessible volume (FAV), gas permeability, and selectivity. The effect of different monomer structure in their gas separation performance is also discussed in detail. Computational simulation powered by Material Studio, especially the FAV value simulation, is proved as a more accurate method to analyze and predict gas separation performance. Acknowledgment The authors would like to thank NUS and A*Star for funding this work with the grants of R-398-000-044-305 (NUS) and 092

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ReceiVed for reView December 2, 2009 ReVised manuscript receiVed March 15, 2010 Accepted March 30, 2010 IE901906P