Synthesis and Gas Transport Properties of Polyesters and a

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Ind. Eng. Chem. Res. 2010, 49, 12060–12066

Synthesis and Gas Transport Properties of Polyesters and a Copolyester Obtained from 4,4′-(9-Fluorenylidene) Bisphenol and 4,4′-(1-Phenylethylidene) Bisphenol M. Isabel Lorıá-Bastarrachea and Manuel Aguilar-Vega* Unidad de Materiales, Centro de InVestigacioń Cientı´fica de Yucatań, A.C., Calle 43 No. 130, Chuburna´ de Hidalgo, C.P. 97200, Me´rida, Yucatań, Me´xico

Two isophthalic polyesters synthesized from 4,4′-(9-fluorenylidene) bisphenol, BF, and 4,4′-(1-phenylethylidene) bisphenol, BAP, and a random copolyester made from BF and BAP containing 50 mol % of these aromatic diols were prepared by interfacial polymerization. The resulting polyesters show glass transition temperatures, Tg, above 200 °C and onset of decomposition temperatures above 440 °C. The values of these thermal properties in the copolyester BAP50/ISO fall between those of the polyesters. It was also found that the thermal stability and maximum on the R-transition of the damping factor, tan δ, of BF/ISO were higher than those of BAP/ISO which is an indication of higher rigidity in the former. Wide-angle X-ray diffraction measurements indicated that both the homopolyesters and copolyester were amorphous. The copolyester showed an amorphous pattern with maxima that fell between those of the polyesters. Pure gas permeation measurements in films cast from solution show that gas permeability coefficients increase as the fractional free volume, FFV, of the polyester increases. This is attributed to a lower packing in BF/ISO as compared to BAP/ISO which increases the free volume available for gas permeation. The copolyester shows FFV and permeability coefficients that are intermediate between those exhibited by the homopolyesters. It was also found that ideal gas separation factors for O2/N2, CO2/CH4, and CO2/N2 are higher for BF/ISO which also shows the highest permeability coefficients. Introduction The separation of gases by membranes offers an alternative to traditional processes such as cryogenic distillation or pressure swing adsorption.1 In the last decades there has been an increased interest in developing new materials for achieving the separation of certain gases of industrial interest such as carbon dioxide from natural gas or oxygen from nitrogen, sometimes at harsh physical conditions. Gas transport properties of polymers with rigid structures such as aromatic polyesters, socalled polyarylates, polysulphones, polyimides, polyamides, and polyethers have been tested because they offer some of the highest glass transition temperatures of organic polymer and they also show flame and oxidative resistance.2-4 Unfortunately rigid structures tend to be insoluble and crystallize easily which complicates the preparation of membrane films from these materials. Some studies show that substitutions of bulky lateral groups lead to an enhanced solubility and inhibit the crystallization in some polymers. This has led to the study of the relationship between structure and gas transport properties of families of these polymers.5,6 These studies are directed to understand the role of structural substitution of different functional groups, in the polymer chain and on the gas permeability and diffusion coefficients as well as the selectivity for gas separation in different polymers. In this work, the synthesis, physicochemical characterization, and gas transport properties of two polyesters and a copolyester were performed. The two isophthalic polyesters, poly(phenylethylidene biphenyl isophthalate), BAP/ISO, poly(fluorenylidene biphenyl isophthalate), BF/ISO, and a 50 mol % BAP/ISO-coBF/ISO random copolymer that in short will be called BAP50/ ISO, were synthesized by interfacial polymerization. Both polyesters bear bulky lateral phenyl groups pending from the quaternary carbon in the bisphenol hinge; however, BF presents * Author to whom correspondence should be addressed. E-mail: [email protected].

two phenyl groups bridged in the middle which produce a higher restriction on the mobility of the chain as compared to BAP/ ISO which presents a phenyl ethylidene group pending from the quaternary carbon in the bisphenol hinge, see monomers in Table 1. Films cast from solution of these polyesters and the copolyester were tested for pure gas permeation for different gases at 35 °C. Thermal and dynamical mechanical properties were also determined in the films. This study is aimed to compare the role of the phenyl pendant groups present in these polyesters and to determine the influence that copolymerization has in the final properties, particularly the possibility of using them as membranes for gas separation. Experimental Section Materials. The polyesters and copolyester were prepared from 4,4′-(1-phenylethylidene) bisphenol (BAP, 99%; Aldrich Chemical Co., Milwaukee, WI) and 4,4′-(9-fluorenylidene) bisphenol (BF, 99%; Aldrich Chemical Co., Milwaukee, WI). They were used as received. Isophthaloyl dichloride (ISO, 98%; Aldrich Chemical Co., St. Louis, MO) was purified by distillation under reduced pressure. Benzyltriphenyl phosphonium chloride (BTPC, 99%; Fluka Chemie, Buchs, Switzerland) was used as the interfacial catalyst. The reactant sodium hydroxide (NaOH, 98%; J.T. Baker, Me´xico) and solvents chloroform (CHCl3, 99+%, Mallinckrodt Baker, Me´xico), 1,2-dichloroethane (DCE, 99+%; Aldrich, Chemical Co., Milwaukee, WI), and methanol (98%; Fisher Scientific, Fair Lawn, NJ) were used as received. Polymerization. The synthesis of the polyesters and copolyester was performed by interfacial polymerization according to the method described by Morgan.7,8 The isophthalic homopolyesters synthesized from 4,4′-(1-phenyliethylidene) bisphenol and 4,4′-(9-fluorenylidene)diphenol are designated BAP/ISO and BF/ ISO, respectively. The random copolyester BAP50/ISO, was synthesized using 50 mol % of BAP and 50 mol % of BF. The

10.1021/ie100478v  2010 American Chemical Society Published on Web 08/16/2010

Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010

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Table 1. Reactants Used in the Synthesis of Aromatic Polyesters and Copolyester

polymerization reactions were carried out in a 250-mL threeneck European-type flask equipped with a mechanical stirrer under nitrogen atmosphere. A schematic reaction for this synthesis is given in Figure 1. In a typical reaction, for example, synthesis of BAP50/ISO, a mixture of BAP and BF (0.005 mol each) was dissolved in a sodium hydroxide, aqueous solution (0.02 mol of NaOH) prepared with 100 mL of deionized water. The mixture was stirred until the diols had dissolved completely. After that, BTPC (2 wt %, with respect to the expected final weight of polymer) was added in the reaction system until it dissolved completely. At this point, a solution of 0.01 mol of isophthaloyl dichloride in 25 mL of 1,2-dichloroethane (DCE) was added. After a reaction period of 1 h at room temperature, the aqueous phase was separated from the organic phase, and

the organic phase was dispersed in 75 mL of chloroform. Finally, the copolyester was precipitated in 300 mL of methanol by stirring vigorously using a Waring laboratory blender (Waring Products, New Hartford, CT). The white precipitate was collected by filtration and then washed several times with methanol. The copolymer as obtained was dried in a vacuum oven at 100 °C for 24 h with a polymer yield between 89 and 91. Films for each polyester and the also the copolyester were cast by dissolving the polymer in chloroform, a 5% w/v solution. These solutions were poured into stainless steel rings on the surface of a glass plate and maintained under chloroform atmosphere. The films obtained were vacuum-dried at 100 °C

Figure 1. Schematic reaction for synthesis of homopolyesters and copolyester by interfacial polycondensation.

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Table 2. Thermal and Physicochemical Properties of BAP/ISO, BF/ISO, and BAP50/ISO polymer

Tg (°C)

Td (°C)

weight loss at 600 °C (%)

TR (°C)

inherent viscosity (ηinh) (g/dL)

density (F) (g/cm3)

FFV

BAP/ISO BAP50/ISO BF/ISO

202 245 274

451.0 447.3 434.5

51.3 52.6 41.7

205.7 232.1 275.9

0.514 0.591 0.620

1.228 1.222 1.211

0.151 0.173 0.198

for 24 h. A second drying step was performed at 160 °C for 24 h to make sure that all the solvent was eliminated from the films. Polymer Characterization. Glass transition temperatures (Tg) of the polyesters and copolyester were measured in a differential scanning calorimeter, DSC-7 (Perkin-Elmer Inc., Cetus Instruments, Norwalk, CT), in samples containing between 6 and 10 mg of the polymer at a scanning rate of 20 °C/min, between 50 and 350 °C under nitrogen atmosphere. The Tg is reported as the midpoint of the enthalpy change in the second run. The onset of decomposition temperature (Td) for all synthesized polymers was determined by thermogravimetrical analysis in a Pyris one analyzer, (Perkin-Elmer Inc., Cetus Instruments, Norwalk, CT). Samples between 5 and 7 mg of each polymer were tested between 50 and 800 °C at a heating rate of 10 °C/min under nitrogen atmosphere. Dynamic mechanical properties of polyesters and copolyester as a function of temperature were determined in 15 × 2.2 mm film strips, with thickness (0.15 and 0.19 mm thick). Dynamic mechanical properties measurements were carried out in a DMA-7 (Perkin-Elmer Inc., Cetus Instruments, Norwalk, CT), using the extension mode between -120 and 350 °C, at a frequency of 1 Hz and a heating rate of 4 °C/min under nitrogen atmosphere. Inherent viscosity (ηinh) of polymers was determined in chloroform at a concentration of 1.0 g/dL with an Ubbelohde viscometer No. 50 at 30 °C. Wide-angle X-ray diffraction (WAXD) measurements of polymer films were performed on a Philips 1140 X-ray diffraction instrument (Philips, The Netherlands), using Cu KR (λ ) 1.54 Å) at 40 kV and 15 mA. The measurements were made between 5 and 60° 2θ at a scanning rate of 0.5°/min. Density measurements for the polymer films were performed by a density gradient column (Techne Corp., Princeton, NJ), filled with Ca(NO3)2 (Aldrich Chemical Co., Milwaukee, WI) aqueous solutions, between 1.20 and 1.30 g/cm3 at 25 °C. Calibration of the column was done using standards of known density. Gas permeability coefficients (P) for five pure gases, helium (He), carbon dioxide (CO2), oxygen (O2), nitrogen (N2), and methane (CH4), were measured at 35 °C and 2 atm upstream pressure using a permeation cell of the constant volume type built in our laboratories.7 Apparent gas diffusion coefficients (D) were determined using the time-lag (θ) method at the same temperature. Apparent diffusion coefficients for He were not determined since its time lag was too short to be measured. Ideal gas separation factors were calculated from the ratio of pure gas permeability coefficients using the following equation:3,8-11 RA/B )

PA PB

(1)

where PA and PB are gas permeability coefficients of the pure gases A and B, respectively. Result and Discussion The polyesters and copolyester synthesized were collected by precipitation in the form of white fibers. A representation of the structures obtained is given in Figure 1. The polyesters and copolyester were cast readily from CHCl3 solutions to form

translucent, tough, and flexible films that were used for their characterization. These polymers were amorphous as denoted by the transparency of the films and there was no evidence of a melting endotherm in the DSC thermogram of any of the polymers. The inherent viscosities (ηinh) of BAP/ISO, BF/ISO, and random copolymer BAP50/ISO as given in Table 2 are an indirect measurement of the molecular weight of the isophthalic polyesters and copolyester. Relatively high inherent viscosities, between 0.51 and 0.62 dL/g, were obtained for these polymers. As can be seen, the inherent viscosity increases as the content of the BF moiety increases in the polymers. As for the density determinations, see Table 2, it is seen that BAP/ISO has higher density followed by BAP50/ISO and BF/ ISO which presents the lowest density of the three polymers. As expected from the fact that random copolymers have densities that depend generally in a linear form on the relative amounts of the concentration of comonomers in the copolymer, BAP50/ISO shows a reasonable agreement with this rule. It is also seen that BF/ISO which bears two large lateral connected phenyl groups from the main chain has a lower density. This result is attributed to an inhibition on packing of the polymer chains due to the bulky fluorene group. The measured density for each polymer was used to calculate the fractional free volume, FFV, from FFV )

(V - V0) V

(2)

where V is the specific volume of the polymer, which is obtained from experimental measurement of the polymer density; V0 is the specific volume occupied by the polymer chains, calculated from van der Waal’s volume (Vw), estimated by Bondi’s group contribution method according to the relation V0 ) 1.3Vw.12 The occupied volume (Vo-Cop) of the copolymer BAP50/ISO was predicted using a direct mixing rule approach as reported elsewhere:8,10 Vo-Cop ) w1V01 + w2V02

(3)

where V01 and V02 are the occupied volumes of each polyester, also w1 and w2 are the weight fractions of each comonomer in the copolymer. The FFVs calculated by Bondi’s method are listed in Table 2. It was found that BF/ISO shows the largest FFV while BAP/ISO has the lowest, a further indication of lower packing of BF/ISO polymer chains. In copolyester BAP50/ISO, FFV falls in between that presented for the two homopolyesters. In Table 2 also, thermal properties are presented for the three polymers synthesized. It is seen that BAP/ISO has the lowest glass transition temperature Tg and BF/ISO has the highest, which could be an indication of higher rigidity of the polymer chains of the latter. The copolymer presents a Tg that is intermediate between those found for the homopolymers. This result is expected in a random copolymer since Tg depends on the relative concentrations of the different comonomers.8,10,11 In Figure 2 the thermograms showing the mass lost by the homopolyesters and the copolyester as a function of temperature between 50 and 800 °C are presented. In the second graph of Figure 2, the first derivative of the thermograms is also shown.


Figure 2. TGA thermograms of BAP/ISO, BF/ISO, and BAP50/ISO.

It also observed, as presented in Table 2, that the three polyesters have an onset of decomposition temperature (Td), taken as the temperature where the first derivative slope drops to form the minimum, under nitrogen atmosphere which is quite close to around 440 °C. However, BF/ISO weight loss starts at lower temperature with 2% weight loss at 400 °C while BAP/ISO and BAP50/ISO remain without appreciable weight loss. The polyesters show a drastic weight loss between 470 and 600 °C where BF/ISO presents a 40% weight loss and BAP/ISO presents 50% weight loss. At temperatures above 600 °C BF/ ISO shows a faster thermal decomposition that reaches a final mass of 10% at 800 °C. On the other hand, BAP/ISO reaches a 40% residual mass at 800 °C. The behavior of the copolyester falls in between that of the homopolyesters presenting a 30% residual mass at 800 °C. The decomposition temperature is the highest for BF/ISO when it is calculated as the middle point of the first derivative dw/dT. It is also seen that the decomposition of BF/ISO reaches a plateau around 600 °C with 60% residual mass, and the weight loss increases quite rapidly between 600 and 800 °C. The BF/ISO mass loss is steeper than the ones presented by BAPISO and BAP 50/ISO, both exhibiting quite similar weight loss behavior in this temperature range. This is attributed to the presence of a large number of phenyl groups in the structure of the polyester and copolyester that are known to increase thermal stability in organic polymers.3,13 Wide-angle X-ray diffraction patterns for isophthalic polyesters and copolyester BAP50/ISO are shown in Figure 3. All polyesters show an amorphous halo in their diffraction pattern with maximum between 17 and 22 deg 2θ. The diffraction pattern of BAP/ISO shows an amorphous halo with a 2θ maximum at 18, whereas the pattern of BF/ISO also shows an amorphous halo with a maximum at 20° and a small shoulder at around 12°. These results are the same as those reported by Charati et al.14 and Pixton and Paul.15 The BAP50/ISO copolyester shows an amorphous diffraction pattern with maxima between those of the homopolyesters. Figure 4 shows values of tan δ for BAP/ISO, BF/ISO, and copolymer BAP50/ISO. Each curve has been shifted by a factor of 1, to exhibit the data more clearly. All polymers show a welldefined maximum for the R-transition (TR) that in BAP/ISO appears at 205 °C, whereas that of BF/ISO appears at 275 °C and the copolyester BAP50/ISO at 232 °C. BAP50/ISO shows a single maximum in tan δ that falls between those of the homopolyesters as expected for a 50 mol % random copolymer. The R-transitions in this polyesters are the result of the same processes that give rise to the glass transition, Tg, and the single

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Figure 3. WAXD scattering patterns for BAP/ISO, BF/ISO, and BAP50/ ISO.

Figure 4. Tan δ as a function of temperature for BAP/ISO, BF/ISO, and BAP50/ISO.

intermediate maximum temperature value for the R-transition in the copolymer confirms that it is a random copolymer. Gas Transport. Gas permeability coefficients, P, at 2 atm and 35 °C are presented in Table 3. The general trend for the polyesters and the copolyester is gas permeability coefficients in the order PHe > PCO2 > PO2 > PCH4 > PN2 showing a decrease as the kinetic diameter of the gas increases. For all gases polyester BAP/ISO shows the lowest P while BF/ISO shows the largest P. The copolymer BAP50/ISO presents P values for all gases that are intermediate between those of the homopolyesters. It was also found (see Figure 5) that the gas permeability coefficients for the copolymer, Pcop are well predicted by a simple mixing rule of the form ln Pcop ) φ1 ln P1 + φ2 ln P2

(4)

where P1 and P2 are gas permeability coefficients for BF/ISO and BAP/ISO and φ1, φ2, are the volume fractions of each comonomer in the copolymer. The trend presented by P in the polyesters and copolyesters is attributed to the FFV available for permeation. As reported in Table 2 BF/ISO has the largest FFV and the highest gas permeability coefficients, while BAP/ ISO, which has the lowest FFV, shows the lowest gas permeability coefficients. The dependence of P with FFV was related by Lee16 with an Arrhenius-type equation:17,18

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Table 3. Pure Gas Permeability Coefficients and Ideal Separation Factors of BAP/ISO, BF/ISO, and BAP50/ISO Random Copolyester at 2 atm Pressure and 35 °C RA/B

a

polymer

PHe

PO2

PN2

PCH4

PCO2

PO2/N2


23.0 25.5 30.2

2.58 2.84 3.92

0.54 0.56 0.64

0.59 0.65 0.71

13.1 16.3 20.1

4.7 5.0 6.1

NP

CO2/CH4

22.2 25.2 28.3

P

CO2/N2

24.2 29.2 31.4

Permeability is expressed in barrer, where 1 barrer ) (10-10 cm3 (STP) cm)/(cm2 s cm Hg).

Figure 5. Comparation of the predicted copolymer permeability with experimental values.

P ) A exp[-B/FFV]

(5)

where A and B are characteristic constants of the polymerpenetrant system and which assumes that solubility would not be a strong function of free volume. The possibility of being able to calculate FFV by group contribution methods makes it a useful relationship. In several amorphous polymer families and its copolymers,8,10,11,17,18 it has been found that the logarithm of P decreases in a linear form with increasing reciprocal FFV. As is evident from the plots in Figure 6, the dependence of P with the reciprocal FFV for BAP/ISO, BF/ ISO, and BAP50/ISO follows closely the behavior described by eq 5. While the behavior of P for BAP/ISO, BF/ISO, and BAP50/ ISO with respect to the fractional free volume and copolymer composition adjusts to the simple linear mixing rule, a deviation was found in the expected behavior of the ideal separation factors RA/B. The usual trade-off found in polymer gas permeability is that as P increases RA/B should decrease. As it is reported in Table 3, in this particular case, BF/ISO presents the highest P and also the highest RA/B while BAP/ISO, which reports the lowest P, has RA/B values that are around 30 to 40% lower than those presented by the BF/ISO for the commercially important pairs O2/N2, CO2/CH4, and CO2/N2. It was also found that BAP50/ISO has RA/B values which are intermediate to the ones presented by the homopolymers. This indicates that copolymerization not only has increased P but has made the polymer more selective to gas separation, in particular those pairs involving CO2 gas. Knowing that permeability coefficients based on the solution diffusion model are the product of the diffusion coefficient and the solubility coefficient: P ) DS

(6)

The ideal separation factor can be factorized in their diffusivity selectivity, and solubility selectivity factors of the form:

Figure 6. Gas permeability coefficients as a function of reciprocal fractional free volume for BAP/ISO, BF/ISO, and BAP50/ISO at 2 atm and 35 °C.

RA/B )

( )( )

PA DA SA ) PB DB SB

(7)

Apparent gas diffusion, D, and solubility coefficients, S, are given in Table 4 for the polyester and copolyester BAP/ISO, BF/ISO, and BAP50/ISO. As reported D for all gases is larger for BF/ISO followed for BAP50/ISO which in turn agrees with a more open structure for these polymers according to FFV values as compared to BAP/ISO. This is in agreement with the fact that according to Cohen and Turnbull gas diffusivity D is assumed to depend on free volume following the equation:18,19

[

D ) AD exp -

BD FFV

]

(8)

Where D is the diffusion coefficient, AD and BD are the characteristic constants of the polymer-penetrant system that are independent of penetrant concentration. Figure 7 shows the dependence of D with reciprocal FFV. As it can be observed in Figure 7, the apparent diffusion coefficients correlate well with the experimental values, an indication that the increase in FFV can be used to predict diffusion coefficients in this kind of aromatic copolyesters. It is also seen that diffusivity selectivities DA/DB are very similar in the polyesters and copolyesters for the gas pairs CO2/ CH4 and CO2/N2 indicating that, even though diffusion increases with free volume, selectivity by diffusion in these gas pairs is similar in all polyesters. Diffusion selectivity of the gas pair O2/N2 increases as FFV increases in the polyester which indicates that the increase in selectivity in this gas pair is due to diffusivity selectivity. Apparent solubility coefficients estimated as the ratio of permeability coefficients, P, and apparent diffusion coefficients, D, follow the order SCO2 > SCH4 > SO2 > SN2 where CO2 is the more soluble of the gases tested. Apparent solubility values

Ind. Eng. Chem. Res., Vol. 49, No. 23, 2010 a

b

Table 4. Apparent Gas Diffusion and Solubility Coefficients, Diffusivity Selectivities, and Solubility Selectivities for BAP/ISO, BF/ISO, and BAP50/ISO Random Copolyester at 2 atm Upstream Pressure and 35 °C DA/DB polymer

DO2

DN2

DCH4

DCO2

DO2/N2

D CO2/CH4


2.10 2.70 2.96

0.86 0.90 0.97

0.35 0.39 0.42

1.28 1.35 1.37

2.44 3.00 3.05

3.65 3.46 3.26

D

CO2/N2

1.48 1.50 1.41

SA/SB polymer

SO2 SN2 SCH4 SCO2 SO2/N2 SCO2/CH4 SCO2/N2

BAP/ISO 1.22 0.62 BAP50/ISO 1.05 0.62 BF/ISO 1.32 0.66

1.68 1.66 1.68

10.24 12.13 14.65

1.96 1.69 2.00

6.09 7.30 8.72

16.51 19.56 22.19

a D has units of 10-7 cm2/s and was determined from time lag measurements. b S has units of 10-3(cm3 (STP))/(cm3 cmHg). Determined as the ratio P/D.

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and 274 °C while the copolyester shows a Tg that falls between those of the polyesters according to the relative concentration of the comonomers. Td values are above 450 °C for all the polyesters and copolyester. Wide-angle X-ray diffraction measurements reported amorphous patterns for all polymers with the copolyester BAP50/ISO showing an amorphous pattern with a maximum between those of the homopolyesters. Density measurements indicate that BAP/ISO presents the highest density while BF/ISO shows the lowest one. It was also found that as density increasesthe permeability and diffusion coefficients decrease because FFV present in the polyester decreases. The latter was attributed to higher intermolecular chain packing that in turn slows the permeation process in a polymer. The copolymer shows permeability coefficients that are intermediate between those of the parent homopolyesters, and they fall in line with the values expected from the relative concentration of them. It was also found that there exists a departure from the usual trade-off that is found in gas permeation through polymers, which indicates that as gas permeability coefficients increase in polymers the selectivity or ideal separation factor decreases. Given the fact that BF/ISO is the most permeable polyester of the ones tested in this work, in order to follow the usual trade off, it should present the lowest separation factor; however, it is the one that presents the largest ideal separation factors and permeability coefficients, a fact that is attributed to an increase in solubility selectivity particularly for the gas CO2. Acknowledgment

Figure 7. Apparent diffusion coefficients as a function of reciprocal fractional free volume for BAP/ISO, BF/ISO, and BAP50/ISO at 2 atm and 35 °C.

The present work was performed under Grants 43173 from CONACYT and 108920 from FOMIX-CONACYT. The authors are grateful to Dr. Humberto Va´zquez-Torres for running the WAXD measurements and Dr. Jose´ Manuel Cervantes for kindly running TGA measurements in the homopolyesters and copolyester. Literature Cited

increase for CO2 as the concentration of BF/ISO increased in the polyesters, while in O2, N2, and CH4 they present similar values for all polymers. The later values may arise from the method used to calculate S as the ratio of P/D. They were reproducible when retested in homopolymers and a copolymer, showing a close value among them. In all cases BF/ISO shows the highest solubility coefficient, while the values for BAP/ISO and BAP50/ISO are lower. We believe that the larger solubility for CO2 in BF/ISO arises from a larger unpaired electron density due to the bridged benzene rings in the fluorene moiety in this polyester. On the basis of the DA/DB and SA/SB values given in Table 4, the increase in ideal separation factors observed for BF/ISO as compared to that of BAP/ISO could be ascribed to an increase in solubility selectivity, particularly for the pairs in which CO2 is involved, CO2/CH4 and CO2/N2, and is an indication of a better affinity of CO2 with BF/ISO which presents the highest apparent solubility coefficient for this gas. Conclusions Two polyesters and a copolyester were synthesized by interfacial polymerization. The aromatic polyesters formed transparent membranes and they present Tg values between 200

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ReceiVed for reView March 3, 2010 ReVised manuscript receiVed July 25, 2010 Accepted July 29, 2010 IE100478V

Synthesis and Gas Transport Properties of Polyesters and a

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