Preparation and Characterization of Sulfonated Copolyamides Based

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Preparation and Characterization of Sulfonated Copolyamides Based on Poly(hexafluoroisopropylidene) Isophthalamides for Polymer Electrolytic Membranes Yamile Perez-Padilla,† Mascha. A. Smit,‡ and Manuel J. Aguilar-Vega*,† †

Unidad de Materiales and ‡Unidad de Energías Renovables, Centro de Investigacion Científica de Yucatan A.C., Calle 43 No. 130, Chuburna de Hidalgo, 97200, Merida, Yuc. Mexico ABSTRACT: A series of sulfonated random copolyamides with increasing sulfonation degree were synthesized from the aromatic diamine 4,40 -(hexafluoroisopropylidene)dianiline, HFA, reacted with 2,4-diaminobenzenosulfonic acid, DABS, by direct polycondensation using isophthalic diacid, ISO, as comonomer. Thin films of the sulfonated copolyamides were prepared by the solution-evaporation method using dimethylacetamide, DMAc. The structure of the resulting sulfonated copolyamides was confirmed by FTIR and 1H NMR which evidenced the presence of amide and sulfonic groups in the proposed concentrations. Thermal stabilities as characterized by TGA, showed the onset of decomposition at 320 °C for the ionic moieties and above 600 °C for the nonsulfonated part of the copolymers. The values of water uptake and ion exchange capacity, IEC, at room temperature for the copolymer with 50 mol % of DABS, HFAS55 membrane were higher than those obtained for Nafion 115. HFAS55 also presented a proton conductivity value of 3.3 (mS cm1) which was similar to the one of Nafion 115 tested under the same conditions which situates it as a possible material for PEM membrane preparation.

1. INTRODUCTION The development of proton exchange membranes (PEM) for fuel cells has received extensive attention lately, because they are known to be one of the most promising clean energy conversion technologies. PEM fuel cells are regarded to be the next step into producing efficient and clean energy since they offer many advantages such as high efficiency, high energy density, quiet operation, and environmental friendliness and they can be used in the generation of power for laptops, cell phones, transportation, and residential use. Perfluorosulfonate ionomer membranes, such as Nafion membranes, are the standard membranes used in fuel cells presently. Although Nafion shows superior performance, the application of these membranes is limited by their high cost, loss of proton conductivity at high temperature and loss of humidity and poor barrier properties to methanol crossover.16 The most successful alternative (PEM) membranes studied to date are based on high performance polymeric backbones containing sulfonic acid groups, such as polysulfones,2,5,79 polyimides,1015 poly(arylene-ether)s1619 and poly (etherether-ketones).20,21 Sulfonation of these polymers is achieved by introducing sulfonic groups (SO3H); either by treating an existing polymer with sulfonating agents (postsulfonation) or by synthesizing polymers with monomers containing sulfonated moieties, which render these polymers proton conductive. The hydrophobic backbones enhance physical properties and the sulfonic acid groups provide the ionic conductivity, and the resulting polymers exhibit high ionic conductivity, good mechanical strength, and high temperature resistance.15,811Preparation of materials by postsulfonation procedures not only affects mechanical and thermal stability but also could make control of the sulfonation process difficult. On the other hand, when sulfonation is achieved by synthesis of the polymer with monomers bearing sulfonic groups, it is possible to control the concentration and position of the sulfonic r 2011 American Chemical Society

group in the polymer structure. Moreover, it is important to control the sulfonation degree (SD); high sulfonation degrees generally lead to high swelling and even dissolution in water of the membranes, while low sulfonation degrees generally result in poor proton conductivity.2224 Aromatic polyamides are considered a class of engineering polymers that are thermally stable, with excellent mechanical properties. The incorporation of ionic groups into the structure of these polymers could impart them with ion exchange properties.22 Because of the presence of amide groups, aromatic polyamides tend to retain water. Amorphous aromatic polyamides, particularly those containing fluorine groups such as poly(hexafluoroisoporpylidene) isophthalamide, HFAISO, are reported to absorb 4 to 10% water; they are also reported to have a high fractional free volume due to the presence of the lateral CF3 groups that hinder rotation and increase chain rigidity.22,25 Depending on their chemical composition, polyamides may show enhanced water absorption and proton conduction capacity by the introduction of sulfonic acid side groups.22,26,27 Since fluorinated polymers such as Nafion are known to be resistant to degradation in a PEM cell environment due to their perfluorinated hydrophobic backbone membranes, the possibility of sulfonating HFAISO to render it proton conductive should be explored to determine if it could reach acceptable properties for PEM cell applications. In this work, we report a series of sulfonated copolyamides, with sulfonation degrees, SD, between 20 and 50 mol % that are synthesized from the diamines 4,40 -(hexafluoroisopropylidene)dianiline, HFA, and 2,4-diaminobenzensulfonic acid, Received: November 30, 2010 Accepted: June 23, 2011 Revised: June 17, 2011 Published: June 23, 2011 9617

dx.doi.org/10.1021/ie102409d | Ind. Eng. Chem. Res. 2011, 50, 9617–9624

Industrial & Engineering Chemistry Research DABS, as comonomer. The mixture of diamines is reacted by polycondensation with isophthalic diacid, ISO, in order to produce random copolymers poly(hexafluoroisopropylidene-co2,4-diaminobenzensulfonic) isophthalamides (HFA-co-DABS/ ISO). Membranes from the sulfonated polyamides were prepared using solvent evaporation. Their structure is characterized by FTIR and 1H NMR spectroscopy. Their thermal stability was assessed as well as their ionic exchange capacity (IEC), water uptake, and proton conductivity in order to determine the effect of an increased concentration of sulfonic acid groups on these properties. At the same time, we would like to assess if sulfonated HFA-co-DABS/ISO as prepared membranes present characteristics similar to those membranes used in PEM cells.

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Table 1. Molar Concentration of Monomers Used for Synthesis HFA Polyamide and Copolyamides HFA-coDABS/ISO

2. EXPERIMENTAL SECTION 2.1. Materials. Monomers for sulfonated copolyamide preparation, 4,40 -(hexafluoroisopropylidene)dianiline (HFA), 2,4diaminobenzensulfonic acid (DABS), and isophthalic acid (ISO), were obtained from Aldrich Chemical Co. Solvents such as pyridine (Py), triphenyl phosphite (TPP), and dimethyl acetamide (DMAc) were used as received without further purification. Isophthalic acid (ISO) and DABS were dried at 100 °C for 24 h before used. N-Methyl-2-pyrrolidone, NMP, was stored over molecular sieves. Calcium chloride (CaCl2) was dried at 180 °C under vacuum for 24 h before use. All reactants and solvents were purchased from Aldrich Chemical, Co. 2.2. Synthesis of Homopolyamides and Sulfonated Copolyamides. Polyamide poly(hexafluorosiopropylidene)isophthalamide, HFA, and the sulfonated copolyamides were synthesized by direct polycondensation following a procedure described first by Yamazaki28 with some small modifications due to the ionic nature of comonomer DABS. The polycondensation reaction was performed with equimolar amounts of the diamine, HFA, and diacid, ISO, for HFA preparation, or a controlled combination of nonsulfonated and sulfonated diamines, HFA and DABS with isophthalic diacid, ISO, for copolymers preparation, HFA-co-DABS/ISO, adjusting the concentration of diamine monomer HFA to monomer DABS (HFA/DABS) as shown in Table 1 which in turn adjusts the sulfonation degree, SD. The reaction was carried out using NMP as a solvent, with Py and TPP as the transfer reactants. In a typical polymerization reaction into a three neck roundbottom flask, equipped with mechanical stirrer and nitrogen flow, a mixture of diamines HFA and DABS, 1.25 mmol, and isophthalic acid, 1.25 mmol, were dissolved in 2.8 mL of NMP, 0.7 mL of TPP, and 0.7 mL of Py. When the mixture was completely dissolved, CaCl2 was added and the solution was heated to 100 °C and kept at this temperature for 23 h. After this time, the reaction was cooled to 70 °C and the solution was precipitated into methanol with constant stirring. The polymer obtained was filtered off and washed with cold methanol and cold water continuously, and dried at 120 °C under vacuum for 24 h. Thin membranes of the polyamide and sulfonated copolyamides were cast using dimethylacetamide, DMAc, as the solvent by the solution-evaporation method. The as obtained polyamide and sulfonated copolyamides were dissolved in DMAc, (8 wt/vol %), the solution was filtered and the membrane cast onto an aluminum plate enclosed in a 70 mm aluminum ring, the solvent was evaporated slowly at 70 °C. The remaining solvent was fully removed under vacuum at 180 °C for 48 h. The structure of the resulting membranes was analyzed by Fourier transform infrared

spectroscopy using the method of attenuated total reflectance (ATR), with an optical microscope coupled to a Fourier transform infrared spectrometer, Nicolet Protege 460, taking an average of 50 scans per sample between 600 and 4000 cm1. 1 H NMR spectra for HFA and HFA-co-DABS/ISO copolyamides were obtained in a Bruker 400 MHz NMR spectrometer using deuterated dimethyl sulfoxide (DMSO-d6) as the solvent and tetramethylsilane (TMS) as an internal standard. Inherent viscosities (ηinh) of all polymers were determined in DMAc using a Cannon-Ubbelohde viscometer No. 50 at 30 °C at a polymer concentration of 0.2 g/dL. Thermal decomposition of the as cast membranes was followed by thermogravimetric analysis in a TGA-7 Perkin-Elmer Inc. under nitrogen atmosphere. The scans for thermal decomposition were taken between 50 and 800 °C at a heating rate of 10 °C min1. To make sure that all the solvent was eliminated in the membrane, all polyamides were previously heated from 50 to 320 °C at a heating rate of 10 °C/min, if solvent presence was detected, they were further dried in a vacuum oven at 230 °C, and the thermogravimetrical measurements repeated in the dried samples from 50 up to 800 °C. The ability of the membranes to absorb water was estimated by gravimetric analysis. The membrane was dried under vacuum at 120 °C for 48 h to eliminate moisture content. Then, it was immersed in deionized water for 48 h at a fixed temperature (at 25, 45, 65, and 75 °C). At the end of this time the membrane was extracted, wiped with blotting paper, and immediately weighed to determine its wet membrane weight (Ws). The next step was to dry the membrane at 120 °C under vacuum for 48 h and weight it again to determine the dry membrane weight (Wd). The water uptake, S, was calculated according to the following equation: S¼

Ws Wd 100 Wd

ð1;Þ

Water uptake for polyamides and copolyamides is reported as the average value of at least two measurements. A potentiometric titration technique was used to determine the ion exchange capacity (IEC) of the membranes at room temperature. The dry membrane was weighed and immersed in 1 M HCl solution for 24 h and then was washed with deionized water until the pH was neutral. The membrane in the acid form, H+, was then converted to the sodium form, Na+, by immersing the membrane in NaCl 2 M solution for 9618

dx.doi.org/10.1021/ie102409d |Ind. Eng. Chem. Res. 2011, 50, 9617–9624

Industrial & Engineering Chemistry Research

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Table 2. Sulfonation Degree Determined by 1H NMR and Inherent Viscosity of Polyamide and Sulfonated Copolyamides HFA-co-DABS/ISO

Scheme 1. Schematic Reaction to Obtain the HFA-coDABS/ISO Sulfonated Copolyamides

SD (%) polymer

theoretical

HFA

a

24 h. The exchanged H+ ions were titrated using NaOH 0.01 M solution. The IEC of the membranes was determined by21 IEC ðmmol=gÞ ¼

CV W

ð2;Þ

where C (M) and V (mL) are the concentration and volume of NaOH, respectively. W (g) is the weight of the dry membrane. IEC is reported as the average of at least two measurements. Mechanical properties under uniaxial tension for wet membranes were determined at 25 °C in a microtensile machine Minimat (Rheometrics Inc.) at a testing speed of 1 mm min1 with a 100 N load cell. The samples were cut into strips 6.5 mm long 3.2 mm wide with a thickness between 0.08 and 0.15 mm. The strips were dried under vacuum at 120 °C for 48 h. Then the dried strips were immersed in deionized water at 25 °C for 48 h prior to testing. For each homopolymer and copolymer sample the average value is reported for at least three measurements. Proton conductivity was measured by electrochemical impedance spectroscopy using a potentiostat-galvanostat AUTOLAB model PGSTAT12/30/302 with the EIS module over a frequency range of 106 to 1 Hz with oscillating voltages of 50 mV. Prior to the measurement, and in order to test their stability, the membranes were immersed in water for 96 h, then they were immersed in HCl 1 M for 12 h to activate them, and finally they were washed with deionized water until neutral pH. To have a direct comparison with a commercial material, Nafion 115 was also subjected to the same treatment, and its proton conductivity was measured under the same conditions. The membranes were sandwiched between two stainless steel alloy 20 electrodes (13 mm diameter) in a Teflon cell. The measurements were carried at room temperature. The samples were cut into circular shape with a 13 mm diameter using a hollow chisel. The resistance value associated with the membrane conductance was determined from the high frequency intercept of the impedance with the real axis. The proton conductivity (σ) of each membrane was calculated from the impedance data according to the follow equation: σ ¼

l RA

ð3;Þ

where σ is the proton conductivity (S cm1), l is the membrane thickness (cm), R is the membrane resistance (Ω), and A is the membrane area (cm2).

H RMN

ηinha

1

-

0.35

HFAS82

20

18.00

0.27

HFAS73

30

28.60

0.25

HFAS64

40

39.71

0.18

HFAS55

50

49.26

0.19

ηinh = inherent viscosity at a polymer concentration of 0.2 g/dL.

3. RESULTS AND DISCUSSION Sulfonated copolyamides HFA-co-DABS/ISO with different sulfonation degrees, SD were synthesized through the high temperature polycondensation reaction of the monomers: HFA, DABS, and ISO in the presence of NMP, TPP, and Py. The sulfonation degree, SD, was controlled by adjusting the ratio of diamines HFA and DABS (HFA/DABS). A schematic of the reaction is given in Scheme 1. The HFA polyamide and HFA-coDABS/ISO copolymers obtained were white rigid fibers. The fiber size increased with decreasing concentration of sulfonated diamine in the copolymer. In contrast, the fibers will tend to swell in water with increasing concentration of sulfonic acid groups. The amount of CaCl2 used in the reaction had a great effect on the polymerization, as it was necessary to add a larger amount (8 to 10% more) of CaCl2 as the amount of sulfonated diamine increases in the copolymer polymerization reaction. It was also necessary to increase the reaction times described in the literature,29 during the synthesis of sulfonated copolyamides; otherwise a yield reduction in the reaction or low molecular weight products were obtained. The resulting polyamides showed moderated inherent viscosities which decrease with increasing concentration of sulfonated monomer and they are in the range of 0.350.18 dL/g in DMAc with ηinh being quite similar for the copolyamides with the highest concentration of sulfonic acid groups HFAS64 (40 mol % of sulfonic acid groups) and HFAS55 (50 mol % of sulfonic acid groups). As can be observed in the Table 2, in general, as the degree of sulfonation increased, the inherent viscosity decreased. This effect had been observed in several sulfonated polymers.22 Films of copolymers elaborated using the solution evaporation method were translucent with a yellowish coloration that intensified as SD increased. The rigidity of the films also increased with increasing sulfonation degree. The thickness of all membrane films was in the range 0.080.15 mm. In Figure 1 the IR spectra of the synthesized HFA polyamide and HFA-co-DABS/ISO copolyamides are shown. These spectra were used to analyze characteristic bands of the SO3H group in the polymer chains. There are several bands present that are expected for the formation of the sulfonated polyamides. The characteristic broad bands’ for the amide group between 3270 and 3310 cm1 (NH), the amide carbonyl groups at 1660 cm1. On the other hand, the bands at 1077, 1020, and 690 cm1 appeared only in sulfonated copolymers. The absorption band at 690 cm1 is assigned to CS stretching vibration and those at 1077 cm1 and 1020 cm1 are characteristic of the symmetric and asymmetric OdSdO stretching vibrations of the sulfonic acid group, respectively. These three characteristic bands 9619



Figure 1. FTIR of HFA polyamide and sulfonated copolyamides HFAco-DABS/ISO.

Figure 2. 1H NMR of HFA and HFA-co-DABS/ISO copolymers with increasing sulfonation degree.

have a gradual increase with an increase in the content of sulfonated diamine, SD, in the copolymer. Figure 2 shows the 1H NMR spectra of (a) HFA polyamide, and (b) HFA-co-DABS/ISO sulfonated copolyamides respectively. The integration area ratio of each proton agrees with the structure of HFA polyamide, Figure 2a, and HFA-co-DABS/ISO copolyamides, Figure 2b. Initially there appear two new proton interactions when the sulfonic acid groups are introduced to the polyamide, the peak at 8.95 ppm only appears in the spectra of sulfonated copolymers. This signal corresponds to the ortho proton Hg with respect to the sulfonic acid group, appearing at downfield due to the electron-withdrawing effect of the sulfonic acid group. The aromatic hydrogen atoms Hh and Hi near the SO3H group were detected at 8.63 and 7.54 ppm, respectively. We also observed that with an increasing amount of sulfonated

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Figure 3. Thermal decomposition thermograms of HFA and HFA-coDABS/ISO sulfonated copolyamides.

monomer there is an increase in the intensity of the signal of these protons. All the other peaks correspond to other aromatic hydrogen atoms of the chemical structure of the sulfonated copolyamides that are also present in the hompolyamide HFA. To determine the sulfonation degree, from Figure 2 (b) the relationship between the areas of protons which does not change in HFA or the copolymers, Hf, and the proton ortho to the SO3H group in the copolymer Hg, was calculated.27 The resulting concentration of sulfonated moieties using the ratio of the areas for protons Hf and Hg for sulfonated copolyamides agrees well with the concentration of sulfonated monomer initially charged for the synthesis, with a minimum drift, indicating that the structures proposed with different sulfonation degrees, SD, were obtained, as described in Table 2. Figure 3 shows the thermal decomposition thermograms of HFA and HFA-co-DABS/ISO sulfonated copolyamides. The thermograms have been shifted by 2% from each other starting from the one corresponding to HFA to make it easier to follow their decomposition. All polymers show an initial weight loss up to 100 °C that is associated with the loss of absorbed water. The mass loss of the nonsulfonated polyamide, HFA, is 2% at around 100 °C, as was expected from other similar aromatic polyamides found in the literature,25,29 while that of the sulfonated copolyamides could reach up to 5%. The initial weight loss of the sulfonated copolyamides is related to desorption of moisture bound by the polymer that increases with the presence of the hydrophilic sulfonic group. The polyamide HFA presents the highest thermal stability as compared with the sulfonated polyamides since it shows the onset of decomposition temperature, Td, at around 480 °C which is attributed to the onset of the aromatic polymer chain degradation. The resulting high decomposition temperature is due to the large number of aromatic groups as well as CF3 groups present in the structure that results in a high thermal stability. On the other hand, all the thermograms for the sulfonated copolymers HFA-co-DABS/ ISO show similar profiles which include two decompositions at different temperatures from that presented in the nonsulfonated HFA, one of them below 320 °C and a slope change above 550 and up to 600 °C. The initial weight loss starting just below 320 °C, is attributed to the onset of sulfonic acid group, SO3H, decomposition. The weight loss that occurs in HFA-co-DABS/ ISO starting at around 320 and up to 550 °C until ∼600 °C 9620



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Table 3. Thickness and Mechanical Properties under Tension of HFA, Nafion 115, and Sulfonated Copolyamides HFA-co-DABS/ ISO polymer

thickness (mm)

tensile strength (MPa)

Young’s modulus (MPa)

elongation at break (%)

HFA

0.08

27.4 ( 8.6

494.3 ( 23.4

6.5 ( 3.4

HFAS82 HFAS73

0.11 0.15

34.8 ( 8.0 20.0 ( 5.6

427.1 ( 38.2 277.5 ( 13.6

11.9 ( 5.7 10.9 ( 1.6

HFAS64

0.12

21.0 ( 9.9

286.8 ( 11.8

9.1 ( 3.5

HFAS55

0.14

29.7 ( 1.5

410.8 ( 11.8

12.6 ( 1.7

Nafion 115

0.12

13.2 ( 0.1

89.3 ( 11.6

181 ( 54.0

Table 4. IEC and Water Stability of Polyamide HFA and Sulfonated Copolyamides HFA-co-DABS/ISO IEC (mmol/g) polymer

theoreticala

HFA

experimentalb 0.15

T (°C)

t (h)

water stability O

25

120

75

48

O

HFAS82

0.59

0.58

25

120

O

HFAS73

0.87

0.49

75 25

48 120

O O

HFAS64

1.02

0.83

HFAS55

1.39

1.6

Nafion 115

1.28

75

48

O

25

120

O

75

48

25

120

O

75

48

25

120

O

75

48

O

Theorical IEC values were obtained as in ref 1; IEC = (SD 1000)/(318 + 80SD). b Experimental measured by titration; T, temperature; t, time. (O) Mechanical strength maintained; () somewhat brittle.

Figure 4. Water uptake as temperature increase for membranes of HFA polyamide and HFA-co-DABS/ISO sulfonated copolyamides.

a

increases with increased concentration of sulfonic moieties in the copolymer. At 580 °C there is a change of slope in the ionic copolymers which is attributed to thermal decomposition of HFA aromatic moieties on the copolymer main chain since it follows closely the decomposition thermogram of HFA. The thermograms clearly indicate that the sulfonated copolymer onset of thermal stability is situated at around 320 °C, where the sulfonated moieties onset of thermal decomposition occurs, well above the servicing temperature for a PEM cell application. It is also clear in Figure 3 that all HFA-co-DABS/ISO membranes present similar onsets of decomposition temperature, weight losses, and thermal stability, which are above those shown by Nafion 115 that decomposes quite fast in the interval between 320 and 500 °C. Since ionic membranes tend to be used in environments with high humidity such as PEM cells or in contact with water solutions, it is important to determine their mechanical properties when they are hydrated. Table 3 lists the thickness, tensile strength, Young's modulus, and elongation at break of HFAISO and HFA-co-DABS/ISO membranes. As a reference, Nafion 115 was also subjected to the same test. The values of Young’s modulus are between 280 and 494 MPa, and tensile strength values are between 20 and 34 MPa; they also do not show a general trend with increasing sulfonation degree. Since water uptake at 25 °C is already equilibrated at 48 h in the membranes,

the mechanical tests values dispersion is attributed to differences in the plasticization of the test samples particularly for HFAS55, which shows higher mechanical tensile modulus and strength. It is also found that the values for tensile strength and Young's modulus are at least 2 times larger than those found in Nafion 115 at the same testing conditions. The hydrolytic stability of the membranes was determined by subjecting them to four cycles of 48 h immersion in deionized water and drying in a vacuum oven at 120 °C. The cycles were performed at four different temperatures of 25, 45, 65, and 75 °C, reported in Table 4 as water stability at 75 °C for 48 h, or up to 120 h at 25 °C . Under this conditions they did not dissolve and were observed only slightly bent and with a little stiffness but not broken. It is also expected that membranes with lower mechanical stability should have higher water uptake.23 The results found are summarized in Table 4 and they indicate that membranes with a larger SD tend to be more sensitive to hydrolysis. The amount of water uptake of polyamide HFA and copolyamide HFA-co-DABS/ISO membranes at different temperatures (25°, 45°, 65° and 75 °C) is shown in Figure 4. A commercial Nafion 115 membrane was also tested at the same conditions. As can be seen in Figure 4, HFA-co-DABS/ISO membranes with higher sulfonation degree values, HFAS64 and HFAS55, tended to have larger water uptake at all temperatures and the water uptakes increase with increasing SD. It is also seen that the hydrophylicity in HFAS64 and HFAS55 is larger than the one observed in Nafion 115 which is attributed to a larger free volume due to the presence of CF3 groups and swelling of the membrane due to the water bonded by the SO3H groups. Those two effects combined are credited for the increase in water 9621



Figure 5. Proton conductivity and IEC of HFA and HFA-co-DABS/ ISO membranes as SD increases.

uptake even though the aramide polymer backbone is less flexible than that of the Nafion 115. It also seen that at low temperatures (25 °C), there is a jump in the water uptake between copolymers containing 30 and 40 mol % sulfonic acid groups, it is taken as an indication that there is a critical concentration of SO3H groups that given the opening of the structure and vicinity of them allows for an increase in water uptake, as have been reported in other similar polyamides.22 The water uptake of HFA is the lowest observed and it does not change appreciably with temperature. The sulfonated membranes with high concentrations of sulfonic acid groups’, HFAS55 and HFAS64, present water uptake values that increase between 25 and 75 °C while membranes HFA and HFAS82, present water uptake values below those presented by Nafion 115, and HFAS73 presented water absorption values that run close to those found for Nafion 115 for the temperature interval evaluated. At elevated temperatures, the polymer chain mobility increases and the free volume in these copolymers could increase leading to the water adsorption increment in the membranes for all rigid membrane copolyamides.18,21 If only based on water absorption characteristic membranes HFAS64 and HFAS55 present the best characteristics of water uptake, even above those of Nafion 115. It has been reported in the literature that the water up-take and ion exchange capacity, IEC, play important roles in the conductivity of a sulfonated polymer. In fact, both IEC and water uptake contribute to the proton transport through the dense membrane. Therefore, it is important to consider how ion transport performance of the membrane correlates to its water retention capacity and also to the IEC value.8,14,18,19,21,30,31 Table 4 shows theoretical ion exchange capacity, IEC, values for the synthesized copolymers and they are compared to experimentally obtained values of IEC for HFA-co-DABS/ISO membranes with different sulfonation degrees evaluated by titration. The theoretical IEC values are higher than those found experimentally with the exception of the ones for HFAS55 which presents an IEC value that is larger than the theoretical one. There is also a lower than expected value of IEC for HFAS73 which is the lowest of all measured values. The low IEC for HFAS73 seem to correlate well with a low water uptake at 25 °C and also a lower proton conductivity which is discussed later. The membranes obtained from copolyamides HFA-co-DABS/ ISO showed an increase in IEC with increasing degree of

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sulfonation; this is attributed to the increase on water concentration and retention of sulfonated polyamides due to the increased concentration of sulfonic groups, which in turn opens the sites available for ionic exchange in the structure, and increases the possibilities of an enhanced proton transfer capacity. This behavior is better illustrated by Figure 5, where the proton conductivity and the relationship between SO3H group concentration and IEC are depicted. There is a clear indication that an increase in IEC and sulfonation degree in HFA-co-DABS/ISO membranes provides an increase in conductivity. It is also seen that proton conductivity in membranes HFAS82 and HFAS73 remains below or at 0.04 mS cm1, while there is a marginal increase to 0.09 mS cm1 for HFAS64. The best proton conductivity is found for the HFAS55 membrane which shows a value of 3.3 mS cm1 quite close to that found for Nafion 115 (6.6 mS cm1) which goes hand-in-hand with increases in IEC. The proton conductivity, σ, for membranes at 25 °C is shown in Figure 5. Nafion 115 was used for comparison with the membranes synthesized in this work. Following the trend observed for ion exchange capacity, the proton conductivity of the copolyamides membranes shows a trend, of increasing proton conductivity with increasing SD as in other reports,4,31 see Figure 5. Membranes with higher IEC tend to have higher proton conductivity although differences for HFA-co-DABS/ ISO copolymers are small (0.04 to 0.09 mS cm1) up to 40 SO3H mol % with a marked increase for HFAS55. The explanation of this behavior is related to a threshold in which, based on a gradual growth of SD domains, water clusters are formed in close proximity and once formed they give rise to an increase in proton conductivity. This mechanism seems to be followed closely by the sulfonated HFA copolyaramide membranes and has been reported for other sulfonated polymers.5,23 The membrane HFAS55 had the highest IEC value and therefore greater proton conductivity compared to other membranes synthesized. In addition, although HFAS55 presents slightly larger IEC values than Nafion 115, its proton conductivity is roughly in the same level as the Nafion 115, tested under the same conditions. As shown in Figure 5, an increase in SD from 20 to 50% results in an increase of up to 2 orders of magnitude in proton conductivity values. These results indicate that it is possible to render proton conductive copolymers based on fluorinated polyaramides such as those prepared in this work. By controlling the sulfonation degree, it is possible to obtain IEC and proton conductivities similar to those presented by standard ionic membranes of Nafion 115, with better thermal stability. Further tests should be performed, in particular oxidative stability, to determine if HFAS55 is a suitable candidate for proton exchange membranes with improved thermal stability as compared to the ones now being used.

4. CONCLUSIONS Membranes based on rigid fluorinated copolyaramides, HFA, and a sulfonated monomer DABS were obtained with increasing degree of sulfonation by direct polycondensation, the resulting poly(hexafluoroisopropylidene-co-diaminobenzensulfonic) isophthalic copolyamide, HFA-co-DABS/ISO, was cast from DMAc; the thicknesses of all membrane films were in the range 0.080.15 mm, and the rigidity of the films increased with increasing sulfonation degree. Sulfonated copolyamides showed good thermal stability up to 320 °C which is attributed to the onset of sulfonic acid groups decomposition. The mechanical 9622


Industrial & Engineering Chemistry Research tests showed larger Young's modules and tensile strength as compared to those presented by Nafion 115. The analysis of water uptake, IEC, and proton conductivity, generally show that they increase with increasing concentration of sulfonated diamine in the copolyamides, although there is a threshold were the water uptake and IEC jump to higher values at a concentration between 30 and 40 mol % of sulfonic acid groups. The membranes HFAS55 show values of water uptake and IEC higher than that of Nafion 115 at room temperature. Proton conductivity values at room temperature obtained with the membrane HFAS55 are in the same order of magnitude as those obtained for Nafion 115. Overall membranes prepared from HFAS55 should be considered good candidates for proton exchange membranes with improved thermal stability as compared to the ones currently in use, although further testing for oxidative and acid stability should be made.

’ AUTHOR INFORMATION Corresponding Author

*E:mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by The National Council for Science and Technology, CONACyT (Project Grants 108920 FomixYucatan and CB 83295). Yamile Perez-Padilla thanks CONACyT for a scholarship (170273). The authors are grateful to MMP Maria Isabel Loría Bastarrachea and MMP Wilberth Herrera Kao, for helpful advice in the synthesis reaction and Dr. Daniela Esperanza Pacheco Catalan for help in electrochemical measurements. ’ REFERENCES (1) Zhong, S.; Liu, C.; Dou, Z.; Li, X.; Zhao, C.; Fu, T.; Na, H. J. Synthesis and properties of sulfonated poly(ether ether ketone ketone) containing tert-butyl groups as proton exchange membrane materials. J. Membr. Sci. 2006, 285, 404–411. (2) Parcero, E.; Herrera, R.; Nunes, S. P. J. Phosphonated and sulfonated polyhphenylsulfone membranes for fuel cell application. J. Membr. Sci. 2006, 285, 206–213. (3) Basile, A.; Paturzo, L.; Iulianelli, A.; Gatto, I.; Passalacqua, E. Sulfonated PEEK-WC membranes for proton-exchange membrane fuel cell: Effect of the increasing level of sulfonation on electrochemical performances. J. Membr. Sci. 2006, 281, 377–385. (4) Li, C.; Liu, J.; Guan, R.; Zhang, P.; Zhang, Q. Effect of heating and stretching membrane on ionic conductivity of sulfonated poly(phenylene oxide). J. Membr. Sci. 2007, 287, 180–186. (5) Wiles, K .B; De Diego, C. M.; De Abajo, J.; McGrath, J. E. Directly copolymerized partially fluorinated disulfonated poly(arylene ether sulfone) random copolymers for PEM fuel cell systems: Synthesis, fabrication, and characterization of membranes and membrane electrode assemblies for fuel cell applications. J. Membr. Sci. 2007, 294, 22–29. (6) Hickner, M.; Ghassemi, H.; Seung, K. Y.; Einsla, B. R.; McGrath, J. E. Alternative polymer systems for proton exchange membranes (PEMs). Chem. Rev. 2004, 104, 4587–4612. (7) Zeng, R.; Chen, Y.; Zhou, W.; Xiao, S.; Song, C. Synthesis and characterization of poly(ether sulfone ether ketone ketone) grafted poly(sulfopropyl methacrylate) for proton exchange membranes via atom transfer radical polymerization. J. Mater. Sci. 2010, 45, 1610–1616. (8) Benavente, J.; García, J. M.; Riley, R.; Lozano, A.; De Abajo, J. Sulfonated poly(ether ether sulfones): Characterization and study of

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Preparation and Characterization of Sulfonated Copolyamides Based

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