Mechanical properties and morphology of polystyrene-co-acrylic acid synthesized as membranes for fuel cells

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Mechanical properties and morphology of polystyrene-co-acrylic acid synthesized as membranes for fuel cells L. Melo a, R. Benavides a,*, G. Martı´nez a, D. Morales-Acosta a, M.M.S. Paula b, L. Da Silva a Centro de Investigacion en Quı´mica Aplicada, Blvd. Enrique Reyna H. 140, Saltillo, Coahuila, 25294, Mexico Universidade do Sul de Santa Catarina, Programa de pos-graduacao em Ciencias da Saude, Av. Jose Acacio Moreira 787, Tubarao, Santa Catarina, Brazil a

b

article info

abstract

Article history:

Two copolymers of poly(styrene-co-acrylic acid) (PS-AA) were synthesized in solution by

Received 30 May 2016

radical polymerization and partially crosslinked by adding, either trimethylol propane

Received in revised form

trimethacrylate (TMPTMA) or divinylbenzene (DVB) to improve mechanical resistance.

3 February 2017

Copolymers were sulfonated with theoretical molar quantities of sulfuric acid

Accepted 28 February 2017

(H2SO4 ¼ 170%) and two different amounts of silver sulfate (Ag2SO4 ¼ 0.11 or 0.055%).

Available online xxx

Materials were dissolved in three different solvent compositions: copolymer þ THF, copolymer þ THFþ55% DMSO and copolymer þ THFþ110% DMSO and used to prepare

Keywords:

membranes prepared by casting. Membranes were characterized by Thermomechanical

Copolymer membranes

Analysis (TMA), Scanning Electron Microscopy (SEM), Ionic Exchange Capacity (IEC) and

Casting

Water Uptake (WU) capacity. Addition of DMSO to THF during casting procedure has an

Flexure modulus

important increment on IEC and WU results; however, mechanical resistance is consid-

Porosity

erably reduced. SEM images show almost no pores in the membranes casted from THF alone, while increasing the amount of DMSO enhances porosity. Such phenomenon is responsible for reduced mechanical property (brittleness), as seen by TMA. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction The most common and widely used membrane for PEM fuel cells is the Dupont's product Nafion, and it is also well known that it lacks for mechanical stability at temperatures of 80e100  C. Moreover, it not useful for direct methanol fuel cells due to its permeability. Such drawbacks have induced a great interest in discovering new polymeric ionic materials for the preparation of ion exchange membranes. Traditionally, a

membrane is evaluated according to their polyelectrolyte performance, characteristics as ion exchange conductivity (IEC), water uptake (WU or U) and proton conductivity. However, membranes must also possess mechanical stability to withstand the processes to which they are exposed; those include processing (extrusion, casting, etc.), incorporation of catalysts, compression during formation of the MEA, pressure during PEMFC closure to test performance, dimensional changes during hydration and dehydration of the membrane, and pressure generated by the fuel, either gaseous or liquid.

* Corresponding author. E-mail address: [email protected] (R. Benavides). http://dx.doi.org/10.1016/j.ijhydene.2017.02.210 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Melo L, et al., Mechanical properties and morphology of polystyrene-co-acrylic acid synthesized as membranes for fuel cells, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.210

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The mechanical properties of polymeric membranes depend on its own chemical nature as well as by some external factors. A polymeric material tends to be brittle if has low molecular weight, a rigid chemical structure or no branching. Furthermore, an increase in the degree of sulfonation, needed to improve water uptake and conductivity, reduces mechanical performance on the other side. The latter also reduce the long-term stability of the membrane in the fuel cell [1]. Mechanical degradation in polymeric membranes occur in many forms, such as cracks, tears, punctures, or pinhole blisters; all of them reducing their service life. Hence, adequate care must be taken with membranes during assembly of MEA to prevent no uniform pressure and between bipolar plates during operation [2]. Mechanical properties of Nafion are influenced by hydration, although high conductivities require the presence of moisture. Nevertheless, when Nafion is soaked in water or any solvent, its Young's modulus decreases, since the solvent have a direct impact on the stressestrain relationship. The high solvent (water) content enhance swelling of the membrane, reduces the intermolecular forces and increase the degree of elongation; such conditions make the Nafion membrane more ductile and susceptible to permanent deformation, gradual weakening, and eventual failure in fuel cell when exposed to pressure gradients and pressure pulses [3,4]. It has been reported [5] proton conductivity and methanol permeability (T ¼ 20e60  C) for membranes cast from Sulfonated Poly(styrene) (SPS); the results showed that both properties depend on sulfonation degree. The sulfonated membrane (sulfonic acid groups content of about 15% mol) exhibited conductivity equal to that of Nafion. However the mechanical properties were not evaluated. Our research group has been working on sulfonated cheap hydrocarbon copolymers, based on styrene and acrylic acid, as PEM membranes for low temperature applications. Such copolymers are intrinsically rigid and brittle structures, which mechanically improve after partial crosslinking with byfunctional (divinylbenzene-DVB) or a tryfunctional (trimetilolpropane trimetacrylate-TMPTMA) agent [6,7], including their degradation reactions evaluated during sulfonation procedure [8]. Considering all the above aspects, it is of high importance the evaluation of mechanical properties for new synthesized materials, alternative to Nafion, pretending to be employed as ion exchange membranes.

Experimental Materials Styrene monomer (St, 99%, Aldrich) was purified with NaOH, dried with CaCl2 and distilled at reduced pressure. Acrylic acid monomer (AA, 99%, Aldrich) was added with phenothiazine and distilled at reduced pressure. Benzoyl peroxide (BPO, Aldrich). Nafion 117 membrane (Aldrich), Sulfuric acid 98% (J.T.Baker), Hydrochloric acid 36.5e38% (Sigma Aldrich), Nitric acid 70% (CTR Scientific), Silver Nitrate 99.0% (CTR Scientific), Sodium chloride (J.T. Baker), Sodium hydroxide (Aldrich), inhibitor free Tetrahydrofuran 99.9% (THF, Aldrich), Dimethylsulfoxide 99.5% (DMSO, Sigma Aldrich) and anhydrous

dichloromethane 99.8% (Aldrich). Trimethylol propane trimethacrylate (TMPTMA, Aldrich) and Divinyl benzene (DVB, Aldrich) as crosslinking agents.

Methods Polymerization procedure Two different copolymers of poly(styrene-co-acrylic acid) (PSAA) were previously synthesized with 94% mol of St and 6% mol of AA. The reactions were carried out by conventional solution free radical polymerization, using diethylbenzene as solvent. Benzoyl peroxide was used as radical initiator at 0.045% mol, and partially crosslinked (crosslinking agent at 0.25% mol) with divinylbenzene (DVB) or trimethylol propane trimethacrylate (TMPTMA) to improve the mechanical resistance. The initiator and crosslinking agent concentrations used were selected from previous experiments made in our research group. The random PS-PAA copolymer D (crosslinked with DVB) exhibits a Mn ¼ 68,012, Mw ¼ 259,095, and the random PS-PAA copolymer T (crosslinked with TMPTMA) presents a Mn ¼ 54,068, Mw ¼ 302,607. The polymerization procedure was described and reported previously [8].

Sulfonation procedure D and T copolymers were sulfonated with a theoretical molar quantity of sulfuric acid (H2SO4 ¼ 170% mol) employing silver sulfate as catalyst (Ag2SO4 ¼ 0.11 or 0.055% mol). Each copolymer was dissolved in dichloromethane by means of stirring at 200 rpm and 40  C during 40 min under nitrogen atmosphere. The theoretical amount of Ag2SO4 was dissolved in the H2SO4 and subsequently added to the dissolved copolymer, the sulfonation reaction was left to proceed during 2 or 4 h (for T and D copolymer, respectively). The reaction was ended removing the solvent and adding cold distilled water. The sulfonated copolymer was washed with more distilled water until reaching pH z 7. Finally, the polymer was dried at room temperature with an airstream during 48 h.

Casting procedures Three membranes were prepared by casting from each copolymer, employing a specific solvent or mixture of solvents. The ratio copolymer mass/solvent volume/area membrane employed was 0.4 g/2 mL/16 cm2. The composition for the first casting was: sulfonated copolymer þ THF; the second was sulfonated copolymer þ THF þ DMSO (55 % wt.) and the third was sulfonated copolymer þ THF þ DMSO (110 %wt). Evaporation of the solvent proceeded gradually at room temperature during 7 days, keeping the molds partially covered to allow solvent vapor to escape. Membranes thickness ranged from 0.18 to 0.22 mm, quite similar to Nafion's membrane (0.18 mm).

Membrane activation Membranes were activated before further characterization. The membranes were unmolded and immersed in distilled water by two days, changing the water every 4 h to eliminate the DMSO. Subsequently, the membrane was immersed in HNO3 0.5 M during 24 h, then immersed 1 h in H2O2 (5% vol) at 80  C, 1 h in H2SO4 0.5 M at 80  C and finally 1 h in distilled water at 80  C.

Please cite this article in press as: Melo L, et al., Mechanical properties and morphology of polystyrene-co-acrylic acid synthesized as membranes for fuel cells, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.210

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Ion-exchange capacity (IEC) and water uptake (WU) for membranes

Morphology of the membranes

Ion-exchange capacities were measured using a titration method. The cation-exchange membrane was soaked in 1 M HCl solution for 24 h. After washing with distilled water they were immersed into 1 M NaCl solution for 24 h. The number of displaced protons from the membrane was determined by titration of the NaCl solution with 0.005 M NaOH solution using a pH-meter. The membranes were then soaked in 1 M HCl solution for 24 h or more. After that, the membrane was washed with distilled water and water on the surface was wiped off with tissue paper to measure the wet weight of membrane (Wwet). Finally the membranes were placed in an oven at 44  C for 24 h and then the dry weight of membrane was then measured (Wdry). The percentage of water absorption was calculated by differences between dry/wet weights. The IEC and water uptake (WU) values for membranes were calculated using Eqs. (1) and (2) [9e12].

IEC ¼ a  b  Wdry

(1)

WU (%mass) ¼ (Wwet  Wdry/Wdry)  100

(2)

where a is the concentration of the NaOH solution (mmol/ mL ~ meq/mL), b is the volume of NaOH solution consumed during titration (mL), Wdry is the dry weight of the membrane (g) and Wwet is the wet weight of the membrane (g).

Thermomechanical Analysis (TMA) The complex modulus was determined by the flexural test in rectangular bars directly cut from the membranes and following the ASTM D790. These test method use a three-point bending accessory applied to a simple supported beam. The Modulus is the ratio, within the elastic limit, of stress to the corresponding strain, and uses a ramp force (0.1e1 Nw, at a 0.15 N/min ramp) while measuring deflection of sample. It was calculated by drawing a tangent to the steepest initial straightline portion of the load-deflection curve and using Eq (3) [13].

E* ¼ L3m4bD3

(3)

where E* is the Young's modulus by flexure as deformation (MPa), L is the length (m), b the width, d is the depth of beam tested (m), and m is the slope of the tangent to the initial straight-line portion (Nw/m).

Morphology surface topography of the membranes was observed using scanning electron microscopy (SEM) JEOL JSM7401F at 3 kV with a working distance of 5.7 mm.

Results and discussion Ion exchange capacity and water uptake The IEC value is an important factor related to the conductivity and ion transport properties of the membranes. The WU and IEC of the synthesized D and T membranes, casted from using mixture of solvents are summarized in Table 1. Generally the IEC increases with higher degree of sulfonation, due to the presence of more hydrophilic sulfonic acid groups in the polymer matrix. However, it is clear from Table 1 that is also possible to get different IEC values for the same sulfonated copolymer by simple change of the casting procedure. The higher concentration of DMSO in the mixture with THF for preparation of membranes by casting generates higher values of IEC and WU. This effect seems to be related with chemical structure size of solvents, since DMSO is heavier than THF (78.13 and 72.11 g/ mol) and their boiling point as well (189  C and 66  C, respectively). DMSO evaporates slower than THF, even some DMSO remains among the chains of the sulfonated copolymer membranes, and that is the reason to remove it by immersing the membranes in distilled water for 2 days before activation. The polymer membranes morphology depends on the chemical nature of the polymer, but it is also affected by other external factors, including the method of membrane preparation (solution casting, compression molding and extrusion) [14]. Peckham et al. observed that the more akin is the solvent with the copolymer in the casting, a better morphological arrangement in the membrane is generated. However, no direct correlation between solvent properties and morphology adopted by the copolymer was mentioned [15]. Silva et al. also observed that conductivity and WU of membranes are different depending on the solvent used during casting [16]. They casted Nafion 117 commercial solutions from ethylene glycol (EG), DMSO and dimethylformamide (DMF), maintaining the same conditions of casting (180  C for 90 min). Their results indicate that the use of different solvents promotes different conductivity values (DMF > EG > DMSO) and water absorption (DMF > DMSO > EG). The hypothetical reason they proposed consisted in a continuity of agglomerates of ions, which are able to absorb more

Table 1 e IEC and WU of the membranes casted from different solvent composition. Sulfonated copolymer D

T

Nafion 117

Solvent composition to the casting procedure

Label

IEC (meq/g)

WU (%)

THF-0% DMSO THF-55% DMSO THF-110% DMSO THF-0% DMSO THF-55% DMSO THF-110% DMSO e

D-0% D-55% D-110% T-0% T-55% T-110% Nafion

0.1439 0.4231 0.5524 0.0795 0.6405 0.9015 0.87

15.2210 64.9186 105.1195 13.6132 80.7000 138.9580 23.2

Please cite this article in press as: Melo L, et al., Mechanical properties and morphology of polystyrene-co-acrylic acid synthesized as membranes for fuel cells, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.210

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water that separated ions. A similar effect is observed in our results (THF < THFþ55%DMSO < THFþ110%DMSO), where the composition with higher DMSO solvent content, apparently increase the accessibility of the functional groups of the copolymers, which in turn enhance IEC values.

Physical performance of the polymers The tests were made in a thermomechanical analyzer (TMA) with a flexure accessory and calculating Complex Modulus in order to evaluate differences when dry or wet; also to envisage mechanical changes induced by the use of different solvent composition during casting of membranes. Figs. 1 and 2 show the curves of Force vs Deformation and the Modulus of Elasticity at different membranes obtained from both D and T copolymers. Nafion 117 mechanical behavior is also shown as a reference. Results indicate that the Modulii values (E*) for dry and wet membranes obtained from the D and T copolymers are higher than Nafion, which means that all those copolymer membranes are stiffer. Another clear observation is that E* values from the sulfonated membranes are smaller than neat (unsulfonated) copolymers D and T; this effect is observed by comparing the values of membrane D-neat vs D-0%, as well as T-neat vs T-0%; all those membranes were casted from THF solutions. Such difference in the mechanical behavior could be attributed to a different chain arrangement of the neat polymer comparing with the sulphonated polymer, since both types of materials (neat and sulphonated) have different steric hindrance, chemical environment and chemical interaction with the solvent, basically due to the presence of the sulfonic group. There is another observation in the Force vs Deformation graphs clearly seen (Figs. 1 and 2) regarding linearity of traces; while neat materials get an straight line, sulfonated copolymers are more related to curved traces. The reason, according to literature, could be related with the amount of pores in the sulfonated membranes, which in turn results in a stochastic cracking [17]. Such fracture type implies multiple cracking of inter-porous bridges, resulting in nonlinear stressestrain diagrams for these materials. In the flexural test, the load is applied slowly in the middle of the membrane

supported in two points; in the upper face of the membranes exist compression forces, meanwhile in the opposite face tension force is generated. During the compression process the pore walls of porous materials can be crumbled under the pressure while increasing the compression strain, becoming more compact. Porous materials have the characteristics of volume variability, inhomogeneous deformation and densification depending on porous pattern, which lead to a complex deformation pattern [18]. Regarding the use of DMSO with THF for casting procedures, results indicate that the higher content of DMSO the lower E* value, suggesting they become less brittle membranes, for both D and T copolymers; such effect must also depend on pore formation. Other characteristic observed is the plasticizer effect generated by the presence of water in the membranes and Fig. 3 show results for D and T copolymers when dry or wet for intercomparison. Plasticization causes higher deformation or less modulus values in the wet membranes in comparison with dry membranes at the same force applied. This effect is also observed for Nafion membranes. In general, during dry state T membranes are stiffer (except 110% DMSO) and such effect could be attributed to the higher level of crosslinking for T copolymer as a result of the three functional groups in its molecule, comparing with the two reactive groups from the DVB molecule. However, same membranes in wet state show an opposite behavior, where D membranes have higher E* values (stiffer). Such result can be rationalized in terms of how water enters into the polymer structure, since TMPTMA crosslinks three polymer chains and form an esther group at each point, which is a quite mobile group. On the other hand, DVB crosslinks only two polymer chains and the links are very close to the aromatic ring, which is a rigid group with steric effect. Considering that water enters easily into T crosslinked copolymer, an enhanced water plasticization is induced.

Structural analysis Fig. 4 shows SEM images of the membranes casted from D and T sulfonated copolymers employing different solvent

Fig. 1 e Mechanical behavior of D membranes, neat and sulfonated copolymer obtained from casting procedures (dry and wet state). Please cite this article in press as: Melo L, et al., Mechanical properties and morphology of polystyrene-co-acrylic acid synthesized as membranes for fuel cells, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.210

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Fig. 2 e Mechanical behavior of T membranes, neat and sulfonated copolymer obtained from casting procedures (dry and wet state).

Fig. 3 e Comparision of the mechanical behavior between D and T copolymers in wet and dry state.

Fig. 4 e SEM images of D and T sulfonated copolymers obtained by casting method employing different solvent composition.

composition (THF-0%DMSO, THF-55%DMSO and THF-110% DMSO). The difference in the solvent employed, as expected, causes changes in the topography and porosity of the membranes for the same copolymer. As mentioned earlier, high-porous materials tend to fracture by stochastic cracking. Such fracture type implies

multiple cracking of interporous bridges and in consequence, nonlinearity of the stressestrain diagrams of these materials is produced. Membranes consist of porous with different sizes; moreover, the membranes obtained using 110% of DMSO show irregular pore shapes. The critical defect size is associated with the crack size, according to the theory of

Please cite this article in press as: Melo L, et al., Mechanical properties and morphology of polystyrene-co-acrylic acid synthesized as membranes for fuel cells, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.210

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fracture mechanics; pores with shape different from the ideal sphere shape, means a critical defect. With the application of load during TMA experiments, the bridges break between the nearest pores at first and then as the load increases, the pores located at a greater distance are involved in the cracking process, and so on until the complete fracture of the material [17]. It is known that materials with small pores also fracture by the propagation of a single crack, while materials with middle and large pores suffer fracture by stochastic cracking of separated interporous bridges. A brittle crack in a porous material has two alternatives for propagation: by a linear or a curved trajectory, via the break of interparticle bridges and the confluence of the neighboring pores. Considering copolymers were synthesized from two brittle co-monomers (styrene and acrylic acid), therefore a brittle copolymer made of PS and PAA is expected. Besides that, the membranes prepared have a large quantity of pores. The mechanism for crack propagation in brittle high-porous materials changes with the amount of porosity. Therefore, the presence of pores could increase the IEC and WU of the membranes but this feature decreases the resistance of the materials during bending and in turn causes fragility.

Conclusions Membranes of partially crosslinked PSAA copolymers were prepared and sulfonated; casting procedure involving a mixture of solvents had a notorious influence and induced different morphologies. The method also increases porosity in membranes and IEC and WU values as well, as a consequence of an enhanced accessibility to the functional groups on the polymer structure. Finally, mechanical properties are reduced and membranes become brittle. Following experiments will be directed to enhance such undesirable mechanical properties.

[2] [3]

[4]

[5]

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

Acknowledgments [14]

n The authors would like to thank M.C. Marı´a Concepcio  lez Cantu´ for her assistance in the laboratory, M.C. Gonza Marcelina Sanchez Adame for technical support in TMA analysis and L. Melo would also like to express thanks to CONACYT (National Council of Science and Technology) for her PhD scholarship.

[15]

[16]

[17]

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Please cite this article in press as: Melo L, et al., Mechanical properties and morphology of polystyrene-co-acrylic acid synthesized as membranes for fuel cells, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.02.210