Influence of the membrane treatment on structure and properties of sulfonated poly(etheretherketone) semi-interpenetrating polymer network

Abstract

Semi-Interpenetrating Polymer Networks (semi-IPNs) combining a fluorinated network with a linear sulfonated poly(etheretherketone)(S-PEEK) were synthesized in presence of DMAc as solvent. The effects of the evaporation conditions of solvent during fabrication process on water swelling, proton conductivity and thickness measurements suggest a morphology modification of the semi-IPN. SAXS and SANS measurements confirmed the structural modification of the S-PEEK and fluorinated phases leading to a decrease of the hydrophilic/hydrophobic domain nanoseparation when the drying time increases along with lower water swelling ability, proton conductivity, water self-diffusion coefficient. On the contrary, the immersion time of this semi-IPN into an acid solution to carry out the cation exchange does not affect its morphology.

Introduction

Fuel cells are electrochemical converters considered as promising power sources. Most of fuel cell prototypes are based on Nafion^® type membranes, i.e. a perfluorosulfonated ionomer, used as solid proton conducting electrolyte and gas separator. During the last twenty years, many alternative membranes mainly based on sulfonated polyarylene materials have been developed to replace Nafion^® with the main aims of decreasing the production costs, improving the methanol and gas permeation properties and increasing the operating temperature [1]. However, these materials such as sulfonated poly(etheretherketones) (S-PEEK) usually exhibit excessive water uptakes at elevated temperatures [2], [3]. Indeed, interesting proton conductivities were obtained by increasing significantly the sulfonate content at the expense of the water uptake. In order to avoid excessive water uptakes, different strategies such as the introduction of inorganic particles [3], polymer blending [4] or partial cross-linking [5] were explored. Up to now, these modifications did not lead to expected improvements and were often accompanied of a critical loss of proton conduction. Finally, the synthesis of semi-interpenetrating polymer network (semi-IPN) appears as an interesting pathway [6].

Semi-IPN architecture is defined as the combination of cross-linked polymers in which one linear polymer is entrapped, these polymers being synthesized in juxtaposition [7], [8]. The cross-linking of one of partners limits water uptake and the entanglements compel miscibility unlike the usual incompatible polymer blends, and the resulting materials exhibit a good stability of morphology. Only few examples of semi-IPN architectures combining fluorinated network and polyelectrolyte are reported [6], [9], [10], certainly because of the important polarity difference of these compounds which makes the association particularly difficult to carry out without macroscopic phase separation. Recently, the synthesis of semi-IPN based on S-PEEK and a partially fluorinated appears as a promising strategy [11]. S-PEEK phase ensures the proton conductivity while the fluorinated network phase limits the water swelling of the material. The semi-IPN morphology was described as a continuous fluorinated network phase with a characteristic size between 0.3 and 1 μm, entrapped in a S-PEEK-rich phase matrix, both phases being continuous on the whole material.

Due to the polarity difference of the combined partners, the semi-IPN synthesis, like most of polymer blends, often needs solvent to mix the different partners. This solvent must be then eliminated after synthesis and many different procedures of drying are reported in the literature. For example, evaporation of the synthesis solvent can be carried out at temperatures varying from 50 to 180 °C, from 1 to 24 h [12], [13], [14], in a forced convection air oven [15], under vacuum [16], [17] or ambient atmosphere [12]. The synthesis solvent can be also removed from the polymer membrane by immersion in water after the synthesis is completed [18]. Nevertheless, regardless of the way chosen to eliminate the solvent, this drying step affects the material morphology and its properties. For instance, synthesis solvent of sulfonated poly(arylene ether sulfone)/poly(ether sulfone) semi-IPNs was dried prior to the cross-linking reaction, at temperatures ranging from 60 to 140 °C [19]. As the drying temperature decreases, the phase separation is delayed leading to a decrease in the domain size. Therefore, the morphology of co-continuous phases is more developed in the whole material leading to an increase in the proton conductivity.

In fuel cell applications, polymer membranes are usually based on sulfonated polymers [20]. The counter-ion, generally an alkaline ion, must then be exchanged after the synthesis to obtain a membrane in the acid form. Different protocols are also reported for this step. The sample is generally immersed in acid aqueous solution of either hydrochloric acid [17] or sulfuric acid [21] at different concentrations varying from 0.5 [22] to 4 mol/L [16]. Immersion time can range from 3 [14] to 48 h [17], [23] at temperatures varying from room temperature [17] to 100 °C [21]. Kim et al. highlighted that the morphology of S-PEEK based copolymer depends on the hydrothermal treatment conditions [24], [25]. Indeed, two solid-state membrane morphologies can be observed: (i) hydrophilic domains were isolated in a continuous hydrophobic matrix (“closed” structure) when the membrane was immersed in 1.5 mol/L H₂SO₄ solution for 24 h at 30 °C (followed by immersion in deionized water at 30 °C for 24 h); and (ii), a co-continuous phase morphology (“open” structure) was obtained when the material was immersed in 0.5 mol/L boiling H₂SO₄ solution for 2 h (followed by treatment with boiling deionized water for 2 h). Consequently, different morphologies could be clearly observed according to the cation exchange procedure used, inducing thus strongly different properties such as proton conductivity as well as water uptake [23], [26]. Finally, morphological modifications can also appear during the final drying step in which the membrane in the acid form can be heated up to 100 °C under vacuum [17].

As described previously, the process conditions to dry and exchange a polymer membrane can affect its morphology. Thus, the impact of drying and exchange conditions on the morphology of fluorinated network/linear S-PEEK semi-IPNs was characterized, at different scales, by analyses of soluble fractions, measurements of water uptake and proton conductivity, by scanning electron microscopy (SEM), small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS).