Abstract
Abstract. Proton-conducting polymers, such as sulfonated poly(ether ether ketone) (SPEEK), are of great industrial interest. Such proton exchange membranes show high tendencies for water and water vapor uptake. The incorporation of water not only leads to mass and dimensional changes, but also to changes in conductivity by several orders of magnitude. Both properties highly impact the potential application of the materials and, therefore, have to be known precisely. As hydration is diffusion controlled, thin films may behave differently to bulk specimens. However, the determination of small mass changes occurring in thin-film samples is very challenging. In this work, a new measurement setup is presented to simultaneously characterize the mass change and the conductivity of thin polymer films. The mass change is measured by resonant piezoelectric spectroscopy (RPS) with a nanobalance, which is based on high-precision piezoelectric resonators operating in thickness-shear mode (TSM). The mass resolution of this nanobalance is ±7.9 ng. Electrochemical impedance spectroscopy and an interdigitated electrode array are used for conductivity measurements. The approach is validated by comparing two SPEEK films with different degrees of sulfonation (DS). The relative humidity (RH) in the measurement setup was changed stepwise within the range ∼ 2 % < RH < ∼ 85 %. For both material compositions, DS = 0.5 and DS = 0.9, the mass uptake, the hydration number and the proton conductivity are presented and discussed depending on RH. This newly designed experimental setup allows for in situ characterization of the properties mentioned above; it can monitor not only the data for the stationary state, but also the dynamics of the hydration. To the authors' knowledge this is the first simultaneous and in situ measurement device for simultaneously sensing mass and conductivity change due to hydration of polymeric thin-film materials.
Highlights
The resulting ion-conducting materials are very useful for many applications, including acid–base or humidity sensors (Alberti and Casciola, 2001; Ruzimuradov et al, 2018), water purification (Bauer et al, 1990) or separation membranes for electrochemical energy technologies, such as proton exchange membrane (PEM) (Alberti et al, 2001; Mehta et al, 2003) or anion exchange membrane (AEM) fuel cells (Sun et al, 2018; Varcoe et al, 2014)
We investigate a well-known ionomer, sulfonated poly(ether ether ketone) (SPEEK) (Bauer et al, 2000; Di Vona et al, 2009; Kaliaguine et al, 2003; Xing et al, 2004; Zhong et al, 2007), prepared by drop coating in thin-film form on the analysis platform
Each relative humidity (RH) level is applied after the resonance frequency shows no significant changes anymore, indicating that the SPEEK film is saturated and in equilibrium with the constant level of water vapor pressure in the surrounding atmosphere
Summary
Ionomers are fascinating polymeric materials, in which electrolytic groups, such as sulfonic acid (Alberti et al, 2001; Jones and Roziere, 2001; Kreuer, 1997; Kreuer et al, 2004; Li et al, 2003) or quaternary ammonium (Arges and Zhang, 2018; Bauer et al, 1990; Couture et al, 2011; Elattar et al, 1998; Sun et al, 2018; Varcoe and Slade, 2005), are anchored on the polymer chains and dissociated counter-ions can migrate in hydrated nanometric channels inside the hydrophobic polymer matrix. Thickness-shear mode (TSM) resonators based on langasite (La3Ga5SiO14, LGS) (Fritze and Tuller, 2002; Seh et al, 2007) can be used for the in situ analysis of thin films in many fields, such as non-stoichiometry determination at high temperatures (Schröder et al, 2018), chemical vapor deposition (Grate, 2000; Habuka and Tanaka, 2013), aerosol mass detection (Czanderna and Lu, 1984), electrochemical deposition (Leppin et al, 2021), mass changes in polymers (Marx, 2006) or humidity control (PascalDelannoy et al, 2000) They measure very small mass changes in a material deposited on the resonator by the shift of the resonance frequency (Lucklum and Eichelbaum, 2006; Johannsmann, 2015). The conductivity and WU data are obtained as a function of temperature and relative humidity (RH) and discussed in dependence on the polymer’s degree of sulfonation (DS)
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