Proton-Conducting Membranes from Polyphenylenes Containing Armstrong’s Acid

This study demonstrates the use of 1,5-naphthalenedisulfonic acid as a suitable building block for the efficient and economic preparation of alternating sulfonated polyphenylenes with high ion-exchange capacity (IEC) via Suzuki polycondensation. Key to large molar masses is the use of an all-meta-terphenyl comonomer instead of m-phenyl, the latter giving low molar masses and brittle materials. A protection/deprotection strategy for base-stable neopentyl sulfonates is successfully implemented to improve the solubility and molar mass of the polymers. Solution-based deprotection of polyphenylene neopentyl sulfonates at 150 °C in dimethylacetamide eliminates isopentylene quantitatively, resulting in membranes with high IEC (2.93 mequiv/g) and high proton conductivity (σ = 138 mS/cm). Water solubility of these copolymers with high IEC requires thermal cross-linking to prevent their dissolution under operating conditions. By balancing the temperature and time of the cross-linking process, water uptake can be restricted to 50 wt %, retaining an IEC of 2.33 mequiv/g and a conductivity of 85 mS/cm. Chemical stability is addressed by treatment of the membranes under Fenton’s conditions and by considering barrier heights for desulfonation using density functional theory (DFT) calculations. The DFT results suggest that 1,5-disulfonated naphthalenes are at least as stable as sulfonated polyphenylenes against desulfonation.

Solid state NMR spectroscopy.Solid-state nuclear magnetic resonance spectroscopy (S-NMR) was performed at 9.4 T on a Bruker Avance 400 spectrometer equipped with double-tuned probes capable of MAS (magic angle spinning).The finely powdered samples were packed into 3.2 mm rotors (OD) made of zirconium oxide spinning at 15 kHz. 13C-{ 1 H}-CP-MAS NMR spectra were acquired using cross polarization (CP) technique with contact time of 3 ms to enhance sensitivity, a recycle delay of 1.5 s, and 1 H decoupling during acquisition using a TPPM (two pulse phase modulation) puls sequence.The spectra are referenced with respect to tetramethyl silane (TMS) using TTSS (tetrakis(trimethylsilyl)silane) as a secondary standard (3.55 ppm for 13 C, 0.27 ppm for 1 H).All spectra were acquired at room temperature (25 °C).

Size exclusion chromatography. SEC measurements of all samples were carried out on
PolarGel-M columns (300 x 7.5 mm from Agilent Technologies, US), connected in series with a HPLC-Pump 1200(Agilent Technologies, US), a KNAUER K2301 RI detector (Knauer GmbH, DE), and a MiniDAWN-LS detector "TREOS II" (Wyatt Technology) at 25°C.N,N-Dimethylacetamide containing 3 g/L LiCl was used as eluent at a flow rate of 1.0 mL/min.The absolute molar masses and molar mass distributions were calculated using Astra 7.3.2software (Wyatt Technology, US).The refractive increment values dn/dc for calculation of the molar mass were calculated using the same software and assuming full separation of the injected samples or using external determination if required.Differential scanning calorimetry.DSC measurements were carried out on a DSC 2500 (TA Instruments) under nitrogen atmosphere.Heating and cooling rates were 10 K/min.The mass of the samples for each measurement was approx.5 mg.
Thermogravimetric analysis.TGA measurements were done on a TGA/DSC3+ (Mettler-Toledo) within the temperature range 30 °C to 650 °C at a heating rate of 10 K/min under argon.

Small-and wide-angle X-ray scattering (SAXS and WAXS). WAXS and SAXS experiments
were performed in a SAXSLAB laboratory (Retro-F) equipped with an AXO microfocus X-ray source and an AXO multilayer X-ray optics (ASTIX) as a monochromator for Cu-Kα radiation (λ = 0.15418 nm).A PILATUS3 R 300K detector from DECTRIS was used to record the 2D scattering patterns.The measurements were performed in transmission geometry under vacuum at room temperature; the sample to detector distance was approximately 85 mm for WAXS and 1035 mm for SAXS.The isotropic SAXS and WAXS patterns were integrated over the azimuthal angle to generate the 1D curves of I(q).Thermogravimetric analysis coupled to mass spectrometry TG/MS (Netzsch STA 449 F3 Jupiter, Pfeiffer Omnistar) measurements were conducted in Al2O3 crucibles using a heating rate of 3 K/min.During the measurements, helium (Air Liquide, 99.999%) was flushed through the system at 40 mL/min using mass flow controllers (Bronkhorst EL-FLOW).Background correction was conducted by subtraction of blank measurements under identical conditions.The ion current for m/z = 70 and m/z = 81 signals was used as an indicator for isopentylene and sulphurous acid, respectively.MALDI-TOF MS.Matrix-assisted laser desorption/ionization mass spectra were taken using BRUKER autoflex MALDI-TOF instrument in negative ion and reflector operating modes.The laser of this instrument is a smartbeam-II with a wavelength of 355 nm.The software for measuring and evaluating the spectra is flexControl 3.4 and flexAnalysis 3.4.
Samples were prepared on a standard sample plate (Bruker "MTP 384 target plate ground steel BC").Sample spot preparation was as follows.The Sample (1.0 mg/mL a suitable solvent) was hand-spotted onto a MALDI sample plate and air-dried.Afterwards the substance was spotted on top of the sample spot as a matrix and air dried before analysis.
Density functional theory calculations.DFT calculations 2,3 were performed at the B3LYP level 4-7 using the ORCA program package 8,9 , version 5.0.3, with def2-TZVP basis sets 10 , a D3 dispersion correction with Becke-Johnson damping 11,12 and TightSCF convergence criteria.To account for solvent effects, the conductor-like polarizable continuum model (CPCM) 13 was applied for implicit water solvation.After geometry optimizations of the two model compounds M1 and M2, their respective Wheland complexes, H2O and H3O + were carried out applying a BFGS optimizer and NormalOpt convergence criteria, reaction energies for the protonation of M1 and M2 were obtained as the differences in energy of the relaxed product (Wheland complex of M1/M2 + H2O) and reactant (M1/M2 + H3O + ) structures.

Cross-linking under inert atmosphere
The membranes were cut into 1x1 cm 2 pieces, placed in quartz-glass vials and flushed with argon three times.The vials were sealed and placed into an oven, following a thermal protocol from Di Vona et al.. 14 After cross-linking, the vials were opened and the membranes were taken out cautiously.

Water uptake
To determine water uptake (WU), cast membranes P(AA-alt-mTP) from DMSO were dried at 60 °C for 24 h under vacuum, weighted and immersed in deionized water at 80 °C for 24 h.The membranes were carefully dried and weighted, and WU was calculated using the following equation with wsoaked representing the weight of the immersed membrane and wdry being the weight of the dried membrane.

Ion exchange capacity (IEC)
The ion exchange capacity was determined via titration.Therefore, the air-dried, acidified membrane (1M HCl, 24 h) was set in a brine solution for 24 h.The membrane was taken out of the vessel and the solution was back titrated with 0.05 M NaOH against cyanidin.With the obtained data, IEC was calculated using the following equation: where V(NaOH) is the amount of NaOH consumed during the titration.

Mechanical properties
Tensile Testing were carried out on a Linkham TST-350.Prior to testing, the samples were prepared with a standard shape (DIN 53504 type 3, equal to ISO 37 type 4), with a thickness between 100 and 150 µm.The observed stress is defined as the engineering stress, which is calculated based on the initial cross-sectional area of the specimen at its midpoint when it experiences 0 % strain.It's important to note that the actual or true stress just before fracture is anticipated to be significantly greater.This is attributed to the ongoing thinning of the specimen as it undergoes stretching.The presented strain is defined as engineering strain, which quantifies the percentage elongation concerning the initial gauge length of the specimen, set at 15 mm.The strain rate applied during the test was 2 mm/min.

Proton conductivity measurements
Proton conductivity was measured via a modified through-plane setup reported in the literature. 15Instead of platinum, 5x1 cm 2 gold plates were used as electrodes.The membranes were cut into 1x1 cm 2 films and cross-linked, if needed, immersed in 1M HCl for at least 24 h, taken out and rinsed with deionized water carefully.The thickness of the membranes was measured 10 times with a layer thickness measurement device from List-Magnetik and the calculated mean value was used for further calculations.Then, the membrane was fixed in the proton conductivity setup and the whole was placed into a humidity chamber from Binder.
Proton conductivity was measured at 90 % RH at 80 °C, using electrochemical impedance spectroscopy (EIS) over a frequency range from 100 Hz to 7 MHz and a voltage amplitude of 10 mV.The resistance was read out at the high frequency intercept of the obtained curve in the Nyquist plot with the x-axis and the measurement was stopped once this value converged.The proton conductivity was calculated based on the following equation: Where is the proton conductivity (S/cm), l is the thickness of the membrane (cm), A is the area of the membrane (cm 2 ) and R is the obtained resistance (Ω).

Fenton's Test
Cross-linked membranes, were immersed in a 3 wt.-%solution of H2O2 containing 4 ppm Fe 2+ (with the source of Fe 2+ being FeSO4 * 7 H2O).The solution was then heated up to 80 °C for 1 h.Afterwards, the membranes were carefully rinsed with deionized water and dried with a filter paper before further analysis.
To this solution, pyridine (100 mL) was added and the solution was allowed to stir for 6 h.
Product 2 was obtained yellow powder, which was isolated by filtration and dried in a vacuum oven over night at 30 °C (113.12 g, 187.2 mmol, 90 %).The synthesis was adapted from a previously reported procedure. 17 a 500 mL round bottom flask, 2 (41.88 g, 69.3 mmol) was added to chlorosulfonic acid (100 mL, large excess) and the whole was heated to 100 °C.After 2 hours, the resulting dispersion was added dropwise onto ice water.The resulting solid was filtered, washed with cold water and cold ethanol afterwards and dried under reduced pressure for 12 h at 40 °C.