Anion Exchange Membranes: Enhancement by. Addition of Unfunctionalized Triptycene Poly(Ether. Sulfone)s

Transcription

1 Supporting Information Anion Exchange Membranes: Enhancement by Addition of Unfunctionalized Triptycene Poly(Ether Sulfone)s Yoonseob Kim, Lionel C.H. Moh and Timothy M. Swager* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, MA USA * To whom correspondence should be addressed. tswager@mit.edu S-1

2 Materials: Chloromethyl methyl ether (CMME), 1-methylimidazole (Im), and N-methyl-2- pyrrolidone (NMP) were purchased from Sigma Aldrich and used as received. Bisphenol A, bis(4- chlorophenyl) sulfone, anhydrous potassium carbonate, and zinc chloride (ZnCl 2 ) were purchased from Sigma Aldrich and dried in 60 C vacuum oven overnight before use. Triptycene-1,4,- hydroquinone was purchased from Triton Systems, Inc and used after recrystallization from acetone. N,N -dimethylacetamide (DMAc), toluene and 1,1,2,2-Tetrachloroethane (TCE) were dried over 3 Å molecular sieves for at least 24 h. Synthesis of Triptycene Poly(Ether Sulfone) (TrpPES): TrpPES and its copolymers, designated as TrpPES(x,y) (Table S1 and S2), were synthesized through aromatic nucleophilic substitution (SNAr) reaction between x equivalents of triptycene hydroquinone, y equivalents of bisphenol A, and (x+y) equivalents of bis (4- fluorophenyl) sulfone (Scheme 1). The general procedure of polymerization, illustrated using TrpPES(1,1), is as follows: In a flamed dried two-necked round bottom flask under nitrogen atmosphere with a Dean-Stark apparatus, triptycene hydroquinone (2.863 g, 10.0 mmol), bisphenol A (2.283 g, 10.0 mmol) and bis (4-fluorophenyl) sulfone (5.085 g, 20.0 mmol) was dissolved in anhydrous DMAc (60 ml) and anhydrous toluene (10 ml). Potassium carbonate ( g, 23.0 mmol) was added and the reaction was heated to 140 C for azeotropic distillation to remove water that was generated in the reaction. Once all the toluene was collected the reaction stirred at 140 C for 72 hours before heated to 165 C (reflux) for another 18 hours. The reaction was then precipitated in boiling water and filtered. The resulting polymer was purified by dissolving in S-2

3 DMAc and precipitating twice in boiling water and twice in methanol, yielding an off-white polymer powder that was dried in the vacuum oven for two days at 75 C. Synthesis of Chloromethylated BPAPES (Cl-BPAPES): BPAPES was chloromethylated using CMME with ZnCl 2 as reported (ref. 27 in the main text, Scheme 1). The equivalence of reactants were with respect to every active phenyl rings on the bisphenol A repeat unit in the polymer. In a flamed dried flask under nitrogen, BPAPES was dissolved in TCE at a concentration of 3 w/v%. Concentration of the polymer was kept low to prevent crosslinking of the polymer. CMME was added to the reaction and mixed homogenously before ZnCl 2 was added. The reaction was then heated to 60 C and stirred for 3 h. The product was precipitated in methanol and filtered, yielding a white polymer that had 95 % (calculated from 1 H NMR spectra) of the active aromatic rings in the polymer functionalized. For Cl-BPAPES, the peaks in the range of δ were chemical shifts of protons on benzene rings and the peak at δ 4.65 was a chemical shift of protons on the chloromethyl groups (Figure S1b) (Bruker AXS 400 MHz with DMSO-d6 as solvent at 20 C). The degree of chloromethylation was calculated from comparing the ratio of the integration of peaks in δ and peak on δ 4.65, to the number of protons on the benzene rings and protons on the chloromethyl groups. Degree of chloromethylation can be reduced by reducing the reaction time, the amount of CMME, or reaction temperature. S-3

4 Synthesis of 1-methylimidazolium functionalized BPAPES with chloride anion (Im- BPAPES): Im-BPAPES was made via nucleophilic substitution of the benzyl chloride group in Cl- BPAPES with 1-methylimidazole. Cl-BPAPES was dissolved in NMP to make a 10 w/v % solution.1-methylimidazole was added in excess (10 equivalence) and left to stir at room temperature for 18 h. The reaction was then precipitated in acetone, filtered and washed with acetone. The filtrate was dried in a vacuum oven overnight at 60 C to yield off white polymers of Im-BPAPES. Degree of functionalization was calculated by comparing the integration peaks on δ 5.4 and δ 4.7 to be 85% (Figure S1c). (Bruker AXS 400 MHz with DMSO-d6 as solvent at 20 C) Formation of anion exchange membranes: Im-BPAPES (200 mg) was dissolved to form 10 wt% solution in NMP:DMSO 50:50 (v/v) at 90 C for 1 h. In a separate vial, additives (TrpPES(x,y), 100 mg) were also dissolved to form 10 wt% solution in NMP at 90 C for 1 h. The solutions were mixed and stirred at 90 C for additional 30 mins and dispensed into a glass petri dish with 5 cm-diameter. The samples were placed in the vacuum oven and heated to 90 C for 20 hours at atmospheric pressure to remove solvents slowly for homogeneous thickness. The chamber was heated to 120 C and placed under vacuum for another 6 hours to completely dry out the membranes. Free standing membranes with thicknesses between µm were obtained and cut to sizes needed for further experiments. Prior to characterization, membranes were soaked in 2.0 M NaOH for 24 h, changing the soaking solution 3 times to exchange the chloride ions with hydroxide ions, forming the AEMs with hydroxide ions. Membranes with additives of BPAPES, TrpPES(1,4), TrpPES(1,1), TrpPES(4,1), S-4

5 and TrpPES(1,0) are named as TRP-0, TRP-20, TRP-50, TRP-80, and TRP-100, respectively (named depending on the mole fraction of the triptycene to the bisphenol A). General characterization methods: 1 H NMR spectra were obtained using a Bruker 400MHz. Molecular weights of polymers were obtained, either with the soluble fraction in THF using gel permeation chromatography with polystyrene as a standard, or by gel permeation chromatography using an Agilent LC system with a Wyatt minidawn TREOS multi-angle light scattering detector and DMF (doped with 0.02 M LiBr) as eluent. Thermogravimetric analysis was performed using TA Discovery TGA from 25 C to 1000 C at a heating rate of 10 C/min. Tensile test were performed on Zwick/Roell Z010 with a 10 kn load cell with a tensile rate of 1 mm/s. For all experiments, mean values from triplicate experiments are used. Anion conductivity measurements: Anion conductivity at various temperature was obtained through electrochemical impedance spectroscopy using electrochemical interface (Solartron 1287) with an impedance analyzer (Solartron 1260). Films were placed across two platinum electrodes spaced 3 cm apart and the setup is placed in a water bath placed in an oven. Measurements were taken over the temperature range of C with a mean AC voltage 0 V and an amplitude of 0.1 V over a frequency range of 10 MHz to 0.1 Hz. Conductivity was then calculated using an equation below where l is the length between the electrodes, R is the resistance of the membrane acquired from a Nyquist plot, t is the thickness of the membrane and w is the width of the membrane. The S-5

6 membranes were equilibrated in N 2 gas purged deionized water for at least 24 h at room temperature before measurements. σ = l R t w (Eq. S1) Ion exchange capacity: IEC was determined by a back titration method with 0.01 M sodium hydroxide solution to prevent errors that can occur from the reaction of free hydroxide ions with carbon dioxide in the air. The membranes were first soaked in 2.0 M sodium hydroxide solution for 24 hours, rinsed with deionized water and soak in deionized water for another 24 hours to ensure that all the mobile ions in the membrane are hydroxide ions. The membranes were then transfer into 20 ml (V HCl ) of 0.10 M hydrochloric acid (M HCl ) for exchange the hydroxide ions with chloride ions. The acid was then titrated with 0.01 M (M NaOH ) sodium hydroxide solution (standardized with M potassium hydrogen phthalate) until ph 7.0 using a ph meter and the volume of base (V NaOH ) was used to calculate amount of acid that was neutralized by the hydroxide ions (n OH- ) in the membrane which was then used to determine IEC with equation below, where W dry is the weight of dried membrane. IEC = n 34 5 W 789 = V 4;< M 4;< V?@34 M?@34 W 789 (Eq. S2) Small Angle X-ray Scattering: Transmission small angle X-ray scattering measurements were performed on SAXSLAB instrument. Hydrated membranes were sandwiched by Kapton tapes and mounted on the stage. The instrument was equipped with a Rigaku 002 microfocus X-ray source with an Osmic staggered S-6

7 parabolic multilayer optics to focus the beam and two sets of JJ X-ray 4 jaw single crystal collimation slits (aperture sizes of two slits are 0.7, 0.3 mm). Samples were introduced into a large vacuum chamber that was pumped down to 0.08 mbar. The transmitted X-rays were detected with a DECTRIS PILATUS 300K, placed at mm away to measure X-rays that were scattered within q = Å -1. The internal domain spacing (the Bragg spacing), d, was calculated according to the following equation: d = 2π q (Eq. S3) Transmission Electron Microscopy: For better contrast, the membranes were loaded with iodide ions by immersing them into a 2.0 M potassium iodide aqueous solution for more than 48 h. Then the samples were embedded in epoxy, to be sliced to a thickness of 60 nm using a Leica EM UC6 ultramicrotome. TEM images were obtained with a 120 kv FEI Tecnai Multipurpose TEM. Water uptake: Water uptake was determined by measuring the weight difference of the membrane before and after immersing in deionized water. The samples were placed in deionized water at desired temperatures for more than 4 h, dried with a kimwipe to remove surface water and immediately weighed to determine the weight of the wet membrane. Then the wet membranes was subsequently dried at 60 C under vacuum for 20 h and weighed to obtain weight of dried samples. The weights of the hydrated, W wet, and dry, W dry, membrane were then used to calculate water uptake using the equation below. S-7

8 WU % = W IJK W 789 W (Eq. S4) Swelling Ratio: All lengths and widths were measured with a Vernier caliper and thicknesses were measured with a micrometer screw gauge to improve the precision of the measurements. Samples were placed in deionized water at desired temperatures for more than 4 h, dried with a kimwipe to remove surface water and immediately measured to determine the dimensions of the wet membrane. The wet membranes was subsequently dried at 60 C under vacuum overnight before dimensions of dried samples were measured. The surface area of the hydrated, A wet, and dry, A dry, membrane were then used to calculate water in-plane swelling ratio using the equation below. In Plane Swelling Ratio % = A IJK A 789 A (Eq. S5) Oxidative Stability: The oxidative stability was studied by estimating the weight changes of the membranes (3 cm x 3 cm) in Fenton s reagent (4 ppm of iron(ii)sulfate heptahydrate in 3% H 2 O 2 ) at 80 C under stirring. The immersed samples were taken out at regular intervals to be dried with a kimwipe to remove surface liquid, and immediately weighed. The Fenton s reagent was refreshed every 10 h. S-8

9 Table S1. M w and PDI of polymers. BPAPES TrpPES(1,4) TrpPES(1,1) TrpPES(4,1) TrpPES(1,0) M w (kda) PDI Table S2. Average molecular weight and fraction of triptycene hydroquinone in the polymers. Mole ratio bisphenol A, (y) Average molecular weight per repeat unit (g/mol) triptycene hydroquinone, (x) bis (4- fluorophenyl) sulfone, (x+y) BPAPES TrpPES(1,4) TrpPES(1,1) TrpPES(4,1) TrpPES(1,0) Fraction of triptycene hydroquinone over bisphenol A(%) S-9

10 Figure S1. a-c. Chemical structures and 1 H-NMR spectra (400 MHz, DMSO-d6, 20 C) of BPAPES, Cl-BPAPES and Im-BPAPES, respectively. BPAPES was not soluble in DMSO, while functionalized ones were soluble in DMSO. To calculate of degree of chloromethylation, integration for all protons on benzene rings (chemical shifts in the range of ) and that for protons on chloromethyls (chemical shift around 4.65) were compared. S-10

11 Figure S2. Photographic images of the samples. S-11

12 Figure S3. Ln σ vs 1000/T plot of Figure 1a. The dotted lines indicate linear regression. Linear regression equations and R 2 values for each sample are on the right side of the figure (from top to bottom: TRP-100, TRP-80, TRP-50, TRP-20 and TRP-0, respectively.) Calculated E a using the Arrhenius equation for TRP-100, TRP-80, TRP-50, TRP-20 and TRP-0 are 16.4, 16.8, 17.3, 17.7 and 18.4 kj/mol, respectively. S-12

13 Figure S4. Stress strain curves of the hydrated samples. Data for elongation at break, Young's modulus and tensile strength are summarized in Table S3. Table S3. Elongation at break, Young's modulus and tensile strength of the samples. TRP-100 TRP- 80 TRP-50 TRP-20 TRP-0 Elongation at break (%) Young's modulus (MPa) Tensile strength (MPa) S-13

14 Figure S5. TGA analysis of the samples. Samples were heated at 10 C/min from 50 C to 1000 C S-14