Polyelectrolyte membrane (PEM) is an important part of PEM fuel cell. Nafion is a commercially known membrane which gives the satisfactory result in PEM fuel cell operating at low temperature. Present research paper includes functionalization of Poly(ether ether ketone) (PEEK) polymer with phosphonic acid group. The functionalization was done with the help of nickel-based catalyst. Further, the polymer was characterized by the FTIR, EDAX, DSC, TGA, and 1H NMR, and it was found that PEEK polymer was functionalized with phosphonic acid group with good thermal stability in comparison to virgin PEEK. Finally, the thin films of functionalized polymer were prepared by solution casting method, and proton conductivity of film samples was measured by impedance spectra whose value was found satisfactory with good thermal stability in comparison to commercially available Nafion membrane.
PEEK is a semicrystalline aromatic polymer having high melting temperature (335°C) and very low or no solubility in ordinary solvents at room temperature. Sulfuric acid is the common solvent which dissolves and also sulfonates PEEK at room temperature. Dilute solutions at high temperatures can also be obtained in hydrofluoric acid, trifluoromethanesulfonic acid, dichlorotetrafluoroacetone monohydrate, phenol 1,2,4-trichlorobenzene, and benzophenone [
The above efforts have been done on polysulfones polymer. However, in present paper we have done phosphonation of PEEK polymer by utilizing the well-known Michaelis-Arbuzov Reaction route [
The commercial available GatoneTM PEEK polymer was obtained by Gharda Chemical India; however, reagent grade NiCl2 and Triethyl phosphate were purchased from Sigma-Aldrich, India. Liquid bromine was obtained from MERK India Ltd, India. Dimethyl acetamide (DMAC) and
All glass wares and PEEK polymer were completely dried at 110°C overnight before use. The reaction assembly was established in fume-hood with help of three necks round bottom flask, mechanical stirrer and gas washing bottle attached with nitrogen supply. 15 g (0.052 mol.) of completely dried PEEK powder were charged in flask containing a mixture of 4 mL (0.156 mol.) liquid bromine and 150 mL of concentrated H2SO4. The reaction mixture was stirred at ambient temperature for 24 h. The brominated samples were collected at 3 h, 6 h, 9 h, and 24 h time intervals and attributed as 3 h, 6 h, 9 h, and 24 h respectively. The polymer was precipitated into ice cold distilled water and further washed with several portions of distilled water, methanol, respectively. To ensure bromine free sample, the polymer was extracted with methanol for 8 h. A white modified polymer was obtained which was finally dried in vacuum oven. To obtain the powder sample, the polymer was grinded in Retsch MM 400 Mixture Mill. Modified polymer was readily soluble in a common organic solvent such as Dimethyl sulfoxide (DMSO), DMAC, Chloroform, and Tetrahydrofuran (THF).
The characterizations of polymers were done by the following equipment—elemental analysis by EDAX detector equipped in FEI Quanta 200FC SEM system, IR spectra by Thermo-Nicolet Nexus FT-IR spectrometer, 1H NMR by Bruker AC500 MHz NMR spectrometer, and thermal analysis by EXSTAR TG/DTA 6300. Proton conductivity of P-PEEK film sample was determined by Impedance Gain Phase Analyzer (Solartron SL1260) in which Julabo S25 was equipped for temperature variation.
The wt% of C, O, and Br in mono- and dibrominated PEEK was calculated theoretically (Table
Calculated wt% of elements in Br-PEEK.
Element | PEEK | Br-PEEK | (Br)2-PEEK |
---|---|---|---|
C | 82.61 | 64.06 | 52.32 |
O | 17.39 | 13.48 | 11.01 |
Br | 0 | 22.45 | 36.66 |
Measured wt% of elements in Br-PEEK.
Element | wt% | ||||
---|---|---|---|---|---|
PEEK | 3 h | 6 h | 9 h | 24 h | |
C | 86.63 | 66.71 | 59.48 | 54.33 | 48.27 |
O | 13.37 | 12.86 | 11.73 | 10.8 | 8.31 |
Br | — | 20.43 | 28.79 | 34.87 | 43.42 |
EDAX of 3 h Br-PEEK.
Bromine level in Br-PEEK polymer.
FTIR spectrum of pure PEEK polymer was reported by the Nguyen and Ishida [
FTIR of PEEK and 9 h Br-PEEK.
Since pure PEEK is insoluble in organic solvents except in strong acids, therefore its 1H-NMR spectrum could not be recorded. The 1H NMR of brominated sample is shown in Figure
1H NMR of 3 h Br-PEEK.
Further, a distinct signal at 8.00 (d) ppm and 7.62 (dd) ppm was obtained in 9 h and 24 h samples which may be associated with the neighbouring proton of substituted benzophenone segment of Br-PEEK. The substitution in benzophenone segment of PEEK polymer was also reported by the Conceição et al. [
P-PEEK was characterized by the FTIR spectrometer. The main reported absorption bands are as follows. González et al. [
FTIR of P-PEEK ester (20 m).
The wt.% of different elements in P-PEEK ester derivatives were determined by EDAX (Figure
Element % in P-PEEK ester by EDAX.
Element | Calculated wt% | Measured wt% | |||
---|---|---|---|---|---|
P-PEEK ester | |||||
5 m | 10 m | 15 m | 20 m | ||
C | 68.49 | 65.94 | 67.34 | 70.41 | 72.01 |
O | 23.82 | 13.69 | 15.06 | 18.46 | 19.93 |
P | 7.69 | 1.73 | 2.98 | 4.15 | 6.02 |
Br | — | 18.46 | 13.86 | 6.07 | 0.82 |
Ni | — | 0.18 | 0.76 | 0.91 | 1.22 |
EDAX of P-PEEK ester (20 m).
The solubility of P-PEEK ester and P-PEEK acid were determined by visual inspection in various solvent at room temperature and also at high temperature >150°C. It was found that both are insoluble in almost all the organic solvents. NMP sparingly dissolved the P-PEEK acid and ester both only at high temperature. Further, addition of 2-3 drops of acid such as HBr or H2SO4 fairly dissolved the both form of polymer (Table
Solubility of P-PEEK acid and ester in various solvents.
Solvent | Phosphonated PEEK | |
---|---|---|
Room temp. | Temp. > 150°C | |
DMSO | C | C |
DMAC | C | C |
NMP | C | B |
DMF | C | B |
DMSO+ | C | B |
DMAC+ | C | B |
NMP+ | C | A |
DMF+ | C | C |
4-Chlorophenol+ | C | C |
THF+ | C | C |
Cyclohexanone+ | C | C |
Solubility of P-PEEK ester was not found in any normal available deuterated solvent. Therefore, characterization of P-PEEK in liquid stage was somewhat difficult. To resolve the problem of solubility, initially a concentrated solution of P-PEEK ester in NMP at higher temperature was prepared which was further utilized in DMSO-
1H NMR of P-PEEK ester 5 m, 10 m, 15 m, and 20 m.
Solid state 31P NMR of P-PEEK acid.
P-PEEK (20 m) showed
DSC of P-PEEK (20 m).
TGA of P-PEEK (20 m).
Proton conductivity of P-PEEK was determined by casting the film on Indium-tin oxide (ITO) coated glass sheet using the solvent NMP. Coated glass sheet was further dipped in water for 24 h. The thickness of polymer film was determined by taking the difference in glass sheet thickness before and after the coating. The polymer coated sheet was sandwiched with ITO coated glass sheet, and impedance was measured by varying the frequency 100 Hz to 32 MHz, AC voltage 50–500 mV in Impedance Gain Phase Analyzer (Solartron SL1260). Julabo S 25 was used for variation in temperature during the impedance measurement. Proton conductivity was calculated by the following equation
Proton conductivity of polymer membranes.
Membrane | Proton conductivity (S Cm−1) | ||
---|---|---|---|
Nafion | 0.06–0.100 |
0.19 |
0.210 |
| |||
SPEEK | 0.008 |
0.076 |
0.033 |
| |||
P-PEEK (20 m) | 0.0069 |
0.0550 |
0.114 |
Proton conductivity of P-PEEK samples with temperature.
Nyquist plot of P-PEEK (20 m).
Commercial PEEK polymer can be brominated with only a small excess of bromine in H2SO4. However, no catalyst was used during the reaction. FTIR, EDAX, and 1H-NMR indicated that PEEK can be brominated readily. Further, conversion of Br-PEEK in to P-PEEK was carried out in presence of nickel-based catalyst. P-PEEK was characterized by various techniques, and it was found that P-PEEK showed high thermal stability in relation with PEEK polymer and solubility in NMP at higher temperature. The proton conductivity of P-PEEK was compared with Nafion at >100°C which showed slightly low value but thermal stability of P-PEEK was found good. Thus, it is expected that P-PEEK will show better durability with acceptable proton conductivity in comparison to both Nafion and S-PEEK at high temperature.
One of the authors is thankful for financial support from the Ministry of Human Resource Development (MHRD), India, and also for NCL Pune, India, for carrying out solid state NMR.