Macromol. Rapid Commun. 19,135-137 (1998) 135

Synthesis of poly(ether ether ketone) containing sodium sulfonate groups as gas dehumidification membrane material

Feng Wang*, Tianlu Chen, Jiping Xu

Changchun Institute of Applied Chemistry, Changchun 130022, P. R. China

(Received: September 23, 1997; revised manuscript of November 10, 1997)

SUMMARY Poly(ether ether ketone) (PEEK) containing sodium sulfonate groups can be prepared directly from sodium 2,5-dihydroxybenzenesulfonate and 4,4'-difluorobenzophenone. This method is more advanta- geous than sulfonation of PEEK, avoiding possible degradation and crosslinking. The polymer shows high permeability of water vapor and high selectivity of permeation of water vapor versus nitrogen and can be used as gas dehumidification membrane material.

Introduction During the last two decades, aromatic poly(ary1 ether ketone)s have received considerable attention. They are high performance thermoplastics known for excellent environmental resistance and good mechanical properties. Among these polymers, poly(ether ether ketone) (PEEK) is the most widely used material. It is desirable to modify the chemical nature of PEEK while maintaining its excel- lent physical properties, to find membrane applications where hydrophilicity is required.

It has been shown that the sulfonation of polymers is an effective method to increase both the permeation rate of water vapor and the separation factor of water vapor over gases'). Several methods have been employed to sul- fonate poly(ether ether ketone)s, including sulfonation with concentrated sulfuric acid2v3), with chlorosulfonic acid4), with pure or complexed sulfur trioxideb7), and with methanesulfonic acidconcentrated sulfuric acid8). As reported in previous literature^^.^'^), sulfonation of PEEK is believed to induce degradation and crosslinking. Jin et aL2) and Bishop et al.9) have reported that sulfona- tion of PEEK in H2S04 is essentially free of degradation and crosslinking reactions, provided the concentration of the acid is kept below loo%, due to decomposition of pyrosulfonate intermediates by water. However, this is a less than ideal method, because of the entailed longer time than that in 100% H2S04. According to Jin et al., 33 days were needed to reach a sulfonation degree of 1 .O per repeating unit. In this communication, we describe a suc- cessful synthesis of poly(ether ether ketone) bearing one sodium sulfonate group per repeating unit via polycon- densation of sodium S5-dihydroxybenzene sulfonate and 4,4'-difluorobenzophenone. By introducing of sodium sulfonate group to hydroquinone before polycondensa- tion, side reactions during sulfonation, including cross- linking and degradation, were avoided. Moreover, the sul- fonated poly(ether ether ketone) was obtained at a tem-

0 1998, Hiithig & Wepf Verlag, Zug

perature of 175 "C that was much lower than that for pre- paring PEEK (around 300°C).

Experimental part

Sodium 2,5-dihydroxybenzenesulfonate Hydroquinone (10 g) was dissolved in 36 ml HzS04 (100%) and kept at 18°C for 5 h. Then the solution was poured into ice water (300 ml). The sulfonated compound was neutra- lized with NaCl (90 g), filtered, and dried. Recrystallization from methanovwater (9: 1 v/v) gave white needles. The yield was 15.6 g (81%).

C6H505SNa (212.1) Calc. C 33.96 H 2.36 S 15.10 Found C34.10 H2.20 S 15.16

'H NMR (DMSO-ds, Varian Unity 400 MHz): 6 = 9.942 (s, hydroxyl, ortho to -S03Na); 9.956 (hydroxyl, meta to -SO,Na>; 6.989, 6.982 (d, phenyl ortho to -S03Na, 1H); 6.744, 6.737; 6.721, 6.714 (2d, phenyl para to -S03Na, 1 H); 6.693,6.672 (d, phenyl meta to -S03Na, 1 H).

IR (KBr): 3 301 (phenolic - O H stretching); 1506, 1447 (C=C aromatic stretching); 1 196, 1030 (O=S=O stretch- ing); 1 087 (aromatic ring vibration); 71 1 cm-' (C-S stretch- ing).

Synthesis of polymer

In a 100 ml three-necked round bottom flask, fitted with a Dean-Stark trap, a condenser, a nitrogen inlet, and a thermo- meter, sodium 2,5-dihydroxybenzenesulfonate (2.122 g, 10 mmol), 4,4'-difluorobenzophenone (2.182 g, 10 mmol) and anhydrous potassium carbonate (1.52 g) were dissolved in a mixture of DMSO (20 ml) and toluene (10 ml). The mix- ture was heated with continuous stimng until the toluene began to reflux, and then heated under reflux at 150°C until water was essentially removed from the reaction mixture by azeotropic distillation. After the water was removed, the temperature was raised to 175 "C and kept for 6 h. The reac- tion mixture was filtered and poured into 500 ml acetone to

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precipitate the polymer. The crude polymer was washed twice with excess acetone and dialyzed. For the dialysis, SPECTRUM cellulose acetate dialysis tubes with molecular weight cut off value of 2000 were used. The resulting sulfo- nated PEEK (P-Na) (yield: 99%) was acidified with excess 1 N HCI aqueous solution to its free acid form (P-H), which was dialyzed with cellulose acetate dialysis tubes (MWCO 2000) until no C1- was detected.

Preparation of membranes

The P-Na and P-H membranes were prepared by casting a 8% solution in DMF on a glass plate. The membranes were dried at room temperature for 24 h and then dried at 120°C in a vacuum oven for 48 h.

Results and discussion As shown in Scheme 1, sodium 2,5-dihydroxybenzene- sulfonate was prepared by the sulfonation reaction of hydroquinone with concentrated sulfuric acid. The che- mical structure of the monomer was assigned on the basis of elemental analysis, NMR and IR spectra.

Scheme 1:

I 11 IN SOsNa

P-Na

SOqH

P-H

Poly(ether ether ketone) containing pendant sodium sulfonate groups was synthesized by the aromatic nucleo- philic substitution polycondensation (Scheme 1). The polymerization proceeded smoothly and afforded 99% yield of P-Na. The reduced viscosity of P-Na was 1 .O dL/ g, as measured in 0.5 g/dL DMF solution at 25 f 0.1 "C, showing that P-Na was a high molecular weight polymer. The DSC diagram, conducted on a Perkin-Elmer differen- tial scanning calorimeter (DSC 7) in nitrogen atmosphere at a heating rate of 10"C/min, shows typical amorphous structure of the polymer. Compared with unsubstituted PEEK, P-Na lost its crystallinity due to the introduction of sodium sulfonate groups into the polymer chain. P-Na was a pale brown powder and soluble in several aprotic polar organic solvents such as dimethylformamide (DMF), dimethylacetamide (DMAc), N-methyl-2-pyrroli- done (NMP), and dimethyl sulfoxide (DMSO).

I 0.8

2 O L I& 16w I 1400 I 1200 I IWO I BM) I 600 I 400 1

Wavenumbers

Fig. 1. IR spectrum of P-Na

The introduction of sodium sulfonate was confirmed by the IR spectrum (Fig. I), where the strong characteris- tic peak at 1026 cm-I assigned to the S=O stretching vibration was observed. The other bands of sulfur-oxygen are assigned as follows: asymmetric O=S=O stretch at 1250 cm-', symmetric O=S=O stretch at 1081 cm-', and S-0 stretch at 709 cm-'. No peaks attributable to the sul- fone bond were observed, as shown in Fig. 1. These results suggest that no detectable sulfone crosslinking occurred in the polycondensation process.

Thermogravimetry of P-Na and P-H was conducted on a Perkin-Elmer 7 Series Thermal Analysis apparatus at a heating rate of lO"C/min in a nitrogen atmosphere. As shown in Fig. 2, P-H shows three weight loss stages. The first one between 80°C and 200°C runs to 14%, which corresponds to the loss of adsorbed water and the dehy- dration from a sulfonic acid and a phenyl proton to form a sulfone bond. The second weight loss of ca. 25% of the original weight between 250 "C and 450 "C is attributed to the splitting of two C-S bonds of sulfone group. The last thermal degradation above 500°C is the result of

Synthesis of poly(ether ether ketone) containing sodium sulfonate groups ... 137

303 305 310 315 3P 325 33 335 340

M x l a n ( v l g

Fig. 3. Relationship between lg P, and 1IT for P-Na and P-H (Pw: permeability coefficient of water; 1 barrer = 1 x 1 @ ' O cm3 * cm/(cm2 - s * cmHg))

0.05

k

o . o l ~ . l , l , , , l . l . l . l . l l 3.00 3.05 3.10 3.15 3.20 3.25 3.30 3.35 3.40

1m x 1000(1/K)

Fig. 4. (PN : permeability coefficient of nitrogen)

Relationship between lg PN and 1IT for P-Na and P-H

decomposition of the main chain. Only one weight loss stage, between 390°C and 550°C was observed in the TGA curve for P-Na, indicating that P-Na has better ther- mal stability than P-H. This result proves that an alkali metal counter ion can improve the thermal stability of a polymeric electrolyte.

The permeability of nitrogen was measured on a model K3 15-N-03 manometric permeation apparatus (Japan), and the permeability of water vapor was measured by the cup method"). As shown in Fig. 3, both P-Na and P-H exhibit high water vapor permeation (Pw), 3.14 x lo4 and 4.18 x lo4 barrers at 25 "C, respectively. The water vapor permeation shows an approximate Arrhenius-type temperature dependence with E, = 25.3 kJ/mol for P-Na and E, = 19.3 kJ/mol for P-H. Arrhenius plots for the per- meability of nitrogen (PN) are presented in Fig. 4. It is obvious that a linear relationship exists between lg Z" and 1/T in each case. The apparent activation energies for nitrogen permeation through P-Na and P-H are 16.0 and 17.9 kJ/mol, respectively. P-Na exhibits a high selectivity of permeation of water vapor versus nitrogen, 1.52 x 106, which is about 10 times higher than that of sulfo- nated PPO'O).

Acknowledgement: This project was supported by the National Natural Science Foundation of China (NNSFC).

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