Modification of perfluorinated sulpho-cationite membranes with amines

13olymerScienceU.S.S.R.Vol.26, ~o. 2, pp. 456-462,1984 Prated in Poland

0032-3950184 $I0.00+.00 O 1985PergamonPressLtd.

MODIFICATION OF PERFLUORINATED SULPHO-CATIONITE MEMBRANES WITH AMINES* S. N. GLADKIKH, R. R. SmFRn,IA, YU. M. POPKOV, S. F. TIMASHEV, V. P. BAZOV and S. V. TIMOFEYEV L. Ya. Karpov Scientific Research Institute of Physico-Chemistry

(Received 23 September 1982) tR spectroscopy was used to examine the interaction of amines with unsaponified perfluorinated membranes. Amination kinetics of these polymers with ethylenediamine and diethylenetriamine were studied. Processes taking place in aminated layers of membranes during heat-treatment were investigated. IN ORDER to increase the selectivity (current efficiency) of perfluorinated sulpho-cationexchange resin membranes during electrolytic preparation of alkali from c o m m o n salt, their surface is aminated on the side of catholyte (when treating with amines to a depth o f up to 10 pro), followed by heat-treatment [1-3]. It is assumed [1--4] that crosslinks are formed during heat-treatment as a result of unreacted amino-groups, resulting in a reduction of moisture content in the modified layer. A study is made in this paper of the mechanism of processes taking place in perfluorinated unsaponified polymers, which are the source for sulpho-cation-exchange resin membranes (MF-4SK), using amination and heat-treatment; formation kinetics and the type of possible erosslink in the aminated layer of perfluorinated membranes were examined. Interaction of unsaponified polymer films (the thickness of samples varied between 10 and 25/tin) with amines (monoamines-allylamine and n-butylamine, ethylenediamine (EDA) and diethylenetdamine (DETA)) was studied by IR spectroscopy, according to treatment time and conditions of subsequent heat-treatment (temperature and duration of the process). IR spectra of samples were measured using a Perkin-Elmer spectrophotometer (model 580). When carrying out kinetic experiments in the case of EDA and DETA the unsaponified polymer film was consequently placed into a concentrated amine solution (98.5 ~)) (1-2 rain for EDA, 1 hr for DETA), carefully washed with distilled water to remove the unconverted absorbed amines, dried with filter paper and spectra of films treated by this method were then recorded. Subsequent heat-treatment was carried out at 100-200° for 14--240rain. As shown by Fig. 1, the absorption band at 1470 cm -1 is typical of vibrations v,s S = O in the sulpho-fluoride group, and disappears practically completely during aminat ion. At the same time a band appears at 1380 era-1, which is typical of vos S = O in the * VysokomoL soyed. A26: No. 2, 409-415, 1984. 456

Modification of perfluorinated sulpho'cationite membranes with amines

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sulphonamide group and absorption bands dependent on bond-stretching and deformation vibrations of the N - H bond of the sulphonamide group (3160, 1520 em -~) and fixed N H + ions (structureless band in the range of 2300-2800 cm-1 and 3=s and ~ at 1620 and 1520 c m - a) [5]. Similar changes were observed in IR spectra of a film treated with DETA. This suggests that amination using tri- and diamines involves the transformations of two amino-groups, one of which substituted fluorine in the sulphofluoride group, the other takes part in the secondary process of forming fixed N H ~ ions NH2--R--NH2 + F--SO2--Rt ~ N H ~ R - - N H ~ S O 2 ~ R 1 + HF'-+ --*F-NH ~+--R--NH---SO2--Rt, where R = - C H 2 - C H 2 - , -CH2-CH2-NH-CH2-CH2-; nated matrix of the membrane.

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FIG. 2. Dependences of optical densities of absorption bands at 1470 (1) and 795 cm-~ (2) on the time of amination of perfluorinated films 25 pm thick using EDA (a) and DETA (b). Figure 2 shows the dependence of optical density for bands at 1470 cm -1 and one o f the bands of the doublet at 825-795 cm -1, typical of bond-stretching vibrations of S--F on the time of amination using EDA and DETA. The rate constant of amination o f EDA (assuming that this is a first order reaction) kEoA =(1"65 +0"20) X 10-a s e c - i , while the corresponding constant kD~TAis lower than kEDA by two orders of magnitude; kD~a-A= (4"77 ___0"17) X 10 -5 sec -~ for a composition of 98~o D E T A + 2 ~ HzO and kDETA=(5"21+___0"08)X 10 -5 sec -1 for 88~o D E T A + 1 2 ~ H20. It should be n o t e d that during amination of D E T A the following reactions may take place: NH2 R--lqH--R--NH2+2F--SO2--Rx~F-NH+3--R--N--R

NH--SO2--RI+HF,

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SO2

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R1

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where R = - C H 2 - C H 2 - , to form crosslinks in the polymer structure. One of the t w o amino-groups taking part in reaction (lb) may be secondary, since it is characterized .[6] by higher basicity and therefore, higher reactivity. Because of the appearance of a

Modification of perfluorinated sulpho-cationite membranes with amines

459

crosslinked structure the coefficient of diffusion of DETA molecules decreases as amination goes on, which apparently reduces the effective rate constant korTA. The --R

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N - S O 2 - group obtained by reaction (lb) is not ionogenic, which reduces swell/ -R ing of the polymer and therefore, hinders the :diffusion of amine molecules during ~amination. It should be noted that the rate constant of amination of the DETA membrane is comparable in value with the rate constant of saponification of the initial polymer [7]. Therefore, when processing DETA polymers a secondary reaction takes place-fluorine in the sulphofluoride group is replaced by hydroxyl to form a sulphonic acid group (an absorption band appears at 1053 err/-1 in the IR spectrum of the aminated film, which is typical of vibrations v s S = 0 of the sulphonic acid group o f the membrane in H + form) [8]. During amination of EDA this band is also observed, but it is of much lower intensity. During interaction of the polymer film with monoamines (allylamine, n-butylamine) bands, typical of charged fixed ions are absent from IR spectra of aminated films, since amination only takes place with the formation of sulphonamide and sulphonic acid groups. Aminated films on heating behave differently, according to the amine used (mono-, di- or triamine). No changes were observed in IR spectra of membranes treated with monoamines and heat-treated at 170° for 3 hr, except for the disappearance of a small band at 3500 cm - 1 (bond-stretching vibrations of OH in a H20 molecule), which points to a reduction in moisture content. With similar treatment of films aminated using EDA and DETA considerable changes are observed in IR spectra: both the absorption band at 3160 cm- ~ and the band in the range of 2300-2800 cm-x decrease. At the same time, band intensity increases in the range of 3300 cm -1, characterizing bond-stretching vibrations of the N - H bond of the nonassociated sulphonamide group and band intensity also increases at 1380 cm-1. Results of measuring optical densities of absorption bands at 1380, 2500 and 3500 cm -~ in IR spectra of films aminated using EDA and DETA and heat-treated at 170 and 200 ° are shown in Fig. 3. It can be seen that on heating at 170° for 1 hr main changes occur in the range of absorption of the sulphonamide group (1380 cm-~)and of associated water molecules (3500 cm-~). During the two subsequent hours of heat-treatment at the same temperature the intensity of absorption only decreases at 2500 cm -1, due to vibrations of charged fixed ions. For films treated with DETA changes in absorption at 2500 cm -~ are also observed at the beginning of hea t-treatment. Therefore, results of IR spectroscopy confirm the formation mechanisms proposed [l, 4] or crosslinks in polymer films during heat-treatment due to free amino-groups [4] by reaction (2a); a slight proportion of sulphofluoride groups [1] unsubstituted in ~mfination may also take part in the formation crosslinks heating

R1--SO2--NH--R--NH3+ F- +F-NHs4- --R--NH--SO2--RI -2HF, -NHs ~ Rt --SO2--NH--R--NH--R--NH--SO2--R1

(2a)

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heating

Rx--SOz--NH--R--NH3*F- +F--SO2--R1 ~

R1--SO2 NH

R--NH--SO2--R1, (2b),

where R------CH2--CH2, --CH2--CH2--NH--CH2--CH2--. If amination of DETA takes place by reaction (lb), heat treatment may produce the following structures: R t- - S 0 2 - - - N H - - R - - N - - R - - N H - - R - - N - - R - - N H - - S O 2 - - R t

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The possibility of reaction (2b) is confirmed by results of heat-treatment of an incompletely aminated film (Fig. 4), which indicates that the formation of sulphonamide groups and the disappearance of sulphofluoride groups during heat treatment take place simultaneously.

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FIG. 3. Dependences of optical densities D13so (1), D250o (2) and D35oo (3) of films aminated using EDA (a) and DETA (b) on the temperature and duration of heat-treatment. A marked increase in absorption band intensity at 1380 cm-1 during the first hour of heating at 170° is also, apparently, due to the elimination of water molecules hydrating the sulpho-groups [9]. We have not observed band v S =O of the hydrated sulphonamide group (it is in a lower frequency range and is overlapped by intense absorption of fluorine-containing polymer chains). During heat-treatment associates of sulphonamide groups with water break dqwn therefore, band intensity at 1380 cm -~ corresponding to v S = O vibrations in the non-associated sulphonamide group, increases. Reactions (2a) and (2b) are the main reactions of crosslink formation in the polymer structure during heat-treatment. In films aminated using DETA this reaction takes place during the first hour of heating at 170° (Fig. 3b). This is, apparently, due to the

Modification of perfluorinated sulpho-cationite membranes with amines

461

fact that the amount of fixed charged ions ( - N H J - and - N H +) on films aminated using DETA is higher than when using EDA, since with secondary amino-groups in the case of amination using DETA, in view of their higher reactivity, the probability of forming fixed charged ions is higher. The large size of DETA molecules also con tributes to the formation of lateral bridges (equations (2a) and (2c))during heat-treatment.

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FIG. 4. D e p e n d e n c e s o f o p t i c a l d e n s i t i e s o f a b s o r p t i o n b a n d s a t 1380 c m -1 (1) a n d 1470 c m -~ (2) fo r a film p a r t i a l l y a m i n a t e d u s i n g E D A on t h e d u r a t i o n of h e a t - t r e a t m e n t a t 100 a n d 170 °.

An attempt was made in this study to evaluate the rates of the reactions during amination using EDA and DETA and during heat-treatment for 2 hr. Results of the relative content of sulphonamide and ethylene groups calculated from structures according to reactions (la), (lb), (2a)-(2d) and results of measuring relative optical density of absorption bands at 1380 and 2940 cm- ~ are tabulated (vas C - H in the - C H 2 group). Since the band at 2940 cm-1 is considerably overlapped by the absorption band in the range of 3160-3300 cm -1, typical of bond-stretching vibrations of N - H bonds of the sulphonamide group, its optical density 0 2 9 , , 0 w a s determined from spectra of membranes saponified using a 6N solution of NaOH, when absorption bands 31603300 era- x were absent from spectra. The optical density of the band at 1380 cm-1 of an aminated film was measured after treatment with alkali, careful washing with water and drying at 50°. This resulted RELATIVE CONTENT OF SULPHONAMIDE AND ETHYLENE GROUPS IN MEMBRANES AFTER AMINATION AND HEAT TREATMENT

Type of reaction Amination by reaction (la) Amination by reaction (1b) Heat-treatment by reactions (2a)-(2c) Heat-treatment by reactions (2b) and (2d)

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0-5

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* The relative optical density D1aso/D2s¢o (proportional according to the Lambert-Beer law, to the relative contem of sulphonamide and ethylene groups) of the membrane aminated using EDA, is taken as one.

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S . N . GLADKIKHet aL

in the breakdown of associates of the sulphonamide groups and enabled the optical density of the b a n d Dxaso to be accurately evaluated. Analysis of results suggests that the assumption made about possible amination of D E T A according to reaction (lb) is confirmed, since the ratio Dlzso/D294o =0.8 exceeds the value of 0.5, which corresponds to reaction (la). Furthermore, a comparatively slight increase in the relative content of sulphonamide groups after heat-treatment in the case o f D E T A (from 0"8 to 0-97) is due to the fact that (2a) and (2c) are the main reactions during heat-treatment. Reactions (2b) and (2d) seem to result in ratio D13ao/ JD294o > 1. Results of investigation show that crosslink formation in the m e m b r a n e structure by reactions (2a), (2b) and (2c) takes place during heat-treatment for 3 hr at 170 °. Heating aminated m e m b r a n e s from 100 ° to 160 ° does not result in the formation of crosslinkages in the polymer structure by reactions (2a) and (2c). At higher temperature (200 °) processes take place which are due to polymer breakdown. (Band intensity at 1380 cm -1, corresponding to ionogenic sulphonamide groups, decreases during heating for 3.5 hr at 170 ° and at 200 ° (Fig. 3). Therefore, heat-treatment of m e m b r a n e s aminated using E D A and D E T A for 3 hr a t 170 ° results in the formation o f crosslinks in their structure and therefore, a reduction in moisture content in the aminated layer of the m e m b r a n e [10]. It should be emphasized once more that a necessary condition of forming transverse crosslinking units in a modified layer is the treatment of initial perfluorinated films with di- or polyamines. When using allylamine having a vinyl group no crosslinks are formed as a result o f opening of double bonds during heat-treatment, although this process seems quite probable. Translated by E. SEMERE REFERENCES

1. British Pat. 1455070 2. British Pat. 2030152 3, S. F. BURKHARD and D. E. MALONEY, U.S. Pat. 4168216. Publ. in R.ef. Zh. Khim., No. 9, T499P, 1980 4. M. Jo CO~/'ITCH, S. R. LOWRY, C. L. GRAY and B. BLACKFORD, Polymer Preprints 21: 120, 1980 5. R. S1L'¥ERSTE1N, G. BASSLER and T. MORRIL, Spektrometricheskaya identifikatsiya organicheskikh soyedinenii (Spectrometric Identification of Organic Compounds). p. 196, Mir, Moscow, 1977 6. N. G. KORZHENEVSKAYA, Dis. na soiskaniye uch. st. kand. khim. nauk, ln-t fiziko-organieheskoi khimii i ugle~khimii AN USSR, Donetsk, 1979 7. L. P. BOCdDVTSEVA, R. R. SHIFRINA, Yu. M. POPKOV, N. A. DREIMAN and S. F. TIMASHEV, Vysokomol. soyed. 24: 262, 1982 (Not translated in Polymer Sci. U.S.S.R..) 8. C. HEITNER-WIRGUIN, Polymer 20: 371, 1979 9. G. TSUNDEL', Gidratatsi~a i mezhmolekulyarnoye vzaimodeistviye, Mir, Moscow, 1972 10. S. F. TIMASHEV and S. N . GLADKIKH, Dokl. AN SSSR. 262: 656, 1982