Synthesis, characterisation and membrane properties of sulphonated poly(aryl ether sulphone) copolymers

Reactive & Functional Polymers 66 (2006) 634–644

REACTIVE & FUNCTIONAL POLYMERS www.elsevier.com/locate/react

Synthesis, characterisation and membrane properties of sulphonated poly(aryl ether sulphone) copolymers R.T.S. Muthu Lakshmi a, Jochen Meier-Haack b, K. Schlenstedt b, H. Komber b, V. Choudhary a, I.K. Varma a,* a

Centre for Polymer Science and Engineering, Indian Institute of Technology Delhi, Hauz Khas, New Delhi 110016, India b Leibniz Institute of Polymer Research, Hohe Strasse 6, D-01069 Dresden, Germany Received 15 February 2005; received in revised form 10 October 2005; accepted 19 October 2005 Available online 13 December 2005

Abstract Novel sulphonated poly(ether ether sulphone) copolymers were synthesised by polycondensation reaction of stoichiometric amounts of silylated hydroquinone sulphonic acid, 4,4 0 -difluorodiphenyl sulphone and bisphenols, i.e., bisphenol-A, 4,4 0 -dihydroxybiphenyl, hydroquinone, phenolphthalein and 2,6-dihydroxynaphthalene. The structure of the copolymers was confirmed by using ATR-FTIR, 1H NMR and 13C NMR techniques. Number average molecular weight (GPC) of the polymers was in the range of 7900–18,000 g/mol. The water uptake of polymer films prepared by casting technique depended on the backbone structure and degree of sulphonation. The proton conductivity of these membranes at room temperature was in the range of 3.3 · 103–6.1 · 106 S/cm.  2005 Elsevier B.V. All rights reserved. Keywords: Poly(ether ether sulphone) copolymers; Silylated bisphenols; Hydroquinone sulphonic acid; Proton conductivity; Sulphonation

1. Introduction Sulphonated high-performance polymers such as poly(aryl ether ketone)s and poly(aryl ether sulphone)s have been extensively investigated in the last few years as possible materials for proton exchange membranes in fuel cell applications. The state-of-the-art perfluorinated membranes [1] such as Nafion, Flemion, etc. are expensive and unable to retain water above 80 C leading thereby to sig-

* Corresponding author. Tel.: +91 11 26591425; fax: +91 11 26591421. E-mail address: [email protected] (I.K. Varma).

nificant decrease in proton conductivity at elevated temperatures. There is a need to develop alternative materials of low cost and better retention of proton conductivity at elevated temperatures. Aromatic polymers such as poly(ether ether sulphone)s (PEES) are important engineering plastics that display excellent resistance to hydrolysis, oxidation, and at the same time, excellent mechanical properties, good thermal stability and toughness. Sulphonation of PEES has been carried out by using conc. or fuming H2SO4, chlorosulphonic acid and methanesulphonic acid/sulphuric acid [2–6]. The process of postsulphonation has several disadvantages. Problems encountered with this process are:

1381-5148/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.reactfunctpolym.2005.10.016

R.T.S. Muthu Lakshmi et al. / Reactive & Functional Polymers 66 (2006) 634–644

(a) the sulphonation takes place at the activated sites of benzene rings which bears the risk of desulphonation under fuel cell conditions; (b) the sulphonic acid groups are randomly distributed along the polymer backbone; (c) the degree of sulphonation is very sensitive to the reaction conditions and therefore hard to control [7]; (d) risk of degradation and cross-linking [8,9] (Scheme 1). Attempts have been made to overcome these shortcomings of direct sulphonation. The morphology of these sulphonated polymers is also expected to play an important role in determining the mechanical strength, water uptake and proton conductivity. In perfluorosulphonate ionomer membranes (Nafion) a ‘‘channel like’’ network of ions accounts for high proton conductivity despite their low IEC [10]. Sulphophenylation of polysulphones has been carried out by converting the lithiated polymer with 2-sulphobenzoic acid cyclic anhydride for attaining nanophase separation. The sulphophenyl units are expected to have high stability against desulphonation because of the proximity of the electron withdrawing ketone links [11]. Phase separated morphologies have also been achieved by block copolymerisation of bisphenol-A sulphone and poly(vinylidene fluoride) followed by post-sulphonation of selected block copolymers [12]. An alternative approach for the preparation of sulphonated polysulphones is the use of sulphonated monomers. This method is advantageous for

O

S

obtaining polymers free from degradation and cross-linking. It also provides an easy route to control the degree and the site of sulphonation. For example, Ueda and co-workers [13] have reported the synthesis and characterisation of poly(ether sulphone)s containing sodium sulphonate groups by polycondensation of 4,4 0 -dichlorodiphenyl sulphone and sodium 5,5 0 -sulphonyl-bis-(2-chlorobenzene sulphonate) with bisphenols in the presence of potassium carbonate and N,N-dimethylacetamide. Synthesis of wholly aromatic, film forming, highly sulphonated arylene ether sulphone copolymers via direct polycondensation of 3,3 0 -disulphonate-4,4 0 -dichlorodiphenylsulphone and various bisphenols have been investigated by McGrath and co-workers [14–17]. The synthesis of sulphonated poly(ether sulphone)s from trimethylsilyl derivatives of bisphenols and sulphonated bisphenols with 4,4 0 -difluorodiphenylsulphone, the so-called ‘‘silyl method’’ has recently been reported by Meier-Haack and co-workers [18], although the silyl method for the synthesis of non-sulphonated poly(ether sulphone)s was first reported by Kricheldorf in 1983 [19]. This method has advantages over methods using the non-silylated counterparts. These include: (a) pure monomers can be easily prepared by silylation followed by vacuum distillation; (b) volatile trimethylsilyl fluoride is the only byproduct and further purification of polymers is not required; (c) silylated bisphenols are more readily soluble in dipolar aprotic solvents such as N-methyl-2-pyrO

O O

635

O n

O

S

O

n

O

ClSO3H

SO3H

ClSO3H

O O

O

S

n

O

- HCl

SO2 O

S

O

S O

O O

O O

SO2Cl

n

O Scheme 1. Possible interchain cross-linking during sulphonation of PEES.

n

+ H2SO4

636

R.T.S. Muthu Lakshmi et al. / Reactive & Functional Polymers 66 (2006) 634–644

rolidone (NMP) bisphenols.

than

the

non-silylated

2. Experimental 2.1. Materials

We now report the synthesis and characterisation of sulphonated PEES copolymers by reacting varying amounts of silylated hydroquinone sulphonic acid and bisphenols, i.e., bisphenol-A (or) 4,4 0 -dihydroxybiphenyl (or) hydroquinone (or) phenolphthalein (or) 2,6-dihydroxynaphthalene with 4,4 0 -difluorodiphenyl sulphone according to the reaction Scheme 2. The main purpose of these studies was to evaluate the effect of bisphenol structure on the properties of SPEES while keeping the degree of sulphonation fixed, i.e., 75 or 60 mol% of silylated hydroquinone sulphonic acid and varying the bisphenols (25 or 40 mol%). CH3

CH3

x H3C Si O Ar O Si CH3 CH3

Hydroquinone, hexamethyl disilazane, chlorotrimethyl silane (Merck chemicals), N,N-dimethylacetamide (DMAc), ethanol, resorcinol and bisphenol-A (CDH), 4,4 0 -dihydroxybiphenyl, phenolphthalein, 2,6-dihydroxynaphthalene, hydroquinone sulphonic acid potassium salt (HQSA-K) and 4,4 0 -difluorodiphenylsulphone (DFDPS) (Acros) were used as received. K2CO3 (Qualigens) was dried at 150 C in vacuum oven for 24 h. NMP (Merck) was distilled under reduced pressure after drying first over P4O10, then K2CO3 followed by P4O10. CH3

CH3 + y H3C Si O

CH3

O

O Si CH3 + (x+y) F

CH3 O

S

O

S

CH3

O

O CH3 Si H3C CH3

K2CO3 (50 mol% excess), 150˚C NMP (15 wt.-%), 24 h precipitation in 10 fold excess ethanol

O

O O Ar O

S

O

O

S O

O SO3H

where Ar =

, SHQ

, SBP

SBA

O O SNA

SPH

Scheme 2. Reaction scheme for the synthesis of sulphonated PEES copolymers.

F

R.T.S. Muthu Lakshmi et al. / Reactive & Functional Polymers 66 (2006) 634–644

2.2. Silylation of bisphenols Silylation of bisphenols was carried out by the procedure reported earlier [18]. 1 mol of bisphenols was first suspended in a mixture of 500 mL dry toluene and 2.5 mol of hexamethyl disilazane (HMDS). The resulting mixture was then refluxed for 7 h until the evolution of ammonia, which is a by-product of silylation, ceased. Toluene and excess of HMDS were completely removed in a rotary evaporator. The final silylated monomers were purified by vacuum distillation (yield > 95%). 2.3. Silylation of hydroquinone sulphonic acid 0.5 mol of HQSA-K was suspended in 1.5 L dry THF and 1.1 mol of an equimolar mixture of HMDS and chlorotrimethyl silane. The mixture was refluxed for 96 h under exclusion of moisture. Ammonium chloride and sodium chloride, which are by-products, were removed by filtration. THF was removed under reduced pressure using a rotary evaporator. The tris-silylated product was purified by vacuum distillation (yield = 78%). 2.4. Synthesis of sulphonated PEES copolymers from sulphonated monomers In a three-necked round-bottom flask, 0.01 mol of desired silylated bisphenols (according to their molar ratio) and 0.01 mol of DFDPS were dissolved in NMP. Then a 50 mol% excess of K2CO3 was added. The flask was fitted with an air condenser,

637

argon inlet and a magnetic stirrer bar. The contents were heated at 150 C with stirring for 24 h. During all operations the flask was flushed with a stream of dry argon. A viscous liquid was obtained. The contents of the flask were cooled to room temperature and diluted with a small amount of NMP. The polymer was precipitated in 10-fold excess of ethanol, filtered and dried in an air oven at 120 C for 2 days. Ten copolymer samples were prepared by changing the ratio of silylated HQSA to silylated bisphenols, i.e., hydroquinone/bisphenol A/4,4 0 -dihydroxybiphenyl/phenolphthalein/2,6-dihydroxynaphthalene. The details of reaction conditions and sample designations of SPEES samples are given in Table 1. 2.5. Characterisation 2.5.1. Molecular weight measurements Reduced viscosity measurements of polymer solution (0.2% in dry DMAc) were done using an automated Ubbelohde viscometer (Schott) thermostated at 25 C. GPC measurements were performed on a Knauer GPC equipped with two PSM {trimodal S columns} and RI detector. A mixture of DMAc with 2 vol% of water and 3 g/L of LiCl was used as eluent. Poly(vinyl pyrrolidone) samples were used as standards for molecular weight calibration. 2.5.2. Structural characterisation Structural characterisation of sulphonated monomers and polymers was done by ATR-FTIR, 1 H NMR and 13C NMR spectroscopy.

Table 1 Reaction conditions for the synthesis of sulphonated PEES copolymers and their designation Amount of DFDPS Amount of silylated bisphenols K2CO3 Solvent Temperature S. No.

1 2 3 4 5 6 7 8 9 10

= = = = =

0.01 mol 0.01 mol 50 mol% excess of bisphenols NMP (20 wt% of the total ingredients) 150 C for 24 h

Silylated bisphenols (molar ratio)

Sample designation

HQSA

HQ

BA

BP

NA

PH

75 60 75 60 75 60 75 60 75 60

25 40 – – – – – – – –

– – 25 40 – – – – – –

– – – – 25 40 – – – –

– – – – – – 25 40 – –

– – – – – – – 25 40

SHQ-1 SHQ-2 SBA-1 SBA-2 SBP-1 SBP-2 SNA-1 SNA-2 SPH-1 SPH-2

638

R.T.S. Muthu Lakshmi et al. / Reactive & Functional Polymers 66 (2006) 634–644

IR spectra of polymer samples were recorded by Attenuated Total Reflectance Infrared spectroscopy (ATR-FTIR) using an IFS 66v/s FTIR-spectrometer (Bruker) equipped with a single reflection ‘‘Golden Gate’’ diamond ATR unit (SPECAC). Spectra were recorded from 4000 to 600 cm1 with a resolution of 4 cm1. 100 scans were accumulated per spectrum. The 1H and 13C NMR spectra were recorded on a DRX 500 NMR spectrometer (Bruker) operating at 500.13 MHz for 1H and 125.75 MHz for 13C using 5 mm o.d. sample tubes. The samples were dissolved in DMSO-d6 and the spectra were referenced on the solvent signal (d(1H) = 2.50 ppm, d(13C) = 39.60 ppm). 2.5.3. Thermal behaviour Thermal characterisation was done using a TA instrument 2100 thermal analyzer having a 951 TG module. Thermogravimetric analysis was done in the temperature range of 50–850 C in N2 atmosphere (flow rate = 60 cm3/min) at a heating rate of 20 C/min. A sample mass of 10 ± 2 mg was used. DSC studies were done using a Perkin–Elmer DSC7 instrument in the temperature range of 50– 250 C in static air atmosphere using a sample mass of 5 ± 2 mg. For these studies, the samples were heated from room temperature to 200 C at a heating rate of 10 C/min, cooled to room temperature and again heated at a rate of 5 C/min upto 250 C. The mid-point of the endothermic shift in the baseline during the second heating cycle was determined and reported as Tg. 2.6. Film casting 10–20% of SPEES solution in N,N-dimethylacetamide was used for casting films on a glass plate using a casting machine in which the level of the blade was maintained at 700 lm. After that, the glass plate was kept at 80 C overnight in an air oven and then at the same temperature in a vacuum oven. The films were then removed from the glass plates by keeping them in a freezer. The acid treatment of these films was carried out by immersing the films in 2% HCl solution for 24 h followed by repeated washing using distilled water. These films were then analysed for their water uptake. 2.6.1. Water uptake Sulphonated film samples were first dried at 120 C for 20 h. The dried polymer films were kept

immersed in water at ambient temperature. Water uptake was then determined by taking out the samples at definite intervals of time and removing surface water by using a filter paper. Water uptake was then calculated as follows: Water uptake ¼

W2W1  100%; W1

where W1 and W2 are masses of the sample before and after keeping in water, respectively. 2.6.2. Proton conductivity measurements Proton conductivity of all samples were tested from 25 to 100 C by the method of impedance spectroscopy over a frequency range of 1–105 Hz with oscillating voltage 5 mV using an EG&G potentiostat/galvanostat (Model 6310) electrochemical impedance analyzer using EIS software. The film samples were kept in water for 24 h prior to test. A sample with the diameter of 20 mm was placed between the spring loaded steel block electrodes which were kept in 100% RH. The conductivity r of the samples in the transverse direction was calculated from the impedance data, using the relation r = d/RS, where d and S are the thickness and face area of the sample and R was derived from the low intercept of the high frequency that produced the minimum imaginary response (i.e., due to diffusion characteristic of the film) on a complex impedance plane with the Re(Z) axis. 3. Results and discussion All the sulphonated polymers were flaky in nature and were highly soluble in dipolar aprotic solvents such as DMAc, DMF, DMSO and NMP. The samples had medium to high molecular weights as indicated by the reduced viscosities (0.3–2.1 dL/g) and results from GPC measurements. The M n was in the range of 7900– 18,100 g/mol whereas the M w of was very high ranging from 26,100 to 74,600 g/mol. This may be due to aggregation of these polymers. Such aggregation of ionic polymers has been reported by several workers [12–15]. The possibility of side reactions (i.e., cross-linking) is not likely with this type of synthesis. In any case the molecular weights were sufficient for the preparation of stable films. These were tough and transparent with a brownish tinge except for films of SNA-1 which were brittle.

R.T.S. Muthu Lakshmi et al. / Reactive & Functional Polymers 66 (2006) 634–644

3.1. Structural characterisation

639

due to the aromatic C–C absorption band of hydroquinone sulphonic acid. The intensity of the absorption band at 1470 cm1 increased while that of 1486 cm1 decreased with increase in sulphonic acid moiety in the sulphonated polymers. In case of phenolphthalein based copolymers, an additional band at 1773 cm1 was observed and is due to the – COO– unit of phthalide. 1 H NMR spectra of different sulphonated PEES samples are depicted in Fig. 2 and the proton designations of these samples are given in Scheme 3. The signal of the SO3H protons is strongly broadened because of fast proton exchange processes. However, the aromatic proton region of the spectra (Fig. 2) confirms the polymer structure and quantifies the content of the different structural units. All samples contain diphenylsulphone units which results in signals between 6.95 and 7.25 ppm for the protons in meta-position (HA) and at 7.8– 8.0 ppm for the protons in ortho-position (HB) to the sulphone group. These large shift regions are mainly caused by the hydroquinone sulphonic acid comonomer with its unsymmetric structure. The sul-

Sulphonated polymers were characterised by using ATR-FTIR spectroscopy (Fig. 1). The characteristic absorption bands along with their assignments are given in Table 2. In addition to the characteristic absorption bands corresponding to the aromatic skeleton, the presence of SO2 in the backbone in all the samples was indicated by absorption at 1145 ± 5 cm1 (symmetric O@S@O stretching), 1320 and 1288 cm1 (doublet resulting from asymmetric stretching of O@S@O groups). Asymmetric C–O–C stretching of aryl ether group appeared at 1215 ± 10 cm1. Absorption band associated with sulphonic acid groups in the hydroquinone sulphonic acid moiety was observed at 3500 cm1 (assigned to the O–H stretch of SO3H as well as to the absorbed moisture). The other bands are due to asymmetric and symmetric stretch of O@S@O (at 1172 ± 5 and 1080 ± 5 cm1) S@O stretch (1026 ± 5 cm1), and S–O stretch (706 ± 5 cm1). The doublets at 1486 and 1470 cm1 and 1410 and 1402 cm1 are

~1028 ~1773

(d) SPH-2

(c) SPH-1

(b) SHQ-2

(a) SHQ-1 2000

1500

1000 cm-1

Fig. 1. ATR-FTIR spectra of: (a) SHQ-1, (b) SHQ-2, (c) SPH-1, and (d) SPH-2.

500

640

R.T.S. Muthu Lakshmi et al. / Reactive & Functional Polymers 66 (2006) 634–644

Table 2 ATR-FTIR absorption bands of SPEES samples and their assignments Absorption bands (cm1 ± 5)

Functional groups

1589 1486 and 1470 and 1410 and 1402 1215 1145 1320 and 1288 3440 1172 1080 1026 706 1760

C@C stretching 1,3,4-trisubstituted aromatic C–C skeletal vibration band of HQSA (asym) C–O–C stretching of aryl ether group (s) O@S@O stretching of SO2 (asym) O@S@O stretching of SO2 9 mO–H > > > (asym) O@S@O stretch = due to –SO3H group (s) O@S@O > > > S@O stretch ; S–O stretch –COO– in SPH copolymers

HA 7.18 (*) 7.00 (x)

HB 7.94 (*) 7.86 (x)

HF 7.23

HE 7.46 #

a

HD : 7.17

HC 7.09 SHQ-1

#

HH 7.75

HG 7.24 SBP-1

b HI 7.08

HK 7.30

HL 1.66 SBA-1

c

HO

HM

HN

8.01

7.72

7.39 SNA-1

d

also HS; HT; HW

also HP

HR 7.40 HU 7.70

SPH-1

e 8.00

7.80

7.60

7.40

7.20

7.00

6.80 (1H) in ppm

Fig. 2. Representative 1H NMR spectra of the different SPEES polymers. Proton designation according to Scheme 3. The signals of the diphenylsulphone and hydroquinone sulphonic acid unit occurring in all samples and of fluorophenyl end groups (#) are only indicated in (a).

R.T.S. Muthu Lakshmi et al. / Reactive & Functional Polymers 66 (2006) 634–644 A

B

O Ar O

*

C

O S

D

O

O

x

641

O

S

O

y n

*

O E

F

G

SO3H

*H

x I

L CH3

K

CH3 where Ar =

SHQ

SBA

SBP

P

M

R N

SPH:

S

O

T

O

U

SNA

O

W

Scheme 3. Proton designations of SPEES samples.

phonic acid group can direct to the phenyl ring of the neighbouring diphenylsulphone unit (x: d(HA) = 7.00 ppm; d(HB) = 7.86 ppm) or not (*: d(HA) = 7.18 ppm; d(HB) = 7.94 ppm) as indicated in Scheme 3 by arrows. Smaller chemical shift effects are caused by the third comonomers in the SPEES samples. The three signals of the sulphonated hydroquinone ring appear at 7.17 (HC), 7.17 (HD) and 7.46 (HE) ppm. Besides these signals which are present in all the samples, additional signals appear due to the different comonomers (i.e., Ar moiety). They were assigned by evaluation of 1H–1H and 1H–13C correlated 2D NMR spectra and the assignments are given in Fig. 2.

Although there is some overlap of proton signals of different monomers, it was possible to select signal regions which allow to calculate the monomer composition of the SPEES samples from their integrals taking into account the type of protons causing these signals. The results are summarised in Table 3. The DFDPS content is larger than the sum of the bisphenols content in most cases which is in accordance with the predominance of fluorophenyl end group signals in the 1H NMR spectra. For the bisphenols the content of HQSA is lower than expected in most cases whereas the content of the second bisphenol is in good agreement with the feed ratio.

Table 3 Polymer compositions derived from 1H NMR Sample designation

SHQ-1 SHQ-2 SBA-1 SBA-2 SBP-1 SBP-2 SNA-1 SNA-2 SPH-1 SPH-2

DFDPS

102 102 106 103 103 102 105 102 99 98

(100) (100) (100) (100) (100) (100) (100) (100) (100) (100)

Silylated bisphenols (molar ratio) HQSA

HQ

BA

BP

NA

PH

66 48 67 53 71 56 66 54 72 55

32 (25) 50 (40) – – – – – – – –

– – 27 (25) 44 (40) – – – – – –

– – – – 26 (25) 41 (40) – – – –

– – – – – – 29 (25) 44 (40) – –

– – – – – – – – 29 (25) 46 (40)

(75) (60) (75) (60) (75) (60) (75) (60) (75) (60)

Numerals within the parenthesis represent the moles of monomers taken in the initial feed.

642

R.T.S. Muthu Lakshmi et al. / Reactive & Functional Polymers 66 (2006) 634–644

IR and NMR studies thus confirmed the structure of these polymers. However, the microstructure of the copolymers in terms of a random or blocklike nature could not be ascertained. The synthesis of SPEES was done by nucleophilic displacement of fluoride by the phenoxide ion having electronwithdrawing sulphonic acid groups in the backbone. The nucleophilicity of sulphonated monomer is expected to be lower than that of the unsulphonated bisphenols. Under these conditions, it is expected that during initial stages of polymerisation, the reaction of bisphenols and DFDPS may be predominant. At later stages HSQA is incorporated to a higher content into the polymer backbone than expected from the monomer composition due to the lack of the unsulphonated bisphenols. This will lead to the formation of a block-like structure rather than a random copolymer. 3.2. Thermal behaviour The glass transition temperature Tg could only be determined for the samples SHQ-1, SHQ-2 and SPH-1. For these samples the Tg was 205, 188 and 184 C, respectively. For all other samples the Tg is supposed to be higher than 230 C and interferes, therefore with the broad endothermic transition observed at that temperature. This transition is attributed to the release of remaining solvent in

the polymer samples which is physically bonded to the sulphonic acid groups by hydrogen bonds. The thermal stability of SPEES samples was investigated by TGA in nitrogen atmosphere. An initial mass loss of 4–7% was observed in the temperature range of 50–150 C (Fig. 3). This has been attributed to absorbed moisture. Polymers containing sulphonic acid groups are hydrophilic in nature and will absorb moisture from the surroundings. However, when these samples were heated upto 200 C, cooled to 50 C and reheated in TGA cell, then there was no mass loss found at this temperature range which lends support that absorbed mass loss is due to moisture. Breakdown of the polymer backbone takes place above 450 C (Table 4). Table 4 Characteristic decomposition temperatures during major mass loss step Sample designation

Ti (C)

Tmax (C)

Tf (C)

SHQ-1 SHQ-2 SBA-1 SBA-2 SBP-1 SBP-2 SPH-1 SPH-2 SNA-1 SNA-2

470.2 462.1 474.0 468.1 471.3 470.5 458.0 442.1 445.0 464.6

504.2 506.3 497.0 491.5 513.4 512.5 493.1 484.0 488.7 503.7

545.2 543.5 523.5 516.4 553.8 559.6 548.1 539.6 541.3 556.5

Fig. 3. TG curves of: (a) SHQ-1, (b) SHQ-2, (c) SPH-1, and (d) SPH-2.

R.T.S. Muthu Lakshmi et al. / Reactive & Functional Polymers 66 (2006) 634–644

In order to compare the relative thermal stability of various SPEES samples, the characteristic decomposition temperatures, i.e., the initial decomposition temperature (Ti), the temperature of the maximum rate of decomposition (Tmax) and the final decomposition temperature (Tf) of major mass loss step were determined. Thermal stability of the samples was not affected by an increase in degree of sulphonation from 60 to 75 mol%. However, the initial decomposition temperature (Ti) of SPH and SNA samples was 30 C lower (i.e., 440 C) than for the rest of the samples (470 C).

643

Table 6 Proton conductivity of SPEES samples at different temperatures Sample designations

Proton conductivity (· 103 S/cm) Temperature (C)

SHQ-1 SHQ-2 SBP-1 SBP-2 SBA-1 SBA-2 SPH-1 SPH-2 SNA-2

25

50

75

100

3.3 0.942 0.211 0.00511 3.05 1.23 0.414 0.00612 0.0103

6.75 2.49 0.595 0.0185 4.57 1.51 1.59 0.00926 0.038

7.51 0.532 0.865 0.0309 6.46 2.45 2.61 0.0563 –

9.74 0.39 1.94 – 5.85 0.0373 0.229 – –

3.3. Water uptake The water uptake of the SPEES samples determined at ambient temperatures varied with the ion exchange capacity (IEC) as well as with the chemical structure. Samples with the highest IEC showed the highest water uptake (Table 5). The effect of the chemical structure on the water uptake becomes clearer when taking the ratio of mole water per mole sulphonic acid groups into consideration. Here, the samples deriving from hydroquinone (SHQ-1 and SHQ-2) and to a lesser extent also the sample SBA-1 showed the highest ratio, 20, 21 and 17, respectively, among the polymers discussed in this study. For all other polymers the ratio is between 13 and 14. Furthermore, samples SHQ-1 and SBA-1 were highly swollen in water at 80 C, whereas the other samples retained the same amount of water at this temperature as given in Table 5.

Table 5 Water uptake of SPEES samples Sample designation

Water uptake (after 24 h) (at room temperature)a

Theoretical IECb (mmol/g)

H2O/SO3H (mol/mol)

SHQ-1 SHQ-2 SBA-1 SBA-2 SBP-1 SBP-2 SNA-1 SNA-2 SPH-1 SPH-2

70.5 42.4 54.3 32.5 43.3 37.5 – 30.4 – 31.8

1.95 1.61 1.81 1.43 1.86 1.49 1.89 1.53 1.72 1.32

20 21 17 13 13 14 – 10 – 13

a b

Room temperature varied from 30 to 40 C. Calculated from initial monomer composition.

3.4. Proton conductivity measurements The proton conductivity of the samples was in the range of 3.3 · 103–6.1 · 106 S/cm at 25 C. The conductivity of Nafion117 was 4.91 · 103 S/ cm under the similar experimental conditions. The conductivity increased with increasing content of sulphonated monomer (HQSA) in the backbone (Table 6). An increase in temperature from 25 to 75 C also resulted in an increase in conductivity. In samples having hydroquinone or bisphenol-A (75% DS), conductivity increased upto 100 C, whereas in other samples, a decrease was observed above 75 C. A reduction in proton conductivity of Nafion above 80 C has also been reported in the literature. 4. Conclusions Sulphonated PEES samples containing varying amounts of hydroquinone sulphonic acid (75 or 60 mol%) and varying bisphenols (25 or 40 mol%) were successfully synthesised using the silyl method. Molecular weight of the samples was high enough for making films. The structure of these polymers was confirmed by ATR-FTIR and 1H NMR spectra. DSC studies showed that the Tg of these sulphonated samples were higher than the hydroquinone-based polymers. Thermal stability of the samples was not affected when the degree of sulphonation was increased from 60% to 75%. However, the initial decomposition temperatures of SPH and SNA samples were 30 C lower than that of other samples. Water uptake of the samples could be minimised by incorporating 4,4 0 -dihydroxybiphenyl or phenolphthalein or naphthalene moiety into the backbone. Conductivity of copolymers based on

644

R.T.S. Muthu Lakshmi et al. / Reactive & Functional Polymers 66 (2006) 634–644

hydroquinone or bisphenol-A (75% DS) was only marginally lower than Nafion membranes and was found to increase with increase in temperature up to 100 C. Acknowledgements Reliance Industries Limited, India is gratefully acknowledged for creating a chair at IIT Delhi (I.K. Varma). The Council of Scientific and Industrial Research (CSIR, Government of India) for providing a Senior Research Fellowship (SRF) and the Leibniz Institute of Polymer Research (IPF) Dresden, Germany for financial support during the stay in Germany to one of the authors (R.T.S. Muthu Lakshmi) is gratefully acknowledged. References [1] K.D. Kreuer, J. Membr. Sci. 185 (2001) 29. [2] J.P. Quentin, Sulphonated Polyarylether Sulfones, US 3,709,841, Rhone-Poulene, 1973. [3] P. Genova-Dimitrova, B. Baradie, D. Foscallo, C. Poinsignon, J.Y. Sanchez, J. Membr. Sci. 185 (2001) 59. [4] A. Noshay, L.M. Robenson, J. Appl. Polym. Sci. 20 (1976) 1885.

[5] A. Al-Omran, J.B. Rose, Polymer 37 (1996) 1735. [6] A. Bunn, J.B. Rose, Polymer 34 (1993) 1114. [7] J. Kerres, A. Ullrich, M. Hein, J. Polym. Sci. A 39 (2001) 2874. [8] J.F. Blanco, Q.T. Nguyen, P. Schaetzel, J. Appl. Polym. Sci. 84 (2002) 2461. [9] B.C. Johnson, I. Yilgor, C. Tran, M. Iqbal, J.P. Wightman, D.R. Lloyd, J.E. McGrath, J. Polym. Sci., Polym. Chem. Edn. 22 (1984) 721. [10] C.H. Wirguin, J. Membr. Sci. 120 (1996) 1. [11] B. Lafitte, L.E. Karlsson, P. Jannasch, Macromol. Rapid Commun. 23 (2002) 896. [12] Y. Yang, Z. Shi, S. Holdcroft, Macromolecules 37 (2004) 1678. [13] M. Ueda, H. Toyota, T. Ouchi, J.-I. Sugiyama, K. Yonetake, T. Mosuko, T. Teramoto, J. Polym. Sci., Polym. Chem. Edn. 31 (1993) 853. [14] F. Wang, M.A. Hickner, Q. Ji, W.L. Harrison, J. Mecham, T.A. Zawodzinski, J.E. McGrath, Macromol. Symb. 175 (2001) 387. [15] F. Wang, M.A. Hickner, Y.S. Kim, T.A. Zawodzinski, J.E. McGrath, J. Membr. Sci. 197 (2002) 231. [16] W.L. Harrison, F. Wang, J.B. Mecham, V.A. Bhanu, M. Hill, J.E. McGrath, J. Polym. Sci., Polym. Chem. Edn. 41 (2003) 2264. [17] Y.S. Kim, L. Dong, M.A. Hickner, T.E. Glass, V. Webb, J.E. McGrath, Macromolecules 36 (2003) 6281. [18] A. Taeger, C. Vogel, D. Lehmann, W. Lenk, K. Schlenstedt, J. Meier-Haack, Macromol. Sym. 210 (2004) 175. [19] H.R. Kricheldorf, G. Bier, J. Polym. Sci., Polym. Chem. Edn. 21 (1983) 2283.