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Structural and electrical analysis of ion conducting composites
European Polymer Journal 36 (2000) 2551±2558
Structural and electrical analysis of ion conducting composites A. Linares *, J.L. Acosta Instituto de Ciencia y Tecnologõa de Polõmeros (CSIC), c/. Juan de la Cierva 3, 28006 Madrid, Spain Received 25 October 1999; accepted 9 February 2000
Abstract In this work, new polymer composites based on polyvinylidene ¯uoride and ethylene-polypropylene-diene terpolymer have been prepared. Sulphonated styrene-co-divinylbenzene and its sodium salt (as organic proton conductors) and antimonic acid (as inorganic proton conductor) have been incorporated at dierent contents. Photoelectron spectroscopy, infrared spectroscopy, glass transition temperature (DSC and DMA), isothermal crystallization and electrical properties have been studied in order to characterize the obtained materials. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polyvinylidene ¯uoride; Ethylene-polypropylene-diene terpolymer; Composites; Proton conductors; Crosslinked polystyrene derivatives
1. Introduction Conducting protonic materials have been given a considerable number of applications of late [1,2] especially in devices meant for storing or generation of electric current, apart from the ®eld of sensors. Within the group of protonic conductors, those of a polymeric nature are acquiring growing relevance, due to their remarkable chemical and thermal stability, low cost and property to form membranes, the best known of which are per¯uorinated membranes. However, their high market price as well as the diculty of eliminating them from urban and industrial wastes, have increased the eorts to replace them by more environment-friendly materials. On these lines of research sulphonated polystyrene has proved to possess extraordinary conducting properties, but matched with very poor technological bene®ts, taking into account its high water solubility and the impossibility to use it in the manufacture of mem-
branes. There exist, however, certain procedures which inhibit its water solubility, for instance, when slightly crosslinked with small amounts of divinylbenzene. In this work, some new polymeric composites are described, based on the sulphonated acid or saponi®ed form of a crosslinked copolymer (polystyrene-co-divinylbenzene), incorporated into a commercial thermoplastic matrix, such as polyvinylidene ¯uoride (PVDF) or an elastomer thermoplastic (EPDM). In addition, some other composites are described and studied, based on the same components but having incorporated an inorganic protonic conductor (antimonic acid) for the purpose of reinforcing and improving the technological properties. In this research, the eects which are exerted by each single component on the structural parameters of the resulting composites and their properties are examined.
2. Experimental 2.1. Materials *
Corresponding author. Tel.: +34-1-562-2900; fax: +34-1562-1829. E-mail address: [email protected] (A. Linares).
All the polymers were commercial products and used without further puri®cation. PVDF was Solef 6010
0014-3057/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 0 6 2 - 8
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A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
supplied by Solvay, Germany. Ethylene-polypropylenediene terpolymer EPDM was Vistalon 9500 delivered by Exxon Chemical, France. Polystyrene (crosslinked), sulphonic acid sodium form (PS-co-DVB-SNa) and polystyrene (crosslinked), sulphonic acid hydrogen ion form (PS-co-DVB-SH) were delivered by Scienti®c Polymer, USA. The antimonic acid (HSb) was synthesized by oxidation of Sb2 O3 (Merck) with an excess of 35% H2 O2 (Merck). The suspension was vigorously stirred and heated at 65°C for 30 h. Once the antimonic acid was obtained, it was washed with de-ionized water and then centrifuged. Finally, the product was dried at 40°C under low pressure [3]. Blends were prepared in a thermoplastic mixing chamber, preheated at 180°C for PVDF-based samples and 160°C for EPDM-based samples. In both the cases, rotor speed was set at 80 rpm, and 10 min of mixing were enough to get a steady-state torque response indicating uniform dispersion of the components. Subsequently, samples were obtained from compression moulding. 2.2. Methods Photoelectron spectra (XPS) were acquired with a VG Escalab 200R spectrometer equipped with a hemispherical electron analyzer and AlKa (hm 1486:6 eV, 1 eV 1:6302 10ÿ19 J) 120 W X-ray source. A DEC PDP 11/53 computer was used for collecting and processing the spectra. The polymer samples were mounted on a sample rod placed in a pretreatment chamber and outgassed at 10ÿ5 Torr prior to being transferred to the analysis chamber. Before recording the spectra, the sample was maintained in the analysis chamber under a residual pressure of 2 10ÿ9 Torr for 2 h. This pressure was almost unchanged during data acquisition. The spectra were collected at pass energy of 20 eV, which is typical of high-resolution conditions. The intensities were estimated by calculating the integral of each peak, after smoothing, subtraction of the ``S-shaped'' background, and ®tting the experimental curve to a combination of Lorentzian and Gaussian lines of variable proportion. All binding energies were referenced to the C 1s line at 284.9 eV. This reference gave binding energy values with an accuracy of 0.2 eV.
Calorimetric measurements and isothermal crystallizations were carried out on a Mettler TA4000 dierential scanning calorimeter (DSC) operating in N2 atmosphere. Before recording DSC thermograms, at a heating rate of 10°C/min, samples were quenched at low temperature from the melt. The midpoint of the slope change of the heat capacity plot was taken as the glass transition temperature. The following procedure was used to study the isothermal crystallization: the samples were kept for 5 min at 220°C to destroy their thermal history and then quenched to their crystallization temperature Tc . The melting temperatures Tm of each of the samples after isothermal crystallization at Tc were calculated by heating the sample directly from Tc to Tm at 10°C min. Films of blends based on PVDF were compression moulded at 180°C. The dynamic mechanical properties were analyzed in DMTA Polymer Laboratory in the tensile mode, at deformation frequencies of 10 and 30 Hz and the temperature range was between ÿ150°C and 150°C. Dynamic mechanical measurements were performed on EPDM-based samples using a Viscoanalyseur RA C815A (Metravib). Specimens were compression moulded at 160°C and the sample cross-sectional areas were 2 cm2 . Samples were analyzed in the compression mode at deformation frequencies of 10 and 20 Hz and the temperatures ranged from ÿ100°C to 100°C. Samples were compression moulded in a COLLIN press. Complex impedance spectroscopy was carried out using a Hewlett Packard impedance analyzer, model 4192A, at room temperature and the frequency range used was 0.01±10,000 kHz. Composites were dispersed in KBr to form a pellet and the infrared spectra were recorded with a Nicolet 520 FT-IR spectrometer.
3. Results and discussion 3.1. Photoelectron spectroscopy Photoelectron spectroscopy is generally used to determine the chemical condition of the elements in the
Table 1 Binding energies (eV) of core electrons and surface atomic ratios of the sulphonated polystyrene-co-divinylbenzene and its sodium saponi®ed sample Sample
C1s
S2p
Na1s
S/C atoms
Na/S atoms
±SO3 H (%)
PS-co-DVB-SH PS-co-DVB-SNa
284.9 284.9
168.5 168.4
± 1070.8
0.117 0.112
± 0.670
96.2 ±
A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
samples, as well as to quantify their surface abundance, assuming that it mirrors their abundance inside the material. Table 1 shows the comparative analysis of two commercial materials supplied by Scienti®c Polymer, the ®rst being a sulphonated polystyrene/divinylbenzene copolymer (PS-co-DVB-SH) and the second the sodium salt of the ®rst sample (PS-co-DVB-SNa). For each sample, the table compiles the binding energies of each element, as well as the atomic ratios S/C and Na/S in order to measure the abundance of sulphonic groups and the sodium atoms in the samples. Atomic ratios were computed from intensity ratios normalized by tabulated atomic sensitivity factors [4], as no signi®cant dierences in kinetic energy of the C1s , S2p and Na1s photoelectrons exist and the eciency of the detector was assumed to be constant irrespective of the peak scanned. The atomic S/C ratios found for the two samples, 0.111 and 0.112, respectively, indicate that practically all styrene units in both the samples contain sulphonic groups (or similar organic groups). The value 0.670 of the Na/S atomic ratio determined in the saponi®ed sample of the sulphonated styrene copolymer allows for the assumption that either not all SO3 H clusters are saponi®ed (only 67%) or else they contain additional non-saponi®able chemical groups. Depending on the experimental conditions [5], the sulphonation reaction of aromatic groups is known to be liable to progress towards sulphone (±SO2 ±) formation, which in our case would imply additional polymer crosslinking and hence the modi®cation of some of its physical properties, e.g. its water swelling properties. As in our case, absence of swelling has been observed and the two crosslinked samples were analyzed using IR
2553
spectroscopy and compared to the non-sulphonated copolymer (Fig. 1) for the purpose of ®nding out whether, apart from the sulphonic group, other sulphone-type groups could be detected. In fact, Fig. 1 shows the typical bands of each sample, especially those associated to the sulphonated benzene group (1000 and 1250 cmÿ1 ), as well as the three absorption bands of the ±SO3 H group (1250±1160, 1080±1000 and 700±610 cmÿ1 ), the presumable sulphone (1340±1300 and 1160±1135 cmÿ1 ) or the sulphonate groups (1420±1330 and 1200±1145 cmÿ1 ). Due to the complexity of the zone between 1000 and 1500 cmÿ1 , particularly when the sample is a polymer containing sulphonic groups, sulphonate and/or sulphones, it is very dicult indeed to assign bands to each group. But the IR spectra allow to intute the presence of the ±SO3 H (sulphonic) group and that of the ±SO2 ± group (sulphones) from the acid nature of the crosslinked copolymer (Fig. 1(b)). This presence is further con®rmed by other types of analysis, such as XPS spectroscopy, the absence of swelling in water and by the low conductivity achieved. Hence it is legitimate to conclude that this copolymer is strongly crosslinked, that it does not swell and thus inhibits free interior proton movement. 3.2. Glass transition temperature (Tg ) from DSC analysis In Table 2, the glass transition temperature values are compiled, as well as the speci®c heat values reached by the samples and obtained according to the procedure detailed in Section 2. Taking into account the data compounded in Table 1, it is observed that the glass transition temperatures of the dierent composites are in¯uenced both by the presence of the crosslinked copolymer and the antimonic acid. In general terms, the Tg s of the PVDF composites containing PS-co-DVB-SH are much higher (approximately 3°C) than those containing the saponi®ed copolymer (PS-co-DVB-SNa). No signi®cant deviations from this behaviour are observed when varying the antimonic acid portion in the system. Nevertheless, these dierences are not observed in the EPDM composites, i.e. independent of the HSb or crosslinked copolymer content, both in acid and saponi®ed form and the Tg of the system practically does not vary, except for the highest PS-co-DVB-SH concentrations, whose Tg s are 1±2°C lower as compared to those of the saponi®ed copolymer. 3.3. Isothermal crystallization
Fig. 1. FTIR spectra of (a) PS-co-DVB, (b) PS-co-DVB-SH and (c) PS-co-DVB-SNa.
From a theoretical point of view, it was ®rst Avrami [6] and then Evans [7] who demonstrated that
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A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
Table 2 Glass transition temperature and heat capacity obtained from DSC of the samples Tg (°C)
Cp (J/g K)
± ±
ÿ38:2 ÿ42:5
0.15 0.37
± ± ± ± 10 20 30
30 20 10 ± 20 10 ±
ÿ40:3 ÿ36:3 ÿ40:4 ÿ39:2 ÿ43:1 ÿ43:0 ÿ43:2
0.12 0.12 0.13 0.20 0.19 0.18 0.22
± ± ± ± 10 20 30
30 20 10 ± 20 10 ±
ÿ43:8 ÿ43:9 ÿ45:0 ÿ45:0 ÿ43:8 ÿ43:8 ÿ42:7
0.27 0.28 0.24 0.25 0.26 0.26 0.30
Samples
Composition (wt.%) PVDF
EPDM
PS-co-DVB-SH
PS-co-DVB-SNa
HSb
PVDF EPDM
100 ±
± 100
± ±
± ±
PF-10 PF-11 PF-12 PF-13 PF-21 PF-22 PF-23
70 70 70 70 70 70 70
± ± ± ± ± ± ±
± 10 20 30 ± ± ±
EP-10 EP-11 EP-12 EP-13 EP-21 EP-22 EP-23
± ± ± ± ± ± ±
70 70 70 70 70 70 70
± 10 20 30 ± ± ±
Table 3 Kinetic studies: Avrami parameters of the blends Samples
Composition (wt.%)
Tc (°C)
Tm (°C)
n
log K
±
151 152 153 154
176.2 176.3 176.3 176.2
2.5 2.6 2.8 2.8
ÿ2:00 ÿ2:51 ÿ3:01 ÿ3:44
±
30
151 152 153 154
174.2 174.1 176.0 176.2
2.5 2.7 2.4 2.5
ÿ0:67 ÿ1:08 ÿ1:44 ÿ1:78
70
10
20
151 152 153 154
173.7 173.5 173.9 174.3
2.4 2.5 2.4 2.4
ÿ0:94 ÿ1:27 ÿ1:52 ÿ1:84
PF-12
70
20
10
151 152 153 154
174.4 174.5 174.4 174.7
2.5 2.6 2.5 2.3
ÿ1:14 ÿ1:45 ÿ1:99 ÿ2:40
PF-13
70
30
±
151 152 153 145
174.2 174.2 175.2 175.5
2.5 2.3 2.4 2.6
ÿ1:22 ÿ1:34 ÿ2:07 ÿ2:46
PVDF
PS-co-DVB-SH
HSb
PVDF
100
±
PF-10
70
PF-11
in isothermal conditions the change in crystallinity with time can be monitored by thermal analysis through the evolution of crystallization heat, and can be readily expressed in the form of the equation
X 1 ÿ exp ÿKtn ;
1
where X is the volume fraction of material crystallized at time t, K is the temperature-dependent constant that
A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
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Table 4 Kinetic studies: Avrami parameters of the samples Samples
Composition (wt.%)
Tc (°C)
Tm (°C)
n
log K
PVDF
PS-co-DVB-SNa
HSb
PVDF
100
±
±
151 152 153 154
176.2 176.3 176.3 176.2
2.5 2.6 2.8 2.8
ÿ2:00 ÿ2:51 ÿ3:01 ÿ3:44
PF-20
70
±
30
151 152 153 154
174.2 174.1 176.0 176.2
2.5 2.7 2.4 2.5
ÿ0:67 ÿ1:08 ÿ1:44 ÿ1:78
PF-21
70
10
20
151 152 153 154
173.2 174.8 174.6 174.7
2.4 2.4 2.3 2.4
ÿ0:71 ÿ1:64 ÿ1:38 ÿ1:87
PF-22
70
20
10
151 152 153 154
174.5 173.5 174.5 174.9
2.6 2.4 2.5 2.4
ÿ0:86 ÿ1:19 ÿ1:47 ÿ1:89
PF-23
70
30
±
151 152 153 145
174.5 175.9 174.9 175.3
2.6 2.7 2.5 2.7
ÿ1:16 ÿ1:31 ÿ1:70 ÿ1:99
contains nucleation and crystal growth rate (rate constant), and n is a number whose value depends on the crystallization mechanism and the shape of crystal growth (®bril, disc, spherulite). The Avrami parameters
Fig. 2. Representation of AvramiÕs equation, at the tested temperatures, for PF-11 sample.
n and K are determined by taking the double logarithm of Eq. (1) to yield a plot log [ÿln (1 ÿ X )] versus log t. Tables 3 and 4 show the results obtained from the isothermal crystallization kinetics of the experimental samples. Fig. 2 reproduces, in terms of representative example, AvramiÕs equation applied to sample PF-11. In all cases, the values of exponent n are fairly similar to those reached for pure PVDF, and they remain practically invariable, independent of variations in HSb content or the dierent crosslinked copolymers. This suggests that PVDF adopts the same growth geometry, independent of Tc values and blend composition. Considering the crystallization rate K, it logically diminishes in all cases as a function of increasing crystallization temperature. However, when HSb is incorporated or any other type of crosslinked copolymer, the panorama changes signi®cantly. For any of the experimental Tc s, it is observed that when the HSb portion is rised in the system, the crystallization rate of PVDF increases, both for PS-co-DVB-SH and for PS-co-DVB-SNa composites, the increase being more signi®cant, however, in the presence of the acid form of the crosslinked copolymer when compared to its saponi®ed form. At any temperature at which the
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A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
Table 5 Glass transition temperatures, at dierent frequencies, obtained from dynamic mechanical measurements HSb
Tg (°C) 10 Hz
Tg (°C) 30 Hz
± ±
ÿ34:6 ÿ41:5
ÿ29:6 ÿ40:8a
± ± ± ± 10 20 30
30 20 10 ± 20 10 ±
ÿ36:0 ÿ36:5 ÿ37:1 ÿ37:4 ÿ39:1 ÿ40:6 ÿ36:2
ÿ34:8 ÿ34:3 ÿ35:6 ÿ36:3 ÿ36:0 ÿ37:0 ÿ32:5
± ± ± ± 10 20 30
30 20 10 ± 20 10 ±
ÿ39:0 ÿ44:1 ÿ41:8 ÿ42:5 ÿ43:0 ÿ41:1 ÿ40:2
ÿ37:6a ÿ43:6a ÿ39:1a ÿ40:9a ÿ41:4a ÿ39:4a ÿ38:7a
Samples
Composition (wt.%) PVDF
EPDM
PS-co-DVB-SH
PS-co-DVB-SNa
PVDF EPDM
100 ±
± 100
± ±
± ±
PF-10 PF-11 PF-12 PF-13 PF-21 PF-22 PF-23
70 70 70 70 70 70 70
± ± ± ± ± ± ±
± 10 20 30 ± ± ±
EP-10 EP-11 EP-12 EP-13 EP-21 EP-22 EP-23
± ± ± ± ± ± ±
70 70 70 70 70 70 70
± 10 20 30 ± ± ±
a
The frequency used was 20 Hz.
system crystallizes isothermally, this phenomenon is present, which can be interpreted as HSb producing a nucleating eect on PVDF crystallization, coadjuvated by the presence of any of the crosslinked copolymers [8,9].
3.4. Mechanical dynamic analysis Table 5 lists the Tg data obtained from dynamic mechanical analysis of the dierent samples tested at two frequencies. In Fig. 3, by way of illustration, the dynamic mechanical spectra of samples FP-12 and EP-12 are compared, each sample showing, apart from the Tg of the material, some further transitions [10]. The eects exerted by the dierent components of the composite on the Tg of the material are similar to what was observed in thermal analysis (DSC), i.e. the Tg s of the PVDF composites are higher when containing the saponi®ed copolymer, as compared to its acid form, but no major dierences are noted attributable to the presence or absence of antimonic acid at any concentration. This means that the interactions produced by the acid form of the crosslinked copolymer are stronger than those activated by the saponi®ed component. In the EPDM composites, the same global eect is observed with the dierence that the higher HSb concentrations shift the composite Tg towards lower temperatures,
Fig. 3. Variation of tan d versus temperature, at 10 Hz, for EP12 and FP-12 samples.
which translates into the triggering of stronger interactions. On comparing the Tg values obtained from DSC and DMA, the same global trends can be observed. But the Tg values derived from DMA are approximately 2±4°C higher than the DSC values in the great majority of the samples, which may be interpreted as a characteristic inherent to the respective test method.
A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
3.5. Complex impedance The electrical properties were determined by means of complex impedance spectroscopy. Samples were hydrated by immersion in de-ionized water at 50°C for dierent times. In previous research, the insulating nature (even in a hydrated condition) of the samples containing PS-coDVB-SNa has been proven. The conductivity data of the remaining samples are compiled in Table 6.
2557
Fig. 4 shows the arcs obtained for sample EP-12 at dierent hydration degrees. The y-intercept (zero frequency) determines the conductivity of each arc. As can be observed, conductivity increases notably with increasing hydration time. If the PVDF and EPDM-based samples are compared in global terms, they both show fairly similar general trends, in spite of displaying quite dierent behaviour. In one and the other family, conductivity reaches signi®cant levels only in the samples containing
Table 6 Conductivity of the samples from complex impedance spectroscopy analysis Samples
Composition (wt.%)
Hydration time (min)
Conductivity (S/cm)
PVDF
EPDM
PS-co-DVBSH
HSb
PVDF
100
±
±
±
0
8:1 10ÿ10
EPDM
±
100
±
±
0
6:3 10ÿ10
PF-10
70
±
±
30
0 60 120 180
2:1 10ÿ10 2:9 10ÿ10 2:8 10ÿ10 2:6 10ÿ10
PF-11
70
±
10
20
0 60 120 180
4:6 10ÿ9 2:6 10ÿ8 3:2 10ÿ8 5:5 10ÿ8
PF-12
70
±
20
10
0 60 120 180
3:5 10ÿ8 7:8 10ÿ6 6:9 10ÿ6 5:6 10ÿ6
PF-13
70
±
30
±
0 60 120 180
1:2 10ÿ9 1:5 10ÿ6 5:0 10ÿ6 4:9 10ÿ6
EP-10
±
70
±
30
0 60 120 180
6:9 10ÿ10 4:5 10ÿ10 3:4 10ÿ10 3:3 10ÿ10
EP-11
±
70
10
20
0 60 120 180
3:3 10ÿ10 5:1 10ÿ10 7:5 10ÿ10 4:6 10ÿ10
EP-12
±
70
20
10
0 60 120 180
4:9 10ÿ10 6:1 10ÿ9 2:5 10ÿ8 4:0 10ÿ8
EP-13
±
70
30
±
0 60 120 180
6:0 10ÿ10 7:7 10ÿ10 4:7 10ÿ10 5:9 10ÿ10
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A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
20% PS-co-DVB-SH and 10% HSb, but does not show any alteration in the rest of the samples, independent of hydration time. In contrast, if the absolute conductivity values achieved in the two samples as a function of hydration time (Fig. 5) are compared, the conductivity values of PVDF composites are observed to be higher than those of EPDM. In other words, the motility of the proton derived from the acid form of the crosslinked copolymer and from the antimonic acid is more strongly constrained in the EPDM composites than in the PVDF-based materials, probably as a consequence of de®cient hydration, basically due to the presence of HSb, as the samples with the highest concentrations of this inorganic proton conductor are also the ones that present the lowest conductivity values. Fig. 4. Complex plane diagram for EP-12 at dierent hydration times (min): (a) 0, (b) 60, (c) 120 and (d) 180.
References
Fig. 5. Variation of conductivity versus hydration time for PF12 and EP-12 samples.
[1] Kreuer KD. Chem Mater 1996;8:610. [2] Appleby AJ. In: Doughty D, Vyas B, Takamura T, Hu JR, editors. Materials Research Society Symposium Proceedings, vol. 393. PA, USA: Materials Research Society, 1996. p. 11. [3] Amarilla JM, Rojas RM, Rojo JM, Cubillo MJ, Linares A, Acosta JL. Solid State Ionics 2000;127:133. [4] Wagner CD, Davies LE, Zeller MV, Taylor JA, Raymondond RH, Gale LH. Surf Interf Anal 1981;3:211. [5] Fitzgerald JJ, Weis RA. JMS-Rev Macromol Chem Phys 1988;C28(1):99. [6] Avrami M. J Chem Phys 1939;7:1103. [7] Evans UR. Trans Faraday Soc 1945;41:365. [8] Acosta JL, Ojeda MC, Morales E, Linares A. J Appl Polym Sci 1986;32:4119. [9] Jurado JR, Moure C, Duran P, Rodriguez M, Linares A, Acosta JL. J Mater Sci 1991;26:4022. [10] Linares A, Acosta JL. Polym Bull 1996;36:241.
Structural and electrical analysis of ion conducting composites A. Linares *, J.L. Acosta Instituto de Ciencia y Tecnologõa de Polõmeros (CSIC), c/. Juan de la Cierva 3, 28006 Madrid, Spain Received 25 October 1999; accepted 9 February 2000
Abstract In this work, new polymer composites based on polyvinylidene ¯uoride and ethylene-polypropylene-diene terpolymer have been prepared. Sulphonated styrene-co-divinylbenzene and its sodium salt (as organic proton conductors) and antimonic acid (as inorganic proton conductor) have been incorporated at dierent contents. Photoelectron spectroscopy, infrared spectroscopy, glass transition temperature (DSC and DMA), isothermal crystallization and electrical properties have been studied in order to characterize the obtained materials. Ó 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polyvinylidene ¯uoride; Ethylene-polypropylene-diene terpolymer; Composites; Proton conductors; Crosslinked polystyrene derivatives
1. Introduction Conducting protonic materials have been given a considerable number of applications of late [1,2] especially in devices meant for storing or generation of electric current, apart from the ®eld of sensors. Within the group of protonic conductors, those of a polymeric nature are acquiring growing relevance, due to their remarkable chemical and thermal stability, low cost and property to form membranes, the best known of which are per¯uorinated membranes. However, their high market price as well as the diculty of eliminating them from urban and industrial wastes, have increased the eorts to replace them by more environment-friendly materials. On these lines of research sulphonated polystyrene has proved to possess extraordinary conducting properties, but matched with very poor technological bene®ts, taking into account its high water solubility and the impossibility to use it in the manufacture of mem-
branes. There exist, however, certain procedures which inhibit its water solubility, for instance, when slightly crosslinked with small amounts of divinylbenzene. In this work, some new polymeric composites are described, based on the sulphonated acid or saponi®ed form of a crosslinked copolymer (polystyrene-co-divinylbenzene), incorporated into a commercial thermoplastic matrix, such as polyvinylidene ¯uoride (PVDF) or an elastomer thermoplastic (EPDM). In addition, some other composites are described and studied, based on the same components but having incorporated an inorganic protonic conductor (antimonic acid) for the purpose of reinforcing and improving the technological properties. In this research, the eects which are exerted by each single component on the structural parameters of the resulting composites and their properties are examined.
2. Experimental 2.1. Materials *
Corresponding author. Tel.: +34-1-562-2900; fax: +34-1562-1829. E-mail address: [email protected] (A. Linares).
All the polymers were commercial products and used without further puri®cation. PVDF was Solef 6010
0014-3057/00/$ - see front matter Ó 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 4 - 3 0 5 7 ( 0 0 ) 0 0 0 6 2 - 8
2552
A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
supplied by Solvay, Germany. Ethylene-polypropylenediene terpolymer EPDM was Vistalon 9500 delivered by Exxon Chemical, France. Polystyrene (crosslinked), sulphonic acid sodium form (PS-co-DVB-SNa) and polystyrene (crosslinked), sulphonic acid hydrogen ion form (PS-co-DVB-SH) were delivered by Scienti®c Polymer, USA. The antimonic acid (HSb) was synthesized by oxidation of Sb2 O3 (Merck) with an excess of 35% H2 O2 (Merck). The suspension was vigorously stirred and heated at 65°C for 30 h. Once the antimonic acid was obtained, it was washed with de-ionized water and then centrifuged. Finally, the product was dried at 40°C under low pressure [3]. Blends were prepared in a thermoplastic mixing chamber, preheated at 180°C for PVDF-based samples and 160°C for EPDM-based samples. In both the cases, rotor speed was set at 80 rpm, and 10 min of mixing were enough to get a steady-state torque response indicating uniform dispersion of the components. Subsequently, samples were obtained from compression moulding. 2.2. Methods Photoelectron spectra (XPS) were acquired with a VG Escalab 200R spectrometer equipped with a hemispherical electron analyzer and AlKa (hm 1486:6 eV, 1 eV 1:6302 10ÿ19 J) 120 W X-ray source. A DEC PDP 11/53 computer was used for collecting and processing the spectra. The polymer samples were mounted on a sample rod placed in a pretreatment chamber and outgassed at 10ÿ5 Torr prior to being transferred to the analysis chamber. Before recording the spectra, the sample was maintained in the analysis chamber under a residual pressure of 2 10ÿ9 Torr for 2 h. This pressure was almost unchanged during data acquisition. The spectra were collected at pass energy of 20 eV, which is typical of high-resolution conditions. The intensities were estimated by calculating the integral of each peak, after smoothing, subtraction of the ``S-shaped'' background, and ®tting the experimental curve to a combination of Lorentzian and Gaussian lines of variable proportion. All binding energies were referenced to the C 1s line at 284.9 eV. This reference gave binding energy values with an accuracy of 0.2 eV.
Calorimetric measurements and isothermal crystallizations were carried out on a Mettler TA4000 dierential scanning calorimeter (DSC) operating in N2 atmosphere. Before recording DSC thermograms, at a heating rate of 10°C/min, samples were quenched at low temperature from the melt. The midpoint of the slope change of the heat capacity plot was taken as the glass transition temperature. The following procedure was used to study the isothermal crystallization: the samples were kept for 5 min at 220°C to destroy their thermal history and then quenched to their crystallization temperature Tc . The melting temperatures Tm of each of the samples after isothermal crystallization at Tc were calculated by heating the sample directly from Tc to Tm at 10°C min. Films of blends based on PVDF were compression moulded at 180°C. The dynamic mechanical properties were analyzed in DMTA Polymer Laboratory in the tensile mode, at deformation frequencies of 10 and 30 Hz and the temperature range was between ÿ150°C and 150°C. Dynamic mechanical measurements were performed on EPDM-based samples using a Viscoanalyseur RA C815A (Metravib). Specimens were compression moulded at 160°C and the sample cross-sectional areas were 2 cm2 . Samples were analyzed in the compression mode at deformation frequencies of 10 and 20 Hz and the temperatures ranged from ÿ100°C to 100°C. Samples were compression moulded in a COLLIN press. Complex impedance spectroscopy was carried out using a Hewlett Packard impedance analyzer, model 4192A, at room temperature and the frequency range used was 0.01±10,000 kHz. Composites were dispersed in KBr to form a pellet and the infrared spectra were recorded with a Nicolet 520 FT-IR spectrometer.
3. Results and discussion 3.1. Photoelectron spectroscopy Photoelectron spectroscopy is generally used to determine the chemical condition of the elements in the
Table 1 Binding energies (eV) of core electrons and surface atomic ratios of the sulphonated polystyrene-co-divinylbenzene and its sodium saponi®ed sample Sample
C1s
S2p
Na1s
S/C atoms
Na/S atoms
±SO3 H (%)
PS-co-DVB-SH PS-co-DVB-SNa
284.9 284.9
168.5 168.4
± 1070.8
0.117 0.112
± 0.670
96.2 ±
A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
samples, as well as to quantify their surface abundance, assuming that it mirrors their abundance inside the material. Table 1 shows the comparative analysis of two commercial materials supplied by Scienti®c Polymer, the ®rst being a sulphonated polystyrene/divinylbenzene copolymer (PS-co-DVB-SH) and the second the sodium salt of the ®rst sample (PS-co-DVB-SNa). For each sample, the table compiles the binding energies of each element, as well as the atomic ratios S/C and Na/S in order to measure the abundance of sulphonic groups and the sodium atoms in the samples. Atomic ratios were computed from intensity ratios normalized by tabulated atomic sensitivity factors [4], as no signi®cant dierences in kinetic energy of the C1s , S2p and Na1s photoelectrons exist and the eciency of the detector was assumed to be constant irrespective of the peak scanned. The atomic S/C ratios found for the two samples, 0.111 and 0.112, respectively, indicate that practically all styrene units in both the samples contain sulphonic groups (or similar organic groups). The value 0.670 of the Na/S atomic ratio determined in the saponi®ed sample of the sulphonated styrene copolymer allows for the assumption that either not all SO3 H clusters are saponi®ed (only 67%) or else they contain additional non-saponi®able chemical groups. Depending on the experimental conditions [5], the sulphonation reaction of aromatic groups is known to be liable to progress towards sulphone (±SO2 ±) formation, which in our case would imply additional polymer crosslinking and hence the modi®cation of some of its physical properties, e.g. its water swelling properties. As in our case, absence of swelling has been observed and the two crosslinked samples were analyzed using IR
2553
spectroscopy and compared to the non-sulphonated copolymer (Fig. 1) for the purpose of ®nding out whether, apart from the sulphonic group, other sulphone-type groups could be detected. In fact, Fig. 1 shows the typical bands of each sample, especially those associated to the sulphonated benzene group (1000 and 1250 cmÿ1 ), as well as the three absorption bands of the ±SO3 H group (1250±1160, 1080±1000 and 700±610 cmÿ1 ), the presumable sulphone (1340±1300 and 1160±1135 cmÿ1 ) or the sulphonate groups (1420±1330 and 1200±1145 cmÿ1 ). Due to the complexity of the zone between 1000 and 1500 cmÿ1 , particularly when the sample is a polymer containing sulphonic groups, sulphonate and/or sulphones, it is very dicult indeed to assign bands to each group. But the IR spectra allow to intute the presence of the ±SO3 H (sulphonic) group and that of the ±SO2 ± group (sulphones) from the acid nature of the crosslinked copolymer (Fig. 1(b)). This presence is further con®rmed by other types of analysis, such as XPS spectroscopy, the absence of swelling in water and by the low conductivity achieved. Hence it is legitimate to conclude that this copolymer is strongly crosslinked, that it does not swell and thus inhibits free interior proton movement. 3.2. Glass transition temperature (Tg ) from DSC analysis In Table 2, the glass transition temperature values are compiled, as well as the speci®c heat values reached by the samples and obtained according to the procedure detailed in Section 2. Taking into account the data compounded in Table 1, it is observed that the glass transition temperatures of the dierent composites are in¯uenced both by the presence of the crosslinked copolymer and the antimonic acid. In general terms, the Tg s of the PVDF composites containing PS-co-DVB-SH are much higher (approximately 3°C) than those containing the saponi®ed copolymer (PS-co-DVB-SNa). No signi®cant deviations from this behaviour are observed when varying the antimonic acid portion in the system. Nevertheless, these dierences are not observed in the EPDM composites, i.e. independent of the HSb or crosslinked copolymer content, both in acid and saponi®ed form and the Tg of the system practically does not vary, except for the highest PS-co-DVB-SH concentrations, whose Tg s are 1±2°C lower as compared to those of the saponi®ed copolymer. 3.3. Isothermal crystallization
Fig. 1. FTIR spectra of (a) PS-co-DVB, (b) PS-co-DVB-SH and (c) PS-co-DVB-SNa.
From a theoretical point of view, it was ®rst Avrami [6] and then Evans [7] who demonstrated that
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A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
Table 2 Glass transition temperature and heat capacity obtained from DSC of the samples Tg (°C)
Cp (J/g K)
± ±
ÿ38:2 ÿ42:5
0.15 0.37
± ± ± ± 10 20 30
30 20 10 ± 20 10 ±
ÿ40:3 ÿ36:3 ÿ40:4 ÿ39:2 ÿ43:1 ÿ43:0 ÿ43:2
0.12 0.12 0.13 0.20 0.19 0.18 0.22
± ± ± ± 10 20 30
30 20 10 ± 20 10 ±
ÿ43:8 ÿ43:9 ÿ45:0 ÿ45:0 ÿ43:8 ÿ43:8 ÿ42:7
0.27 0.28 0.24 0.25 0.26 0.26 0.30
Samples
Composition (wt.%) PVDF
EPDM
PS-co-DVB-SH
PS-co-DVB-SNa
HSb
PVDF EPDM
100 ±
± 100
± ±
± ±
PF-10 PF-11 PF-12 PF-13 PF-21 PF-22 PF-23
70 70 70 70 70 70 70
± ± ± ± ± ± ±
± 10 20 30 ± ± ±
EP-10 EP-11 EP-12 EP-13 EP-21 EP-22 EP-23
± ± ± ± ± ± ±
70 70 70 70 70 70 70
± 10 20 30 ± ± ±
Table 3 Kinetic studies: Avrami parameters of the blends Samples
Composition (wt.%)
Tc (°C)
Tm (°C)
n
log K
±
151 152 153 154
176.2 176.3 176.3 176.2
2.5 2.6 2.8 2.8
ÿ2:00 ÿ2:51 ÿ3:01 ÿ3:44
±
30
151 152 153 154
174.2 174.1 176.0 176.2
2.5 2.7 2.4 2.5
ÿ0:67 ÿ1:08 ÿ1:44 ÿ1:78
70
10
20
151 152 153 154
173.7 173.5 173.9 174.3
2.4 2.5 2.4 2.4
ÿ0:94 ÿ1:27 ÿ1:52 ÿ1:84
PF-12
70
20
10
151 152 153 154
174.4 174.5 174.4 174.7
2.5 2.6 2.5 2.3
ÿ1:14 ÿ1:45 ÿ1:99 ÿ2:40
PF-13
70
30
±
151 152 153 145
174.2 174.2 175.2 175.5
2.5 2.3 2.4 2.6
ÿ1:22 ÿ1:34 ÿ2:07 ÿ2:46
PVDF
PS-co-DVB-SH
HSb
PVDF
100
±
PF-10
70
PF-11
in isothermal conditions the change in crystallinity with time can be monitored by thermal analysis through the evolution of crystallization heat, and can be readily expressed in the form of the equation
X 1 ÿ exp ÿKtn ;
1
where X is the volume fraction of material crystallized at time t, K is the temperature-dependent constant that
A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
2555
Table 4 Kinetic studies: Avrami parameters of the samples Samples
Composition (wt.%)
Tc (°C)
Tm (°C)
n
log K
PVDF
PS-co-DVB-SNa
HSb
PVDF
100
±
±
151 152 153 154
176.2 176.3 176.3 176.2
2.5 2.6 2.8 2.8
ÿ2:00 ÿ2:51 ÿ3:01 ÿ3:44
PF-20
70
±
30
151 152 153 154
174.2 174.1 176.0 176.2
2.5 2.7 2.4 2.5
ÿ0:67 ÿ1:08 ÿ1:44 ÿ1:78
PF-21
70
10
20
151 152 153 154
173.2 174.8 174.6 174.7
2.4 2.4 2.3 2.4
ÿ0:71 ÿ1:64 ÿ1:38 ÿ1:87
PF-22
70
20
10
151 152 153 154
174.5 173.5 174.5 174.9
2.6 2.4 2.5 2.4
ÿ0:86 ÿ1:19 ÿ1:47 ÿ1:89
PF-23
70
30
±
151 152 153 145
174.5 175.9 174.9 175.3
2.6 2.7 2.5 2.7
ÿ1:16 ÿ1:31 ÿ1:70 ÿ1:99
contains nucleation and crystal growth rate (rate constant), and n is a number whose value depends on the crystallization mechanism and the shape of crystal growth (®bril, disc, spherulite). The Avrami parameters
Fig. 2. Representation of AvramiÕs equation, at the tested temperatures, for PF-11 sample.
n and K are determined by taking the double logarithm of Eq. (1) to yield a plot log [ÿln (1 ÿ X )] versus log t. Tables 3 and 4 show the results obtained from the isothermal crystallization kinetics of the experimental samples. Fig. 2 reproduces, in terms of representative example, AvramiÕs equation applied to sample PF-11. In all cases, the values of exponent n are fairly similar to those reached for pure PVDF, and they remain practically invariable, independent of variations in HSb content or the dierent crosslinked copolymers. This suggests that PVDF adopts the same growth geometry, independent of Tc values and blend composition. Considering the crystallization rate K, it logically diminishes in all cases as a function of increasing crystallization temperature. However, when HSb is incorporated or any other type of crosslinked copolymer, the panorama changes signi®cantly. For any of the experimental Tc s, it is observed that when the HSb portion is rised in the system, the crystallization rate of PVDF increases, both for PS-co-DVB-SH and for PS-co-DVB-SNa composites, the increase being more signi®cant, however, in the presence of the acid form of the crosslinked copolymer when compared to its saponi®ed form. At any temperature at which the
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A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
Table 5 Glass transition temperatures, at dierent frequencies, obtained from dynamic mechanical measurements HSb
Tg (°C) 10 Hz
Tg (°C) 30 Hz
± ±
ÿ34:6 ÿ41:5
ÿ29:6 ÿ40:8a
± ± ± ± 10 20 30
30 20 10 ± 20 10 ±
ÿ36:0 ÿ36:5 ÿ37:1 ÿ37:4 ÿ39:1 ÿ40:6 ÿ36:2
ÿ34:8 ÿ34:3 ÿ35:6 ÿ36:3 ÿ36:0 ÿ37:0 ÿ32:5
± ± ± ± 10 20 30
30 20 10 ± 20 10 ±
ÿ39:0 ÿ44:1 ÿ41:8 ÿ42:5 ÿ43:0 ÿ41:1 ÿ40:2
ÿ37:6a ÿ43:6a ÿ39:1a ÿ40:9a ÿ41:4a ÿ39:4a ÿ38:7a
Samples
Composition (wt.%) PVDF
EPDM
PS-co-DVB-SH
PS-co-DVB-SNa
PVDF EPDM
100 ±
± 100
± ±
± ±
PF-10 PF-11 PF-12 PF-13 PF-21 PF-22 PF-23
70 70 70 70 70 70 70
± ± ± ± ± ± ±
± 10 20 30 ± ± ±
EP-10 EP-11 EP-12 EP-13 EP-21 EP-22 EP-23
± ± ± ± ± ± ±
70 70 70 70 70 70 70
± 10 20 30 ± ± ±
a
The frequency used was 20 Hz.
system crystallizes isothermally, this phenomenon is present, which can be interpreted as HSb producing a nucleating eect on PVDF crystallization, coadjuvated by the presence of any of the crosslinked copolymers [8,9].
3.4. Mechanical dynamic analysis Table 5 lists the Tg data obtained from dynamic mechanical analysis of the dierent samples tested at two frequencies. In Fig. 3, by way of illustration, the dynamic mechanical spectra of samples FP-12 and EP-12 are compared, each sample showing, apart from the Tg of the material, some further transitions [10]. The eects exerted by the dierent components of the composite on the Tg of the material are similar to what was observed in thermal analysis (DSC), i.e. the Tg s of the PVDF composites are higher when containing the saponi®ed copolymer, as compared to its acid form, but no major dierences are noted attributable to the presence or absence of antimonic acid at any concentration. This means that the interactions produced by the acid form of the crosslinked copolymer are stronger than those activated by the saponi®ed component. In the EPDM composites, the same global eect is observed with the dierence that the higher HSb concentrations shift the composite Tg towards lower temperatures,
Fig. 3. Variation of tan d versus temperature, at 10 Hz, for EP12 and FP-12 samples.
which translates into the triggering of stronger interactions. On comparing the Tg values obtained from DSC and DMA, the same global trends can be observed. But the Tg values derived from DMA are approximately 2±4°C higher than the DSC values in the great majority of the samples, which may be interpreted as a characteristic inherent to the respective test method.
A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
3.5. Complex impedance The electrical properties were determined by means of complex impedance spectroscopy. Samples were hydrated by immersion in de-ionized water at 50°C for dierent times. In previous research, the insulating nature (even in a hydrated condition) of the samples containing PS-coDVB-SNa has been proven. The conductivity data of the remaining samples are compiled in Table 6.
2557
Fig. 4 shows the arcs obtained for sample EP-12 at dierent hydration degrees. The y-intercept (zero frequency) determines the conductivity of each arc. As can be observed, conductivity increases notably with increasing hydration time. If the PVDF and EPDM-based samples are compared in global terms, they both show fairly similar general trends, in spite of displaying quite dierent behaviour. In one and the other family, conductivity reaches signi®cant levels only in the samples containing
Table 6 Conductivity of the samples from complex impedance spectroscopy analysis Samples
Composition (wt.%)
Hydration time (min)
Conductivity (S/cm)
PVDF
EPDM
PS-co-DVBSH
HSb
PVDF
100
±
±
±
0
8:1 10ÿ10
EPDM
±
100
±
±
0
6:3 10ÿ10
PF-10
70
±
±
30
0 60 120 180
2:1 10ÿ10 2:9 10ÿ10 2:8 10ÿ10 2:6 10ÿ10
PF-11
70
±
10
20
0 60 120 180
4:6 10ÿ9 2:6 10ÿ8 3:2 10ÿ8 5:5 10ÿ8
PF-12
70
±
20
10
0 60 120 180
3:5 10ÿ8 7:8 10ÿ6 6:9 10ÿ6 5:6 10ÿ6
PF-13
70
±
30
±
0 60 120 180
1:2 10ÿ9 1:5 10ÿ6 5:0 10ÿ6 4:9 10ÿ6
EP-10
±
70
±
30
0 60 120 180
6:9 10ÿ10 4:5 10ÿ10 3:4 10ÿ10 3:3 10ÿ10
EP-11
±
70
10
20
0 60 120 180
3:3 10ÿ10 5:1 10ÿ10 7:5 10ÿ10 4:6 10ÿ10
EP-12
±
70
20
10
0 60 120 180
4:9 10ÿ10 6:1 10ÿ9 2:5 10ÿ8 4:0 10ÿ8
EP-13
±
70
30
±
0 60 120 180
6:0 10ÿ10 7:7 10ÿ10 4:7 10ÿ10 5:9 10ÿ10
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A. Linares, J.L. Acosta / European Polymer Journal 36 (2000) 2551±2558
20% PS-co-DVB-SH and 10% HSb, but does not show any alteration in the rest of the samples, independent of hydration time. In contrast, if the absolute conductivity values achieved in the two samples as a function of hydration time (Fig. 5) are compared, the conductivity values of PVDF composites are observed to be higher than those of EPDM. In other words, the motility of the proton derived from the acid form of the crosslinked copolymer and from the antimonic acid is more strongly constrained in the EPDM composites than in the PVDF-based materials, probably as a consequence of de®cient hydration, basically due to the presence of HSb, as the samples with the highest concentrations of this inorganic proton conductor are also the ones that present the lowest conductivity values. Fig. 4. Complex plane diagram for EP-12 at dierent hydration times (min): (a) 0, (b) 60, (c) 120 and (d) 180.
References
Fig. 5. Variation of conductivity versus hydration time for PF12 and EP-12 samples.
[1] Kreuer KD. Chem Mater 1996;8:610. [2] Appleby AJ. In: Doughty D, Vyas B, Takamura T, Hu JR, editors. Materials Research Society Symposium Proceedings, vol. 393. PA, USA: Materials Research Society, 1996. p. 11. [3] Amarilla JM, Rojas RM, Rojo JM, Cubillo MJ, Linares A, Acosta JL. Solid State Ionics 2000;127:133. [4] Wagner CD, Davies LE, Zeller MV, Taylor JA, Raymondond RH, Gale LH. Surf Interf Anal 1981;3:211. [5] Fitzgerald JJ, Weis RA. JMS-Rev Macromol Chem Phys 1988;C28(1):99. [6] Avrami M. J Chem Phys 1939;7:1103. [7] Evans UR. Trans Faraday Soc 1945;41:365. [8] Acosta JL, Ojeda MC, Morales E, Linares A. J Appl Polym Sci 1986;32:4119. [9] Jurado JR, Moure C, Duran P, Rodriguez M, Linares A, Acosta JL. J Mater Sci 1991;26:4022. [10] Linares A, Acosta JL. Polym Bull 1996;36:241.