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More studies on the PVAl+H3PO2+H2O proton conductor gels
Electrochimica Acta 45 (2000) 1399 – 1403 www.elsevier.nl/locate/electacta
More studies on the PVAl+H3PO2 +H2O proton conductor gels M.A. Vargas a, R.A. Vargas b,*, B.-E Mellander c a
Departamento de Fı´sica, Uni6ersidad Popular del Cesar, Apartado Aereo 590, Valledupar, Colombia b Departamento de Fı´sica, Uni6ersidad del Valle, Apartado Aereo 25360, Cali, Colombia c Physics Department, Chalmers Uni6ersity of Technology, 42196, Gothenburg, Sweden Received 31 October 1998; received in revised form 23 March 1999
Abstract Solid polymer proton conductors made of poly(vinyl alcohol) (PVAl), hypophosphorous acid (H3PO2) and water with conductivities as high as 0.1 S cm − 1 at room temperature were studied by means of differential scanning calorimetry (DSC), X-ray diffraction and thermogravimetry (TG). The DSC curves for membranes with the highest acid concentrations show on heating a very low glass transition temperature (revealing a separate acid/water phase), a cold crystallization after this glass transition, a melting of crystallizes or of the freezing water in the polymer network and a recrystallization above ambient temperatures as the membranes lose water. The X-ray spectra for the raw samples at room temperature indicate that the amorphousness of PVAl complexes increases with the concentration of H3PO2; but its degree of crystallinity increases with the annealing time of samples above room temperature. The TG traces confirm that membranes with the highest acid concentrations have the highest water content and that the maximum rate of water removal is at about 50°C for all samples. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polymer proton conductor; DSC; X-ray; TG; Degree of crystallinity
1. Introduction Since the pioneering papers of Wright and co-workers [1,2] a lot of facts have been found out about the ionic conductivity in polymeric complexes. One of them is that the ionic conductivity occurs in the amorphous phase of the polymer [3,4]. The amorphousness percentage of a polymer depends on its average molecular weight (MW), its stereoregularity (tacticity) of its monomeric units chains, its thermal history and temperature [5]. If the polymer is complexed with a salt or an acid for obtaining an ionic conductor, the amor* Corresponding author. Tel.: +57-2-3394610; fax: +57-23393237. E-mail address: [email protected] (R.A. Vargas)
phous phase of the polymer could be changed by the presence of the salt or the acid [3,4]. When we want a solid polymeric ionic conductor at ambient temperature, MW is a critical variable to be considered: when it has certain low value, the polymer is liquid or, if it is solid, its mechanical properties will be not so good. An approach to overcome this problem has been done by mixing polymers [6]. On the other hand, in 1985, a solid protonic conductor based on PVAl and phosphoric acid was obtained [7], broadening the scope of the basic and applied research on polymer ionic conductors. Proton conducting membranes such as Nafion have been very successful for a number of applications where a high proton conductivity is needed. These membranes as well as gel-type proton conductors consist of a polymer matrix swollen with water and/or electrolyte
0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 3 5 0 - 3
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M.A. Vargas et al. / Electrochimica Acta 45 (2000) 1399–1403
solutions and the proton transport takes place primarily in the electrolyte which is entrapped in the polymer matrix [8,9]. The conductivity is thus not directly linked to the polymer dynamics and, as a consequence, high conductivity is possible below the glass transition temperature, Tg, of the polymer. As a comparison, for solid cation conducting polymer electrolytes, e.g. poly(ethylene oxide) complexed with LiCF3SO3, the salt is dissolved in the polymer and the ion transport takes place basically in the amorphous regions of the complex [3,4,8,9]. In this case the cation transport is directly linked to the flexibility of the polymer chains; below Tg of the polymer no conductivity is detected and the temperature dependence of the conductivity, s, can often be described by the VTF equation [10]: s=s0 exp[B/(T−T0)]
(1)
where s0, B and T0 are fitting parameters and s0 depends on T as A/T − 1/2 and T0 :Tg +50 K. Instead of this behavior, for water swollen membranes, the temperature dependence of the conductivity can often be described by the Arrhenius equation [10]: s=s0 exp( −Ea/kT)
(2)
Trying to understand the behavior of this type of protonic conductors or/and to improve its performance (related to its electrical conductivity, its current vs. voltage fuel cell curve, its thermal stability, etc.), a number of earlier studies on distinct combinations of polymers and acids or acid salts have been reported (see, for example Refs. [7–9,11–17] and Refs. therein). In a recent paper, we reported that a commercial glue, consisting primarily of PVAl and PVAc, mixed with H3PO2 and KHSO4, shows an appreciable conductivity at room temperature [18]. We also reported recently that dissolution of H3PO2 in PVAl and water produces proton conductor polymer membranes whose highest room temperature conductivity is of the order of 10 − 1 S cm − 1. The highest fuel cell voltage when one of the samples is used as electrolytic separator is about 440 mV [19]. Furthermore, the prepared membranes preserved the dimensional stability of a solid system even for the highest acid content samples, which reach the highest electrical conductivities and fuel cell voltages. The studies we have done up to now show that our samples are thermally stable, at least in the temperature window of 8–70°C. It is important to stress that although the crystalline fraction of PVAl increases with its hydrolysis content [20], the polymer blends were prepared from high hydrolizad (98–99%) and high molecular weight PVAl (average MW 85 000–146 000), in order to improve water insolubility [20] and bulk modulus [21] of the resulting membranes. However, the water content in the PVAl/H3PO2 (host) blend enhanced both its electrical
conductivity and the fuel cell voltage when the membrane is used as an electrolytic separator [22], while preserving the dimensional stability of a solid. In this paper, we report a study of the phase behavior of the system PVAl/H3PO2/H2O by means of X-ray diffraction, thermogravimetric and differential scanning calorimetry measurements, which are related with our previously reported charge transport measurements [19].
2. Experimental methods
2.1. Sample preparation We have previously reported details about sample preparation [22]. We prepared seven PVAl/H3PO2 concentrations, e.g. OH/P: 0.02, 0.05, 0.075, 0.1, 0.15, 0.25 and 0.5 (in this paper they will be designated as samples 1, 2, …, 7, respectively).
2.2. DSC measurements The thermal phase behavior of the raw samples was characterized using a Mettler DSC 30 calorimeter controlled by a microcomputer; the coolant was liquid nitrogen. All samples weighed around 10 mg before each sweep. The DSC thermograms were taken on increasing temperature with a heating rate of 10 K min − 1 and under a flow of nitrogen gas of 50 ml min − 1. In all cases, the samples were first cooled to −150°C from ambient temperature as fast as possible (in a few minutes).
2.3. X-ray measurements The X-ray study of the samples was performed with a Philips X-ray diffractometer, with Ni filtered Cu – Ka radiation. The membranes were attached to a Pt sample holder. The angle limits were (10°, 53°) with D(2u) = 0.2° and the exposure time on each angle was 4 s. Air was used as purge gas. The spectra were taken at ambient temperature (20°C). Sample 4 was annealed at 70°C during 12 h and the evolution of the X-ray diffraction spectra was recorded each 2 h.
2.4. Thermogra6imetric characterization Thermogravimetric characterization was carried out with a TA Instruments-2050 TGA microbalance controlled by a microcomputer. All sweeps ran from T = 27°C to T= 130°C at a rate of 5 K min − 1; dry nitrogen was used as purge gas. The weight of all samples before each sweep was about 10 mg.
M.A. Vargas et al. / Electrochimica Acta 45 (2000) 1399–1403
3. Results and discussion
3.1. DSC results The DSC traces for all samples which were first heated from −150°C to 100°C at a rate of 10 K min − 1 are shown in Fig. 1. A number of thermal events are recorded related to recrystallization, melting and glass transition. A well-resolved step anomaly which is associated to a glass transition is observed for the highest acid concentration samples (1, 2, 3 and 4) at about −130°C. Tg of pure PVAl is around 80°C [20]; the plastification effect
Fig. 1. DSC curves of the samples studied; Sample 1 (OH/P: 0.02), 2 (0.05), 3 (0.075), 4 (0.1), 5 (0.15), 6 (0.25), 7 (0.5).
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of water in PVAl is well known [21], but our results show that the addition of H3PO2/water makes to drop Tg of PVAl even further. The effect of increasing the H3PO2 concentration on Tg was negligible for samples with the highest acid contents, indicating the presence of a polymeric phase with constant composition (e.g. PVAl+ H3PO2 + H2O) in blends with acid contents greater than that of Sample 4. As a consequence of this separation, a second phase H3PO2/water is also present within these blends. In the others OH/P ratios the glass transition is not clear. Above the glass transition, the polymer chains get enough mobility making possible the formation of crystallizes; as the heating continues, the crystallizes are annealed and the crystallinity increases being possible to observe a cold crystallization [23]. Note that for the highest acid concentration samples (1, 2, 3 and 4), an exothermic peak is observed at about −80°C which is associated with the cold crystallization transition. The endothermic peaks for the samples 1, 2, 3 and 4 may be attributed to the melting of the freezing-bound [23] or intermediate [24] water in the system or to the melting of polymer crystallizes that have been formed before or to both processes. For higher temperatures, the DSC traces show a rapid downward variation indicating a fast endothermic process in the samples that may be associated with the liberation of water by the gels, as we have been seen from the conductivity measurements above 60°C [19].
3.2. X-ray spectra Fig. 2a shows the X-ray diffraction spectra, taken at T =20°C, of some of the raw samples studied and a membrane of PVAl. This membrane was made by dissolving pure PVAl in deionized water and casting the water during several days at atmospheric pressure and room temperature. Fig. 2b shows four X-ray spectra, taken at 70°C and at different annealing times of Sample 4. It is clearly seen from Fig. 2a that the height of the peak at about 2u= 20° decreases as the acid content is increased; on the other hand, Fig. 2b reveals that this peak increases with annealing time. These results can be interpreted by considering the Hodge et al. criterion [21], which establishes a correlation between the height of this peak and the degree of crystallinity of PVAl, e.g. the amorphousness of our PVAl complexes increases with the concentration of H3PO2, or its degree of crystallinity increases as the sample is annealed.
3.3. TG results Fig. 2. (a) X-ray diffraction at 20°C for some of the samples studied and of a membrane of pure PVN. (b) Evolution of the X-ray diffraction spectra of Sample 4 at 70°C as a function of annealing time at this temperature.
TG traces for samples with various acid concentrations confirm what we have conjectured about the effect of water content on our conductivity [19], DSC and
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M.A. Vargas et al. / Electrochimica Acta 45 (2000) 1399–1403
shown by the conductivity versus acid concentration plot reported elsewhere [19]. Thus, these observations explain why the electrical conductivity of the samples at room temperature shows a sensitive variation with the acid concentration, between 10 − 6 and 10 − 1 S cm − 1 as the relative acid content is increased, and support our suggestion that the presence of a separate acid/water phase in the samples is responsible for this anomalous behavior in charge transport.
4. Conclusions
Fig. 3. (a) TG curves of the samples studied showing their mass loss at heating rate of 5 K min − 1. (b). TG derivative curves for the main curves shown in (a) (c) Percentage of total mass loss that reaches each sample at 120°C versus the OH/P ratio.
X-ray results for the same samples. Fig. 3a shows the percentage of actual mass when the samples are first heated at 5 K min − 1 in the 30–120°C temperature range. Fig. 3b shows the TG derivatives of the curves shown in Fig. 3a. These curves clearly reveal some interesting characteristics of the water removal from the samples: the onset of liberation of water is at temperatures slightly above room temperature (provided that the atmosphere is dry) and that the maximum rate of this removal is at around 50°C. Therefore, the drop of conductivity on the linear Arrhenius behavior (log s vs. T − 1) observed above 50°C for all samples [19] is due to water removal. Fig. 3c shows the percentage of the total mass loss that reaches each sample at 120°C, indicating that the water removal decreases rapidly with decreasing acid concentration. Note the relative minimum for mass loss at OH/P=0.075 (Sample 3) and its slow variation for the samples of low acid concentration (Samples 5, 6, 7). The profile shown by the total mass loss versus acid concentration plot (Fig. 3c) resembles what it was
In summary, we have found by DSC, X-ray and TG measurements that the water content of the membranes prepared by mixing PVAl, H3PO2 and H2O is an important parameter for its phase behavior and the anomalous high conductivity observed in some acid doping concentrations. For the highest acid concentration samples the glass transition temperature does not vary substantially, which was observed at about −130°C. This indicates the presence of a H3PO2/water phase with the observed Tg besides to the polymeric phase. In the DSC traces for the samples with high acid content were also detected cold crystallization peaks at about − 80°C and an unexpected endothermic peak at about −40°C which remains to be explained. Both DSC and TG measurements in dry atmosphere show that the maximum rate of water removal is at around 50°C (by conductivity measurements the water content seems to remain constant in the bulk of the sample below approximately 60°C [19]). The X-ray diffraction spectra confirm that the amorphousness of membranes increases as the H3PO2 concentration increases, and its crystallinity increases when they are annealed above room temperature.
Acknowledgements The authors are grateful for the financial support the International Science Program, of Uppsala University, Sweden, and the Colombian Research Council (Colciencias) has granted to this work.
References [1] D.E. Fenton, J.M. Parker, P.V. Wright, Polymer 14 (1973) 589. [2] P.V. Wright, Br. Polymer J. 7 (1975) 319. [3] C. Berthier, W. Gorecki, M. Minier, M.B. Armand, J.M. Chabagno, P. Rigaud, Solid State Ion. 11 (1983) 91. [4] M. Armand, Solid State Ion. 9/10 (1983) 745.
M.A. Vargas et al. / Electrochimica Acta 45 (2000) 1399–1403 [5] F. Billmeyer Jr., Texthook of Polymer Science, Wiley, Singapore, 1984. [6] F. Gray, Solid Polymer Electrolytes, Fundamentals and Technological Applications, VCH, New York, 1991. [7] K.-C. Gong, H. Shou-Cai, Mater. Res. Soc. Symp. Proc. 135 (1989) 377. [8] S. Petty-Weeks, A.J. Polak, Sens. Actuators 11 (1987) 377. [9] A.J. Polak, S. Petty-Weeks, A.J. Beuhler, Sens. Actuators 9 (1986) 1. [10] D. Fauteux, M.D. Lupien, C.D. Robitaille, J. Electrochem. Soc. 134 (1987) 2761. [11] S. Petty-Weeks, J.J. Zupancic, J.R. Swedo, Solid State Ion. 31 (1988) 117. [12] D. Schoolmann, O. Trinquet, J.C. Lasse`gues, Electrochim. Acta 37 (1992) 1619. [13] J. Grondin, D. Rodriguez, J.C. Lasse`gues, Solid State Ion. 77 (1995) 70. [14] R. Tanaka, H. Yamamoto, S. Kawamura, T. Iwase, Electrochim. Acta 40 (1995) 2421. [15] S. Miachon, P. Aldebert, J. Power Sour. 56 (1995) 31.
.
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[16] J.R. Stevens, W. Wieczorek, D. Raducha, K.R. Jeffrey, Solid State Ion. 97 (1997) 347. [17] G. Casalbore-Miceli, N. Camaioni, M.J. Yang, M. Zhen, X.W. Zhan, A. D’Aprano, Solid State Ion. 100 (1997) 217. [18] R.A. Vargas, M.A. Vargas y, M. Garcı´a, Electrochim. Acta 43 (1998) 1271. [19] M.A. Vargas, R.A. Vargas, B-E. Mellander, Fourth International Symposium on Electrochemical Impedance Procedures, Rio do Janeiro, Brazil, 1998, pp. 121. [20] F.L. Marten, in: H.F. Mark, N.M. Bikales, C.G. Overberger, G. Menges, J.Y. Kroschwitz (Eds.), Encyclopedia of Polymer Science and Technology, vol. 17, Wiley, New York, 1989, p. 167. [21] R.M. Hodge, G.H. Edward, G.P. Simon, Polymer 37 (1996) 1371. [22] M.A. Vargas, R.A. Vargas, B.-E. Mellander, Electrochim. Acta (accepted). [23] T. Hatakeyama, F.X. Quinn, Thermal Analysis, Fundamentals and Applications to Polymer Science, Wiley, New York, 1995. [24] W.-I. Cha, S.-H. Hyon, Y. Ikada, Makromol. Chem. 194 (1993) 2433.
More studies on the PVAl+H3PO2 +H2O proton conductor gels M.A. Vargas a, R.A. Vargas b,*, B.-E Mellander c a
Departamento de Fı´sica, Uni6ersidad Popular del Cesar, Apartado Aereo 590, Valledupar, Colombia b Departamento de Fı´sica, Uni6ersidad del Valle, Apartado Aereo 25360, Cali, Colombia c Physics Department, Chalmers Uni6ersity of Technology, 42196, Gothenburg, Sweden Received 31 October 1998; received in revised form 23 March 1999
Abstract Solid polymer proton conductors made of poly(vinyl alcohol) (PVAl), hypophosphorous acid (H3PO2) and water with conductivities as high as 0.1 S cm − 1 at room temperature were studied by means of differential scanning calorimetry (DSC), X-ray diffraction and thermogravimetry (TG). The DSC curves for membranes with the highest acid concentrations show on heating a very low glass transition temperature (revealing a separate acid/water phase), a cold crystallization after this glass transition, a melting of crystallizes or of the freezing water in the polymer network and a recrystallization above ambient temperatures as the membranes lose water. The X-ray spectra for the raw samples at room temperature indicate that the amorphousness of PVAl complexes increases with the concentration of H3PO2; but its degree of crystallinity increases with the annealing time of samples above room temperature. The TG traces confirm that membranes with the highest acid concentrations have the highest water content and that the maximum rate of water removal is at about 50°C for all samples. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Polymer proton conductor; DSC; X-ray; TG; Degree of crystallinity
1. Introduction Since the pioneering papers of Wright and co-workers [1,2] a lot of facts have been found out about the ionic conductivity in polymeric complexes. One of them is that the ionic conductivity occurs in the amorphous phase of the polymer [3,4]. The amorphousness percentage of a polymer depends on its average molecular weight (MW), its stereoregularity (tacticity) of its monomeric units chains, its thermal history and temperature [5]. If the polymer is complexed with a salt or an acid for obtaining an ionic conductor, the amor* Corresponding author. Tel.: +57-2-3394610; fax: +57-23393237. E-mail address: [email protected] (R.A. Vargas)
phous phase of the polymer could be changed by the presence of the salt or the acid [3,4]. When we want a solid polymeric ionic conductor at ambient temperature, MW is a critical variable to be considered: when it has certain low value, the polymer is liquid or, if it is solid, its mechanical properties will be not so good. An approach to overcome this problem has been done by mixing polymers [6]. On the other hand, in 1985, a solid protonic conductor based on PVAl and phosphoric acid was obtained [7], broadening the scope of the basic and applied research on polymer ionic conductors. Proton conducting membranes such as Nafion have been very successful for a number of applications where a high proton conductivity is needed. These membranes as well as gel-type proton conductors consist of a polymer matrix swollen with water and/or electrolyte
0013-4686/00/$ - see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 1 3 - 4 6 8 6 ( 9 9 ) 0 0 3 5 0 - 3
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M.A. Vargas et al. / Electrochimica Acta 45 (2000) 1399–1403
solutions and the proton transport takes place primarily in the electrolyte which is entrapped in the polymer matrix [8,9]. The conductivity is thus not directly linked to the polymer dynamics and, as a consequence, high conductivity is possible below the glass transition temperature, Tg, of the polymer. As a comparison, for solid cation conducting polymer electrolytes, e.g. poly(ethylene oxide) complexed with LiCF3SO3, the salt is dissolved in the polymer and the ion transport takes place basically in the amorphous regions of the complex [3,4,8,9]. In this case the cation transport is directly linked to the flexibility of the polymer chains; below Tg of the polymer no conductivity is detected and the temperature dependence of the conductivity, s, can often be described by the VTF equation [10]: s=s0 exp[B/(T−T0)]
(1)
where s0, B and T0 are fitting parameters and s0 depends on T as A/T − 1/2 and T0 :Tg +50 K. Instead of this behavior, for water swollen membranes, the temperature dependence of the conductivity can often be described by the Arrhenius equation [10]: s=s0 exp( −Ea/kT)
(2)
Trying to understand the behavior of this type of protonic conductors or/and to improve its performance (related to its electrical conductivity, its current vs. voltage fuel cell curve, its thermal stability, etc.), a number of earlier studies on distinct combinations of polymers and acids or acid salts have been reported (see, for example Refs. [7–9,11–17] and Refs. therein). In a recent paper, we reported that a commercial glue, consisting primarily of PVAl and PVAc, mixed with H3PO2 and KHSO4, shows an appreciable conductivity at room temperature [18]. We also reported recently that dissolution of H3PO2 in PVAl and water produces proton conductor polymer membranes whose highest room temperature conductivity is of the order of 10 − 1 S cm − 1. The highest fuel cell voltage when one of the samples is used as electrolytic separator is about 440 mV [19]. Furthermore, the prepared membranes preserved the dimensional stability of a solid system even for the highest acid content samples, which reach the highest electrical conductivities and fuel cell voltages. The studies we have done up to now show that our samples are thermally stable, at least in the temperature window of 8–70°C. It is important to stress that although the crystalline fraction of PVAl increases with its hydrolysis content [20], the polymer blends were prepared from high hydrolizad (98–99%) and high molecular weight PVAl (average MW 85 000–146 000), in order to improve water insolubility [20] and bulk modulus [21] of the resulting membranes. However, the water content in the PVAl/H3PO2 (host) blend enhanced both its electrical
conductivity and the fuel cell voltage when the membrane is used as an electrolytic separator [22], while preserving the dimensional stability of a solid. In this paper, we report a study of the phase behavior of the system PVAl/H3PO2/H2O by means of X-ray diffraction, thermogravimetric and differential scanning calorimetry measurements, which are related with our previously reported charge transport measurements [19].
2. Experimental methods
2.1. Sample preparation We have previously reported details about sample preparation [22]. We prepared seven PVAl/H3PO2 concentrations, e.g. OH/P: 0.02, 0.05, 0.075, 0.1, 0.15, 0.25 and 0.5 (in this paper they will be designated as samples 1, 2, …, 7, respectively).
2.2. DSC measurements The thermal phase behavior of the raw samples was characterized using a Mettler DSC 30 calorimeter controlled by a microcomputer; the coolant was liquid nitrogen. All samples weighed around 10 mg before each sweep. The DSC thermograms were taken on increasing temperature with a heating rate of 10 K min − 1 and under a flow of nitrogen gas of 50 ml min − 1. In all cases, the samples were first cooled to −150°C from ambient temperature as fast as possible (in a few minutes).
2.3. X-ray measurements The X-ray study of the samples was performed with a Philips X-ray diffractometer, with Ni filtered Cu – Ka radiation. The membranes were attached to a Pt sample holder. The angle limits were (10°, 53°) with D(2u) = 0.2° and the exposure time on each angle was 4 s. Air was used as purge gas. The spectra were taken at ambient temperature (20°C). Sample 4 was annealed at 70°C during 12 h and the evolution of the X-ray diffraction spectra was recorded each 2 h.
2.4. Thermogra6imetric characterization Thermogravimetric characterization was carried out with a TA Instruments-2050 TGA microbalance controlled by a microcomputer. All sweeps ran from T = 27°C to T= 130°C at a rate of 5 K min − 1; dry nitrogen was used as purge gas. The weight of all samples before each sweep was about 10 mg.
M.A. Vargas et al. / Electrochimica Acta 45 (2000) 1399–1403
3. Results and discussion
3.1. DSC results The DSC traces for all samples which were first heated from −150°C to 100°C at a rate of 10 K min − 1 are shown in Fig. 1. A number of thermal events are recorded related to recrystallization, melting and glass transition. A well-resolved step anomaly which is associated to a glass transition is observed for the highest acid concentration samples (1, 2, 3 and 4) at about −130°C. Tg of pure PVAl is around 80°C [20]; the plastification effect
Fig. 1. DSC curves of the samples studied; Sample 1 (OH/P: 0.02), 2 (0.05), 3 (0.075), 4 (0.1), 5 (0.15), 6 (0.25), 7 (0.5).
1401
of water in PVAl is well known [21], but our results show that the addition of H3PO2/water makes to drop Tg of PVAl even further. The effect of increasing the H3PO2 concentration on Tg was negligible for samples with the highest acid contents, indicating the presence of a polymeric phase with constant composition (e.g. PVAl+ H3PO2 + H2O) in blends with acid contents greater than that of Sample 4. As a consequence of this separation, a second phase H3PO2/water is also present within these blends. In the others OH/P ratios the glass transition is not clear. Above the glass transition, the polymer chains get enough mobility making possible the formation of crystallizes; as the heating continues, the crystallizes are annealed and the crystallinity increases being possible to observe a cold crystallization [23]. Note that for the highest acid concentration samples (1, 2, 3 and 4), an exothermic peak is observed at about −80°C which is associated with the cold crystallization transition. The endothermic peaks for the samples 1, 2, 3 and 4 may be attributed to the melting of the freezing-bound [23] or intermediate [24] water in the system or to the melting of polymer crystallizes that have been formed before or to both processes. For higher temperatures, the DSC traces show a rapid downward variation indicating a fast endothermic process in the samples that may be associated with the liberation of water by the gels, as we have been seen from the conductivity measurements above 60°C [19].
3.2. X-ray spectra Fig. 2a shows the X-ray diffraction spectra, taken at T =20°C, of some of the raw samples studied and a membrane of PVAl. This membrane was made by dissolving pure PVAl in deionized water and casting the water during several days at atmospheric pressure and room temperature. Fig. 2b shows four X-ray spectra, taken at 70°C and at different annealing times of Sample 4. It is clearly seen from Fig. 2a that the height of the peak at about 2u= 20° decreases as the acid content is increased; on the other hand, Fig. 2b reveals that this peak increases with annealing time. These results can be interpreted by considering the Hodge et al. criterion [21], which establishes a correlation between the height of this peak and the degree of crystallinity of PVAl, e.g. the amorphousness of our PVAl complexes increases with the concentration of H3PO2, or its degree of crystallinity increases as the sample is annealed.
3.3. TG results Fig. 2. (a) X-ray diffraction at 20°C for some of the samples studied and of a membrane of pure PVN. (b) Evolution of the X-ray diffraction spectra of Sample 4 at 70°C as a function of annealing time at this temperature.
TG traces for samples with various acid concentrations confirm what we have conjectured about the effect of water content on our conductivity [19], DSC and
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shown by the conductivity versus acid concentration plot reported elsewhere [19]. Thus, these observations explain why the electrical conductivity of the samples at room temperature shows a sensitive variation with the acid concentration, between 10 − 6 and 10 − 1 S cm − 1 as the relative acid content is increased, and support our suggestion that the presence of a separate acid/water phase in the samples is responsible for this anomalous behavior in charge transport.
4. Conclusions
Fig. 3. (a) TG curves of the samples studied showing their mass loss at heating rate of 5 K min − 1. (b). TG derivative curves for the main curves shown in (a) (c) Percentage of total mass loss that reaches each sample at 120°C versus the OH/P ratio.
X-ray results for the same samples. Fig. 3a shows the percentage of actual mass when the samples are first heated at 5 K min − 1 in the 30–120°C temperature range. Fig. 3b shows the TG derivatives of the curves shown in Fig. 3a. These curves clearly reveal some interesting characteristics of the water removal from the samples: the onset of liberation of water is at temperatures slightly above room temperature (provided that the atmosphere is dry) and that the maximum rate of this removal is at around 50°C. Therefore, the drop of conductivity on the linear Arrhenius behavior (log s vs. T − 1) observed above 50°C for all samples [19] is due to water removal. Fig. 3c shows the percentage of the total mass loss that reaches each sample at 120°C, indicating that the water removal decreases rapidly with decreasing acid concentration. Note the relative minimum for mass loss at OH/P=0.075 (Sample 3) and its slow variation for the samples of low acid concentration (Samples 5, 6, 7). The profile shown by the total mass loss versus acid concentration plot (Fig. 3c) resembles what it was
In summary, we have found by DSC, X-ray and TG measurements that the water content of the membranes prepared by mixing PVAl, H3PO2 and H2O is an important parameter for its phase behavior and the anomalous high conductivity observed in some acid doping concentrations. For the highest acid concentration samples the glass transition temperature does not vary substantially, which was observed at about −130°C. This indicates the presence of a H3PO2/water phase with the observed Tg besides to the polymeric phase. In the DSC traces for the samples with high acid content were also detected cold crystallization peaks at about − 80°C and an unexpected endothermic peak at about −40°C which remains to be explained. Both DSC and TG measurements in dry atmosphere show that the maximum rate of water removal is at around 50°C (by conductivity measurements the water content seems to remain constant in the bulk of the sample below approximately 60°C [19]). The X-ray diffraction spectra confirm that the amorphousness of membranes increases as the H3PO2 concentration increases, and its crystallinity increases when they are annealed above room temperature.
Acknowledgements The authors are grateful for the financial support the International Science Program, of Uppsala University, Sweden, and the Colombian Research Council (Colciencias) has granted to this work.
References [1] D.E. Fenton, J.M. Parker, P.V. Wright, Polymer 14 (1973) 589. [2] P.V. Wright, Br. Polymer J. 7 (1975) 319. [3] C. Berthier, W. Gorecki, M. Minier, M.B. Armand, J.M. Chabagno, P. Rigaud, Solid State Ion. 11 (1983) 91. [4] M. Armand, Solid State Ion. 9/10 (1983) 745.
M.A. Vargas et al. / Electrochimica Acta 45 (2000) 1399–1403 [5] F. Billmeyer Jr., Texthook of Polymer Science, Wiley, Singapore, 1984. [6] F. Gray, Solid Polymer Electrolytes, Fundamentals and Technological Applications, VCH, New York, 1991. [7] K.-C. Gong, H. Shou-Cai, Mater. Res. Soc. Symp. Proc. 135 (1989) 377. [8] S. Petty-Weeks, A.J. Polak, Sens. Actuators 11 (1987) 377. [9] A.J. Polak, S. Petty-Weeks, A.J. Beuhler, Sens. Actuators 9 (1986) 1. [10] D. Fauteux, M.D. Lupien, C.D. Robitaille, J. Electrochem. Soc. 134 (1987) 2761. [11] S. Petty-Weeks, J.J. Zupancic, J.R. Swedo, Solid State Ion. 31 (1988) 117. [12] D. Schoolmann, O. Trinquet, J.C. Lasse`gues, Electrochim. Acta 37 (1992) 1619. [13] J. Grondin, D. Rodriguez, J.C. Lasse`gues, Solid State Ion. 77 (1995) 70. [14] R. Tanaka, H. Yamamoto, S. Kawamura, T. Iwase, Electrochim. Acta 40 (1995) 2421. [15] S. Miachon, P. Aldebert, J. Power Sour. 56 (1995) 31.
.
1403
[16] J.R. Stevens, W. Wieczorek, D. Raducha, K.R. Jeffrey, Solid State Ion. 97 (1997) 347. [17] G. Casalbore-Miceli, N. Camaioni, M.J. Yang, M. Zhen, X.W. Zhan, A. D’Aprano, Solid State Ion. 100 (1997) 217. [18] R.A. Vargas, M.A. Vargas y, M. Garcı´a, Electrochim. Acta 43 (1998) 1271. [19] M.A. Vargas, R.A. Vargas, B-E. Mellander, Fourth International Symposium on Electrochemical Impedance Procedures, Rio do Janeiro, Brazil, 1998, pp. 121. [20] F.L. Marten, in: H.F. Mark, N.M. Bikales, C.G. Overberger, G. Menges, J.Y. Kroschwitz (Eds.), Encyclopedia of Polymer Science and Technology, vol. 17, Wiley, New York, 1989, p. 167. [21] R.M. Hodge, G.H. Edward, G.P. Simon, Polymer 37 (1996) 1371. [22] M.A. Vargas, R.A. Vargas, B.-E. Mellander, Electrochim. Acta (accepted). [23] T. Hatakeyama, F.X. Quinn, Thermal Analysis, Fundamentals and Applications to Polymer Science, Wiley, New York, 1995. [24] W.-I. Cha, S.-H. Hyon, Y. Ikada, Makromol. Chem. 194 (1993) 2433.