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Engineering polymer electrolytes with enhanced ionic conduction
Solid State Ionics 136–137 (2000) 549–558 www.elsevier.com / locate / ssi
Engineering polymer electrolytes with enhanced ionic conduction A.R. Kulkarni* Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology ( Bombay), Powai, Mumbai 400076, India
Abstract This paper describes our strategies to engineer amorphous polymer–salt complexes and investigate their electrical behavior. In the first approach, semicrystalline polymer polyoxyethylene (POE) was plasticised using amorphous polyoxypropyleneglycol (POPG) in the presence of sodium salts. In the second approach, amorphous polymers, poly(bis(methoxyethoxyethoxy phosphazene)) (MEEP) and polysiloxane, were used to synthesise electrolytes. These were complexed with sodium salts and a small amount of polyoxyethylene. The nature of each complex was ascertained from optical microscopy, X-ray diffraction (XRD) and Differential Scanning Calorimetry (DSC) in these systems. The polymer–salt complexes, in the first approach, revealed a well-defined glass formation region in which all the compositions were amorphous and showed unusually high ionic conductivity | 10 24 (V cm)21 at ambient temperature. The electrolytes in the second approach showed a maximum conductivity of | 10 25 (V cm)21 . However, truly amorphous compositions could not be obtained. The first approach of engineering reduction in crystalline phases appeared more effective in enhancing ionic conductivity. 2000 Elsevier Science B.V. All rights reserved. Keywords: Polymer electrolytes; Ionic conductivity; Amorphous polymers; Plasticisers
1. Introduction Poly(oxyethylene)-based salt complexes are one of the widely studied polymers in the field of electrolytes [1], due to their wide ranging applications in batteries [2], sensors [3] and display devices [4]. These electrolytes, however, suffer due to low conductivity at room temperature that arises mainly due to the semicrystalline nature with ease of ion transport only in the amorphous phase (liquid-like) generated by the melting of the spherulites. There are two possibilities for the design a polymer electrolyte with high ionic conductivity: (a) to increase amor*Fax: 191-22-578-3480. E-mail address: [email protected] (A.R. Kulkarni).
phous domains in POE; or (b) make a completely amorphous material. Both of these approaches have been widely investigated and among the successful ones are inorganic polymer-based binary systems [5] and plasticiser-based ternary systems [6] which show promise with reduced crystallinity and high roomtemperature conductivity. The latter class of materials also includes blends and polymer mixtures exhibiting enhanced ionic conduction in systems like PEO–polymethylmethacrylate (PMMA) [7], POE– polystyrene [8], POE–siloxanes-g [9], POE–poly(2vinylpyridine) [10] and most recently POE–poly(vinylpyridine) [11]. The blends have been found to be elastomeric in certain composition ranges and showed conductivity enhancements of approximately two orders when compared with polymer salt com-
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plexes of individual polymers. Various techniques such as reduction in crystallinity, reduction in glass transition temperature, using unusual salts and varying the salt concentration have been proposed in order to improve conductivity [12]. The objective, while attempting the modifications, is not to compromise the solvating and mechanical properties offered by the oxyethylene (OE) group. Another solution is to synthesise a comb-like polymer with an amorphous backbone and POE side chains of optimum length to improve mechanical strength of the complex. Such comb-like structures have been reported for both poly(phosphazenes) and polysiloxanes and are found to give room-temperature conductivity of the order of 10 25 (V cm)21 , which is two orders of magnitude higher than those for POEbased electrolytes. Our research group has been involved in developing amorphous polymer salt complexes. In studies reported here, two strategies to design an amorphous polymer or to maximise the content of amorphous phase are considered. The first approach is to investigate phase and conductivity behaviour of a ternary electrolyte system consisting of an amorphous polymer (polyoxypropylene glycol), (POPG), a semicrystalline polymer, POE and an alkali salt (NaI). In the second approach, amorphous polymers were used to synthesise electrolytes. The polymers chosen were MEEP and polysiloxane. Both polymers being glutinous, a minimum quantity of high molecular weight POE was added to achieve dimensional stability and free standing films. These complexes are thermally stable, mechanically strong and completely amorphous in a certain composition range and show higher ionic conductivity than most twocomponent systems hitherto reported. Another interesting observation is that the addition of salt provides an easy method for enhancing the compatibility range of two or more polymers. Our study focuses on the engineering and characterisation of these ternary polymer electrolytes.
2. Experimental Synthesis of polymer electrolyte POE (MW 300 000, Aldrich), POPG (MW 4000, Aldrich) NaSCN and NaI (BDH, UK) were used as received.
MEEP was synthesised in our laboratory [13]. Mixed electrolytes were prepared by dissolving MEEP and PEO (Aldrich, MW 5 4 3 10 6 ) in the weight ratio 55:45 in acetonitrile. The polymer studied in the siloxanes system contained polysiloxanes backbones with randomly substituted side chains composed of three EO monomeric units. Appropriate quantities of raw materials were dried under vacuum and were separately dissolved in minimum quantity of acetonitrile. The solutions were mixed with constant stirring. The solvent was allowed to evaporate at room temperature in a desiccator. The transparent, bubble free, nearly 100-mm thick films were stored in a desiccator. For DSC measurements 10 mg sample was used. The thermograms were recorded from 2 100 to 1 1008C at a heating rate of 108C min 21 using a Du Pont Thermal Analysis System. The calorimeter was flushed with dry nitrogen. The T g values were read from intersection of the tangent drawn to transition curve and base line. AC electrical conductivity of the polymer films was measured in the frequency range from 10 Hz to 5 MHz and the temperature range 20–1008C using a Solartron SI1260 Impedance / Gain Phase Analyzer. Data at various temperatures and frequencies were recorded using a data acquisition facility developed in our laboratory. The films were coated with roomtemperature-drying silver paste and the sample was sandwiched between two spring-loaded SS plates. X-ray diffractograms for the films were recorded using a Philips 1820 X-ray diffractometer in the 2u range 20–408 and matched with diffraction patterns reported in the literature.
3. Results and discussion
3.1. Differential scanning calorimetric studies Fig. 1 depicts DSC thermograms obtained for pure POE, POPG and blends of POE–POPG and POE– POPG–NaI electrolyte samples. Two prominent features to be noted in all the samples are: (i) a glass transition temperature, T g , at which the glassy phase becomes rubbery and amorphous; and (ii) a relatively sharp endotherm for some of the samples. The endotherm is generally attributed to the melting of
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Fig. 1. DSC curves for polymer electrolyte samples. (a) POE; (b) POPG; (c) POE–5% POPG; (d) POE–6% POPG–NaI; (e) POE– 9% POPG–NaI; (f) POE–10% POPG–NaI; (g) POE–12% POPG–NaI; (h) POE–12% POPG–NaI, O / Na510; and (i) POE– 12% POPG–NaI, O / Na512. The O / Na ratio in samples (d)–(g) is 4.
the pure crystalline POE and is sometimes due to a eutectic phase of crystalline POE and a POE–salt complex [14–16]. Reported values of glass transition temperatures of POE and POPG are around 2 608C and 2 758C; these are very close and cannot be used as an indicator to find the extent of the blend formation in sample (c) where T g lies in between these temperatures. However, the absence of two separate glass transition temperatures in the complexes (d)–(g) indicates the coordination of sodium ion with both polymers. At this point it is important to mention that for samples (d)–(g) (Fig. 1) the O / Na ratio was maintained at 4 even though the POPG content was changed from 0 to 15%. On the other hand, for samples (h) and (i) in the same figure
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the POPG content was held constant at 12% and the O / Na ratio was varied from 4 to 12. The complexes show higher T g values depending on the amount of POPG in the system. The increase in T g for the polymers has been attributed to the reduced flexibility of the polymer chains on complexing with the salts. Further addition of POPG increases the flexibility of the chains and hence shifts in T g to lower temperature [17]. The endothermic peak around 608C in Fig. 1 corresponding to melting of crystalline POE shifts from 608C to 558C with the addition of POPG. It may be noted that curves (d)–(f) show a third relatively small endotherm around 2008C. This hightemperature endotherm in our studies is due to the melting / dissolution process of a salt-rich POE–NaI phase. This is consistent with the observations of Berthier et al. [18]. Another prominent feature is the decrease in the area under the endotherm that also serves as a measure of the crystallinity in the system. The percentage crystallinity for each composition was calculated according to the procedure described by Richardson [19]. The most interesting feature in this figure is the absence of the melting endotherm for curves (g) and (h). The absence of the melting endotherm and the presence of a glass transition with no other transition indicates that these materials do not contain crystalline components. The salt and the POPG content together control the overall amorphous phase content of the polymer and an optimum mix leads to completely amorphous polymers. X-ray diffraction studies and conductivity measurements provide additional evidence that the salt plays a major role in the phase relationships.
3.2. X-ray diffraction studies Two types of crystalline structures are noted in the present studies. X-ray diffractograms of a few samples are compared in Fig. 2. In sample (a) POE is semicrystalline in nature and the Bragg reflection lines are in agreement with those reported by Yao [20]. The most intense peak was observed at 2u 5 23.38. The addition of 5% POPG did not change the position of these peaks. However, for the threecomponent samples [curve (c)] the peak has shifted to 228 indicating the formation of a crystalline complex. It was also observed that with the progres-
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A.R. Kulkarni / Solid State Ionics 136 – 137 (2000) 549 – 558 Table 1 Physical constants for POE, POPG and NaI
Fig. 2. X-ray diffraction patterns for polymer electrolyte samples. (a) POE; (b) POE–2% POPG; (c) POE–5% POPG–NaI; (d) POE–13% POPG–NaI. The O / Na ratio in samples (c) and (d) is 4.
sive addition of POPG the intensity and the area under the curve for the most prominent peak decreased in magnitude, and an amorphous profile emerged as a halo. The halo seen in curve (d) (Fig. 2) is typical of amorphous polymers indicating reduced long-range crystalline order in the system. The extent of crystallinity in the samples was calculated using procedure reported by Yao [20] and is tabulated in Table 1. The X-ray diffraction thus substantiates the role of NaI as a crucial component for formation of a complex in ternary compositions.
3.3. Ternary pseudo-phase diagram of the POE– POPG–NaI system To understand the nature of the phases in the ternary system it is represented as a ternary pseudophase diagram as usually used to determine amor-
Property
POE
POPG
NaI
Ref.
T g (K) T m (K) DHf (kJ mol 21 ) CED (kJ mol 21 ) Lattice energy (kJ) DWf with NaI (kJ mol 21 )
206 342 8.67 16.885 – 375
– 198 8.40 17.33 – 350
– 600
[28] [19] [29] [30] [31] [32]
– 680 –
phous formation regions in inorganic ternary systems [21]. Fig. 3 shows a ternary pseudophase diagram, which includes the nature of the various phases formed in the individual binary systems, POE–NaI, POPG–NaI and POE–POPG, and also the ternary system under investigation. Different phases have been indicated by different symbols. Open circles indicate truly amorphous, filled circles show partially crystalline and the open squares show semi-solid materials. Before representing a composition in the diagram using the symbols mentioned above, the nature of its phase was confirmed from DSC and X-ray diffraction, as shown for some of the samples in Figs. 1 and 2. In the binary POE–POPG system the two polymers are compatible only up to 5 mole%. This agrees well with the reported data of Booth and Pickles [22]. However, even in this miscible binary region the compositions are semicrystalline. However, when a small amount of salt is added the compatibility range of the two polymers increases from 5 to 15 mole%. In the present case not only is the compatibility of the polymers increased, but also the crystallinity observed in the binary system is destroyed. The quantities estimated using DSC and X-ray data show that a 60% semicrystalline binary of POE–NaI becomes completely amorphous in the ternary POE–POPG– NaI. This has been indicated in the ternary diagram by a region (circle with dotted lines) in which all the compositions are amorphous. Outside this region the compositions are either partially crystalline or sticky (semi-solid). When POPG exceeds 15 mole% the ternary films show solid–liquid phase separation. At higher salt concentration the films are brittle, resembling polymer in salt systems. Observation of miscibility enhancements, due to the addition of a salt mixture of polymers, has been reported for ion conducting polymers. Hara and Eisenberg [23] re-
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Fig. 3. Ternary phase diagram in the POE–POPG–NaI system. Different symbols show the phases present: s, amorphous; d, partially crystalline; and h, semi-solid.
ported miscibility enhancements in polystyrene ionomer / poly(alkylene oxide) systems from DSC studies and attributed the enhancement to ion–dipole interactions. Li et al. [10] observed POE and poly(2vinylpyridine) to be miscible in a 3:1 ratio in the presence of LiClO 4 . The reason for the miscibility was not understood. Recently, Li and Khan [11] have carried out a systematic study on the POE–poly(4vinylpyridine)–LiClO 4 system where the miscibility enhancement was observed, and this was explained on the basis of the simultaneous interaction of lithium ions with oxygens of POE and nitrogens from the pyridine unit. Our observations are consistent with these observations and reinforce earlier reports confirming that the salt plays a major role in enhancing the miscibility limits of the polymers. Although a considerable number of miscible binary blends have been studied in detail only a few studies on ternary polymer blends have been reported in the literature [24]. These are classified as: Solvent 1–Solvent 2–Polymer, Solvent–Polymer 1–Polymer 2 and Polymer 1–Polymer 2–Polymer 3. To the best of our knowledge miscibility studies on Polymer 1–Polymer 2–salt have not been reported, or in such studies the systems have not been considered as
ternary. The concept that the third component in the ternary system, an inorganic salt, may act as a compatibiliser for two immiscible polymers is important. Although the concept has merit the thermodynamic considerations are not straightforward. Flory theory [25] is widely used to predict and explain miscibility in binary and ternary polymeric blends. In the present work we have extended this theory to polymer electrolytes. The detailed thermodynamic arguments and calculations are worked out elsewhere [26]. The following assumptions are made in addition to the existing ones in the Flory–Huggins theory: 1. The ionic interaction between the elements in the salt is considered to be similar to that between the solvent molecules. 2. The lattice energy is considered to be the energy required to separate the salt pairs. 3. The entropic contribution is not taken into account. The primary step in using the theory involves calculating the interaction parameter for the polymer–salt interaction. Based on the above assump-
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tions and using various experimental values listed in Table 1, the interaction parameter for POE–NaI interaction is found to be 0.077, and that for POPG– NaI is found to be 2 0.063. Free energy contours were obtained for polymer electrolytes by using these values in Eq. (1):
F
S
D
1 2 DGm 5 RT ln(1 2 wn ) 1 1 2 ] wn 1 xw n x
G
(1)
Here the dimensionless parameter x 5 zDe /RT is known as the Flory–Huggins interaction parameter, n is the amount of solvent, and x 5 (V 2 /V 1) is the number average for the degree of polymerisation of the polymer. Fig. 4 shows the variation of DG for POE–NaI and POPG–NaI. A strong interaction between the polymer and the salt is evident in both cases. Experimental results also agree with this, as the complex between the polymer and the salt is formed in the entire range of composition. However, these diagrams do not throw any light on the different phases present in the electrolyte. Thus, the term miscibility in polymer electrolytes will mean strong interaction between the polymer and salt. Based on the Flory–Huggins theory for the free
energy of mixing, the phase behaviour of ternary system can be calculated. The free energy of mixing of a three-component system can be expressed as:
w1 w2 w3 DGm 5 ] ln w1 1 ] ln w2 1 ] ln w3 1 x12 w1 w2 r1 r2 r3 1 x23 w2 w3 1 x31 w3 w1
(2)
For a mixture to be single phase, the requirement that DGm ,0 must be fulfilled. Additionally, the second derivative of DGm with respect to wi must be zero or positive. Thus, the spinodal for a ternary blend must be of the form: ≠ 2 DGm ]] 5 r 1 w1 1 r 2 w2 1 r 3 w3 ≠w 2 2 2[r 1 r 2 ( x1 1 x2 )w1 w2 1 r 2 r 3 ( x2 1 x3 )w2 w3 r 3 r 1 ( x3 1 x1 )w3 w1 ] 14r 1 r 2 r 3 ( x1 x2 1x2 x3 1x3 x1 )w1 w2 w3 50 (3) The critical point is given by:
Fig. 4. Free energy diagram for the POE–NaI and POPG–NaI binary complexes. Note complete miscibility of the polymer and the salt.
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555
Fig. 5. Theoretical calculation of the miscibility limits in the ternary POE–POPG–NaI blends. The solid line calculated from theory demarcates the miscible and the immiscible regions.
≠DGm ≠ 2 DGm ≠ 3 DGm ]] 5 ]] 5 ]] 50 ≠w ≠w 2 ≠w 3
(4)
Fig. 5 shows the free energy contour for the POE–POPG2000–NaI ternary electrolyte system obtained using x POE–POPG50.067, x POE–NaI5 20.077 and x POPG–NaI5 20.063. The curve obtained shows area of miscibility. Experimental observation from DSC and optical microscopy (WAX diffractograms) indicate a similar result. The results are shown in Fig. 3. It is interesting to note the miscibility window observed from experiments falls within the theoretically predicted window. These calculations substantiate our arguments that salt plays an active role in enhancing compatibility of the polymers and also provides a method for predicting the miscibility window in the polymeric systems.
3.4. Conductivity of ternary polymer electrolytes The ionic conductivity of the ternary electrolytes has been measured by impedance spectroscopy from
25 to 1008C. Fig. 6 shows the variation of conductivity as a function of inverse temperature for ternary compositions at various concentrations of POPG and constant O / Na ratio of 4. It is observed that as the amount of POPG increases to 12 mole% in the ternary system, the conductivity increases continuously from 4310 25 (V cm)21 to 2310 24 (V cm)21 at 258C. It may be recalled from Figs. 1 and 2 and Table 1 that for this composition variation the crystallinity decreases drastically. The increase in conductivity is therefore a direct consequence of the reduced crystalline order in the ternary. The values of conductivity obtained in present studies, at room temperature, are approximately two orders of magnitude higher than the best sodium ion conducting binary systems. It may be noted that the sample with 3% POPG shows a conductivity curve with two regions of different slopes. Such a discontinuity is a common occurrence in the binary system and is attributed to the melting of the semicrystalline polymer. In our case this temperature matches exactly with the melting endotherm in the DSC. The
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Fig. 6. Conductivity behaviour of ternary polymer electrolytes in which the POPG content has been varied. The compositions are indicated in the legends.
progressive addition of POPG eliminates the semicrystalline phase and the conductivity behavior becomes that of typical amorphous (liquid-like) polymer. This has been reported earlier [27]. The high room-temperature conductivity, 2310 24 (V cm)21 , for sodium ions is nearly two orders of magnitude higher than that of the best conductivity reported for binary linear POE–sodium salt systems. This value of conductivity is approximately one order of magnitude higher than those reported by Li and Khan [11] for the miscible POE–poly(4-vinylpyridine)–LiClO 4 system; they reported a maximum conductivity of 1310 25 (V cm)21 for the lithium ion at room temperature with elastomeric materials. The blends discussed in this paper showed progressive reduction in crystallinity till truly amorphous compositions were obtained. The highest conductivity is for this optimised amorphous composition. Let us now turn our attention to the phase and conductivity behavior of the complexes obtained in the MEEP and siloxanes-based systems. As mentioned earlier, both polymers being glutinous, a
minimum quantity of POE was added to fabricate freestanding films. Plots of electrical conductivity, in Arrhenius coordinates, for all the compositions in the MEEP systems are shown in Fig. 7. The conductivity shows little variation with composition and the room-temperature conductivity is almost independent of the composition. The conductivity at room temperature is excellent, 5310 25 (V cm)21 at 258C for MEEP–POE–(NaSCN)0.1431. This value is nearly two orders of magnitude higher than for the POEbased electrolytes. Another feature in the same figure is the abrupt change in the conductivity around 608C. This has been attributed to the melting of the semicrystalline POE phase. We have used high molecular weight POE to induce mechanical stability to the films and it has given crystallinity to the otherwise amorphous polymer. Polysiloxane complexes also showed a conductivity of 10 25 (V cm)21 at room temperature. The maximum conductivity was mainly due to the reduction in the crystallinity on complexation. Although electrolytes in both these systems show reasonably high conductivity and appear attractive for application, the complexes are semicrystalline. The second approach to engineering amorphous complexes, starting from an amorphous polymer, does not appear to be feasible for producing amorphous polymer electrolytes.
4. Conclusion Two different design strategies have been used to obtain amorphous polymer salt complexes and high ionic conductivity. Plasticised electrolytes show a reduction in the crystalline phase content and optimised compositions are truly amorphous. For the first time ternary polymers have been represented in a pseudophase diagram to determine the amorphous composition. The Flory–Huggins theory was extended to include inorganic salt as the third component and explained the enhanced miscibility of the two polymers in the presence of salt. This value is two orders of magnitude higher than the best binary linear POE–sodium conducting polymers so far reported. The addition of salt resulted in the enhancement of the miscibility of the POE–POPG polymer blends and resulted in the formation of truly amorphous polymeric electrolytes with high con-
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557
Fig. 7. Arrhenius plots of the bulk conductivity for the mixed electrolytes. The compositions are indicated in the legends.
ductivity, nearly 2310 24 (V cm)21 at room temperature. The second design strategy resulted only in semicrystalline electrolytes and was not found to be useful.
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Engineering polymer electrolytes with enhanced ionic conduction A.R. Kulkarni* Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology ( Bombay), Powai, Mumbai 400076, India
Abstract This paper describes our strategies to engineer amorphous polymer–salt complexes and investigate their electrical behavior. In the first approach, semicrystalline polymer polyoxyethylene (POE) was plasticised using amorphous polyoxypropyleneglycol (POPG) in the presence of sodium salts. In the second approach, amorphous polymers, poly(bis(methoxyethoxyethoxy phosphazene)) (MEEP) and polysiloxane, were used to synthesise electrolytes. These were complexed with sodium salts and a small amount of polyoxyethylene. The nature of each complex was ascertained from optical microscopy, X-ray diffraction (XRD) and Differential Scanning Calorimetry (DSC) in these systems. The polymer–salt complexes, in the first approach, revealed a well-defined glass formation region in which all the compositions were amorphous and showed unusually high ionic conductivity | 10 24 (V cm)21 at ambient temperature. The electrolytes in the second approach showed a maximum conductivity of | 10 25 (V cm)21 . However, truly amorphous compositions could not be obtained. The first approach of engineering reduction in crystalline phases appeared more effective in enhancing ionic conductivity. 2000 Elsevier Science B.V. All rights reserved. Keywords: Polymer electrolytes; Ionic conductivity; Amorphous polymers; Plasticisers
1. Introduction Poly(oxyethylene)-based salt complexes are one of the widely studied polymers in the field of electrolytes [1], due to their wide ranging applications in batteries [2], sensors [3] and display devices [4]. These electrolytes, however, suffer due to low conductivity at room temperature that arises mainly due to the semicrystalline nature with ease of ion transport only in the amorphous phase (liquid-like) generated by the melting of the spherulites. There are two possibilities for the design a polymer electrolyte with high ionic conductivity: (a) to increase amor*Fax: 191-22-578-3480. E-mail address: [email protected] (A.R. Kulkarni).
phous domains in POE; or (b) make a completely amorphous material. Both of these approaches have been widely investigated and among the successful ones are inorganic polymer-based binary systems [5] and plasticiser-based ternary systems [6] which show promise with reduced crystallinity and high roomtemperature conductivity. The latter class of materials also includes blends and polymer mixtures exhibiting enhanced ionic conduction in systems like PEO–polymethylmethacrylate (PMMA) [7], POE– polystyrene [8], POE–siloxanes-g [9], POE–poly(2vinylpyridine) [10] and most recently POE–poly(vinylpyridine) [11]. The blends have been found to be elastomeric in certain composition ranges and showed conductivity enhancements of approximately two orders when compared with polymer salt com-
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plexes of individual polymers. Various techniques such as reduction in crystallinity, reduction in glass transition temperature, using unusual salts and varying the salt concentration have been proposed in order to improve conductivity [12]. The objective, while attempting the modifications, is not to compromise the solvating and mechanical properties offered by the oxyethylene (OE) group. Another solution is to synthesise a comb-like polymer with an amorphous backbone and POE side chains of optimum length to improve mechanical strength of the complex. Such comb-like structures have been reported for both poly(phosphazenes) and polysiloxanes and are found to give room-temperature conductivity of the order of 10 25 (V cm)21 , which is two orders of magnitude higher than those for POEbased electrolytes. Our research group has been involved in developing amorphous polymer salt complexes. In studies reported here, two strategies to design an amorphous polymer or to maximise the content of amorphous phase are considered. The first approach is to investigate phase and conductivity behaviour of a ternary electrolyte system consisting of an amorphous polymer (polyoxypropylene glycol), (POPG), a semicrystalline polymer, POE and an alkali salt (NaI). In the second approach, amorphous polymers were used to synthesise electrolytes. The polymers chosen were MEEP and polysiloxane. Both polymers being glutinous, a minimum quantity of high molecular weight POE was added to achieve dimensional stability and free standing films. These complexes are thermally stable, mechanically strong and completely amorphous in a certain composition range and show higher ionic conductivity than most twocomponent systems hitherto reported. Another interesting observation is that the addition of salt provides an easy method for enhancing the compatibility range of two or more polymers. Our study focuses on the engineering and characterisation of these ternary polymer electrolytes.
2. Experimental Synthesis of polymer electrolyte POE (MW 300 000, Aldrich), POPG (MW 4000, Aldrich) NaSCN and NaI (BDH, UK) were used as received.
MEEP was synthesised in our laboratory [13]. Mixed electrolytes were prepared by dissolving MEEP and PEO (Aldrich, MW 5 4 3 10 6 ) in the weight ratio 55:45 in acetonitrile. The polymer studied in the siloxanes system contained polysiloxanes backbones with randomly substituted side chains composed of three EO monomeric units. Appropriate quantities of raw materials were dried under vacuum and were separately dissolved in minimum quantity of acetonitrile. The solutions were mixed with constant stirring. The solvent was allowed to evaporate at room temperature in a desiccator. The transparent, bubble free, nearly 100-mm thick films were stored in a desiccator. For DSC measurements 10 mg sample was used. The thermograms were recorded from 2 100 to 1 1008C at a heating rate of 108C min 21 using a Du Pont Thermal Analysis System. The calorimeter was flushed with dry nitrogen. The T g values were read from intersection of the tangent drawn to transition curve and base line. AC electrical conductivity of the polymer films was measured in the frequency range from 10 Hz to 5 MHz and the temperature range 20–1008C using a Solartron SI1260 Impedance / Gain Phase Analyzer. Data at various temperatures and frequencies were recorded using a data acquisition facility developed in our laboratory. The films were coated with roomtemperature-drying silver paste and the sample was sandwiched between two spring-loaded SS plates. X-ray diffractograms for the films were recorded using a Philips 1820 X-ray diffractometer in the 2u range 20–408 and matched with diffraction patterns reported in the literature.
3. Results and discussion
3.1. Differential scanning calorimetric studies Fig. 1 depicts DSC thermograms obtained for pure POE, POPG and blends of POE–POPG and POE– POPG–NaI electrolyte samples. Two prominent features to be noted in all the samples are: (i) a glass transition temperature, T g , at which the glassy phase becomes rubbery and amorphous; and (ii) a relatively sharp endotherm for some of the samples. The endotherm is generally attributed to the melting of
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Fig. 1. DSC curves for polymer electrolyte samples. (a) POE; (b) POPG; (c) POE–5% POPG; (d) POE–6% POPG–NaI; (e) POE– 9% POPG–NaI; (f) POE–10% POPG–NaI; (g) POE–12% POPG–NaI; (h) POE–12% POPG–NaI, O / Na510; and (i) POE– 12% POPG–NaI, O / Na512. The O / Na ratio in samples (d)–(g) is 4.
the pure crystalline POE and is sometimes due to a eutectic phase of crystalline POE and a POE–salt complex [14–16]. Reported values of glass transition temperatures of POE and POPG are around 2 608C and 2 758C; these are very close and cannot be used as an indicator to find the extent of the blend formation in sample (c) where T g lies in between these temperatures. However, the absence of two separate glass transition temperatures in the complexes (d)–(g) indicates the coordination of sodium ion with both polymers. At this point it is important to mention that for samples (d)–(g) (Fig. 1) the O / Na ratio was maintained at 4 even though the POPG content was changed from 0 to 15%. On the other hand, for samples (h) and (i) in the same figure
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the POPG content was held constant at 12% and the O / Na ratio was varied from 4 to 12. The complexes show higher T g values depending on the amount of POPG in the system. The increase in T g for the polymers has been attributed to the reduced flexibility of the polymer chains on complexing with the salts. Further addition of POPG increases the flexibility of the chains and hence shifts in T g to lower temperature [17]. The endothermic peak around 608C in Fig. 1 corresponding to melting of crystalline POE shifts from 608C to 558C with the addition of POPG. It may be noted that curves (d)–(f) show a third relatively small endotherm around 2008C. This hightemperature endotherm in our studies is due to the melting / dissolution process of a salt-rich POE–NaI phase. This is consistent with the observations of Berthier et al. [18]. Another prominent feature is the decrease in the area under the endotherm that also serves as a measure of the crystallinity in the system. The percentage crystallinity for each composition was calculated according to the procedure described by Richardson [19]. The most interesting feature in this figure is the absence of the melting endotherm for curves (g) and (h). The absence of the melting endotherm and the presence of a glass transition with no other transition indicates that these materials do not contain crystalline components. The salt and the POPG content together control the overall amorphous phase content of the polymer and an optimum mix leads to completely amorphous polymers. X-ray diffraction studies and conductivity measurements provide additional evidence that the salt plays a major role in the phase relationships.
3.2. X-ray diffraction studies Two types of crystalline structures are noted in the present studies. X-ray diffractograms of a few samples are compared in Fig. 2. In sample (a) POE is semicrystalline in nature and the Bragg reflection lines are in agreement with those reported by Yao [20]. The most intense peak was observed at 2u 5 23.38. The addition of 5% POPG did not change the position of these peaks. However, for the threecomponent samples [curve (c)] the peak has shifted to 228 indicating the formation of a crystalline complex. It was also observed that with the progres-
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A.R. Kulkarni / Solid State Ionics 136 – 137 (2000) 549 – 558 Table 1 Physical constants for POE, POPG and NaI
Fig. 2. X-ray diffraction patterns for polymer electrolyte samples. (a) POE; (b) POE–2% POPG; (c) POE–5% POPG–NaI; (d) POE–13% POPG–NaI. The O / Na ratio in samples (c) and (d) is 4.
sive addition of POPG the intensity and the area under the curve for the most prominent peak decreased in magnitude, and an amorphous profile emerged as a halo. The halo seen in curve (d) (Fig. 2) is typical of amorphous polymers indicating reduced long-range crystalline order in the system. The extent of crystallinity in the samples was calculated using procedure reported by Yao [20] and is tabulated in Table 1. The X-ray diffraction thus substantiates the role of NaI as a crucial component for formation of a complex in ternary compositions.
3.3. Ternary pseudo-phase diagram of the POE– POPG–NaI system To understand the nature of the phases in the ternary system it is represented as a ternary pseudophase diagram as usually used to determine amor-
Property
POE
POPG
NaI
Ref.
T g (K) T m (K) DHf (kJ mol 21 ) CED (kJ mol 21 ) Lattice energy (kJ) DWf with NaI (kJ mol 21 )
206 342 8.67 16.885 – 375
– 198 8.40 17.33 – 350
– 600
[28] [19] [29] [30] [31] [32]
– 680 –
phous formation regions in inorganic ternary systems [21]. Fig. 3 shows a ternary pseudophase diagram, which includes the nature of the various phases formed in the individual binary systems, POE–NaI, POPG–NaI and POE–POPG, and also the ternary system under investigation. Different phases have been indicated by different symbols. Open circles indicate truly amorphous, filled circles show partially crystalline and the open squares show semi-solid materials. Before representing a composition in the diagram using the symbols mentioned above, the nature of its phase was confirmed from DSC and X-ray diffraction, as shown for some of the samples in Figs. 1 and 2. In the binary POE–POPG system the two polymers are compatible only up to 5 mole%. This agrees well with the reported data of Booth and Pickles [22]. However, even in this miscible binary region the compositions are semicrystalline. However, when a small amount of salt is added the compatibility range of the two polymers increases from 5 to 15 mole%. In the present case not only is the compatibility of the polymers increased, but also the crystallinity observed in the binary system is destroyed. The quantities estimated using DSC and X-ray data show that a 60% semicrystalline binary of POE–NaI becomes completely amorphous in the ternary POE–POPG– NaI. This has been indicated in the ternary diagram by a region (circle with dotted lines) in which all the compositions are amorphous. Outside this region the compositions are either partially crystalline or sticky (semi-solid). When POPG exceeds 15 mole% the ternary films show solid–liquid phase separation. At higher salt concentration the films are brittle, resembling polymer in salt systems. Observation of miscibility enhancements, due to the addition of a salt mixture of polymers, has been reported for ion conducting polymers. Hara and Eisenberg [23] re-
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Fig. 3. Ternary phase diagram in the POE–POPG–NaI system. Different symbols show the phases present: s, amorphous; d, partially crystalline; and h, semi-solid.
ported miscibility enhancements in polystyrene ionomer / poly(alkylene oxide) systems from DSC studies and attributed the enhancement to ion–dipole interactions. Li et al. [10] observed POE and poly(2vinylpyridine) to be miscible in a 3:1 ratio in the presence of LiClO 4 . The reason for the miscibility was not understood. Recently, Li and Khan [11] have carried out a systematic study on the POE–poly(4vinylpyridine)–LiClO 4 system where the miscibility enhancement was observed, and this was explained on the basis of the simultaneous interaction of lithium ions with oxygens of POE and nitrogens from the pyridine unit. Our observations are consistent with these observations and reinforce earlier reports confirming that the salt plays a major role in enhancing the miscibility limits of the polymers. Although a considerable number of miscible binary blends have been studied in detail only a few studies on ternary polymer blends have been reported in the literature [24]. These are classified as: Solvent 1–Solvent 2–Polymer, Solvent–Polymer 1–Polymer 2 and Polymer 1–Polymer 2–Polymer 3. To the best of our knowledge miscibility studies on Polymer 1–Polymer 2–salt have not been reported, or in such studies the systems have not been considered as
ternary. The concept that the third component in the ternary system, an inorganic salt, may act as a compatibiliser for two immiscible polymers is important. Although the concept has merit the thermodynamic considerations are not straightforward. Flory theory [25] is widely used to predict and explain miscibility in binary and ternary polymeric blends. In the present work we have extended this theory to polymer electrolytes. The detailed thermodynamic arguments and calculations are worked out elsewhere [26]. The following assumptions are made in addition to the existing ones in the Flory–Huggins theory: 1. The ionic interaction between the elements in the salt is considered to be similar to that between the solvent molecules. 2. The lattice energy is considered to be the energy required to separate the salt pairs. 3. The entropic contribution is not taken into account. The primary step in using the theory involves calculating the interaction parameter for the polymer–salt interaction. Based on the above assump-
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tions and using various experimental values listed in Table 1, the interaction parameter for POE–NaI interaction is found to be 0.077, and that for POPG– NaI is found to be 2 0.063. Free energy contours were obtained for polymer electrolytes by using these values in Eq. (1):
F
S
D
1 2 DGm 5 RT ln(1 2 wn ) 1 1 2 ] wn 1 xw n x
G
(1)
Here the dimensionless parameter x 5 zDe /RT is known as the Flory–Huggins interaction parameter, n is the amount of solvent, and x 5 (V 2 /V 1) is the number average for the degree of polymerisation of the polymer. Fig. 4 shows the variation of DG for POE–NaI and POPG–NaI. A strong interaction between the polymer and the salt is evident in both cases. Experimental results also agree with this, as the complex between the polymer and the salt is formed in the entire range of composition. However, these diagrams do not throw any light on the different phases present in the electrolyte. Thus, the term miscibility in polymer electrolytes will mean strong interaction between the polymer and salt. Based on the Flory–Huggins theory for the free
energy of mixing, the phase behaviour of ternary system can be calculated. The free energy of mixing of a three-component system can be expressed as:
w1 w2 w3 DGm 5 ] ln w1 1 ] ln w2 1 ] ln w3 1 x12 w1 w2 r1 r2 r3 1 x23 w2 w3 1 x31 w3 w1
(2)
For a mixture to be single phase, the requirement that DGm ,0 must be fulfilled. Additionally, the second derivative of DGm with respect to wi must be zero or positive. Thus, the spinodal for a ternary blend must be of the form: ≠ 2 DGm ]] 5 r 1 w1 1 r 2 w2 1 r 3 w3 ≠w 2 2 2[r 1 r 2 ( x1 1 x2 )w1 w2 1 r 2 r 3 ( x2 1 x3 )w2 w3 r 3 r 1 ( x3 1 x1 )w3 w1 ] 14r 1 r 2 r 3 ( x1 x2 1x2 x3 1x3 x1 )w1 w2 w3 50 (3) The critical point is given by:
Fig. 4. Free energy diagram for the POE–NaI and POPG–NaI binary complexes. Note complete miscibility of the polymer and the salt.
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Fig. 5. Theoretical calculation of the miscibility limits in the ternary POE–POPG–NaI blends. The solid line calculated from theory demarcates the miscible and the immiscible regions.
≠DGm ≠ 2 DGm ≠ 3 DGm ]] 5 ]] 5 ]] 50 ≠w ≠w 2 ≠w 3
(4)
Fig. 5 shows the free energy contour for the POE–POPG2000–NaI ternary electrolyte system obtained using x POE–POPG50.067, x POE–NaI5 20.077 and x POPG–NaI5 20.063. The curve obtained shows area of miscibility. Experimental observation from DSC and optical microscopy (WAX diffractograms) indicate a similar result. The results are shown in Fig. 3. It is interesting to note the miscibility window observed from experiments falls within the theoretically predicted window. These calculations substantiate our arguments that salt plays an active role in enhancing compatibility of the polymers and also provides a method for predicting the miscibility window in the polymeric systems.
3.4. Conductivity of ternary polymer electrolytes The ionic conductivity of the ternary electrolytes has been measured by impedance spectroscopy from
25 to 1008C. Fig. 6 shows the variation of conductivity as a function of inverse temperature for ternary compositions at various concentrations of POPG and constant O / Na ratio of 4. It is observed that as the amount of POPG increases to 12 mole% in the ternary system, the conductivity increases continuously from 4310 25 (V cm)21 to 2310 24 (V cm)21 at 258C. It may be recalled from Figs. 1 and 2 and Table 1 that for this composition variation the crystallinity decreases drastically. The increase in conductivity is therefore a direct consequence of the reduced crystalline order in the ternary. The values of conductivity obtained in present studies, at room temperature, are approximately two orders of magnitude higher than the best sodium ion conducting binary systems. It may be noted that the sample with 3% POPG shows a conductivity curve with two regions of different slopes. Such a discontinuity is a common occurrence in the binary system and is attributed to the melting of the semicrystalline polymer. In our case this temperature matches exactly with the melting endotherm in the DSC. The
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Fig. 6. Conductivity behaviour of ternary polymer electrolytes in which the POPG content has been varied. The compositions are indicated in the legends.
progressive addition of POPG eliminates the semicrystalline phase and the conductivity behavior becomes that of typical amorphous (liquid-like) polymer. This has been reported earlier [27]. The high room-temperature conductivity, 2310 24 (V cm)21 , for sodium ions is nearly two orders of magnitude higher than that of the best conductivity reported for binary linear POE–sodium salt systems. This value of conductivity is approximately one order of magnitude higher than those reported by Li and Khan [11] for the miscible POE–poly(4-vinylpyridine)–LiClO 4 system; they reported a maximum conductivity of 1310 25 (V cm)21 for the lithium ion at room temperature with elastomeric materials. The blends discussed in this paper showed progressive reduction in crystallinity till truly amorphous compositions were obtained. The highest conductivity is for this optimised amorphous composition. Let us now turn our attention to the phase and conductivity behavior of the complexes obtained in the MEEP and siloxanes-based systems. As mentioned earlier, both polymers being glutinous, a
minimum quantity of POE was added to fabricate freestanding films. Plots of electrical conductivity, in Arrhenius coordinates, for all the compositions in the MEEP systems are shown in Fig. 7. The conductivity shows little variation with composition and the room-temperature conductivity is almost independent of the composition. The conductivity at room temperature is excellent, 5310 25 (V cm)21 at 258C for MEEP–POE–(NaSCN)0.1431. This value is nearly two orders of magnitude higher than for the POEbased electrolytes. Another feature in the same figure is the abrupt change in the conductivity around 608C. This has been attributed to the melting of the semicrystalline POE phase. We have used high molecular weight POE to induce mechanical stability to the films and it has given crystallinity to the otherwise amorphous polymer. Polysiloxane complexes also showed a conductivity of 10 25 (V cm)21 at room temperature. The maximum conductivity was mainly due to the reduction in the crystallinity on complexation. Although electrolytes in both these systems show reasonably high conductivity and appear attractive for application, the complexes are semicrystalline. The second approach to engineering amorphous complexes, starting from an amorphous polymer, does not appear to be feasible for producing amorphous polymer electrolytes.
4. Conclusion Two different design strategies have been used to obtain amorphous polymer salt complexes and high ionic conductivity. Plasticised electrolytes show a reduction in the crystalline phase content and optimised compositions are truly amorphous. For the first time ternary polymers have been represented in a pseudophase diagram to determine the amorphous composition. The Flory–Huggins theory was extended to include inorganic salt as the third component and explained the enhanced miscibility of the two polymers in the presence of salt. This value is two orders of magnitude higher than the best binary linear POE–sodium conducting polymers so far reported. The addition of salt resulted in the enhancement of the miscibility of the POE–POPG polymer blends and resulted in the formation of truly amorphous polymeric electrolytes with high con-
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Fig. 7. Arrhenius plots of the bulk conductivity for the mixed electrolytes. The compositions are indicated in the legends.
ductivity, nearly 2310 24 (V cm)21 at room temperature. The second design strategy resulted only in semicrystalline electrolytes and was not found to be useful.
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