lithium perchlorate solid polymer electrolytes

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Electrochimica Acta 53 (2007) 1503–1511

Ionic conductivity in polyethylene-b-poly(ethylene oxide)/lithium perchlorate solid polymer electrolytes L.A. Guilherme a , R.S. Borges b , E. Mara S. Moraes b , G. Goulart Silva b,∗ , M.A. Pimenta c , A. Marletta a , R.A. Silva a,∗ a

Instituto de F´ısica, Universidade Federal de Uberlˆandia, Bloco X, Campus Santa Mˆonica, CP: 593, Uberlˆandia, Minas Gerais, Brazil b Departamento de Qu´ımica, ICEx, Universidade Federal de Minas Gerais, CP: 702, Belo Horizonte, Minas Gerais, Brazil c Departamento de F´ısica, ICEx, Universidade Federal de Minas Gerais, CP: 702, Belo Horizonte, Minas Gerais, Brazil Received 29 December 2006; received in revised form 19 March 2007; accepted 2 April 2007 Available online 7 April 2007

Abstract The ionic conductivity and phase arrangement of solid polymeric electrolytes based on the block copolymer polyethylene-b-poly(ethylene oxide) (PE-b-PEO) and LiClO4 have been investigated. One set of electrolytes was prepared from copolymers with 75% of PEO units and another set was based on a blend of copolymer with 50% PEO units and homopolymers. The differential scanning calorimetry (DSC) results, for electrolytes based on the copolymer with 75% of PEO units, were dominated by the PEO phase. The PEO block crystallinity dropped and the glass transition increased with salt addition due to the coordination of the cation by PEO oxygen. The conductivity for copolymers 75% PEO-based electrolyte with 15 wt% of salt was higher than 10−5 S/cm at room temperature and reached to 10−3 S/cm at 100 ◦ C on a heating measurement. The blend of PE-b-PEO (50% PEO)/PEO/PE showed a complex thermal behavior with decoupled melting of the blocks and the homopolymers. Upon salt addition the endotherms associated with PEO domains disappeared and the PE crystals remained untouched. The conductivity results were limited at 100 ◦ C to values close to 10−4 S/cm and at room temperature values close to 3 × 10−6 S/cm were obtained for the 15 wt% salt electrolyte. Raman study showed that the ionic association of the highly concentrated blend electrolytes at room temperature is not significant. Therefore, the lower values of conductivity in the case of the blend with 50% PEO can be assigned to the higher content of PE domains leading to a morphology with lower connectivity for ionic conduction both in the crystalline and melted state of the PE domains. © 2007 Elsevier Ltd. All rights reserved. Keywords: Polyethylene-b-poly(ethylene oxide); Polymer electrolyte; DSC; Micro-Raman; Conductivity

1. Introduction The various electrolytes based on poly(ethylene oxide) (PEO), linear-semicrystalline, modified amorphous and networks, as well as gels and composites of them, are the most exploited materials for salt dissolution and used in electrochemical devices [1–4]. However, the search for new systems in the polymer electrolyte field continues to be an important task, since a successful interplay between high ionic conductivity and good material processability was not yet achieved. Solid polymer electrolytes present a number of desirable properties, such as

∗

Corresponding authors. Tel.: +55 34 32394190; fax: +55 34 32934106. E-mail addresses: [email protected] (G.G. Silva), [email protected] (R.A. Silva). 0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.04.016

simplicity of manufacture in a wide variety of shapes and sizes and greater safety, since non-corrosive or explosive liquids can leak out [4]. Block copolymers are an interesting class of polymeric materials because of their ability to self-assemble into ordered supramolecular structures [5–8]. Block copolymers, both homogeneous and phase separated, have been investigated as polymer matrix for solid polymeric electrolytes [9–18]. In some cases the block copolymers were constituted by a ionophilic block, such as PEO, and an ionophobic block, such as polyisoprene [9,13], polystyrene [13,15], poly(alkylmethacrylate) [14] and polyethylene [12]. Another approach is the use of all blocks with ion interacting moieties [10,16,18]. This work aims to investigate the behavior of solid electrolytes based on polyethylene-b-poly(ethylene oxide) (PEb-PEO) in relation to ionic conductivity and phase arrangement.

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Two block copolymers, with different contents of PEO units, were used to host lithium perchlorate. One of the materials is a blend of PE-b-PEO and homopolymers. The components of the copolymers have very different chemical affinities allowing phase separation and preferential salt interaction take place. A copolymer based on PE and PEO may show crystallinity associated to both blocks. For this kind of material, the most important thermal events are: (i) the melting of the crystalline blocks, (ii) the glass transitions of the amorphous blocks and (iii) the order–disorder transition temperature (TODT ) [7,8,13]. Unconfined and confined crystallization have been observed for diblock copolymers and blends of them [5,6]. In the case of unconfined crystallization, the crystals did not arrange into preexisting microdomains. On the other hand, the confined crystals organized themselves inside the phase-separated copolymer morphologies. 2. Experimental PE-b-PEO copolymers (Aldrich) molar masses and PEO contents as reported by the supplier are presented in Table 1. The proposed chemical structure is CH3 –CH2 –[–CH2 CH2 –]n –[–OCH2 CH2 –]m –OH The polymer matrices were characterized 1 H and 13 C NMR (Varian 200 MHz). These analyses confirmed the proposed chemical structure and show indications of the presence of contaminants as, for instance, homopolymer chains. 1 H NMR allowed the evaluation of molar mass by using the content of protons in the end-group –CH3 in relation to the protons in the blocks. The molar masses and PEO unit content assessed from the NMR characterization are shown in Table 1. Size exclusion chromatography (Column GPC 303D Shimadzu, tetrahydrofurane as eluent) indicated that these samples have a single one but very broad molar mass distribution. The thermal characterization by differential scanning calorimetry (DSC) indicates that the material with 50% PEO is actually a blend of homopolymers and copolymer, as will be discussed later in this work. Solutions in the concentration range between 1 and 15 wt% of LiClO4 (Aldrich) were prepared by dissolving both salt and polymer in acetonitrile. This range of composition represents in [O/Li] ratio values of 194, 37, 18 and 11 for the 75% PEO copolymer and values of [O/Li] = 135, 26, 12 and 8 for the electrolytes based on the blend with 50% PEO copolymer. The copolymers have small PE blocks and were completely soluble in acetonitrile. The films of copolymers were obtained by casting these solutions in Petridishes, removing the solvent slowly at ambient

pressure, and followed by vacuum (∼10−1 bar) evaporation for several days (at least 7 days) at 60 ◦ C. The samples were stored in a desiccator. The pure polymers are composed by a hydrophobic part (PE) and a hydrophilic moiety (PEO). The copolymer electrolytes showed a hygroscopic character characterized by thermogravimetric analysis (TA Instruments SDT 2960 at 10 ◦ C/min) exhibiting a loss of up to 5 wt% of volatiles for samples which had contact with the environment. Besides, the thermal stability of all materials is superior to 300 ◦ C in nitrogen atmosphere. DSC measurements were carried out with TA Instruments 2920 DSC in three scanning experiments: heating from 25 to 120 ◦ C, cooling and second heating between 100 and 150 ◦ C, at heating rates of 10 ◦ C/min, under He atmosphere (100 ml/min). Samples with masses of approximately 5 mg were sealed in aluminum DSC pans. Glass transition (Tg ) values were quoted on the second heating scan as the extrapolated onset and the melting temperatures (Tm ) were determined on peak positions. Raman spectra were recorded in a triple monochromator spectrometer (Dilor XY) equipped with a multiarray (Gold) and CCD detectors. A microscope (Olympus BH-2) was coupled to the spectrometer, allowing a Raman analysis with spatial resolution of about 1 ␮m (micro-Raman technique). Argon laser was used operating in the green line (λ = 514.5 nm) with a power of ∼50 mW. The total ionic conductivity was measured with a ECO CHEMIE potentiostat/impedance frequency analyzer Autolab PGSTAT 30 in an experimental cell including two stainless steel block electrodes inside a heater system to control the temperature between 25 and 110 ◦ C and to avoid contact with the environment. The frequency range was 0.5 to 5 × 105 Hz at 0 V with 50 mV amplitude. The films were carefully dried before measurement and heating/cooling runs were carried out to guarantee that there is no influence of moisture and solvents. In order to avoid leakage at high temperatures under pressure, a separator consisting of a Teflon disc was used to fix the thickness of the samples. 3. Results and discussion 3.1. DSC results DSC curves for the PE-b-PEO (75%) and (50%):LiClO4 systems are shown in Figs. 1 and 2 and Table 2 presents a summary of the main data extracted from these curves. The two systems show very different thermal behavior. The results for the system based on the copolymer with 75% of PEO units are dominated by the PEO phase. This copolymer has molar mass of PEO block of

Table 1 Properties of the polymers used in this study Copolymer

PE-b-PEO (50%) PE-b-PEO (75%)

Evaluated by NMR 1 H and 13 C

Reported by the supplier Molar mass (g mol−1 )

Content of PEO units (%)

Molar mass (g mol−1 )

Content of PEO units (%)

920 2250

50 80

780 1790

50 75

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Table 2 DSC results for the materials based on copolymers PE-b-PEO with 75 and 50% of PEO units and polymer electrolytes wt% LiClO4

Copolymer 75% PEO Tg

0 1 5 10 15

(◦ C)

−62, −11 −54, −5 −42, −3 −34, ∼12 −40, ∼29

Blend with copolymer 50% PEO Tm

(◦ C)

57, 90a 56 54 48 ∼43 (small)

Tg (◦ C) b

−65 −60 −61, −42 −64, −36

PEO Tm,block (◦ C)

9 9 ∼8 – –

PEO Tm,homopolymer (◦ C)

TmPE (◦ C)

41 33, 41 38 – –

71, 79, 90 77, 89 75, 89 77, 90 78, 90

Temperatures of glass transition (Tg ) and melting (Tm ). a Small and broad thermal event, also present in the electrolyte curves. b Not observed.

1450 g mol−1 and presents the melting temperature characteristic of PEO, Tm = 57 ◦ C with enthalpy of fusion H = 138 J/g (value corrected for the PEO mass in the copolymer). The enthalpy of fusion for low molar mass PEO crystals is 179.3 J/g [6], thus, the PEO block presents 77% of crystallinity in the

copolymer. The melting temperature and crystallinity decreases with salt addition, as usually observed for polymer electrolytes [1,16,19]. For instance, in the case of the phase diagram proposed for the high molar mass PEO with LiClO4 electrolyte, the range of compositions studied in the present work are located before the first eutectic near [O/Li] = 10 [19]. Thus, when the polymer host is a low molar mass matrix, the increase of salt addition up to the eutectic composition progressively inhibits the crystallization as observed in Fig. 1a. A broad and relatively small thermal event in the region between 70 and 90 ◦ C is observed in Fig. 1a. As mentioned before, the molar mass of these copolymers is widely distributed, thus, PE block crystals may contribute for this high temperature event. Furthermore, the temperature-dependent Flory–Huggins interaction parameter, χE/EO , can be written as proposed by Sun et al. [8]: χE/EO = −0.2802 + 177.4/T

Fig. 1. DSC curves for the PE-b-PEO-based solid polymer electrolytes. (a) Copolymer PE-b-PEO (75% PEO):LiClO4 and (b) blend PE-b-PEO (50% PEO)/PEO/PE:LiClO4 . Salt concentrations (wt%) are indicated in the figure.

(1)

From the above equation, the TODT for the copolymer with 75% PEO can be predicted to be 70 ◦ C, assuming the mean field (χN)ODT = 10.495, where N is the degree of polymerization. Two glass transitions were observed for the pure copolymer 75% PEO, and its electrolytes on analyzing the region between −80 ◦ C and ∼30 ◦ C in higher magnification curves as showed in Fig. 2. The expected glass transition for PE is close to −130 ◦ C. Therefore, the two observed glass transitions can be assigned to PEO amorphous phases. This assignment is confirmed by the fact that both glass transitions increase with salt addition, which is a consequence of ion–dipole interaction between the cation (Li+ ), and the oxygen atoms of the host polymer [11,12,20]. PEO chains adopt specific conformations around the cation which depends on the composition and salt investigated. These structures have been studied on the crystal polymer–salt complexes and used as the basis to analyze the results of the analogous amorphous phases [21]. The presence of two glass transitions for PEO has been already reported [22–24] and is associated with a more constraint amorphous phase at interfaces with crystals, besides the typical disordered region. The increase of both glass transitions with salt addition showed in Table 2 and Fig. 2 indicates that the existence of two PEO amorphous phases is characteristic of all electrolytes prepared with the copolymer 75% PEO. More restricted PEO amorphous phase was expected to disap-

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Fig. 2. DSC curves for the PE-b-PEO-based solid polymer electrolytes in the range of glass transition. (a) Copolymer PE-b-PEO (75% PEO):LiClO4 and (b) blend PE-b-PEO (50% PEO)/PEO/PE:LiClO4 . (c) Glass transitions as a function of composition. Salt concentrations (wt%) are indicated in the figure.

pear when the PEO crystallization was disrupted, which was not observed. This result might be associated with the influence of PE block crystals remaining in the sample. Nevertheless, the general behavior of melting and glass transition changes for the copolymer PE-b-PEO 75% PEO electrolytes can be con-

sidered as similar to electrolytes based on a low molar mass PEO homopolymer. The system based on the PE-b-PEO copolymer with 50% of PEO units shows a very distinct and complex thermal behavior. Note that the curves presented in Fig. 1 correspond to the

Fig. 3. Isotherms of conductivity as a function of slat concentration for PE-b-PEO (75% PEO):LiClO4 and blend PE-b-PEO/PEO/PE (50% PEO):LiClO4 . Isotherm temperatures are indicated in the figure. Data correspond to heating run.

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second heating run in the DSC measurements, and the influence of volatiles was suppressed by the thermal treatment. Sun et al. [7] have previously reported that these copolymers may contain some contamination of homopolymer. The PEO block for this copolymer would present a molar mass of approximately 440 g mol−1 and for this kind of PEO the reported melting temperature is between 4 and 8 ◦ C [25]. The DSC curves in Fig. 1b indicate that this sample can be classified as a blend of PE-b-PEO with the homopolymers PE and PEO. The effect of salt addition on this material produces the disappearance of the two endotherms at lower temperature. The introduction of salt should have consequences only for the PEO block and homopolymer. Therefore, the three main endothermic events observed in Fig. 1b were assigned as shown in Table 2. The broad endotherms between 60 and 90 ◦ C are present for the overall range of electrolyte composition. Thus, these overlapped endotherms are assigned to PE homopolymer and block present in the material. These conclusions are based on the fact that the homopolymers are preferentially dissolved in the similar block microdomains [6,26]. The melting events associated to PEO block (∼8 ◦ C) and PEO homopolymer (∼41 ◦ C) are very sensitive to salt addition. For the copolymers with 10 and 15 wt% salt there is no more PEO crystals in the materials. On the other hand, the PE block and homopolymer kept a similar thermal response and can be considered as crystalline microdomains which help in enhancing the dimensional stability of these electrolytes. The glass transition temperature of the electrolytes based on the blend material with 50% PEO units (Fig. 2 and Table 2) slightly increases with addition of salt between 1 and 5 wt%

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salt. Above this concentration, a second Tg is detected which also show a small increase with the salt concentration between 10 and 15 wt%. The comparison between glass transitions trends with salt addition between the two systems studied in this work is showed in Fig. 2c. The overall behavior of the blend based on copolymer with 50% PEO units is quite complex. However, some important conclusions can be established to help on the discussion of the conductivity results: (a) PE crystalline domains corresponding to a ∼50% of the polymer units are present in all electrolyte compositions; (b) PEO domains rapidly lose crystallinity upon salt addition; (c) PEO domains show small changes of glass transition upon salt addition in comparison with typical polymer electrolytes. These observations indicate that the PEO/salt microdomains are restraint by the PE crystalline phase. A relatively strong microphase separation occurs in this material. The PEO and PEO/salt microdomains may be not fully interconnected. 4. Conductivity Conductivity measurements as a function of the composition for both systems were obtained for four samples with 1, 5, 10 and 15 wt% of salt as shown in Fig. 3. As mentioned in Section 2, this range of composition represents in [O/Li] ratio values of 194, 37, 18 and 11 for the 75% PEO copolymer and values of [O/Li] = 135, 26, 12 and 8 for the electrolytes based on the blend with 50% PEO copolymer. Fig. 3 presents results obtained by impedance on heating the samples. The isotherms at 50 and 100 ◦ C, for both systems, exhibit a sharp increase in conduc-

Fig. 4. Arrhenius plots of conductivity for the copolymer PE-b-PEO (75% PEO):LiClO4 electrolytes. Salt concentrations (wt%) are indicated in the figure.

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tivity between 1 and 5 wt% of salt followed by a plateau in the range 5–15 wt% of salt. At 100 and 50 ◦ C the copolymer (75% PEO)-based electrolytes show one order of magnitude higher conductivity on the plateau (Fig. 3) than the blend electrolytes. Robitaille and Fauteux [19] reported the conductivity isotherms for high molar mass PEO/LiClO4 at temperatures higher than 65 ◦ C in the range of composition of [O/Li] = 100 to 3. The results presented here agree with these authors in relation to the range of compositions of the plateau. The conductivity will decrease for electrolytes with concentration of salt higher than the ratio [O/Li] = 8 [19]. The values of conductivity obtained for the copolymer with 75% PEO at 100 ◦ C are similar to the values obtained for Robitaille and Fauteux [19] at 85 ◦ C. For temperatures below 65 ◦ C the high molar mass PEO crystallizes and conductivity drops two or three orders of magnitude below the values obtained in the present work (Fig. 3). The conductivity for the polymer electrolytes based on the copolymer with 75% PEO units as a function of temperature (25–110 ◦ C) is shown in Fig. 4. Conductivity curves obtained on heating are presented for all compositions. The low concentrated, 1 and 5 wt% salt, exhibit the typical discontinuity associated with melting of PEO domains on heating [2,3]. Appreciable values of conductivity, superior to 10−5 S cm−1 at room temperature, were obtained for the electrolytes with 10 and 15 wt% salt as shown in Fig. 4, which is an interesting result in comparison with other block copolymer electrolytes [11,12,14]. The conductivity curves for the high concentrated and low crystalline samples (see Fig. 1 for DSC curves) can be easily adjust with a linear function which may indicate an Arrhenius type of mechanism and not a Vogel–Tamman–Fulcher (VTF) [1] behavior. However, it is possible that the temperature interval is not large enough to allow an evaluation of the mechanism by this analysis.

Conductivity depends directly on concentration and mobility of charge carriers. Furthermore, the mobility of ionic species depends, in the case of the systems studied in this work, on the polymer chain dynamics or segmental mobility in the amorphous phase. The cyclic conductivity measurements on heating/cooling revealed an interesting effect for the 75% PEO copolymer-based electrolytes. Fig. 4 shows that on cooling the conductivity presents an anomalous drop between 80 and 90 ◦ C, which can be associated with crystallization of the PE block and consequent decrease in the mobility of ions on the amorphous PEO domains. The conductivity for the materials based on a blend of a copolymer with 50% PEO units and the homopolymers will now be discussed in detail. Fig. 3 shows that this system does not surpass the value of 10−4 S/cm of conductivity at 100 ◦ C, whilst the copolymer with 75% PEO-based electrolytes reaches 10−3 S/cm. Moreover, the conductivity values on heating remain close to 3 × 10−6 S/cm at room temperature for the electrolyte with salt content of 15 wt%, as shown in Fig. 5. The cyclic measurements of conductivity presented in Fig. 5 exhibit a special trend in the case of the electrolytes with 10 and 15 wt% LiClO4 based on the blend host polymer. The curve feature shows saturation for the increase in conductivity close to the temperatures of melting the PE domains. Furthermore, the 15 wt% salt electrolyte shows lower values of conductivity on cooling. These results appear to indicate that, for the more conducting samples, after PE domain melting the disappearance of rigid phase-separated crystals disturbs even more the PEO conductivity pathways. In this context the morphology after heating seems to be less favorable for the ion displacements. The ion transport mechanism in the case of the block copolymer and blend electrolytes should be compared to the behavior

Fig. 5. Arrhenius plots of the conductivity for the blend PE-b-PEO (50% PEO)/PEO/PE:LiClO4 . Salt concentrations (wt%) are indicated in the figure.

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of high molar mass PEO/LiClO4 . The hydrophobic block of PE imposes a trend of solid polymer electrolyte for both systems. This means that the materials have the advantage of dimensional stability and, as consequence, conductivities close to low molar mass PEG (liquid aprotic electrolytes) were not obtained at all. Thus, one can propose that the ion transport mechanism is defined by the PEO segmental dynamic at the same time that it suffers an influence of the microdomains of PE chains. The experimental evidences for that are: (i) the increase of conductivity with concentration and the concentration range of the plateau of conductivity are similar to the results reported by Robitaille and Fauteux [19] for high molar mass PEO/LiClO4 ; (ii) the drops of the conductivity on cooling and the saturation in the conductivity of the blend in the range of PE melting/crystallization clearly demonstrates the influence of the hydrophobic PE segments. The PEO microdomains are the pathways of conductivity and the degree of connectivity between them determines the level of ionic conductivity delivered by the materials.

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Fig. 6. Raman spectra at room temperature for the blend PE-b-PEO (50% PEO)/PEO/PE:LiClO4 polymer electrolytes in the 600–1350 cm−1 range. Salt concentrations (wt%) are indicated in the figure.

5. Raman study Fig. 6 shows micro-Raman spectra for the system based on PE-b-PEO (50% PEO)/PEO/PE:LiClO4 in the region between 600 and 1350 cm−1 . These materials showed an intricate behavior of phase arrangement and lower conductivity values in the range of compositions studied, as discussed above. Thus, further investigation of the samples in relation to the Raman features associated to the polymer components and salt appears to be of interest. Frequencies of the Raman bands observed in the spectra at room temperature and assignments of the main polymer host

bands, established using Raman studies of PEO [27,28] and PE [29,30], are presented in Table 3. A first interesting feature which appears in the Raman spectra, by changing the concentration of salt, is the significant decrease of the intensity of the bands marked with a star in Fig. 6. These are the Raman bands at 844, 1147, 1237 and 1284 cm−1 which are assigned as the important vibrational modes of the PEO segments in Table 3 [27,28]. Therefore, the Raman spectra show very clearly the decrease of crystallinity of the PEO domains by salt addition. All bands assigned to the PE segments (Table 3)

Fig. 7. The Raman peak associated with the anion symmetric stretching mode for the electrolytes based on the blend PE-b-PEO (50% PEO)/PEO/PE; different salt concentrations of the electrolytes and pure salt fitted by the sum of Lorentzian lines. Salt concentration (wt%) is indicated in the figure.

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Table 3 Frequencies observed on Raman spectra at room temperature for the blend based on the copolymer (PE-b-PEO) with 50% PEO units Frequency (cm−1 )

PEO [25,26]

PE [27,28]

Assignment

844/857 913/932 1064 1133 1147 1171 1237 1284 1295

Strong Weak Medium – Medium – Medium Strong –

– – Medium Medium – Weak – – Strong

r(CH2 )a r(CH2 )s , ␯(COC)a ␯(CC)a , r(CH2 )s , ␯(CO)s w(CH2 ), ␯(CC)s w(CH2 )s , ␯(CC) r(CH2 ) t(CH2 )s , t(CH2 )a t(CH2 )a , t(CH2 )s t(CH2 )s , t(CH2 )a

Assignments of Raman bands based on Refs. [25,26] for PEO and [27,28] for PE. r, Rocking; ␯, stretching; t, torsion; w, wagging; s, symmetric; a, antisymmetric.

[29,30] are present with similar intensities in all electrolyte spectra at room temperature. Fig. 6 shows an increase in the intensity of ClO4 − symmetric stretching ␯1 (A1 ) at 934 cm−1 with an increase in salt concentration. Ion association in polymer electrolytes is frequently studied by investigating this mode of the anion [31,32]. Fig. 7 presents the corresponding band shape analysis for the pure blend and LiClO4 salt, as well as for two different electrolyte concentrations. In the fitting procedure, each previous Lorentzian line is kept at both fixed frequency and line width. For salt concentrations below 10 wt% the band for ClO4 − is fitted with a single Lorentzian at 934 cm−1 (line width = 8 ± 2 cm−1 ), which is attributed to “free ions” or solvent-separated ions [32]. When the concentration is increased from 10 to 15 wt%, the addition of a second Lorentzian line is necessary to perform the ClO4 − band fitting. This Lorentzian was introduced at 936 cm−1 (line width = 8 ± 2 cm−1 ) and was attributed to the presence of contact ion pairs. The electrolyte based on the blend with 15 wt% of salt in Fig. 7 corresponds to a ratio [O/Li] = 8. PEG 400/LiClO4 electrolytes previously studied by our group showed the limit for the formation of ion pairs close to [O/Li] = 7.5. Therefore, the behavior of salt association in the blend electrolytes is quite similar to what was observed for a pure low molar mass PEG. No special influence of the presence of PE crystalline phase on the ion association was characterized. This is an interesting result taken into account that the crystalline PE block and hompolymer domains constitute ∼50% of the polymer chains in this material. These domains did not change in crystallinity through the addition of salt. 6. Conclusion The phase transition changes observed for the copolymer (75% PEO) electrolytes indicate strong coordination between cation and polymer. This was characterized by the increase of glass transitions and decrease of PEO domains crystallinity. The electrolytes based on a blend of PE-b-PEO (50% PEO)/PEO/PE also showed decrease in PEO crystallinity upon salt addition. However, the glass transition increase was much less pronounced and the PE domains maintained a defined crystalline phase in

the blend electrolytes. The well-defined PE crystalline structure in the blend electrolytes was also confirmed by Raman spectroscopy, which, besides that, indicated a low ionic association in these electrolytes. Cyclic conductivity measurements on heating and cooling allowed the observation of interesting effects. The samples were always previously thermal annealed for drying purposes and stored at room temperature. On heating, the conductivities were in some cases greater than on cooling for both systems. Cooling conductivity curves for the copolymer with 75% PEO units showed abrupt drops in the region of PE crystallization. Furthermore, the system based on the blend with 50% PEO units showed lower values of conductivity in the overall range of temperature. This seems to indicate that the PE crystalline domains did perturb the favorable PEO salt pathways leading to a less well-connected morphology of the conducting domains. Acknowledgements The authors thank the Brazilian agencies CNPq, CAPES and FAPEMIG for financial support. R.A. Silva thanks C.A. Furtado and Rodrigo L. Lavall for the help with the conductivity measurements. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]

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