Influence of molar mass on the thermal ... - Wiley Online Library

Research Article Received: 28 September 2016

Revised: 26 December 2016

Accepted article published: 5 January 2017

Published online in Wiley Online Library: 14 February 2017

(wileyonlinelibrary.com) DOI 10.1002/pi.5322

Influence of molar mass on the thermal properties, conductivity and intermolecular interaction of poly(ethylene oxide) solid polymer electrolytes Fatin Harun, Chin Han Chan* and Tan Winie Abstract The sample preparation pathway of solid polymer electrolytes (SPEs) influences their thermal properties, which in turn governs the ionic conductivity of the materials especially for systems consisting of a crystallizable constituent. Majority of poly(ethylene oxide) (PEO)-based SPEs with molar masses of PEO well above 104 g mol−1 (where PEO is crystallizable and should reach an asymptote in thermal behaviour) display molar mass dependence of the thermal properties and ionic conductivities in non-equilibrium conditions, as reported in the literature. In this study, PEO of different viscosity-molar masses (M𝛈 = 3 × 105 , 6 × 105 , 1 × 106 , 4 × 106 g mol−1 ) and LiClO4 salt (0 to 16.7 wt%) were used. The SPEs were thermally treated under inert atmosphere above the melting temperature of PEO and then cooled down for subsequent isothermal crystallization for sufficient experimental time to develop morphology close to equilibrium conditions. The thermal properties (e.g. glass transition temperature, melting temperature, crystallinity) according to differential scanning calorimetry and the ionic conductivity obtained from impedance spectroscopy at room temperature (𝝈 DC ∼ 10−6 S cm−1 ) demonstrate insignificant variation with respect to the molar mass of PEO at constant salt concentration. These findings are in agreement with the PEO crystalline structures using X-ray diffraction and ion − dipole interaction by Fourier transform infrared results. © 2017 Society of Chemical Industry Keywords: poly(ethylene oxide); differential scanning calorimetry; ionic conductivity; X-ray diffraction; Fourier transform infrared

INTRODUCTION Since the 1980s, studies of poly(ethylene oxide) (PEO) polymer host added with lithium (Li) salt have attracted extensive research interest as solid polymer electrolytes (SPEs) for highly efficient batteries.1 – 12 It is widely accepted that ionic transport mainly takes place in the amorphous region of semicrystalline PEO.13 – 16

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We note here that there are studies on the conductivity (ca 10−7 S cm−1 ) in the crystalline region of low molar mass PEO (M = 103 g mol−1 ).17,18 The literature on the molar mass dependence properties of SPEs is scant and the effects on conductivity are inconsistent.14,19 – 24 High molar masses of PEO (M > 105 g mol−1 ) are often used to obtain appreciable ionic conductivity and acceptable mechanical properties.3,5,7,10 In general, the thermal properties of the polymer become almost independent of molar mass when M > 104 g mol−1 .25,26 This may imply that PEO/Li systems with relatively high molar masses of PEO should also demonstrate insignificant variation in thermal properties and ionic conductivities at constant salt concentration when the systems are in conditions close to equilibrium. In the studies of Chan and colleagues,14,19,21 PEO/Li SPEs (Mη = 6 × 105 to 4 × 106 g mol−1 ) prepared by the solution casting method were left to dry at room temperature and then further dried under vacuum at 50 ∘ C. It was reported that at mass fraction of salt W S = 0.13, PEO with Mη = 4 × 106 g mol−1 records ionic conductivity 𝜎 DC = 7 × 10−4 S cm−1 at 25 ∘ C, two orders of Polym Int 2017; 66: 830–838

magnitude higher than PEO with Mη = 6 × 105 g mol−1 which shows 𝜎 DC = 8 × 10−6 S cm−1 . The influence of molar mass on 𝜎 DC for PEO/Li may be due to the existence of two-phase system (crystalline PEO and amorphous mixture of PEO/Li) which is not in equilibrium at room temperature under the experimental conditions. These studies indicate that consistency in sample preparation for SPEs is crucial for reproducible properties. The thermal history during sample preparation influences the thermal properties and morphologies of the samples, which in turn governs the 𝜎 DC of the system.19,27 – 29 In this paper, PEO with different molar masses (Mη = 3 × 105 to 4 × 106 g mol−1 ) was added with various mass fractions of Li salt (W S = 0 to 0.167). In order to have a comparable and consistent sample preparation pathway, all the solution casted samples were thermally treated under inert atmosphere above the melting temperature of PEO for an extended period of time (to erase thermal memory of the sample during the solution casting procedure) and then isothermally crystallized at 50 ∘ C under vacuum. Here, the systems under discussion are more towards equilibrium conditions. We focus mainly on the influence of molar mass of PEO and mass

∗

Correspondence to: CH Chan, Faculty of Applied Sciences, Universiti Teknologi MARA, 40450 Shah Alam, Selangor, Malaysia. E-mail: [email protected] Faculty of Applied Sciences, Universiti Teknologi MARA, Shah Alam, Selangor, Malaysia

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© 2017 Society of Chemical Industry

Poly(ethylene oxide) solid polymer electrolytes

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Table 1. Characteristics of materials Material Mη a (g mol−1 ) T g b (∘ C) T m c (∘ C) ΔHref (J g−1 )

PEO2

PEO1 3 × 105 −58 68

6 × 105 −57 68

PEO3

PEO4

1 × 106 −55 68

4 × 106 −55 68

LiClO4 − − 236d 272.3f

188.3e

H2 C

Molecular structure

H2 C

O

O n

O

Cl

Li+ O–

O Supplier a

Sigma-Aldrich Chemical Co. (St Louis, MO, USA)

¯ Organics Co. (Geel, Antwerp, Belgium) Acros

Viscosity-average molecular weight provided by the supplier.

b Glass transition temperature during the first heating cycle as determined in this work. c Melting temperature during the first heating cycle as determined in this work. d Melting temperature adopted from O’Neil.32 e Melting enthalpy of 100% crystalline PEO adopted from Cimmino et al.33 f Melting enthalpy adopted from Lide.34

fraction of salt on the ionic conductivity, thermal properties, crystalline structure and intermolecular interaction.

MATERIALS AND METHODS Different molar masses of PEO were purified before further use by dissolution in chloroform (Merck, Darmstadt, Hesse, Germany) followed by precipitation in n-hexane (Merck). LiClO4 salt was dried at 100 ∘ C for 24 h prior to use. LiClO4 salt was chosen due to its advantages, such as low lattice energy that ease the complexation with PEO, high mobility of charge carriers, stable electrochemical properties and absence of crystalline complex formation at low salt concentration.30,31 The characteristics of the polymers and the salt used are summarized in Table 1. Preparation of samples As shown in Table 2, PEO with different LiClO4 salt concentrations, Y S = (mass of LiClO4 )/(mass of PEO), were dissolved in tetrahydrofuran (Merck) and stirred for 48 h at 50 ∘ C. The solution was poured into Teflon® dish and left at room temperature until the solvent evaporated. Then the sample was dried at 50 ∘ C for 24 h for removal of residual solvent. This was followed by thermal treatment under nitrogen atmosphere at 80 ∘ C for 30 min. Finally, the samples were isothermally crystallized at 50 ∘ C for 24 h and kept in desiccators. Samples were vacuum dried again at 50 ∘ C for 24 h before sample characterizations.

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X-ray diffraction (XRD) PANalytical X’pert Powder XRD (Almelo, Overijssel, The Netherlands) was used to record the diffraction patterns with 2𝜃 ranging from 5∘ to 50∘ in reflection mode using Cu K𝛼 radiation with 𝜆 = 1.5406 Å, operated at 40 kV and 30 mA, with a step size of 0.013∘ . The d-spacing between diffraction lattice planes of PEO was determined by Bragg’s relation 𝜆 = 2d sin 𝜃, whereas the interchain separation R was evaluated from R = 5𝜆/(8 sin 𝜃) where 𝜃 is expressed in radians.35,36 The values of the 2𝜃 peak position were obtained using HighScore Plus software (PANalytical). Attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy Spectra were recorded using ATR accessory on Nicolet 6700 FTIR spectrometer (Thermo Fisher, Madison, WI, USA) equipped with a diamond crystal in the transmittance mode over the range 600 − 4000 cm−1 by averaging 16 scans at a maximum resolution of 2 cm−1 on two spots per sample. Ionic conductivity The ionic conductivity (𝜎 DC ) at 25 ∘ C was determined from AC impedance measurements using Hioki 3532-50 Hi-Tester (Nagano, Chubu, Japan) interfaced with a computer for data acquisition over the frequency range between 50 Hz and 2 MHz. The thin film was sandwiched between two stainless steel disc electrodes (2 cm diameter), which acted as blocking electrodes for ions. The value of 𝜎 DC was calculated by using the equation 𝜎 DC = L/Rb A. The 𝜎 DC value was estimated either from Nyquist plots or from the values of the real (Z’) and imaginary parts (Z") of the impedance at frequency


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Differential scanning calorimetry (DSC) TA Q2000 (TA Instruments, New Castle, Delware, USA) equipped with RCS90 refrigerator cooling system (TA Instruments) and nitrogen gas purging was used to determine the values of glass transition temperature (T g ), change in heat capacity (ΔC p ), melting temperature (T m ), melting enthalpy (ΔHm ) and crystallinity (X*) of the systems in the heating cycle. The differential scanning calorimeter was calibrated with indium and sapphire standard. The sample was quench-cooled to −90 ∘ C for 5 min and heated to 80 ∘ C at a rate of 10 ∘ C min−1 . A similar thermal procedure was applied in the sapphire standard run and baseline run. The value of T g is taken at half ΔC p value. To determine the C p value of sample, the heat flow

signal from sample was compared to the baseline of sapphire standard which is of known C p . Both curves were corrected by zero or baseline correction experiment whereby an empty reference and an empty sample pan were placed in the furnace. The quantity of ΔC p was estimated from the glass transition of the onset and endset of the heat flow curve. The melting peak maximum was taken as the T m value. The X* value of PEO was calculated from the melting enthalpy in the DSC trace, X* = ΔHm /ΔHref .

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F Harun, CH Chan, T Winie

Table 2. Concentration of LiClO4 salt added to PEO YS 0 0.02 0.05 0.07 0.10 0.12 0.15 0.20

Mass fraction (W S )

Mole fraction (X S )

0 0.0196 0.0476 0.0654 0.0909 0.107 0.130 0.167

0 0.00820 0.0203 0.0281 0.0397 0.0473 0.0584 0.0764

PEO1 at YS = 0.12

PEO1 at YS = 0

Free standing film becomes sticky in nature with increasing salt concentration (thickness ca 0.02 cm; diameter 5 cm)

for fully stabilized network (f o ) [Z ′ (f o ) and Z ′′ (f o )] at maximum of Z ′′ .37 Values of 𝜎 DC reported were the average calculations of four impedance analyses from different spots of the sample with errors in 𝜎 DC values at approximately 10%. To determine the power law dependence, 𝜎 = 𝜎o YS x

(1)

double-logarithmic plots of 𝜎 DC versus Y S were plotted which yielded 𝜎 o and exponent x; these were then substituted in 𝜎 = NA e 𝜇𝛼

𝜌P x Y MS S

(2)

to compute the mobility 𝜇𝛼. The terms used in Eqn (2) can be defined as follows: NA is Avogadro’s constant, e represents the elementary charge, 𝜌p is the density of PEO (1.2 g cm−3 ), MS is the molar mass of LiClO4 and x is the extent of correlation between salt and polymer segment.21 The diffusion coefficient of charge carrier, D was calculated using D = kB T𝛼𝜇/e.

between polymer and salt. Figure 1 presents the DSC traces of the glass transition of PEO1 and PEO4 at Y S = 0 and 0.12. The T g values are almost independent of molar mass and increase gradually with increasing LiClO4 salt mass fraction up to W S = 0.065 before levelling off when the solubility limit of PEO is reached, as illustrated in Fig. 2. Meanwhile, data from Chan et al.14 inserted in Fig. 2 display the molar mass influence, particularly at high salt compositions. This can be attributed to the non-equilibrium morphologies developed during sample preparation in their studies where PEO SPEs were solvent casted at room temperature and dried under vacuum at 50 ∘ C for 48 h. Now, turning back to this study, it may be suggested that for SPEs prepared in conditions towards equilibrium, the solvation of salt in the polymer is independent of the molar mass of PEO at W S = const. Hence, there are minor differences in T g as well as ΔC p (see Fig. 3) for different molar masses of PEO, as observed. ΔC p value is a measure of the mass fraction of amorphous microphase (PEO and salt) in the mixtures. Below the solubility limit of PEO, ΔC p increases linearly with W S . In short, for lower salt contents, the SPEs of these systems consist of an amorphous phase (PEO and salt) that is almost independent of the molar mass of PEO, which is in equilibrium with the PEO crystalline phase.

RESULTS AND DISCUSSION

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Glass transition temperature Since ionic conduction takes place mainly in the amorphous phase, related parameters such as T g and ΔC p were examined to study the dissolution of LiClO4 salt in the amorphous phase of PEO. It is well established that T g increases with salt addition due to loss of polymer chain flexibility from the intermolecular interaction


Crystallinity DSC thermograms of the melting endotherm of PEO1 and PEO4 at Y S = 0 and 0.12 are presented in Fig. 4. Selected values of T m and ΔHm from the first heating cycle for PEO SPEs are listed in Table 3. Selected data from the second heating cycle of PEO1 (as well as for PEO2 , PEO3 and PEO4 which are not shown in Table 3) display


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Figure 1. DSC traces of the glass transition of PEO1 and PEO4 at Y S = 0 and 0.12.

Figure 2. T g values of PEO2 and PEO4 against LiClO4 mass fraction obtained from this study (dashed curve is for visual aid) and data from Chan et al.14

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Figure 4. DSC traces of the melting endotherms of PEO1 and PEO4 at Y S = 0 and 0.12

Data from Nasir et al.21 were included in Fig. 5 for illustration. In their study, a similar trend of observation is noted. At low salt content, the crystallinity of PEO stays constant to a good approximation and is more or less independent of the molar mass of PEO. At higher salt content, a slight deviation from constancy of PEO crystallinity occurs. The non-equilibrium morphologies developed during sample preparation may lead to PEO of lower molar mass to undergo a greater deviation. It can be concluded that under equilibrium conditions, the extent of phase separation between crystalline PEO and the amorphous phase (PEO and Li) is almost independent of molar mass. Melting behaviour As PEO and Li salt are completely miscible in the molten state (when Y S < 0.15), the apparent melting temperature depression of PEO can be used to estimate the deviation from perfect behaviour, as demonstrated by Nasir et al.21 and shown as Tm = Tmo +


( )2 ( ) R Tmo ln 𝛾P + 1 ln XP ΔHref ln XP

(3)


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reproducible values of T m (±1 ∘ C) and ΔHm (±1 J g−1 ). This suggests that precise control in the thermal pre-treatment of PEO-based SPEs during their preparatory stage leads to the development of morphologies of SPEs which are close to equilibrium under the experimental conditions. PEO and Li salt are completely miscible in the molten state. Below the melting temperature of PEO, the SPEs undergo phase separation, segregating them into pure crystalline PEO and amorphous phase (PEO and salt). The extent of this phase separation can be evaluated from the degree of crystallinity. Figure 5 shows the crystallinity of PEO as a function of the mass fraction of Li salt. At Mη = const, the crystallinity of PEO at low salt content stays constant to a good approximation, while at higher salt concentration, drastic deviation occurs. At W S = const, the X* values do not change significantly with the molar mass of PEO. These results conform to the ΔC p − salt content relationships discussed in the previous section. Reports in the literature14,38,39 have affirmed that the interaction of Li salt with PEO chains slows down the rate of crystallization of PEO at T c = const and increases the content of amorphous microphase in mixtures with increasing salt concentration.

Figure 3. ΔC p as a function of the mass fraction of salt. The dashed curve is the linear regression curve for PEO1 (r2 = 0.997).

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Table 3. Selected T m and ΔHm for PEO SPEs PEO1 YS 0 0.05 0.07 0.10 0.12

T m (∘ C) 67.9 (67.1) 64.8 (64.6) 63.8 (63.6) 61.9 (61.1) 59.6 (59.1)

PEO2

ΔHm

(J g−1 )

131.0 (129.4) 116.7 (115.6) 102.2 (101.5) 85.1 (84.1) 66.1 (65.1)

T m (∘ C)

PEO3

ΔHm

(J g−1 )

T m (∘ C)

PEO4

ΔHm

(J g−1 )

T m (∘ C)

ΔHm (J g−1 )

67.6

126.4

68.0

129.6

67.7

123.5

64.9

111.5

65.5

110.3

65.1

110.6

63.9

101.4

64.2

99.2

63.5

97.1

62.0

81.4

62.1

80.5

61.4

78.8

62.9

66.9

60.0

65.6

60.0

65.6

Data provided in parentheses is extracted from the second heating cycle of the DSC measurement.

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Figure 5. Crystallinity of PEO2 and PEO4 as a function of the mass fraction of salt obtained from this study and data from Nasir et al.21 The dashed curves represent the loci of constancy in the crystallinity of PEO2 .

Figure 6. Melting temperature of PEO2 and PEO4 as a function of the mole fraction of salt obtained from this study and data from Nasir et al.21 The dashed curves give the linear regression for PEO2 at low salt content.

where T m ∘ symbolizes the melting temperature of PEO, ΔHref represents the melting enthalpy of 100% crystalline PEO, X p denotes the mole fraction of PEO in the molten state, X S is the mole fraction of Li salt added to the PEO and 𝛾 p is the corresponding activity coefficient. For low Li salt concentration, ln X p = −X S can be approximated to determine the activity coefficient 𝛾 p . Equation (3) is considered valid as long as pure PEO crystallizes out from the molten mixture. The validity of this term for this system was corroborated by XRD results that demonstrate insignificant changes in the PEO crystalline structure (see the next section). From the linear plot of T m versus X S as illustrated in Fig. 6, the melting point depression ΔT(X S ) was determined and is tabulated in Table 4. At low salt content, the melting point depression is not influenced by the molar mass of PEO, as indicated by the 𝛾 p values (1+ 0.4X S ). Data from Nasir et al.21 (included in Fig. 6) suggest a relationship between molar mass and melting point depression of PEO systems for the non-equilibrium morphologies developed during sample preparation. PEO with higher molar mass behaves to a good approximation, displaying a very small 𝛾 p value, whereas PEO with lower molar mass displays pronounced deviation. Hence, it may be deduced that the solubility of salt in PEO prepared close

to equilibrium is independent of molar mass. This observation is in agreement with the results discussed in Chan et al.14 , where estimation of the equilibrium melting temperature (T m o ) by the Hoffman − Weeks method from the linear relationship between crystallization and melting temperature (T m = 𝛾T c + (1 − 𝛾)T m o ) was carried out for PEOs with Mη = 6 × 105 to 4 × 106 g mol−1 at different concentrations of LiClO4 . The quantities T c and T m denote the isothermal crystallization temperature and the melting temperature during the heating cycle after isothermal crystallization using DSC. The quantity 𝛾 represents the derivative function of T m (T c ), (dT m /dT c ).40 In that study, the T m o depression as a function of mole fraction of salt concentration in PEO yields 𝛾 p values close to unity (1 + 0.45X S ). This may imply that the systems under the described experimental conditions behave nearly perfectly.


Crystalline structure XRD may provide insight into the crystalline structure of PEO. The XRD diffractograms of PEO2 and PEO4 with and without salt are illustrated in Figs 7(a) and 7(b). Pure PEO exhibits two distinct peaks at 19∘ (assigned to the (120) plane) and at 24∘ (assigned to the (032) and (112) planes), indicating a monoclinic crystalline


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Table 4. Melting point depressions and activity coefficients from Eqn (3) in the range of low salt concentration

Polymer

Melting point depression from Eqn (3) (correlation)

As determined in this study ΔT (K) = 159.8X S (0.981) PEO1 PEO2 ΔT (K) = 160.1X S (0.982) PEO3 ΔT (K) = 160.9X S (0.981) PEO4 ΔT (K) = 161.2X S (0.993) As reported by Nasir et al.21 PEO2 ΔT (K) = 68.7X S (0.991) PEO3 ΔT (K) = 103.7X S (0.999) PEO4 ΔT (K) = 115.9X S (0.989)

Activity coefficient (𝛾 p )

1 + 0.37X S 1 + 0.37X S 1 + 0.38X S 1 + 0.38X S 1 + 0.42X S 1 + 0.12X S 1 + 0.01X S

structure. Both of the peaks move slightly to lower 2𝜃 values with incorporation of salt. Based on Table 5, it can be deduced that PEO undergoes minor changes in 2𝜃 (±0.1∘ ), d-spacing (±0.1 Å) and interchain separation values (±0.1 Å) when Y S ≤ 0.15. At Y S = const, the parameters are almost unaffected by the molar mass of PEO. To sum up, the lattice structure of PEO is barely disturbed by the presence of salt up to Y S = 0.15, indicating that salt does not enter the crystalline phase. This is in agreement that SPE undergoes phase separation into pure PEO crystalline phase and amorphous phase (PEO + Li). Intermolecular interaction FTIR was used to study the intermolecular interaction between PEO and LiClO4 . The spectra of pure and salt-doped PEO2 and PEO4 are illustrated in Fig. 8. Since the characteristic vibrational modes exhibited by PEO1 , PEO2, PEO3 and PEO4 are similar in both wavenumber and shape, the spectrum of pure PEO1 is used as a reference in the following discussion (see Table 6). Figure 8(a) shows a sharp doublet at 1359 and 1341 cm−1 , the 𝜔(CH2 ) mode of PEO representing the crystalline phase of PEO. When PEO is added with Li salt, no shifting in wavenumber is observed. This agrees with the XRD results that the crystalline structure of PEO is preserved with addition of salt. The absorption bands at v(C − O − C) mode of PEO and free ClO4 − anions are often associated with the polymer − cation interaction.41 – 43 In Fig. 8(b), the v(C − O − C) absorption band of pure PEO appears as a triplet at 1144, 1094 and 1061 cm−1 . When Y S ≥ 0.10, the maximum of the C − O − C stretch which represents the PEO amorphous phase shifts down to 1079 cm−1 , causing changes in the shape of the triplet band. This observation supports the interaction between PEO and salt in the amorphous phase. Meanwhile, the two shoulders at 1144 and 1061 cm−1 corresponding to the crystalline phase of PEO do not shift, implying absence of interaction in the crystalline phase. The free ClO4 − absorption band in Fig. 8(c) was detected when salt was added to the PEO, implying dissolution of Li salt in PEO. Thus, the absence of shifting in the wavenumber of this absorption band up to Y S = 0.15 suggests that the extent of PEO − Li interaction is not influenced by the molar mass of PEO at W S = const and salt concentration at Mη = const.

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semicircle of the Nyquist plot for pure PEO1 and PEO4 is not observed. This implies that there is no percolation network in pure PEO. In the Z ′ − Z" plane of salt-added PEO, the impedance (Z) forms a semicircle with its centre and radius at Rb /2. ( )2 |2 | |Z − Rb | = Rb (4) | 2 || 2 | The semicircle of PEO1 at Y S = 0.07 estimated from Eqn (4) with Rb = 2.9 × 104 Ω was inserted in Fig. 9. It is clear that the Rb values at Y S = 0.10 are very close for both molar masses of PEO. As the quantity 𝜎 DC ∝ 1/Rb , this suggests that 𝜎 DC of PEO is of insignificant difference at W S = const for different molar masses of PEO. Figure 10 depicts double-logarithmic plots of conductivity versus LiClO4 concentration. For all molar masses of PEO, values of 𝜎 DC increase at elevated salt concentration until W S = 0.107. For samples prepared with morphologies close to equilibrium as in this study, there is no significant difference of 𝜎 DC for PEO with different molar masses at W S = const. However, for samples



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Ionic conductivity Figure 9 shows the Nyquist plots of PEO1 and PEO4 at Y S = 0, 0.07 and 0.10. For pure PEO1 and PEO4 (as well as for PEO2 and PEO3 which are not shown in Fig. 9), capacitors dominate over the entire frequency range studied. Hence, the

Figure 7. XRD diffractograms at (a) 2𝜃 = 19∘ and (b) 2𝜃 = 24∘ for PEO1 and PEO4 at salt concentrations Y S = 0, 0.05 and 0.12.

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Table 5. Parameters of the PEO crystalline peak 2𝜃 (∘ ) YS (120) plane 0 0.02 0.05 0.12 (032) and (112) plane 0 0.02 0.05 0.12

d (Å)

R (Å)

PEO1

PEO4

PEO1

PEO4

PEO1

PEO4

19.4 19.4 19.3 19.3

19.4 19.4 19.3 19.3

4.57 4.57 4.61 4.61

4.59 4.59 4.61 4.60

5.71 5.71 5.76 5.76

5.74 5.73 5.75 5.75

23.5 23.5 23.4 23.3

23.4 23.4 23.4 23.3

3.78 3.78 3.81 3.81

3.85 3.74 3.75 3.82

4.73 4.72 4.75 4.76

4.74 4.74 4.76 4.77

Figure 8. FTIR spectra of (a) 𝜔(CH2 ), (b) v(C − O − C) and (c) free ClO4 − for PEO1 and PEO4 at salt concentrations Y S = 0, 0.05 and 0.12. The dash dot curve represents the deconvoluted band corresponding to the maximum of the v(C − O − C) triplet band.

Table 6. FTIR assignment of PEO and LiClO4 Assignment PEO 43,44 𝜌(CH2 ) v(C − O − C) 𝜔(CH2 ) 𝛿 as (CH2 ) LiClO4 42,43 Free ClO4 − Bound ClO4 −

Wavenumber (cm−1 )

842, 947, 961 1061, 1094, 1144 1341, 1359 1455, 1474 622 635

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prepared with non-equilibrium morphologies as described by Nasir et al.21 , the influence of molar mass on 𝜎 DC at W S = const was demonstrated, especially at high salt concentration. Linear regression after Eqn (2) allows estimation of the exponent x, mobility 𝜇𝛼 and diffusion coefficient of charge carriers D, as illustrated in Fig. 10. The results of these parameters are listed in Table 7. The mobilities and diffusion coefficient of the charge carriers in PEO do not depend on the molar mass of PEO if the morphologies of the systems are close to equilibrium. For systems with non-equilibrium morphologies, the diffusion coefficient increases by two orders of magnitude for PEO with Mη = 4 × 106 g mol−1 compared to PEO with Mη = 6 × 105 g mol−1 .


Figure 9. Nyquist plots of PEO1 (solid marker) and PEO4 (open marker) at Y S = 0, 0.07 and 0.10. The solid curve is the semicircle fitting of PEO1 at Y S = 0.07, estimated from Eqn (4).

CONCLUSIONS SPEs of PEO and LiClO4 salt were prepared by the solution casting method where the morphologies developed are close to equilibrium under the experimental conditions. The DSC results reveal insignificant variation of properties with respect to the molar mass


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Table 7. Mobilities 𝜇𝛼, exponent x and diffusion coefficients D from Eqn (2) for PEO/LiClO4 systems Polymer

Regression function (S cm−1 )

As estimated in this study 𝜎 DC = 5.74 × 10−5 Y S 1.73 PEO1 PEO2 𝜎 DC = 9.10 × 10−5 Y S 1.97 PEO3 𝜎 DC = 9.37 × 10−5 Y S 1.94 PEO4 𝜎 DC = 1.19 × 10−4 Y S 1.99 As reported by Nasir et al.21 PEO2 𝜎 DC = 4.09 × 10−3 Y S 3.07 PEO3 𝜎 DC = 2.83 × 10−2 Y S 3.45 PEO4 𝜎 DC = 1.10 Y S 4.30

Correlation

𝛼𝜇 (cm2 V−1 s−1 )

x

D (cm2 s−1 )

0.99 0.99 0.99 0.99

5.3 × 10−8 8.4 × 10−8 8.6 × 10−8 1.1 × 10−7

1.73 1.97 1.94 1.99

1.4 × 10−9 2.2 × 10−9 2.2 × 10−9 2.9 × 10−9

0.99 0.99 0.98

3.8 × 10−6 2.6 × 10−5 1.0 × 10−3

3.07 3.45 4.30

9.9 × 10−8 7.0 × 10−7 2.6 × 10−5

REFERENCES

Figure 10. Ionic conductivity of PEO2 and PEO4 against LiClO4 concentration obtained from this study and data from Nasir et al.21 The solid and dashed curves represent the linear regression of PEO2 after Eqn (2).

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ACKNOWLEDGEMENTS

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The authors would like to acknowledge financial support from the Ministry of Education, Malaysia, for Research Acculturation Grant Scheme grant RAGS/1/2014/ST05/UITM/1 and MyPhD scholarship.

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Polym Int 2017; 66: 830–838