Dielectric behaviour of cellulose acetate-based polymer electrolytes
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- Harun, N.I., Ali, R.M., Ali, A.M.M. et al. Ionics (2012) 18: 599. doi:10.1007/s11581-011-0653-0
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The present work deals with the findings on the dielectric behaviour of cellulose acetate (CA) and its complexes consisting of ammonium tetrafluoroborate (NH4BF4) and polyethylene glycol with a molecular weight of 600 g/mol (PEG600) that were prepared using the solution casting method. The highest σ obtained for CA-NH4BF4 film was 2.18 × 10−7 S cm−1 and enhanced to 1.41 × 10−5 S cm−1 with the addition of 30 wt.% PEG600. The dielectric behaviours of the selected samples were analyzed using complex impedance Z*, complex admittance A*, complex permittivity ɛ*, and complex electric modulus M*-based frequency and temperature dependence in the range of 10 Hz–1 MHz and 303–363 K, respectively. The variation in dielectric permittivity (εr and εi) as a function of frequency at different temperatures exhibits a dispersive behaviour at low frequencies and decays at higher frequencies. The variation in dielectric permittivity as a function of temperature at different frequencies is typical of polar dielectrics in which the orientation of dipoles is facilitated with the rising temperature, and thereby the permittivity is increased. Modulus analysis was also performed to understand the mechanism of electrical transport process, whereas relaxation time was determined from the variation in loss tangent with temperature at different frequencies.
KeywordsCellulose acetateDielectric permittivityElectrical modulusLoss tangent
Solid polymer electrolytes (SPEs) have been proved to be prospective candidates for advanced electrochemical device applications because of their characteristics such as viscoelasticity and flexibility, as well as high ionic conductivity. Interest in these materials grew mainly because of the pioneering measurements of the ionic conductivity of polymer salt complexes reported by Wright et al. and the development of ionic conductivity of polymer salt complexes by Armand et al. . Aside from poly(ethylene oxide) —the most frequently used polyether—a few types of biopolymers such as chitosan [3–5] and cellulose with its derivatives [6–10] are also used as polymer matrixes in developing various SPEs. Both chitosan and cellulose films are homogenous with high mechanical strength . However, most biopolymer films have very low electrical conductivity at ambient temperature in their actual state. The salt-solvating power and the sufficient mobility of ions necessary for ionic conduction are imparted by incorporating plasticizer .
Although such systems have evolved a great deal, understanding the ion transport behaviour of these polymer electrolytes is important. Dielectric analysis is an informative technique used to determine the molecular motions and structural relaxations present in polymeric materials possessing permanent dipole moments . In dielectric measurements, the material is exposed to an alternating electric field generated by applying a sinusoidal voltage. This process causes alignment of dipoles in the material, resulting in polarization. The capacitance and the conductance of the material are measured over a range of temperature and frequency and are related to the dielectric constant (εr) and dielectric loss (εi), respectively. εr represents the amount of dipole alignment (both induced and permanent), and εi measures the energy required to align dipoles or move ions. A further analysis of the dielectric behaviour would be more successfully achieved using electric modulus formalism, which is used to suppress the signal intensity associated with electrode polarization. Thus, the electric modulus spectra provide an opportunity to investigate conductivity and its associated relaxation in ionic conductors and polymers.
In this work, SPEs based on cellulose acetate (CA) as host polymer complexes with ammonium tetrafluoroborate (NH4BF4) as doping salt and polyethylene glycol with a molecular weight of 600 g/mol (PEG600) as plasticizer were prepared. Their conductive performances were evaluated using an electrical impedance spectroscopy (IS) instrument.
CA with an acetyl content of 39.8 wt.% (Aldrich), NH4BF4 (Fluka), PEG600 (Fluka), and acetone (Aldrich) were used in this study.
The polymer electrolytes comprising CA as a host polymer, NH4BF4 as a doping salt, and PEG600 as a plasticizer were prepared by the solution casting technique using acetone as solvent. CA (1 g) was dissolved in 30 mL acetone for several hours to obtain the homogenous solution. Subsequently, 5–50 wt.% NH4BF4 and 5–40 wt.% PEG600 were added into the solution. After complete dissolution of the complexes, the solutions were casted in Petri dishes and left to dry at room temperature (~ 30 °C) to form thin films of (a) CA-NH4BF4 and (b) CA-NH4BF4-PEG600. The films were then placed in dry cabinet for further drying before they were used.
Results and discussion
Ionic conductivity at 303 K
Frequency dependence of dielectric behaviour
Dielectric permittivity, εr, and εi
The dielectric constant (εr) as a function of frequency at different temperatures for the highest conducting unplasticized sample—CA-25 wt.% NH4BF4 (A6)—and the highest conducting plasticized sample—CA-25 wt.%–30 wt.% PEG600 (B6)—is given in Fig. 3 and its inset, respectively. The value of εr for B6 is relatively higher than that of the unplasticized sample A6. This observation is also true for the conductivity. The addition of plasticizer is expected to increase the degree of salt dissociation, which alternatively increases the ionic mobility by reducing the potential barrier to ionic motion, resulting in decreased anion–cation coordination of the salt and also a more favourable chain flexibility motion of the polymer host.
The decay of εr at higher frequencies suggests that less ionic polarization occurred in the bulk. At high-frequency regimes, less excess ion accumulating at the electrode–electrolyte interface than that in the bulk is found, resulting in low dielectric constant. This result again reflects on the space charge and electrode polarization effect. At such frequencies, the periodic reversal of the electric field occurs so fast that no excess ion diffusion in the direction of the field is found. Hence, εr decreases with the increase in frequency [1, 19].
At low frequencies (see Fig. 3), εi has great value because of the space charge polarization where there is enough time for the charges to build up at the interface before the applied field changes direction contributing to the large apparent value of εi. In contrast, no time is provided at high frequencies due to increasing rate of electric field to change direction. Consequently the decrease of the space charge polarization leads to decrease in εi value [1, 3].
Modulus formalism, Mr and Mi
The value of Mr is very low and approaching zero in the low-frequency region. As frequency increases (see Fig. 4), the value of Mr increases and reaches a maximum constant value of M∞ = 1/ε∞ at higher frequencies for all temperatures. These observations may be related to a lack of restoring force governing the mobility of charge carriers under the action of an induced electric field. This type of behaviour supports the conduction phenomena because of the long-range mobility of charge carriers .
Mi peaks are not found in the plasticized system, whereas they appear in the unplasticized system (Fig. 5). The peak positions in A6 are shifted toward higher frequencies with increasing temperature. The possible presence of peaks in the modulus formalism at higher frequencies for all the polymer system and temperature indicates that the polymer electrolyte films are ionic conductors . The constancy of the height of the modulus plot suggests the invariance of the dielectric constant and the distribution of relaxation times with temperature. Beyond 333 K, no appearance of relaxation peak is found. The relaxation peaks are expected to be displaced toward higher frequencies and so are not observed in this plot because of the frequencies being in the range exceeding that permitted by the instrument used in the present study.
Temperature dependence of relaxation frequency (fr) and relaxation time (τ) of A6 and B6
(Tan δ) max
4.57 × 103
9.12 × 104
3.48 × 10−5
1.75 × 10−6
1.20 × 104
1.82 × 105
1.33 × 10−5
8.74 × 10−7
2.40 × 104
4.37 × 105
6.63 × 10−6
3.64 × 10−7
3.31 × 104
4.81 × 10−6
3.39 × 104
4.69 × 10−6
4.27 × 104
3.73 × 10−6
7.94 × 104
2.00 × 10−6
Temperature dependence of dielectric behaviour
Dielectric permittivity, εr, and εi
The behaviour of εr with temperature can be explained as follows: at relatively low temperature, the charge carriers on most cases cannot orient themselves with respect to the direction of the applied field. Therefore, they possess a weak contribution to the polarization and εr. As temperature increases, the bound charge carriers obtain enough excitation thermal energy to be able to obey the change in the external field more easily. This effect in turn enhances their contribution to the polarization, leading to an increase in εr of the sample .
Modulus formalism, Mr and Mi
The frequency and temperature dependence of dielectric behaviour for the highest conducting unplasticized sample—CA-25 wt.% NH4BF4 (A6)—and the highest conducting plasticized sample—CA-25 wt.%–30 wt.% PEG600 (B6)—were studied. The frequency-dependent εr and εi show the presence of electrode polarization phenomena at lower frequencies and decay at higher frequencies. The modulus formalism approaching zero at low frequency and reaching maximum constant value at high frequency supports the conduction phenomena because of the long-range mobility of charge carriers. The behaviour of εr and εi with temperature is typical of polar dielectrics in which the orientation of dipoles is facilitated with the rising temperature, and thereby the permittivity is increased. The complex impedance plots (Zi vs. Zr) support the fact that the increase in temperature causes both Mr and Mi to decrease. The nature of variation in tan δ as a function of frequency and temperature indicates the presence of dielectric relaxation in the material.
N.I. Harun would like to thank the Universiti Teknologi MARA for the scholarship (NSF) awarded.