**5. Results and discussion**

#### **5.1 Ac-impedance spectroscopy**

The bulk ionic conductivity of polymer electrolytes is determined by using the equation as shown below.

$$
\sigma = \frac{l}{R\_{\text{b}} \, ^{\text{A}}}
$$

where � is the thickness (cm), �� is bulk resistance (Ω) and *A* is the known surface area (cm2) of polymer electrolyte films. The semicircle fitting was accomplished to obtain �� value. The �� of the polymer electrolyte is calculated from the interception of high–frequency depressed semicircle with low–frequency spike.

Figure 5 depicts the logarithm of ionic conductivity with respect to PC mass fraction. As can be seen, the ionic conductivity increases with PC mass loadings, up to 10 wt% of PC. The optimum ionic conductivity of 5.10×10-7 Scm-1 is achieved with this mass fraction of PC. Plasticizing effect is the main attributor for this phenomenon. This effect would weaken the dipole–dipole interactions in the polymer chains and reduce the solvation of Li cations (Li+) by polymer matrix. Hence, it promotes the ionic decoupling and enhances the dynamic free volume of the polymer system and thereby increases the ionic conductivity. It suggests that the plasticizer is not only weakening the polymer–polymer chain interactions, but also decreasing the dipole–ion interactions in the dopant salt. For the dipole–dipole interactions within the polymer chains, the hydrogen atom from the methyl group of PC may interact with the chloride anions in the polymer backbone. On the contrary, it proposes that the hydrogen from the methyl group of PC would weaken the O–Li bond of the Li2B4O7. As a result, it promotes the dissociation of lithium cations from the bonding and hence favors the ionic transportation within the polymer matrix, improving the ionic conductivity.

In addition, the plasticizing effect lowers the ��. Thus, it softens the polymer backbone and increases the segmental mobility when an electric field is applied onto the polymer electrolytes. Consequently, it disrupts the crystalline phase of polymer side chains and produces voids, which enables the easy flow of ions through polymer membrane when the

TGA is a versatile thermal study in polymer field. It is primarily used to determine thermal stability and thermal degradation of the samples as a function of change in temperature under inert conditions. The main principle of TGA is to monitor the weight of the samples on a sensitive balance (also known as thermobalance) continuously as the sample temperature is increased, under an inert atmosphere or air at a controlled uniform rate. The data were recorded as a thermogram of weight which is in y–axis against sample

The thermal stability of polymer films was performed by Mettler Toledo Thermal Gravimetric Analyser which comprised of TGA/SDTA851@ as main unit and STARe software. Sample weighing 2–3 mg placed into 150 μl of silica crucible. The samples were then heated from 30 °C to 400 °C at a heating rate of 10 °C min-1 under nitrogen flow rate of

The bulk ionic conductivity of polymer electrolytes is determined by using the equation as

� �� �

� �

where � is the thickness (cm), �� is bulk resistance (Ω) and *A* is the known surface area (cm2) of polymer electrolyte films. The semicircle fitting was accomplished to obtain �� value. The �� of the polymer electrolyte is calculated from the interception of high–frequency

Figure 5 depicts the logarithm of ionic conductivity with respect to PC mass fraction. As can be seen, the ionic conductivity increases with PC mass loadings, up to 10 wt% of PC. The optimum ionic conductivity of 5.10×10-7 Scm-1 is achieved with this mass fraction of PC. Plasticizing effect is the main attributor for this phenomenon. This effect would weaken the dipole–dipole interactions in the polymer chains and reduce the solvation of Li cations (Li+) by polymer matrix. Hence, it promotes the ionic decoupling and enhances the dynamic free volume of the polymer system and thereby increases the ionic conductivity. It suggests that the plasticizer is not only weakening the polymer–polymer chain interactions, but also decreasing the dipole–ion interactions in the dopant salt. For the dipole–dipole interactions within the polymer chains, the hydrogen atom from the methyl group of PC may interact with the chloride anions in the polymer backbone. On the contrary, it proposes that the hydrogen from the methyl group of PC would weaken the O–Li bond of the Li2B4O7. As a result, it promotes the dissociation of lithium cations from the bonding and hence favors the

ionic transportation within the polymer matrix, improving the ionic conductivity.

In addition, the plasticizing effect lowers the ��. Thus, it softens the polymer backbone and increases the segmental mobility when an electric field is applied onto the polymer electrolytes. Consequently, it disrupts the crystalline phase of polymer side chains and produces voids, which enables the easy flow of ions through polymer membrane when the

**4.3.3 Thermogravimetry Analysis (TGA)** 

temperature which is in x–axis.

**5. Results and discussion 5.1 Ac-impedance spectroscopy** 

depressed semicircle with low–frequency spike.

10 ml min-1.

shown below.

electric field is applied. The ionic conductivity is eventually enhanced with these higher amorphous and more flexible polymer chains. The increase in ionic conductivity is also owing to the high dielectric constant of PC. High dielectric constant could allow the greater dissolution of Li2B4O7 and offers a result in increasing the number of charge carriers, promoting the ionic hopping mechanism. The ionic conductivity is reduced to 4.20×10-8 Scm-1 with increasing the PC mass loadings further. This is suggestive of the decrease in effective number of charge carriers for ionic transportation as a result of the domain of short–range ion–plasticizer interactions within the polymer matrix (Stephan et al., 2002). Hence, the ionic conductivity is lower than of other plasticized–polymer electrolytes because of the reduced amount of lithium cations.

Fig. 5. The variation of logarithm of ionic conductivity of plasticized–based polymer electrolytes as a function of weight percentage of PC at ambient temperature.

Upon addition of 20 wt% of PC, the ionic conductivity is rising up further to the maximum level of 4.12×10-6 Scm-1 at room temperature. Again, the contribution from plasticizer is the main attributor for this enhancement of ionic conductivity. The incorporation of plasticizer increases the ionic conductivity through two ways. High plasticizer concentration would open up the narrow rivulets of plasticizer–rich region and lead to greater ionic migration. Moreover, it provides a large free volume of a relatively superior conducting region by reducing the crystalline degree of the polymer electrolytes (Rhoo et al., 1997; Stephan et al., 2000b). General expression of ionic conductivity of a homogenous polymer electrolyte is illustrated as below:

$$\sigma(T) = \sum\_{l} n\_{l} q\_{l} \mu\_{l}$$

where �� is the number of charge carriers type of *i*, �� is the charge of ions type of type of *i*, and �� is the mobility of ions type of *i.* Based on the equation above, the quantity and mobility of charge carriers are the main factors that could affect the ionic conductivity of

Characterization of High Molecular Weight Poly(vinyl chloride) –

Fig. 6. Arrhenius plot of SPC5 in the temperature range of 298–373 K.

The FTIR spectra and description of vibration modes of pure PVC, pure Li2B4O7, SPC1, PC and SPC5 are shown in Figures 7(a)–(e) and Table 2, respectively. Comparing SPC1 with pure PVC, there are 10 new peaks have been formed. All of these new peaks are the characteristic bonds of Li2B4O7. Five new peaks have been detected in the wavenumber range of 1200 cm-1–700 cm-1. These peaks are assigned as B–O(B) stretching mode of BO4 tetrahedral shape of Li2B4O7 at 710 cm-1, 815 cm-1, 905 cm-1, 1034 cm-1 and 1120 cm-1. In contrast, for B–O(B) stretching mode of BO3 triangle shape, Li2B4O7 portrays two characteristic peaks at 1246 cm-1 and 1376 cm-1. However, only one peak is observed at 1253 cm-1 for SPC1. This indicates the interaction between PVC and Li2B4O7 and further reveals the decoupling of Li+ from the B–O(B) coordinative bonds. Four new peaks at 451 cm-1, 504 cm-1, 565 cm-1 and 670 cm-1 are designated as O–B–O deformation mode of BO4 tetrahedral in Li2B4O7. All the vibration modes exhibit peak shifting, except the weak peak at 1331 cm-1

2.6 2.8 3 3.2 3.4

**1000/T(K-1)** 

As shown in Figure 7(a), the transmittance peaks at 616 cm-1 and 969 cm-1 are corresponding to cis and trans C–H wagging modes, respectively. Upon addition of Li2B4O7, these peaks are shifted towards higher wavenumber to 637 cm-1 and 973 cm-1, respectively. Apart from that, they exhibit changes in shape. For cis wagging mode, it has been changed from weak peak to shoulder peak, whereas a medium peak has been changed to a broad band for trans wagging mode. A sharp peak is observed at 1067 cm-1 in Figure 7(a), which designated as C– H rocking mode of PVC. However, it turns to a broad band with inclusion of Li2B4O7 and manifests a downward shift to 1062 cm-1. The peak at 833 which corresponds to C–Cl

**5.3 Fourier Transform Infrared (FTIR) studies** 





**log [**

*σ*

**(Scm-1)]** 





which is denoted as CH2 deformation of PVC.

Lithium Tetraborate Electrolyte Plasticized by Propylene Carbonate 179

polymer electrolytes as the charge of the mobile charge carriers are negligible. Therefore, it can be concluded that the mobility and concentration of mobile charge carriers have been optimized in SPC5 as it achieves the highest ionic conductivity compared to other polymer complexes. However, the ionic conductivity is drastically declined with increasing the PC concentration further. It is ascribed to the restricted ionic and segmental mobility of mobile charge carrier in a rigid polymer matrix (Cha et al., 2004).

#### **5.2 Temperature dependence-ionic conductivity studies**

The temperature dependence study of ionic conductivity is further investigated in order to understand the mechanism of ionic conduction in this plasticized–polymer electrolyte. SPC5 is chosen as it achieves the highest ionic conductivity. Figure 6 illustrates the logarithm of ionic conductivity against reciprocal absolute temperature of SPC5, from ambient temperature to 373 K. As expected, the ionic conductivity increases with temperature. Polymer expansion effect plays an important role in this phenomenon. The polymer matrix expands with temperature, which in turn to the formation of local empty spaces and voids for the segmental migration. Therefore, it facilitates the migration of ions and diminishes the ion clouds effect between the electrodes and electrolyte interface (Ramesh et al., 2010). The enhancement of charge carriers and segmental motions could assist the ionic transportation and compensate for the retarding effect of the ion cloud virtually, inducing to higher ionic conductivity.

A linear relationship is perceived in the figure with regression value of 0.99. Therefore, it can be concluded that SPC5 follows Arrhenius rules as its regression value is close to unity. In this thermally activated principle, the conductivity is expressed as below:

$$
\sigma = A \exp\left(\frac{-E\_\mathrm{a}}{kT}\right)
$$

where � is the pre–exponential factor which is proportional to the amount of charge carriers, �� is the activation energy, � is Boltzmann constant and � is the absolute temperature. The Arrhenius relationship indicates the presence of the hopping mechanism. This theory states the ion jumps from its normal position on the lattice to an adjacent equivalent but empty site. As the temperature increases, the vibrational modes of polymer segments are also increased. Thus, it weakens the interaction between the polar group of the polymer backbone and Li+, and promotes the decoupling process of charge carriers from the segmental motion of polymer matrix, leading to formation of vacant sites in the polymer chain. Hence, the neighboring ions from adjacent sites tend to occupy these vacant sites and coordinate with the polymer chain again. Eventually, the ionic hopping mechanism is generated. In this study, it implies that the methyl group from PC and the C–H group from PVC could weaken O–Li coordinative bond of Li2B4O7 through the hydrogen bonding. As a result, it initiates the decoupling of Li+ from the bond and therefore generates the ionic hopping process. In order to probe the ion dynamic of polymer electrolytes further, activation energy (��) is determined by fitting it in Arrhenius equation as shown above. �� is defined as the energy required to overcome the reorganization and reformation of the polymer chain with Li+. Based on the calculation, the �� of SPC5 is 0.08eV. This activation energy is considered low. Therefore, it can be concluded that Li+ would break and re–bind the coordination bond easily with lower energy barrier.

polymer electrolytes as the charge of the mobile charge carriers are negligible. Therefore, it can be concluded that the mobility and concentration of mobile charge carriers have been optimized in SPC5 as it achieves the highest ionic conductivity compared to other polymer complexes. However, the ionic conductivity is drastically declined with increasing the PC concentration further. It is ascribed to the restricted ionic and segmental mobility of mobile

The temperature dependence study of ionic conductivity is further investigated in order to understand the mechanism of ionic conduction in this plasticized–polymer electrolyte. SPC5 is chosen as it achieves the highest ionic conductivity. Figure 6 illustrates the logarithm of ionic conductivity against reciprocal absolute temperature of SPC5, from ambient temperature to 373 K. As expected, the ionic conductivity increases with temperature. Polymer expansion effect plays an important role in this phenomenon. The polymer matrix expands with temperature, which in turn to the formation of local empty spaces and voids for the segmental migration. Therefore, it facilitates the migration of ions and diminishes the ion clouds effect between the electrodes and electrolyte interface (Ramesh et al., 2010). The enhancement of charge carriers and segmental motions could assist the ionic transportation and compensate for the retarding effect of the ion cloud virtually, inducing to higher ionic

A linear relationship is perceived in the figure with regression value of 0.99. Therefore, it can be concluded that SPC5 follows Arrhenius rules as its regression value is close to unity.

� � � ��� �

where � is the pre–exponential factor which is proportional to the amount of charge carriers, �� is the activation energy, � is Boltzmann constant and � is the absolute temperature. The Arrhenius relationship indicates the presence of the hopping mechanism. This theory states the ion jumps from its normal position on the lattice to an adjacent equivalent but empty site. As the temperature increases, the vibrational modes of polymer segments are also increased. Thus, it weakens the interaction between the polar group of the polymer backbone and Li+, and promotes the decoupling process of charge carriers from the segmental motion of polymer matrix, leading to formation of vacant sites in the polymer chain. Hence, the neighboring ions from adjacent sites tend to occupy these vacant sites and coordinate with the polymer chain again. Eventually, the ionic hopping mechanism is generated. In this study, it implies that the methyl group from PC and the C–H group from PVC could weaken O–Li coordinative bond of Li2B4O7 through the hydrogen bonding. As a result, it initiates the decoupling of Li+ from the bond and therefore generates the ionic hopping process. In order to probe the ion dynamic of polymer electrolytes further, activation energy (��) is determined by fitting it in Arrhenius equation as shown above. �� is defined as the energy required to overcome the reorganization and reformation of the polymer chain with Li+. Based on the calculation, the �� of SPC5 is 0.08eV. This activation energy is considered low. Therefore, it can be concluded that Li+ would break and re–bind

��� �� �

In this thermally activated principle, the conductivity is expressed as below:

the coordination bond easily with lower energy barrier.

charge carrier in a rigid polymer matrix (Cha et al., 2004).

**5.2 Temperature dependence-ionic conductivity studies** 

conductivity.

Fig. 6. Arrhenius plot of SPC5 in the temperature range of 298–373 K.
