Thermal Analysis
The thermal effect of glycerin on a plasticized polymer electrolyte is investigated using differential scanning calorimetry (DSC). In Fig. 1, the DSC curves of pure PVA, PVA + 20wt% CuSO4, and PVA + 20wt% CuSO4 plasticized by 1, 2 and 3 mL glycerin are presented. An endothermic peak corresponding to the melting temperature (Tm) and the glass transition temperature (Tg) of pure PVA is discovered at 73.12°C and 66.02°C, respectively. When 20 wt% of CuSO4 salt was added to the polymer matrix, the melting temperature (Tm), glass transition temperature (Tg), and degree of crystallinity (χc) were all increased.
The glass transition temperature (Tg) moved towards the lower temperature side when different amounts of glycerin were added to the prepared electrolyte membrane PVA + 20wt% CuSO4 polymer electrolyte. The decrease in Tg of the polymer electrolyte with increased glycerin content suggests a weaker intermolecular connection between the glycerin, CuSO4 salt, and PVA, allowing the polymer network to move more segmentally by making the polymer matrix more flexible [12, 13]. Glycerin's plasticizing action produces a reduction in the Tm and Tg of polymer gel electrolyte membranes when it is added. The degree of crystallinity (χc) and melting temperature increased when 20 wt% of CuSO4 salt was added to the polymer matrix (Tm) were initially elevated, but this increased crystallinity and melting temperature was successfully controlled by the addition of glycerin and achieved the lowest value (approximately 11.65%) for the membrane contains a higher amount of glycerin (3mL).
The DSC parameters are presented in Table 2. The relative percentage of crystallinity (χc) has been calculated with the Eq. (1).
$$\chi = \frac{{\varDelta H}_{f} }{{\varDelta H}_{fo}} \times 100\dots \dots \dots \dots \dots \left(1\right)$$
Where ∆Hfo= 2.65 (J/g). The calculated heat of fusion (Hf), melting temperature (Tm), and percentage crystallinity (χc) values are shown in Table 2.
Table 2
DSC parameters of pure PVA, PVA + 20wt.% of CuSO4 and PVA + 20wt.% of CuSO4 plasticized by 1, 2 and 3 mL of glycerin.
samples | Melting temperature (°C) | Glass transition temperature (°C) | ∆Hf (J/g) | percentage of crystallinity (𝜒c%) |
Pure PVA | 73.12 | 66.02 | 85.23 | 32.12 |
PVA + 20wt.%CuSO4 | 74.24 | 67.34 | 87.34 | 32.95 |
PVA + 20wt.%CuSO4/1 mL glycerin | 64.78 | 53.12 | 50.12 | 18.91 |
PVA + 20wt.%CuSO4/2 mL glycerin | 59.89 | 42.62 | 34.22 | 12.91 |
PVA + 20wt.%CuSO4/3 mL glycerin | 54.34 | 37.22 | 30.89 | 11.65 |
The heat of fusion (Hf), melting temperature (Tm), and percentage of crystallinity all decreased when plasticized with 1, 2 and 3 mL of glycerin. This is owing to a decrease in PVA polymer electrolyte crystallinity, which is a well-known favorable condition for boosting ionic conductivity.
TGA thermographs of pure PVA, PVA + 20 wt% CuSO4, and PVA + 20 wt% CuSO4 plasticized by 1, 2, and 3 mL glycerin gel polymer electrolytes are shown in Fig. 2. The plot clearly shows two stages of weight loss, the first of which is a 5% weight loss at 54.3 oC and the second of which is a maximum weight loss at 285.8 oC for pure PVA and PVA + 20 wt% CuSO4, both of which can be attributed to evaporation of water and degradation of PVA by the polymer chain's dehydration reaction [13]. Water absorption, heat degradation of functional groups, and thermal oxidation of the polymer backbone are the processes in which the PVA + 20wt% CuSO4/Glycerin gel polymer electrolyte loses weight [14].
All membranes lose weight before reaching 50 oC in the first phase, which can be attributed to the evaporation of bound water in the samples. The release of the quaternary ammonium group's degraded product causes weight loss in the second phase in the range 124 to 180 oC [12]. Weight loss in the third phase at 200–230 oC is assumed to be caused by the release of residual quaternary ammonium group. At 270–300 oC, the fourth and final weight loss was discovered, which was caused by polymer chain breakdown [12, 14].
The weight loss of the polymer electrolyte increases as the glycerin concentration rises, which is due to scission monomers and bonds in the polymeric backbone cracking and loss of dopant due to heat energy [15]. The decomposition of organic polymer chains, both the hard segment of PVA linkage and the soft segment from glycerin, was attributed to the decomposition of plasticizer with polymer [14, 16].
The DTG thermograms of pure PVA, PVA + 20wt% CuSO4, and PVA + 20wt% CuSO4 plasticized by 1, 2, and 3 mL glycerin are shown in Fig. 3. The maximal decomposition temperature of PVA is 283.4 oC, and this temperature drops when 20 wt% of CuSO4 is added and the plasticizer concentration is increased from 1 to 3 mL. For PVA + 20 wt.% CuSO4 had a Tmax of 272°C, which dropped to 261°C for the electrolyte PVA + 20 wt.% CuSO4/3mL glycerin. This phenomenon has been linked to the low Tg value of the plasticized gel polymer electrolyte. The dipole-dipole interaction of polymer chains was reduced when glycerin was added, because it softens the polymer chain's backbone and lowers the polymer's Tg. The same pattern was found by Abdulkadir et al., 2020 [17].
Figure 4 shows the results of separating CuSO4 salt to cations and anions in PVA GPE. The GPEs were created by injecting conducting salt (CuSO4) into the polymer host (PVA) and plasticizing it with glycerin, as described in the experimental section. PVA and glycerin with hydroxyl or polar groups (-OH) produced a covalent dative bond with the CuSO4 cations in the electrolytes [17, 18]. This is because polymer materials (PVA) and plasticizers (glycerin) have -OH groups in their macromolecular chains and three-dimensional networks that can react with various inorganic salts [19, 20]. The presence of polar -OH groups enable chemical (complexing processes) and physical connections (via H bonding, Van der Waals dipole–ion interactions, or dipole–dipole interactions) [21]. CuSO4 dissociates into cations (Cu+ 2) and anions (SO4 − 2) when dissolved in solvent is shown in Eq. 2.
$$CuS{O}_{4}\to {Cu}^{+2}+S{{O}_{4}}^{-2}\dots \dots \dots \dots ..\left(2\right)$$
The cations formed when coordination with the polar groups (-OH) of the plasticized polymer matrix, resulting in a complex molecule, as shown in Fig. 4. Because of the interaction between the plasticized polymer polar groups and the cations from the salts, there are more ion-conducting sites and a better interfacial contact, resulting in a higher ionic conductivity for the electrolyte [19, 22]. Metal ions from salts interact with each other and polar groups (-OH) of polymers by electrostatic interactions, resulting in the formation of coordinating bonds [17, 23]. The type of functional groups attached to the polymer backbone, their compositions and distances between them, molecular weight, branching degree, metal cation nature and charge, and counter ions are all important factors that might affect polymer–metal ion interactions [20, 24].
The dielectric constant decreases with increasing frequency and reaches a stable value at high frequencies (100 kHz), as shown in Figs. 5 and 6. A rapid decline in dielectric constant can be seen throughout a frequency range of 1 to 100 kHz, because the charge carriers do not have enough time to orient themselves in the field direction. The periodic reversal of the field occurs so quickly at very high frequencies, resulting in the frequency-independent ε′ behaviour observed [21, 25]. The ions are capable of migrating in the direction of the electric field, but due to the blocking electrodes in the low frequency area, they are unable to reach the external circuit, resulting in a dispersion with large ε′ values. As a result, ions become trapped along the electrode–electrolyte contact, forming an electrode polarization layer [ 22,26]. This indicates that electrode polarization and space charge effects are dominant in the low frequency region.
The dielectric constant of the system increases when 20 wt% CuSO4 is added to pure PVA. This could be due to a high dielectric constant combined with a strong dissociation capacity to avoid the formation of ion pairs or a high efficiency in shielding the interionic columbic attraction between cations and anions, resulting in a high dielectric constant [23, 27].
The effect of glycerin concentration on ε′ of the gel polymer electrolytes was investigated in the frequency range from 1 to 100 kHz is shown in Figs. 5 and 6. The dielectric constant ε′ decrease with increasing glycerin content for pure PVA and PVA + 20wt.% CuSO4 in the above frequency range. 5 and 6. It achieves a high value at 3 mL glycerin concentration. This type of behaviour has also been observed in electrical conductivity investigations (see Table 3). The decrease in ε′ is due to a reduction in mobile charge carrier density. The addition of plasticizer to the polymer salt system introduces additional ions, lowering the density of charge carriers and thereby lowering the gel polymer electrolyte system's dielectric constant [21].
We can see from Table 3 that when pure PVA and PVA + 20wt.% CuSO4 are exposed to an electric field, the cations from the salts can migrate from one coordinated site to another. This is due to the weak coordinates of the cations with sites along the polymer chain [16]. According to prior research [21], ions, primarily cations, connected to functional groups of the host polymer chains can move along the polymer backbone by recoordination. After that, the polymer chains are folded to form tunnels in which the functional groups locate and coordinate the cations. Cations can readily flow via these tunnels, which form channels [14, 23].
Conducting salts have also been found to reduce the number of active centers in polymer chains, diminishing intermolecular and intramolecular interactions [14]. As a result, the stiffness of the host polymer will be lowered, and the mechanical and thermomechanical properties of the polymer will be altered [18, 23]. Similarly, introducing high conducting salts reduces the glass transition temperature (Tg) of the polymer system (as would be stated in DSC data) [14, 28]. As a result, crystallinity will decrease and salt dissociation capacity will increase, resulting in enhanced charge carrier transport and greater ionic conductivity [22, 28].
An increase in glycerin concentration, which results in the formation of a complex between the polymer matrix and the conducting salt (PVA + CuSO4), would raise entropy, which will improve the composite's segmental motion. Reduced crystallinity (greater flexibility) and enhanced electrolyte ionic conductivity will result from increased segmental motion [23, 30].
Table 3
ionic conductivity of pure PVA, PVA + 20wt.%CuSO4 and PVA + 20wt.%CuSO4 plasticized by 1, 2 and 3 mL glycerin.
| | Pure PVA | PVA + 20wt.%CuSO4 |
Ionic conductivity (S/cm) | Frequency (kHz) | 0 mL glycerin | 1 mL glycerin | 2 mL glycerin | 3 mL glycerin | 0 mL glycerin | 1 mL glycerin | 2 mL glycerin | 3 mL glycerin |
1 | 7.43×10ˉ⁸ | 3.48×10ˉ⁶ | 5.46×10ˉ⁵ | 1.83×10ˉ⁵ | 1.38×10ˉ⁸ | 2.02×10ˉ⁴ | 2.24×10ˉ⁴ | 1.57×10ˉ⁴ |
10 | 3.39×10ˉ⁷ | 4.68×10ˉ⁶ | 7.87×10ˉ⁵ | 2.31×10ˉ⁵ | 1.17×10ˉ⁷ | 3.93×10ˉ⁴ | 4.54×10ˉ⁴ | 4.95×10ˉ⁴ |
20 | 6.48×10ˉ⁷ | 4.87×10ˉ⁶ | 8.48×10ˉ⁵ | 2.42×10ˉ⁵ | 2.27×10ˉ⁷ | 4.55×10ˉ⁴ | 5.48×10ˉ⁴ | 6.25×10ˉ⁴ |
30 | 1.01×10ˉ⁶ | 4.98×10ˉ⁶ | 8.77×10ˉ⁵ | 2.48×10ˉ⁵ | 3.59×10ˉ⁷ | 4.94×10ˉ⁴ | 5.92×10ˉ⁴ | 7.08×10ˉ⁴ |
40 | 1.16×10ˉ⁶ | 5.07×10ˉ⁶ | 8.96×10ˉ⁵ | 2.53×10ˉ⁵ | 4.66×10ˉ⁷ | 5.21×10ˉ⁴ | 6.31×10ˉ⁴ | 7.68×10ˉ⁴ |
50 | 1.29×10ˉ⁶ | 5.16×10ˉ⁶ | 9.09×10ˉ⁵ | 2.57×10ˉ⁵ | 5.67×10ˉ⁷ | 5.42×10ˉ⁴ | 6.61×10ˉ⁴ | 8.14×10ˉ⁴ |
60 | 1.40×10ˉ⁶ | 5.25×10ˉ⁶ | 9.19×10ˉ⁵ | 2.60×10ˉ⁵ | 6.57×10ˉ⁷ | 5.58×10ˉ⁴ | 6.86×10ˉ⁴ | 8.49×10ˉ⁴ |
70 | 1.48×10ˉ⁶ | 5.33×10ˉ⁶ | 9.28×10ˉ⁵ | 2.63×10ˉ⁵ | 7.41×10ˉ⁷ | 5.72×10ˉ⁴ | 7.07×10ˉ⁴ | 8.77×10ˉ⁴ |
80 | 1.56×10ˉ⁶ | 5.42×10ˉ⁶ | 9.36×10ˉ⁵ | 2.66×10ˉ⁵ | 8.17×10ˉ⁷ | 5.84×10ˉ⁴ | 7.24×10ˉ⁴ | 9.01×10ˉ⁴ |
90 | 1.62×10ˉ⁶ | 5.51×10ˉ⁶ | 9.42×10ˉ⁵ | 2.68×10ˉ⁵ | 8.86×10ˉ⁷ | 5.95×10ˉ⁴ | 7.39×10ˉ⁴ | 9.21×10ˉ⁴ |
100 | 1.68×10ˉ⁶ | 5.60×10ˉ⁶ | 9.48×10ˉ⁵ | 2.70×10ˉ⁵ | 9.51×10ˉ⁷ | 6.04×10ˉ⁴ | 7.53×10ˉ⁴ | 9.39×10ˉ⁴ |