**3.3 Propylene carbonate (PC)**

172 Recent Advances in Plasticizers

PVC is mainly produced by radical polymerization (Endo K, 2002). In this polymerization, it associates the vinyl chloride molecules and thus forms the polymeric chains of macromolecules. From the scientific point of view, lone pair of electrons at the chlorine atoms is the main reason for choosing PVC as host polymer. Thus, it can form solvation onto lithium salts easily (Ramesh and Chai, 2007). PVC is chosen due to its high compatibility with the liquid electrolyte, good ability to form homogeneous hybrid film, commercially available and inexpensive (Li et al., 2006). Other unique characteristics are easy processability and well compatible with a large number of plasticizers (Ramesh and Ng, 2009). It plays an important role as mechanical stiffener because of the dipole–dipole

Several types of lithium salt has widely been used, such as lithium hexaflurorophosphate (LiPF6), lithium hexafluoroarsenate (LiAsF6), lithium bis(trifluoromethanesulfonyl) imide (LiTFSI), LiTf, LiClO4 and LiBF4. Lithium tetraborate (also known as boron lithium oxide or dilithium borate) (LBO or more commonly known as Li2B4O7) was employed in this study. This compound is generally defined as one type of dopant used to provide lithium cations in the preparation of polymer electrolytes. It is constructed of lithium cations and tetraborate anions, where its stoichiometric ratio is two cations to one anion, as illustrated as below.

B B

In addition, it is a new non–ferroelectric piezoelectric substrate material with a congruent melting point of 917 °C. LBO single crystal is also a superior substrate for surface acoustic wave (SAW) and bulk acoustic wave (BAW) devices as it has a specific crystallographic plane (110). Thus, it provides a zero temperature coefficient of frequency and a fairly high electromechanical coupling factor. According to Byrappa and Shekar (1992), LBO occurs naturally as diomignite, i.e. colourless crystals in fluid inclusions in the mineral spodumene. It appears as a loosely attached crystal with a very fine–grained structure. These intrinsic and unique properties make it as excellent material by comparing with other lithium salt. The fine structure of Li2B4O7 would enhance the solubility in the polymer matrix and eventually, speed up the salt dissociation process. Other factors to choose it as doping salt are abundant availability of raw materials and no environmental pollution (Xu et al., 2004). As aforementioned, Li2B4O7 is naturally obtained from mineral. Therefore, it is a cost effective material compared to those synthetic lithium salts, for instance LiBF4, LiTFSI and LiTf. High toxicity of LiAsF6 and poor chemical and thermal stabilities of LiPF6 are not good choices as doping salt in polymer electrolyte. Likewise, LiClO4 reacts with most organic species readily in violent ways under certain conditions such as high temperature and high current charge because of high oxidation state of chlorine (VII) in perchlorate. Moreover, the corrosion of a key component of the cell by TFSI anions restricts the possible application of LiTFSI greatly in the polymer electrolytes. Poor ionic conductivity of LiTf in non–aqueous

OLi OLi

O O O

B

interactions between the hydrogen and chlorine atoms (Ramesh and Chai, 2007).

B O O

Fig. 2. Chemical structure of lithium tetraborate.

**3.2 Lithium tetraborate (Li2B4O7)** 

As aforesaid, plasticizer could enhance the ionic conductivity of polymer electrolytes. Propylene carbonate (PC) is used in this study. PC is an organic, colourless and odourless organic compound. It is also well known as highly polar and aprotic solvent. Furthermore, it is a byproduct of the synthesis of polypropylene carbonate from propylene oxide and carbon dioxide. It can be obtained from the synthesis of urea and polypropylene glycol in the presence of zinc–iron double oxide catalysis. It is composed of twofold ester of propylene glycol and carbonic acid as illustrated as below.

Fig. 3. Chemical structure of propylene carbonate.

PC is a preferred solvent as it exhibits many unique characteristics. These properties are static stability with lithium, wide liquid range and low freezing point (Kang, 2004). Many researchers have drawn interests onto this plasticizer due to its high dielectric constant (ε= 64.9). This high dielectric constant could help in the dissociation of the charge carriers, especially the cations from the dopant salt. The electrical performance would be improved as it manifests excellent plasticizing effect (Tobishima and Yamaji, 1984). Cations are more readily to be obtained as the salt is dissolved in PC through electrolysis process, because of its high molecular dipole moment of 4.81 D (Jorné and Tobias, 1975). Moreover, high polarity of this plasticizer creates an effective solvation shell around the cations and hence forms a conductive electrolyte. PC displays the highest dielectric constant in comparison with *DEC (*ε= 2.8), *ethyl methyl carbonate (EMC*) *(*ε= 2.9), *DMC (*ε= 3.1), DOA *(*ε= 4–5), DBP *(*ε= 6.4), DMP *(*ε= 8.5) and benzyl acetate (BC) *(*ε= 53) (Kang, 2004; Gu et al., 2006). Despite EC illustrates higher dielectric constant than PC, however PC exhibits wider range of liquidity than EC as its melting point is up to –48.8 °C. In contrast, high melting temperature of 36.4 °C is the main shortcoming of EC. High dipole moment of PC than EC (4.61 D) with low vapor pressure also compensates the obstacle of lower dielectric constant of PC (Kang, 2004). Therefore, PC is becoming an attractive prospect as plasticizer compared to other plasticizers.

### **3.4 Tetrahydrofuran (THF)**

Tetrahyrdofuran (THF) is a colorless, water–miscible organic liquid with low viscosity at standard temperature and pressure. It is one of the most polar ethers with a wide liquid range and is widely been used as solvent. In addition, it is an aprotic and highly volatile solvent with

Characterization of High Molecular Weight Poly(vinyl chloride) –

spectroscopy (FTIR) and thermogravimetry analysis (TGA).

**4.3 Instrumentation** 

**4.3.1 Ac-impedance spectroscopy** 

current (Selvasekarapandian et al., 2006).

be determined.

the transmittance mode.

under spring pressure with the configuration SS/SPE/SS.

**4.3.2 Fourier Transform Infrared Spectroscopy (FTIR)** 

Lithium Tetraborate Electrolyte Plasticized by Propylene Carbonate 175

After the samples were prepared, the characterizations have been employed to investigate the electrical, structural and thermal properties of the samples. These analytical and evaluative methods include ac–impedance spectroscopy, Fourier Transform Infrared

Impedance spectroscopy (IS) is a powerful analytical tool to characterize the electrical properties of materials and their interfaces with electronically conducting electrodes. It is also widely been used to envisage the dynamics of bound or mobile charge in the bulk or interfacial regions of any kind of solid or liquid material: ionic, semiconducting, mixed electronic–ionic and even insulator (dielectric) (Barsoukov & Macdonald, 2005). The principle of the impedance spectroscopy is based on the ability of a medium to pass an alternating electrical or frequency current. It is well functioned by conducting current and measuring the potential difference created by the circulation of this current. When an electric field is applied across the sample, the polar group might be activated as dipoles which always interact with the corresponding ions due to the Coulombic electric force. Thus, these dipole moments will rearrange themselves under the influence of the external electric field, depending on the mobility of backbone. So, lithium cations can travel faster along these activated or polarizing areas to reach opposite of the electrode and generate

The prepared samples were subjected to ac–impedance spectroscopy. The thickness of the samples was measured by using micrometer screw gauge. The ionic conductivities of the samples were determined, by using HIOKI 3532–50 LCR HiTESTER, over a frequency range between 50 Hz and 1 MHz. The ionic conductivity was measured from ambient temperature to 100 °C. Samples were mounted on the holder with stainless steel (SS) blocking electrodes

The main fundamental of FTIR is to determine structural information about a molecule. The main principle of FTIR is related to the interferometry which is an optic study. It separates infrared beam of light which serves as light source radiation into two ray beams. Once the beam of infrared is passed through the sample, the molecules would absorb the infrared radiation and then excite to a higher energy state. Thus, the energies associated with these vibrations are quantized; within a molecule, only specific vibrational energy levels are allowed. The amount of energy absorbed at each wavelength was recorded. The frequencies which have been absorbed by the sample are determined by detector and the signal is amplified. Hence, IR spectrum was obtained. FTIR spectroscopy is not only applied in the crystalline region of complexation, whereas the complexation in amorphous phase can also

FTIR analysis was performed by using Perkin–Elmer FTIR spectroscopy RX 1 in the wave region between 4000 and 400 cm-1. The resolution of the spectra was 4 cm-1 and recorded in

dielectric constant of 7.6. Another virtue of this solvent is its good solvent properties. It can dissolve a wide range of non–polar and polar chemical substances. Therefore, it is being chosen as solvent as it shows a well–dissolution with PVC, Li2B4O7 and PC in this study. It is also a heterocyclic compound with chemical formula of (CH2)4O, as shown in Figure 4.

Fig. 4. Chemical structure of tetrahydrofuran.
