**3. Materials**

170 Recent Advances in Plasticizers

diethyl carbonate (DEC), and high dielectric constant solvent such as N,N– dimethylformamide (DMF), N,N–dimethylacetamide (DMAc) and *γ*–butyrolactone are widely used as main components of GPE (Pradhan et al., 2005; Suthanthiraraj et al., 2009; Ning et al., 2009). Other examples of common plasticizers are dibuthyl phthalate (DBP),

The effect of plasticizers on the polymer electrolytes entirely depends on the specific characteristics of the plasticizer, for example, viscosity, dielectric constant, the interaction between polymer and plasticizer, and the coordinative bond between ion and plasticizer (Rajendran and Sivakumar, 2008b). The incorporation of plasticizer not only enhances the salt solvating power, but it also provides sufficient mobility of ions with a better contact between polymer electrolytes and electrodes (Ramesh and Arof, 2001; Rajendran et al., 2004). Apart from that, plasticizer is an attractive additive due to its superior miscibility with polymer, high dielectric constant and low viscosity (Ramesh and Chao, 2011b). Inclusion of plasticizer is the most successful skill to enhance the ionic conductivity without compromising the thermal, electrochemical and dimensional stabilities (Ganesan et al., 2008). Plasticizer are expected to improve the ionic conductivity through some important intrinsic modifications such as significant changes in local structure, enhancement of amorphous fraction and changes in local electric field distribution in the polymer matrix.

The principal function of a plasticizer is to reduce the modulus of polymer at the desired temperature by lowering its *Tg*. In this theory, the increase in concentration of plasticizer causes the transition from the glassy state to rubbery region at progressively lower temperature. Moreover, it reduces the viscosity of polymer system and then facilitates the ionic migration within the polymer matrix. Besides, it weakens the interactions within the polymer chains and thus improves the flexibility of polymer chains in the polymer matrix (Ganesan et al., 2008). As a result, it increases the free volume of polymer and enhances the long–range segmental motion of the polymer molecules in the system. In an approach, the polymer matrix is swollen in a plasticiser, the latter being an aprotic solvent with a high dielectric constant. A new mobile pathway for ion migration is being introduced upon addition of plasticizer by dissociating the charge carriers. Hence, it increases the amount of mobile charge carriers and promotes the ionic transportation, enhancing the ionic conductivity. The polymer component would, on the other hand, render necessary stability to the lithium anode electrolyte interface, which thereby reduces the chance of dendrite growth on the lithium anode (Rajendran et al., 2000a). In general, plasticizers are having conjugated double bond which initializes the delocalization of electrons. Therefore, it

improves the donor capacity of oxygen atom which facilitates the binding of cations.

As reported in Michael et al. (1997), three types of ester class plasticizers, namely dioctyl phthalate (DOP), dibutyl phthalate (DBP) and dimethyl phthalate (DMP), were employed to examine its effect on ionic conductivity in the PEO–LiClO4 polymer complex. Among these

**2.7.3 Literature review of plasticized-gel polymer electrolytes** 

diocthyl adipate (DOA) and polyethylene glycol (PEG) (Suthanthiraraj et al., 2009).

**2.7.1 Advantages of plasticizers** 

**2.7.2 Roles of plasticizers** 

In this study, poly (vinyl chloride) (PVC), lithium tetraborate (Li2B4O7) and propylene carbonate (PC) were employed as host polymer, dopant salt and plasticizer, respectively. In this section, we discuss about the general description of the materials and the reasons for choosing the materials.
