**2.7.2 Roles of plasticizers**

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.

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

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 plasticizers, DOP was found to be the excellent plasticizer in term of thermal stability as proven in differential thermal analysis (DTA). The results show the reduced weight loss as increases the plasticizer concentration. Ali et al. (2007) studied the plasticized–polymer electrolytes that composed of PMMA, propylene carbonate (PC) or ethylene carbonate (EC) as plasticizer and LiTf or LiN(CF3SO2)2 as dopant salt. According to this literature, the ionic conductivity increases with the concentration of the plasticizer. They also declare that the PC–based plasticized–polymer electrolytes exhibit higher ionic conductivity compared to the EC–based plasticized–polymer electrolytes (Ali et al., 2007).

Rajendran et al. (2004) also incorporated few types of plasticizers in the polymer electrolytes containing PVA/PMMA–LiBF4. The highest ionic conductivity of 1.29 mScm-1 had been observed for EC–based complex because of higher dielectric constant of EC (*ε*=85.1) (Rajendran et al., 2004). The addition of the plasticizer has been reported to reduce the crystallinity of the polymer complexes which in turns to a better ionic conductivity (Kelly et al., 1985). Kelly and co–workers assert that the presence of plasticizer exhibits downward shift in *Tg* due to the weaker interactions between the ions and polymer chain which in accordance with higher ion dissociation. A maximum electrical conductivity of 2.60×10−<sup>4</sup> Scm−1 at 300 K has been observed for 30wt.% of PEG as plasticizer compared to the pure PEO–NaClO4 system of 1.05×10−6 Scm−1. This can be explained that the addition of plasticizer enhances the amorphous phase in with concomitant the reduction in the energy barrier. Eventually, it results higher segmental motion of lithium ions (Kuila *et al*., 2007).
