**2.4 Mixed salt system**

168 Recent Advances in Plasticizers

break up the regular helical structure of PEO and suppress the crystalline region of polymer matrix. A similar random copolymer had been synthesized by replacing the methylene oxide with dimethyl siloxy units. This resultant polymer matrix illustrated higher flexibility

In general, comb polymers contain pendant polymer chain and they are structurally related to grafting copolymers. The comb–branched system containing of low molecular weight of polyether chain is grafted to a polymer backbone. Thus, it lowers the glass transition temperature (ܶ) and then helps to optimize the ionic conductivity by improving the flexibility of polymer chain into the system. The elastic poly[ethylene oxide–co–2–(2– methoxyethoxy)ethyl glycidyl ether] [P(EO/MEEGE)]–based polyether comb polymer electrolytes were synthesized by Nishimoto and co–workers. The degree of crystallinity was decreased with increasing the composition of MEEGE in copolymers, which in accordance with higher ionic conductivity. The introduction of the side chain of MEEGE in the copolymers enhances the flexibility of polymer matrix and hence improves the ion mobility. The highest ionic conductivity of 10-4 Scm-1 was achieved at room temperature (Nishimoto et al., 1998). Until today, this technique is still being employed in this area. Recently, many researchers have keen of interest on polyvinylidene–co–hexafluorophosphate (PVdF–co– HFP) copolymer. Composite polymer electrolytes prepared by adding SiO2 nanowires into (PVdF–co–HFP) are described by Zhang et al. The ionic conductivity of this composite polymer electrolyte is up to 1.08×10−3 Scm−1 with the electrochemical window of 4.8 V

Polymer blending is physical means to mix two or more different polymers or copolymers which are not linked by covalent bonds. This polymer blend is a new macromolecular material with special combinations of properties. For polymer blends, a first phase is adopted to absorb the electrolyte active species, whereas the second phase is tougher and sometimes substantially inert. It is a feasible way to increase the ionic conductivity because it offers the combined advantages of ease of preparation and easy control of physical properties within the definite compositional change (Rajendran et al., 2002). Polymer blending is of great interest due to their advantages in properties and processability compared to single component. In industry area, it enhances the processability of high temperature or heat–sensitive thermoplastic in order to improve the impact resistance. Besides, it can reduce the cost of an expensive engineering thermoplastic. The properties of polymer blends depend on the physical and chemical properties of the participating polymers and on the state of the phase, whether it is in homogenous or heterogeneous phase. If two different polymers able to be dissolved successfully in a common solvent, this polymer blends or intermixing of the dissolved polymers will occur due to the fast establishment of the thermodynamic equilibrium (Braun, 2005). Sivakumar et al. (2006) observed that PVA (60 wt%)–PMMA (40 wt%)–LiBF4 complex exhibits the maximum conductivity of 2.8×10−5 Scm−1 at ambient temperature. It is also higher than the pure PVA

which assists in ionic conduction (Gray, 1997b).

**2.2 Comb polymers** 

(Zhang et al., 2011).

**2.3 Polymer blending** 

system which has been reported to be 10−10 Scm−1.

The conductivity of the mixed salts in polymer electrolyte is higher than single salt electrolyte. It is due to the addition of second salt may prevent the formation of aggregates and clusters. Therefore, it increases the mobility of ion carriers (Gray, 1997b). An approach had been done by Ramesh and Arof (2000). In this research, we synthesized poly (vinyl chloride) (PVC)–based polymer electrolytes with lithium trifluoromethanesulfonate (LiTf) and lithium tetrafluoroborate (LiBF4) as doping salts. The ionic conductivity is increased by four orders of magnitude in comparison with single salt system. It is attributed to the increase in the mobility of charge carriers by avoiding the aggregation process.
