**2.2 Comb polymers**

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 (Zhang et al., 2011).
