Plasticised polymer electrolytes based on PMMA grafted natural rubber–LiCF 3 SO 3 – PEG200 K. S. Yap, L. P. Teo, L. N. Sim, S. R. Majid and A. K. Arof* Polymer electrolyte membranes that consist of 70 wt-% natural rubber grafted with 30 wt-% poly(methyl methacrylate) (MG30) and 30 wt-% lithium trifluoromethane sulphonate (LiTf) salt have been investigated with various concentrations of poly(ethylene glycol) 200 (PEG200). The formation of polymer–salt complexes has been confirmed by Fourier transform infrared spectral studies. X-ray diffraction studies further confirmed that all the samples prepared are amorphous. The highest conducting sample has the composition of 63 wt-%MG30–27 wt-%LiTf–10 wt-%PEG with conductivity of 3?65610 24 S cm 21 at 298 K. From differential scanning calorimetry studies, the glass transition temperature T g is found to increase from 263 to 254uC with the increase in PEG concentration at 30 wt-%PEG200. The ionic transference number of mobile ions has been estimated using Wagner’s polarisation method, and the results reveal that the conducting species are mainly ions. Keywords: Natural rubber, PMMA, SEM, Complexation, Conductivity Introduction A polymer electrolyte is a solid ionic conductor consisting of salt dissolved in a polymer. 1 Polymer based electrolytes have been studied since they were discovered by Fenton et al. 2 The potential use of these materials as a ‘solid electrolyte’ was first announced by Armand et al. in 1978. 3 Solid polymer electrolytes have shown great potential for application as ionic conduc- tors in solid state electrochemical devices. 4,5 These devices include batteries, 6 electrochromic windows, 7 fuel cells 8 and supercapacitors. 9 Solid polymer electrolytes are non-corrosive and non-volatile and can be easily formed into thin films. 10 Many researchers have contributed great effort to synthesise high ion conducting polymer electrolytes over the decades; however, the desired value of conductivity at ambient temperature has not yet been achieved. 5,10 The incomplete understanding and the lack of experi- mental data to support theory on the ion conduction mechanism in polymer electrolytes are the main barriers to the realisation of the usage of solid polymer electrolytes. 11 For application in electrochemical devices, amorphous or non-crystalline polymer electro- lytes are gaining attention because crystalline polymer electrolytes are believed to be the cause of the significant decrease in ionic conductivity. 12 It is understood that conduction in polymer electrolytes takes place through migration of ions along the coordination sites in the host material and is associated with the polymeric chain motion. 13,14 The motion of the chains assists in the breaking and renewal of coordination bonds of the cation to the polar atom in the polymer and allows the ion species to jump to other coordination sites. 13,14 Thus, materials with low glass transition temperature T g are usually chosen as polymer host. One of the methods to improve the conductivity is by adding plasticisers. The addition of a plasticiser, in general, helps to soften the polymer backbone and increase segmental motion, which leads to conductivity enhancement. Furthermore, the high dielectric constant of the plasticiser helps to dissociate the salt and ion aggregates, providing more free ions. The low viscosities of these plasticisers also lead to high ionic mobility, which, in turn, enhances the conductivity. Poly(ethylene glycol) 200 (PEG200) can also act as a plasticiser. 15–17 The PEG helps in the ion transport mainly by increasing the amorphous content, which is reported to be the high conducting phase, and also by providing more polar groups from the hydroxyl end and ethylene groups, which can act as electron donors to form a dative bond with the cations of the incorporated salt. Thus, this prompted the authors to use PEG200 as plasticiser while natural rubber (NR) grafted with 30 wt-% of poly(methyl methacrylate) (PMMA, MG30) as the polymer host doped with 30 wt-% of lithium trifluoromethane sulphonate (LiTf) as an electrolyte to improve the ionic conductivity of the lithium based polymer electrolytes. The NR has good mechanical properties, 18 but being an insulator, its conductivity at ambient temperature is rather low. However, PMMA is Centre for Ionics, Physics Department, University of Malaya, Kuala Lumpur 50603, Malaysia *Corresponding author, email akarof@um.edu.my S2-34 ß W. S. Maney & Son Ltd. 2011 Received 13 June 2010; accepted 1 April 2011 DOI 10.1179/143307511X13031890747534 Materials Research Innovations 2011 VOL 15 SUPPL 2