ABSTRACT
Solid solutions of epoxidized natural rubber with 25 and 50 mol % epoxidation, ENR-25 and ENR-50, respectively, added with LiClO4 were prepared by solution casting technique. Glass transition temperature (Tg)values obtained using differential scanning calorimetry (DSC) and the ionic conductivity evaluated from bulk resistance (Rb) determined using the impedance spectroscopy point towards higher solubility of the lithium salt in ENR-50 when the ratio of the mass of salt to the mass of polymer (Y) 0.15. This ramification correlates with spectroscopic results demonstrated in FTIR spectra. Ionic conductivity (σ) is observed to increase with ascending values of Y. When Y 0.15, ENR-50 exhibits higher ionic conductivity than ENR-25 but the σ values of ENR-25 increase sharply with increasing salt content to above that of ENR-50 when Y 0.20. Higher ion mobility, better salt molecule-chain segment correlation and higher charge carrier diffusion rate account for the higher σ value for ENR-50 at 0.00 < Y 0.15. However, restricted ion transport for ENR-50 and relatively flexible segmental motion for ENR-25 at Y 0.20 cause the conductivity of ENR-25 to be higher than that of ENR-50. Therefore, conductivity of the epoxidized natural rubber is primarily governed by the segmental motion of the elastomer rather than charge carrier density, since the discovery of ion-conducting polymer by Fenton et al. [1] followed by the application of polymer electrolyte in lithium batteries by Armand et al. [2], solid polymer electrolyte (SPE) has been widely studied especially on the enhancement of ionic conductivity. To date, SPE has become the focus of extensive research in pursue for a new generation of power source to cater for the latest development in electrochemical devices. Polymer electrolyte is a complex formed by dissolving an inorganic salt in a polymer matrix with polar groups acting as an immobile solvent. It is generally accepted that the charge carrier density and ion mobility are the two important parameters contributing to the ionic conductivity of a SPE [3-5]. For SPE with a semi-crystalline polymer matrix like poly(ethylene oxide) (PEO), ion mobility is attributed to the segmental motion of the amorphous phase of the macromolecular chain. Epoxidized natural rubber (ENR) is derived from natural rubber by converting different percent of the C=C bonds on the macromolecular backbone to the polar epoxy groups. ENR has good potential to be polymer host in SPE because of their distinctive characteristic such as low glass transition temperature (Tg), soft elastomeric characteristics at room temperature [5]
and good electrode-electrolyte adhesion. Furthermore, the highly flexible macromolecular chain and the polar epoxy oxygen provide excellent segmental motion and coordination sites for Li+ ion transport, respectively, in ENR-based polymer electrolytes [4, 6]. However, Chan and Kammer [7], in their study on the properties of PEO/ ENR/lithium perchlorate (LiClO4) solid solutions concluded that the solubility of the ionic salt is comparatively higher in PEO than in ENR. Therefore, the conductivity mechanism as well as the role of the oxirane ring in the dissociation of ionic salt in ENR-salt complex will be investigated in detail. Other than ENR with 25
and 50 mol percent of epoxidation, ENR-25 and ENR-50, respectively, deproteinized natural rubber (DPNR) is used as a polymer reference [8].
