FTIR spectra of plasticized grafted natural rubber–LiCF3SO3 electrolytes

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Abstract

Chemical modification of natural rubber (NR) has frequently been attempted to improve the performance in specific application. 30% poly(methyl metacrylate) (PMMA) grafted into NR (MG30) has been explored as a potential candidate for polymer electrolytes. The complexation effect of salt and plasticizer in polymer host electrolytes had been investigated using FTIR. The carbonyl stretch of MG30 locates at 1729 cm−1, with the addition of lithium trimethanesulfonate (LiCF3SO3) salt, new band evolves at lower frequency region at 1643–1645 cm−1. The nondegenerate vibrational mode of νs(SO3) of salted electrolytes appearing at 1031–1034 cm−1 comes from ‘free’ trimethanesulfonate anions and the 1040–1046 cm−1 absorption from the monodentate ion paired with triflates. These indicate MG30–salt interaction. When MG30 and ethylene carbonate (EC) formed film, the CH3 asymmetric bend of MG30 appearing at 1447 cm−1 is shifted to 1449 cm−1 in the EC–polymer complex. The Cdouble bondO stretching at 1729 cm−1 also shifted to 1728 cm−1. Hence, the EC–MG30 system is complexed to each other. EC–LiCF3SO3 interactions are indicated by the shifting of Cdouble bondO bending band of EC from 718 cm−1 being shifted to 720 cm−1 in the complex. In Li+–EC interaction where the ring breathing region at 897 cm−1 in EC has shifted to 899 cm−1 in EC–salt spectrum. The band appearing at 1643–1645 cm−1 due to the coordination of Li+  O–C is still under observation and new peaks at 1779 and 1809 cm−1 are responsible to the carbonyl stretches of EC in plasticized salt–polymer electrolytes.

Introduction

The discovery of ionic conductivity in polymer based on hosts complexed with salts [1], [2], [3], [4] has generated research activities that lead to significant advances in lithium batteries. Studies [5], [6] have sought to investigate on the factors governing ionic conduction, such as ion pairing and ion aggregation. The introduction of plasticizers with low molecular weight such as propylene carbonate (PC), ethylene carbonate (EC), dibuthyl phthalate (DBP) and diocthyl adipate (DOA) plays an important role in conducting material with sufficient mobility of ionic conduction. In most cases the role of plasticizer is as a conductivity enhancer, but its mode of operation is complex [7].

Polymers such as poly-vinyl chloride (PVC), poly-methylmethacrylate (PMMA), poly-ethylene oxide (PEO) and poly-vinylidene fluoride (PVdF) have been used as matrices for plasticized polymer electrolytes. Blending of different polymers is the most promising and feasible approach [8], [9] and has become technically important material. Although a large number of combinations of polymers are possible, there are a few that lead to a totally miscible systems [10], [11], [12], [13], [14], [15].

Razali Idris et al. [16], [17] explored 25% epoxidised NR, 50% epoxidised NR and PMMA grafted NR as a potential candidate for polymer electrolytes in lithium batteries. NRs various chemical modifications have been attempted to modify its properties and extend its use [18], [19], [20], [21]. One such modification is the graft polymerization with methyl methacrylate monomer. The polar PMMA provided the path for ion conduction and the nonpolar NR gave the desired properties in its lightweight [14] and elasticity. MG30 gives good retention at elevated temperatures and good electrical properties [22]. Owing to their interesting properties, MG30 is employed in this work.

FTIR spectroscopy studies were done to establish the interactions between the polymer and salt which identify any changes in the electronic levels of the atoms and to investigate the influence of the plasticizer (EC), on the salted polymer electrolyte. The studies of polymers FTIR consist of analyzing and identifying polymeric compositions and subtle structural variations such as tacticity and neighboring group interactions.

Section snippets

Experimental

MG30 was obtained commercially and LiCF3SO3 was obtained from Aldrich. Prior to the preparation of polymer electrolytes, LiCF3SO3 were dried at 100 °C for 2 h in order to eliminate any trace amount of water. All the electrolyte samples were prepared by solvent cast method. MG30 was cast on glass to obtain a dry rubber film. The dried rubber film was sliced into grain size to ease the dissolution process. The required amounts of grafted polymer were prepared by dissolving it in stoppered flask

Results and discussion

FTIR spectroscopy is a powerful tool to monitor the vibrational energy levels in the region of different molecules. The IR spectra of PMMA, MG30 and polyisoprene are shown in Fig. 1. In this study, interests have been shown on oxygen atoms, which acted as electron donor atoms in the structure of the polymer host. When salt is added to the polymer, the oxygen atoms with lone pair of electron form a dative bond with Li+ ion from the salt and hence a polymer–salt complex would be formed. In the

Conclusions

The complex formations in MG30–LiCF3SO3 and LiCF3SO3–EC systems had been confirmed from the FTIR studies. There is no interaction between MG30 and EC at the Cdouble bondO bending and stretching modes and the ring breathing region. It was also concluded that the plasticizer molecules had penetrated the salted polymer matrix in MG30–LiCF3SO3–EC complex.

Acknowledgements

The authors would like to thank the government of Malaysia for the PJP grant and vote F0185/2004A and F0710/2004D that enable this work to be carried out.

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      Functional groups corresponding to the major absorption peaks of the FTIR spectra are listed in Table 8. It should be noted that the absorption peaks 2924 and 2852 cm−1 are typical of EPDM rubber arising from the saturated hydrocarbon chain backbone of aliphatic alkyl symmetric/asymmetric CH2 stretching vibration [59–61]. The absorption bands 1461 and 1380 cm−1 are assigned to CH2 scissoring vibration and CH bending vibration of CH3 from the propylene and ethylene unit, respectively [60,61].

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