This paper was presented at VIIIth National Symposium On Polymeric Materials 2008 (NSPM 2008), Naza Hotel. Penang.

Preparation and Characterization of Solid Polymer Electrolytes

49% Poly(Methyl Methacrylate)-Grafted Natural Rubber - Poly(Methyl Methacrylate) - Lithium Perchlorate Salts.

¹M.S. Su’ait, ^1*A. Ahmad, ¹H. Hamzah and ²M.Y.A. Rahman

¹ School of Chemical Sciences and Food Technology, Faculty of Sciences and Technology, Universiti Kebangsaan Malaysia, 43600 Bangi, Selangor, Malaysia.

² College of Engineering, Universiti Tenaga Nasional, 43009 Kajang, Selangor, Malaysia.

^*Corresponding Author : azizan@ukm.my

Abstract: The potential of hybrid polymer 49% poly(methyl methacrylate)–grafted natural rubber (MG49) and poly(methyl methacrylate) (PMMA) as a polymer host in solid polymer electrolyte (SPE) film for rechargeable batteries system has been investigated. The hybrid films were prepared by solution casting technique. The ionic conductivity was investigated by alternating current electrochemical impedances spectroscopy (EIS). The highest conductivity is 1.47×10^-8 S.cm^-1 at 20 wt. % of LiClO₄. The observation on structural studies done by X-ray diffraction (XRD) showed that the crystallinity phase is reduced at the highest conductivity. The Fourier transform infrared spectroscopy (FTIR) analysis showed that the interaction between lithium ion and oxygen atoms occurred at carbonyl (C=O) and ether (C-O-C) groups.

Keywords: Ionic conductivity, MG49, PMMA, Lithium perchlorate, Solid polymer electrolyte

1. Introduction

Research on polymer electrolyte was firstly conducted by Fenton et al. [1]. They found that the non-conducting polymer, polyethylene oxide (PEO) become conductive when lithium salt was added into the polymer matrix. However, research on PMMA based SPE was only conducted in 1985 by Iijima and co-workers [2]. The finding of ionic conductivity in these polymer material complexes with salts has led to the development of electrochemical devices such as rechargeable batteries, electrochromic windows and sensing devices for chemicals/gases [3].

PMMA has been used as a polymer host due to its high transparency in the visible region as preferred solid electrolytes in electrochromic window [4], high stability at lithium-electrolyte surface [5] and able to be diluted in various organic solvents [6]. PMMA based electrolyte is also less reactive towards the lithium electrode. It induces more favourable passivation film on the electrode surface. Therefore, it is expected that a higher cyclibility of lithium electrode in PMMA based electrolyte [7]. PMMA also acts as a stiffener that fast ion transport occurs through a continuous conduction path which does not affect the electrochemical stability of the electrolyte [8]. The increasing in surface exposure of PMMA could increase the mechanical properties and increase the melting level of lithium salts due to the high dielectric properties of lithium salts. In addition, PMMA has a polar functional group in their main polymer chain that shows high affinity for lithium ions to transport. Oxygen atom from carbonyl group (C=O) will form a coordinate bond with lithium ion from doping salts [7]. The main drawback of plasticized gel-based PMMA system is its poor mechanical properties. Its mechanical properties can only be improved by modifying the ratio between polymer host and plasticizer/solvent. However, this will adversely affect the ionic conductivity and easily corrode the lithium metal anode in an electrochemical cell [9]. Polymer electrolyte must have enough mechanical strength to hold the pressure between anode and cathode, as gel polymer electrolyte (GPE) is unable to do so as compared to solid polymer electrolyte.

To overcome the drawback of GPE and mechanical properties of PMMA film, PMMA was hybrided with elastomer material such as modifier natural rubber to improve the segmental motion in polymer hybrid systems and hence a more flexible and elastic material. Natural rubber and synthetic rubber like poly(styrene-co-butadiene) (SBR) and poly(acrylonitrie-co-butadiene) (NBR) rubber were not suitable because natural rubber has sticky properties and not compatible with PMMA [10]. Besides, natural rubber does not has polar group to enhance the ion mobility in the SPE system. Whereas, synthetic rubber gives a poor mechanical properties at low polymer concentrations [11]. Recently, modified natural rubber such as epoxidized natural rubber (ENR) and PMMA–grafted natural rubber (MG) based SPE had drawn the attention of many researchers [9,11-13,15,17-19]. This is due to the attractive attributes such as low glass transition temperature (T_g), soft elastomer characterization at room temperature, and good elasticity. Suitable elasticity can result in flat, thin, and flexible film. Furthermore, modified NR gives excellent contact between an electrolytic layer and an electrode in batteries system. It can also act as a polymeric solvent and the ionic conductivity is higher as compared to glassy or crystalline state of polymer [3]. On the other hand, modified NR has oxygen atoms, which can act as electron donor atoms in the structure of the polymer host. Research conducted by Kamuta et al. [12], Alias et al. [15] and Ali et al. [19] found that the oxygen atoms with lone pair of electron formed a coordinate bond with lithium ion from perchlorate salt and hence a polymer-complex. However ENR based SPE shows a drawback to its mechanical properties such as slightly sticky and difficult to peel off from substrate [9,13,14] as compared to MG film which is more free standing, elastic and flexible. Previous studies on various MG was conducted elsewhere [3,12,15-18]. Fig. 1 shows the structure of MG monomer [16].

Fig. 1. Structure of MG monomer

In this work, polymer hybrid MG49-PMMA with ratio 30:70 is doped with LiClO₄ salt to prepare SPE by solution casting technique. All the sample were characterized by using AC electrochemical impedences spectroscopy (EIS), X-ray diffraction (XRD) and Fourier transform spectroscopy (FTIR). It is expected that LiClO₄ salt gives an optimum value for ionic conductivity studies in polymer hybrid (30:70) MG49-PMMA.

2. materials and methods

2.1 Materials.

MG49 was commercially obtained. PMMA low molecular weight and LiClO₄ salt were supplied by Fluka. All the materials were used without further purification.

2.2 Sample preparation.

All the polymer electrolyte samples were prepared by solution casting technique. MG49 rubber was sliced into a grain size. The quantity of MG49 was dissolved in stopped flasks containing toluene. After 24 hours, the solution was stirred with efficient magnetic stirring for the next 24 hours until complete dissolution of MG49 into clear viscous solution. PMMA solution was prepared in another stopped flask containing toluene and stirring for 24 hours. These two solutions were then mixed together for 24 hours to obtain a homogenous solution. LiClO₄ salt was dissolved in THF solution for 12 hours and doped to the solutions for the next 24 hours with continue stirring to obtain a homogenous solution. The electrolyte solutions were casted onto a glass Petri dish and the solvent were allowed to slowly evaporate in a fume hood at room temperature. A free standing film was obtained when the solvent completely evaporated. Residual solvents were then removed in vacuum oven for 48 hours at 50C. The samples were then stored in a desiccator until further use. The same experimental procedure was repeated for different weight percent salts loading.

2.3 Characterization.

The ionic conductivity measurements were carried out by AC EIS using High Frequency Resonance Analyzer (HFRA) model 1255 with applied frequency from 6500 Hz to 0.1 Hz at perturbation voltage of 2500 mV. The disc shaped sample of 16 mm in diameter was sandwiched between two stainless steel block electrodes. XRD model D-5000 Siemen was used to observe on the appearance and disappearance of crystalline or amorphous phase as a function of salt content. The data was collected from the range of diffraction angle 2θ from 2° to 80° at rate 0.04° s^-1. FTIR spectrum was recorded by computer interfaced Perkin Elmer GX Spectrometer. The electrolyte was casted onto NaCl windows and was analyze in the frequency range of 4000 cm^-1 to 400 cm^-1 with scan resolution of 4 cm^-1. All the analysis was conducted at room temperature.

3. Results and Discussion

3.1. Ionic conductivity

Typical impedance plots are shown in Fig. 2. According to Rajendran et al. [7] and Kim et al. [29], the complex impedance plots showed two well-defined regions, there is a semicircle in the high frequency range which is related to conduction process in the bulk of complex and the linear region in the low frequency range that is attributed to the effect of blocking electrodes. At low frequency, the complex impedance plot shows a straight line parallel to the imaginary axis, but the double layer at blocking electrodes causes the curvature.

Fig. 2. Typical impedence plot of 30/70 MG49-PMMA 20 wt. % LiClO₄

The bulk resistance (R_b) was obtained from the intercept on real impedance axis (Z’ axis). The ionic conductivity (σ) was calculated from the R_b, the film thickness (l) and contact area of the thin film (A=2πr), according to the equation σ= [l/(A.R_b)]. Ionic conductivity and O/Li ratio of SPE MG49-PMMA-LiClO₄ is showed in Table 1. Ionic conductivity at 0 wt. % of LiClO₄ salt content is 4.07×10^-12 S.cm^-1 and the highest ionic conductivity is 1.47×10^-8 S.cm^-1 at 20 wt. % LiClO₄ salt. The ionic conductivity increased as the salt loading increases up to its optimum level in polymer host. This is due to the increase of the number in conducting species in the electrolyte. This optimum value indicates the maximum and an effective interaction between oxygen atoms and Li⁺ ion in the electrolyte.

The interaction that occurs was explained by FTIR investigation from elsewhere [7,9,12,15,17,19,20,24]. It was founded that a coordinate bond was formed in the polymer-salt complexes between Li⁺ and oxygen atoms. The O/Li ratio for the optimum LiClO₄ salt loading is 7 of oxygen atoms to 1 lithium ion or can be simply written as 7/1. The different value in O/Li is due to the difference of weight percent (wt. %) of the lithium salt. The higher ionic conductivity in addition of 20 wt. % LiClO₄ salt was caused by the large anion size and low lattice energy of LiClO₄ salt generally expected to promote greater dissociation of salts, thereby providing higher concentration of ions [21]. Nevertheless, the ionic conductivity decreases after the optimum salt loading due to the ions association or ions aggregation [18] and the effect of lithium salt’s crystallization in the polymer host as shown in the XRD diffractograms in Fig. 3.

However these findings are slightly lower than the finding by Latif et al. [9], Idris et al. [11] and Ali et al. [18] because in this research, there is no plasticizer such as polypropylene carbonate (PC) and ethylene carbonate (EC) added into SPE and a different type of lithium salt used. The presence of PC and EC in polymer electrolyte can easily corrode the lithium metal electrode in electrochemical cell [14].

Table 1. The ionic conductivity and [O/Li] ratio of SPE MG49-PMMA-LiClO₄

3.2. Structural studies

The XRD analysis is used to determine the structure and crystallization of polymer-salts complex by observing the appearance and disappearance of crystalline or amorphous region. Fig. 3 shows the XRD diffractograms of SPE MG49-PMMA-LiClO₄ in the angle range 2 to 80°. The bell shaped intense curve in Fig. 3 shows the semi-crystalline region occurs in the polymer host (PMMA) while the amorphous region occurs when the intensity of the peak became broader at 20 wt. % LiClO₄ salt loading [25].

However, the system is not fully amorphous based on the presence of LiClO₄ peak at 23.0° and PMMA single peak at 13.2°. In addition, a high ionic conductivity level do still occurs due to the reduction in the intensity of the PMMA crystallization peak from bell shaped curve to a broadening shape. This finding approved the suggestion from elsewhere [1,13,18,20-26] that either the amorphous region or the reduction of crystalline region gives high ionic conductivity as compared to the crystalline or semi-crystalline region. The presence of LiClO₄ peaks at angle 23.0°, 31.4° and 35.3° in Fig. 3 showed that the crystalline phase occurred in SPE MG49-PMMA doped with 25 wt. % LiClO₄ salt. This is because of the recrystallization of LiClO₄ salt due to the ion association between Li⁺ and ClO₄^- in the electrolyte at the high salt concentration. There are no significant changes from 5 wt. % to 15 wt. % salt loading in SPE MG49-PMMA. The salt affect the overall ionic conductivity through crystalline complexes formation, intramolecular crosslinking of the polymer chains and degree of salts dissociation-number of charge carriers [1].

Fig. 3. XRD diffractograms of 30/70 MG49-PMMA-LiClO₄ from 2 to 80°

3.3. FTIR Spectrum studies

FTIR spectroscopy is used to observe the vibration energy of covalent bond in the polymer host and the interaction occurs in the polymer-salt complexes. Since each type of bonds has a different natural frequency of vibration, so the identification of absorption peak in the vibration portion of infrared region will give a specific type of bonding [27,28]. In this research, the main interests are shown on the oxygen atoms of the carbonyl (C=O) (1750 cm^-1-1730 cm^-1) and ether group (C-O-C) (1300 cm^-1-1000 cm^-1) from PMMA and MG49 [27]. According to the literature [5,7,12,17-20,22,24], the oxygen atoms acted as electron donor atoms in the structure of polymer host and form a coordinate bond with lithium ion from doping salts to form polymer-salt complexes. Carbonyl from ester group shows a very strong peak appearing in the range of 1750 cm^-1 to 1735 cm^-1 for simple aliphatic esters and shift to lower wavenumbers by about 15 cm^-1 to 25 cm^-1 in the polymer salt complexes [18,27].

From the experiment conducted, the C=O symmetrical stretching frequency of PMMA and MG49 in polymer-salt complexes gives rise to an intense, very strong and sharp peak at 1733 cm^-1 and 1734 cm^-1 respectively. With addition of lithium salt loading, the intensity of C=O symmetric stretching of MMA peak reduced and shifted to the lower wave number from 1732 cm^-1 to 1735 cm^-1. The shifting of C=O symmetric stretching of MMA peak are demonstrated in Fig. 4(a). The shifting of the intensity peaks confirmed the interaction between lithium ion from doping salt and oxygen atoms in the structure of polymer host to form a coordinate bond and subsequently forming polymer-salt complexes. Previous study reported that the shifting of the intensity peaks still occur even though in insignificant range. Kamuta et. al [17] reported that the C=O stretching of MMA at 1729 cm^-1 is shifted to 1728 cm^-1 in the MG30-EC with LiCF₃SO₃ salt complex. The inconsistent changes in wavenumbers are observed in O-CH₃ asymmetric deformation of MMA and C-O symmetric stretching of MMA. However, in term of peak intensities illustrated in Fig. 4(b), the intense, strong and sharp peak became weak and broader with addition of lithium salt. This is due to the weak interaction between oxygen atoms and lithium ion from doping salt. This change has not been reported before. It was also observed, that there is no significant changes at C=C stretching of polyisoprene and CH₃ asymmetric stretching of MMA/rubber structure.

(a)

(b)

Fig. 4. FTIR spectrum for (a) carbonyl group and (b) ether group

4. Conclusion

SPE MG49-PMMA doped with LiClO₄films have been successfully prepared by solution casting technique. The highest conductivity is 1.47×10^-8 S.cm^-1 at 20 wt. % of LiClO₄ salt loading. The observation on structural studies done by XRD showed that the crystallinity phase is reduced at the highest conductivity and FTIR analysis showed that the interaction between lithium ion and oxygen atoms occurred at carbonyl (C=O) and ether (C-O-C) groups.

5. ACKNOWLEDGEMENTS

The authors would like to extend their gratitude towards Universiti Kebangsaan Malaysia for allowing this research to be carried out. This work is supported by the MOSTI grant 03-01-02-SF0423.

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