PEO-Mg(NO3)2 착화합물의 열분해 특성

조영신, 심미자*, 김상욱

서울시립대학교 화학공학과

*서울시립대학교 생명과학과

Thermal Decomposition Characteristics of PEO Complex with Mg(NO3)2

Young-Shin Cho, Mi-Ja Shim*, Sang-Wook Kim

Dept. of Chem. Eng., The Univ. of Seoul

*Dept. of Life Sci., The Univ. of Seoul

Introduction

This age is an information-oriented society. So, the electronic devices like portable computer, codeless telephone have been very important. Such devices need for becoming smaller, thinner and more flexible [1]. In that case, rechargeable battery is the most important part in electronic device. With the development of the minimization of electric device, solid polymer electrolytes are applied for the electro-chemical devices such as batteries because of its high reliability without leakage of electrolyte, high energy density and easy formation of thin films. By the way, the ionic conductivity of the polymer electrolyte is too low for the application of polymer battery at room temperature [2]. Many researchers have tried to develop a good electrolyte and the complex of PEO and inorganic compound was considered as one of the best candidate [1,3]. The PEO based electrolytes using divalent cationic salts, especially Mg2+-cation conducting polymers, are of interest due to their possible applications as solid electrolytes. The divalent charge and the consequent increase in anion to cation ratio of 2:1. The thermal stability of electrolyte should be considered for the application of battery electrolyte which is operated at the elevated temperatue. In this study, the thermal properties of the new ion conducting polymer electrolyte based on PEO complexed with MG(NO3)2 salt were discussed by using TA of TGA and DSC.

Experiment

Anhydrous magnesium nitrate (Mg(NO3)2) was provided from Duksan Pharmaceutical Co. Ltd. PEO (polyethylene oxide) with a molecular weight of 6×105 was supplied by Aldrich (CTLG No. 18,202-8) and the structure is follows:

PEO and Mg(NO3)2 were each dissolved in methyl alcohol at a concentration of 15 w%. Mixing these two solutions in different proportions formed complexes PEO/Mg(NO3)2 in the PP (polypropylene) dishes. After the methyl alcohol was evaporated at room temperature and atmospheric pressure for 12 hrs. The sample was dried in a vacuum oven at 40℃ for at least 12 hrs and the scanty residue of methyl alcohol was removed. The samples were then kept in a dry glove bag filled with nitrogen. The thermal behaviour was investigated using DSC 4000 (Solomat, USA) in the temperature range of RT to 300℃ at a heating rate of 10 ℃/min under nitrogen atmosphere. The glass transition temperature (Tg) was taken as the midpoint of the baseline shift. The melting temperature (Tm) was obtained from the onset location of the endothermic peak. Thermal decomposition characteristics were obtained using TGA 1000 (ISI, USA) in the temperature range of RT to 500℃. Initial decomposition temperature (IDT) was measured from the onset location of the base line shift. The thermal decomposition activation energy was calculated from the Flynn and Wall equation.

Results and Discussion

Fig. 1. TG thermogram of PEO and PEO complex with Mg(NO3)2.

Fig. 1 shows the thermogravimetric (TG) curves of PEO and PEO complex with Mg(NO3)2. The initial decomposition temperature of pure PEO was 418.5℃. The TG curve showed one step thermal degradation. As the PEO was complexed with Mg2+-cation, the initial decomposition temperature increased from 418.5 to 432℃. Mg2+-cation acted as the thermal heat absorber. It is thought that the Mg2+-cation could bind the PEO compound and the thermal stability increased under the pure thermal stress.

Fig. 2 shows the DSC curves of PEO and PEO complex with Mg(NO3)2. The melting point of the pure PEO was 62℃. The DSC curve showed one step melting process. As the PEO was complexed with Mg(NO3)2, the melting temperature decreased to 59℃. The mixing ratio between PEO and Mg(NO3)2 was 8:1. The melting range increased. The melting range of PEO was 52 to 70℃ but for the case of the complex was 30 to 80℃. The DSC curve of PEO-Mg(NO3)2 complex also

Fig. 2. DSC thermogram of PEO and PEO complex with Mg(NO3)2.

Fig. 3. DTG thermogram of PEO complex with Mg(NO3)2.

showed one step melting process. It showed that the two components were well complexed.

Fig. 3 shows the differential thermogravimetric curve of PEO complex with Mg(NO3)2. The peak temperature is the most rapidly decomposing temperature. The curve has only one peak in the DTG curve. From the DTG peak data, the decomposition reaction activation energy and pre-exponential factor can be obtained by using the revised Kissinger expression:

[eqn: - ln left ( beta over {T_p}^2 right ) = E_a over R · 1 over T_p - ln AR over E_a]

where □: heating rate, Tp: temperature at the peak value of the DTG curve, Ea: decomposition activation energy, A: pre-exponential factor, R : gas constant

The ionic conductivity depends on the morphology of the complex and chemical structure[4]. So the surface configuration and chemical structure will be examined by using SEM, XRD and FT-IR. The crystalline morphology of the sample should be photographed using a polarizing microscope. The detail results will be discussed in the presentation.

References

1. S. Ramalingaiah, D. Srinivas Reddy, M. Jaipal Reddy, E. Laxminarsaiah, U.V. Subba Rao, "Conductivity and Discharge Characteristic Studies of Novel Polymer Electrolyte Based on PEO Complexed with Mg(NO3)2 Salt", Materials Letters, 29, 285-289 (1996).

2. J.M. Ko, "Materials and Research Trend on Lithium Ion Polymer Battery", Polymer Science and Technology, 9(2), 104-110 (1998).

3. W. Xu, X.Z. Zhang, Z.H. Deng, Y.G. Zheng, G.X. Wan, "Ionic Conduction of Lithium Oligo(ethyleneoxy) Phenylsulfonate and its Complex with Poly(ethylene oxide)", Solide State Inoics, 111, 219-226 (1998).

4. H.J. Kim, J.S. chung, "Resent Trend of Lithium Polymer Battery Technology", Polymer Science and Technology, 9(2), 111-116 (1998).