Nonlinear conductivity spectra of new polymer blend films with high ion conductivity

Abstract

Thin secondary batteries using ion-conducting polymers are now being developed. In order to realize secondary batteries of good performance, recently, the study of ionic transport processes in ion-conducting polymers, has been intensively pursued and the findings have become a focus of attention [1, 2]. Detailed information regarding the correlation between ionic transport and dynamic behavior of the polymer chain [2, 3] is expected to be obtained through the investigation of the frequency dependence of conductivity. In order to obtain useful information on this correlation, many researchers have measured the frequency dependence of permittivity and conductivity in ion-conducting polymers in the linear region where the current response is proportional to the applied electric field [2–8]. On the other hand, we have taken note of the possibility that studies on nonlinear spectra of conductivity may provide information regarding the elementary processes of ionic transport and the formation of potential energy, which is difficult to obtain from measurements in the linear region [9, 10]. In this work, we extend the measurements of the conductivity spectra to a nonlinear regime in order to gain additional knowledge on the correlation between ionic transport and dynamic behavior of the polymer chain. In addition to this, to attempt this work, we fabricated a new polymer blend film of polypropylene oxide (PPO) and epoxy compound (EPOXY), and including Li+. The outline of blend polymer film fabrication is described below [11]. Tetrahydrofuran (THF) was distilled on sodium and LiCIO4 was dried at 100 ◦C in vacuo before use. First, PPO was dissolved in the THF in which LiCIO4 had been dissolved ([Li+]/[O] = 0.02). The PPO used here is composed of trifunctional oligomers (ADK-G1500 made by Asahi Denka Kogyo. Co., Ltd.), shown in Fig. 1, with the average molecular weight (Mw) of 1500. Next, the epoxy resin was added to the solution. The curing agent, polythiol (polyamine) was then added to the solution. The mixture was quickly stirred and poured into flat-bottomed Teflon vessels and allowed to stand for a few days in a dry atmosphere at room temperature. After the film was formed, it was transferred into another Teflon vessel. Then it was heated at 100 ◦C in vacuo for 24 h. Finally, blend polymer films with 40 wt% of PPO and 60 wt% of EPOXY (PPO/EPOXY) were obtained. Next, we studied the high-order structures of PPO/EPOXY using transmission electron microscope (TEM) images. PPO particle dispersion in PPO/EPOXY was then determined by TEM graphic image data processing using a microcomputer. As a result, two phases were found to exist in PPO/EPOXY: one is made up of PPO particles with diameters of 70– 100 nm which form clusters of about 400–500 nm in size, and the other contains EPOXY. The values obtained from TEM graphic image data processing are listed in Table I. Next, the transition temperatures of PPO/EPOXY were determined to be about −50 ◦C and 50 ◦C by differential scanning calorimeter (DSC) measurements. The low-temperature of −50 ◦C (Tg1) corresponds to the glass transition temperature of PPO, and the high-temperature of 50 ◦C (Tg2) corresponds to that of EPOXY. The existence of Tg1 and Tg2 is consistent with the following results. The complex elastic constant, C∗ = C ′ + jC ′′ was determined by increasing the temperature at a frequency of 10 Hz. The results are shown in Fig. 2. The relaxation of C∗ is observed at around −50 ◦C and 50 ◦C, which correspond to Tg1 and Tg2, respectively. When the current I for a nonlinear system is expanded by odd powers of the electric field E ,