NMR and Conductivity Studies of Ethylene Oxide-Epichloridrine Copolymer Doped with LiClO 4 A. E. Wolfenson, ²,‡ R. M. Torresi,* ,§ T. J. Bonagamba, ² M. A. De Paoli, | and H. Panepucci ² Instituto de Fı ´sica de Sa ˜ o CarlossUSP, Caixa Postal 369, 13560-970 Sa ˜ o Carlos (SP), Brazil, Instituto de Quı ´mica de Sa ˜ o CarlossUSP, Caixa Postal 780, 13560-970 Sa ˜ o Carlos (SP), Brazil, and Instituto de Quı ´micasUNICAMP, Caixa Postal 6154, 13083-970 Campinas (SP), Brazil ReceiVed: July 1, 1996; In Final Form: February 5, 1997 X The copolymer ethylene oxide and epichloridrine doped with lithium perchlorate shows interesting properties as an ionic conducting polymer mainly because of its amorphous structure at room temperature and its low glass transition temperature (T g )- 40 °C). 13 C, 7 Li, and 1 H nuclear magnetic resonance studies were performed in order to analyze the interactions between the polymer matrix and the dopant salt with solvent molecules. These studies and conductivity measurements were used to investigate the influence of the preparation method and the properties of the solvent on the physicochemical characteristics of the polymer electrolyte. Experimental results show that solvent molecules are retained in the solid polymer electrolyte, strongly modifying its ion transport dynamic. This fact can be related to the increase in the ionic conductivity when solvent molecules rest inside the polymer matrix, producing a plasticizing effect. Introduction Polymeric electrolytes are composed of a polar matrix polymer and lithium salts forming a solid salt solution. These systems have a high ionic conductivity and they have received considerable attention in the past decade because of possible technological applications in advanced electronic devices (solid state lithium batteries, supercapacitors, electrochromic displays, etc.). 1,2 The family of poly(ethylene oxide) (PEO) polymers has been extensively studied because of its properties as a polar matrix material. 3 However, its high transition temperature to the amorphous phase (∼65 °C) and its high tendency to crystallize lead to low ionic conductivity and poor mechanical properties at room temperature. 4-6 To obtain better properties at room temperature, the PEO must be modified to have a larger proportion of the amorphous phase. 7 This structural modifica- tion of the PEO matrix can be obtained using substituted monomers in order to produce another polymer, a copolymer, or a terpolymer. Another method to improve the conductivity and mechanical properties of the solid electrolyte is the addition of inorganic and organic soluble additives to increase the segmental motion of the polymer matrix in order to aid the ion transport in the solid electrolyte. 8-10 The copolymer of PEO with epichloridrine, poly(epichlori- drine-co-ethylene oxide) (PEPI-PEO) has been proposed as a solid electrolyte because of its capacity to combine with the salt cations to form a homogeneous solid solution with good conductivity at room temperature. 11,12 The glass transition temperature of PEPI-PEO is ca. -40 °C, 13 and it is amorphous at room temperature and shows a high thermal stability in a wide temperature range. For this reason, good ionic conductiv- ity is expected when inorganic salts are dissolved into the PEPI- PEO matrix. The elastomer PEPI-PEO shows interesting characteristics such as high elasticity at low temperatures and good resistance to solvents, and it allows for a wide range of working temperature (-40 to 150 °C). 13 In this work, nuclear magnetic resonance (NMR) and conductivity measurements have been performed with PEPI- PEO copolymer doped with lithium perchlorate salt in different [O/Li] ratios. Tetrahydrofuran (THF) and acetone were used as solvents in the preparation process. The aim of these studies is to identify the role of the solvent and the effect of the dopant on the conductive properties of the solid electrolyte. 13 C, 7 Li, and 1 H NMR spectra were used, together with conductivity measurements, to characterize the chemical process involved in the formation of the solid electrolyte. Experimental Section PEPI-PEO copolymer (1:1) was provided by DINACO (Sa ˜o Paulo, Brazil) under the commercial denomination Hydrin C. The PEPI-PEO was purified by dissolution in THF (Merck) followed by coagulation in ethanol (Merck). The process was repeated three times, and the PEPI-PEO obtained was dried in vacuum. Solid solutions (purified (PEPI-PEO)-LiClO 4 (Riedel-de Hae ¨n)) were prepared from 1.0 g of PEPI-PEO + 30 mL of solvent by three different methods: Method A. Samples were prepared by dissolution of PEPI- PEO copolymer and different quantities of LiClO 4 in THF. The drying process was carried out at room temperature under vacuum (10 -4 Torr) for 72 h. Method B. Samples were prepared by dissolution of PEPI- PEO copolymer and different quantities of LiClO 4 in acetone (Merck). In this case, the copolymer was not completely dissolved by the solvent, and the incorporation of LiClO 4 was produced by diffusion. The drying process was carried out at room temperature under vacuum in the same way as in method A. Method C. Samples were prepared in the same way as method B, but in order to improve the diffusion process, the temperature was maintained at 40 °C for 72 h and the samples kept covered with acetone. Then the drying process was carried out at 55 °C for 120 h under a nitrogen stream. All samples were stored under nitrogen atmosphere. The [O/Li] ratio range studied was 600-0.3. The high-resolution solid-state NMR experiments were made in a 2 T home-built spectrometer using a Doty double-resonance ² Instituto de Fı ´sica de Sa ˜o CarlossUSP. ‡ Permanent address: FaMAF-UNC, Ciudad Universitaria, 5000 Co ´r- doba, Argentina. § Instituto de Quı ´mica de Sa ˜o CarlossUSP. | Instituto de Quı ´micasUNICAMP. X Abstract published in AdVance ACS Abstracts, April 1, 1997. 3469 J. Phys. Chem. B 1997, 101, 3469-3473 S1089-5647(96)01926-8 CCC: $14.00 © 1997 American Chemical Society