Local Free Volume and Structure of Polymer Gel Electrolytes on the Basis of Alternating Copolymers

Abstract

The relationship between the structure of the polymer and the charge carrier mobility and the ionic conductivity has been studied for a new class of gel electrolytes on the basis of alternating copolymers. These gel electrolytes were prepared by photopolymerization of maleic anhydride and oligo(ethylene glycol) 4 divinyl ether in the presence of various oligo(ethylene glycol) n dimethyl ethers with molar masses between 134 and 2000 g/mol, and LiCF 3 SO 3 . Thermal properties of the materials were studied by differential scanning calorimetry and dynamic mechanical analysis, which additionally gives structural information. Changes in the free volume as a function of the content of the plasticizer and the salt were studied by positron annihilation lifetime (PAL) spectroscopy. The self-diffusivity of charge carriers and plasticizer in the gel electrolytes was investigated by pulsed field gradient NMR. The ionic conductivity and its pressure dependence were determined by the impedance technique. The gel electrolytes studied are heterogeneous materials composed of a highly crosslinked polymer network (M c ≈ 600 g/mol) with a T g ≈ 100 °C and of a plasticizer-salt solution with a T g ≈ -70 °C. Hence, two hole size distributions were measured by PAL spectroscopy around 0.26 and 0.34 nm related to the network and the liquid phase, respectively. The activation volume V* calculated from the pressure dependence of the ionic conductivity was V* ) 22.7 cm 3 /mol. It is concluded that the charge carrier transport occurs in the liquid phase of the gel electrolytes. However, the network is too dense to provide sufficient distribution and mobility of the plasticizer-salt solution. Self-diffusivity and conductivity of the gel electrolytes studied are not related according to the Nernst-Einstein equation. The ortho-positronium (o-Ps) lifetime τ 3 and its intensity I 3 in the gel were found to be changed if salts were added to the gel. The o-Ps lifetime is discussed in terms of o-Ps bubbles in the plasticizer-salt solution. The o-Ps intensity I 3 , which decreases with the salt concentration, mirrors inhibition reactions of the o-Ps formation attributed to the anions of the salt.