Local free volume and structure of polymer gel electrolytes on the basis of alternating copolymers

The relationship between the structure of the polymer and the charge carrie r mobility and the ionic conductivity has been studied for a new class of g el electrolytes on the basis of alternating copolymers. These gel electroly tes were prepared by photopolymerization of maleic anhydride and oligo(ethy lene glycol)(4) divinyl ether in the presence of various oligo(ethylene gly col), dimethyl ethers with molar masses between 134 and 2000 g/mol, and LiC F3SO3. Thermal properties of the materials were studied by differential sca nning calorimetry and dynamic mechanical analysis, which additionally gives structural information. Changes in the free volume as a function of the co ntent of the plasticizer and the salt were studied by positron annihilation lifetime (PAL) spectroscopy. The self-diffusivity of charge carriers and p lasticizer in the gel electrolytes was investigated by pulsed field gradien t NMR. The ionic conductivity and its pressure dependence were determined b y the impedance technique. The gel electrolytes studied are heterogeneous m aterials composed of a highly crosslinked polymer network (M-c approximate to 600 g/mol) with a T-g approximate to 100 degrees C and of a plasticizer- salt solution with a T-g approximate to -70 degrees C. Hence, two hole size distributions were measured by PAL spectroscopy around 0.26 and 0.34 nm re lated to the network and the liquid phase, respectively. The activation vol ume V* calculated from the pressure dependence of the ionic conductivity wa s V* = 22.7 cm(3)/mol. It is concluded that the charge carrier transport oc curs in the liquid phase of the gel electrolytes. However, the network is t oo dense to provide sufficient distribution and mobility of the plasticizer -salt solution. Self-diffusivity and conductivity of the gel electrolytes s tudied are not related according to the Nernst-Einstein equation. The ortho -positronium (o-Ps) lifetime tau(3) and its intensity I-3 in the gel were f ound to be changed if salts were added to the gel. The o-Ps lifetime is dis cussed in terms of o-Ps bubbles in the plasticizer-salt solution. The o-Ps intensity I-3, which decreases with the salt concentration, mirrors inhibit ion reactions of the o-Ps formation attributed to the anions of the salt.