Molecular Weight Dependence of Ion-Mode Relaxation and DC Conduction in Polypropylene Oxide Complexed with LiClO₄

Broadband dielectric spectroscopy has been employed to investigate the ion dynamics in polypropylene oxide (PPO) complexed with 0.1% LiClO 4 over a molecular weight range of M =425–3500. It is found that ions undergo two types of motions: local fluctuation (ion-mode relaxation) and long-range diffusion (dc conduction). The dc conductivity σ dc decreases markedly with increasing molecular weight (∝ M -2 ) whereas the ion-mode relaxation time τ ion and its strength Δ ε ion are nearly independent of M . The ion-mode relaxation is an indication of some structural inhomogeneity that confines ions temporarily in a certain domain. Analyses of these experimental results based on a dynamic percolation model lead us to propose the following ionic processes in PPO/LiClO 4 systems. The ionic motion originates in the elementary jumps for a distance equivalent to a monomer length at a frequency of segmental mode molecular motion. Due to structural inhomogeneity, such an elementary motion is confined in a domain of approximately 1.2 nm in diameter to result in a relaxational nature specified by the time constant τ f . Structural renewal occurring at a rate of 1/τ r allows ions to jump into adjacent domains and to undergo long-range diffusion. Below M =400, τ r equals τ f . In a high M range, τ r becomes much larger than τ f to control σ dc according to σ dc =Δ ε ion /τ r . Structural renewal is suggested to occur in association with a large-scale molecular motion.