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% LiClO4 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 sigma (dc) decreases markedly with increasing molecular weight (proportional to M-2) whereas the ion-mode relaxation time and its strength Delta epsilon (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 an a dynamic percolation model lead us to propose the following ionic processes in PPO/LiClO4 systems. The ionic motion originates in the elementary jumps for a distance equivalent to a monomer length at a frequency of se.-mental 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 rf. Structural renewal occurring at a rate of 1/tau (r) allows ions to jump into adjacent domains and to undergo long-range diffusion. Below M = 400, tau (r) equals tau (f). In a high M range, tau (r), becomes much larger than rf to control sigma (dc) according to sigma (dc) = Delta epsilon (ion)/tau (r). Structural renewal is suggested to occur in association with a large-scale molecular motion.