Network viscoelastic behavior in poly(ethylene oxide) melts: Effects of temperature and dissolved LiClO4 on network structure and dynamic behavior

Low Frequency Dielectric Relaxation and Conductance of Solid Polymer Electrolytes with PEO and Blends of PEO and PMMA

Solid polymer electrolytes are mixtures of polymer and inorganic salt. There are quite a number of studies dealing with the relationship between electric conductivity and structural relaxation in solid polymer electrolytes. We present a phenomenological approach based on fluctuation-dissipation processes. Phase heterogeneity appears in poly(ethylene oxide) (PEO) of high molecular mass and its blends due to crystallization and accompanying phase segregation. Addition of salt hampers crystallization, causing dynamic heterogeneity of the salt mixtures. Conductivity is bound to amorphous phase; the conductivity mechanism does not depend on content of added salt. One observes dispersion of conductivity relaxation only at low frequency. This is also true for blends with poly(methyl methacrylate) (PMMA). In blends, the dynamics of relaxation depend on glass transition of the system. Glassy PMMA hampers relaxation at room temperature. Relaxation can only be observed when salt content is sufficiently high. As long as blends are in rubbery state at room temperature, they behave PEO-like. Blends turn into glassy state when PMMA is in excess. Decoupling of long-ranging and dielectric short-ranging relaxation can be observed. Conductivity mechanism in PEO, as well as in blends with PMMA were analyzed in terms of complex impedance Z*, complex permittivity, tangent loss spectra and complex conductivity.

Characterization of polymer electrolytes by dielectric response using electrochemical impedance spectroscopy

Authors present a phenomenological view on dielectric relaxation in polymer electrolytes, which is monitored by electrochemical impedance spectroscopy. Molecular interaction of polymer chains with salt molecules (or dipole-dipole interaction between segments and salt molecules) leads to dipolar molecular entities. Frequency-dependant impedance spectra are the key quantities of the interest for determination of electric properties of materials and their interfaces with conducting electrodes. Salt concentration serves as parameter. Bulk and interfacial properties of the samples are discussed in terms of impedance (Z *) and modulus (M *) spectra. We focus on two different classes of systems, i.e. high molar mass of poly(ethylene oxide) (PEO)+lithium perchlorate (LiClO4) (i.e. the inorganic salt) and epoxidized natural rubber (ENR-25) with 25 mol% of epoxide content+LiClO4. Impedance spectra with salt content as parameter tell us that we have interaction between dipolar entities leading to dispersion of relaxation times. However, as scaling relations show, dispersion of relaxation times does not depend on salt content in the PEO system. The relaxation peak for the imaginary part of electric modulus (M″) provides information on long-range motion of dipoles. Summarizing the results from imaginary part of impedance spectrum (Z″), tan δ (imaginary/real of permittivities) and M″ for the two systems under the discussion, PEO behaves like a mixture of chains with dipoles. There are interactions between the dipoles, but they are relaxing individually. Therefore, we see PEO-salt system as a polymer electrolyte where only a tiny fraction of added salt molecules becomes electrically active in promoting conductance. However, ENR-25-salt system behaves just as a macroscopic dipole and it can not display electrode polarization or electric relaxation because there is no mobility of individual dipoles. Hence, ENR-25-salt does not form a polymer electrolyte in the classic sense.

Molecular dynamics simulation of the polymer electrolyte poly(ethylene oxide)/LiClO4. II. Dynamical properties

The dynamical properties of the polymer electrolyte poly(ethylene oxide) (PEO)LiClO(4) have been investigated by molecular dynamics simulations. The effect of changing salt concentration and temperature was evaluated on several time correlation functions. Ionic displacements projected on different directions reveal anisotropy in short-time (rattling) and long-time (diffusive) dynamics of Li(+) cations. It is shown that ionic mobility is coupled to the segmental motion of the polymeric chain. Structural relaxation is probed by the intermediate scattering function F(k,t) at several wave vectors. Good agreement was found between calculated and experimental F(k,t) for pure PEO. A remarkable slowing down of polymer relaxation is observed upon addition of the salt. The ionic conductivity estimated by the Nernst-Einstein equation is approximately ten times higher than the actual conductivity calculated by the time correlation function of charge current.

An investigation of a sol-gel/melt transition: The poly(ethylene oxide)/methanol/LiClO4 system

The crossover behavior of 50 000 molar mass poly(ethylene oxide)/methanol solutions from dilute solution to the melt/gel was examined. At first this behavior was investigated without LiClO(4) and then reexamined with LiClO(4). To better understand this behavior, the dependencies of dynamic light scattering (specifically, photon correlation spectroscopy) measurement results on polymer concentration, on the scattering wave vector and on temperature, and the dependence of static light scattering results on the scattering wave vector were studied. This study produced interesting and important results about network structure and behavior in poly(ethylene oxide) solutions and melts generally and about the effects of LiClO(4) on this structure and behavior more particularly.