Temperature dependence of relative modulus in filled polymer systems

The temperature dependence of relative modulus observed in filled thermoset, thermoplastic, and polyelectrolyte salt matrices is explained on the basis of induced stresses produced by the differences in the thermal expansion coefficients of the constituent materials. The analysis is based on the assumption that the modulus of the matrix in a filled polymer is less than that of the unfilled polymer. The temperature dependence of relative modulus is expressed as a function of the difference in thermal expansion coefficients, the volume fraction, the relative modulus in the unstressed state, and mechanical properties of the phases. Agreement is good between the analysis and experimental results for three systems: epoxy and glass, polyethylene and wollastonite, and a polyelectrolyte salt with mica and asbestos.     

Anomalous temperature dependence of a dielectric relaxation rate

A polypropylene film containing 2,4,6-triiodophenol had, at 8 K, a dielectric absorption peak at 13.5 Hz. The peak moved to 22 Hz at 20 K, back to 15.5 Hz at 48 K, then rapidly to higher frequency with further increase in temperature. This probably results from a change in regime as the thermal phonon wavelengths become comparable to the size of the molecules.     

Ion dynamics and relaxation behavior of NaPF6-doped polymer electrolyte systems

The present paper discusses the ion dynamics of a novel polymeric system prepared by doping of NaPF₆ in a lab-prepared polymer matrix. Ion dynamics of the system is analyzed by presenting the impedance data in different formalisms. Mobility (and hence the conductivity) continuously increases with salt concentration, and the phenomenon is correlated with salt’s plasticization nature, which is reconfirmed by the shifting of minima in ∂logε′/∂logω vs. logω curve towards high frequency. It has been observed that number of charge carriers (N′) estimated from conductivity data do not represent the real charge concentration in the system. Within the experimental frequency (∼MHz) range, three different regions are identified in ∂logσ vs. ∂logω curves namely (i) dc conductivity/free hopping, (ii) correlated ion hopping, and (iii) caged movement of ions. In the present case, also a scaled master conductivity curve is obtained by estimating the σ ₀ and ω _p (exclusively in Jonscher Power Law or JPL region) according to our previously proposed method. Scaling is realized with respect to salt concentration and temperature, which is an indication that salt concentration and temperature are only governing the number of charge carriers and mobility without affecting the underlying ion transport mechanism. Non-Debye-type relaxation phenomenon is indicated by KWW exponent β (<1). Relaxation times, obtained from tanδ vs. logω curves, inversely follow the conductivity, indicating strongly correlated ion-polymer segmental motions.

     

Anisotropy of the dielectric relaxation of a crystalline polymer

A theory which relates the change in the strength of absorption to the change in crystal orientation function is presented for the anisotropy of dielectric relaxation for the dipolar orientation change in an oriented crystalline polymer. Experimental measurements are presented for the α dielectric absorption of a stretched ethylene–carbon monoxide copolymer and compared with the orientation of crystals of this copolymer as determined by x-ray diffraction.     

Temperature dependence of vibrational relaxation in carbon tetrachloride

The temperature dependence of vibrational relaxation in carbon tetrachloride has been investigated from near the melting point to near the boiling point. The method of spontaneous Brillouin scattering has been used to determine the hypersonic frequencies and velocities as functions of temperature. The dispersion in the velocities are then compared with theoretical predictions in order to investigate the character of the relaxation.     

Temperature dependence of dephasing relaxation in dense media

Temperature dependence of dephasing relaxation is studied within the framework of impact theory. The temperature dependence of a linewidth calculated by perturbation theory is non-monotonic; as T rises, the spectral line first narrows, then broadens. Motion narrowing of the line is attributed to the effect of attraction forces.     

Temperature dependent dielectric relaxation study of ethylene glycol-water mixtures

Using the picosecond time domain reflectometry method, dielectric relaxation measurements for 13 ethylene glycol (EG)-water mixtures have been studied from 0 to 40°C. The dielectric relaxations in the mixtures show a Debye-type behavior, whereas the relaxation in pure EG can be described by the Cole-Cole model. The static dielectric constant ₀, the relaxation time and the dielectric constant at high frequency have also been determined at various temperatures. The dielectric relaxation data suggests that there is no tendency to form hydrogen bonds with the addition of water to EG unlike other alcoholic systems but this tendency becomes increasingly important with decreasing temperature. The activation energy decreases with increased water content in the mixture as expected.     

Temperature dependence of phase behavior for ternary systems composed of ionic liquid + sucrose + water

In this work, temperature dependence of phase behaviors for the [Bmim]BF 4 + sucrose + water system was investigated. It was found that interaction of [Bmim]BF 4 with sucrose is exothermic, and lowering temperature is favorable for phase separation. In addition, a "[Bmim] (+)-induced structural changes" model was developed and used to interpret the temperature effect, whereby the salting-out effect was thought to be an entropy driving process through analysis of the structural interaction and the electrostatic interaction.     

Temperature dependence of vibrational energy relaxation in liquid oxygen

The energy relaxation of the lowest vibrational level (υ = 1) of liquid oxygen in the electronic ground state was investigated within a wide temperature range (53.4 K ? T ? 96 K). The relaxation time exhibits a peak value of τ′ ≈ 3.1 ms around 65 K and is shorter at lower and higher temperatures. The observed temperature behavior is discussed in view of theoretical models of energy relaxation in liquids.     

Theory of relaxation spectra and dielectric relaxation of rigid rodlike particles incorporated in a polymer network

The theory of molecular mobility and relaxation spectra is developed for rodlike particles embedded in a polymer network with allowance for the involvement of the particles in collective network dynamics through topological entanglements with network fragments. A regular cubic coarse-grained network model is used, where the motion of junctions describes the mobility of large fragments (domains) of the initial network with a size equal to the distance between adjacent rodlike particles. The involvement of the rods in collective network dynamics is taken into account by introducing an effective quasi-elastic potential acting between the rods and junctions of the coarse-grained network and preventing long-distance diffusion of the embedded particles. The viscoelastic parameters of the coarse-grained (“renormalized”) network are functions of the viscoelastic characteristics of the initial network. The relaxation time spectra are calculated as well as the frequency dependences of the dielectric loss factor of the embedded particles that possess a permanent dipole moment directed along the major axis of each rod. Depending on the ratio between the viscoelastic characteristics of the rods and the network, the frequency dependence of the dielectric loss factor may have two maxima. The high-frequency maximum corresponds to local orientational movements of particles at fixed junctions of the coarse-grained network, which correspond to the position of the domain centers in the initial network. The low-frequency maximum corresponds to movements of particles involved in large-scale dynamics of network fragments. The dependence of the dielectric loss factor on the ratio between the viscoelastic parameters of the rods and the network is studied.     

Finite-element calculations for dielectric relaxation of one-sphere systems in a parallel-electrode measuring cell

Using the finite-element method (FEM), the frequency dependence of conductivity and relative permittivity was calculated for a heterogeneous specimen in which a sphere was placed in a medium. The FEM calculations were made for the whole system in which the specimen was located in a measuring cell comprising two parallel electrodes and a spacer with a cylindrical sample cavity, as in practical measurements. Values of the conductivity and the permittivity of the sphere and the medium were chosen so as to simulate specimens of water-in-nitrobenzene (W/N) and oil-in-water (O/W) types. When the volume ratio of the sphere to the sample cavity p was below 0.1, the frequency dependence of conductivity and permittivity was in good agreement with that described by Wagner's theory for both the W/N and O/W specimens. When p was greater than 0.1, deviations from Wagner's theory were found for limiting values of the conductivity and permittivity at low and high frequencies for both the W/N and O/W specimens, and for the relaxation time for the W/N specimens. Received: 15 July 1998 Accepted in revised form: 5 November 1998     

Temperature dependence of excess enthalpies for systems containing normal hexadecane

Excess enthalpies, and heat capacities derived therefrom, have been obtained between 25 and 65 or 75°C at a constant concentration for cyclohexane and octamethylcyclotetrasiloxane mixed with normal hexadecane and with a highly branched C₁₆ isomer, 2,2,4,4,6,8,8-heptamethylnonane, and also forcis-andtrans-decalin mixed withn-C₁₆. Theh ^E values withn-C₁₆ are positive and much larger than with the branched-C₁₆. They decrease rapidly withT so thatc _p ^E is large and negative. These results imply the presence of orientational order in then-C₁₆, which is destroyed on mixing with the other component and which decreases withT. Theh ^E fortrans-decalin+n-C₁₆ is much smaller than forcis-decalin+n-C₁₆, and becomes negative with increase ofT. This change of sign, which is unexplained by current theory, is interpreted as due to an interference of the flat, plateliketrans-decalin molecule with the molecular motion of then-C₁₆ chain.     

Analysis of the cure-dependent dielectric relaxation behavior of an epoxy resin

The dielectric relaxation behavior of an epoxy-amine resin was investigated using the Williams-Watts relaxation function. Phenomenologically, the dielectric features of the resin during cure are similar to those of stable materials. The distribution parameter of the dipole relaxation decreases from the onset of cure to a conversion near the gel point and then maintains a constant value. Based on the experimental observations and theoretical considerations, a single-frequency approach has been proposed for extracting the relaxation time of maximum loss. The relaxation data so obtained are independent of the measurement frequency and are in agreement with those acquired directly from the dipole loss peaks. © 1994 John Wiley & Sons, Inc.     

Temperature dependence of the conformational relaxation time of polymer molecules in elongational flow: Invariance of the molecular weight exponent

Using an elongational flow technique, we have investigated the relationship between the molecular conformational relaxation time and the molecular weight for a solvent whose quality was altered thermally from near θ to a good solvent state. The materials used were closely monodisperse samples of atactic polystyrene. The results show that the relaxation time τ varies with the molecular weight M as τ ∝ M^1.5, independent of the solvent quality, a result which apparently is at variance with the observed molecular weight dependence of intrinsic viscosity. Despite this invariance of the molecular weight exponent with solvent quality, our results also show that the coils do expand when the solvent quality was increased in agreement with the mean-field theory of Flory.     

Simulation of dielectric relaxation in periodic binary systems of complex geometry

Dielectric relaxation in binary mixtures containing particles or lamellae with complex geometry has been simulated within the quasielectrostatic approximation by a three-dimensional finite-difference method. The method was tested using simple models corresponding to water-in-oil (W/O) and oil-in-water (O/W) emulsions with volume fraction P up to 0.5. The dielectric spectra calculated by the finite difference method agreed with those expected from Wagner's equation at P < or =0.3 and approached those from Hanai's equation at P>0.4. This method was applied to more complicated binary mixtures of oil and water: a bicontinuous cubic structure, a suspension of particles with projections, and a planar bilayer with a rippled or interdigitated interface. The bicontinuous phase that is supposed to appear near the transition between W/O and O/W emulsions showed dielectric properties similar to those of the O/W emulsion. The undulation of the particle surface and the interface of the planar bilayer affected all parameters of dielectric relaxation, especially the relaxation intensity.     

Conductivity and dielectric relaxation in various polyvinyl alcohol/ammonium salt composites

We studied electrical conductivity and dielectric relaxation in polyvinyl alcohol/ammonium chloride and polyvinyl alcohol/ammonium acetate composite films. Infrared absorbance showed the presence of H-bonding interaction between the salt and the polymer. X-ray diffraction showed the reduction of the grain size of ordered regions in the polymer matrix after adding salt. Thermo gravimetric analysis (TGA) showed water wt% content between 4.2 and 5.8%. Differential Scanning Calorimetry (DSC) showed the decrease of the glass transition due to retained water indicating its plasticizer effect. The ac conductivity studied in the frequency range from 10^?1 Hz to 1 MHz and the temperature range from 10 to 150°C is described by the universal law of Jonsher characterizing the charge transport in disordered materials. With NH₄Cl inclusion, the dc conductivity showed a higher value in the vicinity of 4% but with NH₄CH₃CO₂ the dc conductivity decreases monotonically by increasing the salt amount. By using the dielectric permittivity and dielectric modulus we detected three relaxation processes which we attributed to electrode/sample polarization, alpha relaxation and conductivity relaxation respectively.     

Nuclear spin relaxation times dispersion in disordered polymer systems

Nuclear spin relaxation reveals a wealth of information concerning motional dynamics and morphology within a polymer system. The conventional breakdown of relaxation into discrete decay components is usually arbitrary and often ambiguous due to strong coupling and spin diffusion between domains. To avoid being limited by pre-determined models, we have explored the utility of maximum entropy regularization scheme to reconstruct the complete NMR spin relaxation distribution continuum. Case studies showed that the three characteristics of the distribution correlate with various aspects of polymer morphology and with physical properties. These positive results suggested that this approach has the potential of yielding profiles on polymer dynamics and information concerning phonon coupling between domains in complex polymer systems which are not previously accessible. Furthermore, the nondestructive nature of NMR gives this approach an extra advantage over conventional tests for “in-situ” studies of polymers.     

Nonlinear rheological behavior of polymer systems for several shear-flow histories

A stiffened and automated Weissenberg rheogoniometer has been used to measure time-dependent nonlinear properties of several polymer solutions in simple shear flow. Departures from finite linear viscoelasticity are correlated with molecular structure. It is shown that certain features of behavior at high shear rates can be explained if the Doi–Edwards equilibration process is taken into account. A method is outlined for calculating the characteristic time τ_e for this process as a function of concentration, molecular weight, temperature, and branch length. This characteristic equilibration time is incorporated into a simplified BKZ constitutive equation. The proposed equation is found to adequately describe a wide range of experimental results both in step-strain and continuous shear flows.     

Resistance of dielectric polymer films with fillers in metal-polymer-metal systems

The effect of physical factors on the resistance of thin films of nonconjugated polymers was studied. The resistance of polymer films with the metal type of conductivity decreases with an increase in the amount of interchain hydrogen bonds and of metal nanoparticles in the polymer matrix, and also in the presence of a plasticizing additive. Introduction of paramagnetic admixtures and formation of interchain covalent cross-links, on the contrary, increase the resistance of polymer films.