Broadband Dielectric Measurements Research Articles

Dynamic mechanical and dielectric relaxation characteristics of poly(trimethylene terephthalate)

The dynamic mechanical and dielectric relaxation properties of a commercial poly(trimethylene terephthalate) [PTT] have been investigated for both quenched and isothermally melt-crystallized specimen films. The relaxation characteristics of PTT were consistent with those of other low-crystallinity semiflexible polymers, e.g. PET and PEEK. While the sub-glass relaxation was largely unperturbed by the presence of the crystalline phase, both calorimetric and broadband dielectric measurements across the glass transition indicated the existence of a sizeable rigid amorphous phase (RAP) fraction in melt-crystallized PTT owing to the constraining influence of the crystal surfaces over the crystal–amorphous interphase region. A strong increase in measured dielectric relaxation intensity (Δ ɛ) with temperature above T g indicated the progressive mobilization of the RAP material, as well as an overall loss of correlation amongst the responding dipoles.

Dielectric secondary relaxations in polypropylene glycols

Broadband dielectric measurements of polypropylene glycol of molecular weight M(w)=400 g / mol (PPG 400) were carried out at ambient pressure over the wide temperature range from 123 to 353 K. Three relaxation processes were observed. Besides the structural alpha relaxation, two secondary relaxations, beta and gamma, were found. The beta process was identified as the true Johari-Goldstein relaxation by using a criterion based on the coupling model prediction. The faster gamma relaxation, well separated from the primary process, undoubtedly exhibits the anomalous behavior near the glass transition temperature (T(g)) which is reflected in the presence of a minimum of the temperature dependence of the gamma-relaxation time. We successfully applied the minimal model [Dyre and Olsen, Phys. Rev. Lett. 91, 155703 (2003)] to describe the entire temperature dependence of the gamma-relaxation time. The asymmetric double-well potential parameters obtained by Dyre and Olsen for the secondary relaxation of tripropylene glycol at ambient pressure were modified by fitting to the minimal model at lower temperatures. Moreover, we showed that the effect of the molecular weight of polypropylene glycol on the minimal model parameters is significantly larger than that of the high pressure. Such results can be explained by the smaller degree of hydrogen bonds formed by longer chain molecules of PPG at ambient pressure than that created by shorter chains of PPG at high pressure.

Broadband Dielectric Study of Dynamics of Poly(vinyl pyrrolidone)−Ethylene Glycol Oligomer Blends

Broadband dielectric measurements for blends of poly(vinyl pyrrolidone) (PVP) and ethylene glycol oligomer (EGO) from 0 to 40 wt % PVP were carried out at 25 degrees C in the frequency range from 20 Hz to 20 GHz. The EGOs used in this study were ethylene glycol (EG), diethylene glycol (2EG), and PEG400 (MW = 400). For the PVP-EG, -2EG, and -PEG400 blends, relaxation processes caused by the motion of EGO in the GHz range and the micro-Brownian motion of the PVP chain at 10 kHz-1 MHz were observed. Although the PVP-EGO blend is miscible, relaxation processes caused by the molecular motion of EGO and the local chain motion of PVP were observed individually. The relaxation time of the local chain motion of PVP showed a strong PVP concentration dependence and a solvent viscosity dependence, which are similar to those reported so far for the solutions in nonpolar solvents.

Anomalous Narrowing of the Structural Relaxation Dispersion of Tris(dimethylsiloxy)phenylsilane at Elevated Pressures

Broadband dielectric relaxation measurements of tris(dimethylsiloxy)phenylsilane were made at ambient pressure and at elevated pressures. The data show an anomalous behavior not previously seen in any other glass-formers; namely, the structural alpha-relaxation loss peak narrows with increasing pressure and temperature at constant peak frequency. Interpreted by the coupling model, the effect is due to reduction of intermolecular coupling at elevated pressures. This interpretation has support from the observed decrease of the separation between the alpha-relaxation and the Johari-Goldstein secondary relaxation, as well as the smaller steepness or "fragility" index m of the data obtained at 1.7 GPa than at ambient pressure.

Broadband dielectric study of the glass transition in poly(ethyleneglycol)-water mixture

We performed broadband dielectric measurements of a polyethyleneglycol-water mixture in the frequency range between 10 GHz and 1 microHz and the temperature range between 300 and 133 K. One relaxation process is observed throughout the whole temperature range. The temperature dependence of the relaxation time clearly obeys the Vogel-Fulcher law above 183 K, and the Arrhenius law below 183 K. This observed relaxation process is the secondary process, and the primary process related to the glass transition is masked by the low-frequency ionic contribution below 183 K. The glass transition concerned with the masked primary process leads to the Vogel-Fulcher to Arrhenius transition of the secondary process.

Characterisation of Pb(Mn 1/3Nb 2/3)O 3 ceramics by SEM, XRD, XPS and dielectric permittivity tests

The Pb(Mn 1/3Nb 2/3)O 3 ceramics has been obtained from oxides by sintering in air, using a two-stage process with precursor columbite-like (Mn 0.5Nb)O 3 phase. The PbO oxide was added in the second stage. Analysis of the X-ray diffraction pattern shows that the ceramics consist of 91% of major perovskite phase. A monoclinic distortion of the perovskite structure was found. The cell parameters are a = 12.193(3) Å, b = 11.966(6) Å, c = 12.144(2) Å, β = 90°10.7′. The microanalysis made with SEM exhibited fluctuation in chemical composition of the perovskite phase. Precipitation of MnO 2, PbO and the Pb–Mn–Nb–O phase different from perovskite was found. The X-ray photoelectron spectroscopy was used to study the electronic structure of the Pb(Mn 1/3Nb 2/3)O 3 ceramics. The core levels of lead, manganese, niobium and oxygen were measured. The shape of valence band ridge is influenced by Mn 3d states. The real average chemical composition obtained from the XPS measurement is Pb 0.99(Mn 0.42Nb 0.67)O 2.92. Broadband dielectric measurement was carried out in 10 −2 to 10 6 Hz and within 80–700 K ranges. The dominant relaxation process exhibits characteristic times typical for ionic processes τ 0,H = 1 × 10 −11 s for the higher temperature range and, τ 0,L = 1 × 10 −9 s for lower temperatures. The activation energy of relaxation process, E M,H = 0.43 eV and E M,L = 0.34 eV corresponds to activation energy of electric conductivity. The dielectric relaxation is ascribed to dipoles created by oxygen vacancies and/or Mn– V O complexes.

Ageing of Adhesive Bonds with Various Surface Treatments, Part 2: Acrylic Modified Amine–Cured Epoxy Resin

ABSTRACT Ageing of adhesive-bonded aluminium–diglycidyl ether of bisphenol A based aliphatic amine–cured acrylic modified epoxy resin reinforced with fused silica exposed to 100% humidity and cycled between 42–48–42 °C every hour is reported. Moisture uptake was followed using broadband dielectric measurements. It was found that the adhesive absorbs moisture very rapidly, the adhesive undergoes significant swelling, and there is a consequent reduction in strength consistent with plasticization of the matrix. The dielectric measurements parallel closely the changes occurring in the adhesive bond line and provide a nondestructive method for monitoring the changes in bond strength. The changes that occur are dominated by the effects of moisture on the adhesive, various pretreatments have little effect on the ageing process, and interfacial failure in this case does not make a significant contribution to the failure of these joints.

Dielectric relaxation processes in water mixtures of tripropylene glycol

Broadband dielectric measurements for anhydrous tripropylene glycol (3PG) and 96, 92, 84, 80, 74, 71, and 68 wt % 3PG-water mixtures are performed in the frequency range of 10(-2)-10(7) Hz and in the temperature range of 123-243 K. We examined the effect of adding water into anhydrous 3PG on relaxation dynamics. Apart from the two well-known relaxation processes, i.e., alpha and beta for anhydrous 3PG we observed new relaxation peak (beta') for all aqueous mixtures of 3PG. In addition we found the critical mole fraction of water x(w)=0.67 in which relaxation dynamics changes its behavior. According to the Sudo approach [S. Sudo et al., J. Non-Cryst. Solids 307-310, 356 (2002)], the behavior of relaxation processes was interpreted assuming the existence of three kinds of cooperative domains (CDs): containing only 3PG molecules, including only water molecules, and including both 3PG and water molecules, which molecules of each kind CD are bound by hydrogen bonds.

Primary and secondary relaxations in bis-5-hydroxypentylphthalate revisited

The molecular structure of bis-5-hydroxypentylphthalate (BHPP) is like dihexyl phthalate but having appended to it two hydroxyl end groups, which contribute additional dipole moments and capacity for hydrogen-bond formation. In a previously published dielectric study of the primary and secondary relaxations of BHPP, it was found that all the dynamic properties are normal except for the anomalously large width of the primary relaxation loss peak. There are two secondary relaxations, the relaxation time of the slower one increases with increasing pressure, whereas that of the faster one is practically insensitive to pressure. Hence, the slower secondary relaxation is the "universal" Johari-Goldstein (JG) [J. Chem. Phys. 53, 2372 (1970); 55, 4245 (1971)] relaxation in BHPP. All is well except if the observed large width of the primary relaxation were an indication of a corresponding large coupling parameter n=0.45 in the coupling model. Then the predicted relations between the primary relaxation time tau(alpha) and the JG relaxation time tau(JG) found previously to hold in many glass formers would be violated. It was recognized that this singular behavior of BHPP is likely due to broadening of the primary loss peak by the overlapping contributions of two independent dipole moments present in BHPP, and the actual coupling parameter is smaller. However, at the time of publication of the previous work there were not enough data to support this explanation. By making broadband dielectric measurements of dibutyl phthalate (DBP) and dioctyl phthalate (DOP) that have chemical structures closely related to BHPP but with only one dipole moment, we show that all their dynamic properties are almost the same as BHPP but the widths of their primary relaxation loss peaks are significantly narrower corresponding to a smaller coupling parameter n=0.34. The new data presented here indicate that the coupling parameter of BHPP is about the same as DBP and DOP, and the predicted relations between tau(alpha) and tau(JG) of BHPP are brought back in agreement with the experimental data.

Comment on “Merging ofαand slowβrelaxation in supercooled liquids”

Fujima [T. Fujima, H. Frusawa, and K. Ito, Phys. Rev. E 66, 031503 (2002)] report on broadband dielectric relaxation measurements on two glass-formers. They find that the relaxation times of the beta relaxation follow different temperature dependences above and below the glass-transition temperature, Tg; i.e., there appears to be a crossover at Tg where the activation energy of the beta relaxation change. In this Comment we show that the observed behavior can be explained by analyzing the merging of the alpha and beta relaxations using an approach proposed by Williams. This analysis clearly shows that the low temperature (below Tg) behaviors of the alpha and beta relaxations can be used to describe also the high-temperature behavior (above Tg). The apparent change in activation energy is thus not to be identified with a change in relaxation mechanism.

Relaxation dynamics and colossal magnetocapacitive effect inCdCr2S4

A thorough investigation of the relaxational dynamics in the recently discovered multiferroic CdCr2S4 showing a colossal magnetocapacitive effect has been performed. Broadband dielectric measurements without and with external magnetic fields up to 10 T provide clear evidence that the observed magnetocapacitive effect stems from enormous changes of the relaxation dynamics induced by the development of magnetic order.

A dielectric test of the validity of the Adam–Gibbs equation out-of-equilibrium: Polymers vs. small molecules

The α-relaxation time, both at equilibrium and out-of-equilibrium, has been investigated by means of dielectric relaxation in a series of fragile glass formers including polymers and several small molecules. The dielectric experiments combine both, thermally stimulated depolarization (accessible time range 10–10 6 s) and standard broad-band dielectric relaxation measurements (frequency range 10 −2–10 7 Hz). The analysis of both set of data was based on a stretched exponential response. The experimental temperature dependence so derived for the relaxation times out-of-equilibrium has been found to give account not only of global polarization experiments but also for those thermally stimulated depolarization experiments involving partial polarization. Moreover, the so obtained activation energies in the glassy range have been quantitatively compared with the predictions of the Adam–Gibbs equation. For the small molecule glass formers a good agreement was found, but systematic discrepancies are apparent for polymers. By studying the influence of physical aging on this comparison, we found that after significant annealing also the polymers tends to agree with the predictions of the Adam–Gibbs equation. All these results would support the validity of the Adam–Gibbs equation for describing the temperature dependence of the time scale of the α-relaxation also out-of-equilibrium. However, they would also imply a noticeable qualitative difference between polymers and small molecule systems concerning the configurational entropy behavior. Whereas a significant temperature variation of the configurational entropy would occur for non-annealed polymers some 10 K below the glass transition, this temperature dependence would be much weaker for small molecules and well annealed polymers.

Dynamical behavior of unfreezable molecules restricted in a frozen matrix

Broadband dielectric spectroscopy measurements performed around the freezing temperature showed two different dynamical behaviors for various mixtures of poly(vinyl pyrrolidone) (PVP)/water, poly(vinyl acetate)(PVAc)/benzene, PVP/poly(ethylene glycol) (PEG), and 4,4′- n-pentylcyanobiphenyl (5CB)/benzene. A discontinuous behavior of the temperature dependence of the relaxation time around the freezing temperature was observed for the processes due to water in PVP/water, PVP in PVP/PEG, and 5CB in 5CB/benzene mixtures, and a continuous behavior was observed for PVAc in PVAc/benzene and PEG in PVP/PEG mixtures. These behaviors are explained by a large volume effect from the large motional unit. PVP/ n-alcohol mixtures support that the high hydrogen-bonding ability of water brings about the large volume effect also in PVP/water mixture.

Two secondary modes in decahydroisoquinoline: Which one is the true Johari Goldstein process?

Broadband dielectric measurements were carried out at isobaric and isothermal conditions up to 1.75 GPa for reconsidering the relaxation dynamics of decahydroisoquinoline, previously investigated by Richert et al. [R. Richert, K. Duvvuri, and L.-T. Duong, J. Chem. Phys. 118, 1828 (2003)] at atmospheric pressure. The relaxation time of the intense secondary relaxation tau(beta) seems to be insensitive to applied pressure, contrary to the alpha-relaxation times tau(alpha). Moreover, the separation of the alpha- and beta-relaxation times lacks correlation between shapes of the alpha-process and beta-relaxation times, predicted by the coupling model [see for example, K. L. Ngai, J. Phys.: Condens. Matter 15, S1107 (2003)], suggesting that the beta process is not a true Johari-Goldstein (JG) relaxation. From the other side, by performing measurements under favorable conditions, we are able to reveal a new secondary relaxation process, otherwise suppressed by the intense beta process, and to determine the temperature dependence of its relaxation times, which is in agreement with that of the JG relaxation.

Dielectric study of the alpha and beta processes in supercooled ethylene glycol oligomer-water mixtures.

Broadband dielectric measurements for 65 wt % ethylene glycol oligomer (EGO)-water mixtures with one to six repeat units of EGO molecules were performed in the frequency range of 10 microHz-10 GHz and the temperature range of 128-298 K. In the case of the water-EGO mixtures with one and two repeat units of the EGO molecule (small EGO), the shape of the dielectric loss peak of the primary process is asymmetrical about the logarithm of the frequency of maximum loss above the crossover temperature, T(C). The asymmetric process continues to the alpha process at a low frequency, and an additional beta process appears in the frequency range higher than that of the alpha process below T(C). In contrast, the water-EGO mixtures with three or more repeat units of the EGO molecule (large EGO) show a broad and symmetrical loss peak of the primary process above T(C). The symmetric process continues to the beta process, and an additional alpha process appears in the frequency range lower than that of the beta process below T(C). These different scenarios of the alpha-beta separation related to the shape of the loss peak above T(C) are a result of the difference in the cooperative motion of water and solute molecules. The solute and water molecules move cooperatively in the small EGO-water mixtures above T(C), and this cooperative motion leads to the asymmetric loss peak above T(C) and the alpha process below T(C). For the large EGO-water mixtures, the spatially restricted motion of water confined by solute molecules leads to the symmetric loss peak above T(C) and the beta process below T(C).

The dielectric properties of granular media saturated with DNAPL/water mixtures

We present the results of five experiments investigating the dielectric properties of granular materials partially saturated with trichloroethylene (TCE), a common dense non‐aqueous contaminant. Previous research has investigated the radar signatures of similar solvents in controlled field experiments but no core‐scale measurements have verified the appropriate petrophysical model. Broadband dielectric measurements were performed using a time domain reflectometry (TDR) system coupled to a solvent‐compatible coaxial transmission line. Two synthetic samples and three natural aquifer samples were fully saturated with water and then subjected to an axial TCE injection until breakthrough was observed. The resulting dielectric measurements show good agreement with the empirical complex refractive index model (CRIM) allowing a reasonable prediction of the radar reflectivities and transmission velocities expected in field surveys targeting pools of similar non‐aqueous contaminants.

Input Impedance of a Coaxial Line Terminated With a Complex Gap Capacitance— Numerical and Experimental Analysis

A full-wave numerical analysis was performed for a coaxial line terminated with a complex gap capacitance using a finite-element high-frequency structure simulator. The scattering parameters, input impedance, and spatial distribution of the electromagnetic field have been obtained in the frequency range of 100 MHz to 19 GHz for specimens 8 to 320 /spl mu/m thick, with a dielectric constant of up to 80. It was found that the residual inductance of the specimen affects the impedance characteristic of the network. The inductance-capacitance resonance is coupled with the cavity resonance. The specimen inductance is linearly dependent on the specimen thickness. At frequencies near the cavity resonance, the specimen section can be treated as a network of a transmission line with a capacitance, where the fundamental mode propagates along the diameter of the specimen. Results of the numerical analysis were verified experimentally using water as a model material with a high dielectric constant. Our closed-form formula for input impedance of the network is valid in a wider frequency range than the lumped-element method. The results are useful in improving the accuracy of broadband dielectric measurements in the extended frequency range of thin films with high dielectric constant that are of interest to bio- and nanotechnology.

Structural and Secondary Relaxations in Supercooled Di-n-butyl Phthalate and Diisobutyl Phthalate at Elevated Pressure

Broadband dielectric measurements at very large hydrostatic pressures (up to 1.8 GPa) are used to investigate the effect of molecular structure on the dynamical properties of supercooled di-n-butyl phthalate (DBP) and its isomer, diisobutyl phthalate (DiBP). At atmospheric pressure, both the shape of the α-relaxation loss peak and the fragility are essentially the same for the two materials, although the behavior of their β-relaxations differs. The activation energy for the β-process in DiBP, which is independent of pressure, is larger than that for DBP, while the activation volume of the former is about an order of magnitude larger. In contrast to the similarities of the α-relaxation at low pressure, distinct differences between the two isomers become evident at elevated pressure. The activation volume for the α-relaxation and the pressure coefficient of the glass transition temperature are significantly larger for DBP than DiBP. On the other hand, the fragility of both liquids is invariant to pressure. ...

Structure and dielectric properties of a thermoplastic blend containing dispersed metal

AbstractIn the present work broadband dielectric relaxation spectroscopy measurements were employed to investigate the dielectric properties of polymer composites. A polyethylene/polyoxymethylene (PE/POM) thermoplastic blend was used as a matrix, while the inclusions were iron (Fe) particles. For comparison, the two pure polymers‐ PE and POM‐ were used as a matrix, too. In the PE/POM‐Fe composites, the polymer matrix is two‐phase and the filler particles are localized only in the POM phase, resulting in an ordered distribution of the dispersed filler particles within the blend. In PE‐Fe and POM‐Fe composites, the filler spatial distribution is random. The behaviour of all the composites studied is described in terms of the percolation theory. The PE/POM‐Fe composites, based on the PE/POM blend, demonstrate different electrical behaviour compared to that of POM‐Fe and PE‐Fe systems. The percolation threshold value of the PE/POM‐Fe composites was found much lower than that of the other two systems. The results were related to the microstructure of the composites. A schematic model for the morphology of the composites studied has been proposed. This model explains the peculiar behaviour of the PE/POM‐Fe composites by taking into account the ordered distribution of the filler particles in a binary polymer matrix. Optical microscopy photographs confirm this model.

Test of the Einstein-Debye relation in supercooled dibutylphthalate at pressures up to 1.4 GPa.

Broadband dielectric measurements were carried out on di-n-butyl phthalate (DBP) under isothermal conditions at hydrostatic pressures up to 1.6 GPa. A comparison of the dielectric relaxation times with the viscosity revealed that no breakdown of the Einstein-Debye relation is induced by high compression. This absence of any decoupling is attributed to the weak intermolecular cooperativity of DBP and its negligible change with pressure. Because of the latter, the dielectric spectra conform to time-pressure superpositioning.