Abstract: Recent measurements have suggested that the antiplasticizing effect of glycerol on trehalose can significantly increase the preservation times of proteins stored in this type of preservative formulation. In order to better understand the physical origin of this phenomenon, we examine the nature of antiplasticization in trehalose-glycerol mixtures by dielectric spectroscopy. These measurements cover a broad frequency range between 40 Hz to 18 GHz (covering the secondary relaxation range of the fragile glass-former trehalose and the primary relaxation range of the strong glass-former glycerol) and a temperature (T) range bracketing room temperature (220 K to 350 K). The Havriliak-Negami function precisely fits our relaxation data and allows us to determine the temperature and composition dependence of the relaxation time tau describing a relative fast dielectric relaxation process appropriate to the characterization of antiplasticization. We observe that increasing the glycerol concentration at fixed T increases tau (i.e., the extent of antiplasticization) until a temperature dependent critical "plasticization concentration" xwp is reached. At a fixed concentration, we find a temperature at which antiplasticization first occurs upon cooling and we designate this as the "antiplasticization temperature," Tant. The ratio of the tau values for the mixture and pure trehalose is found to provide a useful measure of the extent of antiplasticization, and we explore other potential measures of antiplasticization relating to the dielectric strength.

Abstract: The beta relaxation is a principal source of information about the dynamics in the glassy state; however, the nature of this process remains a controversial issue. In this paper, we show that properties of the beta relaxation measured below Tg are sensitive to the structure of the glass; that is, the thermodynamic path from the equilibrium liquid strongly affects the beta relaxation times, their distribution, and the activation energy quantifying their temperature dependence. These results support the idea that the Johari-Goldstein beta process is the precursor to the structural relaxation transpiring at longer times. We discuss the experimental findings in light of the heterogeneous and homogeneous scenarios for the beta process.

Abstract: Dielectric measurements on 4,4�-methylenebis(N,N-diglycidylaniline) were carried out over a temperature range from 258 to 335 K, at hydrostatic pressures from ambient to 300 MPa. Both the shape of the alpha-relaxation function and the Tg-normalized temperature dependence of the relaxation times (fragility) were found to be independent of pressure, consistent with the established correlation between these two quantities. Interpretation of the data using an extension of the model of Adam and Gibbs yielded values for the configurational entropy and the excess heat capacity. The implication of this analysis is that these two quantities govern to a substantial extent both the temperature- and pressure-dependences of the structural relaxation times. We also show that all relaxation times can be rescaled onto a single master curve, whose abscissa is simply related to the ratio between the configurational entropy and the excess heat capacity.

Abstract: Dielectric spectroscopy was employed to study the effects of water on the primary alpha -relaxation and the secondary beta -relaxation of xylitol. The measurements were made on anhydrous xylitol and mixtures of xylitol with water with three different water concentrations over a temperature range from 173 K to 293 K. The alpha -relaxation speeds up with increasing concentration of water in xylitol, whereas the rate of the beta -relaxation is essentially unchanged. Some systematic differences in the behavior of alpha -relaxation for anhydrous xylitol and the mixtures were observed. Our findings confirm all the observations of Nozaki et al. [J. Non-Cryst. Solids 307, 349 (2002)]] in sorbitol/water mixtures. Effects of water on both the alpha - and beta -relaxation dynamics in xylitol and sorbitol are explained by using the coupling model.

Abstract: Dielectric measurements of 4,4�-methylenebis (N,N-diglicydilaniline) (MBDGA) have been carried out along both pressure and temperature pathways. The phenomenon of decoupling between the DC-conductivity and the structural (alpha-) relaxation time has been studied. We found that the exponent s in the fractional Debye-Stokes-Einstein relation is both temperature and pressure independent, and its value can be determined from the ratio of activation volumes for the DC-conductivity relaxation and the structural relaxation process, respectively. The results for MBDGA have been compared with results previously obtained for similar epoxies, i.e., N,N-diglycidylaniline and N,N-diglycidyl-4-glycidyloxyaniline. A correlation between the molecular size and the fractional exponent has been established.

Abstract: Dielectric measurements were obtained on poly(propylene glycol) (molecular weight: 4000 Da) at pressures in excess of 1.2 GPa. The segmental (alpha process) and normal-mode (alpha� process) relaxations exhibited different pressure sensitivities of their relaxation strengths, as well as their relaxation times. Such results are contrary to previous reports, and (at least for the dielectric strength) can be ascribed to the capacity for intermolecular hydrogen-bond formation in this material. With equation-of-state measurements, the relative contributions of volume and thermal energy to the alpha-relaxation times were quantified. Similar to other H-bonded liquids, temperature is the more dominant control variable, although the effect of volume is not negligible.

Abstract: Two relaxation processes, involving dc conductivity and the glass (alpha) structural relaxation, respectively, were measured in the low-molecular-weight glass-forming liquid, bisphenol- A-propoxylate(1 PO/phenol)diglycidylether, by dielectric spectroscopy, as a function of temperature and pressure. We focused on the correlation between dc conductivity and the alpha-relaxation time, proving that the fractional Debye-Stokes-Einstein relation is valid for both isobaric and isothermal data. The value of the exponent was independent of both temperature and pressure, and related to the ratio of the apparent activation volume of the relaxation processes being considered. The shape of the dielectric loss alpha peak did not change with either temperature or pressure; i.e., frequency-temperature-pressure superposition was valid in the vicinity of the glass transition.

Abstract: Ionic conductivity data and structural relaxation times for poly(phenyl glycidyl ether)-co-formaldehyde (PPGE) are compared for the supercooled liquid under a wide range of temperatures and pressures. Conformance to the fractional Debye–Stokes–Einstein (fDSE) relation was observed under all conditions. The fDSE exponent, &equal;0.81, provides an estimate of the relative magnitude of the activation volumes for conductivity and dielectric relaxation. The smaller activation volume for the former suggests less free volume is necessary to accommodate ion diffusion in the PPGE than that required for dielectric relaxation. We also fit the combined temperature and pressure dependences of both the conductivity and relaxation times to the Avramov equation.

Abstract: From temperature studies at ambient pressure, it was pointed out for several glass-forming liquids that the alpha-relaxation time (tau) can be related to the dc-ionic conductivity (sigma) through the phenomenological fractional Debye-Stokes-Einstein (DSE) equation. In the present paper we test the validity of fractional DSE equation for relaxation data obtained from pressure variable experiments. To this end we carried out broadband dielectric measurements (10 mHz-10 MHz) in a wide range of pressures (0.1-300 MPa). The material under study were N,N-diglycidyl-4-glycidyloxyaniline and N,N-diglycidylaniline. As a result we found that the fractional DSE equation is also obeyed for pressure pathways.

Abstract: The pressure and temperature dependent dielectric relaxation times are compared with the predictions of the model proposed by Avramov. Remarkable agreement is found in whole P and T plane. It was also pointed out that the tested model predicts no pressure effect on fragility and nonlinear character of pressure dependence of glass transition temperature. Our data provide also an assessment of the generalized Vogel-Fulcher-Tammann (VFT) law. However, it is inferred that the generalized VFT law is not able to accurately reproduce the surface described by experimental relaxation times. Additionally, isothermal data are analyzed by the pressure counterpart of the temperature VFT law. The isothermal data expressed in terms of the reduced relaxation time and reduced pressure form a master curve.

Abstract: Complex impedance and conductivity were measured for regioregular poly(3-hexylthiophene) (P3HT) at alternating current (AC) voltages using a waveform technique. The waveforms were Fourier transformed from time domain to frequency domain and analyzed at fundamental and higher order harmonic frequencies. It was found that the impedance of the semi-conducting P3HT decreases with increasing electric field strength. The non-linear charge transport is dominated by a third harmonic response that originates from extended polarizability of p-type electronic states. The third order non-linear conductivity can be used to quantify the effect of an electric field on the conduction mechanism and to correlate the intrinsic charge carriers mobility with molecular structure

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