Simone Capaccioli

Dielectric response analysis of a conducting polymer dominated by the hopping charge transport

The d.c. conductivity and the electric a.c. response from 100 Hz up to 40 MHz of poly(3n-decylpyrrole) were measured in the 80-330 K interval to characterize the charge transport behaviour of the system. The d.c. conductivity well fitted the variable range hopping model, and the loss factor, after having deducted the d.c. contribution, showed a relaxation peak when the conductivity versus frequency started to rise. The strength of this relaxation increased with temperature and became too large to be related to a dipolar relaxation; moreover, the temperature dependence of the loss peak frequency and d.c. conductivity coincided. The observed relaxation was attributed to the hopping charge transport, as further confirmed by the temperature behaviour of the relaxation strength and by the frequency dependence of the exponents of the power law which locally approximate the conductivity behaviour. As the activation energy of the d.c. conductivity differed from the frequency of the loss peak, the theoretical prediction concerning the selfsimilarity of the a.c. conductivity was roughly verified.

Interdependence of Primary and Johari-Goldstein Secondary Relaxations in Glass-Forming Systems

We report evidence from broadband dielectric spectroscopy that the dynamics of the primary alpha- and secondary Johari-Goldstein (JG) beta-processes are strongly correlated in different glass-forming systems over a wide temperature T and pressure P range, in contrast with the widespread opinion of statistical independence of these processes. The alpha-beta mutual dependence is quantitatively confirmed by (a) the overall superposition of spectra measured at different T-P combinations but with an invariant alpha-relaxation time; (b) the contemporary scaling of the isothermal-pressure and isobaric-temperature dependences of the alpha-and beta-relaxation times as plotted versus the reduced variable Tg(P)/T where Tg is the glass transition temperature. These novel and model-independent evidences indicate the relevance of the JG relaxation phenomenon in glass transition, often overlooked by most current theories.

Glass Transitions in Aqueous Solutions of Protein (Bovine Serum Albumin)

Measurements by adiabatic calorimetry of heat capacities and enthalpy relaxation rates of a 20% (w/w) aqueous solution of bovine serum albumin (BSA) by Kawai, Suzuki, and Oguni [Biophys. J. 2006, 90, 3732] have found several enthalpy relaxations at long times indicating different processes undergoing glass transitions. In a quenched sample, one enthalpy relaxation at around 110 K and another over a wide temperature range (120-190 K) were observed. In a sample annealed at 200-240 K after quenching, three separated enthalpy relaxations at 110, 135, and above 180 K were observed. Dynamics of processes probed by adiabatic calorimetric data are limited to long times on the order of 10(3) s. A fuller understanding of the processes can be gained by probing the dynamics over a wider time/frequency range. Toward this goal, we performed broadband dielectric measurements of BSA-water mixtures at various BSA concentrations over a wide frequency range of thirteen decades from 2 mHz to 1.8 GHz at temperatures from 80 to 270 K. Three relevant relaxation processes were detected. For relaxation times equal to 100 s, the three processes are centered approximately at 110, 135, and 200 K, in good agreement with those observed by adiabatic calorimetry. We have made the following interpretation of the molecular origins of the three processes. The fastest relaxation process having relaxation time of 100 or 1000 s at ca. 110 K is due to the secondary relaxation of uncrystallized water (UCW) in the hydration shell. The intermediate relaxation process with 100 s relaxation time at ca. 135 K is due to ice. The slowest relaxation process having relaxation time of 100 s at ca. 200 K is interpreted to originate from local chain conformation fluctuations of protein slaved by water. Experimental evidence supporting these interpretations include the change of temperature dependence of the relaxation time of the UCW at approximately T(gBSA) approximately = 200 K, the glass transition temperature of protein in the hydration shell, similar to that found for the secondary relaxation of water in a mixture of myoglobin in glycerol and water [Swenson et al. J. Phys.: Condens. Matter 2007, 19, 205109; Ngai et al. J. Phys. Chem. B 2008, 112, 3826]. The data all indicate in hydrated BSA or other proteins that the secondary relaxation of water and the conformation fluctuations of the protein in the hydration shell are inseparable or symbiotic processes.

Two crossover regions in the dynamics of glass forming epoxy resins

Broadband dielectric spectroscopy, heat capacity spectroscopy (3ω method), and viscosimetry have been used to study the dynamic glass transition of two glass-forming epoxy resins, poly [(phenyl glycidyl ether)-co-formaldehyde] and diglycidyl ether of bisphenol-A. In spite of their rather simple molecular structure, the dynamics of these systems is characterized by two well-separated crossover regions where the relaxation times of main transition and the two secondary relaxations β and γ approach each other. The main transition has three parts: The a process at high temperature, the a′ process between the two crossover regions, and the α process at low temperatures. Both the γ-crossover region [around a temperature Tc(γ)∼(1.4–1.5)Tg and a relaxation time τc(γ)≈10−10 s] and the β-crossover region [around Tc(β)∼(1.1–1.2)Tg and τc(β)≈10−6 s] could be studied within the experimentally accessible frequency–temperature window. Different typical crossover properties are observed in the two regions. The γ-crossove...

Critical Issues of Current Research on the Dynamics Leading to Glass Transition

Glass transition is still an unsolved problem in condensed matter physics and chemistry. In this paper, we critically reexamine experimental data and theoretical interpretations of dynamic properties of various processes seen over a wide time range from picoseconds to laboratory time scales. In order of increasing time, the ubiquitous processes considered include (i) the dynamics of caged molecular units with motion confined within the anharmonic intermolecular potential and where no genuine relaxation has yet taken place; (ii) the onset of the Johari-Goldstein secondary relaxation involving rotation or translation of the entire molecular unit and causing the decay of the cages, to be followed by the cooperative and dynamically heterogeneous motions participated by increasing number of molecules or length scale; and (iii) the terminal primary alpha-relaxation with the maximum cooperative length-scale allowed by the intermolecular interaction and constraints of the glass former. Some general and important properties found in each of these processes are shown to be interrelated, indicating that the processes are connected, with one being the precursor of the other following it. Thus, a theory of glass transition is neither complete nor fundamental unless all of these processes and their inter-relations have been accounted. In addition to published data, new experimental data are reported here to provide a limited collection of critical experimental facts having an impact on current issues of glass transition research and servingas a guide for the construction of a complete and successful theory in the future.

Thermodynamic scaling of α-relaxation time and viscosity stems from the Johari-Goldstein β-relaxation or the primitive relaxation of the coupling model

By now it is well established that the structural α-relaxation time, τ(α), of non-associated small molecular and polymeric glass-formers obey thermodynamic scaling. In other words, τ(α) is a function Φ of the product variable, ρ(γ)/T, where ρ is the density and T the temperature. The constant γ as well as the function, τ(α) = Φ(ρ(γ)/T), is material dependent. Actually this dependence of τ(α) on ρ(γ)/T originates from the dependence on the same product variable of the Johari-Goldstein β-relaxation time, τ(β), or the primitive relaxation time, τ(0), of the coupling model. To support this assertion, we give evidences from various sources itemized as follows. (1) The invariance of the relation between τ(α) and τ(β) or τ(0) to widely different combinations of pressure and temperature. (2) Experimental dielectric and viscosity data of glass-forming van der Waals liquids and polymer. (3) Molecular dynamics simulations of binary Lennard-Jones (LJ) models, the Lewis-Wahnström model of ortho-terphenyl, 1,4 polybutadiene, a room temperature ionic liquid, 1-ethyl-3-methylimidazolium nitrate, and a molten salt 2Ca(NO(3))(2)·3KNO(3) (CKN). (4) Both diffusivity and structural relaxation time, as well as the breakdown of Stokes-Einstein relation in CKN obey thermodynamic scaling by ρ(γ)/T with the same γ. (5) In polymers, the chain normal mode relaxation time, τ(N), is another function of ρ(γ)/T with the same γ as segmental relaxation time τ(α). (6) While the data of τ(α) from simulations for the full LJ binary mixture obey very well the thermodynamic scaling, it is strongly violated when the LJ interaction potential is truncated beyond typical inter-particle distance, although in both cases the repulsive pair potentials coincide for some distances.

Dynamics of supercooled and glassy dipropyleneglycol dibenzoate as functions of temperature and aging: Interpretation within the coupling model framework

Dielectric relaxation measurements of a typical small molecular glassformer, dipropyleneglycol dibenzoate show the presence of two secondary relaxations. Their dynamic properties differ in the equilibrium liquid and glassy states, as well as the changes during structural recovery after rapid quenching the liquid to form a glass. These differences enable us to identify the slower secondary relaxation as the genuine Johari-Goldstein (JG) beta-relaxation, acting as the precursor of the primary alpha-relaxation. Agreement between the JG beta-relaxation time and the independent relaxation time of the coupling model leads to predicted quantitative relations between the JG beta-relaxation and the alpha-relaxation that are supported by the experimental data.

CHANGES IN THE DYNAMICS OF SUPERCOOLED SYSTEMS REVEALED BY DIELECTRIC SPECTROSCOPY

The dynamics of monoepoxy, diepoxy, and triepoxy glass-formers from below to above the glass transition temperature, Tg, has been investigated through the temperature behavior of relaxation times, strengths, and conductivity, determined in a wide frequency range (102–2×1010 Hz). In all systems the main and secondary relaxations define a splitting temperature TS∼1.3×Tg; moreover, a crossover temperature TB∼TS is recognized, marking the separation between two different Vogel–Fulcher regimes for the structural dynamics. The strengths behavior reflects the distribution of the overall energy between the relaxation processes and no peculiar behavior is revealed at TS. A strong increase characterizes the strength of the secondary relaxation on crossing the glass transition from the lower temperatures. Conductivity data have been analyzed to test the dynamics in terms of the Debye–Stokes–Einstein (DSE) diffusion law. The prediction of the DSE model is well verified for mono- and diepoxide up to the high viscosity...

The glass transition and dielectric secondary relaxation of fructose-water mixtures.

Broad-band dielectric measurements for fructose-water mixtures with fructose concentrations between 70.0 and 94.6 wt% were carried out in the frequency range of 2 mHz to 20 GHz in the temperature range of -70 to 45 degrees C. Two relaxation processes, the alpha process at lower frequency and the secondary beta process at higher frequency, were observed. The dielectric relaxation time of the alpha process was 100 s at the glass transition temperature, T(g), determined by differential scanning calorimetry (DSC). The relaxation time and strength of the beta process changed from weaker temperature dependences of below T(g) to a stronger one above T(g). These changes in behaviors of the beta process in fructose-water mixtures upon crossing the T(g) of the mixtures is the same as that found for the secondary process of water in various other aqueous mixtures with hydrogen-bonding molecular liquids, polymers, and nanoporous systems. These results lead to the conclusion that the primary alpha process of fructose-water mixtures results from the cooperative motion of water and fructose molecules, and the secondary beta process is the Johari-Goldstein process of water in the mixture. At temperatures near and above T(g) where both the alpha and the beta processes were observed and their relaxation times, tau(alpha) and tau(beta), were determined in some mixtures, the ratio tau(alpha)/tau(beta) is in accord with that predicted by the coupling model. Fixing tau(alpha) at 100 s, the ratio tau(alpha)/tau(beta) decreases with decreasing concentration of fructose in the mixtures. This trend is also consistent with that expected by the coupling model from the decrease of the intermolecular coupling parameter upon decreasing fructose concentration.

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.