Pierangelo Rolla

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.

Unusual Deviations from Bulk Behavior in Ultrathin Films of Poly(tert-butylstyrene): Can Dead Layers Induce a Reduction of T g ?

The temperature and thickness dependence of the thermal expansivity of ultrathin thin films of poly(tert-butylstyrene) capped between aluminum layers revealed an unusual and intriguing confinement scenario. Below 50 nm, both the glass transition temperature and the thermal expansion coefficients decreased. Such a mixed behavior implies an enhancement of the molecular mobility, without the presence of any free surface, but dead layers. On the basis of a careful analysis of averaged quantities measured by capacitive dilatometry, we deduced a profile of thermal expansivity that explains our experimental data and previous observations in polymer nanocomposites. The effect of density-conformation coupling in proximity of a nonattractive interface allows the coexistence of an immobilized fraction in direct contact with the metal and an excess of thermal expansivity, arising from the long-range effects of packing frustration penetrating inside the bulk-like core of the film.

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.

Bond-controlled configurational entropy reduction in chemical vitrification

Glass formation is usually viewed in terms of physical vitrification: a liquid in a metastable state is cooled or compressed so as to avoid crystallization. However, glasses may also be formed by chemical vitrification, a process involving progressive polymerization of the constituent molecules via the formation of irreversible chemical bonds. The formation of most of the materials used in engineering plastics and the hardening of natural and synthetic resins are based on chemical vitrification. Despite the differences in the molecular processes involved in chemical and physical vitrification, surprising similarities are observed in the slowing down of the dynamics and in the thermodynamical properties of the resulting glasses. Explaining such similarities would improve general understanding of the glass transition and may disclose its universal nature. Here we report dielectric and photon-correlation measurements that reveal the origin of the similarity in the dynamical behaviour of physical and chemical glass formers. We find that the evolution of their configurational restrictions proceeds in a similar manner. In particular, we make a connection between the reduction in configurational entropy and the number of chemical bonds, a quantity that can be controlled in experiments.

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...

Dielectric parameters to monitor the crosslink of epoxy resins

Recent developments in dielectric monitoring of cure processes are considered. Direct current (dc) conductivity and dielectric data concerning the crosslinking of an epoxy resin are analyzed and compared with the results of late microwave experiments. The analysis of the dielectric behavior of the system carried out on a rather wide frequency interval (103-1010Hz) has provided a deeper insight into the relationships between dielectric parameters (i.e., dc conductivity, permittivity, relaxation time and shape parameters, and the physical and chemical modifications of the systems). The results confirm the possibility to utilize dielectric quantities to obtain information on relevant parameters such as conversion, viscosity change, sol-gel transition, glass transition temperature, and dynamics of cooperative relaxation phenomena of the system. The required dielectric data can be gathered by simple methods that facilitate in situ applications.

Dielectric monitoring of epoxy cure

The cure process of mixtures of a DGEBA-type epoxy resin and ethylenediamine, in different molar ratios, was investigated by dielectrometry in a wide frequency range, 103-2·1010 Hz, at three temperatures, 25, 60, and 70°C. Two relaxation processes were found whose relevant parameters were obtained by a proper fit procedure. A deeper knowledge and a quantitative description of the observed effects was achieved by analyzing the change of dielectric data caused by the chemical reaction. The change of the dielectric strength of both processes, as well as of other dielectric parameters, reflect the chemical modifications of the system; the permittivity, both static and at microwave frequencies, was linearly related to the conversion. These results support the interpretative model based on the disappearance of dipoles, and confirm the permittivity as a valuable quantity to monitor the kinetics of the reaction.

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.

Local dielectric spectroscopy of nanocomposite materials interfaces

Local dielectric spectroscopy is performed to study how relaxation dynamics of a polyvinyl-acetate ultrathin film is influenced by inorganic nanoinclusions of a layered silicate (montmorillonite). Dielectric-loss spectra are measured by electrostatic-force microscopy in the frequency-modulation mode in ambient air. Spectral changes in both shape and relaxation time are evidenced across the boundary between pure polymer and montmorillonite sheets. Dielectric-loss imaging is also performed, evidencing spatial variations of dielectric properties near nanostructures with nanometer-scale resolution.