Search

We employ calorimetry, Rietveld refinement of X-ray powder diffraction, broadband dielectric spectroscopy and molecular dynamics simulations, to investigate the phase behavior, crystal structure, dc conductivity and dielectric response of the condensed phases of ethanolamine (EAM), an ethane derivative with two distinct hydrogen-bonding groups. EAM is found to exhibit, besides its stable crystal phase, a metastable crystalline phase obtained from the supercooled liquid phase. The metastable phase can only be maintained at low temperature; upon heating an irreversible exothermic transformation to the stable crystal structure takes place at 210 K, with a low transition enthalpy of ca. 2.4 J/g. The observed polymorphism is accompanied by differences in the molecular conformation and degree of structural order. Each EAM molecule participates in six H-bonds in both crystalline phases. While the stable phase is fully ordered, the metastable phase appears to be characterized by a statistical disorder in the position of the hydrogen atoms and thus of the intramolecular conformation and intermolecular hydrogen bonding. Our molecular dynamics simulations suggest that such disorder is dynamic in character, and allow analyzing the dynamic H-bonding motif of the metastable phase in terms of composition, length and energy of the H-bonds, and molecular conformations involved. Charge conduction is mainly ionic in both crystalline phases and exhibits the typical temperature dependence of disordered solid electrolytes, rather than crystalline ones. Dielectric characterization indicates that, upon crystallization of liquid EAM, a small fraction of the sample (approximately 5%) remains amorphous, a situation similar to that reported for other H-bonded systems such as glycerol and nbutanol., Postprint (author's final draft)

We investigated the impact of sample preparation on the glassy dynamics of thin films of poly(4-chlorostyrene), a polymer whose molecular mobility is particularly sensitive to changes in the specific volume. Samples were obtained by spincoating, the technique most commonly used to prepare thin organic layers, which consists of pouring dilute polymer solutions onto a plate rotating at a high rate. Our experimental results demonstrate that filtering the solutions before spincoating affects the value of the segmental relaxation time of the as-prepared films. Thin polymer layers obtained via filtered solutions show accelerated segmental dynamics upon confinement at the nanoscale level, once below 100nm, while the samples obtained via unfiltered solutions exhibit bulk-like dynamics down to 15-20nm. We analyzed these results by means of the cooperative free volume rate model, considering a larger free volume content in thin films obtained via filtered solutions. The validity of the model predictions was finally verified by measurements of irreversible adsorption, confirming a larger adsorbed amount, corresponding to a higher specific volume, in the case of samples obtained via unfiltered solutions. Our results prove that filtering is a crucial step in the preparation of thin films, and it could be used to switch on and off nanoconfinement effects. Graphical abstract: [Figure not available: see fulltext.]., SCOPUS: ar.j, info:eu-repo/semantics/published

The molecular dynamics in the ambient-pressure solid phase of 1,1,2-trichloroethane is studied by means of broadband dielectric spectroscopy and molecular dynamics simulations. The dielectric spectra of polycrystalline samples obtained by crystallization from the liquid phase exhibit, besides a space-charge relaxation associated with accumulation of charges at crystalline domain boundaries, two loss features arising from dipolar molecular relaxations. The most prominent and slower of the two loss features is identified as a configurational leap of the molecules which involves a simultaneous change in spatial orientation and structural conformation, namely between two isomeric forms (gauche+ and gauche–) of opposite chirality. In this peculiar dynamic process, the positions of the three chlorine atoms in the crystal lattice remain unchanged, while those of the carbon and hydrogen atoms are modified. This dynamic process is responsible for the disorder observed in an earlier X-ray diffraction study and confirmed by our simulation, which is present only at temperatures relatively close to the melting point, starting 40 K below. The onset of the disorder is visible as an anomaly in the temperature dependence of the dc conductivity of the sample at exactly the same temperature. While the slower relaxation dynamics (combined isomerization/reorientation) becomes increasingly more intense on approaching the melting point, the faster dynamics exhibits significantly lower but constant dielectric strength. On the basis of our molecular dynamics simulations, we assign the faster relaxation to large fluctuations of the molecular dipole moments, partly due to large-angle librations of the chloroethane species., Postprint (author's final draft)

In this thesis we employ dielectric spectroscopy (in different implementations) to study the dielectric properties of different materials ranging from completely disordered supercooled liquids to low-disorder solids with only ratcheting reorientational motions, to low-dimensional systems such as thin films or needle-like crystals. The probed material properties include the electrical conductivity, the space-charge processes due to sample heterogeneities, molecular dynamics, hydrogen-bond dynamics, and phase-transition temperature and kinetics. To study materials in thin film form, we implement silicon-based interdigital electrode devices, which we calibrate to obtain their equivalent capacity in vacuum. We also probe two-dimensional samples obtained by intercalating (sandwiching) organic molecules between parallel graphite oxide sheets. We study both organic and hybrid (organic-inorganic) films, and compared the results with the bulk counterpart of the same materials. Rhodamine films are deposited by two different procedures starting from rhodamine 6G chloride: solution-deposited films are ionic and ordered, while vacuum-deposited films consist of a different, neutral species, namely rhodamine19, due to decomposition of rhodamine 6G upon sublimation. Both types of films display variable-range hopping electronic conduction, and a conductivity-induced space-charge relaxation. Solution deposited films display a dipolar relaxation, absent in rhodamine 19 films. We assign the dipolar relaxation to some intramolecular motion involving the charged nitrogen of the Rhodamine and the chlorine counter-ion. We also performed studies on hybrid films of cadmium iodide covalently attached to ethylamine (CdI2EA), deposited by the Langmuir-Blodgett process. Bulk CdI2EA material has a solid-to-solid (structural) phase transition, which in the Langmuir-Blodgett films occurs 5 degrees higher in temperature. The films displayed a dielectric relaxation dynamics absent in the bulk, and wh, En esta tesis utilizamos la espectroscopia dieléctrica para estudiar las propiedades dieléctricas de diferentes materiales que van desde líquidos subenfriados completamente desordenados a sólidos de bajo desorden con movimiento de reorientación, hasta sistemas de baja dimensionalidad tales como películas delgadas o cristales casi unidimensionales. Las propiedades estudiadas incluyen la conductividad eléctrica, los procesos de carga espacial debido a las heterogeneidades de la muestra, la dinámica molecular, la dinámica de enlaces por puente de hidrógeno, y la cinética de cristalización. Para el estudio de materiales en forma de película delgada, se han implementado nuevos dispositivos de electrodo interdigital a base de silicio. Muestras bidimensionales han sido obtenidas también por intercalación de moléculas orgánicas entre planos atómicos de óxido de grafito. Estudiamos películas orgánicas e híbridas orgánicas-inorgánicas depositadas mediante procedimientos diferentes, y comparamos los resultados con las fases puras de los mismos materiales. Obtenemos películas ordenadas de rodamina 6G clorada por crecimiento desde solución, y películas amorfas de rodamina 19 por condensación de vapor en vacío. Ambos tipos de películas presentan una conducción electrónica por salto de electrones y una relajación de carga espacial. Las películas de rodamina 6G clorada muestran además una relajación dipolar, que se asigna a movimientos intramoleculares que involucran la carga asociada al nitrógeno de la rodamina 6G y al átomo de cloro. También se han estudiado películas híbridas de ioduro de cadmio covalentemente unido a etilamina (CdI2EA), depositadas mediante el método Langmuir-Blodgett. El material en fase “bulk” presenta una transición estructural, que en la película delgada aparece a una temperatura superior de unos 5 K. Las películas presentan una relajación dinámica ausente en el material puro y que es debida a la existencia de movimientos moleculares en la estructura de la, Postprint (published version)

We employ dielectricspectroscopy and molecular dynamic simulations to investigate the dipolar dynamics in the orientationally disordered solid phase of (1,1,2,2)tetrachloroethane. Three distinct orientational dynamics are observed as separate dielectric loss features, all characterized by a simply activated temperature dependence. The slower process, associated to a glassytransition at 156 ± 1 K, corresponds to a cooperative motion by which each molecule rotates by 180° around the molecular symmetry axis through an intermediate state in which the symmetry axis is oriented roughly orthogonally to the initial and final states. Of the other two dipolar relaxations, the intermediate one is the Johari-Goldstein precursor relaxation of the cooperative dynamics, while the fastest process corresponds to an orientational fluctuation of single molecules into a higher-energy orientation. The Kirkwood correlation factor of the cooperative relaxation is of the order of one tenth, indicating that the molecular dipoles maintain on average a strong antiparallel alignment during their collective motion. These findings show that the combination of dielectricspectroscopy and molecular simulations allows studying in great detail the orientational dynamics in molecular solids., Peer Reviewed, Postprint (author's final draft)

We study the temperature-dependent molecular dynamics, ion conduction, and phase behavior of plastic-crystal electrolytes based on the succinonitrile molecule. We employ calorimetry and dielectric spectroscopy to probe binary mixtures of succinonitrile with glutaronitrile or acetonitrile and also analyze the effect of dissolving lithium salts in these systems. The glutaronitrile-succinonitrile mixture has the highest conductivity, and it is the only plastic-crystal system that displays a perfect correlation between the ion drift and the on-site reorientational dynamics. Doping with lithium ions boosts the conductivity but breaks such perfect correlation. All of these features can be rationalized by assuming that conduction is due to the self-diffusion of a minority of ionized dinitrile molecules. Doping with lithium salts slows down the collective molecular dynamics while leaving the intramolecular relaxation motion unaffected. All samples exhibit a very broad melting transition and exist in a mixed liquid plus plastic state near room temperature. Some mixtures undergo phase segregation below 233 K, the transition temperature between the plastic and the fully ordered solid phase in pure succinonitrile, resulting in the appearance of a space-charge relaxation loss. Phase separation therefore plays an important role in pristine and lithium-doped succinonitrile mixtures., Postprint (published version)

Peer Reviewed, Postprint (author's final draft)

We employ dielectric spectroscopy to monitor the relaxation dynamics and crystallization kinetics of the Biclotymol antiseptic in its amorphous phase. The glass transition temperature of the material as determined by dielectric spectroscopy is Tg=290±1K. The primary (a) relaxation dynamics is observed to follow a Vogel-Fulcher-Tammann temperature dependence, with a kinetic fragility index m=86±13, which classifies Biclotymol as a relatively fragile glass former. A secondary relaxation is also observed, corresponding to an intramolecular dynamic process of the non-rigid Biclotymol molecule. The crystallization kinetics, measured at four different temperatures above the glass transition temperature, follows an Avrami behavior with exponent virtually equal to n=2, indicating one-dimensional crystallization into needle-like crystallites, as experimentally observed, with a time-constant nucleation rate. The activation barrier for crystallization is found to be Ea=115±22kJmol(-1), Postprint (author's final draft)

Interdigital electrodes fabricated by standard lithography on silicon chips are employed to probe the dipolar molecular dynamics and electric conduction properties of thin rhodamine films grown with two different methods. The conductivity is due to electronic charge carriers, and at around room-temperature, it is higher by 1 order of magnitude in solution-deposited films than in thermally evaporated ones. The organic material exhibits two intrinsic dynamic processes, of which the one at higher temperature is due to the orientational motion of the dipole moment of the rhodamine units, while the one at lower temperature is due to the motion of a local dipole associated with the chlorine counterions and is absent in thermally evaporated films. Our results show that thin-film dielectric spectroscopy is an easily implementable and versatile tool to extract valuable information on thin organic films., Peer Reviewed, Postprint (published version)

Impedance spectroscopy is employed to probe the frequency-dependent conductivity and dielectric response of the crystalline C60O24Na24 fulleride, both in its pure form obtained by heating to 473 K and in its bulk-hydrate form stable only below 390 K, of chemical formula C60O24Na24 ·16 H2O. A dielectric loss feature is visible in both the pure material and the hydrate, displaying different strength and activated behavior in different temperature ranges., Peer Reviewed, Postprint (published version)

Interdigital electrodes fabricated by standard lithography on silicon chips are employed to probe the dipolar molecular dynamics and electric conduction properties of thin rhodamine films grown with two different methods. The conductivity is due to electronic charge carriers, and at around room-temperature, it is higher by 1 order of magnitude in solution-deposited films than in thermally evaporated ones. The organic material exhibits two intrinsic dynamic processes, of which the one at higher temperature is due to the orientational motion of the dipole moment of the rhodamine units, while the one at lower temperature is due to the motion of a local dipole associated with the chlorine counterions and is absent in thermally evaporated films. Our results show that thin-film dielectric spectroscopy is an easily implementable and versatile tool to extract valuable information on thin organic films., Peer Reviewed, Postprint (published version)