G. Carini

Relaxation dynamics of H-bonded liquids confined in porous silica gels by Rayleigh wing spectroscopy

Reorientational dynamics of neat liquid propylene glycol molecules is compared with that of confined molecules in 25 and 75 A pores of a sol-gel porous glass by means of depolarized light-scattering spectroscopy. The experimental spectra, performed as a function of temperature, show unambiguously the slowing down of the collective reorientational processes triggered by the confinement effects. Such results confirm the main role played by the strong dipolar interactions on the dynamics of hydrogen-bonded liquids diffusing within nanoscopic spherical pores.

Vibrational dynamics of permanently densified GeO2 glasses: Densification-induced changes in the boson peak

Vitreous GeO(2), one of the main prototypes of strong glasses, was densified at several pressures up to 6 GPa, achieving more than 20% of densification. The density dependence of the vibrational density of states and of the low temperature properties of these glasses was investigated by means of inelastic neutron scattering and calorimetric measurements. With increasing density, both the boson peak and the bump in c(p)/T(3) versus T plot exhibit variations which are stronger than the elastic medium expectation. If one reduces the measured spectra to a common master curve, one finds that this is only possible for the densified samples; the first densification step has an additional effect, similar to other cases in the literature. Nevertheless, the existence of a master curve for the three densified samples proves that the total number of excess modes remains constant on further densification. The experimental data are discussed in the framework of different theoretical models.

Anelastic effects in (AgI)x(Ag2O·B2O3)1−x superionic glasses

Abstract We present here the first measurements of acoustic absorption in (AgI) x (Ag 2 O·B 2 O 3 ) 1− x superionic glasses at ultrasonic frequencies (5–45 MHz) and in the 80–434 K temperature range. The attenuation shows, at decreasing temperature, an unusually high peak which seems to be attributed to a thermally activated relaxation process. A possible microscopic explanation of this anomaly is proposed, in connection with the presence of mobile ions in those materials.

Internal friction due to mobile ions in (AgI)x(Ag2O 2B2O3)1−x superionic glasses

Abstract The behavior of acoustic attenuation in (AgI)x(Ag2O·2B2O3)1−x, where the molar fraction x varies from 0 to 0.6, was measured in the temperature range 80 to 470K and at ultrasonic frequencies (5 to 45 Mhz). The presence of a broad peak, whose position shifts to higher temperatures with increasing frequency and whose height increases with AgI concentration, indicates the existence of thermally activated relaxation processes due to mobile Ag+ ions. A quantitative analysis in terms of a relaxation time distribution, coming from a Gaussian-like distribution function for the activation energies E, gave a good fit of the experimental data. All the results are discussed in connection with the possible microscopic structure of those glasses.

Relaxation in semi-interpenetrating polymers network of linear polyurethane and heterocyclic polymer networks

Abstract New semi-interpenetrating polymers (semi-IPN) of linear polyurethane (PU) and heterocyclic polymer networks prepared by trimerized dicyanate of Bisphenol-A (TDCE) have been analyzed by dynamic mechanical spectroscopy (DMS) in the 130–550 K temperature and 0.3–30 Hz frequency ranges. Single mechanical α a -relaxations suggest a large-scale homogeneity ascribed to the affinity between TDCE and PU. Below the glass transition temperature, the interpenetration affects the local molecular motions as a consequence of modifications in the mutual local environments of pure components.

Microscopic origin of anelastic relaxations in polyethylene oxide‐sodium thiocyanate complexes

The temperature behavior (100–400 K) of the mechanical characteristics (internal friction and resonant frequency) of polyethyleneoxide‐NaSCN polymeric electrolytes reveals the presence of anelastic relaxations, which arise from local and cooperative microscopic motions. The low‐temperature γ relaxation is explained in terms of a relaxation time distribution, which is interpreted as arising from random deviations in the local arrangement of the relaxing particles. Moreover, the elastic and anelastic features, as a function of the salt content, give some insight concerning their microscopic origin and the connection between the structure and the fast ion dynamics in this kind of systems.

Vibrational properties of superionic glasses (AgI)x(Ag2O nB2O3)1-x

Abstract In this letter we refer on the Raman-scattering measurements in superionic glasses (AgI) x (Ag 2 O n B 2 O 3 ) 1- x where 0 ⩽ x ⩽ 0.5. The behaviour of the low-frequency Raman spectra, Δν −1 , has been interpreted as due to a vibrational density of states mainly due to the silver halide. Nonlinear increase of the Raman efficiency with the increase of AgI concentration has been found: a phenomenological explanation is presented.

Molecular mobility in semi-IPNs of linear polyurethane and heterocyclic polymer networks

Abstract A study of the thermal and mechanical properties of new semi-interpenetrating polymer networks (IPNs) based on linear polyurethane (PU) and crosslinked trimerized dicyanate (TDC) reveals the existence of structures characterized by the absence of chemical interactions. Two distinct glass transitions are observed in the thermograms, as an indication of the fact that the two polymeric components preserve their molecular structure. The interpenetration affects markedly the glass transition temperatures revealed in the pure components in consequence of modifications in the local environments of the relaxing molecular units in the two phases. The primary and secondary relaxations of these systems show features which can be explained by accounting for the free-volume decrease due to the inclusion of PU in the network of TDC. Below the glass transition two molecular relaxations have been observed which have been ascribed to the secondary relaxation motions characterizing each polymeric component. Both ...

Locally heterogeneous dynamics in miscible blends of poly(methyl methacrylate) and poly(vinylidene fluoride)

Comparative measurements of specific heat capacities (temperature interval between 2 and 500 K), and of low frequency mechanical spectroscopy (temperature interval between 120 and 400 K) in poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) amorphous blends show the existence of single calorimetric and mechanical glass transition temperatures, as a clear indication of the existence of homogeneous single-state structures. Below T g , the experimental data reveal distinct local relaxation processes within the backbone of the individual components, while the heat capacities below 15 K can be explained in terms of a two-phase model (i.e., a simple linear overlap of the contributions from wholly amorphous PMMA and PVDF, weighted by their proportions). These findings are associated with locally heterogeneous relaxation and vibrational motions, and are regarded as experimental evidence for the existence of a nanoscopic length scale where the dynamics of a blend exhibits a heterogeneous regime.

Ag2O-B2O3 glassy systems: network coherence behaviour as seen from molecular dynamics calculations

A study of molecular dynamics in ionic glasses Ag2O-B2O3 is presented. The analysis of the random structures obtained indicates that they are built up from triangular and tetrahedral distorted units BO3 and BO4. These units are connected by bridging oxygen atoms. The coordination change of the boron from three (typical of the pure boron oxide) to four is due to the addition of the metallic oxide, which acts as a network modifier. The formation rate of the tetrahedral BO4 groups follows the x/(1-x) law, x being the molar fraction of Ag2O, up to approximately 0.25 molar fraction, giving rise to a corresponding increase of the network coherence degree that defines the network connectivity. For a higher content of Ag2O an additional mechanism of non-bridging oxygen formation starts. These two mechanisms work in opposition to each other, decreasing the network coherence. The silver ions seem to be almost homogeneously distributed throughout the network to ensure charge neutrality.