G. Tripodo

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.

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.

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.

Vibrational properties of rare-earth phosphate glasses

Abstract The Raman spectra of metaphosphate glasses, which contain very high concentrations of either Eu3+ or Gd3+ ions, have been measured over a wide range of temperatures (10–300 K). The high-frequency part of the spectrum (above 200 cm−1) is similar to that found for other phosphate glasses, indicating that these glasses share the common structural feature of a skeleton built up from (PO4)2 tetrahedral units. The low-frequency region is determined by two spectral contributions which are characteristic of the disordered topology of vitreous materials: the light-scattering excess and the ‘boson peak’. The former contribution, clearly observed for frequencies below 10cm−1, becomes larger with increasing temperature, while the latter does not change appreciably as the temperature is increased to room temperature. Interpretation of the light scattering excess and the boson peak in terms of the predictions of the soft potential model can account for their frequency and temperature dependences.

Elastic and anelastic properties of rare earth phosphate glasses

Abstract The behaviour of the ultrasonic attenuation between 10 K and 400 K in Sm 2 O 3 P 2 O 5 glasses is characterized by the presence of very broad peaks, due to thermally activated relaxations of structural defects, typical of amorphous materials. A study of these anomalies reveals that the addition of Sm 2 O 3 to P 2 O 5 has little influence on the mean activation energy of the relaxation process, but does cause a decrease in the number of relaxing particles. The effect of temperature on the anomalous negative hydrostatic pressure derivatives of the elastic moduli of samarium phosphate glass is also examined experimentally. Reduction of the temperature below 300 K causes ϖC 11 / ϖP and ϖB/ϖP to increase steeply to more negative values: both longitudinal and shear Gruneisen parameters, which are negative, become much larger. It is suggested that application of pressure drives the samarium ion f → d transition and that the ion size collapse couples to the acoustic modes, strongly enhancing acoustic mode softening.

Low-temperature excess specific heat and fragility in polymers: Crystallinity dependence

A study of the mechanical characteristics and the low-temperature specific heat has been performed on a class of polymers whose crystallinity degree varies in a wide range, from a wholly amorphous to a highly crystalline structure. Typical features of the amorphous phase have been analyzed in order to shed further light on their correlations: the deviation from the exponentiality and the “fragility” obtained by modeling the mechanical αa-relaxation and the excess specific heat over the predictions of the Debye theory. It has been found that increasing crystallinity enhances the nonexponentiality of the αa-relaxation, and decreases both the fragility and the excess specific heat. This behavior is intrinsic to semicrystalline polymers. Consequently, the recently suggested correlation between the degree of fragility and the additional low-energy vibrations causing the low-temperature excess specific heat in glasses fails in semicrystalline polymers.

Low-energy vibrational dynamics of hydrogenated silica gels

Abstract A comparative study of low temperature specific heat (1.5–25 K) and low frequency Raman scattering (

Low-energy vibrations in superionic glasses

Abstract Measurements of low temperature specific heat C p (1.5 K–25 K) have been carried out in AgI–Ag 2 O–B 2 O 3 superionic glasses. It has been revealed that modifications of the network coherence change the magnitude of the hump in the specific heat, which is observed when it is plotted as C p / T 3 : the increasing connectivity (defined as the number of bridging bonds per network forming ion) reduces the deviations of C p from a T 3 behavior. Finally comparative measurements of low temperature C p and low frequency Raman scattering (below 100 cm −1 ) in the same glass permit us to assess the frequency dependence of the photon-vibration coupling coefficient C ( ω ) and the spectral density of low-energy vibrational modes g ( ω ).