Search

We report a compared analysis of inelastic X ray scattering (IXS) and of low frequency Raman data of glassy 3-methylpentane. The IXS spectra have been analysed allowing for the existence of two distinct excitations at each scattering wavevector obtaining a consistent interpretation of the spectra. In particular, this procedure allows us to interpret the linewidth of the modes in terms of a simple model which relates them to the width of the first sharp diffraction peak in the static structure factor. In this model, the width of the modes arises from the blurring of the dispersion curves which increases approaching the boundary of the first pseudo-Brillouin zone. The position of the boson peak contribution to the density of vibrational states derived from the Raman scattering measurements is in agreement with the interpretation of the two excitations in terms of a longitudinal mode and a transverse mode, the latter being a result of the mixed character of the transverse modes away from the center of the pseudo-Brillouin zone.

The physical properties of disordered systems are in the focus of a large research effort. In particular, one of the open problems related to glasses is their excess of modes in the THz frequency range over the vibrational contribution predicted by the Debye model. In fact, one could expect the continuum approximation underlying the Debye model to hold in glasses at long wavelengths at least as well as in crystalline solids. Experiments, instead, seem to indicate that the specific heat at low temperature (a few Kelvin) and the density of vibrational states at low frequency (a few terahertz) are very different from the Debye prediction. We present here a detailed analysis of specific heat measurements of vitreous GeO2, a prototype of strong glasses, and of permanently densified vitreous GeO2. Our data give experimental evidence that glasses do not show any excess of vibrational modes when compared to their crystalline counterparts of similar mass density.

We present a detailed investigation of the terahertz vibrational dynamics of 3-methylpentane performed by means of high-resolution inelastic x-ray scattering (IXS). We probe the dynamics in a large temperature range, which includes the glass, the supercooled liquid, and the liquid phases. The characteristic frequency of the excitations follows a well-defined dispersion curve extending beyond 8 nm(-1) at all the investigated temperatures, indicating the persistence of a solidlike behavior also in the liquid phase. This implies the existence of a pseudo-Brillouin zone whose size compares surprisingly well with the periodicity inferred from the first sharp diffraction peak in the static structure factor. We show that, in the investigated temperature range, both sizes undergo a variation of about 15%-20%, comparable to that of the average intermolecular distance. We finally show that the IXS sound velocity coincides with the infinite frequency sound velocity previously inferred from visible and ultraviolet Brillouin spectroscopy data. This analysis confirms the role of the shear relaxation processes in determining the variation with frequency of the apparent sound velocity.

In order to elucidate the effects of the nature of the cross-linker, of the hydration level and of the hydrophobic/ hydrophilic balance on the macro- and microscopic properties of a paradigmatic model of polysaccharide hydrogel, an integrated approach based on the combined use of low-frequency Raman scattering and FTIR-ATR absorption has been developed. The proposed methodology aims to explore the stability range of the liquid and gel phase diagram of the system. This approach was applied to cyclodextrin based nanosponges (CDNS), prepared by polymerization of α-, β-, and γ-cylcodextrins (CDNS) with an activated derivative of ethylenediamine tetra acetic acid. It revealed successful, on one side, for a comparative analysis of the elasticity, as mesoscopic parameter, of the dry polymeric network, measured by Raman spectroscopy in the low frequency (0 ÷ 150 cm–1) range, and, on the other side, for the investigation of the water holding capacity, as macroscopic parameter, through the analysis of the vibrational dynamics of water molecules in the swollen polymers, monitored by FTIR-ATR spectroscopy in two selected wavenumber regions, i.e. 2800 ÷ 3800 cm–1 and 1500 ÷ 1800 cm–1, where the O-H stretching and H-O-H bending vibrations respectively occur. As main result, the same evolution as a function of the molar ratio n, has been observed for the elasticity on mesoscopic scale, as described by the Boson Peak (ωBP) and the swelling ability on macroscopic scale, expressed by the mass ratio m describing the gel-to-sol transition in these systems. From the results, the way for a rational development of stimuli-responsive systems with specific performances, by modulating the stability range of the liquid suspension and gel phases properly changing the molar ratio n during the synthesis, is opened.