We use the advent of reliable ultrafast optical techniques to explore acoustic modes in glasses in the difficult but crucial frequency region between around 0.1 and 1 THz. Brillouin scattering of light allows to measure velocity and absorption of sound up to frequencies around 100 GHz and coherent inelastic x-ray scattering (IXS) is limited to frequencies larger than 1 THz. A rather universal property of glasses is that a large excess of modes exists, with a maximum density of states near 1 THz, forming the so-called boson peak. The acoustic modes are expected to be strongly affected as their frequency nears the boson peak. The onset of this effect, which is expected to be dominant in the 0.5 to 1 THz range, is observed here for the first time in v-SiO2 as a function of temperature.

Polymerized ionic liquids (PolyILs) are promising candidates for a broad range of technologies. However, the relatively low conductivity of PolyILs at room temperature has strongly limited their applications. In this work, we provide new insights into the roles of various microscopic parameters controlling ion transport in these polymers, which are crucial for their rational design and practical applications. Using broadband dielectric spectroscopy and neutron and light scattering techniques, we found a clear connection between the activation energy for conductivity, fast dynamics, and high-frequency shear modulus in PolyILs at their glass transition temperature (Tg). In particular, our analysis reveals a correlation between conductivity and the amplitude of fast picosecond fluctuations at Tg, suggesting the possible involvement of fast dynamics in lowering the energy barrier for ion conductivity. We also demonstrate that both the activation energy for ion transport and the amplitude of the fast fluctuations depend on the high-frequency shear moduli of PolyILs, thus identifying a practically important parameter for tuning conductivity. The parameters recognized in this work and their connection to the ionic conductivity of PolyILs set the stage for a deeper understanding of the mechanism of ion transport in PolyILs in the glassy state.

The antagonistic effect of processing and thermal annealing on both the filler structure and the polymer matrix is explored in polymer nanocomposites based on natural rubber with precipitated silica incorporated by coagulation from aqueous suspension followed by roll-milling. Their structure and linear and non-linear rheology have been studied, with a particular emphasis on the effect of high temperature thermal treatment and the number of milling passes. Small-angle X-ray scattering intensities show that the silica is organized in small, unbreakable aggregates containing ca. 50 primary nanoparticles, which are reorganized on a larger scale in filler networks percolating at the highest silica contents. As expected, the filler network structure is found to be sensitive to milling, more milling inducing rupture, as evidenced by the decreasing Payne effect. After thermal treatment, the nanocomposite structure is found to be rejuvenated, erasing the effect of the previous milling on the low-strain modulus. In parallel, the dynamics of the samples described by the rheology or the calorimetric glass-transition temperature remain unchanged, whereas the natural latex polymer network structure is modified by milling towards a more fluid-like rheology, and cannot be recovered.

Industrial and model polymer nanocomposites are often formulated with coating agents to improve polymer-nanoparticle (NP) compatibility. Here the localization of silane coating agents in styrene-butadiene nanocomposite is investigated through the segmental dynamics of the polymer matrix by broadband dielectric spectroscopy (BDS), allowing the detection of silanes in the matrix through their plasticization effect. This acceleration of dynamics was followed via the shift of τmax of the α-relaxation induced by the presence of coating agents of different molecular weight and quantity, for different amounts of incorporated colloidal silica NPs (R ≈ 12.5 nm, polydispersity 12%). Any noteworthy contribution of interfacial polymer layers on τmax has been excluded by reference measurements with bare NPs. Our approach allowed quantifying the partition between the matrix and the NP interfaces, and was confirmed independently by calorimetry. As a control parameter, the silane grafting reaction could be activated or not, which was confirmed by the absence (resp. presence) of partitioning with the matrix. Our main result is that in the first steps of material formulation, before any grafting reaction, coating agents both cover the silica surface by adsorption and mix with the polymer matrix-in particular if the latter has chemical compatibility via its functional groups. Silane adsorption was found to be comparable to the grafted amount (1.1 nm-2), and does not increase further, confirming that the plateau of the adsorption isotherm is reached in industrial formulations. These results are hoped to contribute to a better understanding of the surface reactions taking place during complex formulation processes of nanocomposites, namely the exact amounts at stake, e.g., in industrial mixers. Final material properties are affected both through NP-matrix compatibility and plasticization of the latter by unreacted molecules. 2

Using molecular dynamics simulations we investigate the dependence of the structuraland vibrational properties of the surfaces of sodo-silicate glasses on the sodium content as well as the nature of the surface. Two types of glass surfaces are considered:A melt-formed surface (MS) in which a liquid with a free surface has been cooleddown into the glass phase and a fracture surface (FS) obtained by tensile loadingof a glass sample. We find that the MS is more abundant in Na and non-bridgingoxygen atoms than the FS and the bulk glass, whereas the FS has higher concentration of structural defects such as two-membered rings and under-coordinated Si thanthe MS. We associate these structural differences to the production histories of theglasses and the mobility of the Na ions. It is also found that for Na-poor systems thefluctuations in composition and local atomic charge density decay with a power-lawas a function of distance from the surface while Na-rich systems show an exponentialdecay with a typical decay length of ≈ 2.3 Å. The vibrational density of states showsthat the presence of the surfaces leads to a decrease of the characteristic frequenciesin the system. The two-membered rings give rise to a pronounce band at ≈ 880 cm−1which is in good agreement experimental observations.