Molecular Dynamics in Binary Organic Glass Formers

Abstract

We investigated binary low-molecular-weight glass formers as model systems for mixtures of small and large molecules. Tricresyl phosphate (TCP) in oligomeric styrenes (OS), benzene in OS and polystyrene (PS), and benzene in TCP were studied by applying dielectric spectroscopy as well as 1H, 2H, and 31P NMR spectroscopy. Temperatures above and below the glass-transition temperature (T G) are covered. The dielectric loss of the small component appears broader the higher the molecular ratio M/m is, and the lower the TCP concentration and the lower the temperature are chosen. Close to T G, extremely broad distributions of correlation times G(log τ) result, which are similar to those reported in the cases of polymer−plasticizer systems, although for our systems the motional heterogeneities are already established at similar M and m. By applying 1H and 2H (1D and 2D) NMR on benzene in OS and PS, we can demonstrate that the large molecules basically behave as in neat glass formers. However, the small molecules exhibit an isotropic reorientation also well below T G, and the dynamics is rather characterized by a random jump process than by rotational diffusion, the latter being found in neat systems. Furthermore, we can prove that within G(log τ), exchange processes take place, even below T G, which essentially occur on the same time scale as reorientation.

Two-Dimensional 109Ag NMR and Random-Walk Simulation Studies of Silver Dynamics in Glassy Silver Ion Conductors

“…Hence, it is expected that the 109 Ag NMR spectrum consists of a superposition of a Gaussian and a Lorentzian. We note that such ''two-phase spectra'' are also observed when studying the dynamics of plasticizer molecules in polymer-plasticizer systems [3,43,44]. In this case, the line-shape results from a broad continuous Gðlg tÞ:…”

“…Further, small-angle fluctuations of a few degrees attributed to the secondary relaxation of organic glass formers below the glass transition have been studied in detail [10][11][12]. In addition, the dynamics of particles in various matrices, e.g., in zeolithes [13,14], organic glasses [15,16] and metallic glasses [17], have been characterized by 2D NMR. More insight into complex motional processes is available on the bases of three-and four-time correlation functions in NMR.…”

Susceptibility functions for slow relaxation processes in supercooled liquids and the search for universal relaxation patterns

“…This class of glass formers exhibits very broad dielectric loss spectra, in particular, significantly broader than in neat substances. 32,41 In addition, a strong secondary relaxation process may show up if the concentration is chosen properly, although the corresponding neat systems do not show a ␤ process. Explicitly, we consider 2-picoline in o-terphenyl.…”

Reorientational Dynamics and Solid-Phase Transformation of Ammonium Dicyanamide into Dicyandiamide: A 2H Solid-State NMR Study

“…[25][26][27][28] Traditionally, 2 H NMR spectroscopy is used as a powerful technique for the study of dynamics in glassy systems, liquid crystals, and polymers, with the emphasis being on structural phase and glass transitions of essentially amorphous systems. 25,26,[29][30][31][32][33][34][35] In addition, 2 H NMR in conjunction with neutron scattering afforded a better understanding of the cross-link between structural and dynamic phenomena and provided new insights into relaxation and tunneling processes [36][37][38][39] in simple crystalline salts, among which ammonium compounds such as NH 4 Cl 40-43 or (NH 4 ) 2 SO 4 play a prominent role. Accordingly, applying the 2 H NMR technique to the in situ study of the topochemical transformation of ammonium dicyanamide into dicyandiamide seems to be a straightforward issue, which will be discussed in some detail in the present work.…”