Dynamic Light Scattering in Binary Glass Forming Systems -- Concentration Fluctuations, Secondary Processes and Relaxation in Confinement

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The aim of the continuation of the present project is to gain an understanding of the molecular dynamics on different time- and lengthscales in dynamically asymmetric binary mixtures of glass forming molecules, i.e., mixtures of dynamically rather different partners, with a large difference in each components Tg. It turns out that within a certain temperature range, where the large molecules in such a mixture a already dynamically frozen, the smaller component retains a high degree of mobility, and the dynamics of the latter shows so-called intrinsic confinement effects. These effects include a so-called fragile-to-strong transition in the temperature dependence of the time constants and in particular for small molecules close to the rigid matrix a peculiar Type-A glass transition, which was previously predicted by mode-coupling theory for the glass transition in certain confinement situations. In the continuation of the project such intrinsic confinement effects are investigated and systematically compared to effects of other kinds of hard and soft confinement. The investigations will be based on the one hand on the microemulsion systems, which were developed in the first period of the project and provide the opportunity to study soft confinement and also on our experience of light scattering of supercooled binary mixtures and liquids in porous Vycor matrices. Therefore it will be possible to study the same small molecule in different types of hard and soft confinement. Concerning the experimental methods used, it will be crucial to include the multi-speckle correlation technique into the present light scattering setup, in order to be able to efficiently study non-ergodic and partially ergodic systems with polarized and depolarized photon correlation spectroscopy. In this way molecular reorientation as well as the decay of concentration fluctuations can be separately monitored by light scattering. In particular, the multispeckle-technique will be applied to monitor molecular reorientation in non-ergodic systems. In addition, these measurements will be combined with differential scanning calorimetry, broadband dielectric spectroscopy and quasielastic neutron scattering to cover the full dynamic range and be able to disentangle different surface and finite-size effects. At the same time the decay of concentration fluctuations is investigated at different time- and lengthscales, on the one hand to further clarify the influence on the blend dynamics and on the other hand to understand the temperature dependence of these fluctuations. First studies with X-ray photon correlation spectroscopy indicate jamming behavior of partially frozen concentration fluctuations in the vicinity of the mixtures upper glass transition, which is characterized by compressed correlation functions and a so-called ballistic wave-vector dependence of the correlation times. This unusual behavior will be investigated by multispeckle dynamic light scattering.