Dielectric and Mechanical Relaxation of Glass-Forming Liquids in Nanopores
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H. WENDT, R. RICHERT
Max-Planck-Institut fuir Polymerforschung, Ackermannweg 10, D-55128 Mainz, Germany ABSTRACT We have measured the time resolved phosphorescence of different probe molecules in glassforming solvents under the condition of geometrical confinement in porous glasses. This solvation dynamics technique probes the local dielectric relaxation in the case of a dipolar chromophore in polar liquids. In the absence of dipolar interactions, the observed Stokes shifts reflect the local density or mechanical responses. Therefore, both orientational and translational modes of molecular motions can be measured for liquids imbibed in porous silica glasses. The effect of confinement on the relaxations of supercooled liquids is strongly dependent on the surface chemistry and can be rationalized on the basis of the cooperativity concept. As in the bulk case, we find that the relaxations in nano-confined liquids display heterogeneous dynamics. The density relaxation turns out to be more sensitive to the thermal history relative to the orientational features of molecular motion. By selectively positioning the chromophores at the liquid/solid interface, we observe also that the structural relaxation of the liquid in the immediate vicinity of the glass surface is slowed down but not entirely blocked. INTRODUCTION The investigation of molecular dynamics in the case of spatial restrictions is an active area of research with the aim of understanding the confinement and interfacial effects on the behaviour of liquids, supercooled liquids, polymers, or other forms of condensed matter [1,2]. Confining systems are sometimes applied in order to seek for length scales intrinsic in the molecular motion of disordered materials in the viscous regime [1-5]. According to the picture of cooperatively rearranging regions (CRR's) advanced by Adam and Gibbs, such spatial scales range well above the molecular distances and are responsible for the complex temperature dependence of the average relaxation times [6]. Porous silica glasses are attractive materials for such investigations because of their large surface area per sample volume and their availability with pore diameters in the range of several nanometers. The high optical quality of pure silica porous glasses opens the possibilities of applying methods of optical spectroscopy to liquid samples inside the pores. One of the optical methods suitable for studying molecular dynamics is the technique knows as solvation dynamics measurement [7-9]. Such experiments have recently been demonstrated to reveal the relaxation behaviour of nano-confined materials [10-13 ]. In this type of experiments, the liquid under study is doped with chromophores at a low concentration level and the emission spectra of these isolated probe molecules are recorded as a function of time following electronic excitation [7-9]. For dye molecules which display an appreciable change in their dipole moment 1u upon excitation, the observed gradual shift of the emission energy reflects the dielectric polarization of
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