Structural relaxation in the glass transformation range using probe-ion spectroscopy

Relaxation process in glassforming systems near and below the glass transition are of great practical importance, but always are found to be complex, non linear, and difficult to fully interpret. In this thesis insight into the nature of such relaxation processes is sought by application of a probe-ion spectroscopy technique which follows a microscopic element of the total relaxation process. With this method, the structural changes around optically active elements dissolved in glassforming solvents are followed as temperature is changed near or through the glass transition. This can be done either after a sudden jump in temperature (T-jump) is made or a continuous temperature change is applied. Two kinds of host media suitable for these experiments have been studied, inorganic glass-formers containing ZnCl 2, and sugar-based glasses. The optically active centers, detectable by visible spectroscopy, that were chosen for this study were redox couples of colored metal transition ions for the former, and metal complexes with temperature-dependent coordinating spheres, for the latter. Most data were obtained for the second case in which the kinetics of the change of first coordination shell of Co+2 in fragile sugar based-glasses was followed. The results obtained in this work, permitted each of the three canonical features of glass-forming systems (non-Arrhenius relaxation time, non-exponential relaxation and non-linear relaxation) to be explored on the local structure level. The temperature dependence of the average relaxation times for the local relaxation was compared with that of the solvent and found to be smaller. The non-exponentiality for the local relaxation process of the fragile sugar-based glasses obtained from the analysis of the data on isothermal experiments using the Kohlrausch-Williams-Watts (KWW) function, is found to be less pronounced than for the bulk. Finally, the non-linearity of the relaxation process was obtained from the difference in tau parameters, also fitted to the KWW function, obtained from positive and negative temperature-jump experiments of the same magnitude. By comparison of the up-scan and downscans of enthalpy (global relaxation) and absorbance at fixed wavelength (local property) during temperature ramp experiments performed in the same sample it could be shown that there is a broader distribution of relaxation time for the global structural relaxation than for the local elements. This is consistent with the idea that local relaxation process is much less cooperative than the global relaxation processes. Judging by the differences in their characteristics relaxation times, the local probe decouples increasingly from the global process (solvent relaxation) as temperature increases above Tg. At the temperatures characteristic of the classic ligand exchange studied using fast T-jumps by Eigen and coworkers, the relaxation time gap would be several orders of magnitude. Thus our probe species measurements demonstrate the manner in which processes in a solvent, which are activation energy controlled at high temperature, can become diffusion controlled, and develop "glassy dynamics" character at low temperature, as they encounter the solvent relaxation time. This interesting phenomenology is a consequence of the non-Arrhenius temperature dependence of diffusion in the solvent.