0000000000173986
AUTHOR
Werner Köhler
Thermal irreversibility in optically labeled low-temperature glasses.
We present an investigation of irreversible features of thermal broadening of persistent spectral holes. The investigation is based on temperature-cycling hole-burning experiments performed with a variety of organic glasses doped with rather different probe molecules. The results show a rich temperature dependence. They can, however, be interpreted in terms of the well-known spectral diffusion models, in which we introduced a freezing condition to account for thermal irreversiblity. There is a tunneling regime for low temperatures and an activated regime for high temperatures. In the tunneling regime the broadening is linear in T; in the activated regime it increases with ${T}^{3/2}$ and lo…
The effect of the internal architecture of polymer micronetwork colloids on the dynamics in highly concentrated dispersions
Motivated by the finding that colloidal dispersions of polymer micronetwork spheres with a cross-link density of 1:50 (inverse number of monomer units between crosslinks) show significant deviations from the dynamics of hard spheres in the colloid glass as seen by dynamic light scattering (DLS) (E. Bartsch, V. Frenz, H. Sillescu J. Non — Cryst. Solids 172–174 (1994), 88–97), we have undertaken a systematic study of the effect of the crosslink density on the dynamics at high concentrations. Long-time self-diffusion coefficients D S L and collective diffusion coefficients D c were measured for colloids with crosslink densities of 1:10, 1:20 and 1:50 by forced Rayleigh scattering (FRS) and the…
Thermal diffusion and Soret feedback of gold-doped polyorganosiloxane nanospheres in toluene
We have investigated diffusion and thermal diffusion properties of light-absorbing colloidal polyorganosiloxane microgels containing tiny nanometer-sized gold clusters dispersed in toluene. Transient holographic gratings allow for very subtle perturbations in the linear regime where Soret feedback is negligible. Gold-doped colloids of different size and crosslink ratios show different Soret coefficients but identical thermal diffusion coefficients D(T). Undoped colloids tend to aggregate, but a consistent interpretation is obtained if an identical D(T) is assumed for the doped, the undoped, and the aggregated particles. Previously reported Soret feedback measurements on similar systems inci…
Collective and self diffusion of PS microgels in solution as observed by thermal diffusion forced Rayleigh scattering
Thermal diffusion in solutions of polystyrene micro-network spheres (microgels) in toluene has been studied by the holographic scattering technique of thermal diffusion forced Rayleigh scattering (TDFRS) and by photon correlation spectroscopy (PCS). Size distributions of microgels of different crosslink ratios are obtained from TDFRS measurements on dilute solutions at very low q-values around 4000cm−1. At low concentrations a single diffusive mode is observed and the diffusion coefficient increases with concentration. It is attributed to the collective diffusion of the microgels and the solvent. At high concentrations an additional slow mode appears whose diffusion coefficient decreases wi…