Berlin 2008 – scientific programme
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DY: Fachverband Dynamik und Statistische Physik
DY 11: Glasses I (joint session DF/DY)
DY 11.7: Talk
Tuesday, February 26, 2008, 11:15–11:30, EB 407
Secondary relaxations in molecular glasses and polymers studied by 2D 2H NMR — •Björn Micko1, Dieter Bingemann2, and Ernst Rössler1 — 1Experimentalphysik II, Universität Bayreuth, 95440 Bayreuth, Germany — 2Department of Chemistry, Williams College, Williamstown, MA 01267, USA
We present a two-dimensional (2D) 2H exchange NMR study, attempting to clarify the geometry of the molecular motion involved in the secondary relaxation (β-process) of three glass formers: PMMA, polybutadiene and a mixture of decaline and chlorobenzene. Stimulated echo measurements of the orientational correlation function circumscribe the temperature range, in which the β-process is expected to dominate the spectra. In this range we will show by comparison with the spectra of o-terphenyl, which does not show a pronounced β-process, that the β-process is also clearly observable in the 2D NMR spectra below and somewhat above Tg, until upon further heating the structural relaxation (α-process) enters the time window of the experiment and gives rise to a convergence of the spectra. Whilst the time constants for the studied systems (obtained from dielectric spectrocopy) are very similar on the reduced temperature scale Tg/T, the dielectric relaxation strength differs for each system. In contrast the 2D NMR spectra turn out to be practically identical on the Tg/T scale - which implies strong similarities concerning time scale and underlying geometry of the motion. To get further insight on the reorientation angles involved, simple motional models will be compared against the spectra.