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Study of relaxation processes in simple glass formers by means of 2H NMR spectroscopy
(2006)
- In the present work molecular glass formers were investigated by means of multidimensional 2H NMR (solid echo and 2D spectra, stimulated echo decays, spin-lattice and spin-spin relaxation). We focused our attention on glass forming liquids that were composed of simple, rigid molecules in order to study the properties of the secondary relaxation processes (beta-processes) in the glassy state (0.5Tg<T<Tg). For several systems we also acquired NMR data at temperatures corresponding to the supercooled liquid state (T>Tg) providing information on the main relaxation process (alpha-process). In addition, extensive random walk simulations were performed to gain a deeper understanding of the experimental findings. As the molecular motion involved in the beta-process displays a highly restricted angular displacement, its effects on the line shape of the solid echo are subtle. The spectral changes due to molecular motion are quantified by the relative spectral intensity at zero 2H NMR frequency R(tp,T) (tp is the interpulse delay). Random walk simulations employing several simple models for the beta-process were performed in order to estimate the evolution of R(tp,T). They proved that 2H NMR can provide a time constant <ln(taubeta)> at a certain temperature. In addition, it was demonstrated that the R(tp,T) traces contain information about the angular amplitude and the distribution of correlation times G[ln(taubeta)]. The available data of ethanol-d1 and ethanol-d2 were successfully analyzed in the view of the simulation results, and the time constant of ethanol-d1 agrees well with dielectric spectroscopy (DS) data. We studied in detail m-fluoroaniline (m-FAN) by 2H NMR as it is a glass former that displays a relatively peculiar beta-process that is faster and shows a much smaller relative relaxation strength than most of the previously studied materials (e.g. toluene, polybutadiene). The geometry of molecular motion involved in the beta-process in m-FAN was found to be similar with the one in toluene with the difference that in the case of m-FAN a smaller angular amplitude suffices to explain the experimental findings. However, a large discrepancy between the NMR and DS time scale of the beta-process was found. The results support the claim of several authors that m-FAN forms hydrogen-bond-induced-clusters. In the quest of further understanding the secondary relaxation processes, toluene and o-terphenyl (OTP) confined in a nanoporous SBA-15 matrix were investigated. The temperature dependence of spin-lattice relaxation time allows us to differentiate type A (without a secondary dielectric beta-peak) from type B glass formers (with a beta-peak). We find that the alpha-process in both OTP (type A) and toluene (type B) in confinement is governed by a broad heterogeneous distribution of correlation times, not seen in the bulk. Additionally there is evidence that the beta-process in toluene changes under confinement conditions, where it exhibits a distribution of spin-lattice relaxation times that is substantially broadened towards faster times. In contrast, the corresponding low temperature relaxation time for OTP is not altered in confinement (T<Tg). Polybutadiene (PBD) is a widely investigated polymer without side groups. At low temperatures (T<Tg) we identified a further secondary process in addition to the beta-process that was designated as the gamma-process. In addition, a mixture of roughly 10% deuterated benzene in PBD was also measured by 2H NMR. The results proved that the benzene molecules display features very similar to those of neat PBD, thus being sensitive to both beta and gamma-process of its host. In the end we established several constraints for any model to explain the gamma-process ruling out a large angle reorientation of the C- 2H bonds at low temperatures (T<Tg). The supercooled plastically crystalline phase of cyanoadamantane was investigated in order to study the alpha-process (T>Tg). Although the orientationally disordered crystalline phase always coexisted with the orientationally ordered crystalline phase, we were able to single out the signal from the former by selective excitation and it was possible to carry out line shape measurements and 2D experiments. The latter directly reveal 6-fold reorientation of the molecular C3 axis via 90° angles, thus reflecting the symmetry of the lattice. We can reproduce the line shape by random walk simulations properly taking into account the molecular motion. Both line shape and 2D experiments yield time constants which agree with those reported by other techniques. We did not find any indication of a small angle motion as usually found for the alpha-process in structural glasses. Thus, the motional process in the glassy crystal appears to be simple and quite different from that in the structural glasses.
