- Secondary relaxation processes in neat and binary glass formers studied by 2H NMR spectroscopy (2012)
- The present study employs various solid state 2H NMR techniques to elucidate glassy dynamics in plastic crystals and binary mixtures of glass forming substances. We focus on the Johari-Goldstein beta-process, which not only plays a key role for the understanding of glassy dynamics in neat systems, but also in binary mixtures thereof. The first part is devoted to the plastic crystalline (PC) phase of cyanocyclohexane. A 2D 2H NMR assessment regarding the alpha-process demonstrated that dynamics is not governed by the symmetry of the lattice, rather molecular reorientation in cyanocyclohexane can be modelled via the same distribution of small and large angular jumps as reported for many structural glass formers. Although the stimulated echo technique yields a strong temperature dependence of the latter fractions, it was shown that this effect can be rationalized via the time-window of the experiment and the inherent decoupling of jump-times within the distribution of angles. This analysis also holds in case of previously studied structural glass formers and questions the arise of a dynamical crossover at temperatures somewhat above Tg. The beta-process of cyanocyclohexane below Tg is well described by models developed for the structural glass former toluene, where the C-2H bond is confined to the base circle of a cone, i.e. also the secondary relaxation is not significantly affected by the translational symmetry of the PC phase. As alpha- and beta-process do not merge in cyanocyclohexane, pronounced effects were observed at high temperatures: for the first time an additional minimum in the spin-lattice relaxation T1 reflecting the beta-process was found. Furthermore the solid-echo spectra at T>Tg exhibit an articulate and characteristic deviation from a Pake pattern over a broad temperature range. These fast motion limit line-shape effects allow for a direct determination of the spatial restriction: a model based on a Gaussian distribution of cone opening angles proved adequate for a detailed modelling of the spectral evolution. T1 was modelled by means of the spectral density from dielectric spectroscopy, the results in terms of the relaxation strength 1-S are in agreement with the line shape analysis. Hence the pronounced NMR effects naturally arise due to the non-merging beta-process in cyanocyclohexane and are in full agreement with an extension of previous models for the beta-process in structural glass formers at T<Tg. Furthermore the present study emphasizes the important role of the beta-process for glassy dynamics, as it demonstrates that a substantial fraction of correlation is lost via the beta-process at high temperatures in the absence of merging. The second part addresses the dynamics of the glass former toluene in binary mixtures. Below Tg of neat toluene a pronounced concentration dependence of the beta-process was observed in mixtures of toluene and a polychlorinated biphenyl (PCB54). Whereas the distribution of correlation times remains unaltered, the relaxation strength of the process significantly decreases below a threshold toluene concentration of x=0.7. Stimulated echo experiments demonstrated that this attenuation arises from a fraction of toluene molecules, 1-fb, which do no longer exhibit a beta-process. This finding is in accordance with bi-exponential magnetization curves observed around Tg of the mixture. Hence the "islands of mobility" concept for the beta-process, refuted in the case of neat glass formers, was introduced in binary mixtures thereof. A model was developed that links the fraction 1-fb to toluene molecules in a local PCB54 rich environment, which only exhibit the alpha-process of the latter, whereas the remainder of molecules reorient on a faster time scale (alpha'-process) and show a beta-process. This bimodal approach with two distinct toluene sub-ensembles was shown to explain the observed behaviour in virtually all NMR experiments, whereas an adaptation via a single broad distribution for the toluene motion was infeasible. The relative weight of the fractions is a function of concentration and temperature, as 2D NMR spectra demonstrated that exchange between the sub-ensembles exists. 2D NMR experiments furthermore show that toluene molecules in the mixtures reorient via the same mechanism as in the neat system - both sub-ensembles are governed by typical glassy dynamics. To widen the validity of the present study, previous results from other binary mixtures containing toluene were reassessed: all features regarding the beta-process were recovered and its concentration dependence below Tg in diverse mixtures was successfully described within a simple lattice model. This dependence of the beta-process on the local toluene concentration provides strong arguments for a cooperativeness of the process, in contrast to the general perception.