Dynamics studied by Quasielastic Neutron Scattering (QENS)

Abstract

Quasielastic neutron scattering (QENS) allows measurement of the molecular displacements in time and space, from pico- to tens of nanoseconds and from Ångstroms to nanometers, respectively. The method probes dynamics from fast vibrational modes down to slow diffusive motion. Every scattering experiment leads to a dynamic structure factor $$S\left( {\vec Q,\omega } \right)$$ S Q → , ω or its spatial and temporal Fourier transform (van Hove correlation function $$G\left( {\vec r,t} \right)$$ G r → , t ). This shows exactly where the atoms are and how they move. In this manuscript the basics of the QENS method are presented and a few examples highlighting the potentials of QENS are given: (i) diffusion of liquids and gases in nano- and mesoporous materials; (ii) hydrogen dynamics in a high temperature polymer electrolyte fuel cell (HT-PEFC) and (iii) influence of the surface interactions on polymer dynamics in nanopores.

Nanostructure Explains the Behavior of Slippery Covalently Attached Liquid Surfaces

“…The lowest hysteresis samples studied here (K2, W1, Z1) all possessed similar nanostructure, with $${\hat N_{\rm{n}} }$$ ≈40 (≈3 kDa) and d dry ≈3.5 nm (corresponding to Σ=2.5). Previous work has speculated that a minimum in CAH is observed for polymers of intermediate molecular weight as shorter chains imperfectly coat the substrate, and longer chains are above the bulk entanglement length of PDMS, [4,11,22,26,37] which is between 6 and 34 kDa ($${\hat N_{\rm{n}} }$$ =81 to 460) [38,39] . However, our study reveals that the exact value of entanglement length in the bulk is not significant, as entanglement is a function of σ for polymer brushes [40] .…”

Diffusion in Nanoporous Solids in the Focus of IUPAC – A Tribute to Jens Weitkamp

“…Measuring techniques with their focus on the elementary processes of molecular propagation are referred to as submicroscopic. They include the various techniques of solidstate NMR [85,86] and quasi-elastic neutron scattering (QENS [87]).…”

“…Vice versa, on considering, e.g., sufficiently large observation times, PFG NMR diffusion studies [88,89] must, more properly, be referred to as mesoor even macroscopic rather than as microscopic techniques. It is, most importantly, a particular virtue of QENS [87] to monitor, under appropriate conditions, not only the elementary steps of diffusion but also their superposition over distances providing clear evidence of Fickian diffusion, i.e., on displacements over ''microscopic'' dimensions.…”

“…The information provided by dynamic light scattering (DLS [100]) and quasi-elastic neutron scattering (QENS [87]) is based on internal fluctuations within the system. These fluctuations are known to be subject, according to Onsager's regression theorem [118] (see also Sect.…”

“…This concerns in particular the correlation between concentration gradients and diffusive fluxes following Fick's 1st law. It is for this reason that, via DLS [100] and QENS [87], transport diffusivities can be determined with the host-guest system at (macroscopic) equilibrium.…”