Incoherent quasielastic neutron scattering study of molecular dynamics of 4-n-octyl-4′-cyanobiphenyl

Abstract

We report incoherent quasielastic neutron scattering experiments on the thermotropic liquid crystal 4-n-cyano-4'-octylbiphenyl. The combination of time-of-flight and backscattering data allows analyzing the intermediate scattering function over about three decades of relaxation times. Translational diffusion and uniaxial molecular rotations are clearly identified as the major relaxation processes in respectively the nanosecond and picosecond time scales. The comparison with literature data obtained by other techniques is discussed.

Dual translocation pathways in smectic liquid crystals facilitated by molecular flexibility

“…Free energy estimates of the barrier height required for such mechanisms justify this observation. Signatures of these distinct mechanisms could also be inferred from experiments like quasielastic neutron scattering [42,43], by measuring the dynamic structure factor, which is related to the Fourier transform of the VHCF. The appearance of the shoulders in the VHCF (marked by vertical arrows in the left panel of Fig.…”

Physics of fluids confined in nanochannels: A shared space for neutrons and simulation

“…It assumes that i) the surface dynamics is frozen within the experimental resolution (r = R), ii) far from the surface, the dynamics tends to that of the bulk, following a simple exponential decay 1 (r) ∝ exp( R−r wall ) where 1 (r) is the inverse local relaxation time at a distance r from the pore center, R is the pore radius, and wall is the dynamical correlation length that characterizes the range of influence of the surface on the molecular dynamics and iii) the relaxation modes remain qualitatively the same as in the bulk. The input dynamic parameters for the bulk (diffusion coefficient D, rotational correlation time and Debye-Waller factors) were obtained from a previous study of unconfined 8CB, where the intermediate scattering functions were fully analyzed as a combination of vibrational modes, uniaxial rotation and centre-of-mass translational diffusion [36]. A local intermediate scattering function f (r, Q, t) is then defined as a function of the distance r to the pore wall according to eq.…”

Relation between static short-range order and dynamic heterogeneities in a nanoconfined liquid crystal

“…The spatial correlation length ξ stat of the critical fluctuations of these order parameters is directly accessible to the experiment. Although the molecular dynamics of LCs is generally described by a finite set of correlation times and a well-defined relaxation mechanism [11][12][13], they also share many features with supercooled liquids on a time scale where collective critical fluctuations remain very slow [14]. Interestingly enough, this similarity between LCs and supercooled liquids can be emphasized by confining the LC in a nanoporous solid.…”

“…Eight wafers were stacked parallel to each other in a cylindrical aluminum cell, representing a total amount of confined LC of about 20 mg. Complete details of the experimental setup, data processing, intermediate scattering functions calculations, and model fitting are given elsewhere [11]. Figure 1 displays the incoherent intermediate scattering functions F(Q,t) of nanoconfined 8CB compared to those of the bulk at the same temperature (296 K) for different momentum transfer vectors Q.…”

“…Far from the pore wall, it seems reasonable to assume that the molecular dynamics results in the same incoherent scattering function F bulk (Q,t) as in the bulk liquid crystal. We developed a model of this function for bulk 8CB [11] including the afore-mentioned rotational and selfdiffusional elementary relax-ation processes. On the other hand, 8CB molecules close to the silicon wall (at a distance |r| from the pore center with R − |r| < ξ wall ) might experience a significant mobility reduc-tion, resulting in a different local scattering function ƒ(r,Q,t).…”