196 research outputs found
Modeling of Transient Trapping of Fatty Acid Tails in Phospholipids
We present the derivation of a new model to describe neutron spin echo
spectroscopy and quasi-elastic neutron scattering data on liposomes. We compare
the new model with existing approaches and benchmark it with experimental data.
The analysis indicates the importance of including all major contributions into
modeling of the intermediate scattering function. Simultaneous analysis of the
experimental data on lipids with full contrast and tail contrast matched
samples, reveals highly confined lipid tail motion. A comparison of their
dynamics demonstrates the statistical independ-ence of tail-motion and
height-height correlation of the membrane. A more detailed analysis indi-cates
that lipid tails are subject to relaxations in a potential with cylindrical
symmetry, in addition to the undulation and diffusive motion of the liposome.
Despite substantial differences in the chemis-try of the fatty acid tails, the
observation indicates a universal behavior. The analysis of partially
deuterated systems confirms the strong contribution of the lipid tail to the
intermediate scattering function. Within the time range from 5 to 100 ns, the
intermediate scattering function can be de-scribed by the height-height
correlation function. The existence of the fast-localized tail motion and the
contribution of slow translational diffusion of liposomes determines the
intermediate scattering function for t 100 ns, respectively.
Taking into account the limited time window lowers the bending moduli by a
factor of 1.3 (DOPC) to 2 (DMPC) compared to the full range.Comment: 33 pages, 5 figures, published in Soft Matte
Validity of Stokes-Einstein Relation in Soft Colloids up to the Glass Transition
We investigate the dynamics of kinetically frozen block copolymer micelles of
different softness across a wide range of particle concentrations, from the
fluid to the onset of glassy behavior, through a combination of rheology,
dynamic light scattering and pulsed field gradient NMR spectroscopy. We
additionally perform Brownian dynamics simulations based on an ultrasoft
coarse-grained potential, which are found to be in quantitative agreement with
experiments, capturing even the very details of dynamic structure factors S(Q,
t) on approaching the glass transition. We provide evidence that for these
systems the Stokes-Einstein relation holds up to the glass transition; given
that it is violated for dense suspensions of hard colloids, our findings
suggest that its validity is an intriguing signature of ultrasoft interactions.Comment: 5 pages, 4 figures, Supplementary Information, Accepted to Physical
Review Letters (PRL) (2015
Characteristic length scales of the secondary relaxations in glass-forming glycerol
We investigate the secondary relaxations and their link to the main structural relaxation in glass-forming liquids using glycerol as a model system. We analyze the incoherent neutron scattering signal dependence on the scattering momentum transfer, Q , in order to obtain the characteristic length scale for different secondary relaxations. Such a capability of neutron scattering makes it somewhat unique and highly complementary to the traditional techniques of glass physics, such as light scattering and broadband dielectric spectroscopy, which provide information on the time scale, but not the length scales, of relaxation processes. The choice of suitable neutron scattering techniques depends on the time scale of the relaxation of interest. We use neutron backscattering to identify the characteristic length scale of 0.7 Å for the faster secondary relaxation described in the framework of the mode-coupling theory (MCT). Neutron spin-echo is employed to probe the slower secondary relaxation of the excess wing type at a low temperature ( ∼ 1.13Tg . The characteristic length scale for this excess wing dynamics is approximately 4.7 Å. Besides the Q -dependence, the direct coupling of neutron scattering signal to density fluctuation makes this technique indispensable for measuring the length scale of the microscopic relaxation dynamics
Impact of Local Stiffness on Entropy Driven Microscopic Dynamics of Polythiophene
© 2020, The Author(s). We exploited the high temporal and spatial resolution of neutron spin echo spectroscopy to investigate the large-scale dynamics of semiflexible conjugated polymer chains in solutions. We used a generalized approach of the well-established Zimm model of flexible polymers to describe the relaxation mode spectra of locally stiff polythiophene chains. The Zimm mode analysis confirms the existence of beads with a finite length that corresponds to a reduced number of segmental modes in semiflexible chains. Irrespective of the temperature and the molecular weight of the conjugated polymer, we witness a universal behavior of the local chain stiffness and invariability of the bead length. Our experimental findings indicate possibly minor role of the change in π-electron conjugation length (and therefore conjugated backbone planar to non-planar conformational transition) in the observed thermochromic behavior of polythiophene but instead point on the major role of chain dynamics in this phenomenon. We also obtained the first experimental evidence of an existence of a single-chain glass state in conjugated polymers
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