764 research outputs found
Polymer conformation and dynamics in crowded environments: A combined diffusion NMR and small-angle neutron scattering study
The effect of particles on the behavior of polymers in solution is important in a number of important phenomena such as the effect of “crowding” proteins in cells, colloid-polymer mixtures, and nanoparticle “fillers” in polymer solutions and melts. In this talk, I will present a study of the effect of spherical inert nanoparticles (which we refer to as “crowders”) on the diffusion coefficient and radius of gyration of polymers in solution using pulsed-field-gra- dient NMR and small-angle neutron scattering (SANS), respectively. In addition, the role of enthalpic crowder- crowder interactions on the crowding process is unknown: we can control this by varying charge on the crowder particle.
Below a characteristic polymer concentration, which we identify as the overlap threshold concentration c⋆, the diffusion coefficients exhibit a plateau. Above c⋆, in a crossover region between the dilute and semidilute regimes, the (long-time) self-diffusion coefficients are found, universally, to decrease exponentially with polymer concentration at all crowder packing fractions, consistent with a structural basis for the long-time dynamics. When the polymer radius of gyration and crowder size are comparable, the polymer size is very weakly affected by the presence of crowders, consistent with recent computer simulations. We find that crowder charge only weakly affects polymer size and dynamics in the crowding limit, but that local macromolecular mobility depends strongly on molecular flexibility
Self-consistent mode-coupling theory for the viscosity of rod-like polyelectrolyte solutions
A self-consistent mode-coupling theory is presented for the viscosity of
solutions of charged rod-like polymers. The static structure factor used in the
theory is obtained from polymer integral equation theory; the Debye-H\"{u}ckel
approximation is inadequate even at low concentrations. The theory predicts a
non-monotonic dependence of the reduced excess viscosity, , on
concentration from the behaviour of the static structure factor in
polyelectrolyte solutions. The theory predicts that the peak in occurs
at concentrations slightly lower than the overlap threshold concentration,
. The peak height increases dramatically with increasing molecular
weight and decreases with increased concentrations of added salt. The position
of the peak, as a function of concentration divided by is independent
of salt concentration or molecular weight. The predictions can be tested
experimentally.Comment: 9 pages, 9 figures (2 figures added in the revise version
Thermodynamically consistent Reference Interaction Site Model theory of the tangent diatomic fluid
Thermodynamic and structural properties of the tangent diatomic fluid are
studied in the framework provided by the Reference Interaction Site Model
(RISM) theory, coupled with a Modified Hypernetted Chain closure. The
enforcement of the internal thermodynamic consistency of the theory is
described in detail. The results we obtain almost quantitatively agree with
available or newly generated simulation data. We envisage the possibility to
extend the consistent RISM formalism to generic, more realistic molecular
fluids.Comment: Typeset with LaTeX, 6 pages, 3 figures (5 subfigures), 28 references,
submitted to Chem. Phys. Let
Contrasting the dynamics of elastic and non-elastic deformations across an experimental colloidal Martensitic transition
We present a framework to segregate the roles of elastic and non-elastic
deformations in the examination of real-space experiments of solid-solid
Martensitic transitions. The Martensitic transformation of a
body-centred-tetragonal(BCT) to a body-centred-orthorhombic(BCO) crystal
structure has been studied in a model system of micron-scale ionic microgel
colloids. Non-affine fluctuations, i.e., displacement fluctuations that do not
arise from purely elastic(affine) deformations, are detected in particle
configurations acquired from the experiment. Tracking these fluctuations serves
as a highly sensitive tool in signaling the onset of the Martensitic transition
and precisely locating particle rearrangements occurring at length scales of a
few particle diameters. Particle rearrangements associated with non-affine
displacement modes become increasingly favorable during the transformation
process. The nature of the displacement fluctuation modes that govern the
transformation are shown to be different from those predominant in an
equilibrium crystal. We show that BCO crystallites formed through shear may,
remarkably, co-exist with those resulting from local rearrangements within the
same sample
Combining Diffusion NMR and Small-Angle Neutron Scattering Enables Precise Measurements of Polymer Chain Compression in a Crowded Environment
The effect of particles on the behavior of polymers in solution is important in a number of important phenomena such as the effect of “crowding” proteins in cells, colloid-polymer mixtures, and nanoparticle “fillers” in polymer solutions and melts. In this Letter, we study the effect of spherical inert nanoparticles (which we refer to as “crowders”) on the diffusion coefficient and radius of gyration of polymers in solution using pulsed-field-gradient NMR and small-angle neutron scattering (SANS), respectively. The diffusion coefficients exhibit a plateau below a characteristic polymer concentration, which we identify as the overlap threshold concentration
c⋆. Above c⋆, in a crossover region between the dilute and semidilute regimes, the (long-time) self-diffusion coefficients are found, universally, to decrease exponentially with polymer concentration at all crowder packing fractions, consistent with a structural basis for the long-time dynamics. The radius of gyration obtained from SANS in the crossover regime changes linearly with an increase in polymer concentration, and must be extrapolated to c⋆ in order to obtain the radius of gyration of an individual polymer chain. When the polymer radius of gyration and crowder size are comparable, the polymer size is very weakly affected by the presence of crowders, consistent with recent computer simulations. There is significant chain compression, however, when the crowder size is much smaller than the polymer radius gyration
Clusters in sedimentation equilibrium for an experimental hard-sphere-plus-dipolar Brownian colloidal system
In this work, we use structure and dynamics in sedimentation equilibrium, in
the presence of gravity, to examine, confocal microscopy, a Brownian
colloidal system in the presence of an external electric field. The zero field
equation of state (EOS) is hard sphere without any re-scaling of particle size,
and the hydrodynamic corrections to the long-time self-diffusion coefficient
are quantitatively consistent with the expected value for hard spheres. Care is
taken to ensure that both the dimensionless gravitational energy, which is
equivalent to a Peclet number , and dipolar strength are of
order unity. In the presence of an external electric field, anisotropic
chain-chain clusters form; this cluster formation manifests itself with the
appearance of a plateau in the diffusion coefficient when the dimensionless
dipolar strength . The structure and dynamics of this
chain-chain cluster state is examined for a monodisperse system for two
particle sizes
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