294 research outputs found
A transient network of telechelic polymers and microspheres : structure and rheology
We study the structure and dynamics of a transient network composed of
droplets of microemulsion connected by telechelic polymers. The polymer induces
a bridging attraction between droplets without changing their shape. A
viscoelastic behaviour is induced in the initially liquid solution,
characterised in the linear regime by a stretched exponential stress
relaxation. We analyse this relaxation in the light of classical theories of
transient networks. The role of the elastic reorganisations in the deformed
network is emphasized. In the non linear regime, a fast relaxation dynamics is
followed by a second one having the same rate as in the linear regime. This
behaviour, under step strain experiments, should induce a non monotonic
behaviour in the elastic component of the stress under constant shear rate.
However, we obtain in this case a singularity in the flow curve very different
from the one observed in other systems, that we interpret in terms of fracture
behaviour.Comment: 9 pages, 4 figure
Hiding in Plain View: Colloidal Self-Assembly from Polydisperse Populations
We report small-angle x-ray scattering experiments on aqueous dispersions of colloidal silica with a broad monomodal size distribution (polydispersity, 14%; size, 8 nm). Over a range of volume fractions, the silica particles segregate to build first one, then two distinct sets of colloidal crystals. These dispersions thus demonstrate fractional crystallization and multiple-phase (bcc, Laves AB2, liquid) coexistence. Their remarkable ability to build complex crystal structures from a polydisperse population originates from the intermediate-range nature of interparticle forces, and it suggests routes for designing self-assembling colloidal crystals from the bottom up
Packing polydisperse colloids into crystals: when charge-dispersity matters
Monte Carlo simulations, fully constrained by experimental parameters, are found to agree well with a measured phase diagram of aqueous dispersions of nanoparticles with a moderate size polydispersity over a broad range of salt concentrations and volume fractions. Upon increasing volume fraction the colloids freeze first into coexisting compact solids then into a body centered cubic phase (bcc) before they melt into a glass forming liquid. The surprising stability of the bcc solid at high volume fractions and salt concentrations is explained by the interaction (charge) polydispersity and vibrational entropy
Theory for polymer coils with necklaces of micelles
If many micelles adsorb onto the same polymer molecule then they are said to
form a necklace. A minimal model of such a necklace is proposed and shown to be
almost equivalent to a 1-dimensional fluid with nearest-neighbour interactions.
The thermodynamic functions of this fluid are obtained and then used to predict
the change in the critical micellar concentration of the surfactant in the
presence of the polymer. If the amount of polymer is not too large there are
two critical micellar concentrations, one for micelles in necklaces and one for
free micelles.Comment: 12 pages, 5 figure
Compressive consolidation of strongly aggregated particle gels
The compressive yield stress of particle gels shows a highly nonlinear dependence on the packing fraction. We have studied continuous compression processes and discussed the packing-fraction dependence with the particle-scale rearrangements. The two-dimensional simulation of uniaxial compression was applied to fractal networks, and the required compressive stresses were evaluated for a wide range of packing fractions that approached close packing. The compression acts to reduce the size of the characteristic structural entities (i.e., the correlation length of the structure). We observed three stages of compression: (I) Elastic-dominant regime; (II) single-mode plastic regime, where the network strengths are determined by the typical length scale and the rolling mode; and (III) multimode plastic regime, where sliding mode and connection breaks are important. We also investigated the way of losing the fractal correlation under compression. It turns out that both fractal dimension Df and correlation length Îľ start to change from the early stage of compression, which is different from the usual assumption in theoretical models
Binding of molecules to DNA and other semiflexible polymers
A theory is presented for the binding of small molecules such as surfactants
to semiflexible polymers. The persistence length is assumed to be large
compared to the monomer size but much smaller than the total chain length. Such
polymers (e.g. DNA) represent an intermediate case between flexible polymers
and stiff, rod-like ones, whose association with small molecules was previously
studied. The chains are not flexible enough to actively participate in the
self-assembly, yet their fluctuations induce long-range attractive interactions
between bound molecules. In cases where the binding significantly affects the
local chain stiffness, those interactions lead to a very sharp, cooperative
association. This scenario is of relevance to the association of DNA with
surfactants and compact proteins such as RecA. External tension exerted on the
chain is found to significantly modify the binding by suppressing the
fluctuation-induced interaction.Comment: 15 pages, 7 figures, RevTex, the published versio
Bending elasticity of a curved amphiphilic film decorated anchored copolymers: a small angle neutron scattering study
Microemulsion droplets (oil in water stabilized by a surfactant film) are
progressively decorated with increasing amounts of poly ethylene- oxide (PEO)
chains anchored in the film by the short aliphatic chain grafted at one end of
the PEO chain . The evolution of the bending elasticity of the surfactant film
with increasing decoration is deduced from the evolution in size and
polydispersity of the droplets as reflected by small angle neutron scattering.
The optimum curvature radius decreases while the bending rigidity modulus
remains practically constant. The experimental results compare well with the
predictions of a model developed for the bending properties of a curved film
decorated by non-adsorbing polymer chains, which takes into account, the finite
curvature of the film and the free diffusion of the chains on the film.Comment: 30 June 200
Copolymer-induced stabilizing effect of highly swollen hexagonal mesophases
We show quantitatively that tiny amounts of copolymer that decorate a
oil/water interfaces can greatly enhance the stability of swollen surfactant
hexagonal phases, comprising oil tubes regularly arranged in a water matrix.
Such soft composite materials, whose both radius of the tubes and water channel
between the tubes can be controlled independently over large ranges, offer a
potential interest for the synthesis of mesoporous materials
Astrochemistry - Complex organic matter in Titan's aerosols? Reply
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/62938/1/nature05418.pd
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