160 research outputs found
Structure and rheological properties of model microemulsion networks filled with nanoparticles
Model microemulsion networks of oil droplets stabilized by non ionic
surfactant and telechelic polymer C18-PEO(10k)-C18 have been studied for two
droplet-to-polymer size ratios. The rheological properties of the networks have
been measured as a function of network connectivity and can be described in
terms of simple percolation laws. The network structure has been characterised
by Small Angle Neutron Scattering. A Reverse Monte Carlo approach is used to
demonstrate the interplay of attraction and repulsion induced by the copolymer.
These model networks are then used as matrix for the incorporation of silica
nanoparticles (R=10nm), individual dispersion being checked by scattering. A
strong impact on the rheological properties is found for silica volume
fractions up to 9%
Universal scattering behavior of co-assembled nanoparticle-polymer clusters
Water-soluble clusters made from 7 nm inorganic nanoparticles have been
investigated by small-angle neutron scattering. The internal structure factor
of the clusters was derived and exhibited a universal behavior as evidenced by
a correlation hole at intermediate wave-vectors. Reverse Monte-Carlo
calculations were performed to adjust the data and provided an accurate
description of the clusters in terms of interparticle distance and volume
fraction. Additional parameters influencing the microstructure were also
investigated, including the nature and thickness of the nanoparticle adlayer.Comment: 5 pages, 4 figures, paper published in Physical Review
Mechanical Properties of End-crosslinked Entangled Polymer Networks using Sliplink Brownian Dynamics Simulations
The mechanical properties of a polymeric network containing both crosslinks
and sliplinks (entanglements) are studied using a multi-chain Brownian dynamics
simulation. We coarse-grain at the level of chain segments connecting
consecutive nodes (cross- or sliplinks), with particular attention to the
Gaussian statistics of the network. Affine displacement of nodes is not
imposed: their displacement as well as sliding of monomers through sliplinks is
governed by force balances. The simulation results of stress in uniaxial
extension and the full stress tensor in simple shear including the (non-zero)
second normal stress difference are presented for monodisperse chains with up
to 18 entanglements between two crosslinks. The cases of two different force
laws of the subchains (Gaussian chains and chains with finite extensibility)
for two different numbers of monomers in a subchain (no = 50 and no = 100) are
examined. It is shown that the additivity assumption of slip- and crosslink
contribution holds for sufficiently long chains with two or more entanglements,
and that it can be used to construct the strain response of a network of
infinitely long chains. An important consequence is that the contribution of
sliplinks to the small-strain shear modulus is about ⅔ of the
contribution of a crosslink
Effect of Nanoparticle Size on the Morphology of Adsorbed Surfactant Layers
The surface aggregates structure of dimethyldodecylamine-N-oxide (C12DAO) in
three silica dispersions of different particle sizes (16 - 42 nm) was studied
by small-angle neutron scattering (SANS) in a H2O/D2O solvent mixture matching
the silica. At the experimental conditions (pH 9) the surfactant exists in its
nonionic form and the structure of the adsorbed layer is not affected by added
electrolyte. It is found that C12DAO forms spherical surface micelles of 2 nm
diameter on the 16 nm silica particles, but oblate ellipsoidal surface micelles
are formed on the 27 and 42 nm particles. The dimensions of these oblate
surface aggregates (minor and major semi-axes Rn and Rlat) are similar to those
of C12DAO micelles in the aqueous solutions. It is concluded that the
morphological transition from spherical to ellipsoidal surface aggregates is
induced by the surface curvature of the silica particles. A comparison of the
shape and dimensions of the surface aggregates formed by C12DAO and C12E5 on
the 16 nm silica particles demonstrates that the nature of the surfactant head
group does not determine the morphology of the surface aggregates, but has a
strong influence on the number of surface aggregates per particle, due to the
different interactions of the head groups with the silica surface
Stabilization and Controlled Association of Inorganic Nanoparticles using Block Copolymers
We report on the structural properties of mixed aggregates made from
rare-earth inorganic nanoparticles (radius 20 Angstroms) and
polyelectrolyte-neutral block copolymers in aqueous solutions. Using scattering
experiments and Monte Carlo simulations, we show that these mixed aggregates
have a hierarchical core-shell microstructure. The core is made of densely
packed nanoparticles and it is surrounded by a corona of neutral chains. This
microstructure results from a process of controlled association and confers to
the hybrid aggregates a remarkable colloidal stability.Comment: 14 pages, 5 figure
Well dispersed fractal aggregates as filler in polymer-silica nanocomposites: long range effects in rheology
We are presenting a new method of processing polystyrene-silica
nanocomposites, which results in a very well-defined dispersion of small
primary aggregates (assembly of 15 nanoparticles of 10 nm diameter) in the
matrix. The process is based on a high boiling point solvent, in which the
nanoparticles are well dispersed, and controlled evaporation. The filler's fine
network structure is determined over a wide range of sizes, using a combination
of Small Angle Neutron Scattering (SANS) and Transmission Electronic Microscopy
(TEM). The mechanical response of the nanocomposite material is investigated
both for small (ARES oscillatory shear and Dynamical Mechanical Analysis) and
large deformations (uniaxial traction), as a function of the concentration of
the particles. We can investigate the structure-property correlations for the
two main reinforcement effects: the filler network contribution, and a
filler-polymer matrix effect. Above a silica volume fraction threshold, we see
a divergence of the modulus correlated to the build up of a connected network.
Below the threshold, we obtain a new additional elastic contribution of much
longer terminal time than the matrix. Since aggregates are separated by at
least 60 nm, this new filler-matrix contribution cannot be described solely
with the concept of glassy layer (2nm)
Charge-Fluctuation-Induced Non-analytic Bending Rigidity
In this Letter, we consider a neutral system of mobile positive and negative
charges confined on the surface of curved films. This may be an appropriate
model for: i) a highly charged membrane whose counterions are confined to a
sheath near its surface; ii) a membrane composed of an equimolar mixture of
anionic and cationic surfactants in aqueous solution. We find that the charge
fluctuations contribute a non-analytic term to the bending rigidity that varies
logarithmically with the radius of curvature. This may lead to spontaneous
vesicle formation, which is indeed observed in similar systems.Comment: Revtex, 9 pages, no figures, submitted to PR
Modeling of Intermediate Structures and Chain Conformation in Silica-Latex Nanocomposites Observed by SANS During Annealing
The evolution of the polymer structure during nanocomposite formation and
annealing of silica-latex nanocomposites is studied using contrast-variation
small angle neutron scattering. The experimental system is made of silica
nanoparticles (Rsi \approx 8 nm) and a mixture of purpose-synthesized
hydrogenated and deuterated nanolatex (Rlatex \approx 12.5 nm). The progressive
disappearance of the latex beads by chain interdiffusion and release in the
nanocomposites is analyzed quantitatively with a model for the scattered
intensity of hairy latex beads and an RPA description of the free chains. In
silica-free matrices and nanocomposites of low silica content (7%v), the
annealing procedure over weeks at up to Tg + 85 K results in a molecular
dispersion of chains, the radius of gyration of which is reported. At higher
silica content (20%v), chain interdiffusion seems to be slowed down on
time-scales of weeks, reaching a molecular dispersion only at the strongest
annealing. Chain radii of gyration are found to be unaffected by the presence
of the silica filler
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