104 research outputs found
On the absence of structure factors in concentrated colloidal suspensions and nanocomposites
Small-angle scattering is a commonly used tool to analyze the dispersion of
nanoparticles in all kinds of matrices. Besides some obvious cases, the
associated structure factor is often complex and cannot be reduced to a simple
interparticle interaction, like excluded volume only. In recent experiments, we
have encountered a surprising absence of structure factors (S(q) = 1) in
scattering from rather concentrated polymer nanocomposites [A.-C. Genix et al,
ACS Appl. Mater. Interfaces 11 (2019) 17863]. In this case, quite pure form
factor scattering is observed. This somewhat ``ideal'' structure is further
investigated here making use of reverse Monte Carlo simulations in order to
shed light on the corresponding nanoparticle structure in space. By fixing the
target ``experimental'' apparent structure factor to one over a given q-range
in these simulations, we show that it is possible to find dispersions with this
property. The influence of nanoparticle volume fraction and polydispersity has
been investigated, and it was found that for high concentrations only a high
polydispersity allows reaching a state of S = 1. The underlying structure in
real space is discussed in terms of the pair-correlation function, which
evidences the importance of attractive interactions between polydisperse
nanoparticles. The calculation of partial structure factors shows that there is
no specific ordering of large or small particles, but that the presence of
attractive interactions together with polydispersity allows reaching an almost
``structureless'' state
Tuning Structure and Rheology of Silica-Latex Nanocomposites with the Molecular Weight of Matrix Chains: A Coupled SAXS-TEM-Simulation Approach
The structure of silica-latex nanocomposites of three matrix chain masses
(20, 50, and 160 kg/mol of poly(ethyl methacrylate)) are studied using a
SAXS/TEM approach, coupled via Monte Carlo simulations of scattering of fully
polydisperse silica nanoparticle aggregates. At low silica concentrations (1
vol. %), the impact of the matrix chain mass on the structure is quantified in
terms of the aggregation number distribution function, highest mass leading to
individual dispersion, whereas the lower masses favor the formation of small
aggregates. Both simulations for SAXS and TEM give compatible aggregate
compacities around 10 vol. %, indicating that the construction algorithm for
aggregates is realistic. Our results on structure are rationalized in terms of
the critical collision time between nanoparticles due to diffusion in viscous
matrices. At higher concentrations, aggregates overlap and form a percolated
network, with a smaller and lighter mesh in the presence of high mass polymers.
The linear rheology is investigated with oscillatory shear experiments. It
shows a feature related to the silica structure at low frequencies, the
amplitude of which can be described by two power laws separated by the
percolation threshold of aggregates
Micromechanisms of fracture propagation in glassy polymers
While most glassy polymers are nominally brittle at macroscopic scales, they are known to exhibit plastic deformation in indentation, scratching, and microcutting when the loaded region is sufficiently small. The same applies to the micrometer size process zone at the tip of a propagating crack. While the presence and approximate size of this microscale plastic zone is well described by the Dugdale model, the prediction of the toughness of these materials is not possible without accounting for the details of the local large strain field and the work hardening behaviour of these polymers, which can be inferred from their response to compressive tests. Strain localization mechanisms such as crazing or shear banding should also be taken into account to properly model toughness. Finally, viscoplastic creep plays a major role in determining the dependence of the toughness on crack propagation velocity, as well as the important difference between the initiation and propagation toughness, which is responsible for the occurrence of a characteristic stick-slip propagation under some loading conditions.
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How Tuning Interfaces Impacts the Dynamics and Structure of Polymer Nanocomposites Simultaneously
Fundamental understanding of macroscopic properties of polymer nanocomposites
(PNCs) remains difficult due to the complex interplay of microscopic dynamics
and structure, namely interfacial layer relaxations and three-dimensional
nanoparticle arrangements. The effect of surface modification by alkyl
methoxysilanes at different grafting densities has been studied in PNCs made of
poly(2-vinylpyridine) and spherical 20 nm silica nanoparticles (NPs). The
segmental dynamics has been probed by broadband dielectric spectroscopy, and
the filler structure by small-angle X-ray scattering and reverse Monte Carlo
simulations. By combining the particle configurations with the interfacial
layer properties, it is shown how surface modification tunes the attractive
polymer-particle interactions: bare NPs slow down the polymer interfacial layer
dynamics over a thickness of ca. 5 nm, while grafting screens these
interactions. Our analysis of interparticle spacing and segmental dynamics
provides unprecedented insight into the effect of surface modification on the
main characteristics of PNCs: particle interactions and polymer interfacial
layers
Fracture propagation in glassy polymers: From nanometer to centimeter
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Albe 1998 - La grande Motte 2009 : quelles avanc\'ees en diffusion de neutrons aux petits angles en 10 ans ?
The importance of neutron scattering techniques for the characterization of
samples in soft condensed matter has been demonstrated all along the present
book. The fine understanding of the physical properties is closely linked to
progress in the field of instrumentation. This chapter describes the advances
over the last decade in technical domains, such as neutron detection,
electronics and sample environment. The news software for data reduction and
analysis are also discussed before to conclude with the ILL and LLB projects
for new instruments
Rejuvenating the structure and rheological properties of silica nanocomposites based on natural rubber
The antagonistic effect of processing and thermal annealing on both the
filler structure and the polymer matrix is explored in polymer nanocomposites
based on natural rubber with precipitated silica incorporated by coagulation
from aqueous suspension followed by roll-milling. Their structure and linear
and non-linear rheology have been studied, with a particular emphasis on the
effect of high temperature thermal treatment and the number of milling passes.
Small-angle X-ray scattering intensities show that the silica is organized in
small, unbreakable aggregates containing ca. 50 primary nanoparticles, which
are reorganized on a larger scale in filler networks percolating at the highest
silica contents. As expected, the filler network structure is found to be
sensitive to milling, more milling inducing rupture, as evidenced by the
decreasing Payne effect. After thermal treatment, the nanocomposite structure
is found to be rejuvenated, erasing the effect of the previous milling on the
low-strain modulus. In parallel, the dynamics of the samples described by the
rheology or the calorimetric glass-transition temperature remain unchanged,
whereas the natural latex polymer network structure is modified by milling
towards a more fluid-like rheology, and cannot be recovered
Aggregate Formation of Surface-Modified Nanoparticles in Solvents and Polymer Nanocomposites
International audienceA new method based on the combination of small-anglescattering, reverse Monte Carlo simulations, and an aggregate recognition algorithm is proposed to characterize the structure of nanoparticle suspensions in solvents and polymer nanocomposites, allowing detailedstudies of the impact of different nanoparticle surface modifications.Experimental small-angle scattering is reproduced using simulated annealing of configurations of polydisperse particles in a simulation box compatible with the lowest experimental q-vector. Then, properties of interest likeaggregation states are extracted from these configurations and averaged. This approach has been applied to silane surface-modified silica nanoparticles with different grafting groups, in solvents and after casting into polymer matrices.It is shown that the chemistry of the silane function, in particular mono- or trifunctionality possibly related to patch formation, affects the dispersion state in a given medium, in spite of an unchanged alkylchain length. Our approach may be applied to study any dispersion or aggregation state of nanoparticles. Concerningnanocomposites, the method has potential impact on the design of new formulations allowing controlled tuning of nanoparticle dispersion
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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