401 research outputs found
Anisotropic Reinforcement of Nanocomposites Tuned by Magnetic Orientation of the Filler Network
We present a new material which displays anisotropic and mechanical
properties tuneable during synthesis under magnetic field. It is formulated by
mixing aqueous suspensions of polymer nanolatex and magnetic nanoparticles,
coated by a thin silica layer to improve their compatibility with the polymeric
matrix, followed by casting. The magnetic properties of these nanoparticles
enable their pre-orientation in the resulting nanocomposite when cast under
magnetic field. Detailed insight on dispersion by Small Angle Neutron
Scattering (SANS) shows chainlike nanoparticle aggregates aligned by the field
on the nanometer scale. Applying strain to the nanocomposite parallel to the
particle chains shows higher mechanical reinforcement, than when strain is
transverse to field. . SANS from strained samples shows that strain parallel to
the field induce an organization of the chains while strain perpendicular to
the field destroys the chain field-induced ordering. Thus improved mechanical
reinforcement is obtained from anisotropic interconnection of nanoparticle
aggregates
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)
Cross-linking of polyolefins : a study by thermoporosimetry with benzene derivatives swelling solvents
o, m, p-xylene, p-dichlorobenzene, 1,2,4 trichlorobenzene and naphthalene
were calibrated as condensates used in thermoporosimetry technique. Exponential
relationships were found connecting the pore radii (Rp(nm)) and dT (C) on one
side and the apparent energy of crystallization (Wa (J.cm-3)) and dT on the
other side: Pore or mesh size distribution can be derived from DSC thermal
recording using the following equation: All the numerical parameter values were
determined. Polyethylene and polypropylene samples, cross-linked with
high-energy electrons or γ-rays, were submitted to thermoporosimetry
study. The mesh size distributions (MSD) calculated for these polyolefins,
using o, m and p-xylene as solvent, were found in the same sequences that their
degrees of swelling and the irradiation doses they received
Tuning the Mechanical Properties in Model Nanocomposites: Influence of the Polymer-Filler Interfacial Interactions
This paper presents a study of the polymer-filler interfacial effects on
filler dispersion and mechanical reinforcement in Polystyrene (PS) / silica
nanocomposites by direct comparison of two model systems: un-grafted and
PS-grafted silica dispersed in PS matrix. The structure of nanoparticles has
been investigated by combining Small Angle Neutron Scattering (SANS)
measurements and Transmission Electronic Microscopic (TEM) images. The
mechanical properties were studied over a wide range of deformation by
plate/plate rheology and uni-axial stretching. At low silica volume fraction,
the particles arrange, for both systems, in small finite size non-connected
aggregates and the materials exhibit a solid-like behavior independent of the
local polymer/fillers interactions suggesting that reinforcement is dominated
by additional long range effects. At high silica volume fraction, a continuous
connected network is created leading to a fast increase of reinforcement whose
amplitude is then directly dependent on the strength of the local
particle/particle interactions and lower with grafting likely due to
deformation of grafted polymer.Comment: Journal Polymer Science (2011
Structure of interacting aggregates of silica nanoparticles in a polymer matrix: Small-angle scattering and Reverse Monte-Carlo simulations
Reinforcement of elastomers by colloidal nanoparticles is an important
application where microstructure needs to be understood - and if possible
controlled - if one wishes to tune macroscopic mechanical properties. Here the
three-dimensional structure of big aggregates of nanometric silica particles
embedded in a soft polymeric matrix is determined by Small Angle Neutron
Scattering. Experimentally, the crowded environment leading to strong
reinforcement induces a strong interaction between aggregates, which generates
a prominent interaction peak in the scattering. We propose to analyze the total
signal by means of a decomposition in a classical colloidal structure factor
describing aggregate interaction and an aggregate form factor determined by a
Reverse Monte Carlo technique. The result gives new insights in the shape of
aggregates and their complex interaction in elastomers. For comparison, fractal
models for aggregate scattering are also discussed
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
Hierarchical simulations of hybrid polymer-solid materials
Complex polymer-solid materials have gained a lot of attention during the last 2-3 decades due to the fundamental physical problems and the broad spectrum of technological applications in which they are involved. Therefore, significant progress concerning the simulations of such hybrid soft-hard nanostructured systems has been made in the last few years. Simulation techniques vary from quantum to microscopic (atomistic) up to mesoscopic (coarse-grained) level. Here we give a short overview of simulation approaches on model polymer-solid interfacial systems for all different levels of description. In addition, we also present a brief outlook concerning the open questions in this field, from the point of view of both physical problems and computational methodologies
Origin of micro-scale heterogeneity in polymerisation of photo-activated resin composites
Photo-activated resin composites are widely used in industry and medicine. Despite extensive chemical characterisation, the micro-scale pattern of resin matrix reactive group conversion between filler particles is not fully understood. Using an advanced synchrotron-based wide-field IR imaging system and state-of-the-art Mie scattering corrections, we observe how the presence of monodispersed silica filler particles in a methacrylate based resin reduces local conversion and chemical bond strain in the polymer phase. Here we show that heterogeneity originates from a lower converted and reduced bond strain boundary layer encapsulating each particle, whilst at larger inter-particulate distances light attenuation and monomer mobility predominantly influence conversion. Increased conversion corresponds to greater bond strain, however, strain generation appears sensitive to differences in conversion rate and implies subtle distinctions in the final polymer structure. We expect these findings to inform current predictive models of mechanical behaviour in polymer-composite materials, particularly at the resin-filler interface
Molecular dynamics simulations of glassy polymers
We review recent results from computer simulation studies of polymer glasses,
from chain dynamics around the glass transition temperature Tg to the
mechanical behaviour below Tg. These results clearly show that modern computer
simulations are able to address and give clear answers to some important issues
in the field, in spite of the obvious limitations in terms of length and time
scales. In the present review we discuss the cooling rate effects, and dynamic
slowing down of different relaxation processes when approaching Tg for both
model and chemistry-specific polymer glasses. The impact of geometric
confinement on the glass transition is discussed in detail. We also show that
computer simulations are very useful tools to study structure and mechanical
response of glassy polymers. The influence of large deformations on mechanical
behaviour of polymer glasses in general, and strain hardening effect in
particular are reviewed. Finally, we suggest some directions for future
research, which we believe will be soon within the capabilities of state of the
art computer simulations, and correspond to problems of fundamental interest.Comment: To apear in "Soft Matter
- …