Molecular dynamics simulations are used to investigate the diffusion process of nanoparticles in polymer melts. The effects of size, concentration, and mass of the particle, chain length, and polymer−particle interaction on the diffusion of particles in polymer melts are also explored. Our simulated results indicate that the gyration radius of the polymer chain is the key factor determining the validity of the Stokes−Eintsein(SE) relation in describing the particle diffusion at infinite dilution. When the particle size is larger than the gyration radius of the polymer chain, the SE formula can predict the particle diffusion in the polymer melts correctly, and the particle diffusion does not show mass dependence. When the particle size is smaller than the gyration radius of the polymer chain, however, the SE equation becomes invalid in the prediction of particle diffusion, because particle diffusion is exactly related to the nanoviscosity rather than the macroviscosity used in the SE formula. Furthermore, ...

Molecular Dynamics Study on Nanoparticle Diffusion in Polymer Melts: A Test of the Stokes−Einstein Law

Stress-induced crystallization (SIC) and stress-induced melting (SIM) in pure and filled (carbon black, CB) natural rubbers are studied by mechanical analysis, X-ray scattering, and quadrupolar NMR as a function of temperature and filler content. These complementary techniques allow to measure the crystallinity and the local deformation of the amorphous chains during the mechanical cycle. The critical draw ratio for appearance (stretching) and disappearance (recovery) of the crystallinity are compared by these three techniques. It is shown by WAXS that CB particles act as nucleation centers of the SIC. By 2H NMR one finds that during crystallization in both types of materials the remaining amorphous chains relax partially, as predicted by Flory. The amplification factors Aσ, Aχ, and ANMR deduced from the stress crystallinity and NMR splitting measurements are compared. The amplification factors measured by WAXS (Aχ) and NMR (ANMR) are of the same order and are not far from the Bueche prediction. The ampli...

Stress-Induced Crystallization and Reinforcement in Filled Natural Rubbers: 2H NMR Study

The structure and dynamics of filler nanoparticles and macromolecules in polymer nanocomposites determine many aspects of macroscopic behavior, which is why these properties have attracted considerable interest in the scientific literature. In this article, recent progress using scattering techniques for the elucidation of structure and dynamics, of both filler nanoparticles and polymer chains, is reviewed. Small-angle X-ray and neutron scattering, more and more combined with direct imaging methods and simulations, are appropriate methods to investigate nanocomposite structure. While they can usually be applied to standard samples for filler structure, chain structure can only be accessed with neutrons and isotopic substitution, under the zero-average contrast condition to match the filler contribution. Quasi-elastic neutron scattering and X-ray photon correlation spectroscopy cover different time scales with spatial resolution in the nanoscale. The former allows studies of polymer motions from atomic or segmental relaxation to reptation of the entire chain in the presence of filler, while the latter is increasingly used for investigations of nanoparticle dynamics.

Structure and dynamics of polymer nanocomposites studied by X-ray and neutron scattering techniques

Methods of contrast variation are tools that are essential in macromolecular structure research. Anomalous dispersion of X-ray diffraction is widely used in protein crystallography. Recent attempts to extend this method to native resonant labels like sulfur and phosphorus are promising. Substitution of hydrogen isotopes is central to biological applications of neutron scattering. Proton spin polarization considerably enhances an existing contrast prepared by isotopic substitution. Concepts and methods of nuclear magnetic resonance (NMR) become an important ingredient in neutron scattering from dynamically polarized targets.

/pdf/small-angle-scattering-and-its-interplay-with-525d8eqjn2.pdf

Small-angle scattering and its interplay with crystallography, contrast variation in SAXS and SANS.

The unique aspects of neutron scattering in biology rely largely on the extraordinary parameters of interaction of neutrons with hydrogen nuclei. The origin of isotopic and spin incoherent scattering and its possible implication for neutron diffraction from protein crystals are described. The impact of a change in the coherent scattering length of hydrogen nuclei with isotopic substitution and spin polarization is illustrated by a number of applications in structural biology, which show clearly the complementarity to x-ray diffraction. A more detailed account is given of polarized neutron scattering from dynamically polarized spin systems. Experimental evidence for the existence of selectively polarized proton spin domains and their use in structural studies is presented.

https://iopscience.iop.org/article/10.1088/0034-4885/67/7/R02/pdf

Unique aspects of neutron scattering for the study of biological systems

Time-dependent small-angle polarised neutron scattering from domains of polarised protons has been observed at the onset of dynamic nuclear polarisation in a frozen solution of 98% deuterated glycerol-water at 1 K containing a small concentration of paramagnetic centres (EHBA-CrV). Simultaneous NMR measurements show that the observed scattering arises from protons around the CrV-ions which are polarised to ~ 10% in a few seconds, much faster than the protons in the bulk.

Neutron scattering from polarised proton domains

We present first direct results of SANS measurements of the form factor of matrix chains in a cross-linked, silica filled elastomer in the isotropic state. The structure of the secondary filler network of silica is investigated in parallel in the unlabeled analog at identical filling degrees. Limits of the scattering description of these 2-phase/3-components mixture are tested. This investigation is a pilot study in the understanding of the reinforcement process by silica in the deformed state.

Chain deformation in filled elastomers: a SANS approach

We report on time-resolved small-angle polarised neutron scattering from domains of polarised protons created by dynamic nuclear polarisation in frozen deuterated glycerol–water solutions containing a small amount of paramagnetic centres. In order to observe the rapid build-up of the polarisation of the protons around the paramagnetic ions and to separate it from the much slower polarisation change of the protons in the solvent, we have developed techniques that include stroboscopic SANS and NMR synchronised to cyclic microwave irradiation.

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Neutron scattering from polarised proton domains

Chain deformation in filled elastomers: a SANS approach

Neutron scattering from polarised proton domains