Résumé:

cf. Baeza et al. Macromolecules 2012

Résumé:

à venir

Résumé:

Observation of chain structure in nanocomposites

Ref HAL: hal-00700773_v1
DOI: 10.1021/ma3003399
WoS: 000304224700042
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36 Citations
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By means of quasielastic neutron scattering we have investigated the hydrogen dynamics in poly(alkylene oxide)s (PAOs) with different side-chain lengths at temperatures below as well as above the glass-transition. The combination of results from three different spectrometers (a time-of-flight and two backscattering instruments) has allowed covering almost 4 orders of magnitude in timefrom the ps to ns rangewith spatial resolution. The results evidence the simultaneous occurrence of vibrations and localized side-group motions at low temperatures and additional diffusive-like (segmental) dynamics at high temperatures. The localized processes of the side groups show (i) stretching of the scattering function, (ii) associated activation energies similar to those found for single and cooperative bond rotations of polyethylene, and (iii) spatial extents that increase with increasing temperature. Compared with poly(ethylene oxide) (PEO), the diffusive segmental process in PAOs presents (i) the same spectral shape, (ii) slower characteristic timesantiplasticization(iii) similar deviations from Gaussian behavior. For comparison, we also report on backscattering results on the side-group dynamics of poly(n-hexyl methacrylate) in the same temperature range, that show evidence for confinement effects. We suggest that the dynamic asymmetry in systems with intrinsic dynamic heterogeneities between constituent parts is the key ingredient leading to both plasticization and confinement effects.

Ref HAL: hal-00679898_v1
DOI: 10.1021/ma202653k
WoS: 000301397800040
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20 Citations
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We present a study on the structure and dynamics of poly(ethyl methacrylate) (PEMA), one of the polymers with shortest side-group in the poly(n-alkyl methacrylate)s (PnMAs) series. The contributions to the structure factor have been unveiled by combining T-dependent X-ray and neutron diffraction with polarization analysis on samples with different deuteration labelings, accessing thereby different partial structure factors of PEMA. We find clear hints for strong structural ordering with anticorrelated side-groups and main-chain locations, fitting thus in the general scenario of nanosegregation of main chains and side groups found in higher order PnMAs members. Backscattering measurements on a PEMA sample with deuterated side-group have revealed the dynamics of main-chain and α-methyl group hydrogens. The characterization of the rotations of the latter indicates highly disordered environments in the glassy state. Above the glass transition, the incoherent scattering function related to segmental relaxation shows stretching and clear deviations from Gaussian behavior in the accessed window. We also present results on the dynamic structure factor (collective dynamics) at the peaks characteristic (i) for the intermain-chain and interside-group-domain correlations and (ii) for the side-group/side-group atomic correlations within the domains. Viscosity dictates the temperature dependence in both cases, and also that of the dielectric spectroscopy results at high temperatures. However, the α-process followed by dielectric relaxation deviates from the rheological behavior when approaching the calorimetric glass-transition, indicating a strong influence of the faster side-group motions on the dynamics of the dielectric probes.

Ref HAL: hal-00674994_v1
Ref Arxiv: 1202.6466
DOI: 10.1021/ma202308c
WoS: 000300124200060
Ref. & Cit.: NASA ADS
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28 Citations
Résumé:

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 ≈ 8 nm) and a mixture of purpose-synthesized hydrogenated and deuterated nanolatex (Rlatex ≈ 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.

Résumé:

Model systems of nanocomposites are important for our understanding of the structural and dynamical contributions of the constituents to macroscopic properties, and in particular mechanical reinforcement. Systems in which the filler aggregation can be finely tuned thanks to an aqueous casting route were developed1. The control of the initial colloidal solution stability (mixture of nanolatex and colloidal silica) by the pH enabled to obtain various microstructures for given silica fractions. Using Small Angle Neutron Scattering (SANS) and Transmission Electronic Microscopy (TEM) data, aggregation diagrams were obtained and enabled to study separately the effect of the silica fraction and its aggregation state on mechanical properties2 (see left side of the figure). On the other hand, we studied polymer structure and dynamic in such nanocomposites. SANS experiments were performed on samples prepared with a mixture of hydrogenated (H) and deuterated (D) chains in order to match the silica signal and get independently the polymer signal. A first system made with incompatible H and D latex beads enabled to follow the demixing kinetic for various silica fractions. The evolution of H zone size with annealing indicated that the polymer was immobilized in 15% v silica samples (see right side of the figure)2. A second system made with compatible beads highlighted the dissolution kinetic of latex beads with annealing. With sufficient annealing conditions a molecular mixture of H and D chains was obtained and the effect of silica fraction on chain radius of gyration was studied.