In this work we have investigated how the dynamics of poly(vinyl methyl ether), PVME, changes by blending with deuterated polystyrene. The experimental techniques used were dielectric spectroscopy, quasielastic neutron scattering, and 13 C nuclear magnetic resonance. By means of these techniques, the dynamics of the poly(vinyl methyl ether) units in the blends can be selectively investigated in a huge time range (10 1 -10 -11 s). Two different blend compositions have been investigated. The main relaxation processes observed in this range are the secondary -process and the segmental R-relaxation. It turns out that the -relaxation is not affected by blending. The data analysis procedure followed by us in the case of the R-process is based on the assumption that the dynamics of the PVME segments in the blends is a superposition of dynamical processes with the same shape as that in pure PVME, but with the relaxation times distributed due to the presence of concentration fluctuations. From this analysis we found that, in the blends, and in pure PVME as well, the results obtained by means of the different techniques can consistently be described with the same set of parameters. Moreover, the temperature dependence of the distribution of relaxation times in each blend composition can be accounted for by a single, temperature-independent, Gaussian distribution of the Vogel-Fulcher temperature, T 0, the average and the variance of the distribution increasing as the PVME concentration decreases. Our results suggest that a significant number of PVME segments in the blends move faster than in pure PVME. Furthermore, our results strongly indicate that each polymer component of the blend exhibits very different R-relaxation rates, i.e., different "glass transitions". Several implications of these results concerning the usually accepted ideas of polymer blend dynamics are outlined.

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Recent neutron scattering experiments on the microscopic dynamics of polymers below and above the glass transition temperature T(g) are reviewed. The results presented cover different dynamic processes appearing in glasses: local motions, vibrations, and different relaxation processes such as alpha- and beta-relaxation. For the alpha-relaxation, which occurs above T(g), it is possible to extend the time-temperature superposition principle, which is valid for polymers on a macroscopic scale, to the microscopic time scale. However, this principle is not applicable for temperatures approaching T(g). Below T(g), an inelastic excitation at a frequency of some hundred gigahertz (on the order of several wave numbers), the "boson peak," survives from a quasi-elastic overdamped scattering law at high temperatures. The connection between this boson peak and the fast dynamic process appearing near T(g) is discussed.

We relate the dynamical behavior of molecular liquids confined in mesoscopic cylindrical pores to the thermodynamic properties, heat capacity and density and to the static structure by combining different experimental methods (H-NMR, calorimetry, elastic and inelastic neutron scattering, numerical simulations). The crystallization process is greatly reduced or avoided by confinement under standard cooling conditions, instead a glass transition temperature T(g) at the 1000s time scale can be observed. The pore averaged local structure of the confined liquid is not noticeably affected when "excluded-volume" corrections are carefully applied, but follows the density changes reflected by the Bragg peak intensities of the porous matrices. The pore size dependence of T(g) is dominated by two factors, surface interaction and finite-size effect. For the smallest pores ([Formula: see text], [Formula: see text] being the van der Waals radius of a molecule), one observes an increase of T(g) and a broadening of the transition region, related to the interaction with the surface that induces a slowing-down of the molecules close to the wall. This is confirmed by neutron scattering experiments and molecular-dynamics simulations at shorter time scales and higher temperatures, which indicate a remaining fraction of frozen molecules. For larger pore sizes, taking the decrease of density under confinement conditions into account, a decrease of T(g) is observed. This could be related to finite-size effects onto the putative cooperativity length that is often invoked to explain glass formation. However, no quantitative determination of this length (not to mention its T-dependence) can be extracted, since the interaction with the wall itself introduces an additional length that adds to the complexity of the problem.

Energy-resolved, elastic neutron backscattering was employed to investigate the methyl group dynamics in polyisoprene between T = 2 K and room temperature. The use of partially deuterated samples (PI-d6, PI-d3, and PI-ds) and of a fully protonated sample (Pl-hg) allowed the separation of the dynamics arising from the methyl group and from the backbone. A two-step relaxation is observed and attributed to the methyl group rotation at low temperatures and to the main-chain relaxation close to the glass transition.An Arrhenius-like increase of the methyl group rotational correlation time r = ro exp(EtcJkT), with E^Jk = 1550 K ~12 kJ/mol and 0 ~1/ 0 = 23.5 meV (r0 ~1.76 X 10-13 s) describes well the midposition of the first elastic intensity decrease but not its breadth. A 3-fold jump model with a broad Gaussian distribution of activation energies (dE/E ~25%) around 1500 K can account for the observed temperature decrease. Inconstancies in the Q-dependence might be due to disorder effects. The torsional mode of the methyl group rotation is directly observed at = 23.5 meV by time-of-flight. Near the glass transition temperature a further decrease of the elastic scattering is observed due to the onset of a fast dynamics of the backbone in the picosecond range.

We study the changes in the low-frequency vibrational dynamics of poly(isobutylene) under pressure up to 1.4 GPa, corresponding to a density change of 20%. Combining inelastic neutron, x-ray, and Brillouin light scattering, we analyze the variations in the boson peak, transverse and longitudinal sound velocities, and the Debye level under pressure. We find that the boson peak variation under pressure cannot be explained by the elastic continuum transformation only. Surprisingly, the shape of the boson peak remains unchanged even at such high compression.

International audienceA comprehensive study of commercially available and newly synthesized proton conducting perfluorinated sulfonic acid (PFSA) surfactantsand polymeric systems is reported, specially designed in a bottom-up search to improve the basic understanding of PFSA polymers used as benchmark electrolytes in fuel cells. Hydration-dependent mesomorphous phases are formed by the self-assembly of these molecules in water. The impact of the hydrophobic chain length, the density of charge, the molecular architecture on the nanostructure, and the dynamics of confined water were studied by combining small-angle X-ray scattering, quasielastic neutron scattering, and pulsed-field gradient NMR. We introduce a hydration-dependent structural parameter, dw (mean size of water domains), that allows to establish the structure−transport relationship in PFSA materials. This multiscale study reveals that (i) the dynamical behavior of confined protons and water molecules are rather insensitive to the topology of the host matrix and (ii) the main parameter driving the performance of fuel cell electrolytes is the total water content required for swelling the domains above a value of 1 nm

2 H NMR and quasi-elastic neutron scattering techniques have been used to study the rotational dynamics of the 1,4-benzene-dicarboxylate (BDC) linkers in the porous cubic UiO-66(Zr) metal−organic framework (MOF). The rotation of the benzene rings in the BDC linkers is at the limit of detection of the neutron technique, but it fits perfectly on the 2 H NMR time scale. The aromatic rings in the UiO-66 framework exhibit the lowest rotational barrier compared to other MOFs, the activation energy for π-flips being 30 kJ mol −1 . However, instead of having well-defined flipping rates like in MOF-5, MIL-47, or MIL-53, UiO-66(Zr) shows a distribution of flipping correlation times, probably due to local disorder in the structure. Because of the rotational motion of the benzene rings, the effective size of the microporous windows in UiO-66(Zr) appears to be temperature dependent.