Depercolation of aggregates upon polymer grafting in simplified industrial nanocomposites studied with dielectric spectroscopy

The dynamics of polymer and filler in simplified industrial silica-styrene-butadiene nanocomposites (silica Zeosil 1165 MP, volume fraction 0-21%v) have been studied with broadband dielectric spectroscopy (BDS) and nuclear magnetic resonance (NMR). The fraction of graftable matrix chains was varied from 0 to 100%D3. The introduction of silica nanoparticles is shown to leave the segmental relaxation unaffected, an observation confirmed by the measurement of only a thin (some Angstroms thick) immobilized layer by NMR. The low-frequency measurements are resolved in two distinct dielectric Maxwell-Wagner-Sillars (MWS) processes of different behavior with respect to changes of large-scale silica structures induced by variations of filler fraction and grafting. It is found that increasing grafting leaves the first MWS-process unaffected, while it decreases the strength of the (slower) second MWS by about a decade. At constant silica volume fraction, this indicates depercolation of the filler, thereby providing a microscopic explanation of the evolution of rheological reinforcement. The sensitivity to large-scale reorganizations together with a characterization of local polymer dynamics provides insight over many length- and time-scales into structure and dynamics of nanocomposites, and thus the physical origin of the reinforcement effect.We are thankful for a “Chercheur d'Avenir” grant by the Languedoc-Roussillon region (J.O.) and Ph.D. funding “CIFRE” by Michelin (G.P.B.). The authors acknowledge financial support from the European Commission under the Seventh Framework Program by means of the grant agreement for the Integrated Infrastructure Initiative N. 262348 European Soft Matter Infrastructure (ESMI).Peer Reviewe

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

Dynamics of poly(ethylene oxide) in a blend with poly(methyl methacrylate): A quasielastic neutron scattering and molecular dynamics simulation study

In this paper, we have addressed the question of the dynamic miscibility in a blend characterized by very different glass-transition temperatures, T-g, for the components: poly(ethylene oxide) and poly(methyl methacrylate) (PEO/PMMA). The combination of quasielastic neutron scattering with isotopic labeling and fully atomistic molecular dynamics simulations has allowed us to selectively investigate the dynamics of the two components in the picosecond-10 nanoseconds scale at temperatures close and above the T-g of the blend. The main focus was on the PEO component, i.e., that of the lowest T-g, but first we have characterized the dynamics of the other component in the blend and of the pure PEO homopolymer as reference. In the region investigated, the dynamics of PMMA in the blend is strongly affected by the alpha-methyl rotation; an additional process detected in the experimental window 65 K above the blend-T-g can be identified as the merged alpha beta process of this component that shows strong deviations from Gaussian behavior. On the other hand, pure PEO displays entropy driven dynamics up to very large momentum transfers. Such kind of motion seems to freeze when the PEO chains are in the blend. There, we have directly observed a very heterogeneous and moreover confined dynamics for the PEO component. The presence of the hardly moving PMMA matrix leads to the creation of little pockets of mobility where PEO can move. The characteristic size of such confined islands of mobility might be estimated to be of approximate to 1 nm. These findings are corroborated by the simulation study, which has been an essential support and guide in our data analysis procedure

Local structure of syndiotactic poly(methyl methacrylate). A combined study by neutron diffraction with polarisation analysis and atomistic molecular dynamics simulations

The local structure of syndiotactic poly( methyl methacrylate) ( PMMA) has been investigated by combining neutron scattering and fully atomistic molecular dynamics simulations. Selectively deuterating parts of the PMMA monomer, we have accessed five different partial structure factors in the glassy state by neutrons, and polarization analysis has allowed isolating the coherent contribution to the total scattering. In addition, the temperature dependence of the static structure factor has been determined on the fully deuterated sample. The different measured partial structure factors show qualitatively different features with respect to peak positions and heights and provide a very critical check to validate the simulated structure. To gain deep insight into the structure, we have grouped the simulation results in terms of three molecular substructures: the main chain, the alpha-methyl group, and the ester side group. The study of the resulting partial structure functions has revealed the origin of the diffraction peaks, including those in the X-rays pattern reported in the literature. In addition, a real-space evaluation of the characteristic radial distribution functions has allowed separating intra- and interchain contributions to the total correlation functions. We have found that (i) PMMA exhibits a strong local order with an average main-chain distance of approximate to 8.6 angstrom, ( ii) this is the only average interchain distance and thus no precursor effect of a layered structure is found, (iii) the main chain shows a persisting all-trans structure, and (iv) a strong anticorrelation between the main chain and the ester side groups, together with an interdigitation of the side groups, suggests a marked separation between the backbone and the side-group spatial arrangements

A high-temperature dielectric process as a probe of large-scale silica filler structure in simplified industrial nanocomposites

The existence of two independent filler-dependent high-temperature Maxwell-Wagner-Sillars (MWS) dielectric processes is demonstrated and characterized in detail in silica-filled styrene-butadiene (SB) industrial nanocomposites of simplified composition using Broadband Dielectric Spectroscopy (BDS). The uncrosslinked samples are made with 140 kg mol-1 SB-chains, half of which carry a single graftable end-function (50% D3), and Zeosil 1165 MP silica incorporated by solid-phase mixing. While one high-temperature process is known to exist in other systems, the dielectric properties of a new silica-related process - strength, relaxation time, and activation energy - have been evidenced and described as a function of silica volume fraction and temperature. In particular, it is shown that its strength follows a percolation behavior as observed with the ionic conductivity and rheology. Moreover, activation energies show the role of polymer layers separating aggregates even when they are percolated. Apart from simultaneous characterization over a broad frequency range up to local polymer and silanol dynamics, it is believed that such high-temperature BDS-measurements can thus be used to detect reorganizations in structurally-complex silica nanocomposites. Moreover, they should contribute to a better identification of dynamical processes via the described sensitivity to structure in such systems.We are thankful for a ‘‘Chercheur d’Avenir’’ grant by the Languedoc-Roussillon region (J.O.) and PhD funding ‘‘CIFRE’’ by Michelin (G.P.B.). The authors acknowledge financial support from the European Commission under the Seventh Framework Program by means of the grant agreement for the Integrated Infrastructure Initiative No. 262348 European Soft Matter Infrastructure (ESMI).Peer Reviewe

Dynamics in poly(n-alkyl methacrylates): A neutron scattering, calorimetric, and dielectric study

13 páginas, 15 figuras, 4 tablas.-- El pdf del artículo es la versión post-print.Combining neutron diffraction, neutron spin echo, differential scanning calorimetry, and dielectric spectroscopy, we have investigated the structure and dynamics of poly(n-butyl methacrylate) (PBMA) and poly(n-hexyl methacrylate) (PHMA). Signatures of the occurrence of a glass transition associated with the freezing of the intermolecular correlations within alkyl nanodomains are present in the structural data. Exploiting isotopic labeling, neutron scattering has revealed collective dynamics at the main-chain and side-group levels for both polymers and the self-motions of hydrogen atoms in the side groups of PHMA, adding valuable microscopic information to comprehensive relaxation maps and putting the relaxation results into a perspective. Moreover, we find exotic dynamical behavior for the side groups, characterized by extremely stretched (nearly logarithmic-like) decays of the correlation functions. For PHMA, a complete dynamical decoupling of side-group dynamics from the main-chain motions is found. The side groups of this polymer show an extremely “strong” temperature dependence of the structural relaxation time and much faster characteristic times for self than collective motions. The analogies found between the self-motions of the side-group H atoms in PHMA and the γ-relaxation process in semicrystalline polyethylene (PE) strengthen the picture of confined PE-like dynamics within alkyl nanodomains. We discuss possible origins for the observed phenomenology.This research project has been supported by the European Commission NoE SoftComp, Contract NMP3-CT-2004-502235, the “Donostia International Physics Center”. A.A. and J.C. acknowledge support from the projects MAT2007-63681, IT-436-07 (GV), and the Spanish Ministerio de Educacion y Ciencia (Grant CSD2006-53).Peer reviewe

Self- and Collective Dynamics of Syndiotactic Poly(methylmethacrylate). A Combined Study by Quasielastic Neutron Scattering and Atomistic Molecular Dynamics Simulations

We have investigated the molecular dynamics of syndiotactic poly(methyl methacrylate) well above the glass transition by combining quasielastic neutron scattering and fully atomistic computer simulations. The incoherent scattering measured by backscattering on a sample with deuterated ester methyl groups has revealed the single-particle motions of hydrogens in the main chain and in the R-methyl groups. Moreover, with neutron spin-echo experiments on the fully deuterated sample we have accessed the collective motions at the two first maxima of the structure factor. The simulated cell, which has been previously validated regarding the structural properties [Genix, A.-C.; et al. Macromolecules 2006, 39, 3947], shows a dynamical behavior that, allowing a shift in temperature, reproduces very accurately all the experimental results. The combined analysis of both sets of data has shown that: (i) The segmental relaxation involving backbone atoms deviates from Gaussian behavior. (ii) The dynamics is extremely heterogeneous: in addition to the subdiffusion associated with the R-process and the methyl group rotations, we have found indications of a rotational motion of the ester side group around the main chain. (iii) At a given momentum transfer and depending on the molecular groups considered, the time scales for collective motion are spread over about 1 order of magnitude, the correlations involving the main chain decaying much more slowly than those relating side groups. (iv) At the length scale characteristic for the overall periodicity of the system (that corresponding to the first structure factor peak), the experimentally observed collective dynamics relates to the backbone motions and is of interchain character; there, coherency effects are observed for all correlations, though side groups display weaker collectivity. (v) At the second structure factor peak, coherency remains only for correlations involving the main chains

Holographic study of heterogeneous dynamics by Amplitude Time Resolved Correlation (ATRC)

International audienceATRC is a new ligth scattering technique that measures by holography the field amplitude scattered by a sample, and analyze its spatial correlations. ATRC outperforms other light scattering techniques like DLS, DWS and TRC. Dynamic light scattering (DLS) [1], diffusing wave spectroscopy (DWS) [2] and time resolved correlation (TRC) [3] are well-established techniques for investigating the dynamics of a wide variety of systems in physics, chemistry, biology , and medicine. DLS and DWS consider the intensity time fluctuations of the scattered light, and study samples, whose dynamics are homogeneous in time. TRC considers the spatial fluctuations, and is able to study slow or temporally heterogeneous dynamics. We propose, in this work, to replace the TRC multi pixel detector by an holographic one, and to consider the spatial fluctuations of the field complex amplitude E as a new quantity of interest. We introduce thus a new tool called ATRC for Amplitude Time Resolved Correlation, which outperforms TRC and open new perspectives. Fig. 1. (a) TRCA typical setup. (b) Reconstructed image of a nanocomposite sample. Fig.1 (a) shows a simplified diagram of the TRCA setup. The sample S is illuminated by the laser beam L1. The light scattered by the sample is split by the beam splitter BS into two imaging paths, corresponding to cameras C1 and C2. Lens L made the image of the sample S on C1, which records the light intensity image I of S. On the other hand, the camera C2 records the interference pattern (i.e. the hologram) of the scattered light with the reference laser light L2, and the complex amplitude image E of S is calculated by holographic reconstruction. The optimize the detection sensitivity [4], holography is heterodyne [5] and off-axis. Fig.1 (b) shows the intensity reconstructed image (I = |E| 2) of a 30 × 10 mm 2 Styrene-Butadiene/Silica nanocom-posite sample, which is observed in reflection. Because 2 phase detection is used, both the +1 and-1 image of the sample are seen (right and left hand side images of Fig.1 (b)). To analyze the dynamic of the sample, TRC consider

Holographic study of heterogeneous dynamics by Amplitude Time Resolved Correlation (ATRC)

International audienceATRC is a new ligth scattering technique that measures by holography the field amplitude scattered by a sample, and analyze its spatial correlations. ATRC outperforms other light scattering techniques like DLS, DWS and TRC. Dynamic light scattering (DLS) [1], diffusing wave spectroscopy (DWS) [2] and time resolved correlation (TRC) [3] are well-established techniques for investigating the dynamics of a wide variety of systems in physics, chemistry, biology , and medicine. DLS and DWS consider the intensity time fluctuations of the scattered light, and study samples, whose dynamics are homogeneous in time. TRC considers the spatial fluctuations, and is able to study slow or temporally heterogeneous dynamics. We propose, in this work, to replace the TRC multi pixel detector by an holographic one, and to consider the spatial fluctuations of the field complex amplitude E as a new quantity of interest. We introduce thus a new tool called ATRC for Amplitude Time Resolved Correlation, which outperforms TRC and open new perspectives. Fig. 1. (a) TRCA typical setup. (b) Reconstructed image of a nanocomposite sample. Fig.1 (a) shows a simplified diagram of the TRCA setup. The sample S is illuminated by the laser beam L1. The light scattered by the sample is split by the beam splitter BS into two imaging paths, corresponding to cameras C1 and C2. Lens L made the image of the sample S on C1, which records the light intensity image I of S. On the other hand, the camera C2 records the interference pattern (i.e. the hologram) of the scattered light with the reference laser light L2, and the complex amplitude image E of S is calculated by holographic reconstruction. The optimize the detection sensitivity [4], holography is heterodyne [5] and off-axis. Fig.1 (b) shows the intensity reconstructed image (I = |E| 2) of a 30 × 10 mm 2 Styrene-Butadiene/Silica nanocom-posite sample, which is observed in reflection. Because 2 phase detection is used, both the +1 and-1 image of the sample are seen (right and left hand side images of Fig.1 (b)). To analyze the dynamic of the sample, TRC consider