Direct small angle neutron scattering observation of stretched chain conformation in nanocomposites: more insight of polymer contributions in mechanical reinforcement

International audienceIn this paper we present a direct measurement of stretched chain conformation in polymer nanocomposites in a large range of deformation using a specific contrast-matched SANS method. Whatever the filler structure and the chain length the results show a clear identity of chain deformation in pure and reinforced polymer and offer a new insight of the polymer chain contribution in the mechanical reinforcement. It suggests that glassy layer or glassy paths, recently proposed, should involve only a small fraction of chains. As a result, the remaining filler contribution appears strikingly constant with deformation as explained by continuous locking-unlocking rearrangement process of the particles

In Situ SAXS and SANS Monitoring of Both Nanofillers and Polymer Chain Microstructure under Uniaxial Stretching in a Nanocomposite with a Controlled Anisotropic Structure

International audienceWe present a combined detailed monitoring of the respective evolutions of the structure of fillers by SAXS and conformation of polymeric chains by SANS under uniaxial stretching at various elongation ratios in a nanocomposite made of spherical magnetic nanoparticles of γFe2O3 dispersed in a matrix of polystyrene (PS) chains. We can make the structure of fillers in the nanocomposite before stretching very anisotropic, as we demonstrated in reference (Macromolecules,2011,44(22), 8858–8865), thanks to the appliance of a magnetic field during the nanocomposite processing that induces the formation of nanoparticle chains aligned along the direction of the field, either parallel or perpendicular to the subsequent stretching. This gives rise to very anisotropic mechanical properties, and the structure of fillers evolves very differently. In the parallel case, there is a rupture of the chains of nanoparticles into smaller subunits that progressively align completely in the stretching direction. In the perpendicular case, the chains first rotate, to eventually reorient along the stretching direction, gradually breaking themselves. Finally, at a very large elongation rate (λ = 6), the organization of nanoparticles in both cases converge toward a common structure made of small chains of nanoparticles completely oriented along the stretching. The chain conformation is directly obtained by SANS as we probed samples containing 25% PSH/75% PSD chains, taking benefit from the fact that the neutron scattering length density of PSD is similar to the one of γFe2O3. Strikingly, the deformation of the polymer chains is the same as in the reference PS matrix without fillers, whatever stretching is parallel and perpendicular to the chains of nanoparticles at λ = 3. The chains scattering exhibit the typical features of those of the former studies of relaxation during or after stretching on pure melts, accounting for the relatively slow deformation rate, the distance from Tg (15 °C) their average masses, and their large polydispersity

Polymer-Grafted-Nanoparticles Nanocomposites: Dispersion, Grafted Chain Conformation, and Rheological Behavior

International audienceWe investigate the dispersion mechanisms of nanocomposites made of well-defined polymer (polystyrene, PS) grafted-nanoparticles (silica) mixed with free chains of the same polymer using a combination of scattering (SAXS/USAXS) and imaging (TEM) techniques. We show that the relevant parameter of the dispersion, the grafted/free chains mass ratio R tuned with specific synthesis process, enables to manage the arrangement of the grafted nanoparticles inside the matrix either as large and compact aggregates (R \textless 0.24) or as individual nanoparticles dispersion (R \textgreater 0.24). From the analysis of the interparticles structure factor, we can extract the thickness of the spherical corona of grafted brushes and correlate it with the dispersion: aggregation of the particles is associated with a significant collapse of the grafted chains, in agreement with the theoretical models describing the free energy as a combination of a mixing entropy term between the free and the grafted chains and an elastic term of deformation of the grafted brushes. At fixed grafting density, the individual dispersion of particles below the theoretical limit of R = 1 can be observed, due to interdiffusion between the grafted and the free chains but also to processing kinetics effects, surface curvature and chains poly dispersity. Mechanical analysis of nanocomposites show the appearance of a longer relaxation time at low frequencies, more pronounced in the aggregated case even without direct connectivity between the aggregates. Correlation between the local structure and the rheological behavior suggests that the macroscopic elastic modulus of the nanocomposite could be described mainly by a short-range contribution, at the scale of the interactions between grafted particles, without significant effect of larger scale organizations

Correction to “Polymer-Grafted Magnetic Nanoparticles in Nanocomposites: Curvature Effects, Conformation of Grafted Chain, and Bimodal Nanotriggering of Filler Organization by Combination of Chain Grafting and Magnetic Field”

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Polymer-Grafted Magnetic Nanoparticles in Nanocomposites: Curvature Effects, Conformation of Grafted Chain, and Bimodal Nanotriggering of Filler Organization by Combination of Chain Grafting and Magnetic Field

International audienceWe present the synthesis and structural characterization of new nanocomposites made of linear tetramers of spherical magnetic nanoparticles of maghemite (?-Fe2O3), grafted by tethered polystyrene (PS) chains with an intermediate grafting density (?0.15 chains/nm2), dispersed in a PS matrix. First, we studied by combination of SAXS and TEM the dispersion state of the grafted objects within the matrix for various ratio R of the mass of the grafted chains N to the mass of free chains P ranging from R = N/P = 0.09 to R = 2.83. For R \textless 0.4, we obtained ramified aggregates of a few tens of grafted objects whose compactness slightly depends on R. For R = 1, the objects are well dispersed, a case that we have studied by recovering the free untethered residual chains resulting from the synthesis of the PS-grafted nanoparticles. For R \textgreater 1, the objects are also well dispersed although there are some remaining aggregates, arising probably from the fact that we have used matrix? chains below the entanglement mass to reach such a high R values. This enabled to determine the threshold of the ?wet-to-dry? transition, between the wetting of the brush by the free chain and the expulsion of them from the brush due to entropic effects, between 0.4 and 1, a value higher than the one (0.24) obtained on a very similar system on silica spherical particles by Chevigny et al. [Macromolecules 2011, 44 (1), 122?133]. This shift highlights the influence of the curvature of the grafted objects on the threshold?s transition. Second, we have made a direct measurement of the conformation of the grafted chains in the case of aggregates, benefiting from the fact that the scattering length density (SLD) of maghemite has almost the same value as the one of deuterated PS chains. They are strongly collapsed in comparison with their conformation in bulk, as shown by a significant reduction of their radius of gyration accompanied by the deviation from a Gaussian behavior. Finally, by applying a magnetic field during casting on samples for R \textless 0.4, we showed that the aggregates can be aligned in the direction of the magnetic field. We demonstrate thus the possibility of tuning the structure of the fillers by coupling a double control: chemical and magnetic

Zirconocene Dichloride: An Efficient Cleavable Photoinitiator Allowing the in Situ Production of Zr-Based Nanoparticles Under Air

Cp₂ZrCl₂ is presented as both an effective photoinitiator and additive for radical photopolymerization reactions in aerated conditions. This compound is characterized by remarkable properties: (i) an efficiency higher than that of a reference Type I photoinitiator (2,2-dimethoxy-2-phenylacetophenone, DMPA), (ii) an excellent ability, when added to DMPA, to overcome the oxygen inhibition of the polymerization, and (iii) a never reported in situ photoinduced and oxygen-mediated formation of zirconium-based nanoparticles (diameter ranging from 50 to 70 nm). The photochemical properties of Cp₂ZrCl₂ are investigated by steady state photolysis and electron spin resonance (ESR) experiments. The high reactivity of this compound is ascribed to a bimolecular homolytic substitution S_H2 (clearly characterized by molecular orbital calculations) which converts the peroxyls into new polymerization-initiating radicals and oxygenated Zr-based nanoparticles

3D Dispersion of Spherical Silica Nanoparticles in Polymer Nanocomposites: A Quantitative Study by Electron Tomography

cited By 15International audienceA simple methodology is proposed to quantitatively discuss the quality of the 3D dispersion of grafted silica nanoparticles (NPs) mixed with free chains in nanocomposites. 3D observations of NPs are obtained by transmission electron tomography (TEMT). The NPs are individualized from the tomograms by segmentation and watershed processing. Equivalent dispersions of spherical objects are then generated. By computing 3D Voronoi tessellation from the spheres, the local environment is investigated for each NP through geometrical measurements. Two types of populations of NP can be extracted: either densely packed or isolated. As the length of the free chains decreases, when the length of the free chains are close to the ones of the grafted chains, this second population becomes predominant and the quality of the dispersion gradually increases. The width of the local volume fraction distribution surrounding each NP can be considered as a quantitative parameter indicative of the homogeneity of the microstructure. It becomes narrower as the quality of the dispersion increases. © 2014 American Chemical Society