Two-Component Self-Assemblies: Investigation of a Synergy between Bisurea Stickers

It is of interest to develop two-component systems for added flexibility in the design of supramolecular polymers, nanofibers, or organogels. Bisureas are known to self-assemble by hydrogen bonding into long supramolecular objects. We show here that mixing aromatic bisureas with slightly different structures can yield surprisingly large synergistic effects. A strong increase in viscosity is observed when a bisurea with the sterically demanding 2,4,6-trimethylbenzene spacer is combined with a bisurea bearing no methyl group in position 2 of the aromatic spacer (i.e., 4-methylbenzene or 4,6-dimethylbenzene). This effect is the consequence of a change in the supramolecular assembly triggered by the composition of the mixture. The mixture of complementary bisureas forms rodlike objects that are more stable by about 1 kJ/mol and that are thicker than the rodlike objects formed by both parent systems

Unusual, pH-Induced, Self-Assembly Of Sophorolipid Biosurfactants

An increasing need exists for simple, bioderived, nontoxic, and up-scalable compounds with stimuli-responsive properties. Acidic sophorolipids (SL) are glucose-based biosurfactants derived from the yeast broth of Candida bombicola (teleomorph: Starmerella bombicola). The specific design of this molecule, a sophorose head with a free end-COOH group at the end of the alkyl chain, makes it a potentially interesting pH-responsive compound. We have specifically investigated this assumption using a combination of small angle neutron scattering (SANS), transmission electron microscopy under cryogenic conditions (Cryo-TEM), and nuclear magnetic resonance (NMR) techniques and found a strong dependence of SL self-assembly on the degree of ionization, α, of the COOH group at concentration values as low as 5 and 0.5 wt %. At least three regimes can be identified where the supramolecular behavior of SL is unexpectedly different: (1) at low α values, self-assembly is driven by concentration, C, and micelles are mainly identified as nonionic objects whose curvature decreases (sphere-to-rod) with C; (2) at mid α values, the formation of COO– groups introduces negative charges at the micellar surface inducing an increase in curvature (rod-to-sphere transition). Repulsive electrostatic long-range interactions appear at this stage. In both regimes 1 and 2, the cross-section radius of the micelles is below 25 Å. This behavior is concentration independent. (3) At α = 1, individual micelles seem to favor the formation of large netlike tubular aggregates whose size is above 100 nm. Such a complex behavior is very unique as it is generally not observed for common alkyl-based surfactants in concentration ranges below 5–10 wt %

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

We 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

Amphiphilic Diblock Copolymers with a Moderately Hydrophobic Block: Toward Dynamic Micelles

Amphiphilic Diblock Copolymers with a Moderately Hydrophobic Block: Toward Dynamic Micelle

Role of Filler Shape and Connectivity on the Viscoelastic Behavior in Polymer Nanocomposites

We compare the rheological behavior of three classes of polymer nanocomposites (PNCs) to understand the role of particle shape and interactions on mechanical reinforcement. The first two correspond to favorably interacting composites formed by mixing poly­(2-vinylpyridine) with either fumed silica nanoparticles (NPs) or colloidal spherical silica NPs. We show that fumed silica NPs readily form a percolated network at low NP volume fractions. We deduce that the NPs act as network junctions with the effectively irreversibly bound polymer chains serving as the connecting bridges. By comparing with colloidal spherical silica, which has a significantly higher percolation threshold, we conclude that the fractal shape of the fumed silica is responsible for its unusually low percolation threshold. The third system corresponds to polystyrene grafted colloidal silica nanoparticles (PGNPs) in a polystyrene matrix. These PNCs have an even lower percolation threshold probably because the grafted chains increase the effective volume fraction of the NPs. When we take these different thickness of the polymer layers in the two cases into account (i.e., grafted layer vs adsorbed layer thickness), the percolation threshold for the fumed and the grafted system occurs at similar effective loadings, but the NP network with fumed silica has a higher low-frequency plateau modulus than that formed with the PGNPs. These findings can be reconciled by the fact that the fumed silica NPs are composed of fused entities, thus ensuring that they have a higher modulus than the PGNPs where the modulus is largely attributed to interactions between the grafts. Our results systematically stress the important role of the nanofiller shape and connectivity on the mechanical reinforcement of PNCs

Insight into Asphaltene Nanoaggregate Structure Inferred by Small Angle Neutron and X-ray Scattering

Complementary neutron and X-ray small angle scattering results give prominent information on the asphaltene nanostructure. Precise SANS and SAXS measurements on a large q-scale were performed on the same dilute asphaltene–toluene solution, and absolute intensity scaling was carried out. Direct comparison of neutron and X-ray spectra enables description of a fractal organization made from the aggregation of small entities of 16 kDa, exhibiting an internal fine structure. Neutron contrast variation experiments enhance the description of this nanoaggregate in terms of core–shell disk organization, giving insight into core and shell dimensions and chemical compositions. The nanoaggregates are best described by a disk of total radius 32 Å with 30% polydispersity and a height of 6.7 Å. Composition and density calculations show that the core is a dense and aromatic structure, contrary to the shell, which is highly aliphatic. These results show a good agreement with the general view of the Yen model (Yen, T. F.; et al. Anal. Chem. 1961, 33, 1587–1594) and as for the modified Yen model (Mullins, O. C. Energy Fuels 2010, 24, 2179–2207), provide characteristic dimensions of the asphaltene nanoaggregate in good solvent

Self-Assembly of Monodisperse versus Bidisperse Polymer-Grafted Nanoparticles

We systematically compare the dispersion and self-assembly of silica nanoparticles (NPs) grafted with either a sparse monomodal long chain length polystyrene (PS) brush or a bimodal brush comprised of a sparse grafting of long PS chains and a dense carpet of short poly­(2-vinylpyridine) (P2VP) chains. These two different types of NPs are placed in pure PS matrices of varying molecular weights in a series of experiments. We first show that NP dispersion is generally improved in the case of bimodal brushes. More interestingly, at low PS grafting densities the bimodal brushes give different self-assembled structures relative to the monomodal brushes; we conjecture that the presence of the short P2VP chains in the bimodal brush reduces the effective core–core attractions and thus allows these bidisperse NPs to display self-assembly behavior that is less likely to be kinetically trapped by the strong intercore attractions that control the behavior of monomodal NPs. In this low PS grafting density limit, where we expect the spatial coverage of the brush to be the most nonuniform, we find the formation of “vesicular” structures that are representative of highly asymmetric (“tadpole”) surfactants. Our results therefore show that reducing the inter-NP attractions gives rise to a much richer ensemble of NP self-assemblies, apparently with a smaller influence from kinetic traps (or barriers)

Controlling the Morphology in Epoxy/Thermoplastic Systems

Thermosets are frequently toughened by a high-Tg thermoplastic (TP). Blend morphologies obtained by curing induced phase separation with scales of a few hundreds of nanometers are relevant for high-performance applications, but no quantitative description for obtaining these morphologies exists yet. We propose such a quantitative approach for predicting and controlling the final morphology. The key is the degree of curing and the corresponding Tg of the blend and both phases when phase separation takes place. It is controlled by the Flory interaction parameter χ of the constituents and their respective Tg’s. We show that if phase separation takes place too early during curing, the Tg is too low and the morphologies grow to reach sizes of a few micrometers or more. Our study of different systems allows us to propose the relevant range of Flory interaction parameter χ and temperature window T–Tg for which the sizes of interest may be obtained. Our work opens the way for devising thermoplastics–thermosets couples with the appropriate affinity and Tg’s in order to make blends with tailored morphologies

Homogeneous Dispersion of Magnetic Nanoparticles Aggregates in a PS Nanocomposite: Highly Reproducible Hierarchical Structure Tuned by the Nanoparticles’ Size

We present here the synthesis and structural characterization of new nanocomposites made of spherical magnetic nanoparticles of maghemite (γ-Fe2O3) dispersed in a polystyrene (PS) matrix. The γ-Fe2O3 nanoparticles, synthesized in aqueous media, were first gently transferred by dialysis in dimethylacetamide (DMAc), a polar solvent which is a good solvent for PS. Electrostatic repulsions enable to keep colloidal stability in DMAc. The nanocomposites were then processed by a controlled evaporation of DMAc of binary mixtures of γ-Fe2O3 nanoparticles and PS chains. The size of the nanoparticles ranges from 3.5 to 6.5 nm and can be changed without any modification of the nanoparticles’ surface. The structural organization of the nanoparticles inside the polymer was determined as a function of the nanoparticles’ size. It was performed by combining very high resolution SAXS measurements which permit to decrease the nanoparticles content down to very low values (Φmag ∼ 10−5) and TEM microscopy. Whatever the size, the nanoparticles are organized with a hierarchical structure that shows that their aggregation has been driven by a two-step process. At low spatial scale, dense primary aggregates composed of some tens of nanoparticles are formed whatever Φmag, resulting from the first aggregation step. For Φmag > 10−4, these primary aggregates underwent a second aggregation step and are organized at larger scale in fractal aggregates of finite size of ∼200 nm of radius, with a dimension of 1.7. The size of the dense primary aggregates is almost constant when changing the nanoparticles radius; i.e., the mean aggregation number of primary aggregates decreases with an increase of the radius