We investigate the formation/re-dissociation mechanisms of hybrid complexes made from negatively charged PAA2k coated γ-Fe2O3 nanoparticles (NP) and positively charged polycations (PDADMAC) in aqueous solution in the regime of very high ionic strength (I). When the building blocks are mixed at large ionic strength (1 M NH4Cl), the electrostatic interaction is screened and complexation does not occur. If the ionic strength is then lowered down to a targeted ionic strength Itarget, there is a critical threshold Ic = 0.62 M at which complexation occurs, that is independent of the charge ratio Z and the pathway used to reduce salinity (drop-by-drop mixing or fast mixing). If salt is added back up to 1 M, the transition is not reversible and persistent out-of-equilibrium aggregates are formed. The lifetimes of such aggregates depends on Itarget: the closer Itarget to Ic is, the more difficult it is to dissolve the aggregates. Such peculiar behavior is driven by the inner structure of the complexes that are formed after desalting. When Itarget is far below Ic, strong electrostatic interactions induce the formation of dense, compact and frozen aggregates. Such aggregates can only poorly reorganize further on with time, which makes their dissolution upon resalting almost reversible. Conversely, when Itarget is close to Ic more open aggregates are formed due to weaker electrostatic interactions upon desalting. The system can thus rearrange with time to lower its free energy and reach more stable out-of-equilibrium states which are very difficult to dissociate back upon resalting, even at very high ionic strength.
Herein, the synthesis of a novel nitroxide-mediated polymerization (NMP) initiator bearing a photolabile ortho-nitrobenzyl (oNB) group allowing surface-initiated NMP preparation of well-defined photoresponsive polystyrene grafted on silica nanoparticles is described. The photocleavable and photoresponsive properties of the prepared materials are demonstrated using small angle X-ray scattering (SAXS) characterization.
Nanoparticles , Silicon Dioxide , Nitrogen Oxides , Polymerization , Polystyrenes , Scattering, Small Angle , X-Ray Diffraction
In this study, poly(ethylene oxide) monomethyl ether (MPEO) of molecular weight of 5000, 10 000, and 20 000 g mol-1 were grafted onto colloidal silica nanoparticles (NPs) of a 27.6 nm diameter using two distinct "grafting to" processes. The first method was based on the coupling reaction of epoxide-end capped MPEO with amine-functionalized silica NPs, while the second method was based on the condensation of triethoxysilane-terminated MPEO onto the unmodified silica NPs. The influence of PEO molecular weight, grafting process and grafting conditions (temperature, reactant concentration, reaction time) on the PEO grafting density was fully investigated. Thermogravimetric analysis (TGA) was used to determine the grafting density which ranged from 0.12 chains per nm2 using the first approach to 1.02 chains per nm2 when using the second approach. 29Si CP/MAS NMR characterization indirectly revealed that above a grafting density value of 0.3 PEO chains per nm2, a dendri-graft PEO network was built around the silica surface which was composed of PEO chains directly anchored to the silica surface and those grafted to silica NPs by intermediate of >CH-O-Si- bonds. The colloidal stability of the particles during different steps of the grafting process was characterized by small-angle X-ray scattering (SAXS). We have found that the colloidal systems are stable whatever the achieved grafting density due to the strong repulsions between the NPs, with the strength of repulsion increasing with the molecular weight of the grafted MPEO chains.
We investigate the structure of gluten polymer-like gels in a binary mixture of water/ethanol, 50/50 v/v, a good solvent for gluten proteins. Gluten comprises two main families of proteins, monomeric gliadins and polymer glutenins. In the semi-dilute regime, scattering experiments highlight two classes of behavior, akin to standard polymer solution and polymer gel, depending on the protein composition. We demonstrate that these two classes are encoded in the structural features of the proteins in very dilute solution, and are correlated with the presence of proteins assemblies of typical size tens of nanometers. The assemblies only exist when the protein mixture is sufficiently enriched in glutenins. They are found directly associated to the presence in the gel of domains enriched in non-exchangeable H-bonds and of size comparable to that of the protein assemblies. The domains are probed in neutron scattering experiments thanks to their unique contrast. We show that the sample visco-elasticity is also directly correlated to the quantity of domains enriched in H-bonds, showing the key role of H-bonds in ruling the visco-elasticity of polymer gluten gels.
Glutens , Polymers , Gels/chemistry , Gliadin/chemistry , Glutens/chemistry , Polymers/chemistry , Proteins , Viscosity
HYPOTHESIS: Thermoresponsive polymers are important materials for various applications. However, the number of polymers that exhibit this property in the temperature range of interest remains limited. The development of novel rational design strategies through the understanding of the thermal transition's origin is therefore of utmost importance. EXPERIMENTS: Bisurea-functionalized water-soluble polymers were synthesized by RAFT polymerization. After direct dissolution in water, the supramolecular assemblies were analyzed by cryo-TEM and SANS. Their temperature-dependent water-solubility was characterized by various techniques, namely DLS, SANS, DSC, IR, to understand the origin of the temperature sensitivity. FINDINGS: The supramolecular assemblies exhibit an unexpected temperature-dependent solubility. For instance, a cloud point of only 39 °C was measured for poly(N,N-dimethylacrylamide) assemblies. This property is not restricted to one specific polymer but is rather a general feature of bisurea-functionalized polymers that form supramacromolecular bottlebrushes in water. The results highlight the existence of two distinct transitions; the first one is a visually perceptible cloud point due to the aggregation of individual micelles, presumably driven by the hydrophobic effect. The second transition is related to the dissociation of intermolecular bisurea hydrogen bonds. Finally, we show that it is possible to widely tune the cloud point temperature through the formation of co-assemblies.
Environmental iron-organic matter (Fe-OM) aggregates play a major role in the dynamic of pollutants. Nowadays, there is a lack of information about the control exerted by their structural organization on their reactivity towards metal(loid)s and in particular, the impact of major ions, such as calcium. The sorption capacity of mimetic environmental Fe-OM-Ca aggregates was investigated relative to the Fe/organic carbon (OC) and Ca/Fe ratios using As as a probe. It was shown that Fe speciation is the key factor controlling the reactivity of Fe-OM-Ca aggregates regarding the high affinity of Fe(III)-oligomers towards As and the high sorption capacity of ferrihydrite-like nanoparticles. Moreover, when it occurs at high concentration, Ca competes with Fe for OM binding leading to an increase in the amount of ferrihydrite-like nanoparticles and binding site availability. As a consequence, Ca not only impacts the ionic strength but it also has a dramatic impact on the structural organization of Fe-OM aggregates at several scales of organization, resulting in an increase of their sorption capacity. In the presence of high amounts of Ca, Fe-OM-Ca aggregates could immobilize pollutants in the soil porous media as they form a micrometric network exhibiting a strong sorption capacity.
Polymer membranes are critical to many sustainability applications that require the size-based separation of gas mixtures. Despite their ubiquity, there is a continuing need to selectively affect the transport of different mixture components while enhancing mechanical strength and hindering aging. Polymer-grafted nanoparticles (GNPs) have recently been explored in the context of gas separations. Membranes made from pure GNPs have higher gas permeability and lower selectivity relative to the neat polymer because they have increased mean free volume. Going beyond this ability to manipulate the mean free volume by grafting chains to a nanoparticle, the conceptual advance of the present work is our finding that GNPs are spatially heterogeneous transport media, with this free volume distribution being easily manipulated by the addition of free polymer. In particular, adding a small amount of appropriately chosen free polymer can increase the membrane gas selectivity by up to two orders of magnitude while only moderately reducing small gas permeability. Added short free chains, which are homogeneously distributed in the polymer layer of the GNP, reduce the permeability of all gases but yield no dramatic increases in selectivity. In contrast, free chains with length comparable to the grafts, which populate the interstitial pockets between GNPs, preferentially hinder the transport of the larger gas and thus result in large selectivity increases. This work thus establishes that we can favorably manipulate the selective gas transport properties of GNP membranes through the entropic effects associated with the addition of free chains.
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Janus cylinders are one-dimensional colloids that have two faces with different compositions and functionalities, and are useful as building blocks for advanced functional materials. Such anisotropic objects are difficult to prepare with nanometric dimensions. Here we describe a robust and versatile strategy to form micrometer long Janus nanorods with diameters in the 10-nanometer range, by self-assembly in water of end-functionalized polymers. The Janus topology is not a result of the phase segregation of incompatible polymer arms, but is driven by the interactions between unsymmetrical and complementary hydrogen bonded stickers. Therefore, even compatible polymers can be used to form these Janus objects. In fact, any polymers should qualify, as long as they do not prevent co-assembly of the stickers. To illustrate their applicative potential, we show that these Janus nanorods can efficiently stabilize oil-in-water emulsions.
Improving cell-material interactions of nonadhesive scaffolds is crucial for the success of biomaterials in tissue engineering. Due to their high surface area and open pore structure, sponges are widely reported as absorbent materials for biomedical engineering. The biocompatibility and biodegradability of polysaccharide sponges, coupled with the chemical functionalities of supramolecular dimers, make them promising combinations for the development of adhesive scaffolds. Here, a supramolecular tactic based on (UPy)-modified polysaccharide associated with three-dimensional structure of sponges was developed to reach enhanced cellular adhesion. For this purpose, three approaches were examined individually in order to accomplish this goal. In the first approach, the backbone polysaccharides with noncell adhesive properties were modified via a modular tactic using UPy-dimers. Hereupon, the physical-chemical characterizations of the supramolecular sponges were performed, showing that the presence of supramolecular dimers improved their mechanical properties and induced different architectures. In addition, small-angle neutron scattering (SANS) measurements and rheology experiments revealed that the UPy-dimers into agarose backbone are able to reorganize in thinning aggregates. It is also demonstrated that the resulted UPy-agarose (AGA-UPy) motifs in surfaces can promote cell adhesion. Finally, the last approach showed the great potential for use of this novel material in bioadhesive scaffolds indicating that neural stem cells show a spreading bias in soft materials and that cell adhesion was enhanced for all UPy-modified sponges compared to the reference, i.e. unmodified sponges. Therefore, by functionalizing sponge surfaces with UPy-dimers, an adhesive supramolecular scaffold is built which opens the opportunity its use neural tissues regeneration.
Adhesives , Neural Stem Cells , Biocompatible Materials , Polymers , Tissue Engineering , Tissue Scaffolds
The structure of adsorbed surfactant layers at the equilibrium state has already been investigated using various experimental techniques. However, the comprehension of the formation of structural intermediates in nonequilibrium states and the resulting adsorption kinetics still remain a challenging task. The temporal characterization of these intermediate structures provides further understanding of the layer structure at equilibrium and of the main interactions involved in the adsorption process. In this article, we studied the adsorption kinetics of AOT vesicles on silica at different pHs at ambient temperature. The AOT vesicles were formed in a brine solution. Quartz crystal microbalance with dissipation monitoring (QCM-D) was used to obtain information on the kinetics of surfactant adsorption and on the structure of the adsorbed layer at the equilibrium state. Additionally, neutron reflectivity experiments were performed to provide a detailed description of the mean surfactant concentration profile normal to the surface at equilibrium. Results suggest that vesicles in the bulk influence the adsorption mechanisms. In acidic conditions, after a time-dependent structural rearrangement step, followed by the rupture of initially adsorbed vesicles, the formation of a bilayer was observed. At an intermediate and basic pH, in spite of the electrostatic repulsion between the negatively charged surfactants and silica, results demonstrated the existence of an adsorbed layer composed of AOT vesicles. Vesicles are more or less closely packed depending on the pH of the solution. Results show a non-negligible influence of NaCl addition at pH values where adsorption is initially inhibited. Vesicle adsorption at the intermediate and basic pH is probably due to the combination of attractive van der Waals interactions promoted in high ionic strength systems and the formation of hydrogen bonds. Interpretation of adsorption kinetics gave insight into adsorption mechanisms in an electrostatic repulsion environment.
We previously showed that nanoparticles (NPs) could be ordered into structures by using the growth rate of polymer crystals as the control variable. In particular, for slow enough spherulitic growth fronts, the NPs grafted with amorphous polymer chains are selectively moved into the interlamellar, interfibrillar, and interspherulitic zones of a lamellar morphology, specifically going from interlamellar to interspherulitic with progressively decreasing crystal growth rates. Here, we examine the effect of NP polymer grafting density on crystallization kinetics. We find that while crystal nucleation is practically unaffected by the presence of the NPs, spherulitic growth, final crystallinity, and melting point values decrease uniformly as the volume fraction of the crystallizable polymer, poly(ethylene oxide) or PEO, ÏPEO, decreases. A surprising aspect here is that these results are apparently unaffected by variations in the relative amounts of the amorphous polymer graft and silica NPs at constant Ï, implying that chemical details of the amorphous defect apparently only play a secondary role. We therefore propose that the grafted NPs in this size range only provide geometrical confinement effects which serve to set the crystal growth rates and melting point depressions without causing any changes to crystallization mechanisms.
The tracer diffusion coefficient of six different permanent gases in polymer-grafted nanoparticle (GNP) membranes, i.e., neat GNP constructs with no solvent, show a maximum as a function of the grafted chain length at fixed grafting density. This trend is reproduced for two different NP sizes and three different polymer chemistries. We postulate that nonmonotonic changes in local, segmental friction as a function of graft chain length (at fixed grafting density) must underpin these effects, and use quasielastic neutron scattering to probe the self-motions of polymer chains at the relevant segmental scale (i.e., sampling local friction or viscosity). These data, when interpreted with a jump diffusion model, show that, in addition to the speeding-up in local chain dynamics, the elementary distance over which segments hop is strongly dependent on graft chain length. We therefore conclude that transport modifications in these GNP layers, which are underpinned by a structural transition from a concentrated brush to semidilute polymer brush, are a consequence of both spatial and temporal changes, both of which are likely driven by the lower polymer densities of the GNPs relative to the neat polymer.
Introduction of competing interactions in the design of a supramolecular polymer (SP) creates pathway complexity. Ester-bis-ureas contain both a strong bis-urea sticker that is responsible for the build-up of long rod-like objects by hydrogen bonding and ester groups that can interfere with this main pattern in a subtle way. Spectroscopic (FTIR and CD), calorimetric (DSC), and scattering (SANS) techniques show that such ester-bis-ureas self-assemble into three competing rod-like SPs. The previously unreported low-temperature SP is stabilized by hydrogen bonds between the interfering ester groups and the urea moieties. It also features a weak macroscopic alignment of the rods. The other structures form isotropic dispersions of rods stabilized by the more classical urea-urea hydrogen bonding pattern. The transition from the low-temperature structure to the next occurs reversibly by heating and is accompanied by an increase in viscosity, a rare feature for solutions in hydrocarbons.
Naphthalene-diimide (NDI)-containing nanocylinders were formed by supramolecular self-assembly in water through cooperative hydrogen bonds between bis(urea) units, reinforced by hydrophobic and aromatic-stacking interactions. The nanocylinders, decorated with poly(ethylene oxide) arms ensuring their solubility in water, exhibit a huge aspect ratio (diameter 13 nm, length 300 nm) and are extremely stable.
The plant-derived biosurfactant aescin is naturally present in many plants and is used for treatment of disorders such as varicose veins and inflammation of veins. The hemolytic activity of this saponin is attributed to its interaction with cholesterol in the red blood cell membrane. This work investigates the phase and aggregation behavior of saponin-containing model membranes consisting of the phospholipid 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC). The aescin concentrations studied range from 1â¯mol% to 7â¯mol% with respect to the total lipid content. The methods of choice to elucidate the structural picture are small-angle scattering of X-rays (SAXS) and neutrons (SANS) and cryogenic transmission electron microscopy (cryo-TEM). SANS and SAXS revealed that at lower aescin contents vesicular structures are conserved and vesicles tend to aggregate already at aescin contents of around 1â¯mol%. Aggregation and vesicle deformation effects are found to be stronger when the phospholipids are in the L [Formula: see text] phase. With increasing aescin content, mixed structures, i.e. aggregated and deformed vesicles and solubilized bilayer fragments, are present. This was proven for a sample with 4â¯mol% aescin by cryo-TEM. An increasing aescin amount leads to membrane decomposition and free standing bilayers which tend to build stacks at high temperature. These stacks are characterized by SAXS using the modified Caillé theory. Analyses and model dependent fitting reveal formation of well-defined structures beginning at 7â¯mol% aescin.
Dimyristoylphosphatidylcholine/chemistry , Escin/chemistry , Membranes, Artificial , Microscopy, Electron, Transmission , Scattering, Small Angle , X-Ray Diffraction
Understanding the structure and dynamics of the bound polymer layer (BL) that forms on favorably interacting nanoparticles (NPs) is critical to revealing the mechanisms responsible for material property enhancements in polymer nanocomposites (PNCs). Here we use small angle neutron scattering to probe the temporal persistence of this BL in the canonical case of poly(2-vinylpyridine) (P2VP) mixed with silica NPs at two representative temperatures. We have observed almost no long-term reorganization at 150 °C (â¼Tg,P2VP + 50 °C), but a notable reduction in the BL thickness at 175 °C. We believe that this apparently strong temperature dependence arises from the polyvalency of the binding of a single P2VP chain to a NP. Thus, while the adsorption-desorption process of a single segment is an activated process that occurs over a broad temperature range, the cooperative nature of requiring multiple segments to desorb converts this into a process that occurs over a seemingly narrow temperature range.
The orientation of cytochrome c molecules at the surface of silica nanoparticles was studied in a wide pH range by combining small-angle neutron scattering, adsorption measurements, and molecular dynamics simulations. The results indicate a reorientation of the ellipsoidal protein from head-on to side-on as the pH is increased. This is attributed to changes in the surface charge distribution of both the protein and the nanoparticles.
Cytochromes c/chemistry , Nanoparticles/chemistry , Silicon Dioxide/chemistry , Enzymes, Immobilized/chemistry , Hydrogen-Ion Concentration , Molecular Dynamics Simulation , Static Electricity
The evolution of rare earth element (REE) speciation between reducing and oxidizing conditions in a riparian wetland soil was studied relative to the size fractionation of the solution. In all size fractions obtained from the reduced and oxidized soil solutions, the following analyses were carried out: organic matter (OM) characterization, transmission electron microscopy (TEM) observations as well as major and trace element analyses. Significant REE redistribution and speciation evolution between the various size fractions were observed. Under reducing conditions, the REEs were bound to colloidal and dissolved OM (<2µm size fractions). By contrast, under oxidizing conditions, they were distributed in particulate (>2µm size fraction), colloidal (<2µm size fraction), organic and Fe-enriched fractions. In the particulate size fraction, the REEs were bound to humic and bacterial OM embedding Fe nano-oxides. The resulting REE pattern showed a strong enrichment in heavy REEs (HREEs) in response to REE binding to specific bacterial OM functional groups. In the largest colloidal size fraction (0.2µm-30kDa), the REEs were bound to humic substances (HS). The lowest colloidal size fraction (<30kDa) is poorly concentrated in the REEs and the REE pattern showed an increase in the middle REEs (MREEs) and heavy REEs (HREEs) corresponding to a low REE loading on HS. A comparison of the REE patterns in the present experimental and field measurements demonstrated that, in riparian wetlands, under a high-water level, reducing conditions are insufficient to allow for the dissolution of the entire Fe nano-oxide pool formed during the oxidative period. Therefore, even under reducing conditions, Fe(III) seems to remain a potential scavenger of REEs.
Membranes made purely from nanoparticles (NPs) grafted with polymer chains show increased gas permeability relative to the analogous neat polymer films, with this effect apparently being tunable with systematic variations in polymer graft density and molecular weight. To explore the structural origins of these unusual transport results, we use small angle scattering (neutron, X-ray) on the dry nanocomposite film and to critically examine in situ the structural effects of absorbed solvent. The relatively low diffusion coefficients of typical solvents (â¼10-12 m2/s) restricts us to thin films (≈1 µm in thickness) if solute concentration profiles are to equilibrate on the 1 s time scale. The use of such thin films, however, renders them as weak scatterers. Inspired by our nearly two decades old previous work, we address these conflicting requirements through the use of a custom designed flow cell, where stacks of 10 individual ≈1 µm thick supported films are used, while ensuring that each film is individually exposed to solvent vapor. By using isotopically labeled solvents, we study the solvent distribution within the film and show surprisingly that the solvent homogeneously swells the polymer under all conditions that we examined. These results are not anticipated by current theories, but they suggest that, at least under some conditions, the free volume increases due to the grafting of chains to nanoparticles is apparently distributed isotropically in these materials.
