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International audience; A bidentate chiral dithiol (diBINAS) is utilised to bridge Au 25 nanoclusters to form oligomers. Separation by size allows the isolation of fractions that are stable thanks to the bidentate nature of the linker. The structure of the products is elucidated by smallangle X-ray scattering and calculated using density functional theory. Additional structural details are studied by diffusion-ordered nuclear magnetic resonance spectroscopy, transmission electron microscopy and matrix-assisted laser desorption/ionization time of flight mass spectrometry. Significant changes in the optical properties are analysed by UV/Vis and fluorescence spectroscopies, with the latter demonstrating a strong emission enhancement. Furthermore, the emergent chiral characteristics are studied by circular dichroism. Due to the geometry constraints of the nanocluster assemblies, diBINAS can be regarded as a templating molecule, taking a step towards the directed selfassembly of metal clusters.

Langmuir-Blodgett (LB) technique allows the deposition of gold nanoparticles and nanoclusters (atomically precise nanoparticles below 2 nm in diameter) onto solid substrates with an unprecedented degree of control and high transfer ratios. Nanoclusters are expected to follow the crinkle folding mechanism, which promotes the formation of trilayers of nanoparticles but kinetically disfavors the formation of the fourth layer.LB films of AuNanocluster bilayers and trilayers exhibited the elastic moduli of ca. 1.2 GPa and 0.9 GPa respectively. Films transferred in the 20-25 mN/m surface pressure regime displayed a particular propensity to form highly vertically organized trilayers. Further compression resulted in disorganization of the layers. Crucially, the use of two cantilevers of contrasting stiffness for bimodal AFM measurements has demonstrated a new approach to quantify the mechanical properties of ultrathin films without the use of deconvolution algorithms to remove the substrate contribution.

International audience; The formation of dense protein interfacial layers at afree air−water interface is known to result from both diffusion andadvection. Furthermore, protein interactions in concentrated phasesare strongly dependent on their overall positive or negative net charge,which is controlled by the solution pH. As a consequence, aninteresting question is whether the presence of an advectionflow ofwater toward the interface during protein adsorption producesdifferent kinetics and interfacial structure of the adsorbed layer,depending on the net charge of the involved proteins and, possibly, onthe sign of this charge. Here we test a combination of the followingparameters using ovalbumin and lysozyme as model proteins: positiveor negative net charge and the presence or absence of advectionflow. The formation and the organization of the interfacial layers arestudied by neutron reflectivity and null-ellipsometry measurements. We show that the combined effect of a positive charge oflysozyme and ovalbumin and the presence of advectionflow does induce the formation of interfacial multilayers. Conversely,negatively charged ovalbumin forms monolayers, whether advectionflow is present or not. We show that an advection/diffusionmodel cannot correctly describe the adsorption kinetics of multilayers, even in the hypothesis of a concentration-dependent diffusioncoefficient as in colloidalfiltration, for instance. Still, it is clear that advection is a necessary condition for making multilayers througha mechanism that remains to be determined, which paves the way for future research.

Langmuir-Blodgett technique is utilized to deposit ultrathin films of Au

International audience; Langmuir–Blodgett technique is utilized to deposit ultrathin films of Au$_{38}$(SC$_2$H$_4$Ph)$_{24}$ nanocluster onto solid surfaces such as mica and silicon. The morphologies of the films transferred at various surface pressures within the mono/bi/trilayer regime are studied by atomic force microscopy (AFM). The time spent on the water surface before the deposition has a decisive effect on the final ordering of nanoclusters within the network and is studied by fast AFM, X-ray reflectivity, and grazing-incidence wide-angle X-ray scattering.

Nanofilms of thickness of about two nanometers have been formed at the air-water interface using functionalized castor oil (ICO) with cross-linkable silylated groups. These hybrid films represent excellent candidates for replacing conventional polymeric materials in biomedical applications, but they need to be optimized in terms of biocompatibility which is highly related to protein adsorption. Neutron reflectivity has been used to study the adsorption of two model proteins, bovine serum albumin and lysozyme, at the silylated oil (ICO)-water interface in the absence and presence of salt at physiologic ionic strength and pH and at different protein concentrations. These measurements are compared to adsorption at the air-water interface. While salt enhances adsorption by a similar degree at the air-water and the oil-water interface, the impact of the oil film is significant, with adsorption at the oil-water interface three-to four-fold higher compared to the air-water interface. Under these conditions, the concentration profiles of the adsorbed layers for both proteins indicate multilayer adsorption: The thickness of the outer layer (oil-side) is close to the dimension of the minor axis of the protein molecule, ~ 30 {\AA}, suggesting a side-way orientation with the long axis parallel to the interface. The inner layer extends to 55-60 {\AA}. Interestingly, in all cases, the composition of oil film remains intact without significant protein penetration into the film. The optimal adsorption on these nanofilms, 1.7-2.0 mg$\bullet$m-2 , is comparable to the results obtained recently on thick solid cross-linked films using quartz crystal microbalance and atomic force microscopy, showing in particular that adsorption at these ICO film interfaces under standard physiological conditions is non-specific. These results furnish useful information towards the elaboration of vegetable oil-based nanofilms, in direct nanoscale applications or as precursor films in the fabrication of thicker macroscopic films for biomedical applications. 2, Comment: Langmuir, 2019

The behavior of compound 2 [1,3,5-tri(2,2′-bipyridin-5-yl)benzene] with three bipyridine units arranged in a star geometry is investigated in the presence and absence of Ni(ClO4)2. Its properties at the air–water interface as well as after transfer onto a solid substrate are studied by several techniques including Brewster angle microscopy, X-ray reflectivity, neutron reflectivity, X-ray photoelectron spectroscopy, Rutherford backscattering spectrometry, and atomic force microscopy combined with optical microscopy. It is found that compound 2 within the monolayers formed stays almost vertical at the interface and that at high Ni2+/2 (Ni2+/2 = 4000, 20′000) ratios two of the three bipyridine units of 2 are complexed, resulting in supramolecular sheets that are likely composed of arrays of linear metal–organic complexation polymers.

International audience; Solvent penetration in porous layers was analyzed using dynamical and mechanical characterization. Spreading dynamics of a solvent drop in a porous substrate provided parameters of the porous medium such as permeability and porosity. These measurements are relevant for many porous systems, for example paintings or porous varnishes and resins… We present direct visualizations of the drop as well as of the wet zone during the imbibition process and we evidence three distinct regimes. Experiments performed with various porous systems and different solvents highlight a universal behavior. The mechanical properties during the imbibition process are deduced through indentation testing measurements. We show that solvent penetration is responsible for the appearance of a viscous component in the system. A characteristic time depending on the solvent and on the porous medium is then deduced. The system recovers its initial mechanical properties and no swelling nor cracking is observed contrary to the case of paintings. This result tends to prove that visco-plastic properties are required to observe swelling or cracking.

International audience; During evaporative drying of a colloidal dispersion, the structural behavior at the air–dispersion interface is of particular relevance to the understanding of the consolidation mechanism and the final structural and mechanical properties of the porous media. The drying interface constitutes the region of initial drying stress that, when accumulated over a critical thickness, leads to crack formation. This work presents an experimental study of top-down drying of colloidal silica dispersions with three different sizes (radius 5, 8, and 13 nm). Using specular neutron reflectivity, we focus on the structural evolution at the free drying front of the dispersion with a macroscopic drying surface and demonstrate the existence of a thick concentrated surface layer induced by heterogeneous evaporation. The reflectivity profile contains a strong structure peak due to scattering from particles in the interfacial region, from which the interparticle distance is deduced. A notable advantage of these measurements is the direct extraction of the corresponding dispersion concentration from the critical total reflection edge, providing a straightforward access to a structure-concentration relation during the drying process. The bulk reservoir of this experimental configuration renders it possible to verify the evaporation–diffusion balance to construct the surface layer and also to check reversibility of particle ordering. We follow the structural evolution of this surface layer from a sol to a soft wet-gel that is the precursor of a fragile skin and the onset of significant particle aggregation that precedes formation of the wet-crust. Separate complementary measurements on the structural evolution in the bulk dispersion are also carried out by small-angle neutron scattering, where the particle concentration is also extracted directly from the experimental curves. The two sets of data reveal similar structural evolution with concentration at the interface and in the bulk and an increase in the degree of ordering with the particle size.

Aromatic dithiol linkers were used to prepare aggregates of Au25(SR)18 clusters (SR: thiolate) via ligand exchange reactions. Fractions of different aggregate sizes were separated by size exclusion chromatography (SEC). The aggregates were characterized by UV-vis absorption spectroscopy, matrix assisted laser desorption ionization (MALDI) mass spectrometry, nuclear magnetic resonance (NMR) spectroscopy (including diffusion-ordered spectroscopy, DOSY) and small angle X-ray scattering (SAXS). At a 2 : 1 cluster : dithiol ratio, small aggregates (dimers, trimers) and also larger aggregates consisting of 10-20 Au25 clusters were formed, according to DOSY, besides unreacted (monomeric) Au25(SR)18. MALDI mass spectrometry shows signals consistent with dimers and trimers (doubly charged). The SAXS curves for the small aggregates can be well fitted by a pearl-necklace model. For the bigger aggregates the SAXS curves evidence a characteristic separation distance between the clusters within the aggregates, which is imposed by the length of the linker. The SAXS curves of these larger aggregates can be well fitted with a core-shell sphere model with a sticky hard-sphere structure factor, in agreement with closely packed aggregates. The absorption spectra of smaller aggregates resemble those of individual Au25(SR)18 clusters; however, and most importantly, the larger aggregates show completely different, less structured spectra with a new band emerging at 840 nm. We assign this drastic change in the absorption spectra and the new band to the electronic coupling between the clusters through the all aromatic linker. In accordance with this view, the aggregates formed with a linker containing methylene groups, thus breaking conjugation, do not show the band at 840 nm. By the addition of monothiols to the larger aggregates their size can be reduced through an "unlinking" reaction. This reaction also affects the band at 840 nm, which moves to higher energy when reducing the aggregate size, as would be expected within a particle in a box model. The electronic coupling between the clusters through the linker is the basis for future applications in nanoelectronics.

International audience; Shrinkage cracks are observed in many materials, particularly in paintings where great interest lies in deducing quantitative information on the material with the aim of proposing authentication methods. We present experimental measurements on the crack opening induced by the drying of colloidal layers and compare these results to the case of a pictorial layer. We propose a simple model to predict the crack width as a function of the thickness of the drying layer, based on the balance between the drying stress buildup and the shear frictional stress with the substrate. Key parameters of the model include the mechanical properties that are measured experimentally using micro-indentation testing. A good agreement between theory and experimental data for both colloidal layers and the real painting is found. These results, by comparing the shrinkage cracks in model layers and in pictorial layers, validate the method based on the use of colloidal systems to simulate and to reproduce drying cracks in paintings.

Atomic force microscopy in the pulsed force mode (PFM) is applied in this work to the study of thin dewetting patterns formed by drying an aqueous solution of poly(N-isopropylacrylamide) (PNIPAM) and sodium dodecyl sulfate (SDS) on mica. This technique allows the automated acquisition of typically 4 x 10(6) force-distance curves on the sample surface together with maps showing nanodomains differentiated by their stiffness and adhesion to the tip. Topography images of dry films revealed a morphology formed by droplets distributed on the substrate. Adhesion and stiffness images with good lateral resolution show droplets containing polymer and surfactant contrasting with the substrate and also nanosized heterogeneities inside these droplets. They also revealed very small dewetted structures which could not be observed in the topography map by noncontact AFM. Adhesion interactions between the AFM tip and the polymer or the dewetted mica substrate were measured in terms of adhesion force and detachment energy, and can be used as new information to understand dewetting patterns containing silica particles, PNIPAM, and SDS. Other surface mechanical parameters such as stiffness, maximum indentation, hardness, compliance, hysteresis, and Young's modulus were obtained by sampling many points and used to characterize the PNIPAM/SDS films formed in the dewetting process.

Gold nanoparticles grafted with poly(N-isopropylacrylamide) (PNIPAM) are rendered amphiphilic and thermosensitive. When spread on the surface of water, they form stable Langmuir monolayers that exhibit surface plasmon resonance. Using Langmuir balance and contrast-matched neutron reflectivity, the detailed structural properties of these nanocomposite monolayers are revealed. At low surface coverage, the gold nanoparticles are anchored to the interface by an adsorbed PNIPAM layer that forms a thin and compact pancake structure. Upon isothermal compression (T=20 degrees C), the adsorbed layer thickens with partial desorption of polymer chains to form brush structures. Two distinct polymer conformations thus coexist: an adsorbed conformation that assures stability of the monolayer, and brush structures that dangle in the subphase. An increase in temperature to 30 degrees C results in contractions of both adsorbed and brush layers with a concomitant decrease in interparticle distance, indicating vertical as well as lateral contractions of the graft polymer layer. The reversibility of this thermal response is also shown by the contraction-expansion of the polymer layers in heating-cooling cycles. The structure of the monolayer can thus be tuned by compression and reversibly by temperature. These compression and thermally induced conformational changes are discussed in relation to optical properties.

The effect of hydrogen-bonding complexation on the interfacial behavior of poly(isoprene)-b-poly(ethylene oxide) (PI-b-PEO) diblock copolymer at the air-water interface has been investigated by Langmuir balance and neutron reflectivity. PI-b-PEO forms Langmuir monolayers with PI as the anchoring block. Introduction of a second diblock, poly(isoprene)-b-poly(acrylic acid) (PI-b-PAA) yields PI-b-PEO/PI-b-PAA mixed layers with interfacial behavior that is pH-dependent. At pH 10.0 and 5.7, the compression (pi-A) isotherms exhibit three regions that are characteristic of PEO-type tethered layers, (i) a low-pressure 2-D "pancake" region (region I), (ii) a pseudoplateau where PEO segments desorb and are immerse in the subphase (region II), and (iii) a steep pressure rise region commonly considered as the "brush" regime (region III). At pH 2.5, on the other hand, the pi-A isotherm shows only two regions, (I) and (III). This novel behavior is attributed to hydrogen-bonding complexation between the undissociated carboxylic acids and the PEO, forming very compact layers. It appears that desorption of PEO segments is hindered as a consequence of this complexation. Furthermore, no brush-like structure is observed in region III of the isotherml; thus, the steep rise in surface pressure in this case arises primarily from interactions of the anchoring block. The hydrogen-bonded complex of PI-b-PEO/PI-b-PAA monolayers thus shows enhanced surface stability.

The interfacial behavior of silica nanoparticles in the presence of an amphiphilic polymer poly( N-isopropylacrylamide) (PNIPAM) and an anionic surfactant sodium dodecyl sulfate (SDS) is studied using neutron reflectivity. While the nanoparticles do not show any attraction to hydrophilic and hydrophobic surfaces in pure water, presence of the amphiphilic polymer induces significant adsorption of the nanoparticles at the hydrophobic surface. This interfacial behavior is activated due to interaction of the nanoparticles with PNIPAM, the amphiphilic nature of which leads to strong adsorption at a hydrophobic surface but only weak interaction with a hydrophilic surface. The presence of SDS competes with nanoparticle-PNIPAM interaction and in turn modulates the interfacial properties of the nanoparticles. These adsorption results are discussed in relation to nanoparticle organization templated by dewetting of charged polymer solutions on a solid substrate. Our previous studies showed that nanoparticle assembly can be induced to form complex morphologies produced by dewetting of the polymer solutions, such as a polygonal network and long-chain structures. This approach, however, works on a hydrophilic substrate but not on a hydrophobic substrate. These observations can be explained in part by particle-substrate interactions revealed in the present study.

Monolayer sheets have gained attention due to the unique properties derived from their two-dimensional structure. One of the key challenges in sheet modification/synthesis is to exchange integral parts while keeping them intact. We describe site-to-site transmetalation of Zn(2+) in the netpoints of cm(2)-sized, metal-organic sheets by Fe(2+), Co(2+), and Pb(2+). This novel transformation was done both randomly and at predetermined patterns defined by photolithography to create monolayer sheets composed of different netpoints. All transmetalated sheets are mechanically strong enough to be spanned over 20 × 20 μm(2) sized holes. Density functional theory calculations provide both a model for the molecular structure of an Fe(2+)-based sheet and first insights into how transmetalation proceeds. Such transmetalated sheets with random and patterned netpoints can be considered as two-dimensional analogues of linear copolymers. Their nanoscale synthesis presents an advance in monolayer/polymer chemistry with applications in fields such as surface coating, molecular electronics, device fabrication, imaging, and sensing.

Covalent monolayer sheets in 2 hours: spreading of threefold anthracene-equipped shape-persistent and amphiphilic monomers at the air/water interface followed by a short photochemical treatment provides access to infinitely sized, strictly monolayered, covalent sheets with in-plane elastic modulus in the range of 19 N/m.

Elemental distribution maps of Hevea brasiliensis natural rubber gels have been obtained using electron energy-loss spectroscopy imaging in a low-energy (80 kV) electron spectroscopy imaging transmission electron microscope. Two types of gels have been investigated: a microgel contained within the natural rubber particle, and a macrogel prepared by equilibrating dry natural rubber in toluene. Both types of gels are found to contain a high amount of calcium. The intraparticle microgel is dense and rich in calcium but poor in nitrogen, indicating the predominant role of calcium in cross-link formation. The macroscopic gel is inhomogeneous, with dense calcium-rich microgels interspersed in a matrix of a less dense gel. The significant level of nitrogen associated with the matrix of the less dense gel supports the role of proteinaceous materials in the formation of the macroscopic gel.

The interaction of poly(N-isopropylacrylamide) (PNIPAM) with an anionic (SDS) and three cationic (dodecyl and hexadecyltrimethylammonium bromides, DTAB and CTAB, and dodecylammonium chloride, DAC) surfactants has been studied using isothermal titration calorimetry between 17 and 29 °C. The high sensitivity of this technique allows precise determination of the parameters that characterize polymer−surfactant interactions. Based on these results, it is possible to conclude that the interaction of PNIPAM with ionic surfactants is significantly affected by temperature. This is attributed to changes in polymer hydration, which become more important near the lower critical solution temperature (≈32 °C). Interaction of PNIPAM with SDS is more favorable than with cationic surfactants. Surfactant hydrophobicity is also an important factor for this interaction, with CTAB displaying a more intense interaction than DTAB. The contribution from the size of the headgroup was also investigated: DAC interacts more strongl...

Dewetting of thin films of a volatile aqueous solution of charged polymer is investigated using atomic force microscopy. Spontaneous dewetting of films below 10 nm shows a wide range of morphology ranging from holes, polygonal networks, and bicontinuous structures to droplets, depending on film thickness. These morphologies are also shown to depend on polymer charge density and the rate of dewetting. At high polymer charge density, unusual features are observed, including very long fingers around hole rims and cylindrical structures that resist fragmentation. These features, attributed to viscous effects, are enhanced when the dewetting rate is increased.

The structures of aqueous copolymer solutions have been examined through small angle neutron scattering. The copolymers contained mostly N-isopropylacrylamide (NIPAM) monomers. Poly (NIPAM) solutions have a lower critical solution temperature (LCST), above which the macromolecules separate from water. A small fraction of ionizable N,N-[(dimethylamino) propyl] methacrylamide (MADAP) monomers was introduced into the macromolecules. This had dramatic consequences on the solution behavior at temperatures above the LCST of PNIPAM, where phase separation would have been expected for the homopolymer. When all MADAP monomers were ionized, it was found that the solutions resisted the phase separation. At short spatial scales, the chains were collapsed but at large scales they formed branched aggregates that did not separate out of water. When only half of the MADAP monomers are ionized, the electrical charges were able to redistribute themselves along the chains. In this case, the rise in temperature caused a microphase separation where the electrical charges were relocated on a fraction of the chains that remained in solution.The other chains (or section of chains) formed large nodules of a polymer rich phase.

The behavior of poly (N-isopropylacrylamide) (PNIPAM) at the air–water interface above the lower critical solubility temperature and the effects of sodium dodecyl sulfate (SDS) have been studied by neutron reflectivity. In the absence of SDS, PNIPAM adsorbs at the air–water interface and forms a thick and dense multilayer which can be characterized by two regions: a first region made up of pure polymer, and a second containing the same volume fraction of polymer as in the bulk precipitate. In the presence of SDS, polymer adsorption is decreased, and the structure of the adsorbed polymer can be described as a layer of colloidal particles of collapsed PNIPAM stabilized by SDS molecules. At higher SDS concentrations where the PNIPAM is completely resolubilized into charged chains, most, but not all, of the polymer is displaced from the surface, and the remaining adsorbed polymer forms a thin and dilute layer.

International audience; In plant cell walls, lignins are closely associated with hydrophilic polysaccharides. In these associations, the interface between the two polymers may have a strong influence on the molecular architecture of plant cell walls. To model such hydrophilic/hydrophobic and anisotropic media, two DHPs (dehydrogenation polymer models of lignins, one guaiacyl type (G) and one mixed guaiacyl/syringyl type (GS)) were spread at the air/water interface. The investigation of these two DHP layers at the air/water interface by neutron reflectivity has demonstrated that both DHPs form a nonhomogeneous structure with DHP distribution going from a concentrate structure at the air side to a dilute one at the water side. When the amount of spread polymer is increased, the GS DHP layer shows a regular increase of the DHP volume fraction whereas the G; DHP forms steeper density profile structures, implying that molecular structural differences might be involved in these different behaviors.

Poly(N-isopropylacrylamide) (PNIPAM) precipitates out of water around 32 °C. This critical temperature is raised when hydrophilic acrylamide sequences are present on the polymer chain. We have used neutron scattering to study the structural properties of a statistical copolymer containing acrylamide and N-isopropylacrylamide segments at different temperatures and its interactions with an anionic surfactant, sodium dodecyl sulfate (SDS). At low temperatures, the copolymer behaves as a swollen polymer coil. With an increase in temperature, intermolecular attractions are observed, and close to the critical temperature of the copolymer, microphase separation is observed. Here, the structure consists of dense nodules of hydrophobic sequences stabilized by hydrophilic sequences. In the presence of a small amount of SDS, additional colloidal stability is observed: the nodule size is decreased. At high SDS concentration, the copolymer is completely solubilized at all temperatures studied and the structure of the polymer–surfactant complex resembles the “necklace” structure obtained for the homopolymer PNIPAM–SDS system.

The effects of sodium dodecyl sulfate (SDS) on the adsorption behavior of poly(N-isopropylacrylamide) (PNIPAM) at the air−water interface have been studied using neutron reflectivity and surface tension measurement. PNIPAM adsorbs strongly at the air−water interface due to its partially hydrophobic nature. It is also a thermosensitive polymer; thus a variation in temperature can modify its adsorption behavior. In the presence of SDS below the critical aggregation concentration (CAC), the polymer adsorption is unaffected. Above the CAC however, the polymer is progressively displaced from the surface. This loss of polymer from the surface layer can be explained by the complexation of the polymer with the surfactant in solution and an equilibrium of the charged complex at the surface and in the bulk phase. Furthermore, the thermal response of the polymer adsorption is attenuated and shifted to higher temperatures. This effect is equivalent to the increased solubilization of the polymer chain by charged micel...

The interaction between a microbial lipase and an anionic and a cationic surfactant at the air−water interface has been studied by neutron reflectivity. Sodium dodecyl sulfate (SDS) and tetradecyltrimethylammonium bromide (TTAB) were used as anionic and cationic surfactant, respectively. The same enzyme−surfactant systems were also studied at the interface between a hydrophobic solid surface and water by ellipsometry, and the results from the two techniques were compared. Surface tension measurements were also performed in order to monitor complex formation in bulk. The data obtained from neutron reflectivity and from ellipsometry were in very good agreement with each other. Both techniques show that lipase adsorbs readily at the interfaces and that SDS at low concentration does not interact strongly with this protein layer. At higher SDS concentration, the protein is displaced from the surface. On the other hand, when TTAB is added at low concentration, a thick lipase−surfactant layer is formed at the su...

Recent studies of bulk and interfacial properties of polymer–surfactant systems using neutron scattering and neutron reflectivity are presented, with some discussions on a few selected systems. In bulk, the principal interests are centred on thermosensitive and hydrophobically modified associative polymers, where structural information has been used to interpret the effects of surfactants on the solubilization behavior, phase separation and gelation processes of these polymers. Conversely, the effects of polymers anchored in surfactant layers and membranes and the resulting phase changes in microemulsion systems have also received much interest. At the interface, information obtained on the structure and composition of mixed polymer–surfactant layers is discussed in relation to the surface tension and stability of these layers.

Poly(N-isopropylacrylamide) macromolecules (PNIPAM) dissolve in water at temperatures below 33 °C; at higher temperatures, due to hydrophobic interactions, they precipitate out of solution. Addition of a small quantity of charged surfactant resolubilizes the precipitated polymer. We have used small angle neutron scattering to determine the structures of aqueous solutions of PNIPAM and sodium dodecyl sulfate (SDS). At temperatures above the precipitation temperature of the pure polymer solution, two different structures have been found, depending on the ratio of surfactant to polymer. At high ratios, SDS micelles bind to individual macromolecules and pull them into solution; the distance between consecutive micelles in a “necklace” is found to remain constant at around 60 A. At low ratios, the macromolecules collapse to form small colloidal particles rather than forming necklaces with micelles spaced further apart. The collapse transition is abrupt in the sense that no intermediate states between the stret...

We have used specular neutron reflectivity to study adsorbed layers of a modified polysaccharide bearing lateral cholesterol anchors (cholesterylpullulan, CHP) at the air−water interface. In this system, the otherwise non surface active polysaccharide is attached, at several points along the backbone, to the surface by the hydrophobic cholesterol groups. The properties of these adsorbed polymer layers have been studied for different degrees of cholesterol substitution varying from 0.6 to 1.4 mol % and for different bulk concentrations. Using a parabolic profile to describe the adsorbed layer, it is found that the thickness of the layer decreases with surface concentration. These results are in contrast to those reported for end-attached tethered polymer layers. We attribute this behavior to the associative properties of the polymer as the interacting cholesterol groups are increased in the layer. Furthermore, the amount of polymer adsorbed decreases with the degree of cholesterol substitution. Surface ten...

Using specular neutron reflectivity, we have measured the structure of tethered polymer layers immersed in a solution of mobile chains of the same chemical composition. The tethered polymer layer is a stable Langmuir film of a diblock copolymer spread on the air—liquid interface of a selective solvent. Due to the restricted conformational freedom of the grafted chain, a depletion layer at the grafting surface is expected. In our system, this depletion zone is enhanced and thus observable experimentally due to a repulsion between the polymer and the surface. The structure of the tethered layer is adequately described by a parabolic profile followed by an exponential tail, with a depletion layer at the surface. In the presence of mobile chains in solution, the grafted polymer layer is compressed: the thickness decreases and the monomer volume fraction increases. This compression may result from a re-equilibration of the osmotic pressure of the polymer layer, or from a screening of the monomer excluded volume interaction due to penetration of the mobile chains into the grafted layer. In this paper, we present our experimental data on the effects of mobile polymer concentration and chain length on the structure of the tethered chain. From these data, we construct a phase diagram to identify the regions of interpenetration of the mobile chains into the grafted layer.

International audience; The optical properties of a series of gold nanoparticles (D ≈ 2.3−8 nm) grafted with a stabilizing polymer with a wide range of chain lengths (Mw ≈ 6.5K−29.5K) have been studied quantitatively in different dielectric solvents. Mie−Drude dipolar theory was applied to model the localized surface plasmon resonance (SPR) peak position, as well as the peak width of the absorption spectra, using the dielectric function of gold. The modeled spectra yielded information on the dielectric function of the polymer shell, ϵs, from which the average polymer concentration in the shell was deduced. Combining information from optical modeling and structural properties obtained from small-angle neutron scattering (SANS) on the polymer shell thickness and from transmission electron microscopy (TEM) on the gold core size, the SPR peak shifts and their attenuated sensitivity to solvent refractive index were characterized. The SPR behaviors for all of the gold colloids with different core sizes and graft chain lengths were thus expressed as a function of the effective polymer volume fraction, p,of the composite nanoparticle. It was found that sensitivity to the solvent dielectric property decreased until, for p > 0.9 (corresponding to an effective shell thickness on the order of the core radius), the SPR mode was "frozen" in by the polymer shell and lost sensitivity to the solvent.

Langmuir-Blodgett films of mesomorphic side-chain poly(methacrylates) are investigated from the point of view of their phase change versus temperature via X-ray and neutron reflectivity together with atomic force microscopy measurements. Upon heating, «thick» films undergo an irreversible transition toward the wellknown smectic Ar lamellar phase. In contrast, a bilayer deposited onto a solid substrate appears to be very stable upon annealing. Such a behavior is believed to induce an epitaxial mechanism when cooling «thick» films from the isotropic melt phase

Langmuir-Blodgett films of mesomorphic side-chain polymethacrylates are investigated from the point of view of their phase change versus temperature via X-ray and neutron reflectivity. Upon heating, “thick” films undergo an irreversible transition towards the well known smectic Al lamellar phase. In contrast a bilayer deposited onto a solid substrate appears to be very stable upon annealing. Such a behaviour is believed to induce an epitaxial mechanism when cooling “thick” Rims from the isotropic melt phase.

We report observations of chain stretching in diblock copolymer monolayers on the surface of a selective solvent. Using neutron reflectivity, we have studied the concentration profile of the submerged block over a large range of surface density \ensuremath{\sigma} (chains per area) for two different molecular weights. The observed increase in the layer thickness is weaker than the ${\mathrm{\ensuremath{\sigma}}}^{1/3}$ prediction of mean-field and scaling theories for the limiting behavior, but is in agreement with recent numerical self-consistent-field calculations by Whitmore and Noolandi [Macromolecules 23, 3321 (1990)].

Simultaneous measurements of the concentration profile and the free energy of the submerged PS blocks of PDMS-PS diblock copolymers spread as monolayers at the air surface of ethyl benzoate are carried out as a function of surface concentration. A parabolic concentration profile is found to give the best fit to neutron reflectivity curves among the set of two-parameter models examined. The maximum extension h * /R g is comparable to previously reported values obtained from a variety of techniques for similar values of the reduced surface density σ * and indicates some stretching perpendicular to the surface

Thin liquid foam films stabilized with the addition of a temperature-sensitive neutral polymer, poly(N-isopropylacrylamide) (PNIPAM), in the presence of varying amounts of an anionic surfactant, SDS, were investigated using the thin-film balance. Data were analyzed in light of new and previously reported neutron reflectivity data describing the adsorption behavior of PNIPAM/SDS mixtures at the single air-water interface. In the case of thin films prepared from pure polymer solutions, unexpectedly thick and stable films were observed, and increases in solution temperature or polymer molecular weight induce corresponding increases in the thin-film thickness. This behavior is attributed to a stabilization of the films by repulsive entropic forces between long dangling polymer tails adsorbed at the interfaces. The addition of SDS causes a striking decrease in the thin-film thickness that is very well correlated with the desorption of polymer chains from the interface resulting from polymer-surfactant interactions in the bulk. Upon SDS introduction, repulsive forces stabilizing the films vary from purely entropic forces for small amounts of surfactant to supramolecular oscillatory forces at high SDS concentrations, whereas a combination of electrostatic and entropic forces is expected in intermediate concentration regions. Properties of thin films made from PNIPAM and SDS are thus highly sensitive to parameters such as the polymer-surfactant ratio, temperature, and molecular weight, offering precise control of the thin-film thickness that could be used to tailor foam characteristics.

The interfaces between water and alkanes in the presence of a monolayer of nonionic surfactant (polyethylene glycol alkyl ether) have been studied as a function of temperature for surfactants of different chain lengths. At low temperatures and moderate surfactant concentration, the aqueous phase is an oil in water microemulsion; at high temperatures, the oil phase is a water in oil microemulsion. In our study, the middle of the studied temperature range corresponds to the inversion temperature where a middle phase microemulsion coexists with both oil and water. — The interfacial tensions, which are very low, have been measured with surface light scattering and the interfacial film roughness with ellipsometry. From these data, the bending elastic constant of the surfactant monolayer is determined. The calculated persistence length of the monolayer is compared to the characteristic size in the middle phase microemulsion measured with neutron scattering.

Dewetting of thin films of charged polymer solutions produces complex patterns that can be applied to direct nanoparticle organization on solid substrates. The morphology produced by dewetting can be controlled by the solution properties, temperature, and substrate wetting. In this work, new results on this liquid-template self-assembly system are presented, with special emphasis on producing large arrays of organized nanoparticles. On a hydrophilic substrate with complete wetting, the patterns include polygonal networks and parallel-track arrays that extend over several hundreds of microns. These large structures are formed under well-controlled drying conditions and characterized by scanning electron microscopy, which is better suited for the examination of large as well as small areas than atomic force microscopy. On partial wetting substrates, new patterns are observed, including a complex set of parallel curved bands with variable particle number densities.

The gas permeabilities of thin liquid films stabilized by poly(N-isopropylacrylamide) (PNIPAM) and PNIPAM-SDS (sodium dodecyl sulfate) mixtures are studied using the "diminishing bubble" method. The method consists of forming a microbubble on the surface of the polymer solution and measuring the shrinking rates of the bubble and the bubble film as the gas diffuses from the interior to the exterior of the bubble. PNIPAM-stabilized films exhibit variable thicknesses and homogeneities. Interestingly, despite these variable features, the gas permeability of the film is determined principally by the structure of the adsorbed polymer layer that provides an efficient gas barrier with a value of gas permeability coefficient that is comparable to that of an SDS Newton black film. In the presence of SDS, both the film homogeneity and the gas permeability coefficient increase. These changes are related to interactions of PNIPAM with SDS in the solution and at the interface, where coadsorption of the two species forms mixed layers that are stable but that are more porous to gas transfer. The mixed PNIPAM-SDS layers, studied previously for a single water-air interface by neutron reflectivity, are further characterized here in a vertical free-draining film using X-ray reflectivity.

Two polymer-surfactant mixtures have been studied at the air-water interface using neutron reflectivity and surface tension techniques. For the noninteracting system poly(N-isopropylacrylamide) (PNIPAM)/octaethyleneglycol mono n-decyl ether (C10E8), the adsorption behavior is competitive and driven purely by surface pressure (pi). When pi(polymer) > pi(surfactant), the surface layer consists of almost pure polymer, and for pi(polymer) < pi(surfactant), the polymer is displaced from the surface by the increasing pressure of the surfactant. Beyond the CMC, the polymer is completely displaced from the surface. For the interacting system PNIPAM/sodium dodecyl sulfate (SDS) where the two species interact strongly in the bulk beyond the critical aggregation concentration (CAC), the surface behavior is more original. Earlier neutron reflectivity studies investigated PNIPAM adsorption behavior where the SDS was contrast-matched to the solvent. In the present study, complementary measurements of SDS adsorption where PNIPAM is contrast-matched to the solvent give a complete view of the surface composition of the mixed system. At a constant polymer concentration, with increasing SDS, three main regimes are obtained. For C(SDS) < CAC, adsorption is governed by simple competition and PNIPAM is predominant at the interface. At intermediate SDS concentration (CAC < C(SDS) < x2, where x2 indicates the predominance of free SDS micelles), interfacial behavior is governed by bulk polymer-surfactant interaction. Adsorbed polymer is displaced from the interface to form PNIPAM-SDS complex in the bulk. SDS adsorption remains weak since most of the SDS molecules are used to form bulk polymer-surfactant aggregates. Further increase in SDS concentration results in continued displacement of PNIPAM and an abrupt increase in SDS adsorption. This is a result of saturation of bulk polymer chain with adsorbed micelles. Interestingly, beyond x2, PNIPAM is not completely displaced from the surface. A mixed PNIPAM-SDS adsorbed layer with enhanced packing of the SDS monolayer is formed.

In this paper, we present an alternative approach for controlled nanoparticle organization on a solid substrate by applying dewetting patterns of charged polymer solutions as a templating system. Thin films of charged polymer solutions dewet a solid substrate to form complex dewetting patterns that depend on the polymer charge density. These patterns, ranging from polygonal networks to elongated structures that are stabilized by viscous forces during dewetting, serve as potential templates for two-dimensional nanoparticle organization on a solid substrate. Thus, while nanoparticles dried in pure water undergo self-assembly to form close-packed arrays, addition of charged polymer in the dispersion leads to the formation of open structures that are directed by the dewetting patterns of the polymer solution. In this study, we focus on the application of elongated structures resulting from dewetting of high-charge-density polymer solutions to align nanoparticles of silica and gold into long chains that are several micrometers in length. The particle ordering process is a two-step mechanism: an initial confinement of the nanoparticles in the dewetting structures and self-assembly of the particles within these structures upon further drying by lateral capillary attractions.

In this paper, the effects of drying and aging of natural rubber particles of Hevea brasiliensis are studied. The evolutions of the particle morphology and of the elemental distribution are investigated using electron-energy-loss spectroscopy imaging in a low-energy transmission electron microscope (ESI-TEM). It is found that when the sample is aged, calcium salt crystallites are formed around the particles. Fusion of these crystallites with time to form larger crystals shows evidence of ion mobility in the dry rubber matrix. Electron diffraction patterns and elemental mapping analysis indicate the crystals to be calcium sulfate. These crystallites are closely associated with membrane materials of the rubber particle and are surprisingly compatible with the hydrocarbon matrix of the rubber particle. It is proposed that polar sites on the membrane materials provide nucleation sites for the crystallization of calcium and sulfate ions from the latex serum.

When a dilute dispersion of relatively monodisperse silica particles is dried on a solid substrate, close-packed arrays are formed. The ordering of the particles results from attractive capillary forces between neighboring particles, and convective transport of particles to the ordered domain during drying. In the presence of adsorbed polymer layers on the particles, the close-packed arrays are disrupted and the particles are disperse upon drying. This is due to steric repulsions of adsorbed polymer layers that prevent close contact of particles during drying, decreasing the probability for formation of the nucleus phase for close-packed ordering. Using electron spectroscopy imaging in a transmission electron microscope, we directly observed the adsorbed polymer layers on the dry dispersion. The thickness of the adsorbed layer and its effect on particle dispersion are investigated as a function of polymer concentration, chain length and solvent quality.

Cream and skim fractions of freshly tapped natural rubber latex have been studied using atomic force microscopy and scanning electric potential microscopy to elucidate the topology and charge properties in film formation. Elemental distribution maps of the particles have also been obtained using electron energy-loss imaging in a low-energy transmission electron microscope. The two rubber fractions are obtained by centrifugation. The cream fraction is stable while rapid coagulation occurs in the skim fraction. After removal of the coagulum, no further coagulation occurs and the remaining skim rubber particles are stable. The rubber particles from the cream rubber particles contain higher amount of adsorbed protein–phospholipid materials compared to those in the “self-cleaned” skim fraction. This difference in membrane property has a significant impact on the spreading of the cis -1,4-polyisoprene cores, their coalescence and film formation behavior. Coalescence of cream particles appears to be hindered by the membrane materials, forming a rough film that retains the topology of individual particles. Skim particles coalesce more readily, forming relatively smooth films.

The interaction between a charged surfactant and a lipase has been investigated by several methods. Interactions in aqueous bulk phase was studied by NMR and by microcalorimetry. Surface tension and neutron reflectivity were used for studies at the air–water interface. Interactions at the interface between a hydrophobic solid surface and water was investigated by ellipsometry. The results obtained are as follows. The cationic surfactant, tetradecyltrimethylammonium bromide (iodide in the NMR experiments), showed strong interaction at the air–water and the hydrophobic solid–water interfaces but no clear indication of an interaction in bulk phase was seen. The anionic surfactant showed no interaction with the lipase neither at the interfaces, nor in bulk. The difference in behavior of the system cationic surfactant–lipase in bulk and at the interfaces may be due to the change in enzyme conformation that is known to occur at interfaces between water and an apolar phase.