Jean-Paul Chapel

Interfacial Activity of Phosphonated-PEG Functionalized Cerium Oxide Nanoparticles

In a recent publication, we have highlighted the potential of phosphonic acid terminated PEG oligomers to functionalize strong UV absorption cerium oxide nanoparticles, which yield suspensions that are stable in aqueous or organic solvents and are redispersible in different solvents after freeze-drying. In the present work, we highlight the interfacial activity of the functional ceria nanoparticles and their potential to modify hydrophobic surfaces. We first investigated the phosphonated-PEG amphiphilic oligomers behavior as strong surface active species forming irreversibly adsorbed layers. We then show that the oligomers interfacial properties translate to the functional nanoparticles. In particular, the addition of a small fraction of phosphonated-PEG oligomers with an extra C16 aliphatic chain (stickers) into the formulation enabled the tuning of (i) the nanoparticles adsorption at the air/water, polystyrene/water, oil/water interfaces and (ii) the particle/particle interaction in aqueous solutions. We also found that dense and closely packed two-dimensional monolayers of nanoceria can be formed by spontaneous adsorption or surface compression using a Langmuir trough. A hexagonal organization controlled by reversible and repulsive interaction has been characterized by GISAXS. Mono- or multilayers can also be stably formed or transferred on solid surfaces. Our results are key features in the field of polymer surface modification, solid-stabilized emulsions (Pickering), or supracolloidal assemblies.

Plasma-Treated Superhydrophobic Polyethylene Surfaces: Fabrication, Wetting and Dewetting Properties

Superhydrophobic surfaces correspond to hydrophobic surfaces whose water contact angle is higher than 150°. Two new routes involving plasma technique are proposed to prepare such surfaces: the first one is a one-step process (CF4 plasma modification of low density polyethylene), and the second involves two steps (O2 plasma treatment followed by CF4 plasma treatment). The plasma parameters are defined to optimize the degradation and/or the functionalization. The O2 plasma allows to create a variable roughness while CF4 plasma accentuates this roughness and creates a non-polar layer. With the two-step treatment, plasma parameters were found which produced superhydrophobic surfaces with a controlled roughness and whose chemical structure was close to a Teflon-like structure. The wetting and dewetting of these superhydrophobic surfaces are compared to those of only hydrophobic polyethylene. The behavior of surfaces, whether dry or prewetted with water vapor, is found to be different. The dewetting of the dried surface previously prewetted is found to be discontinuous, and slower than that of the dry one. This specific behavior is interpreted as a roughness effect on trapped water. However, its dewetting is still faster than that of the corresponding hydrophobic surface such as PTFE.

Nanoscale morphology of melt crystallized polyethylene/graphite (HOPG) interphase

High density polyethylene (PE) was crystallised from the melt on freshly cleaved surface of highly oriented pyrolitic graphite (HOPG) or mica. Atomic force microscopy (AFM) studies of structure of the polymer surface adjacent to the graphite or mica were performed after peeling of from the substrate. Significant differences of crystalline structure on the interface were found between PE crystallised on graphite and mica. The surface of polyethylene crystallised on graphite shows large areas with regularly arranged rectangular structures. These objects (ca 20-80 nm big) probably represent the nucleation centres of the lamellar growth. The surface of polyethylene crystallised at mica surface shows some dot-like structures showing no particular arrangement.

Synthesis and grafting ability of silane-terminated polyethylenes on silica surfaces

Chlorosilane-terminated polyethylenes having different molar masses are prepared to be used as connecting molecules between a silica surface and a neat polyethylene matrix. The synthesis and the grafting ability of such functionalized polyethylenes is reported. The grafting efficiency is demonstrated by means of 29 Si NMR and wetting measurements according to the hydrophobic nature of the grafted molecules in comparison with the hydrophilic character of the silica surface. The resulting thickness and refractive index of the dry grafted layers are characterized using spectroscopic ellipsometry. The grafting ratios are given from microanalysis measurements. Surface topography is observed using AFM. All of these techniques are in a good agreement and a general trend of the organization of the tethered layers at the surface is proposed.

Preparation of well-structured organosilane layers on silica

An efficient grafting process of monofunctional alkylchlorosilanes (general formula: CH3-(CH2) n-1-Si(CH3)2Cl with n varying from 4 to 30) onto silica nanoparticules was developed by varying the surface preparation and the solvent used for the deposition process. A vapor phase deposition method was considered as reference and silicon wafers with a native SiO2 layer were used as a model surface of the silica particles. The grafting method was evaluated by studying the wettability and the grafting densities of the resulting monolayers. The chain conformation of the monolayers was determined by comparing the thickness measured by SE ellipsometry and AFM. By comparing the solvent and vapor phase deposition methods, it was demonstrated that the deposition process had a large influence on the structure of the grafted monolayers. The same structure as from a vapor phase method can be obtained from a solvent deposition process by a suitable choice of the solvent and by a strict cleaning of the surface before deposition. The grafting of much longer chains of such silane-terminated polyethylenes with different molar mass on the silica surface was also investigated in order to study the effects of the chain length on the grafting density and the layer structure. For both the short alkylchlorosilanes and polymeric grafted chains, the proposed organization of the grafted chains at the silica surface is found to be strongly dependent on the length of the alkyl chains.

Surfactant-triggered disassembly of electrostatic complexes probed at optical and quartz crystal microbalance length scales.

A critical advantage of electrostatic assemblies over covalent and crystalline bound materials is that associated structures can be disassembled into their original constituents. Nanoscale devices designed for the controlled release of functional molecules already exploit this property. To bring some insight into the mechanisms of disassembly and release, we study the disruption of molecular electrostatics-based interactions via competitive binding with ionic surfactants. To this aim, free-standing micrometer-size wires were synthesized using oppositely charged poly(diallyldimethylammonium chloride) and poly(acrylic acid) coated iron oxide nanoparticles. The disassembly is induced by the addition of sodium dodecyl sulfates that complex preferentially the positive polymers. The process is investigated at two different length scales: the length scale of the particles (10 nm) through the quartz crystal microbalance technique and that of the wires (>1 μm) via optical microscopy. Upon surfactant addition, the disassembly is initiated at the surface of the wires by the release of nanoparticles and by the swelling of the structure. In a second step, erosion involving larger pieces takes over and culminates in the complete dissolution of the wires, confirming the hypothesis of a surface-type swelling and erosion process.

Importance of the Surface Layer for Polymer Materials

The surface layer of polymers exhibits different composition, morphol ogy and structure, also in the nanoscale, from the bulk material. The paper demonstrates the clearest examples, discussing consequences of the gradient nature of the materials from the aspect of their exploitation (mainly hardness profile and friction). The ability of low molecular weight substances to migrate in a polymer matrix and the surface segregation occurring in polymer blends, are studied by FTIR, AFM, XPS, microindentation, microfriction, contact angle and DSC techni ques. It has been proved that the surface layer of polymer materials can be shaped during both compounding and process ing.