Viscoelasticity-Induced Onset of Slip at the Wall for Polymer Fluids.

The progressive onset of slip at the wall, which corresponds to a slip length increasing with the solicitation time before reaching a plateau, has been investigated for model viscoelastic polymer solutions, allowing one to vary the longest relaxation time while keeping constant solid-fluid interactions. A hydrodynamic model based on a Maxwell fluid and the classical Navier's hypothesis of a linear response for the friction stress at the interface fully accounts for the data. In the limit of the linear viscoelasticity of the fluid, we could postulate a Newtonian response for the interfacial friction coefficient, reflecting the local character of solid-liquid friction mechanisms. Deviations between the experiments and our model are observed when the fluid is far from its linear viscoelastic behavior.

Nanorheology with a Conventional Rheometer: Probing the Interfacial Properties in Compatibilized Multinanolayer Polymer Films.

Measuring the viscoelastic behavior of polymers in the vicinity of a surface or under confinement is an experimental challenge. Simple rheological tests of nanolayered films of polyethylene/polyamide 6 compatibilized in situ during the coextrusion process enabled the probing of these interfacial properties. Taking advantage of the different melting points and of the multiplication of the number of interfaces, a drastic increase of dynamic moduli was reported when increasing the interphase volume fraction in the films. A solid-like behavior for the interphase was identified. The complex viscosity of nanolayered films as a function of angular frequency was quantitatively captured for all samples using a weighted mixing law of bulk and interphase viscosities, without additional adjusting parameters, highlighting the interfacial synergy developed in nanolayered polymer films.

Temperature-Controlled Slip of Polymer Melts on Ideal Substrates.

The temperature dependence of the hydrodynamic boundary condition between a polydimethylsiloxane melt and two different nonattractive surfaces made of either an octadecyltrichlorosilane self-assembled monolayer or a grafted layer of short polydimethylsiloxane chains has been characterized. We observe a slip length proportional to the fluid viscosity. The temperature dependence is deeply influenced by the surfaces. The viscous stress exerted by the polymer liquid on the surface is observed to follow exactly the same temperature dependences as the friction stress of a cross-linked elastomer sliding on the same surfaces. Far above the glass transition temperature, these observations are rationalized in the framework of a molecular model based on activation energies: increase or decrease of the slip length with increasing temperatures can be observed depending on how the activation energy of the bulk viscosity compares to that of the interfacial Navier's friction coefficient.

Sensing adsorption kinetics through slip velocity measurements of polymer melts.

The evolution over time of the nonlinear slip behavior of a polydimethylsiloxane (PDMS) polymer melt on a weakly adsorbing surface made of short non-entangled PDMS chains densely end-grafted to the surface of a fused silica prism has been measured. The critical shear rate at which the melt enters the nonlinear slip regime has been shown to increase with time. The adsorption kinetics of the melt on the same surface has been determined independently using ellipsometry. We show that the evolution of slip can be explained by the slow adsorption of melt chains using the Brochard-de Gennes's model.

Friction of Polymers: from PDMS Melts to PDMS Elastomers.

The slip behavior of polydimethylsiloxane (PDMS) polymer melts flowing on weakly adsorbing surfaces made of short nonentangled PDMS chains densely end-grafted to silica has been characterized. For high enough shear rates, slip lengths proportional to the bulk fluid viscosity have been observed, in agreement with Navier's interfacial equation and demonstrating that the interfacial Navier's friction coefficient is a local quantity, independent of the polymer molecular weight. Comparing the interfacial shear stresses deduced from these measured slip lengths to available friction stress measured for cross-linked PDMS elastomers, we further demonstrate the local character of the friction coefficient and compare its value to the monomer-monomer friction.

Influence of grafting on the glass transition temperature of PS thin films.

We present an investigation of the effect of the interaction between a thin polystyrene film and its supporting substrate on its glass transition temperature ([Formula: see text]). We modulate this interaction by depositing the film on end-tethered polystyrene grafted layers of controlled molecular parameters. By comparing [Formula: see text] measurements versus film thickness for films deposited on different grafted layers and films deposited directly on a silicon substrate, we can conclude that there is no important effect of the film-subtrate interaction. Our interpretation of these results is that local orientation and dynamic effects substantial enough to influence [Formula: see text] cannot readily be obtained by grafting prepolymerized chains to a surface, due to intrinsic limitation of the surface grafting density.

Effect of surface elasticity on the rheology of nanometric liquids.

The rheological properties of liquids confined to nanometer scales are important in many physical situations. In this Letter, we demonstrate that the long-range elastic deformation of the confining surfaces must be taken into account when considering the rheology of nanometric liquids. In the case of a squeeze-flow geometry, we show that below a critical distance D(c), the liquid is clamped by its viscosity and its intrinsic properties cannot be disentangled from the global system response. Using nanorheology experiments, we demonstrate that picometer elastic deflections of the rigid confining surfaces dominate the overall mechanical response of nanometric liquids confined between solid walls.

Hydrodynamic interaction between a spherical particle and an elastic surface: a gentle probe for soft thin Films.

We study the hydrodynamic interaction between a sphere and an elastic surface at a nanoscale with a dynamic surface force apparatus. We show that the interplay between viscous forces and elastic deformations leads to very rich scaling properties of the force response, providing a unique signature of the surface elastic behavior. These properties are illustrated on three different examples: a thick elastomer, a thin elastomer film, and a layer of micrometric bubbles. We show that this fluid probing allows one to measure the Young's modulus of surfaces and soft thin layers at distance, without any direct solid-solid contact.

Sliding friction at soft micropatterned elastomer interfaces.

In this paper, we present an experimental study of the friction between a smooth elastomer lens and an elastomer substrate micropatterned with hexagonal arrays of cylindrical pillars. Depending on the normal load, the surfaces can be in top or mixed contact. The friction force can be interpreted in terms of friction stresses in the full contact and top contact zones. The latter is higher than that on smooth surfaces evidencing the role of the elastic deformations of the surfaces in the dissipation processes.

Click chemistry grafting of poly(ethylene glycol) brushes to alkyne-functionalized pseudobrushes.

A versatile method for the grafting of azide-terminated polymer chains to alkyne-functionalized pseudobrushes by the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition has been developed. First, poly[(propargyl methacrylate)-r-(glycidyl methacrylate)-r-(methyl methacrylate)] random copolymers with monomer ratios of respectively 27/27/46, 41/31/28, and 45/55/0 were synthesized by RAFT polymerization. Then, dense alkyne-functionalized pseudobrushes were grafted in melt by thermal ring-opening of the glycidyl groups by the silanols from the silicon substrate. Finally, the grafting of tailor-made alpha-methoxy-omega-azido-poly(ethylene glycol)s (M(w) approximately 5000, 20,000, and 50,000 g/mol) by Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition was performed in sealed reactors at 60 degrees C for 72 h using a polymer weight fraction of 10% in tetrahydrofuran and Cu(PPh(3))(3)Br/DIPEA as the catalytic system. Alkyne-functionalized pseudobrushes and poly(ethylene glycol) brushes were characterized by ellipsometry, scanning probe microscopy, and water contact angle measurements. This "grafting-to" approach represents a fast and versatile method to provide thick and homogeneous polymer brushes with a high surface coverage. A major benefit of this strategy is the tunable and versatile tethering of alkyne functionalities to silicon substrates using a straightforward spin-coating procedure.

Contact angle and contact angle hysteresis measurements using the capillary bridge technique.

A new experimental technique is proposed to easily measure both advancing and receding contact angles of a liquid on a solid surface, with unprecedented accuracy. The technique is based on the analysis of the evolution of a capillary bridge formed between a liquid bath and a solid surface (which needs to be spherical) when the distance between the surface and the liquid bath is slowly varied. The feasibility of the technique is demonstrated using a low-energy perfluorinated surface with two different test liquids (water and hexadecane). A detailed description of both experimental procedures and computational modeling are given, allowing one to determine contact angle values. It is shown that the origin of the high accuracy of this technique relies on the fact that the contact angles are automatically averaged over the whole periphery of the contact. This method appears to be particularly adapted to the characterization of surfaces with very low contact angle hysteresis.

Capillary bridge formation and breakage: a test to characterize antiadhesive surfaces.

In order to characterize very weak adhesive surfaces, we have developed a quantitative test inspired by the Johnson, Kendall, and Roberts adhesion test for soft adhesives, which relies on the formation and then the rupture of a capillary bridge between the surface to be tested and a liquid bath. Both the shape and the kinetics of breakage of the capillary bridge for various coatings put into contact with liquids of various viscosities and surface tensions have been studied. Several pull off regimes can be distinguished. For low pull off velocities, a quasi-static regime is observed, well described by capillary equations and sensitive to the hysteresis of the contact angle of the fluid on the coating. Above a critical pull off velocity that depends on the fluid viscosity, a dynamic regime is observed, characterized by the formation of a flat pancake of fluid on the coating that recedes more slowly than the capillary bridge itself. After the breakage of the capillary bridge, a small drop can remain attached to the surface. The volume of this drop depends on the dynamical regime and is strongly affected by very small differences between the coatings. The aptitude of this test in characterizing very weakly adhesive surfaces is exemplified by a comparison between three different perfluorinated coatings.

Polymer brushes grafted to "passivated" silicon substrates using click chemistry.

We present herein a versatile method for grafting polymer brushes to passivated silicon surfaces based on the Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition (click chemistry) of omega-azido polymers and alkynyl-functionalized silicon substrates. First, the "passivation" of the silicon substrates toward polymer adsorption was performed by the deposition of an alkyne functionalized self-assembled monolayer (SAM). Then, three tailor-made omega-azido linear brush precursors, i.e., PEG-N3, PMMA-N3, and PS-N3 (Mn approximately 20,000 g/mol), were grafted to alkyne-functionalized SAMs via click chemistry in tetrahydrofuran. The SAM, PEG, PMMA, and PS layers were characterized by ellipsometry, scanning probe microscopy, and water contact angle measurements. Results have shown that the grafting process follows the scaling laws developed for polymer brushes, with a significant dependence over the weight fraction of polymer in the grafting solution and the grafting time. The chemical nature of the brushes has only a weak influence on the click chemistry grafting reaction and morphologies observed, yielding polymer brushes with thickness of ca. 6 nm and grafting densities of ca. 0.2 chains/nm2. The examples developed herein have shown that this highly versatile and tunable approach can be extended to the grafting of a wide range of polymer (pseudo-) brushes to silicon substrates without changing the tethering strategy.

Adhesion mechanisms at soft polymer interfaces.

Based on several significant examples, we analyse the adhesion mechanisms at soft polymer interfaces with a special emphasis first on the role of connector molecules, that is, polymer chains bound to the interface and which transmit stress through a stretching and extraction mechanism, and second on the necessary relay that must be taken by additional dissipation mechanisms acting at larger scales if one wants to reach typical fracture toughnesses in the range of a few 10 J m(-2). Examples of such bulk dissipation mechanisms will be discussed for interfaces between polymer melts and for pressure-sensitive adhesives in contact with a solid surface. We shall particularly point out the fact that the level of adhesion results from a competition between adhesive failure usually driven by both the interactions and the friction properties of the interface and bulk strong deformations which take place in the bulk of the adhesive layer. Controlling the friction properties of the interface then becomes a tool to finely tune adhesive properties.

Effect of nanometric-scale roughness on slip at the wall of simple fluids.

It is commonly acknowledged that roughness decreases the aptitude of simple liquids to exhibit flow with slip at solid interfaces. Most available studies have, however, been conducted on substrates for which both the surface chemistry and the roughness were varied simultaneously, making it difficult to identify their respective role on wall slip. To overcome this difficulty, we have developed a series of surfaces formed by grafting hyperbranched polymeric nanoparticles on a smooth, dense, self-assembled monolayer of SiH-terminated short poly(dimethylsiloxane) oligomers, allowing us to vary independently the surface density, the height, and the width of the grafted nanoparticles, and thereby the roughness parameters, while keeping similar surface chemistry. On such substrates, the boundary condition for the flow velocity of hexadecane has been characterized through near-field laser velocimetry. We demonstrate that decreasing the wavelength of the roughness at a fixed height strongly decreases slip, while increasing the height of the nanoparticles at a fixed aspect ratio of the roughness also dramatically affects slippage.

Effect of plasticizers (water and glycerol) on the diffusion of a small molecule in iota-carrageenan biopolymer films for edible coating application.

Translational diffusion of a fluorescein probe has been measured in iota-carrageenan edible films containing different amounts of glycerol (0, 15, 30, and 45%), using fluorescence recovery after photobleaching (FRAP) experiments. The effects of this plasticizer as well as the plasticizing effect of water on the diffusion of fluorescein have been studied in this edible coating mainly composed of natural biopolymer. Diffusion coefficients of about 10(-13) m2 s(-1) have been measured in these films for water activity (aw) lower than 0.7. Above this water content threshold, fluorescein translational diffusion coefficient increases up to 10(-12) m2 s(-1). Another interesting information obtained from FRAP experiments on this system is the ratio of the diffusing molecules which are immobilized in the carrageenan matrix at aw lower than 0.98. Moreover, films containing more than 30% glycerol (w/w carrageenan) present a huge increase of the diffusion coefficient of fluorescein at high water activity (about 2 orders of magnitude), this effect being less pronounced at low water activity. The increase of diffusion seems to be only related to the water content, and glycerol only acts through the enhancement of water adsorption. Therefore, in biopolymer films containing polyol plasticizers, the gain in mobility could be devoted to the effect of the ubiquitous plasticizing molecule, water, whose adsorption is increased by the plasticizer.

Sliding friction at a rubber/brush interface.

We study the friction of a poly(dimethylsiloxane) (PDMS) rubber network sliding, at low velocity, on a substrate on which PDMS chains are end-tethered. We thus clearly evidence the contribution to friction of the pullout mechanism of chain ends that penetrate into the network. This interfacial dissipative process is systematically investigated by probing the velocity dependence of the friction stress and its variations with the grafting density and molecular weight of the tethered chains. This allows us to confirm semiquantitatively the picture of arm retraction relaxation of the grafted chains proposed in models of slippage at a network/brush interface.

Unsteady-State Flow of Flexible Polymers in Porous Media.

In this paper we report an investigation of the unsteady-state flow of polymer solutions through granular porous media. The experiments were performed using high-molecular-weight nonionic and anionic polyacrylamides dissolved in water containing NaCl and model porous media obtained by packing silicon carbide (SiC) grains having a narrow grain size distribution. Before injection in porous media, the polymer solutions were carefully filtered according to a method that was proved to be efficient in removing any possibly remaining microgels. The SiC grain surface was passively oxidized by a controlled thermal treatment in order to obtain a surface partially covered by a thin silica layer having adsorption properties similar to those of quartzitic sand. By packing SiC grains of different sizes, porous media having identical adsorption properties and well-known pore throats sizes can be obtained with a good reproducibility. Parameters investigated include pore size, velocity gradient, polymer concentration, and adsorption energy. A striking unsteady-state flow behavior (pressure build-up at constant flow rate) is observed when three conditions are fulfilled: (a) the velocity gradient is larger than that known to be able to induce a coil-stretch transition, (b) the polymer adsorbs on the pore surfaces, and (c) the length of stretched macromolecules is larger than the effective pore throat diameter. When one of these conditions is not satisfied the flow remains steady. These observations are interpreted by a mechanism involving the adsorption and bridging across pore restrictions of elongated chains. We propose to refer to this peculiar mode of polymer adsorption as bridging adsorption. Copyright 2001 Academic Press.