Lionel Bureau

Nanoconfined ionic liquids: effect of surface charges on flow and molecular layering

Ionic liquids have remarkable physico-chemical properties that make them highly attractive as new solvents or electrolytes in applications ranging from solar cells to MEMS lubrication, in which their flow properties in the vicinity of solid surfaces are crucial. We have performed a study of the nanorheology of two ionic liquids confined in molecularly thin films which shows how surface charges drastically affect their molecular structure and flow properties.

Rheological aging and rejuvenation in solid friction contacts.

Abstract:We study the low-velocity (0.1-100 μm s-1) frictional properties of interfaces between a rough glassy polymer and smooth silanized glass, a configuration which gives direct access to the rheology of the adhesive joints in which shear localizes. We show that these joints exhibit the full phenomenology expected for confined quasi-2D soft glasses: they strengthen logarithmically when aging at rest, and weaken (rejuvenate) when sliding. Rejuvenation is found to saturate at large velocities. Moreover, aging at rest is shown to be strongly accelerated when waiting under finite stress below the static threshold.

Rate effects on layering of a confined linear alkane.

We perform drainage experiments of a linear alkane fluid (n-hexadecane) down to molecular thicknesses, and we focus on the role played by the confinement rate. We show that molecular layering is strongly influenced by the velocity at which the confining walls are approached: under high enough shear rates, the confined medium behaves as a structureless liquid of enhanced viscosity for a film thickness below approximately 10 nm. Our results also lead us to conclude that a rapidly confined film can be quenched in a metastable disordered state, which might be related with recent intriguing results on the shear properties of confined films produced at different rates [Zhu and Granick, Phys. Rev. Lett. 93, 096101 (2004)].

The Conformation of Thermoresponsive Polymer Brushes Probed by Optical Reflectivity

We describe a microscope-based optical setup that allows us to perform space- and time-resolved measurements of the spectral reflectance of transparent substrates coated with ultrathin films. This technique is applied to investigate the behavior in water of thermosensitive polymer brushes made of poly(N-isopropylacrylamide) grafted on glass. We show that spectral reflectance measurements yield quantitative information about the conformation and axial structure of the brushes as a function of temperature. We study how parameters such as grafting density and chain length affect the hydration state of a brush, and provide one of the few experimental evidences for the occurrence of vertical phase separation in the vicinity of the lower critical solution temperature of the polymer. The origin of the hysteretic behavior of poly(N-isopropylacrylamide) brushes upon cycling the temperature is also clarified. We thus demonstrate that our optical technique allows for in-depth characterization of stimuli-responsive polymer layers, which is crucial for the rational design of smart polymer coatings in actuation, gating, or sensing applications.

AN INERTIAL TRIBOMETER FOR MEASURING MICROSLIP DISSIPATION AT A SOLID-SOLID MULTICONTACT INTERFACE

An apparatus has been built to measure the shear response of a multicontact interface between flat-ended solid bodies, rough at the micron scale. The device makes use of inertia to apply a steady sinusoidal shear force to a slider without direct mechanical drive. Both elastic compliance and damping losses are deduced from the in-phase and out-of-phase components of the submicronic shear displacement. Operating frequencies range between 15 Hz and 1 kHz, while below 100 Hz quasistatic motion of the slider is achieved. Acceleration amplitudes range typically between 0.1 and 7 m s−2, where gross sliding occurs. The resolution of the microslip detection is 1 nm. Apparatus design and operation are described, and the application and limitation of the method to a weakly nonlinear response are discussed and illustrated by experimental results with a polymer glass.

Surface force apparatus for nanorheology under large shear strain.

We describe a surface force apparatus designed to probe the rheology of a nanoconfined medium under large shear amplitudes (up to 500 microm). The instrument can be operated in closed loop, controlling either the applied normal load or the thickness of the medium during shear experiments. Feedback control allows us to greatly extend the range of confinement/shear strain attainable with the surface force apparatus. The performances of the instrument are illustrated using hexadecane as the confined medium.

Non-Amontons behavior of friction in single contacts

Abstract.We report on the frictional properties of a single contact between a glassy polymer lens and a flat silica substrate covered either by a disordered or by a self-assembled alkylsilane monolayer. We find that, in contrast to a widely spread belief, the Amontons proportionality between frictional and normal stresses does not hold. Besides, we observe that the velocity dependence of the sliding stress is strongly sensitive to the structure of the silane layer. Analysis of the frictional rheology observed on both disordered and self-assembled monolayers suggests that dissipation is controlled by the plasticity of a glass-like interfacial layer in the former case, and by pinning of polymer chains on the substrate in the latter one.

Probing the surface properties of a polymer glass with macroscopic friction

We show how macroscopic friction can be used as a sensitive probe of chain dynamics at the surface of a glassy polymer. We present experiments in which a smooth poly(methylmethacrylate) (PMMA) solid slides on flat surfaces presenting different densities of pinning sites available for polymer/substrate bond formation. These experiments indicate that: (i) at high pinning level, frictional dissipation occurs through the sudden flips of molecular-sized bistable regions localized in a nm-thick layer of confined chains, which responds to shear as an elasto-plastic solid, and (ii) in situations of weak pinning, dissipation appears to be governed by a process akin to that proposed for rubber friction. This suggests that some glass-to-rubber transition occurs at the polymer surface when its interaction with the substrate goes from strong to weak. The temperature-dependence of friction provides further support for the presence of a nm-thick layer at the polymer surface, which exhibits a rubberlike response in situation of weak interaction with the countersurface. This behavior results from the interplay between viscous flow in this surface layer, and shear induced depinning of adsorbed surface chains. Moreover, a quantitative analysis of the results indicates that the pinning dynamics of polymer chains is controlled by localized β rotational motions at the interface.

Correction to “The Conformation of Thermoresponsive Polymer Brushes Probed by Optical Reflectivity”

We describe a microscope-based optical setup that allows us to perform spaceand time-resolved measurements of the spectral reflectance of transparent substrates coated with ultrathin films. This technique is applied to investigate the behavior in water of thermosensitive polymer brushes made of poly(N-isopropylacrylamide) grafted on glass. We show that spectral reflectance measurements yield quantitative information about the conformation and axial structure of the brushes as a function of temperature. We study how molecular parameters (grafting density, chain length) affect the hydration state of a brush, and provide one of the few experimental evidence for the occurrence of vertical phase separation in the vicinity of the lower critical solution temperature of the polymer. The origin of the hysteretic behavior of poly(N-isopropylacrylamide) brushes upon cycling the temperature is also clarified. We thus demonstrate that our optical technique allows for in-depth characterization of stimuli-responsive polymer layers, which is crucial for the rational design of smart polymer coatings in actuation, gating or sensing applications.