Laboratoire de Physique des Solides - UMR 8502

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Home > Members > Frédéric Restagno

Research

Formation Mechanisms of Polymer Interfaces (Coll. L. Léger)

Polymer brushes have many interesting physical properties that gave rise to intensive theoretical and experimental studies in recent years. Those systems can, for example, promote adhesion between a solid substrate and a compatible polymer network, control the friction with a melt, and modify the interfacial energy or even the interfacial rheology of an interface. The conformation of the grafted chains determines the macroscopic properties of the interface and thus the properties of composite materials. The precise characterization of the formation and the structure of an interphase in which macromolecules are involved has been performed on different systems.

From a fundamental point of view, we studied the swelling dynamics of end-grafted hydrogenated polystyrene (h-PS) chains confined between a solid substrate and a deuterated polystyrene (d-PS) melt, using neutron reflectometry experiments. The kinetics of relaxation was quantified by measuring the inter-digitation dynamics of dry chains put in contact with different melts as a function of the molecular parameters. It was possible to measure the dependence of this kinetics versus the molecular weight of both the grafted and the melt chains (Figure 3), and the grafting density and to propose a model based on the slow dynamics of an end-tethered polymer chain [Chennevière 2013].

Polymers slip at solid interfaces (Coll. L. Léger,ANR ENCORE)

The no-slip boundary condition at a solid-liquid interface is at the center of our description of fluid mechanics. A consequence of this no-slip boundary condition is that when a liquid is forced to flow in a narrow channel, the force required to push the liquid diverges when the channel thickness tends to zero. As a consequence, when considering the rheology of liquids confined at nanometric scale measured by forcing a liquid between two approaching surfaces, we demonstrated that the picometric elastic deformation of the rigid confining surfaces dominate the overall mechanical response of nanometric liquids films confined between solid walls [Villey 2013].

However, this no-slip boundary condition is an assumption that cannot be derived from first principles. In order to gain a better understanding of the friction at liquid-solid interface and to be able, as a long-term goal, to develop an interfacial rheology of the solid-liquid interface, we used polymer systems as a way to measure molecular mechanisms of friction. We developed an experimental method allowing to quantify slip at the wall in viscous polymer fluids, based on the observation of the evolution under simple shear flow of a photo-bleached pattern within a fluorescent labeled polymer melt [Hénot 2018a]. We demonstrated the potential of this method with measurements of the slip length for an entangled polydimethylsiloxane (PDMS) melt, as a function of the shear rate, in contact with several weakly adsorbing surfaces. In particular, 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 Navier’s interfacial 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 (Figure 5), we further demonstrate the local character of the friction coefficient and compare its value to the monomer–monomer friction [Hénot 2018b].

The case of a polymer fluid slipping on a surface with end-tethered chains is another interesting situation. In this case, we investigated the effect of shear on the conformation of hydrogenated polystyrene chains end-grafted on a silicon wafer and covered by a deuterated polystyrene melt by neutron reflectivity. The flow-induced distorted conformation of the end-tethered chains was evidenced by a change in the density profile of the end-tethered chains due to the shear. We have shown that the effect of the shear is a decoupling between the grafted chains and the bulk chains which leads to a strong slip of the polymer melt at the solid interface [Chennevière 2016]. Finally, we obtained some preliminary result on the effect of polymer solution flowing over a polymer brush [Korolkovas 2017] showing also a change conformation of the polymer chains when they are submitted to the shear exerted by a polymer solution instead of that by a polymer melt.

Copolymer at interfaces for industrial applications (Coll. L. Léger, Arkema)

Another studied example comes from everyday life and in particular food packaging where multilayer polymer films formed by co-extrusion processes are produced, in order to benefit from the complementary properties of different polymers. Most polymer pairs being incompatible, it is then necessary to reinforce the mechanical strength of the interface. This is usually achieved by forming diblock copolymers during the extrusion process by in situ interfacial reaction between functionalized polymer molecules incorporated into each film. Collaborating with Arkema, we developed a new FTIR-based protocol to directly measure the quantity of copolymer formed during the extrusion process.

Film generation and rupture (Coll. E. Rio)

In the last years, our strategy has been to start with the study of isolated foam films. We thus developed an experiment allowing to pull foam films at controlled velocity and to measure the time at which the film ruptures thanks to a force sensor positioned at the top of the film. This experiment is coupled with a spectrometer, which gives us the possibility to measure the film thickness in time.

A first benchmark has been to describe the generation the foam film by describing the influence of the stabilizing surfactant on the film thickness. We have been able to describe quantitatively the film thickness using numerical simulations and independent measurements of the surface elasticity [Champougny 2015]. A second benchmark has been to fully describe the process of film generation and rupture with a pure liquid [Champougny 2017]. Again numerical simulations have been combined with experiments obtained with silicon oils. This pure liquid can form non-stationary films, which drain and rupture. Their dynamics has been shown to be very well described by a simple hydrodynamical model where the liquid flow has two components: an extensional one and a shear one. The simulations, which led to the results of the preceding paragraphs have been performed in close collaboration with Benoît Scheid (ULB, Bruxelles, Belgium), who co-supervised the PhD work of Lorène Champougny.

We then have been able to perform rupture experiments on foam films stabilized by surfactants. During the development of this work, the importance of evaporation became obvious so that we chose to study the influence of the atmospheric humidity on foam film rupture [Champougny 2018]. We thus measured the variation of the maximum length of a foam film versus the pulling velocity at different ambient humidity. We complemented these measurements with thinning measurements. Unsurprisingly, the film lifetime is much longer at high humidity but our main result is that the thinning dynamics is completely independent of the humidity. We deduced from this result that the contribution of the foam film evaporation to the film thinning becomes important only when the film reaches a few hundreds of nanometers.

Mechanics of a rough elastic contact (Coll. L. Léger and C. Poulard)

When two solids are put into contact, the roughness of the two contacting surfaces plays a crucial role in the contact formation and as a consequence in determining whether adhesion or friction will develop. The exact evolution of the real area of contact with the normal load is however a delicate question, involving the mechanical properties of the solids, the exact geometry of the two surfaces, and the adhesion between the solids. The contact between a smooth elastomer lens and series of elastomer substrates patterned with regular hexagonal arrays of cylindrical pillars has been analyzed experimentally in a lab-developed JKR test apparatus. The transition from a “top contact” with the lens only touching the patterned substrate at the top of the pillars and a “full contact” when the asperities are flattened has been modelled taking into account the balance between adhesion and elastic energies inside the zone of full contact [Dies 2015]. The role of the density of pillars has later been more systematically analyzed [Ledesma-Alonso 2017].