Elie Raphaël

Surface-Anchored Polymer Chains: Their Role in Adhesion and Friction

Polymer surfaces and interfaces have many technological applications. In the present article we review some recent experiments conducted on model systems with the aim of understanding the role played by surface-anchored polymer layers in adhesion and friction. We also discuss some of the related theoretical models. The key parameter for both situations is the degree of interdigitation between the surface layer and the bulk polymer system (an elastomer in the case of adhesion, a molten polymer in the case of friction). We analyze how this degree of interdigitation governs the optimum enhancement in the adhesion energy between the solid wall and an elastomer, and how it is at the origin of the various wall slip regimes observed experimentally.

Conformation of Adsorbed Comb Copolymer Dispersants

Comb copolymers with an adsorbing backbone and nonadsorbing side chains can be very effective dispersants, particularly when a high ionic strength strongly penalizes electrostatic stabilization. For this reason, they have become essential components of concrete over the past decade. This article examines the steric hindrance characteristics of such polymers through the use of atomic force microscopy (AFM) on calcium silicate hydrate, the main hydration product of Portland cement. It is found that solution and surface properties (hydrodynamic radius, radius of gyration, surface coverage, steric layer thickness) and force-distance curves obtained during AFM measurements can be well described by a scaling approach derived in this paper. This represents the first real quantitative step in relating these properties directly to the molecular structure of such comb copolymer dispersants.

A Direct Quantitative Measure of Surface Mobility in a Glassy Polymer

Polymer Film Behavior An ongoing debate in the understanding of the behavior of thin-film glassy polymers is whether there is nanoconfinement of large molecules or enhanced mobility near a free surface. Chai et al. (p. 994; see the Perspective by Chen et al.) prepared polymer films with a sharp step in the profile by depositing broken film fragments onto a uniform underlay. Atomic force microscopy revealed changes to the overall film profile with time at various temperatures. A transition was observed from localized motions to relaxation of the entire film at a temperature close to that of the bulk glass transition temperature. A step geometry is used to probe the behavior of a glassy polymer above and below its glass transition temperature. [Also see Perspective by Chen et al.] Thin polymer films have striking dynamical properties that differ from their bulk counterparts. With the simple geometry of a stepped polymer film on a substrate, we probe mobility above and below the glass transition temperature Tg. Above Tg the entire film flows, whereas below Tg only the near-surface region responds to the excess interfacial energy. An analytical thin-film model for flow limited to the free surface region shows excellent agreement with sub-Tg data. The system transitions from whole-film flow to surface localized flow over a narrow temperature region near the bulk Tg. The experiments and model provide a measure of surface mobility in a simple geometry where confinement and substrate effects are negligible. This fine control of the glassy rheology is of key interest to nanolithography among numerous other applications.

Comb-like polymers inside nanoscale pores

Abstract A new method of polymer characterization, based on permeation studies using nanoscale pores, was recently proposed by Brochard and de Gennes. In the present paper, we study (at the level of scaling laws) how this method, initially developed for star polymers, can be extended to comb-like polymers.

Polymers in confined environments

Phase Transitions of Polymer Blends and Block Copolymer Melts in Thin Films.- Flexible Polymers in Nanopores.- Polymer-Silicate Nanocomposites: Model Systems for Confined Polymers and Polymer Brushes.- Normal and Shear Forces Between Polymer Brushes.- Surface-Anchored Polymer Chains: Their Role in Adhesion and Friction.- Molecular Transitions and Dynamics at Polymer / Wall Interfaces: Origins of Flow Instabilities and Wall Slip.

Adhesion of nanoparticles.

We have developed a new model of nanoparticle adhesion which explicitly takes into account the change in the nanoparticle surface energy. Using combination of the molecular dynamics simulations and theoretical calculations we have showed that the deformation of the adsorbed nanoparticles is a function of the dimensionless parameter beta proportional, variant gamma(p)(GR(p))(-2/3)W(-1/3), where G is the particle shear modulus, R(p) is the initial particle radius, gamma(p) is the polymer interfacial energy, and W is the particle work of adhesion. In the case of small values of the parameter beta < 0.1, which is usually the case for strongly cross-linked large nanoparticles, the particle deformation can be described in the framework of the classical Johnson, Kendall, and Roberts (JKR) theory. However, we observed a significant deviation from the classical JKR theory in the case of the weakly cross-linked nanoparticles that experience large shape deformations upon particle adhesion. In this case the interfacial energy of the nanoparticle plays an important role controlling nanoparticle deformation. Our model of the nanoparticle adhesion is in a very good agreement with the simulation results and provides a new universal scaling relationship for nanoparticle deformation as a function of the system parameters.

Dynamics of wetting with nonideal surfaces. The single defect problem

Under static conditions, the macroscopic contact angle θ between a (partially wetting) liquid, a solid, and air, lies between two limiting values θr (receding) and θa (advancing). If we go beyond these limits (e.g., θ=θa+e, e>0) the contact line moves with a certain macroscopic velocity U(e). In the present paper, we discuss U(e) (at small e) for a very special situation where the contact line interacts only with one defect at a time. (This could be achieved inside a very thin capillary, of radius smaller than the average distance between defects.) Using earlier results on the elasticity and dynamics of the contact line in ideal conditions, we can describe the motions around ‘‘smooth’’ defects (where the local wettability does not change abruptly from point to point). For the single defect problem in a capillary, two nonequivalent experiments can be performed: (a) the force F is imposed (e.g., by the weight of the liquid column in the capillary). Here we define e=(F−Fm)/Fm, where Fm is the maximum pinning...

Kelvin wake pattern at large Froude numbers

Gravity waves generated by an object moving at constant speed at the water surface form a specific pattern commonly known as the Kelvin wake. It was proved by Lord Kelvin that such a wake is delimited by a constant angle

Dewetting of thin polymer films near the glass transition.

{\simeq }19. 4{7}^{\circ }

Indentation of a rigid sphere into an elastic substrate with surface tension and adhesion

. However a recent study by Rabaud and Moisy based on the observation of airborne images showed that the wake angle seems to decrease as the Froude number