Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session U3: Simple Views on Polymers at Surfaces and Interfaces: Symposium Honoring P G de Gennes |
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Sponsoring Units: DPOLY Chair: E. Raphael Room: Morial Convention Center RO2 - RO3 |
Thursday, March 13, 2008 8:00AM - 8:36AM |
U3.00001: Polymer adsorption Invited Speaker: The aim of this talk is to review Pierre-Gilles deGennes' work on polymer adsorption and the impact that it has now in our understanding of this problem. We will first present the self-consistent mean-field theory and its applications to adsorption and depletion. De Gennes most important contribution is probably the derivation of the self-similar power law density profile for adsorbed polymer layers that we will present next, emphasizing the differences between the tail sections and the loop sections of the adsorbed polymers. We will then discuss the kinetics of polymer adsorption and the penetration of a new polymer chain in an adsobed layer that DeGennes described very elegantly in analogy with a quantum tunneling problem. Finally, we will discuss the role of polymer adsorption for colloid stabilization. [Preview Abstract] |
Thursday, March 13, 2008 8:36AM - 9:12AM |
U3.00002: Polymer brushes Invited Speaker: The polymer brush is an ensemble of macromolecules end-tethered to a substrate. The so-called Alexander-de Gennes polymer brush model (S. Alexander, J. Physique 1977, and P.-G. de Gennes, Macromolecules 1980) opened a new field in polymer science and provided a theoretical framework to look at brush-like polymer systems. In this presentation, we will first briefly review the ideas and concepts behind the Alexander-de Gennes model of a planar polymer brush and summarize its major findings. We will then focus on the general impact of this seminal work by demonstrating numerous theoretical developments initiated by the Alexander-de Gennes model with specific emphasis on polyelectrolyte brushes and biological brush-like systems. [Preview Abstract] |
Thursday, March 13, 2008 9:12AM - 9:48AM |
U3.00003: Adhesion Invited Speaker: Adhesion is a highly practical subject in which the vast majority of published work is either chemical in nature, concerned with chemistry that is thought to occur at an adhesive junction or chemistry of adhesives, or essentially mechanical, concerned with the mechanics of testing and failure of adhesive systems. The role of polymer physics in general and de Gennes' work in particular is to discover what happens at the scale of the polymer chain and hence form a bridge between these two approaches. A distinguishing feature of Gennes' work in adhesion is the way he developed simple models that permitted us to see the essential physics of the situation. This is particularly true in his work in viscoelastic effects on toughness (the de Gennes trumpet) where more sophisticated mechanics had been done but the physical situation was obscure. Much of his work was concerned with the effects of connector molecules in toughening an interface in both elastomeric and glassy materials. This work has been extended by a number of authors and forms the basis of our current understanding of the area. [Preview Abstract] |
Thursday, March 13, 2008 9:48AM - 10:24AM |
U3.00004: Slippage Invited Speaker: After a brief overview of the usual assumptions made to fix the boundary conditions for the flow velocity at a solid wall, we shall present the early conjecture made by Pierre Gilles de Gennes in 1979, predicting huge slip at the wall for polymer melts flowing against smooth non adsorbing surfaces. We shall then discuss how interplay between theory and experiments has allowed to produce a refined picture of the molecular mechanisms of friction at polymer interfaces, which quantitatively accounts for the three different friction regimes which have been experimentally identified when increasing the shear rate in simple shear experiments of polymer melts. The key idea (Brochard and de Gennes, 1992) focuses on the effect of a few surface anchored chains. If these chains are not rigid, they will deform under the effect of the friction forces resulting from entanglements between surface and bulk chains. When increasing the shear rate, the surface chains are thus progressively stretched, and can disentangle from bulk chains: a dynamic decoupling between surface and bulk polymer occurs. We shall discuss the available sets of experimental data (and the corresponding techniques which have been developed to either directly characterize wall slip or yield friction forces measurements). At present, the low surface density regime is fully understood, while series of data in the case of large grafting densities are available, but still lack of an adequate model. We shall finally draw lines of possible extensions of the ideas to other systems which start to be investigated as regards to friction: polyelectrolytes - grafted polyelectrolyte chains, rigid polymers, other complex fluids. We shall also discuss what is known at present on slip at the wall in the case of simple fluids, a situation which appear much more difficult to model than the polymer case. [Preview Abstract] |
Thursday, March 13, 2008 10:24AM - 11:00AM |
U3.00005: Polymers in Confined Geometry Invited Speaker: Thanks to P. G. de Genne's famous~``$n$ = 0'' theorem (relating the configuration of a polymer chain to a magnetic phase transition with an order parameter of $n$ = 0 component), the swelling exponent \textit{$\nu $} ($R=N^{\nu }a)$ was calculated in terms of space dimension $d$ and fitted the qualitative Flory calculation. Using scaling laws, or the~``blob'' picture, it became possible to derive the conformation of a chain confined in a tube or a slit. The dynamics of confined polymers followed immediately. The new feature was the screening of the hydrodynamic interactions in confined geometry leading to Rouse-like behaviour. In a third step, de Gennes focused on the forced penetration of polymers (linear, branched, stars, neutral or charged) in narrow tubes, when they are submitted to a flow (or an electrical field for DNA). From all these calculations, the conclusion was that the threshold velocity (or field) corresponds to the penetration of the first blob. Afterwards, the friction force increases linearly with the penetrated length, whereas the confinement force remains constant. In the last few years, with his collaboration we studied the penetration of DNA in ~``soft'' tubes. We predicted a transition from extended to globular DNA observed experimentally. We also showed that semi flexible polymer chains, like DNA, are ideal in 3d, but swollen effects are more pronounced in confined geometries. We will also discuss experiments inspired by his work on confined polymers, in the last thirty years. [Preview Abstract] |
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