Polymer Adsorption
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P
hOLYMER ADSORFPTION.
M. DAOUD Laboratoire LUon Brillouin (C.E.A.-C.N.R.S.), C.E.-Saclay, 91191 Gif-sur-Yvette cedex, France 1n. INTRODUCT]ION. Because of both their practical importance and their fundamental properties [I], polymers at interfaces have been under constant study. In this paper, we will consider the adsorbtion of linear chains on flat surfaces. Theoretical interest in this question was renewed recently by the introduction of scaling methods by de Gennes [2], Pincus [3], Binder, Kremer and Eisenriegler [4]. This allowed a new approach to the problem. New experimental methods such as neutron reflectivity [5] and small angle neutron scattering [6] also provided a much more detailed information than was previously available. This combined progress in theory and technique improved tremendously our understanding of the adsorption of linear chains on a flat impenetrable surface when the surface attraction is not too large. Many important questions however remain open. These are related both to the quality of the surface and to the nature of the solvent. Actual surfaces are never completely flat or homogeneous. Work is in progress to take into account surface roughness [7,8] and chemical imperfections [9,10]. Similarly, the quality of the solvent was considered in many directions. These include surface driven phase separation [11], or the effect of a mixed solvent on a copolymer. A somewhat different solvent effect will be considered here and is related to the length of the solvent molecules. Previous studies were concerned with solutions in a simple solvent [12]. It is possible both theoretically and experimentally to consider long polymers, made of N monomers, dissolved in shorter chains, with P units, of similar nature and slightly different interaction with the surface. The interest in this question is twofold. First by changing P, it is possible to have either adsorbed or desorbed probe chains as we shall see. Second, a generalization of this problem is to consider a mixture of both long and short chains in a simple solvent. The question then is the displacement of an adsorbed polymer by another one. In the following, we will only consider the equilibrium properties for the first case, in the absence of any simple solvent. As we shall see, this question is already very rich. We will first recall the various bulk regimes for long probe chains dissolved in shorter polymers. Section 3 will review the case of adsorbed chains in the presence of a molecular solvent Adsorption from a dilute solution in a matrix of short chains will be considered section 4. Finally, we discuss very briefly in section 5 the kinetics of the exchange between long chains adsorbed and in the bulk of the solution. 2. M•IXTURE OF LONG POLYMERS AND SHORT CIHAIINS. We consider first the structure of a long chain, made of N monomers, dissolved in a matrix of shorter polymers of a similar species but with P units. P may be varied from 1 to N. When P is of order unity, we assume the monomer to be a good solvent of the probe chain. There
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