Adsorption of polymers at solid substrates

Cavity model used for studies of the adsorption of single polymers at solid substrates on the simple-cubic lattice. The lower of the two parallel surfaces Is attractive to the polymer, the upper Is steric only. The distance between the surfaces is/ , lattice units. From [302]. 

Adsorption of polymers on surfaces plays a key role in numerous technological applications and is also relevant to many biological processes. During the last three decades it has been constantly a focus of research interest. The theoretical studies of the behavior of polymers interacting with solid substrate have been based predominantly on both scaling analysis [49] as well as on the self-consistent field (SCF) approach [50]. The close relationship between theory and computer experiments in this field [27, 51] has proved especially fruitful. Most investigations focus on the determination of the critical adsorption point (CAP) location and on the scaling behavior of a variety of quantities below, above, and at the CAP. 

Let us consider now the interaction of a single polymer chain with a solid substrate. The main effects particular to the adsorption of polymers (as opposed to the adsorption of simple molecules) are due to the reduetion of eonformational states of the polymer at the substrate, which is due to the impenetrability of the substrate for monomers [13-18], The second factor determining the adsorption behavior is the substrate-monomer interaction. Typically, for the ease of an adsorbing substrate, the interaction potential p(z) between the substrate and a single monomer has a form similar to the one shown in Fig. 6, where z measures the distance of the monomer from the substrate siuface ... 

With this we conclude the investigation of folding properties of individual polymer or peptide chains and the aggregation of multiple chains, and turn our interest to another large class of structure-formation processes that accompany the adsorption of polymers or peptides at solid substrates. Hybrid systems composed of organic and inorganic matter are of particular relevance for micro-scale technological applications. 

While the bulk behavior of polyampholytes has been investigated for some time now, studies of interfacial performance of polyampholytes are still in their infancy. There are several reasons for the limited amount of experimental work the major one being the rather complex behavior of polyampholytes at interfaces. This complexity stems from a large array of system parameters governing the interaction between the polymer and the substrate. Nearly all interfacial studies on polyampholytes reported to-date involved their adsorption on solid interfaces. For example, Jerome and Stamm and coworkers studied the adsorption of poly(methacryhc acid)-block-poly(dimethyl aminoethyl methacrylate) (PMAA-fc-PDMAEMA) from aqueous solution on sihcon substrates [102,103]. The researchers found that the amount of PMAA-fo-PDMAEMA adsorbed at the solution/substrate interface depended on the solution pH. Specifically, the adsorption increased... 

A very versatile approach to the formation of multilayer films has been developed by Decher, based on polyelectrolytes. If a solid substrate with ionic groups at the surface is dipped into a solution of a complementary polyelectrolyte, an ultrathin, essentially monomolecular film of the polyion is adsorbed [340]. The adsorption is based on pairing of surface bound ionic sites with oppositely charged ions, bound to the macromolecule. The polymers adsorb in an irregular flattened coil structure and only part of the polymer ions can be paired with the surface ions (Figure 29a). Ionic sites which remain with small counterions provide anchor points for a next layer formed by a complementary polyelectrolyte [342,343]. This way multilayer polyelectrolyte films can be prepared layer-by-layer just by dipping a suitable substrate alternately in an aqueous solution of polyanions and polycations. The technique can be employed with nearly all soluble charged polymers and results in films with a... 

Secondly, the fact that polymers are emchored to the surface by many points of attachment severely reduces their mobility. Very slow surface processes can occur, as has been demonstrated by means of competitive adsorption studles -S). Polyelectrolytes, which can form very strong ionic bonds with solid substrates, are also found to relax very slowly, or not at all. Addition of small ions (salt) can facilitate relaxation somewhat. All this Implies that in Judging adsorption data for polymers, one should be aware of the possibility that off-equillbrlum states may persist over longer periods of time. 

Fig. 13 Velocity dependence of frictional stress for a soft gel sliding on a smooth adhesive solid substrate. The result is based on the molecular picture in Fig. 12, which considers the thermal fluctuation of adsorption and desorption of the polymer chain, (a) The elastic term of the frictional stress of a gel. See text for a description of parameter u. (b) Summation of the elastic term and the viscous term. When v -C Vf, the characteristic polymer adsorption velocity, the elastic term is dominant. At v 2> the viscose term is dominant. Therefore, transition from elastic friction to lubrication occurs at the sliding velocity characterized by the polymer chain dynamics. (Modified from figure 1 in [65])...

These forces and hence the stability of the dispersions can be altered/controlled by the adsorption of ions, surfactants, or polymers at the solid-liquid interface. Adsorption of surfactants and polymers at the solid-liquid interface depends on the nature of the surfactant or polymer, the solvent, and the substrate. Ionic surfactants adsorbing on oppositely charged surfaces exhibit a typical four-region isotherm. Such adsorption can alter the dispersion stability mainly by changing the double layer interaction, which depends on the extent of adsorption. Thus, it is seen that alumina suspensions are destabilized by the adsorption of SDS when the zeta potential is reduced to zero. At higher concentrations, bilayered surfactant adsorption can occur with changes in wettability and flocculation of the particles by altering the hydrophobic interactions. 

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