Polymer adsorbed layers

The observed chaige reversal can prove the presence of two types of the PE adsorption sites on the capillary surface. At low concentration, the electrostatic adsorption of positively charged PE molecules predominantly occurs on the negatively charged sites of quartz surface. Thereafter (or simultaneously), on the surface of a capillary covered with a polymer adsorbed layer, the adsorption of the PE molecules can occur due to the forces of molecular attraction and attraction between hydrophobic sites of polyelectrolyte and surface (e.g. siloxane groups). Their competition with the electrostatic repulsion forces that increase in the course of further adsorption of PE molecules determines the completion of the adsorption and the formation of equilibrium (with the solution) adsorbed layer. 

Inoue, H., Fukke, H., and Katsumoto, H., Effect of polymer adsorbed layer on magnetic particle dispersion, IEEE Trans. Magn., 26, 75, 1990. 

Chibowski, S., Zeta potential and thickness of a polymer adsorbed layer in the system dispersed sohd-electrolyte, Pol. J. Chem., (H. 1137, 1993. 

The polymer concentration and the macromolecule conformation in the adsorption layer depend on the forces between the macromolecule and the solid surface. Therefore, an investigation of this phenomenon is very important for describing the polymer adsorption. Some information has been obtained by studying the forces between two polymer adsorbed layers as a function of the distance between them. 

The structure of polymer adsorbed layers at the air-liquid interface is similar to that at the solid-liquid interface. Adsorption at air-liquid interface has been studied by ellipsometry (72), X-ray and neutron reflectivity (73,74), surface tension measurements (75), X-ray evanescent wave-induced fluorescence (76), and Langmuir trough techniques (73). Neutron reflectivity measurements indicate that in the case of homopolymers the segment density decreases as in good agreement with scaling predictions for homopolymers at the solid-liquid interface (73). 

The polymer concentration profile has been measured by small-angle neutron scattering from polymers adsorbed onto colloidal particles [70,71] or porous media [72] and from flat surfaces with neutron reflectivity [73] and optical reflectometry [74]. The fraction of segments bound to the solid surface is nicely revealed in NMR studies [75], infrared spectroscopy [76], and electron spin resonance [77]. An example of the concentration profile obtained by inverting neutron scattering measurements appears in Fig. XI-7, showing a typical surface volume fraction of 0.25 and layer thickness of 10-15 nm. The profile decays rapidly and monotonically but does not exhibit power-law scaling [70]. 

A disadvantage of AES is that the intense electron beam easily causes damage to sensitive materials (polymers, insulators, adsorbate layers). Charging of insulating samples also causes serious problems. 

Adsorption of dispersants at the soHd—Hquid interface from solution is normally measured by changes in the concentration of the dispersant after adsorption has occurred, and plotted as an adsorption isotherm. A classification system of adsorption isotherms has been developed to identify the mechanisms that may be operating, such as monolayer vs multilayer adsorption, and chemisorption vs physical adsorption (8). For moderate to high mol wt polymeric dispersants, the low energy (equiUbrium) configurations of the adsorbed layer are typically about 3—30 nm thick. Normally, the adsorption is monolayer, since the thickness of the first layer significantly reduces attraction for a second layer, unless the polymer is very low mol wt or adsorbs by being nearly immiscible with the solvent. 

Adsorption of macromolecules has been widely investigated both theoretically [9—12] and experimentally [13 -17]. In this paper our purpose was to analyze the probable structures of polymeric stationary phases, so we shall not go into complicated mathematical models but instead consider the main features of the phenomenon. The current state of the art was comprehensively summarized by Fleer and Lyklema [18]. According to them, the reversible adsorption of macromolecules and the structure of adsorbed layers is governed by a subtle balance between energetic and entropic factors. For neutral polymers, the simplest situation, already four contributor factors must be distinguished ... 

This exchangeability of adsorbed layers should be considered for better understanding of the irreversible adsorption of polymers. Apparently, penetration by the macromolecules adsorbed later through the layer of the initially adsorbed ones will include a slow exchange between the positions of segments and take a longer time. 

Rheological methods of measuring the interphase thickness have become very popular in science [50, 62-71]. Usually they use the viscosity versus concentration relationships in the form proposed by Einstein for the purpose [62-66], The factor K0 in Einstein s equation typical of particles of a given shape is evaluated from measurements of dispersion of the filler in question in a low-molecular liquid [61, 62], e.g., in transformer oil [61], Then the viscosity of a suspension of the same filler in a polymer melt or solution is determined, the value of Keff is obtained, and the adsorbed layer thickness is calculated by this formula [61,63,64] ... 

Aid in the uniform dispersion of additives. Make powdered solids (e.g. particulate fillers with high energy and hydrophilic surface) more compatible with polymers by coating their surfaces with an adsorbed layer of surfactant in the form of a dispersant. Surface coating reduces the surface energy of fillers, reduces polymer/filler interaction and assists dispersion. Filler coatings increase compound cost. Fatty acids, metal soaps, waxes and fatty alcohols are used as dispersants commonly in concentrations from 2 to 5 wt %. 

For Gg (b), a reasonable (although not strictly correct) procedure is to replace the Stern potential in one of the standard equations for Gg by the zeta potential of the polymer-coated particles this assumes that the plane of hydrodynamic shear corresponds to the periphery of the adsorbed layer. 

Adsorption of polyelectrolyte on interfaces is concerned with various applications such as flocculation and steric-stabilization of colloidal particles in an aqueous phase, oil recovery, and soil conditioning. In these cases, both the adsorbance of polyelectrolytes and the conformation of the adsorbed polymer, which is connected with the thickness of the adsorbed layer, are very important. 

For homopolyelectrolyte, we first studied the ellipsometric measurement of the adsorption of sodium poly(acrylate) onto a platinum plate as a function of added sodium bromide concentration (5). We measured the effect of electrolyte on the thickness of the adsorbed layer and the adsorbances of the polyelectrolyte. It was assumed that the Donnan equilibrium existed between the adsorbed layer and the bulk phase. The thickness was larger and the adsorbance of the polyelectrolyte was lower for the lower salt concentration. However, the data on the molecular weight dependence of both the adsorbance and the thickness of the adsorbed polyelectrolyte have been lacking compared with the studies of adsorption of nonionic polymers onto metal surfaces (6-9). 

The aim of this paper is to offer experimental results for the molecular weight dependence of adsorption of polystyrene-sulfonate) onto a platinum plate from aqueous NaCl solution at 25 °C. Measurements of poly(styrenesulfonate) adsorption were carried out by ellipsometry. The dependences of molecular weight and added salt concentration on the thickness of the adsorbed layer and also the adsorbances of polymer and salt are examined. 

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