Polymer adsorption layers

Up to date, besides the SFA, several non-interferometric techniques have been developed for direct measurements of surface forces between solid surfaces. The most popular and widespread is atomic force microscopy, AFM [14]. This technique has been refined for surface forces measurements by introducing the colloidal probe technique [15,16], The AFM colloidal probe method is, compared to the SFA, rapid and allows for considerable flexibility with respect to the used substrates, taken into account that there is no requirement for the surfaces to be neither transparent, nor atomically smooth over macroscopic areas. However, it suffers an inherent drawback as compared to the SFA It is not possible to determine the absolute distance between the surfaces, which is a serious limitation, especially in studies of soft interfaces, such as, e.g., polymer adsorption layers. Another interesting surface forces technique that deserves attention is measurement and analysis of surface and interaction forces (MASIF), developed by Parker [17]. This technique allows measurement of interaction between two macroscopic surfaces and uses a bimorph as a force sensor. In analogy to the AFM, this technique allows for rapid measurements and expands flexibility with respect to substrate choice however, it fails if the absolute distance resolution is required. 

In this chapter specific theories and experimental set-ups for interfacial relaxation studies of soluble adsorption layers are presented. A general discussion of relaxation processes, in bulk and interfacial phases, was given in Chapter 3. After a short introduction, in which the important role of mechanical properties of adsorption layers and the exchange of matter for practical applications are discussed, the main differences between adsorption kinetics studies and relaxation investigations are explained. Then, general theories of exchange of matter and specific theories for different experimental techniques are presented. Finally, experimental setups, based on harmonic and transient interfacial area deformations, are described and results for surfactant and polymer adsorption layers discussed. 

The influenee of the adsorption layer thiekness on doublet lifetime is shown in Fig. 5 for one value of the Hamaker eonstant. There is high specificity in the thiekness of a polymer adsorption layer.p-Casein adsorbed on to polystyrene latex causes an increase in the radius of the particle of 10-15 nm (79). Alayer of P-Mactoglo-bulin appears to be in the order of 1-2 nm thick, as compared to 10 nm for the caseins (80). 

The quantitative contribution of the polymer constituent into the interaction energy of particles is a function of both polymer adsorption layer parameters on the surface (share of elementary links of a macromolecule contacting the surface, degree of its covering by the polymer, amount of the polymer in the first (dense) layer, layer thickness, and so on) and those of macromolecules in the solution. 

Adsorbed on the surface of dispersed particles, polymer chains lessen the attraction energy by steric reasons (the minimum distance to which particles can approach increases) and because they change the efficient Hamaker s constant value. The attraction energy in expressions for Ur dependence on A is the function of not only interaction constants of dispersed phase A, dispersion medium A2 and the phase with the medium A 2, but of Gamaker s constant for adsorption layer 3 too. The effect of polymer adsorption layers on molecular attraction of particles has been described theoret-ically. Below is an expression for Ur, based on the Lifshits macroscopic theory... 

The surface influence is explained by the theory of adsorption and the structure of the polymer adsorption layer. However, one should not think that this factor is prevailing, as the thickness of the adsorption layer is low compared with the thickness of the surface layer, as it will be shown below. 

The forces between two polymer adsorption layers depend on the concentration of monomer links of polymer in the space between two plates. As was shown in Section in.C, the distribution of chain links in adsorption layer can be described by... 

Attached forces between the plate, free from the polymer and the plate with polymer adsorption layer determined by the number of new bonds forming between the macromolecule and the plate, free from the polymer or the number of monomer links of polymer bound with this... 

Hereafter, we assume the polymers to form an adsorbed layer around the colloidal particles, with a typical thickness much smaller than the particle radius, such that curvature effects can be neglected. In that case, the effective interaction between the eolloidal particles with adsorbed polymer layers can be traced back to the interaction energy between two planar substrates covered with polymer adsorption layers. In the case when the approaeh of the two particles is slow and the absorbed polymers are in full equilibrium with the polymers in solution, the interaction between two opposing adsorbed layers is predominantly attractive [45, 46], mainly beeause polymers form bridges between the two siufaees. Recently, it has been shown that there is a small repulsive eomponent to the interaetion at large separations [47]. 

A logical division is made for the adsorption of nonelectrolytes according to whether they are in dilute or concentrated solution. In dilute solutions, the treatment is very similar to that for gas adsorption, whereas in concentrated binary mixtures the role of the solvent becomes more explicit. An important class of adsorbed materials, self-assembling monolayers, are briefly reviewed along with an overview of the essential features of polymer adsorption. The adsorption of electrolytes is treated briefly, mainly in terms of the exchange of components in an electrical double layer. 

The drawback of the described adsorbents is the leakage of the bonded phase that may occur after the change of eluent or temperature of operation when the equilibrium of the polymer adsorption is disturbed. In order to prepare a more stable support Dulout et al. [31] introduced the treatment of porous silica with PEO, poly-lV-vinylpyrrolidone or polyvinylalcohol solution followed by a second treatment with an aqueous solution of a protein whose molecular weight was lower than that of the proteins to be separated. Possibly, displacement of the weakly adsorbed coils by the stronger interacting proteins produce an additional shrouding of the polymer-coated supports. After the weakly adsorbed portion was replaced, the stability of the mixed adsorption layer was higher. 

According to the concepts, given in the paper [7], a significant difference between the values of yield stress of equiconcentrated dispersions of mono- and polydisperse polymers and the effect of molecular weight of monodisperse polymers on the value of yield stress is connected with the specific adsorption on the surface of filler particles of shorter molecules, so that for polydisperse polymers (irrespective of their average molecular weight) this is the layer of the same molecules. At the same time, upon a transition to a number of monodisperse polymers, properties of the adsorption layer become different. 

The situation becomes most complicated in multicomponent systems, for example, if we speak about filling of plasticized polymers and solutions. The viscosity of a dispersion medium may vary here due to different reasons, namely a change in the nature of the solvent, concentration of the solution, molecular weight of the polymer. Naturally, here the interaction between the liquid and the filler changes, for one, a distinct adsorption layer, which modifies the surface and hence the activity (net-formation ability) of the filler, arises. Therefore in such multicomponent systems in the general case we can hardly expect universal values of yield stress, depending only on the concentration of the filler. Experimental data also confirm this conclusion [13],... 

Highly branched polymers, polymer adsorption and the mesophases of block copolymers may seem weakly connected subjects. However, in this review we bring out some important common features related to the tethering experienced by the polymer chains in all of these structures. Tethered polymer chains, in our parlance, are chains attached to a point, a line, a surface or an interface by their ends. In this view, one may think of the arms of a star polymer as chains tethered to a point [1], or of polymerized macromonomers as chains tethered to a line [2-4]. Adsorption or grafting of end-functionalized polymers to a surface exemplifies a tethered surface layer [5] (a polymer brush ), whereas block copolymers straddling phase boundaries give rise to chains tethered to an interface [6],... 

Moreover, the interaction of the surface of the fillter with the matrix is usually a procedure much more complicated than a simple mechanical effect. The presence of a filler actually restricts the segmental and molecular mobility of the polymeric matrix, as adsorption-interaction in polymer surface-layers into filler-particles occurs. It is then obvious that, under these conditions, the quality of adhesion can hardly be quantified and a more thorough investigation is necessary. 

Clay Polymer Adsorption Drag Clay layers separate... 

For h < 26, the situation is much more complex. One not only needs to know 4>(z) for each layer, but how 4>(z) changes as the two particles approach, i.e. 4>(z,h) this may well depend on the time-scale of the approach, i.e. the equilibrium path may not be followed. Scheutjens and Fleer (25) in an extension of their model for polymer adsorption have analysed the situation for two interacting uncharged parallel, flat plates carrying adsorbed, neutral homopolymer, interacting under equilibrium conditions. Only a semi-quantitative picture will be presented here. 

The theory of polymer adsorption is complicated for most situations, because in general the free energy of adsorption is determined by contributions from each layer i where the segment density is different from that in the bulk solution. However, at the critical point the situation is much simpler since the segment density profile is essentially flat. Only the layer immedia-... 

Adsorption behavior and the effect on colloid stability of water soluble polymers with a lower critical solution temperature(LCST) have been studied using polystyrene latices plus hydroxy propyl cellulose(HPC). Saturated adsorption(As) of HPC depended significantly on the adsorption temperature and the As obtained at the LCST was 1.5 times as large as the value at room temperature. The high As value obtained at the LCST remained for a long time at room temperature, and the dense adsorption layer formed on the latex particles showed strong protective action against salt and temperature. Furthermore, the dense adsorption layer of HPC on silica particles was very effective in the encapsulation process with polystyrene via emulsion polymerization in which the HPC-coated silica particles were used as seed. 

According to this concept, it is expected that polymer molecules, especially high molecular weight polymers, give an increased adsorption at a temperature close to the cloud point, and particles with the thick(or dense) adsorption layer of polymer formed out of a poor solvent would show strong protection against flocculation. 

Also, here, the effect of the adsorption layer of HPC on encapsulation of silica particles in polymerization of styrene in the presence of silica particles has been investigated. Encapsulation is promoted greatly by the existence of the adsorption layer on the silica particles, and the dense adsorption layer formed at the LCST makes composite polystyrene latices with silica particles in the core (7.). This type of examination is entirely new in polymer adsorption studies and we believe that this work will contribute not only to new colloid and interface science, but also to industrial technology. 

It was apparent that the dense adsorption layer of HPC which was formed on the silica particles at the LCST plays a part in the preparation of new composite polymer latices, i.e. polystyrene latices with silica particles in the core. Figures 10 and 11 show the electron micrographs of the final silica-polystyrene composite which resulted from seeded emulsion polymerization using as seed bare silica particles, and HPC-coated silica particles,respectively. As may be seen from Fig.10, when the bare particles of silica were used in the seeded emulsion polymerization, there was no tendency for encapsulation of silica particles, and indeed new polymer particles were formed in the aqueous phase. On the other hand, encapsulation of the seed particles proceeded preferentially when the HPC-coated silica particles were used as the seed and fairly monodisperse composite latices including silica particles were generated. This indicated that the dense adsorption layer of HPC formed at the LCST plays a role as a binder between the silica surface and the styrene molecules. 

In this paper we present results for a series of PEO fractions physically adsorbed on per-deutero polystyrene latex (PSL) in the plateau region of the adsorption isotherm. Hydro-dynamic and adsorption measurements have also been made on this system. Using a porous layer theory developed recently by Cohen Stuart (10) we have calculated the hydrodynamic thickness of these adsorbed polymers directly from the experimental density profiles. The results are then compared with model calculations based on density profiles obtained from the Scheutjens and Fleer (SF) layer model of polymer adsorption (11). 

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