Polymer adsorption, experimental techniques

A major advantage of the simple model described in this paper lies in its potential applicability to the direct evaluation of experimental data. Unfortunately, it is clear from the form of the typical isotherms, especially those for high polymers (large n) that, even with a simple model, this presents considerable difficulty. The problems can be seen clearly by consideration of some typical polymer adsorption data. Experimental isotherms for the adsorption of commercial polymer flocculants on a kaolin clay are shown in Figure 4. These data were obtained, in the usual way, by determination of residual polymer concentrations after equilibration with the solid. In general, such methods are limited at both extremes of the concentration scale. Serious errors arise at low concentration due to loss in precision of the analytical technique and at high concentration because the amount adsorbed is determined by the difference between two large numbers. 

Many experimental techniques are used to study the adsorption ability of fillers and the adsorption of polymer chains at surfaces. Different types of information are provided by these methods. 

For the polymer to be effective, it must adsorb to the interface and maintain a certain configuration. Thus the following discussion describes various experimental techniques used for the study of adsorption density and configuration of polymer at the interface. After adsorption occurs, the main mechanisms of flocculation are due to the adsorption of a single polymer molecule on separate particles, interaction through the interpenetration of adsorbed polymer, and interactions due to the loss of freedom of movement of the polymer chains. 

The aim of this chapter is to present the fundamentals of adsorption at liquid interfaces and a selection of techniques, for their experimental investigation. The chapter will summarise the theoretical models that describe the dynamics of adsorption of surfactants, surfactant mixtures, polymers and polymer/surfactant mixtures. Besides analytical solutions, which are in part very complex and difficult to apply, approximate and asymptotic solutions are given and their range of application is demonstrated. For methods like the dynamic drop volume method, the maximum bubble pressure method, and harmonic or transient relaxation methods, specific initial and boundary conditions have to be considered in the theories. The chapter will end with the description of the background of several experimental technique and the discussion of data obtained with different methods. 

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. 

Although ellipsometry is well established as an experimental technique for the investigation of adsorbed layers, the number of studies at fluid/liquid interfaces is relatively small. Ellipsometry was used for investigation of the layer thickness between two immiscible liquids near the critical point (254, 255). This technique was also quite often used for in situ studies of the adsorption kinetics at an air/protein solution surface or polymer monolayers at an air/water interface (251, 256). It was also shown that ellipsometric re-... 

Topics covered include the nature and properties of the surface of a polymer melt, the structure of interfaces between different polymers and between pol5miers and non-polymers, adsorption from polymer solutions, the molecular basis of adhesion and the properties of polymers at liquid surfaces. Emphasis is placed on the common physical principles underlying this wide range of situations. Statistical mechanics based models of the behaviour of polymers near interfaces are introduced, with the emphasis on theory that is tractable and applicable to experimental situations. Experimental techniques for studying polymer surfaces and interfaces are reviewed and compared. 

Abstract The physical and experimental technique of gas adsorption measurements by slow oscillations of a rotational pendulum or, hkewise, the relaxational motion of a freely floating rotator are described. Combinations of the pendulum with either gravimetric or volumetric measurements are outhned. These especially are suited to measure the absorption or solubility of gases in non-rigid or swelling sorbent materials like polymers. Pros and cons of these methods are discussed in brief. List of symbols. References. 

Cohen and Christ (unpublished) presented a new experimental technique for determining mobility reduction resulting from polymer retentions in porous media. Their method was designed to separate the contributions of adsorption and non-adsorptive retention to be measured in their flow experiments using HPAM. This was done by using a silane treatment of the silica in their sandpacks, which changed the surface such that it no longer adsorbs HPAM. 

The structure of the adsorbed layer is described in terms of the segment density distribution, p(z). As an illustration, Fig. 5 shows some calculations by Scheutjens and Fleer [17] for loops and tails with r = 1000,4>. = 10 , and X = 0.5. In this example, 38% of the segments are in trains, 55.5% in loops, and 6.5% in tails. This theory demonstrates the importance of tails which dominate the total distribution in the outer region of the adsorbed layer. As we will discuss in the next section on experimental techniques for characterization of the adsorption and conformation of polymers at the solid liquid interface, determination of the segment density distribution is not easy and usually assigns a value for the adsorbed layer thickness 6. This increases with increase of the molecular weight of the polymer and increase of solvency of the medium for the chains. 

The study of acid-base interaction is an important branch of interfacial science. These interactions are widely exploited in several practical applications such as adhesion and adsorption processes. Most of the current studies in this area are based on calorimetric studies or wetting measurements or peel test measurements. While these studies have been instrumental in the understanding of these interfacial interactions, to a certain extent the interpretation of the results of these studies has been largely empirical. The recent advances in the theory and experiments of contact mechanics could be potentially employed to better understand and measure the molecular level acid-base interactions. One of the following two experimental procedures could be utilized (1) Polymers with different levels of acidic and basic chemical constitution can be coated on to elastomeric caps, as described in Section 4.2.1, and the adhesion between these layers can be measured using the JKR technique and Eqs. 11 or 30 as appropriate. For example, poly(p-amino styrene) and poly(p-hydroxy carbonyl styrene) can be coated on to PDMS-ox, and be used as acidic and basic surfaces, respectively, to study the acid-base interactions. (2) Another approach is to graft acidic or basic macromers onto a weakly crosslinked polyisoprene or polybutadiene elastomeric networks, and use these elastomeric networks in the JKR studies as described in Section 4.2.1. 

Polymer adsorption has been reviewed by many authors28-37. An earlier volume of this journal presented an article which dealt with polymer adsorption studies made before 196429. This paper gives a review of subsequent advances in this field of study. In Chap. B, the principal theories are described, confining ourselves to those which are amenable to experimental teste. Chapter C gives a brief survey of typical measuring techniques. In Chap. D, important experimental data on the thickness of the adsorbed polymer layer and the fraction of adsorbed segments are summarized and discussed, along with their comparison with relevant theories. 

As has been depicted in Fig. 1, various conformations are possible for adsorbed polymers, depending on polymer-polymer, polymer-solvent, and polymer-interface interactions and the flexibility of polymers. To determine experimentally the conformation of adsorbed polymers only adsorption isotherm data are insufficient. The average thickness of the adsorbed polymer layer, the segment density distribution in this layer, the fraction of adsorbed segments, and the fraction of surface sites occupied by adsorbed segments must be measured. Recently, several unique techniques have become available to measure these quantities. 

Protein Adsorption. The development of medical implant polymers has stimulated interest in the use of ATR techniques for monitoring the kinetics of adsorption of proteins involved in thrombogenesis onto polymer surfaces. Such studies employ optical accessories in which an aqueous protein solution (93) or even ex - vivo whole blood (94-%) can be flowed over the surface of the internal reflection element (IRE), which may be coated with a thin layer of the experimental polymer. Modem FT-IR spectrometers are rapid - scanning devices, and hence spectra of the protein layer adsorbed onto the IRE can be computed from a series of inteiferograms recorded continuously in time, yielding ah effective time resolution of as little as 0.8 s early in the kinetic runs. Such capability is important because of the rapid changes in the composition of the adsorbed protein layers which can occur in the first several minutes (97). 

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