Conformation of polymers at interfaces

Several experimental methods can be applied to study adsorption and conformation of polymers at interfaces. The amount of polymer adsorbed, F, can be directly determined by equilibrating a known amount of the disperse phase (particles or droplets) with a polymer solution of known concentration, Cj. When the system reaches equilibrium (that may take hours or even days with high-molecular weight polymers), the disperse phase is separated (by filtration or centrifugation), and the equilibrium concentration of the polymer, C, is determined using a suitable analytical method. From Cj and C2 and the amount of disperse phase m and its surface area A (m /g, which may be obtained from a knowledge of the particle size distribution), F can be calculated, that is. 

Several experimental methods can be applied to study adsorption and conformation of polymers at interfaces. The amount of polymer adsorbed, T, can be directly determined by equilibrating a known amount of the disperse phase (particles or droplets) with a poly-... 

This area of research is still at its beginning and many aspects are not resolved. This includes in particular the structure and conformation of polymers at an interface as well as the modification of polymer dynamics by the interface. We have given several examples of the potential of surface and interface analytical techniques. They provide information on surface roughness, surface composition, lateral structure, depth profiles, surface-induced order and interfacial mixing of polymers on a molecular and sometimes subnanometer scale. They thus offer a large variety of possible surface and interface studies which will help in the understanding of polymer structure and dynamics as it is modified by the influence... 

The S parameter is a function of the segment density distribution of the stabilizing chains. The conformation, and hence the segment density distribution function of polymers at interfaces, has been the subject of intensive experimental and theoretical work and is a subject of much debate (1). Since we are only interested in qualitative and not quantitative predictions, we choose the simplest distribution function, namely the constant segment density function, which leads to an S function of the form (11) ... 

To understand the behavior of polymers at interfaces, it is essential to gain a clear picture of then-behavior in solution first. Modem polymer solution studies were pioneered by Flory. Since then, there has been considerable effort to elucidate solution behavior of polymers and many models have been proposed. The theoretical treatment of polymer adsorption characteristics and conformation characteristics received considerable attention, mainly because of the significant applications. 

Therefore, complete unfolding of the polypeptidic chain at the interface leading to a train-loop-tail model, similar to the conformation of copolymers at interfaces, is to be considered as a limiting case only. Even for a disordered and uncross-linked protein such as /3-casein, this type of conformation at air-water or oil-water interface may not in fact be totally realistic [200]. It provides however a convenient basis to derive a thermodynamical analysis of protein interfacial layers from polymer theories [201-203]. Recent specular neutron reflectance studies on protein adsorption layers at the air-water interface [204] show that the protein density profile normal to the interface is qualitatively similar for /3-casein and BLG and consists of a protein-rich layer, about 15 A thick, close to the interface, and a... 

Many uses of polymers are concerned with the properties of polymers at interfaces. Chapter 2 presents a summary of theories of polymer adsorption and discusses the properties and state of polymers at interfaces and methods for determining the details of their structure, conformations, and so on. The basic theory of colloid-interaction forces in terms of DL VO theory is presented, together with a discussion of the different basic stabilization mechanisms of colloids. 

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. 

Even from those first remarks it is evident that our knowledge of polymers at surfaces and interfaces depends largely on analytical techniques. They should yield information on chemical composition, density, roughness, chain conformation, end distribution etc. across the interface with subnanometer resolution. In Sect. 2... 

On the basis of the above experimental results, the expected conformations of polymer-surfactant complexes at the oil-water interface are depicted in Fig. 2.19. In case I, the added polymer associates with excess surfactants present in the bulk solution, but the complexes prefer to remain in the bulk phase. Alternately, the polymer-surfactant complexes are unable to displace the adsorbed surfactant molecules from the liquid-liquid interface. Irrespective of the amount of polymer-surfactant concentration in the bulk, the experimental decay length values remain comparable to the Debye lengths, corresponding to the concentration of ion species in the bulk solution (Eq. (2.11)). This means that the force profile is... 

The system described in this investigation is polystyrene-14C adsorbed on Graphon carbon black (graphitized Spheron 6) from six solvents comprising a wide spectrum from good to poor solvent power. Well-characterized materials were selected to elucidate the conformation of polymer molecules at the solid/liquid interface. So far two models have been postulated to describe the conformation of the adsorbed polymer molecules at the solid/liquid interface (9, 13, 14, 18, 19, 21, 27). In the first model the polymer assumes a loop or coil structure in which only a fraction of the polymer segments are attached directly at the interface, and in the second model the polymer forms a relatively flat and compressed interfacial layer with many segments attached to the solid substrate. 

The process of polymer adsorption is quite different in many aspects from that of small molecules the latter has been studied extensively in the past. These differences in their adsorption characteristics arise in turn from the obvious flexibility of the larger polymer molecules, so that in addition to the nsnal adsorption factors considered, such as the adsorbate-adsorbent, adsorbate-solvent, and adsorbent-solvent interactions, a major aspect to be understood is the conformation of polymer molecnles at the interface and its role in dispersion. Polymers have a large number of functional groups, each of which can potentially adsorb at the surface, whereas smaller molecules are mostly monofunctional. 

The conformation of polymer chains (in effect, their size) in solution is an important characteristic of a system in solution and at an interface. The detailed analysis of polymer conformations is a very complex process requiring powerful computer facilities. A simpUfied treatment based on random-flight (or random-walk) statistics allows for the estimation of chain dimensions to a degree adequate for most practical situations. 

Wei, G. 2006. Radius and chirality dependent conformation of polymer molecule at nanotube interface. Nano Letters 6 1627-1631. 

In view of the experimental findings on filler-matrix interactions and their effects on conformations of polymer chains, it is suggested that non-covalent filler-matrix interactions and the resulting increases of the exposure ofless polar groups and chains segments at the interfaces reduce the polarity of the droplets, which in turn reduces the interfacial tension. 

Understanding the adsorption and conformation of polymeric surfactants at interfaces is key to knowing how these molecules act as stabilizers. Most basic ideas on adsorption and conformation of polymers have been developed for the solid/liquid interface [9-17]. As mentioned above, the same concepts may be applied to the liquid/liquid interface, with some modification, whereby some parts of the molecule may reside within the oil phase, rather than simply staying at the interface. Such modification does not alter the basic concepts, particularly when one deals with stabilization by these molecules. 

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