Segmental adsorption energy, solvent

Such displacement effects, although often very pronounced, have not yet been studied systematically. They will be the subject of the present paper. We will discuss the adsorption of polymer from a mixture of two solvents and we will see that in some cases drastic effects occur as a function of the mixture composition. Also, we explore some consequences and practical applications of displacement. It turns out that displacement studies not only increase our insight on the role of the solvent in polymer adsorption but can also be used to determine the segmental adsorption energy. So far, experimental data for this quantity were very scarce. Some illustrative experiments will be discussed briefly. 

Determination of the segmental adsorption energy. The determina-tion of x ° is also possible. Since x d can be found from Equation 5 if Xsc an the solvency terms are known, we can add xf° and find x ° by Equation 1. The determination of xf° calls for a separate experiment, e.g., an adsorption isotherm of the displacer from solvent, in the absence of polymer. Following such a scheme we used the values of cr obtained from the displacement isotherms of Figure 3 and 4 to determine segmental adsorption energy parameters Xg° for PVP on silica. The required additional information on xdo was obtained from the initial slopes of dis-... 

The shape of the adsorption isotherm is clearly affected by the magnitude of the segmental adsorption energy %s defined above. The thermodynamics of the bulk solution also influence the adsorption for polymers in 6 solutions the amount adsorbed is higher and generally this tends to increase continuously with increasing solution concentration. If the solvent quality is worse still, then one may have a very thick adsorbed layer indeed effectively one has bulk phase separation initiated at the surface. This situation is discussed in section 5.3. 

NEP and PVP. In the polymer where monomers are linked in a chain, hydrophobic parts are largely screened from interactions with the solvent. For free monomers such screening is not possible so that they experience more unfavourable interactions with the solvent. The adsorption energy parameter xs is not affected by the different chemical surrounding of free monomer and polymer segments, since the mechanism for interaction with the surface is hydrogen bonding in both cases (8).)... 

In most situations the experimental system is more complicated than one (homodisperse) polymer adsorbing from a single solvent. In multicomponent systems preferential adsorption always plays a role. A common example is the adsorption of a polydisperse polymer, where usually long chains adsorb preferentially over short ones, even if the adsorption energy per segment is the same. 

Similar considerations are important in theories of polymer adsorption, although here the relative magnitudes of surface-solvent and surface—segment interaction energies have to be taken into account. 

The process of polymer adsorption involves a number of interactions that need to be carefully considered. Three main interactions are obvious, i.e. the surface/solvent, the chain/solvent and the chain/surface. For adsorption to take place, then unfavourable entropy loss when a polymer molecule approaches a surface must be compensated for by an energy of adsorption term. One usually describes the adsorption energy in terms of the value per segment in direct contact with the surface, addition, the polymer molecule... 

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