Polymer concentration interface

This article is not intended to give a comprehensive overview over the entire field, but will concentrate on some recent developments and highlights as perceived by the author. Only most recent references will be given where most of the previous work will be found. There are several books, proceedings and review articles available on earlier work and on specific aspects of polymers at interfaces [1-13]. In particular the area of tethered chains in solution or melt is covered by another article in this book [14]. 

On the other hand, Doblhofer218 has pointed out that since conducting polymer films are solvated and contain mobile ions, the potential drop occurs primarily at the metal/polymer interface. As with a redox polymer, electrons move across the film because of concentration gradients of oxidized and reduced sites, and redox processes involving solution species occur as bimolecular reactions with polymer redox sites at the polymer/solution interface. This model was found to be consistent with data for the reduction and oxidation of a variety of species at poly(7V-methylpyrrole). This polymer has a relatively low maximum conductivity (10-6 - 10 5 S cm"1) and was only partially oxidized in the mediation experiments, which may explain why it behaved more like a redox polymer than a typical conducting polymer. 

Adsorption of the polymer molecule causes a reduction of its conformational entropy (Norde 2003b). Flence, adsorption takes place only if the loss in conformational entropy is compensated by sufficient favorable interactions between polymer segments and the interface. Because the polymer molecule attaches with many segments at the interface, it adsorbs tenaciously with a very high affinity, even if the interaction of the individual segments with the interface is rather weak. The high affinity manifests itself by the adsorption being irreversible with respect to variations of the polymer concentration in solution. 

The rheological properties of a fluid interface may be characterized by four parameters surface shear viscosity and elasticity, and surface dilational viscosity and elasticity. When polymer monolayers are present at such interfaces, viscoelastic behavior has been observed (1,2), but theoretical progress has been slow. The adsorption of amphiphilic polymers at the interface in liquid emulsions stabilizes the particles mainly through osmotic pressure developed upon close approach. This has become known as steric stabilization (3,4.5). In this paper, the dynamic behavior of amphiphilic, hydrophobically modified hydroxyethyl celluloses (HM-HEC), was studied. In previous studies HM-HEC s were found to greatly reduce liquid/liquid interfacial tensions even at very low polymer concentrations, and were extremely effective emulsifiers for organic liquids in water (6). 

F. Rondelez, D. Ausserre, and H. Hervet, Experimental studies of polymer concentration profiles at solid-liquid and liquid-gas interfaces by optical and X-ray evanescent wave techniques, Annu. Rev. Phys. Chem. 38, 317-347 (1987). 

Rgure 2.10. (a) Influence of the polymer molecular weight on the force-distance profiles between air-water (filled symbols) and oU-water interfaces (empty symbols). Polymer concentration = 0.5 wt%. Average molecular masses of the polymers oil-water interface (0) Mw = 10000 g/mol ( ) = 55000 g/mol (Q) = 155000 g/mol. air-water... 

In ISFETS utilizing polymeric ion-selective membranes, it has been always assumed that these membranes are hydrophobic. Although they reject ions other than those for which they are designed to be selective, polymeric membranes allow permeation of electrically neutral species. Thus, it has been found that water penetrates into and through these membranes and forms a nonuniform concentration gradient just inside the polymer/solution interface (Li et al., 1996). This finding has set the practical limits on the minimum optimal thickness of ion-selective membranes on ISFETS. For most ISE membranes, that thickness is between 50-100 jttm. It also raises the issue of optimization of selectivity coefficients, because a partially hydrated selective layer is expected to have very different interactions with ions of different solvation energies. 

A more realistic situation for diffusion in a laminate is illustrated in Fig. 7-14b, which shows the solute concentration profile in the barrier layer after a short contact time t=tj. In this illustration the concentration profile of the solute just reaches the polymer/liquid interface and cL.t = 0. If we now consider a similar case with a semi-infinite polymer system with the initial solute concentration (cP>e) at the distance x < xQ = a+b/2 and cP=0 at x>x0 and t=0 (Fig.7-14c), then the possible concentration profiles for the three different times, tctj, t=t, and t>tj can be illustrated in Fig. 7-14d. If we assume a mass transfer through the interface A at x=x at t=t, in Fig. 7-14d, then mpt/A = 0.5cpepp(d-X ), which corresponds to mP, /A= cPepp(x0-a) = cPeppb/2 in Fig. 7-14c. If we combine this result with Eq. (7-54) for t=t, then we obtain the time... 

This type of diffusion/reaction mechanism has been treated semi-analyti-cally by Albery et al. [42, 44, 45], under steady-state conditions and its applications to amperometric chemical sensors has been described by Lyons et al. [46]. In both models, only diffusion and reaction within a boundary layer is considered, while the effect of concentration polarisation in the solution is neglected. Thus, to apply the model to an experimental system it is necessary to be able to accurately determine the concentration of substrate at the polymer/solution interface. Assuming that the system is in the steady state, the use of the rotating disc electrode allows simple determination of the substrate concentration at the interface from the bulk concentration and the experimentally determined flux using [47]... 

Because of these uncertainties, equations 1, 2, 3, 4, and 5 may not be relied upon as a means of quantitative evaluation of A until more data for other polymer-solvent systems become available. The equation-of-state thermodynamics is, however, useful in its ability to give us insight into the physical factors and their relative magnitudes which contribute to the polymer-polymer interaction parameter. The results in this work clearly show that the dependence of A on concentration and temperature is moderate. This gives a justification as a good approximation to the use of a constant polymer-polymer interaction parameter in the polymer interface theories where the polymer concentration encompasses the whole range Wi = 0 to 1 across the phase boundary. 

The left branch of hw(Cei) dependence is dominated by electrostatic repulsion. The copolymers are non-ionic and charge creation may be attributed to preferential adsorption of OH ions at the air/water interface [173,186,188]. If this is true (small) variations in the copolymer concentration should not affect this part of the dependence. Indeed, the two sets of data (Fig. 3.32) were taken at different F108 polymer concentration but their descending branches coincide. Moreover, the data for both copolymers located below their Cei,a roughly followed the same trend (Fig. 3.33). 

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