Polymer Solution-Air Interface

The free energy of mixing may, of course, be written F = AHm - TASm 

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It can be anticipated that further studies of the isoelectric state at the PEO polymer solution/air interface would indicate that the steric component of disjoining pressure (see Section 3.3.3) appearing in foam films from long chain PEO polymers would prevent the foam films from rupturing. 

The breath figures technique is one of the most widely employed methods for the fabrication of organized porous polymer films [30, 31] and, as fiuther depicted in detail, in this approach the template consists of an ordered array of water droplets that can be removed by simple evaporation. Indeed, the simultaneous evaporation of a volatile solvent and condensation of water vapor in combination with thermocapillary effects and Marangoni convection allow the formation and precise organization of water droplets at the polymer solution-air interface [30]. This array of water droplets will evaporate upon complete evaporation of the solvent of the polymeric solution, and the surface will reflect its presence in the form of pores. 

The vicinity of total reflection of a polymer solution - air interfaces is investigated using a neutron radiation. Experiments and calculations reveal a significant singular behaviour, from which the exact nature of the polymer concentration profile can be determined. 

We discuss here the use of neutron reflectivity at a polymer solution-air interface, for the experimental determination of the polymer concentration profile. The method consists of analyzing the neutron reflectivity near total reflection, as a function of the incident inverse wavelength k. This method was developed at the LLB in... 

Smith [113] studied the adsorption of n-pentane on mercury, determining both the surface tension change and the ellipsometric film thickness as a function of the equilibrium pentane pressure. F could then be calculated from the Gibbs equation in the form of Eq. ni-106, and from t. The agreement was excellent. Ellipsometry has also been used to determine the surface compositions of solutions [114,115], as well polymer adsorption at the solution-air interface [116]. 

Frothing is an imwanted effect of surface active water soluble pol5nners. D5mamic surface properties of the solution-air interface due to the presence of the polymer play an important role in foam formation and stability. The surface tension decrease due to adsorbed polymer plays a lesser role (see for example Ref. [84]). 

Among plasma proteins we studied at solution/air interfaces, fibrinogen behaved most remarkably. Compressed in a surface film balance (5), fibrinogen tended to form a cohesive film that could be lifted off the interface with a hydrophobic slide (6). The finding suggests that fibrinogen may form polymer-like complexes at certain interfaces even in absence of thrombin (7), the enzyme normally needed to polymerize fibrinogen into fibrin. 

Figure 5. Adsorption measuring devices. (A) For adsorption at solution-air interface (B) for adsorption on polymer or mica surfaces. (1), (3) Supports ensuring reproducibility of geometrical conditions and tightness (2) glass container (4) polymer or mica film.

The reflectivity of the solution-air interface was measured for polydimethylsiloxane solutions in toluene. In such systems, the polymer is known to be attracted at the surface because of the negative increment of the surface tension (--S mN/m). Two isotopic compositions were tested, and they are characterized respectively by ya = 4.321 and yb = -0.98 (see Eq.(H)). Ideally in the experiment, the polymer molecular weight Mu,)i and the monomer volume fraction (j>i should be independent of composition. This was however not exactly achieved ... 

Comparison between Figs. 5 and 6 also suggests that the bulk concentrations 4> introduced in the calculation of kc)i could have been underestimated. However, even this interpretation cannot explain all observed features of the experimental result. Therefore, the data shown in Fig. 5 reveal information on the polymer concentration profile due to the polymer adsorption at the solution-air interface. This remains to be determined more precisely. 

A preliminary interpretation of the data suggests that adsorption at the solution-air interface creates a deep layer in which the polymer concentration is higher than that in the bulk and varied slowly. This is not predicted by the scaling approach. 

We have measured the interfacial film pressure of various water-soluble polymers at several hy(J oge1-water interfaces and at the polyethylene-water interface. For the latter, II. was positive and approximately equal to the surface tension depression of the solvent (II ) at the solution-air interface. When the soluble polymer was more hydrophilic than the gel... 

The influence of the surfactant in the modified polymers of Figure 1 on n (aqueous solutions, Table I) is not overpowering. The surfactant s influence is diminished by the amphiphilic oxyethylene units which lie interfacially flat at the aqueous-air interface. The hydrophobes are structurally similar to the surfactants providing stability to commercial latices and should be capable of competing with the classical surfactants at the latex surface, but this ability is not reflected in 7T values. The oxyethylene units have been demonstrated(18) to provide osmotic stabilization to latex particles. 

In the presence of anionic surfactants, it is reasonable to expect that the hydrophobic groups of the poljrmer and of the surfactant would combine to form a mixed film at the liquid-air interface. The interactions between the cationic groups of the polymer and the anionic groups of the surfactant would further strengthen the interactions in the monolayer. These effects can be expected to increase the surface and sub-solution viscosity in lamellae and in turn enhance their stability. 

Negative adsorption is in most cases very small compared to positive adsorption and therefore not easily detected directly. A sophisticated optical technique (evanescent-wave-induced fluorescence, EWIF) has been used to prove the reduction in segment concentration close to a non-adsorbing surface Depletion also has an effect on the flux of polymer solutions through pores since the viscosity of the liquid near the non-adsorbing surface is lower than that of the polymer solution, the flux is then higher than would be expected on the basis of the bulk viscosity. Negative adsorption at a liquid-air interface leads to a measurable increase In surface tenslon ... 

Neutron reflectivity has established the structure of polymer layers at interfaces (air-solution and solution-solid), determined the surface ordering in block copolymer films, and revealed the nature of inter-diffusion in mixed polymer films. 

It is known that, in a polymer blend, thermodynamic incompatibility between polymers usually causes demixing of polymers. If the polymer is equilibrated in air, the polymer with the lowest surface energy (hydrophobic polymer) will concentrate at the air interface and reduce the system s interfacial tension as a consequence. The preferential adsorption of a polymer of lower surface tension at the surface was confirmed by a number of researchers for a miscible blend of two different polymers. Based on this concept, surface modifying macromolecules (SMMs) as surface-active additives were synthesized and blended into polymer solutions of polyethersulfone (PES). Depending on the hydro-phobic or hydrophilic nature of the SMM, the membrane surface becomes either more hydrophobic or more hydrophilic than the base polymeric material. ... 

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