Lamellar films

The distribution of chains in a lamellar film formed by a binary blend of symmetric PS-PMMA diblocks has been probed using neutron reflectivity by Mayes et al. (1994). The microphase self-assembled parallel to the substrate, giving a lamellar stack, as for pure diblocks (see Section 2.5.1). By blending mixtures of unlabelled and selectively labelled long and sort diblocks, it was determined that short diblock copolymer chains are localized at the PS-PMMA interface, while the longer chains are selectively located at the domain centres. These results... 

The first mechanism, gas diffusion, is due to the difference in gas pressure inside the bubble as a result of differences in curvature of the lamellar films. As a simple example, consider the system of two contacting bubbles shown in Figure 12.2. The Laplace equation states that the pressure difference, Ap, on either side of a curved interface will be given by... 

Thus, the internal pressure of the bubbles is just balanced by the interfacial forces acting across the lamellar film. The most important interfacial interactions contributing to n(S) are electrostatic repulsion between charged interfaces and steric interactions due to adsorbed species. Those topics have already been discussed in the context of colloidal stability and will not be treated further here. 

FIGURE 12.5. Thin lamellar films in foams will develop a disjoining pressure, 7r h), between the opposing interfaces when electrostatic or steric stabilization mechanisms are operative. The stabiUty of the film (and the foam) will depend on the value of n, among other things. 

The fundamental impact of surfactant concentration and diffusion rate in lamellar films can be viewed roughly as follows (Fig. 12.6) as the lamellar... 

FIGURE 12.7. The presence of surfactant liquid crystals may add stability to a normal foam (a) by forming a semirigid structure in the plateau border regions (b) and/ or thick lamellar films that provide mechanical as well as colloidal resistance to drainage. 

The second procedure consists of LBF deposition on the surface of stabilized nanosize particles followed by insertion of these particles into polylayers. Fimctional groups participate, including groups on inorganic layers. For example, a LBF based on amphiphilic Ru complexes was immobilized on a monolayer of hectorite. Lamellar films and polylayers were formed. LBFs with a specified organization were formed either directly on the surface of colloidal particles or on the water surface. Then... 

PP blends with EVAl, and maleated polypropylene (PP-MA), can be formed by biaxial stretching into lamellar films or sheets. Their overlapping layer morphology result is significantly reduced permeability by gases or liquids [8]. 

Huang, E., Mansky, P. et al. (2000) Mixed lamellar films evolution, commensurability effects, and preferential defect formation. Macromolecules, 33(1), 80-88. 

The collapse of foam is attributed to (a) the diffusion of gas molecules from a small bubble with higher internal pressure to a large one with lower internal pressure or into the bulk gas phase surrounding the foam system, (b) coalescence of bubbles due to capillary flow that results in rupture of the lamellar film between the adjacent bubbles (usually slower than (a) and occurring even in stabilized foam system), and (c) rapid hydrodynamic drainage of liquid between bubbles that leads to rapid collapse of bubbles [35], In most nonrigid foam systems, all three mechanisms are operative simultaneously to some extent during the foam collapse process. 

Opitz R., Lambreva D. M., and de Jeu W. H. (2002) Confined crystallization of ethylene-oxide butadiene diblock copolymers in lamellar films. Macromolecules 35 6930-6936. 

Thus, the internal pressure of the bubbles is just balanced by the interfacial forces acting across the lamellar film. 

If rupture and coalescence take place (Figure 9.7c), they occur in an area of the adsorbed monolayer thinned out by the mechanical action of the approaching drops in stretching the monolayer as a result of deformation and increases in the interfacial area and/or of depletion of emulsifier due to desorption. Understanding the behavior of such lamellar films and the role of surfactants in their action requires an understanding of the forces involved in interactions across the film and the kinetic aspects of film fluctuations. 

The coalescence of liquid droplets, therefore, is intimately related to the nature of the thin lamellar film formed between them as they are brought into close encounters as a result of thermal convection. Brownian motion, or mechanical agitation. It is important to understand the nature of the forces acting across the film in order to obtain information about the thermodynamic stability, metastability, or instability of the film, and the kinetic processes that will control the rate of film breakdown. Comprehensive reviews of those aspects of emulsion stability can be... 

If the lamellar film between approaching emulsion droplets thins beyond d>26 (Figure 9.3), with no drop coalescence taking place, interactions generally referred to as steric repulsions come into play (Figure 9.8a). Such repulsions normally result in a steep maximum in the potential-energy curve. Interfacial film rupture. 

The exact role of interfacial forces in emulsion stabilization is the subject of some question. In the view of many researchers in the field of emulsion stabilization, the rate of failure of the adsorbed lamellar film at the droplet interfaces as they approach and touch is a primary factor in the stabilization of an emulsion system. Such phenomena are related to the elasticity of the film and have been addressed in the theories of Gibbs and Marangoni. As pointed out previously, the Gibbs adsorption isotherm relates the lowering of the interfacial tension between two phases, da, resulting from the presence of a surface-active solute to the concentration C, of the solute in the system... 

The experimental determination of the elasticity of lamellar films has, until relatively recently, been difficult, as have studies of the rates of diffusion of surfactant molecules to newly formed interface. It is difficult, therefore, to determine the relative importance of each mechanism in the stabilization of OAV emulsions. New techniques using photon correlation spectroscopy, which can measure the duration and amplitude of surface and interfacial waves, promise to provide a great deal of useful information about the physical properties of such regions as surfaces, interfaces, and lamellar films. 

In the discussion of foams and foam stability in Chapter 8 it was shown that the presence of small amount of a surface-active impurity can contribute greatly to foam stability as a result of its effect on lamellar film elasticity. Such a dramatic effect has not been found in the case of emulsion stability against droplet coalescence. Although a limited amount of data are available on the interfacial tension of ternary mixtures, it is generally assumed that surface-active impurities of this type are probably too soluble in the oil phase to remain at the interface. They will be extracted into the oil phase, where they are not particularly surface-active and are not strongly adsorbed back into the interface. 

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