Swollen lamellar phases

This chapter is concerned with experiments and theory for semidilute and concentrated block copolymer solutions.The focus is on the thermodynamics, i.e. the phase behaviour of both micellar solutions and non-micellar (e.g. swollen lamellar) phases. The chapter is organized very simply Section 4.2 contains a general account of gelation in block copolymer solutions. Section 4.3 is concerned with the solution phase behaviour of poly(oxyethylene)-containing diblocks and tri-blocks. The phase behaviour of styrenic block copolymers in selective solvents is discussed in Section 4.4. Section 4.5 is then concerned with theories for ordered block copolymer solutions, including both non-micellar phases in semidilute solutions and micellar gels. There has been little work on the dynamics of semidilute and concentrated block copolymer solutions, and this is reflected by the limited discussion of this subject in this chapter. 

Another force [57, 58] occurs in a multilayered system, like a swollen lamellar phase of surfactant bilayers or phospholipid vesicles. Shape fluctuations in the bilayers can give rise to steric effects that are supposed to stabilise such systems where the van der Waals and double-layer forces are very weak, as they often are. The magnitude of such fluctuations depends on the "stiffness" of die bilayer. The status of these forces is the subject of an active debate and imclear. 

This potential force occurs in microstructured fluids like microemulsions, in cubic phases, in vesicle suspensions and in lamellar phases, anywhere where an elastic or fluid boundary exists. Real spontaneous fluctuations in curvature exist, and in liposomes they can be visualised in video-enhtuiced microscopy [59]. Such membrane fluctuations have been invoked as a mechanism to account for the existence of oil- or water-swollen lamellar phases. Depending on the natural mean curvature of the monolayers boimding an oil region - set by a mixture of surfactant and alcohol at zero -these swollen periodic phases can have oil regions up to 5000A thick With large fluctuations the monolayers are supposed to be stabilised by steric hindrance. Such fluctuations and consequent steric hindrance play some role in these systems and in a complete theory of microemulsion formation. 

It is important to note that the lamellar phase is thus stabilized by the balance of a negative interfacial tension (of the free oil/water interface covered by an amphiphilic monolayer), which tends to increase the internal area, and a repulsive interaction between interfaces. The result, Eq. (48), indicates that the scattering intensity in a lamellar phase, with wave vector q parallel to the membranes, should have a peak at nonzero q for d > d due to the negative coefficient of the q term in the spectrum of Eq. (40). just as in the microemulsion phase. This effect should be very small for strongly swollen lamellar phases (in coexistence with excess oil and excess water), as both very small [96]. Very similar behavior has been observed in smectic liquid crystals (Helfrich-Hurault effect) [122]. Experimentally, the lamellar phase under an external tension can be studied with the surface-force apparatus [123,124] simultaneous scattering experiments have to be performed to detect the undulation modes. 

In summary, in the ternary mixtures water-alcohol-SDS, the main effect of increasing the alcohol chain length from five to ten carbon atoms is to reduce the extent of the isotropic micellar domain to the benefit of the lamellar phase. This behavior reflects an increase in the elastic modulus k as the alcohol chain length increases. The addition of salt to these ternary solutions produces very swollen lamellar phases, sponge phases, and vesicle phases. However, differences are observed when the alcohol chain length is varied which also demonstrate the role of flexibility of the interface. Sponge phases do not occur with long-chain alcohols, whereas vesicle phases are not formed with short-chain alcohols. 

Capillary waves do not only broaden the width of the interface but they can also destroy the orientational order in highly swollen lamellar phases (see Fig. 1 for a phase diagram extracted from Monte Carlo Simulations). Those phases occur in mixtures of diblock-copolymers and homopolymers. The addition of homopolymers swells the distance between the lamellae, and the self-consistent field theory predicts that this distance diverges at Lifshitz points. However, general considerations show that mean-field approximations are bound to break down in the vicinity of lifshitz points [61]. (The upper critical dimension is du = 8). This can be quantified by a Ginzburg criterion. Fluctuations are important if... 

Hydrating under shear, eg by vigorous stirring, reduces the vesicle size to the submicron range. Details of the shear-induced formation of vesicles from swollen lamellar phases have been investigated (92). 

Fig. 8. Block copolymer vesicles budding-off from a swollen lamellar phase. After separation, there is no further exchange of block copolymers between vesicles, from Ref 50, with permission from ACS Publications Division.

Ionic Amphiphiles. A variety of ionic amphiphiles were investigated for their phase behavior in water (W). A swollen lamellar phase formed by SDS, pentanol, and NaBr brine was investigated in the presence of Co(III) coordination compounds the analysis of the AvqS of H, Na, Br and Co nuclei... 

A special case is that of rigid interfaces with a spontaneous curvature close to zero, such as the case of oil-swollen lamellar phases, when long-chain doubletailed surfactants form oil-swollen layers separated by water layers. These phases present strong geometrical constraints and the thickness of layers are easily measurable with precision by means of scattering experiments. The variation of composition associated with the addition of a solute can be detected by visual inspection between crossed polarizers and by chemical analysis. However, to our knowledge, no experimental results on curvature variation, area per molecule variation and maximum admissible concentration of a solute in these systems is yet available. 

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