Worm-like micelles solution structure

Macrophase separation after microphase separation has been observed in an AB block copolymer/homopolymer C blend (Hashimoto et al 1995). Blends of a PS-PB starblock copolymer (75wt% PS) and PVME homopolymer were prepared by solvent casting. Binary blends of PS and PVME exhibit a lower critical solution temperature (LCST), i.e. they demix at high temperatures. The initial structure of a 50% mixture of a PS-PB diblock and PVME shown in Fig. 6.20(a) consists of worm-like micelles. Heating led to macrophase separation as evident... 

FIG. 2.7 Relevant length scales for the colloidal structure of worm-like micelles contour length, Lc, entanglement length, /e, mesh size, m, persistence length, Zp and cross-sectional radius, rcs. Values shown are those measured for a solution with 1.5% total surfactant at a cetyltrimethylammonium tosylate (CTAT)/sodium dodecylbenzenesulfonate (SDBS) ratio of 97/3 with 0.10% added sodium tosylate. (Reproduced from Schubert, B.A., Kaler, E.W., and Wagner, N.J., Langmuir, 19, 4079 -089, 2003. With permission.)... 

The microstructure in solutions of the asymmetrical geminis m-s-rri, 2Br" was systematically investigated by various techniques [55,57]. For small values ot m + or large values ot m — m only spherical or worm-like micelles were formed. For large values oim + m or small values otm m lamellar and tubular structures were observed (Fig. 14). 

Nordskog et al. observed a behavior similar to that found by Bronstein et al. for mixed systems of PEO62- -PB40 with DTAB [29]. In this work, worm-like BCP micelles were characterized by SANS, DLS, and Cryo-TEM, Upon addition of the cationic surfactant, the apparent hydrodynamic radius (/ h) calculated for these block copolymer solutions started to decrease until a plateau value of approximately 15 nm was reached. Further addition of surfactant did not lead to any further change in size of the mixed polymer/surfactant micelles. Using SANS it was possible to show that above a certain threshold concentration, excess micelles of the surfactant were formed. Below this threshold, all added surfactant is incorporated into mixed BCP/DTAB micelles and, hence, the change in size is due to structure reorganization induced by the DTAB. 

Surfactants, when dissolved in water, can form aggregates called micelles. The micelles are generally spherical at small surfactant concentration and become rod-like at higher concentrations. The evolution is faster when salt is added to an aqueous solution of an ionic surfactant the salt screens the electrostatic interactions between the polar parts of the surfactant molecules, and lowers the aggregates curvature. In some cases, very long aggregates can form.2> Because of thermal fluctuations, they adopt, like polymers, worm-like structures. These structures have also been observed in microemulsions (oil-water-surfactant mixtures), where their occurrence was predicted earlier... 

Fig. 9 is a schematic phase diagram of a dilute aqueous cationic surfactant solution showing temperature and concentration effects on its microstructures. When the temperature is lower than the Krafft point [the temperature at which the solubility equals the critical micelle concentration (CMC)], the surfactant is partially in crystal or in gel form in the solution. At temperatures above the Krafft point and concentrations higher than the CMC, spherical micelles form in the surfactant solution. With further increase in concentration and/or on addition of counterions, the micelles form cylindrical rods or threads or worms with entangled thread-like and sometimes branched threadlike structures. 

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