Polymers, gels micellar systems

The phenomenology of physical organogels and jellies is extremely rich, and their comportments are similar in some aspects to those of both surfactants in solution (e.g., lyotropism and crystallization) and polymer solutions (6 (e.g.. swelling/shrinking behaviors and microscopic mass motion). Gels can be considered as being at the interface between complex fluids (i.e.. micellar systems) and phase-separated states of matter. The main properties and concepts appropriate to describe the gels and the basic principles of techniques for their study will be reviewed here. 

The extreme sensitivity of the visible absorption spectrum to small changes in the surrounding medium has made this betaine dye a useful molecular probe in the study of micellar systems [298, 299, 443-445], mieroemulsions and phospholipid bilayers [299], model liquid membranes [300], polymers [301, 446], organic-inorganie polymer hybrids [447], sol-gel matrices [448], surfaee polarities [449-451], and the retention behaviour in reversed-phase liquid chromatography [302]. Using polymer membranes with embedded betaine dyes, even an optical alcohol sensor has been developed [452]. 

Scandium triflate-catalyzed aldol reactions of silyl enol ethers with aldehyde were successfully carried out in micellar systems and encapsulating systems. While the reactions proceeded sluggishly in water alone, strong enhancement of the reactivity was observed in the presence of a small amount of a surfactant. The effect of surfactant was attributed to the stabiMzation of enol silyl ether by it. Versatile carbon-carbon bondforming reactions proceeded in water without using any organic solvents. Cross-linked Sc-containing dendrimers were also found to be effective and the catalyst can be readily recycled without any appreciable loss of catalytic activity.Aldol reaction of 1-phenyl-l-(trimethylsilyloxy) ethylene and benzaldehyde was also conducted in a gel medium of fluoroalkyl end-capped 2-acrylamido-2-methylpropanesulfonic acid polymer. A nanostmctured, polymer-supported Sc(III) catalyst (NP-Sc) functions in water at ambient temperature and can be efficiently recycled. It also affords stereoselectivities different from isotropic solution and solid-state scandium catalysts in Mukaiyama aldol and Mannich-type reactions. 

Abstract This chapter discusses the potential of fluorescence correlation spectroscopy (PCS) to study polymer systems. It introduces the technique and its variations, describes analysis methods, points out advantages and limitations, and summarizes PCS studies of molecular and macromolecular probes in polymer solutions, polymer gels, polymer nanoparticles, and polymeric micellar systems. In addition, a comparison with other experimental methods is presented and the potential of a combination with simulations discussed. 

Keywords Pluorescence correlation spectroscopy Pluorescence microscopy Pluorescent probes Polymer solutions Polymer gels Polymer nanoparticles Polymeric micellar systems Translational diffusion... 

In the illustrated micellar system, each polymer molecule is obviously constrained in sampling the conformations compared to an ideal homogeneous system, because of the tethering in space. If the constraints are very strong, only a few conformations will remain, and the mean-field picture will break down. But, nevertheless, in almost all of these applications the interactions are weak, the gel is soft and fragile, enough of the coil volume remains open for other chains and solvent, and the mean field will still be a very good approximation. 

Another actively developing and interesting direction is the investigation of interactions of polyelectrolytes and other polymers with surfactants. At Moscow State University a significant contribution to this area has been made by the associates of the scientific school founded by V.A. Kargin and V.A. Kabanov. In their studies a number of systems with peculiar structures were described, such as those containing micellar beads attached to the macromolecular chains, or systems in which gels form at very low polymer and surfactant concentrations. 

Fig. 2 (a-c) Physical polymer-network cross-linking provided by mixed micelles in hydrogels formed via hydrophobic interactions in surfactant solutions. Mixed micelles are formed by aggregation of hydrophobic blocks of per-se hydrophilic polymers and surfactant alkyl tails, (b) Nonionic polymer and ionic surfactant gel system at the state of preparation. For clarity, charges are not shown, (c) Ionic polymer and oppositely charged surfactant gel system after extraction of free micelles, (d) Structure of the hydrophobic monomers used in the micellar polymerization... 

A fringed-micellar structure was proposed in 1930 for the sttuctures of colloids and gels [19]. By adding the possibility of chain-folded crystals, as illustrated in the center of Fig. 5.42, all three limiting macroconformations are combined. Semicrystalline polymers, thus, are a system consisting of folded-chain crystals, intercrystalline amorphous, and possibly extended-chain subsystems (see Fig. 2.80). The latter are expected particularly in fibers drawn to large extension. Most samples have a macroconformation somewhere within the triangle of Fig. 5.42. 

The di-block system of PLLA-PEG/PDLA-PEG enantiomers also showed thermo-responsive sol-gel transition. However, the solution of individual polymers does not show any gelation with temperature variation. In an aqueous solution, individual copolymers of PLLA-PEG or PDL A-PEG form micelles with a core of PLLA/PDLA and a shell of PEG chains on mixing with each other, these lead to hexagonal crystal formation of polymer chains and gelation at room temperature. With a rise in temperature, hydrophobic interactions increase, which enhances micellar aggregation. These polymers exhibit irreversible gel-to-sol conversion at 75 °C. At higher temperatures... 

---