Micelles of PEO Amphiphilic Macromonomers

An emulsifier is a molecule that possesses both polar and nonpolar moieties, i.e., it is amphiphilic. In very dilute water solutions, emulsifiers dissolve and exist as monomers, but when their concentration exceeds a certain minimum, the so-called critical micelle concentration (CMC), they associate spontaneously to form aggregates - micelles. Micelles are responsible for many of the processes such as enhancement of the solubility of organic compounds in water, catalysis of many reactions, alteration of reaction pathways, rates and equilibria, reaction loci for the production of polymers, etc. 

The outer-core region of the micelle, commonly referred to as the palisade layer, may provide a medium of intermediate polarity that affects the energetics of transition state formation. The primary influence of micelles is to concentrate 

An important group of surface-active nonionic synthetic polymers (nonionic emulsifiers) are ethylene oxide (block) (co)polymers. They have been widely researched and some interesting results on their behavior in water have been obtained [33]. Amphiphilic PEO copolymers are currently of interest in such applications as polymer emulsifiers, rheology modifiers, drug carriers, polymer blend compatibilizers, and phase transfer catalysts. Examples are block copolymers of EO and styrene, graft or block copolymers with PEO branches anchored to a hydrophilic backbone, and star-shaped macromolecules with PEO arms attached to a hydrophobic core. One of the most interesting findings is that some block micelle systems in fact exists in two populations, i.e., a bimodal size distribution. 

With nonionic PEO emulsifiers, intermolecular interactions vary with temperature and types of metal ions and solvents. At low temperatures, nonionic emulsifiers are hydrophilic and form normal micelles. At higher temperatures they are lipophilic and form reverse micelles. A weak interaction with metal ions favors the stability of associates against moisture. On the other hand, a strong interaction may lead to a completely amorphous system. Ethanol as a co-solvent is a moderate solvent for PEO at low temperatures, but its power improves as the temperature is raised [34]. This means that solutions of the PEO copolymers in water and ethanol have opposing temperature coefficients of solubility negative for water and positive for ethanol. 

The water solubility of R-(EO)n types of nonionic emulsifiers is derived from the weak interaction between the ether oxygen of EO unit and water. It was suggested that each EO unit in the PEO chain, requires three molecules of water to form a hydrated complex [35]. This hydrogen bond complex is destroyed if the solution is taken above the melting point of the PEO. Water usually acts as plasticizer when present in hydrophilic PEO polymers and Tg values decrease with increasing water contents [36]. This phenomenon in the PEO-water system is observed up to 1 mol water/ether group. Beyond this a rise in Tg is observed and water acts as an antiplasticizer. 

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