Polymer micelles water solubility

Chain-Growth Associative Thickeners. Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain-growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles (50). Although the initiation and propagation occurs primarily in the aqueous phase, when the propagating radical enters the micelle the hydrophobically modified monomers then polymerize in blocks. In addition, the hydrophobically modified monomer possesses a different reactivity ratio (42) than the unmodified monomer, and the composition of the polymer chain therefore varies considerably with conversion (57). The most extensively studied monomer of this class has been acrylamide, but there have been others such as the modification of PVAlc. Pyridine (58) was one of the first chain-growth polymers to be hydrophobically modified. This modification is a post-polymerization alkylation reaction and produces a random distribution of hydrophobic units. 

Water-soluble initiator is added to the reaction mass, and radicals are generated which enter the micelles. Polymerization starts in the micelle, making it a growing polymer particle. As monomer within the particle converts to polymer, it is replenished by diffusion from the monomer droplets. The concentration of monomer in the particle remains as high as 5—7 molar. The growing polymer particles require more surfactant to remain stable, getting this from the uninitiated micelles. Stage I is complete once the micelles have disappeared, usually at or before 10% monomer conversion. 

Emulsion Polymerization. In this method, polymerization is initiated by a water-soluble catalyst, eg, a persulfate or a redox system, within the micelles formed by an emulsifying agent (11). The choice of the emulsifier is important because acrylates are readily hydrolyzed under basic conditions (11). As a consequence, the commonly used salts of fatty acids (soaps) are preferably substituted by salts of long-chain sulfonic acids, since they operate well under neutral and acid conditions (12). After polymerization is complete the excess monomer is steam-stripped, and the polymer is coagulated with a salt solution the cmmbs are washed, dried, and finally baled. 

The core of reversed micelles can be transformed to a highly viscous domain (nanogel) by entrapping appropriate species, such as viscous solvents and hydrophilic macromolecules, or by performing in situ appropriate polymerization reactions or intramolecular cross-linking of water-soluble polymer chains [232-234]. 

A carboxylate derivative of a fully aromatic, water-soluble, hyperbranched polyphenylene is considered as a unimolecular micelle due to its ability to complex and solubilize non-polar guest molecules [23]. The carboxylic acid derivative of hyperbranched polyphenylene polymer (HBP) (My,=5750-7077, Mn=3810-3910) consists of 40-60 phenyl units that branch outward from a central point forming a roughly spherical molecule with carboxylates on the outer surface. The free acid form of HBP was suspended in distilled water and dissolved by adding a minimum quantity of NaOH. The solution was adjusted to pH 6.2 with aqueous HCl. Calcium carbonate crystals were growth from supersaturated calcium hydrogencarbonate solution at room temperature. HBP gave... 

Surfactants and Colloids in Supercritical Fluids Because very few nonvolatile molecules are soluble in CO2, many types of hydrophilic or lipophilic species may be dispersed in the form of polymer latexes (e.g., polystyrene), microemulsions, macroemulsions, and inorganic suspensions of metals and metal oxides (Shah et al., op. cit.). The environmentally benign, nontoxic, and nonflammable fluids water and CO2 are the two most abundant and inexpensive solvents on earth. Fluorocarbon and hydrocarbon-based surfactants have been used to form reverse micelles, water-in-C02... 

Fig. 30 Types of nanocarriers for drug delivery, (a) Polymeric nanoparticles polymeric nanoparticles in which drugs are conjugated to or encapsulated in polymers, (b) Polymeric micelles amphiphilic block copolymers that form nanosized core-shell structures in aqueous solution. The hydrophobic core region serves as a reservoir for hydrophobic drugs, whereas hydrophilic shell region stabilizes the hydrophobic core and renders the polymer water-soluble.

The monomers get absorbed in micelles resulting in their swelling. Water soluble initiators are used which form free radicals. Inorganic persulphates are commonly used as initiators. The initiator diffuses into a micelle and polymerisation proceeds. As more monomer is polymerised monomers from outside the micelle diffuse inside and the process continues when another radical enters the micelle the polymerisation stops. This technique can give high Molecular weight polymers. 

Charged polysoaps (polymer micelles) combine within a molecule structural characteristics of the conventional micelles and polyelectrolytes, and supposedly adopt globular conformations in aqueous media with the hydrophobic region inside and charged groups outside as in water-soluble proteins. Thus,... 

Micelle solutions of PlPAAm-Ci8H35 was prepared by direct dissolution of the polymer in cold water (4°C) due to its good water solubility [23]. Each solution of PIPAAm-PSt, PlPAAm-PBMA, and PIPAAm-PLA was prepared by dissolving each copolymer in DMF, A-ethylacetamide, and DMAc, respectively. The solutions were put into a dialysis bag (MWCO = 13,000) and dialyzed against distilled water at 10°C, 20°C, and 4°C, respectively, for 24 hours. The micelles were purified with ultrafiltration membrane of 200,000 molecular weight cut off at 4°C. The aqueous solution was lyophilized to leave a white powder of micelles. 

Recently, Mecking et al. reported the synthesis of inverse micelles based on a hy-perbranched polyglycerol polymer. Terminal -OH groups were modified with palmi-toyl chloride and gave a polymeric catalyst soluble in organic solvents with hydrophilic core to immobilize water-soluble guest molecules such as PdCl2 or Pd(OAc)2. 

The particle number remains the same in interval III as in interval II, but the monomer concentration decreases with time, since monomer droplets are no longer present. The decrease in 4>m is slower with the more water-soluble monomers as the monomer in solution acts as a reservoir. The presence of a gel effect continues in interval IE. The quantitative interplay of a decreasing monomer concentration with the gel effect determines the exact behavior observed in this interval (GF or H). Polymerization continues at a steadily decreasing rate as the monomer concentration in the polymer particles decreases. Final conversions of essentially 100% are usually achieved. The final polymer particles, spherical in shape, usually have diameters of 50-300 nm, which places them intermediate in size between the initial micelles and monomer droplets. 

Microemulsion polymerization is an emulsion polymerization with very much smaller monomer droplets, about 10-100 nm compared to 1-100 pm. Micelles are present because the surfactant concentration is above CMC. The final polymer particles generally have diameters of 10-50 nm. Although many of the characteristics of microemulsion polymerization parallel those of emulsion polymerization, the details are not exactly the same [Co et al., 2001 de Vries et al., 2001 Lopez et al., 2000 Medizabial et al., 2000]. Water-soluble initiators are commonly used, but there are many reports of microemulsion polymerization with... 

One of the possible alternative to micelles are spherical dendrimers of diameter generally ranging between 5 and 10 nm. These are highly structured three-dimensional globular macromolecules composed of branched polymers covalently bonded to a central core [214]. Therefore, dendrimers are topologically similar to micelles, with the difference that the strnctnre of micelles is dynamic whereas that of dendrimers is static. Thus, unlike micelles, dendrimers are stable nnder a variety of experimental conditions. In addition, dendrimers have a defined nnmber of fnnctional end gronps that can be functionalized to prodnce psendostationary phases with different properties. Other psendostationary phases employed to address the limitations associated with the micellar phases mentioned above and to modnlate selectivity include water-soluble linear polymers, polymeric surfactants, and gemini snrfactant polymers. 

However, emulsion polymerizations involve the formation of colloidal polymer particles that are essentially permanently suspended in the reaction medium. The reaction mechanism involves the migration of monomer molecules from liquid monomer droplets to sites of polymerization that originate in micelles consisting of surface-active agent molecules surrounding monomer molecules. Emulsion polymerizations are usually characterized by the requirement of surfactants during the initiation of the process and by the use of water-soluble initiators. This process also permits good control of the exothermic nature of the polymerization. 

Emulsion polymerization is applicable only to monomers that are relatively insoluble in water, such as styrene. A coarse emulsion of monomer in aqueous surfactant is prepared with a water-soluble initiator, say, H202 in the solution. The surfactant concentration is above the CMC, so surfactant molecules are present as monomers, micelles, and emulsifiers at the oil-water interface. Even an insoluble liquid like styrene dissolves in water to some extent. Therefore the monomer is present in coarse emulsion drops, solubilized in micelles, and as dissolved molecules in water. A schematic illustration of the distribution of surfactant, monomer, and polymer in an emulsion polymerization process is shown in Figure 8.14. 

Preparation of hydrophobically modified, water-soluble polymer in aqueous media by a chain growth mechanism presents a unique challenge in that the hydrophobically modified monomers are surface active and form micelles. 

---