Micellar hydrophobe association polymers

Hydrophobically associating polymers consist primarily of water-soluble monomer units with a small number of water-insoluble monomer units. Synthesis of high-molecular-weight random copolymers of acrylamide and alkylacrylamides required a novel aqueous surfactant micellar solution polymerization (2-4) because of the mutual immiscibility of the water-soluble and hydrophobic monomers. The use of surfactant micelles enabled solubilization of the hydrophobic monomer (alkylacrylamide [R]) into the aqueous phase containing the water-soluble monomer (acrylamide [AM]). The resulting RAM polymer after isolation provided homogeneous aqueous solutions. 

In this chapter, recent developments in the design of hydrogels formed via hydro-phobic interactions are reviewed, with a special emphasis on the function of surfactant in their dynamic and mechanical properties. Micellar copolymerization is a simple technique for the synthesis of such hydrogels. A particular advantage of this technique is the blocky structure of the resulting hydrophobically modified polymers, significantly enhancing their associative properties. 

Hydrophobically associating acrylamide based polymers were explored as a means of alleviating the salt sensitivity observed in the block systems. A micellar polymerization technique was developed to enable preparation of random copolymers of acrylamide and N-n-alkylacrylamide. When these copolymers were dissolved in an aqueous solvent, the hydrophobic groups associated to minimize their exposure to water. The hydrophobic associations provided an additional dimension to polymer molecular weight and chain expansion by ionic groups for the control of aqueous fluid rheology. 

Flynn and Goodwin prepared copolymers of acrylamide and dodecyl methacrylate using micellar copolymerization [96]. The hydrophobe content was varied from zero (polyacrylamide) to 0.7 mol%. Polymer solutions contained O.IM NaCl, and sodium azide was added as a biocide. Figure 38 depicts the variation of the reduced viscosity with polymer concentration at various hydrophobe contents. At 0 mol% hydrophobe, the reduced viscosity increased linearly with polymer concentration indicating no hydrophobic association. At 0.2 mol% hydrophobe, the reduced viscosity increased linearly with polymer concentration, but at a polymer concentration of 600 ppm there was an upward variation in the reduced viscosity. Similar results were obtained at hydrophobe contents of 0.4 and 0.7 mol%. However, the polymer concentration at which the upward variation occurred decreased with increasing hydrophobe content. 

Fig. 10.14 Pictorial representation of a telechelic polymer network made up of hydrated polymer chains. The polymer chains are cross-linked by the micellar junctions formed by hydrophobic association of the end chains. Chain association and hydration are expected to be independent except in the region near the junctions.

Figure 19 Association structures of hydrophobically substituted polymers (clockwise from left) comb, end-substituted, end-substituted associating via a surfactant micellar bridge. (Courtesy D. R. Bassett.)... 

These polymers by virtue of their chemistry self assemble in aqueous media,driven by the entropy gain associated with the liberation of water bounding the previously unassociated hydrophobic entities — the so called hydrophobic association. Self assembly yields extremely stable entities with critical micellar concentrations in the iM range, compared to the mM range seen for surfactants such as Triton-X 100 and the pluronic block copolymers. This exceptional stability stems from the ability of each amphiphilic molecule to engage in multiple hydrophobic associations proof of this multiple hydrophobic contact hypothesis is provided by evidence that more stable micelles (with a... 

Micelles are nano or micro structures with a hydrophobic core and a hydrophilic shell. They form above a eritieal eoneentration called the eritical micellar concentration (CMC) as a result of the self-association of amphiphilic molecules to minimize hydrophobic interactions. Micellar surfactants and polymers are widely used in the pharmaceutical industry as formulation aids in solids, semi-solids, suspensions, emulsions, and solutions. One of the... 

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