Polymer-bound hydrophobes, association

The first pathway is the formation of mixed micelles or hemimicelles, composed of polymer-bound hydrophobes comicellized with surfactant molecules. Intermolecular physical cross-links often enhance the viscosity of the micellar solutions. The second pathway is intramolecular comicellization so that the hydrodynamic size of the associates contracts. 

Because of their chemical similarity, the polymer-bound hydrophobes have a tendency to interact with the hydrophobic part of the surfactant molecule. If the surfactant concentration in the system is high enough, micelles are formed. If there are enough micelles in the system, then all the hydrophobes will get bound to micelles (Figure 14). As a result, there will be no intermolecular hydrophobic association (Figure 2) and no viscosity... 

Surfactant concentration (varied after polymerization) greatly affects the viscosity of associating polymer systems. Iliopoulos et al. studied the interactions between sodium dodecyl sulfate (SDS) and hydrophobically modified polyfsodium acrylate) with 1 or 3 mole percent of octadecyl side groups [85]. A viscosity maximum occurred at a surfactant concentration close to or lower than the critical micelle concentration (CMC). Viscosity increases of up to 5 orders of magnitude were observed. Glass et al. observed similar behavior with hydrophobically modified HEC polymers. [100] The low-shear viscosity of hydrophobically modified HEC showed a maximum at the CMC of sodium oleate. HEUR thickeners showed the same type of behavior with both anionic (SDS) and nonionic surfactants. At the critical micelle concentration, the micelles can effectively cross-link the associating polymer if more than one hydrophobe from different polymer chains is incorporated into a micelle. Above the CMC, the number of micelles per polymer-bound hydrophobe increases, and the micelles can no longer effectively cross-link the polymer. As a result, viscosity diminishes. 

The association of polymer-bound hydrophobes can occur either within a single polymer chain or between different polymer chains, or both at a time, depending on the type of amphiphilic polymer. In Ughly dilute aqueous solutions, in general, hydrophobic associations may occur witiiin a polymer chain, but with an increase in the polymer concentration, a tendency for inteipolymer association may increase. In the case of hydrophobically-modified nonionic polymers, tiie association of polymer-bound hydrophobes can occur even if the hydrophobe content in a polymer is very low (4-6). In the case of hydrophobically-modified polyelectrolytes, however, a relatively Ugh content of hydrophobes is necessaiy for the association to occur, because... 

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... 

Hydrophobic functional-molecules can be incorporated into the higher-order unimer micelles either via chemical bonding of the functional molecules to amphiphilic polymers or by physically solubilizing tiie mol ules into the unimer micelle. For example, when an AMPS-DodMAm copolymer and a hydrophobic small-molecule are dissolved together in ethanol, and the ethanol solution is poured into excess water, then the hydrophobic molecules can be entrapped in a hydrophobic microdomain during the process of the hydrophobic self-association of polymer-bound dodecyl groups. However, it is practically difficult to precisely control the number of the sm l... 

Figure 7.15 shows that the apparent intrinsic viscosity of modified polymer (14-4) is higher (10.9 dl/g) than that of unmodified HEC (6.6 dl/g). This fact is consistent with the existence of hydrophobic association formed by the bound fluorocarbon chains in this system. Apparently, intermolecular association of sample 14-4 still prevails at the lowest concentration ( 0.06 gr/dl). 

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