Structure hydrophobically associating polymers

The polymer is hydrophobically associating water soluble, meaning it contains one or more water-soluble monomers (acrylamides) and a small fraction (0.5 to 4%) of water-insoluble (hydrophobic) monomers. A typical hydrophobically associating polymer (HAP) structure is... 

Bock, J., D. B. Siano, P. L. Valint, Jr., S. J. Pace, Structure and Properties of Hydrophobically Associating Polymers, in Polymers in Aqueous Media Performance through Association, Glass, J. E., Ed., Advances in Chemistry Series No. 223, American Chemical Society, Washington, DC, 1989, pp. 411-424. 

Candau, F., S. Biggs, A. Hill, and J. Selb. 1994. Synthesis, structure and properties of hydrophobically associating polymers. Progress in Organic Coatings 24(1 ) 11-19. 

Hydrophobically associating polymers are water soluble polymers containing a small amount of hydrophobic functionality. The conformation of the polymer in solution is influenced by a variety of structural parameters. Polymer architecture, such as random or blocky arrangement of the hydrophobic groups will have a significant influence on polymer conformation and interactions in solution. The presence of hydrophobic functionality can result in inter- or intramolecular association or some combination of both. The relative amounts of these types of association will affect the conformation of the molecules in solution and in turn the properties of the solution. While many chemically different systems, such as hydrophobically modified cellulose [19], urethanes [15], and others [11] have been described in the literature, we will focus on only a few critical structures. To illustrate the relationships between structure and the observed solution phenomena, three types of hydrophobically associating polymers will be discussed as shown in Fig. 3.1. These are ... 

Fig. 3.1. Synthetic approaches and structures for hydrophobically associating polymers. Reprinted with permission from Ref. [4]...

We now know that emulsion polymerization is not just another polymer synthesis method and that the complexity of the interactions, whether chemical or physical, must he considered before any control is possiUe over the outcome of the reaction. The creation and nucleation of particles, for example, is not necessarily and simply explained by the presence or or absence of micelles, but needs the understanding of interactions of all the ingredients present. Variables such as hydrophilic and hydrophobic associations or repulsions, polarity of the monomers, chemical structure of the surfactants, have to lx taken into account. 

Hydrophobically modified polymers can associate in aqueous media to form micelle-like structures above their critical association concentrations (CACs). The nanosized self-aggregates were prepared using modified natural polysaccharides such as pullulan, curdlan, and glycol chitosan. The modified polysaccharides provide excellent biocompatibility, biodegradability, low immunogenicity, and biological activities. 

Both of the types of polymer mentioned above can be modified by the incorporation of hydrophobic monomers onto the essentially hydrophilic acrylate backbone. The effect of this is to modify their characteristics by giving them so-called associative properties. These hydrophobes can interact or associate with other hydrophobes in the formulation (e.g., surfactants, oils, or hydrophobic particles) and thus build additional structures in the matrix [3-11]. These associative polymers are termed cross-polymers when they are based on carbomer-type chemistry [12] and hydrophobically modified alkali-soluble emulsions (HASEs) when based on ASE technology. 

The nature of hydrophobic interactions and their effects on the structure and properties of water have been extensively studied, particularly for small molecules (i 3). In contrast, the introduction of hydrophobic associations into synthetic water-soluble polymers to control solution rheology has received rather limited and recent study (4-7). To better understand the relationships between polymer structure and solution properties, we have synthesized and characterized a series of copolymers of acrylamide and N-substituted alkylacrylamides and terpolymers containing anionically charged carboxyl groups. Solution properties of these systems have been obtained in both the dilute and semidilute concentration regime, to probe the influence of intra- and intermolecular interactions. In addition, the influence of the shear field and solvent quality on the associations was studied. 

The associative mechanism of thickening has been variously described, but is generally thought to result from nonspecific hydrophobic association of water-insoluble groups in water-soluble polymers 34, 35). For associative ASTs, the terminal hydrophobes of the ethoxylated side chains are considered to be the primary interactive components. These hydrophobes can interact with each other via intermolecular association, and can also interact with hydrophobic particle surfaces when present. The specific interaction with dispersed-phase components such as latex particles has been shown to be one of surface adsorption (36). In essence, the associative component of thickening in dispersed-phase systems also has dual character resulting from the building of structure within the aqueous phase and interaction with particle surfaces. 

MYL Mylonas, Y., Bokias, G., lliopoulos, L, and Staikos, G., Interpolymer association between hydrophobically modified poly(sodium aciylate) and poly(Y-isopropyl-aciylamide) in water The role of hydrophobic interactions and polymer structure,... 

Figure 7.6. Indices of association of azobenzene onto hydrophobic domains, (a) Representative spectra of an azo-modified polymer in water (under blue exposure) with increasing concentration of surfactant Cl 2E4 (0-0.4 g/L). Polymer structure cf. left-hand side in Fig. 7.1, with n = 11 andx = 2%. (b) Variation of the fraction of bound azobenzene upon addition of Triton X 100 in a solution of polymer at fixed concentration (polymer similar as in (a), with n = 5 and x = 4%).

---