Hydrophobically associating copolymers

Varadaraj R, Branham KD, McCormick CL, Bock J (1994) Analysis of hydrophobically associating copolymers utilizing spectroscopic probes and labels. In Dubin P, Bock J, Davis R, Schulz DN, Thies C (eds) Macromolecular complexes in chemistry and biology. Springer-Verlag, Berlin, p 15... 

Hydrophobically Associating Copolymers. Hydrophobically modified cellulose derivatives (28) and N-alkylacrylamido copolymers (24, 25, 27) were among the first nonionic associative thickeners reported in the patent literature. The concentration of hydrophobic units allowed for dissolution in aqueous solution is usually less than 1-2 mol %. Like conventional polymers, apparent viscosity is proportional to molecular weight and concentration. However, with associative copolymers, a very dramatic increase in apparent viscosity occurs at a critical concentration, C, which clearly is related to a phenomenon other than simple entanglement. Viscosity dependence on hydrophobe concentration, size, and distribution suggests mi-croheterogeneous phase formation. Surfactants enhance viscosity behavior in some instances (24), yet clearly reduce viscosity in others (i). 

Terpolymer solution behavior is dependent on compositional microstructure, polymer concentration, ionic strength, pH, temperature, and shear history. The viscosities of these low charge density terpolymers would be expected to be insensitive to changes in ionic strength below the critical overlap concentration, C above C intermolecular attractive forces might be expected to cause enhanced viscosities similar to those of hydrophobically associating copolymers. 

Three different approaches were used for preparing the hydrophobically associating copolymers shown in Figure 1. 

Formation of hydrophobically associated copolymers of polyacrylic acid with long-chain amines by use of a carbodiimide [129]. 

Analysis of Hydrophobically Associating Copolymers Utilizing Spectroscopic Probes and Labels... 

Varadaraj et al. [29] have demonstrated that the Ej 30) probe can be used as an extrinsic probe to study the hydrophobic microdomains formed due to polymer association in the hydrophobically associating copolymer system acrylamide-alkylacrylamide. The probe was solubilized in the aqueous polymer solution and absorption spectra recorded. Results are given in Table 2.1. 

The solubility characteristics of hydrophobically associating copolymers of p(alkyl acrylamide-co-acrylamide), RAM, have been studied by a number of... 

Zhou, H., Huang, R.H., 1997. Study of the solution properties of water-soluble hydrophobically associating Acrylamide/n-Octylacrylate copolymers. Oilfield Chemistry 14 (3), 252—256. 

This conclusion has been reached in the copolymerization of acrylic acid (AA) and methacrylic acid (MAA) with N-vinylpyrrolidone (34, 35) (a monomer noted for its thermal stability (36) and discussed in Chapter 9) and with acrylamide (37). The incorporation of the acid monomer in the copolymer decreases with increasing solution pH. The r values with MAA are particularly low at pH <5 because of hydrophobic associations of the methyl groups. Laser-Raman studies (38) have also indicated intramolecular association among the methyl groups of syndiotactic poly(methacrylic acid) (PMAA) in aqueous solution. The addition of NaCl, at a moderate pH, increases the amount of neutralized, weak acid monomer incorporated (Table III). A more gradual change is observed with pH in the MAA-AM combination than in the AA-AM pair because of the hydrophobic interactions cited. The relationships are nearly quantitative with the ionization of the acids as reflected by the pK of the monomer and polymer acid sequences. 

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. 

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. 

Conceptual Model. The behavior of the hydrophobically modified acrylamide copolymers of this work has led us to propose the model illustrated in Figure 10. The hydrophobic groups on the N-alkylacrylam-ide-acrylamide copolymers exhibit concentration-dependent association. In region 1, low hydrophobe concentration on the copolymer, low copolymer concentration, or both conditions are insufficient to bring about significant association. In region 2, hydrophobic association increases, but associations... 

Domain associations in hydrophobically modified copolymers and associations of macroions in aqueous solution can lead to remarkable solution properties in properly tailored systems. An example of each type and selected observations of experimental findings were presented herein. The challenge now for the scientific community is to thoroughly understand the balance of structural forces responsible for macroscopic behavior so that systems may be properly tailored for specific commercial application. 

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