Hydrophobe level

Effect of Hydrophobe Level on Solution Behavior of HMHEC... 

As the hydrophobe content of the HMHEC increases, the viscosity first increases, reaches a maximum, and then decreases rapidly. The viscosity finally approaches that of the solvent as the polymer becomes insoluble. A minimum hydrophobe level (discussed in detail later) is necessary to achieve an observable increase in viscosity as compared to that of the unmodified HEC. Above this threshold, viscosity increases rapidly with increase in the hydrophobe content. After reaching the peak viscosity at an appropriate hydrophobe level, the viscosity decreases upon further increase in hydrophobe level. This decrease is not caused by a diminution in the degree of hydrophobic association, but by the incomplete solubility of the sample. That is, a given HMHEC sample has a distribution of polymer chains with different hydrophobe levels. The proportion of insoluble species increases as the overall hydrophobe content of the sample increases. The hydrophobe... 

Figure 10 shows a plot of intrinsic viscosity versus hydrophobe level for two HMHEC samples that have different hydrophobes. As the hydrophobe content at fixed molecular weight increases, the intrinsic viscosity decreases. This behavior is even more striking for CieHMHEC. In dilute solutions, the HMHEC molecules are separate and untangled, and no intermolecular association occurs. However, to minimize the disruption of water structure, hydrophobes on the same chain tend to cluster. This clustering results in a much reduced hydrodynamic volume, and a lower viscosity is observed. Therefore, the dilute solution properties of HMHEC are consistent with intramolecular hydrophobic association (10). Because of intramolecular association and incomplete solubility, the intrinsic viscosity of HMHEC cannot be appropriately correlated with its molecular weight. Hence, to obtain meaningful intrinsic viscosity data, intramolecular association must be com-... 

Polymer Synthesis. Copolymers of alkylacrylamide (R) and acrylamide (AM), which we called RAM, were prepared with a micellar polymerization technique (4). A micellar surfactant solution was used to disperse the hydrophobic alkylacrylamide monomer into an aqueous phase that contained acrylamide. The monomers were polymerized with a standard free-radical initiator (e.g., potassium persulfate) or a redox initiator to yield the desired random copolymer. Varied temperature and initiator concentrations were used to provide polymers of different molecular weights. Polymerizations were taken to essentially complete conversion. Compositions, in terms of hydrophobe level reported in this chapter, were based on amounts charged to the reactor. Further details on the synthesis and structure of these RAM polymers... 

Semidilute Viscometrics. Solution viscometrics at concentrations above the overlap concentration (C ) indicated dramatic effects caused by the associative nature of the hydrophobic groups in the polymer. As shown by the reduced viscosity-concentration profiles of Figure 3, the introduction of only 1.0 mol % N-n-octylacrylamide to polyacrylamide can increase the viscosification efficiency dramatically. Increasing the hydrophobe level to 1.25 mol % further increased solution viscosity. At 2000 ppm, the presence of the hydrophobe caused a greater that 10-fold increase in viscosity. This result was in contrast to the behavior of these polymers in dilute solution see the box in Figure 3). The presence of hydrophobic functionality on the polymer resulted in a decrease in the reduced viscosity at concentrations below C. In dilute solution, intramolecular hydrophobic associations decreased the hydrodynamic radii of the polymer coils and thus reduced the... 

Figure 5. Effect of hydrophobe level on the shear rate dependence of viscosity. Solvent is 2.0 wt % NaCl. Hydrophobe monomer is N-n-octylacrylamide. Hydrolysis level is 18 mol %. Polymer concentration is 2000 ppm.

Figure 8. Dependence of solution viscosity on hydrophobe level for different hydrophobe structures.

Effect of hydrophobe level on polymer solution viscosity. 

Figure 40. Effect of hydrophobe level on Polymer Solution Viscosity. Acrylamide/N-octyl acrylamide copolymer [89].

Figure 42. Dependence of Solution Viscosity on Hydrophobe Level for Different Hydrophobe Structures [93],...

On the other hand, and CNMR [31] has been successfully used to determine the hydrophobe level for hydrophobically modified poly(sodium acrylates) of lower molecular weights (i.e. < 10 g/mol) and high (up to 30%) hydrophobe contents, provided the pH = 8 and the alkyl group is kept small ( < Cg). A deaggregating but nonprecipitating solvent pair (i.e. D2O/CD3OD, to 70/30) is used to improve resolution. 

The Ej(30) has also been used to explore polymer association as a function of the hydrophobe level. As shown in Fig. 2.15, the increase in A(max) tracks the increase in hydrophobe content in the polymer. This indicates increased hydrophobe character for the microdomains and stronger polymer association with increasing hydrophobe levels. It is noteworthy that the increase in A(max) dovetails the increase in solution viscosity (Fig. 2.16). Thus, the dye probe provides microscopic domain information which complements the macroscopic solution viscosity characterization of the polymer system. 

A further complication in understanding the rheology of hydrophobically associating polymers is the unique response to shear rate and solvent quality (e.g. salt content). As shown in Fig. 3.8, the viscosity can be independent (Newtonian), decrease (pseudoplastic) or even increase (dilatant) with shear rate depending on the polymer concentration and shear rate. As the hydrophobe level and polymer concentration increase beyond a critical value, a shear rate region in... 

The solution associations of some hydrophobically modified polyethylene oxides have been investigated. It was found that as the hydrophobe level increased in the pol nners, the apparent molecular weight and intrinsic viscosity increased, indicative of increased intermolecular association. Additionally, the solution associations appear to be augmented in salt solution. 

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