Solution Rheology and Microstructure of Associative Polymers

Abstract

Water-soluble associative polymers are widely used in a variety of applications because of their ability to modulate rheology and material microstructures. This study focuses on understanding the structure-property relationship for hydrophobically modified alkali soluble emulsion (HASE) polymers with emphasis on their microstructure and rheological properties. These polymers have a complex comb-like structure that is a polyelectrolyte backbone, a copolymer of acrylic or methacrylic acid and alkyl acrylate, with a few hydrophobic macromonomers randomly grafted to this backbone. The hydrophobic macromonomer consists of hydrophobic groups that are separated from the polymer chains by polyethylene oxide (PEO) spacers. Upon neutralization, the polymer backbone adopts a more extended conformation allowing the hydrophobic groups to associate forming a transient network structure that enhances the solution rheological properties.

In the first part of this study, we investigate the effect of the polymer composition on their microstructures and rheological properties. In particular, the effects of the concentrations of methacrylic acid (MAA) and macromonomers on the solution rheology are examined. We find that polymers with low MAA content have smaller hydrodynamic size and weaker network structures compared to larger hydrodynamic size and stronger network structure for polymers with high MAA content. However, due to chain increased stiffness at higher MAA and the lower contribution from the aggregation of ethyl-acrylate groups, a broad maximum in the viscoelastic properties of the polymer solution is observed at about 40 mole% MAA. Moreover, the material functions of polymers with different MAA content show different concentration dependences.

In the second part of this study, co-solvents of water and propylene glycol (PG) in different proportions are used to investigate the effect of the solvent quality on the solution rheology of these polymers. The steady and dynamic properties show the presence of two regimes with respect to the solvent composition. In "water-rich" solvents, the hydrophobic association dominates the solution rheology. In contrast, in "PG-rich" solvents, the hydrophobic association is suppressed due to the lower tendency of the hydrophobes to aggregate, the smaller coil size of the polymer chains and changes in the PEO spacer conformations. These two different types of behavior are discussed and confirmed by the different concentration dependences in each regime.

In the third part of the study, the ability of using diffusing wave spectroscopy (DWS) to probe the dynamics of HASE polymers is examined. We find that DWS accurately probes the structural changes induced by the change in the solvent quality or the polymer concentration. Moreover, comparison with conventional mechanical rheometry data reveals excellent qualitative agreement between the data obtained from the two techniques.

Quantitatively, however, there is a discrepancy between the data obtained from each technique. Several reasons for the discrepancy are discussed, including the possibility that the dynamics at the micro-level could be different from the bulk properties. The scaling of the creep compliance, high–frequency elastic modulus and relaxation time with polymer concentration show power-law dependences. The power-law exponents are discussed in light of theoretical predictions and available experimental data.

An approach to modulate the hydrophobic association is presented in the last part of the study. The first step in this approach involves the addition of inclusion compound forming hosts (α- or β- cyclodextrin) to the polymer solution. The encapsulation of the hydrophobic groups leads to significant reduction in the solution viscosity and viscoelastic properties The second step requires the addition of surfactants to reactivate the hydrophobic groups and thus recover the solution rheological properties. We are able to recover the solution properties using different nonionic surfactants.