Hydrophobically-modified, alkali-soluble associative polymers (HASE) are a class of water soluble associative polymers that are becoming materials of significant interest because of their potential utilization in many applications: rheology modifiers, flocculants for waste-water treatment, coatings, paints, inks and aircraft anti-icing fluids. These polymeric systems are characterized by their ability to form both intra- and inter-molecular association in aqueous solution. They exhibit a multitude of unusual phenomena with respect to physically entangled systems (sol-gel transition, shear thickening followed by extreme shear thinning, strain hardening), the underlying mechanisms of which remain poorly understood.

Our group's contributions towards understanding the microstructure and macroscopic properties of the solution of these polymers include:

 

Using a powerful but infrequently-used rheological method that combines superposition of steady shear with dynamic oscillatory experiments, we have probed the network topology and mode of chain coupling in these associative systems under different rates of deformation. The unusual shear thickening at intermediate shear rates is attributed to a shear-induced transition from a state where considerable intrachain association exists to one in which interchain effects predominate. The catastrophic shear thinning observed at higher shear rates is attributed to the breakdown of the dynamic network formed from such associations.

The phase behavior and aggregation number of macro-surfactants (nonylphenol ethoxylates) exert a strong influence on the rheology of these systems. We have been able to propose a possible mechanism for their interactions with the polymer. In one case, the surfactants (NP6) strengthen the hydrophobic associations leading to a gel-like behavior. In the other case, the surfactants (NP10) wrap around the individual hydrophobes, preventing them from associating. This leads to a significant decrease in viscosity and dynamic moduli. Freeze fracture microscopy has been used to validate our hypothesis.

We have been able to show that by changing the molecular architecture of the polymers, the sample can be made to form microgels. This raises the question as to when these systems behave as true solutions versus microgels!

The behavior of these polymers in the presence of co-solvent is highly dependent on the solvent quality. This was controlled by using mixtures of water and propylene glycol (PG) as cosolvent. In water-rich solvents, both intra- and inter-molecular hydrophobic association contribute to the solution behavior. In PG-rich solvent, both intra- and inter-molecular association become insignificant and the polymer behaves as the polymer without hydrophobes. This is reflected in different dependences of the relative viscosity on the solubility parameter, as shown in the figure below.

Using a new light scattering technique, Diffusing Wave Spectroscopy (DWS), we were able to extract the dynamic properties of semi-dilute HASE solutions over a very wide frequency range. The extracted dynamic properties were in excellent agreement with those measured using conventional mechanical rheometers (see adjacent figure)

Using a new light scattering technique, Diffusing Wave Spectroscopy (DWS), we were able to extract the dynamic properties of semi-dilute HASE solutions over a very wide frequency range. The extracted dynamic properties were in excellent agreement with those measured using conventional mechanical rheometers (see adjacent figure)

The ability to reversibly control the rheological response of concentrated HASE solution was achieved by means of using inclusion compounds ( - and - cyclodextrins) and surfactants. The addition of about 15 moles cyclodextrin per hydrophobe reduced the solution viscosity about 3 decades. With subsequent addition of suitable surfactant, the original solution viscosity was recovered.