Network formation and its consequences for the physical behaviour of associating polymers in solution

The physical properties of hydrophobically modified polyurethane thickeners have been studied in solution. Such polymers form transient networks via association of their hydrophobic end groups. The rheological response of such systems is characterized by a Maxwellian viscoelastic response. It is shown that this is consistent with a picture in which stress relaxation occurs via disengagement of a chain end from an association followed by rapid Rouse relaxation of the stressed chain. This mechanism in principle allows the preparation of fluids with designed relaxation spectra. In this way it is possible to control the viscosity/shear-rate profile of a system thickened with such macromolecules. The concentration dependence of the shear modulus of such solutions can be described by a simple model in which there is an interplay between chains taking up loop and Link configurations. A theory developed on such principles shows excellent agreement with experiment. This model is also shown to shed light on the interaction of associating polymers with surfactants. The phase behaviour of associating polymer mixed with unmodified homopolymer of the same monomer composition is also discussed. The observed phase diagram is well-described by a modified form of Flory-Huggins theory to account for the associations between the hydrophobic end groups of the associating polymer (AT) chains. It is argued that phase separation is driven by entropic rather than enthalpic considerations. This paper highlights the fact that many of the physical features of AT can be accounted for by a simple picture of the network topology combined with an understanding of the mechanism of stress relaxation. It is shown that the capability of association between chains has considerable implications regarding the physical response of polymer solutions.