The interactions between various surfactants (anionic, cationic, and nonionic) and a nonionic ethyl(hydroxyethyl)cellulose (EHEC) polymer or a hydrophobically modified analogue (HM-EHEC) have been examined. The study has been performed as a comparative investigation between the hydrophobically modified polymer and the unmodified parent polymer using rheology. The rheological results have been analyzed with the aid of a simple modified Maxwell model. Information about the low-frequency behavior was extracted from this model, and the rheological features were presented in terms of the complex viscosity and a characteristic relaxation time. In the presence of an ionic surfactant, the rhelogical measurements revealed significant polymer/surfactant interaction for both EHEC and HM-EHEC, but the interaction peak, observed in the different rheological quantities, was more pronounced and located at a lower surfactant concentration for the hydrophobically modified polymer. In the presence of a nonionic surfactant, the polymer/surfactant interaction was weaker. By considering the temperature effect, an important difference in the polymer dynamics between the hydrophobically modified polymer and the unmodified analogue was established. The hydrophobically modified polymer shows a normal temperature dependency, while the motion of the unmodified analogue is slowed down with increasing temperature. The finding for the unmodified polymer is attributed to increased polymer/polymer attractions with an increased temperature. It has also been shown that the hydrophobically modified polymer has features regarding the formation and breakdown of the polymer/surfactant complex in common with other hydrophobically modified polymers (both nonionic and ionic). At a certain surfactant concentration the network starts to lose its connectivity, however, because of an increased surfactant aggregation number of the mixed micelles, the breakdown process is postponed when the surfactant chain length is increased.

The associative character of monodisperse amphiphilic copolymers of the same type, one with a diblock (DB) structure (hydrophobic tail on one end) and the other with a triblock (TB) structure (hydrophobic tails on both ends), has been studied in aqueous solution. The macroscopic properties of these systems have been investigated by rheological methods and correlated to properties on the microscopic level, as revealed from pulsed field gradient NMR and dynamic light-scattering (DLS). The results suggest that, in aqueous solution, both polymers associate, but the thickening effect is much more pronounced for TB due to the gradual formation of bridges between the micellar-like clusters as the concentration increases. This connectivity effect has been surveyed by mixing the polymers in different proportions. The rheological measurements showed that the concentration induced viscosification effect is considerably stronger for TB than that for the DB system; the dynamic moduli were, even at the highest TB concentration, successfully fitted to a single Maxwell element over the experimentally accessible frequency window. The NMR self-diffusion data revealed a much stronger slowing down of the dynamics for the TB system, and a gradually broader distribution of self-diffusion coefficients was observed for this polymer as the concentration increased. The DLS results for all the solutions, except for those of the two highest TB concentrations, indicate initially an exponential decay (always diffusive) followed by a stretched exponential at longer times. For the two highest TB concentrations an additional very slow stretched exponential mode appears in the profile of the correlation function. The slow mode exhibits an approximately q 3 (q is the wave vector) dependence for all the DB solutions and for the dilute TB solutions, while at higher TB concentrations this mode becomes q independent (the viscoelastic effect). The very slow mode shows a strong q dependence (q 5). The overall picture that emerges from this study is that, at low or moderate TB concentrations and over the considered concentration range for DB, the solution consists of a collection of large clusters of various sizes, slightly interconnected to each other, while at higher TB concentrations the structure of the solution is changed to a transient network, where the connectivity is provided by bridging chains.