Dynamics and Rheology in Aqueous Solutions of Associating Diblock and Triblock Copolymers of the Same Type

Abstract

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 q3 (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 (q5). 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.