Dynamics and rheology in aqueous solutions of associating diblock and triblock copolymers of the same type

The associative character of monodisperse amphiphilic copolymers of the sam e 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 pro perties on the microscopic level, as revealed from pulsed field gradient NM R and dynamic light-scattering (DLS). The results suggest that, in aqueous solution, both polymers associate, but the thickening effect is much more p ronounced for TB due to the gradual formation of bridges between the micell ar-like clusters as the concentration increases. This connectivity effect h as been surveyed by mixing the polymers in different proportions. The rheol ogical measurements showed that the concentration induced viscosification e ffect is considerably stronger for TB than that for the DB system; the dyna mic moduli were, even at the highest TB concentration, successfully fitted to a single Maxwell element over the experimentally accessible frequency wi ndow. The NMR self-diffusion data revealed a much stronger slowing down of the dynamics for the TB system, and a gradually broader distribution of sel f-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 (alw ays diffusive) followed by a stretched exponential at longer times. For the two highest TB concentrations an additional very slow stretched exponentia l mode appears in the profile of the correlation function. The slow mode ex hibits 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 concentrat ions this mode becomes q independent (the viscoelastic effect). The very sl ow mode shows a strong q dependence (q(5)). The overall picture that emerge s from this study is that, at low or moderate TB concentrations and over th e considered concentration range for DB, the solution consists of a collect ion of large clusters of various sizes, slightly interconnected to each oth er, while at higher TB concentrations the structure of the solution is chan ged to a transient network, where the connectivity is provided by bridging chains.