Self-diffusion, thermal effects and viscosity of a monodisperse associative polymer: self-association and interaction with surfactants

Abstract

Self-diffusion, calorimetry and rheology measurements have been performed on a monodisperse associative polymer (C₁₂EO₂₀₀C₁₂). The self-association of this polymer was found to show the same general features as non-ionic C_nE_x surfactants. A strongly endothermic self-association starts at 0.014 wt.%, but is a somewhat less cooperative process than micellization of surfactants, probably due to the steric constraints of the polymer backbone. Our measurements are consistent with a model in which the primary aggregates, formed at low associative polymer contents, assemble into clusters as the polymer content increases. Subsequent cluster growth gives rise to an increasing distribution of the size of the diffusing species.

Surfactants were found to be solubilized primarily in the hydrophobic domains consisting of associated polymer end-groups, thereby enhancing the strength of the polymer association. These domains are easily saturated with surfactants, and by a concentration of 2 mmol kg^–1, sodium dodecyl sulfate (SDS) starts to associate with the ethylene oxide (EO) repeat units of the polymer backbone, in a similar way to unmodified poly(ethylene oxide). Since polymers of this chain length will, when saturated with SDS, on average accommodate only one SDS aggregate, it is feasible that both end-groups of a given C₁₂EO₂₀₀C₁₂ are then associated with the same SDS aggregate. As a consequence, the probability that the end-groups will act as bridges between the aggregates will then be low.