Shear thickening mechanism in aqueous solutions of associating polymers with well-controlled molecular architecture.

摘要

Over the last decade, associating polymers have attracted widespread interest both theoretically and experimentally. Despite such efforts, the molecular origin of shear thickening in associating polymer solutions remains a subject of controversy. The main objective of this study is to reveal the mechanism of shear thickening in associating polymer solutions at a molecular level using a model system with a well-defined structure.; Hydrocarbon and fluorocarbon end-capped PEOs with strictly telechelic structure were synthesized and used as model systems in this study. The effects of various parameters (i.e., concentration, temperature, solvent polarity, solvent viscosity, association strength, and molecular weight) on rheological properties of these model systems, especially shear thickening behavior, were investigated. The experimental results were compared to the predictions of the various theories of shear thickening.; Hydrocarbon end-capped PEO of high molecular weight (35k) shows only shear thinning behavior whereas PEO of lower molecular weights (10k, 20k) shows mild shear thickening over a range of concentrations and temperatures. The scaling factors of zero-shear viscosity, network relaxation time, critical reduced shear rate and critical shear stress were found to quantitatively agree with the Free Path model predictions. Using the same scaling factors as found in the hydrocarbon system, the onset of shear thickening of fluorocarbon end-capped PEO solutions can be predicted. Furthermore, the characteristics of shear thickening of the bimodal systems, consisting of two monodisperse polymers (10k and 35k), were found to be the same as in the monodisperse polymer with the lower molecular weight (the 10k, component of the bimodal polymer mixture), instead of that of the monodisperse polymer with the same average molecular weight (20k). Among all the mechanisms proposed, only the Free Path model is consistent with this experimental observation. It is believed that shear thickening observed in the bimodal systems results from the non-Gaussian chain stretching of the shorter polymer chain.; Overall, shear thickening observed in our experiments is in semiquantitative agreement with Free Path model predictions, where shear thickening is due to the non-Gaussian chain stretching.