Novel Hydrophobically Modified Ethoxylated Urethanes End-Capped by Percec-Type Alkyl Substituted Benzyl Alcohol Dendrons: Synthesis, Characterization, and Rheological Behavior

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Novel dendron hydrophobically modified ethoxylated urethanes (DHEUR) with almost the same molecular weights, molecular weight distributions, and identical hydrophilic portion but different terminal hydrophobic group numbers were prepared by using Percec-type alkyl substituted benzyl alcohol dendrons as new end-cappers. These DHEUR polymers in solution possess interesting associative and rheological behavior. For DHEUR-1 with 4-mono(decyloxy)benzyl alcohol (2), the solutions are dominantly composed of the isolated and separated micelles and exhibit Newtonian behavior in a wide shear rate range accompanied by shear thinning at high shear rate region. DHEUR-2 with 3,5-di(decyloxy)benzyl alcohol (4) in solutions form a relatively more complete network through dominant micellar junctions and process a relatively higher solution viscosity and similar solution viscosity behavior to DHEUR-1. However, shear thinning behavior shifts to a lower shear rate region due to a relatively longer relaxation time. Interestingly, the solutions of DHEUR-3 with 3,4,5-tri(decyloxy)benzyl alcohol (6) have developed a complete physical network and show pronounced shear thinning behavior over the whole shear rate range. The oscillatory measurements further confirm that a gradually developing associative network leads to their different solution rheological behavior, i.e., viscous fluid (DHEUR-1), viscoelastic fluid (DHEUR-2), and elastic body (DHEUR-3) with increasing the hydrophobic tail number of dendrons. Furthermore, the rheological activation energy of these DHEUR polymers increases with the increase of terminal hydrophobic group numbers, indicating that DHEUR polymers with more hydrophobic tail chains need more energy potential barrier for the disengagement of hydrophobes from micelles due to stronger association strength. In general, the results demonstrate that the terminal hydrophobic tail number of dendrons plays a key role in determining the associative and rheological behavior of DHEUR in solutions. This work opens a new perspective for more efficient thickeners and also promises the potential of these DHEUR polymers in waterborne coating, cosmetics, dyestuff, medicines, and so on for the first time.