Amphiphilic Isocyanate-Free Polyurethanes Synthesized via Transurethane Polycondensation: Self-Assembly and Rheological Behavior in Aqueous Solutions

This study reports a more sustainable and cost-effective preparation of isocyanate-free hydrophobically modified ethoxylated poly(ether-urethane)s (TU-IFHEURs) with internal and terminal hydrophobes via a melt transurethane polycondensation of PEG (M–n = 400–20,000 g mol–1) with a fatty diamine-based bis(2-hydroxyethyl carbamate). This method reduced the need for expensive PEG bis(glycidyl ether)s and toxic (di)amines, as well as reaction steps and time. Calculated green chemistry indicators showed reduced Efactor and energy consumption compared to previously reported poly(hydroxy-urethane) analogues. Obtained TU-IFHEURs exhibited M–n ranging from 2,200 to 35,800 g mol–1 and varying molar ratios of hydrophobic-to-hydrophilic moieties, as elucidated by SEC and NMR. For the first time, their hierarchical self-assembly behavior in aqueous solution was visualized by cryo-TEM. Depending on the PEG chain length, hydrophobe content, and solution concentration, spherical core-corona-like, worm-like, and vesicle-like assemblies were observed. DLS, cryo-TEM, and rheological results confirmed that the associative behavior of TU-IFHEUR aqueous solutions was concentration- and structure- dependent. The solutions demonstrated a particularly good match for rheology modifiers, providing structural integrity at rest, high shear-viscosity at low shear rates, and shear-thinning behavior at higher shear rates, desirable for coatings, paints, and other waterborne formulations.