Unique viscosity mutation of multi-generation hyperbranched waterborne polyurethane

Different generation hyperbranched polyurethanes (HBPUs) were synthesized through the A2+B’B2 strategy. The molecular structures and viscosity were characterized by nuclear magnetic resonance (NMR) spectroscopy and gel permeation chromatography (GPC). The NMR results confirmed the formation of the branched units between the segments contained in the HBPU structure. The GPC with a four-capillary bridge design differential viscometer and the Ubbelohde viscometer showed a different viscosity. For the HBPUs, the viscosity first increased to higher values then decreased to lower values, and finally increased with increasing weight, which can be attributed to the spherical structure. The growth degree of HBPUs spherical structure depends on the forming hydrogen bonds; when the hydrogen bonds reach a certain degree, the spherical structure becomes compact.