Rheology of a spiropyran functionalized hydrophobically modified ethoxylated urethane in aqueous solution

Hydrophobically modified ethoxylated urethanes (HEURs) have been widely used as effective thickeners for improving the solution rheological properties in a variety of applications for over 40 years. We designed and prepared a novel photosensitive spiropyran-functionalized HEUR polymer ( SP-HEUR ), which was used to investigate the influence of hydrophobicity change of hydrophobes induced by photoisomerization of spiropyran (SP) group on the solution aggregation and rheological properties. The concentrated SP-HEUR solutions show an interesting photoresponsive rheological behavior upon alternative exposure to ultraviolet and visible light due to a reversible SP- to merocyanine-form conversion and back. A reversible rearrangement of micellar junctions through loop-bridge or bridge-loop transitions is proposed to interpret the influence of photoisomerozation of spiropyran groups on the associative structure and rheological properties of the polymer solution. This work will not only provide new insights into the influence of hydrophobicity change of hydrophobes on the associative structure and rheological properties of HEUR solutions but also opens a new perspective for the development of some special applications of HEURs in the manufacturing and transmission of soft materials, waterborne coating, inks, medicines, cosmetics, and so on.