Diffusion-Weighted PFGNMR Study of Molecular Level Interactions of Loops and Direct Bridges of HEURs on Latex Particles

Viscosity building in latex coatings to provide desirable shear thinning rheological properties is a key property commercially achieved with hydrophobically modified ethylene oxide urethane (HEUR) rheology modifiers (RMs). Prior studies focused on the aqueous solution properties of HEURs, resulting in the well-known transient network model that describes solution rheology reasonably well. Relatively fewer studies have probed the molecular level interactions between the hydrophobe groups of HEUR and latex surfaces under conditions of realistic latex volume fractions and HEUR concentration. The presence of ubiquitous surfactant and oligomer molecules in the latex aqueous phase makes it difficult to detect these interactions directly for any off-the-shelf (industrial) materials. In this work, we outline the use of pulsed field gradient (PFG) NMR spectroscopy as diffusion-weighted filter to remove the signals of low molecular weight species in order to detect hydrophobe end groups and urethane linkers. This in situ approach does not have any perturbation issues that are inherent in prior methods involving centrifugation and avoids the questions raised by the use of custom pyrene hydrophobes in fluorescence spectroscopy. From this study we conclude that there are no HEUR transient network structures present in HEUR–latex composites with less than about 2% HEUR and 30% latex relevant for coatings applications. Our results explain the shear thinning rheology of latex–HEUR composites based on molecular level interactions between hydrophobe end groups and urethane linkers of HEURs and latex particles to produce HEUR PEO loops on latex and direct bridges between pairs of latex particles.