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.

Charged particles in aqueous suspension form an electrical double layer at their surfaces, which plays a key role in suspension properties. For example, binder particles in latex paint remain suspended in the can because of repulsive forces between overlapping double layers. Existing models of the double layer assume sharp interfaces bearing fixed uniform charge, and so cannot describe aqueous binder particle surfaces, which are soft and diffuse, and bear mobile charge from ionic surfactants as well as grafted multivalent oligomers. To treat this industrially important system, we use atomistic molecular dynamics simulations to investigate a structurally realistic model of commercial binder particle surfaces, informed by extensive characterization of particle synthesis and surface properties. We determine the interfacial profiles of polymer, water, bound and free ions, from which the charge density and electrostatic potential can be calculated. We extend the traditional definitions of the inner and outer Helmholtz planes to our diffuse interfaces. Beyond the Stern layer, the simulated electrostatic potential is well described by the Poisson-Boltzmann equation. The potential at the outer Helmholtz plane compares well to the experimental zeta potential. We compare particle surfaces bearing two types of charge groups, ionic surfactant and multivalent oligomers, with and without added salt. Although the bare charge density of a surface bearing multivalent oligomers is much higher than that of a surfactant-bearing surface at realistic coverage, greater counterion condensation leads to similar zeta potentials for the two systems.

A normal coordinate anaiysis was carried out based on the force field of Schacht-Schneider and Snyder in order to calculate all amplitudes of vibration and shrinkage corrections for n-butane. The resufts are tabulated to aid diffraction anaIyses of related substances. A vapor-phase electron diffraction reinvestigation of n-butane led to experimental measurements of the principal amplitudes of vibration and to the following molecular parameters (*3a): r&C-C) = l-531(2)& r&-H) = l.l17(5)A.~CCC (truns. gauche average)= 113.8(4)", LCCH (ave) = lll.O[5)".gauche CCCC dihedral angle 65(6)O, % tmns conformer = 54 * 9%, and AG" (gauche-truns) = 497 5 220 cd mol-'.

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