Shear-Dependent Interactions in Hydrophobically Modified Ethylene Oxide Urethane (HEUR) Based Rheology Modifier–Latex Suspensions: Part 1. Molecular Microstructure

We have studied the microstructure of latex suspensions formulated with hydrophobically modified ethylene (oxide) urethane (HEUR) thickener (or rheology modifier, RM) using small-angle neutron scattering under shear (rheo-SANS). Within the shear rate range studied (0–1000 s^–1), the neutron scattering profiles are consistent with a polydisperse core–shell model, with the latex particles comprising the core and an adsorbed layer of water-swollen RM on the latex surface forming the shell. The core–shell structure is isotropic under quiescent conditions but becomes anisotropic under shear (with the major axis along the vorticity direction). During shear, the solvent (D₂O/H₂O) is expelled (hydrodynamic squeezing) from the swollen polymer chains, and the shell structure becomes denser. The <i>anisotropic</i> shell is a result of differing degrees of compression along the flow and vorticity directions. With increasing shear rate, the shell thickness (in both the flow and vorticity direction) tends toward asymptotic values (with the shell thickness in the vorticity direction greater than the shell thickness in the flow direction) independent of the RM hydrophobe density (defined as the average number of hydrophobes per polymer chain). The RM concentration (w/w) in the adsorbed layer varies from ∼0.05–0.1 (at low shear) to ∼0.25–0.4 (high shear, ∼1000 s^–1) with higher values for the RM polymer with higher hydrophobe density. The swollen RM-water shell substantially increases the effective volume fraction of the dispersed latex particles. We find, however, that accounting for this increase within the conventional effective hard-sphere (Krieger–Dougherty) dispersion rheology model does not fully explain the higher viscosity of the formulated mixture. We hypothesize the existence of latex–latex interactions mediated by RM polymer bridges even at high shear. The large-scale structure of the particle assembly will be reported in a subsequent manuscript

Formulation-Controlled Positive and Negative First Normal Stress Differences in Waterborne Hydrophobically Modified Ethylene Oxide Urethane (HEUR)-Latex Suspensions

Hydrophobically modified ethylene oxide urethane (HEUR) associative thickeners are widely used to modify the rheology of waterborne paints. Understanding the normal stress behavior of the HEUR-based paints under high shear is critical for many applications such as brush drag and spreading. We observed that the first normal stress difference, <i>N</i>₁, at high shear (large Weissenberg number) can be positive or negative depending on the HEUR hydrophobe strength and concentration. We propose that the algebraic sign of the <i>N</i>₁ is primarily controlled by two factors: (a) adsorption of HEURs on the latex surface and (b) the ability of HEURs to form transient molecular bridges between latex particles. Such transient bridges are favored for dispersions with small interparticle distances and dense surface coverages; in these systems; HEUR-bridged latex microstructures flow-align in high shear and exhibit positive <i>N</i>₁. In the absence of transient bridges (large interparticle distances, low surface coverage), the dispersion rheology is similar to that of weakly interacting spheres, exhibiting negative <i>N</i>₁. The results are summarized in a simplified phase diagram connecting formulation, microstructure, and the <i>N</i>₁ behavior

Shear-Dependent Interactions in Hydrophobically Modified Ethylene Oxide Urethane (HEUR) Based Coatings: Mesoscale Structure and Viscosity

We have investigated the in situ mesoscale structure of paint formulations under shear using ultra small-angle neutron scattering (rheo-USANS). Contrast match conditions were utilized to independently probe the latex binder particle aggregates and the TiO₂ pigment particle aggregates. Two different latex chemistries and two different hydrophobically modified ethylene oxide urethane (HEUR) rheology modifiers were studied. The rheo-USANS data reveal that both the latex particles and the TiO₂ particles form transient aggregates which are fractal in nature. The structures depend on the chemistry of the binder particles, the type of rheology modifier present and the shear stress imposed upon the formulation. The aggregate size of both the latex and pigment generally decreases with increasing shear stress. In two of the formulations studied, the latex and TiO₂ correlation lengths remain large even at high shear stress and are characteristic of TiO₂ crowding. In a third formulation, shear induces string-like aggregate structures of TiO₂, and a further increase in shear leads to pigment particles becoming more uniformly dispersed. The changes in the latex and pigment transient aggregate structures correlate with the changes observed in their viscosity flow curve profiles. We have used this correlation to develop an elementary viscosity prediction model based on the structural parameters extracted from the rheo-USANS data. Using a single fitting parameter and only the latex transient fractal aggregate structural parameters, good agreement between the measured and calculated viscosity is obtained. This implies that the structural parameters extracted from the scattering data are representative of the colloidal structure under shear and that energy dissipation from transient fractal aggregates of latex is the predominant mechanism of viscosity creation in HEUR thickened latex paints

Modeling the Adsorption of Rheology Modifiers onto Latex Particles Using Coarse-Grained Molecular Dynamics (CG-MD) and Self-Consistent Field Theory (SCFT)

We model the adsorption of hydrophobically ethoxylated urethane (HEUR) thickeners onto two hydrophobic surfaces separated by a 50 nm gallery using coarse-grained molecular dynamics (CG-MD) with implicit solvent and three-dimensional self-consistent field theory (SCFT) with explicit solvent. The CG-MD simulations can be readily extended to encompass very long HEUR chains (up to 540 EO groups) but cannot with current computer speed simulate adsorption of HEURs with hydrophobes longer than 12 carbons (C12). The SCFT method can readily simulate HEURs with longer, C16, hydrophobes but has a greater challenge simulating very long EO chains. For HEURs with 180 EO units and C8 and C12 hydrophobes, both methods can be applied, allowing a combination of the two methods to span much of the parameter space of interest to experimentalists. It is demonstrated that depending on the hydrophobe strength and the HEUR concentration, HEUR chains can adsorb to the surfaces directly or indirectly (as adsorbed micelles or admicelles). We show that for hydrophobes as large or larger than C12 micellization and subsequent adsorption of the micelles play an important role in accurate prediction of adsorption isotherms and the structure of adsorbed layers and that micelles in solution form nodes that allow two or more HEUR chains to bridge the gallery between the two surfaces. The study suggests the need to investigate the influence of admicelles on the effective steric interaction potential, which, in turn, will influence both colloidal stability and rheology of HEUR thickened latex paints

Structure and Properties of Aqueous Methylcellulose Gels by Small-Angle Neutron Scattering

Cold, semidilute, aqueous solutions of methylcellulose (MC) are known to undergo thermoreversible gelation when warmed. This study focuses on two MC materials with much different gelation performance (gel temperature and hot gel modulus) even though they have similar metrics of their coarse-grained chemical structure (degree-of-methylether substitution and molecular weight distribution). Small-angle neutron scattering (SANS) experiments were conducted to probe the structure of the aqueous MC materials at pre- and postgel temperatures. One material (MC1, higher gel temperature) exhibited a single <i>almost</i> temperature-insensitive gel characteristic length scale (ζ_c = 1090 ± 50 Å) at postgelation temperatures. This length scale is thought to be the gel blob size between network junctions. It also coincides with the length scale between entanglement sites measured with rheology studies at pregel temperatures. The other material (MC2, lower gel temperature) exhibited two distinct length scales at all temperatures. The larger length scale decreased as temperature increased. Its value (ζ_c1 = 1046 ± 19 Å) at the lowest pregel temperature was indistinguishable from that measured for MC1, and reached a limiting value (ζ_c1 = 450 ± 19 Å) at high temperature. The smaller length scale (ζ_c2 = 120 to 240 Å) increased slightly as temperature increased, but remained on the order of the chain persistence length (130 Å) measured at pregel temperatures. The smaller blob size (ζ_c1) of MC2 suggests a higher bond energy or a stiffer connectivity between network junctions. Moreover, the number density of these blobs, at the same reduced temperature with respect to the gel temperature, is orders of magnitude higher for the MC2 gels. Presumably, the smaller gel length scale and higher number density lead to higher hot gel modulus for the low gel temperature material