Influence of terminal hydrophobe branching on the aqueous solution behavior of model hydrophobically modified ethoxylated urethane associative thickeners

The influence of terminal hydrophobe branching on the micellar properties of Hydrophobically modified Ethoxylated URethanes (HEURs) is addressed through fluorescence, dynamic light scattering (DLS), solution rheology, and Raman spectroscopy. Model HEURs used in this study are monodisperse and fully substituted with hydrophobic groups of different structures. Two linear hydrophobes (l-C12H25 and l-C16H33) and three branched hydrophobes [b-(C12H26), b-(C16H34), and b-(C(20)H(42)A)] are coupled to the hydroxyls of POE670 and POE195 through 4,4-methylene bis(dicyclohexyl)diisocyanate (H12MDI) units. Prior low molecular weight surfactant studies observed that a CH2 group, introduced as a branch from a linear hydrophobe, contributes approximately half to the surfactant's hydrophobicity that the same CH2 unit would add to the linear chain. Within hydrophobe equivalent comparisons, greater hydrophobic domains are indicated in pyrene's I-1/I-3 emission ratio for the linear hydrophobe HEURs, denoting a more hydrophilic core in branched HEURs. An increase in the number of -CH2- groups notably influences aqueous solution viscosities, as expected, but among equivalent hydrocarbon comparisons, the moderate-size linear hydrophobe is more viscosifying than the branched hydrophobe at high concentrations. Viscosity increases correlate with the aggregation sizes estimated from DLS studies as aggregate sizes approach 500 nm, covering small to large hydrophobes and different oxyethylene spacer lengths. Conformational differences (Raman spectroscopy) of poly(oxyethylenes) and HEURs under stagnant and flow conditions are also examined. The presence of byproduct impurities can markedly influence these results.