SYNTHESIS AND CHARACTERIZATION OF STEP-GROWTH HYDROPHOBICALLY-MODIFIED ETHOXYLATED URETHANE ASSOCIATIVE THICKENERS

The influences of molecular weight, the presence of internal alkyl groups, and the size of the external alkyl group among hydrophobically-modified, ethoxylated urethanes (HEURs) are examined through the synthesis of compositionally defined thickeners. The polymers were prepared by a step growth polymerization of poly(oxyethylenes) with diisocyanates and possess broad molecular weight distributions. HEURs synthesized with only internal alkyl groups do not effect viscosity increases despite variances in the structure of the alkyl group sizes of the diisocyanates, and evidence of hydrophobic aggregation is not observed by fluorescence spectroscopy. The size of the terminal alkyl groups is important. Dramatic increases in hydrophobic associations and viscosity as a function of thickener concentration are observed with increasing ''effective'' terminal hydrophobe size. The ''effective'' hydrophobe size can be altered by increasing the hydrophobicity of the alkyl amine or the alkyl diisocyanate used to couple the amine to the poly(oxyethylene) units. For example, with a constant amine hydrophobe size a larger diisocyanate, dicyclohexylmethane diisocyanate (H(12)MDI), increases the effective hydrophobe size of the terminal alkyl group to a greater extent than the smaller hexamethylene diisocyanate (HDI). Contrary to viscosity build through chain entanglements, lowering the molecular weight of HEUR thickeners effects larger viscosity increases. This also is related to hydrophobic aggregation through the increase in concentration of hydrophobes.