Influence of prepolymers molecular weight on the viscoelastic properties of aqueous HEUR solutions

A series of hydrophobically modified ethoxylated urethanes (HEURs) were synthesized by a step growth polymerization of polyethylene glycol with dicyclohexylmethane diisocyanate (H12MDI). The thickeners were produced with different sizes of the hydrophilic section by changing the molar ratios of reactants. The size of the hydrophobic ends was constant for all prepared samples. The changes in hydrophilic lengths were correlated with the rheological properties of HEURs aqueous solutions. The intrinsic viscosity measurements showed that associates are present even at very low concentration. The response of these HEUR systems in aqueous solution to both steady shear and oscillatory shear was determined as a function of hydrophilic chain length and polymer concentration. Dramatic increases in viscosity are observed with decreasing molecular weight of the prepolymer (with a decrease of the hydrophilic components' size and at the same time an increased ratio between hydrophobic and hydrophilic sections of HEURs). Also, a steep increase in viscosity with increasing thickener concentration is obtained. The rheological properties of aqueous solutions of HEUR polymers can be described using a simple Maxwell model with a single relaxation. The dynamic measurements verified the results obtained from the steady state measurements about the hydrophilic section size and its effect on the association phenomenon.