Rheological properties of hydrophobic ethoxylated urethane (HEUR) in the presence of methylated β-cyclodextrin

Hydrophobic ethoxylated urethane (HEUR) is one type of hydrophobically modified polyethylene glycol, which could form a transient network structure in aqueous solution via intermolecular hydrophobic associations. The viscosity of HEUR solution is dependent on two parameters; namely the effective elastic chain density and the relaxation time. In the present study, different amounts of methylated β-cyclodextrin (m-βCD) were added into aqueous HEUR solutions, and significant reduction in the solution viscosity was observed when the m-βCD concentration exceeds a critical value. Oscillatory shear indicated that the plateau modulus does not decrease appreciably with the addition of m-βCD, however a large reduction in relaxation time was observed. At low m-βCD concentrations, addition of m-βCD does not decouple the hydrophobic association. The ring-shaped hydrophobic inner cores of m-βCD interact with the hydrophobic moiety of looping HEUR chains, leading to a reduction in the relaxation time, which weakens the transient network. However, at higher m-βCD concentrations, bridging chains are decoupled, thereby disrupting the transient network and produce smaller isolated aggregates.