End chain length effect of hydrophobically end-capped poly(ethylene oxide)s on their self-assemblies in solution

The associating behaviors of hydrophobically end-capped poly(ethylene oxide) urethane resin (HEUR) as a function of the number of hydrocarbons on both ends were studied by static fluorescence, dynamic light scattering, small angle neutron scattering, and molecular dynamics (MD) simulation. HEUR 4(8), HEUR 4(12), and HEUR 4(18), which have different end chain lengths of C8, C12, and C18, were used, respectively. HEUR 4(12) and HEUR 4(18) formed well-defined micelles and transient networks, but HEUR 4(8) did not. Dynamic light scattering showed that HEUR 4(8) did not form micelle structures and that HEUR 4(18) micelles are more stable than those of HEUR 4(12). Small-angle neutron scattering was used to analyze the structures of HEUR 4(12) and HEUR 4(18) micelles and large aggregates in aqueous (D2O) solution. The micelle size of HEUR 4(12) is smaller than that of HEUR 4(18). The interaction radii among HEUR 4(12) micelle decrease with increasing HEUR concentration while the interaction radii among HEUR 4(18) micelle remain constant. The MD simulation shows that the association of HEUR micelle is affected by not only the ratio of hydrophobicity but also the chain flexibility of the molecules. HEUR 4(12) has the greatest chain flexibility and hydrophobicity, thus, it is likely to form aggregates more than micelle. HEUR 4(8) has insufficient chain flexibility, thus, it cannot make hydrophobic interactions.