Lower Critical Solution Temperature-type Phase Separation Behavior of Aqueous Solutions of Structurally Uniform Stereoregular Dense Triazole Oligomers Bearing N -ethyl- N -methylamide Side Chains

Abstract The lower critical solution temperature (LCST)-type phase separation behavior of aqueous solutions of structurally uniform stereoregular dense triazole oligomers bearing N-ethyl-N-methylamide side chains was investigated using two series of oligomer samples: TBDMS-syndio-ME nOH (n = 4, 6, and 8), bearing a terminal t-butyldimethylsilyl group, and syndio-ME nOH (n = 2, 4, 6, and 8), which were synthesized by stepwise copper(I)-catalyzed azide–alkyne cycloaddition (CuAAC). Transmittance measurements in the heating and cooling processes with stirring at 500 rpm demonstrated that TBDMS-syndio-ME nOH (n = 4, 6, and 8) exhibited LCST-type phase separation with an unusual apparent “inverse” hysteresis, in which the transmittance recovered in the cooling process at temperatures higher than the clouding point temperature (Tcloud) observed in the heating process. On the other hand, syndio-ME nOH samples remained completely soluble over the temperature range examined. Steady-state fluorescence measurements using Nile Red as a fluorescence probe indicated that TBDMS-syndio-ME nOH (n = 4, 6, and 8) samples formed micelles with lower apparent critical micelle concentrations (0.0047, 0.015, and 0.040 g L–1, respectively) in water. Dynamic light scattering measurements revealed that the micelles with apparent hydrodynamic radii of ca. 1–4 nm rapidly aggregated into micrometer-sized species around Tcloud in the heating process and were regenerated at temperatures < Tcloud in the cooling process, showing no intrinsic inverse hysteresis. Preliminary molecular dynamics simulations further indicated gradual dehydration and compaction of the micelles with increasing temperature. Based on the experimental and simulation data, we propose that the apparent “inverse” hysteresis originates from the formation of highly hydrated amorphous microdroplets produced through aggregation, rearrangement, and fusion of micelles during phase separation. The subsequent decrease in interfacial area and refractive index contrast leads to recovery of transmittance without complete dissolution of the microdroplets of the condensed phase.