Temperature Response of Self-Assembled Micelles of Telechelic Hydrophobically Modified Poly(2-alkyl-2-oxazoline)s in Water

Hydrophobically end-modified (HM) poly(2-ethyl-2-oxazolines) (PEtOx) and poly(2-isopropyl-2-oxazolines) (PiPrOx) bearing an n -octadecyl chain on both termini or on one chain end only were prepared by cationic ring-opening polymerization of 2-ethyl-2-oxazoline and 2-isopropyl-2-oxazoline, respectively, and subsequent end-group modification. The polymers had a molar mass ( M n ) ranging from 7000 to 13 000 g mol −1, a size distribution M w / M n < 1.20, and end-group functionality > 0.97. All polymers, except the semitelechelic sample C 18 -PiPrOx-OH 13K ( M n = 13 000 g mol −1 ), formed core−shell micelles in cold water with a hydrodynamic radius ( R H ), measured by dynamic light scattering, between 7 and 12 nm and a core of radius ( R c ), determined by analysis of small-angle X-ray scattering (SAXS) data, of ∼1.3 nm. Aqueous solutions of all polymers underwent a heat-induced phase transition detected by an increase in solution turbidity at a temperature ( T cp, cloud point) ranging from 32 to 62 °C, depending on polymer structure and size. Temperature-dependent light scattering (LS) measurements and fluorescence depolarization studies with the probe diphenylhexatriene (DPH) revealed that extensive intermicellar bridging takes place in solutions heated in the vicinity of T cp leading to large assemblies ( R H ≥ 1 μm). Further heating caused these assemblies to shrink into objects with R H ∼ 300−700 nm, depending on the size and structure of the polymer. The formation of H-bonds between water molecules and the main-chain amide nitrogen atoms imparts distinct features to the flower/star micelles formed by telechelic/semitelechelic PiPrOx and PEtOx, compared to the micelles formed by other hydrophobically end-modified water-soluble polymers, such as poly(ethylene oxide) or poly( N -isopropylacrylamide).