Heat-Induced Phase Transition and Crystallization of Hydrophobically End-Capped Poly(2-isopropyl-2-oxazoline)s in Water

The phase diagrams of semitelechelic and telechelic hydrophobically modified poly(2-isopropyl-2-oxazolines) (HM-PiPrOx) in water were characterized in terms of an extensive set of parameters: the cloud point (T-cp), obtained from turbidity measurements, the transition temperature (T-M) and enthalpy (Delta H), derived from high-sensitivity differential scanning calorimetry (HS-DSC), the coefficient of thermal volume expansion, alpha, measured by pressure perturbation calorimetry (PPC), together with assessment by X-ray diffraction of the extent of polymer crystallization from hot water. Data were recorded as a function of the molecular weight of the polymers (7000. 10 000, and 13 000 g mol(-1)), the end-group functionality (alpha-n-octadecyl-omega-hydroxyl- or alpha,omega-di-n-octadecyl-), the polymer concentration (0.1-10 g L-1), and the solvent (H2O or D2O). For all polymer solutions, the cloud point temperature was lower than the transition temperature, in agreement with previous studies of telechelic HM-poly(ethylene oxides) and HM-poly(N-isopropylacrylamides) aqueous solutions. In solutions of all telechelic HM-PiPrOx and of the semitelechelic HM-PiPrOx 7000 and 10 000 g mol(-1), the onset of turbidity was accompanied by an endotherm attributed to it rigid-to-fluid phase transition of the hydrophobic core of the polymeric micelles formed in aqueous solutions kept below T-cp. Semitelechelic HM-PiPrOx samples crystallized from water upon prolonged heating at 70 degrees C, whereas telechelic HM-PiPrOx samples remained amorphous under these conditions. The results are discussed in terms of the dipole-dipole interactions between adjacent PiPrOx chains in the micelle corona, which restrict the motion of n-octadecyl chains in the micellar core and favor intermicellar interactions. Water molecules acting as hydrogen-bound cross-linker between polymer chains also contribute to intra- and intermicellar PiPrOx chain association.