Poly(2-isopropyl-2-oxazoline)-b-poly(lactide) (PiPOx-b-PLA)Nanoparticles in Water: Interblock van der Waals Attraction OpposesAmphiphilic Phase Separation

摘要

Poly(2-isopropyl-2-oxazoline)-b-poly(lactide) (PiPOx-b-PLA) diblock copolymers comprise two miscible blocks: the hydrophilic and thermosensitive PiPOx and the hydrophobic PLA, a biocompatible and biodegradable polyester. They self-assemble in water, forming stable dispersions of nanoparticles with hydrodynamic radii (Rh) ranging from ∼18 to 60 nm, depending on their molar mass, the relative size of the two blocks, and the configuration of the lactide unit. Evidence from ¹H nuclear magnetic resonance spectroscopy, light scattering, small-angle neutron scattering, and cryo-transmission electron microscopy indicates that the nanoparticles do not adopt the typical core–shell morphology. Aqueous nanoparticle dispersions heated from 20 to 80 °C were monitored by turbidimetry and microcalorimetry. Nanoparticles of copolymers containing a poly(dl-lactide) block coagulated irreversibly upon heating to 50 °C, forming particles of various shapes (Rh ∼ 200–500 nm). Dispersions of PiPOx-b-poly(l-lactide) coagulated to a lesser extent or remained stable upon heating. From the entireexperimental evidence, we conclude that PiPOx-b-PLAnanoparticles consist of a core of PLA/PiPOx chains associated viadipole–dipole interactions of the PLA and PiPOx carbonyl groups.The core is surrounded by tethered PiPOx loops and tails responsiblefor the colloidal stability of the nanoparticles in water. While thecore of all nanoparticles studied contains associated PiPOx and PLAblocks, fine details of the nanoparticles morphology vary predictablywith the size and composition of the copolymers, yielding particlesof distinctive thermosensitivity in aqueous dispersions.