RESUMO
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 (R h) ranging from â¼18 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 (R h â¼ 200-500 nm). Dispersions of PiPOx-b-poly(l-lactide) coagulated to a lesser extent or remained stable upon heating. From the entire experimental evidence, we conclude that PiPOx-b-PLA nanoparticles consist of a core of PLA/PiPOx chains associated via dipole-dipole interactions of the PLA and PiPOx carbonyl groups. The core is surrounded by tethered PiPOx loops and tails responsible for the colloidal stability of the nanoparticles in water. While the core of all nanoparticles studied contains associated PiPOx and PLA blocks, fine details of the nanoparticles morphology vary predictably with the size and composition of the copolymers, yielding particles of distinctive thermosensitivity in aqueous dispersions.
RESUMO
At room temperature, poly(N-isopropylacrylamide) (PNIPAM) is soluble in water and methanol, but it is not soluble in certain water/methanol mixtures. This phenomenon, known as cononsolvency, has been explored in great detail experimentally and theoretically in an attempt to understand the complex interactions occurring in the ternary PNIPAM/water/co-nonsolvent system. Yet little is known about the effects of the polymer structure on cononsolvency. To address this point, we investigated the temperature-dependent solution properties in water, methanol, and mixtures of the two solvents of poly(2-cyclopropyl-2-oxazoline) (PcyPOx) and two structural isomers of PNIPAM (M n â¼ 11 kg/mol): poly(2-isopropyl-2-oxazoline) (PiPOx) and poly(2-n-propyl-2-oxazoline) (PnPOx). The phase diagram of the ternary water/methanol/poly(2-propyl-2-oxazolines) (PPOx) systems, constructed based on cloud point (T CP) measurements, revealed that PnPOx exhibits cononsolvency in water/methanol mixtures. In contrast, methanol acts as a cosolvent for PiPOx and PcyPOx in water. The enthalpy, ΔH, and temperature, T max, of the coil-to-globule transition of the three polymers in various water/methanol mixtures were measured by high-sensitivity differential scanning calorimetry. T max follows the same trends as T CP, confirming the cononsolvency of PnPOx and the cosolvency of PiPOx and PcyPOx. ΔH decreases linearly as a function of the methanol content for all PPOx systems. Ancillary high-resolution 1H NMR spectroscopy studies of PPOx solutions in D2O and methanol-d 4, coupled with DOSY and NOESY experiments revealed that the n-propyl group of PnPOx rotates freely in D2O, whereas the rotation of the isopropyl and cyclopropyl groups of PiPOx and PcyPOx, respectively, is limited due to steric restriction. This factor appears to play an important role in the case of the PPOxs/water/methanol ternary system.
