Abstract
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 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.
Highlights
In selective solvents, diblock copolymers (BCPs) tend to selfassemble into core−corona structures above their critical micelle concentration,[1] as it is the case for surfactants, their low molecular weight counterparts
We recently reported that poly(L-lactide)-b-poly(2-isopropyl-2-oxazoline) (PiPOx-b-PLLA) BCPs are miscible in the bulk.[3]
The core of PiPOx-b-PLA nanoparticles dispersed in water consists of associated PiPOx/PLA chains held together by dipole/dipole interactions
Summary
Diblock copolymers (BCPs) tend to selfassemble into core−corona structures above their critical micelle concentration (cmc),[1] as it is the case for surfactants, their low molecular weight counterparts. The dimensionless parameter ρ = Rg/Rh, which reflects the mass distribution of the scattering object, is an indicator of the morphology of nanoparticles and of single chain conformation.[38] The values of the ρ parameter vary from 0.9 to 1.1 for all of the BCPs dispersions, except 2DL1 (Table 3) This range of ρ values encompasses 0.926, the ρ value predicted for collapsed globes, fully permeable to the solvent and with uniform segment distribution.[39] The fact that the ρ parameter does not change over a wide range of BCP molecular characteristics is quite remarkable, considering that the molar fraction of the PiPOx units changes by a factor of 3 and the solubility properties of PLLA and PDLLA are entirely different.[39] For core/corona particles, such a large change of the hydrophilic block fraction is accompanied by significant changes in the extent and density of the corona detectable through the evolution of the ρ parameter. WAXS analysis of the 3L3 particles before and after treatment indicated that the PiPOx chains did not crystallize upon heat treatment (see Figure S6-1)
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