We present a facile and versatile
platform approach for the synthesis
of submicrometer-sized hybrid particles based on an oil-in-water emulsion.
These particles comprise organosiloxanes and/or organosilsesquioxanes
formed via hydrolysis and polycondensation of alkyl- or aryltrimethoxysilane,
and polystyrene or poly(methyl methacrylate) formed through radical
polymerization of styrene and methyl methacrylate, respectively. In
this synthesis, the alkyl- or aryltrimethoxysilane fulfills three
different roles: (i) it is part of the oil phase, (ii) serves as monomer
for the formation of the organosiloxane network, and (iii) forms a
surface active species that stabilizes the emulsion. Size, composition
and architecture of the resulting hybrid particles are programmable
in this synthetic approach, as demonstrated for the combination phenyltrimethoxysilane/styrene.
The versatility of the approach is demonstrated by preparing hybrid
particles based on following precursor/monomer combinations: phenyltrimethoxysilane/methyl
methacrylate, methyltrimethoxysilane/styrene, (3-acryloxypropyl)trimethoxysilane/styrene
and (3-mercaptopropyl)trimethoxysilane/styrene. Latter combination
yields hybrid spheres with thiol groups suited for further functionalization.
a Miscible block copolymers (BCPs) are rarely studied. When one or both components of such BCPs are semi-crystalline polymers, strong effects on the crystallization behavior can be expected. We present a study of 18 miscible BCPs comprised of poly(lactide) (PLLA, semi-crystalline and PDLLA, amorphous) and poly(2-isopropyl-2-oxazoline) (PiPOx, semi-crystalline) with PiPOx volume fractions of 0.14 < ϕ PiPOx < 0.82. All BCPs exhibit a single glass transition and form a homogeneous melt. Mixing has a plasticizing effect on PiPOx and increases its crystallization rates (DSC). In contrast, the crystallization rates of PLLA are dramatically reduced, or in most cases entirely prevented. During isothermal crystallization at 130°C, the crystallization rates of the BCPs were inverted in comparison with those of the parent homopolymers. Crystallization drives the BCPs to phase separate and the formed crystalline structure is that of the parent homopolymers. The fast crystallization of PiPOx confines the observed superstructure. The BCPs were studied on multiple length scales -from the atomic level (WAXS, IR spectroscopy) to the meso level (AFM, SAXS) and the macroscopic superstructure ( polarized optical microscopy). A mechanism of the structure evolution is presented.
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 ∼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
1
H 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
∼ 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.
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