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@PHDTHESIS{Chen:783219,
author = {Chen, Zhi},
othercontributors = {Möller, Martin and Pich, Andrij},
title = {{S}ilica-based microcapsules: preparation and release
properties},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
reportid = {RWTH-2020-02373},
pages = {1 Online-Ressource (VI, 150 Seiten) : Illustrationen,
Diagramme},
year = {2020},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, RWTH Aachen University, 2020},
abstract = {In this work, silica microcapsules of different structures
containing hydrophobic substances were prepared using
hyperbranched polyethoxysiloxane (PEOS) and its PEGylated
derivatives (PEG-PEOS) as both silica precursor and
stabilizer of oil-in-water emulsions. PEOS is a hydrophobic
liquid, and it exhibits a pronounced interfacial activity in
an oil/water system induced by hydrolysis at the interface
or partial PEGylation. The influence of reaction conditions
on the morphology and release property of the capsules was
systematically investigated. In the first instance, paraffin
was encapsulated with quantitative efficiency in
mechanically strong submicron silica capsules simply by
emulsifying the mixture of molten paraffin and PEOS in water
under ultrasonication or high-shear homogenization. It is
shown that the size of the capsules can be controlled by
emulsification energy as for a miniemulsion process. The
silica shell, whose thickness can be easily tuned by varying
the paraffin-to-PEOS ratio, acts as an effective barrier
layer retarding significantly the evaporation of enclosed
substances; meanwhile the microencapsulated paraffin
maintains the excellent phase change performance. Further, a
new type of heterophase polymerization that allowed
synthesizing in one step monodisperse polymer@SiO2
core-shell particles in a wide size range from tens to
hundreds of nanometers has been worked out. The strategy
utilizes PEG-PEOS, which can reduce the interfacial tension
between oil and water close to zero. An oil phase containing
such kind of surfactants can be emulsified in water
spontaneously or just under low-energy stirring.
Polymerization of styrene in the resulting emulsions using
an oil-soluble initiator leads to the formation of
monodisperse polystyrene@SiO2 particles, whose size can be
precisely adjusted by the PEGylation degree of the precursor
molecules and can reach as small as 30 nm. It is
demonstrated that the PEGylation degree, i.e. the HLB,
dictates the reaction mechanism that varies from suspension
polymerization with breakup of monomer droplets and
miniemulsion polymerization to microemulsion polymerization
leading to exact “copying” of initial emulsion droplets.
PEG-PEOS derivatives act as very efficient emulsifier to
stabilized oil-in-water emulsions even for polar organic
liquids. After a sol-gel reaction, oil droplets stabilized
by PEG-PEOS of lower PEGylation degrees are converted to
oil-containing aerogel particles. In the case of PEG-PEOS
with higher degrees of modification, core-shell particles
are obtained, where a liquid oil core is surrounded by a
thin silica layer. Remarkably, the presence of an oil phase
increases the porosity of the aerogel particles, acting as
porogen, but has no influence on the porosity of the hollow
particles. After freeze-drying the aerogel particles, acting
as matrix-type microcapsules, can retain encapsulated
volatile hydrophobic liquid, however, the liquid is
evaporated from the hollow particles (core-shell-type
microcapsules). The encapsulation efficiency of hydrophobic
liquids in the aerogel particles can reach as higher as 99
$\%$ after drying. The encapsulation efficiency as well as
barrier property of the aerogel particles are higher due to
bigger particle size and higher meso- and micro-porosity.
Finally, it was attempted to strengthen the shell of silica
nanocapsules using monosilicic acid coating technique. For
this purpose, silica hollow nanoparticles, octyl
acetate@SiO2 as well as polystyrene@SiO2 nanocapsules were
prepared using PEG-PEOS, where 10 $mol.\%$ ethoxy-groups are
replaced by PEG, as the silica precursor, and were coated
with an aqueous dispersion of silicic acid under
hydrothermal conditions at pH close to 9. It is demonstrated
that the silica shell obtained under these conditions
exhibits a high degree of condensation and a low porosity,
and the silica nanocapsules after such a treatment show an
enhanced barrier property. Nevertheless, the silica shell is
still not dense enough to hinder the dissolution of
encapsulated polystyrene by THF.},
cin = {154005 / 150000},
ddc = {540},
cid = {$I:(DE-82)154005_20140620$ / $I:(DE-82)150000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2020-02373},
url = {https://publications.rwth-aachen.de/record/783219},
}
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