Adjustment of swelling characteristics of hydrogels via microstructural control assisted by droplet-based microfluidics
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Abstract
The properties of hydrogels result from both the synthesis method and their composition. In the
present work droplet-based microfluidics is used as the main synthesis method. It allows the
creation of monodisperse hydrogel microparticles from designed emulsions. These hydrogel
microparticles can in the following be investigated to determine the interplay of the composition
and properties of the resulting hydrogel microparticles.
Nonetheless, one major drawback of droplet-based microfluidics is its output. Which is mainly
determined by the flowrates of the two fluids through the microfluidic device, creating the
emulsion. Thus, in the first part of this work an existing microfluidic experimental setup is
modified to maximize the yields. To polymerize the droplets a photo-initiator and UV-radiation
is used, thereby determining the area of improvement in the microfluidic setup.
The second study focuses on the creation of core-shell particles with a temperature sensitive
core and a temperature non-sensitive shell with different degrees of interconnectivity of the
two. Therefore, in a first step thermo-responsive poly(N-isopropylacrylamide) microgels are
synthesized via droplet-based microfluidics, which are later used as cores in core-shell particles.
To gain monodisperse core-particles in sufficient quantities different microfluidic setups are
investigated. The resulting cores are analyzed concerning their usability for the synthesis of
core-shell particles with a controlled interface between core and shell. For the analysis of the
cores optical microscopy is used.
As shell-precursor polymer a linear copolymer, made of acrylamide (AAm) and N-(2-(3,4-
dimethyl-2,5-dioxo-2,5-dihydro-1H-pyrrol-1-yl)ethyl)acrylamide (DMMIAAm) is
synthesized. DMMIAAm allows the controlled crosslinking of the linear shell-precursor
polymer via UV-irradiation in the presence of the photosensitizer thioxanthone-2,7-disulfonate
(TXS) which is also synthesized as part of this work.
After successful preparation of core-particles and shell-precursor polymer the core-shell
synthesis using droplet-based microfluidics is focused on. Challenges during the synthesis of
the core-shell particles are identified and resolved. Furthermore, methods are developed to gain
control over the interpenetration depth of the shell and core. Lastly diffusion simulations of
linear polymer strands into a polymer gel network are analyzed in a tandem-project with XXX
to gain insight into the diffusion mechanism and further ideas on how to control
the interface between core and shell.
In the third part of the thesis the focus shifts to microgels made of crosslinked poly(acrylic acid)
containing different hydrophobic crosslinkers. The microgels are synthesized using dropletbased
microfluidics to keep the influence of the synthesis method on the polymer network as
minimal as possible. In swelling experiments, the effect of the hydrophobicity of the crosslinker
on the swelling kinetics of the microgels is then analyzed. To further ensure the comparability
of the different microgel crosslinkers the reactivity ratios of the corresponding
n-alkyl acrylamides are determined.
The last part of the thesis presents two side-projects. The first of which is about light scattering
experiments done on macro gels with similar composition, as the microgels of the previous part
to determine the heterogeneity of the different gels.
In the second side-project a microfluidic experimental setup for the synthesis of
carbomer974p-agarose hydrogels crosslinked with hyaluronic acid is developed. Since the
polymerization reaction is initiated prior to the start of the microfluidic experiment it poses a
unique challenge to the experimental setup.