Browsing by Author "Stubenrauch, Cosima (Prof. Dr.)"

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Open Access
Emulsion templating: unexpected morphology of monodisperse macroporous polymers
(2021) Koch, Lukas; Stubenrauch, Cosima (Prof. Dr.)
The polymerization and drying of monodisperse water-in-styrene/divinylbenzene (DVB) high internal phase emulsions (HIPEs) leads to monodisperse macroporous polystyrene (PS)/polydivinylbenzene (polyDVB). When the monomer-soluble azobisisobutyronitrile (AIBN) is used as initiator, spherical and interconnected pores and porous pore walls are obtained. In contrast, when the water-soluble potassium peroxodisulfate (KPS) is used, polyhedral and closed pores are obtained and the pore walls are comprised of two similar looking outer layers and one different inner layer. The aim of this work was to identify the mechanism (1) that transforms spherical droplets into polyhedral pores and (2) that creates a three-layered pore wall when the polymerization is initiated from the water/monomer interface with KPS. The styrene/DVB mass ratio and the KPS mass fraction were varied to test the existing hypothesis, i.e. an osmotic transport of DVB. Scanning electron microscopy (SEM) pictures revealed that the morphology of the samples does not change in the way it is expected if osmotic transport of DVB was the acting mechanism. Therefore, the existing hypothesis was rejected and a new explanation had to be found. Experiments in which the surfactant mass fraction βsurfactant was varied revealed that the relative size of the inner layer increases and the relative size of the outer layers decreases when βsurfactant is increased. Moreover, it was found that the outer layers are non-porous and that the inner layer is porous. With the help of a model ternary phase diagram consisting of styrene, surfactant, and PS, it was shown that the surfactant is not soluble in partially polymerized styrene/PS mixtures. The experimental results allow suggesting a mechanism that is based on surfactant diffusion. Since the polymerization starts at the water/monomer interface with KPS, a partially polymerized layer forms close to the interface. From this layer, surfactant molecules that are dissolved in the continuous phase diffuse either (1) to the water/monomer interface or (2) to the interior of the continuous phase. (1) Surfactant diffusion to the interface induces an overpopulation of surfactant. This enables the interface to increase its area, which, in turn, transforms the spherical droplets to polyhedral pores. (2) Surfactant diffusion to the interior of the continuous phase leads to an accumulation of surfactant, while the regions close to the interface become surfactant-free. When the surfactant is washed out during purification, a porous inner and two non-porous outer layers are obtained. Additionally, the mechanical properties of monodisperse macroporous PS/polyDVB were investigated. It was found that the samples are only elastomeric when the amount of DVB is low, while they are elastic-brittle for all other monomer compositions.
Open Access
Gelatin-based hydrogel foams - morphology and polymer functionality
(2022) Dehli, Friederike; Stubenrauch, Cosima (Prof. Dr.)
Open Access
Gelled bicontinuous microemulsions : a new type of orthogonal self-assembled systems
(2013) Laupheimer, Michaela; Stubenrauch, Cosima (Prof. Dr.)
In this work a new type of orthogonal self-assembled systems, namely a gelled bicontinuous microemulsion, was investigated with a set of complementary physico-chemical methods. Orthogonal self-assembly means that different structures self-assemble simultaneously in a system and coexist independently. In the chosen model system H2O – n-decane / 12-hydroxyoctadecanoic acid (12-HOA) – tetraethylene glycol monodecyl ether (C10E4) the organogelator 12-HOA forms a network which is surrounded by bicontinuous microemulsion domains. This was proved by comparing characteristic properties and the microstructure of the gelled bicontinuous microemulsion with those of the two ‘base systems’, i.e. the non-gelled bicontinuous microemulsion H2O – n-decane – C10E4 and the binary gel n-decane / 12-HOA. Firstly, phase studies were carried out which showed that the microemulsion phase boundaries are maintained upon gelation, merely shifted by about 6 K to lower temperatures. Likewise, a sol-gel transition occurs in the gelled microemulsion just as in the binary gel. Differential scanning calorimetry and temperature-dependent oscillating shear rheometry measurements revealed that the sol-gel transition temperature is about 20 K lower when a microemulsion, instead of pure n-decane, surrounds the gelator network. This reflects that part of the surface-active 12-HOA molecules adsorb at the water-oil interface instead of forming gelator fibers when a microemulsion is present. Accordingly, studying the linear viscoelastic range and the frequency-dependence of the storage and the loss modulus it was found that the gelator network is somewhat weaker in the gelled bicontinuous microemulsion than in the binary gel, although both systems are strong gels. In the following the focus turned to the microstructure of the gelled bicontinuous microemulsion. To begin with, the bicontinuity of the microemulsion domains in the middle of the one-phase region was verified determining the relative self-diffusion coefficients of water and n-decane with Fourier transform pulsed-gradient spin-echo 1H-NMR measurements. Subsequently, the coexistence of the bicontinuous microemulsion domains and the gelator network in the gelled bicontinuous microemulsion was evidenced by means of small angle neutron scattering. Finally, a visualization of the coexisting microstructures with freeze-fracture transmission electron microscopy complemented the work.
Open Access
Gelled non-toxic microemulsions
(2021) Peng, Ke; Stubenrauch, Cosima (Prof. Dr.)
Bicontinuous microemulsions gelled with a low molecular weight gelator (LMWG) have been found to be an orthogonally self-assembled system, i.e. the bicontinuous microstructure of the microemulsion and the fibrillar gel network form independently but simultaneously. Gelled non-toxic microemulsions have great potential to work as transdermal drug delivery systems because the non-toxic microemulsion provides optimum solubilization for drugs, while the gel network provides mechanical stability. In this work, we formulated gelled non-toxic bicontinuous microemulsions with an LMWG and investigated their formation via orthogonal self-assembly. Moreover, we provided a prototype for novel drug delivery systems.
Open Access
Monodisperse highly ordered and polydisperse biobased solid foams
(2018) Andrieux, Sébastien; Stubenrauch, Cosima (Prof. Dr.)
The aim of this work was the synthesis of monodisperse highly ordered biobased polymer foams and a comparison with their polydisperse counterparts. We used the biobased and biodegradable polymer chitosan, which we cross-linked with genipin. The polymer foams were synthesised via foam templating, i.e. via a liquid foam whose continuous phase contains a polymer and can be solidified. In order to obtain monodisperse highly ordered polymer foams, one first has to generate monodisperse highly ordered liquid foam templates. We did so by using microfluidics, which allows to produce monodisperse liquid foams with bubble sizes from 200 µm to 800 µm and polydispersities below 5%. The monodisperse foams were collected outside of the microfluidic channels and left to self-order under the influence of gravity and confinement. We studied the kinetics of the cross-linking reaction to find the optimal storage conditions during cross-linking. Once cross-linked we freeze-dried the gelled foams to obtain solid chitosan foams. We compared the morphological properties of the solid foams with those of the liquid templates in order to test the efficiency of the developed templating route. We observed how modifying the cross-linking and drying conditions can strongly affect the morphology of the solid foams. The main issue was to maintain the key properties of the liquid foam template throughout the solidification process, namely the bubble size distribution, the structural order and the density. We then compared the synthesised monodisperse polymer foams with their polydisperse counterparts. Although easy foaming methods exist for the generation of polydisperse foams, they do not allow the control over the polydispersity. We thus used microfluidics to generate liquid chitosan foams with tunable polydispersities from below 5% up to 26%. Microfluidics allows to match the average bubble size and density of the polydisperse liquid chitosan foam with those of the monodisperse counterpart. After solidifying the liquid templates we obtained solid foams with controlled polydispersities and studied the in uence of the polydispersity on the mechanical properties. However, we observed that not the polydispersity but the foam density was the main parameter at play. Moreover, the solid chitosan foams had weak mechanical properties with elastic moduli below 100 kPa. To overcome this issue, we incorporated cellulose nanofibres to the original chitosan solution and followed the developed route for foam templating. We had to adapt the microfluidic parameters to account for the viscosity changes brought about by the nanofibres. However, we managed to produce monodisperse liquid foams having the same bubble size, i.e. ~300 µm, but different amounts of cellulose nanofibres. The cellulose content had a strong influence on the solid foam morphology in general and on the pore connectivity in particular.
Open Access
Successive formation of a gel network and a lyotropic liquid crystal: does the chronology play a role?
(2019) Steck, Katja; Stubenrauch, Cosima (Prof. Dr.)
In this work lyotropic liquid crystals are gelled with low molecular weight gelators (LMWG). The focus is thereby on two fundamental questions. (1) Does the chronology of LLC and gel formation influence the size and orientation of the LLC domains and the alignment of the gel fibers? (2) Do the gel network and the LLC form simultaneously but independently, i.e. are gelled LLC orthogonal self-assembled systems?
Open Access
Tailor-made methacrylate-based polymer foams via emulsion and foamed emulsion templating
(2020) Dabrowski, Miriam Lucia; Stubenrauch, Cosima (Prof. Dr.)
Inspired by studies on tissue engineering, this PhD thesis aims at synthesizing and characterizing methacrylate-based polymer foams with tunable pore structures. To reach this goal, two templating routes, namely emulsion templating and foamed emulsion templating, were combined with microfluidics. Two different templating routes were chosen to get access to different pore sizes without changing the monomer. The monomer used was 1,4-butanediol dimethacrylate (1,4-BDDMA). The liquid templates and the solid polymer foams were characterized via light microscopy and scanning electron microscopy (SEM) to prove the “templating effect”.