Browsing by Author "Koch, Lukas"

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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
Porous polymers via emulsion templating : pore deformation during solidification cannot be explained by an osmotic transport!
(2020) Koch, Lukas; Drenckhan, Wiebke; Stubenrauch, Cosima
Using microfluidics, we were able to synthesize monodisperse water-in-monomer emulsions with styrene and divinylbenzene (DVB) as monomers. When polymerizing and drying these emulsions, we found that the structure of the resulting macroporous polymer strongly depends on the type of initiator. With the oil-soluble azobisisobutyronitrile (AIBN), an open-cell structure with spherical pores was obtained. However, with the water-soluble potassium peroxydisulfate (KPS), a closed-cell structure with rhombic dodecahedron-shaped pores and thick, layered pore walls was formed. In the latter case, a yet unexplained mechanism counteracts the capillary pressure arising from surface minimization: the surface area of a rhombic dodecahedron is ~ 10% larger than that of a sphere. In our previous work, we suggested that the underlying mechanism may be osmotic transport of DVB from the plateau borders to the films. We argued that this transport also explains the layered pore walls, i.e., the formation of two outer poly-DVB-rich layers and one inner polystyrene-rich layer. In order to prove or disprove this mechanism, we carried out additional experiments. However, none of those experiments corroborated our hypothesis of osmotic transport! This study provides clear experimental evidence that our previously suggested mechanism via which spherical droplets become polyhedral pores is incorrect. We will describe (a) the rationale behind the additional experiments, (b) our expectations, and (c) our findings. Last but not least, we will discuss all of this in the light of the proposed osmotic transport.