123 research outputs found
Scattering Techniques to Study the Microstructure of Microemulsions
No full textHellweg T. Scattering Techniques to Study the Microstructure of Microemulsions. In: Stubenrauch C, ed. Microemulsions - Background, New Concepts, Applications, Perspectives. 1st ed. Chichester: Blackwell Publishing Ltd.; 2009: 48-83
Microfluidics: A tool to control the degree of polydispersity
No full textThis contribution deals with a foam templating route to produce solid monodisperse chitosan foams via microfluidics. We firstly produced a monodisperse liquid foam from a 4 wt % chitosan solution. Subsequently, we cross-linked the chitosan with genipin and freeze-dried the resulting monodisperse foamed hydrogel to obtain solid monodisperse chitosan foams. In order to obtain a desired polydispersity we also used microfluidics, i.e. we produced polydisperse foams with a controlled bubble size distribution by applying a periodic pressure to the gas phase. The resulting polydisperse templates were cross-linked and solidified by the same procedures we used for the monodisperse chitosan foams, which, in turn, leads to solid polydisperse chitosan foams with a controlled polydispersity. Having control over both the bubble size and the polydispersity allowed us to study the influence of the pore size distribution on the mechanical properties since the generated solid foams have the same average pore size and the same composition (gas phase included). We focus our attention on the structure of the resulting solid monodisperse and polydisperse chitosan foams (SEM and μ-CT images) and on their compressive mechanical properties and discuss the differences between the two systems
3D printing of functionally graded porous materials using on-demand reconfigurable microfluidics
No full textTailoring the morphology of macroporous structures remains one of the biggest challengesin material synthesis. Here, we present an innovative approach to fabricatecustommacroporous materials with pore size varying throughout the structure by up to an order of magnitudeusingon-demand reconfigurable microfluidics. We employavalve-based flow-focusing junction(vFF)in which the size of the orifice can be adjusted in real-time (within tens of milliseconds)to generate foams withon-linecontrolled bubble size. We use the junction tofabricatelayered and smoothly graded porous structureswith pore size varying in the range 80-800 μm.Additionally, to further exploit thecapacityof the technique, we mount vFF on top of an extrusion printer and 3D-print constructscharacterized by a predefined 3D geometryand controlled spatiallyvaryinginternal porous architecture, such asa model of a bone. The presented technology opensup new possibilities in macroporous material synthesis with potential applications ranging from tissue engineering to aerospace industry and construction
Successive formation of a gel network and a lyotropic liquid crystal: does the chronology play a role?
No full textIn 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
Gelled non-toxic microemulsions
No full textBicontinuous 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
Tailor-made methacrylate-based polymer foams via emulsion and foamed emulsion templating
No full textInspired 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”
Gelierte bikontinuierliche Mikroemulsionen : ein neuer Typ orthogonal-selbstorganisierter Systeme
No full textIn 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.In dieser Arbeit wurde ein neuer Typ Orthogonal-Selbstorganisierter Systeme, nämlich eine gelierte bikontinuierliche Mikroemulsion, mit einer Reihe komplementärer physikochemischer Methoden untersucht. Orthogonale Selbstorganisation bedeutet, dass sich verschiedene Strukturen in einem System in simultanen Selbstorganisationsprozessen bilden und unabhängig voneinander koexistieren. Im gewählten Modellsystem H2O – n-Dekan / 12-Hydroxyoktadekansäure (12-HOA) – Tetraethylenglycolmonodecylether (C10E4) bildet der Organogelator 12-HOA ein Netzwerk, das von bikontinuierlichen Mikroemulsionsdomänen umgeben ist. Dies wurde nachgewiesen, indem charakteristische Eigenschaften und die Mikrostruktur der gelierten bikontinuierlichen Mikroemulsion mit denen der zwei „Basissysteme“, der ungelierten bikontinuierlichen Mikroemulsion H2O – n-Dekan – C10E4 und des binären Gels n-Dekan / 12-HOA, verglichen wurden.
Zunächst zeigte eine Phasenstudie, dass die Mikroemulsionsphasengrenzen bei der Gelierung erhalten bleiben. Sie verschieben sich lediglich um etwa 6 K zu tieferen Temperaturen. Ebenso findet in der gelierten Mikroemulsion, ganz wie im binären Gel, ein Sol-Gel-Übergang statt. Laut Ergebnissen der dynamischen Differenzkalorimetrie und temperaturabhängiger Oszillationsmessungen am Scherrheometer ist die Sol-Gel-Temperatur um etwa 20 K erniedrigt, wenn das Gelatornetzwerk von einer Mikroemulsion anstatt von reinem n-Dekan umgeben ist. Dies lässt sich darauf zurückführen, dass ein Teil der grenzflächenaktiven 12-HOA-Moleküle in Anwesenheit einer Mikroemulsion an die Wasser-Öl-Grenzfläche adsorbiert anstatt Gelatorfibrillen zu bilden. Entsprechend zeigte sich auch bei der Untersuchung des linear-viskoelastischen Bereichs und der Frequenzabhängigkeit von Speicher- und Verlustmodul, dass das Gelatornetzwerk in der bikontinuierlichen Mikroemulsion etwas schwächer ist als im binären Gel, obgleich beide Systeme starke Gele sind. Im Folgenden wurde der Fokus auf die Mikrostruktur der gelierten bikontinuierlichen Mikroemulsion gelenkt. Als Erstes wurde die Bikontinuität der Mikroemulsionsdomänen in der Mitte des Einphasengebiets bestätigt, indem die relativen Selbstdiffusionskoeffizienten von Wasser und n-Dekan durch Fourier-transformierte Spinecho-1H-NMR-Messungen mit Magnetfeldgradientenpulsen bestimmt wurden. Anschließend wurde die Koexistenz der bikontinuierlichen Mikroemulsionsdomänen und des Gelatornetzwerks in der gelierten bikontinuierlichen Mikroemulsion durch Kleinwinkelneutronenstreuexperimente nachgewiesen. Die Visualisierung der koexistierenden Mikrostrukturen durch Gefrierbruch-Transmissionselektronenmikroskopie komplettierte schließlich die Arbeit
Emulsion templating: unexpected morphology of monodisperse macroporous polymers
No full textThe 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
Monodisperse highly ordered and polydisperse biobased solid foams
No full textThe 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
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