665 research outputs found
Microscale modeling of the growth of microgels by precipitation polymerization
No full textIn this work, a predictive model to describe quantitatively the three main formation mechanisms, i.e., formation of oligomers in the liquid, precipitation of gel particles and individual particle growth, during the synthesis of thermally sensitive microgels from N-isopropylacrylamide (NIPAAm) and N-vinylcaprolactam (VCL) monomers by precipitation polymerization in water is presented. Monomer conversion as well as the growth evolution of individual gel particles (particle size distribution) during the synthesis in a reactor are predicted. They are modeled as a function of reaction and transport kinetics, thermodynamics of the liquid-gel particle system, as well as as a function of the process operating conditions in the reactor. The resulting mathematical model of this work can form the basis for rational and model-based decision making in the optimization of the microgel synthesis process and the control of the microgel end-product properties, in particular, particle size distribution. The present model is developed in joint work with the group of Prof. Andrij Pich at the Institute of Technical and Macromolecular Chemistry (ITMC), RWTH Aachen University and at the DWI Leibniz Institute for Interactive Materials e.V., Aachen, Germany, as well as with the group of Prof. Gabriele Sadowski at Lehrstuhl für Thermodynamik, TU Dortmund University. At the mentioned institutes, diverse experimental measurements during the microgel synthesis as well as numerical simulations of phase equilibrium between the liquid and gel particle phases have been carried out by Michael Kather [30] and by Markus C. Arndt [8], respectively, in order to support the development of the present model in this doctoral thesis. As a result of the mentioned joint work, the critical chain length η, at which the precipitation of PVCL gel particles occurs, is experimentally determined and the volume evolution of PNIPAAm and PVCL gel particles is modeled during their synthesis by applying the Perturbed-Chain Statistical Associating Fluid Theory (PC-SAFT) equation of state. In addition, experimental measurements of the monomer conversion and of the averaged hydrodynamic radius of gel particles during the synthesis have been carried out at ITMC and DWI to support the present predictive model [30]. Diverse modeling approaches at different time and length scales are applied to model the polymerization and mass transfer processes in the gel particles and the liquid as well as the thermodynamics of the gel particle-liquid system. Namely, the formation of oligomers in the liquid is modeled on a low level of detail by formulating molar balances for the oligomers with chain length i (i=1,…,η) in the reactor. The precipitation of gel particles is modeled also on a low level of detail by applying population balances which simulate the number of precipitated gel particles which are born in generation g (g=1,…,Ζ) as a function of polymerization time. In this work, the use of generations of particles is the way to mathematically observe and follow the complex processes of the microgel formation mechanisms. In order to apply the framework of population balances, a particle birth rate function is developed in this doctoral thesis which models the precipitation of gel particles at the critical chain length η. The individual particle growth is modeled on a high level of detail by applying local molar balances between single gel particles and their surrounding liquid as well as the stochastic simulation algorithm (SSA) and the PC-SAFT equation of state. In addition to the local molar balances, a Monte Carlo (MC) simulation algorithm is developed in this work which numerically evaluates the oligomer absorption to the gel particles. The SSA algorithm simulates the polymerization of monomer in individual gel particles at time and length scales which are much finer than the scales covered by the molar and population balances. The molar and population balances (liquid model) as well as the MC, SSA algorithms and PC-SAFT equation of state (individual particle model) are depending on each other. For this reason, the molar and population balances of the liquid model are linked to the individual particle model which is cast into the MC, SSA algorithms and PC-SAFT equation of state. Complex integration and simulation strategies are developed in this work which performs the integration of the liquid model with the individual particle model to allow for the prediction of monomer conversion and the growth evolution of individual gel particles (particle size distribution) in the reactor as a function of polymerization time. Simulation results are compared to experimental data of monomer conversion and averaged hydrodynamic radius of gel particles evaluated at different polymerization temperatures. The influence of the polymerization temperature on monomer conversion, on the birth of gel particles as well as on the growth evolution of individual gel particles is evaluated. The polymerization rate coefficients of the synthesis of PNIPAAm and PVCL microgels by precipitation polymerization in water are estimated. The effect of other particle formation mechanisms, i.e., the absorption of oligomer molecules and their polymerization in single gel particles, on the birth and individual growth of gel particles is also studied. The experimental data and the simulation results suggest that the phenomena of absorption and polymerization of oligomers in the gel particles are important mechanisms which contribute enormously to the individual growth of PNIPAAm and PVCL gel particles during their synthesis by precipitation polymerization in water
Pros and Cons: Supramolecular or Macromolecular: What Is Best for Functional Hydrogels with Advanced Properties?
Get PDFHydrogels are fascinating soft materials with unique properties. Many biological systems are based on hydrogel-like structures, underlining their versatility and relevance. The properties of hydrogels strongly depend on the structure of the building blocks they are composed of, as well as the nature of interactions between them in the network structure. Herein, gel networks made by supramolecular interactions are compared to covalent macromolecular networks, drawing conclusions about their performance and application as responsive materials.ChemE/Advanced Soft Matte
Behavior of temperature responsive microgels at interfaces : deformability and interfacial activity
No full textThe interfacial activity is an important property of microgels which consist of intramolecular cross-linked polymer networks, as it is involved in applications like the fabrication of films, emulsions or capsules. To investigate the influence of the chemical composition, cross-linking density and temperature on the interfacial behavior of microgels, thermo-sensitive poly(N-vinylcaprolactam-co-N-isopropylacrylamide) (PVCL/NIPAm), poly(N-vinylcaprolactam-co-N-isopropyl-methacrylamide) (PVCL/NIPMAm) copolymer microgels and homopolymer analogues were synthesized, and their properties were determined. The interfacial activities of PVCL/NIPAm and PVCL/PNIPMAm copolymer microgels were measured with a pendant drop tensiometer. The interfacial behaviors of the copolymer microgels fall between that of homopolymer microgels. It means that the chemical structure contributes to the variation in the interfacial tension. The dynamic interfacial tension of microgels with lower cross-linking densities evolves faster than that of microgels with higher cross-linking densities. This is consistent with the simulation results that loosely cross-linked microgels spread faster at the air/liquid and liquid/liquid interfaces than densely cross-linked ones. Meanwhile, the dimensions of the adsorbed microgels on a solid substrate characterized with atomic force microscopy indicate that the deformability of the microgels at the air/solid interface also increases with the decreasing cross-linking degree. Besides, the evolution of the dynamic interfacial tension for PVCL/NIPAm and PVCL/NIPMAm microgels at TVPTT because of the reducing microgel deformability with increasing temperature. Furthermore, the comparison between the dimensions of microgels on the solid substrate and in solution shows that adsorption of microgels induces higher deformation than the swelling-induced expansion
Synthesis, morphology, and dynamics of microgels with different architectures
No full textMy research work is focused on the characterisation of novel microgel systems using theoretical and experimental analytical tools. The Flory theory for the swelling of microgels was generalised in order to include a bimodal heterogeneous morphology in terms of crosslink distribution in the microgel particle. Further extension to the Flory theory was developed for random copolymer microgels in the heterogeneous crosslinking morphology approximation. The core-shell architecture of microgels was also investigated with an extended Flory swelling theory in the homogeneous approximation. The theories developed allow the determination of specific microgel structural parameters, valuable to the field of study. The distribution of monomer units in a copolymer microgel was investigated using 1H transverse relaxation measurements, establishing a new approach to determine the location of different components specific for microgel particles. The phase transition of microgel as response to the alcohol concentration in water/alcohol mixtures was also investigated. Two definitions of the cononsolvency effect were proposed and exemplified for the case of two different types of microgels. The findings presented, their explanation and the novel methods established for the characterisation of microgels are valuable for the scientific community. Most of the studies presented in this work are peer reviewed and published in high ranking journals
Synthesis, morphology, and dynamics of microgels with different architectures
No full textMy research work is focused on the characterisation of novel microgel systems using theoretical and experimental analytical tools. The Flory theory for the swelling of microgels was generalised in order to include a bimodal heterogeneous morphology in terms of crosslink distribution in the microgel particle. Further extension to the Flory theory was developed for random copolymer microgels in the heterogeneous crosslinking morphology approximation. The core-shell architecture of microgels was also investigated with an extended Flory swelling theory in the homogeneous approximation. The theories developed allow the determination of specific microgel structural parameters, valuable to the field of study. The distribution of monomer units in a copolymer microgel was investigated using 1H transverse relaxation measurements, establishing a new approach to determine the location of different components specific for microgel particles. The phase transition of microgel as response to the alcohol concentration in water/alcohol mixtures was also investigated. Two definitions of the cononsolvency effect were proposed and exemplified for the case of two different types of microgels. The findings presented, their explanation and the novel methods established for the characterisation of microgels are valuable for the scientific community. Most of the studies presented in this work are peer reviewed and published in high ranking journals
Microgel polymer composite fibres
No full textIn this thesis some novel ideas and advancements in the field of polymer composite fibres, specifically microgel-based polymer composite fibres have been achieved. The main task was to investigate and understand the electrospinning process of microgels and polymers and the interplay of parameter influences, in order to fabricate reproducible and continuously homogenous composite fibres. The main aim was to fabricate a composite material which combines the special properties of polymer fibres and thermo-sensitive microgels, as well as properties given by the specific choice of the fibre polymer and the microgels co-monomers. Furthermore, these fibres are supposed to enable a macroscopic access to the microgel properties, because their usual dispersion state is not applicable for many tasks, but a macroscopic fibre nonwoven consisting of microscopic fibres decorated with nanoscopic microgels will provide this opportunity, without losing the “nano” aspect. In a first step, using PVA it was already shown that the microgels retain their thermo-sensitive, smart swelling properties in the fibre structure, which gives the fibres tuneable swelling properties as well. Additional ways to crosslink these fibres chemically or physically are shown. In the next step, PCL, a polymer with more special properties (hydrophobic, degradable), was chosen to achieve fibres with these properties and to show how much these properties can be influenced by the addition of microgels. Moreover, different fibre morphologies have been fabricated, fibres with microgels located only in the core and fibres with microgels located only on the surface, which not only show differences in the tuneable swelling behaviour and the degradation process, but it also opens opportunities to more specific applications. The different morphologies were achieved by using different solvent systems: methanol/toluene and chloroform/DMF. Additionally, it should be mentioned that the simple one step electrospinning process of hydrophilic microgels and hydrophobic PCL gives access to an elegant way to completely change the hydrophobicity of the general polymer fibres. To give a possibility for a better exploitation of the newly achieved PCL fibres with microgel exclusively on the fibre surface, microgels with a special property combination have been created: microgels, crosslinked with a star-shaped acrylate-functionalised poly(epsilon-caprolactone) crosslinker, that are degradable due to the same functionality as PCL, having additionally hydrophobic domains to immobilise hydrophobic drugs. The synthesis was done via a specialised miniemulsion polymerisation and uptake as well as release of ibuprofen was shown. Fibres with these microgels on the surface could deliver drugs targeted to specific places and are completely degradable under physiological conditions. The results of a preliminary study for a project with the aim of creating PLA based stents, with a neutral degradation process for a higher tolerance in the human body. A combination of VIm modified microgels with polylactide fibres was chosen to achieve this. The fibres are successfully realised and analysed regarding their swelling properties, in the same manner as the other composite fibres presented in this work. Two small preliminary studies about different topics, which are still in an early stage, but that already show promising results are also presented in this thesis. Fibres with iron(III) oxide nanoparticles and phosphazene microsphere have also been fabricated using the same technique shown for the PCL-microgel fibres with microgel exclusively on the surface. These fibres give an insight in the process and show its limitations and possibilities. Furthermore, hollow fibre membranes with microgels as additive have been prepared using a wet spinning process, to show other options to fabricate composite materials with microgels, accessing the field of filtration and separation
Microgel polymer composite fibres
No full textIn this thesis some novel ideas and advancements in the field of polymer composite fibres, specifically microgel-based polymer composite fibres have been achieved. The main task was to investigate and understand the electrospinning process of microgels and polymers and the interplay of parameter influences, in order to fabricate reproducible and continuously homogenous composite fibres. The main aim was to fabricate a composite material which combines the special properties of polymer fibres and thermo-sensitive microgels, as well as properties given by the specific choice of the fibre polymer and the microgels co-monomers. Furthermore, these fibres are supposed to enable a macroscopic access to the microgel properties, because their usual dispersion state is not applicable for many tasks, but a macroscopic fibre nonwoven consisting of microscopic fibres decorated with nanoscopic microgels will provide this opportunity, without losing the “nano” aspect. In a first step, using PVA it was already shown that the microgels retain their thermo-sensitive, smart swelling properties in the fibre structure, which gives the fibres tuneable swelling properties as well. Additional ways to crosslink these fibres chemically or physically are shown. In the next step, PCL, a polymer with more special properties (hydrophobic, degradable), was chosen to achieve fibres with these properties and to show how much these properties can be influenced by the addition of microgels. Moreover, different fibre morphologies have been fabricated, fibres with microgels located only in the core and fibres with microgels located only on the surface, which not only show differences in the tuneable swelling behaviour and the degradation process, but it also opens opportunities to more specific applications. The different morphologies were achieved by using different solvent systems: methanol/toluene and chloroform/DMF. Additionally, it should be mentioned that the simple one step electrospinning process of hydrophilic microgels and hydrophobic PCL gives access to an elegant way to completely change the hydrophobicity of the general polymer fibres. To give a possibility for a better exploitation of the newly achieved PCL fibres with microgel exclusively on the fibre surface, microgels with a special property combination have been created: microgels, crosslinked with a star-shaped acrylate-functionalised poly(epsilon-caprolactone) crosslinker, that are degradable due to the same functionality as PCL, having additionally hydrophobic domains to immobilise hydrophobic drugs. The synthesis was done via a specialised miniemulsion polymerisation and uptake as well as release of ibuprofen was shown. Fibres with these microgels on the surface could deliver drugs targeted to specific places and are completely degradable under physiological conditions. The results of a preliminary study for a project with the aim of creating PLA based stents, with a neutral degradation process for a higher tolerance in the human body. A combination of VIm modified microgels with polylactide fibres was chosen to achieve this. The fibres are successfully realised and analysed regarding their swelling properties, in the same manner as the other composite fibres presented in this work. Two small preliminary studies about different topics, which are still in an early stage, but that already show promising results are also presented in this thesis. Fibres with iron(III) oxide nanoparticles and phosphazene microsphere have also been fabricated using the same technique shown for the PCL-microgel fibres with microgel exclusively on the surface. These fibres give an insight in the process and show its limitations and possibilities. Furthermore, hollow fibre membranes with microgels as additive have been prepared using a wet spinning process, to show other options to fabricate composite materials with microgels, accessing the field of filtration and separation
Stimuli responsive hybrid microgels functionalized with metal nanoparticles
No full textThis dissertation deals with the synthesis and modification (in a direct or an indirect way) of microgels, which can act as containers for metal ions or nanoparticles. The first part of the work focuses on the synthesis of new catalytic colloidal reactors. The challenge was to combine the advantages of the homogeneous and heterogeneous catalysis and create hybrid microgels with defined properties. For this the application of the thermo-sensitive hybrid microgels, loaded with catalytic active centers was used. At the reaction temperature, hybrid microgels have a big surface area, are well dispersed and swollen in the solvent. Each catalytic active center was open and accessible for the reactants (advantage of the homogeneous catalysis). After the catalytic reaction, the catalyst can be easily removed from the reaction mixture (advantage of the heterogeneous catalysis). This was achieved through the application of microgels with defined CAT (critical aggregation temperature) properties. After the implementation of the reaction and cooling down of the reaction mixture, aggregated hybrid microgel can be easily removed by decantation. The hybrid microgels loaded with noble metals complexes and nanoparticles show high catalytic activity and can be reused several times. The second part of the work focuses on the synthesis of conductive inks based on water dispersed conductive hybrid microgels. This can be used for the fabrication of smart textiles. In this work it was possible to create continuous connections between hardware components or to create high-flexible electrical circuits for different applications. An approach to use a combination of the microgel flexibility and metal conductivity has significantly reduced the production costs and wearability of the functional textiles. The electrical conductivity of the hybrid microgel films loaded with silver was close to the conductivity of bulk silver. Due to the high mechanical properties, films can be bent for 5000 times without losing conductivity
Polyelectrolyte microgels with controlled number and distribution of charges : from synthesis to application
No full textResearch in microgels focuses more than ever on mastering increasingly complex molecular structures and superstructure assemblies. The studies aim at novel properties, multiple responsiveness, and targeted application in different fields. In this work, I introduce charged microgels with complex architectures and defined localization of different ionizable groups. The challenge is to synthesize polyampholyte microgels with ionizable groups of opposite charges, and the production of surface-modified polyelectrolyte microgels of superior colloidal stability which respond to pH and temperature changes for defined application as drug carriers for biological guest molecules and as building blocks to assemble polyampholyte superstructures. As the control over the charge distribution of the acidic and basic moieties within the microgel network is crucial not only for the fundamental research but also plays an important role in the interaction with guest molecules, polyampholyte microgels with controlled architectures and defined localization of ionizable groups (random, core-shell, and Janus-like) were synthesized by modified precipitation polymerization. A new facile and straightforward approach to produce Janus-like polyampholyte microgels was proposed based on the coacervation process of oppositely charged precursor particles under a specific mixing time. In addition, surface modified polyelectrolyte core-shell microgels with functional GMA (Glycidyl methacrylate) groups located only on the surface of the microgels were synthesized via two-step precipitation polymerization with a delayed GMA addition. Polyelectrolyte microgels have been used as carriers for the transport and protection of guest molecules for decades due to their response to pH changes. So far, studies exposed that the interaction between the polyelectrolyte microgels and oppositely charged guest molecules are strongly dependent on the type of charges within the microgel as well as the kind of guest molecules. However, other factors such as the distribution of opposite charged ionizable groups and the structural formation alone or in combination have not been discussed. Therefore, investigation on the interaction of different pre-synthesized charged microgel carriers (polyampholyte microgels and surface modified polyelectrolyte microgels) with biological guest molecules of a different kind (protein and peptide) were accomplished. The effect of the charge distribution and the structural design of the microgels as well as the change of the environment (pH of the surroundings) on the uptake and release procedure was revealed. A striking discovery was made showing that the distribution of ionizable groups in polyampholyte microgels (random and core-shell) as well as the surface modification of the polyelectrolyte core-shell microgels with fuctional GMA groups controls the interactions with the captured proteins from entrapment and “levitation” to accelerated release. Furthermore, polyampholyte colloidal superstructures using polyampholyte and polyelectrolyte microgels as efficient building blocks was introduced. Regarding their temperature and pH responsiveness, polyampholyte colloidal superstructures with unique properties was designed. In specific, polyampholyte hydrogels via additional physical crosslinking with tannic acid under a dense state, and polyampholyte assemblies via the self-assembly process of two opposite charged microgels was synthesized. The formation of the polyampholyte superstructures were confirmed by means of different techniques. Both superstructures will not only open a new pathway for the fundamental research on studies of gel-based superstructures but are also promising candidates for wound healing or as functional materials that enable multiple loading and release of guest molecules which can be applied in the biomedical entity
Functional microgel coatings
No full textMicrogels are three-dimensional crosslinked polymer networks and are characterized by their mechanical properties and versatility, making them an interesting material for biological applications. Microgels have proven to be particularly suitable as formulations for active ingredients or as coatings for surfaces. In particular, poly(N-vinylcaprolactam) and poly(N-isopropylacrylamide) microgels are interesting candidates for biological applications, as they have a volume-phase transition temperature around 32 °C, which is close to body temperature. In the first part of this work, nanostructured microgel arrays on a solid surface are fabricated and used to align cells and control their movement. These nanostructured microgel lines are produced in a printing process using microwrinkled poly(dimethylsiloxane) stamps. For this purpose, poly (N-isopropylacrylamide) microgels with different sizes and crosslinking degrees are synthesized, in order to identify the ideal parameters for microgel printing. The printed microgel arrays have a defined line spacing and are crosslinked by argon plasma, to stabilize them against aqueous media. The movement of cells is then examined on microgel lines with different line spacing. As a further development of this system, reactive microgels containing glycidyl methacrylate, and thereby epoxy groups, are synthesized. The position of the epoxy groups in the microgel is controlled by variation of the addition time of the glycidyl methacrylate during the reaction and subsequently the reactivity of the microgels is investigated. The reactive microgels are printed using the microwrinkled structured stamps and covalently attached to thiosilanized surfaces, leading to solvent stable microgel lines. After the printing process, the microgel arrays remain reactive and cells align in the direction of the line. In the second part of the work, microgels are developed that are used as coatings for crop protection. The synthesized microgels are loaded with pesticides, which can be released under defined conditions. Those microgels also contain glycidyl methacrylate and thereby epoxy groups, whereby the microgels can be decorated with tailored anchor peptides, which specifically bind to the target plant surface. When the leaves of apple plants or sugar beets are coated with this formulation, they are protected against the fungi Venturia inaequalis or Cercospora beticola. The applied amount of pesticides is lower than the current standard used in agriculture. Thereby, the microgel system can contribute to a more sustainable crop protection
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