13 research outputs found
Isolated ethylene units in isotactic polystyrene chain: stereocontrol of an isospecific post-metallocene titanium catalyst
No full textCopolymerization of styrene with small amounts of ethylene using the catalyst system dichloro[1,4-dithiabutanediyl-2,2'-bis(4,6-di-tert-butyl-phenoxy)] titanium/methylaluminoxane results in the unprecedented formation of isotactic polystyrene containing isolated ethylene units. C-13 NMR spectroscopic analysis of the copolymer indicates that an "enantiomorphic site" control mechanism for isospecific propagation is operating. DSC measurements also indicate the presence of isolated ethylene units which modify the crystallization behavior of the isotactic polystyrene
Core-shell particles and their application for superhydrophobic surfaces
No full textDuring the last years great effort has been devoted to the fabrication of superhydrophobic surfaces because of their self-cleaning properties. A water drop on a superhydrophobic surface rolls off even at inclinations of only a few degrees while taking up contaminants encountered on its way. rnSuperhydrophobic, self-cleaning coatings are desirable for convenient and cost-effective maintenance of a variety of surfaces. Ideally, such coatings should be easy to make and apply, mechanically resistant, and long-term stable. None of the existing methods have yet mastered the challenge of meeting all of these criteria.rnSuperhydrophobicity is associated with surface roughness. The lotus leave, with its dual scale roughness, is one of the most efficient examples of superhydrophobic surface. This thesis work proposes a novel technique to prepare superhydrophobic surfaces that introduces the two length scale roughness by growing silica particles (~100 nm in diameter) onto micrometer-sized polystyrene particles using the well-established Stöber synthesis. Mechanical resistance is conferred to the resulting “raspberries” by the synthesis of a thin silica shell on their surface. Besides of being easy to make and handle, these particles offer the possibility for improving suitability or technical applications: since they disperse in water, multi-layers can be prepared on substrates by simple drop casting even on surfaces with grooves and slots. The solution of the main problem – stabilizing the multilayer – also lies in the design of the particles: the shells – although mechanically stable – are porous enough to allow for leakage of polystyrene from the core. Under tetrahydrofuran vapor polystyrene bridges form between the particles that render the multilayer-film stable. rnMulti-layers are good candidate to design surfaces whose roughness is preserved after scratch. If the top-most layer is removed, the roughness can still be ensured by the underlying layer.rnAfter hydrophobization by chemical vapor deposition (CVD) of a semi-fluorinated silane, the surfaces are superhydrophobic with a tilting angle of a few degrees. rnrnrnAufgrund der selbstreinigenden Eigenschaft von superhydrophoben Oberflächen wurde in den letzten Jahren viel Arbeit in deren Herstellung gesteckt. Ein Tropfen Wasser perlt von einer um nur wenige Grad geneigten superhydrophoben Oberfläche ab, während er Verunreinigungen auf seinem Weg mitnimmt.rnSuperhydrophobe, selbstreinigende Beschichtungen sind eine mögliche Lösung für den kostengünstigen Erhalt vieler Oberflächen. Idealerweise sind diese Beschichtungen einfach in der Herstellung, mechanisch resistent und Langzeitstabil. Bisher konnte keine Methode gefunden werden alle diese Kriterien zur gleichen Zeit zu erfüllen.rnSuperhydrophobizität steht in direkter Beziehung zur Oberflächenrauigkeit. Das Lotusblatt mit einer zweifach skalierten Oberflächenrauigkeit ist ein Beispiel für eine der effizientesten superhydrophobe Oberflächen.rnIn der vorliegenden Arbeit wurde eine neue Technik zur Herstellung hydrophober Oberflächen vorgestellt. Zwei Längenskalen der Rauheit wurden durch das Aufbringen von Silikapartikel (~ 100 nm im Durchmesser) auf Mikrometer große Polystyrolpartikel durch die bekannte Stöber Synthese dargestellt. Mechanische Stabilität der Partikel wurde durch die dünne Silika Schale auf der Oberfläche der resultierenden „Himbeeren“ erzeugt. Neben der einfachen Darstellung und Handhabbarkeit bieten diese Partikel die Möglichkeit einer verbesserten technischen Anwendung: zum einen sind sie in Wasser dispergierbar und zum anderen können Doppelschichten durch Auftropfmethoden auf selbst sehr rauen Oberflächen erzeugt werden. Die Lösung des Hauptproblems – der Stabilität der Multilagen – liegt im Design der Partikel: Die Schalen, obwohl mechanisch stabil, sind porös genug um Polystyrol aus dem Kern durchzulassen. Im THF Dampf können sich Polystyrolbrücken zwischen den Partikeln ausbilden, die durch das Verkleben der Partikel untereinander für eine Stabilität der Multilagenschicht sorgen. Multilagenschichten sind vielversprechend im Einsatz für kratzbeständige Oberfläche. Wenn die oberste Schicht beschädigt wird, kann die definierte Rauigkeit immer noch durch die darunterliegende Schicht gewährleistet werden. rnChemical Vapour Deposition (CVD) mit semi-fluorierten Silanen bewirkt eine Superhydrophobisierung der Oberflächen mit einem Neigungswinkel von wenigen Grad.rnr
Halloysite@Polydopamine Nanoplatform for Ultrasmall Pd and Cu Nanoparticles: Suitable Catalysts for Hydrogenation and Reduction Reactions
No full textThe design of sustainable nanomaterials for catalysis is a key challenge in green chem-istry. Herein, we report the synthesis of halloysite nanotube (Hal)-based nanomaterials selectively functionalized with a bioinspired polydopamine (PDA) coating, which ena-bles the controlled anchoring of palladium and copper nanoparticles (PdNPs and CuNPs). This mild and ecofriendly strategy yields highly dispersed and ultrasmall (<5 nm) metal nanoparticles without the need for surfactants or harsh reagents. The re-sulting materials, Hal@PDA/PdNPs and Hal@PDA/CuNPs, were evaluated in two well-established model reactions commonly employed to probe catalytic performance: cinnamaldehyde hydrogenation and 4-nitrophenol reduction. Hal@PDA/PdNPs dis-played complete conversion and >90% selectivity toward hydrocinnamaldehyde at low Pd loading (0.8 wt%) and maintained its efficiency over six catalytic cycles. Hal@PDA/CuNPs proved to be an excellent recyclable catalyst for the reduction of 4-nitrophenol, retaining high activity through eight consecutive runs. Overall, this study introduces a robust and modular approach to fabricate halloysite-based nano-catalysts, demonstrating their potential as green platforms for metal nanoparti-cle-mediated transformation
Durability of Superamphiphobic Polyester Fabrics in Simulated Aerodynamic Icing Conditions
No full textFabrics treated to repel water, superhydrophobic, and water and oil, superamphiphobic, have numerous industrial and consumer-level benefits. However, the liquid repellency decreases in the course of time. This is largely due to chemical or physical changes of the coating due to prolonged exposure to relatively harsh environments. To develop more durable fabric treatments for specific applications, it is necessary to measure the extent to which the treated fabrics retain their low-wettability after being subjected to controlled aggressive environmental conditions. In this study, plain weave fabrics made from polyester filaments and coated with silicone nanofilaments in-solution were exposed to aerodynamic icing conditions. The coated fabrics showed superhydrophobic behavior, or superamphiphobic for those that were fluorinated. The wettability of the fabrics was progressively evaluated by contact angle and roll-off-angle measurements. The coated fabrics were able to maintain their low-wettability characteristics after exposure to water droplet clouds at airspeeds up to 120 m/s, despite damage to the silicone nanofilaments, visible through scanning electron microscopy.Circular Product DesignNovel Aerospace Material
Flow-Induced Long-Term Stable Slippery Surfaces
No full textSlippery lubricant-infused surfaces allow easy removal of liquid droplets on surfaces. They consist of textured or porous substrates infiltrated with a chemically compatible lubricant. Capillary forces help to keep the lubricant in place. Slippery surfaces hold promising prospects in applications including drag reduction in pipes or food packages, anticorrosion, anti-biofouling, or anti-icing. However, a critical drawback is that shear forces induced by flow lead to depletion of the lubricant. In this work, a way to overcome the shear-induced lubricant depletion by replenishing the lubricant from the flow of emulsions is presented. The addition of small amounts of positively charged surfactant reduces the charge repulsion between the negatively charged oil droplets contained in the emulsion. Attachment and coalescence of oil droplets from the oil-in-water emulsion at the substrate surface fills the structure with the lubricant. Flow-induced lubrication of textured surfaces can be generalized to a broad range of lubricant–solid combinations using minimal amounts of oil
When and how self-cleaning of superhydrophobic surfaces works
No full textDespite the enormous interest in superhydrophobicity for self-cleaning, a clear picture of contaminant removal is missing, in particular, on a single-particle level. Here, we monitor the removal of individual contaminant particles on the micrometer scale by confocal microscopy. We correlate this space- and time-resolved information with measurements of the friction force. The balance of capillary and adhesion force between the drop and the contamination on the substrate determines the friction force of drops during self-cleaning. These friction forces are in the range of micro-Newtons. We show that hydrophilic and hydrophobic particles hardly influence superhydrophobicity provided that the particle size exceeds the pore size or the thickness of the contamination falls below the height of the protrusions. These detailed insights into self-cleaning allow the rational design of superhydrophobic surfaces that resist contamination as demonstrated by outdoor environmental (>200 days) and industrial standardized contamination experiments.</p
Tuning the mechanical properties of silica microcapsules
No full textHeat treatment is a standard method to increase the hardness of silica in various applications. Here, we tested the effect of high temperature annealing on the mechanical properties of silica microcapsules by force spectroscopy under point loads applied to the particle shell. The Young's modulus of the shells moderately increases after annealing at temperatures above 500 °C. Temperatures over 850 °C result in a much stronger increase and the Young's modulus is close to that of fused silica after annealing at 1100 °C. NMR analysis revealed that in untreated microcapsules synthesized by seeded growth using the Stöber method only 55% of the silicon atoms form siloxane bonds with four neighbors, whereas the remaining ones only form three or less siloxane bonds each and, thus, a large number of ethoxy and silanol groups still exist. During annealing at 500 °C, these are successively transformed into siloxane bonds through condensation reactions. This process correlates with only a moderate increase in Young's modulus. The strong increase at temperatures above 850 °C was associated with a densification which was associated by a decrease in capsule size and shell thickness while the shells remained homogenous and of spherical shape. The main strengthening of the shells is thus mainly due to compaction by sintering at length scales significantly larger than that of local siloxane bonds
Hollow silica spheres : synthesis and mechanical properties
No full textCore-shell polystyrene-silica spheres with diameters of 800 nm and 1.9 μm were synthesized by soap-free emulsion and dispersion polymerization of the polystyrene core, respectively. The polystyrene spheres were used as templates for the synthesis of silica shells of tunable thickness employing the Sto¨ber method. The polystyrene template was removed by thermal decomposition at 500 °C, resulting in smooth silica shells of well-defined thickness (15-70 nm). The elastic response of these hollow spheres was probed by atomic force microscopy (AFM). A point load was applied to the particle surface through a sharp AFM tip, and successively increased until the shell broke. In agreement with the predictions of shell theory, for small deformations the deformation increased linearly with applied force. The Young’s modulus (18 + - 6 GPa) was about 4 times smaller than that of fused silica [Adachi and Sakka J. Mater. Sci. 1990, 25, 4732] but identical to that of bulk silica spheres (800 nm) synthesized by the Stöber method, indicating that it yields silica of lower density. The minimum force needed to irreversibly deform (buckle) the shell increased quadratically with shell thickness
