Synthesis and NMR-based network structure analysis of cationic hydrogels for seawater applications

Superabsorbent polymers (SAPs) are hydrophilic polymer networks (i. e., hydrogels) that contain charged monomer units along the polymer backbone. Poly(sodium acrylate) (PSA), which has negatively charged carboxylate groups, is the most prominent chemical structure of SAPs. Recent studies have shown that PSA can be used as a separation medium for the desalination of salt water. This approach, however, is limited to NaCl solutions because the divalent Mg$^{2+}$ and Ca$^{2+}$ cations in seawater interact electrostatically with the anionic polymer backbone of PSA, inducing thereby a network collapse. To overcome this limitation, this dissertation explores the swelling and desalination capacity of cationic SAPs in seawater. Two cationic SAP model systems with distinct functional groups were synthesized. The first one is based on a poly(acrylamide) (PAM) derivative with a trimethyl quaternary ammonium group as the positively charged monomer unit. The second model system is based on poly(vinyl amine) (PVAm) bearing positively charged ammonium groups. Swelling capacity measurements reveal that divalent SO$_{4}^{2-}$ anions in seawater induce a network collapse of PVAm similar to the PSA analogue. In contrast, the swelling behavior of PAM hydrogels is fully unaffected by seawater, suggesting that the quaternary ammonium moiety is essential to provide seawater resistant swelling properties of SAPs. In addition to swelling capacity studies, this dissertation aims to advance our understanding of the intriguing interplay between macroscopic, mechanical properties and molecular dynamics of the hydrogel network. Due to the inherent multi-length scale structural complexity of hydrogels, the quantitative correlation of mechanical properties with molecular dynamics remains a longstanding challenge. An advanced rheometer setup consisting of a portable low-field NMR unit that is integrated into a rheometer was used to study the gelation kinetics of acrylic acid (AAc) hydrogels. The elastic modulus G′ was studied by small amplitude oscillatory time sweeps whereas the local molecular mobility of polymer network chains was probed by T$_{2}$ relaxation measurements. From the in-situ G′ and T$_{2}$ correlation plots, it can be concluded that the elastic plateau modulus is inversely proportional to the T$_{2}$ relaxation time of the hydrogel. Consequently, the mechanical strength of hydrogels can be predicted based on the segmental mobility of polymer network chains, which has important implications for the further development of non-invasive and forceless mechanical characterization techniques

A dynamic copula approach to recovering the index implied volatility skew

Equity index implied volatility functions are known to be excessively skewed in comparison with implied volatility at the single stock level. We study this stylized fact for the case of a major German stock index, the DAX, by recovering index implied volatility from simulating the 30 dimensional return system of all DAX constituents. Option prices are computed after risk neutralization of the multivariate process which is estimated under the physical probability measure. The multivariate models belong to the class of copula asymmetric dynamic conditional correlation models. We show that moderate tail-dependence coupled with asymmetric correlation response to negative news is essential to explain the index implied volatility skew. Standard dynamic correlation models with zero tail-dependence fail to generate a sufficiently steep implied volatility skew.Copula Dynamic Conditional Correlation, Basket Options, Multivariate GARCH Models, Change of Measure, Esscher Transform

Desalination of Seawater Using Cationic Poly(acrylamide) Hydrogels and Mechanical Forces for Separation

In this study, the ability of cationic poly(acrylamide-co-(3-acrylamidopropyl)trimethylammonium chloride) hydrogels to desalinate seawater is explored, where the salt separation is based on the partial rejection of mobile salt ions by the fixed charges along the polymer backbone. Water absorbency measurements reveal that artificial seawater-containing divalent ions (Mg$^{2+}$, Ca$^{2+}$, and SO$_{4}$$^{2-}$) drastically decrease the swelling capacity of previously employed anionic poly(acrylic acid-co-sodium acrylate) hydrogels, whereas no influence on the swelling behavior of the synthesized cationic hydrogels is found. The swelling behavior and mechanical properties are studied by varying the degree of crosslinking and degree of ionization systematically in the range of 1–5 and 25–75 mol%, respectively. Finally, artificial seawater (c$_{sea}$= 0.171 mol L$^{-1}$) is desalinated in a custom-built press setup with an estimated efficiency of E$_{m³}$= 17.6 kWh m$^{-3}$ by applying an external pressure on the swollen hydrogels

In Situ RheoNMR Correlation of Polymer Segmental Mobility with Mechanical Properties during Hydrogel Synthesis

Understanding polymer gelation over multiple length-scales is crucial to develop advanced materials. An experimental setup is developed that combines rheological measurements with simultaneous time-domain $^{1}$H NMR relaxometry (TD-NMR) techniques, which are used to study molecular motion (<10 nm) in soft matter. This so-called low-field RheoNMR setup is used to study the impact of varying degrees of crosslinking (DC) on the gelation kinetics of acrylic acid (AAc) and N,N′-methylene bisacrylamide (MBA) free radical crosslinking copolymerization. A stretched exponential function describes the T$_{2}$ relaxation curves throughout the gelation process. The stretching exponent β decreases from 0.90 to 0.67 as a function of increasing DC, suggesting an increase in network heterogeneity with a broad T$_{2}$ distribution at higher DC. The inverse correlation of the elastic modulus G′ with T2 relaxation times reveals a pronounced molecular rigidity for higher DC at early gelation times, indicating the formation of inelastic, rigid domains such as crosslinking clusters. The authors further correlate G′ with the polymer concentration during gelation using a T$_{1}$ filter for solvent suppression. A characteristic scaling exponent of 2.3 is found, which is in agreement with theoretical predictions of G′ based on the confining tube model in semi-dilute entangled polymer solutions

Reversible photodissipation of composite photochromic azobenzene-alginate supramolecular hydrogels

Supramolecular smart materials can quickly elicit macroscopic changes upon external stimulation. Here we report that an azobenzene-containing cyclic dipeptide can form composite supramolecular hydrogels with alginate based on the charge complementarity, at lower loading than the critical gelation concentrations of either component. The gels can reversibly dissipate to fluids with UV light. They can also encapsulate and photorelease fluorescent cargo. Upon treatment of the gels with aqueous calcium salts, the alginate component is permanently cross-linked and the photochromic component is solubilized

Reprocessing of Instruments for Minimal Invasive Surgery - State of the Art and Prospects

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich.This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively.Die Weiterentwicklung des Instrumentariums der minimal-invasiven Chirurgie (MIC) vollzieht sich in Richtung einer Miniaturisierung mechanischer Komponenten, einer Kombination mehrerer Funktionen in einem Gerät und der Einführung neuer Techniken, insbesondere zur blutungsarmen und thermisch schonenden Durchtrennung von Blutgefäßen. Diese Entwicklungstendenzen haben Auswirkungen auf die Aufbereitungsfähigkeit der Geräte. Sie wirken sich in der Regel erschwerend aus. Hierbei stellt insbesondere die Reinigung, also die Entfernung der Anschmutzungen aus kleinen Lumina, filigranen Gelenken, von der Waschflotte schwer erreichbaren Toträumen etc. besonders hohe Anforderungen. Hinzu kommt, daß der Nachweis des Reinigungserfolges schwer zu führen ist und bisher kein in der Routine praktikables, allgemein akzeptiertes Standardnachweisverfahren eingeführt werden konnte. Es werden ein Überblick über die im Zusammenhang mit der Aufbereitung von Instrumenten der MIC auftretenden Problemstellungen gegeben und Lösungsansätze diskutiert.The development of instruments for minimal invasive surgery (MIS) is moving in the direction of the miniaturization of mechanical components, a combination of multiple functions in a single instrument, and the introduction of new techniques, in particular those reducing bleeding and thermal damage when cutting blood vessels. These tendencies have consequences for the reprocessability of the instruments, usually making reprocessing more difficult. In particular cleaning the removal of contaminations from tiny lumina, joints, etc., is highly demanding. In addition, proof of successful cleaning is difficult, and no standardised method of doing this in practice is currently available. An overview of the problems associated with the reprocessing of instruments for minimal invasive surgery is given, and a numbers of possible solutions are discussed

Fluorinated Azobenzenes Switchable with Red Light

Molecular photoswitches triggered with red or NIR light are optimal for photomodulation of complex biological systems, including efficient penetration of the human body for therapeutic purposes (“therapeutic window”). Yet, they are rarely reported, and even more rarely functional under aqueous conditions. In this work, fluorinated azobenzenes are shown to exhibit efficient E→Z photoisomerization with red light (PSS660nm >75 % Z) upon conjugation with unsaturated substituents. Initially demonstrated for aldehyde groups, this effect was also observed in a more complex structure by incorporating the chromophore into a cyclic dipeptide with propensity for self-assembly. Under physiological conditions, the latter molecule formed a supramolecular material that reversibly changed its viscosity upon irradiation with red light. Our observation can lead to design of new photopharmacology agents or phototriggered materials for in vivo use

Direct assessment of microcirculation in shock:a randomized-controlled multicenter study

Purpose!#!Shock is a life-threatening condition characterized by substantial alterations in the microcirculation. This study tests the hypothesis that considering sublingual microcirculatory perfusion variables in the therapeutic management reduces 30-day mortality in patients admitted to the intensive care unit (ICU) with shock.!##!Methods!#!This randomized, prospective clinical multicenter trial-recruited patients with an arterial lactate value above two mmol/L, requiring vasopressors despite adequate fluid resuscitation, regardless of the cause of shock. All patients received sequential sublingual measurements using a sidestream-dark field (SDF) video microscope at admission to the intensive care unit (± 4 h) and 24 (± 4) hours later that was performed blindly to the treatment team. Patients were randomized to usual routine or to integrating sublingual microcirculatory perfusion variables in the therapy plan. The primary endpoint was 30-day mortality, secondary endpoints were length of stay on the ICU and the hospital, and 6-months mortality.!##!Results!#!Overall, we included 141 patients with cardiogenic (n = 77), post cardiac surgery (n = 27), or septic shock (n = 22). 69 patients were randomized to the intervention and 72 to routine care. No serious adverse events (SAEs) occurred. In the interventional group, significantly more patients received an adjustment (increase or decrease) in vasoactive drugs or fluids (66.7% vs. 41.8%, p = 0.009) within the next hour. Microcirculatory values 24 h after admission and 30-day mortality did not differ [crude: 32 (47.1%) patients versus 25 (34.7%), relative risk (RR) 1.39 (0.91-1.97); Cox-regression: hazard ratio (HR) 1.54 (95% confidence interval (CI) 0.90-2.66, p = 0.118)].!##!Conclusion!#!Integrating sublingual microcirculatory perfusion variables in the therapy plan resulted in treatment changes that do not improve survival at all