A Contribution to the Scanning Electron Microscope Based Microcharacterization of Semi-Insulating Gallium Arsenide Substrates

The macroscopic behaviour of semiconducting materials is determined by the distribution of microscopic defects like dislocations, impurities and intrinsic defects. Therefore, microanalytical methods are necessary to control the influence of technological process parameters on the materials properties. In the case of GaAs substrates, measurements of the cathodoluminescence (CL) and the electron beam induced voltage (EBIV) as well as the new charging technique seem to be promising methods to perform this task. CL-micrographs of as-grown GaAs substrates show bright cellular structures, which correspond to dislocation networks. Comparative investigations by use of the new charging contrast technique indicate an increased conductivity in the bright areas. CL-measurements of annealed substrates reveal additional characteristic island-like structures in the cell interior. Both, cellular and island-like structures can also be visualized by the EBIV technique. These results can be explained by a homogeneous conductivity and an inhomogeneous distribution of the excess carrier lifetime

Direct observation of the morphological changes in a carbon fiber composite by means of in-situ micro-CT compression experiment

Carbon fiber reinforced polymers are widely used due to their lightweight and strong structural properties and understanding their mechanical behavior is crucial for their reliable use in structural components. In this study, we evaluated the mechanical properties of a carbon fiber reinforced composite made of a dynamic polymer based on benzoxazine and polyetheramine Jeffamine(R) ED 600. Standard compression tests were performed on specimens with layers oriented longitudinally or transversely to the loading direction. In-situ mechanical testing using micro-computed tomography (micro-CT) imaging was conducted to capture detailed images of the internal microstructure during deformation. Changes in porosity and pore shape were observed as the composite underwent compression. Digital Volume Correlation (DVC) analysis was applied to quantify displacements and strains within the material and enabled the visualization and quantification of strain patterns, which agree with kinking failure.Comment: 20 pages, 11 figure

Benzoxazine/amine-based polymer networks featuring stress-relaxation and reprocessability

Amines as additives in benzoxazines are known to beneficially affect the polymerization temperature and furthermore to allow for partially reversible reaction steps yielding however a non-dynamic polybenzoxazine network. This contribution proves that the polymerization behavior of a two-component polymer of the polyetheramine Jeffamine® ED-600 and a bisphenol-A-based benzoxazine features stress-relaxation and reprocessability usually known from vitrimers. With the aim to gain a deeper understanding of the material properties of this system and the corresponding polymer structure, the reaction mechanism of a monofunctional benzoxazine and monoamine model system was studied revealing at first primary, and then secondary amine induced opening of oxazine rings, leading at first to linear polymer chains and then to covalently crosslinked networks. Both consist of repeated phenolic benzoxazine/amine motifs with permanently incorporated polyetheramine chains that do not impact the mechanical properties, compared to pure polybenzoxazine. Thermal, spectroscopic, and extraction analyses show that the addition of Jeffamine® reduces the polymerization temperature and introduces material properties such as reprocessability at the same time. Stress-relaxation measurements support the assumption that the reprocessability point to vitrimer-like molecular processes. The material shows rapid stress-relaxation of up to 11 s, a corresponding bond-exchange activation energy of 146 kJ/mol, and a topology freezing temperature of 97°C

Highly Crosslinked Polybenzoxazines from Monobenzoxazines : The Effect of Meta-Substitution in the Phenol Ring

It is possible to control the crosslink density of polymers derived from monobenzoxazines by switching the type of substituents in the phenolic ring and their relative position with respect to the phenol group. We prepared several substituted monobenzoxazines in the para and meta positions of the phenolic ring and studied how these substituents affected the polymerization temperature of monomers and the thermal stability of the final polymers and, more extensively, how they affected the crosslink network of the final polymers. Gel content and dynamic mechanical analysis confirm that ortho- and para-orienting substituents in the meta position generate highly crosslinked materials compared to para ones. This fact can lead to the design of materials with highly crosslinked networks based on monobenzoxazines, simpler and more versatile monomers than the commercial bisbenzoxazines currently in use

The effects of manufacturing processes on the physical and mechanical properties of basalt fibre reinforced polybenzoxazine

The present work provides a comparative investigation between different methods of manufacturing basalt fibre reinforced polybenzoxazines (BFRP), including vacuum infusion, hand laminating, dynamic fluid compression moulding and autoclave curing processes. In comparison to the high pressure based autoclave-cured and compression-moulded BFRPs, vacuum-infused BFRPs showed similar or even higher mechanical properties. Despite the low pressure curing, vacuum infusion yielded BFRPs with a 10% higher tensile strength and a 24% higher strain at failure compared to its autoclave-cured counterparts. Thus, it is possible to gain BFRPs with near-zero porosity and high mechanical properties without the need of high pressure curing methods

American palm ethnomedicine: A meta-analysis

<p>Abstract</p> <p>Background</p> <p>Many recent papers have documented the phytochemical and pharmacological bases for the use of palms (<it>Arecaceae</it>) in ethnomedicine. Early publications were based almost entirely on interviews that solicited local knowledge. More recently, ethnobotanically guided searches for new medicinal plants have proven more successful than random sampling for identifying plants that contain biodynamic ingredients. However, limited laboratory time and the high cost of clinical trials make it difficult to test all potential medicinal plants in the search for new drug candidates. The purpose of this study was to summarize and analyze previous studies on the medicinal uses of American palms in order to narrow down the search for new palm-derived medicines.</p> <p>Methods</p> <p>Relevant literature was surveyed and data was extracted and organized into medicinal use categories. We focused on more recent literature than that considered in a review published 25 years ago. We included phytochemical and pharmacological research that explored the importance of American palms in ethnomedicine.</p> <p>Results</p> <p>Of 730 species of American palms, we found evidence that 106 species had known medicinal uses, ranging from treatments for diabetes and leishmaniasis to prostatic hyperplasia. Thus, the number of American palm species with known uses had increased from 48 to 106 over the last quarter of a century. Furthermore, the pharmacological bases for many of the effects are now understood.</p> <p>Conclusions</p> <p>Palms are important in American ethnomedicine. Some, like <it>Serenoa repens </it>and <it>Roystonea regia</it>, are the sources of drugs that have been approved for medicinal uses. In contrast, recent ethnopharmacological studies suggested that many of the reported uses of several other palms do not appear to have a strong physiological basis. This study has provided a useful assessment of the ethnobotanical and pharmacological data available on palms.</p

Design of natural fiber composites utilizing interfacial crystallinity and affinity

Butanediol initiated poly(e-caprolactone) (PCL) has recently been reported as a toughening agent for cationically curing cycloaliphatic epoxides providing plasticized thermosets with excellent properties (Lützen et al., 2013). In this contribution that promising toughening approach was applied for the first time for the development of novel natural fiber composites (NFC). NFCs based on conventional brittle thermosetting polymers often suffer from poor interfacial adhesion and stress cracking. Composites made up of the novel plasticized thermosets and woven flax fiber preserved the elastomer-like properties and increased tensile strength and elongation at break up to 60 MPa and 5%, respectively. Furthermore, PCL was shown not only to toughen the epoxide but also to modulate the affinity of the matrix to the fiber. In conclusion, improved interfacial adhesion and the resulting excellent mechanical properties of cationically curable NFCs were achieved by both interfacial crystallization and affinity

Generation of multivalent peptide-polymer conjugates based on thioester-HPMA- copolymers to address biological issues in vitro and in vivo

Die Konjugation eines Wirkstoffs an ein biokompatibles Polymer generiert Systeme, die die bioaktive Substanz nicht nur zielgerichtet transportieren, sondern bei Bedarf kontrolliert abgeben können. Im Gegensatz zu ihrer freien Form verfügen polymergebundene Liganden, sowohl peptidischer als auch nicht- peptidischer Natur, über eine veränderte Pharmakokinetik und diverse Vorteile. Auf dem Gebiet der Polymertherapeutika ist Poly(N-2-Hydroxypropylmethacrylamid) (poly(HPMA)) gegenwärtig eines der aussichtsreichsten biokompatiblen Polymere, dessen Vielseitigkeit im Rahmen dieser Arbeit aufgezeigt wird. Auf Basis von poly(HPMA) wurde ein thioesterfunktionalisiertes Copolymer entwickelt, welches an die vorliegenden biochemischen und medizinischen Fragestellungen flexibel angepasst werden konnte. RAFT-vermittelte Copolymerisationen von HPMA mit einem thioesterfunktionalisierten Monomer lieferten Thioester-HPMA-Copolymere, die mit cysteinylierten Liganden in NCL-Reaktionen beladen wurden und so einen schnellen und flexiblen Zugang zu maßgeschneiderten Peptid-Polymerkonjugaten lieferten. Die zelluläre Aufnahme von Peptid-HPMA Konjugaten wurde in Abhängigkeit von ihrer Oberflächenladungen untersucht. Ausschließlich positiv geladene Peptid-Polymerkonjugate wurden in HeLa-Zellen internalisiert. Mit einer exzellenten Biokompatibiliät und effizienten zellulären Aufname, stellten argininbeladende HPMA-Konjugate eine Alternative zum toxischen Zell- penetrierenden Peptid Nonaarginin dar und könnten als effiziente Trägersysteme biologisch relevanter Liganden für intrazelluläre Studien eingesetzt werden. Die positiven Auswirkungen der Peptidkonjugation an die Thioester-HPMA- Copolymere im Hinblick auf den gezielten Transport an den Wirkort und den Schutz vor proteolytischem Abbau wurden in einem Projekt zur Früherkennung von Pankreaskrebs mit nichtinvasiven bildgebenden Verfahren genutzt. Für die Matrix-Metalloprotease 11 (MMP-11) als Biomarker für Bauchspeicheldrüsenkrebs wurden Substrat-Polymerkonjugate konzipiert, deren Umsatz bei der Spaltung durch das Enzym MMP-11 ein auf dem FRET-Effekt beruhendes Fluoreszenzsignal lieferte. Die ersten in vivo-Versuche an MIA PACA-Xenograft-Mäusen waren vielversprechend. Zur Untersuchung der amyloidbildenden Eigenschaften von Modellpeptiden in Neuroblastomzellen wurde ein photolysierbarer Cumarinlinker zwischen das zu transportierende Peptid und den polymeren Träger eingeführt, der eine kontrollierte, photoinduzierte Freisetzung des bioaktiven Liganden am Wirkort ermöglichte. Die Konjugation der fluoreszenzmarkierten Peptide an ein HPMA-Linker-System sorgte nicht nur für den Transport, den Schutz und die photoinduzierte Freisetzung, sondern maskierte zudem die amyloidbildenden Eigenschaften. Nach erfolgter Internalisierung zeigte das Modellpeptid nach Freisetzung Auswirkungen auf die Zellvitalität. Im Hinblick darauf, dass viele biologische Prozesse auf Protein-Protein-Interaktionen beruhen, stellt die Konjugation mehrerer Kopien monovalenter Liganden an ein biokompatibles Polymer einen wichtigen Zugang zu multivalenten Systemen dar. Der Einfluss der polymeren Trägersysteme poly(HPMA), hyperverzweigtes Polyglycerin und Dextran auf die Interaktion mit den Tandem WW-Domänen (tWW) des forminbindenden Proteins FBP21 wurde kalorimetrisch bestimmt und in MD-Simulationen modelliert. Die multivalente Präsentation der Erkennungssequenz resultierte für alle betrachteten Polymere in einer signifikanten Steigerung der Bindungsaffinitäten, die in Abhängigkeit vom verwendeten Polymer und der Peptidbeladungsdichte variierten. Im Rahmen dieser breit gefächerten Projekte konnte die Vielseitigkeit der HPMA-basierten Copolymere beispielsweise als Transportsystem biologisch relevanter Liganden für in vitro und in vivo Anwendungen oder als Trägersystem zur Darstellung multivalenter Liganden gezeigt werden.The conjugation of a drug on a biocompatible polymer generates drug delivery systems which transport and deliver the therapeutic agent into the tissue of interest. Polymer bound drugs of a peptidic or non-peptidic nature exhibit modified pharmacokinetics and diverse advantages compared to their free state. In the field of polymertherapeutics poly(N-2-hydroxypropylmethacrylamide) is one of the most promising biocompatible polymers. The versatile applications of poly(HPMA) will be shown within the scope of this work. A thioester functionalized copolymer was developed based on poly(HPMA) which could be adapted to particular biochemical or medical problem at hand. RAFT-mediated copolymerisation of HPMA with a thioester functionalized monomer provided thioester-HPMA copolymers that enabeled an NCL-reaction of cysteinylated ligands. Accordingly, thioester-HPMA copolymers provided fast and flexible access to tailor-made peptide-polymer conjugates. The surface charge dependence of the cellular uptake of peptide-HPMA conjugates was studied. Exclusively positively charged conjugates were internalized into HeLa cells. Arginine loaded HPMA-conjugates showed not only an efficient internalization but exhibit an excellent biocompatibility in comparison to the cell- penetrating peptide nonaarginine. Accordingly, positively charged HPMA- conjugates could be applied as transport systems for biologically relevant ligands in intracellular studies. The advantagous impact of peptide conjugation with respect to drug targeting and the stealth effect was used in pilot studies for the early diagnosis of pancreatic cancer using noninvasive imaging. The matrix-metallo-protease 11 (MMP-11) is a biomarker for pancreatic cancer. FRET based substrate-polymer conjugates have been developed that present a fluorescence signal upon enzymatic cleavage of MMP-11. Preliminary in vivo experiments on MIA PACA-xenograft mice showed promising results. Targeted delivery was instrumental for determining the amyloid forming properties of model peptides in neuroblastoma cells. Therefore, a photocleavable coumarine linker was incorporated into the peptide-polymer conjugate which enabled a controlled, photoinducable delivery of the peptide onto the site of action. The conjugation of fluorescence labeled peptides on an HPMA-linker-system provided the transport, protection and photoinduced delivery of the peptide. Additionally, the polymer conjugation disabled amyloid formation. After internalization, the delivery of the peptide affected the cell viability. As most biological processes are based on protein-protein- interactions, the conjugation of multiple copies of monovalent ligands on a biocompatible polymer provides an important access point to multivalent systems. The impact of the polymer scaffolds poly(HPMA), hyperbranched polyglycerol, and dextran on the interaction with tandem WW-domains of the formin binding proteine FBP21 was examined by isothermal calorimetry and was calculated in MD-simulations. The multivalent presentation of the recognition sequence resulted in a significant increase in the binding affinity for all polymers. The affinity varied slightly with a dependence on the polymer scaffold and the peptide loading density. In the scope of the diverse projects, the versatility of the HPMA-based copolymers as drug carriers for in vitro and in vivo applications, or as polymer scaffold for the design of multivalent ligands was shown