Research

Current Research Topics and Topics for PhD and Masters

Research Topics Prof. Dr. Michael R. Buchmeiser

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Several projects entail the use of tailored N-heterocyclic carbenes (NHCs) for use in polymerization reactions. Taking advantage of their high basicity and pronounced nucleophilic character, NHCs can also be protected by different means, e.g., by the addition of CO2 or by suitable transition metal ions. The thus prepared systems then serve as latent catalysts that can be activated by application of heat, e. g., for the synthesis of poly(urethane)s or poly(amide)s via reaction injection molding of one-component systems, or for the polymerization of lactones or acrylates. Based on this principle, it is also possible to create fully air-stable and latent single-component curing systems for epoxy resins for use in the preparation of advanced composite materials. Complementary, NHCs are used for the synthesis of the corresponding transition metal-NHC complexes, which serve as excellent catalysts for C-C coupling reactions including Heck-, Suzuki-, Sonohashira-Hagihara, carbonyl-hydrocyanation, -arylation and hydrosilylations, olefin hydrosilylation reactions as well as various metathesis-based reactions. Finally, suitable immobilization technologies allow for generating immobilized catalysts for continuous flow applications. 

Selected Publications:

  1. S. Naumann, M. R. Buchmeiser, Catal. Sci. Technol., 4, 2014, 2466-2479.
  2. B. Bantu, G. M. Pawar, U. Decker, Klaus Wurst, A. M. Schmidt, M. R. Buchmeiser, Chem. Eur. J.,15,  2009, 3103-3109.
  3. S. Naumann, S. Epple, C. Bonten, M. R. Buchmeiser, ACS Macro Lett., 2, 2013, 609-612.
  4. S. Naumann, F. G. Schmidt, M. R. Buchmeiser, Polym. Chem., 4, 2013, 4172-4181.
  5. S. Naumann, F.-G. Schmidt, R. Schowner, W. Frey, M. R. Buchmeiser, Polym. Chem., 4, 2013, 2731-2740.
  6. S. Naumann, M. Speiser, R. Schowner, E. Giebel, M. R. Buchmeiser, Macromolecules, 47, 2014, 4548-4556.

A series of latent, thermally- or UV-triggerable pre-catalysts based on CO2­­-protected N-heterocyclic carbenes (NHCs) and NHC-Sn+II, -Zn+II, -Mg+II, -Al+III as well as NHC-Ru and NHC-Mo alkylidene complexes for step- and chain-growth polymerization have been synthesized.

The CO2-, Sn+II-, Zn+II-, Mg+II-, Al+III-protected NHCs exhibit very good thermal latency in a wide range of applications including the ring opening polymerization of lactams and lactones, polyurethane synthesis with tailored of isocyanurate content, thereby improving the thermal and mechanical properties, polymerization of methyl methacrylate (MMA) and curing of highly crosslinked anhydride-hardened epoxy resins. The active catalyst species can only be formed at elevated temperatures, an advantage that allows premixing of batch and diluted systems, important for a broad spectrum of applications, e.g. for reaction injection molding (RIM) and resin transfer molding (RTM). CO2-protected NHCs do not only show excellent activity, but also allow for metal free synthesis and therefore circumvent toxic heavy metals such as Hg+II or organotin compounds, which have sometimes toxicities comparable to cyanide. 

Complementary, the ring-opening metatheses polymerization (ROMP) triggered by cationic NHC-Ru+II complexes in the presence of various norborn-2-ene substituted monomers can selectively be activated by UV irradiation. These initiators are of particular interest in technical applications of ROMP, they allow for premixing of a monomer/pre-catalyst mixture and its storage over long periods of time at elevated temperatures. Most importantly, these initiators allow for shaping and profiling of such mixtures prior to polymerization (“curing”).

The characteristic coalescence temperature, Tc, at which the complexes exhibit the highest activity, makes them interesting targets for applications that require latent catalyst systems. Poly(dicyclopentadiene) at present is synthesized by a two-component catalyst system, that is mixed immediately prior to use. The employment of a one-component system, which can be premixed and stored within the substrate, i.e. Poly(dicyclopentadiene) would significantly facilitate the process. Molybdenum imido alkylidene NHC complexes, e.g., based on triazol-4-ylidenes and mesoionic carbenes are also most suitable latent pre-catalysts for the polymerization of Poly(dicyclopentadiene). Storage of the catalysts with the highly reactive substrate is possible and activation of the catalysts can be implemented by simple heating of the mixture.

Selected Publications:

  1. S. Naumann, S. Epple, C. Bonten, M. R. Buchmeiser, ACS Macro Lett., 2, 2013, 609-612.
  2. S. Naumann, F. G. Schmidt, W. Frey, M. R. Buchmeiser, Polym. Chem., 4, 2013, 4172-4181.
  3. B. Bantu, G. M. Pawar, K. Wurst, U. Decker, A. M. Schmidt, M. R. Buchmeiser, Eur. J. Inorg. Chem.2009, 1970-1976.
  4. B. Bantu, G. M. Pawar, U. Decker, K. Wurst, A. M. Schmidt, M. R. Buchmeiser, Chem. Eur. J.2009, 3103-3109.
  5. S. Naumann, F. G. Schmidt, R. Schowner, W. Frey, M. R. Buchmeiser, Polym. Chem.2013, 2731-2740.
  6. S. Naumann, M. Speiser, R. Schowner, E. Giebel, M. R. Buchmeiser, Macromolecules, 47, 2015, 4548-4556.
  7. M. R. Buchmeiser, J. A. Kammerer, S. Naumann, J. Unold, R. Ghomeshi, S. K. Selvarayan, P. Weichand, R. Gadow, Macromol. Mater. Eng., 9, 2015, 937-943.
  8. D. Wang, K. Wurst, W. Knolle, U. Decker, L. Prager, M. R. Buchmeiser, Angew. Chem., 120, 2008, 3311-3314; Angew. Chem. Int. Ed., 47, 2008, 3267-3270.
  9. D. Wang, K. Wurst, M. R. Buchmeiser, Chem. Eur. J., 16, 2010, 12928-12934.
  10. J. Beerhues, S. Sen, R. Schowner, G. M. Nagy, M. R. Buchmeiser, invitation to a special issue celebrating Prof. R. H. Grubbs 75th birthday, J. Polym. Sci. A: Polym. Chem. 55, 2017, 3028-3033.

SPAN, a sulfur-containing poly(acrylonitrile)-derived composite material containing up to 55 wt.-% of chemically bound S, is used as cathode material for Li-S batteries. Both fibrous and pellicular structures are employed. The structure of SPAN has been fully elucidated und correlated with the electrochemical performance of Li-S batteries built therefrom. In particular, their discharge / charge chemistry and the role of the electrolyte during cycling are investigated. These investigations entail the electrochemical characterization as well as physico-chemical measurements including WAXS, ESCA, electron microscopy, FT-IR and Raman measurements and MALDI-TOF, to name just a few. Based on that knowledge, Li-S batteries stable for >1200 cycles displaying energy densities up to 2 mA.h/cm2 @ 0.5C have already been realized. In summary, a comprehensive picture of the chemistry and electrochemistry of SPAN-based Li-sulfur batteries is to be created that ultimately allows for designing high-capacity devices with good cycling stability (>1500 cycles) and high energy density (>3.5 mA.h/cm2 @ 1C ).

Selected Publications:

  1. J. Fanous, M. Wegner, J. Grimminger, Ä. Andresen, M. R. Buchmeiser, Chem. Mater., 23, 2011, 5024-5028.
  2. J. Fanous, M. Wegner, J. Grimminger, M. Rolff, M. B. M. Spera, M. Tenzer and M. R. Buchmeiser, J. Mater. Chem., 22, 2012, 23240-23245.
  3. J. Fanous, M. Schweizer, D. Schawaller, M. R. Buchmeiser, Macromol. Mater. Eng., 297, 2012, 123-127.
  4. J. Fanous, M. Wegner, M. B. M. Spera, M. R. Buchmeiser, J. Electrochem. Soc., 160(8), 2013, A1169-A1170.
  5. M. Frey, R. Zenn, S. Warneke, K. Müller, A. Hintennach, R. E. Dinnebier and M. R. Buchmeiser, ACS Energy Lett., 2, 2017, 595−604.
  6. S. Warneke, M. Eusterholz, R. Zenn, A. Hintennach, R. E. Dinnebier, M. R. Buchmeiser, J. Electrochem. Soc., 165, 2018, A6017-A6020.
  7. S. Warneke, R. K. Zenn, T. Lebherz, K. Müller, A. Hintennach, U. Starke, R. E. Dinnebier, M. R. Buchmeiser, Adv. Sust. Systems, 2, 2018, 1700144.
  8. Fluor-basierte Elektrolyte für Lithium-Schwefel Batterien, M. Frey, M. R. Buchmeiser, A. Hintennach (Daimler AG), DE102016004643.0.
  9. Elektrolyt und elektrochemischer Energiespeicher, S. Warneke, M. R. Buchmeiser, A. Hintennach (Daimler AG), DE102016011782.6.
  10. Kathodenmaterial und Verfahren zu dessen Herstellung (2), M. Frey, M. R. Buchmeiser, A. Hintennach (Daimler AG), Offenlegungsschrift DE 10 2015 00220 A1 (25. 08. 2016).
  11. Kathodenmaterial und Verfahren zu dessen Herstellung (1), M. Frey, M. R. Buchmeiser, A. Hintennach (Daimler AG), DE 102014012468.1.

The synthesis of the first molybdenum imido alkylidene N-heterocyclic carbene (NHC) bistriflate catalysts in 2014 and the following mechanistic investigations resulted in the fast development of a large catalyst library. Variations in the metal or in the ligand sphere, such as the imido ligand, the alkoxide ligand and the NHC moiety paved the way for numerous applications. Remarkably, due to the excellent σ-donor properties of the NHCs, the first highly active cationic group 6 metal alkylidene NHC were prepared that allowed running various olefin metathesis reactions with turnover numbers (TONs) >500,000. These olefin metathesis reactions entail ring opening metathesis polymerization (ROMP), cross-metathesis (CM) and ring closing metathesis (RCM), thereby making the catalysts interesting targets even beyond polymer chemistry. Their immobilization on silica, their application in biphasic metathesis and their high functional group tolerance further highlight the versatile properties of this new class of olefin metathesis catalysts and led to cooperations, further widening the scope of the catalyst system.

The unprecedented catalytic activity of 16-electron molybdenum and tungsten complexes in olefin metathesis reactions raised the question about mechanistic details, since the usual Schrock type olefin metathesis catalysts display a 14-electron architecture. In situ 19F NMR measurements revealed the reason for the unexpected catalytic properties. The synergistic effect of the excellent donor ligand NHC and the excellent leaving group character of the triflates results in the formation of the catalytically active cationic species.

Molybdenum imido alkylidene NHC bistriflate complexes display a characteristic coalescence temperature, Tc, at which the catalysts have a square pyramidal geometry and the trans-effect of the NHC ligand on the triflate ligand becomes fully operative, enabling the fast formation of the cationic active species. Variations of the carbene ligand, covering carbenes with large Tolman electronic parameters (TEP) and therefore less pronounced σ-donor properties, such as 1,2,4 - triazol-4-ylidenes, and carbenes with small Tolman electronic parameters, e.g., mesoionic carbenes, allowed for the fine tuning of Tc. This opened the way to tailor-made catalysts with special operating windows.

Molybdenum imdo, tubgsten imido and tungsten oxo alkylidene NHC complexes can be converted into cationic complexes by the replacement of one anionic X ligand (e.g., triflate, chloride, alkoxide) by a weakly coordinating anion like BF4 or B(ArF)4 (tetrakis-(3,5-trifluoromethylphenyl)borate). The stable tetracoordinated cationic species exhibit high activity in olefin metathesis reactions, which sets them apart from the few examples of cationic alkylidene complexes published by the Schrock group. The influence of the weakly coordinating anion on stability and activity of the cationic systems is currently investigated in our laboratories.

Stereoselectivity in olefin metathesis reactions can be addressed by forcing the intermediate metallacyclobutane into a preferred configuration. This can most possibly be realized by the employment of sterically demanding alkoxides, chiral alkoxides, chiral carbenes and bidentate (chiral) ligands. Several complexes with such ligands have already been synthesized in our laboratories. Furthermore, imido alkylidene NHC complexes of group 6 metals exist in two configurations, the syn- configuration with the substituent R at the metal carbon double bond pointing in the direction of the imido ligand and the corresponding anti- configuration. Syn- and anti- isomers can be interconverted by simple irradiation with UV- light. Studies on the rate of syn/anti-interconversion in group 6 imido alkylidene NHC complexes and the influence on the selectivity in olefin metathesis are ongoing.

Molybdenum imido alkylidene NHC bistriflate catalysts have been immobilized via coordination of a silica-bound NHCs to standard bistriflate complexes. Employment of those immobilized analogues in RCM and CM lead to the isolation of metal-free olefin metathesis products. Furthermore, tungsten oxo alkylidene complexes and molybdenum imido alkylidene mono triflate mono alkoxide complexes have been immobilized on silica by replacement of the X ligand with a Si-O bond in cooperation with the Copéret group at the ETH Zurich. Especially the immobilized tungsten based catalyst 2@SiO2 exhibits unprecedented activity in self metathesis reactions with turnover numbers >1,200,000. In depth solid-state NMR investigations allowed for anylzing the geometry of the intermediary metal cyclobutanes.

Selected Publications:

  1. M. R. Buchmeiser, S. Sen, J. Unold, W. Frey, Angew. Chem. Int. Ed., 532014, 9384-9388.
  2. D. A. Imbrich, W. Frey, S. Naumann, M. R. Buchmeiser, Chem. Commun., 522016, 6099-6102.
  3. S. Sen, R. Schowner, D. A. Imbrich, W. Frey, M. Hunger, M. R. Buchmeiser, Chem. Eur. J., 21, 2015, 13778-13787.
  4. K. Herz, J. Unold, J. Hänle, R. Schowner, S. Sen, W. Frey, M. R. Buchmeiser, Macromolecules, 48, 2015, 4768-4778.
  5. M. Pucino, V. Mougel, R. Schowner, A. Fedorov, M. R. Buchmeiser, C. Copéret, Angew. Chem. Int. Ed., 55, 2016, 4300-4302.
  6. M. R. Buchmeiser, S. Sen, C. Lienert, L. Widmann, R. Schwoner, K. Herz, P. Hauser, W. Frey, D. Wang, ChemCatChem, 8, 2016, 2710-2713.
  7. J. Beerhues, S. Sen, R. Schowner, G. M. Nagy, M. R. Buchmeiser, invitation to a special issue celebrating Prof. R. H. Grubbs 75th birthday, J. Polym. Sci. A: Polym. Chem. 55, 2017, 3028-3033.
  8. I. Elser, W. Frey, K. Wurst, M. R. Buchmeiser, Organometallics 201635, 4106-4111.
  9. R. Schowner, W. Frey, M. R. Buchmeiser, J. Am. Chem. Soc., 1372015, 6188-6191.
  10. I. Elser, R. Schowner, W. Frey, M. R. Buchmeiser, Chem. Eur. J. 23, 2017, 6398-6405.
  11. W.-C. Liao, T.-C. Ong, D. Gajan, G. Casano, M. Yulikov, M. Pucino, R. Schowner, M. Schwarzwälder, M. R. Buchmeiser, G. Jeschke, O. Ouari, P. Tordo, A. Lesage, L. Emsley, C. Copéret, Chem. Sci., 8, 2017, 416-422.

Our group recently synthesized novel molybdenum alkylidyne complexes containing N-heterocyclic carbenes. Mechanistic studies based on NMR spectroscopy, allowed postulating two different active species for alkyne metathesis, depending on the nature of the incorporated carbene. In the case of strong σ-donors (e.g. 1,3-dimethylimidazol-2-ylidene) a cationic active species was hypothesized, whereas for less donating carbenes (e.g. 1,3-dimethyl-4,5-dicyanoimidazol-2-ylidene or thiazolylidenes) the NHC free complex was postulated to be the active species. The catalysts were tested in benchmark alkyne metathesis reactions and showed moderate to good activity.

Furthermore, immobilization of the corresponding molybdenum alkylidyne NHC complexes on silica proved to enhance reactivity, most probably due to prevention of bimolecular decomposition pathways. To increase reactivity, the first cationic molybdenum NHC alkylidyne complexes with chelating carbenes that provide additional stability to the highly electrophilic metal center were isolated. However, the cationic complexes were unstable in presence of substrate. Investigations concerning stabilization, reactivity and further immobilization experiments as well as the application in alkyne polymerizations are under way.

Selected Publications:

  1. M. Koy, I. Elser, J. Meisner, W. Frey, K. Wurst, J. Kästner, M. R. Buchmeiser, Chem. Eur. J.2017, 15484-15490.
  2. P. Hauser, Master Thesis, “Immobilization of Molybdenum-Alkylidyne-NHC Complexes “, University of Stuttgart, 2017.
  3. M. Koy, Master Thesis, “Group 6 N-Heterocyclic Carbene Metal Alkylidyne Complexes“, University of Stuttgart, 2016.
  4. J. Groos, Master Thesis, “Chelating N-Heterocyclic Carbene Group 6 Metal Alkylidyne Complexes“, University of Stuttgart, 2017.

The replacement of one triflate ligand in the standard systems by betaine-type ligands led to catalysts that could be applied in olefin metathesis reactions under biphasic conditions in a mixture of pyrrole and heptane. The biphasic reaction setup offers access to metal free products, which are of high importance in, for example, pharmaceutical industry.

Selected Publications:

  1. I. Elser, R. Schowner, W. Frey, M. R. Buchmeiser, Chem. Eur. J., 23, 2017, 6398-6405. 
  2. S. Sen, R. Schowner, D. A. Imbrich, W. Frey, M. Hunger, M. R. Buchmeiser, Chem. Eur. J., 21, 2015, 13778-13787.
  3. M. Pucino, V. Mougel, R. Schowner, A. Fedorov, M. R. Buchmeiser, C. Copéret, Angew. Chem. Int. Ed., 55, 2016, 4300-4302.
  4. W.-C. Liao, T.-C. Ong, D. Gajan, G. Casano, M. Yulikov, M. Pucino, R. Schowner, M. Schwarzwälder, M. R. Buchmeiser, G. Jeschke, O. Ouari, P. Tordo, A. Lesage, L. Emsley, C. Copéret, Chem. Sci., 8, 2017, 416-422.

Sensory fiber-reinforced plastics are classified as a novel kind of composite materials showing sophisticated properties. These materials are designed in the course of an interdisciplinary project within the SFB 1244 dealing with the investigation and development of tools and methods for the planning, construction and operating of tomorrow’s built environment. In this context, the overall aim of the project is defined by the integration of transparent and non-transparent sensory fiber-reinforced plastics into the facade of a ten-storied demonstrator tower enabling the detection of external stress such as surface cracks or strain.

In this regard, the fiber fabric is embedded covalently by an appropriate polymer network which enhances tremendously the mechanical parameters of the polymer such as elasticity and strength. The sensory properties, which are the key features of the system, are generated by imprinting a conductive interdigital structure on the surface of the composite material. Thus, a continuous current applied on the interdigital surface structure can be detected consistently as electrical signal. In this context, the generated electrical signal is highly dependent on length variations within the interdigital structure that might be caused by external stress or even damage on the surface. Therefore, the detected electrical signal will remain constant without external stress or will change measurably in the presence of stress.

Carbon fibers are made of anisotropic carbon with at least 92 wt.-% and up to 100 wt.-% carbon. Carbon fibers have high tensile strengths up to 7 GPa with very good creep resistance, low densities (ρ=1.75-2.00 g/cm3) and high moduli up to E ≤ 950 GPa. They lack resistance to oxidizing agents as hot air and flames, but they are resistant to all other chemical species. The good mechanical properties make carbon fiber attractive for use in composites in the form of woven textiles as well as of continuous or chopped fibers. The composite parts can be produced through filament winding, tape winding, pultrusion, compression molding, vacuum bagging, liquid molding, and injection molding. For the automotive industry, carbon fiber-reinforced polymeric composites allow for a significant reduction in weight, which is a prerequisite for battery-driven cars. More recently, carbon fibers moved into the center of interest for carbon-fiber-reinforced concrete for houses, bridges, etc. as well as for carbon-fiber reinforced compounds for, there increasingly replacing steel.

The most important precursor in the market is poly(acrylonitrile) (PAN) for HT (high strength) and IMS (intermediate modulus) type carbon fibers for high strength CFRP applications, while pitch is used for most other carbon fiber types as HM (high modulus) or UHM (ultra high modulus) for CFRP parts with high stiffness requirements. However, in search of renewable and energy-efficient precursors and processes have moved into the center of interest. Current projects focus on all these precursors, where new precursors systems and processes are developed for PAN, cellulose, lignin, polyethylene and others. Targets are better and more efficient processes for carbon fibers as well as improved fiber properties. Research is carried out at the DITF Denkendorf, where these activities are allocated in the established High-Performance Fiber Center (HPFC), in which pilot lines for fiber preparation and processing can prepare precursor and carbon fibers in kg amounts.

References:

  1. Carbon Fibers E. Frank, M. R. Buchmeiser in "Fiber, films, resins and plastics", Enzyclopedia in Polymeric Nanomaterials (S. Kobayashi, K. Müllen, Eds.), Vol.1, 2015, 306-310, ISBN: 978-642-29647-5.
  2. E. Frank, D. Ingildeev, L. M. Steudle, J. M. Spörl, M. R. Buchmeiser, Angew. Chem. 126, 2014, 5364-5403; Angew. Chem. Int. Ed. 53, 2014, 5262-5298.
  3. E. Frank, F. Hermanutz, M. R. Buchmeiser, Macromol. Mater. Eng., 297, 2012, 493-501.
  4. E. Frank, E. Giebel, M. R. Buchmeiser, Techn. Text., 2, 2015, E53-55; Chem. Fibers, Int., 4, 2015, 216-218.
  5. High-Performance Poly(acrylonitrile) (PAN)-Based Carbon Fibers. In: Structure and Properties of High-Performance Fibers (G. Bhat, Ed.), 1stEd. E. Frank, D. Ingildeev, M. R. Buchmeiser, Woodhead Publishing Ltd., Elsevier, 187, 2016, 7-30,  ISBN 978-0-08-100550-7.
  6. J. M. Spörl, A. Ota, R. Beyer, T. Lehr, A. Müller, F. Hermanutz, M. R. Buchmeiser, J. Polym. Sci. A: Polym. Chem., 52, 2014, 1322-1333.
  7. J. M. Spörl, A. Ota, S. Sun, K. Massonne, F. Hermanutz, M. R. Buchmeiser, Mater. Today Commun. 7, 2016, 1-10
  8. Method for production of carbon fibers from cellulose fibers, S. Son, K. Massonne, F. Hermanutz, J. Spoerl, M. R. Buchmeiser, R. Beyer (BASF E), PCT Int. Appl. (2015), WO 2015173243 A1 20151119
  9. L. Steudle, E. Frank, A. Ota, U. Hageroth, S. Henzler, W. Schuler, R. Neupert, M. R. Buchmeiser, Macromol. Mater. Eng., 302, 2017, 1600441.
  10. Precursor-Fasern von Lignin-basierten Carbonfasern, deren Herstellung und Verwendung, M. R. Buchmeiser, L. Steudle, E. Frank, patents pending (2013).
  11. Verfahren zur Herstellung einer Lignin-basierten Zusammensetzung, E. Frank, M. Clauss, M. R. Buchmeiser (DITF Denkendorf), patents pending (2015), 10 2015 120.377.4.
  12. Verfahren zur Herstellung modifizierter Formkörper sowie deren Verwendung zur Herstellung von Carbonformkörpern, E. Frank, E. Muks, M. R. Buchmeiser (ITCF Denkendorf), DE 10 2015 106 348 A1 (2016.10.27).
  13. J. W. Krumpfer,E. Giebel, A. Müller, L. Ackermann, C. Nardi-Tironi, J. Unold,  M. Klapper, M. R. Buchmeiser, K. Müllen, Chem. Mater., 29, 2017, 780-788.
  14. M. Speiser, S. Henzler, U. Hageroth, A. Renfftlen, A. Müller D. Schawaller, B. Sandig, M. R. Buchmeiser, Carbon, 63, 2013, 554-561.
  15. M. R. Buchmeiser, J. Unold, K. Schneider, E. B. Anderson, F. Hermanutz, E. Frank, A. Müller, S. Zinn, J. Mater. Chem. A, 1, 2013, 13154-13163.

Fibers are classified into two main groups, commodity and high performance. High performance fibers are designed for specialized technical applications with unique physical properties, including high specific stiffness, high temperature resistance, flame retardancy, and/or chemical resistance. These fibers are continuously designed to be stronger, lighter, and safer, and their unique, specialized attributes are essential for automotive, aerospace, construction, and protection applications compared to high-volume, cheaper commodity fibers. 

Poly(aromatic)s, including for example meta- and para-aramid fibers such as Nomex, Kevlar and PBO, are particularly well-known for their highly-oriented rigid structures which contribute to their excellent mechanical properties and high-temperature resistance. They are widely-used in reinforcement, protective, safety, and ballistic applications. However, many high-strength fibers are difficult to process. Although some types of high performance fibers may have excellent chemical resistance, other types are not that UV stable and display poor chemical resistance but may have superior properties for a specific application such as their strength or limiting oxygen index. To solve these problems, we are designing novel modified monomers and comonomers that provide these polymeric fibers with improved processing and/or resistance without sacrificing on mechanical properties. Our goals are to design fibers that outperform the industry standards for strength, density, and thermal and chemical resistance and to facilitate the expansion of their technical applications.

Oxide ceramic fibers are key components of ceramic matrix composites (CMCs), which form a new class of light-weight high temperature resistant materials with exceptional properties. As CMCs combine the advantages of a monolithic ceramic material (corrosion resistance, high strength and high temperature stability) with a non-brittle fracture behavior, a high damage tolerance and an extreme thermo-shock resistance, there is an increasing interest in their industrial applications, particularly by replacing highly legated steel. Important technical fields with growing requirements are: power generation with stationary gas turbines as well as combustion chambers and engines of aircrafts, rockets and space vehicles.

Oxide ceramic fibers determine the properties of CMCs and therefore have to meet special requirements such as high strength, long-term high temperature stability as well as excellent resistance against oxidation, corrosion and creep. Generally, the creep rate of the polycrystalline ceramic fibers increases with decreasing grain size and the creep resistance of the commercially available ultrafine grained fibers is comparatively low. Particularly under mechanical stress and at high temperatures exceeding 1100 °C, they tend to creep and brittleness increases due to grain growth, which can ultimately lead to failure of the entire device. The optimization of the ceramic fiber properties with regard to high creep resistance while maintaining strength and associated long-term high temperature resistance still represents a major topic in ongoing research in the field of ceramic fibers.

Research at the DITF Denkendorf focuses on the development of continuous oxide ceramic fibers of various compositions and started as early as in 1989. The complete production process has been studied intensively comprising the design of spinning dopes, the development of the dry spinning process as well as the thermal treatment including pyrolysis, calcination and sintering processes. Corundum and mullite fibers have achieved a high level of development in the past years. Recent investigations of the fiber properties by others have shown the high potential of these two fiber types. Currently, the transfer of the technology into industrial scale is under progress.
Current research projects focus on the improvement of creep resistance, the reduction of grain growth in long time applications and the improvement of the textile processability of oxide ceramic fibers. For this purpose, the chemical compositions of the oxide ceramic fibers are further varied in order to optimize the structures and the mechanical properties.

Yttrium aluminum garnet (YAG) fibers are high performance fibers with high temperature stability, high modulus and strength, high oxidation resistance and excellent creep resistance. As YAG is characterized by a very high melting point of 1940 °C it is very attractive for high temperature applications. Furthermore, it is chemically inert in reducing and oxidizing atmosphere and it is the oxide with the highest creep resistance. Therefore, YAG fibers have the potential to outperform the commercial oxide ceramic fibers in terms of creep resistance.
The microstructural optimization of corundum fibers by the incorporation of zirconia results in zirconia toughened alumina (ZTA) fibers with a substantially inhibited grain growth at high temperatures. ZTA exhibits enhanced fracture toughness and almost doubled flexural strength in comparison to alumina. ZTA fibers with these properties not only enable more complex structures due to improved textile processability but also exhibit a higher corrosion resistance compared to other fiber types.

References:

  1. D. Schawaller, B. Clauß, M. R. Buchmeiser, Macromol. Mater. Eng., 297, 2012, 502-522.
  2. S. Pfeifer, M. Bischoff, R. Niewa, B. Clauß, M. R. Buchmeiser, J. Eur. Ceram. Soc., 34, 2014, 1321-1328.
  3. S. Pfeifer, P. Demirci, R. Duran, H. Stolpmann, A. Renfftlen, S. Nemrava, R. Niewa, B. Clauß, M. R. Buchmeiser, J. Eur. Ceram. Soc., 36, 2016, 725-731.

Non-oxide Si-C-N ceramic fibers possess interesting properties and are predestined for applications in fiber reinforced ceramics (CMCs). These are materials with outstanding properties like high resistance against heat-shock and damage tolerance, which is completely different from conventional monolithic ceramics. Hence, new technical fields are accessible for these fiber ceramics like aerospace, power engineering and automotive applications. The production of such fibers can be conducted by a feasible melt spinning procedure, if thermoplastic precursor materials with proper rheological properties can be synthesized. By using different chlorosilanes as starting compounds meltspinnable polycarbosilazanes and polysilazanes have been produced. As a part of the research work new strategies for the synthesis for high molecular weight precursors with good processability have been developed. Based on the rheological studies and NMR spectroscopy possible structures of the polycarbosilazanes are derived. These precursors are spun to fibers in a melt spinning process (above). To avoid melting during pyrolysis the green fibers are crosslinked by electron irradiation. Pyrolysis under inert conditions at 1100 °C leads to Si-C-N ceramic fibers. These fibers are stable up to very high temperatures. Beside the formation of a thin oxidation layer no further fiber degradation is observed after temperature exposition at 1500 °C in air (below).

Research Topics Dr. Stefan Naumann

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The development of polymerization catalysts is not only a merit in itself, but ideally provides materials with novel properties. In this regard, we try to implement N-heterocyclic olefins (NHOs) as organocatalysts for the synthesis of polymer architectures which allow for creating a tunable, porous platform material. We have recently demonstrated the first NHO-mediated organopolymerizations. On this basis, it is possible to use a solvent- and metal-free polymerization process for the preparation of very well-defined amphiphilic polyethers. These in turn can be converted into micro- or mesoporous carbon materials. We aim at preparing these carbon materials with predictable pore sizes, all depending on the properties of the underlying polyether structures. If successful, this will allow for creating tailor-made materials for electrodes or catalytic supports. Characterization, application and development of battery/supercapacitator devices as well as catalytic experiments using these materials are part of our investigations.
Funding: Deutsche Forschungsgemeinschaft, Projekt NA 1206/2

Selected Publications:

  1. P. Walther, S. Naumann, Macromolecules 2017, 50, 8406-8416.
  2. S. Naumann, Dongren Wang, Macromolecules 2016, 49, 8869-8878.
  3. S. Naumann, P. B. V. Scholten, J. A. Wilson, A. P. Dove, J. Am. Chem. Soc. 2015, 137, 14439–14445.
  4. S. Naumann, F. G. Schmidt, W. Frey, M. R. Buchmeiser, Polym. Chem. 2013, 4, 4172-4181.

The development of polymerization catalysts is not only a merit in itself, but ideally provides materials with novel properties. In this regard, we try to implement N-heterocyclic olefins (NHOs) as organocatalysts for the synthesis of polymer architectures which allow for creating a tunable, porous platform material. We have recently demonstrated the first NHO-mediated organopolymerizations. On this basis, it is possible to use a solvent- and metal-free polymerization process for the preparation of very well-defined amphiphilic polyethers. These in turn can be converted into micro- or mesoporous carbon materials. We aim at preparing these carbon materials with predictable pore sizes, all depending on the properties of the underlying polyether structures. If successful, this will allow for creating tailor-made materials for electrodes or catalytic supports. Characterization, application and development of battery/supercapacitator devices as well as catalytic experiments using these materials are part of our investigations.

Selected Publications Organocatalysis:

  1. S. Naumann, K. Mundsinger, L. Cavallo, L. Falivene, Polym. Chem. 2017, 8, 5803-5812.
  2. S. Naumann, A. W. Thomas, A. P. Dove, ACS Macro Lett. 2016, 5, 134 – 138.
  3. S. Naumann, A. W. Thomas, A. P. Dove, Angew. Chem. Int. Ed. 2015, 54, 9550 – 9554.

Current Thesis Topics

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  1. Synthesis of cationic molybdenum / tungsten alkylidene complexes for olefin metathesis reactions under continuous biphasic conditions
  2. Syn/anti interconversion of Mo- and W-alkylidene NHC complexes
  3. Cis/trans selectivity of Mo and W alkylidene NHC complexes in olefin metathesis reactions
  4. Ring-opening metathesis polymerization (ROMP) with Mo- and W-alkylidene NHC complexes
  5. Stereoselective olefin metathesis with chiral Mo- and W-alkylidene NHC complexes
  6. Polymeric electrolytes for Li-Sulfur batteries
  7. Highly porous cathode materials for Li-Sulfur batteries
  8. Latent catalysts for the polymerization of dicyclopentadiene
  9. Stereospecific polymerization of tricyclic olefins
  10. NHC-based metal complexes as catalysts for polyaddition reactions
  11. Preparation of high-creep resistant YAG ceramic fibers
  1. CO2 activation using monolith-supported metal nanoparticles
  2. Highly porous cathode materials for Li-Sulfur batteries
  3. Polymeric electrolytes for Li-Sulfur batteries
  4. Synthesis of cationic molybdenum and tungsten alkylidene complexes for olefin metathesis reactions under continuous biphasic conditions
  5. Syn/anti interconversion of Mo- and W-alkylidene NHC complexes
  6. Cis/trans selectivity of Mo and W alkylidene NHC complexes in olefin metathesis reactions
  7. Stereo- and regioselective polymerization of chiral norbornenes, norbornadienes, 1-6-heptadiynes, and 1,7-octadiines using chiral molybdenum and tungsten alkylidene complexes with asymmetric NHC ligands
  8. Stereoselective olefin metathesis with chiral Mo- and W-alkylidene NHC complexes
  9. Preparative and selective enrichment of biogenic carboxylic acids using monolithic anion exchangers
  10. Processing of m-aramids from ionic liquids and characterization of the solution and the fiber structure
  11. Development of fibers/films/coatings from cellulose/aramid or cellulose /PAN blends using ionic liquid technology
  12. Preparation of Poly(phtalamide) fibers
  13. Preparation of high-creep resistant YAG ceramic fibers
  1. Preparation of monolithic cathodes for Li-S batteries
  2. Synthesis of cationic molybdenum and tungsten alkylidene complexes for olefin metathesis reactions under continuous biphasic conditions
  3. Stereo- and regioselective polymerization of chiral norbornenes, norbornadienes, 1-6-heptadiynes, and 1,7-octadiines using chiral molybdenum and tungsten alkylidene complexes with asymmetric NHC ligands
  4. Stereoselective olefin metathesis with chiral Mo- and W-alkylidene NHC complexes
  5. (Chiral) N-heterocyclic carbenes as catalysis for polymerization
  6. Preparative separation/enrichment of rare earth metals
  7. Preparative enrichment of biogenic carboxylic acids using monolithic carriers with a fermentation system
  8. Preparation of polyheterocyclic high-performance fibers using ionic liquids
  9. Preparation of high-strength carbon fibers from biogenic sources (cellulose, lignin)
  10. Preparation of high-creep resistant YAG ceramic fibers
  1. Flammhemmend ausgerüstete Textilien: Ziel ist die Entwicklung eines neuen Ausrüstungs- und Beschichtungsprodukts aus der Reaktion von Cellulose mit Phosphoriger Säure. Hierzu sind im Projekt die Bedingungen bei der Cellulosederivatisierung, die verarbeitungstechnischen Eigenschaften der ausgerüsteten Stoffe zu klären.
    1. Rheologie
    2. Beschichtungstechnik von Textilien
    3. Infrarotspektroskopie, Elementaranalytik
    4. Flammtests, LOI-Bestimmung
  2. Flammschutz modifiziertes Caprolactam für die anionische ringöffnende Polymerisation: Flammschutz ist ein wichtiges Thema für die Anwendung von im Gussverfahren hergestellten Bauteilen aus Polyamid. Bisherige Forschung hat gezeigt, dass bei Polyamid in der Polymerkette gebundene Flammschutzmonomere deutlich effektiver im Vergleich zu ungebundenen Additiven sind. Eine große Herausforderung bei Gusspolyamid ist es kostengünstige Monomere zu finden, die bei der anionischen Polymerisation des Caprolactams in die Polymerkette eingebaut werden können und die Reaktion nicht abbrechen. Um dies zu erreichen sollen Caprolactammoleküle modifiziert werden. Das Projekt beinhaltet die Monomersynthese und Analytik, Polymerisation und Polymeranalytik. Insbesondere auch die Untersuchung des Brennverhaltens der hergestellten Polymere.
  3. Synthese von hochfesten high-performance Cellulosefasern durch Nassspinnprozesse: Einsatz als Verstärkungsfasern zur Anwendung in Verbundmaterialien.
  4. Lignin-basierte Carbonfasern: Vernetzungsmethoden und reaktive Sizings für die chemische Prä-Vernetzung von Lignin-Präkursor-Fasern zur Herstellung von Carbonfasern, um den Schritt der Faserstabilisierung zu beschleunigen und fiber fusing zu verhindern. Mitarbeit bei Versuchsreihen im Rahmen des EU-Projekts „LIBRE“ (Lignin-based carbon fibers).

Contact

 

Chair of Macromolecular Materials and Fiber Chemistry

Pfaffenwaldring 55, D-70569, Stuttgart