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Bifunctional carbanionic synthesis of bio-based thermoplastic elastomers in methyl tert-butyl ether
Get PDFThis thesis presents the synthesis of fully bio-based thermoplastic elastomers (TPEs) via anionic polymerization. TPEs are intrinsically more sustainable compared to classical irreversibly vulcanized elastomers. Consequently, bio-based TPEs and novel solvent options are investigated within this work. The first objective is to establish a more efficient synthesis route based on bifunctional initiation, wherein the solvent methyl tert-butyl ether (MTBE) is investigated for its ability to ensure a reliable and controlled anionic polymerization. Additionally, the utilization of renewable monomers as building blocks for TPEs addresses the urgent societal and political demand for increased sustainability. From this perspective, natural building blocks based on terpenes offer great structural diversity and are therefore used within this work to install different material properties. Thus, this dissertation aims to contribute to a more sustainable future of polymer materials.
Aiming at a general overview of the relevance of diene monomers in the anionic polymerization, Chapter ① gives a comprehensive compilation of findings of recent research in the form of a Perspective article. It features how the shift from established fossil fuel-based elastomers, produced from only three primarily used monomers, i.e., styrene, butadiene, and isoprene, towards bio-based feedstocks changes the perspective of research. These recently developed bio-based monomers in terms of polymer properties, mechanistic insights, and behavior in statistical copolymerization are covered.
As some relevant theoretical concepts for this thesis are not included in this first Perspective article, Chapter ② provides further insights. The general concept of TPEs is introduced, covering the origin of the reprocessability and the general concepts of phase behavior in the bulk of block copolymers. Different synthesis pathways towards ABA-type triblock copolymers are discussed. Since peculiar polymer architectures can be achieved by using bifunctional initiators, these are introduced in greater detail. Challenges and former reports of bifunctional initiators are evaluated. Furthermore, as statistical copolymerization is a key method in producing block-like tapered structures in an even more efficient manner, a general introduction is given regarding the concept of reactivity ratios as well as their determination.
The main objective of this work is to develop an efficient synthesis pathway for bio-based TPEs. As the greatest efficiency for the synthesis of ABA-type triblock structures is based on bifunctional initiators, suitable media have to be identified. The poor solubility of bifunctional initiators in hydrocarbons necessitates the use of more polar solvents. Traditionally, tetrahydrofuran (THF) is used on lab-scale. However, translation to industry is prohibited, as the formation of peroxides and proton abstraction at ambient temperatures are major drawbacks of THF. Therefore, Chapter ③ covers the investigation of MTBE as an alternative solvent that comprises solubility of bifunctional initiators, no peroxide formation, and ideally a higher stability towards alkyl lithium compounds. The latter is demonstrated by the 50 times longer half-life of n butyl lithium in MTBE, compared to THF. As little is known about its general performance as reaction medium in the anionic polymerization, key kinetic parameters, i.e., propagation rates of the polymerization of 1,3-dienes are investigated as well. The trend of higher reaction rates compared to the traditionally used hydrocarbon solvent cyclohexane is determined for isoprene and β-farnesene – one “traditional” monomer and a renewable structure. Further, the “living” polydienyl lithium chains are predominantly present as non-aggregated unimers in MTBE. In addition to the homopolymerization, the established copolymerization system of styrene and isoprene is examined. The effect of an increasing MTBE fraction is investigated via in situ near-infrared (NIR) kinetics. The gradient structure, found in pure cyclohexane, successively changes to random monomer incorporation and ultimately yields an inverted, yet still close to random comonomer incorporation in pure MTBE. This is reflected by the reactivity ratios rSMTBE = 1.82 and rIMTBE = 0.55. The great potential identified for MTBE in carbanionic polymerization is used in the ensuing chapters of this work.
In Chapter ④, MTBE is utilized as a reaction medium for the bifunctional initiator 1,3 diisopropylene benzene. The excellent solubility of the bifunctional initiator in MTBE allows easy access to ABA-type triblock copolymers. To alleviate the current environmental challenges and to overcome the shrinking fossil fuel feedstock, a fully bio-based TPE is targeted. Therefore, β-farnesene is used for the middle block, as it can be derived from sugar cane and has a microstructure-independent low glass temperature (Tg). By controlled termination of the telechelic “living” polyfarnesene chains, hydroxyl groups are introduced via adding ethoxy ethyl glycidyl ether (EEGE) to the reaction. This affords low dispersity (Đ = 1.07 to 1.10) and telechelic polyfarnesene macroinitiators, bearing two hydroxyl groups at each chain end. Subsequent organocatalyzed ring-opening polymerization (ROP) of LL-dilactide (LLA) yields H-shaped, fully bio-based triblock copolymers. The material features two distinct Tgs at – 66 °C and 51 °C, as well as gyroid or cylindrical morphologies, determined by transmission electron microscopy (TEM) and small-angle Χ-ray scattering (SAXS). A soft elastic material at room temperature is obtained.
Given the advantages of the moderately polar solvent MTBE, ensuring reliable solubility of a bifunctional initiator, and the low Tg of polyfarnesene, Chapter ⑤ describes an even more efficient pathway towards ABA-type structures. To allow for a one-step synthesis and to avoid an elaborate multistep pathway, carbanionic polymerization is exclusively used in this chapter. Nopadiene (Nopa) is chosen as a suitable comonomer, as PNopa exhibits a high Tg of ~ 160 °C and can be derived from the renewable feedstock pine tree. Excellent control of carbanionic copolymerization is confirmed by in situ 1H NMR kinetics. By changing the reaction conditions, the reactivity ratios are tailored to yield a steep gradient comonomer incorporation, rFarMTBE = 10.8 and rNopaMTBE = 0.093. Consequently, symmetric, two-sided tapered ABA-type triblock copolymers are accessible in a one-step approach, capitalizing on a difunctional initiator in MTBE. By changing the comonomer ratio, the material properties are tuned in a broad range: highly elastic materials with elongation exceeding 1300% as well as tough materials with Young modulus’ exceeding 500 MPa are obtained. Characterization via SAXS, temperature-modulated differential scanning calorimetry (TM DSC), rheology, and dielectric spectroscopy are employed to relate the material properties to the phase state of the triblock copolymer. This revealed local phase segregation accompanied by respective Tgs of the domains, albeit in the absence of long-range order. Thus, it could be successfully demonstrated that it is possible to produce fully bio-based TPEs that are similar in their properties to fossil fuel-based materials via a one-pot and one-step synthesis.
Bifunctional initiators and functional initiators face similar challenges in their application in the field of carbanionic polymerization. Therefore, Chapter ⑥ involves the translation of previous findings in terms of solubility issues of bifunctional initiators Chapter 4 to functional initiator structures. The project is the result of a collaboration with the synthetic rubber company Arlanxeo. The incorporation of functional groups at the chain end of polymer chains in elastomers is beneficial to improve filler-rubber interaction and helps to improve the performance in, for instance, tires as it lowers the rolling resistance and hence reduces fuel consumption. Further, functional groups at the chain end can be used in a subsequent reaction to access block structures. To this end, a heterotelechelic initiator for the formation of α functionalized polyisoprene is developed. The initiator is accessed via titration using an iodine alkyl precursor and tert-butyl lithium. First, the functional alkyl lithium initiator, bearing two ketal-protected hydroxyl functionalities, is used for the carbanionic polymerization of isoprene. Low dispersity (Đ < 1.1) polymers with a high 1,4-polyisoprene content (60%) are obtained after optimization of the required amount of polar modifiers. Matrix-assisted laser desorption/ionization time-of-flight mass spectroscopy (MALDI-ToF MS) measurements verify an effective end-group functionalization. Second, LLA is grafted from theses (OH)2-PI macroinitiators to access A2B-type block copolymers. Size-exclusion chromatography (SEC), NMR, and diffusion-ordered spectroscopy (DOSY) measurements are used to demonstrate successful chain-extension via lactide grafting. DSC experiments reveal two distinct Tgs, indicating phase-separation within the block copolymers. Hence, a heterotelechelic initiator for both carbanionic polymerization and ROP is introduced.
In the first appendix Chapter A①, an approach for TPEs based on a pure hydrocarbon medium is presented. The bifunctional initiator 1,3-bis(1-phenyl ethenyl) benzene (PEB) is used for its good solubility in cyclohexane. As such, a high 1,4-content can be achieved in the polymerization of 1,3-dienes, which in turn allows for the use of the readily available monomer isoprene. Subsequently, three hydroxyl groups are introduced simultaneously both in α- and ω position by means of end-functionalization of the living anionic dilithiated PI-chains, with 1,2-isopropylidene glyceryl glycidyl ether (IGG) and subsequent acidic deprotection. These multi-hydroxyl PI-macroinitiators are used to initiate LLA in an organocatalyzed ROP. Thereby, super-H-shaped A3BA3-triblock structures are obtained. Thermal characterization reveals two distinct Tgs, suggesting phase-separation within the bulk material. PI-domains feature a low Tg in the range of - 55 °C to - 59 °C and the PLLA-domains show a Tg of 41 °C to 49 °C. Furthermore, the exact phase-separation and morphology is analyzed via TEM and SAXS, confirming cylindrical and lamellar morphologies. The combination of carbanionic synthesis and ROP of lactones is presented as a feasible pathway for complex polymer architectures.
Chapter A② presents a fundamental investigation on binary blends of polyfarnesene and polyisoprene as a model system towards super soft polymer melts. The expertise in the living diene polymerization gained before is used to synthesize a molar mass series of polyfarnesene samples in the range of 3.5 kg∙mol 1 to 720 kg∙mol 1. To prepare polyfarnesene/polyisoprene blends, commercially available polyisoprene samples of the same molar mass are purchased. The addition of linear polyisoprene to the bottlebrush-architecture of polyfarnesene leads to dilation of the reputational “tube” that further reduces the - already low - plateau modulus of polyfarnesene. The work is based on a close collaboration with Prof. G. Floudas, with the syntheses of the polymer samples being contributed in the context of this thesis.
The research article presented in Chapter 4 is additionally published in a German journal. Appendix Chapter A③ is the German translation of Chapter 4.xiv, 340 Seiten ; Illustrationen, Diagramm
Green perspective drives the renaissance of anionic diene polymerization
No full textPolymers based on 1,3-diene monomers play a pivotal role in many commercial elastomers and thermoplastic elastomers. This perspective summarizes the state of the art and recent developments in the living anionic polymerization of dienes, permitting to finely tune the properties of the resulting polymers. An emphasis is placed on novel biobased diene monomers (myrcene, farnesene, etc.) and polymerization solvents, which bear promise for more sustainable elastomers in the future. Furthermore, statistical copolymerization of dienes with vinyl monomers and the in situ monitoring of monomer gradients and formation of tapered di- and triblock copolymers due to disparate reactivity ratios is also reviewed. Thermoplastic elastomers based on tri- and multiblock architectures as well as recently reported diene-based polymer architectures are discussed as well. A summary of current challenges and future options for carbanionic diene polymerization concludes this short review article
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
Supersoft polymer melts in binary blends of bottlebrush cis-1,4-polyfarnesene and cis-1,4-polyisoprene
No full textBinary blends of polyterpenes are employed comprising cis-1,4-polyfarnesene (PF) with a bottlebrush architecture, and linear cis-1,4-polyisoprene (PI) as model systems toward supersoft polymer melts. The bottlebrush PF results in a low plateau modulus that can further be reduced with the addition of PI. Depending on the fraction of short PI chains in the athermal and nearly isofrictional blends, plateau moduli in the range from 1 to 10 kPa can be achieved. Tube dilation is very efficient in the present binary blends as compared to more common blends comprising long/short or linear/star chains of identical polymer structure
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
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