1,720,971 research outputs found
Challenges in Chain-Shuttling Polymerization: Novel Ethylene-norbornene Copolymers and vs. Commercial Ethylene-1-octene Copolymers,
No full textEthylene–co–norbornene copolymerization in the presence of a chain transfer agent
No full textEthylene (E)-co-norbornene (N) copolymerizations were conducted by using two ansa-metallocenes, isopropyliden(η5-cyclopentadienyl)(η5-indenyl)zirconium dichloride (1) and isopropyliden(η5-3-methylcyclopentadienyl)(η5-fluorenyl)zirconium dichloride (2), activated with dimethylanilinium tetrakis(pentafluorophenyl)borate, in presence of TIBA and variable amounts of Zn(Et)2 as a potential chain transfer agent (CTA). The study evaluated the effect of CTA on microstructure, molecular weight, chain end group and structural, thermal and mechanical properties of poly(E-co-N)s. The analysis of the data revealed that in all copolymerization reactions with metallocenes 1 and 2 in presence of diethyl zinc chain transfer to zinc alkyl occurs along with the typical chain transfers of E-co-N polymerization. The chain transfer to zinc alkyl is more efficient at low N content in copolymers. The two catalysts show different behavior in copolymerization at high N content: molar masses of copolymers produced by 2 are more affected by CTA concentration, that is, the chain shortening with increasing CTA content is greater in copolymers obtained with catalyst 2. The analysis of mechanical properties revealed that CTA affects the polymer chain structure. An increase of CTA caused a change of ductility, rigidity, and mechanical strength; chains produced in presence of Zn(Et)2 are, at least in part, characterized by segments produced as a result of exchange between different metal centers, possibly jointed through ethylene units
Copolimeri sindiotattici propene-1-olefina: analisi della microstruttura mediante spettroscopia 2D NMR.
No full textThe Role of CNTs in Promoting Hybrid Filler Networking and Synergism with Carbon Black in the Mechanical Behavior of Filled Polyisoprene
No full textThe filler networking process promoted by multiwalled CNTs is studied in neat and CB-filled poly(1,4-cis-isoprene) matrices. TEM analysis, tensile, dynamic-mechanical, and electrical measurements reveal that the CNTs form a filler network at low concentration in neat PI and a continuous hybrid filler network at a lower
CNT concentration in the presence of CB, with a remarkable increase of the nonlinear dynamic- mechanical behavior of the nanocomposites at low deformation. A synergistic effect between CB and CNTs is demonstrated. The addition of CNTs to the CB-filled PI matrix leads to initial modulus values much larger than those calcu- lated by simple addition of the two initial moduli of the composites containing only CB and only CNTs, respectively
Cycloolefin copolymers with new transition metal catalysts
Full text linkIl complesso [Ti(k2-N,O-{2,6-F2C6H3N=C(Me)C(H)=C(CF3)O})2Cl2] (1) è stato valutato come catalizzatore per la polimerizzazione vivente di etilene (E) con norbornene (N) dopo attivazione con metilalluminossano dry (d-MAO) in un range di temperature da 25 a 90 °C. La reazione di polimerizzazione eseguita a diversi rapporti [N]/[E] ha dato copolimeri P(E-co-N) stereoirregolari, alternanti e ad alto peso molecolare. La natura vivente del sistema 1/d-MAO è stata dimostrata tramite cinetica a 50 °C. Questo sistema catalitico è stato impiegato per la sintesi di copolimeri a blocchi così come PE-block-P(E-co-N) con un blocco di PE cristalino e un blocco amorfo di P(E-co-N) così come P(E-co-N)1-block-P(E-co-N)2, avente diversi contenuti di norbornene nei segmenti e quindi aventi diversi valori di Tg. Inoltre, uno studio NMR multinucleare è stato effettuato per stabilire la natura delle specie responsabili per la copolimerizzazione vivente E-N impiegando il sistema catalitico 1/d-MAO. L'analisi ha riguardato (i) il sistema catalitico (1 con MAO); (ii) le specie che si formano dopo l'aggiunta simultanea di etilene 13C arricchito e norbornene; e (iii) le specie che si formano durante l'omopolimerizzazione del norbornene. I risultati supportano l'esistenza di interazioni non covalenti dei sostituenti orto-fluoro col centro di titanio, che contribuiscono al carattere vivente della reazione di copolimerizzazione.Complex [Ti(k2-N,O-{2,6-F2C6H3N=C(Me)C(H)=C(CF3)O})2Cl2] (1) was evaluated as catalyst for living copolymerization of ethylene (E) with norbornene (N) upon activation with dried methylaluminoxane (d-MAO) at temperatures between 25 and 90 °C. Copolymerization performed at different [N]/[E] feed ratios afforded stereoirregular alternating high molar mass P(E-co-N) with narrow molar mass distribution. The living nature of E-co-N copolymerization by 1/d-MAO was demonstrated by kinetics at 50 °C. This catalyst system was used for the synthesis of block copolymers such as PE-block-P(E-co-N) with a crystalline PE block and an amorphous P(E-co-N) block as well as P(E-co-N)1-block-P(E-co-N)2, having different norbornene contents in the segments and thus having different Tg values.
Moreover, a multinuclear NMR spectroscopic study was performed to assess the nature of species responsible for living E-N copolymerization reaction using the catalytic system 1/d-MAO. The analysis involved (i) the catalytic system (1 with MAO); (ii) the species formed after simultaneous addition of 13C-enriched ethylene and norbornene and after further addition of 13C-enriched ethylene; and (iii) species formed during norbornene homopolymerization. The results supported the existence of noncovalent interactions of ortho-fluoroaryl substituents with the titanium center, which contribute to the living character of the copolymerization reaction.DIPARTIMENTO DI CHIMICA, MATERIALI E INGEGNERIA CHIMICA GIULIO NATTA27RESNATI, GIUSEPPEFARAVELLI, TIZIAN
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe 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
Full text link“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
Full text linkWe 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
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