1,720,983 research outputs found
The genetics in polymers: Crystallization as a fingerprint of the molecular microstructure
No full textHidden details of the molecular structure of polymers can be revealed from the retro-analysis of the crystallization behavior. The way a polymer crystallizes and the selection of a specific polymorphic form upon crystallization contain all the information about the structure of macromolecules. The intriguing challenge is how to extract this information from the formed crystals. The key is the construction of a model and the availability of model systems of polymer crystallization based on the exploitation of the phenomena of polymorphism and isomorphism and on the understanding of how such phenomena are affected by the presence of microstructural defects. The explanatory case study of isotactic polypropylene (iPP) prepared with Ziegler-Natta catalysts is here described. iPP shows an intricate polymorphic behavior, which may be successfully exploited to build models of the crystallization behavior and reveal hidden details of the complex molecular structure dictated by the heterogenous multi-site Ziegler-Natta catalysts. A model of the crystallization behavior of iPP has been defined through deep analysis of the crystallization of iPP and its copolymers synthesized with soluble organometallic catalysts that produce a simple molecular structure with perfectly random distribution of defects. In these model systems the crystallization of the α and γ forms depend only on the average length of the regular propene sequences that, in turn, depends on the concentration of defects. A precise correlation between the amount of crystallized γ form and the average length of regular isotactic propene sequences is the basic of this model. This correlation can be used in more complex samples where the concentration of defects is not known or when the distribution of defects is not random as in Ziegler-Natta iPPs, as calibration curve to evaluate the length of regular propene sequences from the crystallization behavior by simply measuring the maximum amount of crystallized γ form. The retro-analysis of the crystallization behavior of iPP allows to disclose hidden aspects of the molecular structure of the extremely complex Ziegler-Natta polypropylene and provides evidence of the blocky structure of chains of iPP (and polyolefins in general) synthesized with Ziegler-Natta catalysts. This retro-analysis exploits the presence of polymorphism and is possible only for polymers that can crystallize in different polymorphic forms that exhibit thermodynamic stability and crystallization kinetics which depend on the molecular structure and, in particular, on the presence of defects
Random and Block Copolymers of Isotactic Polypropylene with Higher α-Olefins: The Role of the Chain Topology on the Side-Chains Confined Crystallization
No full textSamples of isotactic propylene-1-octadecene (iPP-co-C18) random copolymers and of diblock copolymers (BCPs) of isotactic polypropylene (iPP) linked to a propylene-1-octadecene block (P-co-C18), iPP-b-P(P-co-C18) with tuned block length and different comonomer content (up to 24 mol %) have been prepared using two different organometallic Hf-based catalysts. Both catalysts ensure good activity, high comonomer incorporation preserving high molecular weight, and a living behavior with the Cs-symmetric pyridylamido-Hf dimethyl complex. The influence of the copolymer topology with random and block architecture on the crystallization behavior and, specifically, the crystallization of side-chains has been analyzed. In random copolymers iPP-co-C18, the main-chains crystallize in the α form and in a mesophase of iPP, whereas at low temperature, the side-chains of the C18 co-units crystallize forming PE-like crystals. The crystallization of the lateral chains is surprisingly observed even in the sample with low C18 concentration and low branches’ frequency, where the randomly placed branches are highly spaced. This indicates that these random copolymers show unexpected nanophase separation between iPP chains and alkyl side-chains, with aggregation of side-chains that facilitates their crystallization even at low branches’ frequency. In the iPP-b-P(P-co-C18) BCPs, the iPP blocks crystallize in the common α form and the side-chains of the C18 co-units in the P(P-co-C18) block crystallize at low temperature, forming polyethylene-like crystals. The confined crystallization at low temperature of side-chains of the C18 co-units is driven by both phase separation of the dissimilar blocks (further driven by iPP crystallization) as well as by phase separation induced by the local incompatibility inside the P(P-co-C18) block between main-chain and C18 side-chains. This cooperative mechanism further facilitates the crystallization of the C18 side-chains compared to random copolymers and provides indirect evidence of microphase separation in these systems. We have demonstrated that branches are intriguingly segregated in nanodomains, favoring crystallization after self-aggregation, and can crystallize in random copolymers with a perfectly uniform and random distribution of C18 co-units even at very low comonomer content, therefore, even for low frequency of branching, when randomly placed branches are highly spaced
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
Effect of stretching on the crystallization of un-crosslinked ethylene/propylene(/diene) random copolymers
No full textThe phase behavior of ethylene/propylene(/diene) (EP(D)M) co- (and ter-)polymers characterized by ethylene content in the range 64–78 wt%, and comparatively low level of crystallinity, in the range 1–15%, is analyzed during stretching, through collection of wide angle X-ray scattering data. All samples experience crystallization by effect of stretching, but whereas for the samples with ethylene content ≥71% crystallinity starts increasing already after yielding, the nearly amorphous sample, with 64 wt% ethylene content, experiences crystallization only at strain above a threshold (≈170%). In all cases, a high degree of orientation of the amorphous phase is achieved already at low deformations. It is shown that for the samples with high ethylene content, the pre-existing crystals undergo initial fragmentation and/or mechanical melting at low deformations, and successive oriented recrystallization at higher deformations. The role of oriented amorphous phase, along with the role of pre-existing crystals as nuclei able to foster further crystallization is highlighted
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
Endowing Polythioester Vitrimer with Intrinsic Crystallinity and Chemical Recyclability
No full textTechnologically important thermosets face a long-standing end-of-life (EoL) problem of non-reprocessability, a more sustainable solution of which has resolved to nascent vitrimers that can merge the robust material properties of thermosets and the reprocessability of thermoplastics. However, the lifecycle of vitrimers is still finite, as they often suffer from significant deterioration of mechanical performance following multiple reprocessing cycles, analogous to mechanical recycling, and they often show undesired creep under working conditions. To address these two key limitations, we have developed a cross-linked semi-crystalline polythioester with both dynamic covalent bonds and intrinsic crystallinity and chemical recyclability, affording a vitrimeric system that exhibits not only reprocessability and crystallinity-restricted creep but also complete chemical recyclability to initial monomer by catalyzed depolymerization in solution or bulk. Therefore, reported herein is an "infinite" vitrimer system that is empowered with a facile closed-loop EoL option once serial reprocessing deteriorates performance and the material can no longer meet the application requirements. Specifically, the polythioester vitrimer was constructed by copolymerization of a bicyclic thioester with a bis-dithiolane, producing dynamically cross-linked polythioesters with excellent property tunability, from amorphous to semi-crystalline states and melting transition temperatures from 91 to 178 °C
Crystallization of Propene-Pentene Isotactic Copolymers as an Indicator of the General View of the Crystallization Behavior of Isotactic Polypropylene
No full textThe crystallization from the melt in isothermal conditions of metallocene random propene-pentene isotactic copolymers (iPPC5) has been studied. All samples with pentene concentration between 0.5 and 10 mol % crystallize at any crystallization temperature in mixtures of α and γforms of isotactic polypropylene (iPP) and the amount of γform increases with increasing crystallization temperature up to a maximum (fγ(max)), which depends on pentene concentration. Pentene defects produce a shortening of the regular propene sequences that in turn induces crystallization of the γform. At concentrations higher than 6-7 mol %, pentene units are incorporated to a high extent in the crystals of α and trigonal forms, which are stabilized over the γform, and fγ(max) decreases. The maximum fraction of γform is, therefore, related to the average length of regular propene sequences and the degree of incorporation of defects in the crystals of α and γforms. The values of fγ(max) that develop in iPPC5 copolymers have been compared with those that develop in copolymers of iPP with ethylene (iPPC2), butene (iPPC4), and hexene (iPPC6) and in stereoirregular iPPs reported in the literature. Stereoirregular iPPs and iPPC2 copolymers give the same relationship between fγ(max) and the average length of regular propene sequences (LiPP), whereas iPPC4, iPPC5, and iPPC6 copolymers show different behaviors. In particular, iPPC5 copolymers exhibit a behavior intermediate between those of iPPC4 and iPPC6 copolymers. The relationship between fγ(max) and LiPP of iPPC5 copolymers fits perfectly between the relationships found for iPPC4 and iPPC6 copolymers, in agreement with the different types and sizes of comonomers and the different efficiencies of their interruption and inclusion effects. These data give evidence of the general view of the crystallization behavior of iPP, based on the definition of a double role exerted by defects, the interruption effect that shortens the regular propene sequences and favors crystallization of γform, and the effect of incorporation of defects into the crystalline unit cells of α and γforms, which favors crystallization of the form that better accommodates the defect into crystals. The relative efficiency of these two effects depends on the type and size of the defect. The different relationships between fγ(max) and LiPP are a result of the equilibrium between interruption and inclusion effects achieved by each defect and confirm that the crystallization of γform is a perfect indicator of the length of the regular propene sequences and may provide very detailed information on the molecular structure of iPP
Polymorphism and form II – form I transformation in Ziegler-Natta isotactic 1-butene-ethylene copolymers having a multiblock molecular structure
No full textA study of the crystallization behavior and kinetics of form II-form I transition in isotactic butene-ethylene copolymers (iPBEt) prepared with Ziegler-Natta catalyst with ethylene concentration in the range 9–44 mol% is presented. The samples have been separated with boiling solvents in different fractions containing chains with different ethylene concentration. Crystallization of form II from the melt is observed in each fraction even at high ethylene concentration, up to nearly 16 mol%. For higher ethylene content the samples do not crystallize from the melt but still crystallize during aging at room temperature in form I′. The comparison with the crystallization behavior of iPBEt copolymers prepared with homogeneous metallocene catalysts, characterized by a random distribution of ethylene units, has shown that the form II-form I transformation in Ziegler-Natta copolymers is much slower than in metallocene copolymers having the same ethylene concentrations and for the metallocene copolymers a low concentration of ethylene units of nearly 6 mol% is sufficient to prevent crystallization from the melt of form II. This indicates that in the chains of Ziegler-Natta iPBEt copolymers the regular butene sequences interrupted by the ethylene units are much longer than those of the chains of copolymers prepared with metallocene catalysts of the same ethylene concentration, giving further evidence of the multiblock structure of Ziegler-Natta iPBEt copolymers
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