1,720,974 research outputs found
The aliphatic counterpart of PET, PPT and PBT aromatic polyesters: effect of the molecular structure on thermo-mechanical properties
No full textThe aliphatic counterparts of the most used aromatic polyesters (PET, PPT, and PBT) have been synthesized by a two-stage polycondensation process, starting from dimethyl 1,4-cyclohexane dicarboxylate and different diols. The fully aliphatic polyesters are characterized by two cis/trans isomeric ratios (50 and 90 mol%) of the 1,4-cycloaliphatic rings. According to the cis/trans content, the properties of the materials notably change. Indeed, polymers rich in trans isomer are semicrystalline, whereas polymers with low trans content are fully amorphous, due to the presence of kinks along the chain. Trans isomer is characterized by higher rigidity than the cis one and the corresponding polymers have high glass transition temperatures. Moreover, the length of the methylene sequences in the diol has a notable influence on the final thermal and mechanical properties. Therefore, tunable properties can be easily obtained. This characteristic, in association with good mechanical performances, potential sustainability of the monomers and biodegradability, makes these aliphatic polyesters an interesting class of polyesters for some specific applications
Effect of 1,4-cyclohexylene units on thermal properties of poly(1,4-cyclohexylenedimethylene adipate) and similar aliphatic polyesters
Full text linkIn the field of aliphatic polyesters there is a great interest in building novel macromolecular structures that may effectively
feature the requiredproperties for specific applications. For this reason, it is extremely important to understand the correlations between chemical structure and final performance. Monomers based on the 1,4-cyclohexylene unit are interesting as they are potentially bio-based and rigid enough to improve glass transition and melting temperatures. The present paper describes the preparation of samples of poly(1,4-cyclohexylenedimethylene adipate), characterized by different cis/trans isomeric ratio of the cyclic units, the analysis of their thermal properties and the comparison with the properties of similar polymers such as poly(butylene adipate), poly(butylene-1,4-cyclohexanedicarboxylate) and poly(1,4-cyclohexylenedimethylene-1,4- cyclohexanedicarboxylate). The effect of the rigidity and of the stereochemistry of the 1,4-cyclohexylene unit on the thermal properties was analysed. Such an effect reaches its maximum level when the cyclic-containing monomers are the diacid or the diester and when the trans isomers predominate. Starting from such a result, the combination of specific cyclic building blocks with the correct cis/trans ratiomakes it possible to build macromolecular structures with the required final properties
Aliphatic/aromatic copolyesters containing biobased omega-hydroxyfatty acids: Synthesis and structure-property relationships
Full text linkA family of unique aliphatic-aromatic copolyesters was prepared by bulk condensation copolymerizations
of bio-derived omega-hydroxytetradecanoic acid (H) with 1,4-butanediol (B) and dimethyl terephthalate (DMT). 1H NMR analysis showed the copolyesters have random repeat-unit sequence distributions. Thermal properties strongly depend on molar composition. Melting temperatures are lower than 70 °C for compositions rich in H-units; Tm varies from about 140 to 180°C for copolymers with high butylene terephthalate (BT) content. Crystal lattice structures shift from the crystal phase of poly(omega-hydroxytetradecanoate) (PH) to that of PBT with increasing BT copolymer content, while the minor component is trapped in the crystallizable domains as defects. The amorphous phase is homogeneous for all compositions and Tg increases from -21 °C (PH homopolymer) to 61 °C (PBT). Also, mechanical properties vary in a continuous way, according to copolymer composition. Therefore, the combination of aliphatic and aromatic units enables molecular design of partially biobased materials with adjustable thermal and physical-mechanical properties. Thus, by judicious selection of copolymer composition, material properties can be fine-tuned to attain the desired balance of material rigidity,
ductility, melting point and biobased content
Photodegradation of aliphatic polyesters and their composites with TiO2
No full textDegradability or, on the contrary, durability of polymers are very important features affecting the performances of the materials in their shelf-life and their impact on the environment. The degradation can be due to different environmental factors, including light exposure, very important for polymers used in outdoor applications.
The knowledge of the photo-degradation mechanisms and the possibility to modify them is a very important topic, especially for the novel biopolymers, as their durability with respect to the petro-plastics is yet an open question.
Among the biopolymers aliphatic polyesters are an emerging class of materials, thanks to the possibility of preparing them from renewable resources and their potential environmental degradability. In this study two aliphatic molecular structures, containing 1,4-cyclohexylene units, have been analysed and used as matrixes for composites.
PBCHD and PCCD were exposed to UV irradiation in an accelerated photo-ageing device and the modifications in their chemical structures were analysed by rheology, UV and IR spectroscopies. It results that the photo durability of PBCHD, containing only one cycle per monomeric unit, is higher than that of PCCD.
Moreover, in order to increase the durability, substances can be incorporated into the polymer. Among all the particles and nanoparticles which can be used, titanium dioxide (TiO2) is known for its capability of absorbing UV radiation. This photo-catalytic effect is strictly connected to its crystalline phases.
Therefore, composites containing different amount of TiO2, in the anatase and rutile crystalline phases separately, have been prepared by using a Brabender mixer. The well dispersion of the particles and the good adhesion between TiO2 and polymer have been checked by SEM observations.
The composites result to have a photo stability higher than those of the homopolymers. The protection of TiO2 against the photo-oxidation increases with the particle content and results higher for the anatase crystalline phase.
As a conclusion, a good increment of the photo-durability of the studied aliphatic polyesters has been obtained by preparing composites with titanium oxide. This result could be particularly significant for outdoor applications of these polymers
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
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe 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
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