1,720,989 research outputs found
Evaluation of the Orientation Coefficient For the C-axis In Poly(ethylene-terephthalate) Fibers
No full textThe literature methods for the determination of the mean of the crystallite orientation distribution for the c axis, that is of the orientation coefficient f(c), for poly(ethylene terephthalate) (PET), based on the azimuthal scan of the (() over bar 105) reflection (which can be applied also to samples with ''tilted orientation''), is proposed. This method implies as a first step the determination of the tilt angle, for which the complete fiber pattern is required. A possible simplifying assumption, which allows use of the sole azimuthal (($) over bar 105) profile and makes the method also applicable to poorly oriented samples (for which the determination of the tilt angle is not easy), is also discussed. (C) 1995 John Wiley and Sons, Inc
Mesomorphic form (beta) of nylon 6
No full textThe structure of poly (epsilon-caprolactam) (nylon 6) in the beta mesomorphic form is here examined. The comparison of the diffraction intensity, calculated on modeled structures, with the experimental profiles, collected by us through an automatic diffractometer, is presented. This analysis has put into evidence the following limiting structural features. (a) The beta form of nylon 6 is made of small mesomorphic aggregates of chains (where the matter scatters coherently) with axes arranged in a hexagonal lattice (a = b = 0.48 nm; gamma = 120 degrees). (b) The chains have disordered conformations (and do not have a definite chirality, as it is the case for the 2(1) helices in the alpha and gamma forms) with the -CH2- chains close to nearly all-trans (antiperiplanar) and the two dihedral angles adjacent to the amide bond, +/-120 degrees) to antiperiplanar (180 degrees). This notwithstanding, the chains are straight and extended. As a result, the mean chain periodicity is close to 0.835 nm. (c) The H-bonds are formed along lines in the [100], [010], and [1(1) over bar0$] directions; they force neighboring chains within the small mesomorphic aggregates to adjust their conformation in such a way that nearly 100% of hydrogen bonds are always formed, in agreement with the IR data. In the case of the alpha and gamma more ordered forms of nylon 6, such lines are all in a unique direction, leading to hydrogen-bonded sheets (parallel to the chain axis) of enantiomorphous anticlined, alternately up and down chains (in the alpha form) or of isomorphous isoclined chains, (in the gamma form), (d) As a consequence, since the amide groups lie all at nearly the same height along z, in the beta form of nylon 6 the lines of hydrogen bonds lie in layers perpendicular to the chain asis and have the same direction within each layer. However, consecutive layers along z may have the lines of H-bonds which are not parallel (e.g., occasionally rotated by +120 or -120 degrees instead of the ''normal'' 180 degrees). (e) In the beta form of nylon 6, disorder arises also from the random substitution of up and down chains in the lattice positions
Effects of Distortional Components In Biaxial Stretching of Poly(ethylene-terephthalate) Sheets On Dimensional Stability and Structure
No full textThin sheets of poly(ethylene terephthalate) were stretched biaxially over a wide range to temperatures below the melting point of the polymer. The linear shrinkage occurring at temperatures between 85 and 100-degrees-C decreased with increasing draw temperature and draw ratio. Specimens taken near the edges of the drawn sheets, which had been subjected to in-plane shear deformations, were found to exhibit linear shrinkage 5-8 times lower than those taken from the middle of the sheet. Subsequent experiments, using purpose-built clamps to achieve a more uniform state of shear in both directions of the biaxially drawn samples, confirmed the universality of the principle of shrinkage suppression by the superposition of shear deformations. X-ray diffraction studies revealed that the phenomenon was not related to differences in type of orientation of the crystals. The information from the X-ray diffraction studies and data from thermal analysis have led to the conclusion that the enhanced dimensional stability of biaxially drawn sheets subjected to superimposed shear deformations results from a combination of a higher rate of stress-induced crystallization and a reduction in the level of orientation within the amorphous phase
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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