1,720,967 research outputs found
GAMMA-ISOTACTIC POLYPROPYLENE - QUANTITATIVE-ANALYSIS OF ELECTRON DIFFRACTION DATA AND PACKING ENERGY CALCULATIONS
No full textAPPLICATIONS OF MOLECULAR MECHANICS TO REFINE AND UNDERSTAND POLYMER CRYSTAL-STRUCTURES
No full textA computational approach to refine and analyze polymer crystal structures is presented, where both intra- and intermolecular interactions are simultaneously optimized by minimizing the total crystal potential energy in cartesian coordinates and utilizing a full-atom force field. The advantages of the present methodology are illustrated through several applications, including very recent results concerning the polymorphism of isotactic polypropylene. Some features of the computer program for molecular mechanics CHAMP, developed in our laboratory, are also described
CONFORMATIONAL STUDY OF ALPHA-POLYPIVALOLACTONEAND GAMMA-POLYPIVALOLACTONE BASED ONINTRAMOLECULAR AND INTERMOLECULAR INTERACTIONS
No full textConformational and packing energy of crystalline alpha- and gamma-polypivalolactone has been analyzed with a computational procedure where both intra- and intermolecular interactions are simultaneously
taken into account within each energy minimization cycle. This methodological improvement yields results in good agreement with the models directly refined by the powder X-ray diffraction profile and by electron diffraction data. In particular intermolecular interactions prove to have a determinant influence on the detailed conformations of the polymer chain in the two crystalline forms
STRUCTURE OF THE ALPHA AND BETA POLYMORPHS OF ISOTACTIC 1,4-CIS-POLY(2-METHYL-1,3-PENTADIENE)
No full textThe structures of two polymorphs of isotactic 1,4-cis-poly(2-methyl-1,3-pentadiene) are investigated through a computational procedure that simultaneously minimizes the conformational and the
packing energy of a microcrystal of suitable dimensions. The results are compared with diffractometric data, and in both cases reliable structures are obtained by properly combining the indications coming from crystallographic analysis and energy calculations
RECENT RESULTS ON THE POLYMORPHIC BEHAVIOR OF ISOTACTIC POLYPROPYLENE
No full textThe more recent results concerning the polymorphic behavior of isotactic polypropylene (iPP) are presented. Specifically contributions on the recently determined γ phase crystal structure, in which non-parallel chains coexist, and the novel results concerning the β phase are summarized. The new data appear to suggest that the normally occurring α-phase is in general neither the kinetically favored nor the thermodynamically more stable phase but rather it represents the best compromise between these two criteria. The implications and the open questions arising from the new findings with respect to morphology and polymer crystal growth are considered
ENERGY CALCULATIONS FOR ISOTACTIC POLYPROPYLENE - A COMPARISON BETWEEN MODELS OF THE ALPHA-CRYSTALLINE ANDGAMMA-CRYSTALLINE STRUCTURES
No full textPacking energy calculations have been performed on both alpha and gamma forms of crystalline isotactic polypropylene with the following objectives: (a) to compare the stability of the two crystal lattices; (b) to compare the effects of the up-down statistical disorder in the two polymorphs; (c) to investigate the stability of a possible alternative model for the gamma phase; (d) to define a compromise structural model for gamma-iPP satisfying both the crystallographic and energetic requirements. The results indicate very nearly identical values of the packing energies of the two lattices, the gamma form and the alpha form displaying a preference for the up-down inversions to occur at closely related sites. While the alternative model for the gamma structure shows a substantially higher packing energy, the minimum-energy structure deviates only marginally from the one refined from X-ray powder diffraction data. The compromise model shows a disagreement factor nearly as low as the value obtained considering only X-ray data and an energy only 0.4 kcal/mol of trimer higher than the minimum value
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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