1,721,518 research outputs found
Crystal structures and order-disorder phenomena in polymers
No full textThe crystal structure of various polymers, presenting symmetry breaking, is discussed. Owing to the presence of disorder, the crystal structure of polymers is generally described in term of ideal modifications, a limit ordered form, characterized by ideal three-dimensional order, and a limit disordered form, characterized by the presence of statistical disorder in the packing, while preserving the order in the parallelism of the chains, The real crystalline forms are generally intermediate between the limit ordered and limit disordered models. The occurrence of conformational disorder, which produce defects frozen in the crystals of syndiotactic polypropylene, is analyzed. © WILEY-VCH Verlag GmbH
Crystalline-crystalline isotactic polypropylene-based block copolymers prepared by Pyridylamidohafnium Catalyst
No full textA structural and morphological characterization at different length scales of cystalline-crystalline block copolymers (BCPs) formed by isotactic polypropylene (iPP) block linked to polyethylene (PE) block1 obtained through metallorganic2 catalysis is reported.
These systems allow studying of the effect of confinement on the crystallization behavior under different crystallization conditions. The final morphology of these systems is path dependent, being affected by the competition between crystallization and phase separation.
The properties of this class of materials can be tailored by combining the wide range of morphologies that develop in crystalline BCPs with the polymorphism of iPP, and choosing the proper crystallization conditions. To this aim, a detailed structural characterization has been performed for samples crystallized in the bulk, thin films and stretched fibers and stress-induced phase transformations have been also analyzed. Moreover, simultaneous time and temperature-resolved WAXS and SAXS experiments have been performed with synchrotron radiation to clarify the melting and crystallization behavior.
Finally, thin films of iPP-block-PE have been epitaxially crystallized onto crystals of organic substrates to achieve a control over the crystallization of both crystalline blocks. The final morphology depends on which polymer block crystallizes first, a sequence that depends on the block copolymer composition and block lengths
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
A Rheological Investigation of the Crystallization Kinetics of Syndiotactic Polypropylene of Varying Degree of Tacticity
No full textThe crystallization behavior of five Syndiotactic Polypropylene (sPP) samples of varying degree of tacticity has been studied by means of rheological techniques and compared to the results of more standard DSC measurements. Small Amplitude Oscillatory Shear (SAOS) measurements have been performed on a controlled stress rotational rheometer equipped with a nitrogen-fed, forced convection oven, which allows for good temperature stability and relatively high heating/cooling rates. Both non-isothermal and isothermal crystallization tests have been carried out. The former proved useful to determine the melting and crystallization temperature of the polymers. The latter were exploited to determine the temperature dependence of the polymer crystallization rate. The results showed the strong influence of the degree of tacticity on the crystallization behaviour of sPP. In particular, it was confirmed that a decrease in the stereo-regularity of the polymer chain considerably shifts the crystallization process to lower temperatures. For the case of the lowest tacticity sample, rheology was crucial in determining the temperature where the maximum (but very small) crystallization rate is attained, a result that could not be achieved by standard calorimetry measurements
Static Response and Buckling Loads of Multilayered Composite Beams Using the Refined Zigzag Theory and Higher-Order Haar Wavelet Method
Full text linkThe paper presents a review of Haar wavelet methods and an application of the higher-order Haar wavelet method to study the behavior of multilayered composite beams under static and buckling loads. The Refined Zigzag Theory (RZT) is used to formulate the corresponding governing differential equations (equilibrium/stability equations and boundary conditions). To solve these equations numerically, the recently developed Higher-Order Haar Wavelet Method (HOHWM) is used. The results found are compared with those obtained by the widely used Haar Wavelet Method (HWM) and the Generalized Differential Quadrature Method (GDQM). The relative numerical performances of these numerical methods are assessed and validated by comparing them with exact analytical solutions. Furthermore, a detailed convergence study is conducted to analyze the convergence characteristics (absolute errors and the order of convergence) of the method presented. It is concluded that the HOHWM, when applied to RZT beam equilibrium equations in static and linear buckling problems, is capable of predicting, with a good accuracy, the unknown kinematic variables and their derivatives. The HOHWM is also found to be computationally competitive with the other numerical methods considered
Materiali polimerici nanostrutturati per il controllo della morfologia su scala nanometrica
Full text linkLa presente attività di ricerca ha avuto l’obiettivo di sviluppare un sistema ibrido innovativo in cui un copolimero a blocchi nanostrutturato è selettivamente caricato con nanoparticelle inorganiche conduttive.
I copolimeri a blocchi (BCPs) consistono in macromolecole distinte di due o più omopolimeri, covalentemente legate e che tendono a segregare in microdomini distinti a causa delle loro mutue repulsioni [1, 2]. Questo genera la formazione spontanea di nanostrutture che sono state considerate utili alla creazione di patterns regolari su scala nanometrica e che sono importanti per diverse tecnologie su film sottile [3, 4, 5]. I microdomini dei BCPs nanostrutturati (sfere, cilindri o lamelle) formati spontaneamente per self-assembly [1-5], possono agire da hosts per sequestrare nanofillers di appropriata affinità chimica e geometrica producendo un ordine a lungo raggio nel posizionamento delle nanoparticelle [6]. I BCPs sono, quindi, materiali ideali per agire da “impalcatura” nell’ingegneria di nuovi nanocompositi, dove la distribuzione di particelle guest è guidata dall’ordine della matrice host. Questo genera un assemblaggio gerarchico di nanostrutture ibride organico-inorganico (polimero-metallo) in cui un livello di self-assembly guida il successivo. La chiave per l’ingegneria di questi materiali è l’abilità nel controllare la morfologia finale delle nanostrutture dei BCP e nell’ottenere un’infiltrazione selettiva di nanoparticelle in microdomini specifici delle nanostrutture. Nel presente progetto è stato utilizzato un copolimero a blocchi amorfo, poli(stirene-b-metilmetacrilato) (PS-b-PMMA), con frazione in volume scelta in modo tale da ottenere morfologia di tipo lamellare. Film sottili sono stati preparati mediante spin-coating o drop-casting a Tamb su un substrato trasparente e conduttivo quale ITO (Indium Tin Oxide). La scelta di questo substrato ha consentito di ottenere una orientazione perpendicolare dei microdomini lamellari di PS e PMMA formati per self-assembly. Infine, è stato realizzato un nanocomposito ibrido mediante incorporazione selettiva di nanoparticelle di oro all’interno dei domini di PS utilizzando un processo innovativo basato sull’evaporazione e condensazione di nanoparticelle metalliche [7, 8], le quali diffondono nei domini preferenziali formando nanoparticelle con selettività spaziale sulla superficie del film, in particolare dopo successivi trattamenti di ricottura
Porous cores in small thermoacoustic devices for building applications
No full textThe thermoacoustic behavior of different typologies of porous cores is studied in this paper with the goal of finding the most suitable solution for small thermoacoustic devices, including solar driven air coolers and generators, which can be used in future buildings. Cores provided with circular pores, with rectangular slits and with arrays of parallel cylindrical pins are investigated. For the type of applications in focus, the main design constraints are represented by the reduced amount of the input heat power and the size limitations of the device. In this paper, a numerical procedure has been implemented to assess the behavior of the different core typologies. For a fixed input heat power, the maximum acoustic power delivered by each core is computed and the corresponding engine configuration (length of the resonator and position of the core) is provided. It has been found that cores with parallel pins provide the largest amount of acoustic power with the smallest resonator length. This conclusion has been confirmed by experiments where additive manufactured cores have been tested in a small, light-driven, thermoacoustic prime mover
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