1,720,971 research outputs found
Hierarchical motion of 4D-printed structures using the temperature memory effect
No full textThe temperature memory effect (TME) refers to the ability of a shape memory polymer to display recovery around the temperature at which its predeformation occurred so that the material expresses its shape memory response not only in terms of shape but also for what concerns the deformation temperature. This peculiar effect, displayed only by certain classes of polymers, allows to control of the triggering temperature for the shape memory effect as well as to provide multiple shape memory responses for specific, properly designed predeformation histories. Moreover, when combined with 3D printing, such an effect opens new powerful perspectives for designing autonomous structures with customized architectures and programmable/controllable shape changes. However, the design of such structures and of their active response is not trivial and requires careful attention at different levels, i.e., during printing, experimental characterization, modeling, and simulation. The topic of the present chapter concerns 4D-printed structures exhibiting the TME, and it aims at providing the reader with both an analysis and discussion, helpful in guiding toward the design of functional structures capable of controlled motions, also in a hierarchical manner. Particularly, a methodological approach is proposed and includes three main stages: evaluation of material properties, experimental characterization of 3D-printed structures, and modeling/simulation. A discussion about the steps of each stage is provided, together with an overview of the current state of the art, and a case study is presented. Potential application fields and future perspectives are also explored and discussed
Stress-Free Two-Way Shape Memory Effect of Poly(ethylene glycol)/Poly(ε-caprolactone) Semicrystalline Networks
Full text linkIn this work, poly(ethylene glycol) (PEG)/poly(ε-caprolactone) (PCL) semicrystalline networks were prepared by photo-cross-linking of methacrylated macromonomers with different molecular weights and in different proportions to obtain amphiphilic materials capable of displaying properly designed shape memory effects. Networks based on PCL 10 kDa and PEG 3 kDa showed suitable thermal and mechanical properties with well-separated crystallization and melting regions to achieve a self-standing two-way shape memory effect. Particularly, after the application of a specific thermomechanical history, these materials are capable of cyclically changing their shape between two configurations upon cooling–heating cycles in the absence of any external load applied. The effect of the composition of the networks and of the employed thermomechanical parameters, such as the applied strain and the actuation temperature, was investigated to shed light on the shape memory mechanism for this class of materials, which are considered promising for applications in the biomedical field and as reversible actuators for soft robotics
Reversible Stress-Driven and Stress-Free Two-Way Shape Memory Effect in a Sol-Gel Crosslinked Polycaprolactone
Full text linkThe two-way shape memory effect is the ability of a material to change its shape between two configurations upon application and removal of a stimulus, and, among shape memory polymers, it is featured only by few systems, such as semicrystalline networks. When studied under tensile conditions, it consists of elongation–contraction cycles along cooling and heating across the crystallization and melting region, typically under the application of a constant load. However, recent studies on crosslinked semicrystalline co-polymers demonstrate that also a completely stress-free, or self-sustained, two-way effect may be achieved through specific thermomechanical cycles. This effect is currently regarded with interest for the development of intrinsically reversible sensors and actuators, and it may also be displayed by simpler materials, as homopolymer-based semicrystalline networks. Only seldom articles investigate this possibility, therefore in this work the two-way shape memory behavior is studied on a poly(ɛ-caprolactone) system, crosslinked by means of a sol-gel approach. The effect is studied both under stress-driven and stress-free condition, by applying properly set-up thermo-mechanical histories. The results allow to describe the effect as a function of temperature, to reveal the dependence on specific testing parameters and to compare the extent of the reversible strain variation under these two conditions
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
Temperature-memory effect in 3D printed photopolymers with broad glass transition
No full textThe recent efforts towards the realization of shape memory polymer-based structures through additive manufacturing techniques (3D-printing) are often referred with the name 4D-printing (the 4th dimension representing the time-dependent shape evolution of the printed element) and aim at providing systems capable of complex shape changes and sequential motions. In this paper the shape memory capabilities of commercial photopolymer systems printed by stereolithography were investigated, allowing to describe the recovery process as a function of temperature. Particular attention was reserved towards the effect of the deformation temperature, which in presence of a broad glass transition region allows to achieve the so called "temperature memory effect", i.e. the possibility to modify the thermal trigger of the shape memory effect through the deformation temperature. The temperature memory features of the printed materials were quantified and a sequential thermally activated deployment was attempted
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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