1,720,980 research outputs found
Instrument for mimicking fast cooling conditions of polymers: Design and case studies on polypropylene
No full textThe complete understanding of polymers crystallization kinetic in real industrial processes is fundamental for designing materials towards specific applications. Hereby, we propose a method for fast cooling preparation of polymers by means of air and water sprays, which presents several enhancements with respect to previous similar devices. The new device was built in order to fulfill safety and cleaning standards required in an industrial environment. Furthermore, the small dimensions and the limited costs enable its implementation in any laboratory. Moreover, preliminary results on selected isotactic-polypropylenes (iPP) were obtained in a cooling range from 0.01 to a few thousand °C/s, in order to validate the present method by comparison with previous literature results and to highlight the large range of studies that can be carried out with this set-up. Ex-situ analysis with WAXS, DSC and AFM techniques were performed to characterize such prepared samples
Local mechanical properties by Atomic Force Microscopy nanoindentations
No full textThe analysis of mechanical properties on a nanometer scale is a useful tool for combining information concerning texture organization obtained by microscopy with the properties of individual components- Moreover, this technique promotes the understanding of the hierarchical arrangement in complex natural materials as well in the case of simpler morphologies arising from industrial processes. Atomic Force Microscopy, AFM, can bridge morphological information, obtained with outstanding resolution, to local mechanical properties. When performing an AFM nanoindentation, the rough force curve, i.e., the plot of the voltage output from the photodiode vs. the voltage applied to the piezo-scanner, can be translated into a curve of the applied load vs. the penetration depth after a series of preliminary determinations and calibrations. However, the analysis of the unloading portion of the force curves collected for polymers does not lead to a correct evaluation of Young’s modulus. The high slope of the unloading curves is not linked to an elastic behavior, as would be expected, but rather to a viscoelastic effect. This can be argued on the basis that the unloading curves are superimposed on the loading curves in the case of an ideal elastic behavior, as for rubbers, or generally in the case of materials with very short relaxation times. In contrast, when the relaxation time of the sample is close to or even much larger than the indentation time scale, very high slopes are recorded.
Where AFM nanoindentations are concerned, one observes a dependence of the penetration, i.e., the relative motion between the sample and the tip (indenter), on the elastic properties of a material when using equivalent loads. This relationship becomes visible on samples that are homogeneous down to the scale of nanoindentation. The elastic modulus can be obtained by applying Sneddon’s elastic contact mechanics approach, since the contact between the tip and the sample is dominated by an elastic behavior with negligible plastic deformation. Under such circumstances, the dependence of the penetration on the load follows an exponent of 1.5, consistent with elastic contact mechanics and justified on the basis of the large elastic range exhibited by polymers, on the constraints due to the geometry of the deformation during indentation and to the critical yielding volume needed in order to induce plasticity. As a result, elastic moduli taken from AFM force curves show a very good agreement with bulk values obtained by macroscopic tensile testing. This is true for a broad range of polymers, from materials with rubbery to semicrystalline, or even glassy behaviors. This result confirms that AFM nanoindentations in polymers take place mostly in the elastic range and opens the possibility of characterizing the mechanical behavior of polymers on an unparalleled small scale as compared to commercial depth sensing instruments (DSIs), which use much blunter indenters.
A further application is discussed where, upon decreasing the load, and consequently the penetration depth, the scale becomes comparable to that of the underlying texture which is probed as opposed to the bulk material. Although this apparently presents a limitation on the resolution of the scale that can be mapped, this feature is discussed and shown to open the possibility of identifying properties of individual phases with their surroundings as well as the role of the connectivity among the phases
Linking structure and mechanical properties via instrumented nanoindentations on well-defined morphology poly(ethylene) Polymer 50 8 1939-1947 2009
No full textSeveral poly(ethylene) samples with a broad range of morphologies were studied in this work using nanoindentations. The samples had degrees of crystallinity ranging from 30 to100% while their Young’s modulus ranged from few tens of MPa up to several GPa. Experimental conditions for the correct evaluation of Young’s modulus were at first identified, choosing a suitable loading rate in order to minimize viscoelastic effects on the unloading. The force curves, i.e. plots of applied load vs. penetration depth, were then analyzed following two common procedures available in the literature. None of these procedures leads to satisfying results when compared to other experimental techniques. However, it was found that a recently proposed correction factor to the Oliver and Pharr procedure allows to evaluate reliably the Young’s modulus of the poly(ethylene) samples exhibiting very different, fine tuned morphologies
Estimating the nucleation ability of various surfaces towards isotactic polypropylene via light intensity induction time measurements
No full textCrystallization of isotactic polypropylene (iPP) at the interface with crystalline films of two commercially employed nucleating agents (sodium benzoate (NaBz) and sodium 2,2'-methylene bis-(4,6-di-tert-butylphenyl)phosphate (NA-11)) and with a glass fiber (GF) was investigated using a polarized optical microscope. The analysis of the light intensity evolution during the crystallization process enabled the successful estimation of the time at which the crystal growth began, i.e., the induction time (ti), at various crystallization temperatures. Meaningful differences in the ti values were observed between the investigated systems. Moreover, the ti data have been analyzed according to different nucleation models proposed in the literature, which consider either the time to form the first crystalline layer in contact with the substrate or the time required to grow a cluster of critical size. It has been found that the two models are applicable in different temperature ranges depending on the efficiency of the given substrate. Therefore, in order to obtain the value of the surface free energy difference function, Δσ, which is directly related to the nucleation energy barrier and useful for the definition of a universal nucleating efficiency scale, a model that considers both the above-mentioned times was fitted to the overall data. The values of Δσ for the nucleation of iPP on the surface of the different substrates are thus obtained and discussed in the framework of the literature results
Nanoscale mechanical characterization of polymers by atomic force microscopy (AFM) nanoindentations: Viscoelastic characterization on a model material
No full textThe Atomic Force Microscope (AFM), apart form its conventional use as a microscope, is also used for the characterization of the local mechanical properties of polymers. In fact, the elastic characterization of purely elastic materials using this instrument can be considered as a well assessed technique while the challenge remains the characterization of the viscoelastic mechanical properties. In particular, one finds the mechanical behavior changing when performing indentations at different loading rates, i.e., on different time scales. Moreover, this apparent viscoelastic behavior can also be due to complex contact mechanics phenomena, with the onset of plasticity and long-term viscoelastic features which cannot be identified by the force curve alone. For this reason, a viscoelastic characterization, and thus the study of the effects of indentation rate and temperature, were done on model materials where such additional phenomena are not observed. Another time-dependence originating from the instrument itself has also been identified and decoupled. In fact, the viscoelastic behavior has been found to be reproducible even if one changes the experimental set-up as far as the preliminary determinations concerning AFM nanoindentations are well performed. The effects of temperature and time scales on the mechanical behavior have also been undertaken. A check on time-temperature superposition is also attempted through the WLF equation and the apparent activation energies for the elementary motions in the rubbery and in the glass transition regions are in good agreement with the expected values
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 modulated DSC for composition analysis of recycled polyolefin blends
Full text linkPost-consumer plastic waste contains blends of numerous types of polyethylene (PE) and isotactic polypropylene (i-PP), whose recycling is challenging due to the complexity of this waste stream. A comprehensive knowledge of the composition of these recyclates is essential to understand the structure-property relationship of these systems and therefore upcycle them for high-value applications. To this aim, we used Temperature Modulated DSC (TMDSC) to develop a quantitative method to evaluate PE and Low-density PE (LDPE) content in recycled polyolefin blends. TM-DSC was carried out on 29 virgin PE materials, spanning densities between 960 and 862 kg/m3, characterizing a wide range of PE microstructures. Moreover, several PE/i-PP model blends were prepared by selecting LDPE, High-density PE (HDPE) and Linear Low-density PE (LLDPE) materials to blend them with i-PP of three types: homopolymer (PP-H), block copolymer (PP-B) and random copolymer (PP-R), mimicking the composition of real recyclates. Results from the TM-DSC analysis of these blends allowed us to establish methods for quantifying the amount of overall PE content and also the LDPE fraction within recyclates. The developed methods were applied to real post-consumer recycled grades, and results were compared with the ones obtained from Cross-Fractionation Chromatography (CFC) analysis and Nuclear Magnetic Resonance (NMR) spectroscopy, displaying good agreement between the latter and the TM-DSC method.We acknowledge the financial support from the REPOL project; this project has received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie Grant Agreement No. 860221. The authors wish to thank Tamara Carmeli and all the Thermal Analysis group for the help with the TM-DSC measurements, Karin Kemper for the help with procuring the materials, Andreas Albrecht for the CFC measurements, and Gerhard Hubner for the NMR measurements
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