1,720,975 research outputs found
Preparation and properties of adhesives based on phenolic resin containing lignin micro and nanoparticles: A comparative study
No full textThis work investigated, for the first time, the role of nanosized lignin (LNP), in comparison with microlignin (LMP), when introduced at two different weight amounts (5% and 10 wt%) in bulk phenol–formaldehyde resol as adhesive. Morphological analysis was performed to check out the dispersion and interfacial bonding of lignin in the phenolic resin. The curing process has been examined by differential scanning calorimetry (DSC), while the thermal stability of the composites has been evaluated by using thermogravimetric (TGA) and thermo-mechanical (TMA) analysis. Results exhibited that small amount of lignin could both favor the thermal cure reaction, due to its abundance of phenylpropane units, and the initial thermal resistance could be consequently improved, especially when the nano-sized lignin was used. Meanwhile, the effect of micro- and nano-modification on tensile shear strength of wood lap joints based on lignin-phenol–formaldehyde resol adhesives was also analyzed. Results showed that 5 wt% of LNP could positively increase the shear strength from 8.7 to 10.9 MPa, opening the possibility of using environmental friendly nanoscale lignin in cross linked traditional phenol wood adhesives with enhanced adhesion performance, strongly related to nanoparticles higher specific surface area and reactivityThis work investigated, for the first time, the role of nanosized lignin (LNP), in comparison with microlignin (LMP), when introduced at two different weight amounts (5% and 10 wt%) in bulk phenol-formaldehyde resol as adhesive. Morphological analysis was performed to check out the dispersion and interfacial bonding of lignin in the phenolic resin. The curing process has been examined by differential scanning calorimetry (DSC), while the thermal stability of the composites has been evaluated by using thermogravimetric (TGA) and thermo-mechanical (TMA) analysis. Results exhibited that small amount of lignin could both favor the thermal cure reaction, due to its abundance of phenylpropane units, and the initial thermal resistance could be consequently improved, especially when the nano-sized lignin was used. Meanwhile, the effect of micro- and nano-modification on tensile shear strength of wood lap joints based on lignin-phenol-formaldehyde resol adhesives was also analyzed. Results showed that 5 wt% of LNP could positively increase the shear strength from 8.7 to 10.9 MPa, opening the possibility of using environmental friendly nanoscale lignin in cross linked traditional phenol wood adhesives with enhanced adhesion performance, strongly related to nanoparticles higher specific surface area and reactivity. (C) 2018 The Authors. Published by Elsevier Ltd
Simultaneously Enhancing Mechanical Strength, Toughness, and Fire Retardancy of Biobased Polyurethane by Regulating Soft/Hard Segments and Crystallization Behavior
No full textIn this work, biobased flame-retardant polyurethanes were designed. First, a vanillin-based diol (VDP), as the hard segment, was prepared by condensation and addition reactions of vanillin with DDM (4,4′-diaminodiphenylmethane) and DOPO (9,10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide). Then, polyurethane materials FRBPU-xP were prepared by a polycondensation reaction of different contents of VDP, crystallized polycaprolactone diol, and HDI by modulating the amount (x) of P. Interestingly, when the amount of VDP was increased from 0 to 14.5 wt % (P = 1.0%) and the amount of PCL diol was decreased from 82.4 to 64.8 wt %, the Tg of the prepared FRBPU-xP was significantly increased from −34.7 to 2.8 °C, while the tensile strength increased from 10.7 to 15.6 MPa and the elongation at break increased from 822% to 930%, showing simultaneous mechanical strengthening and toughening. These behaviors can be ascribed to (1) the increase of the hard VDP diols and the enhancement of intermolecular interactions and (2) the decrease of the crystallinity from 21.2% to 14.5% due to the decreasing content of PCL diols and the inhibiting crystallization behavior induced by VDP, which helped to improve the ductility of the polyurethane material. Furthermore, the LOI value of FRBPU-0.5P reached 29.6%, and this material achieved a UL-94 V-0 rating. In addition, the polyurethanes showed good reprocessability, and FRBPU-1.0P had a retention of 72.6% and 93.3% for its tensile strength and elongation at break after thermal remolding, respectively. This work provides an idea for the preparation of high-performance biobased flame-retardant polyurethane
Synthesis of Vanillin-Based Polyurethane with Super Thermal Stability and Free-Radical-Scavenging Activity
No full textThis work focuses on designing biobased polyurethane systems possessing intrinsic free radical scavenging activity (RSA), exceptional toughness, and thermal stability. Initially, divanillyl alcohol (DVO) was synthesized through oxidative coupling activation of the vanillin precursor, followed by reduction of aldehyde groups. Subsequently, polyurethanes denoted as RSPU-x (where x is the molar ratio of DVO) were synthesized, using polycaprolactone diol (OH-PCL-OH) as the soft segment and isophorone diisocyanate (IPDI) along with DVO as the hard segments. With an increase in the DVO content from 0 to 0.25, the thermal stability and mechanical properties of RSPU-x were significantly enhanced. Specifically, the maximum thermal decomposition temperature rose from 317 to 417 °C, while the toughness shot up from 38.1 MJ/m3 to 223.9 MJ/m3. The improvement of thermal stability is attributed to the biphenyl structure of DVO, which has high bond dissociation energy and exceptional free radical scavenging activity, while enhanced toughness primarily results from the introduction of the DVO rigid skeleton, which disrupts the regularity of PCL chains, thereby inhibiting PCL crystallization behavior. Additionally, RSPU-x also exhibited inherent free radical scavenging activity due to the presence of Ar-OH groups in the skeletons. The RSA of RSPU-0.25 (DPPH and ABTS tests) reached, respectively, 94.21% and 99.24% within 0.5 h, achieved via the H+ transfer mechanism. This work presents a novel strategy for preparing polyurethanes with exceptional thermal stability and RSA properties
A fully healable, mechanical self-strengthening and antibacterial Poly(thiocarbamate-urethane) elastomer constructed via dual reversible dynamic networks
No full textCrosslinked polyurethane elastomers may irreversibly suffer from some external damage such as chemical corrosion, stress and long-term heat treatment, inducing the formation of the microcracks and reduction of mechanical performance. Herein, a novel, fully healable and mechanical self-strengthening crosslinked poly(thiocarbamate-urethane) elastomers (PTU) are constructed by dual reversible dynamic networks, which are based on the thiol-isocyanate click reaction and 2, 4-diamino-6-hydroxy-pyrimidine-Zn2+ (HPM-Znx) coordination effect. Interestingly, the tensile strength and toughness of PTU-HPM-Zn1/4 reached 24.2 MPa and 46.3 MJ/m3, which are 3.6 and 1.6 times higher than those of PTU-HPM, due to the coordination effect between Zn2+ and pyrimidine that formed a denser cross-linking network. Interestingly, the snipped specimen of PTU-HPM-Zn1/4 are fully healable within 12 h at 70 °C. Moreover, the hot-reprocessed PTU-HPM-Zn1/4 sample were further increased to 25.0 MPa and 51.5 MJ/m3, showing a mechanical self-strengthening phenomenon. These impressive mechanical properties of PTU-HPM-Zn1/4 elastomer are attributed to the hierarchical structure evolution including the microphase separation between crystallized and amorphous regions, and the multi reversible dynamic networks reconstruction of thiocarbamate, coordination and hydrogen bonds in the system. Meanwhile, PTU-HPM-Zn1/4 showed excellent antibacterial property against E. Coli. In order to expand their application as flexible and wearable sensor devices, two PTU-HPM-Zn1/4 based conductive films were prepared, which exhibited fast and stable sensing characteristics
Ultra-robust polyurethane/lignin elastomers based on multiscale structural orientation induced by pre-stretching
No full textThis work focuses on a simple approach for designing polyurethane elastomers with ultra-robust properties. Initially, polyurethane/lignin elastomers were synthesized using polytetramethylene ether glycol (PTMG) as a soft segment, hexamethylene diisocyanate (HDI) with 4,4′-dihydroxydiphenyl ether (DO) as hard segments, and lignin nanoparticles (LNP) as a biomacromolecular crosslinker. Subsequently, the resulting elastomers were pre-stretched at certain ratios (λ), and surprisingly, the pre-stretched polyurethane/lignin elastomer (at the ratio of λ = 14) displayed significant improvement in mechanical properties, while the tensile strength and toughness values were sharply enhanced from 32.4 MPa and 298.7 MJ m−3 to 116.2 MPa and 501.3 MJ m−3, respectively. This improvement can be attributed to the obtainment of dense cross-linked networks due to the combined effect of pre-stretching on both multiscale structural orientation and stress-induced crystallization (SIC). The interphase distance (dp) decreased from 15.708 to 14.612 nm, the interplanar crystal distance (dc) increased from 4.387 Å to 4.398 Å, and the Herman orientation factor (f) increased from −0.0066 to 0.0518. This work presents a novel strategy for preparing polyurethane elastomers with exceptional mechanical performance
Advanced hollow ball-cactus-like soft-magnetic LDH@MXeneHT nanohybrid materials towards highly efficient electromagnetic protection
No full textIn order to improve the comprehensive electromagnetic protection and meet the aerospace demands, designing the microstructure of functional nanomaterials is an effective strategy. Inspired by the special structure of the ball-cactus, in this work, soft-magnetic nanohybrid materials are realized for highly efficient electromagnetic protection. Layered double FeNi3 hydroxide (LDH) is in-situ nucleated and grown on the surface of MXene via synchronous electrostatic self-assembling, leading to an intermediate product - LDH@MXene - with a hydrangea microstructure (diameter ∼5 μm). LDH@MXene is subsequently annealed at 500 °C to obtain a hollow ball-cactus-like LDH@MXeneHT500 nanohybrid, by transforming the lamellar “petals” of the hydrangea structure into burr-like structures. LDH@MXeneHT500 has soft-magnetic characteristics, facilitating the transport of induced charges and the consumption of electromagnetic energy. Therefore, LDH@MXeneHT500 nanohybrid exhibits excellent comprehensive electromagnetic protection capabilities, such as high reflection loss (RLmin = −76.1 dB) in a wide absorption bandwidth (EAB = 6.12 GHz), and high electromagnetic interference shielding efficiency (EMI SE = 47.2 dB in X band). To broaden its application, poly(dimethylsiloxane) (PDMS)/LDH@MXeneHT500 nanocomposites are further prepared that exhibit improved mechanical strength (9.48 MPa) and highly efficient electromagnetic protection performances (RLmin = −65.11 dB and EAB = 7.64 GHz, EMI SE > 50 dB in 3–18 GHz). Therefore, both the LDH@MXeneHT500 nanohybrid materials and PDMS/LDH@MXeneHT500 nanocomposites show great potential in electromagnetic protection applications, such as in the area of aerospace and satellite communication
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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