The influence of matrix viscosity on MWCNT dispersion and electrical properties in different thermoplastic nanocomposites

Composites of MWCNTs having each three different levels of matrix viscosity with five different polymers (polyamide 12, polybutylene terephthalate, polycarbonate, polyetheretherketone and low density polyethylene) were melt mixed to identify the general influence of matrix viscosity on the electrical properties and the state of MWCNT dispersion. Huge differences in the electrical percolation thresholds were found using the same polymer matrix with different viscosity grades. The lowest percolation thresholds were always found in the composites based on the low viscosity matrix. The state of primary MWCNT agglomerate dispersion increased with increasing matrix viscosity due to the higher input of mixing energy. TEM investigations showed nanoagglomerated structures in the low viscosity samples which are obviously needed to achieve low resistivity values. The effect of nanotube shortening was quantified using two different viscosity grades of polycarbonate. Due to the higher mixing energy input the nanotube shortening was more pronounced in the high viscosity matrix which partially explains the higher percolation threshold. © 2011 Elsevier Ltd. All rights reserved

Melt mixed nano composites of PA12 with MWNTs: Influence of MWNT and matrix properties on macrodispersion and electrical properties

Nanocomposites containing four different polyamide 12 (PA12) types and three grades of multiwalled carbon nanotubes (MWNTs) were prepared via small-scale melt processing to study the effect of different MWNTs and the influence of polymer properties on the dispersion of the fillers and the electrical properties of the composites. Under the selected mixing conditions the lowest electrical percolation threshold of 0.7. wt.% was found for Nanocyl™ NC7000 in low viscous PA12. Moreover, big influences of the end group functionality (acid or amine excess) and the melt viscosity of the matrix were found. Composites of PA12 with acid excess showed lower percolation thresholds than those based on amine terminated materials. At constant end group ratio low viscous matrices resulted in lower percolation thresholds than high viscous materials. The best MWNT dispersion was obtained in both high viscous PA12 composites. In these systems the mixing speed was varied indicating an optimum concerning electrical conductivity at 150 rpm as compared to 50 and 250 rpm. © 2010 Elsevier Ltd

Interfacial chemistry using a bifunctional coupling agent for enhanced electrical properties of carbon nanotube based composites

A bifunctional coupling agent (BCA) containing one oxazoline and one benzoxazinone group was applied to promote a reaction between polyamide 12 (PA12) and multiwalled carbon nanotubes (MWCNTs) during melt mixing. With this modification, the MWCNT content needed for the electrical percolation was significantly reduced by more than a factor of three. For amino functionalized MWCNT-PA12 composites adding 1 wt.% BCA electrical percolation was reached at only 0.37 wt.% MWCNTs compared to 1.0 wt.% without BCA. With the help of a model reaction, the covalent attachment of the BCA to the MWCNTs could be shown by thermogravimetric analysis (TGA) and via fluorescence spectroscopy. Model compounds were applied containing either only the oxazoline or the benzoxazinone group to show that the better electrical properties in the PA12-MWCNT composites were a result of a covalent bond between the polymer and the nanotube which only takes place when the BCA was used. In addition, significantly higher electrical conductivity values were obtained by the addition of BCA as well with amino functionalized as with nonmodified commercial MWCNTs. This surprising result was attributed to the significant hydroxy group content on the surface of those commercial MWCNTs. © 2013 Elsevier Ltd. All rights reserved

Electrical, mechanical, and glass transition behavior of polycarbonate-based nanocomposites with different multi-walled carbon nanotubes

Five commercially available multi-walled carbon nanotubes (MWNTs), with different characteristics, were melt mixed with polycarbonate (PC) in a twin-screw micro compounder to obtain nanocomposites containing 0.25-3.0 wt.% MWNT. The electrical properties of the composites were assessed using bulk electrical conductivity measurements, the mechanical properties of the composites were evaluated using tensile tests and dynamic mechanical analysis (DMA), and the thermal properties of the composites were investigated using differential scanning calorimetry (DSC). Electrical percolation thresholds (pcs) were observed between 0.28 wt.% and 0.60 wt.%, which are comparable with other well-dispersed melt mixed materials. Based on measurements of diameter and length distributions of unprocessed tubes it was found that nanotubes with high aspect ratios exhibited lower pcs, although one sample did show higher pc than expected (based on aspect ratio) which was attributed to poorer dispersion achieved during mixing. The stress-strain behavior of the composites is only slightly altered with CNT addition; however, the strain at break is decreased even at low loadings. DMA tests suggest the formation of a combined polymer-CNT continuous network evidenced by measurable storage moduli at temperatures above the glass transition temperature (T g), consistent with a mild reinforcement effect. The composites showed lower glass transition temperatures than that of pure PC. Lowering of the height of the tanδ peak from DMA and reductions in the heat capacity change at the glass transition from DSC indicate that MWNTs reduced the amount of polymer material that participates in the glass transition of the composites, consistent with immobilization of polymer at the nanotube interface. © 2011 Elsevier Ltd. All rights reserved

XCR1 expression distinguishes human conventional dendritic cell type 1 with full effector functions from their immediate precursors

Dendritic cells (DCs) are major regulators of innate and adaptive immune responses. DCs can be classified into plasmacytoid DCs and conventional DCs (cDCs) type 1 and 2. Murine and human cDC1 share the mRNA expression of XCR1. Murine studies indicated a specific role of the XCR1-XCL1 axis in the induction of immune responses. Here, we describe that human cDC1 can be distinguished into XCR1$^{-}$ and XCR1$^{+}$ cDC1 in lymphoid as well as nonlymphoid tissues. Steady-state XCR1$^{+}$ cDC1 display a preactivated phenotype compared to XCR1$^{-}$ cDC1. Upon stimulation, XCR1$^{+}$ cDC1, but not XCR1$^{-}$ cDC1, secreted high levels of inflammatory cytokines as well as chemokines. This was associated with enhanced activation of NK cells mediated by XCR1$^{+}$ cDC1. Moreover, XCR1$^{+}$ cDC1 excelled in inhibiting replication of Influenza A virus. Further, under DC differentiation conditions, XCR1$^{-}$ cDC1 developed into XCR1$^{+}$ cDC1. After acquisition of XCR1 expression, XCR1$^{-}$ cDC1 secreted comparable level of inflammatory cytokines. Thus, XCR1 is a marker of terminally differentiated cDC1 that licenses the antiviral effector functions of human cDC1, while XCR1$^{-}$ cDC1 seem to represent a late immediate precursor of cDC1

A single-label phenylpyrrolocytidine provides a molecular beacon-like response reporting HIV-1 RT RNase H activity

6-Phenylpyrrolocytidine (PhpC), a structurally conservative and highly fluorescent cytidine analog, was incorporated into oligoribonucleotides. The PhpC-containing RNA formed native-like duplex structures with complementary DNA or RNA. The PhpC-modification was found to act as a sensitive reporter group being non-disruptive to structure and the enzymatic activity of RNase H. A RNA/DNA hybrid possessing a single PhpC insert was an excellent substrate for HIV-1 RT Ribonuclease H and rapidly reported cleavage of the RNA strand with a 14-fold increase in fluorescence intensity. The PhpC-based assay for RNase H was superior to the traditional molecular beacon approach in terms of responsiveness, rapidity and ease (single label versus dual). Furthermore, the PhpC-based assay is amenable to high-throughput microplate assay format and may form the basis for a new screen for inhibitors of HIV-RT RNase H

Electrical and thermal properties of polyamide 12 composites with hybrid fillers systems of multiwalled carbon nanotubes and carbon black

International audienceHybrid filler systems of multiwalled carbon nanotubes (MWCNTs) and carbon black (CB) were incorporated into two types of polyamide 12 (PA12) using small-scale melt mixing in order to identify potential synergistic effects on the interaction of these two electrical conductive fillers. Although no synergistic effects were observed regarding the electrical percolation threshold, at loadings well above the percolation threshold higher volume conductivities were obtained for samples containing both, MWCNT and CB, as compared to single fillers. This effect was more pronounced when using a higher viscous PA12 matrix. The formation of a co-supporting network can be assumed. The combined use of CB and MWCNTs improved the macrodispersion of MWCNT agglomerates, which can be assigned as a synergistic effect. DSC measurements indicated an effect of the nanofiller on crystallisation temperatures of PA12; however this was independent of the kind or amount of the carbon nanofiller

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements