Wigner Surmise for Domain Systems

In random matrix theory the spacing distribution functions p ⁽ⁿ⁾(s) are well fitted by the Wigner surmise and its generalizations. In this approximation the spacing functions are completely described by the behavior of the exact functions in the limits s→0 and s→∞. Most non equilibrium systems do not have analytical solutions for the spacing distribution and correlation functions. Because of that, we explore the possibility to use the Wigner surmise approximation in these systems. We found that this approximation provides a first approach to the statistical behavior of complex systems, in particular we use it to find an analytical approximation to the nearest neighbor distribution of the annihilation random walk.     

Features,Questions and Future Challenges in Layered Silicates Clay Nanocomposites with Semicrystalline Polymer Matrices

The present features review article discusses the crystallisation of the polymer matrix when containing silicate layers. The accent is put on nylons (polyamides) and poly(ethylene oxide) as typical hydrophilic polymers and, poly(propylene) from the hydrophobic group. The effects of the clay, either intercalated or exfoliated, on the crystallisation behaviour of the matrix are highlighted. In addition, the crucial aspects of the semicrystalline morphology of the matrix in the presence of the clay platelets are also debated. The overall crystallisation rate is reported to slow down for most of the crystallisable polymer matrices on account of a retarding growth effect exerted by the clay platelets. As far as the location of the exfoliated clay platelets in the polymer matrix is concerned, they are assumed to be rejected from the crystalline phase in the interspherulitic space.

     

The effect of canopy roughness density on the constitutive components of the dispersive stresses

How to represent the effects of variable canopy morphology on turbulence remains a fundamental challenge yet to be confronted. Planar averaging over some minimal area can be applied to average-out this sort of spatial variability in the time-averaged mean momentum balance. Because of the multiply connected air-spaces, spatial averaging gives rise to covariance or dispersive stress terms that are produced by the spatial correlations of the time-averaged quantities. These terms are “unclosed” and require parameterization, which to date remains lacking due to the absence of data. Here, flume experiments were conducted to quantify the magnitude and sign of the dispersive stresses for a cylindric canopy where the rod density was varied but the individual rod dimensions (rod height h _c and rod diameter d _r) remained the same. Quadrant analysis was used to explore the genesis of their spatial coherency inside the canopy for a wide range of rod densities. When compared to the conventional turbulent stresses, these dispersive stresses can be significant in the lowest layers of sparse canopies. For dense canopies, the dispersive terms remain negligible when compared to the conventional momentum fluxes at all the canopy levels consistent with previous experiments in vegetated and urban canopies. It was also shown that the spatial locations contributing most to the dispersive terms were in the immediate vicinity downstream of the rods. In the deeper layers of sparse canopies, these positions contributed large and negative stresses, but in the upper levels of the canopy, they contributed large but positive stresses. Because the longitudinal velocity spatial perturbation behind the rods is negative, the switch in sign in these stresses was connected with the sign of the vertical velocity spatial perturbation Simplified scaling arguments, using a reduced mean continuity equation and the vertical mean momentum balance for the flow field near the rods, offer clues as to why in much of the lower canopy levels (about 0.75 h _c ) while in the upper canopy levels.     

Polycaprolactone-b-Poly(ethylene oxide) Biocompatible Micelles as Drug Delivery Nanocarriers: Dynamic Light Scattering and Fluorescence Experiments

Summary: Dynamic light scattering (DLS) and fluorescence experiments were carried out to study PCL₄₄-b-PEO₁₁₄ biocompatible micelles used as nanocarriers in drug delivery. Micelles prepared by a simple procedure (THF removal under nitrogen flow) exhibited a narrow size distribution with an average diameter of 100 nm. For micelles containing a hydrophobic model compound (pyrene) within the PCL core, a smaller average micellar size of 80 nm was observed, with a simultaneous broadening in the size distribution profile. In parallel to DLS results, fluorescence experiments showed evidence of pyrene encapsulation, and that the onset of the micellization process occurs at approximately 10/90 (v/v) THF/water mixtures in the case of PCL₄₄-b-PEO₁₁₄ polymer.     

Effect of graphene flake size on functionalisation: quantifying reaction extent and imaging locus with single Pt atom tags

Here, the locus of functionalisation on graphene-related materials and the progress of the reaction is shown to depend strongly on the starting feedstock. Five characteristically different graphite sources were exfoliated and functionalized using a non-destructive chemical reduction method. These archetypical examples were compared via a model reaction, grafting dodecyl addends, evaluated with TGA-MS, XPS and Raman data. A general increase in grafting ratio (ranging from 1.1 wt% up to 25 wt%) and an improvement in grafting stoichiometry (C/R) were observed as flake radius decreased. Raman spectrum imaging of the functionalised natural flake graphite identified that grafting is directed towards flake edges. This behaviour was further corroborated, at atomistic resolution, by functionalising the graphene layers with bipyridine groups able to complex single platinum atoms. The distribution of these groups was then directly imaged using aberration-corrected HAADF-STEM. Platinum atoms were found to be homogeneously distributed across smaller graphenes; in contrast, a more heterogeneous distribution, with a predominance of edge grafting was observed for larger graphites. These observations show that grafting is directed towards flake edges, but not necessary at edge sites; the mechanism is attributed to the relative inaccessibility of the inner basal plane to reactive moieties, resulting in kinetically driven grafting nearer flake edges. This phenomenology may be relevant to a wide range of reactions on graphenes and other 2d materials.

The flake size and morphology of the starting material strongly effect the degree and location of grafting when using reductive functionalisation.     

77Se, 13C and 1H NMR investigations on ortho-carbonyl benzeneselenenyl derivatives

o-Carbonyl benzeneselenenyl compounds with COCH₃, CHO and COOCH₃ as carbonyl functions and SeCl, SeBr, SeSCN, SeSeCN, SeCN and SeCH₃ as selenium-containing groups, have been studied by ¹H, ¹³C and ⁷⁷Se NMR spectroscopy. The IR CO stretching frequencies of these compounds are also reported. If the SeCH₃ derivatives are excluded, the compounds mainly adopt a planar ‘cis’ conformation, due to an interaction between the CO group and the selenium atom. The range of over 800 ppm for the observed ⁷⁷Se chemical shifts makes ⁷⁷Se NMR spectroscopy a powerful tool for physical organic chemists.     

A Fluoroponytailed NHC–Silver Complex Formed from Vinylimidazolium/AgNO3 under Aqueous–Ammoniacal Conditions

3-(1H,1H,2H,2H-Perfluorooctyl)-1-vinylimidazolium chloride [2126844–17–3], a strong fluorosurfactant with remarkably high solubility in water, was expediently converted into the respective doubly NHC-complexed silver salt with nitrate as counter ion in quantitative yield. Due to its vinyl substituents, [bis(3-(1H,1H,2H,2H-perfluorooctyl)-1-vinylimidazol-2-ylidene)silver(I)] nitrate, Ag(FNHC)₂NO₃, represents a polymerizable N-heterocyclic carbene transfer reagent, thus potentially offering simple and robust access to coordination polymers with crosslinking metal bridges. The compound was characterized by infrared and NMR spectroscopy, mass spectrometry as well as elemental analysis, and supplemented by X-ray single-crystal structure determination. It crystallizes in the monoclinic crystal system in the space group P2₁/c. With 173.3°, the geometry of the Ag-carbene bridge deviates slightly from linearity. The disordered perfluoroalkyl side chains exhibit a helical conformation.     

FASSVid: Fast and Accurate Semantic Segmentation for Video Sequences

Most of the methods for real-time semantic segmentation do not take into account temporal information when working with video sequences. This is counter-intuitive in real-world scenarios where the main application of such methods is, precisely, being able to process frame sequences as quickly and accurately as possible. In this paper, we address this problem by exploiting the temporal information provided by previous frames of the video stream. Our method leverages a previous input frame as well as the previous output of the network to enhance the prediction accuracy of the current input frame. We develop a module that obtains feature maps rich in change information. Additionally, we incorporate the previous output of the network into all the decoder stages as a way of increasing the attention given to relevant features. Finally, to properly train and evaluate our methods, we introduce CityscapesVid, a dataset specifically designed to benchmark semantic video segmentation networks. Our proposed network, entitled FASSVid improves the mIoU accuracy performance over a standard non-sequential baseline model. Moreover, FASSVid obtains state-of-the-art inference speed and competitive mIoU results compared to other state-of-the-art lightweight networks, with significantly lower number of computations. Specifically, we obtain 71% of mIoU in our CityscapesVid dataset, running at 114.9 FPS on a single NVIDIA GTX 1080Ti and 31 FPS on the NVIDIA Jetson Nano embedded board with images of size 1024×2048 and 512×1024, respectively.     

An Efficient Synthesis of Porphyrins with Different meso Substituents that Avoids Scrambling in Aqueous Media

We have developed new conditions that afford regioisomerically pure trans‐A₂B₂‐, A₃B‐, and trans‐AB₂C‐porphyrins bearing aryl and arylethynyl substituents. The porphyrins were prepared by the acid‐catalyzed condensation of dipyrromethanes with aldehydes followed by oxidation with p‐chloranil or 2,3‐dichloro‐5,6‐dicyano‐1,4‐benzoquinone (DDQ). Optimal conditions for the condensation were identified after examining various reaction parameters such as solvent composition, acid concentration, and reaction time. The conditions identified (for aromatic aldehydes: EtOH/H₂O 4:1, [DPM]=4 mM , [aldehyde]=4 mM , [HCl]=38 mM , 16 h; for arylethynyl aldehydes: THF/H₂O 2:1, [DPM]=13 mM , [aldehyde]=13 mM , [HCl]=150 mM , 3 h) resulted in the formation of porphyrins in yields of 9–38 % without detectable scrambling. This synthesis is compatible with diverse functionalities such as ester or nitrile. In total, 20 new trans‐A₂B₂‐, A₃B‐, and trans‐AB₂C‐porphyrins were prepared. The scope and limitations of the two sets of reaction conditions have been explored. The methodological advantage of this approach is its straightforward access to building blocks and the formation of the porphyrin core in higher yields than by any other methodology and by using environmentally benign and nonhazardous chemicals.