Search

One of the challenges for modelling of polymerization kinetics is the detailed description of the molecular build-up of both linear and non-linear chains, specifically those with many grafts and crosslinks. Such grafted/crosslinked (co)polymers are relevant as emulsifiers, surface-modifying agents, coating materials, and compatibilizers. In the present work, we put forward a coupled matrix-based Monte Carlo (CMMC) concept to be successful in this respect, avoiding computational stiffness. The CMMC concept is illustrated for single phase grafting of polybutadiene (PB) with styrene (St) at 343 K by limiting the St conversion to 30%. Considering literature kinetic parameters, a benchmark for average characteristics as obtainable by deterministic method of moments simulations is first presented to then gradually extend the level of modelling output addressing (i) conventional grafting performance indicators (e.g. the grafting yield and mass ratio); (ii) univariate chain length distributions for all macrospecies types (polystyrene, PB, PB with only T grafts, PB with at least one H graft, etc.); (iii) bivariate St–butadiene distributions showing a compositional drift, due to the kinetic tendency to attack longer chains if they are sufficiently present and the competition between grafting and crosslinking; (iv) the explicit molecular build-up of individual molecules predicting the positioning of the St–Bd and St–St connectivity points and the chain formation history. It is demonstrated that the CMMC tool allows the simulation of the contribution and structure of molecules that are hard to access purely experimentally, so that in the long run, novel structure–property relationships are within reach. It is also showcased that consideration of elementary reactions is highly recommended and that even at 343 K, thermal self-initiation with St and related chain transfer reactions matter for a full appreciation of molecular variations. The current work also opens the pathway to identifying pragmatic equations for the experimentalist and online control benefiting from a detailed CMMC solution under any desired conditions.

The quantification of dispersity is an indispensable part of characterizing polymers. In this work, general dispersity equations for “living” polymers obtained by nitroxide-mediated polymerization ...

Cost-efficient deterministic method of moments solvers, as widely used to calculate average characteristics of chemical processes driven by population variations (e.g., average chain lengths), can be a posteriori extended with approximated solutions delivering distributed properties (e.g., chain length distributions). However, these solutions are rarely verified, specifically for complex systems with many population members and strong coupling, as is the case for industrially relevant free-radical-induced grafting (FRIG) toward graft copolymer (GC) synthesis with monomer unit dependent reactions. FRIG, as studied in the present work with polybutadiene at low styrene conversions, is an important chemical process, for example, the production of compatibilizers and high-impact materials. Deterministic model validation is uniquely performed by benchmarking the low to medium molar mass (MM) results (29 topologies) in the log-molar mass distribution with detailed matrix-based kinetic Monte Carlo simulation output, inherently capable of mapping distributions. The GC product is identified to be a heterogeneous mixture in MM, chemical composition, and molecular topology at any styrene conversion. The molecular structural evolution during GC synthesis is further theoretically related to both one-dimensional size-exclusion chromatography (1D-SEC) and two-dimensional liquid chromatography (2D-LC) analysis. It is shown that conventional SEC-even in the absence of broadening-is insufficient for GC separation, mainly due to the unavoidable coelution of topologically different GC species. In any case, the parallel running of advanced modeling tools allows for detailed molecular interpretation.

It is still a big challenge to integrate intrinsic modification and self-standing structure construction into graphene via a rapid and facile approach. Herein, we report a sampling and quick in situ chain reduction technology to prepare nitrogen-doped porous graphene frameworks. In this process, easily dispersible graphene serves as an effective receptor for microwave absorption and initiates the reduction of graphene oxide; the reduced part of GO becomes the microwave-absorbing receptor to cause a “chain reduction”, which allows the NPGFs to form in 3–5 s. The as-prepared graphene frameworks maintain the long-range layered structure of the precursor film, while the holey graphene sheets loaded with nanoparticles form a networked structure in the short range. Furthermore, the hierarchical framework with a high specific surface area shows interesting applications in supercapacitors and oil–water separation. The assembled compressible supercapacitor delivered a volumetric energy density of 12.3 mW h cm−3 at a power density of 0.42 W cm−3 and exhibits stable performance under compression. In addition, the graphene frameworks exhibited reusability with mechanical squeezing and extremely high volumetric absorption capacity of 100–243 g cm−3 for different oils and organic solvents, which is much higher than that in other reports (the performance index is ∼1 g cm−3). This study provides a new way to construct a multi-functional graphene self-standing structure, which presents potential application value in catalysis, energy storage, and environmental protection.

The past decades have witnessed an increasing interest in developing advanced polymerization techniques subjected to external fields. Various physical modulations, such as temperature, light, electricity, magnetic field, ultrasound, and microwave irradiation, are noninvasive means, having superb but distinct abilities to regulate polymerizations in terms of process intensification and spatial and temporal controls. Gas as an emerging regulator plays a distinctive role in controlling polymerization and resembles a physical regulator in some cases. This review provides a systematic overview of seven types of external-field-regulated polymerizations, ranging from chain-growth to step-growth polymerization. A detailed account of the relevant mechanism and kinetics is provided to better understand the role of each external field in polymerization. In addition, given the crucial role of modeling and simulation in mechanisms and kinetics investigation, an overview of model construction and typical numerical methods used in this field as well as highlights of the interaction between experiment and simulation toward kinetics in the existing systems are given. At the end, limitations and future perspectives for this field are critically discussed. This state-of-the-art research progress not only provides the fundamental principles underlying external-field-regulated polymerizations but also stimulates new development of advanced polymerization methods.

A challenge for the design of nonlinear polymerization is the full appreciation of the impact of side reactions, demanding the development of modeling techniques to determine the associated kinetic parameters while using the most important experimental responses. Here the combination of computationally inexpensive method of moments (MoM) kinetic simulations and dedicated principal component analysis (PCA) is put forward as a promising strategy to be successful in this respect. Focus is on (radical) vinyl grafting of chains containing unsaturations, selecting styrene (St) as monomer and polybutadiene (PB) as backbone, and low St conversions accounting for diffusional limitations on termination. It is highlighted that the less studied macropropagation cannot be directly ignored and a combined set of experimental responses related to free polystyrene and grafted copolymer (GC) average product properties is recommended for kinetic parameter estimation. This is supported by regression analysis considering in silico generated experimental data compensated for random noise and considering a validated end-chain approximation.

Bioinspired solar-chemical energy conversion has long been challenged by chemists with searching for effective synthetic photocatalysts. Herein we show that powered by visible light, molecular photocatalysts developed by integrating zinc porphyrin (ZnP) with a quadruply-bonded Mo2 unit enables hydrogen evolution from water in high yields up to 200 mmol g−1 with TON = 640. Remarkably, this unimolecular Mo2-ZnP device catalyzes efficiently photoreduction of nicotinamide adenine dinucleotide (NAD+) to NADH, the reducing equivalent for enzymatic reduction of CO2 in natural photosynthesis. Evidently, this Mo2-ZnP photosystem has the capability of accumulating up to four electrons and two protons for two-electron reduction. Mechanistic investigations reveal that the photoreduced zinc phlorin and chlorin-phlorin anions are the catalytic intermediates for hydride transfer to the substrates, which mimics functionally the hydroquinone state of flavin adenine dinucleotide in photosystem I. The understandings gained in this study are valuable for engineering photocatalysts for solar fuel production.

Four thienylene (C4H2S) bridged Mo2 dimers, [Mo2(DAniF)3]2(μ-OOCC4H2SCOO) (DAniF = N,N′-di(p-anisyl)formamidinate), [Mo2(DAniF)3]2(μ-N(H)SCC4H2SCN(H)S), [Mo2(DAniF)3]2(μ-OSCC4H2SCSO) and [Mo2(DAniF)3]2(μ-SSCC4H2SCSS), have been synthesized and studied in terms of electronic coupling. The subtle structural differences between these compounds vary largely the extent of electron delocalization; consequently, a systematic transition from Class II to Class III via Class II–III is achieved, which is probed using spectral parameters of intervalence charge transfer (IVCT) absorption (band energy, intensity and shape) for the mixed-valence complexes. Significantly, the species in Class II–III displays a low energy, half cut-off and solvent-dependent IVCT band, while a high energy, less asymmetrical IVCT band is observed for the complex in Class III. These results give fresh and detailed understanding of the system transition.

Thermal electron transfer through hydrogen bonds remains largely unexplored. Here we report the study of electron transfer through amide-amide hydrogen bonded interfaces in mixed-valence complexes with covalently bonded Mo2 units as the electron donor and acceptor. The rate constants for electron transfer through the dual hydrogen bonds across a distance of 12.5 Å are on the order of ∼ 1010 s−1, as determined by optical analysis based on Marcus–Hush theory and simulation of ν(NH) vibrational band broadening, with the electron transfer efficiencies comparable to that of π conjugated bridges. This work demonstrates that electron transfer across a hydrogen bond may proceed via the known proton-coupled pathway, as well as an overlooked proton-uncoupled pathway that does not involve proton transfer. A mechanistic switch between the two pathways can be achieved by manipulation of the strengths of electronic coupling and hydrogen bonding. The knowledge of the non-proton coupled pathway has shed light on charge and energy transport in biological systems., Thermal electron transfer across hydrogen bond remains largely unexplored. Here the authors demonstrate that electron self-exchange through hydrogen bonds is highly efficient and can proceed either via the known proton-coupled pathway or an overlooked proton-uncoupled pathway

SummaryOur knowledge on the chemistry across molecular and supramolecular levels is insufficient. Here, we show that inclusion of a guest molecule through quadrupolar interactions leads to a chemical transformation of the molecular system. With covalently bonded dimolybdenum complex units as the electron donor (D) and acceptor (A) and a thienylene group (C4H2S) as the bridge (B), the mixed-valence D-B-A complexes are shaped with clefts in the middle of the molecule. The D-A electron transfer is optically studied under the Marcus-Hush theoretic framework. It is found that in aromatic solvents, the electronic coupling matrix elements (Hab) and electron transfer rate constants (ket) are dramatically reduced. DFT calculations and electrostatic potential map analysis indicate that in the solution, an aromatic molecule is encapsulated in the cleft of the D-B-A array; quadrupole-quadrupole interaction between the guest molecule and the C4H2S group evokes charge redistribution, which increases the HOMO-LUMO energy gap, intervening in the through-bond electron transfer. These results demonstrate that a supramolecular system is unified underlying the characteristics of the assembled molecules through constitutional, electronic and energetic complementarities, thus, sharing the general principles developed in molecular chemistry. This study illustrates the role of site-specific interactions of the aromatic residues in biological systems in governing the charge transfer processes.

In this paper, a wideband low-noise amplifier (LNA) with an improved feedforward distortion cancellation technique is proposed for multi-band and multi-standard RF communication applications. Based on the conventional wideband balun-LNA, the auxiliary path is connected in parallel with the main path to suppress the nonlinearity. The novel topology structure helps implement the scaling factor on the chip without pay extra price. Experimental results have demonstrated a bandwidth of 3.7 GHz bandwidth, a voltage gain of 15 dB, and a noise figure below 6.5 dB in TSMC 65 nm CMOS technology with 1.2 V voltage supply. Moreover, a 3.5 to 7 dB improvement in IIP3 is achieved within the bandwidth in comparison with the conventional LNA.

Four thienylene (C

We hereby report on a pioneering and inspiring solid oxide cell which, assisted by natural gas, utilizes a bifunctional electrolysis cell configuration to effectively consume CO

article i nfo Available online 18 January 2012 Fluorinated diamond-like carbon (F-DLC) films were deposited on Ti6Al4V substrates by radio frequency plasma enhanced chemical vapor deposition (rf PECVD) technique using a mixture of methane (CH4) and tet- rafluoromethane (CF4) gasses. A 100 nm Si interlayer was coated in advance by physical vapor deposition (PVD) to improve the adhesion between F-DLC and Ti alloy. A 200 nm TiN-coated specimen with the same Ti6Al4V substrate was also built by PVD as a benchmark. The structure and surface properties of F-DLC coat- ings, prepared with various fluorine flow ratios, were investigated by using X-ray photoelectron spectrosco- py, scanning electron microscopy, atomic force microscopy, liquid drop goniometry, and electrochemical corrosion tests. The blood compatibility was evaluated by measuring haemolysis ratio and platelet-covered area in vitro. The films' spectroscopic results show that the CFx group and fluorine atomic concentration in- crease as CF4 flow ratio is promoted in the mixture. The surface energy is reduced due to the increased fluo- rine content. The modified surfaces are characterized by higher hydrophobicity, lower thrombogenicity, and better corrosive protection than the virgin and TiN ones.

Fluorinated amorphous diamond-like carbon (F-DLC) films were deposited on Ti6Al4V substrates by radio frequency plasma enhanced chemical vapor deposition (rf PECVD) technique using a mixture of methane (CH4) and tetrafluoromethane (CF4) gasses. A 100 nm Si interlayer was coated in advance by physical vapor deposition (PVD) process to improve the adhesion between F-DLC and Ti alloy. The structure and surface properties of F-DLC coatings, prepared by various fluorine flow ratios, were investigated by using X-ray diffraction spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and scanning electron microscopy. The mechanical properties were evaluated by nano-indentation, and the adhesion, by micro-scratch. The results showed that a moderate incorporation of the fluorine content in the DLC films can still maintain acceptable mechanical properties, which, on the other hand, obtains remarkable benefits of blood compatibility and anti-corrosion.

Signal transduction pathways play extremely important roles in various processes of cell life including metabolism, growth, proliferation, stress, development and apoptosis. Disruption of these pathways may affect normal cell growth and development, and even gives rise to tumors. MicroRNAs (miRNAs) are a newly discovered class of non-coding small RNAs (approximately 22 nucleotides in length) in eukaryotes that negatively regulate gene expression at the post-transcriptional level. With a wide range of target genes, they play essential and pervasive roles in diverse biological processes. Aberrant miRNA expression has recently been discovered in tumor development, indicating that miRNAs may be connected with tumorigenesis, possibly through regulating the expression of oncogenes and tumor-suppressing genes. It has also been noticed that many target genes of miRNA can influence tumor associated signal transduction pathways. The significant roles of miRNAs in tumorigenesis indicate that they may become a powerful tool for diagnosis and treatment of human cancers in the future.

We studied the amount of trace elements in the traditional Chinese medicine Wuji Baifeng Wan by cluster analysis, and found that the clustering results of the trace elements in Twelve Wuji Baifeng Wan (the southern preparation) and Wuji Baifeng Wan (the northern preparation) were not in the same group. In this paper, AAS was used for the determination of trace elements in Wuji Baifeng Wan and its preparation. The results showed that there were great differences between Twelve Wuji Baifeng Wan and Wuji Baifeng Wan , while the difference between Chinese Taihe Chicken and Native Chicken was slight. So we drew a conclusion that the great differences between Twelve Wuji Baifeng Wan and Wuji Baifeng Wan were caused by other medicine, not in its preparation.

We theoretically investigate the optical properties of an ultra-thin InN layer embedded in InGaN matrix for light emitters. The peak emission wavelength extends from ultraviolet (374 nm) to green (536 nm) with InN quantum well thickness increasing from 1 monolayer to 2 monolayers, while the overlap of electron-hole wave function remains at a high level (larger than 90%). Increase of In content in InGaN matrix provides a better approach to longer wavelength emission, which only reduces the spontaneous emission rate slightly compared with the case of increasing In content of the conventional InGaN quantum well. Also, the transparency carrier density derived from gain spectrum is of the same order as that in the conventional blue laser diode. Our study provides skillful design on the development of novel structure InN-based light emitting diodes as well as laser diodes.