From orientation disordered to ordered—An ab initio simulation on ammonia borane phase transition within van der Waals corrections

In this work, we report a detailed theoretical investigation of the phase transition of ammonia borane (NH₃BH₃; AB), from a tetragonal I4mm ( ) phase with disordered orientation of hydrogen to an orthorhombic phase with Pmn2₁ ( ) symmetry, as a function of temperature based on Density Functional Theory calculations with semiempirical dispersion potential correction. We define a series of substructures with the NH₃BH₃ moiety always in C_3v symmetry and the partially occupied high temperature state can be described as a continuous transformation between these substructures. To understand the role of the van der Waals corrections to the physical properties, we use the empirical Grimme's dispersion potential correction (PBE‐D2). Both Perdew–Burke–Emzerhof (PBE) and PBE‐D2 functional yield almost the same energy sequence along the transition path. However, PBE‐D2 functional shows obvious advantage in describing the lattice parameters of AB. The rigid rotor harmonic oscillator approximation is used to compute the free energy and the entropies contribution along the transition pathway. With knowledge of free energy surfaces along rotations of the ? [NH₃] and ? [BH₃] groups, complete transformation paths are mapped out. The phase transition is found to follow the sequence of partially occupied tetragonal system (I4mm) of a mixture of states with monoclinic (Cc), (CM) and orthorhombic (Pmn2₁) symmetries to fully occupied quasitetragonal system (the intermediate phase, Pmn2₁) to fully occupied orthorhombic system (Pmn2₁). © 2014 Wiley Periodicals, Inc.     

The stereocomplex formation and phase separation of PLLA/PDLA blends with different optical purities and molecular weights

In this study, the poly(L-lactide)/poly(D-lactide)(PLLA/PDLA) blends with different optical purities of PLLA and various molecular weights of PDLA are prepared by solution mixing, and the stereocomplex formation and phase separation behaviors of these blends are investigated. Results reveal that optical purity and molecular weight do not vary the crystal structure of PLA stereocomplex(sc) and homochiral crystallites(hc). As the optical purity increasing in the blends, the melting temperature of sc(Tsc) and the content of sc(ΔHsc) increased, while the melting temperature of hc(Thm) hardly changes, although the content of hc(ΔHhm) decreased gradually. The Tsc and ΔHsc are also enhanced as the molecular weight of PDLA reduces, and the ΔHhm reduces rapidly even though the Thm does not vary apparently. With lower optical purities of PLLA and higher molecular weights of PDLA, three types of crystals form in the blends, i.e., PLA sc, PLLA hc and PDLA hc. As molecular weight decreases and optical purity enhances, the crystal phase decreases to two(sc and PDLA hc), and one(sc) finally. This investigation indicates that the phase separation behavior between PLLA and PDLA in the PLLA/PDLA blends not only depends on molecular weights, but also relies on the optical purities of polymers.     

Conversion of phenylacetonitrile in supercritical alcohols within a system containing small volume of water

The reaction of phenylacetonitrile in supercritical methanol and ethanol in a system containing a small volume of water was studied. The effects of various operating conditions, such as reaction temperature, reaction time, the mole ratio of phenylacetonitrile/water/methanol or ethanol on the product yield were systematically investigated. The optimal yield of methyl phenylacetate for phenylacetonitrile in supercritical methanol in a system containing a small volume of water was 70 % at 583 K and 2.5 h. The optimal yield of ethyl phenylacetate for phenylacetonitrile in supercritical ethanol with a small volume of water was 80 % at 583 K and 1.0 h. At the same time, a feasible mechanism was proposed for phenylacetonitrile in supercritical methanol and ethanol in a system containing a small volume of water.     

Enhanced Photocatalytic Reduction of CO2 to CO through TiO2 Passivation of InP in Ionic Liquids

A robust and reliable method for improving the photocatalytic performance of InP, which is one of the best known materials for solar photoconversion (i.e., solar cells). In this article, we report substantial improvements (up to 18×) in the photocatalytic yields for CO₂ reduction to CO through the surface passivation of InP with TiO₂ deposited by atomic layer deposition (ALD). Here, the main mechanisms of enhancement are the introduction of catalytically active sites and the formation of a pn‐junction. Photoelectrochemical reactions were carried out in a nonaqueous solution consisting of ionic liquid, 1‐ethyl‐3‐methylimidazolium tetrafluoroborate ([EMIM]BF₄), dissolved in acetonitrile, which enables CO₂ reduction with a Faradaic efficiency of 99 % at an underpotential of +0.78 V. While the photocatalytic yield increases with the addition of the TiO₂ layer, a corresponding drop in the photoluminescence intensity indicates the presence of catalytically active sites, which cause an increase in the electron‐hole pair recombination rate. NMR spectra show that the [EMIM]⁺ ions in solution form an intermediate complex with CO₂^?, thus lowering the energy barrier of this reaction.     

DABCO-based ionic liquids:Green and efficient catalysts with a dual catalytic role for aza-Michael addition

Four recoverable and reusable ionic liquids based on 1,4-diazobicyclo[2.2.2]octane(DABCO) have been synthesized to catalyze the aza-Michael addition of secondary amines to a,b-unsaturated compounds.Among the catalysts tested,[DABCO-PDO][OAc] was found to be most suitable for the reaction of a wide range of cyclic substrates without any solvent at room temperature,and afford the products in good to excellent yields within an appropriate amount of time.The proposed mechanism for the dual activation of the catalyst was supported by experimental results as well as the DFT calculation.In addition,the ionic liquids used can be regenerated and recycled several times without any loss of activity.     

Acid‐Labile Thermoresponsive Copolymers That Combine Fast pH‐Triggered Hydrolysis and High Stability under Neutral Conditions

Biodegradable polymeric materials are intensively used in biomedical applications. Of particular interest for drug‐delivery applications are polymers that are stable at pH 7.4, that is, in the blood stream, but rapidly hydrolyze under acidic conditions, such as those encountered in the endo/lysosome or the tumor microenvironment. However, an increase in the acidic‐degradation rate of acid‐labile groups goes hand in hand with higher instability of the polymer at pH 7.4 or during storage, thus posing an intrinsic limitation on fast degradation under acidic conditions. Herein, we report that a combination of acid‐labile dimethyldioxolane side chains and hydroxyethyl side chains leads to acid‐degradable thermoresponsive polymers that are quickly hydrolyzed under slightly acidic conditions but stable at pH 7.4 or during storage. We ascribe these properties to high hydration of the hydroxy‐containing collapsed polymer globules in conjunction with autocatalytic acceleration of the hydrolysis reactions by the hydroxy groups.     

用三维弹性力学方法研究圆板的稳定问题

用三维弹性力学方法研究任意边界条件圆板的轴对称稳定问题，利用Ｈ变换和Ｓｔｏｃｋｅｓ变换，导出位移函数及其偏导数的一种新型双重极数式，并由数学弹性定理论的基本方程和边界条件建立的特征方程，求得最小临界载荷的精确解，文末以简支圆板为例进行数字计算，结果表明：在弹性失稳范围内，三维弹性力学方法求得的临界载荷略低于经典理论的结果，对于薄板的弹性稳定问题，经典板理论有足够的精度。