Relations between the aspect ratio of carbon nanotubes and the formation of percolation networks in biodegradable polylactide/carbon nanotube composites

The biodegradable polylactide composites containing carbon nanotubes (CNTs) with high aspect ratio (HAR) and low aspect ratio (LAR) were prepared by melt mixing. The physical properties of those two systems were characterized in terms of rheology, conductivity, and mechanical properties for establishing preliminary structure–property relations. Several viscoelastic models were then used to further describe the relations between aspect ratio and percolation network of CNTs. The results show that these two CNTs present different structural characteristics in the polylactide (PLA) matrix during melt mixing: the LAR CNTs are far stiffer than the HAR CNTs. At low loading levels, the former is dispersed as bent fibers or their small bundles, whereas the latter is dispersed as self‐entangled flocs, presenting far larger hydrodynamic radius than the former. At high loading levels, both are dispersed as flocs due to strong tube–tube interactions. However, the two CNTs show approximate average floc size and mesh size because they present same rigid length and effective aspect ratio. At identical loadings, therefore, the HAR CNTs have more total number of flocs than that of the LAR CNTs, forming network with more compact structure and imparting higher contributions to properties of the composites as a result. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 479–489, 2010     

Highly Efficient System for Reduction of Carboxylic Acids and Their Derivatives to Alcohols by HfCl4/KBH4

HfCl₄/KBH₄ was found to be a facile, efficient, convenient, and chemoselective system for the reduction of carboxylic acids and their derivatives to the corresponding alcohols under mild conditions. HfCl₄/NaBH₄ was also utilized to reduce the same carboxylic acids and their derivatives, and it was found that the reducing ability of HfCl₄/NaBH₄ was similar to that of HfCl₄/KBH₄. The action of HfCl₄/KBH₄ on other types of substrates, such as benzyl chloride, peracid, epoxide, ketone, amide, imine, pyridine-N-oxide, and nitrile, was investigated, too. In addition, some competitive reductions of styrene oxide in the presence of carboxylic acid, carboxylic ester, nitrile, and amide were achieved.     

Properties of the Moreau-Yosida regularization of a piecewise C2 convex function

Received December 14, 1995 / Revised version received May 7, 1998 Published online October 21, 1998     

Rheology of isothermally crystallized poly(butylene terephthalate) nanocomposites with clay loadings under the percolation threshold

This article describes the structural evolution of clay in poly(butylene terephthalate) nanocomposites (PCNs) with clay loadings lower than the percolation threshold during the isothermal crystallization process. The study of the structure and rheological properties has revealed that the intercalation and detachment levels of clay are enhanced in samples crystallized at a high temperature (210 °C), in contrast to those of the original PCN, and this results in the formation of a rheological percolation network. However, for PCNs crystallized at a low temperature (190 °C), the further structural evolution of the tactoids is very small. All the experimental results indicate that the morphologies of clay can further evolve during the crystallization process, but the evolution level is strongly dependent on the crystallization temperature. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 229–238, 2007     

Chemically modified glassy carbon electrode for electrochemical sensing paracetamol in acidic solution

Different organic molecules were covalently grafted on glassy carbon electrodes (GCEs) by an electrochemical reduction or potentiostatic process of several in situ-generated diazonium cations in acidic aqueous solution containing NaNO₂. The cyclic voltammetry implemented in 0.1?M KCl aqueous solution containing 5?mM Fe(CN) ₆ ^3? or Ru(NH₃) ₆ ³⁺ confirmed the blocking properties of the modified GCEs. The electrochemical impedance spectroscopy (EIS) performed in 0.1?M KCl aqueous solution containing 5?mM Fe(CN) ₆ ^3?/4? was used to measure the surface coverage of the modifiers on GCE; the results showed that the modified layers on GCEs are very compact. The linear sweep voltammetry (LSV) was employed to investigate the electrochemical sensing properties of the bare and modified GCEs toward paracetamol (PCT) in sulfuric acid solution of pH?1.02, and the corresponding calibration plots were obtained, respectively. The results indicated there is an oxidation peak of PCT in the linear sweep voltammograms on the bare and modified GCEs with the active terminal groups such as ?OPO₃H₂, ?SO₃H, ?COOH, and so on, but do not appear on GCEs modified with the inert terminal groups such as ?NO₂ and ?Br. These imply that the GCEs modified with the active terminal groups display an electrochemical behavior like bulk GCE; however, those with the inert terminal groups present an electrochemical behavior like microelectrode. The varying electrochemical sensitivity of all the electrodes toward PCT was explained according to electronegativity and pK _a of the terminal groups of the modifiers on the electrodes and hydrogen bond between the modifiers and PCT. Apparent standard rate constants of PCT oxidation reaction on the bare and modified GCEs were obtained from the Laviron’s approach.     

An implementable proximal point algorithmic framework for nuclear norm minimization

The nuclear norm minimization problem is to find a matrix with the minimum nuclear norm subject to linear and second order cone constraints. Such a problem often arises from the convex relaxation of a rank minimization problem with noisy data, and arises in many fields of engineering and science. In this paper, we study inexact proximal point algorithms in the primal, dual and primal-dual forms for solving the nuclear norm minimization with linear equality and second order cone constraints. We design efficient implementations of these algorithms and present comprehensive convergence results. In particular, we investigate the performance of our proposed algorithms in which the inner sub-problems are approximately solved by the gradient projection method or the accelerated proximal gradient method. Our numerical results for solving randomly generated matrix completion problems and real matrix completion problems show that our algorithms perform favorably in comparison to several recently proposed state-of-the-art algorithms. Interestingly, our proposed algorithms are connected with other algorithms that have been studied in the literature.     

Study on rheological behaviour of poly(butylene terephthalate)/montmorillonite nanocomposites

Poly(butylene terephthalate)/montmorillonite composites (PBT/MMT) were prepared by melt intercalation and then investigated using X-ray diffractometer (XRD) and transmission electron microscope (TEM) as well as parallel plate rheometer. It was found that the composites had various phase morphologies with nanoscales and distinct behaviours of a percolation network structure under certain conditions. The linear viscoelastic region of the composites is much narrower than that for PBT matrix, the percolation threshold of the composites is near 3 wt.%, and the percolation network structure is not stable under a shear as well as in a quiescent annealing process. Moreover, PBT/MMT presents the nature of temperature independence of G′ versus G″ whether the internal percolated tactoids network formed or not. The magnitudes of the stress overshoots observed in the reverse flow experiments were strongly dependent on the rest time, which could be inferred that the ruptured network is reorganized under the quiescent annealing process. Furthermore, PBT/MMT shows a strain-scaling stress response to the startup of steady shear, indicating that the formation of the liquid crystalline-like phase structure in the nanocomposites may be the major drive force for the reorganization of the internal network.     

腙型双冠醚对碱金属的配位性能

本文报道了五个腙型双冠醚的合成。电导测定结果表明含苯并-15-冠-5单元的双冠醚与四苯基硼酸钾、铷、铯,含苯并-18-冠-6单元的双冠醚与四苯基硼酸铯生成2:1夹心型配合物(冠醚单元:金属离子)。并用这些双冠醚的氯仿溶液萃取苦味酸碱金属盐水溶液,测定了萃取百分率和计算了萃取平衡常数,结果表明腙型双冠醚的萃取能力及选择性优于相应的单冠醚。     

Nonisothermal cold crystallization behavior and kinetics of polylactide/clay nanocomposites

The nonisothermal cold crystallization behavior of intercalated polylactide (PLA)/clay nanocomposites (PLACNs) was studied using differential scanning calorimetry, polarized optical microscope, X‐ray diffractometer, dynamic mechanical thermal analysis, and Fourier transform infrared spectrometer. The results show that both the cold crystallization temperature (T_cc) and melting point (T_m) of PLA matrix decreases monotonously with increasing of clay loadings, accompanied by the decreasing degree of crystallinity (X_c%) at the low heating rates (≤5 °C/min). However, the X_c% of PLACNs presents a remarkable increase at the high heating rate of 10 °C/min in contrast to that of neat PLA. The crystallization kinetics was then analyzed by the Avrami, Jezioney, Ozawa, Mo, Kissinger and Lauritzen–Hoffman kinetic models. It can be concluded that at the low heating rate, the cold crystallization of both the neat PLA and nanocomposites proceeds by regime III kinetics. The nucleation effect of clay promote the crystallization to some extent, while the impeding effect of clay results in the decrease of crystallization rate with increasing of clay loadings. At the high heating rate of 10 °C/min, crystallization proceeds mainly by regime II kinetics. Thus, the formation of much more incomplete crystals in the PLACNs with high clay loadings due to the dominant multiple nucleations mechanism in regime II, may have primary contribution to the lower crystallization kinetics, also as a result to the higher degree of crystallinity and lower melting point in contrast to that of neat PLA. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1100–1113, 2007