Post-Synthetic Enzymatic and Chemical Modifications for Novel Sustainable Polyesters

Because of their biodegradability, compostability, compatibility and flexible structures, biodegradable polymers such as polyhydroxyalkanoates (PHA) are an important class of biopolymers with various industrial and biological uses. PHAs are thermoplastic polyesters with a limited processability due to their low heat resistance. Furthermore, due to their high crystallinity, some PHAs are stiff and brittle. These features result sometimes in very poor mechanical characteristics with low extension at break values which limit the application range of some natural PHAs. Several in vivo approaches for PHA copolymer modifications range from polymer production to enhance PHA-based material performance after synthesis. The methods for enzymatic and chemical polymer modifications are aiming at modifying the structures of the polyesters and thereby their characteristics while retaining the biodegradability. This survey illustrates the efficient use of enzymes and chemicals in post-synthetic PHA modifications, offering insights on these green techniques for modifying and improving polymer performance. Important studies in this sector will be reviewed, as well as chances and obstacles for their stability and hyper-production.

Optimization of the sintering temperature, cooling time and grain size parameters to reduce residual stresses of copper-aluminum functionally graded material using response surface methodology

One of the urgent needs for the medical, aerospace and military industries is to combine materials with heat-resistant as well as flexible structures. To create such a property, a ceramic must be placed next to metal. FGM materials have such a property in terms of thickness. Functionally graded materials (FGM) are examples of materials with different properties in the thickness direction. In the functionally graded materials, different properties can be created, by changing the percent weight of materials in each layer. It is very important to study the number of residual stresses in these materials due to the fact that several materials with different properties are combined with each other. The purpose of this study is to investigate the effect of production parameters on the number of residual stresses in the aluminum-copper FGM part and also to optimize the production process of these materials. The results indicate that the number of residual stresses decreases with increasing the sintering temperature, cooling time of the sample as well as uniformity along the thickness. In the experiments, the maximum residual stress was 171 MPa, which was obtained for a grain size of 100 microns, sintering temperature of 600°C and cooling time of 24 h and the minimum value of pressure residual stress was 120 MPa, which was obtained for grain size of 20 microns, sintering temperature of 900°C and cooling time of 48 h. Also, finite element modeling of the process was performed and shown a good agreement with experimental results.

Generalized Quantitative Stability Analysis of Time-Dependent Comprehensive Rotorcraft Systems

Rotorcraft stability is an inherently multidisciplinary area, including aerodynamics of rotor and fuselage, structural dynamics of flexible structures, actuator dynamics, control, and stability augmentation systems. The related engineering models can be formulated with increasing complexity due to the asymmetric nature of rotorcraft and the airflow on the rotors in forward flight conditions. As a result, linear time-invariant (LTI) models are drastic simplifications of the real problem, which can significantly affect the evaluation of the stability. This usually reveals itself in form of periodic governing equations and is solved using Floquet’s method. However, in more general cases, the resulting models could be non-periodic, as well, which requires a more versatile approach. Lyapunov Characteristic Exponents (LCEs), as a quantitative method, can represent a solution to this problem. LCEs generalize the stability solutions of the linear models, i.e., eigenvalues of LTI systems and Floquet multipliers of linear time-periodic (LTP) systems, to the case of non-linear, time-dependent systems. Motivated by the need for a generic tool for rotorcraft stability analysis, this work investigates the use of LCEs and their sensitivity in the stability analysis of time-dependent, comprehensive rotorcraft models. The stability of a rotorcraft modeled using mid-fidelity tools is considered to illustrate the equivalence of LCEs and Floquet’s characteristic coefficients for linear time-periodic problems.

Modal Identification of Ultralow-Frequency Flexible Structures Based on Digital Image Correlation Method

Traditional modal testing has difficult accurately identifying the ultralow-frequency modes of flexible structures. Ultralow-frequency excitation and vibration signal acquisition are two main obstacles. Aiming at ultralow-frequency modal identification of flexible structures, a modal testing method based on Digital Image Correlation method and Eigensystem Realization Algorithm is proposed. Considering impulse and shaker excitation are difficult to make generate ultralow-frequency vibration of structures, the initial displacement is applied to the structure for excitation. The ultralow-frequency accelerometer always has a large mass, which will change the dynamics performance of the flexible structure, so a structural vibration response was obtained through the Digital Image Correlation method. After collecting the free-decay vibration signal, the ultralow-frequency mode of the structure was identified by using the Eigensystem Realization Algorithm. Ground modal tests were conducted to verify the proposed method. Firstly, a solar wing structure was adopted, from which it was concluded that the signal acquisition using Digital Image Correlation method had high feasibility and accuracy. Secondly, an ultralow-frequency flexible cantilever beam structure which had the theoretical solution was employed to verify the proposed method and the theoretical fundamental frequency of the structure was 0.185 Hz. Results show that the Digital Image Correlation method can effectively measure the response signal of the ultralow-frequency flexible structure, and obtain the dynamics characteristics.

Noise Attenuation of a Duct-resonator System Using Coupled Helmholtz Resonator - Thin Flexible Structures

Several studies have been devoted to increasing the attenuation performance of the Helmholtz resonator (HR). One way is by periodic coupling of HRs in a ducting system. In this study, we propose a different approach, where a membrane (or a thin flexible structure in general) is added to the air cavity of a periodic HR array in order to further enhance the attenuation by utilizing the resonance effect of the membrane. It is expected that three attenuation mechanisms will exist in the system that can enhance the overall attenuation, i.e. the resonance mechanism of the HR, the Bragg reflection of the periodic system, and the resonance mechanism of the membrane or thin flexible structure. This study found that the proposed system yields two adjacent attenuation peaks, related to the HR and the membrane respectively. Moreover, extension of the attenuation bandwidth was also observed as a result of the periodic arrangement of HRs. With the same HR parameters, the peak attenuation by the membrane is tunable by changing its material properties. However, such a system does not always produce a wider attenuation bandwidth; the resonance bandwidths of both mechanisms must overlap.

Manipulation on Two-Dimensional Amorphous Nanomaterials for Enhanced Electrochemical Energy Storage and Conversion

Low-carbon society is calling for advanced electrochemical energy storage and conversion systems and techniques, in which functional electrode materials are a core factor. As a new member of the material family, two-dimensional amorphous nanomaterials (2D ANMs) are booming gradually and show promising application prospects in electrochemical fields for extended specific surface area, abundant active sites, tunable electron states, and faster ion transport capacity. Specifically, their flexible structures provide significant adjustment room that allows readily and desirable modification. Recent advances have witnessed omnifarious manipulation means on 2D ANMs for enhanced electrochemical performance. Here, this review is devoted to collecting and summarizing the manipulation strategies of 2D ANMs in terms of component interaction and geometric configuration design, expecting to promote the controllable development of such a new class of nanomaterial. Our view covers the 2D ANMs applied in electrochemical fields, including battery, supercapacitor, and electrocatalysis, meanwhile we also clarify the relationship between manipulation manner and beneficial effect on electrochemical properties. Finally, we conclude the review with our personal insights and provide an outlook for more effective manipulation ways on functional and practical 2D ANMs.

MICE-PES: An Algorithm for Accurate Conformational Analysis and its Implementation to Natural Products

<p>Secondary metabolites from natural sources have revolutionized the modern drug industry by acting as lead compounds. Many commercial drugs have evolved originally from natural molecules before being synthesized in the laboratory for commercialization. Because of the importance of natural molecules, it is crucial to determine their structural properties carefully as it is essential for their synthesis and studying their pharmacological behaviour. Many natural molecules have flexible structures and can adopt many different conformations in solution at room temperature. Hence, the determination of their relative configuration is a challenging task with the available experimental techniques. For structural analysis of natural molecules and to study their properties, all conformers which might be responsible for their chemical properties have to be considered. Theoretical chemistry has been very helpful in absolute structure determination of complex and conformationally flexible natural molecules by calculating their theoretical nuclear magnetic resonance, ultraviolet, infra red, and circular dichroism spectra etc. There are a number of software tools that offer conformational analysis by utilizing different molecular mechanics approaches. They produce a large number of possible conformers and are not general purpose, thus compromising accuracy. Apart from that, different force fields available for conformational analysis and minimization have been designed for specific molecular classes and do not produce good results beyond their scope. In the past, there have been reports about a “build-up procedure” for predicting the low energy conformations of peptides by optimising smaller fragments of the molecule under study and then joining them while minimizing their energies using force fields. Later on, this method was extended to predict the structure of DNA from sequences. This method used force field methods and did not gain much popularity due to its various limitations. Here, MICE-PES (Method for the Incremental Construction and Exploration of the Potential Energy Surface) is presented, an algorithm which performs a conformational analysis using high level quantum chemical calculations by building the molecule incrementally from its smallest possible analogue whose conformational degrees of freedom are very well separated than the rest of the molecule. MICE-PES has been validated through studies on known biomolecule 3-epi-xestoaminol whose absolute configuration has been determined already by experimental and theoretical methods. MICE-PES has also been used to assign the relative configuration of a natural product (meroterphenol C) whose configuration could not be established experimentally. Overall, the development of MICE-PES will be very helpful in solving problems in the study of conformationally flexible systems, in all aspects of organic chemistry.</p>

MICE-PES: An Algorithm for Accurate Conformational Analysis and its Implementation to Natural Products

<p>Secondary metabolites from natural sources have revolutionized the modern drug industry by acting as lead compounds. Many commercial drugs have evolved originally from natural molecules before being synthesized in the laboratory for commercialization. Because of the importance of natural molecules, it is crucial to determine their structural properties carefully as it is essential for their synthesis and studying their pharmacological behaviour. Many natural molecules have flexible structures and can adopt many different conformations in solution at room temperature. Hence, the determination of their relative configuration is a challenging task with the available experimental techniques. For structural analysis of natural molecules and to study their properties, all conformers which might be responsible for their chemical properties have to be considered. Theoretical chemistry has been very helpful in absolute structure determination of complex and conformationally flexible natural molecules by calculating their theoretical nuclear magnetic resonance, ultraviolet, infra red, and circular dichroism spectra etc. There are a number of software tools that offer conformational analysis by utilizing different molecular mechanics approaches. They produce a large number of possible conformers and are not general purpose, thus compromising accuracy. Apart from that, different force fields available for conformational analysis and minimization have been designed for specific molecular classes and do not produce good results beyond their scope. In the past, there have been reports about a “build-up procedure” for predicting the low energy conformations of peptides by optimising smaller fragments of the molecule under study and then joining them while minimizing their energies using force fields. Later on, this method was extended to predict the structure of DNA from sequences. This method used force field methods and did not gain much popularity due to its various limitations. Here, MICE-PES (Method for the Incremental Construction and Exploration of the Potential Energy Surface) is presented, an algorithm which performs a conformational analysis using high level quantum chemical calculations by building the molecule incrementally from its smallest possible analogue whose conformational degrees of freedom are very well separated than the rest of the molecule. MICE-PES has been validated through studies on known biomolecule 3-epi-xestoaminol whose absolute configuration has been determined already by experimental and theoretical methods. MICE-PES has also been used to assign the relative configuration of a natural product (meroterphenol C) whose configuration could not be established experimentally. Overall, the development of MICE-PES will be very helpful in solving problems in the study of conformationally flexible systems, in all aspects of organic chemistry.</p>

Lithium and Potassium Cations Affect the Performance of Maleamate-Based Organic Anode Materials for Potassium- and Lithium-Ion Batteries

In this study we prepared potassium-ion batteries (KIBs) displaying high output voltage and, in turn, a high energy density, as replacements for lithium-ion batteries (LIBs). Organic electrode materials featuring void spaces and flexible structures can facilitate the mobility of K+ to enhance the performance of KIBs. We synthesized potassium maleamate (K-MA) from maleamic acid (MA) and applied as an anode material for KIBs and LIBs, with 1 M potassium bis(fluorosulfonyl)imide (KFSI) and 1 M lithium bis(fluorosulfonyl)imide (LiFSI) in a mixture of ethylene carbonate and ethyl methyl carbonate (1:2, v/v) as respective electrolytes. The K-MA_KFSI anode underwent charging/discharging with carbonyl groups at low voltage, due to the K···O bond interaction weaker than Li···O. The K-MA_KFSI and K-MA_LiFSI anode materials delivered a capacity of 172 and 485 mA h g−1 after 200 cycles at 0.1C rate, respectively. K-MA was capable of accepting one K+ in KIB, whereas it could accept two Li+ in a LIB. The superior recoveries performance of K-MA_LiFSI, K-MA_KFSI, and Super P_KFSI at rate of 0.1C were 320, 201, and 105 mA h g−1, respectively. This implies the larger size of K+ can reversibly cycling at high rate.