关键词： METAL ions   CELLULOSE   HYDROGELS   RAMAN spectroscopy   ENERGY dissipation  

摘要： Utilization of reversible interactions as sacrificial bonds in biopolymers is critical for the integral synthesis of mechanically superior biological materials. In this work, cellulose nanofibrils (CNFs) reinforced covalent polyacrylamide (PAAm) composite hydrogels are immersed into multivalent cation (Ca2+, Zn2+, Al2+, and Ce3+) aqueous solution to form ionic association among CNFs, leading to the ionic covalent cross-linke dhydrogels. The cations promote the formation of porous networks of nanofibrils by screening the repulsive negative charges on CNF surface and dominate the mechanical properties and self-recovery efficiency of the hydrogels, resulting in mechanically reinforced ionic hydrogels with stiff (Young's modulus 257 kPa) and tough properties (fracture toughness 386 kJ/m(3)). The in situ Raman spectroscopy during stretching corroborates the stress transfer medium of CNF, and the microscopic morphologies of stable crack propagation validates that the multiple toughening mechanisms occur in a balanced energy dissipation manner, enabling synergistic combination of stiffness and toughness. Moreover, the depth-sensing instrumentation by indentation test also demonstrates that the CNF ionic coordination contributes simultaneous improvement in hardness and elasticity by as much as 600% compared to those pristine gels. This work demonstrates a facile way to transfer nanoscale building blocks to bulk elastomers with tunable dynamic properties and may provide a new prospect for the rational design of CNF reinforced hydrogels for applications where high-bearing capability is needed.

关键词： Crystallization kinetics  

摘要： Although Poly(lactic acid) (PLA) possesses many desirable properties, above all biodegradability, its heat deflection temperature is too low for many desirable applications. Similarly, to any other polymers, also for PLA the physical and mechanical properties in the solid state depend on the morphology and crystallinity degree, which in their turn are determined by the thermomechanical history experienced during solidification. A large crystallinity degree is highly desirable to increase the heat resistance of PLA but is rather difficult to reach during injection molding due to the very slow crystallization kinetics of this material. In this work, the crystallization kinetics of an injection molded PLA grade was assessed in function of the thermal history by using calorimetric analysis. The cold crystallization kinetics (starting from the amorphous glassy sample) turned out to be faster than melt crystallization kinetics. Following the indications gained from crystallization kinetics, some samples were injection molded imposing different thermal histories. The effect of molding conditions on crystallinity was determined. This finding was adopted to develop a postmolding stage which allows obtaining crystalline samples in times much shorter (of a factor about two) with respect to samples injection molded in a hot mold kept at temperatures close to the maximum crystallization rate. (C) 2016 Society of Plastics Engineers

关键词： polyurethane   electrostriction   mechanical relaxation   actuators  

摘要： Among the key parameters, which must be taken into account for the choice of actuators used as electrical to mechanical energy converters, the response to a step function and/or the frequency dependence of this response is extremely important. For polymeric actuators and more generally for electroactive polymers, three mechanisms can be at the origin of energy losses, namely dielectric relaxations, viscoelastic relaxations and possible electrical conductivity. In a previous paper, we studied the electrical behavior of segmented polyurethanes with different weight fractions of hard (MDI-BDO) and soft (PTMO) segments. They were shown to exhibit three main mechanisms, namely, from the fastest to the slowest, a secondary or beta-relaxation, the main or alpha-relaxation associated with the glass-rubber transition of the soft phase, and finally, their electrical conductivity. In the present work, we present the general viscoelastic response (as measured through mechanical spectrometry) of the same polyurethanes and their respective time dependent electrostriction responses, and compare it with the relaxation characteristic times of electrical and mechanical spectroscopy data.

关键词： Extrusion   Hydration   H-1 NMR   Relaxation time  

摘要： The effect of hydration (in the range of aw 0.981-0.090) on changes of macroscopic and mechanical features of triticale snacks produced in different conditions (feed moisture- 16 and 20%;extrusion temperature- 135 and 175 degrees C) was analysed. Changes of snacks depended both on the hydration level and the extrusion process conditions. As a result of storage at the highest hydration level (aw = 0.981) contraction of snacks was observed. The shrinkage was more pronounced in case of extrudates obtained from raw material containing 16% of water regardless of the extrusion temperature. The hardness of extrudates depended both on the feed moisture and hydration. Samples obtained from the raw material containing 20% moisture were harder than those prepared at the feed moisture of 16%. Based on the results of molecular dynamics study by low field NMR technique, a model of hydration was developed and critical hydration value of extruded snacks was calculated. Higher values of the critical hydration were obtained for snacks extruded at temperature 135 degrees C than for those obtained at 175 degrees C. The feed moisture level affected critical hydration values to a lesser extent. Obtained results suggest that the state of water in extrudates depends on extrusion process conditions. (C) 2017 Elsevier Ltd. All rights reserved.

关键词： High-speed motorized spindles   Vibration   Dynamical model   Frequency response function  

摘要： This paper presents a dynamical model of high speed motorized spindles in free state and work state. In the free state, the housing is modeled as a rotor with equivalent masses including bearing pedestals, motor stator and rear end cover. As a consequence, a double rotor dynamics can be modeled for high speed motorized spindles by a bearing element which connects the housing and bearing pedestals. In the work state, the housing is fixed and the system becomes a bearing-rotor dynamical model. An excitation-measurement test in the free state is designed to analyze the cross spectral density and auto spectral density of input and output signals. Then the frequency response function of system and coherence function of input and output signals which are used to analyze the inherent characteristics of the double-rotor model can be obtained. The other vibration test in the work state is designed to research the dynamical supporting characteristics of bearings and the effects from bearings on the inherent characteristics of the system. The good agreement between the experimental data and theoretical results indicates that the dynamical model in two states is capable of accurately predicting the dynamic behavior of high speed motorized spindles.

关键词： Nanostructures   molecular dynamics   mechanical properties   computational modeling   functionalization   elastic properties  

摘要： Carbon nanotubes have been proposed as an ideal reinforcement for the fabrication of nanocomposites. However, because of their chemical inertness, carbon nanotubes have to be functionalized in order to acquire superior properties. In the present paper, we examine the effect of functionalization of single-, double-, and triple-walled carbon nanotubes with ethylene-di-amine, analyzing their elastic properties. Condensed-phase optimized molecular potentials for atomistic simulations studies force field is used to model the interatomic interactions for armchair (5,5), (9,0), and (10,10) configuration carbon nanotubes. Molecular dynamics simulations for carbon nanotubes with various densities of the attached ethylene-di-amine molecules have been performed. This study quantitatively investigates the effect of amine functionalization (up to 12 numbers of ethylene-di-amine groups) on the Young's, bulk, and shear moduli and tensile strengths of different carbon nanotube structures.

关键词： Echocardiography   Hemodynamic characteristics   Mitral valvular disease   Mitral valve replacement  

摘要： Mitral stenosis (MS) and mitral insufficiency (MI) have different pre-operative hemodynamic characteristics. However, it is unclear if there are differences in long-term echocardiographic characteristics of MS and MI patients after mechanical mitral valve replacement. This study is to compare long-term echocardiographic results of mechanical mitral valve prostheses between MS and MI patients. From January 2003 to January 2009, a total of 199 consecutive patients were recruited in this study. Patients were classified as group MS (n = 123) and MI (n = 76) according to the manifestation of mitral valvular disease. The mean age for patients was 50.1 +/- 10.5 years and followup time was 7.2 +/- 2.0 years. The MS after operation were more likely to experience atrial fibrillation (p = 0.002). The New York Heart Association (NYHA) class in MI showed a greater improvement (p = 0.006) than in MS. The left ventricular end-diastolic dimension (LVEDD) (p = 0.010) and stroke volume (SV) (p = 0.000) in MI were still larger than that in MS patients. These differences did not disappear with time after operation. The long-term echocardiographic results of mechanical mitral valve prostheses between MS and MI patients are significantly different. Over a longterm follow up, MI patients still have a larger LVEDD and SV than MS, and associated with a greater improvement of NYHA class.

关键词： Hydrodynamic   Acoustic   Cavitation   Crystallinity   Pretreatment  

摘要： Acoustic and hydrodynamic cavitation were examined as suitable mechanical pretreatments for lignocellulosic biomass. Microcrystalline cellulose and lime-treated sugarcane bagasse were subjected to acoustic cavitation, whereas raw and lime-treated sugarcane bagasse were subjected to hydrodynamic cavitation. Acoustic cavitation successfully increased microcrystalline cellulose enzymatic digestibility by 37% compared to no acoustic cavitation treatment;however, there was no significant effect on lime treated sugarcane bagasse. Hydrodynamic cavitation increased the enzymatic digestibility of both raw and lime-treated sugarcane bagasse. Best results were obtained using cavitation treatment of bagasse followed by lime treatment;the 3-d enzymatic digestion increased by 46% when compared to lime treatment only. (C) 2017 Elsevier Ltd. All rights reserved.

关键词： alpha-quartz   Stress-strain   Grain boundary   Molecular dynamics  

摘要： Uniaxial compression test was performed on twinned crystal (TC) models and monocrystal (MC) alpha-quartz to study the effect of the single grain boundary on the strength and fracture of a- quartz. Based on the stress and strain curves, for any given strain rate, MC has a higher stress than any other TC. Based on the bond length changes, bond breaks are found in the TC but no bond break is found in the MC. Thus, the grain boundary causes stress concentration and leads to bond breakage. Larger transformation of bonds absorbs more energy, which results in lower bonding strength. Moreover, the fracture does not necessarily start from the grain boundary but also depends on the initially large bond length. The averaged mean square displacement (AMSD) is also calculated. For the results of MC, only one set of peaks corresponding to the maximum stress appears on the AMSD and strain curves. For TC, two sets of peaks indicate the largest stretching of bond length and the start of fractures, separately. These characteristics illustrate that the vibration of atoms will intensify when the structure reaches the limit of compressive strength.

关键词： PEPTIDOGLYCANS   PROTEOBACTERIA   BACTERIAL growth   BIOSYNTHESIS   CELL compartmentation  

摘要： Most commonly studied bacteria grow symmetrically and divide by binary fission, generating two siblings of equal morphology. An exception to this rule are budding bacteria, in which new offspring emerges de novo from a morphologically invariant mother cell. Although this mode of proliferation is widespread in diverse bacterial lineages, the underlying mechanisms are still incompletely understood. Here, we report the first molecular-level analysis of growth and morphogenesis in the stalked budding alphaproteobacterium Hyphomonas neptunium. Peptidoglycan labeling shows that, in this species, buds originate from a stalk-like extension of the mother cell whose terminal segment is gradually remodeled into a new cell compartment. As a first step toward identifying the machinery mediating the budding process, we performed comprehensive mutational and localization studies of predicted peptidoglycan biosynthetic proteins in H. neptunium. These analyses identify factors that localize to distinct zones of dispersed and zonal growth, and they suggest a critical role of the MreB-controlled elongasome in cell morphogenesis. Collectively, our work shows that the mechanism of growth in H. neptunium is distinct from that in related, polarly growing members of the order Rhizobiales, setting the stage for in-depth analyses of the molecular principles regulating the fascinating developmental cycle of this species. (C) 2016 John Wiley & Sons Ltd.