Self-organization of gold-containing hydrogen-bonded rosette assemblies on graphite surface

The self-organization of supramolecular structures, in particular gold-containing hydrogen-bonded rosettes, on highly oriented pyrolytic graphite (HOPG) surfaces was investigated by tapping-mode atomic force microscopy (TM-AFM) and scanning tunneling microscopy (STM). TM-AFM and high-resolution STM results show that these hydrogen-bonded assemblies self-organize to form highly ordered domains on HOPG surfaces. We find that a subtle change in one of the building blocks induces two different orientations of the assembly with respect to the surface. These results provide information on the control over the construction of supramolecular nanoarchitectures in 2D with the potential for the manufacturing of functional materials based on structural manipulation of molecular components.     

Highly structured porous solids from liquid foam templates

We demonstrate the generation of highly structured porous solids from liquid foam templates, using ordered foam layers and threads made from hydrogels. For this purpose we separate sufficiently foam generation and solidification: well known and highly controllable liquid foam structures are created, which are thereafter ‘frozen’ in situ through polymerisation and cross-linking. Being extendible to a large range of materials and length scales, such an approach opens up a plethora of opportunities in material development.     

纳米尺度氩液体线物性的分子动力学模拟

运用分子动力学模拟方法,对纳米尺度氩液体线的物理性质进行了研究。文中模拟计算了纳米线的熔点温度以及气液平衡状态下液态区密度、气态区密度和液体线的半径,并分析了模拟盒子尺寸和模拟温度对液体线物性的影响。结果表明,由于在初始结构中增加了气体分子,当模拟温度不变时,随模拟盒子尺寸的增加,液态区密度增大,气态区密度减小。但模拟盒子尺寸较小时,液体线半径不随模拟盒子尺寸发生变化。模拟计算所得的液态区密度十分接近宏观尺度氩液体密度时,模拟盒子的尺寸较合适。当模拟盒子尺寸固定不变时,液态区密度和气态区密度随温度的变化趋势与文献中宏观尺度氩液体和气体密度的变化趋势相同。结论可以为进一步系统地分析纳米尺度液体线的稳定性提供一定的依据。     

Tervalent conducting polymers with tailor-made work functions: preparation, characterization, and applications as cathodes in electroluminescent devices

A series of conducting polymers have been prepared through thermal polymerization of transition-metal diimine complexes. The as-polymerized material is electrochemically converted into its formally zerovalent form. Due to the proximity of the half-wave potentials of the formal 1+/0 and 0/1- couples, there is substantial disproportionation of the redox sites at room temperature, resulting in a conductive tervalent mixed-valent material. The redox processes that give rise to this mixed-valent material are predominantly ligand-based, and therefore are highly sensitive to substitution on the ligand periphery. Solution redox chemistry of the monomer can be used to accurately predict the work function of the corresponding zerovalent conducting polymer, which has been verified by ultraviolet photoelectron spectroscopy. Many of these materials have especially low work functions (<3.6 eV) making them appropriate materials to use as cathode materials in organic light-emitting devices (OLEDs). Working examples of tris(8-hydroxyquinoline)aluminum(III)-based OLEDs have been fabricated using one of these polymers as a cathode.     

Morphological development in PDMS/TEOS hybrid materials

PDMS was used as an additive to TEOS based gels in order to improve macroscopic properties for applications such as thermal insulators. The morphological features developed in the PDMS/TEOS hybrid materials were studied by small-angle x-ray scattering and small-angle light scattering. The thermal properties of the material are controlled by nano-scale morphology. A main feature in the nano-scale is mass-fractal domains. As PDMS is incorporated into the TEOS system, a macroscopic network structure becomes dominant, and toughness is enhanced. A monolithic structure is observed macroscopically. The results indicate that PDMS can be used to develop micron morphologies for toughening these materials. © 1996 John Wiley & Sons, Inc.     

A five-parameter fractional derivative temperature spectrum model for polymeric damping materials

To capture viscoelastic behavior of polymeric damping materials based on limited dynamic mechanical analysis tests, a simple fractional temperature spectrum model representing the viscoelastic materials is proposed in this paper and experimental tests aims at stressing the validity of the model. The storage modulus, the loss modulus, and the loss factor, are established based on the five-parameter fractional derivative model and the time–temperature superposition principle. The dynamic mechanical tests of two polymeric materials are carried out to verify this temperature spectrum model. Results indicate a good agreement between the temperature spectrum model and experimental tests at various temperature conditions. Furthermore, thermodynamic coupling of the viscoelastic material is investigated by temperature rise calculation and vibration experiment test. Comparison analysis shows that the temperature rise model can simulate the temperature rise process for the shear vibration of the constrained damping, which provide references for the damping capability, thermal damage and failure of viscoelastic material.     

Influencing parameters on measurement accuracy in dynamic mechanical analysis of thermoplastic polymers and their composites

Long-term predictions of material properties such as stiffness and creep resistance are important in many engineering applications and require high reliability and accuracy. This is especially true for polymer materials and their composites as their viscoelastic nature results in time-dependent material behaviour and any measurement uncertainties or errors amplify in long-term predictions. To measure this behaviour at smallest loadings, Dynamic Mechanical Analysis (DMA) is frequently declared as an ideal method. However, the measurement accuracy and repeatability of this method is strongly influenced by (i) the testing fixture and corresponding loading mode, (ii) the sample preparation and (iii) the plotting scale to interpret the test results. In this study, relevant experimental parameters were found for DMA and a proper procedure was designed, which was then applied to measure the viscoelastic behaviour of a highly temperature and creep resistant thermoplastic polymer (polyethersulfone) and of a highly graphite filled polypropylene composite. In combination with finite element simulations and in-situ strain measurements by digital image correlation (DIC), the main influences on measurement accuracy of three-point-bending DMA were identified and subsequently used to determine measurement guidelines. Using these guidelines, DMA measurements allow quantitative determination of the viscoelastic response for rigid polymer and composite materials.     

Evidences of non-linear short-term stress relaxation in polymers

Back in 1986, investigating the Space Shuttle Challenger disaster, famous physicist Richard Feynman clearly showed how viscoelastic behavior of a polymeric material is of paramount importance in practical engineering. At present day a definitive universal rheological law is not yet available for polymers, as a consequence both theoretical models and experimental investigations of viscoelastic behavior must be necessarily focused independently on each single polymer or, at least, on well-defined classes of polymers. Accurate experimental evidences are needed in order to properly evaluate the mechanical properties of a polymeric material, as a function of its particular applications. In this paper measurements of the stress relaxation behavior of six polymeric materials under uniaxial tension and uniaxial unconfined compression tests, are performed and experimental results are modelled using a stretched exponential function, known as Kohlraush-Williams-Watts time-decay function. In particular the short-term stress relaxation is investigated, as a function of typical environmental temperature range, in order to assess viscoelastic behavior of tested polymeric materials for peculiar industrial and biomedical applications.     

碳基催化剂:为开发下一代纳米工程催化材料开辟新方法(英文)

This essay analyses some of the recent development in nanocarbons (carbon materials having a defined and controlled nano-scale dimension and functional properties which strongly depend on their nano-scale features and architecture), with reference to their use as advanced catalytic materials. It is remarked how their features open new possibilities for catalysis and that they represent a new class of catalytic materials. Although carbon is used from long time in catalysis as support and electrocatalytic applications, nanocarbons offer unconventional ways for their utilization and to address some of the new challenges deriving from moving to a more sustainable future. This essay comments how nanocarbons are a key element to develop next-generation catalytic materials, but remarking that this goal requires overcoming some of the actual limits in current research. Some aspects are discussed to give a glimpse on new directions and needs for RD to progress in this direction.     

Dissipative water intrusion in hydrophobic MCM-41 type materials

Texture-related features of water intrusion in hydrophobised MCM-41 silicas render these materials especially suitable for energy dissipation in mechanical dampers.     

Investigation of surface changes on mica induced by atomic force microscopy imaging under liquids

Surface changes on muscovite mica induced by tip-surface interactions in atomic force microscopy (AFM) experiments under liquids are described. Investigations have been performed with AFM operated both in contact mode (CM-AFM) and in tapping mode (TM-AFM). Additionally, force-distance measurements have been carried out. In contrast to CM-AFM pronounced surface changes can be observed in TM-AFM experiments. However, TM-AFM images of areas previously scanned in contact mode show that imaging in contact mode changes the surface, too. An evaluation of force-distance measurements reveals that these changes depend on the adhesive interaction between tip and sample, which in turn strongly depends on the surrounding medium. The artefact can be avoided by changing the pH-value of the medium or by working with mixtures of ethanol and water. This greatly enhances the applicability of TM-AFM for in-situ investigation of surface processes on mica, which is a frequently used substrate for many technological and biological applications.     

Fabrication and microstructural characterization of functionally graded porous acrylonitrile butadiene styrene and the effect of cellular morphology on creep behavior

The ability to control material properties in space and time for functionally graded viscoelastic materials makes them an asset where they can be adapted to different design requirements. The continuous microstructure makes them advantageous over conventional composite materials. Functionally graded porous structures have the added advantage over conventional functionally graded materials of offering a significant weight reduction compared to a minor drop in strength. Functionally graded porous structures of acrylonitrile butadiene styrene (ABS) had been fabricated with a solid‐state constrained foaming process. Correlating the microstructure to material properties requires a deterministic analysis of the cellular structure. This is accomplished by analyzing the scanning electron microscopy images with a locally adaptive image threshold technique based on variational energy minimization. This characterization technique of the cellular morphology is analyst independent and works very well for porous structures. Inferences are drawn from the effect of processing on microstructure and then correlated to creep strain and creep compliance. Creep is strongly correlated to porosity and pore sizes but more associated to the size than to porosity. The results show the potential of controlling the cellular morphology and hence tailoring creep strain/compliance of ABS to some desired values. © 2015 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 795–803     

Vibrational studies of [Ni(II)(DIARS)2X]X, (DIARS=o-C6H4(As(CH3)2)2 and X=Cl, Br, I).

Infrared and laser Raman spectra of [Ni(II)(diars)2X]X, (X=Cl, Br and I) have been used as probes to determine the structures of chelated diarsine molecules. It has been observed that the effects of metal chelation and coordination geometry give rise to frequency shifts in these complexes. The variation in vibrational spectroscopic features indicates reduction in the symmetry of the complexes in the crystalline environment. The effect of halogen on the Ni-halogen stretching frequency of these square pyramidal complexes is not as significant as observed in the case of octahedral complexes.     

Stress response and energy dissipation in a linear viscoelastic material under periodic triangular strain loading

A general equation is derived for the stress response of a linear viscoelastic material to periodic strain excitation at constant strain rates. The energy dissipated in any cycle, especially in a steady-state loop, is discussed. The results can be used to analyze test results in determining mechanical properties of polymers. A simple Maxwell model and a three-parameter Maxwell model are used to illustrate the calculation of stress response and energy dissipation under constant-strain-rate loading.     

耗散型石英晶体微天平在生物医用高分子材料中的应用

石英晶体微天平(QCM)是一种基于石英晶体压电效应的分析检测技术,可实时在线提供石英晶体表面吸附层质量、厚度、粘弹性等信息,由此获得表面分子相互作用关系。 耗散型石英晶体微天平(QCM-D)因其独特的对粘弹性的解析,使其在高分子材料中的应用迅速发展,尤其是生物医用高分子材料领域,已用来评价生物医用高分子材料的表界面相互作用,力学和生物相容性等。 本文简单介绍了耗散型石英晶体微天平的基本原理及理论模型,重点综述了近几年QCM-D在高分子链构象、蛋白质吸附、生物大分子相互作用、药物释放以及水凝胶中的应用,并且展望了QCM-D的未来发展趋势。     

Coordination Chemistry of [Co(acac)2] with N‐Doped Graphene: Implications for Oxygen Reduction Reaction Reactivity of Organometallic Co‐O4‐N Species

Hybridization of organometallic complexes with graphene‐based materials can give rise to enhanced catalytic performance. Understanding the chemical structures within hybrid materials is of primary importance. In this work, archetypical hybrid materials are synthesized by the reaction of an organometallic complex, [Co^II(acac)₂] (acac=acetylacetonate), with N‐doped graphene‐based materials at room temperature. Experimental characterization of the hybrid materials and theoretical calculations reveal that the organometallic cobalt‐containing species is coordinated to heterocyclic groups in N‐doped graphene as well as to its parental acac ligands. The hybrid material shows high electrocatalytic activity for the oxygen reduction reaction (ORR) in alkaline media, and superior durability and methanol tolerance to a Pt/C catalyst. Based on the chemical structures and ORR experiments, the catalytically active species is identified as a Co‐O₄‐N structure.     

Identify crystal structures by a new paradigm based on graph theory for building materials big data

Material identification technique is crucial to the development of structure chemistry and materials genome project. Current methods are promising candidates to identify structures effectively, but have limited ability to deal with all structures accurately and automatically in the big materials database because different material resources and various measurement errors lead to variation of bond length and bond angle. To address this issue, we propose a new paradigm based on graph theory(GTscheme) to improve the efficiency and accuracy of material identification, which focuses on processing the "topological relationship" rather than the value of bond length and bond angle among different structures. By using this method, automatic deduplication for big materials database is achieved for the first time, which identifies 626,772 unique structures from 865,458 original structures.Moreover, the graph theory scheme has been modified to solve some advanced problems such as identifying highly distorted structures, distinguishing structures with strong similarity and classifying complex crystal structures in materials big data.     

Polar fluid model of viscoelastic membranes and interfaces

The polar surface fluid model is used to derive the generalized dynamic shape equation and the interfacial rheological material functions for viscoelastic membranes and curved interfaces, taking viscous bending and torsion modes into full account. The materials modeling approach based on the polar surface fluid leads to the integration of bending and torsion dissipative modes with their elastic counterparts that appear in the dynamic shape equation and in the interfacial rheological functions. The covariant bending and torsion rates derived in this paper are shown to be related to the interfacial co-rotational derivative of the curvature tensor. The dynamic shape equation is used to analyze shape fluctuation in planar geometries, and to establish the role of bending dissipation in shape dynamics. The dynamic shape equation generalizes the static Helfrich shape equation by incorporating bending and torsion dissipation, and it generalizes the dynamic shape equation based on the Boussinesq-Scriven model by incorporating bending and torsion elasticity and dissipation.     

The evolution of viscoelasticity near the gel point of end-linking poly(dimethylsiloxane)s

The linear viscoelasticity of polymers near the gel point can be described by two scaling laws. The material at the gel point has a power-law linear viscoelastic relaxation modulus, and the relaxation exponent has been found to vary with the composition of the precursor materials, i.e., it is not universal for gelation. A second scaling law describes the evolution of the linear viscoelastic properties through the gel point. The rate of change of the dynamic mechanical modulus/viscosity is observed to scale as a power-law function of frequency. This power-law function defines a dynamic critical exponent, and this has been found to be independent of precursor composition for end-linking poly(dimethylsiloxane) polymers and equal to κ = 0.21 ± 0.02. This exponent may be a universal measure of gelation. The technique of Time Resolved Mechanical Spectroscopy is used to observe the evolution of linear viscoelastic properties of crosslinking polymers in situ in the rheometer. A stretched exponential relaxation modulus describes the evolution of mechanical properties in the vicinity of the gel point very well. The exponents which characterize the divergence of the zero-shear viscosity and the equilibrium modulus are not universal, since they are related to the relaxation exponent and the dynamic critical exponent.