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1.
    
We have previously reported that small quantities of self‐assembling molecules known as dendron rodcoils (DRCs) can be used as supramolecular additives to modify the properties of polystyrene (PS). These molecules spontaneously assemble into supramolecular nanoribbons that can be incorporated into bulk PS in such a way that the orientation of the polymer is significantly enhanced when mechanically drawn above the glass‐transition temperature. In the current study, we more closely evaluate the structural role of the DRC nanoribbons in PS by investigating the mechanical properties and deformation microstructures of polymers modified by self‐assembly. In comparision to PS homopolymer, PS containing small amounts (≤ 1.0 wt.‐%) of self‐assembling DRC molecules exhibit greater Charpy impact strengths in double‐notch four‐point bending and significantly greater elongations to failure in uniaxial tension at 250 % prestrain. Although the DRC‐modified polymer shows significantly smaller elongations to failure at 1000 % prestrain, both low‐ and high‐prestrain specimens maintain tensile strengths that are comparable to those of the homopolymer. The improved toughness and ductility of DRC‐modified PS appears to be related to the increased stress whitening and craze density that was observed near fracture surfaces. However, the mechanism by which the self‐assembling DRC molecules toughen PS is different from that of conventional additives. These molecules assemble into supramolecular nanoribbons that enhance polymer orientation, which in turn modifies crazing patterns and improves impact strength and ductility.  相似文献   

2.
    
A model of structural transformations of amorphous into quasi‐amorphous BaTiO3 is suggested. The model is based on previously published data and on X‐ray photoelectron spectroscopy data presented in the current report. Both amorphous and quasi‐amorphous phases of BaTiO3 are made up of a network of slightly distorted TiO6 octahedra connected in three different ways: by apices (akin to perovskite), edges, and faces. Ba ions in these phases are located in the voids between the octahedra, which is a nonperovskite environment. These data also suggest that Ba ions compensate electrical‐charge imbalance incurred by randomly connected octahedra and, thereby, stabilize the TiO6 network. Upon heating, the edge‐to‐edge and face‐to‐face connections between TiO6 octahedra are severed and then reconnected via apices. Severing the connections between TiO6 octahedra requires a volume increase, suppression of which keeps some of the edge‐to‐edge and face‐to‐face connections intact. Transformation of the amorphous thin films into the quasi‐amorphous phase occurs during pulling through a steep temperature gradient. During this process, the volume increase is inhomogeneous and causes both highly anisotropic strain and a strain gradient. The strain gradient favors breaking those connections, which aligns the distorted TiO6 octahedra along the direction of the gradient. As a result, the structure becomes not only anisotropic and non‐centrosymmetric, but also acquires macroscopic polarization. Other compounds may also form a quasi‐amorphous phase, providing that they satisfy the set of conditions derived from the suggested model.  相似文献   

3.
    
Mixed Ta2O5‐containing SiO2 particles, 6–14 nm in diameter, with closely controlled refractive index, transparency, and crystallinity are prepared via flame spray pyrolysis (FSP) at production rates of 6.7–100 g h–1. The effect of precursor solution composition on product filler (particle) size, crystallinity, Ta dispersity, and transparency is studied using nitrogen adsorption, X‐ray diffraction, optical microscopy, high‐resolution transmission electron microscopy (HRTEM), and diffuse‐reflectance infrared Fourier‐transform spectroscopy (DRIFTS). Emphasis is placed on the transparency of the composite that is made with Ta2O5/SiO2 filler and dimethylacrylate. Increasing Ta2O5 crystallinity and decreasing Ta dispersity on SiO2 decreases both filler and composite transparencies. Powders with identical specific surface area (SSA), refractive index (RI), and Ta2O5 content (24 wt.‐%) show a wide range of composite transparencies, 33–78 %, depending on filler crystallinity and Ta dispersity. Amorphous fillers with a high Ta dispersity and an RI matching that of the polymer matrix lead to the highest composite transparency, 86 %. The composite containing 16.5 wt.‐% filler that itself contains 35 wt.‐% Ta2O5 has the optimal radiopacity for dental fillings.  相似文献   

4.
    
To evaluate the potential of polyester‐based ionomers as biomaterials, we have characterized them in terms of crystallization behavior, degradation, and in‐vitro cellular interactions. The polymers used are poly(butylene succinate)‐based ionomers (PBSis) with 1 to 5 mol‐% dimethyl 5‐sodium sulfoisophthalate. Even a few incorporated ionic groups significantly decreases the folding surface energy, indicating that folding into crystalline lamellae is more difficult for chains restricted by ionic aggregates. Transmission electron microscopy (TEM) does not reveal any distinct aggregation of ionic clusters following hydrolytic degradation, which suggests that the physical crosslinkage due to ionic interactions is vulnerable to hydrolysis. The in‐vitro cellular interactions of polyester‐based ionomers is assessed by the culture of human dermal fibroblasts with PBSi extracts or in direct contact with the PBSi films. Cells on PBSi films and in their extracts exhibit appropriate specific growth rates and normal metabolic function regardless of the incorporated ionic content compared with poly[(D ,L ‐lactic acid)‐co‐(glycolic acid)] (75:25, PLGA), which is well known to be biocompatible. The cells growing on PBSi films spread to a sufficient extent, displaying relatively active filopodial growth, as compared to that of parent PBS. These results suggest that the conspicuous topology and hydrophilic nature of the ionomer surface affect cellular interactions, and that this ionomer therefore has potential applications as a biomaterial.  相似文献   

5.
    
The reduction of thermal conductivity, and a comprehensive understanding of the microstructural constituents that cause this reduction, represent some of the important challenges for the further development of thermoelectric materials with improved figure of merit. Model PbTe‐based thermoelectric materials that exhibit very low lattice thermal conductivity have been chosen for this microstructure–thermal conductivity correlation study. The nominal PbTe0.7S0.3 composition spinodally decomposes into two phases: PbTe and PbS. Orderly misfit dislocations, incomplete relaxed strain, and structure‐modulated contrast rather than composition‐modulated contrast are observed at the boundaries between the two phases. Furthermore, the samples also contain regularly shaped nanometer‐scale precipitates. The theoretical calculations of the lattice thermal conductivity of the PbTe0.7S0.3 material, based on transmission electron microscopy observations, closely aligns with experimental measurements of the thermal conductivity of a very low value, ～0.8 W m?1 K?1 at room temperature, approximately 35% and 30% of the value of the lattice thermal conductivity of either PbTe and PbS, respectively. It is shown that phase boundaries, interfacial dislocations, and nanometer‐scale precipitates play an important role in enhancing phonon scattering and, therefore, in reducing the lattice thermal conductivity.  相似文献   

6.
    
The electrochemical performances of 1D SnO2 nanomaterials, nanotubes, nanowires, and nanopowders, are compared to define the most favorable morphology when SnO2 nanomaterials are adopted as the electrode material for lithium‐ion batteries. Changes in the morphology of SnO2 are closely related with its electrochemical performance. Some SnO2 nanomaterials feature not only an increased energy density but also enhanced Li+ transfer. The correlation between the morphological characteristics and the electrochemical properties of SnO2 nanomaterials is discussed. The interesting electrochemical results obtained here on SnO2 nanomaterials indicate the possibility of designing and fabricating attractive nanostructured materials for lithium‐ion batteries.  相似文献   

7.
    
The use of cryopreserved arteries for vascular tissue engineering provides a promising way for vessel replacement. Unfortunately cryopreservation induces structural changes that strongly modify the mechanical properties and alter the thrombogenicity of the vessel after implantation. We present here a new procedure to treat the inner coating of cryopreserved arteries with poly(sodium‐4‐styrene sulfonate)/poly(allylamine hydrochloride) polyelectrolyte multilayers. We show that this treatment improves the mechanical properties of the cryopreserved vessel. It also allows the adhesion and spreading of endothelial cells so that the internal structure of the vessel closely resembles that of fresh arteries. Finally, we verify by PECAM‐1 and von‐Willebrand‐factor (vWF) expression that this treatment preserves the phenotype of the endothelial cells. This study should open new routes towards the development of future, new biocompatible tissue substitutes allowing long‐term functionality after implantation.  相似文献   

8.
    
Solid‐state grinding is a simple and effective method to include guest species into the channels of ordered mesoporous materials with a different degree of filling. After calcination, a monolayer or several monolayers of guest species can not only form highly dispersed oxide species and other surface species on the hosts whether the template is occluded in the channels or not, but the guest species can also fill the mesoporous channels in the host and thus lead to nanowires or nanoarrays. Solid‐state salt inclusion is faster and more convenient than other inclusion routes. The absence of a solvent not only saves the time otherwise needed for evaporation but also leads to a higher degree of filling through a simple inclusion step as the void space in the pores is not occupied by the solvent. Also, the lack of competitive adsorption of solvent molecules enhances the interaction between the guest species included and the silica wall, which facilitates the high dispersion of oxide species. However, host–guest interactions that are too strong may disturb the self‐crystallization of guest species in the mesopores leading to imperfect nanocasting of the mesostructure.  相似文献   

9.
    
The relation between the nanoscale morphology and associated device properties in conjugated polymer/fullerene bulk‐heterojunction “plastic solar cells” is investigated. We perform complementary measurements on solid‐state blends of poly[2‐methoxy‐5‐(3,7‐dimethyloctyloxy)]‐1,4‐phenylenevinylene (MDMO‐PPV) and the soluble fullerene C60 derivative 1‐(3‐methoxycarbonyl) propyl‐1‐phenyl [6,6]C61 (PCBM), spin‐cast from either toluene or chlorobenzene solutions. The characterization of the nanomorphology is carried out via scanning electron microscopy (SEM) and atomic force microscopy (AFM), while solar‐cell devices were characterized by means of current–voltage (I–V) and spectral photocurrent measurements. In addition, the morphology is manipulated via annealing, to increase the extent of phase separation in the thin‐film blends and to identify the distribution of materials. Photoluminescence measurements confirm the demixing of the materials under thermal treatment. Furthermore the photoluminescence of PCBM clusters with sizes of up to a few hundred nanometers indicates a photocurrent loss in films of the coarser phase‐separated blends cast from toluene. For toluene‐cast films the scale of phase separation depends strongly on the ratio of MDMO‐PPV to PCBM, as well as on the total concentration of the casting solution. Finally we observe small beads of 20–30 nm diameter, attributed to MDMO‐PPV, in blend films cast from both toluene and chlorobenzene.  相似文献   

10.
    
The elastic behavior of solids with a large concentration of interacting point defects has been analyzed. The analysis predicts that, in such solids, mechanical stress may be partially relieved by a shift in the association/dissociation equilibrium of the point defects. Association/dissociation of the point defects in response to an external stress will proceed until the decrease in elastic energy is balanced by the increased chemical energy of the defect distribution. The resulting change in the linear dimensions may be called “chemical strain”, in analogy to the previously studied “chemical stress”. A solid in which chemical strain may develop in response to external stress should exhibit two distinct Young's moduli: relaxed, on a time scale which allows the defects to reach equilibrium; and unrelaxed, on a time scale which is too short for the defect equilibrium to be established. Our analysis suggests that materials exhibiting the chemical‐strain effect are capable of reversible adaptation to external mechanical constraints. Measurements on a self‐supported film of Ce0.8Gd0.2O1.9 strongly support the theoretical predictions.  相似文献   

11.
    
Tetrakis[(4‐(4′‐(2″,5″‐dioctyloxy‐4″‐(4‴‐(2′‴,5′‴‐dioctyloxy‐4′‴‐styryl)styryl)styryl)styryl)styryl)phenyl]methane (T‐6R‐OC8H17) is an organic chromophore that consists of four optoelectronic fragments (“arms”) connected to a tetrahedral point of convergence (carbon). Bulk samples are amorphous as determined by powder diffraction, while differential scanning calorimetry (DSC) is sometimes ambiguous. Film forming properties were studied by atomic force microscopy (AFM) and fluorescence microscopy as a function of casting solvent and heat treatment. The film forming qualities are useful for the fabrication of light‐emitting diodes with low turn‐on voltages. Device performance is also history dependent. The relationship between bulk morphology, film topology, photoluminescence (PL) properties, and light‐emitting diode (LED) performance is discussed. A comparison of these compounds against the parent oligo(phenylenevinylene) arms, with respect to morphology, topology, and PL properties is also presented.  相似文献   

12.
    
We have characterized the morphology and nanomechanical properties of surface‐grafted nanoscale layers consisting of Y‐shaped binary molecules with one polystyrene (PS) arm and one poly(acrylic acid) (PAA) arm. We examined these amphiphilic brushes in fluids (in‐situ visualization), and measured their microtribological characteristics as a function of chemical composition. Atomic force microscopy (AFM)‐based nanomechanical testing has shown that nanoscale reorganization greatly influences the adhesion and elastic properties of the nanoscale brush layer. In water, a bimodal distribution of the elastic modulus, arising from the mixed chemical composition of the topmost layer, is observed. In contrast, the top layer is completely dominated by PS in toluene. As a result of this reorganization, the Y‐shaped‐brush layer exhibits a dramatic variation in the friction and wear properties after exposure to different solvents. Unexpectedly, the tribological properties are enhanced for the hydrophilic and polar, PAA‐dominated, surface, which shows a lower friction coefficient and higher wear stability, despite higher adhesion and heterogeneous surface composition. We suggest that this unusual behavior is caused by the combination of the presence of a thicker water layer on the PAA‐enriched surface that acts as a boundary lubricant and the glassy state of the PAA chains.  相似文献   

13.
    
Chiral magnets are obtained by inclusion of chiral guest molecules into the channels of an achiral nanoporous ferrimagnet consisting of the Mn3(HCOO)6 ( 1 ) framework. Insertion of the R or the S enantiomer of 2‐chloropropan‐1‐ol (CH3C*HClCH2OH) in the chiral pores of the previously emptied framework (space group P21/c) results in the two corresponding chiral solids ( 1R and 1S , space group P21), while insertion of a racemic mixture of the two enantiomers retains the achirality of the host for the meso solid ( 1RS , space group P21/c). The R guest is ordered in the M channels while the S guest is ordered in the P channels. In contrast, the R guests in the P channels and the S guests in the M channels are disordered on two crystallographic orientations. For the racemic mixture of the two enantiomers in 1RS , random disorder of guests in both channels is observed. Thus, the localization of the guest molecule depends on the nature of the surface to recognize the guest of a particular chirality. The guest inclusion compounds are thermally stable. The 1R and 1S compounds are optically active. All the compounds adopt a ferrimagnetic ground state. Compared to the host framework of 1 without guest, the Curie temperature decreases for both 1R and 1S but increases for 1RS . The additional interactions between the framework and the inserted guest alcohols strengthen the lattice via hydrogen bonds and other electrostatic forces, and it might account for the significant lowering of the lattice contribution as well as the magnetic component to the specific heat capacity upon guest loading.  相似文献   

14.
    
The purpose of this paper is to connect two critical aspects of nanocomposite materials engineering: the knowledge of the orientational distribution of quiescent or flowing anisotropic macromolecules, and homogenization theory of composites with spheroidal inclusions at low volume fractions. The nano‐elements considered herein are derived from the class of high‐aspect‐ratio nematic polymers, either rod‐like or platelet spheroids. By combining the two features, we derive the effective electrical conductivity tensor in closed form. Scaling properties of enhanced conductivity versus volume fraction and weak shear rate become explicit. The most dramatic effect is that the effective conductivity tensor inherits hysteresis, bi‐stability, and discontinuous jumps from the isotropic–nematic first‐order phase transition. These formulas reveal finer estimates that depend on a competition between two inherently extreme parameters in nematic polymer nanocomposites: the molecular aspect ratio and the conductivity ratio of the inclusions and matrix. Herein, we confine our attention to steady monodomain orientational distributions at rest and in weak shear flows, which serve as benchmarks and guides for future extensions and numerical approaches.  相似文献   

15.
    
Films of liquid‐crystal networks with a splayed molecular alignment over their cross‐section display a well‐controlled deformation as a function of temperature. The deformation can be explained in terms of differences in thermal expansion depending on the average molecular orientation of the mesogenic centers of the monomeric units. The thermal expansion of the anisotropic polymers has been characterized as a function of their molecular structure and the polymerization conditions. As a reference, films with an in‐plane 90° twist have also been studied and compared with the splayed, out‐of‐plane molecular rotation. The twisted films show a complex macroscopic deformation owing to the formation of saddle‐like geometries, whereas the deformation of the splayed structured is smooth and well controlled. The deformation behavior is anticipated to be of relevance for polymer‐based microelectromechanical system (MEMS) technology.  相似文献   

16.
    
A thin layer of a vertically aligned nanocomposite (VAN) structure is deposited between the electrolyte, Ce0.9Gd0.1O1.95 (CGO), and the thin‐film cathode layer, La0.5Sr0.5CoO3 (LSCO), of a thin‐film solid‐oxide fuel cell (TFSOFC). The self‐assembled VAN nanostructure contains highly ordered alternating vertical columns of CGO and LSCO formed through a one‐step thin‐film deposition process that uses pulsed laser deposition. The VAN structure significantly improves the overall performance of the TFSOFC by increasing the interfacial area between the electrolyte and cathode. Low cathode polarization resistances of 9 × 10?4 and 2.39 Ω were measured for the cells with the VAN interlayer at 600 and 400 °C, respectively. Furthermore, anode‐supported single cells with LSCO/CGO VAN interlayer demonstrate maximum power densities of 329, 546, 718, and 812 mW cm?2 at 550, 600, 650, and 700 °C, respectively, with an open‐circuit voltage (OCV) of 1.13 V at 550 °C. The cells with the interlayer triple the overall power output at 650 °C compared to that achieved with the cells without an interlayer. The binary VAN interlayer could also act as a transition layer that improves adhesion and relieves both thermal stress and lattice strain between the cathode and the electrolyte.  相似文献   

17.
    
The macroscopic viscoelastic properties of a physical hydrogel are reversibly modulated by tuning the microscopic hydrogen‐bonding interactions with pH. The hydrogel forms at a rather low concentration of the multi‐pyridyl‐based gelator, N, N′, N″‐tris(3‐pyridyl)trimesic amide. The yield stress of the hydrogel is greatly enhanced from 10 to 769 Pa by changing the pH from 7.0 to 5.0. At pH 7.0, the amide molecules are assembled into an ordered structure as a result of the hydrogen bonds between the amide N–H bond and the nitrogen on the pyridyl group (N–H…Py). Fourier transform (FT) IR spectroscopy indicates that hydrogen bonds of N–H…Py are partially broken because the pyridyl groups are partly protonated at pH 5.0. This condition leads to a highly branched and homogeneous fibrillar network, which is confirmed by X‐ray diffraction (XRD) measurements and field‐emission scanning electron microscopy (FESEM) images. Highly branched fibrillar networks create more compartments and greatly increase the interfacial tension that is required to hold the solvent in the gel, thereby increasing the yield stress to 769 Pa. By further increasing the acidity of the hydrogel to pH < 3.0, the gel becomes a sol. Both the change in the viscoelastic properties and the sol–gel transition are reversible and controllable in the material.  相似文献   

18.
    
Nanoflakes of α‐Fe2O3 were prepared on Cu foil by using a thermal treatment method. The nanoflakes were characterized by X‐ray diffraction, scanning electron microscopy, high‐resolution transmission electron microscopy, and Raman spectroscopy. The reversible Li‐cycling properties of the α‐Fe2O3 nanoflakes have been evaluated by cyclic voltammery, galvanostatic discharge–charge cycling, and impedance spectral measurements on cells with Li metal as the counter and reference electrodes, at ambient temperature. Results show that Fe2O3 nanoflakes exhibit a stable capacity of (680 ± 20) mA h g–1, corresponding to (4.05 ± 0.05) moles of Li per mole of Fe2O3 with no noticeable capacity fading up to 80 cycles when cycled in the voltage range 0.005–3.0 V at 65 mA g–1 (0.1 C rate), and with a coulombic efficiency of > 98 % during cycling (after the 15th cycle). The average discharge and charge voltages are 1.2 and 2.1 V, respectively. The observed cyclic voltammograms and impedance spectra have been analyzed and interpreted in terms of the ‘conversion reaction' involving nanophase Fe0–Li2O. The superior performance of Fe2O3 nanoflakes is clearly established by a comparison of the results with those for Fe2O3 nanoparticles and nanotubes reported in the literature.  相似文献   

19.
    
Highly efficient orange and green emission from single‐layered solid‐state light‐emitting electrochemical cells based on cationic transition‐metal complexes [Ir(ppy)2sb]PF6 and [Ir(dFppy)2sb]PF6 (where ppy is 2‐phenylpyridine, dFppy is 2‐(2,4‐difluorophenyl)pyridine, and sb is 4,5‐diaza‐9,9′‐spirobifluorene) is reported. Photoluminescence measurements show highly retained quantum yields for [Ir(ppy)2sb]PF6 and [Ir(dFppy)2 sb]PF6 in neat films (compared with quantum yields of these complexes dispersed in m‐bis(N‐carbazolyl)benzene films). The spiroconfigured sb ligands effectively enhance the steric hindrance of the complexes and reduce the self‐quenching effect. The devices that use single‐layered neat films of [Ir(ppy)2sb]PF6 and [Ir(dFppy)2sb]PF6 achieve high peak external quantum efficiencies and power efficiencies of 7.1 % and 22.6 lm W–1) at 2.5 V, and 7.1 % and 26.2 lm W–1 at 2.8 V, respectively. These efficiencies are among the highest reported for solid‐state light‐emitting electrochemical cells, and indicate that cationic transition‐metal complexes containing ligands with good steric hindrance are excellent candidates for highly efficient solid‐state electrochemical cells.  相似文献   

20.
    
Complex oxides with perovskite structure are the ideal arena to study a plethora of physical properties including superconductivity, ferromagnetism, ferroelectricity, piezoelectricity and more. Among them, transition metal oxides are especially relevant since they present large electronic correlations leading to a strong competition between lattice, charge, spin, and orbital degrees of freedom. In particular, manganese perovskites oxides exhibit half‐metallic character and colossal magnetoresistive response rendering them as the ideal materials to develop novel concepts of oxide‐electronic devices and for the study of fundamental physical interactions. Due to the close similarity between kinetic energy of charge carriers and Coulomb repulsion, tiny perturbations caused by small changes in temperature, magnetic or electric fields, strain and so forth may drastically modify the magnetic and transport properties of these materials. In particular clarifying the role of interfacial strain in manganite thin films is interesting not only for device applications but also for basic understanding of physical interactions. A better comprehension of such strongly correlated systems might lead to control the different degrees of freedom in a near future contributing to the development of the so called orbitronics, i.e. controlling and modifying at will the orbital orientation of the 3d levels in transition metals. Here we reveal the importance of interfacial strain in high quality epitaxial thin films of La2/3Ca1/3MnO3 (LCMO), grown on top of SrTiO3 (STO) and NdGaO3 (NGO) (001)‐oriented substrates. We show that in such systems interfacial strain due to lattice mismatch lifts the degeneracy of the eg and t2g orbitals close to the film/substrate interface inducing Jahn‐Teller like distortions and promoting selective orbital occupancy and the appearance of an orbital glass insulating state in an otherwise ferromagnetic metallic material. These results highlight the role of strain and identify it as a key parameter in orbital control.  相似文献   

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