Fibrous scaffolds, which can mimic the elastic and anisotropic mechanical properties of native tissues, hold great promise in recapitulating the native tissue microenvironment. We previously fabricated electrospun fibrous scaffolds made of hybrid synthetic elastomers (poly(1,3‐diamino‐2‐hydroxypropane‐co‐glycerol sebacate)‐co‐poly (ethylene glycol) (APS‐co‐PEG) and polycaprolactone (PCL)) to obtain uniaxial mechanical properties similar to those of human aortic valve leaflets. However, conventional electrospinning process often yields scaffolds with random alignment, which fails to recreate the anisotropic nature of most of the soft tissues such as native heart valves. Inspired by the structure of native valve leaflet, we designed a novel valve leaflet‐inspired ring‐shaped collector to modulate the electrospun fiber alignment and studied the effect of polymer formulation (PEG amount [mole %] in APS‐co‐PEG; ratio between APS‐co‐PEG and PCL; and total polymer concentration) in tuning the biaxial mechanical properties of the fibrous scaffolds. The fibrous scaffolds collected on the ring‐shaped collector displayed anisotropic biaxial mechanical properties, suggesting that their biaxial mechanical properties are closely associated with the fiber alignment in the scaffold. Additionally, the scaffold stiffness was easily tuned by changing the composition and concentration of the polymer blend. Human valvular interstitial cells (hVICs) cultured on these anisotropic scaffolds displayed aligned morphology as instructed by the fiber alignment. Overall, we generated a library of biologically relevant fibrous scaffolds with tunable mechanical properties, which will guide the cellular alignment. 相似文献
We study how the uniaxial–biaxial nematic phase transition changes its nature when going from a low‐molecular‐weight liquid crystal to a liquid‐crystalline elastomer or polymer (the latter above the Maxwell frequency) and find a qualitative change due to the presence of a coupling to the strain field in these materials. While this phase transition can be of second‐order in low‐molecular‐weight materials, as is also experimentally observed, we show here that the order of this phase transition is changed generically to no phase transition at all or to a first‐order phase transition in mean‐field approximation. We analyze the influence of an external mechanical stress field above the uniaxial–biaxial nematic phase transition and find that either biaxial nematic order is induced, which is linear or quadratic in the stress intensity, or no response to an external stress results at all, depending on the relative orientation of the applied shear with respect to the director of the uniaxial nematic phase. 相似文献
The theoretically predicted optimum length/breadth/width ratio for maximizing shape biaxiality was investigated experimentally by the facile and successful synthesis of cross‐shaped compound 3 , which showed enantiomeric nematic phase behavior. This cross‐like core structure could alternatively be viewed as two fused V‐shaped mesogens, which have recently immerged as a new direction in biaxial nematic research, at the bending tips that can act as a new structure for biaxial investigations. Whilst the thermal analysis data of compound 3 did not meet the expected theoretical values for biaxial nematics, surface‐induced biaxiality was evidenced by optical studies. Cluster‐size analysis within the nematic phase of compound 3 revealed the formation of meta‐cybotactic nematics, which approached the cluster sizes of cybotactic nematics. The split small‐angle 2D X‐ray diffraction patterns of magnetic‐field‐aligned samples indicated that the nematic phase was composed of small smectic C‐like clusters with the tilting of molecules within the clusters. The wide‐temperature‐range enantiomeric nematic phase of cross‐like compound 3 enabled the molecular skeleton to serve as an alternative skeleton to bent‐rod mesogens, which exhibited nematic phases with the potential competition of transitions to higher‐order liquid‐crystalline phases and crystallization, for future biaxial investigations. 相似文献
Two-dimensional materials have been extensively applied because of their unusual electronic, mechanical, and optical properties. In this paper, the electronic structure and optical properties of Hf2CO2 MXene under biaxial and uniaxial strains are investigated by the Heys-Scuseria-Ernzerhof (HSE06) method. Monolayer Hf2CO2 can sustain stress up to 6.453 N/M for biaxial strain and 3.072 N/M for uniaxial strain. Monolayer Hf2CO2 undergoes the transition from semiconductor to metal under −12% strain whether it is under biaxial or uniaxial strain. With the increasing biaxial compressive strain, the blue shift of Hf-d, O-p, and C-p orbitals in valence band maximum results in the metallization of monolayer Hf2CO2, while the red shift of Hf-d and O-p orbitals in conduction band minimum results in the metallization of monolayer Hf2CO2 with increasing uniaxial compressive strain. The analysis of optical properties indicates that uniaxial strain weakens the reflectivity and refractive index of monolayer Hf2CO2 in the visible-light range. In addition, the effective mass and the charge distribution under biaxial and uniaxial strains are also explored. 相似文献
New, bent‐core mesogens are described in which the core of the molecule is a semiflexible, di(4‐aminocyclohexyl)methane spacer. The compounds show nematic, columnar nematic and columnar phases as shown by a combination of X‐ray diffraction and optical microscopy. The potential of these new mesogens as biaxial nematic candidates is considered. 相似文献
Summary: The orientation behavior of a smectic A elastomer is investigated by applying a biaxial mechanical field to the elastomer in a swollen state. The network is composed of a siloxane‐polymer backbone, a bi‐functional cross‐linker, and a monomer with a perfluorinated tail. In this work, biaxial deformation is successfully achieved to macroscopically orient the smectic A phase in a uniform, homeotropic fashion. We describe the orientation process in detail and discuss the microstructure of the smectic A phase organized in the monomer, the linear polymer, and the elastomer determined by using X‐ray diffraction data.
A photograph of the oriented smectic A elastomer E4F (above) and a schematic model of its homeotropic orientation (below). 相似文献
We report the sphere method as a unique characterization technique for the complete study of non-linear optical properties for frequency conversion in new materials belonging to the uniaxial or biaxial optical class. It relies on the use of a single crystal with millimetre dimensions cut as a sphere, combined with a tuneable laser source. With the sphere method we perform direct measurements of phase-matching angles and associated conversion efficiencies for second harmonic, sum- and difference-frequency generation. Furthermore, we follow the orientation of the dielectric frame as a function of the wavelength for monoclinic and triclinic crystals. It also allows the determination of the magnitude of the principal refractive indices in biaxial crystals based on the study of the double refraction affect at the exit of a sphere. By combining the analysis of all these data simultaneously, we determine Sellmeier equations reliable over the whole transparency domain and we are able to get the non-zero elements of the second-order susceptibility tensor of uniaxial or biaxial crystals. Finally, the sphere method is completely self-sufficient for the study of biaxial crystals. 相似文献
2H NMR investigations on the biaxial phase behavior of smectic‐A liquid crystalline side‐chain elastomers are presented. Biaxiality parameters were determined by measuring the quadrupolar splitting of two spin probes, namely benzene‐d6 and hexamethylbenzene‐d18, at various angles between the principal director and the external magnetic field: while for a uniaxial sample the angular dependence can be described by the second Legendre polynomial, an additional asymmetric term needs to be included to fit the data of the two investigated biaxial systems. Two elastomers synthesized from mesogens that differ in the molecular geometry in order to study the molecular origin of biaxiality were compared. Biaxiality is observed for both elastomers when approaching the glass transition, suggesting that the network dynamics dominate the formation of the biaxial phase.
X‐ray diffraction patterns for the uniaxial and biaxial nematic phases exhibited by rigid bent‐core mesogens were calculated using a simple model for the molecular form factor and a modified Lorentzian structure factor. The X‐ray diffraction patterns depend strongly on the extent of the alignment of the molecular axes as well as the orientation of molecular planes. The X‐ray diffraction can be unequivocally used to identify the biaxial nematic phase, study the uniaxial–biaxial phase transition, and estimate the order parameters of the nematic phase. 相似文献
It is shown that biaxial thermotropic nematics have anomalously small parameters of biaxial orientational order of molecules, which are one or two orders of magnitude lower than the limit of their detection by NMR. This accounts for the dramatic discrepancy between the optical and NMR data on biaxiality of these compounds. 相似文献
The effect of biaxial orientation on the mechanical properties and supramolecular structure of the films based on rigid-chain polyimides and copolyimides was studied. The films were oriented in two different ways: by hot drawing and by cold drawing at the prepolymer stage and the subsequent cyclization according to the specified temperature regime. It was shown that the curing of the precursor films after their preliminary biaxial drawing is accompanied by self-orientation, although its degree is well below that after uniaxial drawing under the same conditions. As compared with homopolyimide films, the films based on rigid-chain copolyimide exhibit a lower tendency toward orientation under biaxial drawing. The reasons for such behavior are discussed. 相似文献
Highly filled elastomers present strong nonlinear mechanical behavior. This study proposes a biaxial dynamic mechanical analysis (DMA) experiment to study the prestrain induced nonlinearity. This phenomenon has already been observed for uniaxial tests, revealing an increase of the amplitude of the dynamic modulus with prestrain. The novelty proposed here is to investigate the problem under biaxial conditions. For this purpose, a specific apparatus and an appropriate specimen have been designed. Strains and stresses have been measured using localization formulae and compared with measurements from digital image correlation and finite element computations. Biaxial DMA tests were performed on a propellant specimen, for different values of biaxial prestrain. The material is a highly filled elastomer with an important influence of the prestrain on the global viscoelastic behavior. The results exhibit increasing amplitude of the complex modulus with increasing prestrain, as in uniaxial experiments. Moreover, the dependence can be characterized using the second invariant of the prestrain, and the viscoelastic behavior is modeled using a closed-form spectrum of relaxation times. 相似文献
Mechanical properties of hydrated bacterial cellulose have been tested as a function of fermentation time and following the
alkali treatment required for sterilisation prior to biomedical applications. Bacterial cellulose behaves as a viscoelastic
material, with brittle failure reached at approximately 20% strain and 1.5 MPa stress under uniaxial tension. Treatment with
0.1 M NaOH resulted in minimal effects on the mechanical properties of bacterial cellulose. Fermentation time had a large
effect on both bacterial numbers and cellulose yield but only minor effects on mechanical properties, showing that the fermentation
system is a robust method for producing cellulose with predictable materials properties. The failure zone in uniaxial tension
was shown to be associated with large-scale fibre alignment, consistent with this being the major determinant of mechanical
properties. Under uniaxial tension, elastic moduli and failure stresses are an order of magnitude lower than those obtained
under biaxial tension, consistent with the fibre alignment mechanism which is not available under biaxial tension. 相似文献
This Review covers photonic crystals (PhCs) and their use for sensing mainly chemical and biochemical parameters, with a particular focus on the materials applied. Specific sections are devoted to a) a lead‐in into natural and synthetic photonic nanoarchitectures, b) the various kinds of structures of PhCs, c) reflection and diffraction in PhCs, d) aspects of sensing based on mechanical, thermal, optical, electrical, magnetic, and purely chemical stimuli, e) aspects of biosensing based on biomolecules incorporated into PhCs, and f) current trends and limitations of such sensors. 相似文献