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Cover: Scanning electron microscopy (SEM) image of electrospun poly(ɛ- caprolactone) (PCL) nanofibers after being immersed in phosphate buffered saline (PBS, pH=7.4) for 7 days [X. Jing, H-Y. Mi, M.Salick, T. Cordie, J. McNulty, X-F. Peng, L-S. Turnq: In Vitro Evaluations of Electrospun Nanofiber Scaffolds Composed of Poly(ɛ-caprolactone) and Polyethylenimine for Vascular Tissue Engineering. p. 1808].
Nanoindentation is an effective approach for measuring mechanical properties of nanoscale films coated on substrates, yet results obtained through the classic Oliver–Pharr model require additional consideration due to the existence of a “substrate effect” when the film is much more compliant than the substrate. In this study, different models for removing this substrate effect are compared, with focus on the Gao model, the Saha–Nix model, and the Hay model and the use of a direct finite element (FE) approach is discussed. Validity of these models is examined using load–displacement data obtained from simulated indentation of an elastic–plastic film in FEs. It is found that the performance of the analytical models varies significantly with different testing parameters, including ratio between film modulus and substrate modulus (Ef/Es), indenting ratio (hmax/film thickness), and yield strain. Choices of using a nanoindentation model to process experimental data should be made according to estimated indentation depth and modulus difference between film and substrate. An example of applying substrate removal models to experimental data is also shown.
Microporous carbon nanospheres were prepared from β-cyclodextrin (β-CD) by solvothermal carbonization in o-dichlorobenzene in the presence of various concentrations of p-toluene sulfonic acid (PTSA). The contribution of PTSA toward solvothermal char (STC) was established. The STC showed the highest surface area, porosity, and CO2 sorption capacity at a PTSA to β-CD weight ratio of 2.5. The surface area, pore volume, and CO2 sorption capacity were further increased by an in situ high-temperature activation due to the oxidation of carbon at high temperature by oxygen present in the STC. The high-temperature activation reduces the significance of PTSA concentration, as the activated STC showed surface area, micropore volume, and CO2 adsorption capacity in a close range at the PTSA to β-CD weight ratio in the range of 0.04–2.50. The highest CO2 adsorption capacity of the STC increased from 2.4 to 3.5 mmol/g upon the high-temperature activation. The activated STC adsorbs significant amount (0.35 mmol/g) of CO2 from dry air containing 400 ppm CO2. The activated STC showed excellent regeneration stability and selectivity over nitrogen.
A catalytic depolymerization (a reversible polymerization) of 3D-polymerized C60 phases (including an ultrahard fullerite phase) takes place in the presence of sulfur under the conditions of a large plastic deformation at room temperature. The sulfur atoms remain in the samples of 3D C60 polymers after catalytic synthesis using carbon disulfide (CS2) as a catalyst (the presence of sulfur has a considerable impact on the 3D C60 polymerization by decreasing the polymerization pressure). Raman, infrared, and transmission electron microscope studies show that the depolymerized fullerite samples have a structure typical for dimers, 1D and 2D C60 polymers. The 3D C60 samples with some remaining sulfur can be quenched under ambient conditions if the samples have not undergone a large plastic deformation. There is no depolymerization for pure C60 3D-polymerized phases synthesized without a sulfur-based catalyst.
The local micromechanical properties of two cyclic olefin copolymers (COCs) under an applied strain were measured using quasi-static (QS) and dynamic nanoindentation. Samples were prepared by compression molding and tested at five various applied strain levels, leading to a variation in pileup around the residual indentation impression. The variation in the resulting pileup morphology and the subsequent perceived changes in modulus and hardness as a function of applied strain was quantified for these COCs. The perceived mechanical properties determined using both QS and dynamic tests were influenced by the relative out of plane deformation, and as such provide a method to map local variations in residual stresses and strains without the need to measure residual impression pileup for each indentation. The dynamically measured properties appear to provide a more consistent correlation with both the applied strain and pile up behavior around the indents than the modulus and hardness determined from QS nanoindentation.
Polyurethane open cell (PUOC) composites containing SiO2 and Al2O3 nanoparticles (NPs) were prepared. Scanning electron microscopy and Z-scan methods were used for observing porosity and detecting third-order nonlinear optical properties of related samples. Adding NPs into polymer matrix decreased the cell size and subsequently increased the porosity of samples. The nonlinear effects of samples were increased by adding 1 wt% of NPs into polymer in comparison with blanks. However, those features were decreased again through higher loading (up to 2.0 wt%) of NPs. The nonlinear refractive indices and nonlinear absorption coefficients of the synthesized samples were obtained in the order of 10−8 (cm2/W) with negative sign and 10−5 (cm/W), respectively. All the results suggest that the nonlinear coefficients of the synthesized samples can be controlled by NP contents in PUOC.
Nanocrystalline cellulose (NCC) whisker obtained from acid hydrolysis of cotton was incorporated into the freezing polymerized PNIPA/clay hydrogels to prepare inorganic–organic hybrid nanocomposite hydrogels (named as C-NC gels). The influence of NCC on the properties of C-NC gels was investigated systematically. It was found that all C-NC gels exhibit similar lower critical solution temperature as that of NCC-free gels, being independent of the NCC content. However, with the increase of NCC content in C-NC gels, the swelling ability of gels decreases slightly while the response rate of gels increases gradually, the gels with high content of NCC exhibit an ultrarapid deswelling rate due to the amount of interconnected micropores appeared inside the gels. Moreover, the enhancement effect of increased NCC on the gels is significant, which is also determined by the swelling degree of gels directly. Comparably, for the gels with the same content of NCC, higher strength was found when the gels were kept in lower swelling ratio due to the stronger interaction of NCC through hydrogen bond in the gels.
The work was intended to explore the effect of the widely available cationic polymer polyethylenimine (PEI) on small diameter poly(ɛ-caprolactone) (PCL) blood vessel grafts. PEI was blended with PCL and electrospun into nanofibrous vascular scaffolds. The morphologies, wettabilities, mechanical properties, and biological activities of the PCL/PEI electrospun nanofibers were investigated. It was found that by increasing the content of PEI to 5% within the scaffolds, the fiber diameters decreased from 469.7 ± 212.1 to 282.5 ± 107.1 nm, the water contact angle was reduced from 126.6 ± 1.1° to 27.6 ± 3.9°, while the Young's modulus increased from 2.0 ± 0.2 to 4.1 ± 0.1 MPa, the suture retention strength increased from 4.2 ± 0.4 to 6.1 ± 0.7 N, and the burst pressure increased from 801.2 ± 14.1 to 926.2 ± 22.8 mmHg. The in vitro evaluations demonstrated that the nanofibers containing 2% PEI promoted the attachment and proliferation of human umbilical vein endothelial cells (HUVECs).
While conventional metallic glass (MG) is usually an alloy that contains at least two types of different elements, monatomic metallic glass (MMG) in body-centered cubic metals has recently been vitrified experimentally through ultrafast quenching. In this research, MMG in Ta was vitrified by molecular dynamics simulations and used as a model system to explore the atomistic mechanism of hardening in MG under cyclic loading well below the yield point. It was found that significant structural ordering was caused during the elastic cycling without accumulating apparent plastic strain, which ultimately led to the crystallization of MG that has been long conjectured but rarely directly proved before. It was also revealed that tensile stresses were more likely to induce structural ordering and crystallization in MG than compressive stresses.
The crystal–melt interfacial free energy is an important quantity governing many kinetic phenomena including solidification and crystal growth. Although general calculation methods are available, it is still difficult to obtain the interfacial energies that differ only slightly due to anisotropy. Here, we report such a calculation of Al crystal–melt interfacial energy based on the general framework of the capillary fluctuation method (CFM). The subtle dependence of both the melting temperature and interfacial free energy at melting temperature on the crystal interface orientation was examined. For Al, the average melting temperature is obtained at 934.79 ± 5 K and the orientationally averaged mean interfacial free energy is 98.35 mJ/m2. In addition, the anisotropy of the interfacial free energy is found weak, nevertheless with the values ranked as γ100 > γ110 > γ111.
A new theoretical model is proposed to describe the mechanical properties of bimodal nanocrystalline (BNC) materials. This composite model is comprised of coarse grains evenly distributed in the nanocrystalline (NC) matrix. In this study, we have studied the effect of grain size distribution on the constitutive behavior of BNC materials. During the plastic deformation, effects of nanocracks and dislocation emission from crack tips on the constitutive behavior of BNC materials are also analyzed. Numerical calculations have been carried out according to the model, and it is found that the nanocracks make a positive effect on the strain hardening, and the results show that this model can describe the enhanced strength and strain hardening of BNC materials successfully. The prediction of the bimodal Cu–Ag material is in good agreement with the experimental results.
The M-type hexaferrite Sr1−xLaxFe11.75Co0.10Zn0.15O19 (0 ≤ x ≤ 0.7) magnetic powders and magnets were synthesized by the ceramic process. The phase constituents of the magnetic powders were analyzed by x-ray diffraction. There is a single magnetoplumbite phase in the magnetic powders with La content (0.2 ≤ x ≤ 0.4). For the magnetic powders containing La content (0 ≤ x ≤ 0.1) or (0.5 ≤ x ≤ 0.7), magnetic impurities coexist in the structure. The microstructures of the magnets were characterized by field emission scanning electron microscopy. The magnets consist of homogenously distributed ferrite particles with the hexagonal structures. The magnetic properties of the magnets were measured by a permanent magnetic measure equipment. The remanence, maximum energy product, and Hk/Hcj ratio of the magnets at x = 0.3 reach the maximum values. However, the intrinsic coercivity and magnetic induction coercivity of the magnets at x = 0.2 reach the maximum values.
The kinetic and static friction forces between Al2O3 nanowires (NWs) and a Si substrate were simultaneously determined by the use of bending manipulation, which bent a NW into a “hook” shape, and then let it recover elastically. An analytical model was developed to estimate the kinetic friction force based on the hypothesis that part of the elastic energy stored in the bent NW was consumed by the work of the friction during recovering. The static friction force was also calculated using force equilibrium. Finite element analysis and experimental testing were performed to verify the analytical model. The kinetic and static friction forces per unit area obtained were in the ranges of 1.16–3.4 MPa and 0.68–2.7 MPa, respectively, which agree well with most of the values reported previously for NWs or nanoparticles on flat substrates. It was also found that the NW size had no apparent effect on the interfacial shear stress.
In cyclic nanoindentation of single-crystal silicon, an interesting phenomenon of a secondary pop-out event that closely follows the first pop-out event but with a larger critical load than the first is presented. Cyclic nanoindentation experiments under various loading/unloading rates and various maximum indentation loads were performed to verify the generality of the phenomenon of two pop-out events. Raman spectroscopy results indicate that the secondary pop-out does not induce any new phase, and the dominated end phases after the two pop-out events are still a mixture of Si-XII/Si-III phases. According to average contact pressure analysis, the phase transformation paths and the formation mechanism for the secondary pop-out event are discussed from the viewpoint of crystal nucleation and growth. The results indicate that phase transformations from the Si-I phase to Si-XII/Si-III phases are completed by two pop-out events in two adjacent indentation cycles, and the Si-XII/Si-III phases formed in previous indentation cycles strongly affect the phase transformations in subsequent loading/unloading processes.