Mechanical study on the impact of an effective solvent support-removal methodology for FDM Ultem 9085 parts

The use of support material to produce Fused Deposition Modeling parts is often unavoidable. The support removal task tends to be laborious and time-consuming when no soluble support materials are available, which is the case of the high-performance thermoplastic Ultem™ 9085. This paper investigates the effect of different solvent/solvent mixtures on Ultem’s mechanical properties with the aim to identify a solvent capable of dissolving its support material (a polysulfone) without noticeably damaging the model material. To do so, initial solubility tests have helped narrow the list of solvent candidates. These have been followed by infrared analyses to identify the presence of dissolved polymers in the media, as well as scanning electron microscope micrographs to analyze the surface topography of the treated parts. Finally, tensile and flexural tests have permitted to quantify the change on Ultem’s mechanical properties as a function of the treatment time. Major findings include a reproducible method for softening or eliminating Ultem’s support material with non-significant changes in their mechanical properties. The outcome of this work represents a first step on the lookout for a solution to facilitate the removal of polysulfone and is considered of great interest for the scientific community due to the rise of Ultem as a structural material.     

Damage accumulation and fracture in aged lead-free Sn-3.5Ag solder joints

Abstract  

Owing to concerns regarding the environment and health, lead-containing solders have now been eliminated and substituted by their lead-free counterparts. Hence, the present article is devoted to the clarification of their mechanical strength and reliability. Lead-free Sn-3.5Ag solder joints of various thicknesses were exposed to different thermal treatments in order to study the effect of material changes due to ageing. Thereafter, tensile tests were performed showing a pronounced decrease of strength after excessive heat treatment. The theoretical analysis is facilitated by simulations according to the finite element method. Thereby, the influence of material changes in the solder could be separated from the effect of thermal recovery in the copper base material. Crack initiation in the solder is described by an approach of damage mechanics derived from a thermodynamic framework. Excessive heat treatment leads to Kirkendall voids reducing the ultimate tensile strength of solder joints. Therefrom, one can estimate the reduction of tensile strength as a function of time and temperature.     

Progress in mechanochromic luminescence of gold(I) complexes

Photophysical properties of organic and organometallic luminophors are closely related with their molecular packings, enabling the exploitation of stimuli-responsive functional luminescent molecules. Mechanochromic molecules, which can change their luminescence characteristics after mechanical stimulus, have received an increasing interest due to their promising applications in multifunctional sensors and molecular switches. During the past two decades, the development of gold(I) chemistry has been attracting the attention of plenty of researchers. Indeed, a variety of gold(I) complexes with fascinating photophysical behaviors have been discovered. This review focuses on the research progress in the different types of mechanoluminochromic gold(I) complexes, including mono-, bi- and multi-nuclear gold(I) systems. Their interesting luminescence behaviors of these gold(I)-containing luminogens upon mechanical stimulus and the proposed mechanisms of their observed mechanochromic luminescence are summarized systematacially. Moreover, this review will put forward an outlook about the possible opportunities and challenges in this significative scientific field.     

Self‐reinforced polypropylene composites based on low‐cost commercial woven and non‐woven fabrics

In the present paper, different self‐reinforced polypropylene (PP) composites based on low‐cost commercial woven (w) and non‐woven (nw) fabrics were obtained. Hot compaction (HC) and film stacking (FS) followed by compression molding were used to prepared the composites. The fracture and failure behavior of the different materials was determined under different testing conditions through quasi‐static uniaxial tensile tests, Izod impact experiments and by means of fracture mechanics tests on mode I double‐edge deeply notched tensile specimens. In the case of the composite obtained by film stacking + compression molding (rPP/nw/w‐FS) and the hot‐compacted composite (nw/w‐HC) containing simultaneously woven and non‐woven fabrics, the acoustic emission technique was applied in situ in the tensile tests to determine their consolidation quality and to identify the failure mechanisms responsible for their fracture behavior. It was observed that both composites exhibited relatively similar high consolidation quality. However, the hot‐compacted composite presented a more uniform distribution of failure mechanisms (debonding and fiber fracture) than the film‐stacked composite. The hot‐compacted composite containing both types of reinforcements exhibited the best combination of mechanical (tensile, impact, and fracture) properties. Therefore, this composite appeared as the most promising for structural applications among the different composites investigated.     

Effect of steam on structure and mechanical properties of biomedical block copolymers

The effect of steam on the micro‐phase structure and mechanical properties of different block copolymers used in biomedical devices is investigated via FT‐IR, tensile tests and dynamic mechanical analysis (DMA). Steam sterilization, commonly performed on medical devices and simulated in this work, affects the copolymers' morphology, due to high temperature and humidity conditions. FT‐IR analysis reveals that steam induces a modification in the crystalline conformations of copolymers with a pre‐existing hydrogen bonding network, that is, thermoplastic polyurethanes (TPU) and poly(ether‐block‐amide) (PEBA), while it does not significantly affect the domain conformation in styrenic block copolymers (SEBS), due to weak interaction with water. As a consequence, relevant changes of the mechanical properties, closely related to the microdomain structure, are found for TPU and PEBA after sterilization, while SEBS mechanical behavior remains stable, as demonstrated by tensile tests and DMA results. For this reason, SEBS is suggested as the best choice in terms of durability in biomedical applications. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 1337–1346     

Characterization of hdpe/ha Composites Treated with Titanate and Zirconate Coupling Agents

The development of new materials for bone substitution has been of great interest for the scientific community in the last years. High-density polyethylene (HDPE) and hydroxyapatite (HA) composites have been used in biomedical applications without any inflammatory response. However, the differences in nature of both materials have motivated the use of coupling agents to improve their interfacial interactions. In this work, the effects of adding three different commercial coupling agents (NZ12, Lica01, Lica12) to high-density polyethylene (HDPE)/hydroxyapatite (HA) composites were studied. Composites containing 20 parts per hundred (phr) of HA previously treated with the already mentioned coupling agents were characterized by mechanical tests and their morphologies were analyzed afterwards. Composites with 0.3 and 0.5 wt% of NZ12 unfolded an increase in their Young's modulus and tensile strength, as a consequence of an improved dispersion of the filler into the polymeric matrix. Analysis of the samples by XPS showed that the zirconate coupling agent interacted more with the HA, as reflected in the lower binding energies of the corresponding oxygen atoms, which agrees with their better performance from the mechanical point of view.     

Thickness of hydroxyapatite nanocrystal controls mechanical properties of the collagen-hydroxyapatite interface

Collagen-hydroxyapatite interfaces compose an important building block of bone structures. While it is known that the nanoscale structure of this elementary building block can affect the mechanical properties of bone, a systematic understanding of the effect of the geometry on the mechanical properties of this interface between protein and mineral is lacking. Here we study the effect of geometry, different crystal surfaces, and hydration on the mechanical properties of collagen-hydroxyapatite interfaces from an atomistic perspective, and discuss underlying deformation mechanisms. We find that the presence of hydroxyapatite significantly enhances the tensile modulus and strength compared with a tropocollagen molecule alone. The stiffening effect is strongly dependent on the thickness of the mineral crystal until a plateau is reached at 2 nm crystal thickness. We observe no significant differences due to the mineral surface (Ca surface vs OH surface) or due to the presence of water. Our result shows that the hydroxyapatite crystal with its thickness confined to the nanometer size efficiently increases the tensile modulus and strength of the collagen-hydroxyapatite composite, agreeing well with experimental observations that consistently show the existence of extremely thin mineral flakes in various types of bones. We also show that the collagen-hydroxyapatite interface can be modeled with an elastic network model which, based on the results of atomistic simulations, provides a good estimate of the surface energy and other mechanical features.     

振动注塑PP的拉伸蠕变研究

分别通过改变机械振动注塑机的频率(5~25 Hz)和压力(10~18 MPa)获得不同条件下成型的PP样条,然后在各种成型条件下的PP样条上分别施加相同的拉伸力(F=125 N),进行24 h拉伸蠕变实验.结果表明,在相同的振动频率(10 Hz)和不同的振动压力下成型的PP试样,其24 h蠕变量随着压力的增大而减小;在相同振动压力(12 MPa)和不同的振动频率下成型的PP试样,其24 h蠕变量随着频率的增大而增大.当振动频率达到f=10 Hz的时候,其24 h拉伸蠕变量的变化趋于平缓.同时,也对不同振动条件下注塑的PP试样进行拉伸实验,冲击实验和动态力学性能测试,讨论了成型条件对性能的影响.     

Mechanical behaviour of a poydimethylsiloxane elastomer after outdoor weathering in two different weathering locations

The degradation of maxillofacial prosthetic elastomers that occur during physical weathering is usually responsible for the replacement of the prosthesis. In this study the mechanical behaviour of a polydimethylsiloxane (PDMS) elastomer was investigated, after 1 year outdoor weathering in two different weathering locations in Greece (Thessaloniki, Athens). The hypothesis investigated was that irradiation time did not affect the measured properties. Specimens (Elastomer 42) were prepared according to manufacturer’s instructions and exposed to solar radiation for 1 year. Compression, tensile and nanoindentation tests were performed before and after the exposure. Compression and tensile data were also subjected to analysis of variance (ANOVA) and Tukey Post hoc tests at a level of α = .05. These properties were selected due to their clinical significance for fabrication and maintenance of a facial prosthesis. According to statistical analysis all the measured properties changed significantly after outdoor weathering. More specifically, most of the properties presented significant changes after six months of weathering. The observed changes also depended on the weathering locations. The hypothesis investigated was rejected. Material A became harder and the observed differences in the mechanical behaviour resulted from photo-degradation and hydrolysis that might occur due to weathering. The study also provides new information about maxillofacial prosthetics serviceability obtained from nanoindentation tests.     

High Performance Electrospun Polynaphthalimide Nanofibrous Membranes with Excellent Resistance to Chemically Harsh Conditions

Polynaphthalimide (PNI) with six-membered imide ring (6-PI) has better chemical resistance than five-membered imide ring (5-PI),but is difficult to be processed into nanofibers due to the poor processability.In this work,we proposed a template strategy to fabricate nanofiber 6-PI membranes and their composite membranes.Neat 6-PI and 6-PI composite fibrous membranes were prepared using high-molecular-weight polymers 5-PAA and PVP as templates by electrospinning.FTIR,DMA,TGA and tensile tests were used to characterize their chemical structures,thermal stability and mechanical properties.Further eye-observation,micro-morphology investigation and tensile tests were applied to evaluate the chemical resistance of nanofibrous membranes in strong acid,strong alkaline,and concentrated salt.The results demonstrated that 6-PI nanofibrous membranes possessed the best thermal stability,best acid,alkaline,and salt resistance with the highest mechanical retention.This study will provide basic information for high-performance electrospun 6-PI nanofiber membranes and provide opportunities for applications of PIs in different chemically harsh environments.     

Linear-scaling computation of excited states in time-domain

The applicability of quantum mechanical methods is severely limited by their poor scaling.To circumvent the problem,linearscaling methods for quantum mechanical calculations had been developed.The physical basis of linear-scaling methods is the locality in quantum mechanics where the properties or observables of a system are weakly influenced by factors spatially far apart.Besides the substantial efforts spent on devising linear-scaling methods for ground state,there is also a growing interest in the development of linear-scaling methods for excited states.This review gives an overview of linear-scaling approaches for excited states solved in real time-domain.     

Weibull statistical analysis and experimental investigation of size effects on tensile behavior of dry unidirectional carbon fiber sheets

In this study, experimental tests and a statistical analysis are conducted to verify the size effects associated with length on the tensile behavior of dry unidirectional carbon fiber sheets (DUCFS). As a statistical method, the two-parameters Weibull theory is considered. To verify the validity of the Weibull theory, four sets of direct tensile test were made on DUCFS specimens having different lengths. The direct tensile tests consist to evaluate the size effects associated with length on the mechanical properties of DUCFS such as tensile strength and the Young's modulus. The experimental results show a size effect of length on the tensile strength of DUCFS. Indeed, the tensile strength decreases when increasing the specimen length. In addition, the experimental results demonstrate that there is no size effect on the failure mode and on the Young's modulus of DUCFS. Finally, Weibull weakest link theory is used to predict the tensile strength size effect on DUCFS specimens. A good agreement was shown between the test data and predicted results obtained using the Weibull weakest link theory.     

Flavonoids: biological activities and therapeutic potential

Abstract

Flavonoids have aroused much interest in research, since they present a great diversity of biological activities observed in vitro, such as: antioxidant effect, modulation of the enzymatic activity and inhibition of cellular proliferation, exerting beneficial effects on the organism, as well as the use of its therapeutic potential. With wide distribution in the plant kingdom represent a class of phenolic compounds that differ in their chemical structure and particular characteristics. The objective of this review was to describe the relevant aspects of flavonoids, reporting the different known groups, the probable mechanisms by which they act, their pharmacological properties and to gain a better understanding of the reported beneficial health effects of these substances. This systematic review consisted of research using scientific databases such as Scopus, Science Direct, PubMed, SciVerse and SciELO, without time limitation. Some pharmacological properties of some flavonoids and their health benefits have been confirmed by previous studies.     

Rheological,morphological and mechanical properties of flax fiber polypropylene composites: influence of compatibilizers

In this work, the rheological, mechanical and morphological properties of flax fiber polypropylene composites were investigated. The effect of incorporating a polypropylene grafted acrylic acid or a polypropylene grafted maleic anhydride on these properties has been studied as well. According to scanning electron microscopic observations and tensile tests, the addition of a compatibilizer improved the interfacial adhesion between the flax fibers and the polymer matrix. The tensile modulus of composite containing 30 wt% flax fibers was improved by 200 % and the tensile strength improved by 60 % in comparison with the neat PP. Plasticizing effect of the compatibilizers as a result of their lower melt flow index was also shown to decrease the rheological properties of the composites, even though the effect was not pronounced on the mechanical properties.     

A theory of finite tensile deformation of double-network hydrogels

A continuum damage model was developed to describe the finite tensile deformation of tough double-network (DN) hydrogels synthesized by polymerization of a water-soluble monomer inside a highly crosslinked rigid polyelectrolyte network. Damage evolution in DN hydrogels was characterized by performing loading-unloading tensile tests and oscillatory shear rheometry on DN hydrogels synthesized from 3-sulfopropyl acrylate potassium salt (SAPS) and acrylamide (AAm). The model can explain all the mechanical features of finite tensile deformation of DN hydrogels, including idealized Mullins effect and permanent set observed after unloading, qualitatively and quantitatively. The constitutive equation can describe the finite elasto-plastic tensile behavior of DN hydrogels without resorting to a yield function. It was showed that tensile mechanics of DN hydrogels in the model is controlled by two material parameters which are related to the elastic moduli of first and second networks. In effect, the ratio of these two parameters is a dimensionless number that controls the behavior of material. The model can capture the stable branch of material response during neck propagation where engineering stress becomes constant. Consistent with experimental data, by increasing the elastic modulus of the second network the finite tensile behavior of the DN hydrogel changes from necking to strain hardening.     

Design and Catalytic Application of Functional Porous Organic Polymers: Opportunities and Challenges

This review article encompasses the progress and conventional overview of current research activities of porous organic polymers (POPs), especially in catalysis, as they have garnered colossal interest in the scientific fraternity due to their intriguing characteristic features. Various synthetic strategies with possible modification of functionality of POPs have been used to improve the catalytic efficiency towards value‐added chemicals production. Accordingly, this review article is mainly focused on the design, development of various functionalized POPs by employing Friedel‐Crafts alkylation, FeCl₃ assisted oxidative polymerisation and polymerisation in nonaqueous medium, and a comprehensive understanding in potential catalytic applications namely, acetalization, hydrodeoxygenation (HDO), hydrogenation, coupling, photocatalytic hydrogen evolution and biomass conversion towards the production of value‐added chemicals in biodiesel and chemical industries.     

纳米粒子与细胞相互作用的力学-化学偶联研究进展

纳米粒子在生物医学和大气环境领域的广泛研究使得其生物安全性越来越受到重视。目前已经有许多研究关注纳米粒子与细胞的相互作用及细胞毒性问题。本综述从细胞力学-化学偶联的角度总结了近五年来有关纳米粒子与细胞相互作用的研究进展。首先介绍了与细胞力学-化学偶联性质相关的分子基础以及目前检测细胞机械性质的纳米技术,然后重点讨论了纳米粒子对细胞粘附、骨架、刚度和迁移性质的影响。在此基础上,进一步指出了纳米生物力学-化学偶联的挑战与展望。     

Investigation of nanomechanical and morphological properties of silane-modified halloysite clay nanotubes reinforced polycaprolactone bio-composite nanofibers by atomic force microscopy

There is remarkable interest in the fabrication of polymeric composite nano/micro-fibers by electrospinning for many applications ranging from bioengineering to water/air filtration. In almost all of these applications, the mechanical properties of both the polymer fibers and their assemblies, are significant. In this study, unmodified, 3-Glycidoxypropyltrimethoxysilane (GPTMS) or 3-Aminopropyltriethoxysilane (APTES) modified halloysite clay nanotube (HNT) reinforced polycaprolactone (PCL) nanofibers were successfully synthesized via the electrospinning. The morphology and mechanical features of the obtained electrospun fibers were investigated by atomic force microscopy (AFM) and AFM-based nanoindentation for single fibers in nanoscale, respectively. Besides, scanning electron microscopy and tensile strength tests were used to investigate whole fibrous structures in microscale. The AFMresults, accompanied by SEM and tensile strength, support the conclusion that silane-modification affected positively the morphology and mechanical characteristics of electrospun PCL nanofibers. Therefore, it was concluded that the morphological and mechanical features from the single fibers in the nanofiber mats were related to the whole fibrous structure.     

The effect of temperature on the mechanical properties of a protein-based biopolymer network

Several studies have shown that eggshell membrane (ESM) is a suitable biomaterial with potential applications in biomedicine such as wound repair and cell culture. In order to control and improve the use of ESM for biomedical applications their physical and structural properties must be known. In this paper, we have studied the effect of temperature on the mechanical properties of the ESM. Atomic force microscopy was used to assess the morphology of the ESM. The mechanical properties of the membranes were studied by means of uniaxial tensile tests carried out at four different temperatures. Differential scanning calorimetry and thermo-gravimetrical analysis were used to assess the thermal transitions of the ESM and the influence of the water content on its thermal behavior. The Young’s modulus showed a linear inverse dependence with regard to the temperature of the sample. A peak associated to the thermal denaturation of collagen was observed in the DSC tests of the membrane. These peaks showed a dependence on the water content of the specimens.