Active control of spider silk strength: comparison of drag line spun on vertical and horizontal surfaces

There is widespread interest in producing high-performance fibers that mimic the behavior of natural silks, especially spider drag line. Given the multiple roles of drag line in nature, it is pertinent to explore whether spiders can tailor the tensile properties of this material to match its intended use. Here we distinguish between the ability of spiders to control the quality (intrinsic stress-strain response) versus the amount (load-bearing cross-section) of drag line. The mechanical characteristics of drag line spun during a vertical climb differ from those of drag line spun when the spider crawls on a horizontal surface. Also, the intrinsic stress-strain response of drag line spun during a vertical climb is significantly more reproducible (i.e. dependable) than when this fiber is produced under other conditions. Implications for biomimetic polymer science are discussed.     

Controlled supercontraction tailors the tensile behaviour of spider silk

The interest in the production of fibres that mimic the behaviour of natural silks has been boosted by the first successful attempts of spinning fibres based on spider drag line silk proteins. However, both the processing of biomimetic silk fibres and the basic studies on silk are hampered by the large variability of the fibre properties. Here we show that the tensile behaviour of spider silk can be predictably and reproducibly tailored by controlling the supercontraction effect, a large shrinkage of the longitudinal dimension of the fibre if unrestrained by its ends and immersed in water. This procedure allows to reproduce the tensile behaviour of natural drag line fibres and offers the possibility of obtaining silk fibres with predictable tailored properties in large quantities for experimental use. These results can be interpreted in the frame of the molecular model of drag line silk, as the result of re-orientation of the protein chains, leading to an explanation for the observed variability of natural drag line fibres.     

Tensile properties of Argiope trifasciata drag line silk obtained from the spider's web

The tensile properties of Argiope trifasciata (Argiopidae) drag line silk retrieved from mooring threads in the web were characterized. Scanning electron microscope images were used to determine the cross‐sectional area of the samples, allowing force‐displacement plots to be rescaled as stress–strain curves and to characterize fracture surfaces. Twenty‐eight samples were tested to obtain statistically significant values of the mechanical parameters (elastic modulus, stress and strain at the proportional limit, and tensile strength). The tensile strength of the material was subjected to a Weibull analysis—the first time that this has been attempted with a spider silk. A low value of the Weibull modulus, m = 3.4, was obtained, demonstrating that drag line monofilament does not have a sufficiently reliable tensile strength to function as an engineering material on its own. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 2245–2251, 2001     

Self-tightening of spider silk fibers induced by moisture

Spider dragline silk has a unique combination of desirable mechanical properties—low density, high tensile strength and large elongation until breaking—that makes it attractive from an engineering perspective [Nature 410 (2001) 541]. Nevertheless, this outstanding performance is threatened by the way mechanical properties are affected by a wet environment, particularly if the stress of these fibers can relax when exposed to moisture. Tests on spider dragline silk (Argiope trifasciata) performed by the authors have shown that when the fiber is clamped and exposed to a wet enough environment non-vanishing supercontraction forces develop. When the moisture is removed the residual stresses increase, and this effect has proven long lasting, as the fiber remains stressed for hours. In addition, the tensile properties of the fiber remain unaffected by the residual stresses build up after removing the moisture or after a wetting and drying cycle. These tests give support to the thesis that supercontraction helps to keep the spider webs tight and opens new applications for synthetic analogs.     

Partial deuteration probing structural changes in supercontracted spider silk

Polarized IR-spectroscopic and mechanical measurements are combined to analyse the conformational changes in hydrogenated and partially deuterated major ampullate spider silk of Nephila edulis. Special attention is given to supercontraction and to the case where the latter is hindered by mechanical constraints. The determination of the molecular order parameters of the different moieties proves that the amide hydrogen exchange is a selective process, taking place at the surface of β-sheet nanocrystals, implying that these regions are accessible by water. The mechanical properties are changing dramatically when the fiber is wet (“supercontraction”) due to the fact that the pre-stress of the chains interconnecting the nanocrystals is irreversibly released. In course of this a novel network of H-bonds is formed, a process which can be suppressed if supercontraction is hindered.     

Active control on molecular conformations and tensile properties of spider silk

The mechanical properties of spider dragline silk vary with the spinning conditions, and molecular conformation is one of the important factors for the strength and strain of materials. Four kinds of Araneus ventricosus spider dragline silk fibers, measured by Raman microscopic spectrometry, were produced under different conditions: (1) reeled at the rate of 2 cm/s; (2) secreted by a dropping spider from a 100‐cm‐high table; (3) spun by spiders raised in two different containers. The Raman spectra of these fibers showed that the spinning method and growing environment of spiders had evident influences on the molecular conformations and tensile properties of dragline silk, and the dragline silk obtained from a dropping spider contained the greatest number of molecules with highly oriented β‐sheet structures and gave higher stress/strain values. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 92: 901–905, 2004     

Supercontraction in Nephila spider dragline silk - Relaxation into equilibrium state

Time-resolved Fourier transform infrared (FTIR) spectroscopy is combined with dynamic mechanical measurements in order to unravel the interplay between the amorphous chains and nanocrystals in spider silk under conditions of selective swelling. The sample remains humid after supercontraction, identifying in this way the role of the softened amorphous phase. For the first time, the amount of pre-stress in the fibers can be determined by direct comparison between microscopic and macroscopic stress during humidity increase. Nanocrystal stress decreases by ∼120 MPa, even though the external stress increases at the same time. The current findings suggest that water and its interplay with pre-stress determine the relative contribution of crystals and amorphous matrix to the mechanical properties. Increased water content and release of pre-stress deteriorate the macroscopic mechanical properties, as the system becomes viscoelastic and loses its stiffness.     

Cross-plane thermal transport in micrometer-thick spider silk films

This work reports on the first study of thermal transport capacity in the thickness direction (∼μm scale) for spider silk films. Fresh (minimally processed) and hexafluoroisopropanol (HFIP) films of Nephila clavipes and Latrodectus hesperus major ampullate silk are studied. Detailed Raman spectroscopy reveals that the fresh films have more crystalline secondary protein structures such as antiparallel β-sheets than the HFIP films for N. clavipes. For N. clavipes, the randomly distributed antiparallel β-sheets in fresh films have nearly no effect in improving thermal conductivity in comparison with HFIP films. For L. hesperus, the films mainly consist of α-helices and random coils while the fresh film has a higher concentration of α-helices. The higher concentration of α-helices in fresh films gives rise to a higher heat capacity than HFIP films, while the thermal conductivity shows little effect from the α-helices concentration. Thickened HFIP films are heated at different temperatures to study the effect of heat treatment on structure and thermal transport capacity. These experiments demonstrate that α-helices are formed by thermal treatment and that thermal effusivity increases with the appearance of α-helices in films.     

Exploring the backbone dynamics of native spider silk proteins in Black Widow silk glands with solution-state NMR spectroscopy

Spider dragline silk is an outstanding biopolymer with a strength that exceeds steel by weight and a toughness greater than high-performance fibers like Kevlar. For this reason, understanding how a spider converts the gel-like, aqueous protein spinning dope within the major ampullate (MA) gland into a super fiber is of great importance for developing future biomaterials based on spider silk. In this work, the initial state of the silk proteins within Black Widow MA glands was probed with solution-state NMR spectroscopy. ¹⁵N relaxation parameters, T₁, T₂ and ¹⁵N-{¹H} steady-state NOE were measured for twelve backbone environments at two spectrometer frequencies, 500 and 800 MHz. The NMR relaxation parameters extracted for all twelve environments are consistent with MA silk protein backbone dynamics on the fast sub-nanosecond timescale. Therefore, it is concluded that the repetitive core of spider MA proteins are in an unfolded, highly flexible state in the MA gland.     

Mechanical properties of tape casted Lanthanum Tungstate for membrane substrate application

Advances in the development of asymmetric membrane concepts require the development of porosity optimized, mechanically reliable substrate materials. The current study focuses on the characterization of the room temperature mechanical properties of tape casted Lanthanum Tungstate of different porosities for membrane substrate applications. Elastic modulus and hardness are assessed using indentation testing. Characteristic strength and Weibull modulus are determined from data obtained using a ball-on-3-balls test that is particularly advantageous for the rather thin tape casted material. Particular emphasis is placed on the effect of the surface composition onto the fracture strength.     

Simulation of mechanical properties of oriented glassy polystyrene

Mechanical properties of processed polymers depend sensitively on their microstructure. In order to understand how different processing conditions affect the mechanical properties of polymers, one needs a means to describe the process-induced microstructure. Because the characteristic relaxation times of processed polymer chains often span several orders of magnitude, it is commonly the case that partial relaxation of the chains is frozen into the final product. We report results of molecular simulations by the Semi-Grand Canonical Monte Carlo (SGMC) method to study the orientation-dependent elasticity of glassy polystyrene as a function of both the system-average degree of orientation and the degree of relaxation of chain ends at a constant average orientation, in accord with the tube model of Doi and Edwards. Our simulations reproduce quantitatively the experimentally observed trends in the tensile modulus E₁₁ as a function both of the system-average orientation and of the inhomogeneity of the orientation along the chain due to rapid relaxation of chain ends. The results show that the partial relaxation of the polymer chains is sufficient to explain the observed variation of mechanical properties for samples that differ in processing history, yet have the same observed birefringence.     

Mechanical properties of single‐brin silkworm silk

Mechanical tests were performed on single brins of Bombyx mori silkworm silk, to obtain values of elastic modulus (E), yield strength, tensile breaking strength, and shear modulus (G). Specimen cross‐sectional areas, needed to convert tensile loads into stresses, were derived from diameter measurements performed by scanning electron microscopy. Results are compared with existing literature values for partially degummed silkworm baves. The tensile modulus (16 ± 1 GPa) and yield strength (230 ± 10 MPa) of B. mori brin are significantly higher than the literature values reported for bave. The difference is attributed principally to the presence of sericin in bave, contributing to sample cross‐section but adding little to the fiber's ability to resist tensile deformation. The two brins in bave are found to contribute equally and independently to the tensile load‐bearing ability of the material. Measurements performed with a torsional pendulum can be combined with tensile load‐extension data to obtain a value of E/ that is not sensitive to sample cross‐sectional dimensions or, therefore, to the presence of sericin. The value of E measured for brin can be used together with this result to obtain G = 3.0 ± 0.8 GPa and E/G = 5.3 ± 0.3 for brin. The latter value indicates a mechanical, and therefore microstructural, anisotropy comparable to that of nylon. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 1270–1277, 2000     

Mechanical characterization of SOFC/SOEC cells

Mechanical reliability is one main prerequisite for the long-term operation of Solid Oxide Fuel Cells (SOFCs) and Solid Oxide Electrolysis Cells (SOEC) in stacks and systems. Hence, key mechanical properties were derived for cells to be used in Jülich stacks and systems. Since assembling and joining is typically carried out in oxidized state, whereas operation requires reduction of the anode, mechanical characterizations are performed for cells in oxidized and reduced state with additional consideration of elevated temperature behavior as well as possible mechanical degradation due to subcritical crack growth. In particular, fracture strength, elastic modulus and residual stress for half-cells were assessed. With respect to fracture strength, also subcritical crack growth at different temperatures has been analyzed, being the basis of a derived strength-probability-time plot. Overall, the work provides parameters for determination of failure probability and lifetime prediction.     

The effect of homogeneously dispersed few-layer graphene on microstructure and mechanical properties of Al2O3 nanocomposites

Fully dense few-layer graphene (FG)/Al₂O₃ nanocomposites with homogeneously dispersed FG in matrix are prepared by using a heteroaggregation method followed by spark plasma sintering. It is found that the two dimensional FG has great ability to restrain grain growth in comparison to other inclusions. In addition, the morphology of grain in composite is modified by the addition of FG during densification process compared with monolithic alumina. Thanks to the greatly decreased grain size, the composites are almost as hard as monolithic alumina at low sintering temperature (1573 K) even if graphene content is as high as 1.2 vol.%. However, at higher sintering temperature (1673 K), the hardness of composites decreases further but the change in elastic modulus is very limited. The decline of hardness and elastic modulus mainly arises from the sliding feature of FG, low modulus of reduced graphene oxide in both in-plane and out-of-plane directions.     

试样厚度对聚乙烯拉伸性能的影响

通过对不同厚度聚乙烯试样的拉伸试验进行分析.结果表明,试样厚度对聚乙烯的强度、应变、拉伸模量均有较大影响,对单位面积的屈服吸收能没有影响;现有塑料制品设计中,材料的拉伸性能和制品厚度为独立参数的做法存在隐患.     

Unique mechanical properties of nanostructured transparent MgAl2O4 ceramics

Nanoindentation tests were performed on nanostructured transparent magnesium aluminate (MgAl₂O₄) ceramics to determine their mechanical properties. These tests were carried out on samples at different applied loads ranging from 300 to 9,000 μN. The elastic recovery for nanostructured transparent MgAl₂O₄ ceramics at different applied loads was derived from the force-depth data. The results reveal a remarkable enhancement in plastic deformation as the applied load increases from 300 to 9,000 μN. After the nanoindetation tests, scanning probe microscope images show no cracking in nanostructured transparent MgAl₂O₄ ceramics, which confirms the absence of any cracks and fractures around the indentation. Interestingly, the flow of the material along the edges of indent impressions is clearly presented, which is attributed to the dislocation introduced. High-resolution transmission electron microscopy observation indicates the presence of dislocations along the grain boundary, suggesting that the generation and interaction of dislocations play an important role in the plastic deformation of nanostructured transparent ceramics. Finally, the experimentally measured hardness and Young’s modulus, as derived from the load–displacement data, are as high as 31.7 and 314 GPa, respectively.     

Influence of steel and PP fibers on mechanical and microstructural properties of fly ash-GGBFS based geopolymer composites

In this study, experimental investigations were carried out to estimate the mechanical and microstructural properties of polypropylene (PP) and steel fiber reinforced geopolymer mortar. Two industrial by-products are used as binders to produce the geopolymer composites, i.e., fly ash (FA) and ground granulated blast furnace slag (GGBFS). Different percentages of PP and steel fibers are used in geopolymer mortars to find the mechanical properties such as compressive, splitting tensile and flexural strengths were investigated to understand the strength behavior. However, the compressive elastic modulus values were estimated through the proposed equation based on the compressive strength of the fiber reinforced geopolymer composite samples. Moreover, to understand the geopolymeic reaction, microstructural studies, i.e., scanning electron microscopy (SEM), were conducted. The experimental results revealed that the addition of PP fibers up to 2.0% (volume fraction) enhanced the flexural properties of geopolymer mortar samples. The compressive strength of the steel fiber-reinforced geopolymer composite reached a maximum of 2.5% volume fraction, being a 13.26% improvement over the control mix. The flexural toughness index of the PP and steel fiber reinforced composites improved with increasing the fraction. However, steel fiber reinforced geopolymer samples are shown better flexural toughness compared to PP fibers. The SEM analysis of the geopolymer control mix achieved a good degree of geopolymerization and both the fibers yielded a considerable interfacial bonding with the geopolymer paste.     

Mechanical properties of lightweight aggregates

Lightweight aggregates (LWAs) were successfully produced both in a pilot-scale rotary kiln and in a laboratory chamber furnace. The mechanical properties of LWA were investigated in detail applying the European standard crushing resistance test (CR-test) as well as the single pellet compression test (spc-test). The spc-test showed that LWA pellets with porosities <82% behave similar to solid brittle spheres under compression when considering only the solid fraction of the pellet and the strength may be calculated according to σ_crit = F_crit/d² where σ_crit is a porosity independent strength, F_crit is the measured load at failure and d the solid diameter (assuming zero porosity). It was reasoned that catastrophic failure was due to tensile stresses in the centre of the pellet and the strength was observed to increase exponentially with decreasing sample size. The relationship between the CR- and spc-test has been established facilitating “translation” of strength data between the two different test methods.     

Enhanced properties of pure alumina ceramics by oscillatory pressure sintering

A novel oscillatory pressure sintering (OPS) process to consolidate high-quality pure alumina ceramics is reported. The microstructure of the ceramics prepared by OPS develops into a higher final density, a smaller and a narrower distribution of grain sizes compared with those prepared by conventional pressureless sintering (PS) and hot-pressing (HP) processes. Enhanced mechanical properties of alumina ceramics were investigated by OPS process. The bending strength, hardness and elastic modulus of the OPS specimen reached about 546 MPa, 19.1 GPa and 374 GPa, respectively, i.e values significantly higher than that of the specimens by PS and HP. XRD analysis indicates the strengthening of atomic bonds aided by oscillatory pressure. The results suggest OPS to be an effective technique for preparing high-quality pure alumina ceramics.