Superior reinforcement effect of TEMPO-oxidized cellulose nanofibrils in polystyrene matrix: optical, thermal, and mechanical studies

Polystyrene (PS) composites reinforced with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNs) with various weight ratios were fabricated by casting and vacuum-drying mixtures of PS/N,N-dimethylformamide (DMF) solution and TOCN/DMF dispersion. TOCNs of 3 to 4 nm width were dispersed homogeneously at the individual nanofibril level in the PS matrix, such that the TOCN/PS nanocomposite films exhibited high optical transparencies and their tensile strengths, elastic moduli, and thermal dimensional stabilities increased with increasing TOCN content. Dynamic mechanical analysis showed that the storage modulus of the TOCN/PS films increased significantly with TOCN content above the glass-transition temperature of PS by the formation of an interfibrillar network structure of TOCNs in the PS matrix, based on percolation theory. The outstanding and effective polymer reinforcement by TOCNs results from their high aspect ratio, high crystallinity, and nanodispersibility in the PS matrix.     

Preparation of chitin nanofibril/polycaprolactone nanocomposite from a nonaqueous medium suspension

Chitin nanofibrils are prepared by treatment of commercial chitin in hydrochloric acid. It is found for the first time that the obtained chitin nanofibrils can be well dispersed in an organic solvent of 2,2,2-trifluoroethanol (TFE) due to its strong ability to form hydrogen bonds. Polycaprolactone (PCL), a water insoluble biodegradable polymer, is selected to blend with chitin nanofibrils to achieve chitin nanofibril/polycaprolactone (n-chitin/PCL) nanocomposites using TFE as a co-solvent. The results show the n-chitin/PCL nanocomposites, either in the form of solvent-cast films or electrospun fiber mats, both exhibit reinforced mechanical properties. Thus, the processing technique from a TFE suspension instead of aqueous suspensions is a good alternative to broaden the family of chitin nanofibril-based nanocomposites.     

Plasticized xyloglucan for improved toughness—Thermal and mechanical behaviour

Tamarind seed xyloglucan is an interesting polysaccharide of high molar mass with excellent thermomechanical properties. Several plasticizers were studied in order to facilitate thermal processing and improve toughness (work to fracture) of xyloglucan film materials: sorbitol, urea, glycerol and polyethylene oxide. Films of different compositions were cast and studied by thermogravimetric analysis (TGA), calorimetry (DSC), dynamic mechanical thermal analysis (DMA) and tensile tests. Results are analysed and discussed based on mechanisms and practical considerations. Highly favourable characteristics were found with XG/sorbitol combinations, and the thermomechanical properties motivate further work on this material system, for instance as a matrix in biocomposite materials.     

Colloidal ionic assembly between anionic native cellulose nanofibrils and cationic block copolymer micelles into biomimetic nanocomposites

We present a facile ionic assembly between fibrillar and spherical colloidal objects toward biomimetic nanocomposites with majority hard and minority soft domains based on anionic reinforcing native cellulose nanofibrils and cationic amphiphilic block copolymer micelles with rubbery core. The concept is based on ionic complexation of carboxymethylated nanofibrillated cellulose (NFC, or also denoted as microfibrillated cellulose, MFC) and micelles formed by aqueous self-assembly of quaternized poly(1,2-butadiene)-block-poly(dimethylaminoethyl methacrylate) with high fraction of the NFC reinforcement. The adsorption of block copolymer micelles onto nanocellulose is shown by quartz crystal microbalance measurements, atomic force microscopy imaging, and fluorescent optical microscopy. The physical properties are elucidated using electron microscopy, thermal analysis, and mechanical testing. The cationic part of the block copolymer serves as a binder to NFC, whereas the hydrophobic rubbery micellar cores are designed to facilitate energy dissipation and nanoscale lubrication between the NFC domains under deformation. We show that the mechanical properties do not follow the rule of mixtures, and synergistic effects are observed with promoted work of fracture in one composition. As the concept allows wide possibilities for tuning, the work suggests pathways for nanocellulose-based biomimetic nanocomposites combining high toughness with stiffness and strength.     

Spider silk aging: initial improvement in a high performance material followed by slow degradation

Spider silk possesses a unique combination of high tensile strength and elasticity resulting in extraordinarily tough fibers, compared with the best synthetic materials. However, the potential application of spider silk and biomimetic fibers depends upon retention of their high performance under a variety of conditions. Here, we report on changes in the mechanical properties of dragline and capture silk fibers from several spider species over periods up to 4 years of benign aging. We find an improvement in mechanical performance of silk fibers during the first year of aging. Fibers rapidly decrease in diameter, suggesting an increase in structural alignment and organization of molecules. One-year old silk also is stiffer and has higher stress at yield than fresh silk, whereas breaking force, elasticity, and toughness either improve or are unaffected by early aging. However, 4-year old silk shows signs of degradation as the breaking load, elasticity, and toughness are all lower than in fresh silk. Aging, however, does not reduce the tensile strength of silk. These data suggest initially rapid reorganization and tighter packaging of molecules within the fiber, followed by longer-term decomposition. We hypothesize that possibly the breakdown of amino acids via emission of ammonia gas, as is seen in long-term aging of museum silkworm fabrics, may contribute. Degradation of spider silk under benign conditions may be a concern for efforts to construct and utilize biomimetic silk analogs. However, our findings suggest an initial improvement in mechanical performance and that even old spider silk still retains impressive mechanical performance. J. Exp. Zool. 309A:494-504, 2008. (c) 2008 Wiley-Liss, Inc.     

Drought tolerance of selected Eragrostis species correlates with leaf tensile properties

BACKGROUND AND AIMS: Previous studies on grass leaf tensile properties (behaviour during mechanical stress) have focused on agricultural applications such as resistance to trampling and palatability; no investigations have directly addressed mechanical properties during water stress, and hence these are the subject of this study. METHODS: Critical (lethal) relative water contents were determined for three species of grass in the genus Eragrostis varying in their tolerance to drought. Measurements were taken for leaf tensile strength, elastic modulus, toughness and failure load under different conditions of hydration, and light microscopy and histochemical analyses were undertaken. KEY RESULTS: Leaf tensile strength of fully hydrated leaves for the drought-intolerant E. capensis, the moderately drought-tolerant E. tef and the drought-tolerant E. curvula correlated well with drought tolerance (critical relative water content). Eragrostis curvula had higher tensile strength values than E. tef, which in turn had higher values than E. capensis. Measurements on the drought-tolerant grass E. curvula when fully hydrated and when dried to below its turgor loss point showed that tensile strength, toughness and the elastic modulus all increased under conditions of turgor loss, while the failure load remained unchanged. Additional tests of 100 mm segments along the lamina of E. curvula showed that tensile strength, toughness and the elastic modulus all decreased with distance from the base of the lamina, while again the failure load was unaffected. This decrease in mechanical parameters correlated with a reduction in the size of the vascular bundles and the amount of lignification, as viewed in lamina cross-sections. CONCLUSIONS: The results confirm that leaf mechanical properties are affected by both water status and position along the lamina, and suggest a positive correlation between leaf internal architecture, tensile strength, cell wall chemistry and tolerance to dehydration for grasses.     

Synthesis and properties of degradable poly(urethane urea)s to be used for ligament reconstructions

In the present study we describe the synthesis, wet spinning, mechanical testing, and degradation of poly(urethane urea)s (PUURs) intended for clinical use in anterior cruciate ligament (ACL) reconstruction. The effects of soft segment chemical composition and molar mass and the kind of diamine chain extender on the material properties were investigated. It was found that the fibers made of PUUR with polycaprolactone diol (PCL530) as soft segment and MDI/1,3-DAP as hard segment (PCL530-3) have high tensile strength and high modulus and when degraded keep their tensile strength for the time demanded for the application. In conclusion, from a chemical and mechanical point of view PUUR fibers of PCL530-3, ARTELON, are suitable for designing a degradable ACL device.     

Isolation and characterization of cellulose nanofibrils from wheat straw using steam explosion coupled with high shear homogenization

Cellulose nanofibrils of diameter 10–50 nm were obtained from wheat straw using alkali steam explosion coupled with high shear homogenization. High shear results in shearing of the fiber agglomerates resulting in uniformly dispersed nanofibrils. The chemical composition of fibers at different stages were analyzed according to the ASTM standards and showed increase in α-cellulose content and decrease in lignin and hemicellulose. Structural analysis of steam exploded fibers was carried out by Fourier Transform Infrared (FT-IR) spectroscopy and X-ray diffraction (XRD). Thermal stability was higher for cellulose nanofibrils as compared to wheat straw and chemically treated fibers. The fiber diameter distribution was obtained using image analysis software. Characterization of the fibers by AFM, TEM, and SEM showed that fiber diameter decreases with treatment and final nanofibril size was 10–15 nm. FT-IR, XRD, and TGA studies confirmed the removal of hemicellulose and lignin during the chemical treatment process.     

氧化铝玻璃复合体强度及断裂韧性的研究

全瓷修复材料抗拉强度,脆性大且制作困难,限制了它在牙科临床上应用,随着ＣＡＤ／ＣＡＭ技术和Ｉｈ－Ｃｅｒａｍ技术的完美结合,使得牙科陶瓷强度和断裂韧性很大的提高,给临床全瓷修复体的制作提供了新的途径,本研究采用α型高纯度,超细氧化铝粉末,经等静压处理,在在１３５０℃下烧结成一定强度的多孔氧化铝坯体。最后在多孔氧化铝坯体的表面采用镧硅硼玻璃进行渗透,制作具有一定强度和半透明的氧化铝玻璃复合体。结果表明     

The mechanical properties of hydrated intermediate filaments: insights from hagfish slime threads

Intermediate filaments (IFs) impart mechanical integrity to cells, yet IF mechanics are poorly understood. It is assumed that IFs in cells are as stiff as hard alpha-keratin, F-actin, and microtubules, but the high bending flexibility of IFs and the low stiffness of soft alpha-keratins suggest that hydrated IFs may be quite soft. To test this hypothesis, we measured the tensile mechanics of the keratin-like threads from hagfish slime, which are an ideal model for exploring the mechanics of IF bundles and IFs because they consist of tightly packed and aligned IFs. Tensile tests suggest that hydrated IF bundles possess low initial stiffness (E(i) = 6.4 MPa) and remarkable elasticity (up to strains of 0.34), which we attribute to soft elastomeric IF protein terminal domains in series with stiffer coiled coils. The high tensile strength (180 MPa) and toughness (130 MJ/m(3)) of IF bundles support the notion that IFs lend mechanical integrity to cells. Their long-range elasticity suggests that IFs may also allow cells to recover from large deformations. X-ray diffraction and congo-red staining indicate that post-yield deformation leads to an irreversible alpha-->beta conformational transition in IFs, which leads to plastic deformation, and may be used by cells as a mechanosensory cue.     

Fibronectin matrix polymerization increases tensile strength of model tissue

The composition and organization of the extracellular matrix (ECM) contribute to the mechanical properties of tissues. The polymerization of fibronectin into the ECM increases actin organization and regulates the composition of the ECM. In this study, we examined the ability of cell-dependent fibronectin matrix polymerization to affect the tensile properties of an established tissue model. Our data indicate that fibronectin polymerization increases the ultimate strength and toughness, but not the stiffness, of collagen biogels. A fragment of fibronectin that stimulates mechanical tension generation by cells, but is not incorporated into ECM fibrils, did not increase the tensile properties, suggesting that changes in actin organization in the absence of fibronectin fibril formation are not sufficient to increase tensile strength. The actin cytoskeleton was needed to initiate the fibronectin-induced increases in the mechanical properties. However, once fibronectin-treated collagen biogels were fully contracted, the actin cytoskeleton no longer contributed to the tensile strength. These data indicate that fibronectin polymerization plays a significant role in determining the mechanical strength of collagen biogels and suggest a novel mechanism by which fibronectin can be used to enhance the mechanical performance of artificial tissue constructs.     

Plasticizer Effects on Physical–Mechanical Properties of Solvent Cast Soluplus® Films

Soluplus® is a novel amphiphilic polymer that has been shown to enhance the solubility and drug dissolution rate of poorly soluble drugs. However, there still is a lack of information regarding the physical mechanical properties of Soluplus® with addition of the plasticizers. This study characterized the mechanical properties of Soluplus® with four different plasticizers. The plasticizers selected were polyethylene glycol 6, triethyl citrate, propylene glycol, and glycerin; they were studied at three different levels (15%, 20%, and 25% w/w). The effects of these plasticizers on the glass transition temperature, tensile strength, percent elongation, and Young’s modulus of free films made from Soluplus® were measured and the toughness and ratio of tensile strength to Young’s modulus were calculated. These results showed these four plasticizers are capable to plasticizing Soluplus® as indicated by the glass transition temperature lowering, tensile strength, and Young’s modulus while increasing the percent elongation and film toughness. Among the plasticizers tested, polyethylene glycol 6 showed greatest changed in the mechanical properties studied.     

Effects of light and nutrient availability on leaf mechanical properties of Plantago major: a conceptual approach

BACKGROUND AND AIMS: Leaf mechanical properties, which are important to protect leaves against physical stresses, are thought to change with light and nutrient availabilities. This study aims to understand phenotypic changes of leaf mechanical properties with respect to dry mass allocation and anatomy. METHODS: Leaf lamina strength (maximum force per unit area to fracture), toughness (work to fracture) and stiffness (resistance against deformation) were measured by punch-and-die tests, and anatomical and physiological traits were determined in Plantago major plants grown at different light and nutrient availabilities. A conceptual approach was developed by which punch strength and related carbon costs can be quantitatively related to the underlying anatomical and morphological traits: leaf thickness, dry-mass allocation to cell walls and cell-wall-specific strength. KEY RESULTS: Leaf lamina strength, toughness and stiffness (all expressed on a punch area basis) increased with light availability. By contrast, nutrient availability did not change strength or toughness, but stiffness was higher in low-nutrient plants. Punch strength (maximum force per unit punch area, F(max)/area) was analysed as the product of leaf mass per area (LMA) and F(max)/leaf mass (= punch strength/LMA, indicating mass-use efficiency for strength). The greater strength of sun leaves was mainly explained by their higher LMA. Shade leaves, by contrast, had a higher F(max)/leaf mass. This greater efficiency in shade leaves was caused by a greater fraction of leaf mass in cell walls and by a greater specific strength of cell walls. These differences are probably because epidermis cells constitute a relatively large fraction of the leaf cross-section in shaded leaves. Although a larger percentage of intercellular spaces were found in shade leaves, this in itself did not reduce 'material' strength (punch strength/thickness); it might, however, be important for increasing distance between upper and lower epidermis per unit mass and thus maintaining flexural stiffness at minimal costs. CONCLUSIONS: The consequences of a reduced LMA for punch strength in shaded leaves was partially compensated for by a mechanically more efficient design, which, it is suggested, contributes importantly to resisting mechanical stress under carbon-limited conditions.     

Cassava bagasse cellulose nanofibrils reinforced thermoplastic cassava starch

Cellulose cassava bagasse nanofibrils (CBN) were directly extracted from a by-product of the cassava starch (CS) industry, viz. the cassava bagasse (CB). The morphological structure of the ensuing nanoparticles was investigated by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), presence of other components such as sugars by high performance liquid chromatography (HPLC), thermogravimetric analysis (TGA), and X-ray diffraction (XRD) experiments. The resulting nanofibrils display a relatively low crystallinity and were found to be around 2–11 nm thick and 360–1700 nm long. These nanofibrils were used as reinforcing nanoparticles in a thermoplastic cassava starch matrix plasticized using either glycerol or a mixture of glycerol/sorbitol (1:1) as plasticizer. Nanocomposite films were prepared by a melting process. The reinforcing effect of the filler evaluated by dynamical mechanical tests (DMA) and tensile tests was found to depend on the nature of the plasticizer employed. Thus, for the glycerol-plasticized matrix-based composites, it was limited especially due to additional plasticization by sugars originating from starch hydrolysis during the acid extraction. This effect was evidenced by the reduction of glass vitreous temperature of starch after the incorporation of nanofibrils in TPSG and by the increase of elongation at break in tensile test. On the other hand, for glycerol/sorbitol plasticized nanocomposites the transcrystallization of amylopectin in nanofibrils surface hindered good performances of CBN as reinforcing agent for thermoplastic cassava starch. The incorporation of cassava bagasse cellulose nanofibrils in the thermoplastic starch matrices has resulted in a decrease of its hydrophilic character especially for glycerol plasticized sample.     

Characterization of thallus mechanical and physiological traits of tropical fucoids: A preliminary study

Thallus mechanical strength, as well as production of secondary metabolites as defensive compounds, of tropical macroalgae is often essential for protection from herbivory. In tropical macroalgae, thallus mechanical strength is negatively correlated with productivity — a trade‐off between productivity and thallus toughness. The tropical fucoids, Turbinaria ornata which has defensive morphological traits against herbivory and Sargassum ilicifolium which is expanding its distribution in Japanese waters, were examined to determine thallus traits related to mechanical strength and productivity and their pair‐wise relationships were also examined. These traits that are directly or indirectly related to the trade‐off between productivity and thallus toughness were compared to data for various other temperate macroalgae by regression analyses. We found two strong positive correlations between thallus mechanical strength and thallus mass or thallus thickness, confirming that higher levels of mechanical strength for tropical fucoids is associated with higher biomass or thallus thickness. Also, negative correlations between thallus toughness and productivity were found indicating structural and physiological trade‐offs. However, the tropical fucoids exhibited relatively high productivity regardless of their higher level of thallus toughness. These traits of the tropical fucoids slightly deviate from the typical conservative strategy with higher thallus toughness and lower productivity as a trade‐off between productivity and thallus toughness.     

Superabsorbent hydrogel composite made of cellulose nanofibrils and chitosan-graft-poly(acrylic acid)

Superabsorbent hydrogel composites based on cellulose nanofibrils and chitosan-graft-poly(acrylic acid) copolymer were developed in this work. The FTIR data showed that the copolymerization and the composite formation reaction were successfully performed. In addition, the XRD pattern indicated that the nanofibrils crystallinity was as high as 90%. A 2⁴⁻¹ fractional factorial design was employed to evaluate the effect of acrylic acid/chitosan molar ratio, crosslinker, initiator, and filler in the swelling capacity of hydrogel composites. By the analysis of variance (ANOVA), including F-test and P-values, it was found that the crosslinker and filler correspond to 40% and 30% of the evaluated response, respectively. The addition of nanofibrils provided faster equilibrium conditions as well as improved the swelling capacity in ca. 100 units, from 381 to 486. SEM images showed that the addition of nanofibrils into the hydrogel matrix increased the averaged-dimension of porous. Finally, the composites showed responsive behavior in relation to pH and salt solution. Such characteristics make these smart materials suitable for several technological applications.     

Mechanical response of porous scaffolds for cartilage engineering

Mechanical properties of scaffolds seeded with mesenchymal stem cells used for cartilage repair seem to be one of the critical factors in possible joint resurfacing. In this paper, the effect of adding hyaluronic acid, hydroxyapatite nanoparticles or chitosan nanofibers into the cross-linked collagen I on the mechanical response of the lyophilized porous scaffold has been investigated in the dry state at 37 oC under tensile loading. Statistical significance of the results was evaluated using ANOVA analysis. The results showed that the addition of hyaluronic acid significantly (p<0.05) reduced the tensile elastic modulus and enhanced the strength and deformation to failure of the modified cross-linked collagen I under the used test conditions. On the other hand, addition of hydroxyapatite nanoparticles and chitosan nanofibers, respectively, increased the elastic modulus of the modified collagen ten-fold and four-fold, respectively. Hydroxyapatite caused significant reduction in the ultimate deformation at break while chitosan nanofibers enhanced the ultimate deformation under tensile loading substantially (p<0.05). The ultimate tensile deformation was significantly (p<0.05) increased by addition of the chitosan nanofibers. The enhanced elastic modulus of the scaffold was translated into enhanced resistance of the porous scaffolds against mechanical load compared to scaffolds based on cross-linked neat collagen or collagen with hyaluronic acid with similar porosity. It can be concluded that enhancing the rigidity of the compact scaffold material by adding rigid chitosan nanofibers can improve the resistance of the porous scaffolds against compressive loading, which can provide more structural protection to the seeded mesenchymal stem cells when the construct is implanted into a lesion. Moreover, scaffolds with chitosan nanofibers seemed to enhance cell growth compared to the neat collagen I when tested in vitro as well as the scaffold stability, extending its resorption to more than 10 weeks.     

Model films from native cellulose nanofibrils. Preparation, swelling, and surface interactions

Native cellulose model films containing both amorphous and crystalline cellulose I regions were prepared by spin-coating aqueous cellulose nanofibril dispersions onto silica substrates. Nanofibrils from wood pulp with low and high charge density were used to prepare the model films. Because the low charged nanofibrils did not fully cover the silica substrates, an anchoring substance was selected to improve the coverage. The model surfaces were characterized using atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The effect of nanofibril charge density, electrolyte concentration, and pH on swelling and surface interactions of the model film was studied by quartz crystal microbalance with dissipation (QCM-D) and AFM force measurements. The results showed that the best coverage for the low charged fibrils was achieved by using 3-aminopropyltrimethoxysilane (APTS) as an anchoring substance and hence it was chosen as the anchor. The AFM and XPS measurements showed that the fibrils are covering the substrates. Charge density of the fibrils affected the morphology of the model surfaces. The low charged fibrils formed a network structure while the highly charged fibrils formed denser film structure. The average thickness of the films corresponded to a monolayer of fibrils, and the average rms roughness of the films was 4 and 2 nm for the low and high charged nanofibril films, respectively. The model surfaces were stable in QCM-D swelling experiments, and the behavior of the nanofibril surfaces at different electrolyte concentrations and pHs correlated with other studies and the theories of Donnan. The AFM force measurements with the model surfaces showed well reproducible results, and the swelling results correlated with the swelling observed by QCM-D. Both steric and electrostatic forces were observed and the influence of steric forces increased as the films were swelling due to changes in pH and electrolyte concentration. These films differ from previous model cellulose films due to their chemical composition (crystalline cellulose I and amorphous regions) and fibrillar structure and hence serve as excellent models for the pulp fiber surface.     

石料力学性能分析在旧石器考古学研究中的应用

石料是石器制作过程中的重要因素,石料本身的力学性能不仅与古人类的原料利用方式密切相关,还会影响到石器打制方法、器物组合、甚至石器工业面貌。长期以来,旧石器考古学界对于石料力学性能的判断多基于简单的岩石矿物硬度和结构特征,实际上岩石的力学性能涵盖的内容远远超过其硬度和结构的范畴。本文使用力学研究的方法,通过对石料的单轴抗压强度测试,了解不同原料的力学性能,并将其与遗址出土石制品进行比较研究,尝试探讨岩石本身力学性能与石制品面貌之间的关系。     

Size distribution of amyloid nanofibrils

We consider the size distribution of amyloid nanofibrils (protofilaments) in nucleating protein solutions when the nucleation process occurs by the mechanism of direct polymerization of β-strands (extended peptides or protein segments) into β-sheets. Employing the atomistic nucleation theory, we derive a general expression for the stationary size distribution of amyloid nanofibrils constituted of successively layered β-sheets. The application of this expression to amyloid β_1-40 (Aβ₄₀) fibrils allows us to determine the nanofibril size distribution as a function of the protein concentration and temperature. The distribution is most remarkable with its exhibiting a series of peaks positioned at “magic” nanofibril sizes (or lengths), which are due to deep local minima in the work for fibril formation. This finding of magic sizes or lengths is consistent with experimental results for the size distribution of aggregates in solutions of Aβ₄₀ proteins. Also, our approach makes it possible to gain insight into the effect of point mutations on the nanofibril size distribution, an effect that may play a role in experimentally observed substantial differences in the fibrillation lag-time of wild-type and point-mutated amyloid-β proteins.