Mechanical characterization of nanofiber-reinforced composite adhesives

Tensile and shear strength tests of metal/metal and polymer/polymer joints featuring a new functionalized nanofiber/epoxy composite adhesive were conducted. Strength increase is not as high as we expected (only up to 30%) although we used GCNF-ODA reactive linkers to improve the interface. The moderate strength increase is due to high interfacial stress developed in nanocomposites because of the high stiffness property mismatch, and inefficient interfacial shear stress transfer through shear-lag mechanism. In order to design strong nanocomposite materials, continuous or at least aligned nanofibers/nanotubes should be employed.     

High performance polyimide composite films prepared by homogeneity reinforcement of electrospun nanofibers

Highly aligned polyimide (PI) and PI nanocomposite fibers containing carbon nanotubes (CNTs) were produced by electrospinning. Scanning electron microscopy showed the electrospun nanofibers were uniform and almost free of defects. Transmission electron microscopy indicated that the CNTs were finely dispersed and highly oriented along the CNT/PI nanofiber axis at a relatively low concentration. The as-prepared well-aligned electrospun nanofibers were then directly used as homogeneity reinforcement to enhance the tensile strength and toughness of PI films. The neat PI nanofiber reinforced PI films showed good transparency, decreased bulk density and significantly improved mechanical properties. Compared with neat PI film prepared by solution casting, the tensile strength and elongation at break for the PI film reinforced with 2 wt.% CNT/PI nanofibers were remarkably increased by 138% and 104%, respectively. The significant increases in the overall mechanical properties of the nanofibers reinforced polyimide films can be ascribed to good compatibility between the electrospun nanofibers and the matrix as well as high nanofiber orientation in the matrix. Our study demonstrates a good example for fabricating high performance and high toughness polyimide nanocomposites by using this facile homogeneity self-reinforcement method.     

Viscoelasticity in carbon nanotube composites

Polymer composites reinforced by carbon nanotubes have been extensively researched for their strength and stiffness properties. Unless the interface is carefully engineered, poor load transfer between nanotubes (in bundles) and between nanotubes and surrounding polymer chains may result in interfacial slippage and reduced performance. Interfacial shear, although detrimental to high stiffness and strength, could result in very high mechanical damping, which is an important attribute in many commercial applications. We previously reported evidence of damping in nanocomposites by measuring the modal response (at resonance) of cantilevered beams with embedded nanocomposite films. Here we carry out direct shear testing of epoxy thin films containing dense packing of multiwalled carbon nanotube fillers and report strong viscoelastic behaviour with up to 1,400% increase in loss factor (damping ratio) of the baseline epoxy. The great improvement in damping was achieved without sacrificing the mechanical strength and stiffness of the polymer, and with minimal weight penalty. Based on the interfacial shear stress (approximately 0.5 MPa) at which the loss modulus increases sharply for our system, we conclude that the damping is related to frictional energy dissipation during interfacial sliding at the large, spatially distributed, nanotube-nanotube interfaces.     

Novel nanocomposite concept based on cross-linking of hyperbranched polymers in reactive cellulose nanopaper templates

Cellulosic fibers offer interesting possibilities for good interfacial adhesion due to the high density of hydroxyl groups at the surface. In the present study, the potential of a new nanocomposite concept is investigated, where a porous cellulose nanofiber network is impregnated with a solution of reactive hyperbranched polyester. The polymer is chemically cross-linked to form a solid matrix. The resulting nanocomposite structure is unique. The matrix surrounds a tough nanopaper structure consisting of approximately 20 nm diameter nanofibers with an average interfiber distance of only about 6 nm. The cross-linked polymer matrix shows strongly altered characteristics when it is cross-linked in the confined space within the nanofiber network, including dramatically increased T_g, and this must be due to covalent matrix-nanofiber linkages.     

Novel surfactant-stabilized graphene-polyaniline composite nanofiber for supercapacitor applications

In this work, we present a new synthesis method for surfactant stabilized graphene (SSG) combined with polyaniline nanofiber (PANI-Nf) and apply the composite material as supercapacitor (SC) electrodes by screen-printing technique. Surfactant stabilized graphene polyaniline nanofiber composite (PANI-SSG) was synthesized by electrolytic exfoliation of graphite and subsequent interfacial polymerization. Firstly, graphite was electrolytically exfoliated in an electrolyte containing anionic surfactant. Next, ammonium peroxydisulfate initiator and hydrochloric acid were added to the graphene dispersion to form the aqueous phase for interfacial polymerization of polyaniline nanofiber. This dispersion was then added to the water-insoluble solvent phase containing aniline monomer. The polymerization only occurred at the interface of the two immiscible phases leading to polyaniline nanofiber decorated graphene structures. Characterizations by scanning electron microscopy, transmission electron microscopy, atomic force microscopy and Raman spectroscopy suggested nanocomposite formation with intermolecular π-π bonding of graphene with polyaniline nanofibers. Pastes of the materials were screen printed on stainless steel current collectors and tested for SC performance by cyclic voltammetry (CV) and galvanostatic charge–discharge (GCD) measurements with 2 M H₂SO₄ electrolyte using a home-built two-electrode test-cell. CV results showed redox peaks of polyaniline with wide cyclic loop, indicating large pseudocapacitance of the nanocomposite. From GCD measurement, a high specific capacitance of 690 Fg⁻¹ at 1 Ag⁻¹ was achieved. Therefore, PANI-SSG nano-composite prepared by electrolytic exfoliation and interfacial polymerization is a promising candidate for SC applications.     

Mechanical properties and in vitro behavior of nanofiber-hydrogel composites for tissue engineering applications

Hydrogel-based biomaterial systems have great potential for tissue reconstruction by serving as temporary scaffolds and cell delivery vehicles for tissue engineering (TE). Hydrogels have poor mechanical properties and their rapid degradation limits the development and application of hydrogels in TE. In this study, nanofiber reinforced composite hydrogels were fabricated by incorporating electrospun poly(ε-caprolactone) (PCL)/gelatin 'blend' or 'coaxial' nanofibers into gelatin hydrogels. The morphological, mechanical, swelling and biodegradation properties of the nanocomposite hydrogels were evaluated and the results indicated that the moduli and compressive strengths of the nanofiber reinforced hydrogels were remarkably higher than those of pure gelatin hydrogels. By increasing the amount of incorporated nanofibers into the hydrogel, the Young's modulus of the composite hydrogels increased from 3.29 ± 1.02 kPa to 20.30 ± 1.79 kPa, while the strain at break decreased from 66.0 ± 1.1% to 52.0 ± 3.0%. Compared to composite hydrogels with coaxial nanofibers, those with blend nanofibers showed higher compressive strength and strain at break, but with lower modulus and energy dissipation properties. Biocompatibility evaluations of the nanofiber reinforced hydrogels were carried out using bone marrow mesenchymal stem cells (BM-MSCs) by cell proliferation assay and immunostaining analysis. The nanocomposite hydrogel with 25 mg ml(-1) PCL/gelatin 'blend' nanofibers (PGB25) was found to enhance cell proliferation, indicating that the 'nanocomposite hydrogels' might provide the necessary mechanical support and could be promising cell delivery systems for tissue regeneration.     

Electrophoretic deposition of aramid nanofibers on carbon fibers for highly enhanced interfacial adhesion at low content

Here, an anodic electrophoretic deposition was adopted to facilitate the large-scale uniform coating of nano-fillers onto carbon fibers to enhance the interfacial properties between carbon fibers and epoxy matrix. As interface–reinforcing materials, aramid nanofibers were introduced because of their superior mechanical properties and epoxy matrix-friendly functional groups. Furthermore, aramid nanofibers can be readily coated on carbon fibers via electrophoretic deposition because they are negatively-charged in solution with high electrical mobility. Finally, aramid nanofiber-coated carbon fibers showed significantly improved interfacial properties such as higher surface free energy and interfacial shear strengths (39.7% and 34.9% increases, respectively) than those of a pristine carbon fiber despite a very small amount of embedding (0.025 wt% of aramid nanofibers in a carbon fiber), and the short beam strength of the laminated composite prepared with the aramid nanofiber-coated carbon fibers was also improved by 17.0% compared to a non-modified composite.     

Evaluation of the tensile and fatigue behaviour of ingot metallurgy beryllium/aluminium alloys

The tensile and fatigue behaviour of ingot metallurgy beryllium/aluminium alloys produced by Nuclear Metals, Inc., is determined as a function of temperature. The wrought alloy and the casting alloy are both shown to have a very high stiffness to density ratio compared with common structural materials. The wrought alloy was found to have superior fatigue strength, tensile strength and ductility relative to the casting alloy; it also maintained a greater fraction of its tensile strength as a function of temperature. The stiffness of the materials can be readily explained using standard composite theory, where the material is treated as a discontinuous beryllium-reinforced aluminium matrix composite. The strength of the casting alloy is controlled to a large extent by the strength of its aluminium alloy matrix. In contrast, strengthening increments from both dislocation-based mechanisms and load transfer appear to be operative for the wrought material. Fractographic analysis of tensile specimens showed that preferential failure of the aluminium regions or the beryllium/aluminium interfacial regions occurs under certain circumstances. Fracture analysis of fatigue samples revealed no obvious fracture initiation sites and no evidence of limited/controlled crack growth regions. This revised version was published online in November 2006 with corrections to the Cover Date.     

Nanosized-bismuth-embedded 1D carbon nanofibers as high-performance anodes for lithium-ion and sodium-ion batteries

Bi is a promising candidate for energy storage materials because of its high volumetric capacity,stability in moisture/air,and facile preparation.In this study,the electrochemical performance of nanosized-Bi-embedded one-dimensional (1D) carbon nanofibers (Bi/C nanofibers) as anodes for Li-ion batteries (LIBs) and Na-ion batteries (NIBs) was systematically investigated.The Bi/C nanofibers were prepared using a single-nozzle electrospinning method with a specified Bi source followed by carbothermal reduction.Abundant Bi nanoparticles with diameters of approximately 20 nm were homogeneously dispersed and embedded in the 1D carbon nanofibers,as confirmed by structural and morphological characterization.Electrochemical measurements indicate that the Bi/C nanofiber anodes could deliver a long cycle life for LIBs and a preferable rate performance for NIBs.The superior electrochemical performances of the Bi/C nanofiber anodes are attributed to the 1D carbon nanofiber structure and uniform distribution of Bi nanoparticles embedded in the carbon matrix.This unique embedded structure provides a favorable electron carrier and buffering matrix for the effective release of mechanical stress caused by volume change and prevents the aggregation of Bi nanoparticles.     

Isolation of nanofibers from soybean source and their reinforcing capability on synthetic polymers

The focus of this work is to study nanofibers in three different polymers: polyvinyl alcohol (PVA), polypropylene (PP) and polyethylene (PE). The nanofibers were isolated from a soybean source by combining chemical and mechanical treatments. Isolated nanofibers were shown to have diameter between 50 and 100 nm and the length in micrometer scale which results in very high aspect ratio. The mechanical properties demonstrated an increase in tensile strength from 21 MPa of PVA/UNF5 (untreated-fiber (5 wt%) reinforced PVA) and 65 MPa of pure PVA to 103 MPa of PVA/SBN5 (nanofiber (5 wt%) reinforced PVA). The increased stiffness of PVA/SBN5 nanocomposites was also very promising; it was 6.2 GPa compared to 2.3 GPa of pure PVA and 1.5 GPa of PVA/UNF5. In solid phase melt-mixing, nanofiber was directly incorporated into the polymer matrix using a Brabender. The nanofiber addition significantly changed the stress–strain behavior of the composites: modulus and stress were increased with coated nanofibers by ethylene–acrylic oligomer emulsion as a dispersant; however, elongation was reduced. The dynamic mechanical analysis showed the addition of the soybean nanofiber (SBN) improved the thermal properties for PVA and how the addition of different contents of SBN influenced the tan δ peak and storage modulus of PVA.     

Damping Performances of Carbon Nanotube Reinforced Cement Composite

Multi-walled carbon nanotubes (MWCNTs) were dispersed in an aqueous cement matrix using surfactant decoration, ultrasonic treatment, and, subsequently, intensive mixing to fabricate MWCNT/cement composites with six different MWCNT concentrations. Damping performances of these cured nanocomposites were studied with forced vibration testing, half-power bandwidth, and Morlet wavelet transform identification methods. The micro-crack bridging and interfacial “stick-slip” capacity of nanotubes among cement matrix contributes to balanced enhancement on structural damping capacity and flexural strength of the nanocomposite. With the addition of 2.0% nanotubes, the fundamental frequency, damping ratio, and flexural strength of the nanocomposite increases around 13 Hz, 60, and 32%, as compared to the reference, respectively.     

Interfacial strength and fracture energy of individual carbon nanofibers in epoxy matrix as a function of surface conditions

The interfacial shear strength (IFSS) and fracture energy of individual carbon nanofibers embedded in epoxy were obtained for different surface conditions and treatments by novel, MEMS-based, nanoscale fiber pull-out experiments. As-grown vapor grown carbon nanofibers (VGCNFs) with turbostratic surface and 5 nm peak-to-valley surface roughness exhibited high IFSS and interfacial fracture energy, averaging 106 ± 29 MPa and 1.9 ± 0.9 J/m², respectively. Subsequent high temperature heat treatment and graphitization resulted in drastically reduced IFSS of 66 ± 10 MPa and interfacial fracture energy of 0.65 ± 0.14 J/m². The smaller IFSS values and the reduced standard deviation were due to significant reduction of the fiber surface roughness to 1–2 nm, as well as a decrease in surface defect density during conversion of turbostratic and amorphous carbon to highly ordered graphitic carbon. For both grades of VGCNFs failure was adhesive with clear nanofiber surfaces after debonding. Oxidative functionalization of high temperature heat-treated VGCNFs resulted in much higher IFSS of 189 ± 15 MPa and interfacial fracture energy of 3.3 ± 1.0 J/m². The debond surfaces of functionalized nanofibers had signs of matrix residue and/or shearing of the outer graphitic layer of the VGCNFs, namely the failure mode was a combination of cohesive matrix and/or cohesive fiber failure which contributed to the high IFSS. For all three grades of VGCNFs the IFSS was independent of fiber length and diameter. The findings of this experimental study emphasized the critical role of nanofiber surface morphology and chemistry in determining the shear strength and fracture energy of nanofiber interfaces, and shed light to prior composite-level strength and fracture toughness measurements.     

A semi-continuum finite element approach to evaluate the Young’s modulus of single-walled carbon nanotube reinforced composites

This paper describes a micromechanical finite element approach for the estimation of the effective Young’s modulus of single-walled carbon nanotube reinforced composites. These composite materials consist of aligned carbon nanotubes that are uniformly distributed within the matrix. Based on micromechanical theory, the Young’s modulus of the nanocomposite is estimated by considering a representative cylindrical volume element. Within the representative volume element, the reinforcement is modeled according to its atomistic microstructure while the matrix is modeled as a continuum medium. Spring-based finite elements are employed to simulate the discrete geometric structure and behavior of each single-walled carbon nanotube. The load transfer conditions between the carbon nanotubes and the matrix are modeled using joint elements of changeable stiffness that connect the two materials, simulating the interfacial region. The proposed model has been tested numerically and yields reasonable results for variable stiffness values of the joint elements. The effect of the interface on the performance of the composite is investigated for various volume fractions. The numerical results are compared with experimental and analytical predictions.     

Singular stress fields in interfacial notches of hybrid metal matrix composites

This paper presents results from a study on the singular stress fields in metal matrix composites in which dual matrices exist. The adjoining metallic matrices flanking the interface can deform plastically with powerlaw strain hardening. These matrices may have both different hardening exponents and different yield strengths. An asymptotic analysis coupled with numerical eigen-analysis solved the spatial structure of the singular stress field at radial- and angular-dependent parts: . The dependence of the strength of the singular stresses on the matrix properties is discussed. The effects of local geometry on the nature of singular stresses are addressed. Highlights for interfacial notches are reported here. The drivers for this study are interfacial notches and free-edges in hybrid metal matrix composites (Fig. 1). However, the results can also be applied to other advanced structures which are composed of two or more distinct components or phases such as bone-implant interfaces and surface mounts in electronic packages     

Reinforcing epoxy nanocomposites with functionalized carbon nanotubes via biotin–streptavidin interactions

We report on the preparation of nanocomposites consisting of biofunctionalized single-walled carbon nanotubes (BF-SWCNTs) reinforcing an ultraviolet curable epoxy polymer by means of biotin–streptavidin interactions. The as-produced laser ablation SWCNTs are biofunctionalized via acid oxidization based purification process and non-covalent functionalization using surfactant, followed by grafting the resulting nanotubes with biomolecules. The biotin-grafted nanotubes are capable of interacting with epoxy groups in presence of streptavidin molecules by which chemical bridges between BF-SWCNTs and epoxy matrix are formed. The biomolecules grafted to the nanotubes surface not only facilitate the load transfer, but also improve the nanotube dispersion into the epoxy matrix, as observed by optical imaging and scanning electron microscopy. Mechanical characterization on the nanocomposite microfibers demonstrates considerable enhancement in both strength (by 76%) and modulus (by 93%) with the addition of only 1 wt.% of BF-SWCNTs. The electrical measurements reveal a clear change in electrical conductivity of nanocomposite microfibers reinforced with 1 wt.% of BF-SWCNTs in comparison to the microfibers containing solely purified carbon nanotubes. These multifunctional nanocomposite materials could be used to fabricate macro and microstructures for a wide variety of applications such as high strength polymer nanocomposite and potential easily-manipulated biosensors.     

Multiwalled carbon nanotube reinforced polymer composites

Due to their high stiffness and strength, as well as their electrical conductivity, carbon nanotubes are under intense investigation as fillers in polymer matrix composites. The nature of the carbon nanotube/polymer bonding and the curvature of the carbon nanotubes within the polymer have arisen as particular factors in the efficacy of the carbon nanotubes to actually provide any enhanced stiffness or strength to the composite. Here the effects of carbon nanotube curvature and interface interaction with the matrix on the composite stiffness are investigated using micromechanical analysis. In particular, the effects of poor bonding and thus poor shear lag load transfer to the carbon nanotubes are studied. In the case of poor bonding, carbon nanotubes waviness is shown to enhance the composite stiffness.     

Electrospun nanofibers as a platform for multifunctional,hierarchically organized nanocomposite

Nanofibers are ideally suited to form a scaffold where multi-functional components can be hierarchically organized. Development in electrospinning in terms of fiber construction and organization, materials selection and incorporation, and post-spinning modifications have pathed the way for future developments of advanced composite systems. A nanocomposite system with up to five distinct levels of organization can be constructed using electrospun fibers. At the first level is a composite nanofiber. The second level is a second layer of composite material coated over the core composite nanofiber. Surface modification of the nanofiber will give the third level. The fourth level of organization is by arranging the nanofibers to form an assembly. Finally at the last level, the nanofiber assembly can be encapsulated within a matrix or form a bulk structure of a pre-determined shape. Examples of how hierarchically organized multifunctional nanocomposite can be used in healthcare, environmental and defense and security is discussed.     

Utilizing interfaces in carbon nanotube reinforced polymer composites for structural damping

Carbon nanotube reinforced polymer composites have been extensively researched [Shadler LS, Giannaris SC, Ajayan PM (1998) Appl Phys Lett 73:3842; Ajayan PM, Shadler LS, Giannaris C, Rubio A (2000) Adv Mater 12:750; Wagner HD, Lourie O, Feldman Y, Tenne R (1998) Appl Phys Lett 72:188; Thostenson ET, Chou T-W (2002) J Phys D: Appl Phys 35:L77] for their strength and stiffness properties. The interfaces between nanotubes and polymer matrix can play a critical role in nanocomposites for their mechanical properties, since the interfacial area is order of magnitude more than traditional composites. Unless the interface is carefully engineered, poor load transfer between individual nanotubes (in bundles) and between nanotubes and surrounding polymer chains may result in interfacial slippage [Shadler et al. (1998); Ajayan et al. (2000)] and consequently disappointing mechanical stiffness and strength. Interfacial slippage, while detrimental to high stiffness and strength, could result in very high mechanical damping, which is a hugely important attribute in many commercial applications. In this paper, we show that the mechanical damping is related to frictional energy dissipation during interfacial sliding at the extremely many nanotube-polymer interfaces, and characterize the impact of activation of the frictional sliding on damping behavior.     

Fabrication of Nanofiber Reinforced Protein Structures For Tissue Engineering

Extracellular matrices and degradable nanofibers are two very promising materials in the field of tissue engineering; however both of these structures face limitations as tissue engineering scaffolds. Extracellular matrices, such as collagen, gelatin, and laminin, have excellent biocompatibility and allow cell in growth and survival, but structural weakness makes them difficult to handle and greatly limits their uses. Degradable nanofibers support cell attachment and can provide structural support and directional guidance, but individual degradable nanofibers are fragile and have a tendency to form dense fiber bundles which limit cell penetration into the spaces between the nanofibers, especially in the case of aligned nanofibers. To overcome these difficulties, degradable loose nanofibers were embedded in protein matrix in an attempt to fabricate a hybrid scaffold with improved properties, such as improved strength, guidance, spacing among nanofibers, etc. Polycaprolactone (PCL) was used as a model material for degradable nanofibers. Gelatin was employed as a model protein for matrix structure formation. Thin hybrid films (average thickness = 2.78 um) were fabricated by wetting the loose aligned undirectional nanofiber arrays or loose aligned bi-directional nanofiber grids with a gelatin aqueous solution, which also allows for live cell loading into the nanofiber-protein composite if cell are premixed with protein solution or on the surface of the films. Gelatin film alone without nanofiber reinforcement is difficult to handle due to the weakness of the thin membrane. Gelatin films with a fiber density as low as 3% v/v were structurally robust enough for handling, and manipulation into complex shapes. Mechanical testing confirmed that the addition of nanofibers enhanced the strength of gelatin films, in both dry and hydrated state. In vitro testing confirmed that nanofiber reinforced films were biocompatible and provided cells with directional guidance. Results demonstrate the promise of gelatin/PCL nanofiber composites as a tissue scaffolding material.     

Counterintuitive effect of molecular strength and role of molecular rigidity on mechanical properties of layer-by-layer assembled nanocomposites

Molecular engineering of multilayered composites by layer-by-layer assembly (LBL) made possible easy replication of mechanical properties of nacre. Taking advantage of the ability of LBL to finely control the structure of the composite, one can further improve the mechanical properties of the multilayers, e.g., increase the strength and stiffness, and gain better understanding of the nanoscale and molecular scale mechanics of the materials critical for a variety of advanced technologies. In this study, we have replaced poly(diallyldimethylammonium chloride) (PDDA) (sigmaUTS approximately 12 MPa, E approximately 0.2 GPa) with a much stronger polysaccharide polycation, chitosan (CH, sigmaUTS approximately 108 MPa, E approximately 2 GPa), considering that its superior molecular strength will improve the macroscale mechanical properties of the nanocomposite: strength and stiffness. Free-standing films of the CH and montmorillonite (MTM) have been successfully made, and the resulting films revealed high uniformity with very high loading of MTM closely comparable to that in the natural nacre, approximately 80 wt %. Contrary to our expectations and theoretical predictions, the CH-MTM composite revealed lower strength and stiffness than those of PDDA-MTM and lower strength than CH polymer itself: sigmaUTS approximately 80 MPa and E approximately 6 GPa. Analysis of the morphology of adsorbing CH chains with atomic force microscopy revealed highly elongated molecules, which is opposite to the observations made for PDDA. Plane-to-plane adhesion showed a factor of approximately 4 lower strength when compared to PDDA-MTM nanocomposite. Altogether these facts support the conclusion that CH lacks flexibility necessary for strong adhesion and efficient load transfer between the organic matrix and MTM platelets. High rigidity of the CH chains does not allow them to acquire a conformation necessary for maximizing the interfacial attraction with nanoscale component of the composite. These observations create an important foundation in the experimental design of the high-performance nanocomposite materials.