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Analytical Formulation of Stress Distribution in Cellulose Nanocomposites
Abstract
Cellulose nanofibers are known to possess aspect ratios larger than 200 and mechanical properties comparable to carbon nanotubes. Combined with other significant properties including low cost, low density, and biocompatibility, cellulose nanofibers are an attractive reinforcement material for nanocomposites. The load transfer between embedded fibers and matrix play a major role in designing nanocomposites with ultimate mechanical properties. In this work, we studied a general case where a simple axial loading exists along the axis of a cellulose fiber embedded in a polymer matrix. Then analytical relation between the applied load, the longitudinal stress along the fiber, and shear stresses along the interface of fiber and matrix was derived. It is shown that the maximum longitudinal stress occurs at the middle of the fiber, while maximum shear stress occurs at the extreme ends. Also, it is shown that the shear stress along the cellulose fibers can be approximated as a linear function of applied load. The derived relationships are useful for design of cellulose-based nanocomposites with enhanced mechanical properties.
... [23] This model has been successfully used for modeling 1D nanostructures. [24][25] The conservation of linear momentum in the absence of body forces and acceleration is: ...
... rr << zz for the NW), R>>r o (low concentration of NWs), and using the shear-lag model, the stress along the ZnO NW in terms of the cylindrical coordinate system is calculated. [25] The following dimensionless variables are defined: ...
... where is the applied axial stress and is a parameter with the dimension of [L] -1 . The value of depends on the mechanical and geometrical properties of NW and matrix material.[25] The ...
A nanocomposite electrical generator composed of Zinc oxide nanowires (ZnO NWs) was modeled using continuum mechanics and Maxwell's equations. Axial loading was considered and the optimum aspect ratio of ZnO NWs for getting to maximum electric potential was calculated. The bonding between the ZnO NWs and the polymer matrix was considered to be perfect and the linear piezoelectric behavior was assumed. It was shown that the electric potential has maximum and minimum values of opposite signs at the extreme ends along the nanowire length. The maximum generated electric potential varies from 0.01717 for a NW with an aspect ratio of one to 0.61107 for a NW with an aspect ratio of thirty. The optimum aspect ratio of ZnO NW was defined as the difference between the maximum generated electric potential at which the difference becomes less than 1%, which results in an aspect ratio of 16. The results are a major step toward producing ZnO NWs for nanocomposite electrical generators with maximum performance.
... Carbohydrates are biomolecules widely studied due to numerous applications, which range from pharmacological action, for example, the anticoagulant activity of heparin [194], to technological interest in developing cellulose nanofibers [195,196,197] that can exhibit mechanical properties comparable to carbon nanotubes [198]. Such properties are related to the molecular architecture, which is governed by intra-and intermolecular interactions, such as hydrogen bond. ...
The effect of weak intermolecular interactions on the binding affinity between ligand-protein complexes plays an important role in stabilizing a ligand at the interface of a protein structure. In this review article, we will explore the different ways of taking into account these interactions, mainly intramolecular hydrogen bonds, in docking calculations. Their possible limitations and their suitable application domains are highlighted. Inspection of the outliers of this study probed very stimulating, as it provides opportunities and inspiration to medicinal chemists, being a reminder of the impact that minimal chemical modifications can have on biological activities.
... This adhesion prevents slipping of PZTNP-CNT in PTFE polymer and facilitates strain transfer between them. The high aspect ratio of the CNT also improves the ability to transfer the strain into the polymer, which is consistent with the case of cellulose with similar aspect ratios to CNT [22]. In addition, the friction between the CNT and the polymer increases when bending, and this friction plays an important role in transferring the strain into the PZTNP-CNT. ...
Recently, composite-type nanogenerators (NGs) formed from piezoelectric nanostructures and multi-walled carbon nanotubes (CNTs), have become one of the excellent candidates for future energy harvesting because of their ability to apply the excellent electrical and mechanical properties of CNTs. However, the synthesis of NG devices with a high proportion of piezoelectric materials and a low polymer content, such as of polydimethylsiloxane (PDMS), continues to be problematic. In this work, high-piezoelectric-material-content flexible films produced from Pb(Zr,Ti)O₃ (PZT)-atomically-interconnected CNTs and polytetrafluoroethylene (PTFE) are presented. Various physical and chemical characterization techniques are employed to examine the morphology and structure of the materials. The direct growth of the piezoelectric material on the CNTs, by stirring the PZT and CNT mixed solution, results in various positive effects, such as a high-quality dispersion in the polymer matrix and addition of flexoelectricity to piezoelectricity, resulting in the enhancement of the output voltage by an external mechanical force. The NGs repeatedly generate an output voltage of 0.15 V. These results present a significant step toward the application of NGs using piezoelectric nanocomposite materials.
... nanostructures. [177][178] The conservation of linear momentum in the absence of body forces and acceleration is: ...
Emerging nanogenerators have attracted the attention of the research
community, focusing on energy generation using piezoelectric
nanomaterials. Nanogenerators can be utilized for powering NEMS/MEMS
devices. Understanding the piezoelectric properties of ZnO one
dimensional materials such as ZnO nanobelts (NBs) and Nanowires (NWs)
can have a significant impact on the design of new devices. The goal of
this dissertation is to study the piezoelectric properties of one
dimensional ZnO nanostructures both experimentally and theoretically.
First, the experimental procedure for producing the ZnO nanostructures
is discussed. The produced ZnO nanostructures were characterized using
an in situ atomic force microscope and a piezoelectric force
microscope. It is shown that the electrical conductivity of ZnO NBs is a
function of applied mechanical force and its crystalline structure. This
phenomenon was described in the context of formation of an electric
field due to the piezoelectric property of ZnO NBs. In the PFM studies,
it was shown that the piezoelectric response of the ZnO NBs depends on
their production method and presence of defects in the NB. Second, a
model was proposed for making nanocomposite electrical generators based
on ZnO nanowires. The proposed model has advantages over the original
configuration of nanogenerators which uses an AFM tip for bending the
ZnO NWs. Higher stability of the electric source, capability for
producing larger electric fields, and lower production costs are
advantages of this configuration. Finally, piezoelectric properties of
ZnO NBs were simulated using the molecular dynamics (MD) technique. The
size scale effect on piezoelectric properties of ZnO NBs was captured,
and it is shown that the piezoelectric coefficient of ZnO NBs decreases
by increasing their lateral dimensions. This phenomenon is attributed to
the surface charge redistribution and compression of unit cells that are
placed on the outer shell of ZnO NBs.
Ferritic-martensitic steels, such as T91, are candidate materials for high-temperature applications, including superheaters, heat exchangers, and advanced nuclear reactors. Considering these alloys’ wide applications, an atomistic understanding of the underlying mechanisms responsible for their excellent mechano-chemical properties is crucial. Here, we developed a modified embedded-atom method (MEAM) potential for the Fe-Cr-Si-Mo quaternary alloy system—i.e., four major elements of T91—using a multi-objective optimization approach to fit thermomechanical properties reported using density functional theory (DFT) calculations and experimental measurements. Elastic constants calculated using the proposed potential for binary interactions agreed well with ab initio calculations. Furthermore, the computed thermal expansion and self-diffusion coefficients employing this potential are in good agreement with other studies. This potential will offer insightful atomistic knowledge to design alloys for use in harsh environments.
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A multiscale approach is pursued for modeling the size-scale effect on generated electric potential by nanocomposite electrical generators of ZnO nanowires. A core-surface model is used for capturing the effect of size-scale on elastic modulus of ZnO NWs. In this model, a surface with different elastic modulus as of the core of NW was considered. Using linear elasticity and axisymmetric configuration of this problem, closed form governing equations are derived in cylindrical coordinate system. Parametric studied are performed for sample cases to demonstrate application of the developed model. It is shown that ZnO nanowires with larger aspect ratio and smaller diameters have higher performance and can produce higher electric potential.
Four α- and three β-isomers of the d-glucose were optimized in gas phase using ab initio (MP2) and DFT (ωB97X-D) methods, both using the aug-cc-pVDZ basis set. While earlier works suggest that the orientation of the hydroxyl groups is due to intramolecular hydrogen bonds (H-bonds), the present study reveals that most H-bonds forming five-membered rings are either weak or even do not exist. The quantum theory of atoms in molecules (QTAIM) analysis showed only a few cases of H-bond in d-glucose, particularly for those H-bonds forming six-membered rings, while the non-covalent interactions (NCI) analysis indicated that most intramolecular H-bonds are not strong enough to justify the counter-clockwise arrangement of the OH⋯O chains. Natural bond orbital analysis supported the findings obtained from QTAIM and NCI analyses and indicated that the anomeric effect for d-glucose in the gas phase is governed by a balance of steric, electrostatic, and hyperconjugative interactions.
A nanocomposite electrical generator composed of an array of zinc oxide nanowires is considered. The electric potential distribution along zinc oxide nanowires is modeled using continuum mechanics and Maxwell's equations for the case of axial loading. A perturbation technique is used for decoupling the constitutive equations. The governing differential equations are solved using a finite difference method. It is shown that a gradient of electric potential exists along the axis of the zinc oxide nanowires. Maximum and minimum values of electric potential exist at the extreme ends along the nanowire length and have opposite signs. The positive and negative voltages are separated by a zero-valued electric potential at the middle of the nanowire. It is also shown that the electric potential is a strong function of shear stress at the interface of matrix-nanowire. The proposed system and loading configuration can generate up to 160% more electric potential than the values reported for the nanowire in the bended configuration, which results in a more sustainable energy source.
The structure of cellulose microfibrils in wood is not known in detail, despite the abundance of cellulose in woody biomass and its importance for biology, energy, and engineering. The structure of the microfibrils of spruce wood cellulose was investigated using a range of spectroscopic methods coupled to small-angle neutron and wide-angle X-ray scattering. The scattering data were consistent with 24-chain microfibrils and favored a "rectangular" model with both hydrophobic and hydrophilic surfaces exposed. Disorder in chain packing and hydrogen bonding was shown to increase outwards from the microfibril center. The extent of disorder blurred the distinction between the I alpha and I beta allomorphs. Chains at the surface were distinct in conformation, with high levels of conformational disorder at C-6, less intramolecular hydrogen bonding and more outward-directed hydrogen bonding. Axial disorder could be explained in terms of twisting of the microfibrils, with implications for their biosynthesis.
Cellulose nanofibers are obtained from various sources such as flax bast fibers, hemp fibers, kraft pulp, and rutabaga, by chemical treatments followed by innovative mechanical techniques. The nanofibers thus obtained have diameters between 5 and 60 nm. The ultrastructure of cellulose nanofibers is investigated by atomic force microscopy and transmission electron microscopy. The cellulose nanofibers are also characterized in terms of crystallinity. Reinforced composite films comprising 90% polyvinyl alcohol and 10% nanofibers are also prepared. The comparison of the mechanical properties of these composites with those of pure PVA confirmed the superiority of the former.
In this paper, the macroscopic elastic properties of carbon nanotube reinforced composites are evaluated through analysing the elastic deformation of a representative volume element (RVE) under various loading conditions. This RVE contains three components, i.e. a carbon nanotube, a transition layer between the nanotube and polymer matrix and an outer polymer matrix body. First, based on the force field theory of molecular mechanics and computational structural mechanics, an equivalent beam model is constructed to model the carbon nanotube effectively. The explicit relationships between the material properties of the equivalent beam element and the force constants have been set-up. Second, to describe the interaction between the nanotube and the outer polymer matrix at the level of atoms, the molecular mechanics and molecular dynamics computations have been performed to obtain the thickness and material properties of the transition layer. Moreover, an efficient three-dimensional eight-noded brick finite element is employed to model the transition layer and the outer polymer matrix. The macroscopic behaviours of the RVE can then be evaluated through the traditional finite element method. In the numerical simulations, the influences of various important factors, such as the stiffness of transition layer and geometry of RVE, on the final macroscopic material properties of composites have been investigated in detail.
BACKGROUND: Shape memory polymers are capable of fixing a transient shape and of recovering their original dimensions by the application of an external stimulus. Their major drawback is their low stiffness compared to smart materials based on metals and ceramics. To overcome this disadvantage, nanocellulose was utilized as reinforcement.
RESULTS: Composites were prepared by casting stable nanocellulose/segmented polyurethane suspensions. The heat of melting of the polyurethane soft segment phase increased on cellulose addition. Composites showed higher tensile modulus and strength than unfilled films (53% modulus increase at 1 wt% nanocellulose), with higher elongation at break. Creep deformation decreased as cellulose concentration increased (36% decrease in 60-minute creep by addition of 1 wt% nanocellulose). The nanocomposites displayed shape memory properties equivalent to those of the neat polyurethane, with recoveries of the order of 95% (referred to second and further cycles).
CONCLUSIONS: It is possible to markedly improve the rigidity of shape memory polymers by adding small amounts of well-dispersed nanocellulose. However, this improvement did not have substantial effects on the material shape fixity or recovery. Shape memory behavior seems to continue to be controlled by the polymer properties. Copyright
The following paper summarises a number of international research projects being undertaken to understand the mechanical properties of natural cellulose fibres and composite materials. In particular the use of novel techniques, such as Raman spectroscopy, synchrotron x-ray and half-fringe photoelastic methods of measuring the physical and micromechanical properties of cellulose fibres is reported. Current single fibre testing procedures are also reviewed with emphasis on the end-use in papermaking. The techniques involved in chemically modifying fibres to improve interfacial adhesion in composites are also reviewed, and the use of novel fibre sources such as bacterial and animal cellulose. It is found that there is overlap in current international research into this area, and that there are complementary approaches and therefore further combining of these may make further progress possible. In particular a need to measure locally the adhesion properties and deformation processes of fibres in composites, with different chemical treatments, ought to be a focus of future research.
Carbon nanotubes have mechanical properties that are far in excess of conventional fibrous materials used in engineering polymer composites. Effective reinforcement of polymers using carbon nanotubes is difficult due to poor dispersion and alignment of the nanotubes along the same axis as the applied force during composite loading. This paper reviews the mechanical properties of carbon nanotubes and their polymer composites to highlight how many previously prepared composites do not effectively use the excellent mechanical behaviour of the reinforcement. Nanomechanical tests using atomic force microscopy are carried out on simple uniaxially aligned carbon nanotube-reinforced polyvinyl alcohol (PVA) fibres prepared using electrospinning processes. Dispersion of the carbon nanotubes within the polymer is achieved using a surfactant. Young's modulus of these simple composites is shown to approach theoretically predicted values, indicating that the carbon nanotubes are effective reinforcements. However, the use of dispersant is also shown to lower Young's modulus of the electrospun PVA fibres.
The study and application of composite materials are a truly interdisciplinary endeavour that has been enriched by contributions from chemistry, physics, materials science, mechanics and manufacturing engineering. The understanding of the interface (or interphase) in composites is the central point of this interdisciplinary effort. From the early development of composite materials of various nature, the optimization of the interface has been of major importance. While there are many reference books available on composite materials, few of them deal specifically with the science and mechanics of the interface of fiber reinforced composites. Further, many recent advances devoted solely to research in composite interfaces have been scattered in a variety of published literature and have yet to be assembled in a readily accessible form. To this end this book is an attempt to bring together recent developments in the field, both from the materials science and mechanics perspective, in a single convenient volume.
The central theme of the book is tailoring the interface properties to optimise the mechanical peformance and structural integrity of composites with enhanced strength/stiffness and fracture toughness (or specific fracture resistance). It deals mainly with interfaces in advanced composites made from high performance fibers, such as glass, carbon, aramid, ultra high modulus polyethylene and some inorganic (e.g. B/W, A12O3, SiC) fibers, and matrix materials encompassing polymers, metals/alloys and ceramics. The book is intended to provide a comprehensive treatment of composite interfaces in such a way that it should be of interest to materials scientists, technologists and practising engineers, as well as graduate students and their supervisors in advanced composites. We hope that this book will also serve as a valuable source of reference to all those involved in the design and research of composite interfaces.
The book contains eight chapters of discussions on microstructure-property relationships with underlying fundamental mechanics principles. In Chapter 1, an introduction is given to the nature and definition of interfaces in fiber reinforced composites. Chapter 2 is devoted to the mechanisms of adhesion which are specific to each fiber-matrix system, and the physio-chemical characterization of the interface with regard to the origin of adhesion. The experimental techniques that have been developed to assess the fiber-matrix interface bond quality on a microscopic scale are presented in Chapter 3, along with the techniques of measuring interlaminar/intralaminar strengths and fracture toughness using bulk composite laminates. The applicability and limitations associated with loading geometry and interpretation of test data are compared. Chapter 4 presents comprehensive theoretical analyses based on shear-lag models of the single fiber composite tests, with particular interest being placed on the interface debond process and the nature of the fiber-matrix interfacial bonding. Chapter 5 is devoted to reviewing current techniques of fiber surface treatments which have been devised to improve the bond strength and the fiber-matrix compatibility/stability during the manufacturing processes of composites. The micro-failure mechanisms and their associated theories of fracture toughness of composites are discussed in Chapter 6. The roles of the interface and its effects on the mechanical performance of fiber composites are addressed from several viewpoints. Recent research efforts to augment the transverse and interlaminar fracture toughness by means of controlled interfaces are presented in Chapters 7 and 8.
Three closely related cellulosic acrylic latex films were prepared employing acacia pulp fibers, cellulose whiskers and nanocellulose balls and their respective strength properties were determined. Cellulose whisker reinforced composites had enhanced strength properties compared to the acacia pulp and nanoball composites. AFM analysis indicated that the cellulose whisker reinforced composite exhibited decreased surface roughness.
A review of biocomposites highlighting recent studies and developments in natural fibers, bio-polymers, and various surface modifications of natural fibers to improve fiber-matrix adhesion is presented. One of the most important factors which determine the final performance of the composite materials is the quality of the fiber-matrix interface. A sufficient degree of adhesion between the surface of hydrophilic ligno-cellulosic natural fibers and the polymer matrix resin is usually desired to achieve optimum performance of the biocomposite. Dewaxing, alkali treatment, isocyanate treatment, peroxide treatment, vinyl grafting, bleaching, acetylation, and treatment with coupling agents are useful ways to improve fiber-matrix adhesion in natural fiber composites. Two major areas of biocomposites will be discussed in this article. One is the most predominant biocomposite currently being commercialized for semi-structural use in the durable goods industries, e.g. auto-industries, i.e. natural fiber-polypropylene composites. The second type is the biocomposites from natural fibers and biodegradable plastics. Two major classes of biodegradable plastics are available, one being derived from renewable resources and the second type being synthesized in the laboratory from petrochemical sources which can also be used as matrix materials to make value-added biocomposites.
In this study, the possibility of using silane technology in crosslinking composites of wood flour and polyethylene has been investigated. Composites of vinyltrimethoxy silane grafted high density polyethylene and wood flour were produced by compounding in a twin-screw extruder. Gel content analysis with p-xylene extraction revealed higher gel content in the samples where wood flour was added compared to neat crosslinked matrix. Mechanical analysis of the crosslinked composites showed increased tensile strength with increasing amount of wood flour, which might be an indication of improved adhesion between the matrix and the wood flour. The stiffness increased with increasing amount of wood flour with accompanied decrease in strain at break. Dynamic mechanical thermal analysis of crosslinked plastics and composites showed no significant shift in the γ-transition towards higher temperature for the composites compared to neat plastic. Short-term creep experiments showed reduced creep deformation with increasing amount of wood flour. Crosslinking of the composites reduced the creep deformation further. A boiling test in water followed by tensile testing showed that the crosslinked composites were less susceptible to water uptake compared to the non-crosslinked. Moreover, the decrease in tensile strength of the crosslinked composites was not as significant as for the non-crosslinked composites. Scanning electron microscopy revealed good compatibility and adhesion between the plastic and the wood flour for crosslinked composites.
Nanocomposite polymer electrolytes based on high-molecular weight poly(oxyethylene) (POE) were prepared from high aspect ratio cellulosic whiskers and lithium trifluoromethyl sulfonyl imide (LiTFSI). Prior to the polymer electrolyte characterization, the polymer/whiskers nanocomposites were characterized using wide-angle X-ray scattering (WAXS), differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Interactions between cellulose and POE were evidenced. The main effect of the filler was a thermal stabilization of the storage modulus for the composites above the melting point of the complexes POE/LiTFSI. The ionic conductivity was quite consistent with the specifications of lithium batteries.
The stiffness of 10 nm diameter cellulose nanowhiskers is reported. These whiskers are produced by acid hydrolysis. These whiskers are dispersed in epoxy resin and placed on the surface of a beam of the same material and deformed in tension and compression using a four-point bending device. By following the molecular deformation of the whiskers using Raman spectroscopy it is shown that, by theoretical models of their dispersion and matrix reinforcement, their stiffness can be derived. The effects of debonding, matrix yielding, and buckling of whiskers are also discussed using this method as a means for studying nanocomposite materials.
The nanomechanical properties of Kevlar® 49 fibers are quantitatively evaluated using the interfacial force microscope (IFM). The IFM is capable of differentiating the skin/core structure of the fibers and selectively probing each area. The core and skin regions were found to possess an elastic modulus of 60.8 and 13.4 GPa, respectively. The difference in modulus is attributed to an increased amount of order and hence intermolecular bonding in the core region.
Nano- and micron-sized cellulose crystals were prepared and utilized as reinforcements for polyurethane composites. The cellulose crystals obtained from microcrystalline cellulose (MCC) were incorporated into a polar organic solvent, dimethylformamide (DMF), and ultrasonicated to obtain a stable suspension. The suspension was an effective means for incorporating the cellulose crystals into the polyol-isocyanate mixture, utilized to produce polyurethane composite films. The use of DMF presents an interesting alternative for the use of cellulose crystals as reinforcement of a broad new range of polymers. Moreover, the rheology of the uncured liquid suspensions was investigated, and analysis of the results indicated the formation of a filler structure pervading the liquid suspension. Besides, films were prepared by casting and thermal curing of the stable suspensions. Thermomechanical and mechanical testing of the films were carried out to analyze the performance of the composites. The results indicated that a strong filler-matrix interaction was developed during curing as a result of a chemical reaction occurring between the crystals and the isocyanate component.
The first example of a transparent composite reinforced with bacterial nanofibers was discussed. The composite was optically transparent at a fiber content as high as 70%, with a low thermal-expansion coefficient and a mechanical strength five times that of engineered plastics. It was shown that the significant improvements in the thermal and mechanical characteristics of the composite are due to the web-like network of the semicrystalline extended chains of nanofibers, produced by the bacterium Acetobacter xylinum. It was found that the nanofiber network in a variety of resins led to a very low loss of transparency of the original resin, even at a high fiber content, due to the size effect.
The use of lignocellulose fibers as reinforcement components in Polymer reinforcement composites was investigated. To use the potential of the fibrils, the fragmentation of the cellulose fibers into smaller units was attempted many time. The microcrystalline cellulose (MCC) was generated by chemical treatment of various plant fibers. It was found that the fibers are biodegradable, harmless for the ecological system, have good mechanical properties, and are less expensive than conventional synthetic fibers.
Cellulose microcrystals with dimensions of ∼5 nm × 150–300 nm were obtained from wheat straw. To evaluate the reinforcing effect of these fillers within a thermoplastic matrix, composites with a weight fraction of cellulose ranging from O to 30 wt% were processed by freeze-drying and molding a mixture of aqueous suspensions of microcrystals and poly(styrene-co-butyl acrylate) latex. It was found that these microcrystals, or whiskers, bring a great reinforcing effect at temperatures higher than the glass transition temperature (Tg) of the matrix and improve the thermal stability of the composite. The relaxed modulus increased continuously with the filler content, and for a film containing 30 wt% of whiskers, it was more than a thousand times higher than that of the matrix. This effect is discussed with regard to theoretical calculations based on a mean field approach (Halpin-Kardos model). It is concluded that the great reinforcement observed seems to be due not only to the geometry and stiffness of the straw cellulose whiskers but also to the interactions of the microcrystals, their topological arrangement, and the probable formation of whisker clusters within the thermoplastic matrix, the cellulose fillers probably being linked through hydrogen bonds.
Cellulose whiskers have been used as reinforcement in a copolymer matrix prepared from a latex phase. If a water suspension-mixing procedure is adopted, the fibril breakage that usually occurs during the mixing with a molten polymer can be avoided, and an enhanced filler dispersion can be expected. In this study, different processing methods have been used to prepare composite films, either by film casting (water evaporation) or by freeze drying, followed by classical compression or extrusion processes. The thermomechanical properties of these nanocomposites have been investigated, and the influence of processing conditions and the effect of whisker content have been considered. Processing conditions have a large influence on the mechanical behavior and can be classified in ascending order of their reinforcement efficiency: It can be attributed to a decrease of the apparent whisker aspect ratio, due to gradual breakage and/or orientation of the whiskers when hot pressing or extrusion is used. Below Tg, good agreement is found between experimental moduli and the theoretical predictions of the Halpin-Kardos equation. On the other hand, above Tg, a spectacular reinforcing effect is observed, which is widely underestimated by this short fiber composite model. This is related to the presence of a rigid cellulose network, linked by hydrogen bonds, when the whisker content is above its percolation threshold. The quality of this network (i.e., density and homogeneity) and thus, the magnitude of the reinforcing effect, depend on processing conditions.
Using plant microfiber bundles with a nanometer unit web-like network, a moulded product with a bending strength of 250 MPa was obtained without the use of binders. High interactive forces seem to be developed between pulp fibers owing to their nanometer unit web-like network. In other words, the area of possible contact points per fiber are increased, so that more hydrogen bonds might be formed or van der Waals forces increased. When 2% oxidized tapioca starch, by weight, was added, the yield strain doubled and the bending strength reached 310 MPa. The starch mixed moulded product had a similar stress strain curve to that for magnesium alloy, and three to four times higher Young's modulus and bending strength values than polycarbonate and GFRP (chopped). The mouldings have a combination of environmentally friendly and high strength properties.
The goal of this study was to characterize the nanostructure and the properties of starch based nanocomposites with either
cellulose nano whiskers (CNW) or layered silicates (LS) (synthetic hectorite) as reinforcements. Modified potato starch was
used as matrix with water and sorbitol as plasticizers and with 5wt.% of either of the reinforcements. Two methods were explored
to prepare samples for transmission electron microscopy (TEM) examination; chemical fixation and freeze etching. It was possible
to characterize the nanostructure both parallel and perpendicular to the nanocomposite surface by the freeze etching technique.
Both nanocomposites showed well-distributed reinforcements in the starch matrix. Dynamic mechanical thermal analysis showed
that the storage modulus was significantly improved at elevated temperatures, especially for the layered silicate nanocomposite.
Both nanocomposites showed a significant improvement in tensile properties compared to the pure matrix.
Microchannel devices hold the potential to transform many separation processes. This preliminary study investigated the feasibility
of incorporating cellulose nanocrystals (CNXLs) into polysulfone, a commonly used ultrafiltration membrane polymer. Incorporating
CNXLs into non-water soluble polymers without aggregation has been problematic. A solvent exchange process was developed that
successfully transferred an aqueous CNXL dispersion into the organic solvent N-methylpyrrolidone (NMP), which is a solvent for polysulfone (PSf). Films were prepared from the solution of PSf in NMP with
dispersed CNXLs by a phase inversion process. Films were then examined by scanning electron microscopy and tested for their
transport and mechanical properties. The interaction between the polymer matrix and the CNXL filler was studied by means of
thermogravimetric analysis (TGA), which suggested a close interaction between the polymer and filler at the 2% filler loading.
The tensile modulus showed a large increase beyond 1% filler loading, which could be due to a percolation effect. The water
vapor transport rate increased with increase in filler loading. Agglomeration of the CNXLs seemed to be taking place at filler
loadings >7%.
An estimation, using a Raman spectroscopic technique, of the Young’s modulus of a single filament of bacterial cellulose is
presented. This technique is used to determine the local molecular deformation of the bacterial cellulose via a shift in the
central position of the 1095cm–1 Raman band, which corresponds to the stretching of the glycosidic bond in the backbone of the cellulose structure. By calculating
the shift rate with respect to the applied strain it is shown that the stiffness of a single fibril of bacterial cellulose
can be estimated. In order to perform this estimation, networks of fibres are rotated through 360° and the intensity of the
1095cm−1 Raman band is recorded. It is shown that the intensity of this band is largely independent of the angle of rotation, which
suggests that the networks are randomly distributed. The modulus is predicted from a calibration of Raman band shift against
modulus, using previously published data, and by using Krenchel analysis to back-calculate the modulus of a single fibril.
The value obtained (114GPa) is higher than previously reported values for this parameter, but lower than estimates of the
crystal modulus of cellulose-I (130–145GPa). Reasons for these discrepancies are given in terms of the crystallinity and
structural composition of the samples.
High-strength composites were produced using bacterial cellulose (BC) sheets impregnated with phenolic resin and compressed at 100MPa. By utilizing this unique material synthesized by bacteria, it was possible to improve the mechanical properties over the previously reported high-strength composites based on fibrillated kraft pulp of plant origin. BC-based composites were stronger, and in particular the Youngs modulus was significantly higher, attaining 28GPa versus 19GPa of fibrillated pulp composites. The superior modulus value was attributed to the uniform, continuous, and straight nano-scalar network of cellulosic elements oriented in-plane via the compression of BC pellicles.
Nanocomposites of nitrile rubber (NBR) and cellulose II (Cel II) were prepared by co-coagulation of nitrile rubber latex and
cellulose xanthate mixtures. The effect of the addition of increasing amounts of Cell II, varying from 0 to 30phr, on the
mechanical behavior of a NBR was analyzed. The fracture mechanisms of the nitrile rubber-cellulose II (NBR/Cel II) nanocomposites
after tension and tear tests was investigated by scanning electron microscopy (SEM) and correlated to the test results. It
was found that the addition of Cell II to NBR leads to a gradual change in the stress at break, and the samples with 20phr
of Cel II showed the highest resistance; as the cellulose content is increased to 30phr, the strain at break decreases. The
SEM fractographic analyses of NBR/Cel II nanocomposites, with cellulose content up to 30 support the observed mechanical behavior
of the material. The results are presented and discussed. The obtainment of NBR/Cel II nanocomposites was proven by transmission
electron microscopy (TEM).
Biocomposites consisting of hydroxyapatite (HAp) and natural polymers such as collagen, chitosan, chitin, and gelatin have been extensively investigated. However, studies on the combination of HAp with bacterial cellulose (BC) have not been conducted yet. In this work, our latest results concerning the biomimetic synthesis and characterization of HAp-BC nanocomposites with a 3-dimensional (3-D) network are reported. The present research focuses on characterizing the structure of this novel class of nanocomposites. Scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and Fourier-transformed infrared spectroscopy (FTIR) were employed to characterize the HAp-BC nanocomposites. It is found that HAp crystals are formed when the phosphorylated and CaCl2-treated BC fibers are soaked in a 1.5 simulated body fluid (SBF). XRD reveals that the crystallite sizes of the HAp crystals are nano-sized and their crystallinities are low. The FTIR results show that the HAp crystals are partially substituted with carbonate, resembling natural bones. The nanocomposites containing HAp with structural features close to those of biological apatites are attractive for applications as artificial bones and scaffolds for tissue engineering.
An account is given on natural and man-made cellulose fiber reinforced plastics. Possible applications of this material group are detailed. A survey is also discussed about physical and chemical treatment methods that improve fiber matrix adhesion, as well as their results and effects on the physical properties of composites. The results show that natural fibers can be processed with the already commonly applied methods: glass mat thermoplastic matrix (GMT, sheet moulding compound (SMC) or bulk moulding compound (BMC).
Hydroxyapatite (HAp) and bacterial cellulose (BC) are both excellent materials for use in biomaterial areas. The former has outstanding osteoconductivity and bioactivity and the latter is a high-strength nano-fibrous and extensively used biomaterial. In this work, the HAp/BC nanocomposites with a 3-dimensional (3-D) network were synthesized via a biological route by soaking both phosphorylated and unphosphorylated BCs in 1.5 simulated body fluid (SBF). Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transformed infrared spectroscopy (FTIR), and transmission electron microscopy (TEM) were employed to characterize the HAp/BC nanocomposites. SEM observations demonstrated that HAp crystals were uniformly formed on the phosphorylated BC fibers after soaking in 1.5 SBF whereas little HAp was observed on individual unphosphorylated BC fibers. Our experimental results suggested that the unphosphorylated BC did not induce HAp growth and that phosphorylation effectively triggered HAp formation on BC. Mechanisms were proposed for the explanation of the experimental observations. XRD and FTIR results revealed that the HAp crystals formed on the phosphorylated BC fibers were carbonate-containing with nano-sized crystallites and crystallinities less than 1%. These structural features were close to those of biological apatites.
In previous works, the processing of nanocomposites of cellulose whisker filled thermoplastics was presented as well as their mechanical behaviour in the glassy state, both in the linear and non-linear domains. The purpose of the present work is to evaluate the mechanical properties of these composites in the rubbery state. A simple modelling of the mechanical behaviour, using the classical entropy elasticity theory, is proposed and discussed. Particular attention was paid to damage phenomena, already observed with stretched composite in the glassy state. Damage is mainly explained as a debonding of the matrix at the whisker interface. Small-Angle Neutron Scattering experiments performed on stretched composites provide supplementary information on the matrix microstructure with and without whiskers, and on the evolution of this microstructure with the sample deformation. Results confirm the heterogeneous characteristic of the matrix. Void scattering is identified and interpreted.
The hardness and elastic modulus of the cellulose fiber and polypropylene (PP) matrix in a cellulose fiber-reinforced PP composite were investigated by nanoindentation with a continuous stiffness technique. Nanoindentation with different indentation depths and spacings was conducted to measure hardness and elastic modulus in the interphase region, which was modified by maleic anhydride-grafted PP and γ-amino propyltrimethoxy silane (γ-APS) sizing. A line of indents was produced from the fiber to the matrix. There was a gradient of hardness and modulus across the interphase region. The distinct properties of the transition zone were revealed by 1–4 indents, depending on nanoindentation depth and spacing. Based on the results of nanoindentation, it was assumed that the width of the property transition zone is less than 1 μm. However, three dimensional finite element analysis shows that even a perfect interface without property transition has almost same interphase width as that measured by nanoindentation. Using existing nanoindentation techniques, it will be difficult to calculate exact mechanical properties without the effect of neighboring material property in at least 8 times smaller region than indent size.
We used molecular mechanics and molecular dynamics to study the nature of load transfer in a single walled carbon nanotube (SWCNT) bundle consisting of seven (10,10) SWCNTs: one core tube surrounded by six tubes on the perimeter. The surface tension and the inter-tube corrugation are identified as the two factors that contribute to load transfer. The surface tension effectively acts over a “line” (roughly over the circumference of each tube). The inter-tube corrugation scales linearly with respect to the contact surface area, and increases non-linearly as the inter-tube distance decreases. Relaxation in the nanotube cross-section leads to better inter-tube load transfer as a slight “faceting” develops; the tubes appear to be partially polygonized, rather than perfect cylinders. Compared with parallel bundles, twisting can significantly enhance the load transfer between neighboring tubes; this has been computed as a function of twist angle for this nanotube bundle system.
Nanocomposite materials were prepared from copolymers of polyvinyl alcohol and polyvinyl acetate and a colloidal aqueous suspension of cellulose whiskers prepared from cotton linter. The degree of hydrolysis of the matrix was varied in order to vary the hydrophilic character of the polymer matrix and then the degree of interaction between the filler and the matrix. Nanocomposite films were conditioned at various moisture contents, and the dynamic mechanical and thermal properties were characterized using dynamic mechanical analysis and differential scanning calorimetry, respectively. Tensile tests were performed at room temperature to estimate mechanical properties of the films in the non linear range. All the results show that stronger filler/matrix interactions occur for fully hydrolyzed PVA compared to partially hydrolyzed samples. For moist samples, a water accumulation at the interface was evidenced. The reinforcing effect was found to be all the higher as the degree of hydrolysis of the matrix was high.
The mechanical properties of the material surrounding a single carbon fiber in an epoxy matrix have been studied. Properties were determined within 100 nm of the fiber. A pronounced soft interphase was revealed adjacent to the fiber.A single fiber was embedded in a small supported disk of epoxy matrix. The fiber was loaded in tension. Examination of the surface displacements in the resin revealed that the matrix material within 250 nm of the fiber was substantially softer than the matrix far from the fiber. This interphase material was active in creep. Measurement of indentation properties of the matrix around a single fiber showed that the material close to the fiber exhibited an apparent high modulus due to restriction by the fiber. The implications of these findings are discussed.
An analytical model has been developed to study stress transfer in single walled carbon nanotube (SWNT) reinforced polymer matrix composites. This model can be used to predict axial stress (σn) and interfacial shear stress (τi) along the carbon nanotube (CNT) embedded in matrix materials. A simplified 2D representative volume element (RVE) of CNT has been considered in this analysis. An expression for the effective length (Leff) of the carbon nanotube (CNT) has also been established for studying load transfer efficiency in CNT reinforced composites. The effects of CNT aspect ratio, CNT volume fraction and matrix modulus on axial stress (σn) and interfacial shear stress (τi) has also been analyzed in details. Finally, the results of this analytical model are compared with finite element analysis.
There are many examples where animals or plants synthesize extracellular high-performance skeletal biocomposites consisting of a matrix reinforced by fibrous biopolymers. Cellulose is one, which occur as whiskerlike microfibrils that are biosynthesized and deposited in a continuous fashion. Mostly, this mode of biogenesis leads to crystalline microfibrils that are almost defect-free. In this study, attempts to imitate biocomposites are made by blending cellulose whiskers from the mantles of tunicates with synthetic polymer lattices. The films cast from such mixtures have a nanocomposite organization whose structure and mechanical properties are described in this paper.
Nanocomposite materials were prepared from an elastomeric medium-chain-length poly(hydroxyalkanoate) (Mcl-PHA) latex as semicrystalline matrix using a colloidal suspension of hydrolyzed cellulose whiskers as natural and biodegradable filler. After stirring, the preparations were cast and evaporated. High-performance materials were obtained from this system, preserving the natural character of PHA. However, differences were reported by comparison with amorphous PHA filled systems. These differences were ascribed to a transcrystallization phenomenon of semicrystalline PHA on cellulose whiskers, evidenced by dynamic mechanical analysis. Transcrystallization hindered the mechanical percolation of cellulose whiskers and the formation of a rigid network within the polymer matrix during the film formation by evaporation. The whiskers network can reorganize under thermal aging.
This work describes the static and the dynamic properties of fractionated microcrystal cellulose "whiskers" using elastic and quasi-elastic light scattering techniques. These colloidal particles have been obtained by acid hydrolysis from raw materials (tunicate, a marine animal) that lead to stable aqueous suspensions. The presence of charges along these rod-shaped particles induces a structural order in these suspensions, This has been highlighted, using light scattering, by the existence of several scattering peaks in the structure factor S(q) = 1(q)/P(q) as a function of the wavevector q. We have studied the static structure factor as well as the autocorrelation functions as a function of whisker concentration, C (2.6 x 10(-3) to 1.3 x 10(-2) g/cm(3)) and ionic strength (1 x 10(-6) to 1 x 10(-5) mol/L NaCl). In the absence of added salt, "salt-free" suspensions, the observed maxima, that are due to the electrostatic interactions, depend on the whisker concentration C and scale roughly as C-1/2. These maxima disappear progressively as the ionic strength is increased. At high ionic strength, the intensity scattered by these charged colloidal particles increases and reaches neutral system behavior, before the suspension flocculates. All these properties are found also in the dynamic behavior where the measured effective diffusion coefficient Gamma(q)/q(2), at low ionic strength, presents minima and maxima as a function of q and, as expected, is found to be inversely proportional to S(q).
The cellulose nanofibrils that are found in wood and other natural materials are similar to carbon nanotubes in many ways and could be used to strengthen composites for manufacturing.
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Sulfuric acid hydrolysis of native cellulose fibers produces stable suspensions of cellulose nanocrystals. Above a critical concentration, the suspensions spontaneously form an anisotropic chiral nematic liquid crystal phase. We have examined the effect of reaction time and acid-to-pulp ratio on nanocrystal and suspension properties for hydrolyzed black spruce acid sulfite pulp. Longer hydrolysis times produced shorter, less polydisperse black spruce cellulose nanocrystals and slightly increased the critical concentration for anisotropic phase formation. Increased acid-to-pulp ratio reduced the dimensions of the nanocrystals thus produced; the critical concentration was increased and the biphasic range became narrower. A suspension made from a bleached kraft eucalyptus pulp gave very similar properties to the softwood nanocrystal suspension when prepared under similar hydrolysis conditions.
The ability of the atomic force microscope to measure forces with subnanonewton sensitivity at nanometer-scale lateral resolutions has led to its use in the mechanical characterization of nanomaterials. Recent studies have shown that the atomic force microscope can be used to measure the elastic moduli of suspended fibers by performing a nanoscale three-point bending test, in which the center of the fiber is deflected by a known force. We extend this technique by modeling the deflection measured at several points along a suspended fiber, allowing us to obtain more accurate data, as well as to justify the mechanical model used. As a demonstration, we have measured a value of 78 +/- 17 GPa for Young's modulus of bacterial cellulose fibers with diameters ranging from 35 to 90 nm. This value is considerably higher than previous estimates, obtained by less direct means, of the mechanical strength of individual cellulose fibers.
Polymer nanocomposites are one of the important application areas for nanotechnology. Naturally derived organic nanophase materials are of special interest in the case of polymer nanocomposites. Carboxymethyl cellulose is a polyelectrolyte derived from natural materials. It has been extensively studied as a hydrogel polymer. Methods to modify the mechanical properties of gels and films made from CMC are of interest in our lab and in the commercial marketplace. The effect of nano-sized fillers on the properties of CMC-based composites is of interest in the development of novel or improved applications for hydrogel polymers in general and CMC in particular. This project investigated cellulose nanocrystals (CNXLs) as a filler in CMC and compared the effects to microcrystalline cellulose (MCC). The composite material was composed of CMC, MCC or CNXL, with glycerin as a plasticizer. CNXL and MCC concentrations ranged from 5% to 30%. Glycerin concentrations were kept constant at 10%. CNXLs improved the strength and stiffness of the resulting composite compared to MCC. In addition, a simple heat treatment was found to render the nanocomposite water resistant.
New nanocomposite films were prepared from a suspension of cellulose nanocrystals as the filler and a polycaprolactone-based waterborne polyurethane (WPU) as the matrix. The cellulose nanocrystals, prepared by acid hydrolysis of flax fiber, consisted of slender rods with an average length of 327 +/- 108 nm and diameter of 21 +/- 7 nm, respectively. After the two aqueous suspensions were mixed homogeneously, the nanocomposite films were obtained by casting and evaporating. The morphology, thermal behavior, and mechanical properties of the films were investigated by means of attenuated total reflection Fourier transform infrared spectroscopy, wide-angle X-ray diffraction, differential scanning calorimetry, scanning electron microscopy, and tensile testing. The results indicated that the cellulose nanocrystals could disperse in the WPU uniformly and resulted in an improvement of microphase separation between the soft and hard segments of the WPU matrix. The films showed a significant increase in Young's modulus and tensile strength from 0.51 to 344 MPa and 4.27 to 14.86 MPa, respectively, with increasing filler content from 0 to 30 wt %. Of note is that the Young's modulus increased exponentially with the filler up to a content of 10 wt %. The synergistic interaction between fillers and between the filler and WPU matrix played an important role in reinforcing the nanocomposites. The superior properties of the new nanocomposite materials could have great potential applications.
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