Preparation of Homogeneous Dispersions of Tunicate Cellulose Whiskers in Organic Solvents

pH-Responsive Cellulose Nanocrystal Gels and Nanocomposites

We show that functionalization of the surface of cellulose nanocrystals (CNCs) with either carboxylic acid (CNC-CO₂H) or amine (CNC-NH₂) moieties renders the CNCs pH-responsive. At low pH, where the amine groups are protonated, CNC-NH₂ forms aqueous dispersions in water on account of electrostatic repulsions of the ammonium moieties inhibiting aggregation. However, a transition to hydrogels is observed at higher pH where the CNC-NH₂ are neutral and the attractive forces based on hydrogen bonding dominate. The opposite behavior is observed for CNC-CO₂H, which are dispersible at high pH and form gels in an acidic environment. We further show that these pH-responsive CNCs can be incorporated into a poly(vinyl acetate) matrix to yield mechanically adaptive pH-responsive nanocomposite films.

Influence of magnetic field alignment of cellulose whiskers on the mechanics of all-cellulose nanocomposites

Orientation of cellulose nanowhiskers (CNWs) derived from tunicates, in an all-cellulose nanocomposite, is achieved through the application of a magnetic field. CNWs are incorporated into a dissolved cellulose matrix system and during solvent casting of the nanocomposite a magnetic field is applied to induce their alignment. Unoriented CNW samples, without the presence of a magnetic field, are also produced. The CNWs are found to orient under the action of the magnetic field, leading to enhanced stiffness and strength of the composites, but not to the level that is theoretically predicted for a fully aligned system. Lowering the volume fraction of the CNWs is shown to allow them to orient more readily in the magnetic field, leading to larger relative increases in the mechanical properties. It is shown, using polarized light microscopy, that the all-cellulose composites have a domain structure, with some domains showing pronounced orientation of CNWs and others where no preferred orientation occurs. Raman spectroscopy is used to both follow the position of bands located at ~1095 and ~895 cm(-1) with deformation and also their intensity as a function rotation angle of the specimens. It is shown that these approaches give valuable independent information on the respective molecular deformation and orientation of the CNWs, and the molecules in the matrix phase, in oriented and nonoriented domains of all-cellulose composites. These data are then related to an increase in the level of molecular deformation in the axial direction, as revealed by the Raman technique. Little orientation of the matrix phase is observed under the action of the magnetic field indicating the dominance of the stiff CNWs in governing mechanical properties.

Simple Method for the Melt Extrusion of a Cellulose Nanocrystal Reinforced Hydrophobic Polymer

The rheological properties of a dispersion of cellulose nanocrystals (CNCs) in an aqueous solution of polyoxyethylene (PEO) have been investigated. A peculiar behavior is reported. Upon adding CNC, the viscosity of the suspension first decreases and then increases. Adsorption of PEO chains on the surface of the nanoparticles has been suspected. Freeze-drying of this PEO-adsorbed CNC dispersion was performed, and the ensuing lyophilizate was extruded with low density polyethylene. Compared to neat CNC-based nanocomposites, both improved dispersibility and thermal stability were observed. This simple and physical method constitutes an approach of choice for the melt processing of CNC-based nanocomposites with a hydrophobic polymeric matrix applicable at the industrial scale.

Uncertainty quantification in nanomechanical measurements using the atomic force microscope

Quantifying uncertainty in measured properties of nanomaterials is a prerequisite for the manufacture of reliable nanoengineered materials and products. Yet, rigorous uncertainty quantification (UQ) is rarely applied for material property measurements with the atomic force microscope (AFM), a widely used instrument that can measure properties at nanometer scale resolution of both inorganic and biological surfaces and nanomaterials. We present a framework to ascribe uncertainty to local nanomechanical properties of any nanoparticle or surface measured with the AFM by taking into account the main uncertainty sources inherent in such measurements. We demonstrate the framework by quantifying uncertainty in AFM-based measurements of the transverse elastic modulus of cellulose nanocrystals (CNCs), an abundant, plant-derived nanomaterial whose mechanical properties are comparable to Kevlar fibers. For a single, isolated CNC the transverse elastic modulus was found to have a mean of 8.1 GPa and a 95% confidence interval of 2.7-20 GPa. A key result is that multiple replicates of force-distance curves do not sample the important sources of uncertainty, which are systematic in nature. The dominant source of uncertainty is the nondimensional photodiode sensitivity calibration rather than the cantilever stiffness or Z-piezo calibrations. The results underscore the great need for, and open a path towards, quantifying and minimizing uncertainty in AFM-based material property measurements of nanoparticles, nanostructured surfaces, thin films, polymers and biomaterials.

Toward flexible polymer and paper-based energy storage devices

All-polymer and paper-based energy storage devices have significant inherent advantages in comparison with many currently employed batteries and supercapacitors regarding environmental friendliness, flexibility, cost and versatility. The research within this field is currently undergoing an exciting development as new polymers, composites and paper-based devices are being developed. In this report, we review recent progress concerning the development of flexible energy storage devices based on electronically conducting polymers and cellulose containing composites with particular emphasis on paper-based batteries and supercapacitors. We discuss recent progress in the development of the most commonly used electronically conducting polymers used in flexible device prototypes, the advantages and disadvantages of this type of energy storage devices, as well as the two main approaches used in the manufacturing of paper-based charge storage devices.

Biologically inspired hierarchical design of nanocomposites based on poly(ethylene oxide) and cellulose nanofibers

Attempts to create hierarchically structured, uniaxially oriented nanocomposites comprising cellulose nanowhiskers (CNWs), which promise anisotropic mechanical properties, are exceedingly rare. We report here the fabrication of uniaxially-oriented arrays of microfibers based on poly(ethylene oxide) (PEO) and CNWs by electrospinning. Compared with the neat PEO fibers, the incorporation of CNWs within the fibers increased the storage modulus (E') of arrays along the fiber axis of the PEO/CNW nanocomposite fibers. Successful incorporation of the CNWs within each of the as-spun PEO/CNW nanocomposite fibers in the direction parallel to the fiber axis was verified by both scanning and transmission electron microscopy.

In vivo deployment of mechanically adaptive nanocomposites for intracortical microelectrodes

We recently introduced a series of stimuli-responsive, mechanically adaptive polymer nanocomposites. Here, we report the first application of these bio-inspired materials as substrates for intracortical microelectrodes. Our hypothesis is that the ideal electrode should be initially stiff to facilitate minimal trauma during insertion into the cortex, yet become mechanically compliant to match the stiffness of the brain tissue and minimize forces exerted on the tissue, attenuating inflammation. Microprobes created from mechanically reinforced nanocomposites demonstrated a significant advantage compared to model microprobes composed of neat polymer only. The nanocomposite microprobes exhibit a higher storage modulus (E' = ~5 GPa) than the neat polymer microprobes (E' = ~2 GPa) and can sustain higher loads (~12 mN), facilitating penetration through the pia mater and insertion into the cerebral cortex of a rat. In contrast, the neat polymer microprobes mechanically failed under lower loads (~7 mN) before they were capable of insertion into cortical tissue. Further, we demonstrated the material's ability to morph while in the rat cortex to more closely match the mechanical properties of the cortical tissue. Nanocomposite microprobes that were implanted into the rat cortex for up to eight weeks demonstrated increased cell density at the microelectrode-tissue interface and a lack of tissue necrosis or excessive gliosis. This body of work introduces our nanocomposite-based microprobes as adaptive substrates for intracortical microelectrodes and potentially for other biomedical applications.

Cellulose nanomaterials review: structure, properties and nanocomposites

This critical review provides a processing-structure-property perspective on recent advances in cellulose nanoparticles and composites produced from them. It summarizes cellulose nanoparticles in terms of particle morphology, crystal structure, and properties. Also described are the self-assembly and rheological properties of cellulose nanoparticle suspensions. The methodology of composite processing and resulting properties are fully covered, with an emphasis on neat and high fraction cellulose composites. Additionally, advances in predictive modeling from molecular dynamic simulations of crystalline cellulose to the continuum modeling of composites made with such particles are reviewed (392 references).

TEMPO-oxidized cellulose nanofibers

Native wood celluloses can be converted to individual nanofibers 3-4 nm wide that are at least several microns in length, i.e. with aspect ratios>100, by TEMPO (2,2,6,6-tetramethylpiperidine-1-oxyl radical)-mediated oxidation and successive mild disintegration in water. Preparation methods and fundamental characteristics of TEMPO-oxidized cellulose nanofibers (TOCN) are reviewed in this paper. Significant amounts of C6 carboxylate groups are selectively formed on each cellulose microfibril surface by TEMPO-mediated oxidation without any changes to the original crystallinity (∼74%) or crystal width of wood celluloses. Electrostatic repulsion and/or osmotic effects working between anionically-charged cellulose microfibrils, the ζ-potentials of which are approximately -75 mV in water, cause the formation of completely individualized TOCN dispersed in water by gentle mechanical disintegration treatment of TEMPO-oxidized wood cellulose fibers. Self-standing TOCN films are transparent and flexible, with high tensile strengths of 200-300 MPa and elastic moduli of 6-7 GPa. Moreover, TOCN-coated poly(lactic acid) films have extremely low oxygen permeability. The new cellulose-based nanofibers formed by size reduction process of native cellulose fibers by TEMPO-mediated oxidation have potential application as environmentally friendly and new bio-based nanomaterials in high-tech fields.

Stress-transfer in anisotropic and environmentally adaptive cellulose whisker nanocomposites

Quantitative insights into the stress-transfer mechanisms that determine the mechanical properties of tunicate cellulose whisker/poly(vinyl acetate) nanocomposites were gained by Raman spectroscopy. The extent of stress-transfer is influenced by local orientation (or anisotropy) of the whiskers, which in turn is governed by the processing conditions used to fabricate the nanocomposites. Solution-cast materials display no microscopic anisotropy, while samples that were cast and subsequently compression molded contain both isotropic regions as well as domains of locally oriented whiskers. Polarized optical microscopy showed these regions to have dimensions in the hundreds of mum. Polarized Raman spectroscopy of the 1095 cm(-1) Raman band, associated with C-O ring stretching of the cellulose backbone, was used to quantify the local orientation of the cellulose whiskers. Clear and discernible shifts of this Raman band upon uniaxial deformation of nanocomposite films were further used to determine the level of stress experienced by the cellulose whiskers, ultimately reflecting the levels of stress-transfer predominantly between the poly(vinyl acetate) matrix and the tunicate whiskers, but also between the whiskers within the network. In the isotropic regions, where whiskers form a percolating network, the observed Raman shift rate with respect to strain is smaller than in the regions where the whiskers are uniaxially orientated. The Raman shift is strongly affected by the presence of water, leading to a lack of stress-transfer when the samples are fully hydrated, which is clearly detected by the Raman technique. Heating of the nanocomposites above the glass transition temperature of the poly(vinyl acetate) matrix also reduces the stress experienced by the individual whiskers.

Processing of polymer nanocomposites reinforced with polysaccharide nanocrystals

Aqueous suspensions of polysaccharide (cellulose, chitin or starch) nanocrystals can be prepared by acid hydrolysis of biomass. The main problem with their practical use is related to the homogeneous dispersion of these nanoparticles within a polymeric matrix. Water is the preferred processing medium. A new and interesting way for the processing of polysaccharide nanocrystals-based nanocomposites is their transformation into a co-continuous material through long chain surface chemical modification. It involves the surface chemical modification of the nanoparticles based on the use of grafting agents bearing a reactive end group and a long compatibilizing tail.

Natural biopolymers: novel templates for the synthesis of nanostructures

Biological systems such as proteins, viruses, and DNA have been most often reported to be used as templates for the synthesis of functional nanomaterials, but the properties of widely available biopolymers, such as cellulose, have been much less exploited for this purpose. Here, we report for the first time that cellulose nanocrystals (CNC) have the capacity to assist in the synthesis of metallic nanoparticle chains. A cationic surfactant, cetyltrimethylammonium bromide (CTAB), was critical to nanoparticle stabilization and CNC surface modification. Silver, gold, copper, and platinum nanoparticles were synthesized on CNCs, and the nanoparticle density and particle size were controlled by varying the concentration of CTAB, the pH of the salt solution, and the reduction time.

Cellulose whisker/epoxy resin nanocomposites

New nanocomposites composed of cellulose nanofibers or "whiskers" and an epoxy resin were prepared. Cellulose whiskers with aspect ratios of approximately 10 and approximately 84 were isolated from cotton and sea animals called tunicates, respectively. Suspensions of these whiskers in dimethylformamide were combined with an oligomeric difunctional diglycidyl ether of bisphenol A with an epoxide equivalent weight of 185-192 and a diethyl toluenediamine-based curing agent. Thin films were produced by casting these mixtures and subsequent curing. The whisker content was systematically varied between 4 and 24% v/v. Electron microscopy studies suggest that the whiskers are evenly dispersed within the epoxy matrix. Dynamic mechanical thermoanalysis revealed that the glass transition temperature (T(g)) of the materials was not significantly influenced by the incorporation of the cellulose filler. Between room temperature and 150 degrees C, i.e., below T(g), the tensile storage moduli (E') of the nanocomposites increased modestly, for example from 1.6 GPa for the neat polymer to 4.9 and 3.6 GPa for nanocomposites comprising 16% v/v tunicate or cotton whiskers. The relative reinforcement was more significant at 185 degrees C (i.e., above T(g)), where E' was increased from approximately 16 MPa (neat polymer) to approximately 1.6 GPa (tunicate) or approximately 215 MPa (cotton). The mechanical properties of the new materials are well-described by the percolation model and are the result of the formation of a percolating whisker network in which stress transfer is facilitated by strong interactions between the whiskers.

Polymer nanocomposites with nanowhiskers isolated from microcrystalline cellulose

The ability to produce polymer nanocomposites, which comprise a percolating, three-dimensional network of well-individualized nanofibers, is important to maximize the reinforcing effect of the nanofibers. While microcrystalline cellulose (MCC) has been previously shown to improve the mechanical properties of polymer composites, the formation of fibrous percolating networks within the nanocomposites has been stifled. Through the utilization of a template approach, nanocomposites based on an ethylene oxide/epichlorohydrin copolymer and nanowhiskers isolated from MCC were produced that display the maximum mechanical reinforcement predicted by the percolation model.

Stimuli-responsive mechanically adaptive polymer nanocomposites

A new series of biomimetic stimuli-responsive nanocomposites, which change their mechanical properties upon exposure to physiological conditions, was prepared and investigated. The materials were produced by introducing percolating networks of cellulose nanofibers or "whiskers" derived from tunicates into poly(vinyl acetate) (PVAc), poly(butyl methacrylate) (PBMA), and blends of these polymers, with the objective of determining how the hydrophobicity and glass-transition temperature (Tg) of the polymer matrix affect the water-induced mechanically dynamic behavior. Below the Tg (approximately 60-70 degrees C), the incorporation of whiskers (15.1-16.5% v/v) modestly increased the tensile storage moduli (E') of the neat polymers from 0.6 to 3.8 GPa (PBMA) and from 2 to 5.2 GPa (PVAc). The reinforcement was much more dramatic above Tg, where E' increased from 1.2 to 690 MPa (PVAc) and approximately 1 MPa to 1.1 GPa (PBMA). Upon exposure to physiological conditions (immersion in artificial cerebrospinal fluid, ACSF, at 37 degrees C) all materials displayed a decrease in E'. The most significant contrast was seen in PVAc; for example, the E' of a 16.5% v/v PVAc/whisker nanocomposite decreased from 5.2 GPa to 12.7 MPa. Only a modest modulus decrease was measured for PBMA/whisker nanocomposite; here the E' of a 15.1% v/v PBMA/whisker nanocomposite decreased from 3.8 to 1.2 GPa. A systematic investigation revealed that the magnitude of the mechanical contrast was related to the degree of swelling with ACSF, which was shown to increase with whisker content, temperature, and polarity of the matrix (PVAc>PBMA). The mechanical morphing of the new materials can be described in the framework of both the percolation and Halpin-Kardos models for nanocomposite reinforcement, and is the result of changing interactions among the nanoparticles and plasticization of the matrix upon swelling.

Highly filled bionanocomposites from functionalized polysaccharide nanocrystals

Cellulose and starch nanocrystals obtained from the acid hydrolysis of ramie fibers and waxy maize starch granules, respectively, were subjected to isocyanate-mediated reaction to graft polycaprolactone (PCL) chains with various molecular weights on their surface. Grafted nanoparticles were characterized by X-ray diffraction analysis and contact angle measurements. We observed that the nanoparticles kept their initial morphological integrity and native crystallinity. Nanocomposite films were processed from both unmodified and PCL-grafted nanoparticles and PCL as matrix using a casting/evaporation technique. We showed that mechanical properties of resulting films were notably different. Compared to unmodified nanoparticles, the grafting of PCL chains on the surface results in lower modulus values but significantly higher strain at break. This unusual behavior clearly reflects the originality of the reinforcing phenomenon of polysaccharide nanocrystals resulting from the formation of a percolating network thanks to chain entanglements and cocrystallization.

A versatile approach for the processing of polymer nanocomposites with self-assembled nanofibre templates

The incorporation of nanoparticles into polymers is a design approach that is used in many areas of materials science. The concept is attractive because it enables the creation of materials with new or improved properties by mixing multiple constituents and exploiting synergistic effects. One important technological thrust is the development of structural materials with improved mechanical and thermal characteristics. Equally intriguing is the possibility to design functional materials with unique optical or electronic properties, catalytic activity or selective permeation. The broad technological exploitation of polymer nanocomposites is, however, stifled by the lack of effective methods to control nanoparticle dispersion. We report a simple and versatile process for the formation of homogeneous polymer/nanofibre composites. The approach is based on the formation of a three-dimensional template of well-individualized nanofibres, which is filled with any polymer of choice. We demonstrate that this template approach is broadly applicable and allows for the fabrication of otherwise inaccessible nanocomposites of immiscible components.