Electrostatic assembly of Ag nanoparticles onto nanofibrillated cellulose for antibacterial paper products

Nanofibrillated cellulose offers new technological solutions for the development of paper products. Here, composites of nanofibrillated cellulose (NFC) and Ag nanoparticles (NP) were prepared for the first time via the electrostatic assembly of Ag NP (aqueous colloids) onto NFC. Distinct polyelectrolytes have been investigated as macromolecular linkers in order to evaluate their effects on the building-up of Ag modified NFC and also on the final properties of the NFC/Ag composite materials. The NFC/Ag nanocomposites were first investigated for their antibacterial properties towards S. aureus and K. pneumoniae microorganisms as compared to NFC modified by polyelectrolytes linkers without Ag. Subsequently, the antibacterial NFC/Ag nanocomposites were used as fillers in starch based coating formulations for Eucalyptus globulus-based paper sheets. The potential of this approach to produce antimicrobial paper products will be discussed on the basis of complementary optical, air barrier and mechanical data.     

Hemocompatibility of Ca2+‐Crosslinked Nanocellulose Hydrogels: Toward Efficient Management of Hemostasis

The present work investigates Ca²⁺‐crosslinked nanofibrillated cellulose hydrogels as potential hemostatic wound dressings by studying core interactions between the materials and a central component of wounds and wound healing—the blood. Hydrogels of wood‐derived anionic nanofibrillated cellulose (NFC) and NFC hydrogels that incorporate kaolin or collagen are studied in an in vitro whole blood model and with platelet‐free plasma assays. The evaluation of thrombin and factor XIIa formation, platelet reduction, and the release of activated complement system proteins, shows that the NFC hydrogel efficiently triggered blood coagulation, with a rapid onset of clot formation, while displaying basal complement system activation. By using the NFC hydrogel as a carrier of kaolin, the onset of hemostasis is further boosted, while the NFC hydrogel containing collagen exhibits blood activating properties comparable to the anionic NFC hydrogel. The herein studied NFC hydrogels demonstrate great potential for being part of advanced wound healing dressings that can be tuned to target certain wounds (e.g., strongly hemorrhaging ones) or specific phases of the wound healing process for optimal wound management.     

Solvent impact on esterification and film formation ability of nanofibrillated cellulose

In this study we have manufactured nanofibrillar cellulose and modified the fibre surface with ester groups in order to hydrophobise the surface. Nanofibrillated cellulose was chosen to demonstrate the phenomena, since due to its high surface area the effects at issue are pronounced. The prepared NFC ester derivatives were butyrate, hexanoate, benzoate, naphtoate, diphenyl acetate, stearate and palmitate. X-ray photoelectron spectroscopy, solid state NMR and contact angle measurements were used to demonstrate the chemical changes taking place on the cellulose surface. NFC ester derivatives can be prepared after a careful solvent exchange to a water-free solvent medium has been carried out. Butyl and palmitoyl esters were chosen for film forming tests due to the difference in their carbon chain lengths, and their contact angles and water vapour and oxygen permeation rates were studied. The prepared nanocellulose esters show increased hydrophobicity even at very low levels of substitution and readily form films when the films are prepared from acetone dispersions. The permeation rates suggest a potential use as barrier materials.     

Prediction of elastic properties of nanofibrillated cellulose from micromechanical modeling and nano-structure characterization by transmission electron microscopy

Cellulose-based materials have a great potential in terms of mechanical performance, since crystalline cellulose is known to have excellent stiffness along the main axis. This potential is not completely fulfilled in structural wood materials and in composite materials, due to structural inhomogeneities, misalignment, voids etc. on several length scales. This study investigates the difference in stiffness of nanofibrillated cellulose (NFC) compared to that of cellulose crystallites, based on nanostructural characterization, image analysis and micromechanical modeling. Nanofibrillated cellulose is believed to be composed of a distribution of crystallites in an amorphous matrix, and it is assumed to represent the distribution of the crystalline allomorph I_β. To predict the elastic properties of NFC, a micromechanical model based on a Mori–Tanaka approach and self-consistent scheme was used. The input data, i.e. orientation distribution, aspect ratio and volume fraction of these crystalline regions, were estimated from image analysis of transmission electron micrographs. The model predicts a ca. 56 % loss of stiffness of NFC compared to that of cellulose crystals along the main axis.     

Comparison of nano- and microfibrillated cellulose films

Nanocellulose is an interesting building block for functional materials and has gained considerable interest due to its mechanical robustness, large surface area and biodegradability. It can be formed into various structures such as solids, films and gels such as hydrogels and aerogels and combined with polymers or other materials to form composites. Mechanical, optical and barrier properties of nanofibrillated cellulose (NFC) and microfibrillated cellulose (MFC) films were studied in order to understand their potential for packaging and functional printing applications. Impact of raw material choice and nanocellulose production process on these properties was evaluated. MFC and NFC were produced following two different routes. NFC was produced using a chemical pretreatment followed by a high pressure homogenization, whereas MFC was produced using a mechanical treatment only. TEMPO-mediated oxidation followed by one step of high pressure (2,000 bar) homogenization seems to produce a similar type of NFC from both hardwood and softwood. NFC films showed superior mechanical and optical properties compared with MFC films; however, MFC films demonstrated better barrier properties against oxygen and water vapor. Both the MFC and NFC films were excellent barriers against mineral oil used in ordinary printing inks and dichlorobenzene, a common solvent used in functional printing inks. Barrier properties against vegetable oil were also found to be exceptionally good for both the NFC and MFC films.     

Nanofibrillated cellulose originated from birch sawdust after sequential extractions: a promising polymeric material from waste to films

The residual cellulose of wood processing waste, sawdust, which was leftover after sequential hot-water extraction processes to isolate hemicelluloses and lignin in a novel forest biorefinery concept, was explored as the starting material for preparation of a highly value-added polymeric material, nanofibrillated cellulose (NFC) also widely termed as cellulose nanofiber, which has provided an alternative efficient way to upgrade sawdust waste. The residual cellulose in sawdust was converted to a transparent NFC suspension in water through the 2,2,6,6-tetramethylpiperidine-1-oxyl radical/NaClO/NaBr oxidization approach. The resultant NFC with a dimension of ca. 5 nm in width and hundreds of nanometers in length were further processed into NFC films. The morphological features of the NFC suspension and its films were assessed by transmission electron microscopy and scanning electron microscopy. Highly even dispersion of NFC fibrils in the films originated from sawdust feasibly contributes to the outstanding mechanical performance of the films. NFC suspension with higher carboxylate content and its resultant NFC films were found to show higher transmission of light.     

Interactions between inorganic nanoparticles and cellulose nanofibrils

Nanofibrillated cellulose (NFC) is increasingly utilized in materials and biomedical applications consequently increasing interest in the modification of its surface properties. Besides modification using polyelectrolytes and polysaccharides, NFC can be combined with solid particles enabling formation of fibril network loaded with particles. Use of particles enabling easy functionalization could be beneficial for the development of hybrid structures, and lead to preparation of nanocomposites and functional materials. In order to explore interactions related to preparation of such structures, the interactions between nanosized precipitated calcium carbonate (nanoPCC) and nanoclay particles and NFC were examined by observing adsorption of the particles on NFC substrate using a quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) imaging. By a treatment with carboxymethylated cellulose (CMC), the anionicity of the NFC substrate could be increased, providing an additional tool to affect the interplay between NFC and the inorganic particles. For slightly cationic nanoPCC particles an increase in the anionicity of the NFC by the CMC treatment increased the affinity, while the opposite was true for anionic nanoclay. Additionally, for interactions between nanoclay and NFC, dispersion stability was an important factor. QCM-D was successfully used to examine the adsorption characteristics of nanoparticles although the technique is commonly used to study the adsorption of thin polymer layers. Distinct adsorption characteristics were observed depending on the nanoparticle used; nanoclay particles deposited as a thin layer, whereas nanoPCC particles formed clusters.     

Soy protein–nanocellulose composite aerogels

Organic aerogels based on two important and widely abundant renewable resources, soy proteins (SP) and nanofibrillar cellulose (NFC) are developed from precursor aqueous dispersions and a facile method conducive of channel- and defect-free systems after cooling and freeze-drying cycles that yielded apparent densities on the order of 0.1 g/cm³. NFC loading drives the internal morphology of the composite aerogels to transition from network- to fibrillar-like, with high density of interconnected cells. Composite aerogels with SP loadings as high as ca. 70 % display a compression modulus of 4.4 MPa very close to that obtained from reference, pure NFC aerogels. Thus, the high compression modulus of the composite system is not compromised as long as a relatively low amount of reinforcing NFC is present. The composite materials gain moisture (up to 5 %) in equilibrium with 50 % RH air, independent of SP content. Furthermore, their physical integrity is unchanged upon immersion in polar and non-polar solvents. Fast liquid sorption rates are observed in the case of composite aerogels in contact with hexane. In contrast, water sorption is modulated by the chemical composition of the aerogel, with an important contribution from swelling. The potential functionalities of the newly developed SP–NFC composite green materials can benefit from the reduced material cost and the chemical features brought about the amino acids present in SPs.     

Synthesis and Spatiotemporal Modification of Biocompatible and Stimuli‐Responsive Carboxymethyl Cellulose Hydrogels Using Thiol‐Norbornene Chemistry

Carboxymethyl cellulose (CMC) is functionalized with norbornene groups to undergo thiol‐norbornene cross‐linking reactions. Hydrogels synthesized from a single norbornene‐modified carboxymethyl cellulose (NorCMC) via a light‐initiated thiol‐ene cross‐linking reaction with a variety of dithiol cross‐linkers yield hydrogels with a tunable compression modulus ranging from 1.7 to 103 kPa. Additionally, thermoresponsiveness is spatiotemporally imparted to NorCMC hydrogels by photopatterning a dithiol‐terminated poly(N‐isopropyl acrylamide) cross‐linker, enabling swelling and topological control of the hydrogels as a function of incubation temperature. NorCMC hydrogels are cytocompatible as the viability of encapsulated human mesenchymal stem cells (hMSCs) is greater than 85% after 21 d while using a variety of cross‐linkers. Moreover, hMSCs can remodel, adhere, and spread in the NorCMC matrix cross‐linked with a matrix metalloproteinase‐degradable peptide, further demonstrating the utility of these materials as a tunable biomaterial.     

Liquid ammonia treatment of (cationic) nanofibrillated cellulose/vermiculite composites

Liquid ammonia was used to treat films of nanofibrillated cellulose (NFC), trimethylammonium-modified NFC (TMA-NFC), and their composites with vermiculite. Crystal structure, mechanical properties, water vapor permeation and water vapor adsorption of the resulting materials were investigated. Upon treatment, the crystal structure of (TMA-)NFC both in presence and absence of vermiculite changed from cellulose I to III. With the exception of TMA-NFC/vermiculite composites, pronounced effects on the addressed mechanical properties arose after exposure of the materials to ammonia. Furthermore, treatment of composite films with ammonia led to a distinct decrease in water vapor permeation. Remarkably, TMA-NFC/vermiculite composites films show the best water vapor barrier properties, highest tensile strength and highest elastic modulus after treatment with liquid ammonia. This is regarded to be at least partially a consequence of electrostatic attraction between the positively charged ammonium groups in TMA-NFC and the anionic silicate layers of vermiculite. © 2013 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2013     

All-cellulose multilayers: long nanofibrils assembled with short nanocrystals

Self-organized multilayer films were formed by sequential addition of oppositely charged cellulose I nanoparticles. The all-cellulosic multilayers were prepared via adsorption of cationicially modified cellulose nanofibrils (cat NFC) and anionic short crystalline cellulose (CNC) at pH 4.5 and pH 8.3. The properties and build-up behavior of layer-by-layer-constructed films were studied with microgravimetry (QCM-D) and the direct surface forces in these systems were explored with colloidal probe microscopy to gain information about the fundamental interplay between cat NFC and anionic CNC. The importance of the first layer on the adsorption of the consecutive layers was demonstrated by comparing pure in situ adsorption in the QCM-D with multilayer films made by spin coating the first cationic NFC layer and then subsequently adsorbing the following layers in situ in the QCM-D chamber. Differences in adsorbed amount and viscoelastic behavior were observed between those two systems. In addition, a significant pH dependence of cat NFC charge was found for both direct surface interactions and layer properties. Moreover the underlying cellulose layer in multilayer film was established to influence the surface forces especially at lower pH, where the cat NFC chains extensions were facilitated and overall charge was affected by the cationic counterpart within the layers. This enhanced understanding the effect of charge and structure on the interaction between these renewable nanoparticles is valuable when designing novel materials based on nanocellulose.     

The vane method and kinetic modeling: shear rheology of nanofibrillated cellulose suspensions

We conduct rheological characterization of nanofibrillated cellulose (NFC) suspensions, a highly non-Newtonian complex fluid, at several concentrations. Special care is taken to cope with the prevalent problems of time scale issues, wall depletion and confinement effects. We do this by combining the wide-gap vane geometry, extremely long measurement times, and modeling. We take into account the wide-gap related stress heterogeneity by extending upon mainstream methods and apply a gap correction. Furthermore, we rationalize the experimental data through a simple viscous structural model. With these tools we find that, owing to the small size of the particles subjected to Brownian motion, the NFC suspensions exhibit a critical shear rate, where the flow curve experiences a turning point. This makes the steady state of these suspensions at low shear rates non-unique. To optimize various mixing and pumping applications, such history dependent tendency of NFC suspensions to shear band needs to be taken into account.     

Reinforcing effect of carboxymethylated nanofibrillated cellulose powder on hydroxypropyl cellulose

Bionanocomposites of hydroxypropyl cellulose (HPC) and nanofibrillated cellulose (NFC) were prepared by solution casting. The various NFC were in form of powders and were prepared from refined, bleached beech pulp (RBP) by mechanical disintegration, optionally combined with a pre- or post mechanical carboxymethylation. Dynamic mechanical analysis (DMA) and tensile tests were performed to compare the reinforcing effects of the NFC powders to those of their never-dried analogues. For unmodified NFC powders an inferior reinforcing potential in HPC was observed that was ascribed to severe hornification and reagglomeration of NFC. In contrast, the composites with carboxymethylated NFC showed similar behaviors, regardless of the NFC suspensions being dried or not prior to composite preparation. SEM characterization confirmed a homogeneous dispersion of dried, carboxymethylated NFC within the HPC matrix. These results clearly demonstrate that drying of carboxymethylated NFC to a powder does not decrease its reinforcing potential in (bio)nanocomposites.     

Comparison of multilayer formation between different cellulose nanofibrils and cationic polymers

The multilayer formation between polyelectrolytes of opposite charge offers possibility for creating new tailored materials. Exchanging one or both components for charged nanofibrillated cellulose (NFC) further increases the variety of achievable properties. We explored this by introducing unmodified, low charged NFC and high charged TEMPO-oxidized NFC. Systematic evaluation of the effect of both NFC charge and properties of cationic polyelectrolytes on the structure of the multilayers was performed. As the cationic component cationic NFC was compared with two different cationic polyelectrolytes, poly(dimethyldiallylammoniumchloride) and cationic starch. Quartz crystal microbalance with dissipation (QCM-D) was used to monitor the multilayer formation and AFM colloidal probe microscopy (CPM) was further applied to probe surface interactions in order to gain information about fundamental interactions and layer properties. Generally, the results verified the characteristic multilayer formation between NFC of different charge and how the properties of formed multilayers can be tuned. However, the strong nonelectrostatic affinity between cellulosic fibrils was observed. CPM measurements revealed monotonically repulsive forces, which were in good correspondence with the QCM-D observations. Significant increase in adhesive forces was detected between the swollen high charged NFC.     

Grinding process for the production of nanofibrillated cellulose based on unbleached and bleached bamboo organosolv pulp

Nanofibrillated cellulose (NFC) is a type of nanomaterial based on renewable resources and produced by mechanical disintegration without chemicals. NFC is a potential reinforcing material with a high surface area and high aspect ratio, both of which increase reinforcement on the nanoscale. The raw materials used were unbleached and bleached bamboo organosolv pulp. Organosolv pulping is a cleaner process than other industrial methods (i.e. Kraft process), as it uses organic solvents during cooking and provides easy solvent recovery at the end of the process. The NFC was produced by treating unbleached and bleached bamboo organosolv pulps for 5, 10, 15 and 20 nanofibrillation cycles using the grinding method. Chemical, physical and mechanical tests were performed to determine the optimal condition for nanofibrillation. The delamination of the S2 layer of the fibers during nanofibrillation contributed to the partial removal of amorphous components (mainly lignin), which have low polarity and improved the adhesion of the fibers, particularly the unbleached cellulose. The transverse modulus of elasticity of the unbleached NFC was highest after 10 nanofibrillation cycles. Further treatment cycles decreased the modulus due to the mechanical degradation of the fibers. The unbleached NFC produced by 10 cycles have a greater transverse modulus of elasticity, the crystallite size showed increase with the nanofibrillation, and after 5 nanofibrillation cycles, no differences are observed in the morphology of the fibers.     

Topochemical acetylation of cellulose nanopaper structures for biocomposites: mechanisms for reduced water vapour sorption

Moisture sorption decreases dimensional stability and mechanical properties of polymer matrix biocomposites based on plant fibers. Cellulose nanofiber reinforcement may offer advantages in this respect. Here, wood-based nanofibrillated cellulose (NFC) and bacterial cellulose (BC) nanopaper structures, with different specific surface area (SSA), ranging from 0.03 to 173.3 m²/g, were topochemically acetylated and characterized by ATR-FTIR, XRD, solid-state CP/MAS ¹³C-NMR and moisture sorption studies. Polymer matrix nanocomposites based on NFC were also prepared as demonstrators. The surface degree of substitution (surface-DS) of the acetylated cellulose nanofibers is a key parameter, which increased with increasing SSA. Successful topochemical acetylation was confirmed and significantly reduced the moisture sorption in nanopaper structures, especially at RH = 53 %. BC nanopaper sorbed less moisture than the NFC counterpart, and mechanisms are discussed. Topochemical NFC nanopaper acetylation can be used to prepare moisture-stable nanocellulose biocomposites.     

Preparation and characterization of cationic nanofibrillated cellulose from etherification and high-shear disintegration processes

Oat straw cellulose pulp was cationized in an etherification reaction with chlorocholine chloride. The cationized cellulose pulp was then mechanically disintegrated in two process steps to obtain trimethylammonium-modified nanofibrillated cellulose (TMA-NFC). The materials thus obtained were analyzed by elemental analysis, X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), scanning electron microscopy (SEM) and other techniques. A higher nitrogen content of TMA-NFC samples was found by XPS analysis than by elemental analysis, which indicates that the modification occurred mainly on the surface of cellulose fibrils. XPS also confirmed the existence of ammonium groups in the samples. SEM provided images of very fine network structures of TMA-NFC, which affirmed the positive effect of ionic charge on mechanical disintegration process. According to XRD and SEM results, no severe degradation of the cellulose occurred, even at high reaction temperatures. Because of the different properties of the cationic NFC compared to negatively charged native cellulose fibers, TMA-NFC may find broad applications in technical areas, for instance in combination with anionic species, such as fillers or dyes. Indeed, TMA-NFC seems to improve the distribution of clay fillers in NFC matrix.     

Influence of drying restraint on physical and mechanical properties of nanofibrillated cellulose films

Nanofibrillated cellulose (NFC) is a renewable and biodegradable fibril that possesses high strength and stiffness resulting from high level hydrogen bonding. Films made from NFC shrink and distort as they transition from a wet state (20 wt% solids) to a state of moisture equilibrium (90 wt% solids at 50 % RH, 23 °C). Material distortions are driven by development of moisture gradients within the fibril network and effectively reduce mechanical performance. For this study, NFC was extracted from softwood holocellulose by first employing a chemical pretreatment [(2,2,6,6-tetramethylpiperidin-1-yl)oxyl catalyzed oxidation] followed by mechanical fibrillation using ultrasound energy. To assess the problem of film distortion, neat NFC films were dried at 50 % RH, 23 °C under one of the following three restraint conditions: fully restrained, partially restrained, and uniaxially drawn. The influence of restraint condition on the resulting physical and mechanical properties was evaluated. Raman and X-ray results showed that fibrils in the uniaxially drawn specimens tended to align with the drawing axis, whereas no in-plane orientation effects were observed for the fully or partially restrained specimens. Fully restrained specimens had a respective strength and stiffness of 222 MPa and 14 GPa in every (in-plane) direction. However, samples that were wet-drawn to a 30 % strain level had a respective strength and stiffness of 474 MPa and 46 GPa in the direction of draw. Mechanical properties for axially drawn specimens had both fibril alignment and fibril straightening contributions.     

Wood‐Derived Ultrathin Carbon Nanofiber Aerogels

Carbon aerogels with 3D networks of interconnected nanometer‐sized particles exhibit fascinating physical properties and show great application potential. Efficient and sustainable methods are required to produce high‐performance carbon aerogels on a large scale to boost their practical applications. An economical and sustainable method is now developed for the synthesis of ultrathin carbon nanofiber (CNF) aerogels from the wood‐based nanofibrillated cellulose (NFC) aerogels via a catalytic pyrolysis process, which guarantees high carbon residual and well maintenance of the nanofibrous morphology during thermal decomposition of the NFC aerogels. The wood‐derived CNF aerogels exhibit excellent electrical conductivity, a large surface area, and potential as a binder‐free electrode material for supercapacitors. The results suggest great promise in developing new families of carbon aerogels based on the controlled pyrolysis of economical and sustainable nanostructured precursors.     

Evidence of a Uranium-Paddlewheel Node in a Catecholate-Based Metal–Organic Framework

The interactions between uranium and non-innocent organic species are an essential component of fundamental uranium redox chemistry. However, they have seldom been explored in the context of multidimensional, porous materials. Uranium-based metal–organic frameworks (MOFs) offer a new angle to study these interactions, as these self-assembled species stabilize uranium species through immobilization by organic linkers within a crystalline framework, while potentially providing a method for adjusting metal oxidation state through coordination of non-innocent linkers. We report the synthesis of the MOF NU-1700 , assembled from U⁴⁺-paddlewheel nodes and catecholate-based linkers. We propose this highly unusual structure, which contains two U⁴⁺ ions in a paddlewheel built from four linkers—a first among uranium materials—as a result of extensive characterization via powder X-ray diffraction (PXRD), sorption, transmission electron microscopy (TEM), and thermogravimetric analysis (TGA), in addition to density functional theory (DFT) calculations.