Interpolymer association in aqueous solutions is essential for many industrial processes, new materials design, and the biochemistry of life. However, our understanding of the association mechanism is limited. Classical theories do not provide molecular details, creating a need for detailed mechanistic insights. This work consolidates previous literature with complementary isothermal titration calorimetry (ITC) measurements and molecular dynamics (MD) simulations to investigate molecular mechanisms to provide such insights. The large body of ITC data shows that intermolecular bonds, such as ionic or hydrogen bonds, cannot drive association. Instead, polymer association is entropy-driven due to the reorganization of water and ions. We propose a unifying entropy-driven association mechanism by generalizing previously suggested polyion association principles to include nonionic polymers, here termed polydipoles. In this mechanism, complementary charge densities of the polymers are the common denominators of association, for both polyions and polydipoles. The association of the polymers results mainly from two processes: charge exchange and amphiphilic association. MD simulations indicate that the amphiphilic assembly alone is enough for the initial association. Our proposed mechanism is a step toward a molecular understanding of the formation of complexes between synthetic and biological polymers under ambient or biological conditions.
Interactions between dry cellulose were studied using model systems, cellulose beads, and cellulose films, using custom-built contact adhesion testing equipment. Depending on the configuration of the substrates in contact, Polydimethylsiloxane (PDMS) film, cellulose films spin-coated either on PDMS or glass, the interaction shows three distinct processes. Firstly, molecular interlocking is formed between cellulose and cellulose when there is a soft PDMS thin film backing the cellulose film. Secondly, without backing, no initial attraction force between the surfaces is observed. Thirdly, a significant force increase, ∆F, is observed during the retraction process for cellulose on glass, and there is a maximum in ∆F when the retraction rate is increased. This is due to the kinetics of a contacting process occurring in the interaction zone between the surfaces caused by an interdigitation of a fine fibrillar structure at the nano-scale, whereas, for the spin-coated cellulose surfaces on the PDMS backing, there is a more direct adhesive failure. The results have generated understanding of the interaction between cellulose-rich materials, which helps design new, advanced cellulose-based materials. The results also show the complexity of the interaction between these surfaces and that earlier mechanisms, based on macroscopic material testing, are simply not adequate for molecular tailoring.
Nanocellulose-based membranes have attracted intense attention in bioelectronic devices due to their low cost, flexibility, biocompatibility, degradability, and sustainability. Herein, we demonstrate a flexible ionic diode using a cross-linked bipolar membrane fabricated from positively and negatively charged cellulose nanofibrils (CNFs). The rectified current originates from the asymmetric charge distribution, which can selectively determine the direction of ion transport inside the bipolar membrane. The mechanism of rectification was demonstrated by electrochemical impedance spectroscopy with voltage biases. The rectifying behavior of this kind of ionic diode was studied by using linear sweep voltammetry to obtain current-voltage characteristics and the time dependence of the current. In addition, the performance of cross-linked CNF diodes was investigated while changing parameters such as the thickness of the bipolar membranes, the scanning voltage range, and the scanning rate. A good long-term stability due to the high density cross-linking of the diode was shown in both current-voltage characteristics and the time dependence of current.
Cellulose , Ions , Membranes
Despite extensive research on biobased and fiber-based materials, fundamental questions regarding the molecular processes governing fiber-fiber interactions remain unanswered. In this study, we introduce a method to examine and clarify molecular interactions within fiber-fiber joints using precisely characterized model materials, i.e., regenerated cellulose gel beads with nanometer-smooth surfaces. By physically modifying these materials and drying them together to create model joints, we can investigate the mechanisms responsible for joining cellulose surfaces and how this affects adhesion in both dry and wet states through precise separation measurements. The findings reveal a subtle balance in the joint formation, influencing the development of nanometer-sized structures at the contact zone and likely inducing built-in stresses in the interphase. This research illustrates how model materials can be tailored to control interactions between cellulose-rich surfaces, laying the groundwork for future high-resolution studies aimed at creating stiff, ductile, and/or tough joints between cellulose surfaces and to allow for the design of high-performance biobased materials.
In the context of three-dimensional (3D) cell culture and tissue engineering, 3D printing is a powerful tool for customizing in vitro 3D cell culture models that are critical for understanding the cell-matrix and cell-cell interactions. Cellulose nanofibril (CNF) hydrogels are emerging in constructing scaffolds able to imitate tissue in a microenvironment. A direct modification of the methacryloyl (MA) group onto CNF is an appealing approach to synthesize photocross-linkable building blocks in formulating CNF-based bioinks for light-assisted 3D printing; however, it faces the challenge of the low efficiency of heterogenous surface modification. Here, a multistep approach yields CNF methacrylate (CNF-MA) with a decent degree of substitution while maintaining a highly dispersible CNF hydrogel, and CNF-MA is further formulated and copolymerized with monomeric acrylamide (AA) to form a super transparent hydrogel with tuneable mechanical strength (compression modulus, approximately 5-15 kPa). The resulting photocurable hydrogel shows good printability in direct ink writing and good cytocompatibility with HeLa and human dermal fibroblast cell lines. Moreover, the hydrogel reswells in water and expands to all directions to restore its original dimension after being air-dried, with further enhanced mechanical properties, for example, Young's modulus of a 1.1% CNF-MA/1% PAA hydrogel after reswelling in water increases to 10.3 kPa from 5.5 kPa.
Bioprinting , Nanofibers , Humans , Biocompatible Materials/pharmacology , Hydrogels/pharmacology , Cellulose/pharmacology , Tissue Engineering , Printing, Three-Dimensional , HeLa Cells , Tissue Scaffolds
The unique properties of hydrogels enable the design of life-like soft intelligent systems. However, stimuli-responsive hydrogels still suffer from limited actuation control. Direct electronic control of electronically conductive hydrogels can solve this challenge and allow direct integration with modern electronic systems. An electrochemically controlled nanowire composite hydrogel with high in-plane conductivity that stimulates a uniaxial electrochemical osmotic expansion is demonstrated. This materials system allows precisely controlled shape-morphing at only -1 V, where capacitive charging of the hydrogel bulk leads to a large uniaxial expansion of up to 300%, caused by the ingress of ≈700 water molecules per electron-ion pair. The material retains its state when turned off, which is ideal for electrotunable membranes as the inherent coupling between the expansion and mesoporosity enables electronic control of permeability for adaptive separation, fractionation, and distribution. Used as electrochemical osmotic hydrogel actuators, they achieve an electroactive pressure of up to 0.7 MPa (1.4 MPa vs dry) and a work density of ≈150 kJ m-3 (2 MJ m-3 vs dry). This new materials system paves the way to integrate actuation, sensing, and controlled permeation into advanced soft intelligent systems.
A multifunctional soft material with high ionic and electrical conductivity, combined with high mechanical properties and the ability to change shape can enable bioinspired responsive devices and systems. The incorporation of all these characteristics in a single material is very challenging, as the improvement of one property tends to reduce other properties. Here, a nanocomposite film based on charged, high-aspect-ratio 1D flexible nanocellulose fibrils, and 2D Ti3 C2 Tx MXene is presented. The self-assembly process results in a stratified structure with the nanoparticles aligned in-plane, providing high ionotronic conductivity and mechanical strength, as well as large water uptake. In hydrogel form with 20 wt% liquid, the electrical conductivity is over 200 S cm-1 and the in-plane tensile strength is close to 100 MPa. This multifunctional performance results from the uniquely layered composite structure at nano- and mesoscales. A new type of electrical soft actuator is assembled where voltage as low as ±1 V resulted in osmotic effects and giant reversible out-of-plane swelling, reaching 85% strain.
New sustainable materials produced by green processing routes are required in order to meet the concepts of circular economy. The replacement of insulating materials comprising flammable synthetic polymers by bio-based materials represents a potential opportunity to achieve this task. In this paper, low-density and flame-retardant (FR) porous fiber networks are prepared by assembling Layer-by-Layer (LbL)-functionalized cellulose fibers by means of freeze-drying. The LbL coating, encompassing chitosan and sodium hexametaphosphate, enables the formation of a self-sustained porous structure by enhancing fiber-fiber interactions during the freeze-drying process. Fiber networks prepared from 3 Bi-Layer (BL)-coated fibers contain 80% wt of cellulose and can easily self-extinguish the flame during flammability tests in vertical configuration while displaying extremely low combustion rates in forced combustion tests. Smoke release is 1 order of magnitude lower than that of commercially available polyurethane foams. Such high FR efficiency is ascribed to the homogeneity of the deposited assembly, which produces a protective exoskeleton at the air/cellulose interface. The results reported in this paper represent an excellent opportunity for the development of fire-safe materials, encompassing natural components where sustainability and performance are maximized.
The fundamental understanding concerning cellulose-cellulose interactions under wet and dry conditions remains unclear. This is especially true regarding the drying-induced association of cellulose, commonly described as an irreversible phenomenon called hornification. A fundamental understanding of the mechanisms behind hornification would contribute to new drying techniques for cellulose-based materials in the pulp and paper industry while at the same time enhancing material properties and facilitating the recyclability of cellulose-rich materials. In the present work, the irreversible joining of cellulose-rich surfaces has been studied by subjecting cellulose nanofibril (CNF) films to different heat treatments to establish a link between reswelling properties, structural characteristics as well as chemical and mechanical analyses. A heating time/temperature dependence was observed for the reswelling of the CNF films, which is related to the extent of hornification and is different for different chemical compositions of the fibrils. Further, the results indicate that hornification is related to a diffusion process and that the reswellability increases very slowly over long time, indicating that equilibrium is not reached. Hence, hornification is suggested to be a kinetically limited phenomenon governed by non-covalent reversible interactions and a time/temperature dependence on their forming and breaking.
The development of wood-based thermoplastic polymers that can replace synthetic plastics is of high environmental importance, and previous studies have indicated that cellulose-rich fiber containing dialcohol cellulose (ring-opened cellulose) is a very promising candidate material. In this study, molecular dynamics simulations, complemented with experiments, were used to investigate how and why the degree of ring opening influences the properties of dialcohol cellulose, and how temperature and presence of water affect the material properties. Mechanical tensile properties, diffusion/mobility-related properties, densities, glass-transition temperatures, potential energies, hydrogen bonds, and free volumes were simulated for amorphous cellulosic materials with 0-100% ring opening, at ambient and high (150 °C) temperatures, with and without water. The simulations showed that the impact of ring openings, with respect to providing molecular mobility, was higher at high temperatures. This was also observed experimentally. Hence, the ring opening had the strongest beneficial effect on "processability" (reduced stiffness and strength) above the glass-transition temperature and in wet conditions. It also had the effect of lowering the glass-transition temperature. The results here showed that molecular dynamics is a valuable tool in the development of wood-based materials with optimal thermoplastic properties.
Cellulose , Molecular Dynamics Simulation , Cellulose/chemistry , Plastics/chemistry , Transition Temperature , Water/chemistry
Development of strong cellulose nanofibril (CNF) networks for advanced applications, such as in the biomedical field, is of high importance owing to the biocompatible nature and plant-based origin of cellulose nanofibrils. Nevertheless, lack of mechanical strength and complex synthesis methods hinder the application of these materials in areas where both toughness and manufacturing simplicity are required. In this work, we introduce a facile method for the synthesis of a low solid content (< 2 wt%), covalently crosslinked CNF hydrogel where Poly (N-isopropylacrylamide) (NIPAM) chains are utilized as crosslinks between the nanofibrils. The resulting networks have the capability to fully recover the shape in which they were formed after various drying and rewetting cycles. Characterization of the hydrogel and its constitutive components was performed using X-ray scattering, rheological investigations and uniaxial testing in compression. Influence of covalent crosslinks was compared with networks crosslinked by the addition of CaCl2. Among other things the results show that the mechanical properties of the hydrogels can be tuned by controlling the ionic strength of the surrounding medium. Finally, a mathematical model was developed based on the experimental results, which describes and predicts to a decent degree the large-deformation, elastoplastic behavior, and fracture of these networks.
Fibrillar hydrogels are remarkably stiff, low-density networks that can hold vast amounts of water. These hydrogels can easily be made anisotropic by orienting the fibrils using different methods. Unlike the detailed and established descriptions of polymer gels, there is no coherent theoretical framework describing the elastoplastic behavior of fibrillar gels, especially concerning anisotropy. In this work, the swelling pressures of anisotropic fibrillar hydrogels made from cellulose nanofibrils were measured in the direction perpendicular to the fibril alignment. This experimental data was used to develop a model comprising three mechanical elements representing the network and the osmotic pressure due to non-ionic and ionic surface groups on the fibrils. At low solidity, the stiffness of the hydrogels was dominated by the ionic swelling pressure governed by the osmotic ingress of water. Fibrils with different functionality show the influence of aspect ratio, chemical functionality, and the remaining amount of hemicelluloses. This general model describes physically crosslinked hydrogels comprising fibrils with high flexural rigidity - that is, with a persistence length larger than the mesh size. The experimental technique is a framework to study and understand the importance of fibrillar networks for the evolution of multicellular organisms, like plants, and the influence of different components in plant cell walls.
Nanocelluloses are anisotropic nanoparticles of semicrystalline assemblies of glucan polymers. They have great potential as renewable building blocks in the materials platform of a more sustainable society. As a result, the research on nanocellulose has grown exponentially over the last decades. To fully utilize the properties of nanocelluloses, a fundamental understanding of their colloidal behavior is necessary. As elongated particles with dimensions in a critical nanosize range, their colloidal properties are complex, with several behaviors not covered by classical theories. In this comprehensive Review, we describe the most prominent colloidal behaviors of nanocellulose by combining experimental data and theoretical descriptions. We discuss the preparation and characterization of nanocellulose dispersions, how they form networks at low concentrations, how classical theories cannot describe their behavior, and how they interact with other colloids. We then show examples of how scientists can use this fundamental knowledge to control the assembly of nanocellulose into new materials with exceptional properties. We hope aspiring and established researchers will use this Review as a guide.
Nanoscale infrared (IR) spectroscopy and microscopy, enabling the acquisition of IR spectra and images with a lateral resolution of 20 nm, is employed to chemically characterize individual cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs) to elucidate if the CNCs and CNFs consist of alternating crystalline and amorphous domains along the CNF/CNC. The high lateral resolution enables studies of the nanoscale morphology at different domains of the CNFs/CNCs: flat segments, kinks, twisted areas, and end points. The types of nanocellulose investigated are CNFs from tunicate, CNCs from cotton, and anionic and cationic wood-derived CNFs. All nano-FTIR spectra acquired from the different samples and different domains of the individual nanocellulose particles resemble a spectrum of crystalline cellulose, suggesting that the non-crystalline cellulose signal observed in macroscopic measurements of nanocellulose most likely originate from cellulose chains present at the surface of the nanocellulose particles.
Nanoparticles , Nanoparticles/chemistry , Cellulose/chemistry , Spectrophotometry, Infrared , Microscopy, Atomic Force , Wood
HYPOTHESIS: It is theoretically predicted and hypothesized that the charge density and size of spherical nanoparticles are the key factors for their adsorption onto oppositely charged surfaces. It is also hypothesized that the morphology and charge of the surface are of great importance. In-plane 2D (silica) or a volumetric 3D (regenerated TEMPO-oxidized cellulose model surfaces) distribution of charged groups is expected to influence charge compensation and, thus, the adsorption behavior. EXPERIMENTS: In this work, self-stabilized nanolatexes with a range of cationic charge densities and sizes were synthesized through reversible addition - fragmentation chain-transfer (RAFT) polymerization coupled with polymerization-induced self-assembly (PISA). Their adsorption onto silica and anionic cellulose model surfaces was investigated using stagnation point adsorption reflectometry (SPAR) and quartz crystal microbalance with dissipation (QCM-D). FINDINGS: Experiments and theory agree and show that the size of the nanolatex and the difference in charge density compared to the substrate determine the charge compensation and, thus, the surface coverage. Highly charged or large nanolatexes overcompensate the surface charge of non-porous substrates leading to a significant repulsive zone where other particles cannot adsorb. For porous substrates like cellulose, the vertical distribution of charged groups in the 3D volume prevents overcompensation and thus increases the adsorption. This systematic study investigates the isolated effect of surface charge and size and paves the way for on-demand particles specifically designed for a surface with particular characteristics.
Cellulose , Silicon Dioxide , Adsorption , Surface Properties , Cellulose/chemistry , Silicon Dioxide/chemistry , Quartz Crystal Microbalance Techniques , Cations
The fabrication of reusable, sustainable adsorbents from low-cost, renewable resources via energy efficient methods is challenging. This paper presents wet-stable, carboxymethylated cellulose nanofibril (CNF) and amyloid nanofibril (ANF) based aerogel-like adsorbents prepared through efficient and green processes for the removal of metal ions and dyes from water. The aerogels exhibit tunable densities (18-28 kg m-3), wet resilience, and an interconnected porous structure (99% porosity), with a pH controllable surface charge for adsorption of both cationic (methylene blue and Pb(II)) and anionic (brilliant blue, congo red, and Cr(VI)) model contaminants. The Langmuir saturation adsorption capacity of the aerogel was calculated to be 68, 79, and 42 mg g-1 for brilliant blue, Pb(II), and Cr(VI), respectively. Adsorption kinetic studies for the adsorption of brilliant blue as a model contaminant demonstrated that a pseudo-second-order model best fitted the experimental data and that an intraparticle diffusion model suggests that there are three adsorption stages in the adsorption of brilliant blue on the aerogel. Following three cycles of adsorption and regeneration, the aerogels maintained nearly 97 and 96% of their adsorption capacity for methylene blue and Pb(II) as cationic contaminants and 89 and 80% for brilliant blue and Cr(VI) as anionic contaminants. Moreover, the aerogels showed remarkable selectivity for Pb(II) in the presence of calcium and magnesium as background ions, with a selectivity coefficient more than 2 orders of magnitude higher than calcium and magnesium. Overall, the energy-efficient and sustainable fabrication procedure, along with good structural stability, reusability, and selectivity, makes these aerogels very promising for water purification applications.
Methylene Blue , Water Pollutants, Chemical , Adsorption , Methylene Blue/chemistry , Kinetics , Magnesium , Calcium , Lead , Water Pollutants, Chemical/analysis , Water Pollutants, Chemical/chemistry , Anions , Cations , Hydrogen-Ion Concentration
The structure and dynamics of networks formed by rod-shaped particles can be indirectly investigated by measuring the diffusion of spherical tracer particles. This method was used to characterize cellulose nanofibril (CNF) networks in both dispersed and arrested states, the results of which were compared with coarse-grained Brownian dynamics simulations. At a CNF concentration of 0.2 wt% a transition was observed where, below this concentration tracer diffusion is governed by the increasing macroscopic viscosity of the dispersion. Above 0.2 wt%, the diffusion of small particles (20-40 nm) remains viscosity controlled, while particles (100-500 nm) become trapped in the CNF network. Sedimentation of silica microparticles (1-5 µm) in CNF dispersions was also determined, showing that sedimentation of larger particles is significantly affected by the presence of CNF. At concentrations of 0.2 wt%, the sedimentation velocity of 5 µm particles was reduced by 99 % compared to pure water.
Cellulose , Nanofibers , Cellulose/chemistry , Nanofibers/chemistry , Silicon Dioxide , Viscosity , Water
A novel method is reported for the preparation of a hybrid gibbsite-cellulose nanofibril (CNF) nanocomposite film with improved wet and dry mechanical properties and barrier properties. A gibbsite and cationic CNF dispersion was dewatered at pH 7 to prepare well-ordered films. Thereafter, the charge on gibbsite was reversed by dipping the film in pH 12 water to induce an ionic interaction between CNFs and gibbsite, enhancing the film properties; modulus improved from 9 GPa to 12 GPa, with a maintained strain-at-break of 6 % and tensile strength of 190 MPa. Additionally, the charge-reversed film swelled a factor of 24 less than a film without any gibbsite. At 23 °C and 80 % RH, the oxygen barrier properties were improved by a factor of 28, to a value of 18 ml·µm·m-2·kPa-1·24 h-1 and the water vapour barrier properties were improved by a factor of 12, to a value of 105 g·µm·m-2·kPa-1·24 h-1.
Nanocomposites , Nanofibers , Cellulose/chemistry , Nanocomposites/chemistry , Nanofibers/chemistry , Steam , Tensile Strength
Metal-organic frameworks (MOFs) are hybrid porous crystalline networks with tunable chemical and structural properties. However, their excellent potential is limited in practical applications by their hard-to-shape powder form, making it challenging to assemble MOFs into macroscopic composites with mechanical integrity. While a binder matrix enables hybrid materials, such materials have a limited MOF content and thus limited functionality. To overcome this challenge, nanoMOFs are combined with tailored same-charge high-aspect-ratio cellulose nanofibrils (CNFs) to manufacture robust, wet-stable, and multifunctional MOF-based aerogels with 90 wt% nanoMOF loading. The porous aerogel architectures show excellent potential for practical applications such as efficient water purification, CO2 and CH4 gas adsorption and separation, and fire-safe insulation. Moreover, a one-step carbonization process enables these aerogels as effective structural energy-storage electrodes. This work exhibits the unique ability of high-aspect-ratio CNFs to bind large amounts of nanoMOFs in structured materials with outstanding mechanical integrity-a quality that is preserved even after carbonization. The demonstrated process is simple and fully discloses the intrinsic potential of the nanoMOFs, resulting in synergetic properties not found in the components alone, thus paving the way for MOFs in macroscopic multifunctional composites.
