Swelling and Free-Volume Characteristics of TEMPO-Oxidized Cellulose Nanofibril Films

The effect of pre-treatment and process conditions on the gas barrier properties of fibrillated cellulose films and coatings: A review

Microfibrillated cellulose (MFC) is a bio-material produced by disintegrating cellulose fibres into fibrillar components. MFC could offer a sustainable solution to packaging needs since it can form an excellent barrier to oxygen. However, a comprehensive understanding of how MFC characteristics impact barrier properties of MFC films or coatings is required. This article critically reviews how the extent of separation of fibres into fibrils-and any resulting changes to the crystallinity and degree of polymerisation of cellulose-influences gas barrier properties of MFC films or coatings. Findings from publications investigating the barrier performance of MFC prepared through different processes intending to increase the effectiveness of fibrillation are evaluated and compared. The effects of processing conditions or chemical pre-treatments on barrier properties of MFC films or coatings are then discussed. A comparison of reported results showed that morphology and size polydispersity of the cellulose strongly influence the barrier properties of MFC. However, changing the MFC production process to decrease fibril diameter and polydispersity can result in changes to cellulose crystallinity; reduction in fibril length; introduction of bulky functional groups; or increased fibril surface charge: all of which could have a negative impact on the barrier properties of the final films or coatings.

Direct ink writing of biocompatible chitosan/non-isocyanate polyurethane/cellulose nanofiber hydrogels for wound-healing applications

The demand for new biocompatible and 3D printable materials for biomedical applications is on the rise. Ideally, such materials should exhibit either biodegradability or recyclability, possess antibacterial properties, and demonstrate remarkable biocompatibility with no cytotoxic effects. In this research, we synthesized biocompatible and 3D printable hydrogels tailored for biomedical applications, such as wound healing films, by combining antibacterial double-quaternized chitosan (DQC) with cystamine-based non-isocyanate polyurethane (NIPU-Cys) - a material renowned for enhancing both the flexibility and mechanical properties of the hydrogels. To improve the rheological behavior, swelling attributes, and printability, cellulose nanofibrils were introduced into the matrix. We investigated the impact of DQC on degradability, swelling capacity, rheological behavior, printability, and cell biocompatibility. The slightly cytotoxic nature associated with quaternary chitosan was evaluated, and the optimal concentration of DQC in the hydrogel was determined to ensure biocompatibility. The resulting hydrogels were found to be suitable materials for 3D printing via a direct ink writing technique (DIW), producing porous, biocompatible hydrogels endowed with valuable attributes suitable for various wound-healing applications.

Protonation of Surface Carboxyls on Rice Straw Cellulose Nanofibrils: Effect on the Aerogel Structure, Modulus, Strength, and Wet Resiliency

Rice straw cellulose nanofibrils from the optimal 2,2,6,6-tetramethylpiperidine-1-oxyl oxidation/blending process carrying 1.17 mmol/g surface carboxyls were protonated to varying charged (COO^-Na⁺) and uncharged (COOH) surfaces. Reducing the electrostatic repulsion of surface charges by protonation with hydrochloric acid from 11 to 45 and 100% surface carboxylic acid most prominently reduced the aerogel densities from 8.0 to 6.6 and 5.2 mg/cm³ while increasing the mostly open cell pore volumes from 125 to 152 and 196 mL/g. Irrespective of charge levels, all aerogels were amphiphilic, super-absorptive, stable at pH 2 for up to 30 days, and resilient for up to 10 repetitive squeezing-absorption cycles. While these aerogels exhibited density-dependent dry [11.3 to 1.5 kPa/(mg/cm³)] and reduced wet [3.3 to 1.4 kPa/(mg/cm³)] moduli, the absorption of organic liquids stiffened the saturated aerogels. These data support protonation as a critical yet simple approach toward precise control of aerogels' dry and wet properties.

The influence of temperature on cellulose swelling at constant water density

We have in this paper investigated how water sorbs to cellulose. We found that both cellulose nanofibril (CNF) and cellulose nanocrystal (CNC) films swell similarly, as they are both mainly composed of cellulose. CNF/CNC films subjected to water at 0.018 kg/m³ at 25 °C and 39 °C, showed a decrease in swelling from ~ 8 to 2%. This deswelling increased the tensile index of CNF-films by ~ 13%. By molecular modeling of fibril swelling, we found that water sorbed to cellulose exhibits a decreased diffusion constant compared to bulk water. We quantified this change and showed that diffusion of sorbed water displays less dependency on swelling temperature compared to bulk water diffusion. To our knowledge, this has not previously been demonstrated by molecular modeling. The difference between bulk water diffusion (D_WW) and diffusion of water sorbed to cellulose (D_CC) increased from D_WW - D_CC ~ 3 × 10^-5 cm/s² at 25 °C to D_WW - D_CC ~ 8.3 × 10^-5 cm/s² at 100 °C. Moreover, water molecules spent less successive time sorbed to a fibril at higher temperatures.

Selective Oxidation of Cellulose—A Multitask Platform with Significant Environmental Impact

Raw cellulose, or even agro-industrial waste, have been extensively used for environmental applications, namely industrial water decontamination, due to their effectiveness, availability, and low production cost. This was a response to the increasing societal demand for fresh water, which made the purification of wastewater one of the major research issue for both academic and industrial R&D communities. Cellulose has undergone various derivatization reactions in order to change the cellulose surface charge density, a prerequisite condition to delaminate fibers down to nanometric fibrils through a low-energy process, and to obtain products with various structures and properties able to undergo further processing. Selective oxidation of cellulose, one of the most important methods of chemical modification, turned out to be a multitask platform to obtain new high-performance, versatile, cellulose-based materials, with many other applications aside from the environmental ones: in biomedical engineering and healthcare, energy storage, barrier and sensing applications, food packaging, etc. Various methods of selective oxidation have been studied, but among these, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl) (TEMPO)-mediated and periodate oxidation reactions have attracted more interest due to their enhanced regioselectivity, high yield and degree of substitution, mild conditions, and the possibility to further process the selectively oxidized cellulose into new materials with more complex formulations. This study systematically presents the main methods commonly used for the selective oxidation of cellulose and provides a survey of the most recent reports on the environmental applications of oxidized cellulose, such as the removal of heavy metals, dyes, and other organic pollutants from the wastewater.

An Opto- and Thermal-Rewrite PCM/CNF-IR 780 Energy Storage Nanopaper with Mechanical Regulated Performance

In spite of efforts to fabricate self-assembled energy storage nanopaper with potential applications in displays, greenhouses, and sensors, few studies have investigated their multiple stimuli-sensitivities. Here, an opto- and thermal-rewrite phase change material/cellulose nanofibril (PCM/CNF) energy storage nanopaper with mechanical regulated performance is facilely fabricated, through 5 min sonication of PCMs and CNFs in an aqueous system. The combination of PCM and CNF not only guarantees the recyclability of PCM without leakage, but also offers nanopaper adaptive properties by leveraging the mobility and optical variation accompanying solid-to-liquid transition of PCM. Besides, trace near-infrared (NIR) dye (IR 780) in it imparts a PCM-embedded nanopaper photothermal effect to modulate the local transparency via time- and position-controlled laser exposure, leading to a reusable opto-writing nanopaper. Furthermore, since the synergistic effect of stick-and-slip function attributes from PCMs and pore structures are produced by calcium ions, the PCM/CNF energy storage nanopaper exhibits excellent mechanically regulated performance from rigid to flexible, which greatly enriches their application in energy-efficient smart buildings and displays.

Influence of TEMPO oxidation on the properties of ethylene glycol methyl ether acrylate grafted cellulose sponges

In this study, cellulose nanofibers (CNF) obtained via high-pressure microfluidization were 2,6,6-tetra-methylpiperidine-1-oxyl (TEMPO) oxidized (TOCNF) in order to facilitate the grafting of ethylene glycol methyl ether acrylate (EGA). FTIR and XPS analyses revealed a more efficient grafting of EGA oligomers on the surface of TOCNF as compared to the original CNF. As a result, a consistent covering of the TOCNF fibers with EGA oligomers, an increased hydrophobicity and a reduction in porosity were noticed for TOCNF-EGA. However, the swelling ratio of TOCNF-EGA was similar to that of original CNF grafted with EGA and higher than that of TOCNF, because the higher amount of grafted EGA onto oxidized cellulose and the looser structure reduced the contacts between the fibrils and increased the absorption of water. All these results corroborated with a good cytocompatibility and compression strength recommend TOCNF-EGA for applications in regenerative medicine.

Cellulose nanofibers and the film-formation dilemma: Drying temperature and tunable optical, mechanical and wetting properties of nanocomposite films composed of waterborne sulfopolyesters

HYPOTHESIS

Waterborne sulfopolyesters have gained considerable interest as coating materials due to their excellent film-forming and optical properties. Their commercial use has been limited, however, due to their fragile nature. Incorporating cellulose nanofiber (CNF), a sustainable biopolymer, into the polymer matrix is expected to enhance the mechanical integrity of the nanocomposite as these two components synergistically interact.

EXPERIMENTS

In this study, we have investigated the suspension and film characteristics of three sulfopolyesters varying in charge density, glass transition temperature and molecular weight, as well as their mixtures with CNF. We have performed steady-shear rheology on mixtures with different CNF loading levels, and resulting films have been subjected to quasistatic uniaxial tensile and water contact-angle tests to elucidate the effects of CNF on mechanical and surface properties.

FINDINGS

Addition of CNF to waterborne polyester promotes shear-thinning behavior that remains unaffected by the CNF content. Solid films cast from these suspensions possess enhanced mechanical properties, as well as tailorable surface hydrophilicity, depending on composition and film-drying temperature. Tensile tests reveal that films containing 10 wt% CNF display the greatest mechanical improvements, suggesting the existence of a previously unidentified Goldilocks composition window.

Double-crosslinked cellulose nanofiber based bioplastic films for practical applications

While green bioplastic based on carbohydrate polymers have showed considerable promise, the methods typically used to prepare them in a single material have remained a significant challenge. In this study, a simple approach is proposed to fabricate high performance cellulose films composed of chemically and physically dual-crosslinked 2,2,6,6-tetramethylpiperidine-1-oxy-oxidized cellulose nanofibers (DC TEMPO-CNFs). The hydroxyl groups of TEMPO-CNF suspensions were firstly crosslinked chemically with epichlorohydrin (ECH), and subsequently TEMPO-CNF matrices were crosslinked physically via the strong electrostatic interaction between carboxylate and Ca²⁺ ions. It was found that the optimized DC TEMPO-CNF films exhibit a good transmittance (90 %) and a high tensile strength (303 MPa). Furthermore, these DC TEMPO-CNF films revealed superior thermal stability and excellent water resistance compared to neat TEMPO-CNF films without crosslinked domains. We believe that these results will pave the way to preparing practical polysaccharide bioplastics with simple, environmentally-friendly manufacturing processes.

FACILITIES FOR NANO MATERIALS EXAMINATION AT THE PULSTAR REACTOR

The PULSTAR is a 1-MWth nuclear research reactor located at North Carolina State University. It is fueled by uranium dioxide assemblies enriched to 4% or 6% in U-235 and is currently under licensing for operation at 2-MWth power. The PULSTAR is a center for irradiation testing and pre/post irradiation examination of materials. Among its unique capabilities are positron annihilation spectroscopy (PAS) and neutron powder diffraction (NPD) facilities. The PAS facility provides an intense positron beam reaching 6 × 108 e+/s, which drives two spectrometers; the e+-PAS and the Ps-PAS, used for studies of defects in thin film materials. A Na-22 bulk PAS system is also operational, which is used for studying millimeter scale materials. All spectrometers are capable of performing Doppler Broadening Spectroscopy (DBS) and Positron Annihilation Lifetime Spectroscopy (PALS). To date, the PAS systems have been used to characterize various materials (unirradiated and irradiated) that include graphite, soft matter, and metal-organic frameworks (MOF). The NPD facility uses a double focusing single crystal silicon rotating monochromator producing neutron beams with different energies. A position sensitive detection bank covers a scattering angle of 5°-125°. The facility is used in the examination of unirradiated and irradiated materials including graphitic materials, magnetic materials without rare-earth elements, and anode materials used in lithium batteries. The collected diffraction patterns can be processed to produce atomic pair distribution functions. The PAS and NPD facilities are available through user programs including the US DOE’s Nuclear Science User Facilities (NSUF) and the US NSF’s Research Triangle Nanotechnology Network (RTNN).

Alternating Stacking of Nanocrystals and Nanofibers into Ultrastrong Chiral Biocomposite Laminates

Attaining high mechanical strength and flexibility for chiral nematic biopolymer composites without compromising their vivid optical iridescence is an intriguing but challenging task. Traditional cellulose nanocrystal (CNC) blend nanocomposite films typically lose their coloration and display weak mechanical performance due to poor load transfer between needle-like nanocrystals and the collapse of a twisted organization. Herein, we report a design of robust laminated biocomposites with an alternatively stacked chiral nematic CNC phase and a random cellulose nanofiber (CNF) phase via a hydrogen-bonding-assisted layer-by-layer method. In contrast to the traditional biopolymer blends, the alternating CNC-CNF stacked films possess many-fold enhancement in both mechanical strength and toughness with their vivid structural colors highly preserved. We suggest that the enriched hydrogen bonding and partial limited entanglements at the interfaces between the helicoidal and random phases are responsible for enhancing the mechanical performance of robust biocomposites with brilliant iridescent colors. Such organized cellulose-cellulose biocomposites with alternating helicoidal-random phases fabricated by a facile sequential strategy may facilitate the development of sustainably sourced, damage-tolerant, and photonic films for bioenabled display technologies, security indicators, soft robotics, camouflages, and pressure sensors.

Influence of Free Volume Determined by Positron Annihilation Lifetime Spectroscopy (PALS) on Gas Permeability of Cellulose Nanocrystal Films

Cellulose nanocrystals (CNCs) are of increasing interest for packaging applications because of their biodegradability, low cost, high crystallinity, and high aspect ratio. The objective of this study was to use positron annihilation lifetime spectroscopy (PALS) to investigate the free volume of CNC films with different structural arrangements (chiral nematic vs shear-oriented CNC films) and relate this information to gas barrier performance. It was found that sheared CNC films with higher CNC alignment have lower free volume and hence have more tortuosity than chiral nematic self-assembled films, which lowers gas diffusion throughout the films. The overall barrier performance of the aligned CNC film obtained in this study has a higher barrier performance than high barrier polymer films like PVOH and EVOH. Furthermore, a modified model was developed for single-component CNC films to predict the gas permeability with variation of CNC alignment with validation by the data taken.

Production and Mechanical Characterisation of TEMPO-Oxidised Cellulose Nanofibrils/β-Cyclodextrin Films and Cryogels

Wood-based TEMPO-oxidised cellulose nanofibrils (toCNF) are promising materials for biomedical applications. Cyclodextrins have ability to form inclusion complexes with hydrophobic molecules and are considered as a method to bring new functionalities to these materials. Water sorption and mechanical properties are also key properties for biomedical applications such as drug delivery and tissue engineering. In this work, we report the modification with β-cyclodextrin (βCD) of toCNF samples with different carboxyl contents viz. 756 ± 4 µmol/g and 1048 ± 32 µmol/g. The modification was carried out at neutral and acidic pH (2.5) to study the effect of dissociation of the carboxylic acid group. Films processed by casting/evaporation at 40 °C and cryogels processed by freeze-drying were prepared from βCD modified toCNF suspensions and compared with reference samples of unmodified toCNF. The impact of modification on water sorption and mechanical properties was assessed. It was shown that the water sorption behaviour for films is driven by adsorption, with a clear impact of the chemical makeup of the fibres (charge content, pH, and adsorption of cyclodextrin). Modified toCNF cryogels (acidic pH and addition of cyclodextrins) displayed lower mechanical properties linked to the modification of the cell wall porosity structure. Esterification between βCD and toCNF under acidic conditions was performed by freeze-drying, and such cryogels exhibited a lower decrease in mechanical properties in the swollen state. These results are promising for the development of scaffold and films with controlled mechanical properties and added value due to the ability of cyclodextrin to form an inclusion complex with active principle ingredient (API) or growth factor (GF) for biomedical applications.

Manipulation of Fibril Surfaces in Nanocellulose-Based Facilitated Transport Membranes for Enhanced CO2 Capture

The transition toward sustainable processing entails the use of biobased alternatives as functional materials to reduce the overall carbon footprint. Nanocellulose, due to its natural availability, biodegradability, excellent mechanical properties, tunable surface, and high aspect ratio, is attracting more and more interest as a nanoscale additive in polymeric membranes. In this work, an effective way to modify nanocellulose fibril surfaces for performance enhancement in CO₂ separation membranes has been demonstrated. The functionalization promptly triggered intrinsic property responses in favor of nanofiber dispersion and CO₂ transport. Thin composite membranes containing the modified nanofibers in water-swelling poly(vinyl alcohol) (PVA) as well as in the blend of sterically hindered polyallylamine (SHPAA) and PVA were fabricated and tested using humid gas permeation tests. Defect-free ultrathin (300 nm) hybrid selective layers containing evenly distributed nanofibers were successfully coated. The addition of nanocellulose exhibited enhanced CO₂ permeance and CO₂/N₂ selectivity compared to those of the neat PVA membranes. CO₂ permeance up to 652 GPU and a CO₂/N₂ selectivity of 41.3 with SHPAA/PVA blend were documented. Functionalization plays a categorical role in the dispersion of nanocellulose fibrils in the SHPAA/PVA blend, increasing the steric stabilization and interface compatibility with the polymer matrix. The tuned interface with PEG groups act as sites for water clusters retention and increased CO₂ solubility, thus creating fast diffusion pathways for CO₂ transport.

The Importance of Moisture for Brown Rot Degradation of Modified Wood: A Critical Discussion

The effect of wood modification on wood-water interactions in modified wood is poorly understood, even though water is a critical factor in fungal wood degradation. A previous review suggested that decay resistance in modified wood is caused by a reduced wood moisture content (MC) that inhibits the diffusion of oxidative fungal metabolites. It has been reported that a MC below 23%–25% will protect wood from decay, which correlates with the weight percent gain (WPG) level seen to inhibit decay in modified wood for several different kinds of wood modifications. In this review, the focus is on the role of water in brown rot decay of chemically and thermally modified wood. The study synthesizes recent advances in the inhibition of decay and the effects of wood modification on the MC and moisture relationships in modified wood. We discuss three potential mechanisms for diffusion inhibition in modified wood: (i) nanopore blocking; (ii) capillary condensation in nanopores; and (iii) plasticization of hemicelluloses. The nanopore blocking theory works well with cell wall bulking and crosslinking modifications, but it seems less applicable to thermal modification, which may increase nanoporosity. Preventing the formation of capillary water in nanopores also explains cell wall bulking modification well. However, the possibility of increased nanoporosity in thermally modified wood and increased wood-water surface tension for 1.3-dimethylol-4.5-dihydroxyethyleneurea (DMDHEU) modification complicate the interpretation of this theory for these modifications. Inhibition of hemicellulose plasticization fits well with diffusion prevention in acetylated, DMDHEU and thermally modified wood, but plasticity in furfurylated wood may be increased. We also point out that the different mechanisms are not mutually exclusive, and it may be the case that they all play some role to varying degrees for each modification. Furthermore, we highlight recent work which shows that brown rot fungi will eventually degrade modified wood materials, even at high treatment levels. The herein reviewed literature suggests that the modification itself may initially be degraded, followed by an increase in wood cell wall MC to a level where chemical transport is possible.

Fabrication and Evaluation of Bio-Based Nanocomposite TFC Hollow Fiber Membranes for Enhanced CO2 Capture

Nanocellulose is a promising and sustainable biobased nanomaterial because of its excellent mechanical properties, biocompatibility, natural abundance, and especially its high aspect ratio. Interest in applying nanocellulose as nanofillers in membrane fabrication has been growing rapidly in recent years. In the present work, nanocellulose crystals (CNCs) and nanocellulose fibers (CNFs) were incorporated into polyvinyl alcohol (PVA) to prepare evenly dispersed nanocomposites. The resultant nanocomposite materials containing up to 80 wt % of nanocellulose were coated as defect-free, thin-film-composite selective layers onto hollow fiber membrane substrates via dip-coating for efficient CO₂ capture. Thermogravimetric analysis, Fourier-transform infrared, X-ray diffraction, scanning transmission electron microscopy, scanning electron microscopy, and humid mixed gas permeation test were used to evaluate the nanocomposite materials and the membranes. The resultant PVA/CNC nanocomposite membranes exhibit both higher CO₂ permeance and CO₂/N₂ selectivity compared to the PVA/CNF membranes and neat PVA membranes. The addition of CNCs showed more positive effects on the CO₂ permeation compared to CNFs. Under optimized conditions, CO₂ permeance of 672 GPU with a CO₂/N₂ selectivity of 43.6 was obtained with a PVA/CNC membrane. Excellent long-term stability of the membrane was also documented within a period of up to 1 year. The interface between the polymer phase and charged CNFs is believed to form fast gas transport channels at the humid state and thus enhances CO₂ permeation.