Cellulose Nanofibril Films Research Articles

High‐Resolution Patterned Biobased Thin Films via Self‐Assembled Carbohydrate Block Copolymers and Nanocellulose

AbstractThe exploitation of the effortless self‐assembly behavior of biomass‐based bricks can be seen as a promising route toward the innovative architectures. Here, a straightforward approach is presented where carbohydrate‐based diblock copolymer, polystyrene‐block‐maltoheptaose (PS‐b‐MH), is organized either on a rigid ultrathin film or on a flexible self‐standing film of wood‐derived cellulose nanofibrils (CNFs). During solvent annealing PS‐b‐MH deposited on relatively rough CNF film undergoes spontaneous rearrangement into high‐resolution patterns with a diblock domain spacing of 10–15 nm. The ideal conditions the self‐assembly require weak interactions between block copolymer and the substrate to increase the chain mobility and enable rearrangements. This is exactly how the system behaves. Adsorption studies of PS‐b‐MH on CNF surfaces reveal weak interactions, and the formed PS‐b‐MH layer is soft and mobile. Even the appearance of more challenging vertical orientation formed on smooth CNF substrates is tentatively evidenced by grazing‐incidence small‐angle X‐ray scattering and atomic force microscope indicating favorable surface interactions between CNF and PS‐b‐MH.

Cellulose nanofibrils versus cellulose nanocrystals: Comparison of performance in flexible multilayer films for packaging applications

Abstract Cellulose nanomaterials (CNMs) are a unique type of nanomaterial that are produced via several routes including chemical and mechanical, including the most researched cellulose nanocrystals (CNCs) and cellulose nanofibrils (CNFs). CNM films exhibit excellent oxygen barrier properties in medium to low relative humidity conditions. The oxygen barrier characteristics are desirable for CNM film proposed use in food packaging applications where both performance and biodegradability are of concern. However, the oxygen barrier property of CNM films is reduced if films are exposed to high relative humidity (RH) because of moisture-induced plasticizing and swelling. In this research, CNM films were laminated with polypropylene (PP) film using a polyurethane (PU) adhesive tie layer to form flexible multilayer film packaging. The physical properties of the CNM films indicated that CNC films were denser (∼1.4 g/cm3) than CNF films (1.1–1.3 g/cm3). Casting weight affected the densities of the CNM films and this effect was material type dependent. Optical property evaluation showed that the CNC films were clearer than the CNF films. Laminating CNF films with PU improved the transparency of the CNF films. Mechanical test results showed that CNC and CNF laminates containing thicker CNM films had similar maximum tensile strength as the control PP/PU laminates. Laminating CNM films with PP and PU significantly improved the barrier properties of the CNM films. For example, the water vapor transmission rate of CNC film dropped from 516 to 1.0 g/(m2·day). The oxygen transmission rate of CNC film at 80 % RH decreased from 126 to 6.1 cm3/(m2·day).

High-performance flexible freestanding polypyrrole-coated CNF film electrodes for all-solid-state supercapacitors

Nowadays, supercapacitors based on cellulose nanofibril (CNF) films have attracted extensive interest due to their excellent flexibility, light weight, and unique structure. Herein, we report that highly conductive films are prepared through polypyrrole (PPy) coating on CNF films by a simple and low-cost “soak and polymerization” method. The optimized flexible film features a high electrical conductivity of 23.77 S cm−1, a superior tensile strength of 71.4 MPa, outstanding conductance stability, and a good thermal stability. Additionally, the hybrid film as a freestanding and binder-free supercapacitor electrode can provide a high areal capacitance (2.26 F cm−2 at 2 mA cm−2) and good cyclic stability (a capacitance retention of 70.5% after 5000 cycles). Remarkably, an all-solid-state flexible supercapacitor assembled by two pieces of the optimized PPy-coated CNF (PPy/CNF) film electrodes delivers an excellent areal capacitance of 1.39 F cm−2 at 0.1 mA cm−2, revealing an ultrahigh energy density of 16.95 mWh cm−3 at a power density of 1.2 mW cm−3.

Eco-Friendly Cellulose Nanofibrils Designed by Nature: Effects from Preserving Native State.

Cellulose nanofibrils (CNFs) show high modulus and strength and are already used in industrial applications. Mechanical properties of neat CNF films or CNF-polymer matrix nanocomposites are usually much better than for polymer matrix composite films reinforced by clay, graphene, graphene oxide, or carbon nanotubes. In order to obtain small CNF diameter and colloidal stability, chemical modification has so far been necessary, but this increases cost and reduces eco-friendly attributes. In this study, an unmodified holocellulose CNF (Holo-CNF) with small diameter is obtained from mildly peracetic acid delignified wood fibers. CNF is readily defibrillated by low-energy kitchen blender processing. The hemicellulose coating on individual fibrils in the wood plant cell wall is largely preserved in Holo-CNF. This "native" CNF shows well-preserved native fibril structure in terms of length (∼2.1 μm), diameter (<5 nm), high crystallinity, high cellulose molar mass, electronegative charge, and limited mechanical processing damage. The hemicellulose coating contributes mechanical properties and high optical transmittance for CNF nanopaper, which can otherwise only be achieved with chemically modified CNFs. The CNF nanopaper shows superior mechanical properties with a Young's modulus of 21 GPa and an ultimate strength of 320 MPa. Moreover, hemicellulose imparts recyclability from the dried state. Altogether, this native CNF represents a class of colloidally stable, eco-friendly, low-cost CNF of small diameter for large-scale applications of nanopaper and nanomaterials.

The influence of versatile thiol-norbornene modifications to cellulose nanofibers on rheology and film properties

Cellulose nanofibrils (CNF) can form impressive barrier layers but difficult rheological properties, brittleness, and sensitivity to moisture limit their use. To overcome these challenges, esterification reactions were performed in water without volatile organic solvents to create carbic-functionalized CNFs (cCNFs) that enabled versatile, thiol-norbornene secondary modifications. Chemical analysis determined that on average 5% anhydroglucose repeat units were functionalized with norbornene groups. Thiol-norbornene reactions added molecules with varying polar surface areas to the CNFs. Modifications did not change the film properties to a large extent. All CNF films were excellent grease barriers. The modifications significantly changed the rheology of CNF suspensions as the complex viscosity of the modified CNF was 27 times lower than unmodified CNFs. Modification also reduced the filtration rate by a factor of four. Surface modifications appeared to alter the colloidal forces between fibers in suspension that influence the flow and drainage properties.

Fabrication and electrochemical properties of flexible transparent supercapacitor electrode materials based on cellulose nanofibrils and reduced graphene oxide

AbstractIn this research, flexible transparent supercapacitor electrode materials were fabricated using cellulose nanofibrils (CNFs) and reduced graphene oxide (RGO) via a layer‐by‐layer (LbL) self‐assembly method. First, a transparent film was obtained by vacuum filtration of a CNF suspension, which was isolated from bamboo materials using a combination of 2,2,6,6‐tetramethylpiperidin‐1‐oxyl radical catalytic oxidation and ultrasonic treatment. Subsequently, graphene oxide (GO) was deposited on the surface of the CNF film using Cu2+ as a cross‐linking agent via the LbL self‐assembly technique and then was reduced by L‐ascorbic acid under mild reaction conditions. The degree of reduction of the GO on the CNF film surface was investigated by X‐ray photoelectron spectroscopy (XPS), Raman spectroscopy (Raman), and Fourier transform infrared spectroscopy (FT‐IR); concurrently, the effect of the number of self‐assembly times on the transparency, mechanical properties, and conductivity was also evaluated. The XPS, Raman, and FT‐IR spectral analyses proved that the CNFs/GO composite films were successfully reduced to CNFs/RGO composite films, of which the transparency and mechanical properties decreased with the increase in the number of self‐assembly times, while the conductivity remarkably raised. Based on the analysis of the results, the CNFs/RGO composite film obtained after 18 self‐assembly cycles exhibited excellent transparency, good tensile strength, and a high conductivity. Therefore, the CNFs/RGO composite film was selected to fabricate a supercapacitor electrode material, and the obtained supercapacitor displayed excellent electrochemical properties, in which the areal specific capacitance was 2.25 mF cm−2 at a current density of 0.01 mA cm−2 and the capacitance retention reached 97.3% after 1500 cycles. The presented strategy provides a good reference for the development of transparent and portable energy storage devices.

Hydrophobization, smoothing, and barrier improvements of cellulose nanofibril films by sol–gel coatings

Single-layer films from cellulose nanofibrils on a plastic support were coated with sol–gel coated with inorganic–organic copolymers (ORMOCER®s), consisting of inorganic Si–O–Si-based networks combined with ceramic (Al–O– and Zr–O–) groups and special organic fluoroalkyl chain containing functional groups. Sol–gel coatings decreased the surface hydrophilicity and water vapor transmission rate. The water contact angle of uncoated films was 24°, indicating high affinity between water and the cellulose nanofibrils. All sol–gel coatings tested increased the surface hydrophobicity with the contact angles ranging between 54° and 102°. The water vapor transmission rates varied between 230 and 410 g/m2/day. With UV curable highly organically crosslinked coating, the water vapor transmission rate was decreased by 77% as compared to uncoated film. The uncoated film had oxygen transmission rates of 0.7 and 107 cc/m2/day at 50% and 80% RH, respectively. At high humidity conditions, the films tended to swell, thus allowing permeation to increase. Sol–gel coatings significantly improved the oxygen barrier properties especially at 80% RH. The transmission rates varied between 0.4 and 0.5 cc/m2/day (50% RH) and between 51 and 86 cc/m2/day (80% RH).

Birefringence-based orientation mapping of cellulose nanofibrils in thin films

Determination of nanofibril orientation is crucial for predicting the properties of films and membranes made from cellulose nanofibrils (CNF) because of their inherent anisotropic nature. A novel method is proposed based on image analysis of the polarized light micrographs to quantify and map nanofibril orientation in the film structure. Thin films (average 30 µm in thickness) of CNF were produced using a filtration method and were wet-stretched to two extension levels. Randomly-oriented films were also produced as the control without applying stretch. Samples were imaged at − 45°, 0° and + 45° between crossed polarizers using a polarized light microscope. A BOI was developed based on the interference color changes between the two angles (+ 45° and − 45°). The proposed BOI values range between − 1 and + 1 differentiating orientation in perpendicular directions. The index was shown to work successfully for mapping of the fibril orientation in CNF films. Statistical analysis of the tensile test results confirmed significant difference between tensile modulus of CNF films produced using different stretch ratios. This difference was also supported by the good agreement between the tensile properties of the films, the BOI and directionality results obtained from the surface analysis of scanning electron micrographs. The method was validated by applying to single pulp fibers with known orientation as well as un-stretched and stretched polyvinyl chloride films and oriented cellulose nanocrystals. The advantages of the proposed method over other conventional methods used for orientation analysis are discussed.

Controllable Ag-rGO heterostructure for highly thermal conductivity in layer-by-layer nanocellulose hybrid films

Many researchers have attempted to address the problem of frequency reduction or overheating damage caused by inadequate thermal conductivity of substrates. Developing advanced, green, and practical thermal interface materials (TIMs) is one of the most promising solutions to date. Herein, a facile method is proposed to achieve fascinating thermal conductivity of TIMs via constructing two-dimension (2D) reduced graphene oxide (rGO) decorated with zero-dimension (0D) silver nanoparticles (AgNPs) in hybrid nanofibrillated cellulose (NFC) film. The surface morphology and chemistry of Ag-rGO-functionalized NFC (Ag-rGO/NFC) are significantly altered by adding a small amount of Ag-rGO nanosheets (9.6 wt%), resulting in remarkable improvement in the thermal conductivity. The in-plane thermal conductivity of Ag-rGO/NFC (27.55 W m−1 K−1) showed a massive enhancement of over 1095% compared to pure NFC (2.3 W m−1 K−1), accompanied by 573% increase in through-plane conductivity. These enhancements could be attributed to the robust heterostructural Ag-rGO networks and dense layer-by-layer (LBL) structures. It is likely that these controllable structures come into being due to the intense static adsorption between graphene oxide (GO) and Ag+ as well as the hydrogen bonding interaction between GO and NFC. It is worth noting that exceptionally fast heat transport is achieved for Ag-rGO/NFC in practical application, with values as high as 0.18 °C/s, in sharp contrast to 0.16 °C/s for pure NFC films in the time range of 80–220 s. This could be due to the excellent thermal conduction networks built by Ag-bridged rGO nanosheets. This work provides valuable insights into the fabrication of highly thermal conductive composites in the promising field of electronic equipment, integrating intelligent functionality and environmental compatibility.

Tannin-stabilized silver nanoparticles and citric acid added associated to cellulose nanofibrils: effect on film antimicrobial properties

The aim of this work was to evaluate the potential of cellulose nanofibrils (CNFs) films treated with commercial solution of Ag nanoparticles stabilized with natural tannin and silane groups in the bacteria growth inhibition. The films were produced with commercial pulps of pine and eucalyptus treated with sodium hydroxide (5% w/v) and calcium hydroxide (10% w/v), respectively. The bacteria selected for the inhibition test were the gram-positive Listeria monocytogenes and gram-negative Salmonella enteriditis. The films performance was investigated by the antimicrobial activity using disc diffusion. In addition, the pulps were evaluated by their chemical composition, water retention index (WRI) and X-ray diffraction (XRD). The content of hemicelluloses in the treated pine pulp (11.32%) was lower in comparison to the found in the treated eucalyptus pulp (14.46%). The XRD diffractogram presented higher crystallinity index for the treated eucalyptus pulp (70%) with a characteristic peak of calcium carbonate. The treated eucalyptus pulp showed higher WRI and viscosity values in relation to the treated pine pulp. Growth inhibitory halos were more expressive when the treatment with both antimicrobial solutions were proceeded in the films produced with pine CNFs with addition of the citric acid. The effect of the nanoparticles stabilized with tannin must be highlighted, mainly on the bacterium S. enteriditis.

Influence of chitosan addition on the mechanical and antibacterial properties of carrot cellulose nanofibre film

Films of carrot cellulose nanofibrils (CCNFs) with the addition of low-viscosity chitosan (CHIT) were prepared by the vacuum filtration. The chitosan content in the films varied from 9 to 33% (dry wt. basis). The surface morphology of the films was investigated by scanning electron microscopy, and it was found that chitosan was dispersed in the CCNF matrix. The interaction between CCNFs and CHIT was evaluated in terms of Fourier transform infrared spectroscopy (FTIR). The obtained results suggested physical interactions rather than hydrogen bonding between CCNFs and CHIT. This finding also supports the results of the water wettability experiment. The addition of chitosan to the nanocellulose matrix causes an increase in the water contact angle, i.e., the surface of the composites becomes more hydrophobic. This increase is probably connected to an interaction between nanocellulose and chitosan forming a denser structure. Analyses of thermal properties showed that the composites are stable under high temperature, and the degradation occurred above 300 °C. It was found that the addition of CHIT to CCNF matrices caused a decrease in the Young’s modulus—the higher that the concentration of chitosan in the composite was, the lower the Young’s modulus (decreased from 14.71 GPa for CCNFs to 8.76 GPa for CCNF/CHIT_5). Additionally, the tensile strength of composites, i.e., the maximum force that causes a fracture decreased after the addition of chitosan (decreased from 145.83 MPa for CCNFs to 129.43 MPa for CCNF/CHIT_5). The results indicated the highest inhibitory effect of the investigated composites against E. coli and S. epidermidis. Whereas M. luteus was inhibited only by the higher concentration of chitosan in the tested composites, inhibition was not found against C. krissii and all tested filamentous fungi.Graphic abstract

Cellulose Nanofibril (CNF) Films and Xylan from Hot Water Extracted Birch Kraft Pulps

The effects of xylan extraction from birch kraft pulp on the manufacture and properties of cellulose nanofibril (CNF) films were here investigated. Hot water extractions of bleached and unbleached kraft pulps were performed in a flow-through system to remove and recover the xylan. After the extraction, the pulps were oxidized with 2,2,6,6-tetramethylpiperidine-1-oxyl radical (TEMPO) and fibrillated in a high-pressure microfluidizer. Compared to CNF from bleached kraft pulp, the CNF dispersions obtained from water-extracted pulps were less viscous and generally contained a higher amount of microfiber fragments, although smaller in size. In all cases, however, smooth and highly transparent films were produced from the CNF dispersions after the addition of sorbitol as plasticizer. The CNF films made from water-extracted pulps showed a lower tensile strength and ductility, probably due to their lower xylan content, but the stiffness was only reduced by the presence of lignin. Interestingly, the CNF films from water-extracted bleached pulps were less hydrophilic, and their water vapour permeability was reduced up to 25%. Therefore, hot water extraction of bleached birch kraft pulp could be used to produce CNF films with improved barrier properties for food packaging, while obtaining a high-purity xylan stream for other high-value applications.

Dielectric response of hydrated water as a structural component of nanofibrillated cellulose (NFC) from different plant sources

The current work illuminates the interplay between nanofibrillated cellulose (NFC) films and hydrated water. The NFC films from three sources of technological importance, i.e. cotton, wood and flax, are compared. It is shown that cellulose materials present slight variations in supramolecular structure depending on the plant origin. The structural differences determine both quantity and state of the water adsorbed by cellulose. Dielectric spectroscopy was employed to study the state of hydrated water as a probe of both the overall and specific marks of NFCs’ structure. The measurements, carried out in the wide frequency (10−2Hz -106Hz) and temperature (123 K–293 K) ranges, revealed the formation of non-interactive water clusters at low water content. At high water content, additional states of water were identified: Water in saturated glass-forming solution and bulk. These water states were shown to be determined by the NFC’s structure and morphology.

Thermally-induced cellulose nanofibril films with near-complete ultraviolet-blocking and improved water resistance

In recent years, ultraviolet (UV) protection films made from cellulose nanofibril (CNF) have drawn significant attention, owing to its high transparency, high mechanical strength and relatively high thermostability. Generally, CNF films had poor UV-shielding performance, and required UV absorbent to enhance their UV-shielding ability. Herein, a simple thermal treatment is proposed to directly improve the UV blocking properties of CNF films without incorporating UV absorbent. After thermal treatment at 160 °C, the CNF films exhibited a near-complete UV blocking ability. In particular, they exhibited full absorption of ultraviolet A (UVA) and ultraviolet B (UVB), and also showed a high visible light transmittance of 72%. In addition, the UV-shielding films performed stable UV-blocking when exposed to UV irradiation. Simultaneously, the hornification induced by thermal treatment endowed an improved hydrophobicity for CNF films. However, the tensile strength of the CNF films decreased from 133 MPa to 81 MPa after thermal treatment at 160 °C.

Ion‐Induced Assembly: Explaining the Exceptional Wet Integrity of Transparent Cellulose Nanofibril Films in the Presence of Multivalent Ions—Suitable Substrates for Biointerfaces (Adv. Mater. Interfaces 13/2019)

Ion‐Induced Assembly: Explaining the Exceptional Wet Integrity of Transparent Cellulose Nanofibril Films in the Presence of Multivalent Ions—Suitable Substrates for Biointerfaces (Adv. Mater. Interfaces 13/2019)

Antimicrobial activity of biocomposite films containing cellulose nanofibrils and ethyl lauroyl arginate

Food packaging is tailored to keep food fresh by increasing shelf life and preventing microbial deterioration. However, traditional food packaging is commonly made from non-degradable polymers without antimicrobial properties and that pose an environmental threat if not disposed properly. To address this issue, here we describe the preparation of cellulose nanofibril (CNF) films and hydrogels with antimicrobial activity against common foodborne pathogens such as verotoxigenic E. coli, L. monocytogenes and S. Typhimurium. Furthermore, two grades of negatively charged CNFs with different fibrillation degrees were modified with ethyl lauroyl arginate (LAE), which is an antimicrobial agent. CNF films were able to bind LAE molecules up to a maximum concentration of 145–160 ppm. LAE–CNF biocomposite films exerted a bactericidal activity against a major foodborne pathogen present in ready-to-eat food (L. monocytogenes) even at 1% LAE. Our work describes a novel biopolymer-based strategy that overcomes the current hurdles with LAE incorporation into packaging materials, offering a green and antimicrobial alternative for packaging of ready-to-eat (RTE) meat products.

Mechanical properties of cellulose nanofibril films: effects of crystallinity and its modification by treatment with liquid anhydrous ammonia

The influence of cellulose crystallinity on mechanical properties of cellulose nano-fibrils (CNF) was investigated. Degree of crystallinity (DoC) was modified using liquid anhydrous ammonia. Such treatment changes crystal allomorph from cellulose I to cellulose III, a change which was reversed by subsequent boiling in water. DoC was measured using solid state nuclear magnetic resonance (NMR). Crystalline index (CI) was also measured using wide angle X-ray scattering (WAXS). Cotton linters were used as the raw material. The cotton linter was ammonia treated prior to fibrillation. Reduced DoC is seen to associate with an increased yield point and decreased Young modulus. Young modulus is here defined as the maximal slope of the stress–strain curves. The association between DoC and Young modulus or DoC and yield point are both statistically significant. We cannot conclude there has been an effect on strainability. While mechanical properties were affected, we found no indication that ammonia treatment affected degree of fibrillation. CNF was also studied in air and liquid using atomic force microscopy (AFM). Swelling of the nanofibers was observed, with a mean diameter increase of 48.9%.

Flexible electrically conductive films based on nanofibrillated cellulose and polythiophene prepared via oxidative polymerization

Industrial ecology, sustainable manufacturing, and green chemistry have been considered platform‐based approaches to the reduction of the environmental footprint. Recently, nanofibrillated cellulose (NFC) has gained significant interest due to its mechanical properties, biodegradability, and availability. These outstanding properties of NFC have encouraged the development of a more sustainable substrate for electronics. In this context, the combination of NFC and conductive polymers may create a new class of biocomposites to be used in place of conventional electronics which are not optimally designed for use in flexible and mechanically robust devices. In this study, polythiophene was grafted onto nanocellulose surface at appropriate reaction times to obtain a strong, flexible, foldable films with capacity for electrical conductivity. Nanocomposites films were synthesized by a one-step reaction in which a 3-methyl thiophene monomer was oxidatively polymerized onto nanocellulose backbone. The nature of the fabricated NFC films changed from insulator to semiconductor material upon oxidative polymerization.

Facile preparation of high dielectric flexible films based on titanium dioxide and cellulose nanofibrils

A series of high dielectric composite films based on low-cost and eco-friendly titanium dioxide (TiO2) and cellulose nanofibril (CNF) was prepared under a facile condition. The relative dielectric constants (er) and dielectric loss ( $$ \tan\updelta $$ ) were studied as the function of frequency and filler content. The er of CNF/TiO2 composite film was 19.51 (at 1 kHz) with a relatively low dielectric loss. Compared with pure CNF films (er = 6.92 at 1 kHz), the er of the composite film was improved about three times with the dielectric loss increased slightly. The effects of TiO2 addition and hot-press treatment on microstructure, thermal stability, and dynamic mechanical properties of the composite films were also analyzed. It was found that the addition of TiO2 particles reduces the cellulose–cellulose bonding so generates more pores in the films, which has significant impacts on both dielectric and physical strength properties.

Explaining the Exceptional Wet Integrity of Transparent Cellulose Nanofibril Films in the Presence of Multivalent Ions—Suitable Substrates for Biointerfaces

AbstractCellulose nanofibrils (CNFs) assemble into water‐resilient materials in the presence of multivalent counter‐ions. The essential mechanisms behind these assemblies are ion–ion correlation and specific ion effects. A network model shows that the interfibril attraction indirectly influences the wet modulus by a fourth power relationship to the solidity of the network (Ew ∝ φ4). Ions that induce both ion–ion correlation and specific ion effects significantly reduce the swelling of the films, and due to the nonlinear relationship dramatically increase the wet modulus. Herein, this network model is used to explain the elastoplastic behavior of wet films of 2,2,6,6‐tetramethylpiperidine‐1‐oxyl radical (TEMPO)‐oxidized, carboxymethylated, and phosphorylated CNFs in the presence of different counter‐ions. The main findings are that the aspect ratio of the CNFs influences the ductility of the assemblies, that the bivalency of phosphorylate ligands probably limits the formation of interfibril complexes with divalent ions, and that a higher charge density increases the friction between fibrils by increasing the short‐range attraction from ion–ion correlation and specific ion effects. These findings can be used to rationally design CNF materials for a variety of applications where wet strength, ductility, and transparency are important, such as biomaterials or substrates for bioelectronics.