Nanofibril Films Research Articles

Hydrophobization and smoothing of cellulose nanofibril films by cellulose ester coatings

The Cellulose nanofibrils (CNF), also referred to as nanocellulose, is one of the most studied bio-based material in recent year, which has good potential in the future for packaging applications due to its excellent mechanical strength and oxygen barrier properties. In the future, CNF films may also find new applications for example in printed electronics, if the surface smoothness of CNF films can be improved. One way to improve surface smoothness is to use thin coating solutions with zero porosity, such as molar mass controlled cellulose ester coatings. In this study, we have coated CNF films using molar mass controlled cellulose esters with different side chain lengths forming 3-layer film (ester-CNF-ester). These coatings improved significantly the smoothness of CNF films. The 3-layer films have also good water vapor barrier and mechanical properties and the films are heat-sealable, which enable various new applications in the future.

Improving the mechanical properties of CNF films by NMMO partial dissolution with hot calender activation

Reinforcing of cellulose nanofibril (CNF) films by partial dissolution with N-methylmorpholine-N-oxide (NMMO) was investigated. The method investigated is composed of impregnation of CNF film with liquid solution of NMMO followed by dry heat activation. The heat activation of the impregnated film was carried out using a heated calendering nip, which enabled simultaneous heating and compression. The partial dissolution of cellulose by NMMO caused a significant increase in the transparency of CNF film due to the decrease of film porosity and increased surface smoothness. The dry strength of the reinforced film was increased from 122 up to 195 MPa. Furthermore, the wet strength of the reinforced film was up to 70% greater than the dry strength of pure CNF film. The changes in the fibrillar structure were investigated with topographical imaging (SEM and AFM) and spectroscopically using NMR and FTIR. No significant changes in the fibril structure or cellulose morphology were observed. Moreover, the treated film resisted significant water pressure, highlighting CNF film’s permanent water resistance. The partial dissolution process with NMMO was also capable of reinforcing a CNF composite film with macro scale structural elements (lyocell short-cut fibres). The strategy investigated is a robust and fast method to improve the mechanical properties of fibrillary cellulose films, allowing them utilization in applications where improved water resistance and fully cellulosic character are required properties.

Cellulose Nanofibrils from Nonderivatizing Urea-Based Deep Eutectic Solvent Pretreatments.

Deep eutectic solvents (DESs) are a fairly new class of green solvents applied in various fields. This study investigates urea-based DES systems as novel pretreatments for cellulose nanofibril production. In the experiments, deep eutectic systems having urea and ammonium thiocyanate or guanidine hydrochloride as a second component were formed at 100 °C and then applied to disintegrate wood-derived cellulose fibers. The DES-pretreated fibers were nanofibrillated into three different levels of mechanical treatments with a microfluidizer, and their properties were analyzed. Moreover, nanofibril films were fabricated by solvent casting method. Both DES systems were able to loosen and swell the cellulose fiber structure as indicated by the increase in the lateral dimension of the fibers. Nonpretreated birch cellulose fibers had difficulties in mechanical nanofibrillation as clogging of the chamber occurred often. However, cellulose nanofibrils with widths ranging from 13.0 to 19.3 nm were successfully fabricated from DES-pretreated fibers with both systems. Translucent nanofibril films generated from DES-pretreated cellulose nanofibrils had good thermal stability and mechanical properties, with tensile strengths of approximately 135-189 MPa and elastic modulus of 6.4-7.7 GPa. Consequently, both urea-based DESs showed a high potential as environmentally friendly solvents in the manufacture of cellulose nanofibrils.

High wet-strength, thermally stable and transparent TEMPO-oxidized cellulose nanofibril film via cross-linking with poly-amide epichlorohydrin resin

TEMPO-oxidized cellulose nanofibrils (TOCNs) films cross-linked with different dosages of polyamide epichlorohydrin resin (PAE) show a great water-resistance and thermal stability.

Influence of cellulose viscosity and residual lignin on water absorption of nanofibril films

The quality of cellulose nanofibril (CNF) films is affected by processing and raw material related factors. The causes of potential excessive cellulose degradation during defibrillation that may directly affect the quality of CNF films may be assessed by cellulose viscosity, which need further investigations. This study aims to investigate the influence of cellulose viscosity and residual lignin of four different hardwood species (Eucalyptus grandis - eucalyptus; Brosimum parinarioides - amapá; Parkia gigantocarpa - faveira; and Cordia goeldiana - freijó ) on water absorption of the subsequent CNF films. Viscosity of the unbleached and bleached pulps was measured by capillary viscosimetry after alkali and bleaching pre-treatments of the raw wood sawdust. Residual lignin was also determined for the bleached pulps. The films were obtained by casting method after pulp nanofibrillation through 10, 20, 30 and 40 passages in a mechanical defibrillator. The amount of water absorbed per film area (g/m2) after water immersion of the CNF films for 60 s was determined and normalized by the thickness. Overall, CNF films obtained from pulps with residual lignin and higher viscosity, and hence lower degradation by the alkali and bleaching pre-treatments, showed lower water absorption. The lowest water absorption of B. parinarioides CNF films may be highlighted. For pulps with lower viscosity, water absorption continuously increased with the increase of passages through the defibrillator, possibly due to combined degradation of pre-treatments and nanofibrillation. This tendency was observed from 10 to 40 passages for eucalyptus CNF films and from 10 to 30 passages for P. gigantocarpa CNF films.

Hydrophobization of cellophane and cellulose nanofibrils films by supercritical state carbon dioxide impregnation with walnut oil

Hydrophobization of cellophane and cellulose nanofibrils films by supercritical state carbon dioxide impregnation with walnut oil

Dual Mechanism of Dry Strength Improvement of Cellulose Nanofibril Films by Polyamide-epichlorohydrin Resin Cross-Linking

The chemical and physical properties of cellulose nanofibril (CNF) films cross-linked with wet strength cross-linker polyamide-epichlorohydrin resin (PAE) were investigated. In addition to the expected wet strength improvement, a unique stress–strain behavior for dry cross-linked films was observed. This stress–strain behavior indicates that varying stress-bearing mechanisms arise from the formation of new bonds caused by cross-linking at different levels of cross-linker addition. Cross-linked films showed an approximate 150% increase in dry strength, when compared to non-cross-linked CNF films. The cross-linked films showed a transition of both surface and bulk properties from hydrophilic to hydrophobic. The water contact angle increased from 50° to 110°, while the water retention value decreased by approximately 25% for the cross-linked films. Moreover, water vapor permeability also showed a 2-fold decrease. Possible mechanisms for the changes in behavior are discussed in detail.

Cellulose Nanofibril Film as a Piezoelectric Sensor Material.

Self-standing films (45 μm thick) of native cellulose nanofibrils (CNFs) were synthesized and characterized for their piezoelectric response. The surface and the microstructure of the films were evaluated with image-based analysis and scanning electron microscopy (SEM). The measured dielectric properties of the films at 1 kHz and 9.97 GHz indicated a relative permittivity of 3.47 and 3.38 and loss tangent tanδ of 0.011 and 0.071, respectively. The films were used as functional sensing layers in piezoelectric sensors with corresponding sensitivities of 4.7-6.4 pC/N in ambient conditions. This piezoelectric response is expected to increase remarkably upon film polarization resulting from the alignment of the cellulose crystalline regions in the film. The CNF sensor characteristics were compared with those of polyvinylidene fluoride (PVDF) as reference piezoelectric polymer. Overall, the results suggest that CNF is a suitable precursor material for disposable piezoelectric sensors, actuators, or energy generators with potential applications in the fields of electronics, sensors, and biomedical diagnostics.

Understanding the Dispersion and Assembly of Bacterial Cellulose in Organic Solvents

The constituent nanofibrils of bacterial cellulose are of interest to many researchers because of their purity and excellent mechanical properties. Mechanisms to disrupt the network structure of bacterial cellulose (BC) to isolate bacterial cellulose nanofibrils (BCN) are limited. This work focuses on liquid-phase dispersions of BCN in a range of organic solvents. It builds on work to disperse similarly intractable nanomaterials, such as single-walled carbon nanotubes, where optimum dispersion is seen for solvents whose surface energies are close to the surface energy of the nanomaterial; bacterial cellulose is shown to disperse in a similar fashion. Inverse gas chromatography was used to determine the surface energy of bacterial cellulose, under relevant conditions, by quantifying the surface heterogeneity of the material as a function of coverage. Films of pure BCN were prepared from dispersions in a range of solvents; the extent of BCN exfoliation is shown to have a strong effect on the mechanical properties of BC films and to fit models based on the volumetric density of nanofibril junctions. Such control offers new routes to producing robust cellulose films of bacterial cellulose nanofibrils.

Single-crystal Au microflakes modulated by amino acids and their sensing and catalytic properties

Single-crystal Au microflakes with the planar area over 103μm2 (i.e. being accessible to the human eye resolution) were synthesized in an environment-friendly route by directing two-dimensional growth of Au nanocrystals into macroscopic scales with amino acids as both reducing agents and capping agents. Side groups of amino acids were found to be a determinant parameter to tune the dimension and size of Au single crystals. The successful synthesis of Au microflakes provides an unprecedented opportunity to bridge nanotechnology and macroscopic devices, and hereby to start a new scenario of exploring their unique properties and applications in optoelectronic devices and bio-sensing fields across multiple length scales. For example, Au microflakes respond to air humidity upon depositing on films of chitin nanofibrils, and sense various physiological molecules as electrode materials of biosensors.

Water-resistant and high oxygen-barrier nanocellulose films with interfibrillar cross-linkages formed through multivalent metal ions

Once-dried 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibril (TOCN) films with sodium carboxylate groups (TOCN–COONa) were soaked in aqueous MgCl2, CaCl2, AlCl3 and FeCl3 solutions to change the counterion to TOCNs–COOM films (M: metal ion). Dry TOCN–COOM films had high Young's moduli (11–20GPa) and tensile strength (170–280MPa). In particular, the wet TOCN films with aluminum and iron (III) carboxylates had high Young's moduli and tensile strengths of ~3GPa and 30–40MPa, respectively, even at water contents of ~470%. Moreover, the dry TOCN films with calcium and aluminum carboxylates had extremely low oxygen permeabilities of 0.08 and 0.15mLµmm−2day−1kPa−1, respectively, even at 80% relative humidity, which are outstanding values compared to those of other films reported previously. These results are explained in terms of the high water resistance of the films, which is caused by the formation of interfibrillar cross-linkages through multivalent metal ions.

Effect of high residual lignin on the thermal stability of nanofibrils and its enhanced mechanical performance in aqueous environments

In this study, the effect of high residual lignin (21 % w/w) on the thermal properties of cellulose nanofibrils and the performance of films made from these nanofibrils in aqueous environments have been explored for the first time. Individualised cellulose nanofibrils with diameter <100 nm were obtained from the mechanical fibrillation of bark residue fibers with high lignin content. The mass loss by thermal degradation started at a higher temperature of 306 °C for these nanofibrils compared to 278 °C for those fibrils with low amount of lignin (5 % w/w). The maximum rate of degradation occurred at a temperature of around 390 and 319 °C for high and low lignin containing nanofibrils, respectively. Such a high thermal stability for high lignin containing nanofibrils has never been reported for nanocellulose from any other studies. The films made from these high lignin nanofibrils showed lower water uptake and better wet mechanical properties compared to films made from low lignin containing cellulose nanofibrils. The high lignin nanofibril films retained 38 % of the dry strength properties, while the low lignin nanofibril films were able to retain only 9 % of dry strength.

Nanocellulose-Zeolite Composite Films for Odor Elimination.

Free standing and strong odor-removing composite films of cellulose nanofibrils (CNF) with a high content of nanoporous zeolite adsorbents have been colloidally processed. Thermogravimetric desorption analysis (TGA) and infrared spectroscopy combined with computational simulations showed that commercially available silicalite-1 and ZSM-5 have a high affinity and uptake of volatile odors like ethanethiol and propanethiol, also in the presence of water. The simulations showed that propanethiol has a higher affinity, up to 16%, to the two zeolites compared with ethanethiol. Highly flexible and strong free-standing zeolite-CNF films with an adsorbent loading of 89 w/w% have been produced by Ca-induced gelation and vacuum filtration. The CNF-network controls the strength of the composite films and 100 μm thick zeolite-CNF films with a CNF content of less than 10 vol % displayed a tensile strength approaching 10 MPa. Headspace solid phase microextraction (SPME) coupled to gas chromatography-mass spectroscopy (GC/MS) analysis showed that the CNF-zeolite films can eliminate the volatile thiol-based odors to concentrations below the detection ability of the human olfactory system. Odor removing zeolite-cellulose nanofibril films could enable improved transport and storage of fruits and vegetables rich in odors, for example, onion and the tasty but foul-smelling South-East Asian Durian fruit.

Physical and Mechanical Properties of Cellulose Nanofibril Films from Bleached Eucalyptus Pulp by Endoglucanase Treatment and Microfluidization

A GH5 hyperthermostable endoglucanase (Ph-GH5) from the archaeon Pyrococcus horikoshii and a commercial endoglucanase (FR) were used to treat bleached eucalyptus pulp (BEP) fibers to produce cellulose nanofibrils (CNF) and subsequently to CNF films. TEM imaging indicated that Ph-GH5 produced longer and more entangled CNF than FR with the same number of microfluidization passes. Physical and mechanical properties of CNF films were characterized. Optical opacity of CNF films from FR (10 mg/g) at 40 passes through the microfluidizer can be as low as 3.7 %, compared with 18.2 % from untreated BEP at the same number of passes. CNF films exhibited similar thermal stability with untreated BEP. Highest specific modulus of CNF films was also obtained from FR (10 mg/g), reaching 56 MNm/kg, approximately 271 % of the CNF films from untreated BEP at 40 passes through the microfluidizer. CNF film from Ph-GH5 (1 mg/g) at 40 passes provided the highest specific maximum tensile strength at 120 kNm/kg.

Mechanism of TEMPO-oxidized cellulose nanofibril film reinforcement with poly(acrylamide)

2,2,6,6-Tetramethylpiperidine-1-oxyl (TEMPO)-oxidized cellulose nanofibrils (TOCNs) and poly(acrylamide) (PAM) with sodium carboxylate groups (TOCN-COONa and PAM-COONa, respectively) were converted to TOCN and PAM with protonated carboxyl groups (TOCN-COOH and PAM-COOH, respectively). Transparent and flexible PAM-COOH/TOCN-COOH, PAM-COONa/TOCN-COOH, and PAM-COOH/TOCN-COONa composite films were prepared by mixing aqueous PAM-COOH or PAM-COONa solutions and aqueous TOCN-COOH or TOCN-COONa dispersions with various PAM/TOCN weight ratios, and successive casting/drying of the mixtures. In all cases, the Young’s modulus and tensile strength of the composite film were highest when the PAM content of the composite film was in the range 10–25 %. PAM therefore has the potential to reinforce TOCN films, irrespective of the sodium carboxylate groups or protonated carboxyl groups of TOCN or PAM. Although the FT-IR spectra of the PAM-COOH/TOCN-COOH composite films with various PAM/TOCN weight ratios indicated the formation of hydrogen bonds between PAM-COOH and TOCN-COOH, the formation of these hydrogen bonds could not explain the reinforcing effect achieved by PAM addition to TOCN-COOH. Because the TOCN elements form nematic-ordered or self-aligned domain structures in aqueous dispersions, PAM molecules at the corresponding PAM contents are probably present around the boundary regions of TOCN domains, and fill or cover the defects present between the TOCN domains.

Effect of Graphene Oxide Treatment on the Properties of Cellulose Nanofibril Films Made of Banana Petiole Fibers

In this study, banana petiole-based cellulose nanofibril (CNF) films treated with graphene oxide (GO) were prepared and evaluated by means of Fourier-transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), dynamic mechanical analysis (DMA), and thermogravimetric analysis (TGA). Tensile strengths (TS), dynamic mechanical properties, and thermal stabilities of the films were affected positively when the GO loading was less than 4.4 wt%. From these results, FTIR spectra, and SEM analyses, a strong coupling between the GO and the cellulose matrix could be concluded at lower GO loadings. The TGA and DMA results also suggested that the CNF film treated with 4.4 wt% GO had more char residue, better thermal stability, higher storage modulus, and higher retention ratio when compared to that without treatment. This work provides a new approach for more effective utilization of banana petiole as a feedstock for CNF and GO/CNF composites.

Effect of interfibrillar PVA bridging on water stability and mechanical properties of TEMPO/NaClO2 oxidized cellulosic nanofibril films

TEMPO/NaClO2 oxidized cellulosic nanofibrils (TCNF) were covalently bonded with poly(vinyl alcohol) (PVA) to render water stable films. Pure TCNF films and TCNF–PVA films in dry state showed similar humidity dependent behavior in the elastic region. However, in wet films PVA had a significant effect on stability and mechanical characteristics of the films. When soaked in water, pure TCNF films exhibited strong swelling behavior and poor wet strength, whereas covalently bridged TCNF–PVA composite films remained intact and could easily be handled even after 24h of soaking. Wet tensile strength of the films was considerably enhanced with only 10wt% PVA addition. At 25% PVA concentration wet tensile strengths were decreased and films were more yielding. This behavior is attributed to the ability of PVA to reinforce and plasticize TCNF-based films. The developed approach is a simple and straightforward method to produce TCNF films that are stable in wet conditions.

Substitution of nanoclay in high gas barrier films of cellulose nanofibrils with cellulose nanocrystals and thermal treatment

The aim of this study is to design a nanocellulose based barrier film. For this purpose, cellulose nanofibrils (CNFs) are used as a matrix to create an entangled nanoporous network that is filled with two different nanofillers: nanoclay (reference), i.e. the mineral montmorillonite (MMT) and the bio-based TEMPO-oxidized cellulose nanocrystal (CNC-T), to produce different types of nanocelluloses and their main physical and chemical features were assessed. As expected, films based on neat CNFs exhibit good mechanical performance and excellent barrier properties at low moisture content. The introduction of 32.5 wt% of either nanofiller results in a significant improvement of barrier properties at high moisture content. Finally, thermal treatment of a dried CNF/CNC-T film results in a decrease of the oxygen permeability even at high moisture content (>70 %). This is mainly attributed to the hornification of nanocellulose. A key result of this study is that the oxygen permeability of an all-nanocellulose film in 85 % relative humidity (RH), is similar to CNF film with mineral nanoclay (MMT), i.e. 2.1 instead of 1.7 cm3 µm m−2 day−1 kPa−1, respectively.

Self-assembled optically transparent cellulose nanofibril films: effect of nanofibril morphology and drying procedure

Cellulose nanofibril (CNF) films currently provide great opportunity in many applications with advantages of excellent mechanical strength, high light transmittance, and good barrier properties. However, processes for preparing CNFs are typically tedious and vary, along with their properties. Here, five preparation methods using various combinations of filtration, freeze-drying, and casting are applied to produce CNF films, and their major properties are compared. Three different types of CNFs having a range of fiber diameter and aspect ratio were examined using each of these five preparation methods. Because of limited hydrogen bonds and nanofibril arrangement, the freeze-dried CNF films displayed reduced mechanical strength and light transmittance compared to the other methods, although freeze-drying was relatively fast. Some effects of film production methods on measured crystallinity were also observed with freeze-dried samples having lower crystallinity than films similarly produced by filtration and drying. Free-standing CNF films produced by casting at room temperature required long times and mold growth was sometimes observed, but cast films made from 2,2,6,6-tetramethylpiperidine-1-oxyl radical-oxidized CNFs had the highest light transmittance of any samples. Filtration of CNF suspensions followed by air- or oven-drying produced films with minimal defects, high mechanical strength, and good light transmittance with relatively little effort. Therefore, this filtration procedure is recommended for producing CNF films.

Ag nanoparticle-immobilized cellulose nanofibril films for environmental conservation

A facile method was developed to prepare biodegradable nanocomposite films. Ag nanoparticles stabilized by an NH2-terminated fourth generation poly(amido amine) dendrimer (DENAgNPs) were covalently immobilized on 2,2,6,6-tetramethylpiperidine-1-oxyl radical-oxidized nanofibrillated cellulose (NFC) by using a condensing agent for amide bond formation between NFC and DENAgNPs. Subsequently, the films with different concentrations of DENAgNPs were prepared by filtrating DENAgNPs-NFC suspensions on filter membranes, and the dried films were characterized. As-prepared films possessed the high catalytic efficiency to decoloration of Rhodamine B in water and the excellent antibacterial performance against both Gram-positive and Gram-negative bacteria on agar plates. Thus, this novel concept of film-type nanocomposite reactor demonstrates that it is possible to prepare inexpensive catalytic and bactericidal-active films with good selectivity by easy prospective approach for realizing the environmental conservation and innovation.