Strong and Flexible Nanocomposites of Carboxylated Cellulose

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Strong and Flexible Nanocomposites of Carboxylated Cellulose Nanofibril Dispersed by Industrial Lignin Yingxin Liu* Department of Materials and Environmental Chemistry, Stockholm University, 10691 Stockholm, Sweden Wallenberg Wood Science Center, KTH, 10044 Stockholm, Sweden S Supporting Information *

ABSTRACT: We demonstrated that industrial lignin can be facilely processed with carboxylated cellulose nanofibril (CNF) to obtain strong, flexible, and transparent nanocomposites via film casting of dispersions. The tensile strength and strain to failure of lignin−CNF nanocomposites (245 MPa and 15%, respectively at 7.7 wt % of lignin) are superior to previously reported polymer/nanoparticle−CNF composites with polymer contents below 50 wt %, such as poly(vinyl alcohol)− CNF films and even reduced graphene oxide−CNF films. The excellent mechanical properties of lignin−CNF nanocomposite films are related to the lignin-enhanced colloidal stability and dispersity of CNF in aqueous dispersions supported by measurements of rheology and dynamic light scattering, which accordingly suppresses the excess fibril aggregates during film formation. Moreover, lignin in the nanocomposites benefits an efficient functionalization of gold/iron oxide nanoparticles on the surface of nanocomposites. This study illustrates the great potential of industrial lignin in developing nanocellulose-based materials with advanced properties and functionalities. KEYWORDS: nanocellulose, lignin, high strength, flexibility, multifunctionalities

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INTRODUCTION Nowadays, driven by the raising sustainable and environmental concerns, the attention of materials science is shifting from petroleum-based resources to the utilization of biorenewable alternatives, such as protein-based biopolymers (e.g., gelatin, collagen, and silk fibroin) and lignocellulose biomass (e.g., cellulose, lignin, and hemicellulose).1−4 Cellulose nanofibril (CNF) is a versatile bionanomaterial with a low density (1.6 g cm−3), low coefficient of thermal expansion, and ultrahigh specific strength and stiffness (∼3000 kN·m kg−1 and ∼50 MN·m kg−1 respectively, both higher than that of commodity plastics, metals, and alloys).5−7 CNF nanopapers show a high mechanical strength, optical transmittance, and gas/vapor barrier.6,8 Coassembly of CNF with other materials, e.g., metal oxide nanoparticles,9 graphene,10 carbon nanotube,11 silicates (clays), and polymer/surfactant, has generated a variety of multifunctional nanocellulose-based materials,12 which have been applied in energy and electronic devices, water treatment, and biomedicine.5,13 Due to the hydrophilic character, CNF-based films are usually developed by controlled drying, e.g., casting and filtration of aqueous dispersions, where excess aggregation and flocculation need to be prevented to achieve a better mechanical performance, ́ et al. showed e.g., requiring a high colloidal stability.6,14 Benitez that CNF films produced from dispersions of carboxylated (TEMPO-oxidized) at pH = 9 displayed a tensile strength and Young’s modulus that were 33.5% and 36.4% higher, respectively, compared to films prepared from dispersions of © XXXX American Chemical Society

pH = 7, through strengthening the electrostatic repulsion among CNF.14 Lignin is the second most abundant biopolymer after cellulose.15,16 As the byproduct of pulping, paper-making, and ethanol industry, the global annual production of lignin reaches ∼50 million tons, 98% of which is burnt or discarded as wastes.17 Recently, great efforts have been made to apply industrial lignin in developing high-value products, e.g., energy devices,18 carbon fiber,19 UV screening, and antibacterial materials;20,21 however, the complex molecular structure of lignin and the versatility dependent on the origin and isolation process have dramatically restricted the expansion of lignin utilization. Inspired by wood cell walls, where cellulose nanofibrils are laminated with lignin and hemicellulose,22,23 lignocellulose nanofibril (LCNF, cellulose nanofibril with residual lignin nanoparticles on the fibrils surface) attracts a growing research interest. Amphiphilic LCNF was found to more efficiently stabilize styrene emulsions in water compared to carboxylated CNF.24 Wolfgang et al. reported a 30% increase of tensile strength of the LCNF (residual lignin content 19.2 wt %)− polystyrene (PS) films compared to CNF−PS films.25 In addition, the residual lignin (5−14 wt %) was beneficial for improving the thermal stability and reducing the oxygen permeability of Received: January 25, 2018 Revised: February 26, 2018 Published: February 28, 2018 A

DOI: 10.1021/acssuschemeng.8b00402 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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ACS Sustainable Chemistry & Engineering

strength and strain to failure of the nanocomposites were found significantly higher than the previously reported polymer/ nanoparticle−CNF composites. Rheology and dynamic light scattering (DLS) measurements showed that lignin improved the colloidal stability and dispersion of carboxylated CNF, which possibly accounts for the high mechanical performance of lignin−CNF nanocomposites.

Table 1. Composition of CNF and Lignin−CNF Dispersions sample 1 2 3 4 5 6 7

CNF dispersion (10 g)

lignin dispersion (10 g)

CNF concentration in composite dispersions (wt %)

mass ratio of lignin and CNF (w/w)

0.6 0.6 0.6 0.6 0.6 0.6 0.6

water 0.01 wt % 0.02 wt % 0.05 wt % 0.1 wt % 0.15 wt % 0.2 wt %

0.3 0.3 0.3 0.3 0.3 0.3 0.3

0/100 1.6/98.4 3.2/96.8 7.7/92.3 14.3/85.7 20/80 25/75

wt wt wt wt wt wt wt

% % % % % % %

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EXPERIMENTAL SECTION

Materials. Never dried sulfite softwood pulp (Domsjö dissolving pulp, 60/40 spruce/pine, Sweden), lignosulfonates (Domsjö Lignin DS10, Sweden, sulfur content ∼8.5 wt %), kraft lignin (Sigma-Aldrich, sulfur content 1−3 wt %), TEMPO (2,2,6,6-tetramethyl-1-piperidinyloxy free radical, ≥98%, Alfa Aesar), sodium hypochlorite (Alfa Aesar), sodium bromide (BioUltra, ≥99.5%, Sigma-Aldrich), sodium hydroxide (≥99.2%, VWR Chemicals), FeCl2·4H2O, FeCl3·6H2O, chloroauric acid, and hydrogen chloride (37%, Sigma-Aldrich) were purchased. Carboxylated CNF and Lignin Aqueous Dispersions. The carboxylated CNF dispersion (1.0 wt %) was fabricated by TEMPOmediated oxidation and mechanical homogenization of the pulp according to previous literature.31 The carboxylation degree (carboxyl groups per gram of pulp) was characterized by a conductometric titration. Kraft lignin (4 wt %) aqueous dispersion was adjusted to pH = 11 using sodium hydroxide (NaOH) solution (0.5 mol L−1) and

films.26−29 However, it is still technically difficult to develop lignocellulose particles with a high degree of nanofibrillation and controllable residual lignin content,30 which blocks an elucidation of the effect of lignin on the physiochemical properties e.g., mechanical and thermal-stable properties of LCNFbased materials. Accordingly, how to achieve a synergetic integration of CNF and lignin in one material system remains a challenge. In this work, we have prepared lignin−CNF nanocomposite films with an optical transparency by casting and drying of homogeneous mixtures of lignin and CNF dispersions. The tensile

Figure 1. Rheological properties of lignin−CNF dispersions with an increasing mass fraction of lignin from 0 to 25 wt % in lignin−CNF nanocomposites (constant 0.3 wt % CNF in final dispersions) at 25 °C and pH = 7. (a) Comparison of G′ between KL−CNF dispersions and LS−CNF dispersions at a sweeping strain of 5% and frequency of 10 rad s−1. (b) Comparison of viscosity between KL−CNF dispersions and LS−CNF dispersions at a shear rate of 10 s−1. (c) Damping factor (G″/G′) variation of KL−CNF dispersions with a strain sweep from 0.1% to 200%. (d) Damping factor variation of LS−CNF dispersions with a strain sweep from 0.1% to 200%. B


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ACS Sustainable Chemistry & Engineering mixed for 30 min, while lignosulfonates (4 wt %) were directly dissolved in water. Both dispersions were dialyzed (molecular weight cut off ∼6000, Spectrumlabs) with running deionized water for 5 days, followed by centrifugation (6000 rpm for 20 min, Hettich EBA 21) to remove the sediments. The solid content of supernatants was characterized by gravimetry with a relative error of 2% and prepared into to 1 wt % dispersions (pH = 7). Lignin−CNF Nanocomposites. Lignin dispersions (10 g) with varying concentrations from 0.01 wt % to 0.2 wt % were added into carboxylated CNF dispersions (10 g, 0.6 wt %) to get composite dispersions with lignin contents (wt %) of 1.6, 3.2, 7.7, 14.3, 20, and 25 in lignin−CNF nanocomposites, separately in Table 1, while the CNF solid content was 0.3 wt % in final dispersions. Composite dispersions were homogenized by Ultra-Turrax (IKA T25) and degassed by vacuum. Nanocomposite films were prepared by casting the composite dispersions into round polypropylene Petri dishes (diameter 55 mm) and drying in a humidity chamber (CLIMACELL 111, MMM Medcenter) at 30 °C and 50% relative humidity. Functionalization of Lignin−CNF Nanocomposites. Dry lignin− CNF nanocomposites were first immersed in an aqueous solution of FeCl2 and FeCl3 (20 mmol L−1 and 40 mmol L−1, respectively) for 10 min and then rinsed with deionized water and transferred into ammonia solution (20%), while the whole process was protected with nitrogen. For the reduction of gold nanoparticles, CNF films and lignin−CNF nanocomposites were immersed in 3 mmol L−1 chloroauric acid solution for 10 min at room temperature. Characterization. The molecular weight and distribution of lignin were determined by size exclusion chromatography with SEC 1260 Infinity (Polymer Standard Services, Germany) using DMSO−LiBr (0.5 wt %) as a solvent. The dimension of CNF was characterized by atomic force microscopy (tapping mode, Veeco Multimode V, USA) and analysis of images of approximately 300 fibrils using NanoScope Analysis and ImageJ. Dynamic light scattering was performed with a Zetasizer Nano ZS (Malvern Instruments Ltd., U.K.), equipped with a 530 nm laser and backscatter detector (at an angle of 173°) at pH = 7 and 25 °C. Rheological measurements were performed at 25 °C using a Physica MCR 301 rheometer (Anton Paar) equipped with a smooth cone-on-plate geometry (CP25-2-SN7617, diameter 25 mm, 2° nominal angle and gap height 0.5 mm). The optical

transparency of nanocomposites was characterized by UV/vis/NIR (Lambda 19, PerkinElmer). Thermal gravimetric analysis was measured using thermogravimetric analyzer (Discovery TG, TA Instruments), under flowing nitrogen of 20 mL min−1 with a heating rate of 10 °C min−1. Nanocomposite thickness and cross-sectional structure were characterized after sputter-coating of gold by scanning electron microscope (JSM-7401F, JEOL, Peabody, MA) with an acceleration voltage of 1 kV, a beam current of 15 μA, and a working distance of 10 mm. The porosity, ε, of nanocomposites was calculated by the following equations: ε=1−

ρtheory =

ρsample ρtheory

(1)

1 ω lignin ρ lignin

+

ωCNF ρCNF

(2)

where ρsample is the experimental density of nanocomposites, ρlignin the density of lignin (1.3 g cm−3),26 ρCNF the density of CNF (1.6 g cm−3),5 and ω the mass fraction. The tensile tests of the nanocomposites were carried out with a Universal Materials Testing Machine (Instron, Norwood, MA, a 100 N load cell). Rectangular specimens of 20 × 3 mm were measured with a span length of 10 mm at a strain rate of 1 mm min−1, 50% relative humidity, and 25 °C. X-ray diffraction was collected on a PANalytical X’Pert Pro using Cu Kα1 radiation (45 kV, 40 mA), irradiated length 10 mm, mask fixed 10 mm, and a step size of 0.13°.

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RESULTS AND DISCUSSION According to the isolation process, industrial lignin can be categorized into a sulfur group [kraft lignin (KL) and lignosulfonates (LS)] and sulfur-free group (soda lignin and organosolv lignin),16 among which KL and LS are the most commercially produced. Considering the essence of an aqueous solubility for processing with carboxylated CNF, KL and LS were chosen. The solubility of KL in water results from the dissociation of phenol hydroxyls (pKa ≈ 9.5) by adjusting pH = 11 of the

Figure 2. Morphology and structure of the lignin−CNF nanocomposites. (a) Digital photo of the KL−CNF (left) and LS−CNF (right) (w/w = 14.3/85.7) nanocomposites. The inset figure illustrates the flexibility of KL−CNF (w/w = 14.3/85.7) nanocomposites. (b) SEM image of the cross-section and AFM height images of surface for CNF films. (c) SEM image of cross-section and AFM height images of surface for KL−CNF (w/w = 14.3/85.7) nanocomposites. (d) SEM image of the cross-section and AFM height images of surface for LS−CNF (w/w = 14.3/85.7) nanocomposites. C


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layered architecture of the CNF assembly is not affected by the introduction of lignin. No detectable aggregates of lignin nanoparticles or phase-separation were observed in the fibrous matrix (atomic force microscopy (AFM) images in Figure 2), suggesting a high homogeneity of lignin−CNF nanocomposites. In previous studies, the residual lignin on the CNF surface was reported to lead to a complex thermal degrading behavior of films. Nair et al. found that increasing the residual lignin contents from 5.2 wt % to 21.3 wt %, resulted in an increase of

dispersions, while LS could be directly dissolved. Figure S1 shows the molecular weight (Mw) and the corresponding polydispersity index (PDI) of both KL and LS determined by size exclusion chromatography (SEC), 8.7 kDa with PDI of 2.4 and 11.7 kDa with PDI of 1.8, respectively. Macromolecular lignin fragments show a complex aggregate/assembly behavior in water and usually form colloidal particles and even fractal clusters.32 The dissolved KL displayed a hydrodynamic size of 4.8 ± 1.7 nm, correlating well with previous reports (3−7 nm),33 while LS showed 18.2 ± 1.9 nm at 0.01 wt % and 25 °C characterized by DLS in Figure S2. Both KL and LS nanoparticles were negatively charged with zeta potential of −26.9 ± 0.9 mV and −41.8 ± 02.7 mV, respectively, in a neutral condition. Carboxylated CNF (a charge density of ∼1000 μmol g−1) was prepared through TEMPO-oxidation and mechanical homogenization of wood pulp, showing a well-defined fibril diameter of 2.3 ± 0.6 nm and average aspect ratio of ∼250 (Figure S3a).34 On the basis of the lignin contents (25−35 wt %) in dehydrated wood,35 we have prepared lignin−CNF dispersions (pH = 7) with the lignin mass fraction increasing from 1.6% to 25% (constant 0.3 wt % CNF in final mixtures) by mixing lignin and CNF dispersions (see Table 1 for details), which had a longterm stability with no setting and aggregation (Figure S3b). Figure 1 and Figure S4 show the steady-shear and viscoelastic behavior of lignin−CNF dispersions with increasing lignin mass fraction from 0 to 25 wt %. The shear thinning behavior of CNF dispersions was not influenced by the introduction of KL or LS. Both the storage modulus (G′) and steady-shear viscosity just showed a slight increase after adding lignin in Figure 1a,b. In comparison, an addition of 7 wt % anionic surfactant (sodium lauroyl sarcosinate) into carboxylated CNF dispersions resulted in a significant increase of hydrogel stiffness from 50 Pa to ∼300 Pa due to the micelle-bridged effect.36 Therefore, anionic lignin nanoparticles are assumed to have a relatively weak association with carboxylated CNF, which is related to the electrostatically repulsive interaction. The correlation of the increase in viscous dissipation versus the increase in elasticity of lignin− CNF dispersions is reflected by a decreasing damping factor (tan δ = G′′/G′) with lignin contents increasing in Figure 1c,d. The network of LS−CNF dispersions is relatively more “elastic” than KL−CNF dispersions at the same solid contents, probably because of the higher molecular weight and ionic strength of LS compared with KL. Self-standing lignin−CNF nanocomposite films were obtained by casting and drying of the composite dispersions at a controlled humidity (50% relative humidity, RH) and temperature (30 °C). Figure 2a exemplifies a high optical transparency (detailed data within the visible light wavelength in Figure S5) and flexibility of the lignin−CNF nanocomposites. Table S1 summarizes that the thickness of lignin−CNF nanocomposites increased from 6.2 to 8.4 μm determined by scanning electron microscopy (SEM) as the lignin content increased from 1.6 wt % to 25 wt %; however, the average densities of both KL−CNF and LS−CNF nanocomposites represented an unsystematic dependence on the lignin contents, constant around 1.25 g cm−3 with an oscillation below 5%. The film porosity gradually decreased from 21% of the neat CNF films to 13% and 11% of KL−CNF and LS−CNF nanocomposites, respectively, at 25 wt % of lignin, indicating that lignin nanoparticles are homogeneously filled in voids of the CNF matrix.26 Pristine lignin films could not be obtained because of extensive cracking during drying. The SEM images of the cross-section of lignin−CNF nanocomposites in Figure 2 and Figure S6 demonstrate that the

Figure 3. Mechanical properties of lignin−CNF nanocomposites with the lignin (KL and LS) contents increasing from 0 to 25 wt %. (a) Tensile strength at rupture. (b) Young’s modulus. (c) Strain to failure. All the measurements were performed at 50% RH, 25 °C, and a strain rate of 1 mm min−1. D


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Figure 4. Comparison of mechanical properties of lignin−CNF nanocomposites with previously reported polymer/nanoparticle−CNF/lignin composites with polymer/nanoparticle contents below 50 wt %. PVA, PBS, EG-DMAm, and HEC refer to poly(vinyl alcohol), polybutylene succinate, ethylene glycol methyl ether methacrylate and N,N-dimethylacrylamide copolymer, and hydroxyethyl cellulose, separately.

strength larger than that of the neat CNF films. The stiffness (Young’s modulus) of the lignin−CNF nanocomposites had a similar improvement with the increase of lignin contents until 14.3 wt %, and KL−CNF (w/w = 7.7/92.3) nanocomposites displayed the maximum value of 8.6 GPa, which is comparable with nacre-mimicking montmorillonite (MTM)−polymer composites, e.g., MTM−chitosan films (w/w = 65/35) of 6.8 GPa and MTM−CNF films (w/w = 50/50) of 8.7 GPa, respectively.12,37 More importantly, the improvement of strength and stiffness of nanocomposites by adding lignin is not at an expense of reducing toughness, e.g., keeping a high strain to failure of ∼15%. In comparison, the addition of lignin usually reduced the ductility of lignin−polymer composites because of the brittle nature and poor compatibility of lignin with hydrophobic polymers.18,38,39 Figure 4 summarizes the comparison of tensile strength and strain to failure of lignin−CNF nanocomposites with other polymer/nanoparticle−CNF/lignin composites. Lignin−CNF nanocomposites represented a combination of higher tensile strength and larger strain to failure than previously reported lignin/CNF-based composites (polymer contents below 50 wt %), e.g., starch−CNF films,40 polyacrylamide (PAM)−CNF, and PVA−CNF films,40,41 and specifically even surpass the reduced graphene oxide (RGO)−CNF films and RGO−PVA films.42,43 The decrease of porosity of nanocomposites after adding lignin accompanied by an increase of strength and stiffness indicates a more homogeneous structure of films, e.g., more ́ et al. reported that interfibrillar network junctions.6,14 Benitez by improving the pH of carboxylated CNF dispersions from 7 to 9, the CNF films also showed a remarkable mechanical reinforcement with no decrease of strain to failure.14 We assume that lignin nanoparticles have a colloidal stabilizing effect on carboxylated CNF, possibly suppressing the excess aggregate and flocculation of fibrils during drying. Due to the experimental

Tonset (temperature where mass loss becomes apparent) and Tmax (temperature at the fastest degradation rate) from 261 °C to 306 °C, and 324 °C to 390 °C, respectively.27 Thomas et al. reported that residual lignin on LCNF (no particular value offered) could reduce the Tonset of 30 °C compared to CNF.28 However, the effect of isolated lignin on the thermal stability of CNF has not been evaluated. Figure S7 shows the thermal degradation of lignin−CNF. Tonset of lignin−CNF nanocomposites is dependent on both categories and contents of lignin, which increased from 224 °C to 235 °C with LS contents increasing from 0 to 25 wt %, while KL−CNF nanocomposites almost leveled off at 228 °C. Tmax derived from the differential thermogravimetry (DTG) curves represents a negligible change for both KL−CNF and LS−CNF nanocomposites, which is related to the high nanofibrillation degree of carboxylated CNF and the relatively low Mw of industrial lignin in our case (compared to the original lignin in wood cells). The mechanical properties of LCNF films (13.5 wt % residual lignin) were reported to be weakened compared to neat CNF films,26 where the tensile strength, Young’s modulus, and strain to failure decreased from 164 MPa, 14.3 GPa, and 2.9% to 116 MPa, 12.2 GPa, and 1.7%, respectively. However, how the isolated lignin affects the mechanical performance of nanocellulose-based materials is far from being elucidated. The tensile measurements in Figure 3 show that the mechanical properties of lignin−CNF nanocomposites were remarkably influenced by the lignin contents. CNF films represented a tensile strength (120 MPa) and Young’s modulus (2.9 GPa) within the range of previous reports.14 The tensile strength of lignin−CNF nanocomposites (w/w = 7.7/92.3) was almost twice (245 MPa) higher than that of CNF films and then leveled off around 180 MPa with a further addition of lignin. Within the investigated lignin range (from 1.6 wt % to 25 wt %), both LS−CNF and KL−CNF nanocomposites showed a tensile E


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Figure 5. DLS measurements of CNF and lignin−CNF dispersions. ICF curves of CNF dispersions (a), KL−CNF (w/w = 7.7/92.3) dispersions (b), and LS−CNF (w/w = 7.7/92.3) dispersions (c) with an increasing concentration of CNF from 0.05 wt % to 0.5 wt %. (d) The summary of the average ICF intercepts at zero decay time.

strain to failure of nanocomposites. Indeed, fractured lignin− CNF nanocomposites showed a more pronounced pullout phenomenon compared with CNF films in the SEM images of Figure 2 and Figure S6, possibly related to a weakened interfibrillar interaction. Additionally, the rupture and formation of hydrogen bonding between lignin and CNF during sliding can also be beneficial for energy dissipation.57 However, we acknowledge that firmly stating the cause of high toughness of lignin−CNF nanocomposites would require a more careful investigation. The abundant phenolic groups and sulfonate groups of lignin provide the reducing and trapping ability for metal ions, e.g., gold and iron ion, which furtherly widens the scope of lignin− CNF nanocomposites by surface functionalizations. In Figure 6a, lignin−CNF nanocomposites showed a significantly faster reduction of chloroauric acid molecules into gold nanoparticles compared to CNF films, which is confirmed by X-ray diffraction (XRD). Gold nanoparticles were homogeneously coated on KL−CNF nanocomposites within 10 min as determined by AFM, where no additional reducing agents, e.g., sodium borohydride and polyethylenimine, are involved. By mineralization of iron oxide nanoparticles (Figure 6d,e), the lignin−CNF nanocomposites could acquire magnetic response (Figure 6b). These functionalities of lignin−CNF nanocomposites may find applications in catalysis and as electronic actuators.58,59

difficulty of identifying and comparing the mesoscopic aggregates in dry films, we have investigated the colloidal stability of lignin−CNF nanocomposites by analysis of the intensity correlation function (ICF) in DLS measurements as displayed in Figure 5. In dispersed states, the mobility of nanoparticles is reflected in the decay rate of ICF, and the intercept of ICF curves at zero decay time will decrease with the increase of particle concentrations, which could be utilized for evaluating the colloidal stability of nanoparticles.53,54 For highly diluted dispersions of CNF at 0.05 wt %, lignin−CNF (w/w = 7.7/92.3) nanocomposites displayed a decay rate faster than that of CNF in ICF curves while the intercepts of ICF kept at ∼0.9, suggesting a higher mobility of CNF after adding lignin. Figure 5d shows that CNF dispersions had a significantly faster decreasing rate of ICF intercepts compared to lignin−CNF (w/w = 7.7/92.3) dispersions with an increase of CNF concentration from 0.05 wt % to 0.5 wt %, and it thus clearly proves that the aggregation (constrainment of mobility) of CNF is suppressed by lignin, e.g., a higher colloidal stability.53,54 KL−CNF nanocomposites showed a dispersity higher than that of LS−CNF nanocomposites with CNF concentration increasing, possibly leading to the larger stiffness in Figure 3. On the other hand, the lubricating effect of lignin nanoparticles in the CNF matrix, similar to the toughening mechanisms of polysaccharides/polymer micelles− CNF systems,48,49,55,56 is speculated to contribute to the large F


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a potential candidate in applications such as substrates for optoelectronics.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b00402. Additional materials characterization and thermal degradation analysis (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Yingxin Liu: 0000-0002-6859-6993 Notes

The author declares no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Wallenberg Wood Science Center. The author sincerely thanks Mr. Nicola Giummarella at KTH for the characterization of lignin molecular weight and Mr. Xuan Yang in KTH for the help on the tensile measurements. Prof. Shengjie Ling in ShanghaiTech University is acknowledged for valuable discussion. Prof. Lennart Bergström in Stockholm University is acknowledged for all the support and encouragement on this work.

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REFERENCES

(1) Faruk, O.; Bledzki, A. K.; Fink, H. P.; Sain, M. Biocomposites reinforced with natural fibers: 2000−2010. Prog. Polym. Sci. 2012, 37 (11), 1552−1596. (2) Fernandes, E. M.; Pires, R. A.; Mano, J. F.; Reis, R. L. Bionanocomposites from lignocellulosic resources: Properties, applications and future trends for their use in the biomedical field. Prog. Polym. Sci. 2013, 38 (10), 1415−1441. (3) Ojijo, V.; Sinha Ray, S. Processing strategies in bionanocomposites. Prog. Polym. Sci. 2013, 38 (10), 1543−1589. (4) Himmel, M. E.; Ding, S. Y.; Johnson, D. K.; Adney, W. S.; Nimlos, M. R.; Brady, J. W.; Foust, T. D. Biomass recalcitrance: Engineering plants and enzymes for biofuels production. Science 2007, 315 (5813), 804−807. (5) Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J. Cellulose nanomaterials review: Structure, properties and nanocomposites. Chem. Soc. Rev. 2011, 40 (7), 3941−3994. (6) Benítez, A. J.; Walther, A. Cellulose nanofibril nanopapers and bioinspired nanocomposites: A review to understand the mechanical property space. J. Mater. Chem. A 2017, 5 (31), 16003−16024. (7) Fujisawa, S.; Ikeuchi, T.; Takeuchi, M.; Saito, T.; Isogai, A. Superior reinforcement effect of TEMPO-oxidized cellulose nanofibrils in polystyrene matrix: Optical, thermal, and mechanical studies. Biomacromolecules 2012, 13 (7), 2188−2194. (8) Nogi, M.; Iwamoto, S.; Nakagaito, A. N.; Yano, H. Optically transparent nanofiber paper. Adv. Mater. 2009, 21 (16), 1595−1598. (9) Wicklein, B.; Diem, A. M.; Knöller, A.; Cavalcante, M. S.; Bergström, L.; Bill, J.; Burghard, Z. Dual-fiber approach toward flexible multifunctional hybrid materials. Adv. Funct. Mater. 2017, 1704274. (10) Wicklein, B.; Kocjan, A.; Salazar-Alvarez, G.; Carosio, F.; Camino, G.; Antonietti, M.; Bergström, L. Thermally insulating and fire-retardant lightweight anisotropic foams based on nanocellulose and graphene oxide. Nat. Nanotechnol. 2015, 10 (3), 277−283. (11) Koga, H.; Saito, T.; Kitaoka, T.; Nogi, M.; Suganuma, K.; Isogai, A. Transparent, conductive, and printable composites consisting of TEMPO-oxidized nanocellulose and carbon nanotube. Biomacromolecules 2013, 14 (4), 1160−1165.

Figure 6. Examples of functionalities of lignin−CNF nanocomposites. (a) Digital photo of CNF films and LS−CNF and KL−CNF nanocomposites with a lignin content of 14.3 wt % after soaking in chloroauric acid solution (3 mmol L−1) for 10 min at room temperature. (b) Magnetic responsiveness of KL−CNF nanocomposites functionalized with iron oxide nanoparticles. (c) AFM height, threedimensional image, and element mapping of gold on KL−CNF nanocomposites. (d) AFM height, three-dimensional image, and element mapping of iron on KL−CNF nanocomposites. (e) XRD of KL−CNF nanocomposites with in situ growth of gold and iron oxide nanoparticles.

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CONCLUSIONS In summary, anionic lignin nanoparticles from industrial lignin have been applied with carboxylated CNF to fabricate strong and flexible nanocomposites by casting and drying of mixtures of dispersions. Rheology and dynamic light scattering measurements show that lignin nanoparticles can improve the colloidal stability and mobility of CNF in dispersions due to electrostatic repulsion. Lignin nanoparticles are homogeneously distributed within the matrix of CNF. Lignin−CNF nanocomposites display an integrated combination of higher strength and larger strain to failure, compared to the lignin residual-CNF films and other polymer/nanoparticle−CNF composites. Lignin−CNF nanocomposites are able to be functionalized, e.g., a magnetic property with mineralization of iron oxide nanoparticles. The outstanding mechanical property, the low cost of constituents, and a relatively simple manufacturing method make this material G


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Strong and Flexible Nanocomposites of Carboxylated Cellulose

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