Toward Semistructural Cellulose Nanocomposites: The Need for

Perspective Cite This: Biomacromolecules 2018, 19, 2341−2350

pubs.acs.org/Biomac

Toward Semistructural Cellulose Nanocomposites: The Need for Scalable Processing and Interface Tailoring Farhan Ansari and Lars A. Berglund*

Downloaded via UNIV OF SUSSEX on August 6, 2018 at 11:33:47 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Fiber and Polymer Technology and Wallenberg Wood Science Centre, KTH Royal Institute of Technology, Stockholm SE-10044, Sweden

ABSTRACT: Cellulose nanocomposites can be considered for semistructural load-bearing applications where modulus and strength requirements exceed 10 GPa and 100 MPa, respectively. Such properties are higher than for most neat polymers but typical for molded short glass fiber composites. The research challenge for polymer matrix biocomposites is to develop processing concepts that allow high cellulose nanofibril (CNF) content, nanostructural control in the form of well-dispersed CNF, the use of suitable polymer matrices, as well as molecular scale interface tailoring to address moisture effects. From a practical point of view, the processing concept needs to be scalable so that large-scale industrial processing is feasible. The vast majority of cellulose nanocomposite studies elaborate on materials with low nanocellulose content. An important reason is the challenge to prevent CNF agglomeration at high CNF content. Research activities are therefore needed on concepts with the potential for rapid processing with controlled nanostructure, including well-dispersed fibrils at high CNF content so that favorable properties are obtained. This perspective discusses processing strategies, agglomeration problems, opportunities, and effects from interface tailoring. Specifically, preformed CNF mats can be used to design nanostructured biocomposites with high CNF content. Because very few composite materials combine functional and structural properties, CNF materials are an exception in this sense. The suggested processing concept could include functional components (inorganic clays, carbon nanotubes, magnetic nanoparticles, among others). In functional three-phase systems, CNF networks are combined with functional components (nanoparticles or fibril coatings) together with a ductile polymer matrix. Such materials can have functional properties (optical, magnetic, electric, etc.) in combination with mechanical performance, and the comparably low cost of nanocellulose may facilitate the use of large nanocomposite structures in industrial applications.

1. INTRODUCTION Recently, interest in materials from renewable resources, such as plants, has increased. These materials were, for obvious reasons, dominant in prehistoric and historic times, at least until the 1500s. Metal use then increased, as well as the use of concrete, and during the 20th century, and synthetic polymers reached a strong position. Replacement of synthetic polymers with more eco-friendly materials from renewable resources is desirable but also challenging in terms of the requirements for physical performance, moisture durability, and scalable processing methods. It is almost ironic that the performance of one of the most recent material developments, plant-based nanobiocomposites, relies on cellulose, one of the oldest material constituents on earth. Red algae have cellulose nanofibrils in the cell wall, and fossils date from 1.6 billion years ago.1 However, cellulose has also been found in the cell walls of the much older cyanobacteria,2 and its function to © 2018 American Chemical Society

mechanically stabilize cell walls of organisms has been critical to the development of life on earth. The cell wall of wood contains roughly 40% of cellulose in the form of “microfibrils”, which are long, fibrous nanofibrils of ∼4 nm in diameter. The mechanical properties of wood largely rely on the high crystal modulus (∼136 GPa) and strength (∼3 GPa) of these cellulose nanofibrils (CNF),3,4 which has been an active research field over the past decade.5−11 Recently, socalled microfibrillated cellulose,12 a coarser form of nanocellulose, has become available on a large industrial scale. Because of the specific challenges with small nanofibril components, both applied and more basic research progress Received: January 28, 2018 Revised: March 21, 2018 Published: March 26, 2018 2341

DOI: 10.1021/acs.biomac.8b00142 Biomacromolecules 2018, 19, 2341−2350

Perspective

Biomacromolecules

Figure 1. (a) Micrographs showing the long and flexible structure of cellulose nanofibrils (left) and surface image of a porous CNF network prepared by filtration (right). (b) Ashby plots comparing strength vs modulus (left) and strength vs strain to failure (right) for glass fiber composites (Chopped Strand Mat, CSM and Sheet Molding Compound, SMC),32−35 plant fiber/thermoset composites,17−19 and high volume fraction CNF/ thermoset composites.36−39 CNF/thermoset composites are comparable to industrial glass fiber composites in terms of modulus and have a significant advantage in terms of strength and strain to failure. Data are replotted from refs 17−19 and 34−39.

as reinforcement for a polymer matrix with a low glass transition temperature (Tg). Dynamic mechanical analysis showed strong reinforcement effects above the polymer Tg, which was attributed to a “percolating” network of interconnected CNC rods.20 Thereafter, several studies have investigated nanocellulose as a potential reinforcement for different thermoset and thermoplastic polymers.9,21 An overwhelming majority of these studies show improved modulus and strength with respect to the neat polymer. The modulus and strength are usually highest with a few weight % of nanocellulose and thereafter decrease with further addition of nanocellulose,22−25 presumably due to agglomeration of the nanocellulose reinforcement. The nanocellulose content at which the modulus/strength begin to decrease varies over a wide range (between 0.1 and 25 wt %),22−25 depending on the specific nanocellulose/polymer/solvent system. However, the often observed decrease in modulus with increasing volume fraction (Vf) is somewhat unexpected because this phenomenon is largely unknown in glass and carbon fiber composites. Perhaps the most likely explanation is very strong nanoreinforcement aggregation. The role of a low Vf “percolating” network in providing mechanical stability above the polymer Tg was highlighted in early studies on nanocellulose composites.20 However, semistructural applications require that materials are used well below their Tg, and possess high modulus and strength. This could be achieved by incorporating high contents of nanocellulose in composites with high modulus polymer matrices. One may note that water-soluble thermoplastic polymers can be efficiently reinforced with high volume fractions of nanocellulose, leading to substantial improvements in strength and stiffness.26−31 The main reason is that such polymers can be conveniently processed in a hydrocolloidal

is needed to support the development of nanocellulosic materials. CNFs are long slender fibrils obtained by mechanical disintegration of the wood cell wall.13−15 The structure is shown in Figure 1a. Another form of nanocellulose is the rodlike cellulose nanocrystals (CNCs) produced by acid hydrolysis of cellulose. CNCs will not be discussed in detail because CNF is likely to be more feasible for large-scale industrial applications in load-bearing biocomposites. One reason is that the lower aspect ratio and the acid hydrolysis preparation route mean that the reinforcement efficiency will be lower than for CNF (lower aspect ratio) and the cost will be higher (lower yield and acid handling challenges). Polymer matrix biocomposites with plant fibers are widely used.5,16 The reasons include high and tunable mechanical properties, improved moisture stability, biobased origin of the fiber phase, facile melt processing, and cost advantages. Typical mechanical properties of current polymer matrix biocomposites are Young’s modulus (E) = 5−7 GPa and tensile strength (σ*) = 40−70 MPa with brittleness and low strain to failure (∼2%), see Figure 1b.17−19 Replacing plant fibers with wood CNF is of interest, and this possibility is considered in the present perspective. In the context of engineering applications, CNF films, laminates, or inorganic CNF hybrids without a continuous polymer matrix are likely to suffer from intrinsic moisture sensitivity. The use of a continuous polymer matrix phase is crucial because this polymer transfers stresses between fibrils, reduces moisture sorption, allows for geometrical shaping processes, and provides opportunity for property tailoring. In the mid-1990s, Favier et al. prepared composites using very high aspect ratio (length/diameter) CNCs (from tunicate) 2342


Perspective

Biomacromolecules

Figure 2. Representative changes in in-plane random short fiber composite properties (strength and modulus) with increasing CNF volume fraction (Vf) as typically reported experimental data in literature (solid line) and as theoretically predicted (classical micromechanics) (dashed lines). Upper left figure in the inset shows experimental data from ref 24. Agglomeration of nanoparticles has been suggested as the primary reason for poor mechanical properties at high Vf. The inset sketches depict CNF dispersion in the composites at low Vf (lower left), at high Vf when agglomerated (lower right), and when well dispersed (upper right). Processing pathways to preserve CNF dispersion and obtain good mechanical properties at high Vf are discussed in later sections.

ments.40 Micrometer-scale aggregation and the use of low nanoparticle content have been suggested as the primary reasons.40,41 Several materials science and engineering problems remain to be solved, but cellulose has obvious advantages in terms of cost and eco-friendly attributes42 compared with many nanoscale reinforcements. Any materials and processing concept aiming for semistructural biocomposites based on nanocellulose needs to address the following challenges: •CNF agglomerates: If CNFs agglomerate and form concentrated microscale particles, the advantages of nanoscale fibrils are lost. As the material is subjected to mechanical load, microscale agglomerates serve as defects and will initiate failure at low strain so that ultimate strength is compromised. Agglomeration can also cause heterogeneous packing accompanied by entrapment of air voids, which is negative for optical and barrier properties. Effects from agglomeration are further aggravated due to the hydrophilic character of nanocellulose. The agglomerated regions have a high concentration of cellulose-cellulose interfaces, which are rich in hydroxyl groups and act as sorption sites for moisture.43,44 This will reduce mechanical and physical properties even further. •Moisture stability: Many industrial applications of cellulosic materials require moisture stability. This is to realize the full mechanical property potential and to limit dimensional changes, and therefore, the adsorption of moisture needs to be minimized. Because moisture sorption is dominated by surface hydration of cellulose fibrils, the interface between cellulose and the polymer matrix is critical, as will be discussed. For agglomeration and interface problems to be addressed, several surface modification strategies (based on covalent bonds, electrostatic interactions, or polymer physical sorption)

CNF suspension, and therefore, aggregation effects are not as severe. However, such composites are likely to have limited practical application due to their high moisture sensitivity. 1.1. Preliminary Analysis. The engineering science challenge is to develop nanocellulose biocomposites as semistructural load-bearing materials for use in a society committed to sustainable development. Needs include higher mechanical performance and better moisture stability than for current plant fiber composites. Preliminary analysis shows that properties of biocomposites with high volume fraction (Vf) of CNF can indeed compete with molded, industrially used glass fiber and plant fiber/thermoset composites. The comparison is based on random-in-the-plane fiber orientation distribution.17,18,32−39 CNF/thermoset biocomposites show higher strength and comparable modulus with glass fiber composites (Figure 1b) for semistructural applications (automotive, marine, and general industry). Note that currently used “plant fiber/thermoset” composites using the ∼30 μm diameter fibers, have substantially lower strength than CNF nanocomposites. Another important advantage becomes apparent in a plot of strength vs failure strain, where CNF/thermoset composites not only show higher strength but also preserve matrix strain to failure and show better ductility than existing biocomposites and glass fiber composites, see Figure 1(b). 1.2. Nanostructure and Processing: Challenges. However, mixing and processing of CNF in polymer biocomposites is not straightforward due to the nanoscale size of this reinforcement. Interestingly, despite high initial hopes, large-scale structural nanocomposites for load-bearing structures appear not to have been industrially realized with carbon nanotubes (CNTs), clay or related nanoscale reinforce2343


Perspective

Biomacromolecules

Figure 3. Sketch of filtration-based processing to obtain nanostructured biocomposites with high CNF volume fraction. The starting CNF/water suspension is filtered and subjected to solvent exchange and impregnation steps to obtain the prepreg. The dried composite prepregs can either be stored for later processing or stacked and molded into thicker and more complex shapes via compression molding. As a demonstration, 3−5 layers of such prepregs with ≈40 wt% CNF were successfully molded into a protective smartphone casing.70 Top left image shows the porous structure of a dried CNF mat; the scale bar is 200 nm.

are suggested.45−49 These strategies capitalize on the reactive CNF surface hydroxyl groups (or their ionic derivatives), so that the surface becomes relatively hydrophobic after the modification. Different aspects of nanocellulose surface modification have been detailed in extensive reviews.6,8,50 In biocomposites studies, the strength at a given volume fraction of modified nanocellulose may be higher than for composites with native nanocellulose,51,52 and the maximum strength is typically obtained at a higher volume fraction (Vf). This indicates that the dispersion is relatively better for surfacemodified nanocellulose, but agglomeration problems can still not be avoided above a certain nanocellulose content.22,25,52 The intrinsic nanocellulose moisture stability also increases as shown by reduced moisture sorption53 and increased tensile stiffness of the wet nanocellulose network.54 Strategies based on ionic cross-linking55 and polymer sorption28,43,56,57 have also been reported to affect the dispersion and moisture stability. •Processing concepts need to be scalable: The control of nanostructure during processing, especially with high nanocellulose content, is a limiting factor for engineering biocomposites. Solvent casting is well suited for fundamental studies, but solvent handling and slow drying are major obstacles that hinder industrial processing based on this concept. Melt processing using thermoplastic polymers is directly scalable and is investigated by Yano and co-workers.58−60 This approach does not usually provide sufficient mechanical properties for semistructural applications because so far the CNF content is low due to high melt viscosity arising from physical CNF entanglement effects. The concept proposed in the present perspective instead utilizes thermoset precursors and shaping methods such as compression molding. The strategy to mix CNF with liquid thermoset precursors, and cast or mold a composite from the liquid mixture, has so far met with limited success. Typical data are 3−5 GPa in modulus and ∼50 MPa in strength.9 One reason for this is that most

studies focus on low volume fraction (Vf) of CNF, and if higher Vf is attempted, agglomeration takes place and the mechanical properties are reduced, see Figure 2. This is not only negative from an application perspective, but also the real potential of polymer matrix CNF nanocomposites becomes unclear. Moreover, relative contributions from different factors such as agglomeration, intrinsic CNF properties, and so forth cannot be separated. Important research questions are related to the molecular interactions at the CNF−polymer interface, modifications to the bulk thermoset polymer properties due to the presence of CNF as well as the constrained geometrical nanostructural conditions, and the relative distributions of the polymer matrix and CNFs in a high Vf nanocomposite. The illustrated decrease in properties at high Vf in Figure 2 is, unfortunately, widely supported by experimental data in the literature. The class of biocomposites considered in Figure 2 is short fiber composites (high fiber aspect ratio) with in-plane random fiber distribution, and predictive micromechanics models for stiffness properties are available in standard texts.61 A major change in processing strategy, instead of incremental improvements, is probably required to realize the potential of nanocellulose biocomposites. A prepreg-based approach using thermoset polymers is of interest. This approach is based on the preparation of a porous nanopaper structure of high specific surface area. Porous cellulose nanopaper is in essence a nanoporous CNF mat analogous to a random-in-the-plane glass or carbon fiber mat, which can be impregnated by thermoset polymer precursors. This “prepreg” can be further processed, shaped, and cured to form a CNF/thermoset polymer nanocomposite material.

2. PROCESSING CONCEPT FOR NANOSTRUCTURAL CONTROL The suggested processing strategy has practical resemblance to conventional papermaking (Figure 3). It is based on 2344


Perspective

Biomacromolecules

porous mats. The concept can be used to prepare samples large enough for qualified studies of nanostructural effects on mechanical and hygrothermal performance of engineering biocomposites and is considered for industrial processing of polymer matrix biocomposites of controlled nanostructure.

impregnation of a preformed CNF network and can be used to prepare composites with high nanocellulose content. The starting material/precursor is a CNF network obtained by filtration of a dilute hydrocolloidal CNF suspension.62,63 The filtered CNF/water mat is self-supporting and easy to handle. For epoxies or unsaturated polyester resins, it is subjected to solvent exchange by placement in a bath containing the solvent. The choice of solvent is primarily guided by the solubility of the monomers, but typical organic solvents such as acetone are common and also used in existing industrial thermoset-based processes. The CNF/solvent template is then impregnated in a solution of resin monomers or prepolymers followed by drying and curing. Impregnation by liquid thermoset precursors for epoxy, polyester, or vinylester resins is in general not possible due to the nanoscale porosity of the “nanopaper” and lack of sufficient resin-cellulose compatibility. This strategy results in fairly homogeneous impregnation, which is facilitated by the rapid diffusion of epoxy or unsaturated polyester precursors into the CNF network. Low viscosity of the impregnating resin liquid solution, together with the porosity (pore size ≈ 10−100 nm)64 of the CNF mat, ensures a fast process in which homogeneous impregnation can be achieved without vacuum. Resin content in the final composite can be tuned by varying the concentration of the solution. Thus, the same starting material (CNF fiber mat) is used to prepare composites with different fiber contents. Because this fiber mat is formed by controlled filtration from water, the composite nanostructure is not strongly dependent on CNF/solvent interactions (unlike in solvent cast systems). Composites with very high CNF content and retained nanostructure can thus be produced.37−39,65,66 The starting fiber mat is prepared from a hydrocolloidal suspension where CNF is well-dispersed in water, and the CNF network is formed during filtration. It is critical to start with a well-dispersed nanofiber suspension because the degree of dispersion has a strong influence on the composite nanostructure. The rate of water removal during filtration can be increased by applying pressure.67 Once the fiber mat is prepared, CNF agglomeration can still occur during solvent exchange, evaporation,68 and resin impregnation. A sharp change in the chemical and thermodynamic characteristics of the solvent used may cause the network structure to collapse. To avoid this, stepwise solvent exchange has advantages,69 where the CNF/water template is first placed in a mixture of solvent/water and eventually in pure solvent. 2.1. Potential. The proposed methodology is related to conventional papermaking and has potential as a continuous process (Figure 3). The processing strategy is compatible with well-established molding processes for conventional micrometer-scale fiber/thermoset composites. Another technical prospect is that prepregs can easily be laminated after drying and molded to obtain complex shapes of desired thickness.70,71 Additionally, a simplified approach could involve the use of a dried porous CNF mat as the starting material for impregnation (structure shown in Figure 3).64,72 This may minimize costs for transportation of CNF because CNF is typically isolated and stored as a gel of ∼2−10% in water. The drying method is important and determines the nanostructure of CNF nanopaper. It needs to preserve the high specific surface area of the mat, which is possible by supercritical carbon dioxide evaporation. Even sophisticated drying methods may cause some fibril agglomeration, but the concept is still worthwhile because there are handling advantages associated with dried

3. EFFECTS OF NANOSTRUCTURE There are several studies in the literature in which the prepreg approach has been used. In the work by Yano and co-workers, CNF biocomposites prepared by “prepreg-based” processing showed high optical transparency and in-plane thermal conductivity with acrylics, phenol formaldehyde, or epoxy.66,73,74 They reported favorable physical properties even though the fiber mat was dried after filtration and then impregnated. No ductility data are presented, possibly because the polymers are brittle. Henriksson et al. used solvent exchange (instead of drying) and reported high mechanical properties with melamine formaldehyde and hyperbranched polyesters.37,65 The processing was later extended to unsaturated polyester and epoxy composites with tailored interfaces for reduced moisture sensitivity.38,39,75 Impregnation with thermoplastic polymers has also been studied,76,77 but effects from CNF network pore size and polymer molecular weight are likely to be critical for impregnation with long polymer chains. 3.1. Mechanical Properties. In contrast to many other nanocomposite systems, CNF-thermoset biocomposites from prepreg show improved modulus and strength with increasing CNF content, even up to 60−80% of CNF (Figure 4a).36−39 This indicates that the “prepreg” processing approach allows for high nanocellulose content without significant CNF agglomeration. This is in contrast to numerous studies on solvent-cast CNF composites and can be attributed to the use of a preformed nanostructured CNF mat as the starting material. Moreover, the composites can also have high ductility (strain to failure > 6%) due to the synergy between the CNF network and the ductile thermoset matrix.38,39 An important distinction must be made between the networks discussed here and low Vf “percolating network” widely discussed in the literature. Percolation of CNF is typically achieved at a very low nanocellulose content and implies the first formation of a continuous network of physically connected nanocellulose throughout the composite. The current network structure consists of high Vf (15−80 vol %) of physically entangled, “non-woven” fibrils where fibrils are also connected through strong secondary interactions.63 The mechanical property data are an achievement in the context of semistructural nanocomposites. A closer analysis of modulus using micromechanics models to back-calculate the CNF reinforcement efficiency shows that the effective contribution from CNF decreases as the CNF content increases.38,39 This means that further optimization of the processing stage using specific resin/solvent systems may lead to further improvements in properties. Moreover, if a CNF network with oriented fibrils78−80 is used for impregnation, the modulus and strength of the composites will be much higher than for the currently reported composites with random inplane orientation of CNF. Ductility will decrease for more oriented biocomposites. The in-plane random orientation of CNF fibrils is a practically interesting case, for instance, as molded biocomposite components for transport applications. In terms of property predictions, conventional micromechanics models for short fiber composites have not been very successful, probably because of the strong effects from the 2345


Perspective

Biomacromolecules

Figure 4. Mechanical properties of nanostructured thermoset biocomposites (with high CNF volume fraction) prepared by the prepreg approach. (a) Increasing strength and modulus with increasing CNF content indicate that the CNF was well-dispersed in the composites even at high Vf. The data are replotted from refs 36−39. (b) Mechanical properties at RH 50% and 90% for three different CNF/thermoset systems. At similar CNF volume fraction, the interface was tailored so that the secondary interactions between CNF and unsaturated polyester (UP) were changed to covalent interactions between CNF and epoxy (EP), improving moisture stability (sketch at the bottom). For the favorable CNF/EP system, increasing the CNF content from 15 to 30 vol % increases CNF-CNF agglomeration so that there is an increase in weakly bonded and moisturesensitive CNF/CNF interfaces (sketch on the right). Bar chart data are replotted from refs 38 and 39.

The mechanical properties at low moisture content (ambient conditions, relative humidity (RH) 50%) are relatively independent of interface interactions as nanocomposites based on epoxy and unsaturated polyester (UP) matrices are compared.39 The high intrinsic mechanical performance of the CNF mat is helpful for the UP composite. In addition, the high specific interface area may allow for strong secondary interactions between the UP matrix and the CNF so that mechanical performance is high under ambient conditions. Because the high specific area of the CNF is largely preserved after filtration from water, the prepreg concept can also be used as an independent surface modification strategy to modify the individual CNF surface. In this case, the reactive units are impregnated into the preformed CNF mat; the reactions are carried out, and any free molecules are washed out after the reaction.53,54,81−84 The modification reactions are performed on a preformed CNF network formed by filtration from water. This has advantages because the need for dispersing CNF in less polar organic solvents is avoided (which would be required if the CNF is premodified to increase hydrophobicity and then dispersed for filtration/casting), resulting in a strong CNF

CNF state of dispersion. It is apparent that the stress−strain behavior of the predried neat CNF network is important for the tensile behavior of corresponding polymer matrix composites.9 3.2. Interfacial Tailoring and Moisture Durability. During prepreg-based processing, resin curing occurs in the presence of hydroxyl groups at the cellulosic CNF surface. Because of the high specific surface area of the CNF mat, a large number of hydroxyls are available to interact with the resin monomers either via covalent linkages or by secondary interactions (Figure 4b, bottom panel). In the specific case of epoxy, covalent interactions reduce the moisture sensitivity of the composites when the CNF is uniformly dispersed in the matrix.38,75 CNF agglomeration (at higher CNF content) is detrimental to moisture stability due to higher fraction of moisture-sensitive CNF/CNF bonds. With unsaturated polyester matrices, the CNF−polymer interactions are not covalent, but through secondary bonds and the mechanical performance, are compromised at high humidity.39 Thus, highly nanostructured materials must account for homogeneous CNF dispersion as well as controlled interfacial characteristics. 2346


Perspective

Biomacromolecules network with minimum CNF aggregation.68 The impregnation, reaction, and washing steps are carried out by placing the template in a solvent. This allows for conservative use of chemicals and energy because energy-intensive washing steps at low concentration (such as centrifugation) can be avoided. Cellulose-Epoxy Reactions. The cellulose-epoxy reactions are highly relevant because epoxies are widely used (composites, adhesives, coatings). The cellulose hydroxylepoxy reactions are amine-catalyzed and proceed at a rapid rate as was established by investigating model monoepoxide molecules.81 Such reactions from a solid substrate in a solventfree environment are interesting and possibly due to the high specific surface area of the fibrils. The reaction scheme could further be used as an independent surface modification strategy where epoxide molecules with desired functional groups (such as aromatic, allyl) can be attached to the nanocellulose surface. This may lead to moisture-stable CNF substrates81 or improve nanocellulose compatibility with hydrophobic polymers. Interphase Effects. Another effect from the high specific fibril−matrix interface area is that polymer matrix characteristics are substantially modified. High Vf of well-dispersed CNF fibrils means that the distance between two fibrils is on the order of 10 nm or smaller,37 and a large volume of the thermoset matrix is in close vicinity to the fibril surface. The “interphase” region dominates the matrix properties, which are very different from the bulk matrix state. This can manifest itself as a strong increase in the glass transition temperature37−39 because the mobility of polymer chains in the interphase region is lowered due to the proximity to the CNF fibrils. In reactive systems, the increase in thermoset Tg can be extremely high because the effective cross-link density is increased due to chemical bonds between the thermoset and the fibril surface.37,38 Low amounts of CNF in thermoplastic matrices can also improve the matrix properties beyond bulk polymer data, for instance, by increased physical aging rate52 or increased crystallization rate.85,86 Such changes in polymer matrix structure compared with the bulk polymer reference are often the cause of strong improvements in polymer properties due to the addition of a small amount of nanoparticles. 3.3. Functional CNF Composites. With growing interest in multifunctional materials for electronic device applications, nanoparticles such as carbon nanotubes (CNTs), clay, and graphene are of interest. Functional composites with these nanoparticles can be designed by extending the current processing strategy to either precipitate the functional nanoparticle on CNF or add them to a CNF/water suspension and filter.87 In ternary composites containing CNF as an additional component, CNF network formation aids in homogeneous nanoparticle dispersion and also provides additional mechanical support in the final composite. For instance, highly magnetic CNF/epoxy/cobalt-ferrite nanoparticle composites with up to 70 wt % of the ferrite nanoparticle were prepared using the precipitation approach.87 Incorporation of as little as 20 wt % of CNF led to sufficient structural support during processing and resulted in functional and ductile thermoset nanocomposites that could be shaped by compression molding.87 The CNF + thermoset polymer, in these systems, acts as a high-performance “matrix” where the mechanical load bearing properties of the CNF are synergistically combined with the functional properties of added nanoparticles. This approach could further expand the range of CNF-based “smart” materials88−95 while making them suitable for semistructural applications.

4. CONCLUSIONS AND FUTURE PERSPECTIVES Research interest in CNF nanocellulose biocomposites is driven not only by environmental concerns but also the high mechanical performance potential of these materials and the possibility to add functional properties. Recent research demonstrates the critical importance of avoiding nanocellulose agglomeration and the need for CNF−polymer interface tailoring. For semistructural biocomposites of high mechanical performance, materials design must include high cellulose content with controlled nanostructure so that high ductility and the ultimate strength of the nanocellulose network can be translated into biocomposite properties. A prepreg-based processing approach is a promising route toward realizing this goal. The adaptability of the process allows for convenient control of the nanocellulose volume fraction, and a high volume fraction of well-dispersed nanocellulose is attainable. The in situ curing of thermosets offers possibilities for interface tailoring by chemical reactions with cellulose hydroxyls (i.e., amine-catalyzed epoxide reactions with hydroxyls). Excellent mechanical (modulus ≈ 10 GPa, strength ≈ 150 MPa, strain to failure ≈ 6%) and hygrothermal properties have been realized due to synergistic contributions from random in-plane fiber networks and ductile polymer matrices. Furthermore, composites combining mechanical and functional properties (magnetic, conductive, optical) can be obtained by adding a third component, and this requires only minor adaptations to the process. Further scientific work may investigate different aspects to fully explore the promised potential. The design of complex multiphase materials with several functional nanoparticles could be an interesting route, as would be the choice of other matrices (including in situ polymerization of thermoplastics). High specific surface area of the nanocellulose network creates a large amount of sites for cellulose interaction with the polymer matrix. Interactions with cellulose fibrils and the nanoscale distribution of polymer matrix domains lead to constraint effects on the dynamics of the polymer matrix. Such effects can possibly be used to improve polymer matrix properties compared with data for the polymer in bulk state. The processing approach is scalable because it is related to industrially established methods already used in preparation of fiber composite prepregs and adhesive films based on thermoset precursors. The filtration steps to obtain the CNF nanopaper “mat” are related to conventional papermaking, and CNF nanopaper is already close to market introduction. Consequently, large structures with thick and complex shapes may be obtained by laminating and/or molding the prepregs. Several practical challenges need to be addressed before truly nanostructured polymer matrix biocomposites will be available at large scale. Although the use of organic solvents is feasible (consistent with existing thermosets), selection of the specific solvent system needs to be addressed as well as the modification of existing processing technologies. An alternative to solvent-borne systems is to use thermoset precursor or monomer liquids, which are highly compatible with cellulose, showing instantaneous wetting. Industrial production of biocomposites of high nanocellulose content requires low cost for the nanocellulose component. In Japan, the focus of massive industrial development appears to be small-diameter fibrils, often with modified chemical functionality compared with native cellulose. The objective is advanced applications, and cost estimates can be in the range of 2347


Perspective

Biomacromolecules

(11) Oksman, K.; Aitomäki, Y.; Mathew, A. P.; Siqueira, G.; Zhou, Q.; Butylina, S.; Tanpichai, S.; Zhou, X.; Hooshmand, S. Review of the recent developments in cellulose nanocomposite processing. Composites, Part A 2016, 83, 2−18. (12) Turbak, A. F.; Snyder, F. W.; Sandberg, K. R. Microfibrillated cellulose, a new cellulose product: properties, uses and commercial potential. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1983, 37, 815−827. (13) Saito, T.; Nishiyama, Y.; Putaux, J. L.; Vignon, M.; Isogai, A. Homogeneous suspensions of individualized microfibrils from TEMPO-catalyzed oxidation of native cellulose. Biomacromolecules 2006, 7, 1687−1691. (14) Henriksson, M.; Henriksson, G.; Berglund, L. A.; Lindström, T. An Environmentally Friendly Method for Enzyme-Assisted Preparation of Microfibrillated Cellulose (MFC) Nanofibers. Eur. Polym. J. 2007, 43, 3434−3441. (15) Wågberg, L.; Decher, G.; Norgren, M.; Lindströ m, T.; Ankerfors, M.; Axnäs, K. The build-up of polyelectrolyte multilayers of microfibrillated cellulose and cationic polyelectrolytes. Langmuir 2008, 24, 784−795. (16) Holbery, J.; Houston, D. Natural-fiber-reinforced polymer composites in automotive applications. JOM 2006, 58, 80−86. (17) Gindl-Altmutter, W.; Keckes, J.; Plackner, J.; Liebner, F.; Englund, K.; Laborie, M.-P. All-cellulose composites prepared from flax and lyocell fibres compared to epoxy−matrix composites. Compos. Sci. Technol. 2012, 72, 1304−1309. (18) Mehta, G.; Drzal, L. T.; Mohanty, A. K.; Misra, M. Effect of fiber surface treatment on the properties of biocomposites from nonwoven industrial hemp fiber mats and unsaturated polyester resin. J. Appl. Polym. Sci. 2006, 99, 1055−1068. (19) Zadorecki, P.; Flodin, P. Surface modification of cellulose fibers. II. The effect of cellulose fiber treatment on the performance of cellulose−polyester composites. J. Appl. Polym. Sci. 1985, 30, 3971− 3983. (20) Favier, V.; Chanzy, H.; Cavaille, J. Y. Polymer Nanocomposites Reinforced by Cellulose Whiskers. Macromolecules 1995, 28, 6365− 6367. (21) Benitez, A. J.; Walther, A. Cellulose nanofibril nanopapers and bioinspired nanocomposites: a review to understand the mechanical property space. J. Mater. Chem. A 2017, 5, 16003−16024. (22) Tang, L.; Weder, C. Cellulose Whisker/Epoxy Resin Nanocomposites. ACS Appl. Mater. Interfaces 2010, 2, 1073−80. (23) Al-Turaif, H. A. Relationship between Tensile Properties and Film Formation Kinetics of Epoxy Resin Reinforced with Nanofibrillated Cellulose. Prog. Org. Coat. 2013, 76, 477−481. (24) Chirayil, C. J.; Joy, J.; Mathew, L.; Koetz, J.; Thomas, S. Nanofibril reinforced unsaturated polyester nanocomposites: Morphology, mechanical and barrier properties, viscoelastic behavior and polymer chain confinement. Ind. Crops Prod. 2014, 56, 246−254. (25) Kargarzadeh, H.; Sheltami, R. M.; Ahmad, I.; Abdullah, I.; Dufresne, A. Cellulose nanocrystal: A promising toughening agent for unsaturated polyester nanocomposite. Polymer 2015, 56, 346−357. (26) Peng, X.-w.; Ren, J.-l.; Zhong, L.-x.; Sun, R.-c. Nanocomposite Films Based on Xylan-Rich Hemicelluloses and Cellulose Nanofibers with Enhanced Mechanical Properties. Biomacromolecules 2011, 12, 3321−3329. (27) Wang, M.; Olszewska, A.; Walther, A.; Malho, J.-M.; Schacher, F. H.; Ruokolainen, J.; Ankerfors, M.; Laine, J.; Berglund, L. A.; Ö sterberg, M.; Ikkala, O. Colloidal Ionic Assembly between Anionic Native Cellulose Nanofibrils and Cationic Block Copolymer Micelles into Biomimetic Nanocomposites. Biomacromolecules 2011, 12, 2074− 2081. (28) Lucenius, J.; Parikka, K.; Ö sterberg, M. Nanocomposite films based on cellulose nanofibrils and water-soluble polysaccharides. React. Funct. Polym. 2014, 85, 167−174. (29) Sehaqui, H.; Zhou, Q.; Berglund, L. A. Nanostructured Biocomposites of High ToughnessA Wood Cellulose Nanofiber Network in Ductile Hydroxyethylcellulose Matrix. Soft Matter 2011, 7, 7342.

100 US$ per kg or even more. In North America and Europe, several companies have lower cost products, often termed “microfibrillated cellulose” (MFC), although the fibril diameters can be ∼100 nm. One important application area already developed is MFC as a dry strength additive for paper board. This indicates low material cost. Recent market analyses have assumed a material cost in the range of 4−11 US$ per kg.96 Energy requirements for nanocellulose of MFC production have been discussed in the literature, and values of ∼2000 kWh per ton, or even below, have been reported.97,98 Japanese efforts on thermoplastic cellulose nanocomposites from melt processing99 involve active participation from many industrial companies. In combination with arguments from the present study, this suggests that the ratio between performance and cost for nanocellulose biocomposites is certainly sufficiently high to merit further development work. Furthermore, cellulose has high potential as a more eco-friendly reinforcement phase for polymers than glass fibers.42

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Farhan Ansari: 0000-0001-7870-6327 Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors thank the Knut and Alice Wallenberg (KAW) foundation for funding through the Wallenberg Wood Science Center (WWSC); and SSF through grant number GMT140036 on high performance CNF composites.

■

REFERENCES

(1) Bengtson, S.; Sallstedt, T.; Belivanova, V.; Whitehouse, M. Threedimensional preservation of cellular and subcellular structures suggests 1.6 billion-year-old crown-group red algae. PLoS Biol. 2017, 15, e2000735. (2) Nobles, D. R.; Romanovicz, D. K.; Brown, R. M. Cellulose in Cyanobacteria. Origin of Vascular Plant Cellulose Synthase? Plant Physiol. 2001, 127, 529−542. (3) Sakurada, I.; Nukushina, Y.; Ito, T. Experimental Determination of the Elastic Modulus of Crystalline Regions in Oriented Polymer. J. Polym. Sci. 1962, 57, 651−660. (4) Saito, T.; Kuramae, R.; Wohlert, J.; Berglund, L. A.; Isogai, A. An Ultrastrong Nanofibrillar Biomaterial: The Strength of Single Cellulose Nanofibrils Revealed via Sonication-Induced Fragmentation. Biomacromolecules 2013, 14, 248−53. (5) Berglund, L. A.; Peijs, T. Cellulose biocomposites - From bulk moldings to nanostructured systems. MRS Bull. 2010, 35, 201−207. (6) Habibi, Y.; Lucia, L. A.; Rojas, O. J. Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications. Chem. Rev. 2010, 110, 3479−3500. (7) Klemm, D.; Kramer, F.; Moritz, S.; Lindstrom, T.; Ankerfors, M.; Gray, D.; Dorris, A. Nanocelluloses: a new family of nature-based materials. Angew. Chem., Int. Ed. 2011, 50, 5438−66. (8) Moon, R. J.; Martini, A.; Nairn, J.; Simonsen, J.; Youngblood, J. Cellulose Nanomaterials Review: Structure, Properties and Nanocomposites. Chem. Soc. Rev. 2011, 40, 3941−94. (9) Lee, K.-Y.; Aitomäki, Y.; Berglund, L. A.; Oksman, K.; Bismarck, A. Bismarck, A., On the use of nanocellulose as reinforcement in polymer matrix composites. Compos. Sci. Technol. 2014, 105, 15−27. (10) Kaushik, M.; Moores, A. Review: Nanocelluloses as versatile supports for metal nanoparticles and their applications in catalysis. Green Chem. 2016, 18, 622−637. 2348


Perspective

Biomacromolecules

(53) Cunha, A. G.; Zhou, Q.; Larsson, P. T.; Berglund, L. A. Topochemical acetylation of cellulose nanopaper structures for biocomposites: mechanisms for reduced water vapour sorption. Cellulose 2014, 21, 2773−2787. (54) Sehaqui, H.; Zimmermann, T.; Tingaut, P. Hydrophobic cellulose nanopaper through a mild esterification procedure. Cellulose 2014, 21, 367−382. (55) Benítez, A. J.; Torres-Rendon, J.; Poutanen, M.; Walther, A. Humidity and Multiscale Structure Govern Mechanical Properties and Deformation Modes in Films of Native Cellulose Nanofibrils. Biomacromolecules 2013, 14, 4497−4506. (56) Toivonen, M. S.; Kurki-Suonio, S.; Schacher, F. H.; Hietala, S.; Rojas, O. J.; Ikkala, O. Water-resistant, transparent hybrid nanopaper by physical cross-linking with chitosan. Biomacromolecules 2015, 16, 1062−71. (57) Olszewska, A.; Junka, K.; Nordgren, N.; Laine, J.; Rutland, M. W.; Osterberg, M. Non-ionic assembly of nanofibrillated cellulose and polyethylene glycol grafted carboxymethyl cellulose and the effect of aqueous lubrication in nanocomposite formation. Soft Matter 2013, 9, 7448−7457. (58) Sakakibara, K.; Yano, H.; Tsujii, Y. Surface Engineering of Cellulose Nanofiber by Adsorption of Diblock Copolymer Dispersant for Green Nanocomposite Materials. ACS Appl. Mater. Interfaces 2016, 8, 24893−24900. (59) Sato, A.; Kabusaki, D.; Okumura, H.; Nakatani, T.; Nakatsubo, F.; Yano, H. Surface modification of cellulose nanofibers with alkenyl succinic anhydride for high-density polyethylene reinforcement. Composites, Part A 2016, 83, 72−79. (60) Semba, T.; Ito, A.; Kitagawa, K.; Nakatani, T.; Yano, H.; Sato, A. Thermoplastic composites of polyamide-12 reinforced by cellulose nanofibers with cationic surface modification. J. Appl. Polym. Sci. 2014, 131, 131. (61) Hull, D.; Clyne, T. W. An Introduction to Composite Materials; Cambridge University Press, 1996. (62) Sehaqui, H.; Liu, A.; Zhou, Q.; Berglund, L. Fast Preparation Procedure for Large, Flat Cellulose and Cellulose/Inorganic Nanopaper Structures. Biomacromolecules 2010, 11, 2195−2198. (63) Henriksson, M.; Berglund, L. A.; Isaksson, P.; Lindstrom, T.; Nishino, T. Cellulose Nanopaper Structures of High Toughness. Biomacromolecules 2008, 9, 1579−85. (64) Sehaqui, H.; Zhou, Q.; Ikkala, O.; Berglund, L. A. Strong and Tough Cellulose Nanopaper with High Specific Surface Area and Porosity. Biomacromolecules 2011, 12, 3638−44. (65) Henriksson, M.; Berglund, L. A. Structure and Properties of Cellulose Nanocomposite Films Containing Melamine Formaldehyde. J. Appl. Polym. Sci. 2007, 106, 2817−2824. (66) Nakagaito, A. N.; Yano, H. Novel High-Strength Biocomposites Based on Microfibrillated Cellulose Having Nano-Order-Unit WebLike Network Structure. Appl. Phys. A: Mater. Sci. Process. 2005, 80, 155−159. (67) Ö sterberg, M.; Vartiainen, J.; Lucenius, J.; Hippi, U.; Seppälä, J.; Serimaa, R.; Laine, J. A Fast Method to Produce Strong NFC Films as a Platform for Barrier and Functional Materials. ACS Appl. Mater. Interfaces 2013, 5, 4640−4647. (68) Johansson, L.-S.; Tammelin, T.; Campbell, J. M.; Setala, H.; Osterberg, M. Experimental evidence on medium driven cellulose surface adaptation demonstrated using nanofibrillated cellulose. Soft Matter 2011, 7, 10917−10924. (69) Ishii, D.; Tatsumi, D.; Matsumoto, T. Effect of Solvent Exchange on the Solid Structure and Dissolution Behavior of Cellulose. Biomacromolecules 2003, 4, 1238−1243. (70) Galland, S. Compression-moulded and multifunctional cellulose network materials; KTH Royal Institute of Technology: Stockholm, Sweden, 2013. (71) Mautner, A.; Lucenius, J.; Ö sterberg, M.; Bismarck, A. Multilayer nanopaper based composites. Cellulose 2017, 24, 1759−1773. (72) Nissilä, T.; Karhula, S. S.; Saarakkala, S.; Oksman, K. Cellulose nanofiber aerogels impregnated with bio-based epoxy using vacuum

(30) Svagan, A. J.; Samir, M. A. S. A.; Berglund, L. A. Biomimetic Polysaccharide Nanocomposites of High Cellulose Content and High Toughness. Biomacromolecules 2007, 8, 2556−2563. (31) Benítez, A. J.; Lossada, F.; Zhu, B.; Rudolph, T.; Walther, A. Understanding Toughness in Bioinspired Cellulose Nanofibril/ Polymer Nanocomposites. Biomacromolecules 2016, 17, 2417−2426. (32) S80 and S10 Sheet Molding Compound; Technical Data Sheet; IDI Composites International, 2016. (33) Premi-Glas® 1203-28 Semi Structural SMC; Technical Data Sheet; Premix, 2011. (34) Charles Wittman, G. D. S. Hand Lay-Up Techniques. In Handbook of Composites; Lubin, G., Ed.; Springer, 1982; pp 321−367. (35) Young, R. Thermoset Matched Die Molding. In Handbook of Composites; Lubin, G., Ed.; Springer, 1982; pp 391−448. (36) Nakagaito, A. N.; Yano, H. The effect of fiber content on the mechanical and thermal expansion properties of biocomposites based on microfibrillated cellulose. Cellulose 2008, 15, 555−559. (37) Henriksson, M.; Fogelström, L.; Berglund, L. A.; Johansson, M.; Hult, A. Novel nanocomposite concept based on cross-linking of hyperbranched polymers in reactive cellulose nanopaper templates. Compos. Sci. Technol. 2011, 71, 13−17. (38) Ansari, F.; Galland, S.; Johansson, M.; Plummer, C. J. G.; Berglund, L. A. Cellulose nanofiber network for moisture stable, strong and ductile biocomposites and increased epoxy curing rate. Composites, Part A 2014, 63, 35−44. (39) Ansari, F.; Skrifvars, M.; Berglund, L. Nanostructured biocomposites based on unsaturated polyester resin and a cellulose nanofiber network. Compos. Sci. Technol. 2015, 117, 298−306. (40) Dzenis, Y. A. Structural Nanocomposites. Science 2008, 319, 419−20. (41) Schaefer, D. W.; Justice, R. S. How Nano are Nanocomposites? Macromolecules 2007, 40, 8501−8517. (42) Ashby, M. F. Materials and the Environment: Eco-informed Material Choice, 2nd ed.; Butterworth-Heinemann: Boston, MA, 2013. (43) Terenzi, C.; Prakobna, K.; Berglund, L. A.; Furó, I. Nanostructural effects on polymer and water dynamics in cellulose biocomposites: 2H and 13C NMR relaxometry. Biomacromolecules 2015, 16, 1506−1515. (44) Tammelin, T.; Abburi, R.; Gestranius, M.; Laine, C.; Setala, H.; Osterberg, M. Correlation between cellulose thin film supramolecular structures and interactions with water. Soft Matter 2015, 11, 4273− 4282. (45) Araki, J.; Wada, M.; Kuga, S. Steric stabilization of a cellulose microcrystal suspension by poly(ethylene glycol) grafting. Langmuir 2001, 17, 21−27. (46) Carlmark, A.; Larsson, E.; Malmström, E. Grafting of cellulose by ring-opening polymerisation - A review. Eur. Polym. J. 2012, 48, 1646−1659. (47) Tingaut, P.; Zimmermann, T.; Lopez-Suevos, F. Synthesis and Characterization of Bionanocomposites with Tunable Properties from Poly(lactic acid) and Acetylated Microfibrillated Cellulose. Biomacromolecules 2010, 11, 454−464. (48) Lozhechnikova, A.; Dax, D.; Vartiainen, J.; Willför, S.; Xu, C.; Ö sterberg, M. Modification of nanofibrillated cellulose using amphiphilic block-structured galactoglucomannans. Carbohydr. Polym. 2014, 110, 163−172. (49) Sain, S.; Ray, D.; Mukhopadhyay, A. Improved mechanical and moisture resistance property of in situ polymerized transparent PMMA/Cellulose composites. Polym. Compos. 2015, 36, 1748−1758. (50) Habibi, Y. Key advances in the chemical modification of nanocelluloses. Chem. Soc. Rev. 2014, 43, 1519−1542. (51) Lu, J.; Askeland, P.; Drzal, L. T. Surface Modification of Microfibrillated Cellulose for Epoxy Composite Applications. Polymer 2008, 49, 1285−1296. (52) Ansari, F.; Salajková, M.; Zhou, Q.; Berglund, L. A. Strong Surface Treatment Effects on Reinforcement Efficiency in Biocomposites Based on Cellulose Nanocrystals in Poly(vinyl acetate) Matrix. Biomacromolecules 2015, 16, 3916−3924. 2349


Perspective

Biomacromolecules

(90) Díez, I.; Eronen, P.; Ö sterberg, M.; Linder, M. B.; Ikkala, O.; Ras, R. H. A. Functionalization of Nanofibrillated Cellulose with Silver Nanoclusters: Fluorescence and Antibacterial Activity. Macromol. Biosci. 2011, 11, 1185−1191. (91) Nakagaito, A. N.; Nogi, M.; Yano, H. Displays from transparent films of natural nanofibers. MRS Bull. 2010, 35, 214−218. (92) Spoljaric, S.; Salminen, A.; Luong, N. D.; Seppälä, J. Stable, selfhealing hydrogels from nanofibrillated cellulose, poly(vinyl alcohol) and borax via reversible crosslinking. Eur. Polym. J. 2014, 56, 105−117. (93) Zoppe, J. O.; Ö sterberg, M.; Venditti, R. A.; Laine, J.; Rojas, O. J. Surface Interaction Forces of Cellulose Nanocrystals Grafted with Thermoresponsive Polymer Brushes. Biomacromolecules 2011, 12, 2788−2796. (94) Liu, A.; Walther, A.; Ikkala, O.; Belova, L.; Berglund, L. A. Clay Nanopaper with Tough Cellulose Nanofiber Matrix for Fire Retardancy and Gas Barrier Functions. Biomacromolecules 2011, 12, 633−641. (95) Mittal, N.; Jansson, R.; Widhe, M.; Benselfelt, T.; Håkansson, K. M. O.; Lundell, F.; Hedhammar, M.; Söderberg, L. D. Ultrastrong and Bioactive Nanostructured Bio-Based Composites. ACS Nano 2017, 11 (5), 5148−5159. (96) Cowie, J.; Bilek, E. M.; Wegner, T. H.; Shatkin, J. A. Market projections of cellulose nanomaterial-enabled products - Part 2: Volume estimates. Tappi J. 2014, 13, 57−69. (97) Spence, K. L.; Venditti, R. A.; Rojas, O. J.; Habibi, Y.; Pawlak, J. J. A comparative study of energy consumption and physical properties of microfibrillated cellulose produced by different processing methods. Cellulose 2011, 18, 1097−1111. (98) Bharimalla, A. K.; Deshmukh, S. P.; Patil, P. G.; Vigneshwaran, N. Energy Efficient Manufacturing of Nanocellulose by Chemo- and Bio-Mechanical Processes: A Review. World J. Nano Sci. Eng. 2015, 05, 204−212. (99) Suzuki, K.; Homma, Y.; Igarashi, Y.; Okumura, H.; Yano, H. Effect of preparation process of microfibrillated cellulose-reinforced polypropylene upon dispersion and mechanical properties. Cellulose 2017, 24, 3789−3801.

infusion: Structure, orientation and mechanical properties. Compos. Sci. Technol. 2018, 155, 64−71. (73) Iwamoto, S.; Nakagaito, A. N.; Yano, H.; Nogi, M. Optically Transparent Composites Reinforced with Plant Fiber-Based Nanofibers. Appl. Phys. A: Mater. Sci. Process. 2005, 81, 1109−1112. (74) Shimazaki, Y.; Miyazaki, Y.; Takezawa, Y.; Nogi, M.; Abe, K.; Ifuku, S.; Yano, H. Excellent Thermal Conductivity of Transparent Cellulose Nanofiber/Epoxy Resin Nanocomposites. Biomacromolecules 2007, 8, 2976−2978. (75) Ansari, F.; Sjöstedt, A.; Larsson, P. T.; Berglund, L. A.; Wågberg, L. Hierarchical wood cellulose fiber/epoxy biocomposites − Materials design of fiber porosity and nanostructure. Composites, Part A 2015, 74, 60−68. (76) Jonoobi, M.; Aitomäki, Y.; Mathew, A. P.; Oksman, K. Thermoplastic polymer impregnation of cellulose nanofibre networks: Morphology, mechanical and optical properties. Composites, Part A 2014, 58, 30−35. (77) Capadona, J. R.; Van Den Berg, O.; Capadona, L. A.; Schroeter, M.; Rowan, S. J.; Tyler, D. J.; Weder, C. A Versatile Approach for the Processing of Polymer Nanocomposites with Self-Assembled Nanofibre Templates. Nat. Nanotechnol. 2007, 2, 765−9. (78) Sehaqui, H.; Ezekiel Mushi, N.; Morimune, S.; Salajkova, M.; Nishino, T.; Berglund, L. A. Cellulose nanofiber orientation in nanopaper and nanocomposites by cold drawing. ACS Appl. Mater. Interfaces 2012, 4, 1043−9. (79) Yoshiharu, N.; Shigenori, K.; Masahisa, W.; Takeshi, O. Cellulose Microcrystal Film of High Uniaxial Orientation. Macromolecules 1997, 30, 6395−6397. (80) Tang, H.; Butchosa, N.; Zhou, Q. A Transparent, Hazy, and Strong Macroscopic Ribbon of Oriented Cellulose Nanofibrils Bearing Poly(ethylene glycol). Adv. Mater. 2015, 27, 2070−2076. (81) Ansari, F.; Lindh, E. L.; Furo, I.; Johansson, M. K. G.; Berglund, L. A. Interface tailoring through covalent hydroxyl-epoxy bonds improves hygromechanical stability in nanocellulose materials. Compos. Sci. Technol. 2016, 134, 175−183. (82) Boujemaoui, A.; Carlsson, L.; Malmström, E.; Lahcini, M.; Berglund, L.; Sehaqui, H.; Carlmark, A. Facile Preparation Route for Nanostructured Composites: Surface-Initiated Ring-Opening Polymerization of ε-Caprolactone from High-Surface-Area Nanopaper. ACS Appl. Mater. Interfaces 2012, 4, 3191−3198. (83) Kontturi, K. S.; Biegaj, K.; Mautner, A.; Woodward, R. T.; Wilson, B. P.; Johansson, L.-S.; Lee, K.-Y.; Heng, J. Y. Y.; Bismarck, A.; Kontturi, E. Noncovalent Surface Modification of Cellulose Nanopapers by Adsorption of Polymers from Aprotic Solvents. Langmuir 2017, 33, 5707−5712. (84) Chinga-Carrasco, G.; Kuznetsova, N.; Garaeva, M.; Leirset, I.; Galiullina, G.; Kostochko, A.; Syverud, K. Bleached and unbleached MFC nanobarriers: properties and hydrophobisation with hexamethyldisilazane. J. Nanopart. Res. 2012, 14, 1280. (85) Pei, A.; Zhou, Q.; Berglund, L. A. Functionalized cellulose nanocrystals as biobased nucleation agents in poly(l-lactide) (PLLA) − Crystallization and mechanical property effects. Compos. Sci. Technol. 2010, 70, 815−821. (86) Yang, Q.; Saito, T.; Berglund, L. A.; Isogai, A. Cellulose nanofibrils improve the properties of all-cellulose composites by the nano-reinforcement mechanism and nanofibril-induced crystallization. Nanoscale 2015, 7, 17957−17963. (87) Galland, S.; Andersson, R. L.; Ström, V.; Olsson, R. T.; Berglund, L. A. Strong and Moldable Cellulose Magnets with High Ferrite Nanoparticle Content. ACS Appl. Mater. Interfaces 2014, 6, 20524−20534. (88) 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, 1160. (89) Golmohammadi, H.; Morales-Narváez, E.; Naghdi, T.; Merkoçi, A. Nanocellulose in Sensing and Biosensing. Chem. Mater. 2017, 29, 5426−5446. 2350


Toward Semistructural Cellulose Nanocomposites: The Need for

Recommend Documents