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Chemistry and applications of nanocrystalline cellulose and its derivatives: a nanotechnology perspective

Chemistry and Applications of Nanocrystalline
Cellulose and its Derivatives: a
B. L. Peng,1,2 N. Dhar,1 H. L. Liu2 and K. C. Tam1* 1. Department of Chemical Engineering, Waterloo Institute for Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, ON, Canada N2L 3G1 2. State Key Laboratory of Chemical Engineering and Department of Chemistry, East China University of Science and Technology, Shanghai 200237, P.R. China Nanocrystalline cellulose (NCC) is an emerging renewable nanomaterial that holds promise in many different applications, such as in personalcare, chemicals, foods, pharmaceuticals, etc. By appropriate modification of NCC, various functional nanomaterials with outstanding properties,or significantly improved physical, chemical, biological, as well as electronic properties can be developed. The nanoparticles are stabilised inaqueous suspension by negative charges on the surface, which are produced during the acid hydrolysis process. NCC suspensions can form achiral nematic ordered phase beyond a critical concentration, i.e. NCC suspensions transform from an isotropic to an anisotropic chiral nematicliquid crystalline phase. Due to its nanoscale dimension and intrinsic physicochemical properties, NCC is a promising renewable biomaterial thatcan be used as a reinforcing component in high performance nanocomposites. Many new nanocomposite materials with attractive properties wereobtained by the physical incorporation of NCC into a natural or synthetic polymeric matrix. Simple chemical modification on NCC surface canimprove its dispersability in different solvents and expand its utilisation in nano-related applications, such as drug delivery, protein immobilisation,and inorganic reaction template. This review paper provides an overview on this emerging nanomaterial, focusing on the surface modification,properties and applications of NCC.
Keywords: nanocrystalline cellulose, chemical modification, applications
materials. Compared to cellulose fibres, NCC possesses manyadvantages, such as nanoscale dimension, high specific strength which is renewable, biodegradable, as well as non-toxic.
These amazing physicochemical properties and wide application It is a carbohydrate polymer made up of repeating ˇ-D- prospects have attracted significant interest from both research glucopyranose units and consists of three hydroxyl groups per scientists and industrialists. Gray and co-workers have conducted anhydroglucose unit (AGU) giving the cellulose molecule a high extensive research on NCC, and they have made important contri- degree of functionality. The knowledge of the molecular structure butions to the understanding of NCC. A recent review on NCC was of cellulose is of prime importance as it explains the characteristic reported by Habibi et al. (2010a), where the chemical structure, properties of cellulose, such as hydrophilicity, chirality, biodegrad- composition and their relationship to the optical and mechanical ability and high functionality. As a renewable material, cellulose properties of NCC were discussed. At present, nanoscience and and its derivatives have been widely studied, focusing on theirbiological, chemical, as well as mechanical properties. The mate-rials based on cellulose and its derivatives have been used for ∗Author to whom correspondence may be addressed. more than 150 years in a wide variety of applications, such as E-mail address: [email protected] food, paper production, biomaterials and pharmaceuticals (Coffey Can. J. Chem. Eng. 9999:1–16, 2011 et al., 1995; de Souza Lima and Borsali, 2004).
2011 Canadian Society for Chemical Engineering DOI 10.1002/cjce.20554 Nanocrystalline cellulose (NCC) obtained from acid hydrolysis Published online in Wiley Online Library of cellulose fibres, has been realised as a new class of nano- VOLUME9999,2011

nanotechnology are disciplines that have been highlighted for spe- of crystallinity and the geometrical aspect ratio, i.e. the length- cial focus by many funding agencies and government. Many novel to-diameter (L/d) are very important parameters controlling the products have been created and developed based on nanotechnol- properties of NCC-based materials. An important characteristic ogy. Undoubtedly, the exploitation of NCC will become a bridge of NCCs, when prepared in sulphuric acid, is that they possess between nanoscience and natural resource products, which could negative charges on their surface due to the formation of sul- play a major role in reviving the forest industry in Canada and phate ester groups during acid treatment, which enhances their countries with abundant forest resources.
stability in aqueous solutions. In order to characterise the mor- In this review, recent developments on surface modification, phology of NCC, various types of instruments can be used. The physicochemical properties, and applications of NCC will be con- most conventional and common one is the transmission electron sidered and discussed. Above all, the production and morphology microscopy (TEM) (Araki et al., 1998), which can directly provide of NCC are described, and some of their unique solution prop- high-resolution images. Moreover, scanning electron microscopy erties highlighted. A variety of surface functionalisation routes (SEM) (Miller and Donald, 2003), atomic force microscopy (AFM) will be considered, where the focus is on improving the dis- (Pranger and Tannenbaum, 2008), small angle neutron scatter- persability of NCC in different solvents, thereby expanding their ing (SANS) (Terech et al., 1999) and polarised and depolarised applications in various market sectors. Some potential applica- dynamic light scattering (DLS, DDLS) (de Souza Lima and Bor- tions of NCC include nanocomposite films, drug delivery, protein sali, 2002; de Souza Lima et al., 2003) were also employed to immobilisation and metallic reaction template.
study the morphology of NCC.
Acid treatment (acid hydrolysis) is the main process used to pro- Nanocrystalline cellulose derived from acid hydrolysis of native duce nanocrystalline cellulose, which are the smaller building cellulose possesses different morphologies depending on the ori- blocks released from the original cellulose fibres. Native cellulose gin and hydrolysis conditions. NCCs are rigid rod-like crystals consists of amorphous and crystalline regions, and the amorphous with diameter in the range of 10–20 nm and lengths of a few regions have lower density compared to the crystalline regions, hundred nanometers (Figure 1); e.g. crystallites from tunicates so when cellulose fibres were subjected to harsh acid treatment, and green algae have lengths in the range of a few microm- the amorphous regions break up, releasing the individual crys- eters and crystallites from wood and cotton have lengths of tallites. The properties of NCC depend on various factors, such the order of a few hundred nanometers, while some spherical as cellulose sources, reaction time and temperature, and types of shape NCCs were also produced during the acid treatment (Zhang acid used for hydrolysis. Since R˚anby (1951) first reported that et al., 2007; Wang et al., 2008). Therefore, the relative degree colloidal sulphuric acid-catalysed degradation of cellulose fibres, Figure 1. TEM micrographs of nanocrystals obtained by sulfuric acid hydrolysis of (a) cotton (b) avicel (c-e) tunicate cellulose. The insets of (a) and (b)
provide higher resolution images of some characteristic particles (Elazzouzi-Hafraoui et al., 2008).
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Table 1. Examples of the length (L) and diameter (d) of NCCs from various sources obtained via different techniques
Aspect ratio (L/d) Araki et al. (2001) Araki et al. (2001) Miller and Donald (2003) Pranger and Tannenbaum (2008) de Menezes et al. (2009) de Rodriguez et al. (2006) de Souza Lima et al. (2003) 100–several 1000 Kimura et al. (2005) Beck-Candanedo et al. (2005) a series of NCC products were produced from a variety of sources, they demonstrated that shorter NCC was obtained by increasing such as wood (Table 1) (Beck-Candanedo et al., 2005), cotton the temperature. Sulphuric and hydrochloric acids are extensively (Araki et al., 2001; Miller and Donald, 2003), sisal (de Rodriguez used in the preparation of NCC, however, the dispersability of NCC et al., 2006), tunicate (de Souza Lima et al., 2003; Kimura et al., derived from these two kinds of acids is different. Due to the abun- 2005), bacterial (Roman and Winter, 2004), microcrystalline cel- dance of charged sulphate groups on its surface, NCC obtained lulose (Pranger and Tannenbaum, 2008), ramie (de Menezes et from sulphuric acid hydrolysis dispersed readily in water, while al., 2009) and Valonia cellulose (Revol, 1982).
those produced from hydrochloric acid hydrolysis do not dis- The geometrical dimensions (length, L and diameter, d) of NCC perse as readily, and their aqueous suspensions tend to flocculate.
were found to vary widely, in accordance to the examples shown In addition, differences in the thermal stability and rheologi- in Table 1. The varieties of dimensions depend on the source of cal behaviour between the NCC produced from sulphuric acid cellulosic material and conditions under which the hydrolysis is and those from hydrochloric acid were observed (Araki et al., performed (Habibi et al., 2010a). It was reported that nanocrys- 1998). NCC has also been produced from recycled pulp using talline cellulose derived from tunicate and bacterial cellulose is microwave assisted enzymatic hydrolysis. Filson et al. (2009) usually larger in dimension compared to those obtained from reported a method to produce NCC using endoglucanase enzyme, wood and cotton (Heux et al., 2000; de Souza Lima et al., 2003).
a constituent of cellulases. They observed that microwave heat- This is because tunicate and bacterial cellulose are highly crys- ing produced NCC with greater yield compared to conventional talline, hence there are lower fractions of amorphous regions that heating since microwave heating is more selective and reduces need to be cleaved resulting in the production of larger nanocrys- the reaction time as well.
tals. Beck-Candanedo et al. (2005) compared the properties ofNCC from softwood (black spruce) and hardwood (eucalyptus)produced at the same reaction time, temperature and acid-to-pulp PROPERTIES OF NCC SUSPENSIONS
ratios. They found that these NCC suspensions exhibited simi- Nanocrystalline cellulose derived from acid hydrolysis using var- lar dimensions, surface charge, as well as critical concentrations ious forest product sources can disperse in water due to their required to form anisotropic liquid phases. It was also observed negative charged surfaces. At low concentration, NCC particles that at a higher acid-to-pulp ratio and longer reaction time, shorter are randomly oriented in aqueous suspension as an isotropic nanocrystals with narrow polydispersity index (PDI) were pro- phase, and when the concentration reaches a critical value, they duced and the critical concentration to form an anisotropic phase form a chiral nematic ordering, where NCC suspensions transform was increased. As the cleaving on cellulose chains occurred ran- from an isotropic to an anisotropic chiral nematic liquid crys- domly during the acid hydrolysis process, the dimensions of NCC talline phase (Revol et al., 1992). As the concentration increased are not uniform. Bai et al. (2009) proposed a method to obtain further, aqueous NCC suspensions showed a shear birefringence NCC with a narrow size distribution via the differential centrifu- phenomenon. The critical concentration for sulphated NCC typ- gation technique. Six different NCC fractions with narrow PDI ically ranges between 1 and 10% (w/w), which is a function were produced using differential angular velocities that gener- of aspect ratio of NCC, charge density and osmolarity. Theories ate relative centrifugal force (RCF) for each fraction at a constant based on different parameters have been studied to explain the phenomena (Stroobants et al., 1986).
By using response surface methodology, Bondeson et al. (2006) The phase behaviour of NCC is sensitive to the presence of elec- optimised the reaction conditions for sulphuric acid hydrolysis trolytes and their counter ions, as well as macromolecules. The of MCC from Norway spruce (Picea abies). The concentration effect of added electrolyte on the phase separation of NCC was of MCC and sulphuric acid, hydrolysis time, temperature and studied by Dong et al. (1996), and they found that the addition of ultrasonic treatment time were varied during the process. It was electrolytes, e.g. HCl, NaCl and KCl significantly reduced the vol- found that the reaction time, temperature and acid concentration ume fraction of the anisotropic phase, indicating that addition of were critical factors for the production of NCC. It was possible electrolytes reduces the anisotropic phase. Dong and Gray (1997) to produce cellulose whiskers with a yield of 30% (of initial also studied the effect of counter ions on the phase separation weight) with a sulphuric acid concentration of 63.5% (w/w) in behaviour and stability of NCC suspensions by adding inor- approximately 2 h to yield NCC with length of between 200 and ganic counterions, weakly basic organic counterions and highly 400 nm and diameter of less than 10 nm. Elazzouzi-Hafraoui et al.
basic organic tetraalkylammonium salts. It was observed that the (2008) studied the effect of temperature on the size distribution types of counter ions had a significant effect on phase separation of NCC produced from sulphuric acid hydrolysis of cotton, and behaviour of NCC suspensions. Similar to electrolytes, the phase VOLUME9999,2011

Figure 3. Numbering system for carbon atoms in anhydroglucose unit
of cellulose.
Figure 2. Phase separation observed between cross polars for different
concentrations of NCC (cotton fibers) suspensions at total concentrations
ination of the chiral nematic structure (Figure 2) was recently of (a) 19.8 wt% and (b) 25.0wt% (Elazzouzi-Hafraoui et al., 2009).
reported (Elazzouzi-Hafraoui et al., 2009). Zhou et al. (2009)illustrated that NCC with adsorbed xyloglucan oligosaccharide- separation of NCC suspensions is strongly affected by the addi- poly(ethylene glycol)-polystyrene showed excellent dispersability tion of macromolecules. Gray and co-workers (Edgar and Gray, in non-polar solvents. In addition, the addition of surfactants 2002; Beck-Candanedo et al., 2006, 2007) conducted a detailed enhanced the dispersion of NCC in polystyrene matrix (Kim study on the effect of dextran and ionic dyes on the phase equi- et al., 2009).
librium of NCC suspensions. Surfactant coating used to disperse Recently, the assembling behaviours of NCC under external NCC whiskers in non-polar solvents was first reported by Heux field, such as AC electric field and magnetic field were investigated et al. (2000), where NCC whiskers from cotton and tunicate were (Sugiyama et al., 1992; Fleming et al., 2000; Bordel et al., 2006).
mixed with Beycostat NA (BNA) surfactant. The authors demon- The effect of AC electric field on the alignment and orientation strated that the chiral nematic phases were formed in spite of of NCC was investigated by Habibi et al. (2008a). They observed a layer of surfactants around the NCC whiskers. Detailed exam- that the application of an AC electric field to NCC suspensions Figure 4. Schematic diagram illustrating the various types of chemical modifications on NCC surface (Braun and Dorgan, 2009; Hasani et al., 2008;
Dong and Roman, 2007; Gouss´e et al., 2002; Habibi et al., 2006; Morandi et al., 2009).
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deposited between two metallic electrodes resulted in the homo- ACETYLATION AND ESTERIFICATION
geneous alignment of NCC molecules. Moreover, the alignment of Several methods had been employed to achieve acetylation and cellulose nanocrystals generated films was greatly influenced by esterification of NCC. Based on a non-swelling reaction mecha- the frequency and strength of the applied electric field, while the nism, the reaction only occurred on the cellulose chains located on orientation of cellulose nanocrystals became more homogeneous NCC surface. The limitation on the extent of acetylation lies in the with increasing electric field greater than 2000 V/cm with a fre- susceptibility and accessibility of hydrolysed NCC on the surface.
quency ranging between 104 and 106 Hz. Previous studies (Revol Sassi and Chanzy (1995) employed acetic anhydride and acetic et al., 1994) have shown that NCC suspensions exhibited nega- acid to modify the fibrous and homogenous cellulose. They tive diamagnetic susceptibility anisotropy as they dried under the showed that acetylation commenced from the surface and pro- influence of a magnetic field. The authors also demonstrated that ceeded through a non-swelling mechanism. Due to the low for NCC films, the presence of magnetic field did not facilitate the solubility, the acetylated cellulose on fibrous NCC was observed formation of chiral nematic phase but it only increased the chiral to surround the un-reacted NCC core; compared to homogenous nematic pitch of the suspensions. Similar study was recently con- acetylation that proceeded through dissolvation of the surface ducted by Pan et al. (2010), where they examined various factors acetylated cellulose. Yuan et al. (2006) used straightforward freeze affecting the chiral nematic properties of NCC films.
drying and heating of mixtures of alkyenyl succinic anhydride(ASA) aqueous emulsions and NCC suspensions to obtain acety- CHEMICAL MODIFICATION OF
lated NCC. This process imparted hydrophobicity to the NCC andrendered it soluble in a solvent with low polarity, i.e. DMSO and 1,4-dioxane. Surface acetylation of NCC whiskers (obtained from According to its structure, NCC possesses an abundance of MCC) was undertaken by reacting it with vinyl acetate in the hydroxyl groups on the surface, where chemical reactions can presence of potassium carbonate as catalyst (C ¸ etin et al., 2009).
be conducted. Among the three kinds of hydroxyl groups (Figure By progressively increasing the reaction times, it was observed 3), the OH group on the sixth position acts as a primary alcohol, that crystalline structure of the NCC whiskers was destroyed.
where most of the modification predominantly occurs (Roy et al., In an attempt to avoid complex surface functionalisation routes, 2009). Various chemical modifications of NCC, such as esterifica- Braun and Dorgan (2009) recently combined the synthesis and tion, cationisation, carboxylation, silylation and polymer grafting functionalisation of NCC in a single step. By utilising a mixture have been reported (Figure 4) (Gouss´e et al., 2002; Habibi et al., of acetic acid, HCl and organic acids, NCC whiskers were synthe- 2006; Hasani et al., 2008; Braun and Dorgan, 2009; Morandi et al., sised and functionalised using the Fischer esterification process 2009). Most of these focused on the improvement of its dis- (Figure 5). The presence of acetate and butyrate groups affects persability and compatibility in different solvents or matrices that the hydrophilicity of NCC whiskers making their aqueous solu- are suitable in the production of nanocomposites. There is increas- tions unstable but they possessed better dispersibaility in ethyl ing research focus on modification of NCC because of the increas- acetate and toluene. Apart from the solution process, Berlioz et al.
ing potential applications of modified NCC in various industrial (2009) demonstrated a gas phase process that makes use of evap- sectors, such as personal care, nanocomposites, biomedical, etc.
oration of large excess of palmitoyl chloride to achieve a surface Figure 5. Single step process for cellulose hydrolysis and esterification of hydroxyl groups using a mixture of acetic and hydrochloric acid (Braun and
Dorgan, 2009).
Figure 6. Reaction route for surface fluorescently labeled NCC with FITC (Dong and Roman, 2007).
to core esterification. This method can be extended to esterifica- surface of NCC, they developed a simple method involving a three- tion of different fatty acid chlorides. The nature and treatment of step reaction pathway described by the reaction route shown in the cellulose were found to be an important factor controlling the Figure 6. First, the surface of NCC was decorated with epoxy extent of esterification and the final morphology of NCC. Simi- functional groups via reaction with epichlorohydrin, and then lar concept was applied to esterification by refluxing hydrolysed the epoxy ring was opened with ammonium hydroxide to intro- NCC in organic acid chloride (de Menezes et al., 2009). Graft- duce primary amino groups. Finally, the primary amino group ing organic fatty acids of different aliphatic chain length (up to was reacted with isothiocyanate group of FITC to form a thiourea.
18) was achievable; while the crystalline core was found to be They compared the UV/vis absorption spectrum of unlabelled and un-affected. Improved homogeneity of the extruded LDPE was FITC-labelled NCC in their suspensions, and found the absorp- reported with modified NC grafted with longer aliphatic chain.
tion peaks of FITC in the wavelength range of 450–500 nm in thespectrum of the FITC-labelled NCC.
Gray and co-workers (Hasani et al., 2008) described a one stepmethod to introduce positive charges on the surface of NCC Cellulose whiskers resulting from the acid hydrolysis of tunicate through the grafting of epoxypropyltrimethylammonium chloride were partially silylated by a series of alkyldimethylchlorosilanes, (EPTMAC) onto NCC surfaces. Such surface cationisation proce- with alkyl moieties ranging from isopropyl to n-butyl, n-octyl and dure was conducted via a nucleophilic addition of alkali-activated n-dodrecyl (Gouss´e et al., 2002). The partially silylated whiskers cellulose hydroxyl groups to the epoxy moiety of EPTMAC. This with degree of substitution (DS) of between 0.6 and 1 can readily modification process reversed the surface charge and led to stable redispersed in medium polarity organic solvents, such as ace- aqueous suspensions of NCC with unexpected thixotropic gelling tone and THF. At DS of less than 0.6, the morphological integrity properties. It seems that thixotropic gels inhibit the formation of of the whiskers was preserved, however, it was disrupted when chiral nematic liquid crystalline phase, because no liquid crys- the DS was greater than 1. The authors developed a model with talline chiral nematic phase separation was detected, while shear four silylating agents to describe the experimental observations birefringence was observed. However, mild alkaline cationisation at different DS. Figure 7a shows the NCC whisker at low DS that conditions preserve the original morphology and maintain the is still hydrophilic, where its structure integrity is maintained.
integrity of the crystal.
Such NCC was found to flocculate in THF. Figure 7b shows NCCwhisker with moderate DS rendering the surface hydrophobic.
Some of the chains are derivatised, where the soluble chain ends FLUORESCENTLY LABELED NCC
stabilise the whiskers in organic solvents of low polarity. Figure 7c Fluorescence techniques were extensively used to study the shows highly silylated whiskers where the surface chains are sol- cellular uptake and biodistribution of nanoparticulate deliv- ubilised in the reaction medium and the inner core is exposed to ery systems, by tracking the localisation of the fluorophores.
more silylation thereby compromising its structural integrity and Dong and Roman (2007) described a method to label NCC with crystalline nature. Moreover, the partially silylated whiskers were fluorescein-5-isothiocyanate (FITC) for fluorescence bioassay and found to be more swollen compared to the needle-like images bioimaging applications. To covalently attach FITC moieties to the of unmodified whiskers, indicating the occurrence of slight 6 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING
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Figure 7. Model explaining the silylation of NCC whiskers (a) Onset of surface silylation. (b) Silylated with silylating agents (c) Partially silylated NCC
whiskers (Gouss´e et al. 2002).
silylation of the NCC core. In addition, Grunert and Winter (2002) tion. Figure 8 shows a schematic of NCC whiskers and how the also studied the surface trimethyl silylation of NCC from bacterial surface hydroxyl groups become oxidised to carboxylic functional cellulose, and investigated their reinforcement characteristics in groups using TEMPO-mediated oxidation.
After TEMPO-mediated oxidation, NCC suspension will adopt a birefringence pattern indicating a liquid-crystal like behaviour.
The NCC was found to be better dispersed in an aqueous envi- ronment; attributed to electrostatic stabilisation from negatively Conversion of hydroxymethyl groups into carboxylic groups charged carboxylic groups. This is beneficial for further grafting of can be conducted using 2,2,6,6-tetramethylpiperidine-1-oxyl various types of functionalities. Using a carboxylation-amidation (TEMPO) reagent (Saito and Isogai, 2004; Saito et al., 2010). This procedure, Araki et al. (2001) prepared ‘brush polymer' by graft- is a simple oxidative route that uses TEMPO as nitroxyl free radial ing PEG with amide linkage onto TEMPO oxidised NCC. Habibi to specifically oxidise primary hydroxymethyl in an environment et al. (2006) reported that by using different molar ratio of NaOCl of NaBr and NaOCl; leaving the secondary hydroxyl groups intact over anydroglucose unit of hydrolysed cellulose, the degree of (Habibi et al., 2006). The morphology and crystal axis of the oxidation and solubility could be varied. Higher NaOCl resulted NCC are critical in determining the accessibility of the hydrox- in an increase of carboxyl content. The degree of oxidation was ymethyl group. It is generally accepted that only 50% of the determined from conductometric titration that measures the con- surface hydroxymethyl groups are accessible for the TEMPO reac- centration of carboxylic acid. Importantly, the structural integrity Figure 8. Schematic of the NCC whiskers before and after TEMPO-mediated oxidation (Habibi et al., 2006).
Figure 9. Formation of PDMAEMA grafted on rod-like NCC by surface initiated ATRP (Yi et al., 2009).
of NCC was retained after hydrolysis and TEMPO-mediated oxi- ciencies and molecular weights. Yi et al. (2008) also investigated dation. However, excessive oxidation did affect the structural the surface grafting of NCC using styrene as a monomer, while integrity of the original NCC; since the amorphous region of the another hydrophilic monomer, namely, N,N-dimethylaminoethyl NCC would be degraded (Montanari et al., 2005).
methacrylate (DMAEMA) was also used to study the temperature-induced chiral nematic phase behaviour of suspensions ofPDMAEMA-grafted NCC (Yi et al., 2009) (Figure 9). Similar to these reported studies, Xu et al. (2008) synthesised NCC The methods for polymer grafting onto NCC surface are based whiskers grafted with azobenzene polymers to produce a novel on two approaches, i.e. ‘grafting-to' and ‘grafting-from'. Many amphotropic hairy rod-like system. This amphotropic system techniques for surface modification of NCC whiskers involve exhibited thermotropic and lyotropic liquid crystalline properties.
the ‘grafting-to' approach, where a polymer chain is grafted Ring-opening polymerisation was also used in the ‘grafting-from' to the NCC surface. The ‘grafting-to' approach was adopted by approach to graft polycaprolactone (PCL) polymers onto NCC sur- Habibi and Dufresne (2008) to graft various molecular weights face (Habibi et al., 2008b), where stannous octoate (Sn(Oct)2) was polycaprolactone (PCL) to the NCC via the isocyanate-mediated used as a grafting and polymerising agent. To enhance the grafting coupling reaction. They found that the grafted PCL chains were efficiency, Chen et al. (2009); and Lin et al. (2009) studied simi- able to crystallise at the NCC surface, when the grafting density lar grafting process via microwave irradiation. Recently, Siqueira was sufficiently high. Similar efforts were reported in the graft- et al. (2010) grafted long chain isocyanate onto NCC whiskers ing process of presynthesised waterborne polyurethane onto the using two new novel methods of in situ solvent exchange in surface of NCC (Cao et al., 2009). Peptide coupling reaction was order to prevent dispersion problems in the reaction medium. A also used in the polymer grafting process of NCC. HCl-hydrolyzed summary of various types of modifications on NCC whiskers is NCC was carboxylated by TEMPO-mediated oxidation, and then documented in Table 2.
EDC/NHS carbodiimide chemistry was used to conduct a roomtemperature reaction between the –COOH groups on carboxylated APPLICATIONS OF NANOCRYSTALLINE
NCC and –NH2 groups on PEG-NH2 (Araki et al., 2001). The sameapproach was implemented by Mangalam et al. (2009), who suc- cessfully grafted DNA oligomers on the surface of TEMPO oxidised As a result of their distinctive properties, nanocrystalline cellulose NCC. The temperature dependence and structural morphology of has the potential of becoming an important class of renewable DNA-g-NCC composite on the nanoscale were also reported. Graft- nanomaterials, which could find many useful applications. The ing of DNA to NCC extends the application of NCC to nucleic acid main application of NCC is for the reinforcement of polymeric research. Ljungberg et al. (2005) investigated the process of graft- matrix in nanocomposite materials. Favier et al. (1995) was the ing maleated polypropylene onto the surface of NCC using the first to report the use of NCC as reinforcing fillers in poly(styrene- ‘grafting-to' approach. Recently, Azzam et al. (2010) reported the co-butyl acrylate) (poly(S-co-BuA))-based nanocomposites. Since grafting of thermo-responsive polymers onto NCC via a peptidic then, numerous nanocomposite materials were developed by coupling reaction. The modified NCC possesses unusual prop- incorporating NCC into a wide range of polymeric matrices. The erties, such as colloidal stability at high ionic strength, surface properties of these cellulosic nanocomposites depend on the types activity and thermoreversible aggregation. They also reported a and characteristics of NCC and polymeric matrices (which could thermoreversible aggregation, which could pave the way for the be both natural and synthetic polymers) (Samir et al., 2005).
design of stimuli–responsive biobased nanocomposite materials.
Chemical functionalisation of NCC improves its dispersability in The ‘grafting-from' approach has been used to grow poly- organic solvents and this greatly expands its potential applications meric chains from the NCC surface via the atom transfer radical in various sectors. The following section highlights some recent polymerisation (ATRP). This technique allows for very precise studies on potential applications of NCC from a nanotechnological control over the grafting process that produces well-defined monodispersed particles (Wang and Matyjaszewski, 1995). Sur-face initiated ATRP is a two-step process: the first step is the esterification of hydroxyl groups on NCC surface with 2-bromoisobuturyl bromide (BIBB), which is followed by the The mechanical properties of nanocomposite films mainly depend polymerisation of monomers. Styrene was used as the monomer on the morphology and dimensions of the two constituents, i.e.
for surface initiated ATRP by Morandi et al. (2009), and they NCC and polymeric matrix, as well as the processing techniques.
reported a series of grafting products with different grafting effi- Any other factor that interferes or controls the formation of the 8 THE CANADIAN JOURNAL OF CHEMICAL ENGINEERING
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Table 2. Summary of various types of modifications on NCC whiskers
Type of modification Alkyenyl succinic anhydride (ASA) Yuan et al. (2006) Surface acetylation Vinyl acetate in the presence of potassium carbonate as catalyst ¸ etin et al. (2009) Fischer esterification process Mixture of acetic acid, HCl and organic acids Braun and Dorgan (2009) Surface to core esterification Gas phase evaporation of large excess of palmitoyl chloride Berlioz et al. (2009) Grafting epoxypropyltrimethylammonium chloride (EPTMAC) Hasani et al. (2008) Fluorescently labelled NCC Dong and Roman (2007) Series of alkyldimethylchlorosilanes Gouss´e et al. (2002) TEMPO-media oxidation Oxidation of –OH groups to –COOH using Habibi et al. (2006) Grafting of PEG-NH2 Reaction of PEG-NH2 and TEMPO oxidised NCC Araki et al. (2001) Grafting-to approach using isocyanate-mediated coupling reaction Habibi and Dufresne (2008) (PCL)Grafting of styrene, DMAEMA or Grafting-from approach using surface initiated ATRP Yi et al. (2008); Morandi et al.
azobenzene polymers (2009); Xu et al. (2008) percolating whiskers network will also change the mechanical mer grafting is beneficial for dispersing NCC and to formulate performance of the nanocomposite (Dufresne, 2008).
nanocomposites in non-polar solvents (Ljungberg et al., 2005).
The geometrical aspect ratio, defined as the length-to-diameter However, drying and redispersion of NCC without aggregation (L/d), is a major factor that controls the mechanical properties of in casting-evaporation processing is still challenging. Habibi and nanocompsites and determines the percolation threshold value.
Dufresne (2008) prepared nanocomposite films using unmodi- This factor is related to the original cellulose fibres and production fied and PCL-grafted NCC nanoparticles as filler and PCL as the conditions, which was previously discussed. Fillers with a high matrix, and they that found PCL-grafted nanoparticles were eas- aspect ratio give the best reinforcing effect. It was reported that ily dispersed when compared to the unmodified system. They the highest modulus increase in the rubbery state of the poly(S- demonstrated that the transformation of NCC nanoparticles into a co-BuA) matrix and thermal stability were obtained with tunicin cocontinuous material through long chain surface chemical mod- whiskers (L/d ∼ 67) in comparison with bacterial (L/d ∼ 60) or ification represents a new and promising way for the processing Avicel whiskers (L/d ∼ 10) (Samir et al., 2005). de Rodriguez and of nanocomposite material.
co-workers (2006) studied sisal nanowhiskers with high aspectratio as filler in the nanocomposites with polyvinyl acetate (PVAc) as the matrix. They found that the high aspect ratio could ensure Electrostatic fibre spinning or ‘electrospinning' is a versatile percolation, resulting in mechanical improvements and thermal method for preparing fibres with diameters ranging from sev- stability at lower fibre loadings. Dubief et al. (1999) also reported eral microns down to 100 nm through the action of electrostatic the mechanical behaviour of composites based on amorphous forces (Dufresne, 2010). Electrospinning is a fast and simple pro- poly(ˇ-hydroxyoctanoate) (PHO) when reinforced with tunicin cess to produce polymeric filaments, and this approach has been microcrystals. They proved that tunicin whiskers with high aspect widely studied. Park et al. (2007) reported on the incorporation of ratio led to higher mechanical properties through the formation bacterial cellulose whiskers into polyethylene oxide (PEO) nanofi- of a rigid filler network. Similar effects on the dependence of bres via the electrospinning process. The authors demonstrated mechanical properties of nanocomposites on aspect ratio were that electrospun PEO fibres with a diameter of less than 1 ␮m also reported using carbon nanotubes as fillers (Jiang et al., 2007; were successfully prepared, and the mechanical properties were Wong et al., 2009).
enhanced by the cellulose whiskers. Nanocomposite films from In this section, different processing methods, such as casting polystyrene and cellulose nanowhiskers were also produced by evaporation, electrospinning, as well as extrusion and impregna- Rojas et al. (2009) using electrospinning. They found that sorbitan tion, were evaluated for the preparation of nanocomposite films monostearate, a nonionic surfactant could be used to improve the using water and organic solvents. The resulting thermal prop- dispersion of hydrophilic reinforcing nanoparticle in hydropho- erties, mechanical properties and applications of nanocomposite bic matrix. Electrospun poly (vinyl alcohol) (PVA) (Peresin et al., films will be discussed.
2010) and polycaprolactone (PCL) (Zoppe et al., 2009) nanofi-bres reinforced with NCC were also obtained by electrospinning.
Casting Evaporation The new composites from PCL and NCC showed a significant Casting-evaporation is one of the most common techniques used increase in the storage modulus at all temperatures and possessed to produce nanocomposite films, where the solvent is evapo- a nonlinear stress–strain deformation behaviour. Fibre webs from rated after dispersion casting. To achieve good reinforcement, PCL reinforced with 2.5% unmodified NCC showed ca. 1.5-fold the dispersability of the filler, such as NCC in both the poly- increase in Young's modulus and the ultimate strength compared meric matrix and processing solvents is critical. The water-soluble to PCL webs. The reason was explained in terms of the difference matrix is the simplest system for incorporating NCC as filler, in the fibre diameter, NCC loading, and crystallisation processes.
due to the hydrophilic character of NCC. Schroers et al. (2004) A co-electrospinning technique to produce a core-in-shell nano- studied the properties of nanocomposite films based on ethy- material consisting of a cellulose shell and a core containing lene oxide/epichlorohydrin and NCC, which were produced by the cellulose nanocrystals was first reported by Magalh˜aes et al.
dispersion-casting of NCC fillers in THF/water mixtures. The poly- VOLUME9999,2011
Extrusion and Impregnation the air/liquid interface in the presence of dioctadecyldimethy-lammonium (DODA), a cationic amphiphilic molecule. Dioctade- Extrusion and impregnation are two methods used to prepare cyldimethylammonium (DODA) was used to produce NCC-DODA nano-composites comprising of a polymeric matrix and modified complexes that allowed the transfer of NCC from the air/liquid NCC filler. The main challenge lies in the poor dispersion and interface in an aqueous suspension to hydrophobic solid sub- agglomeration of NCC inside the polymeric matrix, which is due to strates. Atomic force microscopy (AFM) and X-ray photoelectron the hydrophilic nature of NCC and the formation of inter-chain H- spectroscopy (XPS) were employed to characterise the morphol- bonding. These non-optimised conditions thus limit the mechani- ogy and chemical composition of NCC films. It was demonstrated cal properties of the prepared nano-composite. One way to address that both of these two techniques could be used to prepare sta- the agglomeration problem previously reported is by introduc- ble, smooth and robust monolayers of NCC. Hence, these layers ing microcrystalline cellulose at the intermediate stage during the offer an opportunity to investigate the interfacial properties rele- extrusion of poly (lactic acid) (PLA), where the suspension of vant to the chemical and biological transformations of cellulose.
whiskers was pumped into the polymer melt during the extrusion Alternatively, these films can be used as a coating technology to process (Oksman et al., 2006). N,N-dimethylacetamide (DMAc) modify the surface of other materials to achieve unique properties containing lithium chloride (LiCl) was used to better disperse the (Habibi et al., 2010b).
microcrystalline cellulose (MCC); however, it appeared to result The self-assembly of NCC in suspension can produce iridescent in the degradation of the composites at high temperature. The chiral nematic films upon drying (Beck et al., 2011) The irides- mechanical properties did not show improvements compared to cence colour exhibited by NCC film is attributed to the ability pure PLA, due to combined effect of additives and high processing of the NCC to reflect circularly polarised light at specific wave- temperature. Wide angle X-ray diffraction showed that the crystal lengths. It was reported that ultrasound treatment increased the integrity of the cellulose was retained after extrusion. A similar chiral nematic pitch in the suspension resulting in the red-shift process was performed by employing poly (vinyl alcohol) (PVA) of the reflection wavelength of NCC films as the applied energy as the dispersing agent; and the MCC was distributed in the PVA phase (Bondeson and Oksman, 2007). Another method employedis the modification of surface functional groups on NCC prior toextrusion. Hydrolysis followed by grafting of organic aliphatic acid chain of different lengths was used (de Menezes et al., 2009). The The layer-by-layer assembly (LBL) is a unique and facile method mixture of modified NCC and low density polyethylene (LDPE) for fabricating composite films with nanometer precision. The was extruded and improvement in the dispersion resulting in a advantages of this approach are simplicity, universality and thick- more homogeneous mixture was observed; where better disper- ness control at the nano length scale. Two components are sion was reported in formulations with longer aliphatic chain.
alternately deposited by solution-dipping or spin-coating, and Impregnation is another technique that could be used to prepare usually there are strong interactions between the alternative lay- dried film or a mat comprising of NCC, before immersing the thin ers. The self-assembling systems obtained from the LBL technique film in the thermosetting resin (e.g. epoxy or melamine formalde- could be used for a wide range of applications.
hyde). The cellulose thin film can be prepared via membrane LBL assembly has already been applied to prepare multilay- filtration of cellulose solution or by pressing the dried cellulose. At ered films based on nanocrystalline cellulose. Podsiadlo et al.
low pressure, the resin impregnates and fills the cavities within the (2005) first reported the preparation of NCC multilayer composites NCC; followed by curing to produce a composite. This technique is with poly (diallyldimethylammonium chloride) using LBL assem- mostly used for sample preparation for evaluating the mechanical bly, and they observed uniform coverage and densely packed (Henriksson and Berglund, 2007; Iwamoto et al., 2008), thermal NCC surface from AFM and SEM analyses. The feasibility of (Shimazaki et al., 2007), and optical (Nogi et al., 2005) properties LBL assembly of NCC was demonstrated with poly-(allylamine of cellulose-filled composites. Higher degree of fibrillation of cel- hydrochloride) (PAH). The formation of a well-ordered alignment lulose was found to be beneficial for composite strength; due to the from anisotropic NCC suspensions and the smoothening effect of elimination of weaker fibres acting as crack initiator. Nakagaito flexible PAH macromolecules was reported by Jean et al. (2008).
and Yano (2008) used NaOH treated cellulose micro-fibres impreg- Layer-by-layer orientated self-assembled multilayer films contain- nated with phenol–formaldehyde resin and a 20% increase in ing NCC and PAH were also prepared by Cranston and Gray using fracture strain compared to non-treated cellulose micro-fibres was a strong magnetic film (Cranston and Gray, 2006a) and spin observed. Composite made from the impregnation of bis-phenol coating techniques (Cranston and Gray, 2006b, 2008). The LBL A-type epoxy resin into cellulose thin film prepared from mem- technique was also used to prepare self-organised films composed brane filtration was found to have a high thermal conductivity and solely of anionically and cationically modified NCC (Aulin et al., low thermal expansion (Shimazaki et al., 2007). Improvement of 2010). The authors demonstrated that separation of surfaces cov- thermal stability was reported for composites consisting of lay- ered by PEI/anionic microfibrillated cellulose (MFC) multilayers ers of micro-size or nano-size cellulose fibres and polyurethane required more energy than surfaces covered by cationic/anionic Ozg ¨ur Seydibeyo ˘glu and Oksman, 2008). The cellulose was pre- MFC multilayers, because the fibrils in the PEI/anionic MFC sys- pared by filtering the water-cellulose slurry and the H-bonding tem required more energy to be extracted from the multilayer.
enhanced the thin film integrity.
Biobased ‘green' nanocomposite films based on alternating lay-ers of two renewable bio-based materials (i.e. NCC and Chitosan) were prepared using the LBL technique (de Mesquita et al., 2010).
Interfacial behaviours of NCC were studied by Habibi et al. using The driving forces for the growth of the multilayered films are the Langmuir–Blodgett (2007) and Langmuir–Schaeffer (2010) hydrogen bonds and electrostatic interactions between the two techniques. Smooth monolayer films based on NCC were pre- layers. In addition, LBL assembly of multilayers using two bio- pared by Langmuir–Blodgett vertical and Langmuir–Schaeffer materials driven by non-electrostatic interactions was recently horizontal deposition. NCC was found to form stable layers at reported by Jean et al. (2009).
VOLUME 9999, 2011
Table 3. Protein binding capacities of magnetised Avicel samples
The materials for drug delivery seem to be one of the most inter- (Marchessault et al., 2006) esting research fields. Abundant researches were conducted toinvestigate various drug delivery systems, such as liposomes, % Bound proteins (w/w) micelles, microgels (Ha and Gardella, 2005). Considering thesafety and efficacy, nanocrystalline cellulose has attracted increas- ing attention in biomedical applications (such as a drug carrier), due to its attractive properties. The toxicity assessment of NCC inhuman brain microvascular endothelial cells was conducted andNCC was non-toxic to cells and could be used as carriers in the activity, and it is anticipated that the approach can be extended targeted delivery of therapeutics (Roman et al., 2010). Recently, to other enzymes.
a comprehensive assessment involving toxicity tests with rain-bow trout hepatocytes and nine aquatic species were conducted NANOSTRUCTURES VIA TEMPLATING WITH NCC
by a team of Canadian researchers (Kovacs et al., 2010). From theinitial ecotoxicological characterisation of NCC, no serious envi- Since Mobil researchers reported the first synthesis of mesoporous ronmental concerns were observed. However, further testings will material in 1992 (Kresge et al., 1992), this approach has attracted be necessary, such as the evaluation on the fate, potential NCC significant attention in fundamental and applied fields. Due to its uptake and exposure studies, so that a detailed risk assessment of attractive properties, NCC has already been used as a template in NCC can be determined.
the synthesis of mesoporous materials. Porous titania with anatase Recently, Dong and Roman (2007) reported a method to label structure was prepared using NCC as a template and aqueous NCC with fluorescein-5'-isothiocyanate (FITC) for fluorescence Tyzor-LA solution as a cheap and stable titania precursor (Shin bioassay and bioimaging applications. The interaction of labelled and Exarhos, 2007). The titania material possesses high specific NCC with cells and its biodistribution in vivo by fluorescence surface area, and may be applied to many fields, such as catalysis, techniques were evaluated. Shi et al. (2003) used microcrystalline catalyst support and photovoltaics.
cellulose as host beads, where drug nanoparticles dispersion A new NCC-inducing route was proposed for the synthe- was spray coated onto them, and they examined the morphology sis of shape-controlled nanoparticles (Zhou et al., 2007). The using scanning electron and AFM. NCC particles were incorpo- novel cubic-shaped TiO2 nanoparticles (Figure 10) with high rated into hydrogels based on cyclodextrin/polymer inclusion crystallinity and uniform size were prepared using NCC as (Zhang et al., 2010), and they found that the new nanocomposite morphology-inducing and coordinate agent at low temperature.
hydrogels can be used as a controlled delivery vehicle.
Thermal gravimetric analysis (TGA) suggests that NCC is probably Extrusion-spheronisation is one of several methods used to embedded in the TiO2 nanoparticles to promote the development produce pellets, where the excipient plays an important role as of regular anatase nanocubes.
spheronisation aid. Dukic-Ott et al. (2009) discussed the prop- Some metal nanoparticles have been synthesised on NCC sur- erties of different materials, and they proposed alternatives for face via a reduction method, such as Ni nanoparticles (Shin et al., MCC, which is considered as a golden standard to produce pel- 2007) and Au–Ag alloy nanoparticles (Shin et al., 2008). In these lets via extrusion-spheronisation. The behaviour of MCC during processes, NCC serves as a dual role, as a matrix and a stabilis- extrusion-spheronisation process could be explained by two pro- ing template, to produce stable dispersions of nanoparticles on posed models: ‘molecular sponge model' (Fielden et al., 1988; NCC surface, and the crystallinity of NCC was maintained during Ek and Newton, 1998) and ‘crystallite-gel model' (Kleinebudde, the alloy formation (Shin et al., 2008). These reducing processes 1997). In another case study with itraconazole, Van Eerden- could be recognised as ‘green' processes ascribed to the use of NCC brugh et al. (2008) demonstrated the efficiency of MCC as a and applied to the preparation of transition metal nanoparticles, viable alternative for the preservation of the dissolution-rate of which have high oxidising property without additional reducing the nanoparticles upon freeze-drying.
A recent discovery of free-standing mesoporous silica films with tunable chiral nematic structures was made by the research groupof MacLachlan at the University of British Columbia (Shopsowitz et al., 2010). Various types of mesoporous silica films were pro- Marchessault et al. (2006) provided a description of a ‘proteins duced by calcinating the NCC/silica composite systems, and the fishing' phenomenon for magnetic MCC. The first step is the transmission spectra of the mesoporous silica films are shown in ferrite synthesis that yields predominantly magnetite. Two dif- Figure 11(left panel). Photograph of the different colours of meso- ferent approaches have been envisaged for preparing magnetic porous silica films S1–S4 are shown in Figure 11(right panel). The MCC, where the order of magnetisation and oxidation of MCC colours in these silica films arise only from the chiral nematic pore was altered, producing Mag-Oxy-MCC (oxidation first) and Oxy- structure present in the materials. This discovery could lead to the Mag-MCC (magnetisation first). The results of protein binding development of novel materials for applications, such as tuneable capacities of magnetic MCC are summarised in Table 3, which reflective filters and sensors. In addition, NCC could be used as a were determined using bovine serum albumin (BSA) as a model hard template to produce other new materials with chiral nematic protein ligand.
A novel nanocomposite consisting of NCC and gold nanopat- icle was recently investigated as a matrix for enzyme/protein OTHER POSSIBLE APPLICATIONS
immobilisation (Mahmoud et al., 2009). Cyclodextrin glycosyltransferase (CGTase) and alcohol oxidase were used as test mod- As more researchers become interested in NCC, many applica- els, and they showed a phenomenally high loading rate in the tions in diverse fields have been proposed and explored, ranging matrix. The novel matrix also exhibits significant biocatalytic from iridescent pigments to biomolecular NMR contrast agents VOLUME9999,2011

Figure 10. (a) Transmission electron microscopy (TEM) image, (b) High-resolution transmission electron microscopy (HRTEM) image, (c) selected area
electron diffraction (SAED) pattern of TiO2 nanocubes (Zhou et al., 2007).
Figure 11. Optical characterization and properties of NCC/silica mesoporous films (a) Transmission spectra; (b) Photograph showing different colours
of mesoporous silica films S1 to S4 (Shopsowitz et al., 2010).
(Fleming et al., 2001). In addition, NCC may be used in security to explore various modification processes that enhance the prop- paper (Revol et al., 1997, 1998), based on the solidified liquid crys- erties of NCC, making it attractive for use in a wide range of tals property. It is believed that NCC may be used in lithium battery industrial sectors. Thus, the recent decision by Domtar and FP products as a mechanical reinforcing agent for low-thickness poly- Innovations to build a demonstration pilot plant to produce larger mer electrolytes (Samir et al., 2004a, b, c; Schroers et al., 2004).
amounts NCC will accelerate the adoption of this new renewable However, due to the duration of the preparation technique, many industrial applications of NCC have focused on nanocomposites Thus far, most of the studies focus on the mechanical and chiral (Samir et al., 2005).
nematic liquid properties of NCC nanocomposites, however, otherresearch directions are also being explored. In nanocomposite sys-tems, the homogeneous dispersion of NCC in a polymer matrix is CONCLUSIONS AND OUTLOOK
still a challenging issue, as aggregation or agglomeration of NCC is The paper reviews the recent development on the modification commonly encountered. We believe that appropriate modification and applications of nanocrystalline cellulose from a nanotechnol- of NCC to impart functional characteristics to this nanomaterials ogy perspective. It is shown that NCC has a distinct advantage for will be necessary if NCC is to be successfully incorporated into preparing nanocomposites with outstanding thermal and mechan- a specific product system. It is anticipated that nanotechnology ical properties. Moreover, NCC may be used in drug delivery, innovations in renewable resources, such as NCC will not only protein immobilisation and metallic reaction template due to its improve the future viability of the forest industry in Canada, but it attractive properties. The feasibility of its potential applications is will also create a larger market for future products based on NCC.
also discussed.
Nanocrystalline cellulose is an environmentally-friendly forest- based material that could serve as a valuable renewable resourcefor rejuvenating the beleaguered forest industry. New and emerg- B. L. Peng thanks the China Scholarship Council for support- ing industrial extraction processes need to be optimised to achieve ing his research at University of Waterloo, Canada. K. C. Tam more efficient operations and this will require active research would like to acknowledge the support from CFI and NSERC.
participations from the academic and industrial sectors. The H. L. Liu would like to acknowledge the financial support from the application of nanotechnology in developing NCC from the for- National Natural Science Foundation of China (20736002) and the est industry to more valuable products is required, because the 111 Project of China (B08021). We wish to thank Dr. Richard Berry availability of materials based on NCC is still limited. Increas- (Domtar-FPInnovations Joint Venture Company) for his support ing attention is devoted to produce NCC in larger quantities, and and encouragement in the research on NCC.
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