Nanocrystalline Cellulose

8/14/2019 Nanocrystalline Cellulose

1/22

Cellulose Nanocrystals: Chemistry, Self-Assembly, and Applications

Youssef Habibi, Lucian A. Lucia,*, and Orlando J. Rojas,

Department of Forest Biomaterials, North Carolina State University, Box 8005, Raleigh, North Carolina 27695-8005, and Department of ForestProducts Technology, Faculty of Chemistry and Materials Sciences, Helsinki University of Technology, P.O. Box 3320,

FIN-02015 TKK, Espoo, Finland

Received October 12, 2009

Contents

1. Introduction and State of the Art A2. Structure and Morphology of Celluloses B3. Cellulose Nanocrystals E

3.1. Preparation of Cellulose Nanocrystals E3.2. Morphology and Dimensions of Cellulose

NanocrystalsG

4. Chemical Modifications of Cellulose Nanocrystals H

4.1. Noncovalent Surface Chemical Modifications H4.2. TEMPO-Mediated Oxidation I4.3. Cationization I4.4. Esterification, Silylation and Other Surface

Chemical ModificationsI

4.5. Polymer Grafting J5. Self-Assembly and -Organization of Cellulose

NanocrystalsK

5.1. Self-Assembly and -Organization of CNs inAqueous Medium

L

5.2. Self-Assembly and -Organization of CNs inOrganic Medium

N

5.3. Self-Assembly and -Organization of CNs under

External Fields

N

5.4. Self-Assembly and -Organization of CNs inThin Solid Films

N

6. Applications of Cellulose Nanocrystals inNanocomposite Materials

O

6.1. Nanocomposite Processing O6.1.1. Casting-Evaporation Processing O6.1.2. Sol-Gel Processing P6.1.3. Other Processing Methods P

6.2. Mechanical Properties of CN-BasedComposites

Q

6.2.1. Morphology and Dimensions of CNs R6.2.2. Processing Method R

6.2.3. Interfacial Interactions R6.3. Thermal Properties of CN-Based Composites R

7. Conclusions and Outlook S8. Acknowledgments S9. References S

1. Introduction and State of the Art

Cellulose constitutes the most abundant renewable polymerresource available today. As a chemical raw material, it isgenerally well-known that it has been used in the form of

fibers or derivatives for nearly 150 years for a wide spectrumof products and materials in daily life. What has not beenknown until relatively recently is that when cellulose fibersare subjected to acid hydrolysis, the fibers yield defect-free,rod-like crystalline residues. Cellulose nanocrystals (CNs)have garnered in the materials community a tremendous levelof attention that does not appear to be relenting. Thesebiopolymeric assemblies warrant such attention not onlybecause of their unsurpassed quintessential physical andchemical properties (as will become evident in the review)but also because of their inherent renewability and sustain-ability in addition to their abundance. They have been thesubject of a wide array of research efforts as reinforcingagents in nanocomposites due to their low cost, availability,renewability, light weight, nanoscale dimension, and uniquemorphology. Indeed, CNs are the fundamental constitutivepolymeric motifs of macroscopic cellulosic-based fiberswhose sheer volume dwarfs any known natural or synthetic

biomaterial. Biopolymers such as cellulose and lignin and

North Carolina State University.

Helsinki University of Technology.

Dr. Youssef Habibi is a research assistant professor at the Departmentof Forest Biomaterials at North Carolina State University. He receivedhis Ph.D. in 2004 in organic chemistry from Joseph Fourier University(Grenoble, France) jointly with CERMAV (Centre de Recherche sur lesMacromolecules Vegetales) and Cadi Ayyad University (Marrakesh,Morocco). During his Ph.D., he worked on the structural characterizationof cell wall polysaccharides and also performed surface chemicalmodification, mainly TEMPO-mediated oxidation, of crystalline polysac-charides, as well as their nanocrystals. Prior to joining NCSU, he worked

as assistant professor at the French Engineering School of Paper, Printingand Biomaterials (PAGORA, Grenoble Institute of Technology, France)on the development of biodegradable nanocomposites based on nanoc-rystalline polysaccharides. He also spent two years as postdoctoral fellowat the French Institute for Agricultural Research, INRA, where he developednew nanostructured thin films based on cellulose nanowiskers. Dr. Habibisresearch interests include the sustainable production of materials frombiomass, development of high performance nanocomposites from ligno-cellulosic materials, biomass conversion technologies, and the applicationof novel analytical tools in biomass research.

Chem. Rev. XXXX, xxx, 000000 A

10.1021/cr900339w XXXX American Chemical Society

1

2

34

5

6

7

8

9

10

11

12

13

14

1516

17

18

19

20

21

22

23

24

25

26

27

2829

30

31

32

33

34

35

36

37

38

39

40

4142

43

44

45

46

47

48

49

50

51

52

53

54

55

56

57

58

59

60

61

62

63

64

65

66

67

68

ohio2/ycr-ycr/ycr-ycr/ycr99907/ycr2637d07z xppws 23:ver.7 2/23/10 8:58 Msc: cr-2009-00339w TEID: lmh00 BATID: 00000

PAGE EST: 21.2


2/22

in some cases heteropolysaccharides provide the hierarchicalconstructs to bioengineer biological factories that give riseto a variegated distribution of plants and organisms. Surpris-ingly, a focus on nanoscale phenomena involving thesematerials has not been realized until the past few years inwhich a virtual collection of information has becomeavailable.

In the following review, the salient chemical and physicalfeatures of the most dominant fundamental building blockin the biosphere, cellulose nanocrystals, are discussed. Aftera brief introduction to cellulose, three general aspects of CNsare covered, namely, their morphology and chemistry includ-ing their preparation and chemical routes for functionaliza-tion, self-assembly in different media and under differentconditions, and finally their applications in the nanocom-

posites field. While these aspects are by no means compre-hensively inclusive of the vast number of research resultsavailable, they may be regarded as perhaps the mostscientifically and technologically pertinent aspects of CNsthat deserve attention.

2. Structure and Morphology of Celluloses

Cellulose is the most abundant renewable organic materialproduced in the biosphere, having an annual production thatis estimated to be over 7.5 1010 tons.1 Cellulose is widelydistributed in higher plants, in several marine animals (forexample, tunicates), and to a lesser degree in algae, fungi,bacteria, invertebrates, and even amoeba (protozoa), forexample, Dictyostelium discoideum. In general, cellulose isa fibrous, tough, water-insoluble substance that plays anessential role in maintaining the structure of plant cell walls.

It was first discovered and isolated by Anselme Payen in1838,2 and since then, multiple physical and chemical aspectsof cellulose have been extensively studied; indeed, discover-ies are constantly being made with respect to its biosynthesis,assembly, and structural features that have inspired a numberof research efforts among a broad number of disciplines.Several reviews have already been published reporting thestate of knowledge of this fascinating polymer.1,3-13

Regardless of its source, cellulose can be characterizedas a high molecular weight homopolymer of-1,4-linkedanhydro-D-glucose units in which every unit is corkscrewed180with respect to its neighbors, and the repeat segmentis frequently taken to be a dimer of glucose, known ascellobiose (Figure 1). Each cellulose chain possesses adirectional chemical asymmetry with respect to the terminiof its molecular axis: one end is a chemically reducingfunctionality (i.e., a hemiacetal unit) and the other has apendant hydroxyl group, the nominal nonreducing end. Thenumber of glucose units or the degree of polymerization (DP)is up to 20 000, but shorter cellulose chains can occur andare mainly localized in the primary cell walls.

All-D-glucopyranose rings adopt a 4C1chair conforma-tion, and as a consequence, the hydroxyl groups arepositioned in the ring (equatorial) plane, while the hydrogenatoms are in the vertical position (axial). This structure is

Dr. Lucian A. Lucia is currently an associate professor who works in thearea of renewable materials chemistry and engineering in the Departmentof Forest Biomaterials (formerly Wood & Paper Science) at North CarolinaState University. He received his Ph.D. in 1996 under Prof. Kirk S. Schanzefrom the University of Florida (Gainesville, FL) in organic chemistry witha focus on modeling the fundamental electron transfer pathways activein plant photosynthesis. He then spent almost 2 years at the NSF Centerfor Photoinduced Charge Transfer at the University of Rochester(Rochester, NY, under Prof. David G. Whitten) where he explored thedesign of highly functional materials based on photoactive amphiphilesand their behavior in heterogeneous media. He began his independentacademic career as an assistant professor at the Institute of Paper Scienceand Technology at the Georgia Institute of Technology (Atlanta, GA) in1997 where he extensively studied the materials chemistry and photo-chemistry of wood biopolymers. He joined the faculty at North CarolinaState University in 2004 where he began to explore fundamental andapplied aspects of renewable material-based composites. His researchinterests include antibacterial materials, cellulose nanocrystal templates,scaffolds for tissue engineering, drug delivery devices, biomass pretreat-ments, and wood pulping chemistry.

Dr. Orlando Rojas is Associate Professor in North Carolina State Universityand Finland Distinguished Professor in Helsinki University of Technology.He worked as a Senior Scientist in the Department of Chemistry, PhysicalChemistry of the Royal Institute of Technology, KTH, and in the Institutefor Surface Chemistry, YKI. Prior appointments include his tenure asprofessor in the Department of Chemical Engineering of Universidad deLos Andes (Venezuela), after obtaining his Ph.D. in Chemical Engineeringin Auburn University and other graduate degrees that included a Diplomain Paper Engineering from the ETSII of Universidad Politecnica de Cataluna(Spain). He leads the Colloids and Interfaces Group at NC StateUniversity with a research focus on surface and colloid chemistry andthe adsorption behaviors of surfactants and polymers at solid/liquidinterfaces. He has studied the viscoelasticity of adsorbed monolayers bysurface laser light scattering, surface plasmon resonance, and quartzcrystal microgravimetry methods. His work has also involved state-of-the-art interferometric and bimorph surface force techniques, as well asatomic force microscopy, to unveil basic phenomena and interactions atthe nanoscale. His group currently works on nanocellulose structures,the dynamics of lignocellulose degradation, biosensor development, andseparation, derivatization, and use of natural polymers and surfactants.He is Associate Editor of theJournal of Surfactants and Detergentsandmember of the advisory committee of several other journals. Dr. Rojas isthe 2009-2010 Chair of the Division of Cellulose and Renewable

Materials of the American Chemical Society and is the recipient of the2009 American Chemical Society Divisional service Award. He wasappointed 2009-2014 Finland Distinguish Professor by Tekes, the FinnishFunding Agency for Technology and Innovation and the Academy ofFinland.

B Chemical Reviews, XXXX, Vol. xxx, No. xx Habibi et al.

69

70

71

72

73

74

75

76

77

78

79

80

81

82

83

8485

86

87

88

89

90

91

92

93

94

95

96

97

98

99

100

101

102

103

104

105

106

107108

109

110

111

112

113

114

115

116

117

118

119

120

121

122

123



3/22

stabilized by an intramolecular hydrogen bond network

extending from the O(3

)-H hydroxyl to the O(5) ring oxygenof the next unit across the glycosidic linkage and from theO(2)-H hydroxyl to the O(6) hydroxyl of the next residue(Figure 2).

The three most probable rotational positions of thehydroxymethyl group are defined by ascertaining the place-ment of the O6-C6 bond with respect to the O5-C5 andC4-C5 bonds: if O6-C6 isgaucheto O5-C5 andtranstoC4-C5, then the conformation is called gt, while the othertwo conformations are referred to as ggand tg (Figure 3).

In nature, cellulose does not occur as an isolated individualmolecule, but it is found as assemblies of individual cellulosechain-forming fibers. This is because cellulose is synthesizedas individual molecules, which undergo spinning in ahierarchical order at the site of biosynthesis. Typically,approximately 36 individual cellulose molecules assembleare brought together into larger units known as elementaryfibrils (protofibrils), which pack into larger units calledmicrofibrils, and these are in turn assembled into the familiarcellulose fibers. However, celluloses from different sourcesmay occur in different packing as dictated by the biosynthesisconditions. The combined actions of biopolymerization,spinning, and crystallization occur in a rosette-shaped plasmamembrane complex having a diameter of 30 nm (Figure 4)and are orchestrated by specific enzymatic terminal com-

plexes(TCs) that act as biological spinnerets.14 Because allthe cellulose chains in one microfibril must be elongated by

the complex at the same rate, crystallization during cellulosesynthesis follows very closely polymerization of the chainsby the TCs.15,16 TCs are thought to be cellulose synthasecomplexes that belong to the large GT-A family of glyco-syltransferases; however, the reaction mechanism involvedin cellulose synthesis and assembly is still conjectural. The

structure of cellulose microfibrils implies that their synthesesand assembly involve the coordinate activity of approxi-mately 36 active sites.17 However, diverse cellulose structuresin various organisms imply that the enzyme complex ismodular.14,18 Recent evidence from live-cell imaging ofcellulose indicates that microtubules exert a direct effect onthe orientation of cellulose deposition under specific condi-tions, but microtubules are not required for oriented deposi-tion of cellulose under other conditions (Figure 4).

During the biosynthesis, cellulose chains are aggregatedin microfibrils that display cross dimensions ranging from 2to 20 nm, depending on the source of celluloses. Theaggregation phenomenon occurs primarily via van der Waalsforces and both intra- (Figure 2) and inter-molecularhydrogen bonds. If the TCs are not perturbed, they cangenerate a limitless supply of microfibrils having only alimited number of defects or amorphous domains.14,18 Anumber of models for the microfibril hierarchy have beenproposed that attempt to describe the supramolecular struc-ture of cellulose, including the crystalline structure, crystallitedimensions and defects, structural indices of amorphous

domains, dimensions of fibrillar formation, etc. These modelsdiffer mainly in the description of the organization and thedistribution of the amorphous or less ordered regions withinthe microfibril. After many years of controversy, it iscommon practice to acknowledge that the amorphous regionsare distributed as chain dislocations on segments along the

Figure 1. Chemical structure of cellulose.

Figure 2. Intramolecular hydrogen-bonding network in a repre-sentative cellulose structure.

Figure 3. The three most probable rotational positions of the hydroxymethyl group.

Figure 4. Orientation of microtubules controlling the orientationof cellulose in the cell wall where the microtubules act like tracksto guide the cellulose enzymes floating in the cell membrane.Reprinted with permission from Ref 17. Copyright 2002 Elsevier.

Cellulose Nanocrystals Chemical Reviews, XXXX, Vol. xxx, No. xx C

124

125126

127

128

129

130

131

132

133

134

135

136

137

138

139

140

141

142

143

144

145

146

147

148

149

150

151

152153

154

155

156

157

158

159

160

161

162

163

164

165

166

167

168

169

170

171

172

173

174

175

176

177

178

179180

181

182

183

184



4/22

elementary fibril where the microfibrils are distorted byinternal strain in the fiber and proceed to tilt and twist (Figure5).19

In the ordered regions, cellulose chains are tightly packedtogether in crystallites, which are stabilized by a strong andvery complex intra- and intermolecular hydrogen-bondnetwork. The hydrogen-bonding network and molecularorientation in cellulose can vary widely, which can give riseto cellulose polymorphs or allomorphs, depending on therespective source, method of extraction, or treament.1,20 Sixinterconvertible polymorphs of cellulose, namely, I, II, IIII,IIIII, IVI, and IVII, have been identified.

Native cellulose has been thought to have one crystal

structure, cellulose I, but evidence for the existence of twosuballomorphs of cellulose I, termed IR and I, was

established in 1984 by cross-polarization magic angle spin-ning (CP-MAS).21,22 Depending on the origin of cellulose,these two polymorphs exist in different ratios: IR is prevalentin celluloses from algae and bacteria, and both IRand Imay be present in celluloses in higher plants. However, thelatter result is not without controversy. Solid-state NMRstudies reported by Atalla and VanderHart23 has demonstratedseveral anomalies within the spectra of higher plant celluloses

compared with those from algae, bacteria, and tunicates. Theanomalies found in the NMR spectra seem to suggest thathigher plants may contain only cellulose I, instead ofcellulose IR, with a distorted form of Ithat resides belowthe surface.

In both the I and IRstructures, cellulose chains adoptparallel configurations, but they differ in their hydrogen-bonding patterns, which implies a difference in the crystallinestructure (Figure 6). Indeed, IR corresponds to a triclinicP1unit cell (a )6.717 , b )5.962 , c )10.400 , R )118.08, ) 114.80, and ) 80.37) containing only onechain per unit cell,24 whereas Iexists in a monoclinic P21unit cell having two cellulose chains (a ) 7.784 , b )

8.201 , c ) 10.38 , R ) ) 90, and ) 96.5).25 IR,a metastable phase, can be converted to the more thermo-dynamically stable Iphase by high-temperature annealingin various media.26 Cellulose I (R and) has sheets stackedin a parallel-up fashion, and the hydroxylmethyl groupsare oriented in atgconformation so that their O6 atom pointstoward the O2 hydroxyl groups of the neighboring residue,which engenders a second inter-residue hydrogen bond.27 Ifcellulose is bent in a plane orthogonal to the hydrogen-bonded sheets of chains, a horizontal displacement of thesheets with respect to one another is induced.28 A bendingangle of 39has been shown to be sufficient to induce IRand Iinterconversion especially when the curvature of the

chain sheets in the microfibril is modeled as a group ofconcentric circular arcs. Sine qua non, all microfibrils

Figure 6. Hydrogen-bonding patterns in cellulose IRand I: (top) the two alternative hydrogen-bond networks in cellulose IR; (bottom)the dominant hydrogen-bond network in cellulose I(left) chains at the origin of the unit cell and (right) chains at the center of the unitcell according to Sturcova et al.32 Reprinted with permission from ref 32. Copyright 2004 American Chemical Society.

Figure 5. Schematic representation of the elementary fibrilillustrating the microstructure of the elementary fibril and strain-distorted regions (defects). Reprinted with permission from Ref 19.Copyright 1972 John Wiley and Sons.

D Chemical Reviews, XXXX, Vol. xxx, No. xx Habibi et al.

185

186

187

188

189

190

191

192

193

194

195

196

197

198199

200

201

202

203

204

205

206

207

208209

210

211

212

213

214

215

216

217

218

219

220

221222

223

224

225

226

227

228

229

230

231

232

233

234235



5/22

generated from the TCs must torque sharply before they canadopt a parallel configuration with respect to the face of theinner cell wall. Therefore, the crystal form is likely to bedrastically changed before cellulose is incorporated into thecell wall. Such polymorphic differences were first evidencedby IR spectroscopy29 and electron diffraction30 and have beenfurther confirmed more recently by solid-state CP/MAS 13CNMR.21 The crystal structure and hydrogen-bonding pattern

of cellulose I (Rand ) were later studied more deeply bysynchrotron X-ray and neutron fiber diffraction,24,25,31 wherein the latter, the hydrogen atom positions involved inhydrogen bonding were determined from Fourier-differenceanalysis with respect to hydrogenated and deuterated samples.The definition of all atomic spatial coordinates in thecellulose crystal structure was only possible for the first timebecause of the availability of these singular methods.

Cellulose II, the second most extensively studied allo-morph, can be obtained by two different processes:

(i) By chemical regeneration, which consists of dissolvingcellulose I in a solvent, then reprecipitating it in water.Suitable solvents for cellulose include, among others,

solutions of heavy metal-amine complexes, mainlycopper with ammonia or diamine such as cupric hy-droxide in aqueous ammonia (Scheweizers reagentcalled cuoxam)33 or cupriethylenediamine (cuen),34

ammonia or amine/thiocyanate,35 hydrazine/thio-cyanate,36 lithium chloride/N,N-dimethylacetamide(LiCl/DMAc),37,38 and N-methylmorpholine-N-oxide(NMMO)/water39-41 systems.

(ii) By mercerization, a universally recognized process thename of which is derived from its inventor JohnMercer (1844),42 which consists of swelling nativecellulose in concentrated sodium hydroxide solutionsand yielding cellulose II after removing the swelling

agent. Other swelling agents, such as nitric acid (65%),are also able to convert native fibers to cellulose II.43

Some atypical bacterial species are reported to bio-synthesize cellulose II.44

Cellulose II exists in a monoclinic P21phase (a )8.10, b ) 9.03 , c ) 10.31 , R ) ) 90, and )117.10).27,45,46 During conversion (I to II), the hydroxylgroups rotate from the tg to the gtconformation, whichexplicitly requires a change in the hydrogen-bond network.47,48

In contrast to cellulose I, which has a parallel up arrangement,the chains in cellulose II are in an antiparallel arrangementyielding a more stable structure, which makes it preferablefor various textiles and paper materials. The conversion ofcellulose I to cellulose II has been widely consideredirreversible, although (partial) regeneration of cellulose I fromcellulose II has been reported.49,50

If cellulose I or II is exposed to ammonia (gas or liquefied)or various amines,51 cellulose III is formed upon removal ofthe swelling agent. The resulting form of cellulose IIIdepends on whether the starting form is I or II, giving riseto cellulose IIIIor IIIII. Their diffraction patterns are similarexcept for the meridional intensities. Cellulose IIIIexists ina monoclinic P21 form (a ) 4.450 , b ) 7.850 , c )10.31 , R ) ) 90, and ) 105.10) with one chain inthe unit cell, displaying parallel chains as observed incellulose I.52 However, the hydroxymethyl groups are in thegtconformation and the intersheet hydrogen bond networkis similar to cellulose II. The exact structure of cellulose IIIIIis not clearly established yet, but the crystallographic andspectroscopic studies reported recently by Wada et al.53

indicate that cellulose IIIIIis a disordered phase of cellulose.This disordered structure mainly contains a crystalline formhaving a unit cell (space group P21; a )4.45 , b )7.64, c )10.36 , R ) )90, )106.96) occupied byone chain organized in antiparallel fashion, in addition to asecond structure, as revealed by CP/MAS 13C NMR (spacegroup P21; a )4.45 , b )14.64 , c )10.36 , R )) 90, ) 90.05). Furthermore, either of these forms IIII

or IIIII reverts to its parent structure if placed in a high-temperature and humid environment.Polymorphs IVI and IVII may be prepared by heating

cellulose IIIIor IIIII, respectively, up to 260 C in glycerol.54,55

In a like manner to the case of cellulose III, these two formscan revert to the parent structures I or II. In addition to thenative cellulose I, it has been shown that cellulose IV existsin several plant primary cell walls.56,57

3. Cellulose Nanocrystals

In the 1950s, Ranby reported for the first time that colloidalsuspensions of cellulose can be obtained by controlled

sulfuric acid-catalyzed degradation of cellulose fibers.

58-60

This work was inspired by the studies of Nickerson andHabrle61 who observed that the degradation induced byboiling cellulose fibers in acidic solution reached a limit aftera certain time of treatment. Transmission electron microscopy(TEM) images of dried suspensions revealed for the first timethe presence of aggregates of needle-shaped particles, whilefurther analyses of these rods with electron diffractiondemonstrated that they had the same crystalline structure asthe original fibers.62,63 Simultaneously, the development byBattista64,65 of the hydrochloric acid-assisted degradation ofcellulose fibers derived from high-quality wood pulps,followed by sonification treatment, led to the commercializa-tion of microcrystalline cellulose (MCC). Stable, chemically

inactive, and physiologically inert with attractive bindingproperties, MCC offered a significant opportunity for multipleuses in pharmaceutical industry as a tablet binder, in foodapplications as a texturizing agent and fat replacer, and also,as an additive in paper and composites applications. Afterthe acid hydrolysis conditions were optimized, Marchessaultet al.66 demonstrated that colloidal suspensions of cellulosenanocrystals exhibited nematic liquid crystalline alignment.Since the discovery of spectacular improvements in themechanical properties of nanocomposites with cellulosenanocrystals,67,68 substantial research has been directed tocellulose nanocrystal composites because of the growinginterest in fabricating materials from renewable resources.

Cellulose nanocrystals are often referred to as microcrys-tals, whiskers, nanocrystals, nanoparticles, microcrystallites,or nanofibers. Hereafter, they are called cellulose nano-crystals (CNs). In the coming sections, methods for separa-tion of CNs and their morphologies, characterization, modi-fication, self-assembly, and applications will be reviewed.

3.1. Preparation of Cellulose Nanocrystals

The main process for the isolation of CNs from cellulosefibers is based on acid hydrolysis. Disordered or para-crystalline regions of cellulose are preferentially hydrolyzed,whereas crystalline regions that have a higher resistance toacid attack remain intact.69,70 Thus, following an acidtreatment that hydrolyzes the cellulose (leading to removalof the microfibrils at the defects), cellulose rod-like nano-crystals are produced. The obtained CNs have a morphology

Cellulose Nanocrystals Chemical Reviews, XXXX, Vol. xxx, No. xx E

236

237

238

239

240

241

242

243

244245

246

247

248

249

250

251

252

253

254

255

256

257258

259

260

261

262

263

264

265

266

267

268

269

270

271

272

273

274

275

276

277

278

279

280

281

282

283

284

285

286

287

288

289

290

291

292

293

294

295

296

297

298

299

300

301

302

303

304

305

306

307

308309

310

311

312

313

314

315

316

317

318

319

320

321

322

323

324

325

326

327

328

329

330

331

332

333

334

335

336

337

338

339

340

341

342

343

344

345

346347

348

349

350

351

352

353

354

355

356

357

358

359

360



6/22

and crystallinity similar to the original cellulose fibers;examples of such elements are given in Figure 7.

The actual occurrence of the acid cleavage event isattributed to differences in the kinetics of hydrolysis betweenamorphous and crystalline domains. In general, acid hy-drolysis of native cellulose induces a rapid decrease in itsdegree of polymerization (DP), to the so-called level-off DP(LODP). The DP subsequently decreases much more slowly,even during prolonged hydrolysis times.65,71-75 LODP has

been thought to correlate with crystal sizes along thelongitudinal direction of cellulose chains present in theoriginal cellulose before the acid hydrolysis. This hypothesiswas based on the reasonable assumption that disordered orpara-crystalline domains are regularly distributed along themicrofibers and therefore they are more susceptible to acidattack (in contrast to crystalline regions that are moreimpervious to attack). Also, homogeneous crystallites weresupposed to be generated after acid hydrolysis. Theseassumptions were actually confirmed by X-ray crystaldiffraction,76 electron microscopy with iodine-staining,76

small-angle X-ray diffraction,72 and neutron diffractionanalyses.77 It was shown that the LODP values obtained by

acid hydrolysis of cellulose correlated well with the periodiccrystal sizes along cellulose chains. The value of LODP hasbeen shown to depend on the cellulose origin, with typicalvalues of 250 being recorded for hydrolyzed cotton,64 300for ramie fibers,77 140-200 for bleached wood pulp,65 andup to 6000 for the highly crystalline Valonia cellulose.78

However, a wide distribution of DPs is typically observedfor different cellulose sources, even at the LODP. Such adisparity in the quoted distributions stimulates vigorousdiscussion to the present day. In fact, the acid hydrolysis ofbacterial, tunicate, Valonia, or cotton results in a higherpolydispersity in the molecular weight, without any evidenceof the LODP, probably because these cellulosic materialshave no regular distribution of the less-organized domains.

Typical procedures currently employed for the productionof CNs consist of subjecting pure cellulosic material to strongacid hydrolysis under strictly controlled conditions of tem-

perature, agitation, and time. The nature of the acid and theacid-to-cellulosic fibers ratio are also important parametersthat affect the preparation of CNs. A resulting suspension issubsequently diluted with water and washed with successivecentrifugations. Dialysis against distilled water is thenperformed to remove any free acid molecules from thedispersion. Additional steps such as filtration,79 differentialcentrifugation,83 or ultracentrifugation (using a saccharose

gradient)84 have been also reported.Specific hydrolysis and separation protocols have been

developed that depend on the origin of the cellulosic fibers.Most common sources include among others, cellulose fibersfrom cotton,85,86 ramie,81,87,88 hemp,89 flax,90,91 sisal,82,92 wheatstraw,93 palm,94 bleached softwood95 and hardwood96 pulps,cotton linters pulp,97,98 microcrystalline cellulose,99-102 sugarbeet pulp,103 bacterial cellulose,104-106 and Tunicates.69,84,107

Sulfuric and hydrochloric acids have been extensively usedfor CN preparation, but phosphoric108-111 and hydrobromic112

acids have also been reported for such purposes. If the CNsare prepared by hydrolysis in hydrochloric acid, their abilityto disperse is limited and their aqueous suspensions tend to

flocculate.113 On the other hand, when sulfuric acid is usedas a hydrolyzing agent, it reacts with the surface hydroxylgroups of cellulose to yield charged surface sulfate estersthat promote dispersion of the CNs in water, resulting inimportant properties that will be discussed shortly.114 How-ever, the introduction of charged sulfate groups compromisesthe thermostability of the nanocrystals.80 Also, differencesin the rheological behavior have been shown betweensuspensions obtained from sulfuric acid hydrolysis and thoseobtained from hydrochloric acid. In fact, the sulfuric acid-treated suspension has shown no time-dependent viscosity,whereas the hydrochloric acid-treated suspension showed athixotropic behavior at concentrations above 0.5% (w/v) and

antithixotropic behavior at concentrations below 0.3%.113

Post-treatment of CNs generated by hydrochloric acidhydrolysis with sulfuric acid has been studied to introduce,in a controlled fashion, sulfate moieties on their surfaces.86,95

CNs generated from hydrochloric acid hydrolysis and thentreated with sulfuric acid solution had the same particle sizeas those directly obtained from sulfuric acid hydrolysis;however, the surface charge density could be tuned to givenvalues by sulfuric acid hydrolysis. With respect to themorphology of the particles, a combination of both sulfuricand hydrochloric acids during hydrolysis steps appears togenerate spherical CNs instead of rod-like nanocrystals whencarried out under ultrasonic treatment.115,116 These sphericalCNs demonstrated better thermal stability mainly becausethey possess fewer sulfate groups on their surfaces.116

The concentration of sulfuric acid in hydrolysis reactionsto obtain CNs does not vary much from a typical value ofca. 65% (wt); however, the temperature can range from roomtemperature up to 70 C and the corresponding hydrolysistime can be varied from 30 min to overnight depending onthe temperature. In the case of hydrochloric acid-catalyzedhydrolysis, the reaction is usually carried out at refluxtemperature and an acid concentration between 2.5 and 4 Nwith variable time of reaction depending on the source ofthe cellulosic material. Bondenson et al.99,102 investigatedoptimizing the hydrolysis conditions by an experimentalfactorial design matrix (response surface methodology) usingMCC that was derived from Norway spruce (Picea abies)as the cellulosic starting material. The factors that were variedduring the process were the concentrations of MCC and

Figure 7. TEM images of dried dispersion of cellulose nanocrystalsderived from (a) tunicate79 (Reprinted with permission from ref79. Copyright 2008 American Chemical Society), (b) bacterial80

(Reprinted with permission from ref 80. Copyright 2004 American

Chemical Society), (c) ramie81

(From ref 81, Reproduced bypermission of The Royal Society of Chemistry), and (d) sisal82

(Reprinted with permission from ref 82. Copyright 2006 Springer).

F Chemical Reviews, XXXX, Vol. xxx, No. xx Habibi et al.

361

362

363

364

365

366

367

368

369

370371

372

373

374

375

376

377

378

379

380

381

382

383384

385

386

387

388

389

390

391

392

393

394

395

396

397

398

399

400

401

402

403

404

405

406

407

408

409

410

411

412

413

414

415

416

417

418

419

420

421

422

423

424

425

426

427

428

429

430

431

432

433

434435

436

437

438

439

440

441

442

443

444

445

446

447

448

449

450

451

452

453

454

455

456

457

458

459

460

461

462

463



7/22

sulfuric acid, the hydrolysis time and temperature, and theultrasonic treatment time. The responses that were measuredwere the median size of the cellulose particles and the yieldof the reaction. The authors demonstrated that with a sulfuricacid concentration of 63.5% (w/w) over a time of ap-proximately 2 h, it was possible to obtain CNs having alength between 200 and 400 nm and a width less than 10nm with a yield of 30% (based on initial weight). Prolonga-

tion of the hydrolysis time induced a decrease in nanocrystallength and an increase in surface charge.85 Reaction timeand acid-to-pulp ratio on nanocrystals obtained by sulfuricacid hydrolysis of bleached softwood (black spruce, Piceamariana) sulfite pulp was investigated by Beck-Candanedoet al.96 They reported that shorter nanoparticles with narrowsize polydispersity were produced at longer hydrolysis times.Recently, Elazzouzi-Hafraoui et al.79 studied the size distri-bution of CNs resulting from sulfuric acid hydrolysis ofcotton treated with 65% sulfuric acid over 30 min at differenttemperatures, ranging from 45 to 72 C. By increasing thetemperature, they demonstrated that shorter crystals wereobtained; however, no clear influence on the width of the

crystal was revealed.

3.2. Morphology and Dimensions of CelluloseNanocrystals

The geometrical dimensions (length,L, and width,w) of CNsare found to vary widely, depending on the source of thecellulosic material and the conditions under which the hydrolysisis performed. Such variations are due, in part, to the diffusion-controlled nature of the acid hydrolysis. The heterogeneity insize in CNs obtained from hydrolysis, for a given source type,can be reduced by incorporating filtration,79 differential cen-trifugation,83 or ultracentrifugation (using a saccharose gradi-ent)84 steps. The precise morphological characteristics are

usually studied by microscopy (TEM, AFM, E-SEM,117 etc.)or light scattering techniques, including small angle neutronscattering (SANS)118 and polarized and depolarized dynamiclight scattering (DLS, DDLS).119 TEM images of CNstypically show aggregation of the particles, mainly due tothe drying step for the preparation of the specimens afternegative staining. Besides aggregation, additional instru-mental artifacts usually lead to an overestimation of CNdimensions. To overcome these issues, Elazzouzi-Hafraouiet al.79 recently reported the use of TEM in cryogenic mode(cryo-TEM) to prevent aggregation.

Atomic force microscopy (AFM) has been widely usedto provide valuable and rapid indication of surface topog-

raphy of CNs under ambient conditions at length scales downto the angstrom level.117,120-122 However, AFM topographymay show rounded cross-sectional profiles in cases whereother shapes are expected; for example, AFM imaging ofValoniagives shapes different than the square shape crosssection observed under TEM. This is probably due to artifactsthat result from substrate shape perturbations induced byAFM tip and tip-broadening effects. Finally, AFM was alsoreported to be a valuable technique to measure CNs me-chanical properties and interactions, such as stiffness andadhesion or pull-off forces.123

Typical geometrical characteristics for CNs originatingfrom different cellulose sources and obtained with a varietyof techniques are summarized in Table 1. The reported widthis generally approximately a few nanometers, but the lengthof CNs spans a larger window, from tens of nanometers toseveral micrometers. An arresting observation is that there

is a direct correspondence between the length of the CNsand the LODP of the corresponding material because it isgenerally recognized that the rodlike CN consists of fullyextended cellulose chain segments in a perfectly crystallinearrangement.

CNs from wood are 3-5 nm in width and 100-200 nmin length, while those for Valonia, a sea plant, are reported

to be 20 nm in width and 1000-2000 nm in length. Likewise,cotton gives CNs 5-10 nm in width and 100-300 nm long,and tunicate, a sea animal, gives ca. 10-20 nm in widthand 500-2000 nm long.69 The aspect ratio, defined as thelength-to-width (L/w) spans a broad range and can varybetween 10 and 30 for cotton and ca. 70 for tunicate.

The morphology of the cross section of CNs also dependson the origin of the cellulose fibers. The basis of themorphological shape in the cross section may be attributedto the action of the terminal complexes during cellulosebiosynthesis. In fact, depending on the biological origin ofthe cell wall, different arrangements of TCs have beenobserved, which generate cellulose crystals with different

geometries.

14

Despite the fact that acid hydrolysis appearsto erode the crystal by preferentially peeling off angularcellulose sheets, as has been reported by Helbert et al.,124 anumber of analyses of cross sections of CNs have neverthe-less attempted to characterize the inherent CN geometry.Based on TEM observations, Revol125 reported that the crosssection of cellulose crystallites in Valonia Ventricosa wasalmost square, with an average lateral element length of 18nm. In contrast, CNs from tunicate that were analyzed byTEM126 and SANS were found to have a rectangular 8.8 nm18.2 nm cross-sectional shape.118

The morphology of CNs along the axis of the crystal seemsto also present different features, depending on the sourceof the nanocrystal. CNs from bacterial cellulose121 andtunicate79 have been reported to have ribbon-like shapes withtwists having half-helical pitches of 600-800 nm (Micras-terias denticulata) and 1.2-1.6m, respectively. However,

Table 1. Examples of the Length (L) and Width (w) of CNsfrom Various Sources Obtained by Different Techniques

source L(nm) w(nm) technique ref

bacterial 100-1000 10-50 TEM 105100-1000 5-10 30-50 TEM 80, 104

cotton 100-150 5-10 TEM 12770-170 7 TEM 128200-300 8 TEM 129255 15 DDL 119

150-210 5-11 AFM 117cotton linter 100-200 10-20 SEM-FEG 97

25-320 6-70 TEM 79300-500 15-30 AFM 130

MCC 35-265 3-48 TEM 79250-270 23 TEM 101500 10 AFM 100

ramie 150-250 6-8 TEM 8150-150 5-10 TEM 131

sisal 100-500 3-5 TEM 82150-280 3.5-6.5 TEM 92

tunicate 8.8 18.2 SANS 1181160 16 DDL 119500-1000 10 TEM 1071000-3000 15-30 TEM 132100-1000 15 TEM 129

1073 28 TEM 79Valonia >1000 10-20 TEM 125soft wood 100-200 3-4 TEM 95, 113

100-150 4-5 AFM 96hard wood 140-150 4-5 AFM 96

Cellulose Nanocrystals Chemical Reviews, XXXX, Vol. xxx, No. xx G

464

465

466

467

468

469

470

471

472473

474

475

476

477

478

479

480

481

482

483

484

485

486

487

488

489

490

491

492

493

494

495

496

497498

499

500

501

502

503

504

505

506

507

508

509

510511

512

513

514

515

516

517

518

519

520

521

522

523

524

525

526

527

528

529

530

531

532

533

534535

536

537

538

539

540

541

542

543

544

545

546

547

548

549

550

551

552

553

554

555

556

557

558

559

560

561

562

563



8/22

these twisted features have not been clearly evidenced inCNs extracted from higher plants, which are believed to beflat with uniplanar-axial orientation.

4. Chemical Modifications of CelluloseNanocrystals

Because of a natural advantage of an abundance of

hydroxyl groups at the surface of CNs, different chemicalmodifications have been attempted, including esterification,etherification, oxidation, silylation, polymer grafting, etc.Noncovalent surface modification, including the use ofadsorbing surfactants and polymer coating, has been alsostudied. All chemical functionalizations have been mainlyconducted to (1) introduce stable negative or positiveelectrostatic charges on the surface of CNs to obtain betterdispersion (CNs obtained after sulfuric acid hydrolysisintroduce labile sulfate moieties that are readily removed

under mild alkaline conditions) and (2) tune the surfaceenergy characteristics of CNs to improve compatibility,especially when used in conjunction with nonpolar orhydrophobic matrices in nanocomposites. The main challengefor the chemical functionalization of CNs is to conduct theprocess in such a way that it only changes the surface ofCNs, while preserving the original morphology to avoid anypolymorphic conversion and to maintain the integrity of the

crystal.

4.1. Noncovalent Surface Chemical Modifications

Noncovalent surface modifications of CNs are typicallymade via adsorption of surfactants. This approach has beenintroduced by Heux et al.,129,133 who used surfactantsconsisting of the mono- and di-esters of phosphoric acidbearing alkylphenol tails. The obtained surfactant-coated CNsdispersed very well in nonpolar solvents.129 Detailed analyses

Figure 8. Scheme of TEMPO-mediated oxidation mechanism of the hydroxymethyl groups of cellulose (top, reaction scheme) and cross-sectional representation of cellulose nanocrystal indicating the occurrence of the surface TEMPO-mediated oxidation of available hydroxylgroups (bottom, surface crystal representations).

H Chemical Reviews, XXXX, Vol. xxx, No. xx Habibi et al.

564

565

566

567

568

569

570571

572

573

574

575

576

577

578

579

580

581

582

583

584

585

586

587

588

589

590

591

592

593

594

595



9/22

of the data provided by SANS revealed that the surfactantmolecules formed a thin layer of about 15 at the surfaceof the CNs.134 When the surface-modified CNs were incor-porated into isotactic polypropylene, they showed very goodcompatibility and they acted as remarkable nucleating agentsto induce the formation of the rare crystalline form inaddition to the regular crystalline form of isotactic polypro-pylene R.135,136 An anionic surfactant was also used by

Bondeson et al.137 to enhance the dispersion of CNs inpoly(lactic acid) (PLA). Kim et al.138 and Rojas et al.139 usednonionic surfactants to disperse CNs in polystyrene-basedcomposite fibers. Zhou et al.140 recently reported a new andelegant way of CN surface modification based on theadsorption of saccharide-based amphiphilic block copoly-mers. By mimicking lignin-carbohydrate copolymers, theyadsorbed xyloglucan oligosaccharide-poly(ethylene glycol)-polystyrene triblock copolymer onto the surface of CNs. Theresulting CNs showed excellent dispersion abilities in non-polar solvents.

4.2. TEMPO-Mediated Oxidation

(2,2,6,6-Tetramethylpiperidine-1-oxyl)-mediated (orTEMPO-mediated) oxidation of CNs has been used toconvert the hydroxylmethyl groups present on their surfaceto their carboxylic form. This oxidation reaction, whichis highly discriminative of primary hydroxyl groups, isalso green and simple to implement. It involves theapplication of a stable nitroxyl radical, the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO), in the presenceof NaBr and NaOCl (see Figure 8, top). The use of thistechnique has been the subject of a number of reports sinceit was first introduced by De Nooy et al.,141 who showedthat only the hydroxymethyl groups of polysaccharides wereoxidized, while the secondary hydroxyls remained unaffected.

In fact, TEMPO-mediated oxidation of CNs involves atopologically confined reaction sequence, and as a conse-quence of the 2-fold screw axis of the cellulose chain, onlyhalf of the accessible hydroxymethyl groups are availableto react, whereas the other half are buried within thecrystalline particle (Figure 8, bottom).

TEMPO-mediated oxidation of CNs, obtained from HClhydrolysis of cellulose fibers, was first reported by Araki etal.127 as an intermediate step to promote grafting of polymericchains. These authors demonstrated that after TEMPO-mediated oxidation, the CNs maintained their initial mor-phological integrity and formed a homogeneous suspensionwhen dispersed in water. The basis for these latter observa-

tions was the presence of the newly installed carboxyl groupsthat imparted negative charges at the CN surface and thusinduced electrostatic stabilization. Similar observations werereported by Montanari et al.142 who also showed that duringexcessive TEMPO-mediated oxidation, a decrease of thecrystal size occurred resulting from the partial delaminationof cellulose chains that are extant on the surface.

Habibi et al.143 performed TEMPO-mediated oxidation ofCNs obtained from HCl hydrolysis of cellulose fibers fromtunicate and showed that it did not compromise the mor-phological integrity of CNs or their native crystallinity. Onthe basis of the supramolecular structure, morphology, andcrystallographic parameters of the CNs, these authorsdemonstrated that various degrees of oxidation can bepredicted and achieved by using specific amounts of theprimary oxidizing agent, that is, NaOCl (see Figure 8,bottom). When dispersed in water, TEMPO-oxidized or

carboxylated CN suspensions display birefringence patternsand do not flocculate or sediment owing to the polyanioniccharacter imparted by the negative charges on the CNssurfaces (see Figure 9).

4.3. Cationization

Positive charges can also be easily introduced on thesurface of CNs; for example, weak or strong ammonium-containing groups, such as epoxypropyltrimethylammoniumchloride (EPTMAC), can be grafted onto the CN surfaces.144

Such surface cationization proceeds via a nucleophilicaddition of the alkali-activated cellulose hydroxyl groups tothe epoxy moiety of EPTMAC and leads to stable aqueoussuspensions of CNs with unexpected thixotropic gellingproperties. Shear birefringence was observed, but no liquidcrystalline chiral nematic phase separation was detected forthese cationic CNs, most likely owing to the high viscosityof the suspension.

4.4. Esterification, Silylation and Other SurfaceChemical Modifications

Homogeneous and heterogeneous acetylation of modelCNs extracted from Valoniaand tunicate has been studiedby Sassi and Chanzy by using acetic anhydride in aceticacid.126 Their ultrastructural study, carried out by TEMimaging and X-ray diffraction, showed that the reactionproceeded by a reduction of the diameters of the crystals,while only a limited reduction in CN length was observed.It has been suggested that the reaction involved a nonswellingmechanism that affected only the cellulose chains localizedat the crystal surface. In the case of homogeneous acetylation,the partially acetylated molecules immediately partitionedinto the acetylating medium as soon as they were sufficientlysoluble, while in heterogeneous conditions, the celluloseacetate remained insoluble and surrounded the crystallinecore of unreacted cellulose chains. The simultaneous occur-rence of cellulose hydrolysis and acetylation of hydroxyl

Figure 9. Aqueous 0.53% (w/v) suspensions of cellulose nano-crystals observed between crossed polarizers (1) after productionby HCl-catalyzed hydrolysis (left) and (2) after their oxidation viaTEMPO-mediated reactions (right). Reprinted with permission fromref 143. Copyright 2006 Springer.

Cellulose Nanocrystals Chemical Reviews, XXXX, Vol. xxx, No. xx I

596

597

598

599

600

601

602

603

604605

606

607

608

609

610

611

612

613

614

615

616

617

618

619

620

621

622

623

624

625

626

627

628

629630

631

632

633

634

635

636

637

638

639

640

641

642643

644

645

646

647

648

649

650

651

652

653

654

655

656

657

658

659

660

661

662

663

664

665

666

667

668

669

670

671

672

673

674

675

676

677

678

679

680

681

682

683

684

685

686

687

688

689

690

691

692

693



10/22

groups has been also reported. Fischer esterification ofhydroxyl groups simultaneously with the hydrolysis ofamorphous cellulose chains has been introduced as a viableone-pot reaction methodology that allows isolation of acety-lated CNs in a single-step process (Figure 10).145,146

An environmentally friendly CN surface acetylation routewas recently developed by Yuan et al.147 involving a lowreagent consumption and simple-to-apply procedure. Themethod used alkyenyl succinic anhydride (ASA) aqueous

emulsions as a template. The emulsions were simply mixedwith CN suspensions and freeze-dried, and the resulting solidwas heated to 105 C. The obtained derivative conferred tothe acylated CNs a highly hydrophobic character that wasevident because they were easily dispersible in solvents withwidely different polarities as measured by the respectivedielectric constant,; for example, they were dispersible notonly in DMSO having a very high of 46.45 but also in1,4-dioxane that has a quite low of 2.21. Berlioz et al.148

have reported recently a new and highly efficient syntheticmethod for an almost complete surface esterification of CNs,leading to highly substituted CN esters. The reaction of fattyacid chains was carried out on dried CNs via a gas-phase

process. It has been shown by SEM and X-ray diffractionanalyses that the esterification proceeded from the surfaceof the substrate to the crystal core. Under moderate condi-tions, the surface was fully reacted, whereas the originalmorphology was maintained and the core of the crystalremained unmodified. Esterification of CNs by reactingorganic fatty acid chlorides, having different lengths of thealiphatic chain (C12 to C18), has also been reported with agrafting density high enough that the fatty acids withbackbones of 18 carbons were able to crystallize on thesurface of the CNs.131

Cellulose whiskers resulting from the acid hydrolysis oftunicate have been partially silylated by a series of alkyl-dimethylchlorosilanes, with the carbon backbone of the alkylmoieties ranging from a short carbon length of isopropyl tolonger lengths represented by n-butyl, n-octyl, and n-dodecyl.149 It has been demonstrated that with a degree of

substitution (DS) between 0.6 and 1, the whiskers becamereadily dispersible in solvents of low polarity (such as THF)leading to stable suspensions with birefringent behavior,while their morphological integrity was preserved. However,at high silylation (DS greater than 1), the chains in the coreof the crystals became silylated, resulting in the disintegrationof the crystals and consequently the loss of original morphol-ogy. Surface trimethyl silylation of CNs from bacterialcellulose and their resulting cellulose acetate butyrate104,150

or polysiloxane151 based nanocomposites was also investi-gated by Roman and Winter. Finally, coupling CNs with

N-octadecyl isocyanate, via a bulk reaction in toluene, hasalso been reported to enhance their dispersion in organicmedium and compatibility with polycaprolactone, whichsignificantly improved the stiffness and ductility of theresultant nanocomposites.92

4.5. Polymer Grafting

Polymer grafting on the surface of CNs has been carriedout using two main strategies, namely, the grafting-ontoand grafting-from.152 The grafting onto approach involvesattachment onto hydroxyl groups at the cellulose surface ofpresynthesized polymer chains by using a coupling agent.In the grafting from approach, the polymer chains areformed by in situ surface-initiated polymerization fromimmobilized initiators on the substrate.

The grafting onto approach was used by Ljungberg etal.153 to graft maleated polypropylene (PPgMA) onto thesurface of tunicate-extracted CNs. The resulting graftednanocrystals showed very good compatibility and highadhesion when dispersed in atactic polypropylene. Araki etal.127 and Vignon et al.154 studied the grafting of amine-terminated polymers on the surface of TEMPO-mediatedoxidized CNs by using a peptide coupling process catalyzedby carbodiimide derivatives in water. The same approachhas been implemented by Mangalam et al.155 who graftedDNA oligomers on the surface of CNs. The grafting ofpolycaprolactone having different molecular weights on the

Figure 10. Reaction scheme illustrating the one-pot (tandem) cellulose hydrolysis and esterification reactivity of hydroxyl groups. Reprintedwith permission from ref 145. Copyright 2009 American Chemical Society.

J Chemical Reviews, XXXX, Vol. xxx, No. xx Habibi et al.

694

695

696

697

698

699

700

701

702

703704

705

706

707

708

709

710

711

712

713

714

715

716717

718

719

720

721

722

723

724

725

726

727

728

729

730

731

732

733

734

735

736

737

738

739

740

741

742743

744

745

746

747

748

749

750

751

752

753

754

755

756

757

758

759

760

761

762

763

764

765

766

767

768

769



11/22

surface of CNs has been achieved by using isocyanate-mediated coupling.87 These authors reported reaching agrafting density that was high enough that the grafted PCLchains were able to crystallize at the surface of CNs. Similarefforts were made by Cao et al.98 who reported the isocy-anate-catalyzed grafting of presynthesized water-borne poly-urethane polymers via a one-pot process. Such crystallizationprovoked cocrystallizations of the free chains of the respec-

tive polymer matrices during CN-based nanocompositeprocessing. Furthermore, this cocrystallization phenomenoninduced the formation of a co-continuous phase between thematrix and filler, which significantly enhanced the interfacialadhesion and consequently contributed to a highly improvedmechanical strength of the resulting nanocomposites.

The grafting from approach applied to CNs was firstreported by Habibi et al.,81 who grafted polycaprolactoneonto the surface of CNs via ring-opening polymerization(ROP) using stannous octoate (Sn(Oct)2) as a grafting andpolymerization agent. Likewise, Chen et al.156 and Lin etal.157 conducted similar grafting reactions under microwaveirradiation to enhance the grafting efficiency. In situ poly-

merization of furfuryl alcohol from the surface of cellulosewhiskers was studied by Pranger et al.100 In this case, thepolymerization was catalyzed by sulfonic acid residues fromthe CN surface. At elevated temperatures, the sulfonic acidgroups were de-esterified and consequently released into themedium to catalyze in situ the polymerization. Yi et al.158

and Morandi et al.159 propagated polystyrene brushes viaatom transfer radical polymerization (ATRP) on the surfaceof CNs with ethyl 2-bromoisobutyrate as the initiator agent.Similarly, other vinyl monomers, mainly acrylic monomerssuch asN-isopropylacrylamide, were also polymerized fromthe surface of CNs to produce thermoresponsive substr-ates.160,161 Grafting of polyaniline from CNs was achievedbyin situpolymerization of aniline onto CNs in hydrochloricacid aqueous solution, via an oxidative polymerization usingammonium peroxydisulfate as the initiator.162

5. Self-Assembly and -Organization of CelluloseNanocrystals

When sulfuric acid is used as the hydrolyzing agent, italso chemically reacts with the surface hydroxyl groups ofCNs to yield negatively charged (surface) sulfate groups thatpromote a perfectly uniform dispersion of the whiskers inwater via electrostatic repulsions.114 By inference, continuous

removal of the water phase should therefore tend to causethe nanocrystals to adopt configurations that minimize theexisting electrostatic interactions. Indeed, (homogeneous)concentrated suspensions self-organize into spectacular liquidcrystalline arrangements, a phenomenon similar to whatoccurs in nonflocculating suspensions of other rod-likeparticles, such as poly(tetrafluoroethylene) whiskers,163 to-bacco mosaic viruses (TMV),164 DNA fragments,165 or

crystallites extracted from other polysaccharides such aschitin.166

This self-organization phenomenon was revealed by theappearance of fingerprint patterns obtained from suspen-sions observed by polarized optical microscopy, indicativeof a chiral-nematic ordering.114 An even more striking findingis that this chiral nematic structure can be preserved aftercomplete water evaporation to provide iridescent films ofCNs. These solid films, in addition to allowing fundamentalstudies of their striking behavior, have numerous potentialapplications such as coating materials for decorative materialsand security papers (because the optical properties cannotbe reproduced by printing or photocopying).167

An investigation into these systems reveals that CNs arerandomly oriented in the dilute regime (isotropic phase).Indeed, polarized optical microscopy demonstrates that atdilute concentrations, CNs appear as spheroids or ovaloidsand the initial ordered domains are similar to tactoids. Anematic liquid crystalline alignment is adopted when the CNconcentration increases because these tactoids coalesce toform an anisotropic phase, which is characterized by aunidirectional self-orientation of the CN rods. When thesuspension reaches a critical concentration of CNs, it formsa chiral nematic ordered phase displaying lines that are thesignature of cholesteric liquid crystals (see Figure 14). Abovethe critical concentration of chiral nematic phase formation,

aqueous CN suspensions produce shear birefringence, andon standing, they can spontaneously separate into an upperisotropic and a lower anisotropic phase (Figure 11).

These chiral nematic or cholesteric structures in theanisotropic phase consist of stacked planes of CN rodsaligned along a vector (director), with the orientation of eachdirector rotated about the perpendicular axis from one planeto the next as shown in Figure 11. The self-induced parallelalignment phenomenon of the CNs that occurs above acritical concentration is attributed to the well-known en-tropically driven self-orientation phenomenon of rod-like

Figure 11. (left) Aqueous 0.63% (w/w) CN suspension observed between crossed polarizers. Immediately after shearing the suspensionshows many iridescent birefringence patterns; after 1 week, the suspension separates into the upper isotropic and the lower anisotropicphases (Reprinted with permission from ref 105. Copyright 2001 American Chemical Society). (right) Schematic representation of CNorientation in both the isotropic and anisotropic (chiral nematic) phases.

Cellulose Nanocrystals Chemical Reviews, XXXX, Vol. xxx, No. xx K

770

771

772

773

774

775

776

777

778779

780

781

782

783

784

785

786

787

788

789

790

791792

793

794

795

796

797

798

799

800

801

802

803

804

805

806

807

808

809

810

811

812

813

814

815

816

817

818

819

820

821

822823

824

825

826

827

828

829

830

831

832

833

834

835

836

837

838

839

840

841

842

843

844

845

846

847

848849

850

851

852

853

854

855

856

857

858



12/22


13/22

display a pitch that decreases with increasing CN concentra-tion and can vary from 20 to 80 m.

The isotropic-to-anisotropic (chiral nematic phase) equi-librium is sensitive to the presence of electrolytes and thespecific nature of the electrolyte counterions. Quantitativestudies of the changes in composition of the isotropic andanisotropic phases as a function of electrolyte concentrationwhere both phases coexist,128 as well as the effect of thetype of counterion,173 have been conducted. It has been foundfrom the latter study that increasing the amount of addedelectrolyte decreases anisotropic phase formation. Interest-ingly, the chiral nematic pitch was found to decrease, thatis, the phase became more highly twisted, as the electrolyteconcentration increased. Apparently, the decrease in pitchoccurred because the decrease in the electrical double layerthickness increased the chiral interactions between thecrystallites.128 As already indicated, the phase separation ofsulfated CN suspensions also depends strongly on the natureof their counterions.173 For inorganic counterions, the criticalconcentration for ordered phase formation increases ingeneral as a function of increasing van der Waals radii, in

the order H+ < Na+ < K+


14/22

Carboxylated CNs, prepared by TEMPO-mediated oxida-tion, have been shown to form homogeneous dispersions inwater that are strongly birefringent. This shear birefringencewas not uniformly distributed throughout the system, butconsisted instead of domains of various sizes and colorsindicative of local domain orientation within the CNs thatnever reached the chiral nematic order in the form of eithertactoids or fingerprints. The lack of further organization was

ascribed to the high polydispersity among the length of theCNs (that were obtained from tunicate by HCl acid prehy-drolysis) and to the high viscosity of the suspensions.143

However, when carboxylated CNs were prepared from cottonfibers, a reduced CNs length polydispersity was observed,and thus, these suspensions reached a chiral nematic orderwith a pitch of 7 m at a concentration of 5% or more(w/w). Furthermore, when PEG was grafted on the surfaceof the CNs, the resulting PEG-grafted CNs gave rise to achiral nematic mesophase through a phase separation similarto that of the unmodified CNs, but with a reduced spacingof the fingerprint pattern (around 4.0 m).127 Moreover,unlike what has been previously found, PEG-grafted CNs

showed drastically enhanced dispersion stability even at highsolid content and the ability to redisperse into either wateror chloroform from a freeze-dried state. They also demon-strated strong stability at high ionic strength because of areduced electroviscous effect, and no aggregations wereobserved upon addition of electrolyte up to 2 M NaCl.

Shear birefringence was also observed for suspensions ofcationic epoxypropyltrimethylammonium chloride-graftedCNs, but no liquid crystalline chiral nematic phase separationwas detected, most likely because the phase was inhibitedas a result of the high viscosity of the suspension.144

5.2. Self-Assembly and -Organization of CNs in

Organic MediumHeux et al.129 provided the first description of a self-

ordering phenomenon for CNs in apolar solvents. In theirpreliminary study, surfactant coating was used to disperseCNs and thereby obtain a chiral-nematic structure. However,the pitch of the chiral nematic structure was found to beapproximately 4m, a value that is too small compared withthe case of aqueous suspensions (pitch between 20 and 80m). In addition, higher CN concentrations, up to 36%, couldbe achieved. These results were attributed to the stericstabilization exerted by the surfactant coating. In fact, thisstabilization screened out the electrostatic repulsion andconsequently induced stronger chiral interactions between

rods that ultimately allowed for higher packing. Detailedexamination of the structure of this chiral nematic phase hasbeen reported recently by the same group.177,178 They studiedthe correlation of the aspect ratio of CNs extracted fromcotton fibers and their dispersion in an apolar solvent suchas cyclohexane. The critical concentration in which sponta-neous phase separation into a chiral nematic mesophase wasobserved was higher than that in water. Correlation withOnsagers theory for these organophilic suspensions was notpossible because the experimental critical concentrationswere much lower than the predicted ones, probably due toan attractive interaction between the rods in the apolarmedium. These strong interactions also induced a decreasein the chiral nematic pitches that were of the order of 2 mor less. In addition, suspensions prepared with CNs havinghigh aspect ratios did not show any phase separation butinstead produced an anisotropic gel phase at a high concen-

tration.177 Conversely, the chiral nematic pitch was muchlarger (17m) for suspensions of CNs in toluene that werestabilized by xyloglucan oligosaccharide-poly(ethyleneglycol)-polystyrene triblock copolymer.140

5.3. Self-Assembly and -Organization of CNsunder External Fields

Nonflocculated CNs in an aqueous suspension under anexternal field, whether magnetic or electric, can be oriented.They can orient when they are subjected to a magnetic fielddue to the negative diamagnetic anisotropy of cellulose.Although the diamagnetic anisotropy of cellulose is relativelyweak per molecular repeat unit, the cellulose rod-likenanocrystals are long and heavy, so their total diamagneticanisotropy is rather larger compared with other particles suchDNA or TMV.179,180 Sugiyama et al.180 first demonstratedthat dilute aqueous suspensions of crystalline celluloseextracted from tunicate were able to orient when subjectedto a magnetic field of 7 T. Films were obtained in which thecrystals were oriented with their long axes perpendicular tothe magnetic field. An overall gross CN orientation couldbe achieved where the cholesteric axis became parallel tothe magnetic field rather than unwinding the chiral nematicstructure. Fleming et al. demonstrated that the orientationof a liquid crystalline CN suspension in the magnetic fieldof a NMR spectrometer can assist in interpretation of theNMR spectra of proteins added to the suspension.181 Re-cently, Kimura et al.132 showed that the chiral nematicbehavior of CN suspensions could be unwound by applyinga slowly rotating strong magnetic field. Kvien and Oksmanattempted to align CNs in a polymer matrix (e.g., polyvinylalcohol) by using a strong magnetic field to obtain aunidirectional reinforced nanocomposite, and interestinglythe results showed that the dynamic modulus of the nano-

composite was higher in the aligned direction compared withthe transverse direction.182

Aqueous suspensions of CNs extracted from ramie fibersand tunicate have been allowed to dry under an AC electricfield and have showed a high degree of orientation alongthe field vector in the films.183 Colloidal suspensions of CNsfrom ramie fibers in cyclohexane have been also oriented inan AC electric field184 as revealed by TEM and electrondiffraction. Moreover, these suspensions demonstrated abirefringence when observed under polarized light with crossnicols whose magnitude could be increased with increasingfield strength. They also displayed interference Newtoncolors, similar to those obtained for thin films of thermotropic

liquid crystals, an interference pattern that reached a satura-tion plateau at ca. 2 kV/cm. This spectacular result wasobtained at concentrations of CNs below the isotropic/nematic transition, which excluded any cooperative effectbetween the electric field and a possible anisotropic phase.184

5.4. Self-Assembly and -Organization of CNs inThin Solid Films

Revol et al.167,185 have taken advantage of the ability ofsuspensions of CNs to engage in lyotropic chiral nematicordering to give rise to iridescent solid cellulosic films withunique and tunable optical properties by simply controllingthe evaporation of suspending water on a flat surface. Theliquid crystalline order obtained from these suspensions waspreserved in solid films and the chiral nematic pitch in thedried films was on the same length scale as the wavelength

N Chemical Reviews, XXXX, Vol. xxx, No. xx Habibi et al.

1020

1021

1022

1023

1024

1025

1026

1027

10281029

1030

1031

1032

1033

1034

1035

1036

1037

1038

1039

1040

10411042

1043

1044

1045

1046

1047

1048

1049

1050

1051

1052

1053

1054

1055

1056

1057

1058

1059

1060

1061

1062

1063

1064

1065

10661067

1068

1069

1070

1071

1072

1073

1074

1075

1076

1077

1078

1079

1080

1081

1082

108

1084

108

108

1087

108

10891090

109

1092

109

1094

109

109

1097

109

1099

1100

110

1102

110

1104

110

110

1107

110

1109

1110

111

1112

111

1114

111

1111117

111

1119

1120

112

1122

112

1124

112

112

1127

112

11291130

113

1132

113

1134

113

113

1137

113

1139

1140

114

1142

114



15/22

of visible light. This unique system was described as an

interference device having the capacity to reflect circularlypolarized light over a specific wavelength range. Becausethe wavelength of reflected light determines its spectral orintrinsic color, the perceived color of the film depends onthe pitch of the cholesteric order and the angle of incidenceof the light. The microstructure of such films is very sensitiveto the drying conditions. Moreover, the perceived color ofthe reflected polarized light is tunable because the finalpitch can be varied depending on processing variables suchas the aspect ratio of CNs and the electrolyte content. Filmsthat are prepared at ambient conditions generally show apolydomain (polymorphic) structure with the helical axes ofdifferent chiral nematic domains pointing in different direc-tions. In fact, Roman and Gray186,187 reported for the firsttime compelling evidence that a parabolic focal conic (PFC)structure (a symmetrical form of focal conic defects in whichthe line defects from a pair of perpendicular, antiparallel,and confocal parabolas) was trapped in these self-assembledsolid films (see Figure 15).

Finally, it has been shown that a magnetic field duringdrying increased the size of the chiral nematic domains andaffected the orientation of the helical axis with respect tothe plane of the film.179

6. Applications of Cellulose Nanocrystals inNanocomposite Materials

Since the first publication related to the use of CNs asreinforcing fillers in poly(styrene-co-butyl acrylate) (poly(S-co-BuA))-based nanocomposites by Favier et al.,68 CNs haveattracted a great deal of interest in the nanocomposites fielddue to their appealing intrinsic properties such as nanoscaledimensions, high surface area, unique morphology, lowdensity (which is estimated to be 1.61 g/cm3188 for purecrystalline cellulose I), and mechanical strength. In addition,they are easily (chemically) modified, readily available,renewable, and biodegradable. CNs have been incorporatedinto a wide range of polymer matrices, including polysilox-anes,151 polysulfonates,189 poly(caprolactone),81,87,190 styrene-butyl acrylate latex,191 poly(oxyethylene),192-195 poly(styrene-co-butyl acrylate) (poly(S-co-BuA)),68 cellulose acetatebutyrate,104,196 carboxymethyl cellulose,197 poly(vinyl alco-hol),198 poly(vinyl acetate),82,97 poly(ethylene-vinyl acetate)(EVA),199 epoxides,70 polyethylene,131 polypropylene,136 poly-

(vinyl chloride),200-203 polyurethane,204 and water-bornepolyurethane.98 Their incorporation into biopolymers, suchas starch-based polymers,69,89,90,205-208 soy protein,209 chito-san,130 or regenerated cellulose,210 and biopolymer-likepoly(lactic acid),137,211-213 poly(hydroxyoctanoate),214,215 andpolyhydroxybutyrates216 have also been reported.

6.1. Nanocomposite Processing

Processing techniques have an important impact on thefinal properties of the composites. The techniques that areadopted should take into consideration the intrinsic propertiesof CNs, their interfacial characteristics (modified or not), thenature of the polymeric matrix (solubility, dispersibility, anddegradation), and the desired final properties such as geo-metrical shape.

6.1.1. Casting-Evaporation Processing

Casting evaporation has been the main technique totransfer cellulose whiskers from an aqueous dispersion intoan organic polymer matrix. Nanocomposite films are formed

via solution casting, that is, simply allowing the solvent toevaporate. Good dispersibility of the CNs in the polymermatrix, as well as in the processing solvent, is a prerequisiteto create polymer/whisker nanocomposites that display asignificant mechanical reinforcement. CNs without surfacemodification have intrinsically strong interactions and havebeen reported as notoriously difficult to disperse. Moreover,this issue is exacerbated when the CN dispersions are driedbefore nanocomposite processing, which generally impliesthat drying and redispersion of CNs without aggregation ischallenging.

Due to the hydrophilic character of CNs, the simplestpolymer systems that incorporate CNs are water-based.

Never-dried aqueous dispersions of CNs are simply mixedwith aqueous polymer solutions or dispersions. However,these systems suffer from limited utility and are onlyappropriate for water-soluble or dispersible polymers suchas latexes. The combination of aqueous solutions of polymers(subject to moderately strong hydrogen bonding, in misciblecosolvents such as THF) with aqueous CN suspensions hasbeen reported.217 The use of polar solvents, most commonly

N,N-dimethylformamide (DMF), with CNs with no surfacemodification has been explored.218 Solvents nonmiscible withwater and with low polarity, such toluene, have also beenwidely used. However, a drawback is that the processrequires solvent exchange steps. Owing to the fact that

acetone is miscible with water; it usually serves as carrierto transfer CNs from water to organic solvents. Freeze-dryingand redispersion of CNs from tunicate in toluene were usedto integrate these fillers into atactic polypropylene, but strongaggregation occurred.153 However, when formic acid wasused to redisperse dried CNs, very satisfactory dispersionswere achieved.219 Freeze-dried CNs were successfully re-dispersed in dipolar aprotic solvents, such DMSO and DMF,containing small amounts of water (0.1%), and it has beenpossible to obtain films of these suspensions by the casting-evaporation technique.220 More recently van den Berg et al.221

investigated the factors limiting the dispersibility of CNsextracted from tunicate via hydrochloric or sulfuric acidhydrolyses in a series of polar protic and aprotic organicsolvents. HCl-generated CNs, which typically have a pro-nounced tendency to aggregate, did not disperse in polaraprotic solvents after being dried. Only protic solvents such

Figure 15. Square lattice in a solid film of cellulose nanocrystalsbetween crossed polarizers. Scale bar 40 m. Reprinted withpermission from ref 186. Copyright 2005 American ChemicalSociety.

Cellulose Nanocrystals Chemical Reviews, XXXX, Vol. xxx, No. xx O

1144

11451146

1147

1148

1149

1150

1151

1152

1153

1154

1155

1156

1157

1158

1159

1160

1161

1162

1163

1164

1165

1166

1167

1168

1169

11701171

1172

1173

1174

1175

1176

1177

1178

1179

1180

1181

1182

1183

1184

1185

1186

1187

118

1189

1190

119

1192

119

1194

119

119

1197

119

1199

1200

120

1202

120

1204

120120

1207

120

1209

1210

121

1212

121

1214

121

121

1217

1211219

1220

122

1222

122

1224

122

122

1227

122

1229

1230

1231232

123

1234

123

123

1237

123

1239

1240

124

1242

124

1244

124

124

1247



16/22

as formic acid andm-cresol were shown to effectively disruptthe hydrogen bonds in aggregated CNs, dispersing both typesof CNs generated from hydrochloric or sulfuric acid hy-drolyses (Figure 16).

Another approach already considered in this review tochange interactions of CNs is via surface modifications. Suchapproach can break the percolating hydrogen-bonded networkand affect the macroscopic mechanical properties of theresulting nanocomposite. Chemical modifications (discussedabove) have been explored to improve dispersibility of CNs

in wide range of organic solvents, from medium to lowpolarity. This approach also allows manipulation of driedCNs because it facilitates freeze-drying and redispersion.

6.1.2. Sol-Gel Processing

Capadona et al. have recently reported a versatile process-ing approach consisting of forming a three-dimensionaltemplate through self-assembly of well individualized CNsand then filling the template with a polymer of choice.222-224

The first step (Figure 17 left, a and b) is the formation of aCN template through a sol/gel process involving the forma-tion of a homogeneous aqueous whisker dispersion that isfollowed by gelation through solvent exchange with a water-miscible solvent (routinely acetone). In the second step(Figure 17 left, d and e), the CN template is filled with amatrix polymer by immersing the gel into a polymer solution

(Figure 17). It should be noted that the polymer solvent mustbe miscible with the gel solvent andde rigueurnot redispersethe CNs.

6.1.3. Other Processing Methods

The use of twin extrusion as a processing method toprepare CN-based nanocomposites has been attempted.225

The process consists of pumping an aqueous dispersion ofCNs coated with surfactant226 or poly(vinyl alcohol)211 into

a melt polymer (i.e., poly(lactic acid), PLA) during extrusion.However, such systems have unfortunately shown a lack ofcompatibility. Starch and CN nanocomposites were processedby extrusion.227 Moreover, the extrusion of modified CNswith long chained molecules seemed to be much easier andcould be processed in solvent free conditions, especiallywhen the grafted chains can melt at the processing temper-ature. An example has been reported recently where fattyacid-grafted CNs were successfully extruded with low-density polyethylene.131 PCL-grafted CNs with long chainsof PCL have shown good thermoformability when subjectedto compression and injection.156

Electrostatic fiber spinning or electrospinning, a versatilemethod to manufacture fibers with diameters ranging fromseveral micrometers down to 100 nm or less through theaction of electrostatic forces, has emerged as an alternativeprocessing method for CNs in polymer matrices. However,

Figure 16. Photographs of 5.0 mg/mL dispersions of H2SO4-generated CNs viewed through cross polarizers: from left to right, as-preparedin water, freeze-dried, freeze-dried and redispersed in water, DMF, DMSO, N-methyl pyrrolidone, formic acid, and m-cresol. Reprintedwith permission from ref 219. Copyright 2007 American Chemical Society.

Figure 17. (left) Schematic representation of the template approach to obtain well-dispersed polymer/CN composites: (a) a nonsolvent isadded to a dispersion of CNs in the absence of any polymer, (b) solvent exchange promotes the self-assembly of a gel of CNs, (c) the gelled

CNs scaffold is interpenetrated with a polymer by immersion in a polymer solution, before the nanocomposite is (d) dried and (e) compacted.(right) The obtained gel after completing step b above Reprinted with permission from ref 101. Copyright 2009 American Chemical Society.

P Chemical Reviews, XXXX, Vol. xxx, No. xx Habibi et al.

1248

1249

1250

1251

1252

1253

1254

1255

1256

1257

12581259

1260

1261

1262

1263

1264

1265

1266

1267

1268

1269

1270

1271

1272

127

1274

127

127

1277

127

1279

1280

1281282

128

1284

128

128

1287

128

1289

1290

129

1292

129

1294

129

129

1297


8/14/2019 Nanocrystal

Nanocrystalline Cellulose

Documents

Transcript of Nanocrystalline Cellulose