Nanocomposites with functionalized polysaccharide ...370471/UQ370471_OA.pdf · 77 agents (Siqueira,...

Accepted Manuscript

Title: Nanocomposites with functionalized polysaccharidenanocrystals through aqueous free radical polymerisationpromoted by ozonolysis

Author: E. Espino-Perez Robert G. Gilbert S. Domenek M.C.Brochier-Salon M.N. Belgacem J. Bras

PII: S0144-8617(15)00852-8DOI: http://dx.doi.org/doi:10.1016/j.carbpol.2015.09.005Reference: CARP 10307

To appear in:

Received date: 11-4-2015Revised date: 27-8-2015Accepted date: 1-9-2015

Please cite this article as: Espino-Perez, E., Gilbert, R. G., Domenek, S.,Brochier-Salon, M. C., Belgacem, M. N., and Bras, J.,Nanocomposites withfunctionalized polysaccharide nanocrystals through aqueous free radicalpolymerisation promoted by ozonolysis, Carbohydrate Polymers (2015),http://dx.doi.org/10.1016/j.carbpol.2015.09.005

This is a PDF file of an unedited manuscript that has been accepted for publication.As a service to our customers we are providing this early version of the manuscript.The manuscript will undergo copyediting, typesetting, and review of the resulting proofbefore it is published in its final form. Please note that during the production processerrors may be discovered which could affect the content, and all legal disclaimers thatapply to the journal pertain.

http://dx.doi.org/doi:10.1016/j.carbpol.2015.09.005

http://dx.doi.org/10.1016/j.carbpol.2015.09.005

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Nanocomposites with functionalized polysaccharide nanocrystals through aqueous free 1

radical polymerisation promoted by ozonolysis 2

3

E. Espino-Péreza,c, Robert G. Gilbertb,c, S. Domenekd,e, M.C. Brochier-Salona, M.N. 4

Belgacema, J. Brasd* 5

6 a Univ. Grenoble Alpes, LGP2, F-38000 Grenoble, France. 7

CNRS, LGP2, F-38000 Grenoble, France. 8 b Centre for Nutrition & Food Sciences (Building 83/S434), Queensland Alliance for 9

Agriculture and Food Innovation, The University of Queensland, Brisbane, Qld 4072, 10

Australia. 11 c School of Pharmacy, Tongji Medical College, Huazhong University of Science and 12

Technology, Wuhan, Hubei, China, 430030 13 d AgroParisTech, UMR 1145 Ingénierie Procédés Aliments, 1 avenue des Olympiades, F-14

91300 Massy Cedex, France 15 e INRA, UMR 1145 Ingénierie Procédés Aliments, 1 avenue des Olympiades, F-91300 Massy 16

Cedex, France 17

18

* Corresponding author: 19

Julien Bras 20

E-mail: [email protected] 21

Phone: +33(0)4 76 82 69 15 22

Fax: +33 (0)4 76 82 69 33 23

24

Abstract 25

Cellulose nanocrystals (CNC) and starch nanocrystals (SNC) were grafted by ozone-initiated 26

free-radical polymerisation of styrene in a heterogeneous medium. Surface functionalisation 27

was confirmed by infrared spectroscopy, contact angle measurements, and thermogravimetric 28

and elemental analysis. X-ray diffraction and scanning electron microscopy showed that there 29

was no significant change in the morphology or crystallinity of the nanoparticles following 30

ozonolysis. The grafting efficiency, quantified by 13C NMR, was greater for SNC, with a 31

styrene/anhydroglucose ratio of 1.56 compared to 0.25 for CNC. The thermal stability 32

improved by 100 °C. The contact angles were 97° and 78° following the SNC and CNC 33

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grafting, respectively, demonstrating the efficiency of the grafting in changing the surface 34

properties even at low levels of surface substitution. The grafting increased the compatibility 35

with the polylactide, and produced nanocomposites with improved water vapour barrier 36

properties. Ozone-mediated grafting is thus a promising approach for surface functionalisation 37

of polysaccharide nanocrystals. 38

39

Keywords 40

Chemical modification, ozonolysis, radical polymerisation, polysaccharide nanocrystals, 41

starch 42

43

Abbreviations 44

AFM: atomic force microscopy; AGU: anhydroglucose monomer unit; CCD:charge-coupled 45

device; CNC: cellulose nanocrystalls; CNC-OOH: CNC hydroperoxide; CP/MAS: cross-46

polarisation/magic angle spinning; FEG-SEM: field-emission gun scanning electron 47

microscopy; Ic, crystallinity index ; I1, I2 : two X-ray intensities; IR: infrared spectroscopy; l: 48

film thickness; MCC: microcrystalline cellulose; NMR: nuclear magnetic resonance; P: 49

permeability; PLA: poly(lactic acid); PSty: polystyrene; RH: relative humidity; S: exchange 50

surface area; SNC: starch nanocrystals; SNC-OOH: SNC hydroperoxide; Sty: styrene 51

monomer unit; TGA: thermogravimetric analysis; TMS: tetramethylsilane; Wsl: work of 52

adhesion; WVTR: water vapour transmission rate; XRD: X-ray diffraction; γlv: liquid-vapour 53

surface tension; γsl: solid-liquid surface tension; γsv: solid-vapour surface tension; ΔP: 54

gradient for partial pressure of water; θ�: contact angle; χc: crystallinity index 55

56

57

Highlights 58

Starch and cellulose nanocrystals were functionalized with polystyrene 59

The grafting was initiated by ozone 60

SNC functionalization was six times more efficient than that of CNC 61

Grafting with polystyrene improved thermal stability of polysaccharide nanocrystals 62

Water barrier properties of polylactide nanocomposites improved by 90% 63

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1. Introduction 69

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Nanopolysaccharide materials, such as cellulose nanocrystals (CNC) or starch nanocrystals 71

(SNC), exhibit unique features, such as excellent mechanical properties, availability, 72

renewability, tunable morphologies and shapes, and high aspect ratios. These features have 73

attracted significant academic interest, resulting in several reviews and books on bio-based 74

nanomaterials (Dufresne, 2012; Habibi, Lucia & Rojas, 2010; Le Corre, Bras & Dufresne, 75

2010). In particular, nanopolysaccharides are used in composite materials as reinforcing 76

agents (Siqueira, Bras & Dufresne, 2010) and as novel functional materials (Habibi, Lucia & 77

Rojas, 2010), or can be chemically functionalised because of the hydroxyl groups at their 78

surface (Angellier, Molina-Boisseau, Belgacem & Dufresne, 2005; Lam, Male, Chong, Leung 79

& Luong, 2012; Tomasik & Schilling, 2004). The functionalities arising from the grafting on 80

the surface of the polysaccharide nanoparticles produce materials with mechanical 81

adaptability (Capadona, Shanmuganathan, Tyler, Rowan & Weder, 2008), pH responsiveness 82

(Way, Hsu, Shanmuganathan, Weder & Rowan, 2012), support for enzyme immobilisation 83

(Mahmoud, Male, Hrapovic & Luong, 2009), induction of electron conduction in bacterial 84

cellulose (Yoon, Jin, Kook & Pyun, 2006), antimicrobial properties (Drogat et al., 2011), the 85

creation of hybrid CNC-DNA complexes (Mangalam, Simonsen & Benight, 2009; Moss, 86

Moore & Chan, 1981), bioimaging fluorescent properties (Dong & Roman, 2007), and that 87

can be used for drug delivery (Román-Aguirre, Dong, Hirani & Lee, 2010). Functionalised 88

SNCs have also been used in the development of hydrogels (Zhang et al., 2010) and aerogels 89

(Garcia-Gonzalez, Alnaief & Smirnova, 2011). 90

91

There has been considerable effort devoted to the functionalisation of polysaccharide 92

nanoparticles with polymers to develop hydrophobic polysaccharides. This should enhance 93

their compatibility with hydrophobic polymer matrices to improve the mechanical, thermal, 94

and/or barrier properties of the nanocomposites. For example, polymers such as polylactides 95

(Braun, Dorgan & Hollingsworth, 2012; Goffin et al., 2011; Peltzer, Pei, Zhou, Berglund & 96

Jimenez, 2014), polycaprolactone (Goffin, Habibi, Raquez & Dubois, 2012; Tian, Fu, Chen, 97

Meng & Lucia, 2014), poly(hydroxybutyrate) (Wei, McDonald & Stark, 2015), polypropylene 98

(Ljungberg, Bonini, Bortolussi, Boisson, Heux & Cavaillé, 2005), poly(ethylene oxide) 99

(Kloser & Gray, 2010), poly(2-ethyl-2-oxazoline) (Tehrani & Neysi, 2013), polyurethane 100

(Cao, Habibi & Lucia, 2009) and polystyrene (Yi, Xu, Zhang & Zhang, 2008) have been 101

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grafted onto CNC. Furthermore, other grafting techniques, such as click chemistry (Chen, Lin, 102

Huang & Dufresne, 2015) and single-electron transfer living radical polymerisation (Wu, Xu, 103

Zhuang & Zhu, 2015) have been used. Despite the low degradation temperature of the SNC, 104

some success has also been reported for the grafting of polylactide (García, Lamanna, 105

D’Accorso, Dufresne, Aranguren & Goyanes, 2012), polycaprolactone (Labet, Thielemans & 106

Dufresne, 2007; Yu, Ai, Dufresne, Gao, Huang & Chang, 2008), polytetrahydrofuran, and 107

poly(ethylene glycol) (Labet, Thielemans & Dufresne, 2007). 108

Surface chemical modification using polymers can be implemented by using either a 109

“grafting-to” or “grafting-from” method. The first method involves the direct grafting of 110

organic molecules via a reaction with the available hydroxyl groups on the surface of the 111

polysaccharide nanoparticles. The main disadvantage of this method is that the grafting of 112

large molecules, such as polymers, rapidly results in the steric hindrance of reactive sites, and 113

consequently produces a very low yield. For this reason, the “grafting-from” approach has 114

been widely used. Here, the first stage involves the grafting of the monomer onto the 115

polysaccharide surface and the second stage involves its polymerisation. This method 116

achieves a greater grafting density and improved control of the reaction (Odian, 2004). 117

However, most of the published “grafting-from” techniques employ organic solvents and 118

harsh energy-intensive conditions, such as high temperatures or pressures (Roy, Guthrie & 119

Perrier, 2005). This topic has been reviewed for cellulose (Roy, Semsarilar, Guthrie & Perrier, 120

2009), starch (Athawale & Rathi, 1999), and for nanoscale cellulose (Lin, Huang & Dufresne, 121

2012; Missoum, Belgacem & Bras, 2013). 122

To develop reactions that comply with the principles of green chemistry (Anastas & 123

Kirchhoff, 2002), grafting should be performed in an aqueous medium, with the efficient 124

activation of substrates generating few, if any, residues. The use of ozone as an activator to 125

graft polysaccharides was first reported in the 1950s (Borunsky, 1957; Kargin, Koslov, Plate 126

& Konoreva, 1959). Ozone is cheap, used industrially, can be used in aqueous solvents, and 127

leaves no residues. Furthermore, the oxygen-containing radicals produced by ozone react with 128

a large variety of monomers. There are a few reports on the production of cellulose and starch 129

grafted copolymers using ozone activation (Bataille, Dufourd & Sapieha, 1994; De Bruyn, 130

Sprong, Gaborieau, Roper & Gilbert, 2007; Morandi, Heath & Thielemans, 2009; Song, 131

Wang, Pan & Wang, 2008). De Bruyn et al. (De Bruyn, Sprong, Gaborieau, David, Roper & 132

Gilbert, 2006) used this method to graft highly branched starch with various synthetic 133

polymers, such as polystyrene, using free-radical polymerisation where the ozone-activated 134

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amylopectin chains functioned as macro-initiators. Their optimised method yielded a latex 135

where the starch was completely encapsulated by the hydrophobic synthetic polymer. 136

Researchers recently demonstrated that poly(lactic acid) (PLA) exhibits affinity for aromatic 137

structures (Salazar, Domenek & Ducruet, 2014). The grafting of styrene moieties to the 138

surface of polysaccharide nanocrystals may therefore yield a compatibilising effect. The 139

grafting of hydrophobic molecules interferes with hydrogen bonding between the 140

polysaccharide nanocrystals and thus reduces agglomeration. Furthermore, compatibility with 141

hydrophobic polymers can be improved, which aids dispersion. The crystalline structures of 142

cellulose and starch nanoparticles impact on several applications. They provide improved 143

reinforcement or barriers when dispersed in polymer materials. The aim of this study is to 144

functionalise polysaccharide nanocrystals in a heterogeneous system under mild conditions 145

whilst retaining their crystalline structure. The approach uses ozone to add surface peroxide 146

functionality to the CNC and SNC; subsequently, surface-initiated graft polymerisation occurs 147

by thermolysis and redox activation. This is the first time such a strategy has been proposed 148

for SNC and CNC. A comparison between the reactivity of each polysaccharide will be 149

performed. Moreover, both nanocrystals have different shape factors. CNC and SNC exhibit 150

rod-like and platelet-like shapes, respectively; this allows evaluation of the effect of the 151

hydrophobic surface grafting, as well as the effect of the morphological factors on the barrier 152

properties of the nanocomposites. 153

154

2. Experimental 155

156

2.1 Materials 157

Microcrystalline cellulose (MCC) powder was acquired from Sigma-Aldrich (France) and 158

used as a raw material for the production of CNC. For the production of SNC, native waxy 159

maize starch was kindly provided by Cargill (Krefeld, Germany). Sulfuric acid (95%) and 160

styrene were obtained from Sigma-Aldrich. The sodium acetate buffer (0.1 mol L-1) was 161

prepared from sodium acetate (Sigma Aldrich) and glacial acetic acid (Merck). Ethanol, 162

acetone, chloroform, and toluene were purchased from Merck and used as supplied. The 163

styrene inhibitors were removed on a basic alumina column (Al2O3 90 active, Merck); the 164

purified styrene was stored at 4 °C for a maximum of five days before use. 165

166

2.2 Production of cellulose and starch nanocrystals 167

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The preparation of the CNC followed an adaptation of the optimised method developed by 168

Bondeson et al. (Bondeson, Mathew & Oksman, 2006). MCC at 7.1 wt% was dispersed in 169

water. Sulfuric acid was slowly added to a concentration of 64 wt%. The CNC in the acid 170

suspension was subsequently hydrolysed at 44 °C for 130 min under mechanical stirring. The 171

waxy maize hydrolysis followed the method employed by Angellier et al. (Angellier, 172

Choisnard, Molina-Boisseau, Ozil & Dufresne, 2004). In this case, acid hydrolysis is gentler 173

but requires a longer period to produce SNC, i.e. 5 days at 40 °C. A solution of 28 wt% 174

sulfuric acid was prepared before the addition of the waxy maize starch to a concentration of 175

12.8 wt%. In both cases, the excess sulfuric acid was removed by the application of water 176

exchange/centrifugation cycles. Subsequently, the resulting suspensions containing the 177

nanocrystals were dialysed in deionised water over one week for the CNC and two days for 178

the SNC, and homogenised using an Ultra-Turax T25 homogeniser (France). Finally, the 179

samples were subjected to an ultrasound source to achieve satisfactory dispersion of the 180

nanocrystals, and subsequently neutralised and stored at 4 °C. A drop of chloroform was 181

added to prevent microbial development. Considering the SNC, recent studies show the 182

presence of microparticles following the hydrolysis procedure (LeCorre, Bras & Dufresne, 183

2011). Therefore, the suspension was filtered using a coarse filter tissue of 31 �m to remove 184

granule ghosts. 185

186

2.3 Functionalisation of polysaccharide nanocrystals 187

The functionalisation of the SNC and CNC with styrene by radical polymerisation was 188

adapted from previous work on gelatinised starch in an aqueous solution (De Bruyn, Sprong, 189

Gaborieau, David, Roper & Gilbert, 2006). 190

Ozonolysis of polysaccharides. Either CNC or SNC at 2 wt% was dispersed in 200 mL of 191

sodium acetate buffer at pH = 3.5. The ozonolysis used an ozone generator and the electric 192

discharge was controlled by a Wallance and Tiernan (Pennwalt) regulator. Polysaccharide 193

nanocrystals were oxidised by a stream of ozone, using oxygen as the carrier gas. This 194

procedure was conducted at a temperature of 10 °C using 0.85 wt% of ozone at 50 L h –1. 195

Free radical polymerisation of polysaccharide nanocrystals. The hydroperoxide 196

nanocrystals dispersed in the buffer system were placed in a three-necked round-bottomed 197

flask equipped with septa and ports for the addition of reagents and supply of gas. Nitrogen 198

gas was bubbled into the suspension over 1 h. The reaction temperature was set at 77 °C and 199

57 °C for the CNC and SNC, respectively, and the mixture was maintained under stirring. 200

Subsequently, 10 g of purified and nitrogen-purged styrene (Aldrich) and 0.17 g of potassium 201

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persulfate (Merck) were added to the system. Grafting and free-radical polymerisation 202

occurred over 36 h. 203

To remove the unreacted styrene and any free polystyrene, the products (CNC-g-PSty and 204

SNC-g-PSty) were purified by dispersion-centrifugation which was performed three times 205

(for each solvent) with ethanol, acetone, and toluene and chloroform, respectively (at 10,000 206

rpm at 4 °C for 10 min). Generally, Soxhlet extraction efficiently eliminates the adsorption of 207

non-grafted by-products. Unfortunately, this technique has not been adapted for nanosized 208

components because they are not retained in the extraction cartridge. Therefore, dispersion-209

centrifugation washing was implemented following the method used in previous literature 210

(Siqueira, Bras & Dufresne, 2010) which demonstrates a washing efficiency identical to that 211

of the Soxhlet extraction. 212

213

2.4 Characterisation of polysaccharide nanocrystal morphology 214

Field-Emission Gun Scanning Electron Microscopy (FEG-SEM). Unmodified and 215

modified CNC and SNC were characterised by field emission gun scanning electron 216

microscopy (FEG-SEM) using a FEG SEM (FEI Quanta 200, FEI Company, USA) with an 217

accelerating voltage of 12.5 kV and a working distance of 6.2 to 6.7 mm. Samples were 218

placed onto a substrate using carbon tape and coated with a thin gold layer. Digital image 219

analysis (ImageJ Software) was used to measure the dimensions of the nanoparticles. 220

Atomic Force Microscopy (AFM). AFM was used to determine and compare the crystal 221

morphologies of the various CNCs obtained. Measurements were performed on a multimodal 222

AFM (DI, Veeco, USA) with both tapping and conductive modes. Following the spin-coating 223

of the CNC suspension at 0.01 wt% on a mica substrate and drying at room temperature, CNC 224

could be observed. At least five images per sample were captured and a representative image 225

of the samples was selected. 226

X-Ray diffraction (XRD). X-ray analyses were performed using a Panalytical X’Pert Pro 227

MPD-Ray diffractometer equipped with an X’celerator detector and operated with Ni-filtered 228

Cu, K� radiation (� = 1.54 Å) generated at a voltage of 45 kV and current of 40 mA, and 229

scanned from 5° to 60°. The crystallinity index Ic for the unmodified and modified CNC was 230

evaluated using the Buschle-Diller and Zeronian equation (Park, Baker, Himmel, Parilla & 231

Johnson, 2010): 232

1

2

1 cIII

= − (1) 233

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Here I1 is the intensity at the minimum (2θ = 18.8°) and I2 that associated with the crystalline 234

region of the cellulose (2θ = 22.7°). For the SNC samples, the crystallinity index was 235

determined according to a two-phase methodology, which involves the division of the area 236

under the crystalline peaks by the total area of the amorphous halo under the diffractogram. 237

This approach was originally proposed by Hermans and Weidinger (Hermans & Weidinger, 238

1948) and adapted for starch by Lopez-Rubio et al. (Lopez-Rubio, Flanagan, Gilbert & 239

Gidley, 2008). The subtraction of the amorphous portion from the total area of the 240

diffractogram was performed by deconvolution using OriginPro 8.6 software. At least one 241

replicate was performed for all analyses. 242

243

244

a) b) c)

200 nm 200 nm

245 Figure 1. a) Birefringence behaviour of CNC, and atomic force microscopy images of b) 246

CNC and c) SNC. 247

248

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249 Table 1. Crystallinity index (�c), elemental analysis, work of adhesion (Wsl), and water 250

contact angle of neat and modified CNC and SNC. 251

252

Elemental Analysis Sample �c(%) %C %O Wsl (mJ m

-2)

Contact angle (°)

CNC 81a 44.4 49.3 112.2 ± 0.2 57.2 ± 0.2

CNC-OOH 77a ND ND ND ND

CNC-g-PSty 68a 54.8 44.3 87.5 ± 3.8 78.3 ± 3.1

SNC 43b 44.4 49.3 126.6 ± 0.5 42.3 ± 0.6

SNC-OOH 36b ND ND ND ND

SNC-g-PSty 33b 79.6 18.6 63.5 ± 1.2 97.3 ± 0.9

acalculated by peak height method.

bcalculated by two-phase method, following previous

work (Lopez-Rubio, Flanagan, Gilbert & Gidley, 2008) 253

2.5 Evidence of polysaccharide nanocrystals functionalisation 254

Determination of hydroperoxides on CNC and SNC. The quantity of the hydroperoxides 255

on the CNC (CNC-OOH) and SNC (SNC-OOH) was determined by iodometric titration. 256

Either 10 mL of CNC-OOH or SNC-OOH suspension was purged with nitrogen over 30 min. 257

Subsequently, 45 mL of N2-purged sulfuric acid (1 mol L-1), 1 g of sodium iodide (in excess), 258

and three drops of the catalyst ammonium molybdate [(NH4)6Mo7O24] at 3 wt%, were added. 259

Finally, the iodine produced was titrated with a nitrogen-purged sodium thiosulfate solution (5 260

× 10-3 mol L-1) under a nitrogen atmosphere. 261

Infrared spectroscopy (IR). IR spectroscopy was performed on the grafted and neat CNC 262

and SNC in the form of KBr pellets using a Spectrum 65 FT-IR Spectrometer (PerkinElmer, 263

USA). All spectra were recorded in the frequency range of 4000-500 cm-1, with a resolution 264

of 4 cm-1 for 16 scans. The spectra were recorded twice for the various samples of each 265

composition to verify the reproducibility of the results. 266

Solid-state Nuclear Magnetic Resonance (NMR). Solid state 13C NMR experiments were 267

performed on a Bruker AVANCE 400 spectrometer. All spectra were recorded using a 268

combination of cross-polarisation (CP), high-power proton decoupling, and magic angle 269

spinning (CP/MAS). 13C spectra were acquired at 298 K with a 4 mm probe operating at 270

100.13 MHz; the MAS rotation was 12 kHz and the CP contact period varied from 0.5 to 12 271

ms with a repetition period of 3 s. The chemical shift values were measured with respect to 272

tetramethylsilane (TMS) using glycine as a secondary reference, with the carbonyl signal set 273

to 176.03 ppm. 274

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Elemental Analysis. Elemental analysis was performed by the Service Central d’Analyse 275

(Vernaison, France) of the Centre National de la Recherche Scientifique (CNRS). The carbon, 276

hydrogen, and oxygen contents were measured for the neat and modified polysaccharide 277

nanocrystals. The precision of the measurement is usually greater than the standard deviation 278

and is typically determined as ±0.2% at maximum. 279

Thermogravimetric analysis (TGA). TGA was conducted to determine the thermal stability 280

of the neat and modified nanocrystals (STA 6000, Perkin Elmer, USA). The samples were 281

heated from 30–800 °C at a rate of 10 °C min-1, under a stream of nitrogen at 100 mL min-1. 282

Contact angle measurements. The dynamic behaviour of the contact angle for a drop of 283

deionised water (purified by micropore filtration) on the surface of a film incorporating either 284

neat CNC or SNC, and a pressed tablet consisting of modified nanocrystals was measured 285

over 1 min. Thereafter, the water adsorption into the samples was observed. The measured 286

contact angle was used in Young’s equation (Zhang & Kwok, 2002): 287

lv sv slcos γγ θ γ γ= − (2) 288

where �lv, �sv, and �sl are the surface tensions of the liquid-vapour, solid-vapour, and solid-289

liquid interfaces, respectively, and �� is the contact angle. The adhesion work (Wsl) required 290

to separate a unit area of the solid-liquid interface, related to the work of adhesion per unit 291

area of a solid and liquid, is subsequently obtained using the equation: 292

sl lv sv slW γ γ γ= + − (3) 293

Together, equations 2 and 3 give Wsl as a function of just �lv and ��: 294

(1 )sl lvW cos γγ θ= + (4) 295

A contact angle measurement instrument (DataPhysics, Germany) equipped with a charge-296

coupled device camera was used, allowing the determination of the contact angle at 297

equilibrium with a precision of ±1°. Following a few tens of milliseconds after the deposition 298

of the drop, the kinetics of its evolution was observed by capturing 1,000 images per second. 299

Each reported value of Wsl is the average of at least three measurements. 300

301

2.5 Production of nanocomposite films 302

Neat PLA and nanocomposite films were prepared using the casting method. The unmodified 303

and modified CNC and SNC were solvent-exchanged to chloroform by centrifugation and 304

dispersed at 0.5 wt%. The dispersion was conducted by performing sonication three times for 305

5 min at 40% amplitude (Branson 250 Sonifier, Germany). PLA pellets were added to both 306

the CNC and SNC dispersions, in order to achieve a nanofiller concentration of 2 wt%. The 307

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system was stirred at room temperature for 3 h until the PLA was completely dissolved. 308

Subsequently, the dispersion was sonicated for three periods of 5 min at an amplitude of 40%, 309

and cast into glass dishes. The solvent was evaporated at room temperature for one day, and 310

subsequently stored over P2O5 for at least one month to ensure that it was completely dry prior 311

to analysis. The films were 100–150 �m thick. 312

313

2.5.1 Water vapour permeability 314

The water vapour transmission rate (WVTR) of the films was measured according to the 315

standard procedure, NF H 00-030, at 25 °C and 90% relative humidity (RH). The procedure 316

involved measuring the weight uptake (using a Kern 770 balance, Kern, Germany (precision 317

0.01 mg)) of dried CaCl2 within a metal cap sealed by the sample films, tightened with the aid 318

of beeswax. The area of the specific exchange surface was 50.24 cm2. To control the 319

environmental conditions, the cups were placed in a climatic chamber (VCN100, Vötsch 320

Industrietechnik, Germany). The WVTR (kg m m-2 s-1) was obtained from the slope of the 321

weight uptake using equation 5: 322

323

slope lWVTRS

= (5) 324

where slope, S, and l correspond to the slope of the plot of cumulated weight against time, the 325

exchange surface area, and the film thickness determined before measurement, respectively. 326

The given values were the average values obtained from two or three experiments. The 327

WVTR was normalised to the permeability (P), taking into account the pressure gradient 328

between each side of the film using equation 6: 329

330

2 H OWVTRP

P= (6) 331

where ΔP is the gradient for the partial pressure of water (Pa). 332

333

3. Results and discussion 334

335

3.1 Characterisation of polysaccharide nanocrystals 336

The concentration of the SNC and CNC suspensions in water were 1.73% and 2.37%, 337

respectively, resulting in high viscosity. Birefringence of the CNC suspensions using cross-338

polarised light (Fig. 1a) showed that ordered regions are present, indicating a high level of 339

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crystallinity of the CNC. The AFM images showed the rod-like and spherical shapes 340

characteristic of CNC (Fig. 1b) and SNC (Fig. 1c), respectively. The average length and 341

diameter of the CNC were 191.7 ±60.8 nm and 6.4 ±2.0 nm, determined by 70 and 52 342

measurements, respectively. The average length and diameter of the SNC were 53.2 ±9.4 nm 343

and 32.2 ± 4.9 nm, determined by 57 and 35 measurements, respectively. These values 344

correspond to the dimensions reported in literature for both CNC and SNC (LeCorre, Bras & 345

Dufresne, 2011; Namazi & Dadkhah, 2010; Ramires & Dufresne, 2011). Figure 2 shows the 346

corresponding X-ray diffractograms. For the CNC, the peaks located at approximately 15.7° 347

and 22.6° are characteristic of cellulose I (Sèbe, Ham-Pichavant, Ibarboure, Koffi & Tingaut, 348

2012). The SCN diffractograms provided evidence for the A-type crystal structures of starch 349

(Lopez-Rubio, Flanagan, Gilbert & Gidley, 2008). Unlike the CNC, the SNC exhibited an 350

amorphous halo. The degrees of crystallinity for both nanopolysaccharides are given in Table 351

1, and correlate with previous literature results. The SNC has a lower degree of crystallinity 352

than the CNC. In both cases, transparent films were produced as a result of drying the 353

suspensions, consistent with the nanoscale dimensions of the particles. 354

355

3.2 Ozonolysis of polysaccharide nanocrystals 356

Surface grafting on the polysaccharide nanocrystals was performed in two stages, as shown in 357

scheme 1: (i) activation of polysaccharide nanoparticles and (ii) free-radical polymerisation. 358

The oxidation of gelatinised starch to peroxidised starch by ozonolysis has previously been 359

described (De Bruyn, Sprong, Gaborieau, David, Roper & Gilbert, 2006), where a trade-off 360

between starch degradation and hydroperoxide yield was observed following 3.5 h of 361

treatment at 10 °C. De Bruyn et al. (De Bruyn, Sprong, Gaborieau, David, Roper & Gilbert, 362

2006) studied a soluble polysaccharide in a homogeneous system, while here, the 363

polysaccharide nanoparticles were crystalline and insoluble in water. The hydroperoxidation 364

efficiency, defined as the percent of hydroxyl groups converted into hydroperoxides, was 365

measured as a function of the ozonolysis period, as shown in figure 3. The conversion rate 366

was lower than the value of 9.3% for the hydroxyl groups of gelatinised starch following 3 h 367

(De Bruyn, Sprong, Gaborieau, David, Roper & Gilbert, 2006). The ozonolyis efficiency 368

depends on the ozone fraction generated by the particular device used and the nature of the 369

polysaccharide. In this respect, the treatment for the SNC was more efficient than for the 370

CNC. As shown in figure 3, approximately 1.5% of the hydroxyl groups were converted into 371

CNC-OOH when the ozonolysis treatment was performed for 3.5 h, while the same 372

percentage was converted after just 1 h for the SNC-OOH. We consider that the oxidation 373

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efficiency of the ozonolysis treatment is related to the crystallinity index of the nanocrystals. 374

CNC has higher crystallinity (i.e. 81.0%) than SNC (i.e. 43.4%), indicating that the SNC 375

contains a larger amount of amorphous phase than the CNC (Le Corre, 2009), where hydroxyl 376

groups are likely to be much more accessible. 377

378

5 10 15 20 25 30 35 40

2θ (°)

CNC CNC O3CNC‐g‐PSty

5 10 15 20 25 30 35 40

2θ (°)

SNC SNC O3SNC‐g‐PSty

379 Figure 2. X-Ray diffractograms of neat, ozonolysed, and polystyrene-grafted CNC (left) and 380

SNC (right). 381

382

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382 383

Scheme 1. Synthesis of polysaccharide nanocrystals grafted with polystyrene (CNC-g-PSty 384

and SNC-g-PSty) by ozonolysis initiation. 385

2I‐/2H+

N2

N2

= CNC‐OHO

OHO

OHO

OH

O

OH.OH

OH

.

n

CNC‐OHO3 (0.85%/50 Lh

‐1)

10 oCpH=3.5

CNC‐OOH

Δ (77 oC CNC, 57 oC SNC)

S2O82‐

CNC‐O.

CH2

36h

*O‐CNCPolymerization

.

.

CNC‐O

CNC‐OH + I2 + 2H2O

2S2O32‐

2I‐ + S2O62‐

Free radical polymerization Hydroperoxide determination 386 387

0

1

2

3

4

5

0 1 2 3 4 5

% of h

ydroxyl group

s con

verted

into

hydrop

eroxides

Time of ozonolysis treatment (h)

SNC

CNC

388 Figure 3. Effect of time on the hydroperoxidation efficiency of SNC and CNC in a 2 wt% 389

aqueous suspension at pH = 3.5 and 10 °C. 390

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O3b

a

d

c

391 Figure 4. FEG-SEM images of polysaccharide nanocrystals: a) CNC b) SNC, and following 392

the ozonolysis treatment: c) CNC-OOH and d) SNC-OOH. 393

394

Since ozonolysis is commonly used to bleach wood pulp, a potential drawback of the use of 395

ozone is the degradation of the nanoparticles. Some studies show modification of the 396

properties of paper pulp when ozonolysis was used for bleaching (Kaneko, Shuji, Kenji & 397

Junzo, 1983; S. C. Puri & Anand, 1986; Van Nifterik, Xu, Laurent, Mathieu & Rakoto, 1993). 398

Even in a dry state, Bataille et al. (Bataille, Dufourd & Sapieha, 1994) showed damage of the 399

cellulose fibres following direct corona treatment over 10 min using a current of 7–20 mA. As 400

mentioned previously, the objective here is to create a chemically modified surface, while 401

maintaining as much of the intact structure as possible to retain the reinforcement properties 402

of the nanocrystals on dispersion in a polymer material. Figure 4 shows FEG-SEM images of 403

the morphology of the CNC and SNC before and after ozonolysis. Following ozonolysis, the 404

CNC-OOH had a length of 170.6 ± 22.0 nm, which is not significantly different to the original 405

CNC dimensions (initial length 191.7 ± 60.8 nm). The SNC-OOH crystals had a length of 406

43.2 ± 10.3 nm, which was significantly shorter than that of the original SNC (initial length 407

53.2 ± 9.4 nm). The X-ray diffractograms of the SNC-OOH and CNC-OOH are presented in 408

figure 2, along with those of the original SNC and CNC. There was no change in the peak 409

positions, indicating that the crystalline morphology remained essentially unchanged. 410

However, there was a 4% and 7% decrease in the crystallinity index (table 1) between the 411

CNC and CNC-OOH, and between the SNC and SNC-OOH, respectively, indicating the 412

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lower stability of the SNC. Nevertheless, the residual crystallinity was relatively high, 413

suggesting that the crystalline structure within the core of the nanoparticles was preserved. 414

500100015002000250030003500

Transm

ittance

Aromatic C‐Hstretch

Aliphatic C‐Hstretch

Aromatic C‐Cstretch

Aromatic substitution

a

bcd

415 Figure 5. IR spectra of neat cellulose (CNC) and starch (SNC) nanocrystals and styrene-416

grafted nanocrystals. Curves correspond to a) CNC, b) CNC-g-PSty, c) SNC, and d) SNC-g-417

PSty. 418

419

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419 Table 2. Chemical shift assignments in 13C NMR spectra for neat and functionalised 420

polysaccharide nanocrystals. 421

C1

C4

C2, C3, C5

C6R = H

C1

C4

C2, C3, C5

C6

CX

CU, CV, CW

CYCZ CZ

CY

CU, CV, CW

CX

R = H

R = H or

O

OROOR

O

OR

O*

OR

OR

OR*

123

4 56

O

ORO

OR*

OR

O

ORO

OR

OR

*

n

123

4 56

SNCCNC

CNC‐g‐PSty SNC‐g‐PSty

**

n

ZY

XW

VU

422 Figure 6. 13C solid state NMR spectra of neat and styrene-grafted cellulose (CNC) and starch 423

nanocrystals (SNC). 424

425

426

3.3 Evidence of grafting of polysaccharide nanocrystals by free-radical polymerisation 427

After the polysaccharide nanocrystals were oxidised by the ozone, the hydroperoxide 428

nanocrystals were heated and sodium persulfate was added to reduce the peroxide group and 429

create a free radical on the oxygen atom of the hydroperoxide polysaccharide (scheme 1). 430

Styrene was added after the radical formation step, and free-radical polymerisation was 431

performed at 57 °C (SNC-OOH) or 77 °C (CNC-OOH). The growing polystyrene chains are 432

terminated by bimolecular termination or transfer to monomer. Extensive washing of the 433

Chemical shifts (ppm) Sample

C1 C4 C2,3,5 C6 CX CU,V,W CY CZ Sty/AGU

CNC 105 83, 89 71-75 62, 65

CNC-g-PSty 105 83, 89 71-75 62, 65 146 127, 128 40 30 0.24

SNC 98-101 81 70-76 62

SNC-g-PSty 99-101 71-76 62 146 126, 128 40 30 1.56

Sty: styrene monomer, AGU: anhydroglucose unit.

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reaction products via the centrifugation-dispersion method was performed to purify the 434

grafted nanocrystals and remove the free styrene and oligostyrene. 435

The functionalisation of the nanocrystals was confirmed by IR spectroscopy analysis (Fig. 5). 436

The IR spectra showed a broad absorption band characteristic of the glucosidic ring of 437

cellulose and starch between 3500 and 3200 cm-1. The peaks between 3100 and 3000 cm-1 438

(detected only for the grafted samples) were assigned to the aromatic C-H vibrations of 439

styrene, while the peaks at 2922 and 2855 cm-1 corresponded to aliphatic C-H stretches. 440

Bands at 1495 and 1454 cm-1, associated with C-C vibrations from the aromatic ring, were 441

identified for the two grafted samples. For the grafted SNC in particular, a typical peak of 442

several aromatic substitutions can be clearly observed at 700 cm-1. 443

444

Further evidence of grafting was provided by solid state 13C NMR. Figure 6 shows the spectra 445

before and after the modification; table 2 shows the chemical shifts. For the CNC 13C NMR 446

spectra, the region from 61–66 ppm was assigned to carbon C6; this signal was split into a 447

contribution at 62 ppm from the amorphous non-hydrolysed cellulose chains on the surface, 448

and a signal at 65 ppm arising from the crystalline portion. The region at 70–75 ppm was 449

attributed to indistinguishable carbons C2, C3, and C5 which do not contribute to the β(1-4) 450

link. The region from 82–90 ppm was assigned to C4 carbons, the wide signal at 83 ppm to 451

the amorphous cellulose chains, and that at 89 ppm was assigned to the crystallised chains. 452

Finally, the region between 104–106 ppm was attributed to the C1 carbons of the native 453

cellulose; all these signal shifts are identical to those reported in literature (Azzam, Heux, 454

Putaux & Jean, 2010; Shang, Huang, Luo, Chang, Feng & Xie, 2013). Comparing the spectra 455

of the SNC and CNC, it can be observed that the signals were shifted to a lower frequency in 456

the case of C1-3, 5. Simultaneously, the signals for C4 and C6 did not demonstrate split peak 457

behaviour. In this case, we could not determine the relative contributions of the amorphous 458

and crystalline components. 459

Upon grafting, no peak shifts associated with CNC and SNC were observed; however, new 460

signals appeared. Following the grafting of the styrene, four characteristic resonance peaks 461

were detected: two peaks corresponding to aliphatic carbons and two corresponding to 462

aromatic carbons. The small signal at approximately 30 ppm corresponds to the CZ carbon of 463

the methylene group of the backbone chain; the value at 40 ppm is ascribed to methine carbon 464

(CY) which connects the phenyl rings to the backbone chain. The region between 126–128 465

ppm was assigned to the highest polystyrene peak, which corresponds to the protonated 466

aromatic carbons (CU, CV, and CW), while the final peak at 146 ppm is related to the non-467

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protonated aromatic carbon (CX). These results correlate with those available in literature 468

(Pimentel, Durães, Drummond, Schlemmer, Falcão & Sales, 2007), where sulfonated 469

polystyrene (Cánovas, Sobrados, Sanz, Acosta & Linares, 2006) and poly(ethylene glycol) 470

grafted polystyrene (Siyad & Kumar, 2012) were characterised. 471

The sensitivity of the NMR analysis allowed the quantification of the grafting efficiency 472

under high polarisation conditions. For this, the spectra were normalised using C1 signals as 473

an internal standard for both the CNC and SNC. The signals were integrated, compared with 474

the integrated peak of the CX of the polystyrene signal, and corroborated with the CY and CZ 475

integrated signals. Since the styrene polymerisation would have resulted in a wide distribution 476

of molecular weights as typically observed for uncontrolled free-radical polymerisation, it 477

was not possible to determine the percent of functionalized hydroxyl groups, or the average 478

polystyrene molecular weight. However, we can quantify the ratio between the styrene and 479

the anhydroglucose (AGU) units. Table 2 summarises these data. The value of 1.56 indicates 480

that there were 1.56 molecules of styrene for each AGU of the starch chain. For the CNC, 481

there were 0.24 molecules of styrene for each AGU of the cellulose chain. This confirmed the 482

lower grafting efficiency on the CNC, presumed to arise from the higher crystallinity of the 483

CNC. Size exclusion chromatography (SEC) analysis was attempted to measure the graft 484

length. However, the stability of the nanoparticles prevented any measurement despite several 485

tests using enzymes or hydrolysis conditions for the elimination of polysaccharide entities. 486

This NMR analysis proves that grafting occurs either with polystyrene, oligostyrene, or 487

styrene. Based on previous literature, and the process conditions, the authors feel that 488

polystyrene (with low degree of polymerisation) is predominantly present at the surface. To 489

further quantify the grafting of the polysaccharides with styrene, elemental analysis was 490

performed to determine the degree of substitution, following the procedure for single-491

molecule grafting (Vaca-Garcia, Borredon & Gaseta, 2001). In principle, the molecular 492

weight distribution of the starch grafts could be determined by removing any ungrafted 493

polymer, completely digesting the starch moiety with α-amylase, and determining the 494

molecular weight distribution of the residual polystyrene component of the graft. An estimate 495

of the grafting efficiency was obtained from the oxygen/carbon weight ratio. Theoretically, 496

the weight ratio between the oxygen and carbon atoms of the AGU is 1.11, which corresponds 497

to elemental weight fractions of 44.4 and 49.4% for carbon and oxygen, respectively. 498

However, the experimental values for carbon and oxygen are 39.1 and 48.2 for the CNC, and 499

39.5 and 49.8 for the SNC respectively; these results give weight ratios (oxygen/carbon) of 500

1.23 and 1.26 for the neat CNC and SNC, respectively. This difference can be explained by 501

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the presence of certain oxygen-rich impurities and sulfate groups, and by experimental error. 502

A correlation has already been proposed (Missoum, Bras & Belgacem, 2012; Siqueira, Bras & 503

Dufresne, 2010) to correct the differences between the theoretical and experimental results. 504

Table 1 shows the corrected values. The elemental analysis of the grafted polysaccharide 505

nanocrystals shows a clear increase in the carbon content, and the oxygen/carbon ratio 506

decreased from 1.11 (corrected values) to 0.80 for the CNC-g-PSty and 0.23 for the SNC-g-507

PSty. This difference indicates that there were almost four times more aromatic groups 508

grafted on the SNC than on the CNC. A rough calculation demonstrates that approximately 63 509

wt% of excess carbon was linked to the aromatic graft of the SNC-g-PSty and there was 510

approximately 15 wt% of excess carbon in the CNC-g-PSty. 511

Finally, under the assumption of efficient washing, the spectroscopy analysis provides good 512

evidence of the grafting of polystyrene onto the CNC and SNC surfaces. 513

514 Figure 7. TGA thermograms of neat and modified cellulose nanocrystals (a) and starch 515

nanocrystals (b). 516

517

3.4 Thermal and surface properties of grafted polysaccharide nanocrystals 518

The highly localised intermolecular forces between both types of polysaccharide nanocrystal 519

and water resulted in high work of adhesion values and low water contact angles for both 520

types of nanopolysaccharide (Table 1). The Wsl of the SNC is slightly higher than that of the 521

CNC; this can be ascribed to its larger surface area, or the presence of microparticles. 522

Following surface grafting, the Wsl decreased as expected, because of the hydrophobic 523

styrene-based surface graft. The Wsl of the SNC-g-PSty (63.5 mJ m-2

) was lower than that of 524

the CNC-g-PSty (87.5 mJ m-2

), consistent with the higher grafting efficiency on the SNC. It 525

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is, however, interesting that even with a relatively low level of grafting on the CNC-g-PSty 526

(0.24 mol of styrene per mol of AGU), there was still a substantial increase in the contact 527

angle. This could be sufficient to yield improved compatibility and dispersibility of CNC 528

within the hydrophobic matrices. 529

The properties of the grafted polysaccharides were also monitored by TGA to study the 530

feasibility of the further production of bionanocomposites. The thermal degradation behaviour 531

of the pure and grafted CNC and SNC is given in figure 7. For the CNC (Fig. 7a), the 532

unmodified samples showed the greatest weight loss (65.1 wt %) at 266 °C; the second 533

degradation temperature is due to the sulfate groups from the persulfate initiator attached to 534

the CNC. The CNC-g-PSty decomposition peak shifted to high temperatures (to 303 °C), 535

indicating improved temperature resistance. The second weight loss of ~15% corresponded to 536

the grafted polystyrene (at 426 °C), correlating with values obtained by elemental analysis. 537

The neat SNC exhibited weight loss (79.6%) at a lower temperature range (210–530 °C) than 538

the CNC; however, the maximum decomposition rate was achieved at just 306 °C. Following 539

the functionalisation process, a second thermal degradation stage is clearly observed at 434 540

°C, which is assigned to the polystyrene graft. This degradation temperature was slightly 541

higher than that of the PSty graft on the CNC. There was 60% weight loss at 434 °C, which 542

correlates with the values obtained from the elemental analysis. Such a shift should not be 543

possible for styrene grafting alone. This also supports our assumption related to the presence 544

of polystyrene at the surface. 545

546

3.5 Effect of CNC and SNC functionalisation on morphological and barrier properties in 547

nanocomposite films 548

Figure 8 shows images of the neat PLA, as well as the nanocomposites incorporating 2 wt% 549

unmodified and modified CNC and SNC. This percentage was lower than both the percolation 550

threshold of CNC (approximately 2.7 %), (Espino-Perez, Bras, Ducruet, Guinault, Dufresne & 551

Domenek, 2013) and SNC. Previous researchers (Follain, Belbekhouche, Bras, Siqueira, 552

Marais & Dufresne, 2013) demonstrated that the quantity of water absorbed by the films 553

increases when the percolation threshold is reached, which can negatively impact on the water 554

vapour barrier properties. The films were transparent at low concentrations in the case of the 555

CNC; however, for the unmodified nanocrystals a small amount of agglomeration and a slight 556

amount of opalescence were observed. This result was also obtained by other authors (Espino-557

Perez, Bras, Ducruet, Guinault, Dufresne & Domenek, 2013; Fortunati et al., 2012) and may 558

be explained by the partial agglomeration of the nanocrystals in the PLA matrix. Following 559

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the incorporation of 2 wt% neat SNC, many more agglomerates were observed owing to their 560

greater surface hydrophilicity (table 1) compared with the incorporation of the CNC. When 561

the SNC-g-PSty were incorporated, no distinct agglomerates were observed; however, there 562

was a complete loss of transparency and the films became white. 563

Figure 9 shows the water vapour barrier properties of the neat PLA and its nanocomposite 564

films incorporated with 2 wt% CNC or SNC. The water vapour permeability values for the 565

neat PLA and nanocomposites incorporated with CNC are similar to those shown in literature 566

(Delpouve, Stoclet, Saiter, Dargent & Marais, 2012; Espino-Perez, Bras, Ducruet, Guinault, 567

Dufresne & Domenek, 2013; Fortunati, Peltzer, Armentano, Jimenez & Kenny, 2013; Luo & 568

Daniel, 2003; Sanchez-Garcia, Gimenez & Lagaron, 2008); the values obtained in our study 569

are slightly greater since the test was performed at 90% RH. There was a slight improvement 570

of 16 and 32% for the water vapour permeability of the CNC and CNC-g-PSty, respectively, 571

compared with that of the neat PLA. Moreover, much greater differences were observed in the 572

case of the SNC-reinforced nanocomposites. The unmodified SNC agglomerates resulted in 573

very high values of water vapour permeability. However, following the functionalisation of 574

the SNC (SNC-g-Sty), the water vapour permeability decreased by 52%, indicating the 575

potential of the SNC treatment. 576

The shape factors of the CNC and SNC are different, as shown in figure 1. The platelet shape 577

of the SNC is more favourable for the development of a tortuous permeation pathway, which 578

may provide one reason for the improved performance of the grafted SNC. 579

580

b)a) PLA + PLA +

SNC SNC‐g‐PStyCNC CNC‐g‐PSty 581

Figure 8. Photographs of PLA films compared to nanocomposites reinforced with unmodified 582

and unmodified CNC (a) and SCN (b) at 2 wt%. 583

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PLA CNC CNC-g-PSty SNC SNC-g-PSty0.0

5.0x10-15

1.0x10-14

1.5x10-14

2.0x10-14

2.5x10-14

3.0x10-14

2.5x10-13

3.0x10-13

3.5x10-13W

ater

Vap

our P

erm

eabi

lity

(Kgm

m-2s-1

Pa-1)

585 Figure 9. Water vapour of neat PLA and its nanocomposites, unmodified and modified CNC 586

and SNC at 2 wt%. 587

588

4. Conclusions 589

590

Free radical polymerisation initiated by ozonolysis was successfully used to graft polystyrene 591

from SNC and CNC in an aqueous media. Under heterogeneous aqueous conditions, the 592

oxidation and ‘grafting from’ procedure was efficient. The degradation of the crystalline 593

structures of the two different polysaccharides was low, and even small quantities of grafted 594

polystyrene were efficient in reducing the surface contact angles and work of adhesion values. 595

Ozonolysis is therefore a very efficient activator for the generation of free radicals on 596

polysaccharide nanocrystals and, moreover, complies with various principles of green 597

chemistry (use of aqueous media, reduction of by-products, and reduction of purification 598

steps). Following chemical modification, 13C NMR clearly demonstrated that the polystyrene 599

grafting was more than six times more efficient for SNC than for CNC. This allowed an 600

improvement in the thermal stability by 100 °C, showing potential use for the manufacture of 601

nanocomposites by melt processes. 602

This new method for the compatibilisation of polysaccharide nanocrystals with hydrophobic 603

molecules could increase the barrier properties of nanocomposites for possible applications in 604

food packaging, and provide reinforcement applications. 605

606

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Acknowledgements 608

609

The authors wish to express their gratitude to Cecile Sillard for the AFM images and Marie-610

Christine Brochier Salon for her assistance with the NMR analysis and discussion. The 611

authors also acknowledge AgroParisTech/INRA for funding for the travel to Brisbane, 612

Australia. This work was supported by the Mexican Scholarship Council (CONACyT) under 613

grant No. 213840. LGP2 is part of the LabEx Tec 21 (Investissements d’Avenir - grant 614

agreement n°ANR-11-LABX-0030) and the Énergies du Futur and PolyNat Carnot Institutes 615

(Investissements d’Avenir - grant agreements n°ANR-11-CARN-007-01 and ANR-11-616

CARN-030-01). 617

618

619

References 620

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Goffin, A. L., Habibi, Y., Raquez, J. M., & Dubois, P. (2012). Polyester-Grafted Cellulose 894 Nanowhiskers: A New Approach for Tuning the Microstructure of Immiscible Polyester 895 Blends. Acs Applied Materials & Interfaces, 4(7), 3364-3371. 896 Goffin, A. L., Raquez, J. M., Duquesne, E., Siqueira, G., Habibi, Y., Dufresne, A., & Dubois, 897 P. (2011). From Interfacial Ring-Opening Polymerisation to Melt Processing of Cellulose 898 Nanowhisker-Filled Polylactide-Based Nanocomposites. Biomacromolecules, 12(7), 2456-899 2465. 900 Habibi, Y., Lucia, L. A., & Rojas, O. J. (2010). Cellulose Nanocrystals: Chemistry, Self-901 Assembly, and Applications. Chemical Reviews, 110(6), 3479-3500. 902 Hermans, P. H., & Weidinger, A. (1948). Quantitative X‐Ray Investigations on the 903 Crystallinity of Cellulose Fibers. A Background Analysis. Journal of Applied Physics, 19(5), 904 491-506. 905 Kaneko, H., Shuji, H., Kenji, I., & Junzo, N. (1983). Degradation of Lignin with Ozone: 906 Reactivity of Lignin Model Compounds Toward Ozone. Journal of Wood Chemistry and 907 Technology, 3(4), 399-411. 908 Kargin, V. A., Koslov, P. V., Plate, N. A., & Konoreva, I. I. (1959). The synthesis of starch 909 and styrene graft copolymer and the study of their properties. Vysokomol, Soedin, 1, 114-122. 910 Kloser, E., & Gray, D. G. (2010). Surface Grafting of Cellulose Nanocrystals with 911 Poly(ethylene oxide) in Aqueous Media. Langmuir, 26(16), 13450-13456. 912 Labet, M., Thielemans, W., & Dufresne, A. (2007). Polymer grafting onto starch 913 nanocrystals. Biomacromolecules, 8(9), 2916-2927. 914 Lam, E., Male, K. B., Chong, J. H., Leung, A. C. W., & Luong, J. H. T. (2012). Applications 915 of functionalized and nanoparticle-modified nanocrystalline cellulose. Trends in 916 Biotechnology, 30(5), 283-290. 917 Le Corre, D., Bras, J., & Dufresne, A. (2010). Starch Nanoparticles: A Review. 918 Biomacromolecules, 11(5), 1139-1153. 919 Le Corre, D., Bras, J., Dufresne, A. (2009). Starch Nanoparticles for eco-efficient packaging: 920 influence of botanic origin. 2nd International Conference on Biodegradable Polymers and 921 Sustainable Composites (BIOPOL 2009). Alicante, Spain. 922 LeCorre, D. b., Bras, J., & Dufresne, A. (2011). Evidence of Micro- and Nanoscaled Particles 923 during Starch Nanocrystals Preparation and Their Isolation. Biomacromolecules, 12(8), 3039-924 3046. 925 Lin, N., Huang, J., & Dufresne, A. (2012). Preparation, properties and applications of 926 polysaccharide nanocrystals in advanced functional nanomaterials: a review. Nanoscale, 927 4(11), 3274-3294. 928 Ljungberg, N., Bonini, C., Bortolussi, F., Boisson, C., Heux, L., & Cavaillé. (2005). New 929 Nanocomposite Materials Reinforced with Cellulose Whiskers in Atactic Polypropylene: 930 Effect of Surface and Dispersion Characteristics. Biomacromolecules, 6(5), 2732-2739. 931 Lopez-Rubio, A., Flanagan, B. M., Gilbert, E. P., & Gidley, M. J. (2008). A novel approach 932 for calculating starch crystallinity and its correlation with double helix content: A combined 933 XRD and NMR study. Biopolymers, 89(9), 761-768. 934 Luo, J. J., & Daniel, I. M. (2003). Characterisation and modeling of mechanical behavior of 935 polymer/clay nanocomposites. Composites Science and Technology, 63(11), 1607-1616. 936 Mahmoud, K. A., Male, K. B., Hrapovic, S., & Luong, J. H. T. (2009). Cellulose 937 Nanocrystal/Gold Nanoparticle Composite as a Matrix for Enzyme Immobilisation. Acs 938 Applied Materials & Interfaces, 1(7), 1383-1386. 939 Mangalam, A. P., Simonsen, J., & Benight, A. S. (2009). Cellulose/DNA Hybrid 940 Nanomaterials. Biomacromolecules, 10(3), 497-504. 941 Missoum, K., Belgacem, M., & Bras, J. (2013). Nanofibrillated Cellulose Surface 942 Modification: A Review. Materials, 6(5), 1745-1766. 943

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