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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.
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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
64 65
66
67
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68
1. Introduction 69
70
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
249
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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
584
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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
607
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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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Chen, J., Lin, N., Huang, J., & Dufresne, A. (2015). Highly alkynyl-functionalisation of 651 cellulose nanocrystals and advanced nanocomposites thereof via click chemistry. Polymer 652 Chemistry, 6(24), 4385–4395. 653 De Bruyn, H., Sprong, E., Gaborieau, M., David, G., Roper, J. A., III, & Gilbert, R. G. 654 (2006). Starch-graft-copolymer latexes initiated and stabilized by ozonolyzed amylopectin. 655 Journal of Polymer Science Part A: Polymer Chemistry, 44(20), 5832–5845. 656 De Bruyn, H., Sprong, E., Gaborieau, M., Roper, J. A., III, & Gilbert, R. G. (2007). Starch-657 graft-(synthetic copolymer) latexes initiated with Ce4+ and stabilized by amylopectin. Journal 658 of Polymer Science Part A: Polymer Chemistry, 45(18), 4185–4192. 659 Delpouve, N., Stoclet, G., Saiter, A., Dargent, E., & Marais, S. (2012). Water Barrier 660 Properties in Biaxially Drawn Poly(lactic acid) Films. The Journal of Physical Chemistry B, 661 116(15), 4615–4625. 662 Dong, S., & Roman, M. (2007). Fluorescently labeled cellulose nanocrystals for bioimaging 663 applications. Journal Of The American Chemical Society, 129(45), 13810-13811. 664 Drogat, N., Granet, R., Sol, V., Memmi, A., Saad, N., Koerkamp, C. K., Bressollier, P., & 665 Krausz, P. (2011). Antimicrobial silver nanoparticles generated on cellulose nanocrystals. 666 Journal of Nanoparticle Research, 13(4), 1557–1562. 667 Dufresne, A. (2012). Nanocellulose: From Nature to High Performance Tailored Materials. 668 Berlin: Walter de Gruyter & Co. pp. 475. 669 Espino-Perez, E., Bras, J., Ducruet, V., Guinault, A., Dufresne, A., & Domenek, S. (2013). 670 Influence of chemical surface modification of cellulose nanowhiskers on thermal, mechanical, 671 and barrier properties of poly(lactide) based bionanocomposites. European Polymer Journal, 672 49(10), 3144–3154. 673 Follain, N., Belbekhouche, S., Bras, J., Siqueira, G., Marais, S., & Dufresne, A. (2013). Water 674 transport properties of bio-nanocomposites reinforced by Luffa cylindrica cellulose 675 nanocrystals. Journal of Membrane Science, 427, 218–229. 676 Fortunati, E., Armentano, I., Zhou, Q., Puglia, D., Terenzi, A., Berglund, L. A., & Kenny, J. 677 M. (2012). Microstructure and nonisothermal cold crystallisation of PLA composites based on 678 silver nanoparticles and nanocrystalline cellulose. Polymer Degradation and Stability, 97(10), 679 2027–2036. 680 Fortunati, E., Peltzer, M., Armentano, I., Jimenez, A., & Kenny, J. M. (2013). Combined 681 effects of cellulose nanocrystals and silver nanoparticles on the barrier and migration 682 properties of PLA nano-biocomposites. Journal of Food Engineering, 118(1), 117–124. 683 García, N. L., Lamanna, M., D’Accorso, N., Dufresne, A., Aranguren, M., & Goyanes, S. 684 (2012). Biodegradable materials from grafting of modified PLA onto starch nanocrystals. 685 Polymer Degradation and Stability, 97(10), 2021–2026. 686 Garcia-Gonzalez, C. A., Alnaief, M., & Smirnova, I. (2011). Polysaccharide-based aerogels-687 Promising biodegradable carriers for drug delivery systems. Carbohydrate Polymers, 86(4), 688 1425–1438. 689 Goffin, A. L., Habibi, Y., Raquez, J. M., & Dubois, P. (2012). Polyester-Grafted Cellulose 690 Nanowhiskers: A New Approach for Tuning the Microstructure of Immiscible Polyester 691 Blends. Acs Applied Materials & Interfaces, 4(7), 3364–3371. 692 Goffin, A. L., Raquez, J. M., Duquesne, E., Siqueira, G., Habibi, Y., Dufresne, A., & Dubois, 693 P. (2011). From Interfacial Ring-Opening Polymerisation to Melt Processing of Cellulose 694 Nanowhisker-Filled Polylactide-Based Nanocomposites. Biomacromolecules, 12(7), 2456–695 2465. 696 Habibi, Y., Lucia, L. A., & Rojas, O. J. (2010). Cellulose Nanocrystals: Chemistry, Self-697 Assembly, and Applications. Chemical Reviews, 110(6), 3479–3500. 698
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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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