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Supporting Information. 1
Five Different Chitin Nanomaterials from Identical Source with Different 2
Advantageous Functions and Performances 3
Thang Hong Trana,b, Hoang-Linh Nguyena,c, Dong Soo Hwangc, Ju Young Leea, Hyun Gil Chaa, 4
Jun Mo Kooa, Sung Yeon Hwang,b***, Jeyoung Parka,b**, Dongyeop X. Oha,b* 5
a Research Center for Bio-based chemistry, Korea Research Institute of Chemical Technology 6
(KRICT), Ulsan 44429, Republic of Korea 7
b Advanced Materials and Chemical Engineering, University of Science and Technology (UST), 8
Daejeon 34113, Republic of Korea 9
c Environmental Science and Engineering, Pohang University of Science and Technology 10
(POSTECH), Pohang 37673, Republic of Korea 11
12
Key words: chitin nanomaterial; surface charge; physical functionality, chemical properties; 13
biomass source 14
15
*Tel.: +82-52-241-6316 Fax., +82-52-241-6349 Email: [email protected] ORCID: 0000-0003-16
3665-405X 17
***Tel.: +82-52-241-6315 Fax., +82-52-241-6349 Email: [email protected] ORCID: 0000-0002-18
9369-1597 19
***Tel.: +82-52-241-6313 Fax., +82-52-241-6349 Email: [email protected] ORCID: 0000-0002-20
4618-2132 21
22
23
2
Purification of α-chitin 24
To remove the remaining protein, α-chitin powder was immersed in a 0.01 M HCl aqueous buffer 25
for 1 day. Then, the chitin was vigorously washed with deionized water (DIW), and the wet chitin 26
was filtered by a vacuum filter using a Nylon membrane with 0.2 μm pores (SciLab, South Korea). 27
After the protein removal, the chitin was immersed in a 0.01 M NaOH aqueous buffer for 1 day. 28
Then, the chitin was vigorously washed with deionized water (DIW), and the wet chitin was filtered 29
by a vacuum filter using a Nylon membrane with 0.2 μm pores (SciLab, South Korea). The samples 30
were dried at 105 °C for 12 h. 31
Preparation of five different chitin nanomaterials 32
ChNF 33
The α-chitin powder (5 g) was immersed in a 30 wt% NaOH aqueous solution (125 mL) (Oh 34
et al., 2015). The suspension was heated at 80 °C for 4 h in a nitrogen atmosphere. The suspension 35
was centrifuged at 10,000 rpm for 10 min. The supernatant was removed, and the precipitate was 36
re-dispersed in DIW (125 mL). The base dilution processes were repeated 3 times. Then, the 37
suspension was dialyzed with DIW until its pH reached 7. The concentration of the suspension was 38
adjusted to 1 wt% by adding DIW. The pH of the suspension was adjusted to 4 by adding several 39
drops of acetic acid, and the suspension was homogenized using a high-performance grinder 40
(MKCA6-3; Masuko Sangyo Co., Ltd.) with a rotation speed of 1500 rpm. The grinder treatment 41
was performed with a clearance gauge of −1.5 (corresponding to a 0.15-mm shift) from the zero 42
position. After the nanofibrillization, the purified suspension was ultrasonicated for 10 min 43
(amplitude 50%, pulse on 10 s, and pulse off 5 s) by a 750 W probe ultrasonic processor (Sonics, 44
Vibra cell, US). The aqueous ChNF suspension itself or its freeze-dried form was kept at 4 °C. The 45
freeze-drying was performed as follows. The suspension was put in a liquid nitrogen, and it was 46
kept for 1 h. Then, the freeze-drying was conducted at −50 °C under a vacuum of 5 torr for 7 days 47
using a freeze-dryer (TFD5505, ILshin, Korea). The other types of chitin nanomaterials were 48
freeze-dried in the same method. 49
3
ChW 50
ChW was obtained as follows (Gopalan Nair & Dufresne, 2003; Zeng et al., 2012). The α-51
chitin powder (5 g) was immersed in a 3 M HCl aqueous solution (150 mL), and the suspension was 52
heated at 120 °C for 3 h in a nitrogen atmosphere. The suspension was centrifuged at 10,000 rpm 53
for 10 min. The supernatant was removed, and the precipitate was re-dispersed in DIW (150 mL). 54
The acid dilution processes were repeated 3 times. Then, the suspension was dialyzed with DIW 55
until its pH reached 7. The concentration of the suspension was adjusted to 1 wt% by adding DIW. 56
The purified suspension was ultrasonicated for 10 min (amplitude 50%, pulse on 10 s, and pulse off 57
5 s) by the same 750 W probe ultrasonic processor. The aqueous ChW suspension itself or its 58
freeze-dried form was kept at 4 °C. 59
T-ChW 60
T-ChW was obtained as follows (Fan et al., 2008). TEMPO (0.08 g) and NaBr (0.5 g) were 61
dissolved in DIW (500 mL). The α-chitin powder (5 g) was dispersed in the TEMPO aqueous 62
solution (500 mL). The NaClO solution (37 g) was slowly added to the chitin-containing solution. 63
The pH of the chitin-containing reaction medium gradually decreased, and the pH was maintained 64
at 10 by adding drops of 0.5 M NaOH aqueous solution. The reaction was quenched by the addition 65
of ethanol (EtOH) (10 ml) when the pH of suspension did not decrease. The suspension was 66
centrifuged at 10,000 rpm for 10 min. The supernatant was removed, and the precipitate was re-67
dispersed in DIW (500 mL). The dilution processes were repeated 3 times. Then, the suspension 68
was dialyzed with DIW until its pH reached 7. The concentration of the suspension was adjusted to 69
1 wt% by adding DIW. The purified suspension was ultrasonicated for 10 min (750 W, amplitude 70
50%, pulse on 10 s, and pulse off 5 s) by the same probe ultrasonic processor. The aqueous 71
suspension itself or its freeze-dried form was kept at 4 °C. 72
Z-ChW 73
The Z-ChW was obtained as follows. TEMPO (0.08 g) and NaBr (0.5 g) were dissolved in 74
DIW (500 mL). The freeze-dried ChNF (5 g) was dispersed in the TEMPO solution (500 mL). The 75
4
NaClO solution (37 g) was slowly added to the reaction solution. The pH of the ChNF-containing 76
reaction medium gradually decreased, and the pH was maintained at 10 by adding drops of 0.5 M 77
NaOH aqueous solution. The reaction was quenched by the addition of ethanol (EtOH) (10 ml) 78
when the pH of suspension did not decrease. The suspension was centrifuged at 10,000 rpm for 10 79
min. The supernatant was removed, and the precipitate was re-dispersed in DIW (500 mL). The 80
dilution processes were repeated 3 times. Then, the suspension was dialyzed in DIW until its pH 81
reached 7. The purified suspension was ultrasonicated for 10 min (750 W, amplitude 50%, pulse on 82
10 s, and pulse off 5 s) by the same probe ultrasonic processor. The T-ChW suspension itself or its 83
freeze-dried form was kept at 4 °C. 84
85
CsW 86
CsW was prepared as follows (Fan et al., 2010; Pereira et al., 2014). The freeze-dried ChW (5 87
g) was heated in a 30 wt% NaOH aqeous buffer (100 g) at 80 °C for 6 h. The reaction was 88
quenched by the addition of ethanol (EtOH) (10 ml) when the pH of suspension did not decrease. 89
The suspension was centrifuged at 10,000 rpm for 10 min. The supernatant was removed, and the 90
precipitate was re-dispersed in DIW. The dilution processes were repeated 3 times. Then, the 91
suspension was dialyzed with DIW until its pH reached 7. The purified suspension was 92
ultrasonicated for 10 min (750 W, amplitude 50%, pulse on 10 s, and pulse off 5 s) by the same 93
probe ultrasonic processor. The suspension itself or a freeze-dried powder form were kept at 4 °C. 94
95
PVA/chitin nanomaterial composite films 96
PVA/chitin nanomaterial composite films were prepared as follows (Sriupayo, Supaphol, 97
Blackwell, & Rujiravanit, 2005). PVA was used a matrix and the 5 chitin nanomaterials were used 98
as a filler. PVA solution (10 wt%) was prepared by dissolving PVA powder (10 g) into DIW (90 99
mL) at 90 °C for 3 h. The freeze-dried chitin nanomaterials (1 g) were in DIW (99 mL). The chitin 100
nanomaterial aqueous suspension (52.6 g) had the chitin concentration of 1 wt% and were mixed 101
5
with the PVA solution (100 g). The wt% of the chitin nanomaterial against the weight of PVA was 102
approximately 5. The solution was sonicated for 2 h to remove all trapped bubbles and left for 1 h. 103
The solutions were casted in a plastic petri dish and dried at room temperature for 4 days. The dried 104
film was detached from the casting dish, and its thickness was approximately 0.3 mm. 105
106
2.4. Characterization of chitin nanomaterials 107
The chitin weight percentage (wt%) in the chitin nanomaterial-containing aqueous 108
suspensions were determined based on the formula of 100(Ww − Wd)/Ww. Ww is the weight (g) of 109
the chitin nanomaterial aqueous suspension sample before drying. Routinely, the sample of 110
approximately 1 g was used. The chitin nanomaterial aqueous suspension sample was dried at 111
105 °C for 12 h. Then, the weight (g) of the dried sample is Wd. The weight was measured using a 112
balance (Mettler Toledo XS64) with the accuracy of 1 mg. 113
For dimensional analysis, the zeta-size was measured using a zetasizer (Nano ZS, Malvern, 114
UK). The zeta-potential was evaluated using a laser Doppler electrophoresis system (Nano ZS, 115
Malvern, UK). Measurements were repeated three times at 25 °C (Hanif et al., 2017). 116
For scanning electron microscopic (SEM) analysis, the 0.1 wt% suspensions were diluted and 117
dropped onto Si wafers. The Si wafers were dried naturally in a fume hood for water evaporation 118
and further dried in a vacuum oven for 48 h. The wafers were coated with Pt in a sputter device at 119
15 mA for 90 s before SEM analysis was conducted by using a field-emission SEM (FE-SEM; 120
Tescan, Czech Republic). 121
X-ray diffraction (XRD) analysis was conducted using an Ultima IV X-ray diffractometer 122
(Rigaku, Japan) with 40 kV/100 mA Ni-filtered Cu Kα radiation. The scan range and rate for the 123
XRD samples were 5–40° and 4°/min, respectively. From the XRD data, crystallinity indices were 124
calculated from the peak intensity I110 and amorphous intensity Iam at the diffraction angles of 19.6° 125
and 12.6°, respectively (Ifuku et al., 2015; Kumirska et al., 2010). Pellets of chitin and chitin 126
6
nanomaterials were prepared by using a disk apparatus for IR measurement from the freeze-dried 127
forms (Fan et al., 2008). 128
Fourier-transform infrared (FT-IR) spectra were recorded 64 times in the transmittance mode 129
using a Nicolet iS50 FT-IR spectrometer (Thermo Fisher Scientific, USA) at ambient temperature 130
with the range 4000–1000 cm−1 and a resolution of 4 cm−1. The films used for FT-IR measurements 131
were obtained by casting the 0.1 wt% suspensions on petri discs coated with 132
poly(tetrafluoroethylene) and drying in a vacuum oven at 50 °C. For T-ChW and Z-ChW, the 133
carboxylate groups in the films were converted to carboxylate acid groups by immersion in 0.1 M 134
HCl solution for several hours and further drying under the same conditions (Fan et al., 2008; Ifuku 135
et al., 2015). Degree of deacetylation (DD) and content of amine groups were calculated based on 136
ratio at peak 1560 and 1030 cm-1 (Kumirska et al., 2010. Also please refer to the reference 137
(Shigemasa, Matsuura, Sashiwa, & Saimoto, 1996 in reference for Supporting Information). 138
The thermal stabilities of the chitin nanomaterials were evaluated by thermogravimetric 139
analysis (TGA) using a Pyris 1 TGA apparatus (Perkin Elmer, USA). The TGA data of the freeze-140
dried samples were scanned with the temperature range 30–500 °C and a heating rate of 10 °C/min 141
under a continuous nitrogen flow. To study the effects of deprotonation on the thermal stability of 142
chitin, Z-ChW, T-ChW, and ChW, the following experiments were conducted. The freeze-dried Z-143
ChW, T-ChW, and ChW samples (5 g) were immersed in a 0.5 M NaOH aqueous buffer (100 mL) 144
for 6 h. The neutralized samples were fully washed as follows. The suspension was centrifuged at 145
10,000 rpm for 10 min. The supernatant was removed, and the precipitate was re-dispersed with 146
DIW (100 mL). The dilution processes were repeated 3 times. The deprotonated chitin nanomaterial 147
suspensions were dialyzed in DIW for 3 days and then fully dried at 105 ºC for 12 h. The Td5 of 148
chitin, Z-ChW, T-ChW, and ChW were measured by TGA. 149
The light transmittance of 0.1 wt% suspensions at different pH were recorded using a UV-vis 150
2600 spectrometer (Shimadzu, Japan). The 0.1 wt% suspension was prepared by dilution. The pH 151
values were adjusted by 0.1 M NaOH or 0.1 M HCl aqueous solutions. 152
7
For titrating functional groups on the surfaces of the chitin nanoparticles, a freeze-dried 153
sample (0.05 g) was dispersed in DIW (50 mL), and several drops of 0.5 M HCl solution were 154
added to the mixture to set the pH to 2.0. The acidic suspension was titrated with 0.1 N NaOH 155
solution at a 0.1 mL/min flow rate up to pH 12 by using a pH-stat titration system (Ifuku et al., 156
2015; Pang et al., 2017). 157
To determine the antibacterial activities of the chitin nanomaterials, the Kirby–Bauer method 158
was employed (Bauer, Kirby, Sherris, & Turck, 1966; Nguyen et al., 2016). The Gram-negative 159
bacteria E. coli suspension at the optical density at 600 nm (OD600) value ≈ 0.1 was spread on LB 160
agar plates. Sterilized filter papers (7 mm in diameter) wetted by suspensions of 0.5 wt% chitin 161
nanomaterials were put onto the surface of agar plate, and followed by incubating at 37 °C for 24 h. 162
Filter paper with DIW was employed as a negative control. Antibacterial ability was determined 163
based on the dimensions of the hollow zone surrounding the filter paper. The growth curves of E. 164
coli against different chitin nanomaterials were also investigated. The E. coli suspension in log-165
phase were enriched in LB broth at 37 °C until the OD reached ~0.6, filter papers wetted with chitin 166
nanomaterial suspensions (0.5 wt%, 50 µL) were added, and OD values were recorded for 12 h. 167
Characterizations of PVA chitin nanomaterial composite films 168
Tensile tests were performed using a universal testing machine (UTM; Model 5943; Instron) 169
with a crosshead speed of 10 mm/min at room temperature. The PVA nanocomposite films were cut 170
into dog-bone shapes with a width of 3.6 mm and a gap (between grabs) of 27 mm. All samples 171
were conditioned at 23 °C and 50% relative humidity (RH) for 24 h prior to the tests. 172
The coefficients of thermal expansion (CTE) of the PVA nanocomposites were measured by 173
using a thermal mechanical analyzer (TMA) Q800 (TA Instruments, USA). Bar-shaped films with 174
approximate dimensions of 20 mm × 5 mm were prepared as test specimens. The measurements 175
were performed at the heating and cooling rate of 5 °C/min from 20 to 80 °C in a N2 atmosphere 176
(Deng, Li, Yang, & Li, 2014). 177
8
The oxygen barrier properties of the PVA chitin nanomaterial composites were evaluated by 178
the method described in ASTM 1434-82. The PVA nanocomposite films were conditioned in a 179
desiccator for 24 h. The oxygen transmittance rate (OTR) was measured by a manometric gas 180
permeation analyzer (Lyssy L100-5000, Systech Instruments, UK) at 23 °C and 0% RH. The film 181
sample was affixed to the self-adhesive paper holder provided by Systech Instruments. The standard 182
size of the films is 10 × 10 cm2. The sample holder was inserted into the test chamber and separated 183
the upper (pure oxygen) and lower chamber (vacuum). 184
The dimension of the nanomaterials were also measured by an atomic force microscopy 185
(AFM), a multimode V (Veeco) with tapping mode. 0.01 wt% solution samples were prepared. As 186
aforementioned, the solutions were treated with ultrasonication for 10 min. To isolate a ChNF 187
strand, a longer time ultrasonication (30 min) was required. Few drops of the 0.01 wt% solution 188
samples are added on silicon wafer surfaces. The wafers were dried for several hour. The 189
nanomaterials were examined using AFM. 190
191
192
193
9
Table S1. Critical factors of chitin nanomaterials affecting different functions 194
Function Critical factor or remark
Aqueous dispersibility 1) The modification of C6 hydroxyl group
2) naturally occurring amines help chitin
nanomaterial’s dispersion in acid condition
Antibacterial property 1) Positive surface charge 2) Smaller size
Tensile modulus and strength in PVA High aspect ratio
Tensile toughness in PVA Coexistence of positive and negative
charges for percolation network
Thermal degradation stability Alkali treatment (NaOH) as the last process
Oxygen barrier No significant factor in the examined range
Thermal expansion reduction 1) High crystallinity 2) Coexistence of
positive and negative charges for percolation
network
195
196
10
Table S2 Degree of deacetylation (DD) and amine and carboxylate concentration of chitin 197
nanomaterials obtained by FT-IR and titration. (n = 3, mean ± standard deviation) 198
DD (%)a
-NH3+
(mmol/g)
-COO-
(mmol/g)b
Total
[-NH3+]+[-COO-]
(mmol/g)c
FT-IR Titration
ChNF 44% 1.32 ± 0.24 0 1.32 ± 0.24
ChW 21% 0.76 ± 0.09 0 0.76 ± 0.09
CsW 70% 1.51 ± 0.03 0 1.51 ± 0.03
T-ChW 20% 0.76 ± 0.09d 0.80 1.56 ± 0.21
Z-ChW 49% 1.32 ± 0.24e 0.97 2.29 ± 0.55
Pure
chitin
20% - - -
a A1560/A1030 in FT-IR analysis 199
b Titration [Total]-[Amine] 200
c Titration 201
d The data is adapted from that of ChW 202
e The data is adapted from that of ChNF 203
204
205
206
11
Table S3. Mechanical reinforcing performance and their features of PVA-based chitin whiskers 207
composites in literatures. 208
Type of
nanomaterial
Chitin concentration
(wt%)
Feature Mechanical reinforcement
1 Nanowhisker 50-90
Chitin is a major
component, PVA is a
binder
Mechanical property is
improved at a high chitin
concentration (>50 wt%)
Kadokawa
et al, 2011
2 Nanowhisker 3-30 Composite fiber Mechanical property is
improved by a high degree
of fiber orientation
Uddin et
al, 2012
3 Nanowhisker 2.51-11.38 Two types of
composites: 1) fiber 2)
film
1) Mechanical property is
improved by a fiber
orientation
2) Negligible improvement
in the film type
Junkasem
et al A,
2010
4 Nanowhisker 2.55-25.38 Composite fiber Mechanical property is
improved by a fiber
orientation, Mechanical
improvement at < 6 wt%,
but no reinforcement at > 6
wt%
Junkasem
et al B,
2006
5 Nanowhisker 0.74–29.6 Water content (8 wt%) is
controlled
Mechanically improved at
the chitin content of 2.96
wt%
Sriupayo
et al 2005
209
210
12
211
Fig. S1. AFM images of (a) ChNF, (b) ChW, (c) CsW, (d) T-ChW, and (e) Z-ChW. (f) Their 212
diameter (width) analysis by AFM. (n = 10; *: p < 0.05; NS = no significance). 213
214
215
13
216
Fig. S2. AFM images of strands of ChNF isolated from the other fibers by 30 min. 217
218
14
219
Fig. S3. Hydrodynamic size of (a) chitin nanofibers and (b) chitin/chitosan nanowhiskers associated 220
with solvent molecules (yellow circles). 221
222
In suspension, the dispersed polymeric nanoparticles are surrounded with solvent molecules, 223
forming a hypothetical concentric sphere. Hydrodynamic size (zeta-size) measured by dynamic 224
light scattering (DLS) is the size of such a sphere that diffuses together with the particles. In the 225
homologous system, the hydrodynamic size of a polymeric particle is proportional to the actual size 226
of the polymeric particle. 227
228
15
229
Fig. S4. Zoom-outed SEM image of ChNF. 230
231
232
16
233
234
235
Fig. S5. (Top) XRD pattern and (Bottom) SEM image of an electro-spinning based bottom-up 236
processed chitin fibers. 237
238
Chitin was dissolved in hexafluoro-2-propanol (HFIP) (Sigma Aldrich) at concentration of 5 wt%. 239
A high electric potential was applied onto a droplet of the solution at the tip with a syringe needle of 240
0.495 mm. The distance from the syringe tip to an aluminum target was 7 cm. At the voltage of 17 241
kV with a solution flow rate of 4 ml/h, the electro-spun nanofibers were collected on the target 242
using a high voltage power supply. The electro-spinning was carried out at room temperature. As a 243
result, nanofiber mat was obtained. The crystallinity index of the resultant mat was approximately 244
17
42%. The diameter of the bottom-up processed chitin nanofiber was approximately 150 nm. Please 245
refer to the reference (Kim et al, 2015 in references for Supporting Information). 246
247
248
18
249
Fig. S6. TGA graphs of (a) chitin, (b) ChW, (c) T-chW, and (d) Z-ChW after a deprotonation 250
process. (e) The summary of the Td5 data before and after the deprotonation. 251
252
19
253
Fig. S7 OTR of PVA and composite films with different types of chitin nanomaterials (5 wt%). The 254
OTR data (n = 3) is presented by mean ± standard deviation. 255
256
The OTR reductions of PVA by different chitin nanomaterials are investigated. The pristine 257
PVA film exhibits an OTR of 55.35 ± 9.22 mL/m2·day, which is insufficient for use in most food-258
packaging applications. In Fig. , the addition of chitin nanomaterials improves the oxygen barrier 259
properties of the PVA film. The OTR values of the chitin nanomaterials containing PVA 260
composites are between 4.94 ± 0.51 and 7.09 ± 0.39 mL/m2·day, and no significant difference 261
occurs among the OTR values for the composites, based on the fact that a logarithmic change of the 262
OTR number is valid (Wang et al., 2018). Although the oxygen barrier property is mainly affected 263
by the crystallinity and dimensions of nanoparticles (Duncan, 2011), ChNF has an OTR value 264
comparable to the other nanomaterials. This means that the crystallinity (≅67%) of the ChNF is 265
tolerable for oxygen barrier probably because the type and density of charges are also involved in 266
OTR. 267
20
The oxygen barrier performances of the composite films are similar to that of ethylene vinyl 268
alcohol (EVOH) (<5 mL/m2·day), a typical oxygen-barrier polymeric film, and indicate suitability 269
for use as packaging films for meats and cheeses (Nguyen et al., 2018; Wang et al., 2018). It is 270
emphasized that the oxygen barrier properties of the composites are as high as those of cellulose 271
nanomaterials which are also emerging as oxygen barrier materials (Nguyen et al, 2018; Syverud & 272
Stenius, 2009); chitin nanomaterials have antibacterial properties that cellulose lacks. 273
274
275
21
References for Supporting Information 276
Duncan, T. V. (2011). Applications of nanotechnology in food packaging and food safety: Barrier 277
materials, antimicrobials and sensors. Journal of Colloid and Interface Science, 363(1), 1–24. 278
Kim, B. J., Kim, S., Oh, D. X., Masic, A., Cha, H. J., & Hwang, D. S. (2015). Mussel-inspired 279
adhesive protein-based electrospun nanofibers reinforced by Fe (III)–DOPA complexation. Journal 280
of Materials Chemistry B, 3(1), 112-118 281
Nguyen, H. L., Hanif, Z., Park, S. A., Choi, B. G., Tran, T. H., Hwang, D. S., Hwang, S. Y., Park, J., 282
Oh, D. X. (2018). Sustainable Boron Nitride Nanosheet-Reinforced Cellulose Nanofiber Composite 283
Film with Oxygen Barrier without the Cost of Color and Cytotoxicity. Polymers, 10(5), 501. 284
Nguyen, H.-L., Jo, Y. K., Cha, M., Cha, Y. J., Yoon, D. K., Sanandiya, N. D., & Hwang, D. S. 285
(2016). Mussel-Inspired Anisotropic Nanocellulose and Silver Nanoparticle Composite with 286
Improved Mechanical Properties, Electrical Conductivity and Antibacterial Activity. Polymers, 8(3), 287
102–115. 288
Shigemasa, Y., Matsuura, H., Sashiwa, H., & Saimoto, H. (1996). Evaluation of different 289
absorbance ratios from infrared spectroscopy for analyzing the degree of deacetylation in chitin. 290
International Journal of Biological Macromolecules, 18(3), 237–242. 291
Syverud, K., & Stenius, P. (2009). Strength and barrier properties of MFC films. Cellulose, 16(1), 292
75–85. 293
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