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PHYSICAL PROPERTIES OF SILK FIBROIN AND CELLULOSE
NANOCRYSTALS BLENDED FILMS: THERMAL, MECHANICAL, AND
MORPHOLOGICAL CHARACTERIZATION
Wei-Zheng Shen, Michael George, Paolo Mussone & Carlo Montemagno*
Ingenuity Lab
1-070C, 11421 Saskatchewan Drive
Department of Chemical and Materials Engineering
University of Alberta
Edmonton, Alberta T6G 2M9
Tel.: 780.641.1617
E-mail: [email protected]
ABSTRACT
Silk fibroin (SF)-cellulose nanocrystals (C) thin films with various compositions were prepared by a new shearing
method utilizing aqueous solutions of both components. The nanocrystals used were supplied by a pilot scale facility
located in Edmonton, Alberta. These particles were prepared by sulphuric acid hydrolysis of dissolving pulp.
Relatively good homogeneity was obtained with each mixing ratio. Films with higher concentration of silk were
characterized with reduced initial thermal stability when compared to the cellulosic control. On the other hand, there
was a corresponding improvement in percentage thermal degradation when small quantities of silk were added to the
cellulose nanocrystals. The main x-ray diffraction peaks were present in most composites. However, for systems
where silk was added to cellulose nanocrystals (10C:S and 5C:S), there was a distinct improvement in the
crystallinity index. Finally, the addition of silk (high quantities) to the cellulosic material resulted in a sharp decrease
in mechanical properties.
Keywords: Silk, cellulose nanocrystals, thin films, thermal properties, mechanical properties.
1. INTRODUCTION
Biocompatible and biodegradable materials have been extensively studied because of their unique chemical,
physical, and biological properties. To that end, the potential of these materials in the fields of medical treatment,
packaging, agricultural mulching, etc. have been continuously explored. Among natural polymers, cellulose (β-1,4-
glucan) is one of the most ubiquitous and well researched material with implications today. Cellulose is produced as
crystalline micro fibrils with widths of 3.5-30 nm depending on the origin and environmental factors [1]. Cellulose
crystals are characterized with high specific Young’s modulus (100-120 GPA); hence they are a highly demanded
material for nanocomposite applications [2].
On the other hand, silk fibroin (SF) is a fibrous protein isolated from the cocoon fibre of the Bombyx mori (B. mori)
silkworm. Over the past centuries, high value uses of SF have been constantly surveyed and studied. As of the last
few years, regenerated SF solutions have been used in the preparation of biomaterials with specific functions,
inclusive of thin films [1], hydrogels [3,4], scaffolds [5], drug delivery [6], wound dressing [7, 8], and bone tissue
engineering [9], etc.
A few studies have been reported on the use of SF – cellulose based composites [10, 11] that involved molecular
blending of the two individual components. In this communication, we investigated the orientation of cellulose
nanocrystals in silk at different ratios and the influence on the mechanical and thermal properties of the films. To
that end, a shearing method was used to mix and produce thin films, which were subsequently characterized.
2. EXPERIMENTAL
2.1 Materials
Silkworm cocoons were obtained from B.mori silkworms raised on Mulberrry Farms, Fallbrook, California.
Cellulose nanocrystals were obtained from a pilot plant at Alberta Innovates Technology Futures, Edmonton,
Alberta. Sodium carbonate (Mol. wt. 105.99 g/mol, >99%), and lithium bromide (Mol. wt. 86.85 g/mol, >99 %)
were obtained from Sigma-Aldrich (Minnesota, USA). Distilled water was used for all analyses.
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2.2 Methods
2.2.1 Extraction of silk fiboin
The dehydrated pupae were extracted from the cocoons prior to sericin removal. The latter was done to avoid
contamination of the fibroin protein. The silk fibroin solution (8 % w/v) were prepared according to a previously
published method [12] and kept at 4 °C prior to analysis. Briefly, 5 g of cocoon pieces were boiled for 30 minutes in
2 L of 0.02 M Na2CO3 aqueous solution and then rinsed thoroughly with distilled water to extract the glue like
sericin proteins. The extracted silk fibroin fibres were dissolved in 9 mL of 9.3 M LiBr solution at 60 °C for 4 hours,
yielding a 20 % solution. This solution was dialyzed against distilled water using a Slide-A-Lyzer dialysis cassettes
(MWCO 3500, Thermo Scientific) at room temperature for 3 days, to remove any excess LiBr salt. The dialysate
was centrifuged two times at 4 °C for 20 minutes to remove the impurities. In the end, the final concentration of the
silk fibroin solution was determined at roughly 8 % wt. The silk solution was used to mix with 7 % wt. CNC
aqueous solution in desired working ratios for the thin film fabrication.
2.2.2 Preparation of silk and CNC solutions
Solutions of silk and CNC were prepared in their respective weight ratios. Silk and CNC suspension was added to a
dialysis cassette and dialysis was performed overnight with water to make a homogenous solution. The following
day, dialysis was done for 8 hours in PEG solution to concentrate the solution.
It should be noted, for silk only films, dialysis was performed only with water for 48 hours. In the end, the final
concentration of the silk was between 80-89 mg/mL (8-9 %). Table 1 gives an indication of the ratios of each
component for the different films.
Table 1. Approximate formulation of the different films
2.2.3 Casting of films
Control films were casted
uniaxially aligned using cellulose nanocrystals (CNC). The preparation of these thin films (< 100 μm in average
thickness) using a shear casting method has been recently researched, with minor modifications [13-16]. For each
film, a known amount of CNC at high viscosity (at least 5 % concentration in weight) was distributed on an
aluminum foil covered glass slide. These slides were then placed on the film applicator. A Gardner knife was moved
at a constant speed over the solution at a fixed height above the plane. The shear force imparted on the fluid resulted
in alignment of the CNC along the direction of the velocity vector.
Resising et al. (2012) [16] were able to prepare films with an approximate thickness of 40 μm by casting and
shearing 1.2 mL of a 10.3 % weight CNC solution. This was repeated cast shearing for a total of 6 times before
drying the film. The neatly spread solution was then dried and the film removed from the surface for subsequent
analysis.
2.2.4 Thermal characterization
2.2.4.1 Thermogravimetric analysis (TGA)
Each experiment was conducted using a Thermal Analysis Instruments TGA Q50 (TA Instruments) apparatus under
a flow of nitrogen to study the effects of heating on the stability of silk and cellulose nanocrystal (CNC) films.
Platinum pans were used for all analyzes given the high temperatures and ease of cleaning. The temperature range
selected was from room temperature to 600 °C at a rate of 10 °C per minute. For each sample, triplicate runs were
done.
CNC: Silk Weight of CNC (mg) Weight of Silk (mg)
1:1 500 500
1:2 333 667
1:5 167 833
1:10 100 1000
10:1 1000 100
5:1 833 167
3:1 750 129
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2.2.4.2 Differential scanning calorimetry
DSC measurements were performed with a Thermal Analysis Instruments DSC Q2000 (TA Instruments) in the
temperature range of 25-300 °C at a heating rate of 10 °C per minute. Approximately 5 mg of sample was used for
each analysis. For each sample, triplicate runs were done.
2.2.5 X-ray diffraction study of the different films
All experiments were carried out on a Bruker-AXS, D8 discover diffractometer system equipped with a Vantec-500
area detector and Cu Kα1 radiation source ((λ = 1.542 A). The instrument was operated at 40 kV and 30 mA for x-
ray diffraction (XRD) experiments. The XRD patterns were obtained by irradiating the films from phi=00 ~ 180 °
over the angular range of 2θ = 5 ~ 80 ° for 300 seconds.
XRD patterns of the CNC and silk in plane helped visualization of the distribution of the alignment. The azimuthal
profiles of the equatorial reflection (200) allowed quantification of the CNC orientations [17]. The degree of
orientation (Π) was calculated according to equation 4, where FWHM is the full width at half-maximum.
Π = 180−FWHM
180 Equation 4.
The peak deconvolution method was used to distinguish the CNC peaks and silk peaks in XRD spectra, as
previously reported by Hamad and Hu (2010) [18]. Basically, a curve-fitting process from the diffraction intensity
profiles extracted individual crystalline peaks. A peak-fitting program (Fityk 0.9.8; http://fityk.nieto.pl/) was used,
assuming Gaussian functions for each peak and a broad peak at around 21.5 ° assigned to the amorphous
contribution. Iterations were repeated until the maximum F-number was obtained. In all cases, the F-number was
10,000, which corresponds to a R2 value of 0.997 [18].
The curve fitting procedure required assumptions about the shape and number of peaks. CNC’s diffraction peaks
was assigned to 2θ = 16.5, 22.5, 34.6, which are characteristic for planes 110, 200, and 400 [20]. Silk was assigned
to contribute only for the amorphous state, characterized by the presence of a broad peak in the 2 theta scattering
angle range (for wavelength 0.1541 nm) from 7 to 32 ° [21]. After peak de-convolution of the spectrum, the
crystallinity was calculated as the ratio of the area of all crystalline peaks to the total area.
2.2.6 Mechanical property evaluation
An Instron dual column tabletop universal testing (System 3365) equipped with a 5 kN capacity, 1000 mm/min
maximum crosshead speed, and vertical test space of 1192 mm was used for all measurements. The tensile
measurements were conducted according to ASTM D 882 Standard Test Method for the tensile properties of thin
plastic sheeting. Depending on the spread/precision of the measurements, up to 5 tests were done for each sample.
Cast films were cut into 5 by 50 mm pieces. The thickness, ranging from 65 to 90 μm, was determined using a
micrometer. For samples, where the thickness was < 65 μm, an assumption was made that the thickness was 65 μm.
A head speed of 10 mm/min was used for all measurements.
2.2.7 Morphological characterization
Micrographs of the film surfaces were taken using a Zeiss Sigma Field Emission Scanning Electron Microscopy (FE
SEM) equipped with an Everhart-Thornley Secondary Electron Detector (ET-SE), and a state of the art Gemini
column for maximum resolution at 1.5 nm at 5 kV accelerating voltage. A gold sputter coater was used to induce
conductivity for all samples. A resolution of 4 nm was used for all samples. At least 3 pieces of fractured sample
was mounted on conductive adhesive tape, sputtered coated and observed. Portions of the cross sectional and top
view of the plastics were mounted.
3. RESULTS AND DISCUSSION
3.1 Thermal analysis
3.1.1 Thermal stability
Thermogravimetric analysis (TGA) is a very useful method for quantitative determination of the degradation
behaviour and composition of different sample types. In addition, the location and magnitude of the different peaks
found in the derivative thermogravimetric (DTG) curve can provide important information on the component and
mutual effect of the film components on the temperature scale.
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The thermal properties of the different films are presented in Table 2 and 3. CNC (control) when compared to silk
fibroin (control) had a better initial decomposition temperature. In fact, the addition of silk to CNC resulted in
significant reduction in initial decomposition temperature at different ratios (systems 1, 3, and 4, with 2 being an
exception). Bettaieb et al. (2015) [22] previously reported the two max decomposition temperatures presented for
the CNC (control). In addition, for silk (control), the first max temperature and the absence of the second was
previously reported by Lee et al. (2005) [23] when they investigated the thermal properties of silk and poly
(butylene succinate) composites. Sample 1 and 2 were characterized with significant improvement in both max
temperatures. One plausible reason for this may be the good miscibility between the components when both are in
equal proportions.
The degradation patterns of the different samples are presented in Table 2. Silk (control) was characterized with a
higher residual weight or ash content than CNC (control). In fact, any film formulation with silk was characterized
with increased ash content except for those where CNC was the dominant component (systems 5, 6, and 7).
Table 2. Thermal degradation temperatures for different CNC/Silk ratios
Sample Key Thermal properties (°C)
TID
T1Max
T2Max
CNC (C) CNC 264 ± 3.4 281 ± 0.2 389 ± 0.8
Silk (S) Silk 223 ± 2.1 303 ± 2.1 -
C/S 1 243 ± 1.2 363 ± 1.0 435 ± 1.2
C/2S 2 261 ± 4.5 366 ± 1.0 434 ± 2.0
C/5S 3 245 ± 2.0 300 ± 2.1 365 ± 0.5
C/10S 4 249 ± 1.0 300 ± 2.1 365 ± 1.5
10C/1S 5 268 ± 3.6 288 ± 6.8 307 ± 2.1
5C/S 6 265 ± 3.2 300 ± 3.2 389 ± 2.1
3C/S 7 269 ± 2.1 355 ± 4.0 -
TID Initial decomposition temperature
T1Max Max decomposition temperature – phase one
T2Max Max decomposition temperature – phase two
Table 3. Percentage degradation and residue for the different CNC/Silk ratios
Sample Key Degradation temperature for a given % sample Residue wt. (%)
20
40
60
CNC (C) CNC 277 ± 2.1 285 ± 4.4 388 ± 1.5 26.4 ± 0.8
Silk (S) Silk 303 ± 1.5 353 ± 3.8 530 ± 2.5 37.5 ± 0.2
C/S 1 326 ± 2.1 365 ± 3.1 486 ± 3.8 36.9 ± 1.4
C/2S 2 319 ± 3.6 368 ± 3.1 512 ± 6.0 38.6 ± 1.1
C/5S 3 301 ± 2.9 355 ± 4.6 581 ± 18 39.7 ± 1.0
C/10S 4 308 ± 1.5 365 ± 1.5 585 ± 5.9 40.3 ± 1.2
10C/S 5 289 ± 1.7 323 ± 3.2 529 ± 2.1 26.3 ± 4.5
5C/S 6 281 ± 2.5 308 ± 5.5 419 ± 1.5 23.3 ± 7.0
3C/S 7 284 ± 4.6 382 ± 3.1 498 ± 3.0 29.1 ± 2.0
Most systems with any silk fraction were characterized with an increased percentage degradation when compared to
the CNC control. In fact, addition of CNC to the films resulted in a reduced thermal stability, as evident by the films
with high concentrations of CNC (10C/S, 5C/S, and 3C/S). The reason for this is the superior network arrangement
of the silk fibroin, which requires much more energy to disrupt the bonding.
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Films with predominantly silk fibroin were associated with degradation at 200-210 °C (up to 25 %) and about 290-
360 °C (about 40 %). These transformations are characteristic of the beginning of chemical debonding and helix
rupturing, respectively, signalling the degradation of silk. The higher ash content for these films resulted from the
burning of amide/amino rich groups and cleavage of peptide bonds. Some of these are released as gases (H2O, CO2,
and NH3), but a large fraction remains as inorganic N.
In summary, CNC-silk films were characterized with improved thermal properties when compared to CNC alone.
For silk dominated films, the silk component degraded at higher temperatures, but has a higher percentage of
waste/ash.
3.1.2 Thermal transitions
Differential scanning calorimetry was used to determine the main transitions for the controls (CNC and silk only)
and the different blends. Figure 1 depicts the thermographs for the different blends outlining the different transitions.
a)
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b)
c)
Figure 1. Thermal transitions for a) CNC based films, b) Silk based films, and c) all different combinations.
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The different transitions for the blends are presented in Table 4. An exothermic shift at about 220 °C and an
endothermic peak at 278 °C for the silk control are attributed to the heat induced β-sheet crystallization and the
thermal decomposition of the silk fibroin, respectively. In fact, the two broad and large endothermic peaks for most
samples (especially those with higher concentrations of silk) can be attributed to the loss of moisture (80-125 °C)
and the crystallization during heating by forming the β-sheet structure from a random-coil conformation (270-290
°C) [24]. In a similar model of experiments, Moraes et al. (2010) [25] investigated the effect of different ratios of
silk fibroin and chitosan on the thermal properties. They observed the endothermic peak below 150 °C and attributed
it to evaporation of adsorbed water in the films. Also, the endothermic peak (transition B) for silk concentrated films
(1-4) is attributed to crystallization during heating from a silk I to a silk II structure. On the other hand, for CNC
concentrated films (5-7), transition B (endothermic peak) is dominated by the presence of the sulphate ester bonds
within the films will suppresses the effect of the silk component in the films [26]. The notable shifts in transitions
among different films results from the interplay of the concentration of the components present and the success of
mixing during production of the films. To that end, the transitions observed for the different films are characteristic
of the individual components coupled with the holistic properties after mixing.
In summary, the CNC-silk fibroin interaction might have been caused by inter-chain interaction, which could have
enhanced the properties during heating. This would be only when the chain mobility was sufficiently high to allow
further interaction.
Table 4. Thermal transitions for the different blends of film made from CNC and silk.
Sample Key Transitions (°C)
A B C
CNC (C) CNC - 224 ± 2.1 270 ± 1.8
Silk (S) Silk 124 ± 1.0 222 ± 1.3 278 ± 3.9
C/S 1 108 ± 0.7 229 ± 0.4 284 ± 0.4
C/2S 2 103 ± 0.5 225 ± 0.3 283 ± 0.6
C: 5S 3 108 ± 1.1 223 ± 0.9 279 ± 0.5
C/10S 4 105 ± 3.7 225 ± 0.6 271 ± 0.5
10C/S 5 106 ± 0.9 232 ± 1.0 275 ± 0.4
5C/S 6 93.7 ± 0.4 223 ± 0.9 277 ± 0.2
3C/S 7 85.1 ± 0.1 220 ± 1.8 287 ± 3.9
Red and black texts are exothermic and endothermic changes, respectively.
3.2 X-ray diffraction study
The crystallinity index (CI) of the different CNC: silk films are presented in Table 5. A peak deconvolution method
was used to distinguish between the different CNC and silk peaks in each XRD spectra. In this method, the
diffraction intensity profiles were used to extract the individual peaks by a curve fitting process. A peak-fitting
program (PeakFit: www.systat.com) was used. Assuming a Gaussian function for each peak, the broad peak around
21.5 ° was assigned to the amorphous contribution. Iterations were repeated until the maximum F-number was
obtained. In all cases, the F-number was > 10,000, which corresponds to an R2 value of 0.997 [19].
It should be noted, the X-ray diffraction of silk fibroin films were characterized with wide-angle diffraction pattern
that had few peaks, which were broad and weakly expressed. The peaks are from the formation of the crystalline β-
pleated sheet secondary structures within the protein. Hence, the degree of crystallinity, calculated from the relative
areas under these crystal peaks, tends to be inaccurate at low crystalline fraction. One reason might be that many of
the crystals formed are small and imperfect. Such crystals may not contribute measurably to the overall coherent
scattering in the X-ray diffraction pattern.
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Table 5. Crystallinity of the different plastic combinations
Sample Key % Crystallinity
CNC (C) CNC 76
Silk (S) Silk Completely amorphous
C/S 1 82.0
C/2S 2 62.4
C/5S 3 79.7
C/10S 4 Completely amorphous
10C/S 5 92.1
5C/S 6 87.7
3C/S 7 83.6
Films with majority CNC (5, 6, and 7) were characterized with increasing CI as the content of CNC increased. This
is very much expected given that the silk fibroin acts as the matrix for the reinforcing CNC particles. The XRD
pattern for the different CNC: silk combinations and amorphous silk is shown in Figure 2. In Figure 2b, the peaks
for the CNC: silk films were normalized and corrected. It can be observed in Figure 2a, the high counts at
approximately 19.5 ° and moderate counts at 12.2 ° (2θ) for the amorphous silk. According to Noishiki et al. (2002)
[1], this corresponds to the 0.72- and 0.44- nm lattice spacing of silk I. On the other hand, the cellulose nanocrystal
dominated films gave a defined pattern of cellulose Iβ at approximately 22.2 °. Man et al. (2011) [27] previously
reported these dominant peaks when they produced CNC using an ionic liquid.
a)
0
100
200
300
2 12 22 32
Counts
2Theta
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b)
Figure 2. XRD patterns of a) amorphous silk and b) normalized peaks of the different CNC and silk films (CNC to
Silk)
The variation of CI with the different CNC ratios for a few systems is shown in Figure 3. As previously mentioned,
increasing the CNC concentrations in the films resulted in increased CI. This is expected because the crystalline
cellulosic material reinforces the amorphous silk.
Figure 3. Variation of crystallinity index with the different ratios of CNC/Silk
3.3 Mechanical properties
The tensile strength and modulus of the different thin films are presented in Figure 4. Silk only films were too brittle
to be tested. This was contrary to the data reported by Noishiki et al. (2002) [1], when they investigated the
mechanical properties of silk-fibroin with microcrystalline cellulose. In that publication, the tensile strength of silk
-0.05
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
10 15 20 25 30 35
Co
un
ts
2Theta
1to1 2to1 3to1 5to1 10to1
y = -0.022x2 + 1.2629x + 81.681R² = 0.853
80
82
84
86
88
90
92
94
0 2 4 6 8 10 12
Cry
sta
llin
ity
In
de
x
Ratio (CNC/Silk)
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fibroin film was approximately 20-40 MPa. On the other hand, the tensile strength for only CNC films was
comparable to their 100 % cellulose films. The films attained a maximum tensile strength when low fractions of silk
were used in conjunction with CNC (10C:S, 5C:S). Several reasons can account for higher tensile strength for these
films, for example, sample mixing and shearing has been shown to significantly affect the orientation of the CNC
within the films [14, 15]. It is obvious, however, that higher fractions of silk within the films resulted in a significant
reduction in the material strength.
a)
b)
Figure 4. Mechanical properties for the different formulations: a) tensile strength and b) tensile modulus.
It should be noted that most samples were brittle and exhibited little (< 0.5 %) or no elongation. The tensile values
reported here are higher than those reported by Giuliano, Tsukada, and Beretta et al. (1999) [28], when they
combined silk fibroin with polyacrylamide. In summary, the mechanical properties of the films reported in this
communication can be improved based on previous reports and an understanding of the potential of the material
being used. Better mixing, longer drying times, and thicker films (depending on the applications) are some of the
improvements that can be made to improve the mechanical properties.
3.4 Morphological characterization
SEM images of the films are shown in Figure 5. The control CNC film was void of any surface protrusion and was
characterized with a smooth surface (5a). The fractured surface at low concentration of silk (5b) was characterized
with a homogenous and dense structure, indicating good dispersion/mixing within the CNC. However, as the content
0
3
6
9
12
15
Silk CNC C:S C:2S C:5S C:10S 10C:S 5C:S 3C:S
Te
nsi
le s
tre
ng
th (
MP
a)
Different systems
0
200
400
600
800
1000
Silk CNC C:S C:2S C:5S C:10S 10C:S 5C:S 3C:S
Te
nsi
le m
od
ulu
s (M
Pa
)
Different systems
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of silk increased, the fractured surfaces were characteristically more rough and protruding (5c and 5d). Plausible
reasons for this may be the aggregation of CNC and the preferential alignment of the silk along lines of shearing.
Li, Zhou, and Zhang (2009) [29] reported similar properties when they studied the structure of nanocomposites
made from cellulose whiskers and reinforced with chitosan. They cited that poor dispersion resulted in aggregation
of chitosan within the structure, evident from SEM characterization. This had a negative impact on the mechanical
property as the concentration of chitosan increased within the composites.
Similarly, Freddi, Tsukaka, and Beretta (1999) [30] reported SEM images of silk fibroin and polyacrylamide-
blended films. They reported uniform polyacrylamide dispersion, without the presence of silk fibroin. But, when the
fibroins were added, there were large nucleating structures with large variations in shape. In the end, the authors
concluded that the nucleating structures could lead to adhesions between the phases because of better contact area.
In summary, SEM was used to characterize the morphological patterns of the CNC, silk, and CNC + silk films. It
appears that the individual components were uniform, but when mixed, there are irregularities and protrusions.
Figure 5. SEM micrographs for a) CNC, b) 5C: S, c) C: 5S, and d) C: 10S.
3.5 Effect of shearing on the properties of the films
CNCs films are known to possess chiral nematic structure and crystallinity. These materials will have widespread
applications in the areas of optically viable films and ink pigments, if the factors affecting their alignment are fully
understood. Hoeger et al. (2011) [31] reported a relative new method for depositing CNC based materials on to the
surface of a solid material to produce a composite film. Their system included a solid support and a
moving/withdrawal plate. In our case, we used a thin glass film as the solid support and a garter knife as the moving
plane. The behaviour of CNCs in aqueous systems under shear stress depends on the ratio of the two Leslie
viscosities. Firstly, either the particles adopt a stable position with flow or against the flow, where an unstable
conformation results [32]. It has been reported in the past, that controlling the shear rate, the rotation of nematic
particles can be ordered to achieve maximum strength [32, 33].
In this study, no method was used to assess the alignment of the CNC particles within the films. But, it can be
agreed that a comparison of studies with similar feedstock and loadings can be used to decipher whether we have
achieved proper alignment. Liu et al. (2010) [34] investigated the properties of transparent cellulose nanocrystals
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films. Their films were characterized with similar thermal degradation patterns. In fact, CNC degraded between 150-
250 °C while the polymer degraded between 300-450 °C. These distinct degradation patterns are similar to those
exhibited by silk fibroin and CNC.
On the other hand, when compared to a few other studies [28, 31], the mechanical properties reported in this study
are significantly lower. A plausible reason for this may be the poor dispersion of the CNC particles within silk. This
will lead to aggregates, which can create weak points that fail easily, when compared to a thoroughly dispersed
phase.
In the end, future studies need to be focused on optimizing the alignment of CNC particles within films and then
evaluating the properties.
4. CONCLUSION
CNC-Silk films showed improved tensile properties when low concentrations of silk fibroin were added to CNC. In
fact, XRD measurements confirm higher degree of crystallinity for samples characterized with high concentrations
of CNC and low silk fibroin presence. SEM micrographs of CNC + silk films were characterized with aggregates
and heterogeneous distribution within the film structure. In the future, better longer mixing times and mechanical
shearing of the films might account for some of the heterogeneity. Nevertheless, the films produced in this study can
find applications in tissue repair, medical device coatings, and food packaging.
ACKNOWLEDGEMENT
The authors acknowledge the Government of Alberta for the financial support. The authors are grateful to Alberta
Innovates Technology Futures (AITF) for kindly supplying the CNC samples needed for this study.
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