physical properties of silk fibroin and cellulose ...€¦ · material for nanocomposite...

IJRRAS 28 (2) ● August 2016 www.arpapress.com/Volumes/Vol28Issue1/IJRRAS_28_2_03.pdf

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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.

IJRRAS 28 (2) ● August 2016 Shen et al. ● Physical Properties of Silk Fibroin

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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.

http://www.systat.com/


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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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physical properties of silk fibroin and cellulose ...€¦ · material for nanocomposite...

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