Cellulose Nanocrystals

Research Article Cite This: ACS Sustainable Chem. Eng. 2018, 6, 1977−1987

pubs.acs.org/journal/ascecg

In-Situ Nanoreinforced Green Bionanomaterials from Natural Keratin and Montmorillonite (MMT)/Cellulose Nanocrystals (CNC) Manpreet Kaur, Muhammad Arshad, and Aman Ullah* Department of Agricultural, Food and Nutritional Science, 4-10 Agric/For Centre, University of Alberta, Edmonton, Alberta, Canada, T6G 2P5

ACS Sustainable Chem. Eng. 2018.6:1977-1987. Downloaded from pubs.acs.org by UNIV OF TEXAS SW MEDICAL CTR on 10/12/18. For personal use only.

S Supporting Information *

ABSTRACT: Biodegradability and renewability has led renewed interest in protein based films reinforced with nanoparticles. Bionanocomposites have gained attention because of their enhanced material properties with the aid of nanoreinforcements. The effects of two different nanoparticles, montmorillonite (MMT) and cellulose nanocrystals (CNCs), at different loading contents (0%, 1%, 3%, 5%, and 10%) were studied as a reinforcement material in modified chicken feather keratin. Compression molding was employed to prepare bionanocomposites films thermoplastically. The effect of CNC and MMT addition, their disposition and impact on the final material properties, was investigated by differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), tensile testing, and dynamic mechanical analysis (DMA). The morphology of in-situ-modified keratin-based nanocomposites and the extent of nanoparticle dispersion was observed through scanning electron microscopy (SEM), transmission electron microscopy (TEM) and wide-angle X-ray diffraction (WAXD) respectively. The molecular level interactions of CNC’s and MMT’s with keratin biopolymer were investigated by X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) techniques. Results indicated improved thermal stability and shift in glass transition temperature for both nanoreinforced biocomposites. Tensile strength was enhanced significantly with the addition of MMT; however, increased percent elongation was observed in case of CNC-reinforced biomaterials. The changes in the chemical bonding of keratin biopolymer reinforced with MMT/CNC compared to neat keratin biopolymer were observed by XPS spectra. These results suggest that high performance bionanomaterials can be developed from feather keratin through in situ dispersion of MMT and CNC nanoparticles, followed by compression molding. KEYWORDS: Feather keratin, Montmorillonite, Cellulose nanocrystals, Bionanocomposites

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INTRODUCTION The rise in environmental concerns have led to the current interest in replacing traditional materials with greener alternatives.1,2 Because of environmental disturbances and envisaged future shortfall of oil and oil-derived products, interest in the development of environment friendly materials from renewable resources, such as lipids, polysaccharides, and proteins, has increased. Currently, the main focus has been repositioned toward proteins being more robust than carbohydrates.3,4 Several works by numerous researchers reported preparation of plastics from proteins by solvent castings, extrusion, or compression molding techniques.5−9 Biopolymers have numerous advantages over synthetic polymers because of their biodegradability, renewability, and sometimes low-cost and eco-friendly nature.10 Previously protein-based films have been developed from soy protein, whey protein, casein, collagen, corn zein, gelatin, and wheat gluten by many researchers. 11 However, their limited availability as feedstock for bioplastics, brittle nature, and weak material properties act as hindrance to their commercial utilization. Chicken feathers are the natural source of keratin © 2018 American Chemical Society

protein, which can be utilized to develop high-performance biomaterials. Chicken feathers are a renewable cheap feedstock, and poultry processing plants generate over 65 million of feathers worldwide every year.12 Currently, the feathers are either processed into a low nutritional value animal feed7 or sent to a land fill, with few applications in composite preparation and other products.7,13−15 Both of the abovementioned disposal methods contribute to the environmental pollution. So by creating alternative ways to use chicken feathers on and industrial scale will help to reduce environmental impact and health risks, which are directly linked to the disposal of these materials in landfills.14 Feathers consists of over 90% keratin15 and 7% cysteine, which results in α-helix formation in keratin protein through disulfide linkage16 in polypeptide chains. These linkages are responsible for the stiffness and hardness of the keratin protein making it difficult to dissolve in organic solvents.17 Therefore, alkaline and acid Received: September 21, 2017 Revised: December 4, 2017 Published: January 9, 2018 1977

DOI: 10.1021/acssuschemeng.7b03380 ACS Sustainable Chem. Eng. 2018, 6, 1977−1987

Research Article

ACS Sustainable Chemistry & Engineering

mechanical properties. To the best of our knowledge, there are no reports on the preparation of chicken feather (keratin) based bionanocomposite films reinforced with montmorillonite (MMT) and cellulose nanocrystals (CNCs), nor has in situ dispersion of nanoparticles, followed by compression molding techniques, been reported.

hydrolysis, oxidation, and reduction methods have been employed and reported by many researchers for dissolution of keratin.18−20 Previous studies have been done to modify poultry feather, either by surface grafting of synthetic polymers or by blending with plasticizer, to transform them into films using casting, compression molding, or extrusion techniques. Apart from using keratin as a biosorbent,21−23 there are few reports on the formation of composite films using keratin fiber. Jin et al. modified native chicken feather fiber through graft polymerization with methyl acrylate, using K2S2O8/NaHSO3 as redox system, and prepared films by compression molding.24 Results indicated higher tensile strength than soy protein isolate (SPI) and starch acetate (SA). Barone et al. prepared compression molded films from keratin feather fibers using high-density polyethylene (HDPE) and observed increase in stiffness of HDPE with reduced tensile breaking stress.25 Ullah et al. studied the effects of different plasticizers, namely, glycerol, propylene glycol, ethylene glycol, and diethyl tartrate, on bioplastics developed from feather quill processed by extrusion. They found ethylene glycol to be a compatible plasticizer responsible in improvements of mechanical properties.8 However, despite abundant availability of feathers and some efforts to process them into bioplastics, their weaknesses, such as their hygroscopic nature26 and low thermal stability,27 need to be addressed to demonstrate a basis for their use in innovative technologies and high-performance composite applications in different industries, including packaging, leisure, aerospace, sports, automotive, and construction.10,28 Addition of nanofiller/nanoparticles into synthetic29 or natural composite materials30,31 is a common method to improve thermal, mechanical, barrier, and other properties. Recent reports on the improvements of different protein-based composite films describe the addition of montmorillonite into soy protein,32,33 zein nanoparticles, and nano-SiO2 in whey protein isolate34 and whey protein isolate/pululan,35 respectively. More recently, incorporation of cellulose nanofibers in gelatin matrix36 and soy protein isolate37 by solvent casting, as well as graphene oxide in keratin materials38 by extrusion, has been reported. The commonly used methods for protein-based bionanocomposite preparation are solvent casting, melt blending, compression molding, and extrusion. Improvements in physical properties of the prepared bionanocomposites are largely dependent on the degree of nanoparticle dispersion, and homogeneous dispersion of nanoparticles in biopolymers is a challenge. Unique structure and properties of clay minerals have been established as an effective materials to improve various properties of biopolymers, including thermal, barrier, and mechanical.39,40 CNCs are new type of nanoparticle with long crystalline rod-shaped needles with sizes ranging from 1 to 20 nm in width and 1−100 nm in length. Because of its impressive material properties, such as high elastic modulus, surface area, specific strength and Young’s modulus, and low density and coefficient of thermal expansion,41−43 it has been used as reinforcement in various polymers.44−46 Recently, we have reported the dispersion of nanoclay as a reinforcement in a keratin matrix and its regeneration into hybrid fiber.28 Herein, we explore the in situ dispersion of MMT and CNCs in keratin solution and their comparison after preparation of bionanocomposite films by compression molding, where 1,2-butanediol and glycerol were used as plasticizers. The effect of MMT and CNCs concentration on keratin based nanocomposite films has also been investigated to evaluate their impact on thermal and

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EXPERIMENTAL SECTION

Materials and Methods. White chicken feathers from broilers were obtained from Poultry Research Centre (University of Alberta). First, they were cleaned by washing with hand soap and water. Then, they were dried under a closed fume hood for 4 days at room temperature. To remove the remaining moisture, feathers were dried in a ventilated oven at 50 °C for 8 h. Scissors were used for processing, and then, the feathers were ground using Fritsch cutting Mill with sieve of 0.25 mm. (Pulverisette 15, Laval Laboratory, Inc., Laval Canada). Processed feathers (8 g) were washed with hexane solvent for 4 h in a Soxhlet apparatus to remove the lipids, dried, and stored at room temperature for further modification. Urea (99%), sodium sulfite (≥98), EDTA (99%), n-hexane (≥95%), tris-base (≥99.8%), glycerol (99.6%), chitosan, 1,2butanediol (≥98), HCL, and hydrophilic nanoclay/Montmorillonite (≥95%) were purchased from Sigma-Aldrich and used as received. The cellulose nanocrystals (CNCs) were provided by the Alberta Innovates Technology Futures (AITF). In-Situ Modification/Reinforcement of Feather Keratin with MMT and CNCs. Urea solution (8 M) and chicken feathers with weight ratio of 17:1 were taken in a reaction flask and stirred on a hot plate at a temperature of 60−80 °C. Further EDTA (0.438 g), tris-base (12.102 g), and sodium sulfite (10 g) were added in the reaction flask. The pH of the reaction mixture was maintained at ∼9.0 to dissolve protein and monitored regularly during dissolution process. The dissolution of protein took about 7 days. After complete dissolution of chicken feathers, nanoparticles (MMT or CNC individually) were added into the solution on weight percent basis of chicken feathers. This keratin solution containing nanoparticles was stirred again for 20 min and then sonicated for 10 min. Afterward, the keratin solution was precipitated by adjusting the pH to its isoelectric point (4.0−4.2) with the help of 1 M HCl solution. The precipitates were centrifuged (10 min, 10,000 rpm) and thoroughly washed with distilled water. The obtained keratin fibers were then dried at 95 °C for 24 h in an oven, ground, and sieved with mesh (180 μm) affording bionanocomposites in powdered form. Different concentrations (1%, 3%, 5%, and 10%) of MMT and CNCs were prepared separately by following the same procedure each time. Bionanocomposite Film Preparation by Compression Molding. For the preparation of bionanocomposite films, the modified keratin powder was mixed with 20% plasticizers, 10% cross-linking agent, 3% sodium sulfite as reducing agent, and 25% moisture contents. All the ingredients were mixed intensively for 5 min in a beaker and kept for 4 h in a sealed plastic bag for hydration and efficient mixing of plasticizer with the keratin. The function of reducing agent, sodium sulfite in the system was to dissociate disulfide linkage in cysteine residues of the keratin chains to enhance possible interactions among modified keratin and plasticizers. Approximately 3.5 g of each blended mixture was taken for compression molding, where a Carver press (Bench Top Manual Heated Press, Model CH (4386), Carver, Inc. USA) equipped with heated platens and hydraulic pump was used to compress the sample. The sample was compressed between two platens at optimized conditions (15 min, 145 ± 2 °C, 10 MPa). The obtained bionanocomposite films were cooled down to room temperature and cut into required gauge dimensions for further testing. The details about screening of plasticizers and optimization of other parameters are given in Supporting Information (page S2). Film Thickness. Thickness of the film was measured at three different randomly selected locations using a digital Vernier calliper (Digi-Max caliper, Sigma-Aldrich, USA). The average value of film thickness was used in determining mechanical properties and dynamic mechanical properties. Three thickness measurements at different 1978


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Scanning Electron Microscopy (SEM). The morphology of the fractured surfaced of neat CFK and nanoreinforced biocomposites was observed by scanning electron microscopy (SEM, FEI XL30, USA), which was operated at 20 kV. All of the samples were coated with Au/ Pd using hummer 6.2 sputter coater (Anatech Ltd.) prior to analyses (scanning electron microscopy results are given in Figures S1 and S2). Statistical Analysis. All experiments were performed in triplicates. Mechanical properties data was subjected to statistical analysis oneway analysis of variance (ANOVA) followed by Duncan test at a significance level of 0.05. The analysis was done to observe the significance difference among mechanical properties. It was performed by SPSS software (SPSS software, Version 23, Inc. USA).

positions were taken on each specimen being in the range of 70−90 μm in all cases. X-ray Photoelectron Spectroscopy (XPS). High-resolution C 1s spectra and elemental composition of the keratin composites surface were analyzed by XPS. Powdered samples were analyzed using ULTRA spectrometer (Kratos Analytical, U.K.) with an Al Kα X-ray source. The base pressure in the analytical chamber was lower than 3 × 10−8 Pa. The electron flood gun for charge compensation was operated at 150 W. The aperture slot was 400 × 700 μm2. The resolution of the instrument is 0.55 eV for Ag 3d and 0.70 eV for Au 4f peaks. Spectra were recorded at pass-energy of 160 eV for survey scans and 40 eV for high resolution scans. Vision-2 instrument software was applied to process the data. The spectra were calibrated for C 1s binding energy position at 284.8 eV. Compositions were calculated from XPS spectra using linear background and sensitivity factors provided by the instrument database. Differential Scanning Calorimetry (DSC). The thermal properties of bionanocomposite films were investigated by differential scanning calorimeter (2920 Modulated DSC, TA Instruments, USA) under the stream of nitrogen. To calibrate the heat flow and temperature of instrument, pure indium sample was used. All samples were analyzed in a temperature range of 25 to 300 °C at a heating rate of 10 °C/min. The samples weighing between 5 and 10 mg were encapsulated in aluminum pans. Thermogravimetric Analysis (TGA). The thermal stability of CNC and MMT reinforced nanocomposite films were studied by thermogravimetric analysis TGA Q50 (TA Instruments, USA) under nitrogen flow. The temperature of the sample was increased from room temperature to 600 °C at a heating rate of 10 °C/min with sample size between 10 and 12 mg. Weight loss of the sample was measured as a function of temperature. Mechanical Testing. Tensile strength (TS) and percent elongation (%E) at break of the bionanocomposite films were determined by tensile testing using universal testing machine (autograph AGS-X shimadzu, Canada) equipped with 50 N static load cell according to the ASTM standard D882-02 (ASTM standards, 2002). Films were cut into rectangle pieces (50 × 5 mm).The initial grid separation was set at 2.5 cm and the cross-head speed was 50 cm/ min. Tensile strength was calculated by dividing peak load to initial specimen cross-sectional area. Percent elongation at break was calculated as the percentage change in length of the specimen between the grips. Three specimens of each sample were evaluated. Dynamic Mechanical Analysis (DMA). The mechanical properties of bionanocomposites films were analyzed by using a dynamic mechanical analyzer (Q800, TA Instrument) under the flow of nitrogen at a frequency of 1 Hz and amplitude of 15 μm. The length and width of the film sample were 4 and 0.6 cm, respectively. The samples were heated from −50 to 150 °C at a heating rate of 3 °C/ min. The storage modulus (E′), loss modulus (E″) and loss tangent (tan δ = E″/E′) were recorded as a function of temperature. Glass transition temperature (Tg) was determined as, the temperature at which tan δ attained its peak value. Wide-Angle X-ray Diffraction (WAXD). X-ray diffraction studies of all bionanocomposites films were performed with a diffraction unit Rigaku Ultima IV operating at 38 kV and 38 mA. The radiation was generated from a Cu Kα source with a wavelength (λ) of 0.154 nm. The diffraction data were collected from 2θ values of 5 to 45° with a step size of 0.02°, where θ is the angle incidence of the X-ray beam on the sample. Samples of bionanocomposite films were prepared by drying of the films at 70 °C in oven overnight followed by grinding. The ground sample of 5 g was used for X-ray diffraction. Transmission Electron Microscopy (TEM). The structure and morphology of all bionanocomposite films were visualized by a transmission electron microscope (CM20 FEG TEM/STEM Philips) operating at 200 kV. An FEI Morgagni 268 instrument operated at 80 kV, equipped with Gatan Orius CCD camera was used to investigate the nanoreinforced samples. The samples were embedded in a polymer resin and thin slices of thickness 80 nm were prepared by ultramicrotome. A slice of sample was put on a fine mesh of copper support grid for analysis.

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RESULTS AND DISCUSSION X-ray Photoelectron Spectroscopy (XPS). XPS was used to identify and estimate the surface elemental composition and corresponding chemical bonding status of neat MMT, CNC, chicken feather keratin (CFK), and modified MMT−CFK, and CNC−CFK nanocomposites. XPS survey spectra of neat MMT, CFK, and keratin reinforced with MMT (MMT− CFK) are given in Figure 1. Carbon, nitrogen and oxygen are

Figure 1. XPS survey spectra of neat CFK, MMT, and MMT−CFK composites.

major elements in neat keratin (CFK) as well as in keratin reinforced with MMT. However, pure MMT spectrum contains additional elemental peaks related to silicon and aluminum. These additional elemental peaks were not observed in biocomposites reinforced with lower concentration of MMT (1%, 3%, and 5%), but at higher MMT contents (10%) the peaks corresponding to silicon and aluminum are obvious. In addition to that a gradual increase in the atomic concentration of oxygen, sodium and silicon has also been observed with the increase of nanoclay ratio in the modified biocomposites (please see Table S1). The high resolution C 1s spectra (Figure 2) revealed the change in chemical bonding occurred during the modification of keratin in the presence of nanoclay. Modification changed the relative amount of C 1s components. The neat CFK has three distinctive peaks with binding energies (BE) 284.4 (C−C, C−H), 285.6 (C−O, C−N), and 287.6 eV (CO, C−O−C), while MMT has four peaks at 284.2 (C− Si), 284.7 (C−C, C−H), 286.5 (C−O, C−N), and 289.2 eV (−COOR) which are consistent with the reported data.47−50 However, the C 1s spectrum of CFK reinforced with 10% MMT has an additional peak at 284.2 eV corresponding to C− 1979


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Figure 2. High-resolution C 1s spectra of MMT (A), neat biopolymer (B), and keratin biopolymer reinforced with 10% MMT (C).

Figure 4. High-resolution C 1s spectra of CNC (A), neat biopolymer (B), and keratin biopolymer reinforced with 10% CNC.

Si, which was not present in the neat CFK. Instead of that, in 10% MMT-CFK the area and intensities of some peaks have been increased, while a slight shift in their position toward higher BE can also be seen. The peaks at 284.4 (C−C, C−H) and 285.6 (C−O, C−N) of untreated CFK have been shifted to 284.5 and 285.9 eV, respectively, in MMT-CFK composite, which can be attributed to the nature of substituent on the carbon or the change in its environment after modification. The increase in the area of C−C/C−H, C−O−C/CO, and C−Si peaks also illustrate that bonding of C 1s has been increased affirming the diverse interaction of nanoclay with CFK. XPS survey and C 1s high-resolution spectra of CNC, CFK, and modified CNC−CFK composites are given in Figure 3 and 4 respectively. The survey spectrum of CNC, CFK, and CNC− CFK contains mainly five elements S, C, N, O, and Na with binding energies at 168.3, 283, 395.7, 527.3, and 1068.5 eV, respectively. The only distinction which can be seen from Figure 3 is the extent of difference in their intensities. It has been noticed that, on increasing the ratio of CNC in modified

composites, an increase in the atomic concentration of oxygen, nitrogen and sulfur was observed in case of 1%, 3%, and 10% CNC−CFK composites when compared to neat CFK film as given in Table S2. Whereas 5% CNC−CFK displayed inconsistent behavior, which could be attributed to inhomogeneous dispersion of CNC. On the other hand, the high resolution C 1s spectra of 10% CNC−CFK has three additional peaks compared to neat CFK film. Moreover, CNC also has three peaks with BE at 284.6, 286.3, and 287.6 eV, corresponding to C−C/C−H, C−O−C/C−OH, and CO/ O−C−O, respectively.51 The presence of extra peaks in 10% CNC−CFK spectra suggests the interaction of CNC with CFK resulting in the formation of new chemical bonding (Figure 4). The additional peak at 287.8 eV can be attributed to ester,47,48 which might be produced from esterification of −COOH from CFK and −OH from CNC. While the second additional peak at 286.3 eV belongs to C−O−C/C−OH bonding and is due to the presence of CNCs as this bonding exists in neat CNC as well. The third additional peak at 286.0 eV could be attributed to either C−N (quaternary ammonium groups) or CN bonding created during dispersion of CNC. In addition to the presence of extra peaks in 10% CNC−CFK composite, the peak intensities and area of other peaks related to C−C/C−H, C−O/C−N, and CO/O−C−O have been increased in comparison to neat CFK, which represent the greater ratio of carbon bonding in the modified composite. Differential Scanning Calorimetry (DSC). The thermal behavior of neat CFK film and its bionanocomposites reinforced with MMT and CNCs were studied by DSC and are presented in Figure 5a and 5b, respectively. Neat CFK film exhibited two broad endothermic peaks: the first peak at 83 °C refers to the evaporation of residual moisture from keratin matrix, and the second peak at 242 °C is due to the crystalline melting of the protein belongs to α-helix denaturation.52 The DSC plots of bionanocomposites reinforced with MMT showed crystalline melting at lower temperature (195 to 221 °C) as compared to neat CFK film (242 °C). Decrease in melting temperature could be due to addition of chitosan on the crystallinity of biopolymer matrix. Similar influence of chitosan/chitin was observed on the melting point of poly(vinyl alcohol) which shifted to a lower temperature accompanied by

Figure 3. XPS survey spectra of neat CFK, CNC, and CNC−CFK composites. 1980


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plasticizer and protein-rich zone domains, while studying transitions and microstructures of glycerol plasticized soy protein.59 Above results showed that both of the nanoparticles have different effect on the thermal properties of chicken feather based composites. Thermogravimetric Analysis (TGA). To investigate the thermal stability and degradation behavior of the bionanocomposites, TGA analyses were performed. Three stages of weight loss were observed in the TG and DTG curves of CFK-MMT composites (Figure 6). In the DTG curve, the highest peak

Figure 5. DSC plots of neat CFK film and nanocomposites reinforced with different ratio of MMT (a) and CNCs (b).

broadening due to inhomogeneous distribution.53 Also CFKMMT films showed absence of moisture peaks (near 83 °C) which represents the interaction of MMT’s with polar groups of the keratin through hydrogen bonding. This results in almost no free moisture available on the surface of the keratin films. Additional small melting peaks were also observed, showing plasticizer and protein-rich domains present in bionanocomposites. Composite films with 3% and 5% MMT showed lower crystalline melting temperatures (Tm) as compared to 1% MMT which indicates better dispersion of 1% MMT into the keratin. However, in case of 10% MMT, rise in Tm relates to higher ratio of clay contents. Several authors did not observe significant changes in the glass transition temperature (Tg) of composites reinforced with CNC and MMT.54−58 In case of CNC reinforced bionanocomposites, similar moisture loss and crystalline melting peaks were observed as shown in Figure 5b. The films with 1% CNC displayed 12 °C (254 °C) higher stability with delayed moisture peak (near 200 °C), which could be ascribed to well and homogenized dispersion of CNCs along with improved chitosan distribution. While gradual increase in Tm from 229 to 247 °C was observed in case of 3%, 5%, and 10% CNC composites. This indicates that higher concentration of CNC’s yields improved homogeneity of chitosan resulting in higher stability of composites materials. DSC plots of bionanocomposite materials showed some additional small peaks of very low intensity in the region of 160 to 200 °C, which is related to different types of interaction of 1,2-butanediol and glycerol (plasticizers) with keratin biopolymer. Chen and Zhang also observed similar

Figure 6. TGA (a) and DTG (b) curves of MMT−CFK films nanoreinforced with different MMT content under nitrogen flow.

intensity represents the maximum weight loss at that specific temperature. The first weight loss of up to 9% in the temperature range of 50−125 °C belongs to the evaporation of residual moisture, while the second weight loss from 10 to 30% in between 160 and 250 °C can be attributed to 1,2butanediol (plasticizer) evaporation and the final weight loss begins at 250 °C occurs due to the decomposition of keratin and chitosan. It can be depicted form the TGA data that the composite films with 1% and 10% MMT at 600 °C displayed weight loss of 74% and 73%, respectively, which is around 6% less than the neat CFK composite film. However, 3% and 5% MMT composites showed total weight loss of 76% and 77%, respectively, showing around 3% less decomposition. The slightly less weight loss was observed in case of 1% MMT composite film as compared to 3% and 5% MMT composites, which could be due to better dispersion of MMT. On the other hand, 10% MMT composite displayed lower weight loss 1981


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bionanocomposites, the maximum weight loss observed at slightly higher temperature for CNC−CFK composites than MMT−CFK based composites when compared with neat CFK. While MMT−CFK composites provided less weight loss after complete decomposition as compared to CNC−CFK composite materials. X-ray Diffraction (XRD) Spectroscopy. X-ray diffractograms of pure MMT, neat CFK, and MMT−CFK bionanocomposites are given in Figure 8. Pure MMT powder exhibited

compared to all other MMT composites, which could be due to higher content of nanoclay60 as observed in DSC analysis. On the other hand, thermal stability of CFK−CNC was also investigated by TG analysis as shown in Figure 7. TG and DTG

Figure 8. Wide-angle X-ray diffraction spectrogram of neat keratin, MMT, and biomaterials reinforced with different percentages of MMT.

a diffraction peaks at 2θ angle of 6°, 28°, 35°, 54°, 61°, and 73°, respectively. It can be clearly seen from the XRD patterns of MMT−CFK composites that all the crystallinity peaks of MMT have been completely disappeared in 1% and 3% MMT bionanocomposite films, while an unusual peak in 3% MMT composite could be attributed to the formation of new crystallinity region. These MMT crystallinity peaks reduced in 5% and 10% MMT-CFK composites. Also 5% and 10% MMT−CFK composites retain some of the pristine clay peaks which shows incomplete dispersion at higher concentration of layered silicates. It has been observed by many authors that physical blends showed tactoids and aggregates of MMT platelets during nanocomposite preparation.54,63 Overall, these results showed good dispersion of nanoparticles at lower loading of MMT, while poor dispersion was observed at higher loading of MMT. The higher dispersion of nanoparticles in MMT-CFK composites could be attributed to the sonication of MMT-CFK suspension during the process. These results also illustrate that the interlayer spacing of MMT was increased at lower loadings, which represents a high extent of intercalation/ exfoliation of nanosilicate layers in bionanocomposites. A similar phenomenon was observed in case of CNC−CFK biomaterials. XRD graphs of neat CFK film, pure CNC, and CNC−CFK are shown in Figure 9. The diffraction behavior of the composite films is dominated by the keratin phase which is the primary phase of the composite. Crystalline cellulose exists in different allomorphic forms, including cellulose I with chains aligned in parallel and cellulose II having antiparallel chain arrangement.44 The CNC display primary peaks at 2θ = 15.1°, 17.5°, and 22.7° with a weak diffraction peak at 34.4°, which corresponds to cellulose I.64−66 Diffraction peaks were also seen at 2θ = 12.5°, 20.1°, which are consistent with the primary peaks associated with cellulose II found at 2θ of 12.5°, 20.1°,

Figure 7. TGA (a) and DTG (b) curves of CNC−CFK films with different ratio of CNC under nitrogen flow.

curves of all CNC based composites also showed three stages of weight loss except the film with 1% CNC, where moisture loss peak is absent. The first weight loss peak for all CFK and CNC−CFK films occurring below 110 °C is due to the evaporation of moisture, loosely bound to glycerol used as a plasticizer, and keratin molecules. The second stage displays two weight loss peaks in DTG curve, may be due to either weak or strong interaction of CNC, chitosan, and glycerol with keratin. The regions which contain less keratin, CNC and chitosan and more glycerol results in early weight loss due to weak interaction of glycerol with protein molecules, while those which are homogeneous have strong interaction of glycerol and keratin molecules resulting in weight loss at higher temperature. Same kind of degradation behavior has already been noticed by Grevellec et al.61 for cottonseed proteins. The third weight loss corresponds to the degradation of keratin, where DTG curve represents more stability for 5% and 10% CNC−CFK composites as their maximum weight loss was observed at bit higher temperature than neat CFK. The higher extent of CNC dispersion results in the high performance composite materials.62 At 600 °C, all the CNC−CFK composites demonstrated lower weight loss and higher stability as compared to neat CFK film. In DTG curves of both 1982


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Figure 9. Wide angle X-ray diffraction spectrogram of neat keratin, CNC, and biomaterials reinforced with different percentages of CNC. Figure 10. TEM images of MMT−CFK composites reinforced with 1% MMT (a), 3% MMT (b), 5% MMT (c), and 10% MMT (d).

and 22.7° corresponds to the diffraction of (101), (10̅1), and (002) crystallographic plane reflections, respectively.64 The obtained crystal structure of the CNC mat is consistent with the previously reported highly crystalline CNC of both cellulose I and II.64−66 The crystallinity peaks at 2θ of 15°, 22°, 28°, 34°, and 45° assigned to pure CNC dispersion have been completely vanished in 1% and 3% CNC−CFK composites. While in case of 5% and 10% CNC−CFK composites the crystallinity peaks are retained at 2θ of 22° and 34°, although significant decrease in peak intensities was observed. The absence of the crystalline peaks in 1% and 3% CNC−CFK films suggests well dispersion of CNC, which were further supported by their TEM analysis in Figure 11. The peaks at near 22° observed in 5% and 10% CNC−CFK composites are due to the CNC crystallinity (Figure 9). The peak at 2θ of 22° is explicit since CNC content was higher, this shows that at higher concentration CNC retains its crystallinity in polymer structure.64,67,68 The XRD results are in accordance with DSC and TGA analysis. Transmission Electron Microscopy (TEM). TEM studies were essential to verify the extent of dispersion (exfoliation/ intercalation) of nanoparticles. TEM micrographs are consistent with those of the previous XRD pattern studies (Figures 8 and 9). TEM images of MMT-CFK and CNC−CFK composites are displayed in Figures 10 and 11, respectively. MMT−CFK composite films showed well dispersed regions as represented in Figure 10. The images of 1% MMT−CFK composite films displayed the exfoliation of layered structures of MMT. However, in case of 3% and 5% MMT content, the layered silicates were mostly intercalated and exfoliated in keratin matrix. Some aggregate regions were also observed which suggests that only sonication technique was not enough to disperse the nanoparticles. While 10% MMT−CFK composite film contains tactoids and aggregated regions. The dispersion of MMT nanoparticles can be attributed to insertion of keratin chains into the intergalleries of nanoclay. TEM micrographs of CNC−CFK composites also confirmed the well dispersion of cellulose nanocrystals and are present in the form of needles (Figure 11). The extent of dispersion is more obvious in case of 1% and 3% CNC−CFK nanocomposites as compared to 5% and 10% CNC−CFK films. TEM results also support XRD data, where crystallinity peaks of MMT and CNC

Figure 11. TEM images of CNC−CFK composites reinforced with 1% CNC (a), 3% CNC (b), 5% CNC (c), and 10% CNC (d).

have been greatly reduced in bionanocomposites. The length of rod-like shape CNC was around 200−400 nm. Dynamic Mechanical Analysis (DMA). DMA measures the changes in the viscoelastic properties of the polymers as a function of temperature. Thermal transitions occur due to chain mobility during temperature variation. The most important transition includes glass transition which belongs to the beginning of chain motions and determined from the peak of tan delta curve. Normally DMA data for solids is displayed as storage modulus and damping or tan delta versus temperature. The size of tan delta peak reflects the volume fraction of the material undergoing transition. Effect of temperature on tan delta of MMT−CFK films showed in Figure 12. Two transitions were observed in all MMT−CFK composites and could be attributed to two different zones depending upon the strong/weak interaction between keratin, chitosan, and plasticizer. The first could be attributed to 1,2-butanediol and chitosan rich domains, while the second may be nanoparticle rich zone. The similar behavior among plasticizer and 1983


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Figure 12. Effect of temperature on log E′ (a) and tan δ (b) of MMT−CFK films with different MMT contents.

Figure 13. Effect of temperature on log E′ (a) and tan δ (b) of CNC− CFK films with different CNC contents.

biopolymer has already been reported.8,59 The glass transition temperature (Tg) has been improved in all MMT−CFK composites as compared to neat CFK film (45 ± 2 °C), where first peak is occurring near 68 ± 5 °C and second peak exhibiting a Tg of 116 ± 5 °C (Figure 12). The effect of temperature on storage modulus (E′) of MMT-CFK composites with 0%, 1%, 3%, 5%, and 10% MMT is shown in Figure 12a. At −50 °C, neat CFK film has the highest E′ as compared to all MMT−CFK bionanocomposites. With the increase in temperature, a rapid decline in E′ was observed for neat CFK film, while MMT−CFK composites displayed a gradual decrease. This gradual decrease in E′ ranges from −45 to 30 °C, represent the transition from glassy state to rubbery state. The MMT−CFK composites containing 1%, 3%, and 10% MMT demonstrated higher value of E′ as compared to 5% MMT composites. At −11 °C and above, all MMT−CFK composites exhibited higher and sustained E′ as compared to neat CFK film, which could be attributed to strong interaction of keratin with 1,2-butanediol and MMT in this temperature range. Generally stronger interactions results in a higher storage modulus.69 Tan delta of CNC−CFK films are displayed in Figure 13. The Tg for neat CFK, 1%, 3%, 5%, and 10% CNC−CFK composite films are 46, 52, 62, 31, and 39 °C, respectively. Tg increases with the increase in CNC content from 0% to 3%, while it was decreased in case of 5% and 10% CNC−CFK composites. The higher value of Tg in 1% and 3% CNC−CFK films might be attributed to better dispersion of CNC causing hindrance in chain motions and increased cross-linking of

keratin, chitosan and 1,2-butanediol as observed in other analyses. However, lower Tg in 5% and 10% CNC−CFK films represents poor CNC dispersion and less capability of keratin polymer chains to cross-link with the glycerol resulting in a weak bonding. In case of CNC−CFK films, at −46 °C 1% CNC composite demonstrated improved E′ as compared to neat CFK, 3% CNC−CFK composite have E′ similar to neat CFK film, while films with 5% and 10% CNC content exhibited low E′ (Figure 13a). With the increase of temperature from −46 to 55 °C, the E′ remains higher than neat CFK film for 1% and 3% CNC−CFK composite films, while it drops for 5% and 10% CNC−CFK composites. So the higher value of E′ for 1% and 3% CNC bionanocomposites is due to better dispersion of CNCs leading to improved interactions with keratin and reduced mobility of biopolymer chains reinforced with CNCs. Tensile Strength and Percent Elongation. The addition of MMT and CNC’s in biopolymers has significant influence on tensile properties (tensile strength and % elongation) as reported by many researchers. Tensile strength (TS) and percent elongation (%E) obtained for keratin biomaterials reinforced with MMT and CNC’s is given in Tables 1 and 2, respectively. MMT−CFK composite films showed improvement in TS and %E as compared to neat CFK films. Particularly, film with 5% MMT−CFK demonstrated highest TS value. A gradual increase in %E was observed, when the MMT content was increased from 1% to 10%. In case of 3% MMT−CFK film significant difference (p < 0.05) was reported as compared to neat CFK film and CFK−MMT films. However, there was no significant difference on percent 1984


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particles demonstrated divergent effect on tensile strength and elongation at break of the bionanocomposites. MMT enhanced the tensile strength, while CNCs incorporated samples showed higher percent elongation. However, there is a need to further improve the properties of Keratin based films for commercial application.

Table 1. Effect of Type and Content of MMT on the Tensile Strength (TS) and Percent Elongation at Break (%E) of MMT−CFK Filmsa sample neat CFK 1% CFK−MMT 3% CFK−MMT 5% CFK−MMT 10% CFK−MMT

tensile strength (MPa) 6.505 5.892 4.485 6.748 5.985

± ± ± ± ±

0.06b 0.80b 0.99a 0.14b 0.07b

elongation at break (%) 6.834 7.444 4.496 8.734 9.380

± ± ± ± ±

0.10a 1.89ab 2.37b 0.98b 0.49b

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03380. Elemental compositions, FT-IR spectra, and SEM images of bionanocomposites and screening and optimization of contents used in film preparation (PDF)

Values of mean of three replicates ± standard deviation. Means in the same column followed by the same letter are not significantly different (P > 0.05). a

Table 2. Effect of Type and Content of CNCs on the Tensile Strength (TS) and Percent Elongation at Break (%E) of CNC−CFK Filmsa sample neat CFK 1% CFK−CNC 3% CFK−CNC 5% CFK−CNC 10% CFK−CNC

tensile strength (MPa) 4.503 5.314 4.634 4.969 4.678

± ± ± ± ±

0.20a 0.19c 0.04a 0.05b 0.07a

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elongation at break (%) 6.587 24.760 24.493 27.627 21.053

± ± ± ± ±

ASSOCIATED CONTENT

S Supporting Information *

AUTHOR INFORMATION

Corresponding Author

0.22a 3.41c 0.41 cd 0.75b 0.44d

*E-mail: [email protected]. Phone: +1 (780) 492-4845. ORCID

Aman Ullah: 0000-0003-1801-0162 Notes

Values of mean of three replicates ± standard deviation. Means in the same column followed by the same letter are not significantly different (P > 0.05). a

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge the financial support for current work by CPRC, ALMA, and NSERC. The CNC were kindly provided by AITF.

elongation was reported except in 1% MMT−CFK film. Generally, the improvement in mechanical properties of the bionanocomposites films can be attributed to the high aspect ratio and rigidity that results from the strong affinity between the biopolymer and MMT. On the other hand, effect of CNC’s on tensile properties of CNC−CFK composites is given in Table 2. TS was not considerably affected (p > 0.05) but %E was substantially increased. At low CNC content, the TS of the CNC−CFK composite films was comparable to that of neat CFK films. But it has decreased at higher CNC contents. This might be due to incomplete dispersion or presence of aggregates of CNCs in the keratin biopolymer matrix. In CNC based composites 3% and 5% CNC−CFK films showed significant difference (p < 0.05).

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CONCLUSIONS In this study in-situ-modified chicken-feather-based bionanocomposites reinforced with montmorillonite (MMT) and cellulose nanocrystals (CNC) were prepared by compression molding. Two plasticizers 1,2-butanediol for MMT-based and glycerol for CNC-based keratin composites with chitosan as cross-linking agent with sodium sulfite as reducing agent were used. XPS analysis revealed the change in chemical bonding occurred during the modification of keratin in the presence of MMT and CNC’s. Crystalline melting temperatures were altered as indicated by DSC results with improved thermal stability at low nanoparticle content (1 and 3 wt %). Thermal degradation of keratin was slower in MMT-based composites. XRD and TEM results represents well dispersion of both nanofillers, where MMT and CNC was present in exfoliated to intercalated form at lower contents (1% and 3%) in keratin matrix. However, aggregates at higher concentration (10 wt %) of both MMT and CNC nanoparticles were observed. High storage modulus and glass transition temperature were dominated in CNC based composites. Both of the nano1985


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