Improvement of polylactic acid film properties through the addition of cellulose nanocrystals isolated from waste cotton cloth

International Journal of Biological Macromolecules 129 (2019) 878–886

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International Journal of Biological Macromolecules journal homepage: http://www.elsevier.com/locate/ijbiomac

Improvement of polylactic acid film properties through the addition of cellulose nanocrystals isolated from waste cotton cloth Zhanhong Wang a,c,1, Zhengjun Yao a,b,⁎,1, Jintang Zhou a,b,⁎, Meng He c, Qiong Jiang c, Aimin Li c, Shuiping Li c, Manqing Liu c, Sen Luo c, Dewen Zhang c a b c

College of Materials and Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology, Nanjing University of Aeronautics and Astronautics, Nanjing 211100, China School of Materials Engineering, Yancheng Institute of Technology, Yancheng 224051, Jiangsu, China

a r t i c l e

i n f o

Article history: Received 22 July 2018 Received in revised form 3 February 2019 Accepted 3 February 2019 Available online 5 February 2019 Keywords: Cellulose nanocrystals Cotton Acid hydrolysis Polylactic acid Composite films Mechanical properties

a b s t r a c t Cellulose nanocrystals (CNCs) were isolated from waste cotton cloth fibers using a mixed acid hydrolysis method and subsequently used as fillers to reinforce a polylactic acid (PLA) matrix for the construction of high performance and biodegradable PLA/CNC composite films. The morphology, structure, and thermal and mechanical properties of CNCs, PLA, and the composite films were characterized. The length, diameter, and aspect ratio of CNCs ranged from 38 to 424 nm, 2 to 17 nm, and 10–32 respectively. The crystallinity, tensile strength, elasticity modulus, and work-to-break of PLA/CNC composite films were effectively improved by the addition of 0.1 wt% and 0.3 wt% CNCs. However, poor performance parameters were acquired at higher CNC content (0.7 wt%), because the CNCs were not well distributed in the polymer matrix. © 2019 Elsevier B.V. All rights reserved.

1. Introduction Plastic film products have the advantage of being light weight, diaphanous, durable, and being available at low price. However, traditional plastic film products are derived from petroleum and have poor biodegradability [1,2], requiring hundreds of years for their natural decomposition. Landfilling and incineration are two common methods used for the disposal of plastic films; however, these solutions are far from ideal because of their serious environmental implications given that large quantities of carbon dioxide would be produced during the incineration process. Carbon dioxide is a considerable contributor to global warming [3,4]. Thus, given their daily use, it is of paramount importance that an effective method for the decomposition of plastic films is developed. Furthermore, petroleum resources are gradually becoming exhausted in the future, such that the production and application of traditional plastics will be limited due to the shortage of raw materials [5]. Thus, in order to alleviate these problems, research and development of green biodegradable plastic film materials has become the focus of attention in recent years. Biodegradable plastic films can be ⁎ Corresponding authors at: College of Materials and Technology, Nanjing University of Aeronautics and Astronautics, No. 169 Sheng Tai West Road, Jiang Ning District, Nanjing city, Jiangsu province 211100, China. E-mail addresses: [email protected] (Z. Yao), [email protected] (J. Zhou). 1 These authors contributed equally to this work.

https://doi.org/10.1016/j.ijbiomac.2019.02.021 0141-8130/© 2019 Elsevier B.V. All rights reserved.

easily decomposed in a natural environment once disposed of. Consequently, traditional plastic films are being gradually replaced with biodegradable polymers in various applications to reduce the environmental pollution caused by plastic waste [6,7]. Polylactic acid (PLA) is a biodegradable polymer that, due to its environmental friendliness, good transparency, and biological compatibility, is commonly used in the fabrication of various plastic film materials. Nevertheless, PLA has a high hardness, and brittleness, low strength, and poor thermal stability, which severely limit its application [8–10]. Therefore, the modification of the PLA matrix is required to broaden its use. Cellulose nanocrystals (CNCs), which are green and biodegradable reinforcing materials, have been the focus of attention with regard to their application in the development of biodegradable polymer composites. The raw materials for the preparation of CNC are abundant in nature and have low cost. CNC has a high strength, modulus, and flexibility, as well as good dynamic mechanical properties [11]. These excellent properties make CNCs one of the most promising classes of materials for the preparation of polymer composites [12]. Moreover, CNCs are widely available from plant, animal, and bacterial sources [13,14]. The most promising candidates for the preparation of CNCs are waste biomass resources, including wastepaper, waste straw, waste peel, and other industrial and agricultural waste materials [15–21]. Furthermore, various waste biomass resources generated from agricultural or food processing industries have also been used for

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the isolation of CNCs to test their potential use for the preparation of polymer composites. Rhim et al. [22] extracted cellulose nanofibers from onion skins using three different concentrations of sulfuric acid (H2SO4, 45%, 55%, and 65%) to test their potential use as reinforcement fibers in agar-based bio-nanocomposite film formation. Bettaieb et al. [23] isolated CNCs from Posidonia oceanica balls and leaves and used these to reinforce different poly(styrene-co-butyl acrylate) nanocomposite samples. Further, the effect of CNCs loading on the mechanical and thermal properties of composites has also been studied. Kakroodi et al. [24] reinforced polyvinyl alcohol with cellulose nanofibers from Aloe vera rind and assessed the properties of the various reinforced composites in comparison with those of pure polyvinyl alcohol. As an underutilized waste biomass source, cotton waste is of considerable interest since the cellulose content in cotton fibers is extremely high (above 90%). Further, cotton waste is generated in significant amounts from the textile and garment industries, with serious disposal issues [1]. However, only a limited number of efforts have been made to recycle cotton waste material, including the production of mops, plush toys, and lower-grade cotton textiles. To date, the extraction of CNCs from cotton waste has focused on the use of sulfuric acid hydrolysis due to the good dispersion obtained [17,25,26]. The reason of good dispersive stability is that sulfate ion exhibits negative charges and can make the electrostatic repulsion between CNCs molecules [27]. However, sulfuric acid (H2SO4) is a strong acid and can lead cellulose degradation. The yield of CNCs obtained using sulfuric acid hydrolysis method is relatively low. Therefore, other mineral acids have been investigated to replace sulfuric acid. Hydrochloric acid was found to have weak oxidation ability and low thermal degradation of CNCs, but poor dispersion ability for CNCs. The mixed acid solution (sulfuric acid and hydrochloric acid) hydrolysis may be a good choice for the production of CNCs. The dispersion ability for CNCs by the hydrolysis of hydrochloric acid may be improved by the electrostatic repulsion of sulfate ion during the mixed acids hydrolysis. The degradation of CNCs is also weakened because part of sulfuric acid solution was replaced by the hydrochloric acid solution. Nevertheless, to the best of the authors' knowledge, there is still no relevant report in the literature focusing on the extraction and characterization of CNCs from cotton waste using a mixed acid hydrolysis method (H2SO4 and HCl). Furthermore, the effects of using CNCs extracted by mixed acid hydrolysis on the structure and properties of PLA composites remain to be elucidated. The interface bonding state and interaction mechanism between CNCs (using a mixed acid hydrolysis) and PLA matrix are unclear. Based on the points discussed above, the main objective of the present study was to prepare PLA composite films enhanced with CNCs for the value-added utilization of waste cotton cloth. To achieve the above objectives, CNCs were obtained through mixed acid (H2SO4 and HCl) hydrolysis. The morphology, components, and properties of the prepared CNCs were characterized. The effect of varying CNC concentration (0, 0.1, 0.2, 0.5, and 0.7 wt%, basis PLA weight) on the properties of the PLA films were evaluated by Scanning electron microscopy (SEM), Xray diffraction (XRD), thermogravimetric analysis (TGA), and differential scanning calorimetry (DSC). In addition, the effects of CNCs on the tensile strength and transparency of the PLA/CNC composite films were also investigated.

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experimental equipment Co., Ltd., China. Sodium hydroxide (NaOH), glacial acetic acid, dichloromethane and hydrogen peroxide (H2O2) were obtained from Aladdin Industrial Inc. Distilled water was used throughout the experiments. The reagents were used as received without further purification. 2.2. Waste cotton cloth pretreatment The waste cotton cloth was cleaned with detergent to remove oil and other organic impurities, air dried naturally, and cut into small pieces (approximately 2 mm × 2 mm). Approximately 5 g of waste cotton cloth pieces and 300 mL of deionized water were mixed and pulped in the beater (JYL-C91T, Joyoung) at a speed of 22,000 rpm, corresponding to 19,609 g at a the power of 200 W at room temperature for 10 min. The process was stopped every 30 s for 5 s to prevent the system from overheating. The obtained pulp was filtered, rinsed, dried, and stored for the next experiment. 2.3. Extraction of cellulose The residue (mass: 4.998 g) was boiled and treated with 300 mL 15% (wt%) analytical grade NaOH at 70 °C for 2 h, filtered, and washed with distilled water until the solution reached pH 10. The filtered residue was dried and weighed. 4.987 g dry residue was acquired. The dry residue and 250 mL 1.5% (wt%) H2O2 were then added into beaker and magnetically stirred at 70 °C for the discoloration treatment. After an hour, acetic acid was added to adjust the pH value of the solution close to 7. The solution was filtered and washed with distilled water at least three times. The resultant cellulose fibers were used for the isolation of CNCs. 2.4. Isolation of CNCs Various steps are required for the preparation of CNCs from waste cotton cloth cellulose (Fig. 1). CNCs were obtained by acid hydrolysis of the extracted cellulose using a mixed H2SO4 (98 wt%), HCl (37 wt%), and deionized water solution at a volume ratio of 3:1:11. The extracted cellulose was added into the mixed acid solution, and treated at 65 °C for 5 h in an ultrasonic water-bath at a power of 50 W. The process was stopped every 20 min for 2 min to let the ultrasound equipment (KQ-100KDE, Kunshan Ultrasonic Instruments Co., Ltd) rest and prevent it from overheating. The mixture was then diluted with five times the volume of distilled water (room temperature ~25 °C), followed by repeated centrifugation at 10,000 rpm corresponding to 11,400 g for 10 min (CT15RT high speed centrifuge, Techcomp, Ltd., China) to remove the supernatant. The suspension was collected when it began to turn turbid by diluting the precipitate and centrifuging repeatedly. The collection process was stopped once the suspension became clear. The collected turbid suspension was subjected to dialysis against distilled water with dialysis tubes (the molecular weight cut off 8000–14,000, MD34 mm, made in USA) for several days until a neutral pH was obtained. The suspension was then placed in a glass petri dish and frozen in bulk at −20 °C for 2 h in a freezer. The frozen sample was placed in a freeze-dryer (FreeZone 18 L, Labconco) at −60 °C for 24 h until all the water was removed. Finally, the dry CNCs powders were collected.

2. Experimental 2.5. Preparation of PLA/CNC composite films 2.1. Materials The waste cotton cloth used herein was an old, printed and weaved bedsheet of approximately 100% cotton content (92% cellulose, 7% hemicellulose, 1% wax, and a minimum amount of dye impurities). The waste cotton cloth was received from a waste products market in Yancheng city, Jiang Su province, China. PLA (4043D) was purchased from NatureWorks (Blair, Nebraska, USA). HCl (37 wt%), H2SO4 (98 wt %), and anhydrous alcohol were provided by Nantong Kai Mei Ke

PLA films were prepared using a casting method following a previously reported method [28]. Briefly, 2 g of PLA were dried in the oven for 12 h, and then added into 50 mL of dichloromethane. The mixture was stirred and heated on a magnetic stirrer at 40 °C for 30 min to completely dissolve PLA. CNC powders were then scattered in the anhydrous alcohol in an ultrasonic water-bath and dried in the oven at 40 °C for 1 h to improve the dispersion of CNCs. For the preparation of PLA/ CNC composite films, a varying content of CNCs (0, 0.1, 0.3, 0.5, and

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Fig. 1. Schematic representation of the various steps required to prepare CNCs from waste cotton cloth cellulose.

0.7 wt% by weight) was mixed with the above mixture (PLA and dichloromethane) and stirred at 40 °C for 3 h. The final mixed solution was poured into a clean petri dish and dried at room temperature to allow film formation. Before peeling, the film was dried in an oven for 2 h at 40 °C. 2.6. Characterization of materials 2.6.1. Morphology analysis The morphology of CNCs was analyzed using a transmission electron microscope (TEM Philips CM100) at an accelerating voltage of 80 kV. The CNC suspension was sonicated for 30 min, deposited on the grid (a copper grid coated with carbon, 200 mesh, 3.05 mm of diameter) and left for 5 min to dry, and then drained on the filter paper. The grid was inverted and placed on top of a drop of 2% w/v phosphotungstic acid to stain for 5 min. The grids were then dried under infrared lamps and observed by TEM. The length and diameter of CNCs were measured using the Nano Measurer 1.2 software, and the aspect ratio of each fiber was calculated. A total of 200 crystals were analyzed to determine the size ranges (confidence interval) or the aspect ratio value for each experiment. The surface, cross section, and thickness of PLA and PLA/CNC composite films was observed by SEM (FEI Nova NanoSEM 450) at an accelerating voltage of 3 kV. Samples were gold coated to prevent charging. The thickness was determined by the average value of five random positions of the film samples. The thickness of films in this study is about 183.5 ± 5.5 μm. The morphological analysis of film surface was also carried out using AFM (Bruker, Model MULTIMODE 8) equipped with silicon cantilever (spring constant ~42 N/m and resonance frequency of~320 kHz). The AFM images, height profiles and the 3D elevated images were analyzed using its included software. The transparency of the film samples was determined by measuring the percent transmittance at a wave length range of 200 to 800 nm using a UV–Vis spectrophotometer (Model UV-2450, Shimadzu Co., Ltd., Japan). 2.6.2. Zeta potential measurement The zeta potential is an important parameter for examining dispersion stability. The CNC suspensions were prepared and their zeta potential was measured using a Nano ZS Zetasizer (model ZEN3600, Malvern Instruments Ltd., UK). 2.6.3. XRD analysis XRD patterns of CNCs, PLA, and PLA/CNC composite films were analyzed using an XRD diffractometer (D8 Advance, Bruker) at a scanning range of 2θ = 10–45°, a step size of 0.02, and time per step of 30 s at room temperature. The operating voltage was 50 kV, and the current was 40 mA. 2.6.4. Thermal properties (TGA and DSC) TGA and DSC were performed on approximately 8 mg of CNC samples in an alumina crucible using a synchronous thermal analyzer (STA 449C, NETZSCH Inc.), from room temperature to 600 °C, at a heating rate of 10 °C/min, under a nitrogen atmosphere with a flux of 50 mL/min.

Thermal degradation of PLA and PLA/CNC composite films can be analyzed using TGA. This test condition is the same as that of CNC samples. The maximum thermal degradation temperature was obtained from the derivative thermogravimetric (DTG) data. All the experiments were performed at least in duplicate under these experimental conditions. The crystallization behavior of PLA and PLA/CNC composite films were studied using a differential scanning calorimeter (DSC 3500 Sirius, NETZSCH Inc.) under a nitrogen atmosphere (flow rate of 50 mL/min) according the reported method [29]. Each sample (~8 mg) was scanned at 10 °C/min under the heat/cool/heat cycles. Specifically, each specimen was first heated from −25 °C to 190 °C and maintained at that temperature for 5 min to eliminate the previous thermal history. Subsequently, the sample was cooled to −25 °C, held for 5 min, and heated up to 190 °C again. The main thermal parameters, such as glass transition temperature (Tg), cold crystallization temperature (Tcc), melting temperature (Tm), cold crystallization enthalpy (△Hcc) and melting enthalpy (△Hm), were determined from the second heating curves. The crystallinity (χc) of each sample was calculated using the following equation: χc ¼

ð△Hm −△Hcc Þ △H 0m  ω

 100%

where △Hm is the enthalpy of melting, △Hcc is the enthalpy of cold crystallization, △H0m is the melting enthalpy of pure 100% crystalline PLA, as determined to be 93.7 J/g, and ω is the weight fraction of PLA in the film sample. 2.6.5. Tensile testing of PLA/CNC composite films Tensile properties were measured using a Model CMT6104 Universal Testing Machine (MST Co., Ltd., China) according to the ASTM Method D 882-12 (2012). The crosshead speed was 2 mm/min during tensile properties measurement. The film samples were cut into 1 cm wide × 15 cm long strips. Tensile strength, elongation at break, and the elastic modulus were determined from the stress–strain curves. At least five replicates were characterized for each sample. The significant difference of the data was evaluated using the Excel Statistical Software with p b 0.05 confident interval. 3. Results and discussion 3.1. Morphological analysis CNCs were isolated from waste cotton cloth, leading to the formation of a milky turbid suspension (Fig. 2a), indicating that the CNCs were well dispersed in water to form a homogeneous suspension. The repulsion forces in CNC particles having charges on their surfaces are found responsible for the above phenomenon [30]. The zeta potential of the CNC suspensions were measured, showing negative zeta potential values (−38.9 mV) in neutral water. The zeta potential results showed the formation of an electric charge between the shear plane of the final external layer and bulk solution. An array of negative charge emerged on the crystal surface of the CNCs during hydrolysis, leading to the larger value of the zeta potential. Therefore, the CNC suspensions have good dispersibility in water. After vacuum freeze drying, the water in the CNC suspension was removed and dry white, opalescent CNC powders were obtained (Fig. 2b). TEM images of CNCs (Fig. 2c) allowed the

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Fig. 2. Photographs of the CNC suspension (a) and CNC powders (b); TEM image (c), length (d), diameter (e), and aspect ratio distribution (f), of CNCs retrieved from waste cotton cloth.

corresponding length, diameter and aspect ratio distribution of CNCs to be calculated (Fig. 2d–f). The length, diameter and aspect ratio (length/ diameter) of CNCs ranged from 38 to 424 nm, 2 to 17 nm and 10 to 32

respectively. It is worth noting that the aspect ratio value is high compared with values previously obtained [22,31–33]. In general, fillers with a high aspect ratio are expected to offer a good reinforcing effect,

Fig. 3. Morphology and transparency analysis of PLA/CNC composite films. The photographs of PLA/CNC composite films with varying CNC content: 0% (a), 0.1% (b), 0.3% (c), 0.5% (d), 0.7% (e). UV–Vis analysis of PLA/CNC composite films (f) and the schematic illustration of light transmission through the PLA and PLA/CNC films (g).

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playing an important role in improving the physical and mechanical properties of composite materials [22]. Thus, the CNCs extracted from waste cotton cloth are suitable for application as reinforcing fibers in biocomposites given their higher aspect ratio. Moreover, the yield value of CNCs from waste cotton cloth was 49.2 ± 1.6%, with respect to the initial weight of the raw material. It is worth noting that the obtained average CNC yield was higher than that previously reported for other cellulosic fibers [22,25,34,35]. The macrographs of PLA composite films with varying CNC content (Fig. 3a–e) showed that the surface of pure PLA film had some tiny pores (Fig. 3a), indicating that the pure PLA film has a loose structure. When CNCs were used as fillers and modifiers, the structure of the PLA/CNC composite films became denser, with a flat and smooth surface at a CNC content of 0.5 wt% or less (Fig. 3b–d); no apparent aggregation was found on the surface of these films. However, at higher CNC content (0.7 wt%; Fig. 3e), the CNCs were no longer well distributed in the polymer matrix. This was ascribed to the strong hydrogen bond interactions between CNCs due to the presence of large hydroxyl groups on the particle surfaces, thus restricting interfacial adhesion between CNCs and the polymer matrix [22,36]. Therefore, the formation of white larger particles on the surface of the film (Fig. 3e) was due to CNC aggregates. Thus, these results indicate that excess addition of CNCs will hinder the formation of transparent films. In order to verify the effects of excess CNC addition, the transparency of the PLA and PLA/CNC composite films was determined according to percent transmittance between 200 and 800 nm (Fig. 3f). Indeed, the pure PLA film was transparent

with a high transmittance against both UV and visible light (78.4% at λ800). However, a significant decrease in light transmittance was observed for PLA/CNC composite films, with a decreased in transparency by 52.7–66.9% at λ800. Moreover, the transmittance of both UV and visible light by the composite films decreased gradually with the increase in CNC concentration, decreasing from 66.9% to 52.7% when the addition of CNCs changed from 0.1% to 0.7% at λ800. The effect was attributed to the hindrance of light transmittance through the films (Fig. 3g), and was observed to be more intense at higher CNC concentrations due to their agglomeration and entanglement. Similar results, wherein CNCs or cellulose dispersed into biopolymer films lead to a decrease in their transparency, have been previously observed [26,37,38]. Indeed, the film obtained with the lowest CNC concentration showed the best UV–Vis light transmittance without sacrificing transparency, and is therefore the most suitable for transparent and UV-screening packaging applications. The micromorphology of the surfaces and fracture cross-section of the composite films was investigated through SEM and AFM (Fig. 4). A rough and lamellar surface structure was observed for pure PLA films, which provides further indication of the loose surface structure (Fig. 4a and a1). On the contrary, the formation of dense structures was observed for the PLA/CNC composites (Fig. 4b, b1, c, c1, d, d1, e, and e1). The surface structure of the PLA/CNC composite films became increasingly compact with the higher CNC content. However, considerable particle aggregation was observed on the surface of the composite film with the highest CNC content (Fig. 4e and e1) due to the hydrogen

Fig. 4. SEM images of the PLA/CNC composite films surfaces with varying CNC concentration: 0% (a, a1), 0.1% (b, b1), 0.3% (c, c1), 0.5% (d, d1), 0.7% (e, e1); SEM images of the fracture crosssection of the PLA/CNC composite films with varying CNC concentration: 0% (a2), 0.1% (b2), 0.3% (c2), 0.5% (d2), 0.7% (e2); AFM images of the PLA/CNC composite films surfaces with varying CNC concentration: 0% (a3), 0.1% (b3), 0.3% (c3), 0.5% (d3), 0.7% (e3). The magnification of the images (a, b, c, d, e) and the images (a1, b1, c1, d1, e1) are 5000× and 100,000× respectively.

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higher roughness (Ra = 15.10 nm). The larger agglomeration of CNC particles can be seen in Fig. 4d3 and e3, which result in the higher surface roughness. Moreover, SEM images of the fractured cross-section of pure PLA and PLA/CNC composite films are also compared in Fig. 4a2, b2, c2, d2, e2. It can be seen from Fig. 4a2 that the fractured surface of pure PLA is laminar and uniform. When the loading level of CNC is lower than 0.3 wt%, the laminar structure is also clearly presented, which implies the plastic fracture surface for the pure PLA and PLA/0.1% CNC composite film. Fracture surface morphology images of PLA/0.3% CNC and PLA/0.5% CNC are shown in Fig. 4c2 and d2. Indistinct laminar structure have been observed for both of PLA/CNC composite films. But the PLA/0.7% CNC composite film shows smoother surface than others. That indicated that the fracture surface becomes brittle instead of plastic with increasing CNC content. 3.2. X-ray diffraction analyses (XRD)

Fig. 5. XRD patterns of PLA/CNC composite films with 0–0.7 wt% CNC content.

bond formation described above and the binding energy between CNCs and CNCs was higher than that between CNCs and the PLA matrix [36]. Therefore, the CNC particles easily aggregate, with the aggregate size increasing with the increase in CNC content. Thus, a high CNC content had an adverse effect on the surface structure homogeneity of films. Such agglomeration would be expected to affect film properties, including tensile strength, crystallinity, and transparency [26,28]. When the CNC content is below 0.5 wt%, the particles are uniformly dispersed in the PLA composite films (Fig. 4b and b1), suggesting that the film sample with lower CNC content. The AFM results are consistent with those of SEM. From the results of AFM, it can be seen that the surface of the film samples becomes rougher and rougher with the increase of CNC doping. The surface roughness of PLA/CNC composite films has been observed in the corresponding AFM photograph. The pure PLA shows low roughness (Ra = 5.88 nm) while the PLA/0.7% CNC composite film have

The XRD patterns of the CNCs, pure PLA, and PLA/CNC composite films in the 2θ range of 10°–45° (Fig. 5) showed diffraction peaks for CNCs at approximately 14.8°, 16.5°, 22.6°, and 34.1°, indexed to cellulose type I [28,39–41]. The pure PLA showed two major characteristic peaks at 16.9° and 19.3° corresponding to the (200)/(110), and (203) crystallographic planes of the PLA, respectively [42]. For all PLA/CNC composite films, four obvious characteristic peaks were observed at approximately 14.7°, 16.6°, 19.0°, and 22.3°. Compared with pure PLA, all the characteristic peaks shifted to a lower 2θ, indicating that the diffraction angle (θ) became smaller and the PLA crystalline interplanar spacing increased according to the Bragg's equation as follows. 2dsinθ ¼ nλ

ð1Þ

where d is the crystalline interplanar spacing, θ is the diffraction angle, n is the diffraction series, λ is the X ray wavelength. Therefore, the PLA crystal structure changed following the addition of CNCs. The observed increase in crystalline interplanar spacing may be attributed to the incorporation of CNCs into the internal structure of PLA. Moreover, the

Fig. 6. TG-DSC curves of CNCs (a); DSC (b), TG (c) and DTG (d) curves of PLA/CNC composite films with 0–0.7 wt.%.

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intensity of all peaks increased remarkably for almost all PLA/CNC composite films (except the sample with 0.7 wt% CNCs), indicating an increase in crystallinity. This was likely due to the addition of CNCs resulting in heterogeneous nucleation of crystal in PLA composite films and improvement of crystallinity of the composite films [43,44]. Conversely, the PLA composite film with 0.7 wt% CNCs showed a weaker intensity for all peaks, likely due to CNC agglomeration reducing the crystallinity (Figs. 3e and 4e). The CNCs were less efficient in favoring the crystallization, since they were more agglomerated with consequent smaller contact area between the fillers and the polymer matrix [43]. Thus, it is clear that crystallinity can be increased through the addition of moderate amounts of CNCs. 3.3. Thermal properties (TG and DSC) The thermal properties of the obtained CNCs were assessed by TGDSC analysis (Fig. 6a). In this figure, three weight loss stages were observed for the CNCs. Below 150 °C, a slight weight loss (~3%) was found. From the corresponding DSC curves, the initial endothermic peak at 55 °C. That can be attributed to the vaporization of water, which was consistent with the prior reports [45–48]. The main thermal degradation occurred in the second temperature stage of 150–400 °C. The thermal decomposition of CNCs showed the maximum weight loss rate (Tmax) at 331 °C in the DTG curve, and the corresponding weight losses were 74%. After 400 °C, a slow thermal degradation occurs, which can be attributed to the further carbonization of polysaccharide chains caused by the cleavage of C\\C and C\\H bonds [49]. In all, CNCs in our work showed good thermal stability, which was similar to that of the previous report [49]. The CNCs obtained in our study may be a good candidate for the composite strengthening. The crystallization behavior of pure PLA and PLA/CNC composite films was investigated by DSC. The second heating scan was used to access the thermal transitions in the samples. DSC curves were shown in Fig. 6b and the corresponding characteristic thermal data are listed in Table 1 (In order to ensure the accuracy of the DSC data, the second replicate test was also characterized for each sample in Fig. S1 and Table S1). As shown in Fig. 6b, the glass transition, cold crystallization and melting of PLA matrix can be clearly observed, which are typical thermal transitions for a semi-crystalline polymer. The glass transition temperature (Tg), cold crystallization temperature (Tcc), and melting peaks temperature (Tm) of all PLA/CNC composite films were respectively in the temperature range 60.1–63.6 °C, 114–134.4 °C, and 161.5–167.8 °C. As can be observed, there are two melting peaks on the DSC curves of pure PLA samples. This double melting behavior of pure PLA may be related to different mechanisms, such as presence of multiple crystal forms, melt/recrystallization model, and crystalline lamellae populations with different thickness or perfection [29,50]. For the PLA composites we prepared, the presence of double melting peaks may be ascribed to the melt/recrystallization model. The melting peak at lower temperature is probably attributed to less perfect or disordered α′-form crystal, while the one at higher temperature might be assigned to the more perfected α-form crystal. During the DSC heating scan, the α′-form crystals can melt and evolve into highly ordered and perfect α form crystals before remelting at higher temperature [29,50]. In our study, it can be seen that the areas under the first and second melting peaks for pure PLA sample are different. The areas under double melting peaks are directly associated with the crystal perfection. It may indicate that disordered α′-form crystals and highly ordered α form crystals exist in the pure PLA. As we know, PLA has a slow crystallization rate. A large amount of disordered α′-form crystals cannot be transformed into highly ordered α-form crystals in a short period of time. Therefore, the crystallinity can be improved by adding appropriate amount nucleation agents, plasticizer, and annealing treatment. The presence of nanofiller in PLA acts as a nucleation agent for the promotion of crystallization. Different nanofiller characteristics (content, dispersion state) achieved different crystallinity. The nucleating

Table 1 Characteristic DSC parameters of PLA and its composite films. Sample

Tg (°C)

Tcc (°C)

Tm (°C)

△Hcc (J/g)

△Hm (J/g)

χc (%)

PLA PLA-0.1% CNCs PLA-0.3% CNCs PLA-0.5% CNCs PLA-0.7% CNCs

60.1 63.2 63.4 63.6 63.6

114.0 130.1 124.3 133.8 134.4

161.5/167.8 165.9 165.3 166.4 166.5

32.5 23.9 33.3 16.7 14.7

37.0 34.2 42.1 21.0 18.5

4.5 11.0 9.4 4.6 4.4

role of CNC and its inducted crystallization have been widely reported for various polymer matrices [29,50,51,53]. It appears that the nucleating effect is strongly dependent on the uniform distribution of CNC, which results in larger interface area for nucleation. In our study, a small amount of CNCs (0.1 wt% and 0.3 wt%) doping has an obvious nucleating effect due to its good dispersibility within PLA matrix. For PLA/ 0.5% CNC and PLA/0.7% CNC composite films, due to its high aggregation state and limited nucleating role in the PLA matrix. Moreover, the crystallinity degree of PLA and PLA/CNC composite films is calculated and results are also shown in Table 1. As we can see, the χc values of pure PLA is 4.5% while the χc values of PLA/CNC composite films decrease significantly from 11 to 4.4% with the addition of 0.1–0.7% CNCs. That is because the more CNC doping, the less dispersive in polylactic acid. Therefore, the less dispersive can lead lower crystallinity degree. These observations provide further confirmation that an appropriate amount of CNC can work as an efficient nucleating agent so that the nonisothermal crystallization rate of PLA has been largely enhanced. TG and differential TG (DTG) curves of pure PLA and PLA/CNC films are shown in Fig. 6c and d, respectively. It can be seen that the single step thermal degradation occurs for the pure PLA and PLA/CNC films, which shows the intrinsic characteristics of PLA. The corresponding parameters including the onset of thermal degradation (Tonset), the maximum thermal degradation temperature (Tmax) and char reside are summarized in Table S2. Tonset of pure PLA was 260.1 °C, while PLA/ CNC composite films, with the exception of the PLA/0.7 wt% CNC, had Tonset of N267 °C. Tmax of PLA/CNC composite films also displayed a similar trend. The PLA/0.1 wt% CNC composite film had the highest Tonset and Tmax values, indicating that it had better thermal stability. The char yield of PLA/CNC composite films slightly increased with increased CNC content (except for PLA/0.7 wt% CNC composite film). The char yield is directly correlated to the potency of flame retardation for the composites. Moreover, the increased crystallinity (showed as Fig. 6b

Fig. 7. Stress–strain curves of PLA/CNC composite films with different cellulose nanocrystal contents.

Z. Wang et al. / International Journal of Biological Macromolecules 129 (2019) 878–886

885

Table 2 Mechanical properties of CNC, PLA and PLA/CNC composite films. Sample

Tensile strength (MPa)

Elongation at break (%)

Elasticity modulus (GPa)

Work-to-break (J·m−2)

CNC PLA PLA-0.1% CNCs PLA-0.3% CNCs PLA-0.5% CNCs PLA-0.7% CNCs

10,000 30 ± 3.1 294 ± 7.2 128 ± 8.5 63 ± 1.3 23 ± 5.5

– 20.1 ± 0.3 5.2 ± 0.5 4.7 ± 0.3 2.8 ± 0.5 1.6 ± 0.1

130–250 2.8 ± 0.3 12.8 ± 0.3 7.5 ± 0.3 4.1 ± 0.4 3.7 ± 0.2

– 534.5 ± 6.1 981.1 ± 10.5 453.6 ± 5.3 63.2 ± 2.6 33.1 ± 2.2

and Table 1) could also improve the Tmax of the composites. Therefore, the increase of Tmax of PLA/CNC composite films was mostly attributed to the char formation and increased crystallinity in PLA and PLA/CNC composite films.

content (0.1 wt% and 0.3 wt%) will be suitable for the preparation of PLA/CNC composite films.

3.4. Tensile properties

CNCs were successfully isolated from waste cotton cloth fibers using a mixed acid hydrolysis method. The CNCs produced ranged in length, diameter and aspect ratio from 38 to 424 nm, 2 to 17 nm, and 10–32 respectively. Moreover, the yield value of CNCs from waste cotton cloth was 49.2 ± 1.6%. The obtained CNCs have good thermal stability. PLA/ CNC composite films were prepared by blending different CNC concentrations (0–0.7 wt%) into the PLA films. The PLA/CNC films showed good surface, high crystallinity, tensile strength, elasticity modulus, and work-to-break at a lower CNC content (0.1% and 0.3%). However, at higher CNC content (0.7%), the CNCs were not well distributed in the polymer matrix, leading to minimal light transmittance, crystallization, tensile strength, elasticity modulus, elongation at break and work-tobreak. Further, the elongation at break of PLA/CNC composite films decreased with the increase in CNC content due to the restricted mobility of polymer strands caused by the increased film stiffness. Above all, the study has demonstrated that CNCs hydrolyzed using a mixed acid serve as a biomass source for the production of PLA/CNC composite films, while providing a theoretical foundation for the effective extraction and preparing of CNCs and PLA/CNC composite films.

The stress–strain curves of all the PLA/CNC composite films were investigated and the results were shown in Fig. 7. Four mechanical property parameters, including tensile strength, elongation at break, elasticity modulus, and work-to-break derived from the stress-strain curves were also listed in Table 2. For the pure PLA, tensile strength, elongation at break, elasticity modulus, and work-to-break are 30 MPa, 20.1%, 2.8 Gpa, and 534.5 J·m−2. Differently, the tensile strength of PLA/CNC composite films is statistically higher than that of pure PLA except PLA/0.7% CNC composites (statistically distinguishable with p b 0.05). Moreover, the tensile strength properties were greatly influenced by the introduction of CNCs, decreasing significantly from 294 MPa for PLA/0.1% CNC composite film to 23 MPa for PLA/0.7% CNC composite film (p b 0.05). That is attributed to the lower crystallinity of PLA/CNC composite films caused by the agglomeration of the CNCs at higher concentrations. It is worth noting that the PLA/0.1% CNC composite film and the PLA/0.3% CNC composite film have higher tensile strength (294 Mpa and 128 Mpa). That is associated with the good dispersion of CNCs in PLA, higher crystallinity of the PLA/CNC composite films, and the very high stiffness of CNCs (modulus in the range of 130 to 250 Gpa). The mechanical property parameters of CNC are also listed in the Table 2 [52,53]. The elasticity modulus of PLA/CNC composite films showed a similar variation trend, decreasing from 12.8 GPa for PLA/0.1% CNC composite film to 3.7 GPa for PLA/0.7% CNC composite film (p b 0.05). Thus, the crystallinity of the PLA film matrix has an strong influence on the tensile strength and modulus of the composite films. The tensile strength and modulus of the PLA/CNC composite films increased with an increase of crystallinity of film matrix. The tensile strength and modulus of PLA/0.5% CNCs and PLA/0.7% CNCs samples decreased. It was attributed to the aggregation of CNCs at high loading levels in the PLA composite film, which decreased crystallinity of PLA (Table 1). A similar mechanical performance has been reported for the addition of CNCs to other references [29]. The flexibility of the composite films was determined by the elongation at break, which decreased with increasing the loading of stiff CNC nanoparticles. The presence of CNCs obviously decreased the elongation at break from 20.1% for pure PLA to 5.2% for PLA/0.1% CNC composite film and 1.6% for PLA/0.7% CNC composite film (p b 0.05). This reduction in elongation behavior may be attributed to the restricted mobility of polymer strands caused by the increased stiffness of the films. Indeed, similar behavior has been previously described in the reference [28]. The work-to-break is the energy of per new unit area formation because of the spread of crack extension in the process of fracture damage of the material. It represents an important characteristic of the material and is an important index to measure the capacity of thermal stress resistance of the material. In our study, work-to-break of PLA/0.1% CNC composite film is 981.1 J·m−2 while work-to-break of PLA/0.7% CNC composite film is 33.1 J·m−2 (p b 0.05). It indicated the capacity of thermal stress resistance of the PLA/CNC composite film decreased with the decreasing of CNC content. Therefore, a small amount of CNC doping

4. Conclusion

Acknowledgments This research is supported by the National Natural Science Foundation of China (51672129 and 51603179), a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD), the Opening Project of Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology (ASMA201405), “Six Talent Peaks” (2015) Project of Jiangsu Province (YPC16005-PT), the Cooperation Project of Yancheng Institute of Technology between School and enterprise (10424) and the Undergraduate Innovation Program of Yancheng Institute of Technology (2018). Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.ijbiomac.2019.02.021. References [1] S. Chuayjuljit, S. Su-uthai, S. Charuchinda, Poly(vinyl chloride) film filled with microcrystalline cellulose prepared from cotton fabric waste: properties and biodegradability study, Waste Manag. Res. 28 (2) (2010) 109–117. [2] E. Fortunati, S. Rinaldi, M. Peltzer, N. Bloise, L. Visai, I. Armentano, A. Jiménez, L. Latterini, J.M. Kenny, Nano-biocomposite films with modified cellulose nanocrystals and synthesized silver nanoparticles, Carbohydr. Polym. 101 (1) (2014) 1122–1133. [3] G. Mazza, A.E. Agnelli, P. Cantiani, U. Chiavetta, F. Doukalianou, K. Kitikidou, E. Milios, M. Orfanoudakis, K. Radoglou, A. Lagomarsino, Short-term effects of thinning on soil CO2, N2O and CH4 fluxes in Mediterranean forest ecosystems, Sci. Total Environ. 651 (2019) 713–724. [4] J.j. Zhang, Y.F. Li, S.X. Chang, P.K. Jiang, G.M. Zhou, J. Liu, J.S. Wu, Z.M. Shen, Understory vegetation management affected greenhouse gas emissions and labile organic carbon pools in an intensively managed Chinese chestnut plantation, Plant Soil 376 (1–2) (2014) 363–375. [5] R. Cerqueti, Exhaustion of resources: a marked temporal process framework, Stoch. Env. Res. Risk A. 28 (4) (2014) 1023–1033.

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