A Fully Biobased Encapsulant Constructed of Soy Protein and

Jun 14, 2017 - After heating at 60 °C for 2 h and degassing, the SPI/CNC–CHO blend solutions were cast on Teflon plates (15 cm × 15 cm) and dried ...
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Research Article pubs.acs.org/journal/ascecg

A Fully Biobased Encapsulant Constructed of Soy Protein and Cellulose Nanocrystals for Flexible Electromechanical Sensing Dan-Yang Xie, Dan Qian, Fei Song,* Xiu-Li Wang, and Yu-Zhong Wang* Center for Degradable and Flame-Retardant Polymeric Materials (ERCEPM-MOE), College of Chemistry, National Engineering Laboratory of Eco-Friendly Polymeric Materials (Sichuan), State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610064, China S Supporting Information *

ABSTRACT: Presently, flexible electromechanical sensors are of particular interest to next generation mobile applications. To enrich the flexible encapsulants, herein, a fully biobased film is developed by cross-linking soy protein isolate (SPI) with nanocross-linker, aldehydebearing, cellulose nanocrystals (CNC). Thanks to the enhanced interfacial interaction between SPI and CNC resulting from the Maillard reaction, the protein-rich phase becomes more homogeneous with smaller domain size. Compared with neat SPI film, the resultant composite films exhibit an obviously improved mechanical property and water resistance. In particular, the heat-sealing property of such films is well maintained, which guarantees their application as encapsulation layers to construct flexible electromechanical sensors. These results indicate that green composite films hold promising applications as universal encapsulation materials for integrating flexible movement-monitoring electronics. KEYWORDS: Soy protein isolate, Cellulose nanocrystal, Interfacial interaction, Encapsulant

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INTRODUCTION With the fast development of flexible electronics, great and ever-increasing interest has been aroused in wearable devices, roll-up displays, and bendable equipment.1−6 Although significant progress has been made for flexible current collectors, electrodes, solid-state electrolytes, and encapsulation materials, some shortages still exist because the flexible substrates are generally nondegradable or stem from petroleum resources.7−10 Development of flexible, renewable, biobased encapsulants, therefore, is more welcome for global green concerns. Among the big family of biomass, soy protein isolate (SPI) has attracted considerable attention because of its low cost, good processability, film-forming characteristics, and gas barrier property, which exhibits promising applications in the biomedical and food fields.11−16 Nevertheless, in spite of some physical and chemical modifications already performed on SPI, how to fabricate SPI-based films with a high mechanical property and water resistance is still a big challenge.17−24 Cellulose nanocrystals (CNC), prepared by the acidcatalyzed hydrolysis of cellulose, have been regarded as attractive reinforcement fillers over cellulose whisker and microcrystalline cellulose for polymeric materials because of the higher specific surface area, degree of crystallinity, and superior mechanical properties.25−30 Up to now, the enhancement of its mechanical property has been realized by incorporating CNC in different matrices, including poly(lactic acid), bacterial polyester, poly(vinyl alcohol), polyurethane, natural rubber, etc.31−35 Unfortunately, the utilization of CNC © 2017 American Chemical Society

to modify SPI is still limited. Until now, there have been only two reports using CNC as a filler of SPI. Zhang and coworkers36 prepared SPI/CNC composite films with improved tensile strength. The films, however, were brittle because of a lacking interfacial interaction between the filler and the protein matrix. Furthermore, as described by Li, Shi, and their coworkers,37 ethylene glycol diglycidyl ether was used as a crosslinker to connect SPI and silane-modified CNC together. Although their interfacial interaction was improved, the crosslinking reaction has no selectivity, which may result in the selfcross-linking of CNC as well as formation of stress concentration. In addition, considering the requirement of application as artificial skins,38 the mechanical property of SPIbased films still needs improved further. As is well known, periodate oxidation of polysaccharides occurs specifically at the secondary hydroxyl groups of C2 and C3 atoms, giving rise to highly reactive aldehyde groups.39−41 The resulting oxidized cellulose can be used as a macromolecular cross-linker for many biopolymers.42−46 Inspired by this fact, we considered reforming CNC to a novel nanocrosslinker for SPI. In this work, the nanocross-linker was prepared by hydrolysis of ramie fiber and subsequent oxidation with sodium periodate. The solution casting method was exploited to prepare SPI/CNC nanocomposite films with different Received: April 24, 2017 Revised: June 7, 2017 Published: June 14, 2017 7063

DOI: 10.1021/acssuschemeng.7b01266 ACS Sustainable Chem. Eng. 2017, 5, 7063−7070

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ACS Sustainable Chemistry & Engineering

Figure 1. Structural characterization of CNC and CNC−CHO. (a) Schematic illustration of the synthesis of CNC−CHO as well as TEM images of CNC before and after oxidation. (b) FTIR spectra and (c) XRD patterns of CNC and CNC−CHO. SCIENTZ, China) for 30 min. The pH value was adjusted to 10 with 2 M NaOH, at which the SPI molecules were unfolded sufficiently in favor of forming films. After heating at 60 °C for 2 h and degassing, the SPI/CNC−CHO blend solutions were cast on Teflon plates (15 cm × 15 cm) and dried at 45 °C for 24 h to prepare composite films. The asprepared films were hot-pressed at 150 °C for 4 min. Accordingly, the resulted films were named as SPI/CNC−CHO-2.5, SPI/CNC−CHO5.0, SPI/CNC−CHO-7.5, and SPI/CNC−CHO-10.0. In addition, neat SPI and SPI/CNC films were prepared by the same method as the controls. Fabrication of SPI/CNC−CHO Film-Encapsulated Electromechanical Sensors. Typically, 25 mg of SWNT was dispersed by ultrasonication for 6 h in the presence of 25 mg of surfactant. After filtration, the SWNT membrane was obtained and washed several times by acetone. The resulting SWNT membrane was encapsulated by a SPI/CNC−CHO composite film with the assistance of a heat-seal tester (DZ260B, Zhejiang, China) at the heating temperature of 150 °C. Characterization. FTIR analysis was performed on an FTIR spectroscopy (NICOLET 6700) within the wavenumber from 4000 to 400 cm−1. X-ray diffractometer analysis was conducted by an XRD instrument (Philips X’Pert), and the scans were taken over the 2θ range from 5° to 40° with the scanning rate of 2°/min. Sample morphology was investigated by a scanning electron microscope (SEM, JSM-5900LV, JEOL, Japan). Samples were fixed on aluminum stubs and coated with gold. A transmission eletron microscope (Tecnai G2 F20 S-TWIN, FEI, America) was used to investigate sample morphologies at the voltage of 200 kV. Dynamic mechanical analysis was carried out using a dynamic mechanical analyzer (Model Q800, TA Instruments, USA) in tensile mode. Samples (15 mm × 40 mm × 0.3 mm) were first equilibrated at −70 °C for 3 min and then scanned from −70 to 180 °C at the heating rate of 5 °C/min and the frequency of 1 Hz. Mechanical Property. The tensile strength (TS) and elongation at break (ε) of the films were investigated using a Universal Testing Machine (SANS CMT4104, SANS Group, China) with a tensile rate of 20 mm/min according to the standard method of GB/T 1040.32006. Samples were kept under RH of 60% at room temperature

amounts of the nanocross-linker. The microstructure, water resistance, and mechanical property of the films were investigated. In addition, the biobased films were explored as an encapsulant to develop flexible electromechanical sensors.

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MATERIALS AND METHODS

Materials. Ramie fiber (RF) with 65−67% cellulose was purchased from Sichuan JB & Ramie Co., Ltd., China. Soy protein isolate (SPI, protein content 90%) was supplied by Chengdu Protein Food Company (China). Sodium periodate, thymol blue, hydroxylamine hydrochloride, hydrochloric acid, and methyl alcohol were purchased from Kelong Chemicals Reagent Co., Ltd., China. Ethylene glycol, acetic acid, and isopropyl alcohol were purchased from Zhiyuan Chemicals Reagent Co., Ltd., China. Sodium hydroxide was purchased from Ruijinte Chemicals Reagent Co., Ltd., China. Single-walled carbon nanotubes (SWNT) were purchased from Timesnano (Chengdu, China). All other reagents were of analytical grade and used without further purification. Preparation of CNC−CHO. At first, CNC was prepared according to a previous report.47 In brief, smashed RF was treated with 5 wt % NaOH at 80 °C for 2 h to remove hemicellulose, pectin, and lignin, followed by washing with deionized water and hydrolysis with 4 M HCl at 80 °C for 8 h. After that, precipitate was obtained by removing the supernatant. CNC suspension was obtained after the treatment with 0.5 wt % NaOH at 60 °C for 1 h and repeated centrifugation. CNC−CHO was then prepared by oxidizing CNC with 0.2 M sodium periodate (2.2 mol for 1 mol of glucopyranose) at pH 3. The reaction was performed in the dark at 30 °C for 4 h and quenched by ethylene glycol. After centrifugation, the CNC−CHO product was obtained and dried in vacuum before use. Preparation of SPI/CNC−CHO Composite Films. SPI/CNC− CHO composite films were prepared by a solution-cast method. CNC−CHO was mixed with SPI in water at weight ratios of 2.5/97.5, 5.0/95.0, 7.5/92.5, and 10.0/90.0. Total solid content of the filmforming solution was fixed at 4.0%. Glycerol with the final concentration at 1.71% was added as a plasticizer. The mixture was immediately treated using an ultrasonic cell crusher (JY 99-IIDN, 7064

DOI: 10.1021/acssuschemeng.7b01266 ACS Sustainable Chem. Eng. 2017, 5, 7063−7070

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Figure 2. (a) FTIR spectra of CNC−CHO, SPI, and composite films. (b) Schematic illustration of the network formation in SPI/CNC−CHO composite film. (c) Photos of neat SPI film, SPI/CNC, and SPI/CNC−CHO composite films before and after immersion in water: (1) SPI, (2) SPI/CNC-5.0, (3) SPI/CNC−CHO-2.5, (4) SPI/CNC−CHO-5.0, (5) SPI/CNC−CHO-7.5, and (6) SPI/CNC−CHO-10.0. before measurement. Each sample was tested five times, and the testing temperature and RH conditions were 25 °C and 60%, respectively. Water Resistance. Water resistance property was evaluated in terms of swelling test, water uptake, and solubility test. All samples were cut into 2.0 cm × 2.0 cm and dried to constant weight. The swelling characteristic of the film was determined by immersing the dried sample to swell in deionized water at 25 °C for 6 h. The initial and final morphologies of the sample were photographed. The expansion rate was calculated by the following equation: Expansion rate = (Sf − Si)/Si × 100%

where R is the resistance of the sample, and a, b, and l represent width, thickness, and length of the sample, respectively. Statistical analysis. Data was expressed as mean ± SD. Statistical analysis of differences was performed by a two-tailed unpaired Student’s t test. Here, p < 0.05 was considered significant.

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RESULTS AND DISCUSSION At first, acid-catalyzed hydrolysis of cellulose fibers was performed to prepare CNC. As shown in Figure 1a, rod-like CNC is detected with a length−diameter ratio of 7.6. After periodate oxidation, its morphology remains nearly unchanged, and the corresponding length−diameter ratio is slightly decreased to 7.1. In comparison to CNC, the oxidized CNC exhibits a new peak at 1723 cm−1 attributed to a newly formed aldehyde group (Figure 1b). Moreover, the intensity of another peak at 893 cm−1 corresponding to hemiacetal vibration increases. The results confirm the successful occurrence of oxidation of CNC to CNC−CHO.48 To guarantee the enhancement effect of CNC, XRD measurements were conducted to investigate the structural change of CNC after oxidation. As shown in Figure 1c, CNC exhibits a typical XRD pattern assigning to cellulose I, which is the characteristic crystalline structure of native cellulose.49 Four well-defined peaks at 2θ of 14.7°, 16.5°, 22.7°, and 34° are attributed to the planes of 11̅0, 110, 200, and 004, respectively. In addition, two weak peaks located at 2θ of 12.6° (101) and 20.7° (021), which belong to cellulose II,50,51 are determined. This transformation from cellulose I to cellulose II is caused by the alkaline treatment during the preparation process of CNC. 52 Furthermore, as determined from the XRD patterns, the crystallinity of CNC is decreased from 88.4% to 64.8% after oxidation. The decrease in crystallinity results from the opening of glucopyranose rings and destruction of the ordered structure of CNC.53 Figure 2a illustrates the FTIR spectra of SPI, CNC−CHO, and SPI/CNC−CHO composite films, from which we can see the stretching absorption band (1723 cm−1) of free aldehyde of CNC−CHO disappears after the reaction with SPI. This

(1)

where Sf and Si are the final and initial areas of testing samples. The kinetics of water uptake was determined by immersing the dried samples in deionized water at 25 °C. At specific time intervals, the samples were taken out and weighed. The water absorption was calculated by eq 2. The total soluble matter (TSM) of the sample was calculated by eq 3 after reaching the water absorption equilibrium

Water uptake = (mt − mi)/mi × 100%

(2)

TSM = (mi − me)/mi × 100%

(3)

where mt is the weight of samples at time of t; mi is the initial weight of dried samples; me is the final dry matter of samples. All experiments were carried out in triplicate. Light Transmittance Performance. Transparency was determined by a UV−vis spectrometer (Varian Cary50, USA) within the wavelength from 200 to 800 nm. Thickness of samples was 0.30 mm. The measurements were performed five times for each sample. Thermal Stability. TGA measurements were carried out by a TG 209F1 (NETZSCH, Germany) thermogravimetric analyzer at a heating rate of 10 K/min under the nitrogen flow of 50 mL/min with the temperature range from 40 to 700 °C. Electromechanical Property. The real-time I-t curve of the sealed SPI/CNC−CHO encapsulated SWNTs membrane during its bending process was recorded by a Keithley 6487 picoammeter digital meter at a constant voltage of 0.3 V. A Keithley 2400 picoammeter was used to determine the electrical resistance of the composite film. Resistivity was calculated using the following formula: ρ = Rab/l

(4) 7065

DOI: 10.1021/acssuschemeng.7b01266 ACS Sustainable Chem. Eng. 2017, 5, 7063−7070

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Figure 3. TEM images of SPI, SPI/CNC-5.0, and SPI/CNC−CHO-5.0 films. Dark and bright areas represent the protein-rich and glycerol-rich domains, respectively. Marked with red circles are observed independent CNC.

Figure 4. (a) Representative stress−strain curves and (b) fracture toughness of SPI, SPI/CNC-5.0, and SPI/CNC−CHO films. SEM images of the tensile fracture surfaces of (c) SPI/CNC-5.0 and (d) SPI/CNC−CHO-5.0 films.

As previously reported by Zhang’s group,19,54 glycerolplasticized soy protein film has two domains, glycerol-rich and protein-rich, resulting from the relatively high or low compatibility between SPI and glycerol. Therefore, two glass transition temperatures, Tg1 and Tg2 corresponding to the glycerol-rich and protein-rich domains, can be detected by DMA for the neat SPI film (Figure S1 and Table S2). With the introduction of CNC, both temperatures become higher. Compared with neat SPI film, Tg1 and Tg2 of SPI/CNC− CHO composite films became closer, indicating the more homogeneous distribution of glycerol in the SPI matrix. To understand the reason, TEM measurements were conducted. As referenced by previous reports on other protein-based materials,55 the dark and bright areas in the TEM images shown in Figure 3 represent the protein-rich and glycerol-rich phases, respectively. As for the neat SPI film, the protein-rich phase with two distinct morphologies (microscale stripe-like and nanoscale sphere-like domains) is detected dispersing in the glycerol-rich phase. For the SPI/CNC film, these characteristics are still determined. As marked with red ellipses, furthermore, rod-like CNC with obvious boundaries can be seen in the composite. More interestingly, CNC prefers to locate at the

suggests the complete consumption of aldehyde groups, which is required for applications in food and biomedical science. The network formation in the composite films is proposed in Figure 2b, which illustrates the Maillard reaction between SPI and CNC−CHO. In addition, another proof for the Maillard reaction is the color change of the resulting films as shown in Figure 2c, that is, from pale yellow for neat SPI film or SPI/ CNC-5.0 (control group) to dark brown for SPI/CNC−CHO composite films. To present the superiority of the forming network between SPI and CNC, the SPI/CNC−CHO composite films were immersed in water for a sufficient period (6 h) that was required to reach the equilibrium absorption state, as compared with control films (neat SPI and SPI/CNC). In contrast to their obvious shape changes, the expansion behavior of SPI/CNC−CHO composite films is remarkably prohibited (Table S1). In particular, the expansion rate of the SPI/CNC−CHO-10.0 film is decreased by 81.1% compared with the neat SPI film. Moreover, the expansion rate of the SPI/CNC−CHO-5.0 film is lower than that of the SPI/CNC5.0 film, suggesting that the cross-linking is favorable to the enhancement of water resistance. 7066

DOI: 10.1021/acssuschemeng.7b01266 ACS Sustainable Chem. Eng. 2017, 5, 7063−7070

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Figure 5. Schematic illustration of the network structures of (a) SPI/CNC and (b) SPI/CNC−CHO composite films during stretching process.

Figure 6. (a) Swelling behavior, (b) optical transmittance, and (c) thermal stability of SPI, SPI/CNC-5.0, and SPI/CNC−CHO films. (d) Real-time I-t curve of SWNT membrane sealed with SPI/CNC−CHO-5.0 during the bending/stretching cycles (“S” represents “stretching”, and “B” represents “bending”).

4a, from which the TS and ε of the neat SPI film are determined as 6.3 MPa and 200% (listed in Table S3). It should be noted that the TS and ε values are higher than those reported previously14 because of the hot-pressing treatment, which can improve the interdiffusion of denatured SPI chains and result in more molecular entanglements.23 Generally, the utilization of CNC can increase the TS as well as decrease the ε of polymeric materials.30,36,56−60 In our work, similar results have been obtained for the SPI/CNC-5.0 composite film, that is, the TS and ε are increased and decreased to 8.8 MPa and 143%, respectively. In comparison, introducing only 2.5% of CNC−CHO into SPI makes TS and ε increased 49.2% and 13.0% to 9.4 MPa and 226%; this strengthening effect is even more obvious than CNC at higher dosage. Notably, when the CNC−CHO content is increased to 10.0%, the TS of the resulting composite film is achieved at 14.0 MPa and its ε is maintained at 145%. Compared with previous reports on cellulose fiber-reinforced SPI films,24,37,61 the interfacial crosslinking shows a superior effect on enhancement of the mechanical property. Furthermore, fracture toughness defined as the area under the stress−strain curves62 is remarkably

relatively hydrophilic glycerol-rich domains rather than the relatively hydrophobic protein-rich domains. These results indicate a poor interfacial interaction between SPI matrix and CNC. On the other hand, the presence of hydrophilic CNC, which is able to interact with glycerol, may cause the reduced location of the plasticizer at protein-rich domains, thus weakening the plasticizing effect for SPI. Consequently, the Tg1 and Tg2 detected for SPI/CNC-5.0 are reasonably higher than those of the neat SPI modified with glycerol alone. In comparison, the microphase separation behavior is significantly affected when incorporating CNC−CHO into SPI, that is, microscale stripe-like domains disappear and a large number of nanoscale sphere-like domains are detected homogeneously in the glycerol-rich phase. Besides that, rod-like CNC cannot be easily seen any more. Therefore, it can be concluded that the interfacial interaction is obviously increased, and the glycerol molecules disperse more homogeneously in the SPI matrix when using reactive CNC as the filler. For practical applications, the mechanical property is definitely an important issue that should be considered. The typical stress−strain curves of film samples are shown in Figure 7067

DOI: 10.1021/acssuschemeng.7b01266 ACS Sustainable Chem. Eng. 2017, 5, 7063−7070

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ACS Sustainable Chemistry & Engineering

expansion and swelling of the SPI-based films. To further understand the cross-linking degree, TSM was determined for the films. TSM of the neat SPI film is as high as 46.6%, which is decreased to 43.1% after the introduction of CNC, attributing to the lower net content of SPI in the composite film. Notably, as for SPI/CNC−CHO films, their TSM is obviously decreased. Nevertheless, the TSM value remains over 30% even if the amount of CNC−CHO is 10.0%. This is due to the fact that the main soluble matter in the case is glycerol. Although the cross-linking is created in the films, the plasticizer is still free to move and diffuse. As a result, it can be concluded that 5.0% of CNC−CHO is enough to form stable network structures with SPI, protecting the film from corrosion by water. However, for the mechanical property, the higher the CNC−CHO amount is the better the tensile performance is. Transparency, an important property for packaging films, was finally evaluated. As shown in Figure 6b, neat SPI film presents high transparency with a transmittance of about 80% within the wavelengths from 600 to 800 nm. Although the incorporation of CNC−CHO results in a decrease in transparency, the transmittance is still maintained over 60%. Compared with that, the optical transmittance is decreased obviously in the case of adding CNC as the filler because of its poor interfacial interaction with matrix.63 Figure 6c illustrates the TGA curves of these films from which no obvious difference is detected. This indicates that the thermal stability of the SPI-based film is not deteriorated after the incorporation of CNC or CNC− CHO. Thereafter, a typical sample composite film, SPI/CNC− CHO-5.0 with the electrical resistivity of 1.7 × 103 Ω·m, was selected as an encapsulant to prepare an electromechanical sensor; the appearance is shown in Figure 6d. From the highly sensitive current response of the sensor to the bending treatment as well as the comparable flexibility, optical transparency, nontoxic nature, and easy encapsulation of such SPI-based film to conventional polymers (polydimethylsiloxane, for instance),64,65 we can conclude that using the fully biobased film as the encapsulant to construct movementmonitoring electronics is of great potential value.

improved for SPI upon the introduction of CNC−CHO rather than CNC (Figure 4b). To understand the reinforcement effects of CNC−CHO, the microstructures of tensile fracture surfaces of SPI/CNC-5.0 and SPI/CNC−CHO-5.0 were investigated by SEM. From the images shown in Figure 4c, a large number of nanopores (marked with white arrows) are observed for the SPI/CNC-5.0 film, resulting from the pulling out of CNC. Compared with that, the SPI/CNC−CHO-5.0 film shows a smoother tensile fracture surface and white circle dots attributing to the broken CNC. Moreover, the dots disperse homogeneously in the SPI matrix. These results, consequently, well explain why a relatively high mechanical property is realized for SPI-based film materials. A corresponding illustration regarding the failure mechanisms of SPI/CNC and SPI/CNC−CHO composite films is proposed in Figure 5. As for the SPI/CNC, the filler, CNC, causes stress concentration under the stretching, and the mechanical breakage prefers to occur at the interfaces between the CNC and SPI matrix. Meanwhile, the CNC-bearing aldehyde groups act as not only nanoreinforcement to increase mechanical strength but also as a nanocross-linker to maintain the ductility. Because of the strong interfacial interaction between CNC and SPI, the alignment of the filler will orient along with the stretching direction, thereby the cross-section of CNC (dot-like morphology) rather than the whole CNC is observed in the SEM image, and the resulting SPI/CNC−CHO films exhibit promoted mechanical properties. As is well known, the poor water resistance of the SPI film is one of the main obstacles to real applications. Generally, the water resistance is obviously dependent on the filler content as well as the cross-linking structure. Figure 6a shows the swelling behavior of such films. Because of the high hydrophilicity, the neat SPI film can absorb a large amount of water with the equilibrium water uptake (EWU) of 650%. However, EWU is reduced by 33.4% upon the addition of CNC. The water resistance property is further improved when using CNC− CHO as the filler, that is, EWU of the cross-linked composite film (SPI/CNC−CHO-10.0) is decreased to 155%, which is 76.1% lower than the neat SPI film. To quantitatively understand the contribution of the network formation to the water resistance, a diffusion coefficient (D) was calculated according to the following equation:19 mt − m 0 =1− m∞

n=0

∑ ∞

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CONCLUSIONS CNC containing aldehyde groups are prepared and employed as the reinforcement material for SPI. Because of the Maillard reaction, the CNC−CHO filler also acts as a nanocross-linker in composite films, resulting in obvious enhancement of the interfacial interaction between SPI and CNC. The as-prepared SPI/CNC−CHO films exhibit a significantly improved mechanical property and water resistance as compared with neat SPI film as well as the SPI/CNC film without the interfacial reaction. Notably, the SPI/CNC−CHO films have a heat-sealing property, which can be used to encapsulate a conductive SWNT membrane. The resultant electronic skin exhibits a current response to mechanical treatments, indicating that biobased composite films have potential applications in next-generation electromechanical sensing.

⎡ −D(2n + 1)2 π 2t ⎤ 8 exp ⎢ ⎥ ⎦ (2n + 1)2 π 2 ⎣ 4L2 (5)

where m0 and mt are the weights of the samples before and after swelling at time of t; m∞ is the weight of sample at equilibrium swelling state; 2L is the thickness of the film of the film sample. Within a short time, this equation can be written as 1/2 mt − m 0 2 ⎛D⎞ = ⎜ ⎟ t 1/2 m∞ L⎝ π ⎠

(6)

When (mt − m0)/m∞ ≤ 0.5, the plots of ((mt − m0)/m∞)2 as a function of (4t/(πL2)) were drawn for all samples, and the D value was calculated from the slope of the plots. As shown in Table S4, the D values of neat SPI and SPI/CNC-5.0 films are over 2.2, while those of SPI/CNC−CHO films are below 1.7, indicating the positive effect of cross-linking on the suppressed water absorption. In addition, the D values are dependent on the amount of CNC−CHO used in the composite films. As a result, the cross-linking extent is regarded essential to limit the

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b01266. 7068

DOI: 10.1021/acssuschemeng.7b01266 ACS Sustainable Chem. Eng. 2017, 5, 7063−7070

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Calculation of aldehyde content and results of DMA, expansion rate, tensile strength, elongation at break, and water resistance. (PDF)

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Corresponding Authors

*E-mail: [email protected]. Tel: 86-28-85410755. Fax: 86-28-85410755 (F. Song). *E-mail: [email protected]. Tel: 86-28-85410755. Fax: 8628-85410755 (Y.-Z. Wang). ORCID

Fei Song: 0000-0001-5229-4379 Xiu-Li Wang: 0000-0002-2732-9477 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grants 51403136 and 51421061), Program for Changjiang Scholars and Innovative Research Teams in University of China (IRT 1026), and Fundamental Research Funds for the Central Universities of China (2015SCU04A22).

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