Humidity-Sensitive and Conductive Nanopapers from Plant-Derived

Research Article pubs.acs.org/journal/ascecg

Humidity-Sensitive and Conductive Nanopapers from Plant-Derived Proteins with a Synergistic Effect of Platelet-Like Starch Nanocrystals and Sheet-Like Graphene Ge Zhu,† Alain Dufresne,§ and Ning Lin*,†,‡ †

School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, 122 Luoshi Road, Wuhan 430070, P. R. China ‡ Key Laboratory of Recycling and Eco-Treatment of Waste Biomass of Zhejiang Province, Zhejiang University of Science and Technology, Hangzhou 310023, P. R. China § Grenoble INP, LGP2, Université Grenoble Alpes, CNRS, F-38000 Grenoble, France S Supporting Information *

ABSTRACT: In the present study, a multifunctional composite nanopaper was developed on the basis of soy protein isolate (SPI) from the synergistic reinforcement of sheet-like graphene (RGO) and platelet-like starch nanocrystals (SNCs), providing a conductive function as well as enhanced mechanical and barrier properties. As a highly crystalline and rigid nanoparticle derived from a natural polymer, the introduction of SNCs improved the dispersion of graphene nanoparticles at the relative ratio of 15/1 (SNC/RGO, w/w), and therefore promoted the electrical conductivity of the composite nanopapers under various humidity atmospheres. Because of hydrogen-bonding interactions from the surface groups, the effect of SNCs as a dispersing agent for RGO was investigated by rheological analysis of the composite suspensions and meanwhile directly observed by microscopy in the composite films. The proposed strategy of dual-enhanced fillers (with molecular interaction) in the composites can provide remarkable improvement of the properties, with simultaneous strengthening and toughening, water-vapor and oxygen permeability reduction, water absorption reduction, and solvent resistance, which may be a novel idea to solve the critical limitations of SPI-based materials in practical applications. KEYWORDS: Soy protein isolate, Starch nanocrystals, Graphene, Synergistic enhancement, Conductive nanopapers

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INTRODUCTION Since the achievement of its effective isolation and exploration in 2004 by Novoselov and Geim (Nobel Prize in Physics winners in 2010),1 graphene, a monolayer of carbon with a hexagonal packed lattice structure, has become an attractive star in the field of materials science. This is related to its extraordinary physicochemical properties, including excellent electronic conductivity, high carrier mobility, enormous theoretical specific surface area, and superior mechanical and thermal properties.2 On the basis of the integration of graphene and organic polymers, conductive composites can be fabricated to bestow electronic properties to nonconductive polymeric matrixes. However, the inherent high specific surface area and nanoassembly tendency of graphene result in poor dispersibility and weak bonding with polymers, and therefore an interfacial incompatibility that limits the enhancement of both conductivity and mechanical properties.3−5 To overcome this problem, the conventional approach is the chemical treatment to introduce oxygen-containing groups, to obtain so-called graphene oxide (GO). Unfortunately, after surface modification the conductive properties are inevitably affected, because of the © 2017 American Chemical Society

lower electrical conductivity of GO compared with that of pristine graphene.6,7 Considering its numerous advantages, the efforts on the combination of graphene (or derivations) and natural resources as environmentally friendly alternatives to synthetic materials have attracted much attention, especially driven by the environmental and cost reasons during the past several decades. Soy protein isolate (SPI) is a plant-derived biopolymer, which has the advantages of sustainability, renewability, biocompatibility, biodegradability, easy processability, and low cost with a wide range of applications in the food industry, materials science, bioscience, etc.8 Upon use as a material component or matrix, soy protein isolate exhibits “green recycle” and carbon balance (ultimately degraded as water and carbon dioxide) in diverse forms of adhesives, fibers, biomaterials, and bioplastics.9 The pioneering studies on SPI-based plastics were carried out in the early 1930s,10,11 which even attracted Ford Motor Co. to Received: July 28, 2017 Revised: September 4, 2017 Published: September 9, 2017 9431

DOI: 10.1021/acssuschemeng.7b02577 ACS Sustainable Chem. Eng. 2017, 5, 9431−9440

Research Article

ACS Sustainable Chemistry & Engineering

in distilled water under strong stirring and ultrasonically treated for 20 min. SNC aqueous suspension was added, and the mixture was mechanically stirred for 1 h. The GO component in the composite suspension was reduced by hydrazine hydrate (HH) at 95 °C for 3 h with the control of the reagents ratio at HH/GO = 1/1 (w/w).25 The resultant suspension of RGO/SNC was purified by successive centrifugations with distilled water to remove the unreacted HH, and it was followed by freeze-drying to release composite powders. As a comparison, neat RGO was prepared from GO with the similar reduction approach. Regarding the preparation of the various composite nanopapers, the obtained RGO/SNC suspensions were mixed with the prepared SPI aqueous suspensions by mechanical stirring, with different component proportions depending on the SNC loading level ranging from 0 to 45 wt %. The protein denaturation was carried out in the presence of trifluoroethanol (TFE) at pH 10.0, at 90 °C for 3 h, to produce composite suspensions of SPI/RGO/SNC (S/R/S-x-s). The composite powders of SPI/RGO/SNC (S/R/S-x-p) were obtained by freezedrying for 2−3 days. A composite suspension and powder of SPI/ RGO without the addition of SNC were also prepared according to the similar method involving the process of protein denaturation. In all of the composite systems, the RGO concentration based on the total solid content was controlled as 1 wt %. Preparation of Conductive Nanopapers. The nanopapers were molded from the composite SPI/RGO/SNC powders by adding glycerol (Gly) as plasticizer, and hot-pressing at 20 MPa, at 120 °C for 10 min (R-3202, Wuhan Qien Science & Technology Development Co., Wuhan, China). After this molding step, the system was cooled with water, and the pressure was released. Black and opaque nanopapers were obtained with smooth appearance and dimensions of 70 mm × 70 mm. All of the nanopapers were conditioned in a desiccator for 1 week before characterization. For the subsequent analysis and characterization, three series of samples were involved and coded as S/R/S-x-s, S/R/S-x-p, and S/R/ S-x-f, in which the former S = SPI, the latter S = SNC, R = RGO, x represented the loading level of SNC based on the total solid content (ranging from 0 to 45 wt %), s = suspension, p = powder, and f = film. In all of the composite systems, the concentration of RGO was fixed at 1 wt %, and therefore this information was not recorded in the sample codification. Finally, pure SPI materials were also prepared with the same nomenclature, labeled as SPI-s, SPI-p, and SPI-f. Characterization. Fourier transform infrared spectroscopy (FTIR) measurements were carried out on an FTIR iS5 spectrometer (Nicolet, Madison, WI) in the range 4000−400 cm−1. X-ray diffraction (XRD) measurements were performed on freeze-dried powders and films with a D8 Advance X-ray diffractometer (Bruker) using Cu Kα radiation (λ = 0.154 nm) at 40 kV and 60 mA. The diffraction angles (2θ) ranged from 5° to 50°. The morphology of the nanoparticles was observed by transmission electron microscopy (TEM) on a Tecnai G2 F30 instrument (FEI) at 300 kV. Regarding the SNC morphological observation, a drop of diluted suspension was negatively stained with 2% (w/v) uranyl acetate. The dispersing-agent effect of SNCs to RGO was investigated by rheological and ζ-potential measurements of the composite suspensions. The rheological analysis of the apparent viscosity and flow behavior of the suspensions were evaluated using an MCR 102 rheometer (Anton Paar) with a 0.3 mm gap parallel-plate serrated sensor, and the shear rates were varied from 0 to 500 s−1. The ζpotential measurement of the suspension was carried out using a Zetasizer Nano ZS90 (Malvern Instruments Co., Britain) device with the suspension concentration of 0.6 wt % under the controlled pH of 7.0 by 0.2 M NaOH. The microstructure of the nanopapers was investigated by scanning electron microscopy (SEM, Hitachi S-4800 instrument) and atomic force microscopy (AFM, MFP-3D-SA, Asylum Research). All of the composite nanopapers were cryofractured using liquid nitrogen, and coated with gold using a sputter coater to view the cross-sectional morphology at an accelerating voltage of 10 kV. The dispersion of the nanofillers in the composites was further observed by AFM from the surface topography, with the cryofractured treatment and exposed

attempt to use SPI as an automobile decoration or body materials to replace traditional metal-based materials.12 Diverse inorganic or organic nanoparticles were introduced into the SPI-based composites with the aim of mechanical enhancement,13 for instance, the well-studied SPI/montmorillonite composites.14−16 However, the practical application of SPI is always restricted by several limitations, particularly inferior mechanical properties (lower strength and elongation at break for protein than for synthetic polymers), high water vapor permeability (due to protein’s hydrophilic nature), and weak functional properties (low-added-value products). In this study, for improvements in the mechanical properties, barrier properties, and additional functionality of SPI, sheet-like graphene was introduced in SPI-based nanopapers with platelet-like starch nanocrystals (SNCs) as dispersing agent. Starch nanocrystal is a highly crystalline and rigid nanoparticle isolated from native amylopectin with the removal of the amorphous regions.17 Different from other polysaccharide nanocrystals, SNCs hold a special platelet-like morphology with typical dimensions of 5−7 nm in thickness, 20−40 nm in length, and 15−30 nm in width.18 The rigidity and platelet-like morphology of SNCs endow them with a promising potential as the biomass-based nanofiller to enhance the mechanical and barrier performances of composites, taking the significant stress-transferring and obstruction effects.19,20 The most attractive application of SNCs may be their use as a nanoreinforcing filler to partially replace conventional carbon black and silica in making tires, which impart remarkable strength reinforcement and water-vapor barrier improvement for natural rubber-based composites.21,22 In this work, we introduced SNCs into denatured SPI/graphene nanopapers to promote the homogeneous dispersion of graphene as well as for expected improvements of the mechanical and barrier properties of the composites. The reaggregation of reduced graphene oxide (RGO) can be prevented by both the attached SNCs (electrostatic repulsion) and denatured soy protein23 during the in situ reduction of GO, which should retain the stable dispersion and high conductivity of the nanopapers. It was found that the critical concentration of SNCs for the graphene coverage was 15/1 (w/w) to achieve a stable dispersion state and effective enhancement for the nanopapers. On the basis of the synergistic effects of graphene and SNCs, conductive nanopapers based on plant-derived protein (SPI) were developed, which exhibit improved mechanical properties, reduced water-vapor and oxygen permeability, and humiditysensitive electric conductivity.

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EXPERIMENTAL SECTION

Materials. Soy protein isolate (SPI; minimum 92% protein content on dry basis, moisture content 97.5%) were purchased from Saiyi Biological Tech Co., Ltd. (Hebei, China). Graphene oxide (GO) was supplied from Nanjing XFNANO Materials Tech Co., Ltd. (Nanjing, China). Glycerol (Gly), trifluoroethanol (TFE), hydrazine hydrate (HH), sulfuric acid (H2SO4), and sodium hydroxide were purchased from Sigma-Aldrich and used without any treatment. Extraction of Starch Nanocrystals (SNCs) from Native Waxy Maize. Waxy maize starch nanocrystals (SNCs) were extracted by H2SO4 hydrolysis according to the protocol reported elsewhere.24 Briefly, the hydrolysis of native waxy maize starch granules was performed with 3.16 M H2SO4 solution for 5 days at 40 °C. The suspension was washed by successive centrifugation with distilled water, and then dialyzed for 5 days to remove the free acids. Homogenous Dispersion of Reduced Graphene Oxide (RGO), SNCs, and SPI. The raw graphene oxide (GO) was suspended 9432


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Figure 1. Preparation route of SPI-based nanopapers, involving the “dispersion and mixing” and “shape and molding” processes (the concentration of total solid components for the suspensions was controlled as 1 wt % for 48 h).

Figure 2. TEM images of (A) GO, (B) RGO, and (C) waxy maize starch SNCs. cross-section observation. The tapping mode of AFM was performed with a force constant of 20 N/m and resonant frequency of 300 kHz. The mechanical properties of the nanopapers were investigated by tensile tests on a CMT6503 universal testing machine (SANS, Shenzhen, China). The tensile strength (σb), strain at break (εb), and Young’s modulus (E) were determined from the stress−strain curves recorded with a cross-head speed of 10 mm/min. Dynamic mechanical analysis (DMA, Pyris Diamond, PerkinElmer) was used to further analyze the thermo-mechanical performance of the nanopapers with testing temperatures ranging from 25 to 200 °C at a heating rate of 4 °C/min (20 μm amplitude, at 1 Hz). The barrier performance of the nanopapers was investigated through water-vapor permeability (WVP) and oxygen permeability (OP), which were measured using Perme W3/031 and Perme VACV1 instruments (Labthink, Jinan, China), respectively. The samples were placed in the permeation cups with a tested area of 8.04 cm2 under controlled atmosphere of 38 °C, and 96% relative humidity (RH) for WVP measurement and 23 °C, and 0% RH for OP measurement. The nanopapers were cut as 20 mm × 20 mm square films for water absorption measurements, performed at 30 °C and 96% RH in the desiccator. The samples were removed at specific intervals, and the water uptake (WU) was calculated from the weight changes according to the following equation: WU% =

Mt − M0 × 100% M0

conditioned at 96% RH for various durations, which reflected the gradual increase of the water absorption. All of the electrical resistivity measurements were performed repeatedly five times at different positions on the nanopapers to obtain accurate results. The solvent resistance of the nanopapers was determined from the relative weight loss in acetone and water media,21 with the complete immersion of the samples in the solvents for 48 h, and then ovendrying at 40 °C overnight to calculate the relative weight loss. The thermal stability of the nanopapers was determined by thermogravimetric analysis (TGA, STA449F3, TA) at a heating rate of 10 °C/min from 25 to 700 °C under nitrogen atmosphere.

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RESULTS AND DISCUSSION The preparation route of composite nanopapers is illustrated in Figure 1, including the steps of component mixing, graphene oxide reduction, soy protein denaturation, freeze-drying, and melt-compression molding. The homogeneous dispersion of the mixture was achieved using SNCs as dispersing agent and denatured SPI, therefore providing dual-enhancement from SNC and RGO to obtain conductive nanopapers with a smooth appearance and remarkable flexibility. The composition of the various nanopapers is summarized in Table S1, with the control of 1 wt % RGO, and SNC content ranging from 0 to 45 wt %. The obtained SPI-based nanopapers commonly had a thickness of about 500 μm. The sheet-like morphology of GO and RGO nanoparticles as well as the platelet-like morphology of SNCs can be observed by TEM observation, as shown in Figure 2. In comparison with the GO sheets, RGO exhibit a creased sheet-like morphology, which may be attributed to the weak dispersion and possible aggregation of RGO nanoparticles in water. Isolated from high-

(1)

where M0 and Mt are the weights of the samples before and after a time t of conditioning, respectively. The electrical conductivity of the nanopapers was measured using an ST2663 four-probe resistivity instrument (Suzhou Jingye) with a 2 mm distance between two copper electrodes. For an evaluation of the humidity sensitivity to the electrical properties, the nanopapers were 9433


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similar viscosity for S/R/S-40-s and S/R/S-45-s suspensions (very high SNC content) as compared to the pure SPI-s suspension. The ζ-potential values of the suspensions are summarized in Table 2, including the suspensions of the four raw components

content amylopectin waxy maize starch, SNCs show the typical platelet-like morphology with a diameter of 20−60 nm. Regarding the chemical structure of the raw components, the characteristic OH (3418 cm−1) group and stretching of NH (1545 cm−1) groups can be observed in the FTIR spectrum for SPI, ascribed to the amino acid residue in the protein chains26 (as shown in Figure S1). The extracted SNCs exhibit the typical feature of the glucose unit, and raw GO shows the characteristic CO (1739 cm−1) and CC (1620 cm−1) stretching.27 Resulting from the overlap of the predominating component, the composite powders held SPI-based characteristics, except the slight signal of CO (1742 cm−1) stretching. The possible interaction between SNCs and RGO particles in the suspension was investigated by rheological analysis, as shown in Figure S2. It is generally believed that, at a low particle fraction, the energy dissipation during laminar shear flow increases because of the perturbation of the streamlines by particles. According to the Einstein model, the viscosity of the composite liquid is directly proportional to the volume fraction of small solid particles, assuming that the radius of the particles is much smaller than the size of the measuring apparatus, but much bigger than the solvent molecules.28 η ∝ η0 , f ∝ η0 , ωSNC% f=

Table 2. ζ Potential for the Various Suspensions with a Diluted Concentration of 0.6 wt % with the Controlled pH of 7.0 by 0.2 M NaOH sample S/R/S-0-s S/R/S-5-s S/R/S-10-s S/R/S-15-s S/R/S-20-s S/R/S-25-s S/R/S-30-s S/R/S-35-s S/R/S-40-s S/R/S-45-s SPI GO RGO SNC

(2)

1 112/(2.845 + 2.364ωSNC%) + 1

(3)

Table 1. Calculated Volume Fraction (f) and Experimental Apparent Viscosity (η) for the SPI-Based Suspensions Containing Various SNC Contents at the Shear Rate of 300 s−1 f (%)

η (mPa s)

SPI-s S/R/S-0-s S/R/S-5-s S/R/S-10-s S/R/S-15-s S/R/S-20-s S/R/S-25-s S/R/S-30-s S/R/S-35-s S/R/S-40-s S/R/S-45-s

2.48 2.48 2.58 2.68 2.78 2.88 2.98 3.08 3.19 3.27 3.37

1.75 1.95 2.16 2.99 3.35 3.09 2.75 2.70 2.20 1.90 1.52

−21.6 −22.7 −24.1 −27.8 −25.6 −23.5 −21.7 −18.2 −17.3 −15.6 −24.8 −8.1 −3.1 −16.3

± ± ± ± ± ± ± ± ± ± ± ± ± ±

0.5 0.1 0.7 0.7 0.5 0.4 0.5 0.1 0.2 0.4 0.6 0.6 0.2 0.9

and S/R/S-x-s composite suspensions. The suspensions containing GO nanoparticles and SNCs exhibit negative ζpotential values, attributed to the surface-oxidized groups for graphene29 and H2SO4-hydrolyzed process for starch.30 With the reduction treatment applied to GO, the ζ-potential value for the RGO suspension tends to neutrality because of the removal of surface groups and charges. With the increase of the SNC loading level, the ζ-potential value of the S/R/S-x-s suspension gradually increases from −21.6 mV for S/R/S-0-s to the maximum value of −27.8 mV for S/R/S-15-s, indicating the electrostatic repulsion and improved dispersion of particles in the suspension.14 The suspensions containing superfluous SNCs probably induce the self-aggregation of the particles, and therefore result in the gradual reduction of ζ potential to a value close to that of the pristine SNC suspension. On the basis of the results of rheological and ζ-potential analysis, the possible hydrogen-bonding interaction between the platelet-like SNCs and sheet-like RGO nanoparticles together with the aiding dispersion effect induced by electrostatic repulsion were proposed as shown in Figure 3. Considering the specific surface area for SNCs (3.23−230 m2/g)21 and RGO (300−800 m2/ g),31,32 the effective coverage of the 15 wt % SNCs on the surface of 1 wt % RGO in the suspension should be theoretically reasonable. The microstructures of pure SPI, and composites containing 1 wt % RGO and low (5 wt %), moderate (15 wt %), and high (45 wt %) SNC contents, were characterized by SEM. As shown in Figure S4, the introduction of RGO caused the roughness of the cross-sectional morphology of the SPI-based nanopaper, while the incorporation of SNC significantly affected the interfacial miscibility depending on its content. With a focus on the microstructure of the S/R/S-15-f composite, the improved smoothness and absence of microphase separation can be observed despite the presence of a slight crease at higher magnification. On the contrary, the S/R/ S-45-f composite nanopaper shows serious immiscibility and interfacial separation resulting from the obvious aggregation of

In this study, the terms are as follows: η and η0 represent the apparent viscosity and initial apparent viscosity; f is the volume fraction; and ωSNC% is the mass fraction of SNCs based on the total solid content. The deduction process of eq 3 is shown in the Supporting Information, and the calculated values are summarized in Table 1.

sample

ζ potential (mV)

The trend for the values of η for the composite suspensions is consistent with the Einstein model prediction, which reflected an increase of η from 1.75 mPa s for the pure SPI-s suspension to 3.35 mPa s for the S/R/S-15-s composite suspension. The highest apparent viscosity of the S/R/S-15-s suspension (3.35 mPa s) demonstrates the strong interactions between SNCs and RGO, and the improved dispersion stability of the particles in this composite suspension. The subsequent decrease of η for the composite suspensions with higher SNC loading levels (≥20 wt %) indicates the possible aggregation of solid particles in the systems, particularly observed from the 9434


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Figure 3. Proposed mechanism for the SNC-aiding dispersion effect to RGO nanoparticles.

enhanced fillers to the SPI-based composite in this study provided the mechanical performances that are superior to those of composites reinforced by inorganic nanoparticles or rod-like organic nanoparticles, exhibiting the comprehensive enhancement of tensile strength, Young’s modulus, and elongation. The segment relaxation and molecule movement can reflect the microstructural changes of the SPI-based composites, which can be obtained from dynamic mechanical analysis (DMA) experiments.33 The results of temperature dependence of the storage modulus (E′) for the prepared nanopapers are presented in Figure 6. The E′ values for the nanopapers exhibited an increase when increasing the SNC loading level (particularly in the high temperature range 90−180 °C), which reveals the restriction of molecular movements of SPI segments by the rigid SNC nanoparticles in the composites. With the experimental data at the temperature of 120 °C taken as an example, the E′ value of the nanopaper increased by 90% and 238% for S/R/S-15-f (16.3 MPa) and S/R/S-45-f (29.1 MPa), respectively, in comparison with that of pure SPI-f (8.6 MPa). In addition, the drop in E′ in the temperature range 30−70 °C (as shown in the enlarged figure) corresponded to the glass transition of the protein-rich domains in the SPI-based nanopapers at around 40 °C according to a previous report.34 The evolution of the glass transition temperature (Tg) as a function of the content for SPI-based nanopapers (Figure 6B) provided a similar evolution as the tensile tests (three regions according to the SNC loading level). Compared with pure SPI-f material, the presence of rigid RGO and SNCs should restrict the free motion of the SPI segments, and therefore induce an increase of Tg value for the composite nanopapers. We indeed observed the gradual increase of Tg from 40.2 (SPI-f) to 46.5 (S/R/S-0-f) and 58.8 (S/R/S-15-f) °C for moderate SNC loading levels. However, a further increase of the SNC content (>20 wt %) inversely caused the reduction of Tg for the composite nanopapers, which reflected the presence of an inhomogeneous and incompatible microstructure in these composite systems. Especially regarding the superfluous SNC (40 and 45 wt %) introduction, the composite nanopapers exhibited a glass transition temperature similar to that of pure SPI-f material (close Tg values at 40−41 °C), indicating a

superfluous SNCs. The dispersion state of RGO and SNCs in the S/R/S-15-f composite nanopaper was further characterized by observing its cross-sectional topography by AFM. As shown in the phase and height images of Figure 4, numerous particles of nano- and microscale homogeneously covered the crosssection of the film, which proved the good dispersion of the fillers in the composite. With the individual particles taken to perform the scale analysis, the larger particles with a scale ranging from 0.4 to 1.0 μm (the red region and curve) and smaller particles with a scale ranging from 30 to 60 nm (the blue region and curve) were individually measured. They can be ascribed to RGO and SNC components in the S/R/S-15-f nanopaper. The mechanical performance of the SPI-based nanopapers was investigated by tensile tests at room temperature, which indicated the synergistic mechanical enhancement of the SNC and RGO fillers. As shown in Figure 5A, the addition of 1 wt % RGO improved the stiffness of the SPI matrix with an increase in the tensile strength and Young’s modulus, but reduced the strain at break (in comparison with the pure SPI-f material). Regarding the effect of SNCs, three regions of mechanical behavior can be observed, consisting of the simultaneous strengthening and toughening effect at low or moderate SNC loading levels (5−15 wt %, Region I), gradual reduction of mechanical properties at higher SNC loading levels (20−35 wt %, Region II), and sharp decrease of mechanical properties at superfluous SNC loading levels (40−45 wt %, Region III). The stress−strain curves (Figure 5B) for the nanopapers further demonstrated the promising simultaneous strengthening and toughening effect with the introduction of 1 wt % RGO and 15 wt % SNCs to the SPI-based composites. The improved mechanical performance of the S/R/S-15-f nanopaper (46.2%, 31.0%, and 26.8% increase for E, εb, and σb, respectively, in comparison with the SPI matrix) can be attributed to the homogeneous dispersion and significant synergistic enhancement effect of SNCs and RGO in the composite, which is consistent with the morphological observations (SEM and AFM). The comparison of mechanical performance for previously reported studies on various nanoparticle-reinforced SPI composites is summarized in Table S2. Particularly, the introduction of platelet-like SNCs and RGO as the dual9435


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Figure 4. AFM images of the cross-sectional morphology for the S/R/S-15-f nanopaper at different magnifications: (A) height image at 3.0 × 3.0 μm2; (B) phase image at 3.0 × 3.0 μm2; (C) phase image at 1.4 × 1.4 μm2; and (D) height image at 1.4 × 1.4 μm2. Scale analysis of the observed particles from the corresponding regions and nanoparticles: (E) red region; and (F) blue region.

effect to water-vapor molecules, while SNCs were remarkable at preventing the diffusion of oxygen molecules. Water absorption behavior is another important property for functional nanopapers or packaging composites, particularly when involving plant-derived protein as the matrix material. As shown in Figure S6, there was an unnoticeable effect of RGO on the water absorption of the nanopaper (in comparison with SPI-f and S/ R/S-0-f materials), which may be attributed to its low content in the system (only 1 wt %). However, a decrease of the water absorption at equilibrium (WAE%) was clearly observed when adding SNCs. For instance, a 39.5% and 48.8% reduction of WAE% was observed for S/R/S-15-f and S/R/S-45-f nanopaper, respectively, in comparison with the pure SPI-f material. The general reduction of the water absorption for the composite nanopapers may be explained by the presence of SNCs having highly crystalline, rigid, and compact structure, in contrast to native starch. The improved water resistance of nanopapers containing 15 or 20 wt % SNCs can be attributed to the better miscibility of starch and SPI nanoparticles together with the possible plasticization of starch in SPI, while the gradual increase of the water absorption of the nanopapers with

microphase separation of protein and starch domains in the composites. As paper-based materials, gas permeability, e.g., water-vapor permeability and oxygen permeability, is an important property to determine the practical applications of these materials. In fact, the incorporation of nanoparticles has proven to be an effective strategy to improve the barrier properties of polymeric composites, because of the blocking effect induced by the increase of the molecule migration pathways.35,36 The enhancement to the barrier properties of SPI-based composites from the presence of nanoparticles was previously reported by studies containing SiO2,37 nanocellulose,38 and carbon39 nanoparticles. In this study, the presence of both SNCs and RGO should be more effective as dual-enhancing fillers to improve the barrier properties of SPI-based nanopapers, because of their unique platelet- and sheet-like morphology. As shown in Figure S5, regardless of the gas molecule, the presence of 1 wt % RGO and 15−25 wt % SNCs resulted in a 20% reduction of water-vapor permeability and 50% reduction of oxygen permeability for the composite nanopapers in comparison with pure SPI-f. RGO had a favorable barrier 9436


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Figure 5. (A) Mechanical properties of SPI-based nanopapers with 1 wt % RGO and various loading levels of SNCs: tensile strength (σb); Young’s modulus (E); and strain at break (εb). (B) Stress−strain curves. (The arrowheads with dotted lines represent the mechanical properties of pure SPI material and S/R/S-15-f nanopaper as the comparison.)

Figure 6. (A) Storage modulus (E′) as a function of temperature, and (B) glass transition temperature (Tg) for the nanopapers. (Open symbols: pure SPI-f material.)

to black and opaque composite nanopapers as shown in Figure 7B. The electrical properties of the nanopapers in high-humidity atmosphere (98% RH) associated with different water uptakes are shown in Figure 8. It is well-known that water is a good conductor, which undoubtedly should facilitate the reduction of the electrical resistance for the SPI-based nanopapers exposed to moisture absorption. However, it should be pointed out that, different from that of fluid water, the effect of the absorbed moisture to the electrical properties still depended on the dispersion state of the conductive medium (RGO) in the nanopapers. Therefore, S/R/S-15-f nanopaper exhibited higher conductivity regardless of the duration of moisture exposure or water absorption than S/R/S-45-f nanopaper. The specific values of electrical conductivity (EC) for the SPI-based nanopapers conditioned at 98% RH for different durations and water absorptions are summarized in Table S3. The solvent resistance of the nanopapers was evaluated by the relative weight loss upon immersion in water and acetone,

the loading levels of SNCs higher than 25 wt % may be related to poor miscibility of retained starch nanoparticles in SPI. The influence of content on the electrical conductivity of SPI-based nanopapers was analyzed by measuring the electrical resistivity (ER) and electrical conductivity (EC), as shown in Figure 7A. It should be pointed out that pure SPI-f material possessed a very high resistance due to the insulation property of the protein component, and therefore was excluded from the comparison. As the only conductive component, the introduced RGO can apparently improve the electrical conductivity of SPIbased nanopapers and the dispersion state of RGO will definitely determine the electrical properties of the composite nanopapers.40 The gradual increase of EC values and decrease of ER values with the addition of 5−15 wt % SNCs indicated the strong interaction and improved dispersion of RGO in the composite system, whereas the introduction of superfluous lowconductive SNCs inevitably induced the aggregation and reduction of conductivity. Incorporating 1 wt % RGO induced the transformation of the yellow and transparent SPI material 9437


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Figure 7. (A) Electrical properties and (B) appearance of the SPI-based nanopapers with 1 wt % RGO and various loading levels of SNCs (roomtemperature dried samples).

Figure 8. Electrical resistivity for SPI-based nanopapers as a function of (A) the duration of conditioning at 98% RH and 30 °C, and of (B) water absorption.

Figure 9. (A) Relative weight loss and (B) appearance of the SPI-based nanopapers upon immersion in water and acetone for 48 h. (Open symbols: pure SPI-f material.)

Contrary to the weak effect of RGO, the presence of highly crystalline SNCs seems to be beneficial for the structural stability of the nanopapers regardless of the nature of the liquid

as shown in Figure 9A. Because of the hydrophilic nature of SPI, the nanopapers exhibited a quite significant sensitivity to the water medium, but stability in the organic acetone medium. 9438


ACS Sustainable Chemistry & Engineering medium. For instance the highest residual weight was observed for S/R/S-15-f nanopaper. The microphase separation induced by the superfluous amount of nanofiller (45 wt %) results in a discontinuous structure in the composites, and therefore causes the joint abscission of SNCs as SPI dissolves, which can be proven by the apparent defects (exposed white dots) of the tested sample (Figure 9B). Aside from the solvent resistance, the S/R/S-15-f nanopaper showed similar thermal stability as pure SPI-f material, with the only change of an additional 1−2% residues belonging to the graphene component (as shown in Figure S7).

CONCLUSIONS Plant-derived proteins (for example SPI) have shown great promise in material science as an edible and biodegradable natural resource, but there still remain some limitations to promote their practical application. We proposed a strategy involving SNCs as dispersing agent for RGO, and synergistic enhancement of both fillers in SPI-based composites to produce novel functional nanopapers. The electrical conductivity of the nanopaper was ascribed to the homogeneous dispersion of 1 wt % RGO with a value of 65.8 μS/m (S/R/S15-f), while the introduction of 15 wt % SNC resulted in improved mechanical properties of the composite nanopaper with outstanding simultaneous strengthening and toughening. Meanwhile, both platelet-like and sheet-like fillers significantly improved the barrier properties of the SPI-based composites, particularly a 50% reduction of oxygen permeability for the S/ R/S-15-f nanopaper. Because of the hydrophilic nature of SPI, the fabricated nanopapers exhibited humidity sensitivity toward conductivity, but promising solvent resistance to water and acetone due to the presence of highly crystalline SNCs. In this study, the dual-enhancement approach of two interacting fillers may be a common strategy for nanocomposite systems, and the “green” nanopapers developed from plant-based proteins, natural starch nanoparticles, and graphene may be potentially applied as conductive materials or functional supercapacitors in diverse fields.

ACKNOWLEDGMENTS

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REFERENCES

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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.7b02577. Deduction process for the calculation of the volume fraction; additional figures of FTIR spectra, rheological analysis, XRD patterns, SEM images, permeability values, water absorption values, TGA and DTG curves; additional tables of sample properties, and mechanical performance comparison for various composites (PDF)

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This study was supported by the National Natural Science Foundation of China (51603159) and Youth Chenguang Program of Science and Technology in Wuhan (2016070204010102), and Fundamental Research Funds for the Central Universities (Self-Determined and Innovative Research Funds of WUT, 2016IVA084 and 2017IVB023). The authors also wish to acknowledge the financial support of Key Laboratory of Recycling and Eco-Treatment of Waste Biomass of Zhejiang Province (2016REWB15). The authors are also grateful to Dr. Tengfei Deng (Wuhan University of Technology) for the discussion on the rheological analysis.

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Research Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-27-87152611. Fax: +86-27-87152611. ORCID

Alain Dufresne: 0000-0001-8181-1849 Ning Lin: 0000-0002-7367-8037 Notes

The authors declare no competing financial interest. 9439


Research Article


Graphene Nanocomposite for Humidity Sensing. Compos. Sci. Technol. 2016, 131, 67−76.

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Humidity-Sensitive and Conductive Nanopapers from Plant-Derived

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