Ultra-Strong, Ultra-Tough, Transparent, and Sustainable Nanocomposite Films for Plastic Substitute

Abstract

•An ultra-strong, ultra-tough, transparent nacre-inspired film is constructed•The transparent film exhibits excellent mechanical and thermal properties•“Brick and fiber” structure with finer fibers boosts the strength and toughness of film•The film is competitive as transparent substrate of flexible electronics Due to the considerably increasing negative impact of plastics, especially plastic films, fabrication of biodegradable and eco-friendly alternatives with competitive properties for plastic substitute is urgently needed. Through biomimetic design, our nacre-inspired composite constructed from renewable resources through eco-friendly processes not only can degrade in the natural environment but also possesses superior performance to some commercial plastics in terms of optical, thermal, and mechanical properties, which makes it a promising substitute for plastics in many practical applications, such as substrates for flexible electronics. Plastics play a critical role in daily life but possess a considerably increasing negative impact on the environment and human health. Fabrication of biodegradable and eco-friendly alternatives with competitive properties for plastic substitute is urgently needed. Here, inspired by the hierarchical structure of nacre, we firstly developed a high-performance nacre-inspired composite with high transmittance (83.4% at 550 nm) and high haze (88.8% at 550 nm) via an aerosol-assisted biosynthesis process combined with the hot-press technique. The nacre-inspired composite film combines higher strength (482 MPa) and toughness (17.71 MJ m−3) than most other nacre-inspired films, and can be folded into various shapes without visible failure after unfolding. Moreover, compared with most commercial plastic films, it exhibits a lower thermal expansion coefficient (∼3 ppm K−1) and higher maximum service temperature besides better mechanical properties, which makes it a promising alternative to plastics in many technical fields. Plastics play a critical role in daily life but possess a considerably increasing negative impact on the environment and human health. Fabrication of biodegradable and eco-friendly alternatives with competitive properties for plastic substitute is urgently needed. Here, inspired by the hierarchical structure of nacre, we firstly developed a high-performance nacre-inspired composite with high transmittance (83.4% at 550 nm) and high haze (88.8% at 550 nm) via an aerosol-assisted biosynthesis process combined with the hot-press technique. The nacre-inspired composite film combines higher strength (482 MPa) and toughness (17.71 MJ m−3) than most other nacre-inspired films, and can be folded into various shapes without visible failure after unfolding. Moreover, compared with most commercial plastic films, it exhibits a lower thermal expansion coefficient (∼3 ppm K−1) and higher maximum service temperature besides better mechanical properties, which makes it a promising alternative to plastics in many technical fields. Nowadays there is no getting away from plastics, which have been ubiquitously used in varieties of applications, including packaging, electronics, and construction.1Gibb B.C. Plastics are forever.Nat. Chem. 2019; 11: 394-395Crossref PubMed Scopus (74) Google Scholar,2Geyer R. Jambeck J.R. Law K.L. Production, use, and fate of all plastics ever made.Sci. 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Therefore, it is vitally important for plastic substitutes to gain improved mechanical properties of sustainable materials through advanced strategies.10Guan Q.F. Yang H.B. Han Z.M. Zhou L.C. Zhu Y.B. Ling Z.C. Jiang H.B. Wang P.F. Ma T. Wu H.A. Yu S.H. Lightweight, tough, and sustainable cellulose nanofiber-derived bulk structural materials with low thermal expansion coefficient.Sci. Adv. 2020; 6: eaaz1114Crossref PubMed Scopus (130) Google Scholar Biomimetic design is one of the most promising solutions to overcome this obstacle. Through imitating a natural hierarchical structure, bio-inspired materials can be endowed with excellent mechanical properties that can surpass those of artificial materials.11Wegst U.G. Bai H. Saiz E. Tomsia A.P. Ritchie R.O. Bioinspired structural materials.Nat. Mater. 2015; 14: 23-36Crossref PubMed Scopus (2678) Google Scholar, 12Yu Z.L. Yang N. Zhou L.C. Ma Z.Y. Zhu Y.B. Lu Y.Y. Qin B. Xing W.Y. Ma T. Li S.C. et al.Bioinspired polymeric woods.Sci. 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The proposed fabrication procedure takes advantage of coupling the nanomaterial deposition and nanoscale assembly during the in situ growth of cellulose nanofibers through microbial fermentation, whereby the nanomaterial becomes uniformly entangled in the robust three-dimensional (3D) network structure of bacterial cellulose (BC). Benefiting from this structure, the continuous BC fibers in our nacre-inspired composite can be separated by nano-clay platelets, resulting in the decrease of BC fiber diameter and “brick and fiber” architecture through subsequent unidirectional hot pressing. Owing to the decrease of BC fiber diameter and nacre-inspired “brick and fiber” structure, a better mechanical performance than plastics is achieved, which makes our nacre-inspired composite competitive as a plastic substitute. The nacre-inspired transparent composite of nano-clay platelets and BC (Figure S1) is constructed by an aerosol-assisted biosynthesis process combined with the hot-press technique recently developed by our group (Figure 1).18Guan Q.-F. Han Z.-M. Luo T.-T. Yang H.-B. Liang H.-W. Chen S.-M. Wang G.-S. Yu S.-H. A general aerosol-assisted biosynthesis of functional bulk nanocomposites.Natl. Sci. Rev. 2019; 6: 64-73Crossref Scopus (31) Google Scholar,19Guan Q.-F. Han Z.-M. Ling Z.-C. Yang H.-B. Yu S.-H. Sustainable wood-based hierarchical solar steam generator: a biomimetic design with reduced vaporization enthalpy of water.Nano Lett. 2020; https://doi.org/10.1021/acs.nanolett.0c01088Crossref Scopus (100) Google Scholar Firstly, the bacterial strain of BC, i.e., Gluconacetobacter xylinus 1.1812, is inoculated onto the agar culture medium solid substrate to form a thin layer of BC. A continuous and stable aerosol of nano-clay suspension and liquid nutrient are then fed onto the interface of BC and air. During the aerosol-assisted biosynthesis process, the nano-clay platelets reaching the interface are entangled by the cellulose nanofibers that are secreted by the bacteria to form a uniform nanocomposite (Figure 1A). After continuous fermentation, the nano-clay platelets are uniformly distributed in the 3D fiber network of cellulose nanofibers (Figure 1B). The wet pellicles that comprise nano-clay and cellulose nanofibers are harvested from the solid substrate (Figures 1C and 2A ). Finally, the obtained hybrid hydrogel is further hot-pressed into a dense composite film with the naturally formed nacre-inspired laminated “brick and fiber” structure (Figure 1D).Figure 2Characterization of Nacre-Inspired Composite FilmsShow full caption(A) Scanning electron microscopy (SEM) image of the structure of the hybrid hydrogel, showing the uniform distribution of nano-clay platelets in 3D nanofiber network of BC. The inset photograph is the robust and translucent hybrid hydrogel.(B) SEM image of the structure of the hybrid hydrogel, showing the nano-clay platelets bound by multiple nanofibers.(C) SEM image of the structure of the hybrid hydrogel, showing the interaction between nano-clay platelets.(D) Nacre-inspired composite films.(E and F) Surface (E) and cross-sectional (F) topography of nacre-inspired composite.View Large Image Figure ViewerDownload (PPT) (A) Scanning electron microscopy (SEM) image of the structure of the hybrid hydrogel, showing the uniform distribution of nano-clay platelets in 3D nanofiber network of BC. The inset photograph is the robust and translucent hybrid hydrogel. (B) SEM image of the structure of the hybrid hydrogel, showing the nano-clay platelets bound by multiple nanofibers. (C) SEM image of the structure of the hybrid hydrogel, showing the interaction between nano-clay platelets. (D) Nacre-inspired composite films. (E and F) Surface (E) and cross-sectional (F) topography of nacre-inspired composite. To confirm the effectiveness of the aerosol-assisted biosynthesis process, we observed the morphologies of the hybrid hydrogel via scanning electron microscopy (SEM). Benefiting from the aerosol-assisted biosynthesis process, the nano-clay platelets are uniformly distributed throughout the integrated 3D cellulose nanofiber networks (Figure 2A). With the incorporation of nano-clay platelets, the bundling of nanofibers into ribbons is disrupted in the composite, which results in finer cellulose fibers compared with a pure BC sample (Figure S2), giving the hybrid hydrogel robustness and translucence (Figure 2A). Meanwhile, SEM images clearly show that the nano-clay platelets are entangled with several nanofibers and form a continuous 3D interaction network (Figures 2B and 2C), which further strengthens the combination between them. Due to the 3D nanofiber network structure, there is no visible in-plane expansion when the hybrid hydrogel is transformed into a dense film, with the thickness reducing from ∼2 mm to ∼20 μm during the hot-pressing process. In this process, BC and nano-clay platelets are closely integrated together with the synergistic effect of pressure and heat, endowing the nacre-inspired composite with high transparency and high haze (Figure 2D). The microstructure can be observed in the SEM image of the surface of nacre-inspired composite (Figure 2E), where cellulose nanofibers are tightly bound by hydrogen bonding and nano-clay platelets are tightly entangled. Meanwhile, as confirmed by X-ray photoelectron spectroscopy (XPS), there exists interaction between BC and nano-clay platelets, strengthening their combination (Figure S3). The SEM image of cross-sections (Figure 2F) reveals the dense well-ordered lamellar microstructure in nacre-inspired composite (Figure S4), which results from the flat and uniform orientation of nano-clay platelets in the nanofiber network. Through unidirectional pressing, nano-clay platelets are stacked in uniform orientation and construct the “brick and fiber” structure with BC, which can greatly improve the mechanical properties of nacre-inspired composite. Through this bottom-up strategy, our nacre-inspired composite film possesses multiple intriguing macroscopic properties in one material, including unique optical properties and excellent mechanical properties. The unique optical performance combining high optical transmittance and high optical haze is critical for efficient light management in optoelectronic devices, which is challenging for plastics because of their homogeneous structure.20Yao Y. Tao J. Zou J. Zhang B. Li T. Dai J. Zhu M. Wang S. Fu K.K. Henderson D. et al.Light management in plastic-paper hybrid substrate towards high-performance optoelectronics.Energy Environ. Sci. 2016; 9: 2278-2285Crossref Google Scholar Within the wavelength range of 370–780 nm, which covers the whole visible spectrum, the transmittance and the haze of our nacre-inspired composite film are more than 73% and 80%, respectively (Figure 3A). Benefiting from the aerosol-assisted biosynthesis process, our nacre-inspired composite film exhibits approximately 1.5-fold higher transmittance at 550 nm compared with pure BC film,21Zhu H. Parvinian S. Preston C. Vaaland O. Ruan Z. Hu L. Transparent nanopaper with tailored optical properties.Nanoscale. 2013; 5: 3787-3792Crossref PubMed Scopus (205) Google Scholar which results from the diameter of BC fibers decreasing as the bundling of nanofibers is disrupted. Such nacre-inspired composite film with high transparency and haze can be utilized as a potential material for plastic substitute in light management. Meanwhile, with the nano-clay platelets integrated into the 3D nanofibrous network, the mechanical properties of our nacre-inspired composite film are greatly enhanced compared with pure BC film. We thus measured the strain-stress curves of pure BC films and nacre-inspired composite films with different contents of nano-clay (Figure S5) in order to investigate the influence of nano-clay on mechanical properties (Figures 3B and 3C). With low content of nano-clay (∼9 wt %), the tensile strength of nacre-inspired composite film slightly decreases compared with that of pure BC film, while the strain to failure dramatically increases from 3.58% to 5.54%, resulting in a 1.7-fold increased toughness. As nano-clay content of nacre-inspired composite film rises further, the tensile strength and toughness increase largely simultaneously. For high nano-clay content (∼27%) of nacre-inspired composite film, its tensile strength increases from 296 MPa to 482 MPa. Meanwhile, the strain to failure and toughness can reach respectively 6.06% and 17.71 MJ m−3, which are 1.7- and 2.9-fold higher than those of pure BC film. Thus, compared with pure BC film, the nacre-inspired composite film can achieve greatly enhanced tensile strength and toughness simultaneously. In general, strength and toughness are mutually exclusive in most artificial materials. However, the nacre-inspired composite film exhibits excellent strength and toughness at the same time, which can be attributed to the incorporation of nano-clay in the aerosol-assisted biosynthesis process and elaborate nacre-inspired “brick and fiber” structure. As the nano-clay platelets are uniformly added during the aerosol-assisted biosynthesis process, the continuous BC fibers are separated by them (Figures 3E and 3F). As shown in Figure 3D, the mean BC fiber diameter decreases from ∼106 nm to ∼26 nm, which results in the reduced defect size of BC fiber and consequent drastic improvement in its strength.22Zhu H. Zhu S. Jia Z. Parvinian S. Li Y. Vaaland O. Hu L. Li T. Anomalous scaling law of strength and toughness of cellulose nanopaper.Proc. Natl. Acad. Sci. U S A. 2015; 112: 8971-8976Crossref PubMed Scopus (241) Google Scholar In the meantime, with fiber diameter decreasing and surface area increasing, hydrogen bonding increases significantly and more slippage occurs between fibers during stretch, which lead to the simultaneous increase in toughness. Moreover, the elaborate nacre-inspired “brick and fiber” structure plays a key role in the high toughness of the nacre-inspired composite. With the nano-clay uniformly distributed in the dense 3D network of BC, the slippage of cellulose nanofibers is enhanced by forming a different hydrogen-bond network, which also increases the toughness of the film. 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Transparent and flexible nacre-like hybrid films of aminoclays and carboxylated cellulose nanofibrils.Adv. Funct. Mater. 2018; 28: 1703277-1703286Crossref Scopus (51) Google Scholar The tensile strength of our nano-clay/BC film can reach 482 MPa, which surpasses those of other nacre-inspired films listed in Table S1. In addition, the strain to failure of our nano-clay/BC film can reach 6.06%, exhibiting a combination of higher tensile strength and strain to failure. Compared with nacre-inspired films with similar strains to failure, the tensile strength of our nacre-inspired film is higher. Moreover, compared with previous nacre-inspired polymer/inorganic films, our nacre-inspired nano-clay/BC film not only has great advantages in mechanical properties but also can be prepared by a simple and robust aerosol-assisted biosynthesis process, which allows it to be prepared on a large scale and at low cost. Due to the unique optical properties and excellent mechanical properties, our nacre-inspired composite possesses promising potential as a substrate for flexible electronics, which currently are always based on plastics. As shown in Figure 5A, both strength and stiffness of the nacre-inspired composite are higher than those of commercial plastics.42Ashby M. Materials Selection in Mechanical Design.Fourth Edition. Elsevier, 2011Google Scholar The tensile strength of nacre-inspired composite is up to 482 MPa, which is more than 2-fold higher than those of traditional high-performance plastics. In addition, compared with the Young's modulus of commercial plastics ranging from 1 GPa to 5 GPa, a much higher stiffness of about 15 GPa is achieved in our nacre-inspired composite. Benefiting from the intrinsic properties of components, our nacre-inspired composite also exhibits an extremely low thermal expansion coefficient (∼3 ppm K−1) and a high maximum service temperature, which plays a critical role in substrates of flexible electronics in maintaining performance at high or variable temperatures with long-time use. As shown in Figure 5B, both the thermal expansion coefficient and thermal stability of our nacre-inspired composite are much better than those of traditional plastics,42Ashby M. Materials Selection in Mechanical Design.Fourth Edition. Elsevier, 2011Google Scholar indicating that it is safer and more reliable than plastics in daily use. By integrating outstanding mechanical properties, low thermal expansion coefficient, and strong thermal stability into one material, our nacre-inspired composite film is a promising alternative to plastics in many fields. For a sustainable composite the achievement of mass production, which is critical for practical application as a plastic substitute, is of great importance. Through the effective and scalable bottom-up approach, nacre-inspired composite films can be fabricated with an area of 20 × 40 cm2 (Figure 5C), indicating the potential of our material for mass production. Moreover, as shown in Figures 5D and 5E, the obtained nacre-inspired composite film possesses good flexibility, which can be folded into desired shapes and shows no visible damage after unfolding. Based on this feature, the nacre-inspired composite film can be endowed with good electrical conductivity by incorporating an ultra-thin layer of conductive material, which can be maintained even in the case of bending (Figure 5F), demonstrating its promising potential as a substitute for plastic in practical applications. As a substrate for flexible electronics, the nacre-inspired composite film not only possesses performance superior to polyethylene terephthalate (PET) film in terms of optical, thermal, and mechanical properties42Ashby M. Materials Selection in Mechanical Design.Fourth Edition. Elsevier, 2011Google Scholar, 43Taniguchi I. Yoshida S. Hiraga K. Miyamoto K. Kimura Y. Oda K. Biodegradation of PET: current status and application aspects.ACS Catal. 2019; 9: 4089-4105Crossref Scopus (225) Google Scholar, 44Zhu H. Fang Z. Wang Z. Dai J. Yao Y. Shen F. Preston C. Wu W. Peng P. Jang N. et al.Extreme light management in mesoporous wood cellulose paper for optoelectronics.ACS Nano. 2016; 10: 1369-1377Crossref PubMed Scopus (149) Google Scholar (Table 1), but also can degrade in the natural environment. After being placed in soil and left exposed to the elements, the nacre-inspired composite film slowly breaks down and returns to nature, showing little negative environmental impact (Figure S6).Table 1Comparison of Properties between PET and This WorkPropertiesPETThis WorkTensile strength (MPa)48.3–72.4∼482Young's modulus (GPa)2.76–4.14∼15Transparency (%)∼90∼80Haze (%)∼1∼90Melting temperature or glass-transition temperature (°C)58–80∼250Coefficient of thermal expansion (ppm K−1)114–120∼3BiodegradabilityBadgood Open table in a new tab In summary, we have fabricated a high-performance nacre-inspired nano-clay/BC composite film through a bottom-up strategy. Benefiting from the scalable aerosol-assisted biosynthesis process, finer BC nanofibers and a “brick and fiber” structure can be achieved in our nacre-inspired composite films, which results in a combination of outstanding properties including high strength, high toughness, high transmittance, high haze, and good foldability. With better mechanical and thermal properties than petroleum-based plastics, our sustainable nacre-inspired composite film can become a strong competitor for plastics. Furthermore, given the widely available industrial production equipment, large-scale continuous production of our sustainable nacre-inspired composite for practical applications as a substitute for plastics can be expected in the near future.