Bioinspired Design of Strong, Tough and Thermally Stable Polymeric


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Bioinspired Design of Strong, Tough and Thermally Stable Polymeric Materials via Nanoconfinement Pingan Song, Jinfeng Dai, Guorong Chen, Youming Yu, Zhengping Fang, Weiwei Lei, Shenyuan Fu, Hao Wang, and Zhi-Gang Chen ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b04002 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 4, 2018

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Bioinspired Design of Strong, Tough and Thermally Stable Polymeric Materials via Nanoconfinement

Pingan Song,*,†,‡ Jinfeng Dai,† Guorong Chen,§ Youming Yu,† Zhengping Fang,± Weiwei Lei,*,£ Shenyuan Fu,† Hao Wang*,‡, and Zhi-Gang Chen*,‡,‖



Department of Materials, Zhejiang A & F University, Hangzhou, 311300, China



Centre for Future Materials, University of Southern Queensland, Toowoomba, QLD 4350,

Australia §

Research Centre of Nanoscience and Nanotechnology, Shanghai University, Shanghai,

200444, China ±

Ningbo Institute of Technology, Zhejiang University, Ningbo, 315100, China

£

Institute for Frontier Materials, Deakin University, Locked Bag 20000, Geelong, VIC 3220,

Australia ‖

Materials Engineering, the University of Queensland, Brisbane, QLD 4072, Australia

*Address correspondence to [email protected], [email protected], [email protected], [email protected]

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ABSTRACT: The combination of high strength, great toughness and high heat resistance for polymeric materials is a vital factor for their practical applications. Unfortunately, until now it has remained a major challenge to achieve this performance portfolio because the mechanisms of strength and toughness are mutually exclusive. In natural world, spider silk features the combination of high strength, great toughness and excellent thermal stability, which are governed by the nanoconfinement of hydrogen-bonded β-sheets. Here, we report a facile bioinspired methodology for fabricating advanced polymer composite films with a high tensile strength of 152.8 MPa, a high stiffness of 4.35 GPa, and a tensile toughness of 30.3 MJ/m3 in addition to high thermal stability (69 oC higher than that of the polymer matrix) only by adding 2.0wt% of artificial β-sheets. The mechanical and thermostable performance portfolio is superior to that of its counterparts developed to date because of the nanoconfinement and hydrogen-bond crosslink effects of artificial β-sheets. Our study offers a facile biomimetic strategy for the design of integrated mechanically robust and thermostable polymer materials, which hold promise for many applications in electrical devices and tissue engineering fields.

KEYWORDS: nanoconfinement, bioinspired design, mechanical performance, thermal stability, poly(vinyl alcohol)

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Due to their unique features, including high strength-weight ratio, extensibility, and ease of processing,1-3 polymeric materials have shown great potential in the fields of aerospace,4 energy,5,6 and tissue engineering.7-9 Extensive studies have been undertaken to enhance their mechanical properties including strength, toughness and thermal stability of the polymer composites on the basis of different additives,10-21 such as graphene oxides (GO),10-13 and carbon nanotubes (CNTs).14-16 Unfortunately, most of these polymeric materials normally show reduced toughness and ductility owing to mutually exclusive mechanical mechanisms between strength and toughness.22,23 Moreover, reduced GO-reinforced or CNTs-reinforced polymeric materials normally exhibit decreased thermal stability, thereby limiting their engineering applications.12,15 Therefore, to date, the challenge of creating integrated strong, tough and thermostable polymeric materials has remained intractable. Biological materials, such as nacre, bones, teeth, wood and spider silk, have evolved over millions of years and feature a combination of high strength and toughness outperforming many synthetic polymer materials because of their orderly hierarchical microstructures.10,24,25 One of the best-known of these materials is nacre, which combines a tensile strength of 80135 MPa, an elastic modulus as high as 40-70 GPa and a high fracture toughness resulting from its hierarchical “brick-and-mortar” structure.10,24 This has led to the development of many nacre-like strong and stiff polymer nanocomposites based on micro-/nano-scale platelets and sheets, such as nanoclay,26-28 Al2O3 platelets,1,29 GO and its derivatives,3,30-32 and layered double hydroxide (LDH).33 A typical example is that Kotov et al. have fabricated ultrastrong and stiff layered polymer nanocomposites showing a high tensile strength of 400 MPa and an elastic modulus as high as 106 GPa.26 The micro-/nano-scale platelets and sheets additives play key roles in enhancing toughness, tensile strength, thermal stability and ductility.25,26,29 Nevertheless, it remains a major challenge to develop effective additives for the design of strong, tough and thermostable polymeric materials in this field. With respect to nacre, fully biopolymer-based spider silk exhibits a far higher tensile 3 ACS Paragon Plus Environment

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strength (1-2 GPa), a larger break strain (≥30%) and greater toughness in addition to a comparable stiffness (~10 GPa) (Figures 1a and 1b), making it one of the strongest and toughest materials.34-37 Moreover, it also has excellent heat tolerance (up to 350 oC) though it is a fully biological protein material without any inorganic components.38 It has recently become clear that such outstanding mechanical and thermostable performances are mainly attributed to the fact that the highly well-organized and densely hydrogen-bonded β-sheet crystals (1.87 -7.04 nm in length, 2.23 nm in width or height and 1.37 nm in thickness) are confined within a semi-amorphous protein matrix. In other words, the nanoconfinement via multiple mechanically inferior hydrogen bonds (H-bonds) interactions governs this exceptional performance portfolio (Figure 1c).34 This intriguing mechanism offers a potential methodology for the design of strong, tough and thermostable polymeric materials. Until now many scientists have attempted to fabricate H-bonded-confinement polymeric materials by mimicking the hierarchical nanostructure of spider silk. Unfortunately, these efforts usually show relatively weak mechanical strength and stiffness due to ineffective design.39-43 Inspired by the hierarchical nanostructure and molecular mechanisms of spider silk, herein, we have demonstrated the facile fabrication of integrated strong, tough and thermostable polyvinyl alcohol (PVA) composite films via the introduction of artificial β-sheet crystals, namely, graphene oxide quantum dots (GOD). This GOD-reinforced PVA composite film shows a high tensile strength of 152.5 MPa and a high toughness of 30.3 MJ/m3 without compromising the ductility of the polymer matrix. Moreover, the maximum decomposition temperature (Td) of the GODs-reinforced PVA composite film is increased from 286 oC to 355 o

C, which is far higher than that of any other nanofiller-reinforced PVA composite film.10-19

The combination of mechanical and thermostable performances portfolio outperforms the counterpart PVA composite films filled with other fillers. This work provides a facile bioinspired strategy for the design of mechanically strong, tough and thermally stable polymer materials, and these materials are expected to find promising application prospects in 4 ACS Paragon Plus Environment

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electrical devices and tissue engineering.

RESULTS AND DISCUSSION Figure 1 Design and Fabrication of Bioinspired PVA Composite Films. Because its extensive Hbond interactions are similar to the existing biological protein materials,24, 34-37 PVA has been selected as the semi-amorphous polymer matrix in this study. GOD is usually prepared via chemically oxidizing the graphite, and therefore it owns abundant hydroxyl, epoxide and carboxyl groups located at their surface and edge, which enables GOD to form multiple Hbonds with PVA and to crosslink the PVA chains. In addition, the excellent water solubility enables GOD to uniformly disperse within the PVA matrix at a molecular level via a facile solution mixing approach. Because of these features, GOD closely resembles natural β-sheets in spider silk.33 Therefore, we select the atom-thick GOD as the artificial β-sheet to create the nanoconfinement. The fabrication process of GOD-reinforced PVA composites is shown in Figure 1d. Firstly, the artificial β-sheet, herein designated as GOD, is easily synthesized via a two-step reaction of chemical oxidation and cutting.44-46 As-synthesized GOD with good solubility in water is then mixed with the PVA using water as a co-solvent. The bioinspired PVA composite films can be obtained by simply casting the aqueous solutions of PVA/GOD into a glass mold followed by completely drying them. GOD can form multiple H-bonds interactions with PVA because of abundant oxygen-containing groups in the GOD nanosheet and rich hydroxyl groups in PVA macromolecules. This enables GOD to uniformly disperse within the continuous PVA matrix and to create the nanoconfinement via multiple H-bonds. Figure 1e shows that the atom-thick GOD exhibits an average size (length scale) of 6.5 nm and thickness in the range of 0.60 -1.40 nm, as evidenced by the high resolution (HR) TEM image (Figure S1) and the atomic force microscopy (AFM) images (Figures S2a and S2b). In

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addition to a similar size range, the GOD also has a relatively low length-thickness ratio of approximately 6.0, which is structurally analogous to β-sheet.34 This means that GODs are expected to play a similar role in creating nanoconfinement. Moreover, the selective area electron diffraction (SAED) pattern shows that the as-synthesized GOD is semi-amorphous due to abundant oxygen-containing groups (Figure 1e1). In addition, the chemical composition and thermal properties characterization of as-prepared GOD are shown in Figures S3 and S4. As-fabricated GOD-reinforced PVA composite films are homogeneous and transparent because of the quantum size of GOD (Figures S5a and S5b), in addition to their ability to absorb UV. As presented in Figure 1f, the typical TEM image of GOD-2.0 clearly shows a classic microphase-separated nanostructure in which the GOD nanosheets serving as artificial β-sheets are well-dispersed with an average domain size of 6.6 nm within the continuous semi-amorphous PVA matrix. At higher magnification (on the top right corner of Figure 1f), the surface and edge of GOD appear blurry and seem to be covered by polymer chains as compared with the pristine GOD (Figure 1e). This microstructure of as-fabricated PVA composite film is structurally analogous to that of spider silk (Figure 1c). The interaction of GOD and PVA is also characterized by computer-aided simulation. As shown in Figure 1g, GOD sheets are uniformly dispersed within the continuous PVA matrix when the system reaches an equilibrium state. Therefore, GOD-centered nanoconfinement (marked by yellow circle) can form via the multiple H-bonds interactions of GOD with PVA molecules. This nanoconfinement is expected to enable PVA to show high tensile strength and ductility as it acts in spider silk. In addition, the multiple H-bonds can also enable GOD to physically crosslink the PVA chains, thus leading to enhanced thermal stability. Figure 2 Table 1 Mechanical Performances. In order to investigate the loading level of GOD on the mechanical performances of the as-prepared GOD-reinforced PVA composite films, we have 6 ACS Paragon Plus Environment

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examined their tensile properties, with detailed results shown in Figure 2. Figure 2a shows typical stress-strain curves of PVA and its GOD-reinforced PVA composite films. Basically, the tensile strength and elastic modulus of the GOD-reinforced PVA composite films increase with increasing the loading levels of GOD without compromising the ductility of the PVA. In comparison, adding 1.0wt% GO leads to significant reductions in the ductility and toughness of the PVA matrix. As shown in Figure 2b, Figure 2c and Table 1, both the yield strength (σy) and elastic modulus (E) of the GOD-reinforced PVA composite films increase with increasing the loading levels of GOD except for the slight decrease in σy when the loading level reaches 2.96 vol% (or 5.0 wt%). The PVA matrix displays a high σy of 91.8 MPa and a E of 2.32 GPa, whereas adding 0.57 vol% GOD enhances the σy and E up to 136.3 MPa (by 48%) and 3.76 GPa (by 62%), respectively. Upon the loading level of GOD reaching 1.16 vol%, σy and E of the PVA are improved by around 66% (152.5 MPa) and 88% (up to 4.35 GPa), respectively. Moreover, the slopes of the plots of both strength (dσ/dϕ) and modulus (dE/dϕ) as a function of the volume fraction of GOD are 6.4 GPa and 247 GPa, both of which are basically linear to the volume fraction of GOD, despite some deviations for GOD-5.0. This demonstrates the strong reinforcement effect of GOD on the PVA. The enhancement in the modulus is also evidenced by dynamic mechanical analysis (DMA) (Figure S6). Interestingly, the relative modulus change demonstrates a nearly linear relationship to the relative degree of crystallinity, indicating that the promotion effect of GOD on the crystallinity of PVA contributes to the modulus enhancement to some extent (Figure S7). Meanwhile, the theoretical strength of GOD (σg) is calculated based on the C/O ratio to determine the shear strength (τy) between GOD and PVA, and GOD has a relatively lower σg of around 23 GPa than graphene.47 By fitting the experimental yield strength data for PVA/GOD blend films with Eqs. 3 and 4, a τy as high as 1.34 GPa is calculated and this value is far larger than the estimated τy of PVA (~46 MPa, τy = σy/2).1 This implies that the PVA matrix will mechanically fail ahead of the interface between the PVA and GOD under the external load because of the H-bond crosslink 7 ACS Paragon Plus Environment

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interactions.48 Moreover, such a high τy in turn further proves the strong multiple H-bond interactions between GOD and PVA. In order to predict the elastic modulus of GOD-reinforced PVA composite films, we employ the simple linear mixing rule (Eq. 5). As also shown in Figure 2c, the experimental E values are consistent with those calculated theoretically when the volume fraction of GOD is below 1.16 vol.% (or 2.0 wt%), but the deviation appears once the loading level is above 1.16 vol.%. This deviation is most likely due to the modulus error of GOD since its theoretical elastic modulus is dependent on its oxidation degree or C/O ratios.47 In addition, instead of reducing the ductility (or strain at failure, εb) and toughness (τ) (calculated by Eq. 6), the addition of GOD increases both mechanical parameters, and a much higher magnitude of increase in the toughness is observed (Figure 2d). For instance, 1.16 vol% of GOD leads to an 80% improvement in τ (from 16.8 MJ/m2 to 30.3 MJ/m2) besides a slight increase in εb (from 23% to 27%) compared with the PVA matrix. As compared with GOD, incorporating GO results in reductions in ductility (a εb of 6.1%) and toughness (a τ of 5.15 MJ/m2) because of its high aspect ratio (≥2500).11 Therefore, similar to the mechanical failure mechanism of spider silk, the GOD-centered nanoconfinement controls the improvements in σy and E whereas the increased τ and εb are most likely due to the dynamic multiple H-bonds between GOD and PVA. Figure 3 Reinforcement Mechanism. To understand the reinforcement mechanism of GOD in the PVA on the molecular scale, the intermolecular H-bond interactions between them are firstly examined. Figure 3a shows that the storage modulus (G′) value of aqueous solution of PVA increases almost linearly with a concomitant increase in the GOD concentration in the entire frequency range, indicating the reinforcement effect of GODs. The PVA solution itself shows a low slope of 0.34 in the low frequency region, indicating strong and extensive intermolecular H-bond interactions, and a relatively high slope of 0.95 in the middle 8 ACS Paragon Plus Environment

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frequency region. As expected, a “plateau” (marked by a red circle) appears in the low frequency regime below 1 rad/s once the content of GOD is above 1.0wt%, as reflected by small slope values (for instance, a low slope of only 0.18 for GOD-1.0). This plateau has been extensively regarded as an indicator of the formation of a physical crosslink network,42,43,49,50 implying that a certain content of GOD is capable of crosslinking PVA chains via multiple Hbond interactions. It is understandable that this crosslink can lead to the effective formation of GOD-centered nanoconfinement within the PVA matrix based on multiple H-bonds, because of the similar function, size (around 6.5 nm) and structure of GOD to the β-sheet within the protein matrix of spider silk. In comparison, as-observed plateau above cannot be observed in the GO-reinforced PVA composite film despite much higher G′ value than that of PVA/GOD systems. Because of much lower contents of oxygen-containing receptors/donors of H-bond than GOD (XPS in Figure S3b), the extensive H-bond interactions among the PVA chains are isolated by the large GO sheets (0.5-2 µm) with a high aspect ratio. However, the steric hindering effect of GO cannot compensate for the adverse isolation effect on the H-bond interactions, thereby leading to the disappearance of the “plateau”.12 Infrared spectra can provide powerful information for determining the H-bond interactions between GOD and PVA, as shown in Figure 3b and Figure S8. Apparently, the difference between the stretching vibration of hydroxyl groups (∆υO-H) GOD-reinforced PVA composite films and the PVA matrix continuously increases with increasing loading levels of GOD, for instance, a ∆υO-H of 5 cm-1 for GOD-2.0 and 7 cm-1 for GOD-5.0. Such large blue shifts strongly indicate their strong H-bond interactions.42,43 By contrast, there is minor blue shift observed in the GO-reinforced PVA composite film because of weak H-bond interactions and fewer intermolecular H-bonds. The glass transition temperatures (Tgs) of GOD-reinforced PVA composite films are examined as they can reflect the restricting effect of GOD on the mobility of PVA chain segments. Overall, the tendency of Tg differences (∆Tg) is similar to both viscoelastic and IR results, indicating that their results corroborate each other. The Tg 9 ACS Paragon Plus Environment

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value (∆Tg) obtained by both DMA and DSC of the GOD-reinforced PVA composite films steadily increases with increasing GOD content whereas the addition of GO reduces the Tg (Figure 3c). For instance, GOD-1.0 and GOD-5.0 respectively show a ∆Tg of 3.1 and 7.3 oC, as compared with a ∆Tg of -1.1 oC for GO-1.0, as observed by other work.12 For the GODreinforced PVA composite films, strong multiple H-bond interactions and nanoconfinement of GOD can restrict the movement of PVA chain segments, thus leading to higher Tg.51,52 In comparison, the presence of GO isolates and reduces the intermolecular H-bond interactions among the PVA chains, thereby making the PVA chain segments easier to move and resulting in a lower Tg value. However, Liang et al. also reported an increase in Tg of PVA probably because of the higher degree of oxidation of GO and its stronger interfacial interactions with PVA.11 Moreover, both GOD and GO seem to act as nucleating agents for PVA, as evidenced by increased degree of crystallinity and higher crystallization temperature than the PVA matrix while not affecting the crystal forms (Table S1, Figures S9 and S10). Further, Raman spectra of PVA and its composites during tensile deformation are recorded to determine the stress transfer between GOD and PVA via H-bonds. As shown in Figure 3d and Figure S11, the out-of-plane bending vibration of C-O located at ~1095 cm-1 of the PVA matrix only shows a neglected shift when a tensile load is applied. This means that the peak closely correlates with the bending mode of the hydroxyl side groups instead of the stretching of the primary bonds in the PVA backbone.48 In comparison, upon tensile deformation, GOD-2.0 shows a 2.7 cm-1 Raman shift from 1095 to 1092.3 cm-1 when the strain reaches 7%. This indicates that the mechanical deformation is due to the stress transfer between PVA and GOD via H-bond interactions, as illustrated in Figure 3e. By contrast, a smaller shift of 1.43 cm-1 at a strain of 5% is also observed in the GO-1.0 system due to fewer H-bonds formed between GO and PVA (Figure 3d). Moreover, polarized optical microscopy (POM) observations show that the presence of GOD can induce the crystallization of PVA during the tensile process (Figure 3f) whereas the PVA does not show detectable 10 ACS Paragon Plus Environment

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crystallization. Interestingly, the crystallization becomes increasingly obvious with increasing GOD contents and reaches the maximum value when the content of GOD is 2.0 wt%, and then it decreases (Figure S12). This is consistent with the trend change in tensile strength (Figure 2b). Meanwhile, GO can also induce the crystallization of PVA to some extent during tension (Figure S12). Generally, stress-induced crystallization can lead to higher mechanical strength and stiffness.53 The crystallites (indicated by the white arrow) evolved during tensile process can act as physical cross-links which can facilitate an increase in the mechanical strength with macroscopic deformation, as evidenced by the stress-strain curve of GOD-2.0 (Figure 2a). The phenomenon is probably because GOD can guide the rearrangement of PVA chains to form orderly crystallite zones during tension by multiple H-bond interactions. In addition, the fracture morphology of the GOD-reinforced PVA composite film is also visually examined by scanning electron microscope (SEM) to further understand the mechanical reinforcement effect of GOD on the PVA. As shown in Figures 3g-3i, GOD-2.0 shows a typical ductile failure morphology where various fracture modes from the matrix deformation to tearing, to microcracks (1-8 µm long) and larger cracks or voids (approaching 25 µm long) are clearly observed, as marked by yellow arrows (Figures 3g and 3h). The evolution of these combined fracture morphologies is able to absorb and dissipate much fracture energy during the periods of being stretched, thereby enabling the PVA to show increased fracture toughness.54,55 It seems that many small bulges with a size range of 200500 nm (marked by red arrows) will disperse within a network-like fracture surface (yellow frame) at higher magnification (Figure 3i), similar to the fracture structure of the crosslinked elastomer reported recently.56 It should be noted that PVA chains break before the interactions between GOD and PVA because of the much higher shear strength (τy) between PVA and GOD than the τy among PVA chains themselves. When the PVA phase distant from the GOD phase undergoes mechanical failure, the GOD-centered nanoconfinement phase restricts the movement and failure of the PVA phase around GOD because of multiple H-bond interactions, 11 ACS Paragon Plus Environment

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thereby leading to a network-like fracture morphology. Unlike GOD, the presence of GO makes PVA show a typical brittle fracture despite uniform dispersion due to its large aspect ratio and poor interfaces (Figure S13). In brief, based on the above comprehensive analysis, it is reasonable to conclude that the GOD-centered nanoconfinement via multiple H-bond interactions are primarily responsible for the improved mechanical strength, stiffness and toughness without compromising the ductility of the PVA. This is because of the features of dynamic H-bonds between PVA and GOD, which can quickly reform after breakdown during external loads on a molecular scale. The mechanical failure process of as-prepared GOD-reinforced PVA composite films is also characterized by the computer-aided illustration, as presented in Figure 3j. In the relaxed stage (Figure 3j (i), ε=0), the PVA composites, such as GOD-2.0, form a three-dimensional (3D) physical network structure based on multiple H-bond interactions between GOD and PVA. These H-bond interactions lead to the formation of GOD-centered nanoconfinement phase, as marked by a yellow circle. Upon stretching (Figure 3j (ii), 0 < ε < εb), the H-bonds among the PVA chains distant from GOD can break and reform, which allows for a large deformation while maintaining structural stability. The GOD-centered nanoconfinement is also capable of restricting the movement and mechanical failure of the PVA chains around them due to multiple H-bonds, leading to high strength and modulus. Meanwhile, the unfolding of the polymer chains at large deformations enables a high ductility without sacrificing stiffness. Despite this, the microcrack may also initiate at this stage. When the external load further increases, more microcracks within the PVA bulk can further initiate and gradually evolve into macrocracks and even cracks, eventually resulting in the breakdown of the PVA chains and the mechanical failure of the polymer bulk, as illustrated in Figure 3j (iii) (ε = εb). Figure 4 Thermal stability. In order to understand the effect of GOD content and the underlying 12 ACS Paragon Plus Environment

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molecular action mechanism, we have examined the thermal stability of as-prepared GODreinforced PVA composite films, with detailed results shown in Figure 4 and Table S1. As can be seen, both thermal stability parameters, namely the Ti (the temperature where 5 wt% of weight loss occurs) and Td (the temperature where the maximum weight loss happens), steadily augment with increasing GOD contents. For instance, GOD-2.0 and GOD-5.0 increase the Ti of PVA from 264 oC to 272 oC and 290 oC, and the Td from 286 oC to 355 oC and 357 oC, respectively. This means that 2.0wt% of GOD leads to an 8 oC increase in Ti and a 69 oC increase in Td. The results can also be validated by the increased degradation activation energy (Ea) because of the addition of GOD (Table S1). In comparison, GO (also prepared by this work) hardly contributes to the thermal stability of PVA despite its larger aspect ratio (Table S1), which is different from Liang’s findings that GO could slightly increase both Ti and Td values due to different degrees of oxidation.11 In addition, the thermal oxidative stability of the GOD-reinforced PVA composite films in air also shows a similar tendency to that in nitrogen condition (Figure S14). To further understand the H-bond crosslink effect on thermal stability, we assume that both Tical and Tdcal of PVA/GOD composite films obey the linear mixing rules, as shown in Eqs. 1 and 2 below.42

Ti,calc = Ti, PVA × φw, PVA + Ti, GOD × φw, GOD (1) Td,calc = Td, PVA × φw , PVA + Td, GOD × φw , GOD (2) Where Ti, PVA and Ti, GOD respectively refer to the Ti of PVA and GOD, and ϕw,

PVA and ϕw, GOD

represent the mass fraction of PVA and GOD, respectively. The same applies to both Td, PVA and Td, GOD, and the subscript c means the composite. As shown in Figures 4c and 4d, the experimental values for both Ti and Td, namely Tiexp and Tdexp, deviate from their corresponding ones calculated by the mixing rules, especially when the loading level of GOD reaches 1.0wt% and higher. These large deviations between 13 ACS Paragon Plus Environment

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experimental and calculated values of both Ti and Td strongly suggest the suppression effect of H-bond crosslinked nanoconfinement on the elimination of hydroxyl side groups of PVA.57 Therefore, it is reasonable to conclude that it is primarily the H-bond crosslink rather than the thermal barrier of GOD that plays the key role in enhancing the thermal stability of PVA. Figure 5 The thermogravimetric analysis-infrared spectrum (TG-IR) analysis is also employed to explain the suppression effect of GOD on the thermal degradation of PVA. As shown in Figures 5a and 5b, the absorbance intensities of all degradation products, especially the O-H and C=O groups, of GOD-2.0 are much lower than those of the PVA matrix. This means that GOD-2.0 generates much fewer degradation products of the pristine PVA during heating. In particular, the production amount of the main degradation product, H2O, from GOD-2.0 is only 53% of that from the PVA matrix, as can be calculated by the area ratio of the characterization peaks (3580, 3736 and 3870 cm-1) of O-H groups (Figure 5c).57 Similarly, the PVA also shows a detectable absorption peak (3027 cm-1) of the unsaturated H-C=C groups due to the dehydration reaction, whereas this peak cannot be determined for GOD-2.0. Moreover, a similar phenomenon can also be observed for other small degradation products including saturated and unsaturated aldehydes and ketones (Figure S15). Generally, the thermal degradation of PVA generates volatile products through water elimination and chain scission based on a six-membered transition state.57 Therefore, the physical H-bond crosslink effect of GOD delays the elimination process of hydroxyl side groups and the chain scission of PVA by restricting the formation of a six-membered transition state, thus leading to the enhanced thermal stability, as proposed in Figure 5d. Figure 6 Table 2 Performance Comparison. The comprehensive mechanical and thermostable properties of as-prepared bioinspired GOD-reinforced PVA composite films are superior to some typical 14 ACS Paragon Plus Environment

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PVA composite films (Figure 6 and Table 2). As compared with other PVA composite films prepared by the same solution casting approach, our GOD-reinforced PVA composite film (GOD-2.0) shows comparable improvement in tensile strength (∆σ/σm %) (by ~66%, σ: 152.5 MPa), but much larger increases in both break strain (ductility) (∆ε/εm %) (by ~17%, ε: 27%) and thermal stability (especially the ∆Td, by 69 oC, Td: 355 oC) (Figure 6, Table 1, Table S1 and Table 2).11-21 As shown in Figure 6a and Table 2, the fillers such as GO,11 RGO,12 LDH,17 MMT,19 ND20 and sPPTA21 can increase the tensile strength (σ) by a comparable magnitude and unfortunately the resultant PVA nanocomposites show reduced ε to different extents. For instance, GO can increase the σ by 76% (87.6 MPa) but it decreases the ε by 65% (~8%) of PVA in addition to a slight increase in the Td value (5.5 oC).11 Despite a comparable increase in magnitude in the Td, both MMT18 and ND20 show relatively low reinforcement effectiveness in the σ (by 39% and 31%) in addition to reducing the ductility of PVA. Likewise, RGO12 and sPPTA21 respectively result in reduced strain at break (ε) by 68% and 67% as compared with the PVA matrix, although their composites show higher ε and toughness than our GOD-2.0. Moreover, RGO12 reduces the Td by 10 oC and sPPTA21 only leads to a 5 oC enhancement in Td of PVA, as compared to a 69 oC improvement in Td for our system. In fact, their presence makes PVA exihibit much lower Ti and Td values than our system (Table 2), indicating the superiority of GOD in terms of improving the thermal stability of PVA. Interestingly, the addition of 5.0wt% SWNTs is reported to increase the σ by 73% (~148 MPa) and ε by around 6.7%, respectively. Meanwhile, a 16 oC reduction in the Ti and a 20 oC decrease in the Td are also observed relative to the PVA matrix.15 These figures indicate that the presence of SWNTs makes the PVA behave in a thermally unstable manner, suggesting its adverse effect on the thermal stability of PVA. In addition, adding only 0.5wt% of dopamine-coated GO (dG-O) leads to a nearly 100% increase in the σ (up to 82.92 MPa) and a 90% increase in the ε value, respectively.58 Therefore, the above analysis fully demonstrates the advantage of artificial β-sheet, GOD, over other fillers in terms of improving 15 ACS Paragon Plus Environment

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the mechanical and thermal stability of PVA. As compared with some typical nacre-mimetic strong layered PVA nanocomposite films, our GOD-reinforced PVA composite film shows larger toughness (τ = 30.3 MJ/m3) and higher ductility (ε) of 27% in addition to comparable strength (Table 2). For instance, the layered PVA-Al2O3/GO film only shows a σ of 143 MPa, a ε of 17% and a τ of 9.2 MJ/m3,29 and the PVA/RGO system exhibits a relatively low ε of 4.1% and a τ of about 6.0 MJ/m3 despite a high σ value of 222 MPa.32 Similar phenomena are also found in the layered PVA nanocomposite films based on MMT, LDH and other kinds of nanofillers.31,33 Besides, some layered PVA nanocomposite films via chemical crosslinking even exhibit high strength, such as a σ as high as 360.7 MPa for the layered PVA/GO film crosslinked by boric acid (BA),31 and a σ of up to 400 MPa for the layered PVA/MMT film crosslinked by glutaraldehyde (GA).26 Similarly, both systems also show relatively low ductility (ε) and toughness (τ) (Table 2). Therefore, these nacre-mimetic layered polymer composite films show relatively low toughness despite high strength due to the mutually exclusive governing mechanisms between strength and toughness. Furthermore, some layered PVA nanocomposite films based on MMT,28 Al2O3/GO,29 and LDH33 become thermally unstable relative to the PVA matrix. Although the PVA/Al2O3 system shows increased Ti (~250 oC) by 25 oC and a relatively high toughness of 20.5 MJ/m3, it only has a low strength of 53 MPa.29 In brief, as-designed PVA/GOD nanocomposite film shows a better balanced mechanical and thermostable performance portfolio than its counterparts. In addition, the solution casting approach also has the advantages of facility and ease of mass production. Based on the above analysis, it is reasonable to conclude that the nanoconfinement plays a significant role in controlling the mechanical strength and toughness and even the thermal stability performance of the polymer/inorganic sheets nanocomposites.59 For the conventional polymer nanocomposites, the aspect ratio (S) of inorganic sheets determines the mechanical strength, fracture toughness and ductility of the confined polymer nanocomposites although 16 ACS Paragon Plus Environment

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the size and content of sheets also have a certain effect. Generally, the sheet with a S lower than the critical aspect ratio (Sc) enables the polymer nanocomposites to yield improved strength, ductility and toughness, as evidenced by the ductile PVA/LDH (a low S of only 2025)17 and the brittle PVA/GO (a high S of ≥2500).11 In the case of the nacre-mimetic layered polymer nanocomposites, both the nanoconfinement and hierarchical architecture govern the mechanical strength and toughness.58 The S of inorganic sheets plays a more important role in governing the fracture toughness of the nanoconfined polymer phase, in addition to certain contributions originating from such a layered structure. The polymer nanocomposites can show ductile fracture if the S of the inorganic nanosheet is lower than the critical aspect ratio, Sc (here, Sc =σp/τy, σp and τy refer to the tensile strength of nanosheets and the yield shear strength of the polymer phase).1 For this reason, both PVA/Al2O3 and chitosan/Al2O3 systems still show ductile fracture despite high contents of Al2O3 platelets (S < Sc).1,29 Therefore, the S of inorganic sheets generally governs the mechanical strength, toughness and ductility of the confined polymer nanocomposites providing that nanosheets can uniformly disperse within the polymer phase via strong interfacial interactions. Instead of the aspect ratio of the inorganic sheets, the nanoconfinement affects the thermal stability of the confined polymer nanocomposites through the interfacial interactions between inorganic sheets and the polymer phase. Especially for the PVA-based nanocomposites, this is mainly determined by the molecular degradation mechanism of the PVA, namely via the elimination of hydroxyl side groups,57 as observed in the thermostable PVA/ND20 and PVA/GOD systems via multiple Hbond interactions. Hence, mechanically robust and thermally stable PVA materials can be developed by taking advantage of nanoconfinement and H-bond crosslink.

CONCLUSION We have synthesized high-performance GOD-reinforced PVA composite films with good mechanical and thermostable performances by mimicking the H-bonded nanoconfinement

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structure of spider silk. The as-prepared GOD-2.0 shows a tensile strength as high as 152.5 MPa (the ultimate value up to 161.2 MPa), a high stiffness of 4.35 GPa and a great toughness of 30.3 MJ/m3, in addition to a Td as high as 355 oC (69 oC higher than that of the PVA matrix), which is superior to the PVA composite films containing other fillers by different preparation methods. Both nanoconfinement and H-bond crosslink effects are responsible for the combination of good mechanical and thermostable properties of as-developed PVA composite films. This work offers a facile bio-inspired methodology for the design of strong, tough and thermostable polymeric materials, which hold promise for electrical devices and tissue engineering applications, such as acritical skin for robots.

EXPERIMENTAL SECTION Raw Materials. Poly (vinyl alcohol) (PVA) (Mw: 89,000 ~ 98,000, 99% hydrolyzed), sulfuric acid (H2SO4), sodium nitrate (NaNO3) potassium permanganate (KMnO4), hydrochloric acid (HCl) and hydrogen peroxide (H2O2) were purchased from Sigma-Aldrich. Graphite with an average particle size of 45 µm and a purity of 99.8% was bought from Alfa Aesar Co., Ltd. All these chemicals were used as received. Synthesis of GOD. Graphene oxide (GO) was firstly prepared by the modified Hummer’s method from graphite flakes according to our previous work.60 Then, 1.0 g GO was further chemically cut by refluxing in a 100 ml HNO3 (6M) solution at 120 oC for 12 h. After that, GOD was obtained by removing the large size GO by centrifugation and by further dialyzing the resultant light-yellow supernatant using a filtering membrane with a cutoff molecular weight of 3 kDa (Amicon Ultra-4, Millipore) for 24 h.61, 62 Fabrication of PVA/GOD Composite Films. Typically, depending on predesigned formulations, a certain amount of PVA powder was completely dissolved in the distilled water by heating at 90 oC for 1 h, then the desired amount of GOD was added with the aid of magnetic stirring to prepare 8.0 wt% aqueous solution of PVA/GOD. The 8.0 wt% PVA

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aqueous solution was also prepared via the same protocol as a control experiment. After being cooled to room temperature, all the solutions were cast into Petri dishes followed by slowly air drying at room temperature for 7 h and 60 oC under reduced pressure for 24 h. Finally, all polymer films were peeled off for further measurements. As-prepared films are highly transparent, and the presence of GOD hardly affects the high transparency of PVA films, whereas the PVA composite film containing 1.0wt% of GO exhibits a lower transmittance. Characterization. All films and the infrared (IR) spectra of pristine MA using KBr disk method were recorded on a Bruker Vetex-70 IR spectrometer (Germany) using an attenuated total reflectance (ATR) mode to obtain spectra of high resolution. Transmittance spectra were recorded using a MAPADA UV-1800PC UV Spectrophotometer in the UV-visible region (200-1000 nm). A transmission electron microscope (TEM) and high-resolution TEM (HRTEM) imaging were performed on a JEOL 2100F microscope operating at 200 kV. For the TEM observation of composite films, the samples were obtained by freezing ultrathinned sectioning with a thickness of ~80 nm. The morphologies of the cross-section of composite films after tensile testing were observed on a field emission scanning electron microscope (FEI-SEM S4800) at an acceleration voltage of 5.0 kV. The molecular models of PVA/GOD nanocomposite films before, during and after tension were generated by the LAMMPS software package to display the trajectory of the molecular structures with the change in the strain (ε). The AFM measurement was performed on a Cypher atomic force microscope. Raman spectra of various samples were carried out with an ALMEGA-Dispersive Raman (Thermo Nicolet) with 514.5 nm excitation. X-ray diffraction (XRD) was carried out using a Rigaku Xray generator (Cu Ka radiation with λ = 1.54 nm). X-ray photoelectron spectroscopy (XPS) spectra were recorded on a Thermo ESCALAB 250 spectrometer. XRD measurements were performed on a PANalytical X’Pert PRO apparatus operating with Cu Kα radiation. The

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stress-induced crystallization of PVA and its composites were observed on a NIKON ECLIPSE LV100ND polarized optical microscopy (POM). The dynamic frequency scanning tests of all solutions were performed on a stresscontrolled TA Discovery HR-3 Rheometer (USA) equipped with DIN Concentric cylinders, Peltier steel (bob diameter: 27.97 mm, bob length: 42 mm). The steady shear data were collected in the shear rate range of 10-2~103 s-1, and the frequency sweep tests were conducted from 10-2~103 rad/s at a strain amplitude of 1.0% to ensure the measurements within the linear viscoelastic region of 10%. All experiments were carried out at a constant temperature of 23 ± 2 oC. Tensile tests of the films were completed using an Instron 30 kN Tensile Tester at 23 ± 2 o

C, and each sample was tested at a loading rate of 5 mm/min with a gauge length of 15 mm.

All samples were cut into strips with a length of 50 mm and a width of 2 mm. The glass transition temperature (Tg), melting point (Tm), enthalpy of fusion (∆Hf) and degree of crystallinity (χc) were determined on a TA Q200 differential scanning calorimeter (DSC, USA). Samples of about 8.0 mg were first heated to 260 oC at a heating rate of 20 o

C/min and kept at this temperature for 5 min (first scan) to eliminate the heat history,

followed by cooling samples to 25 oC at a cooling rate of 10 oC/min to detect the crystallization (second scan), then the samples were reheated to 260 oC at a heating rate of 10 o

C/min to determine Tg, Tm and ∆Hf. The χc values of PVA and its composite films were

calculated according to the result of DSC. Because GOD or GO had no contribution to the melt therapy, the χc of PVA inside the composite was calculated as χc = ∆Hf/(∆Hf0 × ω), where ∆Hf0 is the enthalpy of 138.6 J/g for a theoretical 100% crystalline PVA and ω is the weight fraction of PVA in the composites.21 Thermogravimetry analysis (TGA) was conducted on a Netzsch STA 409PC thermal analyzer at a heating rate of 20 oC/min in both nitrogen and air conditions at a temperature range of 50 ~ 600 oC. Thermogravimetric analysis coupled with infrared spectroscopy (TG-IR) measurements was performed with a TGA 209 F1 instrument (Netzsch, Germany), coupled 20 ACS Paragon Plus Environment

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with a Thermo Nicolet iS10 FTIR spectroscopy (Thermo Scientific, Germany). The volatiles developed from TGA can be transferred into the gas cell of FTIR through the transfer line by a suitable gas flow. Each specimen with 5 mg was examined by heating up to 600 oC at a heating rate of 10 oC/min. The spectra were obtained with a scan interval of 2.23 s. The resolution of the spectra was 4 cm-1. Theoretical Prediction of Interfacial Shear Strength (τy) and Modulus (Ec).

σ c =αφgσ g + (1 − φg )σ m α=

τ ys

2σ g

(3)

(4)

Ec =φg Eg + (1 − φg ) Em

(5)

As mentioned above, ϕg is the volume fraction of platelets, σ and E refer to the strength and elastic modulus, τy is either the interfacial shear strength (weak platelet/polymer interfaces) or the shear strength of the continuous matrix (strong platelet/polymer interfaces) whereas s (~6) is the aspect ratio of GOD. The subscripts g and m refer to GOD and the polymer matrix, respectively. The tensile strength (σg) can be roughly estimated according to the tendency of strength with oxygen content, giving a σg of 23 GPa (Figure S16).47 The τy value was calculated using above Eqs. 3 and 4.1, 63 According to the literature,47 the elastic modulus (Eg) of GOD was ~250 GPa, and the elastic modulus (Ec) of composite films was predicted by above Eq. 5. Calculation of Tensile Toughness (τ). The tensile toughness (τ) can be calculated by integrating the area under tensile curves, as expressed by following Eq. 6.42, 64 n

τ = ∑ σε i

(6)

i=0

where σ and ε respectively refer to the tensile stress and strain at failure. Supporting Information Available: TEM images, AFM, IR, XPS, Raman, XRD and TGA data of as-synthesized GOD, UV-vis transmittance and DMA of as-designed polymer 21 ACS Paragon Plus Environment

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composite films, the correlation between elastic modulus and degree of crystallinity, IR spectra on hydroxyl groups showing H-bond interactions, DSC, XRD, Raman and POM data of PVA and its composites, TEM and SEM images of GO-1.0, and TGA results (in air) of the PVA composite films, the prediction of theoretical strength of GOD, and detailed thermal properties of PVA/GOD nanocomposite films. This material is available free of charge via the Internet at http://pubs.acs.org. Conflict of Interest: The authors declare no competing financial interest. ACKNOWLEDGEMENTS Thanks are due to Miss F. Fang for helping measure the UV-vis spectra, to Miss L.J. Li for their kind help on the computational models, and to Dr. D. Liu for the chemical structure. This work was financially supported by the Scientific Research Foundation of Zhejiang A&F University (Grant No. 2055210012); the National Natural Science Foundation of China (Grant No. 51873196, 51628302, 51503181); Australia Research Council (Grant No. DP150100056), and Australia Research Council Industrial Transformation Training Centre (Grant No. IC170100032). ZGC thanks the USQ start-up grant and strategic research funds. REFERENCES (1) Bonderer, L. J.; Studart, A. B.; Gauckler, L. J. Bioinspired Design and Assembly of Platelet Reinforced Polymer Films. Science 2008, 319, 1069-1073. (2)

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(42) Song, P. A.; Xu, Z. G.; Guo, Q. P. Bioinspired Strategy to Reinforce PVA with Improved Toughness and Thermal Properties via Hydrogen-Bond Self-Assembly. ACS Macro Lett. 2013, 2, 1100-1104. (43) Song, P. A.; Xu, Z. G.; Lu, Y.; Guo, Q. P. Bio-inspired Hydrogen-Bond Cross-link Strategy Towards Strong and Tough Polymeric Materials. Macromolecules 2015, 48, 3957-3964. (44) Liu, F.; Jang, M. H.; Ha, H. D.; Kim, J. H.; Vho, Y. H.; Seo, T. S. Facile Synthetic Method for Pristine Graphene Quantum Dots and Graphene Oxide Quantum Dots: Origin of Blue and Green Luminescence. Adv. Mater. 2013, 25, 3657-3662. (45) Jin, S. H.; Kim, D. H.; Jun, G. H.; Hong, S. H.; Jeon, S. Tuning the Photoluminescence of Graphene Quantum Dots through the Charge Transfer Effect of Functional Groups. ACS Nano 2013, 7, 1239-1245. (46) Suzuki, N.; Wang, Y. C.; Elvati, P.; Qu, Z-B.; Kim, K.; Jiang, S.; Baumeister, E.; Lee, J.; Yeom, B.; Hahng, J. H.; Lee, J.; Violi, A.; Kotov, N. A. Chiral Graphene Quantum Dots. ACS Nano 2016, 10, 1744-1755. (47) Liu, L. Z.; Zhang, J. F.; Zhao, J. J.; Liu, F. Mechanical Properties of Graphene Oxides. Nanoscale 2012, 4, 5910-5916. (48) Yang, G. H.; Wan, X. J.; Liu, Y. J.; Li, R.; Su, Y. K.; Zeng, X. R.; Tang, J. M. Luminescent Poly(vinyl alcohol)/Carbon Quantum Dots Composites with Tunable Water-Induced Shape Memory Behavior in Different pH and Temperature Environments. ACS Appl. Mater. Interfaces 2016, 8, 34744-34754. (49) Kashiwagi, T.; Du, F. M.; Douglas, J. F.; Winey, K. I.; Harris Jr, R. H.; Shields, J. R. Nanoparticle Networks Reduce the Flammability of Polymer Nanocomposites. Nat. Mater. 2005, 4, 928-933. (50) Song, P.A.; Xu, L. H.; Guo, Z. H.; Zhang, Y.; Fang, Z. P. Flame-Retardant-Wrapped Carbon Nanotubes for Simultaneously Improving the Flame Retardancy and 27 ACS Paragon Plus Environment

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Mechanical Properties of Polypropylene. J. Mater. Chem. 2008, 18, 5083-5091. (51) Rittigstein, P.; Priestley, R. D.; Broadbelt, L. J.; Torkelson, J, M. Model Polymer Nanocomposites

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Exfoliated Graphene-Based Polypropylene Nanocomposites with Enhanced Mechanical and Thermal Properties. Polymer 2011, 52, 4001-4010. (61) Zheng, X. T.; Than, A.; Ananthanaraya, A.; Kim, D. H.; Chen, P. Graphene Quantum Dots as Universal Fluorophores and Their Use in Revealing Regulated Trafficking of Insulin Receptors in Adipocytes. ACS Nano 2013, 7, 6278-6286. (62) Sun, Y. Q.; Wang, S. Q.; Li, C.; Luo, P. H.; Tao, L.; Wei, Y.; Shi, G. Q. Large Scale Preparation of Graphene Quantum Dots from Graphite with Tunable Fluorescence Properties, Phys. Chem. Chem. Phys. 2013, 15, 9907-9913. (63) Qian, M. B.; Sun, Y. Q. Xu, X. D.; Liu, L. N.; Song, P. A.; Yu, Y. M.; Wang, H.; Qian, J. 2D Alumina Platelets Enhance Mechanical and Abrasion Properties of PA-612 via Interfacial Hydrogen-Bond Interactions. Chem. Eng. J. 2017, 308, 760-771. (64) Song, P. A.; Xu, Z. G.; Dargusch, M. S.; Chen, Z. G.; Wang, H.; Guo, Q. P.Granular Nanostructure: A Facile Biomimetic Strategy for the Design of Supertough Polymeric Materials with High Ductility and Strength, Adv. Mater. 2017, 29, 1704661.

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Caption of Figures Figure 1. Bioinspired design of strong, tough and thermostable polymer composite films. Schematic representation for a) the tensile behavior and b, c) hierarchal microstructure of spider silk,20-25 d) the fabrication process of graphene oxides dots (GOD)-reinforced PVA composite films, e) high-resolution transmission electron microscopy (HRTEM) image of GOD with e1) selected area electron diffraction (SAED) inserted on the top-right corner, f) representative TEM image, and g) computational simulation snapshot for the microstructure of the PVA composite film (GOD-2.0). Figure 2. Mechanical performances of as-prepared biomimetic polymer composite films. a) Typical stress-strain curves of as-prepared bioinspired PVA composite films as a function of GOD content as well as the PVA composite film (GO-1.0) containing 1.0 wt% graphene oxides (GO), b) yield strength (σy), c) the plots of experimental moduli (E) and theoretical moduli predicted by the linear mixing rule, and d) strain at failure (εb) and toughness (τ) of PVA and its composite films as a function of GOD volume fraction (Vol.%) as well as GO-1.0. Figure 3. Mechanical reinforcement mechanisms of as-prepared GOD-reinforced PVA composite films. a) Frequency dependence of storage modulus (G′) for the aqueous solutions of both PVA and its composite, b) the wavenumber shift (∆υO-H), and c) the glass transition temperature (Tg) difference (∆Tg) of the PVA composite films with respect to that of the PVA matrix (the ∆Tg values marked by red and blue circles were respectively obtained by dynamic mechanical analysis (DMA) and differential scanning calorimetry (DSC) tests), d) comparison of Raman shift of C-O bending vibration peak (~1095 cm-1) as a function of tensile strain for the PVA, GOD-2.0 and GO-1.0, e) schematic illustration of stress transfer via H-bond interactions between GOD and PVA, f) polarized optical microscopy image of the surface of GOD-2.0 after tensile failure, g-i) scanning electron microscope (SEM) images for the typical fracture morphology of GOD-2.0 at different magnifications after tensile tests, and j) 30 ACS Paragon Plus Environment

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computer-aided illustration for the mechanical failure process models of as-designed PVA/GOD composite films. Figure 4. a) Typical thermogravimetric analysis (TGA) and b) derivative thermogravimetric weight loss (DTG) curves of the PVA bulk and its composite films, c and d) comparison of experimental values with calculated values of both the initial decomposition temperature (Ti) and maximum decomposition temperature (Td) from both experimental (Tiexp, Tdexp) and theoretical values (Tical, Tdcal). Figure 5. Three-dimensional TG-IR spectra for a) the PVA matrix and b) GOD-2.0, c) IR spectra of PVA and GOD-2.0 at 400 oC in N2 atmosphere and the belongings of degradation products, and d) illustration of proposed mechanism for the suppression effect of GOD on the elimination reaction of side hydroxyl groups of PVA. Figure 6. a) Comparisons of strength increment, ∆σ/σ % and ductility change, ∆ε/ε %, and b) the thermostability change (the maximum thermal degradation enhancement, ∆Td) of asprepared GOD-reinforced PVA composite film (GOD-2.0) with the PVA composite films reinforced with other fillers including GO,11 SWNTs,14 LDH,17 MMT,18,19 ND,20 and sPPTA21 prepared by the solution casting approach.

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

Table 1 Detailed tensile performances of as-prepared GOD-reinforced PVA composite films. GOD

GOD

σy a

Ea

εb a

τa

(MPa)

(GPa)

(%)

(MJ/m3)

91.8 ± 2.4

2.32 ± 0.15

23 ± 3

16.8±1.4

Run (wt%) (Vol.%)

a

PVA

0

0

GOD-0.5

0.5

0.29

122.2 ± 5.7 2.95 ± 0.20

30 ± 5

28.2±2.0

GOD-1.0

1.0

0.58

136.3 ± 6.4 3.76 ± 0.19

28 ± 5

28.6±2.4

GO-1.0 a

1.0

0.58

122.7 ± 4.9 3.22 ± 0.23 6.1 ± 4 5.15±1.2

GOD-2.0

2.0

1.16

152.5 ± 8.7 4.35 ± 0.20

27 ± 4

30.3±3.2

GOD-5.0

5.0

2.95

138.8 ± 5.6 5.08 ± 0.25

25 ± 5

26.5±2.5

σy, E, εb and τ refer to the yield strength, elastic modulus, strain at failure and tensile

toughness, respectively; GO-1.0 represents the PVA composite film containing 1.0wt% of GO.

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Table 2 Comparison of mechanical and thermostable performances of as-prepared PVA/GOD film with some typical PVA nanocomposite films based on different fillers prepared by different fabrication approaches. Filler type

σa (MPa)

∆σ/σma (%)

εa (%)

∆ε/εm a (%)

τ (MJ/m3)

Ti a (oC)

∆Ti a (oC)

Td a (oC)

∆Td a (oC)

GOD

152.5

+66

27

+17

30.3

272

+8

355

+69

GO RGO Graphene SOH SWNTs FWNTs LDH MMT ND sPPTA dG-O MMT MMT MMT

87.6 158 ~175 107 148 132.6 98 39 124 169 82.92 150 400 165

+76 +66 +75 +45 +73 +65 +54 +39 +31 +54 +100 +275 +900 —

~8 48 ~7.0 40 48 4 71.6 186 16 44 184 0.70 0.33 1.70

-65 -68 -20 -33 +6.7 -50 -47 -17 -77 -67 +90 -97 -97 —

~2.2 — ~6.8 — ~56 ~2.7 ~35 ~68 ~16 ~63 — ~0.53 ~0.66 ~1.9

274 ~226 — — 219 — ~250 — ~270 155 — ~230 ~205 —

+3 0 — — -16 — -23 — +12 0 — — -25 —

316 ~250 — — ~248 — 270 — ~350 269 — — — —

+5.5 -10 — — -20 — -27 — +50 +5 — — — —

MMT

141



0.50



~0.49

~140



~190



Al2O3 Al2O3/GO GO GO RGO RGO LDH

53.0 143 80.2 360.7 222 160 169

+33 +257 +186 +720 +146 +78 +323

57.1 17 0.25 1.2 4.1 4.4 2.1

-71 -99 -98 -99 -99 -99 -99

20.5 9.2 ~0.15 ~1.8 ~6.0 ~6.1 ~1.77

~250 ~160 ~106 — — — 205

+25 -65 -150 — — — -40

— — ~190 — — — ~292

— — -110 — — — -20

a

Fabrication approach

Ref.

Solution casting

This work 11 12 13 14 15 16 17 18 20 21 58

LBL LBL+GA VAF Doctor-bladed + GA

29

VAF VAF + BA LBL + GA LBL LBL

30 31

break strain, increment (%) of break strain and toughness of PVA nanocomposite films. The symbols “+” and “-” represent the increase and decrease, respectively. LBL and VAF refer to layer-by-layer assembly and vacuum-assisted flocculation, respectively. “+ GA” and “+ BA” mean that the fabrication process contains the chemical crosslink with glutaraldehyde (GA) and boric acid (BA), respectively. The subscript, m, represents the polymer matrix. Ti and Td respectively represent the initial degradation temperature where 5 wt% weight loss takes place, and the maximum degradation temperature where the maximum weight loss occurs. ∆Ti and ∆Td refer to the difference between the Ti and Td of composites and the corresponding value

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LBL

σ, ∆σ/σm, ε, ∆ ε/εm, and τ refer to ultimate tensile strength, increment (%) of tensile strength,

of the PVA matrix, respectively.

26

32

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