Sheet Collapsing Approach for Rubber-like Graphene Papers Youhua Xiao, Zhen Xu,* Yingjun Liu, Li Peng, Jiabin Xi, Bo Fang, Fan Guo, Peng Li, and Chao Gao* MOE Key Laboratory of Macromolecular Synthesis and Functionalization, Department of Polymer Science and Engineering, Key Laboratory of Adsorption and Separation Materials & Technologies of Zhejiang Province, Zhejiang University, 38 Zheda Road, Hangzhou 310027, People’s Republic of China S Supporting Information *
ABSTRACT: Understanding and modulating the conformation of graphene are pivotal in designing graphene macroscopic materials. Here, we revealed the sheet collapsing behavior of graphene oxide (GO) sheets by poor solvents in an analogy with linear macromolecules. Triggered by poor solvents, extended GO sheets in good solvents can collapse to hierarchically wrinkled conformations. The collapsing behavior of GO enabled the fabrication of amorphous selfstanding GO and graphene papers with rich hierarchical wrinkles and folds over mutliple size scales. The collapsed GO and graphene papers had a rubber-like mechanical behavior with viscoelasticity. By our collapsing method, GO and graphene self-standing papers were designed to be stiff with high modulus or to become soft with low modulus of 100 MPa at a remarkably large breakage elongation up to 23%. Our philosophy of treating graphene as a 2D polymer enables the efficient control of molecular conformations of graphene and other 2D polymers and the design of macroscopic materials of 2D nanomaterials as in the polymer industry. KEYWORDS: sheet collapsing, rubber-like, graphene paper, viscoelasticity, wrinkles
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analogy with polymeric materials, the rich conformations of 2D constituent graphene sheets intuitively determine the overall performances of graphene macroscopic materials that are exemplified by fibers,18,21−26 films,27−31 and aerogels.32 In this context, the efficient control of conformations of graphene is becoming critical in designing properties and functionalities of graphene materials; however, this still remains a great challenge. In the fabrication of graphene papers/films, the importance of the conformation of constituent graphene sheets has begun to be recognized in controlling multiscale structures and performances of films. To date, two strategies have been specifically directed to control the conformation of graphene. One is the flow-induced filtration to guide individual GO or chemically reduced graphene (CRG) sheets to form regular and compact alignment, resulting in stiff graphene papers with high strength and modulus.26,27 The other one is the post-buckling of graphene films/papers on prestrained or responsive substrates.13−17,33−36 This strategy has been used to prepare highly wrinkled graphene coatings on stretchable supporting matrix33−35 and thermally responsive substrates36 and selfstanding graphene papers with good flexibility detached from
nderstanding the conformation of 2D membranes and surfaces is of great significance in multiple disciplines extending from biology to materials.1,2 In the past 30 years, the statistical mechanics of membranes and surfaces have been established, and many interesting behaviors have been predicted and understood,3−8 for example, the rigidity-related crumpling transition of tethered surfaces,3−5 adhesion between vesicles, and the swelling of lamellar phases.1 Recently, it has become important to investigate the conformations of 2D macromolecules, such as graphene and other newly emerged 2D nanomaterials, for their synthesis, assembly, and macroscopic materials. Graphene, consisting of repeatedly connected benzene monomers in two-dimensional topology, has emerged as a phenomenal 2D macromolecule with remarkable properties, such as high mechanical strength and the record-high electrical/thermal conductivities.9−12 Graphene and its chemical derivatives have exhibited interesting behaviors of a 2D macromolecule in analogy with 1D linear counterparts. For example, single-layer graphene possesses rich conformations including ripples, wrinkles, and folds that possibly follow a universal scale law of self-avoiding macromolecules.6−8,12−18 In dispersions, the collapsing of GO sheets was observed by tuning pH values to weaken the electrostatic interactions.7,8,19,20 Graphene and GO sheets can organize together to form lyotropic liquid-crystalline mesophases.21−23 As a close © 2017 American Chemical Society
Received: April 27, 2017 Accepted: August 4, 2017 Published: August 4, 2017 8092
DOI: 10.1021/acsnano.7b02915 ACS Nano 2017, 11, 8092−8102
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Figure 1. Analogy between linear and 2D macromolecules and their materials. (A) Linear macromolecule chains have conformations of random coil in good solvents and extended chains. These extended states can be processed into collapsed chains to obtain amorphous polymers. (B) As a close analogy, 2D macromolecules can exhibit an extended state with ripples in their good solvents, just as in the case of GO in water and good polar solvents. The extended state can also endure the process of sheet collapsing to form collapsed sheets full of wrinkles, which can be assembled into amorphous materials. The different colors in the amorphous polymer and amorphous materials have no special meanings and were set to make the figure more distinctive.
Figure 2. (A) Phase diagram of GO concentration versus EA volume percentage (center panel). The corresponding photos of four phases under natural light and between crossed polarizers (surrounding panels labeled by triangle, pentagon, diamond, and star). Optical microscopy images of extended GO sheets deposited from neat DMF dispersion (B) and atomic force microscopy image of GO “crumpled paper ball” deposited from EA/DMF (80 vol % of EA) dispersion on the silicon substrate (C). TEM images of homogeneously dried GO film from pure DMF dispersion (10 mg/mL) (D) and collapsed GO thin film after EA soaking (E).
rubber substrates.17 Beyond these methods aided by external forces, one should wonder that whether it is possible to control the conformation of solvated graphene sheets in a more efficient way by employing its own macromolecular characteristics in the frame of polymer theory and technology.
For linear polymers, the chain collapsing transforms extended polymer chains into collapsed states with more disorder and enhanced entanglements (Figure 1A). For example, expanded chains in good solvents can shrink into compact coils after being switched to poor solvents, and 8093
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Figure 3. (A) Illustration of the preparation process of collapsed GO paper, including the casting of GO/DMF solution with blade, EA soaking, and the drying of GO self-standing gel film. (B) Corresponding photographs of three steps in (A).
picked DMF and ethyl acetate (EA) as good and poor solvents, respectively. We depicted the phase diagram of GO related to the volume percentage of EA and the concentration of GO (CGO), as shown in Figure 2A. To depict the phase diagram, EA was introduced into GO DMF dispersions with CGO ranging from 0.001 to 10 mg/mL, and the dispersive states were examined by polarized optical microscopy (POM) and optical microscopy (see the detailed description of phase diagram and Figures S2−S4 in the Supporting Information). In the phase diagram of GO (Figure 2A), four states were identified, which are isotropic (I), coexistence of isotropic and nematic (I+N), nematic (N), and collapsed solid (S). GO sheets have an average lateral size of 25 μm (Figure S1), and its highly asymmetrical attribute determines the formation of liquid crystals (LCs), which was extensively investigated in many previous studies.21,22 In pure DMF, as CGO increased, the dispersive system evolved from I phase with chaotic distribution to I+N phase and N phase with regular alignment of constituent GO sheets. Previous studies found that GO sheets in these three states can keep its extended planar topology for the strong solvation with polar solvent molecules and longrange repulsive interactions.18,22 We further analyze the phase diagram following the increasing EA ratio (Figure 2A and Figures S2−S4). (i) In dilute dispersions (CGO < 0.01 mg/mL), I phase transits to collapsed S state directly at the EA ratio of 35% approximately. In the CGO range from 0.02 to 0.03 mg/mL, I phase turns into I +N phase first at the EA ratio about 20−30% and then S state at an EA ratio higher than 35%. (ii) I+N phase transits to S state directly at the EA ratio of 35−40%, as demonstrated by the fading of Schilieren optical textures under crossed polarizers. (iii) The N phase also transits to S phase directly, whereas the transition EA ratio increased sharply from 40 to 75% as CGO increased from 0.3 to 3 mg/mL. This trend can be possibly explained by the gelation effect by poor solvents in concentrated GO dispersions. To closely examine the collapsed behavior, it was found that CGO determined the conformation of collapsing sheets, including individual crumpled paper balls, agglomerations, and gels, similar to the collapsed behavior of linear polymers. In extremely dilute dispersions (CGO < 0.02 mg/mL), individual
straightly aligned chains in crystalline plastics can melt into disordered arrangement to form amorphous materials.37 As a close analogy, the sheet collapsing of 2D macromolecules should turn extended sheets merely with ripples into collapsed sheets full of wrinkles and folds, possibly offering an efficient method to modulate the conformation of 2D macromolecules to control the structure and performance of their macroscopically assembled materials (Figure 1B). Here, we revealed the sheet collapsing behavior of GO sheets by poor solvents and present a method to efficiently control the molecular conformation of graphene sheets in macroscopic materials. Employing the collapsing behavior of GO sheets, we prepared their macroscopic papers with rich hierarchical wrinkles and folds ranging from nanoscale to macroscale. The collapsed GO papers possessed an amorphous structural attribute and exhibited a rubber-like mechanical behavior. By our method, GO self-standing papers were designed to be stiff with high modulus of 1 GPa at about 5% elongation or to be soft with low modulus of 100 MPa at a large breakage elongation up to 23%, switching from hard materials like polyimides to soft rubbers. The stretching of wrinkles and their intertwined networks account for their rubber behavior. The philosophy of 2D polymers can be referred to as control of the molecular conformation of graphene sheets and design properties of graphene macroscopic materials in a large scale as in the polymer industry. This method may be applicable to other 2D nanomaterials such as MoS2,38 carbon nitride,39 and newly synthesized 2D polymers,12,40−42 by choosing the combination of their good and poor solvents.
RESULTS AND DISCUSSION Collapsing Behavior of GO by Poor Solvents. As an important derivative of graphene, GO can be regarded as a giant 2D macromolecule with pendant oxygen-containing groups on its planar molecular backbone. Due to the rich functional groups, GO is well-dispersed in good solvents with strong polarity, such as water and N,N-dimethylformamide (DMF), and exhibits an extended conformation by keeping 2D topology with only minor ripples.8,18 In analogy with linear polymers, as poor solvents are introduced, the extended GO sheets should evolve to the collapsed state. In this study, we 8094
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Figure 4. Track of conformation change of GO film during gelation and drying process. POM images of GO sol film cast on the silicon substrate (A, EA soaking 0 min) and the gel film after soaking in EA for 20 min (B). POM image of dried GO self-standing gel film under polarized light transmission (C) and the corresponding transmission image (D) with distinctive shrinkage. Corresponding proposed models of GO film’s cross section in LC solution, gelation, and drying process (E). L and T indicate the length and the thickness of films. The contraction ratios of length during gelation and drying process are about 8 and 13%, respectively.
materials, such as wet-spun fibers and films.31 We designed an experimental program to fabricate collapsed GO and graphene papers from GO gel phases, as shown in Figure 3 and Figure S11. GO/DMF liquid-crystalline dispersions were blade cast onto polytetrafluoroethylene (PTFE) substrates to form jelly films followed by soaking in the EA pool to promote the collapsing process. After gelation in EA, the jelly coating turned into self-standing gel films with a shrinkage in length of about 8%. Self-standing gel films were hanged and dried at 70 °C to get flexible rubber-like GO papers with a further shrinkage in length of about 13% (Figure S12). It was found that thicker GO gel films shrank more than thinner ones (Figure S13). The stretching of self-weight during hang-drying was set to eliminate the random shrinkage and ensured the planeness of GO papers. Aside from EA, acetone can work as another poor solvent of GO, and collapsed GO papers by pure acetone and EA had a high ductility (Figure S14). We tracked the real-time structural evolution of collapsing behavior in the gelation and drying process by POM and optical microscopy. During gelation, the Schilieren texture of the lyotropic liquid-crystalline GO/DMF sol film gradually weakened with the shrinkage of liquid-crystalline domains. The complete gelation still kept the Schilieren texture to some extent, denoting the remaining ordering in the gel (Figure 4A,B). Additionally, the transparent coating became opaque due to the strong light scattering induced by hierarchical wrinkles of collapsed GO sheets, as observed in both reflection and transmission modes under optical microscopy (Figure S15). During the following drying process, the collapse resulting from the EA evaporation generated an intertwined wrinkles network spreading on the surface and inside (Figure 4C,D and Figures S16 and S17). The multiscale hierarchy of the wrinkles originated from the multimodes of collapsing behavior, including the shrinkage-induced microscale folding with the alignment perpendicular to the shrinkage direction,17 the random microfolding by merging boundaries of liquidcrystalline domains,46 and the disordered nanoscale wrinkles of GO sheets.14,15 We proposed a corresponding model of the formation of collapsed GO paper, and this model encompasses the formation of gelation by poor solvent exchange and the
extended GO sheets (Figure 2B) with large width collapsed into “crumpled paper balls” of much smaller size (Figure 2C and Figures S5 and S6). From the statistics in scanning electron microscopy (SEM) inspections (Figure S6), the average width of collapsed GO sheets is around 8 μm, about one-third of that of extended GO sheets (25 μm). The collapsed GO sheets exhibited rich wrinkles and folds when deposited onto silicon substrates, whereas extended GO sheets had smooth morphology (Figure S6). A close inspection by atomic force microscopy (AFM) demonstrated that the wrinkles of collapsed GO sheets have a wide distribution of height from 3 to 20 nm, corresponding to a complicated deformation pattern as collapsed (Figure S7). Aside from the poor solvent-induced collapsing we introduced here, the crumpling and collapsed behavior of GO was earlier observed by Wen et al. in the aqueous dispersion by tuning the pH value to weaken the electrostatic interaction.8 By increasing CGO to 0.05−1 mg/mL, clouded agglomerations emerged under optical microscopy, denoting the interconnection between collapsed individual GO sheets (Figure S8). As CGO scaled up above 3 mg/mL, the collapsed effect reflected in a gelation transition. Interestingly, GO dispersions with CGO higher than 3 mg/mL apparently showed a pseudogel state as a jamming gel, which typically can hold the shape while still but flow under external forces with maintaining the homogeneity.43−45 After introducing EA, a real gelation transition happened and the storage modulus increased by 3 orders of magnitude (Figures S9 and S10). In the gelation state, GO sheets become collapsed with local wrinkles and interconnect together to form a framework, while still keeping the liquid-crystalline alignment order to some extent. As demonstrated by TEM inspection, the collapsed GO gels exhibited rich wrinkles after being dried on the copper grid, whereas the homogeneously dried neat GO/DMF film exhibited smooth morphology with much less wrinkles (Figure 2D,E). Collapsing GO into Rubber-like Self-Standing Papers. In the complete phase diagram of GO, the poor solventinduced gelation at high CGO in EA is an important stage of sheet collapsing behaviors. This gel state with high storage modulus facilitates the fabrication of GO macroscopic 8095
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Figure 5. SEM comparative images of homogeneously dried graphene paper (surface, A; cross section, B and C) and collapsed graphene paper (surface, D; cross section, E and F).
structure of collapsed graphene paper can be analogous to the amorphous polymer (Figure S19). Rubber-like Mechanical Behavior. The tensile tests demonstrated that collapsed GO papers had an outstanding softness and flexibility, and their breakage elongation reached up to 23% (Figure 7A). This high failure elongation is 1 order of magnitude higher than that (2% as usual) of vacuum-filtrated GO papers,28,29 4 times larger than that (about 5%) of homogeneously cast self-standing GO films, and higher than that of nacre mimic GO composite films (10%).30 After chemical reduction, graphene papers still kept the flexibility and held a considerably high breakage elongation up to 15% (Figure S20A). The high ductility of collapsed graphene papers was reflected in the rough fracture lines and distinctive slide fringes of graphene sheets at fracture tips (Figure 7C−F), which are different from the straight fracture lines of the homogeneously dried graphene paper (Figure S21). Collapsed GO and graphene papers exhibited a good softness with moderate moduli of 0.1 and 0.3 GPa, respectively (Figure S20B), which deviate from the high modulus (1 GPa) of homogeneously dried graphene paper and filtrated film (around 30 GPa).29,30 The modulus of GO papers was tuned in a wide range from nearly 1 GPa to 100 MPa through controlling the collapsing behavior of GO sheets in the fabrication of GO papers (Figure S20D). The softness of collapsed graphene papers was confirmed by dynamic thermomechanical analysis (DMA). Their storage moduli (G′) were an order of magnitude lower than that of homogeneously cast graphene paper on a glass substrate (Figure 7B). G′ values increased along with the increasing frequency from 0.5 to 5 Hz, possibly because the adaptation of wrinkled units to stress lags behind the oscillation (Figure S22). The frequency dependence of modulus denotes that the deformation of a multiscale wrinkled structure can be dynamically controlled in a time spectrum, similar to the dynamic behavior of polymer materials.37 The softness and extreme flexibility result from the amorphous structure full of multiscale wrinkles, and the connection thereof will be analyzed in the following semi-in situ SEM inspection. The tensile curves of collapsed GO and graphene papers demonstrate a rubber-like behavior (Figures 7a and S20). In the
collapsed drying to form multiscale wrinkles (Figure 4E). As a comparison, the drying process of GO/DMF cast paper exhibited no fading phenomenon of Schilieren texture at the beginning of 10 h and no formation of multiscale crumples (Figure S18). Structure Analysis. The collapsing of GO sheets by poor solvent generates huge amounts of wrinkles on the surface and in the interior core of papers. Apparently, the collapsed GO and graphene papers have a matte appearance for the diffuse reflection of surface wrinkles. By contrast, the homogeneously dried or vacuum-filtrated GO papers are lustrous because of the smooth surface.28,29 Further examined by SEM, collapsed GO and graphene paper had a highly wrinkled conformation on the surface and a flexural alignment morphology on the section (Figure 5), compared with the planar surface and interior straight alignment of graphene sheets of homogeneously dried papers. The wrinkles of graphene can create local curvatures and disturb the regular stacking and, as a result, brought decreasing densities (0.7−0.9 g/cm3) of collapsed GO papers from the density (1.21 g/cm3) of homogeneously dried GO papers (Table S1). The EA collapsed graphene paper possessed hierarchical wrinkles with fractal characteristics extending from hundreds of micrometers to dozens of nanometers. In spite of different scales, these buckling structures exhibited a good self-similarity in shapes, and their formation possibly conforms to the same pattern (Figure 6A,B).47−50 The buckling structures encompass independent ripples, wrinkles, ridges, and merged vertices, as the instability becomes intensified. We pondered that these buckling attributes were produced by the entropy-derived collapsing of GO sheets and the capillary force induced fluctuation in the dry process, originating from unbalanced inplane elasticity and out-of-plane bending rigidity.48−50 These intertwined wrinkles act as homogeneous flexible units to adapt to external tensile forces (Figure 6C). The collapsed conformation of GO and graphene sheets generated a loose packing, and the solid papers had an amorphous attribute, as shown by their XRD patterns (Figure 6D,E). Notably, these collapsed papers had only approximately 14−16% crystalline degree (calculated from the integrated crystalline peak area) of that of the homogeneously dried graphene papers. Amorphous 8096
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Figure 6. (A) Multiscale crumples of collapsed graphene sheets with a fractal structure in the rubber-like graphene papers. SEM image of the multiscale wrinkles on the surface (B) and the cross section (C), corresponding to the model in A. The circles in B denote the similar merged vertices in different scale. The XRD patterns of homogeneously dried GO paper and collapsed GO paper (D) and the corresponding graphene papers (E).
case of GO papers, the tensile curve can be divided to three stages: the beginning elastic region (
