Reduced Graphene Oxide Nanohybrid Films with

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Ductile Polyimide/Reduced Graphene Oxide Nanohybrid Films with Porous Structure Fabricated by a Green Hydrogel Strategy Zhao Chen, Faqin Tong, Dandan Zhu, Xuemin Lu, and Qinghua Lu ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00234 • Publication Date (Web): 22 Mar 2019 Downloaded from http://pubs.acs.org on March 22, 2019

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ACS Applied Polymer Materials

Ductile Polyimide/Reduced Graphene Oxide Nanohybrid Films with Porous Structure Fabricated by a Green Hydrogel Strategy Zhao Chen,a Faqin Tong,b Dandan Zhu,a Xuemin Lu,a and Qinghua Lu a,b* a

School of Chemistry and Chemical Engineering, State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, China.

b

School of Chemical Science and Engineering, Tongji University, Shanghai 200092, China *Email: [email protected]

ACS Paragon Plus Environment

ACS Applied Polymer Materials 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT. Polyimide/inorganics hybrid films fabricated by conventional approaches, including in-situ polymerization method, the sol-gel process and solution blending method, have a dramatic drop in malleability. In this work, hydrogel strategy for preparing ductile polyimide/reduced graphene oxide (PI/rGO) nanohybrid films with internal porous structures had been proposed. The hydrogel strategy involved two steps: the polyamide acid carboxylate/GO (PAAC/GO) films were firstly prepared by tape casting and followed by a gelation process of PAAC/GO aqueous solution; the PI/rGO films were then obtained after an imidization reaction of the PAAC/GO films. Interestingly, in this strategy, the PAA chains tended to self-assemble into microdomain crystalline structures assisted by their interactions with water, as revealed by XRD and Raman spectroscopy. The present PI/rGO films display more uniform microstructures, without aggregation of the rGO nanosheets. Furthermore, they exhibited an enhanced toughness property, with high elongation at break of 37–55%. In addition, their dielectric property is more stable and less frequency-dependent due to the internal porous structure. This hydrogel strategy would appear to be a promising candidate for fabricating hybrid PI–inorganic composite materials, even other hybrid organic/inorganic composite materials with specific features.

KEYWORDS. Polyimide, GO, hydrogel, organic-inorganic composite, hybrid film.


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Introduction Polyimides (PIs) are materials with good mechanical property, excellent chemical stability, outstanding thermal stability, as well as dramatic dielectric properties.1-3 The PI products have been extensively applied in optical materials, gas separation membranes, aviation, aerospace, microelectronics, and battery applications.4-10 In the past 30 years, with the rapid development of nanoscience and nanotechnology, considerable attention has been paid to preparing hybrid composites of PIs and inorganic materials in order to improve specific properties. These hybrid PI-based materials, benefiting from both the organic and inorganic components, display enhanced mechanical properties, lower coefficients of thermal expansion, and improved thermal resistance.11-17 There are three commonly used methods for fabricating polyimide/inorganics hybrid materials: in situ polymerization, the sol-gel process, and simple solution blending. In most cases, the derived polyimide/inorganic hybrids show improved tensile modulus, a more hydrophobic surface, increased dielectric constant, and enhanced thermal stability.13, 14, 18 Lu et al.19 compared PI/titanium dioxide (PI/TiO2) nanohybrid films prepared by in situ polymerization, sol–gel, and in-sol methods, and found the mechanical and electrical properties to be sensitive to the processing methods and the dispersion of nano-TiO2 in the PI matrix. Commonly, in situ polymerization or the sol–gel method generates good homogeneous dispersion systems. For instance, PI/silica hybrid materials prepared by in situ or sol–gel methods have been widely reported to show improved thermal, mechanical, and dielectric properties.20-23 Combinations with other inorganics, such as metal oxides,24, 25

carbon nanotubes,26 or graphene oxide (GO),27 in the PI matrix have been surveyed

in many cases. The PI/inorganic hybrid materials prepared by in situ polymerization or the sol–gel method have shown the most stable, homogeneous mixture properties of both the organic matter and inorganic components.28-30 In essence, the in-situ polymerization and sol–gel methods for preparing PI/inorganic hybrid materials are based on modifying the inorganic component with functional groups, such as amine or hydroxyl. The functionally modified inorganics



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may be well dispersed into organic solutions and further copolymerized into PI chains (in–situ polymerization) (in-sol method).19,

35-37

31-34

or mixed with PAA solution to form a stable mixture

The modification of inorganic matters with indeterminate

numbers of functional groups leads to polydispersion of the molecular weight or aggregation during the solvent evaporation process or microphase separation of the organic phase and inorganic phases. Consequently, the corresponding compact structures may be impacted by silica spheres or other inorganic matter, which might become stress concentrative points.38-41 Thus, the obtained PI/inorganics composite films normally exhibited brittleness, with a decreased elongation at break of less than 10% as the content of inorganic matter is increased, which has limited their practical applications.42-44 With the aim of circumventing the miscellaneous modification of inorganic matter and obtaining stable homogeneous PI/inorganics composites, in the present work, a novel hydrogel strategy for preparing PI-based composite films has been developed. It is known, polyamide acid (PAA, precursor of PI) can be dissolved in water with triethylamine (TEA) by forming polyamide acid carboxylate (PAAC).45-47 The environment-friendly aqueous solution has been reported to prepare polyimide nanofibers by electrospinning.45, 47 Liu et al. have developed a PI aerogel from the aqueous

solution

by thermoregulation

and

freeze-drying.48

In

our work,

hot-water-soluble PAAC fibers were used to prepare PAAC/GO hydrogel films through forming hydrogen bonds and self-crosslinking of PAA chain. After natural drying of the hydrogel and further imidization under thermal condition, the hydrophilic PAAC/GO hydrogel films could be converted into hydrophobic PI/reduced GO（PI/rGO） films, which are stable in both water and other solvents. This hydrogel method, which only uses water as solvent, represents a green, facile route for the large-scaled preparation of PI-based films. Hydrophilic GO was chosen as the inorganic component of the hybrid PI-based composite films. Benefiting from the hydrogel method and a natural drying process,49 the as-prepared PAAC/GO composite showed a homogeneous porous morphology. After imidization at high temperature, the corresponding PI/rGO composite displayed


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excellent ductility with a high elongation at break of above 35% and less frequency-dependent dielectric properties. To understand the formation mechanism, the PI/rGO films were investigated by XRD, Raman spectroscopy and scanning electron microscopy. The formation of mutilayered structure in the hybrid PI/rGO films accounts for the enhanced toughness and elongation at break. Furthermore, this porous

structure

endows the PI/rGO films

with

low density and

less

frequency-dependent dielectric behavior. This novel hydrogel strategy is promising for

preparing

PI/inorganics

hybrid

composites

and

other

well

dispersed

polymer/inorganics hybrid materials for practical applications. 2. Experimental section 2.1 Materials. N-methyl-2-pyrrolidone (NMP), graphite, and HCl (37 wt%) were purchased from Sigma-Aldrich Shanghai Co., Ltd. 4,4'-oxydianiline (ODA) and 3,3',4,4'-biphenyltetracarboxylic dianhydride (BPDA) were purchased from Chinatech (Tianjin) chemical Co., Ltd. Sulfuric acid, phosphoric acid, and triethylamine (TEA) were provided by Shanghai Lingfeng Chemical Reagent Company. All of these chemicals were used as received. 2.2 Preparation of graphite oxide. Graphite oxide sheets were prepared from natural graphite (Gr) flakes by an improved Hummers method.50 Briefly, a mixture of concentrated H2SO4/H3PO4 (9:1,v/v, 200 mL) was poured in a flask containing a mixture of graphite flakes (1.5 g, 1 wt. equiv) and KMnO4 (9.0 g, 6 wt. equiv). The mixture was then heated to 55 °C, and the reaction was stirred for 15 h. After cooling to room temperature, the mixture was poured into iced water (250 mL) containing 30% H2O2 (1.5 mL). The solution was filtered, and the collected solid was successively washed with water (100 mL), 37 wt. % HCl (100 mL), and ethanol (100 mL) for two times. Then the obtained GO was then further washed with deionized water, and the neutral solid was collected by centrifugation. The prepared GO nanosheets were then freeze-dried and dispersed in deionized water at a concentration of 20 mg/mL for further use. 2.3 Preparation of PI/rGO films from aqueous and organic solutions.



Water soluble PAAC were prepared according to a previously reported method.48 Briefly, equimolar amounts of ODA (2.003 g, 0.01mol) and BPDA (2.944 g, 0.01mol) were dissolved in NMP to give a total solid content of 15%. After mechanical stirring for 5 h under nitrogen atmosphere, TEA (1 g, 0.01mol) was added to produce PAAC solution. (Note: in this case, only half of the carboxyl groups on PAA were converted into PAAC; excess TEA would lead to the PAAC dissolved in water, so that PAAC could not be precipitates. Whereas, inadequate TEA will cause that the precipitated PAAC could not be re-solved in hot water). The PAAC solution was then poured into cold water and PAAC precipitates in fiber form (hereinafter referred to as PAAC

fibers) were collected and further washed with deionized water. PAAC fibers with a high molecular weight of about 7.3k (Figure S1) were obtained after freeze-drying. PAAC fibers (4.8 g) was re-dissolved in aqueous GO solution (100 mL; 2 mL GO aqueous solution with 40 mg GO + 96 mL H2O + 2 mL TEA (1.37g)) at 60 °C and stirred for 2 h to give a stable homogeneous aqueous solution of PAAC/GO. The solution was then poured onto a dust-free glass plate surface at 50 °C. When the PAAC/GO solution covered the whole glass plate, the plate was cooled to 25 °C, whereupon PAAC/GO gel was formed immediately. The PAA/GO hydrogel was allowed to dry naturally for 12 h. The obtained thin PAA/GO film was further thermally imidized at 120 °C, 150 °C, 180 °C, 220 °C for 1 h each and 250 °C for 4 h. The corresponding samples obtained by the hydrogel method with different GO contents of 0 wt%, 0.5 wt%, 1.0 wt%, and 2.0 wt% (mass ratio of PAA to GO were 100:0; 99.5:0.5; 99.0:1.0; 98:2.0) are designated as neat PI, PI/rGO0.5, PI/rGO1.0 and PI/rGO2.0, respectively. (rGO: thermal reduced GO) In addition, to survey the effect of film-forming process on the internal structure of final product, PAAC and PAAC/GO-2.0 wt% films prepared by drying their aqueous solution at 25°C and 50°C, respectively. These films and corresponding final imidized films are named PAAC-25, PAAC-50, PAAC/GO2.0-25, PAAC/GO2.0-50, and PI-25, PI-50, PI/rGO2.0-25, PI/rGO2.0-50, respectively. For comparison, the corresponding PI/rGO films were prepared by in-situ polymerization in an organic medium (NMP) according to previous work.51, 52 Briefly,


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2.000g ODA and a certain amount of GO (0g, 0.025g, 0.050g and 0.100g, respectively) were added into NMP, after ODA was completely dissolved, 2.94g BPDA and NMP were added in the mixed solution to give a solid content of 10%. After mechanical stirring for 5 h under nitrogen atmosphere, the obtained PAA solutions were rested to remove bubbles. The as-prepared PAA solutions were used to prepare PAA/GO films by tape casting (thickness: 300μm, Table S1). Followed by further thermally imidized at 120 °C, 150 °C, 180 °C, 220 °C for 1 h each and 250 °C for 4 h. These samples are designated as neat PI-O, PI/rGO0.5-O, PI/rGO1.0-O, and PI/rGO2.0-O, respectively. 2.4 Characterization. Thermal gravimetric analysis (TGA) was performed on a Q50 apparatus (TA Instruments, New Castle, USA) with a scan range from 50 to 800 °C with heating rate of 20 °C/min under air or nitrogen atmosphere. Dynamic mechanical analysis (DMA) tests of the samples (30 mm×4 mm×30~70 μm) were carried out on a TA-Q800 (TA Instruments, US) thermal analysis system at a heating rate of 5°C/min. Tensile tests of the film samples were carried out at room temperature with a humidity around 45% using an Instron 4456 test machine (Instron Corp, 825 University Avenue Norwood, USA) according to ASTM D882-91 (Standard Test Method for Tensile Properties of Thin Plastic Sheeting). Each sample, with a standard sample size of 30mm×4mm×50μm, was measured for five times to ensure accuracy. Fourier transform infrared (FT-IR) spectrums of PI films were taken on a Spectrum 100 FT-IR spectrometer (Perkin Elmer, Inc., United States) with range at 650-4000 cm-1. Dielectric constants of PI samples were performed in Dielectric/Impedance Spectrometer (Concept 40, Novocontrol Company, Germany) with frequencies changed from 10MHz to 1Hz. PI samples were firstly cleaned and dried in vacuum at 100 °C for 2h. The testing area was a circle with 12mm diameter. Before test, the testing areas on PI/rGO samples were sprayed with gold at both sides, each for 3mins. Wide-angle X-ray diffraction was carried out with a Bruker X-ray diffractometer (Bruker-AXS; Karlsruhe, Germany) over the scan range of 2θ=5–35° for GO, Gr and 2θ=10–40° for PI/rGO with steps increments of 2.0° at room temperature to



investigate the interchain spacing of as-prepared films. Ni-filtered Cu-K radiation with a wavelength of λ=1.5418 Å was applied in this test. The average d spacing was determined according to Bragg’s law: nλ=2dsinθ. Field-emission scanning electron microscopy (Sirion 200 SEM，FEI Company, Hillsboro, USA) with an INCA X-Act (EDS) attachment was applied to investigate the microstructure of the as-prepared materials. Cross-sections of the samples were obtained by wetting-off fracture in liquid nitrogen. All exposed surfaces were cleaned with ethanol, desiccated, and sputtered with gold prior to SEM measurements. Atomic Force Microscopy (AFM) measurements were carried out on a Multimode Nanoscope IIIa instrument (Nanonavi E-Sweep, Germany) by GO dropping nanosheets onto a mica plate and dried at 60 °C. Raman spectra were recorded by means of a Raman Microscope (DRX, Thermo Fisher Scientific, USA) with a laser excitation at wavelength of 780 nm. Densities of PAAC, PAAC/GO2.0, PI and PI/rGO samples prepared from two methods were measured by a solid-liquid densitometer (FBS-120G, Fubus, Xiamen). Before test, PI/rGO samples were dried in vacuum at 120 °C for 2 hours. Each sample was tested for five times with an error range less than 5%. Microstructure observations of PI/rGO samples were performed on a field emission transmission electron microscopy (FE-TEM) (Talos F200X, FEI, USA). PI/rGO films were embedded in epoxy resin and cured at 60°C for 3 days. Ultrathin section of PI/rGO films were obtained with an ultramicrotome (LKB-V, Sweden) at room temperature. 3. Results and Discussion 3.1 Preparation and Characterization of GO and PI/rGO films Chemically exfoliated GO nanosheets were characterized by AFM. As shown in Figure 1a, the marked white line crossing over three pieces of GO shows a similar height of the GO crystallite terrace of about 1.13 nm, corresponding to the thickness of two-layer GO nanosheets, and a size of GO nanosheets of less than 1 μm, indicating the successful delamination GO in water. The XRD patterns of raw graphite (Gr) and GO are given in Figure 1b, the d-spacing value of the raw Gr and


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chemical exfoliated GO (calculated by Bragg equation) was 0.34 nm and 1.13 nm, respectively. The increased layer distance of GO resulted from chemical oxidation reaction,

53

during which the functional group, such as hydroxyl and carbonyl, were

inserted between the graphene interlayers, meaning the Gr nanosheets were desquamated from each other. The thermal stabilities of GO and Gr were surveyed through TGA curves (Figure 1c), from which it can be seen that the Gr was stable under nitrogen atmosphere, whereas a rapid weight loss occurred at about 200°C for GO, originating from the decomposition of functional groups such as hydroxyl, carbonyl, and epoxy groups.

Figure 1. a) AFM images of GO; b, c) XRD and TGA curves of GO and Gr. The procedure for preparing PI/rGO composite films by the hydrogel strategy is illustrated in Scheme 1. Water-soluble PAAC fibers were firstly prepared by pouring a solution of PAAC into cold water，followed by filtration and freeze-drying. The PAAC fibers were re-dissolved in water at 60 °C and mixed with aqueous GO solution to give a homogeneous aqueous solution. This solution was directly cast onto a glass plate at 50 °C. When the plate was allowed to slowly cool to room



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temperature, PAAC molecules were re-precipitated and three-dimensional hydrogel network was formed under hydrogen bonding between PAAC nanofibers and GO, thereby, a PAAC/GO hydrogel was formed. The nanofiber network could be demonstrated by capturing SEM image of the dried hydrogel obtained by supercritical extraction (CO2) as shown in Figure S2, because it is generally believed that the supercritical method can maintain the primary network structure in hydrogel state. The PAAC/GO hydrogel was further dried under natural environment for 12 h and then thermally imidized to generate the PI/rGO composite film. During the drying process, the inorganic GO nanosheets were confined to specific positions within the 3D network of the PAAC hydrogel by hydrogen bonding, thereby avoiding the aggregation of GO in the PI matrix. Scheme 1. Schematic diagram of the preparation of water-soluble PAAC fibers (left) and the fabrication process of PI/rGO composites films by the hydrogel strategy (right).

During thermal treatment process, PAAC was firstly converted into PAA with escaping of TEA (boiling point: 89.5 °C) from the xerogel films. As the temperature further increased from 120 °C to 250°C, PAA was gradually imidized to form PI

54

and GO was also thermally reduced to hydrophobic rGO.55 The obtained final PI/rGO films proved to be stable in water as well as in other solvents.


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FTIR spectra were recorded to confirm the accomplishment of the imidization process, as shown in Figure S3. Compared with PAA, the final PI films showed the characteristic peaks of the symmetric and asymmetric stretching vibrations of the carbonyl groups on the imide ring at 1777 and 1720 cm-1, respectively. Furthermore, the stretching vibration of the C-N-C bond in the imide ring was also observed at =1374 cm-1. The absence of characteristic peaks at  = 3220–3450 cm-1 (N-H stretching vibrations) indicated complete imidization by the thermal treatment. Peaks in the FTIR spectra showed no displacement, indicating there were no hydrogen bonds involving the carbonyl groups in the final PI composite films.56 3.2 Macro/micro Morphologies Optical photographs of the respective products are presented in Figure 2. Figure 2a shows PAAC fibers, which could be further dissolved in hot water to form PAAC hydrogel (Figure 2b) and PAAC/GO hydrogel (Figure 2c) at room temperature. Figure 2d shows the final products of neat PI and PI/rGO films, with tailored shape for mechanical tests. It is obvious that with increasing GO content, the as-prepared films became dark brown. In contrast, the films obtained from organic solution (see Figure S4) showed a darker color than those derived from the hydrogel strategy, which might be attributed to different deposition effect of GO in aqueous and organic solutions. In addition, the PI film obtained from hydrogel method is non-transparent with color of bright yellow. When GO was added and reduced to rGO, PI/rGO show a mixed color of yellow and black, resulting the dark brown of the final product. Metallurgical microscopy images were acquired to investigate the mixing situation of PAAC and GO. The images revealed that PI/rGO obtained from the organic solution showed obvious GO aggregation at 2.0 wt% GO content (Figure S5a, b), whereas the PI/rGO derived from aqueous solution exhibited homogeneous mixing of the polymer and rGO nanosheets (Figure S5c, d) due to the formation of three-dimensional network during



hydrogel process. This homogeneous mixing was important for producing uniform composite films (Figure S6).

Figure 2. Optical photographs: a) PAAC fibers; b) PAAC hydrogel; c) thick PAAC/GO hydrogel and d) Neat PI and PI/rGO samples with standard shape for tensile test. Hydrogels have been widely investigated with regard to their unique inner porous, network structures and consequent specific properties.57 It has been proposed that the hydrogel strategy for preparing composite PI films may endow them with specific internal structures. Hence, scanning electron microscopy (SEM) was employed to further survey the microstructures of the PI/rGO composites films. As shown in Figure 3a1, the neat PI film derived from the hydrogel method showed a smooth, planar surface. As the GO content was increased from 0.5 wt% to 2 wt%, the surface became even coarser (Figure 3b1–d1). Notably, even small bulges appeared on the surface of the PI/rGO2.0 film. Cross-sectional SEM images (Figure 3a2–d2) were also acquired to further investigate the internal structure of the PI/rGO composite films. The fracture surface of the PI/rGO films demonstrated an obvious stratification structure, in which layers were parallel to the film plane (Figure S7 and S8). Moreover, as the GO content was increased from 0 to 2.0 wt%, the spaces between the layers gradually grew and the


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thickness of the layers decreased. This unique structure of the PI/rGO films might originate in the drying process of the PAAC hydrogel. When the aqueous solution of PAAC/GO was cooled down to room temperature, the mixture started to gel and formed a stable hydrogel. In the subsequent natural drying procedure of the PAAC/GO hydrogel, the top surface firstly dried and became a compact layer, which slowed down the escape of water molecules from the interior. Thus, a multilayered structure was formed due to the reduced gradient evaporation rate. In addition, the GO nanosheets in the hydrogel played a role of reinforcing the interior porous structure of the PAAC xerogel, preventing its collapse during the natural drying process.

Figure 3. Surface (upper row) and cross-sectional (bottom row) SEM images of PI/rGO films prepared by the hydrogel strategy: a1) Neat PI; b1) PI/rGO0.5; c1) PI/rGO1.0 and d1) PI/rGO2.0; a2) Neat PI; b2) PI/rGO0.5; c2) PI/rGO1.0 and d2) PI/rGO2.0. 3.3 XRD Patterns and Raman Spectra The PI/rGO films showed unique stratification morphology, distinctly different from that of films prepared by the normal organic solution method. To better understand the packing of the PI chains in the porous composite films, wide-angle X-ray (WAXD) diffraction measurements were performed. The WAXD patterns of PI/rGO composite films showed a broad diffuse peak (Figure 4a) at 2θ=10–30o, indicative of an amorphous structure. This peak is often used to estimate the average interchain spacing distance (d spacing); the d spacing in the amorphous glassy PI/rGO composite films was calculated as 0.363 nm using the Bragg equation.58 Interestingly, neat PI film derived from the hydrogel method clearly showed several sharp peaks at



2θ=15.1° (d=0.56nm), 2θ=17.0° (d=0.52nm), 2θ=19.4° (d=0.46nm), and 2θ=22.6° (d=0.40nm), respectively, whereas the neat PI-O derived from organic method showed only one peak at 2θ18°.59 It is known that the polyimide chains can be packed in an ordered manner due to π-π stacking interactions.60 Even though the formation mechanism of PI microcrystals during the hydrogel process is not clear, the interactions between water and PAA must be conducive to enhance orderly stacking of the PI chains. Therefore, we surmise that these sharp peaks at 2θ=15.1°, 17.0°, 19.4°, and 22.6° might arise from the ordered stacking of PI main chains during the formation of the hydrogels and the natural drying process.

Figure 4. a) XRD patterns of as-prepared PI films derived from hydrogel strategy; b) Raman spectra of GO, rGO, Neat PI, and PI/rGO2.0. To validate our assumption, Raman spectroscopy was applied to characterize the accumulated form of the carbonaceous materials. A Raman study relies on an analysis of the main Raman peaks (D band and G band) in the spectra of graphene materials, including their position, width, and most especially the relative intensity ratio ID/IG.61 From Figure 4b, it can be seen that neat PI prepared by the hydrogel method showed no obvious D band and G band because it contained no ordered carbonized structure. However, the ID/IG ratio of PI/rGO2.0 (2.07) was much larger than that of pure GO (1.41) or rGO (1.46) obtained after subjecting GO to the same heating procedure as PI. The D band of PI/rGO2.0 was greatly enhanced. This result verified our deduction: (1) the PI chains extended through the GO nanosheets, which induced more π-π


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stacking interactions; (2) the hydrogel process caused more ordered stacking of the PI chains.62 3.4 Mechanical Properties Hiroshi et al. studied the properties of PI films prepared by one-pot and two-step imidization methods, and found that they exhibited different mechanical properties and other features due to different crystallinities and phase orientations of the PI chains.63 As mentioned above, the PI/rGO composite films prepared by the hydrogel strategy showed dramatic differences from ordinary PI films in terms of their micromorphology, interchain spacing, and lattice packing. These microstructural features could give rise to distinct physical properties. Hence, mechanical properties were surveyed by tensile tests. As shown in Figure 5, PI/rGO films prepared from hydrogel strategy exhibited obvious ductile fracture characteristics. On the contrary, the PI/rGO samples prepared by the organic solution method presented clear brittle fracture characteristics (Figure 5a, b). The detailed data are summarized in Table S2. The elongation at break of the neat PI derived from hydrogel method was 46.7%, about 2.65 times of the neat PI derived from the organic method (17.6%). With the addition of 2.0 wt% GO, the elongation at break retained a high level of 36.3%, 4.96 times of that PI control derived from organic route. The high ductility of PI and PI/rGO films were also repeatable (Figure S9, Table S3, Video S1 and Video S2). In addition, GO addition improved the tensile modulus and tensile stress of PI. Specifically, PI/rGO2.0 showed a tensile modulus of 1.38 GPa and a tensile stress of 95.1 MPa, 15.9% and 6.2% higher than that of neat PI (1.19 GPa and 89.2 MPa), respectively. The improvement also can be verified by the change of thermal-mechanical property. Figure 5c shows the storage modulus of PI/rGO films as a function of temperature obtained by DMA. The storage modulus of PI/rGO films increased from 982 MPa to 2287 MPa as rGO content increased from 0% to 2 wt%, indicating that the use of GO can improve the rigidity of PI films. It is worth noting that irregular pore structure also may cause a decrease in mechanical properties



(Figure S10). Therefore, in the experiment, the film formation conditions, such as temperature, air pressure and so forth, should be kept as consistent as possible.

Figure 5. Stress-strain curves of PI/rGO composite samples derived from tensile tests: a) obtained by the organic solution method and b) obtained by the hydrogel strategy; c) Storage modulus of PI and PI/rGO films derived from hydrogel strategy as functional of temperature with heating rate of 5 °C /min. 3.5 Proposed Mechanism As established above, the multilayer structure and excellent ductility of PI films obtained by the hydrogel method may be related to the drying process of the PAAC/GO hydrogels. Three-dimensional networks may be generated between PAAC and GO based on hydrogen bonds, thereby forming the PAAC/GO hydrogels. To prove this, PAAC and PAAC/GO2.0 films were prepared by drying their aqueous solution at 25°C and 50°C, respectively. For PAAC-50 and PAAC/GO2.0-50 films dried at 50°C, water was directly evaporated from PAAC and PAAC/GO2.0 solution without process of hydrogels. The obtained films showed higher densities (1.244~1.254 g/cm3) than that obtained by natural drying method (1.205-1.208 g/cm3) (Table S4 and Figure S11). Furthermore, their final product of PI-50 and PI/rGO2.0-50 showed no porous structure, not even layer structures (Figure S12). As the result, gel process is critical to the formation of the pore structure in PI/rGO films, although the gas produced by the reduction of graphene oxide also may play a supporting role in the pore structure formation of the final film. Water in a PAAC/GO hydrogels can be divided into free water confined by the 3D networks and hydrate water connected with PAAC by hydrogen bonds. During the


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natural drying process, free water is gradually evaporated at a relatively constant speed. However, the evaporation of hydrate water is much slower than that of free water. This water loss was verified by an evaporation investigation, as shown in Figure S13. Actually, the hydrate water serves as a crosslinker between PAAC chains, such that the 3D structure would collapse following its volatilization. Capillary tension drew the nearby chains closer, ultimately resulting in a dense layer.64 Therefor, top layer was formed firstly as the water could easily escape from the surface. Thereafter, evaporation of the hydrate water became much more difficult. Thus, a multilayer structure was gradually formed by the gradient evaporation of water, as shown in Scheme 2. Furthermore, by incorporating of GO, hydrogen bonds between GO and PAAC were formed. This multi-layered porous structure was much easier to be retained under the support of GO. The multilayer structure showed excellent ductility due to sliding of PI layers during pull-up process. Scheme 2. Possible mechanism of multilayer structures formation mechanism in PI/rGO films.

3.6 Thermal and Dielectric Properties Thermal stability is one of the most important characteristics that distinguished PI materials from other plastics (Figure S14a, b). Thermal decomposition behavior was investigated by TGA under nitrogen atmosphere. As shown in Figure 6a, a marginally higher decomposition temperature and slightly more residual mass at 800 °C were seen for the PI/rGO samples derived from hydrogel method with increasing GO content. Specifically, the thermal decomposition temperature at 5% weight loss (Td5) of PI/rGO composite film increased from 569 °C to about 580 °C as the GO



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content was increased from 0 to 2.0 wt%. The Td10 also increased by 10 °C. The mass residues at 800 °C of PI/rGO composite films showed slight improvement as the GO content was increased (Table S5). The glass transition temperature (Tg) is an important index in estimating the thermal properties of PI products. When rGO sheets were added to PI films, the movement of PI chain segments were limited due to the π-π stacking interaction between PI chains and rGO, so that Tg increased to some extent, such as for the PI/rGO films prepared by organic solution (Table S6). However, for the PI/rGO hybrid films prepared by the hydrogel method, the existence of the pores increased the mobility of the PI chains at the pore interface, resulting a decrease in Tg (Figure 6b). Combined with the two reverse changes, the PI/rGO samples prepared from hydrogel method showed no distinct changes in Tg values (Table S5). This result is consistent with most reports in the literature,

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that is, when the amount of inorganic additives is

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