Improved Electrical and Mechanical Properties for the Reduced

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Article Cite This: Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Improved Electrical and Mechanical Properties for the Reduced Graphene Oxide-Decorated Polymer Nanofiber Composite with a Core−Shell Structure Nan Zheng,† Yang Song,† Ling Wang,‡ Jie-feng Gao,*,‡,§ Yu Wang,† and Xiaoli Dong*,† †

School of Light Industry and Chemical Engineering, Dalian Polytechnic University, Dalian 116034, P. R. China College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu 225002, P. R. China § State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu, Sichuan 610065, P. R. China Downloaded via NOTTINGHAM TRENT UNIV on August 16, 2019 at 16:16:08 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

S Supporting Information *

ABSTRACT: Conductive polymer nanofiber composites (CPNCs) have promising applications in many fields. However, preparation of the CPNC with excellent mechanical and electrical properties is still challenging. Here, reduced graphene oxide (RGO) was assembled onto the nanofiber surface assisted by ultrasonication, obtaining a nanofiber composite with a polymer core−RGO shell structure. The interfacial collision between polymer and RGO would occur under ultrasonication, during which the flexible RGO was easily deformed or bent and finally formed the core−shell structure. The polymer nanofiber core endowed the composite flexibility, while the RGO shell formed a continuous network, decreasing significantly the electrical resistivity. The electrical conductivity of the RGO-based nanofiber composite was closely related to the solvent used to disperse the RGO and the ultrasonication time. RGO introduction enhanced the tensile strength, Young’s modulus, and elongation at break. The RGO tubes were harvested by thermal degradation of the polymer nanofiber core.

1. INTRODUCTION One-dimensional electrospun polymer nanofibers (EPNFs) have recently aroused tremendous attentions in academia and industry because of their unique microstructure and properties such as the controllable porous structure, light weight, and large specific surface area.1,2 These characteristic features have found promising applications in filtration, tissue engineering, catalysis, and so forth.3−7 However, the low dimensional stability, moderate mechanical performance, and insufficient functionality (e.g., low electrical and thermal conductivity) severely limit their practical applications. Thus, nanosized fillers, especially carbon-based nanofillers, are often introduced into the nanofibers to enhance the comprehensive properties of EPNFs.8−13 Graphene as a single atomic layer of graphite has superior electrical, thermal, and mechanical properties,14−16 and it is a good candidate for the enhancement of overall properties of EPNFs. In general, graphene-based nanofiber composites are prepared by first mixing graphene oxide (GO) or RGO in polymer solution followed by electrospinning.17−21 However, graphene tends to aggregate together inside the nanofibers even at a low concentration, which can greatly degrade the material performance. In addition, some of the excellent properties, such as superior electrical conductivity, cannot be achieved when the graphene is trapped within the nanofiber. Graphene wrapped by an insulating polymer cannot form a © XXXX American Chemical Society

continuous conductive network, leading to a high resistivity of the final nanofiber composite. Hence, it is desired that graphene is decorated on the nanofiber surface instead of entering into the nanofiber, which is usually achieved by decorating GO onto the nanofiber and then reducing GO to RGO. It was reported that carbon nanofiber with graphene on its surface was prepared by spraying the GO onto the polymer nanofibers during electrospinning, followed by calcination.22 GO can also be assembled onto the Nylon 66 (PA66) or polyurethane (PU) nanofiber surface through interfacial hydrogen bonding between the amide groups in the PA66 or PU backbone and the carboxyl or hydroxyl groups on GO, during which a surfactant is used to promote GO adsorption.23−25 However, graphene often stacks together, covering not only the fiber surface but also blocking the pores in the nanofiber mat. In this case, the benefits afforded by a great number of interconnected pores in the EPNF membrane cannot be utilized. Thus, it is desirable that the polymer nanofiber is wrapped by graphene to form a core−shell structure. Meanwhile, the pores are preserved in the nanofiber mat after graphene coating. Graphene-coated polymer microReceived: April 1, 2019 Revised: June 24, 2019 Accepted: August 1, 2019

A

DOI: 10.1021/acs.iecr.9b01766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

voltage of 15 kV. The distance between the metallic needle and an aluminium foil as the collector was 15 cm. 2.4. Preparation of RGO-Based Core−Shell Nanofiber Composites. RGO was first dispersed in three kinds of solvents (ethanol, water, or acetone) (1 mg/mL) by a highintensity ultrasonic horn (JY98-IID, Ningbo Scientz Biotechnology Co., Ltd) for 0.5 h, and then the PU nanofiber mat was dipped in the resultant RGO suspension and continued to ultrasonicate for different times (10 s, 1 min, 5 min, 10 min, and 20 min), during which the RGO nanosheets were gradually assembled on the nanofiber surface to form the core−shell structure. Finally, the RGO-decorated composite mat were obtained after washing with ethanol and drying at 60 °C for 12 h and donated as PU/RGO-X (X: min). Besides, the PU nanofiber mat was also dipped in a GO suspension with or without ultrasonication to investigate if GO can be decorated onto the nanofiber surface. RGO tubes were obtained by calcination of the nanofiber composite prepared using the ethanol as the solvent and under an ultrasonication time of 10 min at 800 °C. 2.5. Characterization. Fourier transform infrared spectra (FTIR) were obtained from a PerkinElmer 100 spectrometer. Raman tests were conducted on a Renishaw inVia Raman microscope with an excitation wavelength of 514 nm. The morphology of the RGO-decorated EPNF mat was observed by a field-emission scanning electron microscope (FESEM; FEG JSM6335) after the specimens were gold sputtered. The core−shell structure of RGO-nanofibers deposited onto a copper grid was also observed by a transmission electron microscope (TEM; Philips FEG CM200). Note that the nanofiber composite used for characterization was prepared under an ultrasonication time of 10 min. 2.6. Electrical Conductivity Test. The electrical conductivity for the nanofiber composite was tested by a 4-probe method. The sample resistance (R) was first measured, and then the electrical resistivity (ρs) was calculated from ρs = R(S/ L); where L represents the length, and S represents the crosssectional area of the nanofiber mat. Before the electrical conductivity test, a conductive silver paste was coated onto the two sides (cross section in the direction of thickness) of the composite nanofiber film (2 cm × 1 cm × 25 μm) to achieve the well contact of the nanofiber surface with electrodes. A precision digital resistor (Model TH2683A, Changzhou Tonghui Electronics Co., Ltd.) was employed to measure the sample resistance (higher than 106 Ω) based on a two-probe technique, while a four-probe method was used to test conductivity of the nanofiber composite whose resistance is lower than 106 Ω. 2.7. Mechanical Property Measurement. The mechanical properties of the nanofibrous membrane and its composite were measured by an Instron universal testing machine. The membranes were cut into dumbbell shapes with a size of 20 mm (length) × 4 mm (width) × 0.2 mm (thickness). The test was performed with a loading speed of 100 mm min−1. Five specimens were used for each test.

spheres with a core−shell structure have been fabricated via electrostatic interaction, hydrogen bonding, or hydrophobic interaction, which is mainly completed in the microsphere suspension.26−29 However, it is very difficult to construct a polymer/graphene core−shell structure for the free-standing nanofiber mat, and that is why little or no related research work has been reported in the open literature. In this study, a novel RGO-based nanofiber composite with a core−shell structure, that is, polymer nanofiber as the core and RGO nanosheets as the shell, was fabricated by ultrasonication-induced RGO assembly. Here, ultrasonication not only assisted RGO dispersion but also drove RGO to uniformly assemble onto the nanofiber surface. Compared with the traditional GO adsorption on the nanofibers that is a timeconsuming process, here, ultrasonication-driven RGO assembly on the nanofiber surface was completed within several minutes. In addition, since no surfactant was required, it can avoid the introduction of impurities, which are detrimental to material performance. The obtained nanofiber composite mat displayed good flexibility due to the polymer core, while the continuous RGO shell enhanced the mechanical strength and electrical conductivity. Moreover, the RGO tube was formed after the polymer core was removed by pyrolysis, and the obtained RGO tube may have potential applications in the field of energy such as a supercapacitor and a lithium-ion battery.

2. EXPERIMENTAL SECTION 2.1. Materials. Graphite flakes were provided by Nanjing XFNANO Materials Tech Co., Ltd. Polyurethane (PU; Desmopan 385S) was purchased from Bayer (Hong Kong, China). N,N-Dimethylformamide (DMF), ethanol, acetone, and hydrazine were obtained from Sigma-Aldrich Corp. 2.2. Synthesis of Graphene Oxide (GO) and RGO. For the preparation of GO, a modified Hummers method was used. In brief, 3.0 g of graphite flakes was mixed with NaNO3 (1.5 g), and then 69 mL of H2SO4 was added. The mixture was stirred at an ice bath. Subsequently, 9 g of KMO4 was added slowly in portions, and the solution was vigorously stirred for another 1.5 h. Note that the reaction temperature was first controlled below 10 °C, increased to 35 °C, and then kept for 3 ;h. Subsequently, the deionized water (138 mL) was added, causing a large exotherm to 95 °C, which was maintained for 15 min. After that, the mixture was cooled in a water bath for 10 min, and then deionized water (420 mL) and 30% H2O2 (3 mL) were added. The yellow solution was then washed and centrifuged until it became neutral, that is, the PH was ∼7. The collected precipitate was finally subjected to vacuum drying to obtain the solid GO. RGO could be obtained by GO reduction using hydrazine. Specifically, GO was dispersed in distilled water (1000 mL) and subjected to ultrasonication for 0.5 h, and the GO concentration was 2 mg/mL. An amount of 100 μL of hydrazine solution was then added into the as-prepared GO solution followed by stirring at 95 °C for 12 h, during which the color of the solution turned from yellow to black. The RGO powder was finally obtained by washing with distilled water and centrifugal separation. 2.3. Preparation of PU Nanofibers. PU solution with a proper viscosity for electrospinning was prepared as follows: PU pellets were mixed with DMF as a solvent and then stirred at 60 °C for 12 h to generate a homogeneous PU solution. The obtained polymer solution was placed in a plastic syringe with a metallic needle. The electrospinning process of PU nanofibers was proceeded at a feed rate of 1 mL/h and a

3. RESULTS AND DISCUSSION 3.1. Morphology and Formation Mechanism for the Core−Shell Structure of the Nanofiber Composite. RGO sizes would play a crucial role for determining whether the core−shell can be formed or not. When the RGO planar size is much larger than the nanofiber diameter, the EPNFs could not be completely wrapped by RGO, and the pores between the B

DOI: 10.1021/acs.iecr.9b01766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

which shows RGO stacks together around the nanofiber, forming a thick and wrinkled shell. It is reported that metal nanoparticles may be anchored onto polymer microspheres assisted by ultrasonication, most likely caused by the sintering of the particles and/or interparticle collisions between polymer and metal nanoparticles. Hence, the polymer microsphere, to some extent, is melted or softened so that the metal particles “dissolve” partially in the polymer.31 It has been reported that cavitation bubbles can be generated in liquid during ultrasonication; a transient temperature as high as 5000 K and a pressure of ∼1000 atm with a cooling rate above 108 K/s32,33 may be achieved by the collapse of the bubbles from cavitation. Hence, microjets and shock waves are generated near the solid surfaces after bubbles collapsed, which can drive RGO toward the nanofiber at high speeds. The PU nanofiber is still viscoelastic under the impact of the fastmoving RGO, accompanied by the occurrence of the interfacial collision between the RGO and the nanofiber. Consequently, the polymer nanofiber would be softened or even melted at the point of impact, and the RGO is also very sticky, both of which facilitate RGO assembly onto the nanofiber. In fact, GNP decoration onto polymer nanofiber was investigated in our previous work.30 However, in that work, the polymer nanofibers were not completely wrapped by GNPs, and some of the GNPs were coated on the nanofiber surface, while others were located inside the nanofiber. GNPs consisting of multiple layers of graphene were much more rigid than RGO. The bending stiffness Dm of few-layer RGO is strongly dependent on its thickness calculated by Dm(eV) = 6.7[N t0(nm)]2, where N represents the number of graphene layer, and t0= 0.34 nm represents the graphene interlayer space.34 Thus, the bending stiffness increases significantly with increasing layer number. It is very difficult for the GNPs with a relatively large thickness to be bent or deformed under ultrasonication. The GNPs, which have already been anchored onto the nanofiber, can, to some extent, prevent other GNPs approaching their surrounding region. In addition, the cylindrical shape makes the nanofiber difficult to be completely wrapped by the unfoldable GNPs. Unlike the GNPs, RGO, a single atomic layer of graphite, is more flexible and can be more easily bent and/or folded. Generally, defects are found in chemically or thermally reduced graphene; these defects can favor the formation of kinks in the nanosheets,35 which in turn are folded onto the nanofiber surface to a low energy state during ultrasonication treatment. The adsorbed RGO on the nanofiber surface can attract more RGO by a van der Waals force and hydrophobic interaction, finally forming a polymer/ RGO core−shell structure. After an RGO shell was formed on the nanofiber, the surface wettability was changed accordingly. The contact angle for the pristine nanofiber mat is ∼114° (Figure 1d), and it is increased to ∼123° (Figure 1e) for the nanofiber composite with a core−shell structure. There are functional groups, such as amide, in the PU macromolecular chain, corresponding to low hydrophobicity. However, the hydrophobicity is increased for the nanofiber composite caused by the RGO shell on the nanofiber surface. XPS results confirmed that the carbon-tooxygen (C/O) ratio has an increase from 2.42 for GO to 8.68 for RGO as displayed in Figure S2, indicating the disappearance of most of the oxygen-containing groups after hydrazine reduction, displaying hydrophobicity. Furthermore, the roughness of the nanofiber composite increased due to the

EPNFs might also be blocked. Hence, the starting raw graphite flakes with a planar size comparable with the nanofiber diameter (around 400 nm) was chosen in the experiment. Apart from the planar size, the thickness of RGO stacks could also affect the final morphology of the nanofiber composite. As reported in our previous work, graphite nanoplatelet (GNP) could be decorated on the surface of the EPNFs assisted by ultrasonication.30 However, the GNPs were either covered on the nanofiber surface or embedded inside the nanofiber, and no core−shell structure was observed. GNPs possess a much larger thickness than a single atomic layer of graphite, that is, RGO, and become rigid as well as less deformable during ultrasonication. Unlike GNPs, RGO with a thickness of ∼1.5 nm (Figure S1) is more flexible and deformable and is thus beneficial to fabricate a nanofiber/RGO with a core−shell structure, which will be discussed in detail later. Figure 1a shows an illustration of an ultrasonication-induced RGO assembly onto the EPNFs. The PU nanofiber mat has a

Figure 1. (a) Illustration of ultrasonication-induced RGO assembly onto the EPNFs. (a, b) Schematic illustration showing the preparation of porous GNP/PES nanocomposites. (b) A single RGO-decorated nanofiber possessing a core−shell structure. (c) Photograph of the EPNF mat after RGO coating. (d, e) SEM images of PU nanofibers before and after RGO decoration. The insets are photos showing water contact angles for mats without and with RGO. (f) TEM image of nanofiber composite having a RGO shell and a nanofiber core. Scale bars: 200 nm for (d) and (e) ; 500 nm for (f).

three-dimensional network structure with randomly distributed PU nanofibers inside. After RGO coating, every nanofiber is densely wrapped with RGO nanosheets, exhibiting a core− shell micronanostructure (Figure 1b). The white PU nanofiber mat turns completely black (see Figure 1c), showing that the RGO has successfully embedded in the mat. Note that the RGO-based nanofiber composite keeps the intrinsic flexibility of the PU nanofiber. Detailed information on the microstructure of the nanofiber and nanofiber composite can be found from the SEM and TEM images displayed in Figure 1d− f. PU nanofibers possess a smooth surface and an average diameter of ∼360 nm (see Figure 1d). However, the nanofiber surfaces become much rougher after the nanofibers are densely wrapped by RGO nanosheets (see Figure 1e). It is also observed some RGO just protrudes out of the shell (see red arrows in Figure 1e). More importantly, most of the pores between individual nanofibers are preserved even after the RGO assembly. The polymer nanofiber/RGO core−shell structure is further revealed in TEM image (Figure 1f), C

DOI: 10.1021/acs.iecr.9b01766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

Figure 2. SEM images of RGO-decorated PU nanofiber prepared at different times: (a) 10 s, (b), 1 min and (c) 5 min. The insets in (b) and (c) are the magnified SEM images. Scale bars: 2 μm for (a), (b), and (c); 200 nm for the insets of (b) and (c). (d) The variation of the conductivity for the RGO-decorated nanofiber composite under different ultrasonic times.

nanofiber mat as displayed in Figure 3a. It suggests that no effective hydrogen bonding was formed between GO and PU

protrusion of some RGO sheets from the shell (see Figure 1e), which also contributed to the increased hydrophobicity. As mentioned, the formation of the polymer/RGO core− shell structure could be divided into two steps, that is, RGO adsorption and subsequent assembly on the nanofiber surface, which was time dependent. Figure 2 shows the SEM images of the RGO-based nanofiber composite prepared in ethanol under different ultrasonication times. After a very short time of 10 s, very few RGOs were observed on the nanofiber surface, and most of the nanofibers maintained its originally smooth surface (Figure 2a). More RGO was attached onto the nanofibers when the ultrasonication time was extended to 1 min (Figure 2b), forming a thin and incomplete shell around the nanofiber as displayed in the inset SEM image of Figure 2b. With the further prolongation of the ultrasonication time to 5 and 10 min, the RGO stacked with each other, increasing the thickness of the RGO shell as displayed in Figures 1e and 2c. As discussed before, RGO adsorption onto the nanofiber surface was caused by the interfacial collision between the RGO and viscoelastic polymer during ultrasonication, while the van der Waals force and hydrophobic interaction were responsible for the subsequent RGO stack and assembly. The variation of the conductivity for the RGO-decorated nanofiber composite under different ultrasonic times is shown in Figure 2d. Naturally, the larger the ultrasonication time, the more the RGO decorated on the nanofiber surface, and thus the higher the conductivity. Pure PU nanofiber mat is insulating with an electrical conductivity of around 10−12 S m−1. The conductivity of PU/RGO-1 significantly increases to 2.91 × 10−5 S m−1, implying the formation of the conductive network in the composite. It further increases to 0.141 and 0.262 S m−1 for the PU/RGO-10 and PU/RGO-20, respectively. The interaction among RGO, nanofiber, and a dispersion medium should also be considered to further understand the RGO assembly. As mentioned above, GO could be decorated onto the PA66 or PU nanofiber surface through hydrogen bonding. In our work, the PU nanofiber was dipped in GO solution for 12 h, but very few GOs were found inside the

Figure 3. SEM images of (a) PU nanofiber mat after immersed in GO solution and (b) PU nanofiber mat in GO solution under ultrasonication for 10 min. Scale bars: 1 μm for (a) and (b).

nanofiber mats while the existence of hydroxyl and carboxyl groups on the GO surface and amino groups on the PU polymer chain. When the PU nanofiber mat was also subjected to ultrasonication in GO solution, no GO attachment could be found on the nanofiber, which kept its original smoothness (Figure 3b). GO with a large number of oxygen-containing groups, like carboxyl and hydroxyl, has a uniform dispersion in water, and it was trapped there rather than assembled onto the nanofiber surface. Hence, poor dispersion was beneficial to RGO decoration because of the weak interaction between RGO and water or other solvents like ethanol and acetone. 3.2. Characterization of the RGO-Based Nanofiber Composite. Figure 4a displays the FTIR spectra of the PU nanofiber and RGO-decorated nanofiber composite. For the PU nanofiber mat, many characteristic peaks assigned to different functional groups can be observed. For example, the two strong vibrations at 1732 and 1596 cm−1 represent free and hydrogen-bonded carbonyl groups in the urethane linkage (−H−N−COO−), and the characteristic N−H stretching band of urethanes gives the peak at 3336 cm−1. Also, the peak at 1596 cm−1 belongs to N−H in plane bending and that at 1531 cm−1 is attributed to the combined N−H bending and C−H stretching. Further, there are two peaks at 1172 and D

DOI: 10.1021/acs.iecr.9b01766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research

RGO content in the nanofiber composite, which was controlled by the RGO decoration on the nanofiber. In turn, this was affected by the solvent used for RGO dispersion. In the experiments, three representative solvents, for example, water, ethanol, and acetone, which have different wettabilities and swelling abilities to the PU nanofiber mat, were used to study their influence on the formation of the core−shell structure and thus the electrical conductivity. Table 1 gives the Table 1. RGO Contents of Nanofiber Composites and their Corresponding Electrical Conductivity using Different Dispersion Mediums for Ultrasonication-Induced RGO Assembly Processes

Figure 4. (a) FTIR and (b) Raman spectra of PU nanofiber and RGO-based nanofiber composite.

1078 cm−1 linked to stretching of C−O and C−O−C bands, respectively.36 The RGO-decorated nanofiber composite almost shares the same characteristic peaks with the PU nanofiber except that the positions of N−H stretching and N− H plane bending are gradually shifted from 3336 and 1596 cm−1 for the PU nanofiber to 3331 and 1594 cm−1 for RGOdecorated nanofiber composites, respectively. These shifts may indicate the potential chemical interaction between N−H groups of PU and RGO possessing small amounts of residual oxygen-containing groups. As mentioned earlier, RGO decoration on the nanofiber to form a core−shell structure was completed by interfacial collisions between RGO and PU nanofibers, during which chemical interaction might happen. Raman spectroscopy was widely used to probe the structural characteristics of RGO and RGO-based composites. The Raman spectra of PU, RGO, and RGO-based nanocomposites are displayed in Figure 4b. The characteristic bands such as δ(C−H) at 1445 cm−1, ν(CC) at 1620 cm−1, and ν(CH2) at 2932 cm−1 could be found in PU nanofibers.37 With respect to the RGO, two typical peaks at 1347 and 1596 cm−1 belong to the D and G bands were observed. It is known that the G peak was assigned to the first-order scattering of the E2g phonon of sp2 C atoms. Also, the D peak refers to the defects in the graphite structure. It was found that the characteristic bands of polymer become less obvious and even invisible after RGO assembly, and RGO-based nanofiber composite almost possesses the same characteristic peaks except that the G band slightly shifted from 1596 to 1592 cm−1. In addition, the intensity ratio of D-to-G band (ID/G) was around 1.10 and 1.14 for RGO and its nanofiber composite, indicating the presence of localized sp3 defects within the sp2 carbon network,38 which could be simultaneously formed during the reduction of GO. According to the results from Raman spectra, the substantial microstructure of the RGO has been preserved after its decoration onto the nanofiber surface upon ultrasonication, which is beneficial for improving the electrical properties of the polymer nanofiber mat. 3.3. Electrical Conductivity of Nanofiber Composite. A popular method used to improve electrical conductivity and reduce percolation threshold of conductive polymer composites is by creating a segregated conductive network, in which conductive nanofillers such as carbon black, carbon nanotubes (CNTs), and RGO are located at the interface of the nanofillers/polymer matrix or polymer blend but not randomly distributed within the overall material.39−43 In our study, the nanofibers with a core−shell structure became effective conductive elements whose high aspect ratios would benefit for forming a conductive network.44,45 Thus, nanofiber composites with high electrical conductivity were obtained. The electrical conductivity, as expected, would depend on the

materials (nanofiber mat)

electrical conductivity (S m−1)

RGO content (wt %)

PU PU/RGO (water) PU/RGO (ethanol) PU/RGO (acetone)

10−12 5.4 × 10−3 3.8 × 10−2 1.35 × 10−1

0 1.2 2.7 4.5

RGO content in the nanofiber composite and their corresponding electrical conductivity using different dispersion solvents during the ultrasonication-induced RGO assembly process for 10 min. The RGO content (Wg) was calculated by W −W Wg = 2W 1 × 100%, where W2 and W1 represent the weight 1

of the nanofiber after and before RGO decoration, respectively. Wg were 1.2, 2.7, and 4.5% when water, ethanol, and acetone were used as the dispersion mediums with corresponding electrical conductivities of 5.4 × 10−3, 3.8 × 10−2, and 1.35 × 10−1 S m−1, respectively. Figure 5a is a SEM image of the

Figure 5. SEM images of microstructures of nanofiber composites prepared using different dispersion mediums: (a) water and (b) acetone. Scale bar: 1 μm.

RGO-based nanofiber composite that was prepared using water as the dispersion medium. Here, the nanofibers are seen uniformly wrapped by RGO. However, the nanofiber composite has quite a smooth surface without protruded RGO sheets compared to the sample fabricated using ethanol as a dispersion medium (Figure 1e), yielding a lower RGO decoration density. The PU nanofiber mat can be more easily wetted by ethanol than water, inducing higher RGO adsorption and higher electrical conductivity. In contrast, the PU nanofiber mat is slightly swollen in acetone and becomes sticky. As a result, a large quantity of RGO can be captured and further assembled onto the sticky nanofiber surface. The pores between the nanofibers also become increasingly small under ultrasonication, leading to the eventual coalescence of these nanofibers (Figure 5b). Therefore, the nanofiber composite has the highest RGO content and electrical conductivity, when the RGO assembly is completed in acetone. E

DOI: 10.1021/acs.iecr.9b01766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 6. (a) Stress−strain curves of pure PU, PU/RGO-10, and PU/RGO-20. SEM images of the PU/RGO-20 at different strains (b) 50%, (c) 80%, and (d) 100%.

3.4. Mechanical Properties. In addition to the improvement of electrical conductivity, the RGO introduction could also enhance the mechanical properties of the nanofibrous mat, as observed from the typical stress−strain curves in Figure 6a. The PU nanofiber mat has a large elongation at break of 300%, displaying excellent stretchability. Generally, the rigid nanofillers could increase the Young’s modulus of the polymer matrix, and the tensile strength is also improved if uniform dispersion of the nanofillers in the matrix and good interfacial interaction are achieved. However, the nanocomposite usually suffers from a large decline of the elongation at break. Interestingly, in this study, the RGO decoration enhances the tensile strength, Young’s modulus, and the elongation at break. The detailed mechanical properties for the PU nanofiber mat and its composite are summarized in Table 2. The Young’s

6b−d shows the SEM images of the PU/RGO-20 under different strains. Stretching-induced deformation for the RGO shell is observed (Figure 6b), and the RGO shell is gradually damaged with the strain. Many visible cracks are accordingly generated with the nanofiber surface exposed (see the red circles) when the strain further increases to 80 and 100% (Figure 6c,d, respectively). The cracks were produced by the RGO slip, and more energy is thus consumed than that for the PU nanofiber mat. It should also be noticed that the orientation of the nanofibers was present during stretching, as demonstrated from the blue arrows in Figure 6c,d. As a consequence, the RGO-decorated nanofibers possess a much more interfacial area, which can, to a certain degree, facilitate the mechanical interlocking between the disconnected RGO shells. As mentioned, the RGO decoration further enhances the stretchability of the elastic PU nanofiber membrane. Figure 7a shows the variation of the conductivity and contact angle (CA) of the PU-based nanofiber composite with the strain. The conductivity displays a continuous decline with the strain, and it decreases from 3.8 × 10−2 S m−1 at the initial state to 8.4 × 10−4 S m−1 at the strain of 100%. CA has a slight decrease from the initial value of 123 to 116° and then fluctuates around this value as the strain increases. The RGO shell is gradually fractured during stretching, and the damaged conductive pathway leads to the resistance increase, while the partially exposed nanofiber surface decreases the CA of the composite. Figure 7b shows the CA and conductivity of the nanofiber composite under different stretching−releasing cycles. Obviously, the CA can go back to its initial value after cyclic stretching tests. However, the conductivity shows a moderate decrease during the first 20 cyclic test and then keeps unchanged during the following tests. Thus, the conductive nanofiber composite has potential applications as wearable strain sensors. 3.5. Preparation of RGO Tubes. The RGO assembly with a hollow structure is promising in the field of energy and

Table 2. Mechanical Properties of the PU and PU/RGO Composites nanofiber-based sample

tensile strength (MPa)

Young’s modulus (MPa)

elongation at break (%)

PU PU/RGO-10 PU/RGO-20

5.16 ± 0.43 8.58 ± 0.19 10.51 ± 0.75

2.11 ± 0.20 2.48 ± 0.19 2.71 ± 0.16

312.7 ± 10.9 355.2 ± 28.2 427.2 ± 27.4

modulus of the PU nanofiber mat is 1.97 MPa, and it increases to 2.54 and 2.71 MPa for PU/RGO-10 and PU/RGO-20, respectively. In addition, the tensile strength of PU/RGO-10 and PU/RGO-20 were 8.61 MPa (67.2% increase) and 10.69 MPa (107.6% increase), respectively, compared to that of the PU nanofiber mat with a tensile strength of 5.15 MPa. It is unexpected that the elongation at break values of PU/RGO-10 and PU/RGO-20 are greatly improved from 302.7% for the pristine PU nanofiber mat to 334.6 and 427.2%, respectively. The comprehensive improvement of mechanical properties mainly comes from the uniform decoration of RGO on the surface of the nanofibers, which promotes efficient stress transfer at the composite interface during stretching. Figure F

DOI: 10.1021/acs.iecr.9b01766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

Figure 7. Variation of the contact angle and conductivity for the RGO-decorated nanofiber composite under (a) different strains and (b) different stretching−releasing cycles.

formation of the core−shell structure. From FTIR results, chemical interaction between the N−H group of the polymer and the residual oxygen-containing group in RGO might occur during RGO assembly. The nanofiber composite combines the flexibility of polymer and functionalities of RGO. Thus, the polymer nanofiber mat (a) became more hydrophobic with the contact angle increasing from ∼114 to 123°; (b) changed from an electrically insulating material to an electrically conductive material owing to the formation of a continuous RGO network in the composite (which strongly depended on the solvent used to disperse the RGO during ultrasonication); (c) possessed largely enhanced tensile strength, Young’s modulus, and elongation at break; and (d) served as a template for the preparation of RGO tubes by pyrolysis, which has potential applications in the supercapacitor and lithium-ion battery.

environmental protection due to their light weight, excellent electrical conductivity, and environmental stability. Template method was often employed for the preparation of hollow RGO microspheres,46 and the macroscopic RGO tube was usually fabricated by a coagulation spinning method.47 Here, in this study, RGO was assembled on the nanofiber surface, possessing a core−shell structure, and thus, the RGO tube could be formed after the removal of the core polymer by pyrolysis at 800 °C. Figure 8 shows the TEM image of RGO

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.9b01766.

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Figure 8. TEM image of RGO tube after removal of PU nanofiber core by pyrolysis. Scale bar: 1 μm.

tubes. The nanotube composed of wrinkled RGO was found, and many black dots with the diameters of a few hundred nanometers were also present, which were amorphous carbons derived from the pyrolysis of PU. The thickness for the RGO shell is around 100 nm (Figure S3). Some tubes with diameters of a few micrometers were also found, which was mainly caused by the fuse between the RGO-based nanofiber composites at the beginning of the pyrolysis. The RGO tube derived from the polymer nanofiber composite may have potential applications in energy such as a supercapacitor and a lithium-ion battery.

AFM images and corresponding height profiles of RGO, XPS results for GO and RGO, TEM image of cross section of the nanofiber composite possessing a polymer nanofiber core/graphene RGO (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (J.-f.G.). *E-mail: [email protected] (X.D.). ORCID

Jie-feng Gao: 0000-0002-6038-9770 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by Natural Science Foundation of China (nos. 51873178, 21878031, 51503179, 21673203), the Opening Project of State Key Laboratory of Polymer Materials Engineering (Sichuan University) (no. sklpme2018-4-31), Qing Lan Project of Jiangsu province, the China Postdoctoral Science Foundation (no. 2016 M600446), the Jiangsu Province Postdoctoral Science Foundation (no. 1601024A), the Priority Academic Program Development of Jiangsu Higher Education Institutions, Natural Science Foundation of Liaoning Province (no. 20180551075), Liaoning Revitalization Talents Program (no. XLYC1802124).

4. CONCLUSIONS In this work, we proposed a facile method, namely, ultrasonication-induced RGO assembly onto electrospun nanofiber surfaces, to prepare RGO-decorated nanofiber composites with a core−shell structure. Ultrasonication not only assisted the dispersion of RGO without surfactant but also forced RGO decoration toward the nanofiber. The interfacial collision between flexible RGO and viscoelastic polymer nanofiber during ultrasonication was responsible for the G

DOI: 10.1021/acs.iecr.9b01766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research

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DOI: 10.1021/acs.iecr.9b01766 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX