An Innovative Approach for the Preparation of High-Performance

Dec 6, 2017 - The aim of this study is to prepare high-performance poly(p-phenylene) (PPP)-based polymer nanofiber belts. The hypothesis is that these...
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Article Cite This: Macromolecules XXXX, XXX, XXX−XXX

An Innovative Approach for the Preparation of High-Performance Electrospun Poly(p‑phenylene)-Based Polymer Nanofiber Belts Wenhui Xu,†,‡ Yichun Ding,‡ Ting Yang,† Ying Yu,† Runzhou Huang,† Zhengtao Zhu,‡ Hao Fong,*,‡ and Haoqing Hou*,† †

College of Chemistry and Chemical Engineering, Jiangxi Normal University, Nanchang, Jiangxi 330022, China Program of Biomedical Engineering, South Dakota School of Mines & Technology, Rapid City, South Dakota 57701, United States

‡

S Supporting Information *

ABSTRACT: The aim of this study is to prepare highperformance poly(p-phenylene) (PPP)-based polymer nanofiber belts. The hypothesis is that these nanofiber belts would possess high mechanical properties, excellent thermal and chemical resistances, and unique electrical and photoelectrical characteristics owing to high rigidity of macromolecular backbones. In general, the unsubstituted PPP polymers are infusible and insoluble in common organic solvents; thus, the synthesis and processing of these polymers are intractable. Although some substituted PPP-based polymers (i.e., PPP derivations) are soluble, their molecular weights are too low to be processed into nanofibers (particularly by the electrospinning technique). To date, there has been no report on the preparation of any kind of PPP-based polymer nanofibers. In this study, four soluble PPP-based oligomers of phthalate-capped poly(2,5-benzophenone) (PBPA) with varied molecular weights were synthesized via Ni(II) complex-catalyzed cross-coupling reaction; subsequently, the blend nanofiber belts of poly(2,5benzophenone)−pyrrolone (PBPY) and polyimide (PI) were made by the combination of electrospinning and molecular coupling assembly techniques followed by heat treatment, wherein the use of poly(amic acid) (PAA, the precursor of PI) as carrier/glue polymer for assisting electrospinning is crucial for successful preparation of the nanofibers. The PBPY/PI nanofiber belts exhibited high mechanical strength, superior thermal stability, and chemical resistance; hence, they could be used as filtration media under high-temperature and/or corrosive conditions, and they could also be used as separators in batteries and supercapacitors. It is important to note that this is the first reported study on the preparation of PPP-based polymer nanofibers; additionally, this study also provides an innovative approach for making nanofibers from the polymers that cannot be electrospun directly.

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INTRODUCTION Poly(p-phenylene) (PPP)-based polymers are a class of conducting polymers with the repeating units containing the rigid-rod component of p-phenyl;1−3 in recent decades, they have attracted extensive research interests because of extraordinary properties such as excellent thermal and chemical resistances, high mechanical and self-lubricating properties, and unique electrical and photoelectrical characteristics.4−9 However, the unsubstituted PPP is infusible and insoluble in common organic solvents due to high rigidity of its macromolecular backbone, making the synthesis and direct processing intractable; as a result, many potential applications are restricted.9,10 To address this problem, numerous approaches have been studied, for example, preparing PPP thin films by electrochemical polymerization11,12 and surface-assisted syntheses,13−15 synthesizing soluble substituted PPP-based polymers (i.e., PPP derivations) via the approaches including design and synthesis of novel monomers,16,17 copolymerization,18,19 and incorporation of other chemical components (e.g., alkyl/ alkoxy,20,21 aryl,22 carboxyl,23 and ester groups24,25) as side chains in the repeating units. Although various PPP-based © XXXX American Chemical Society

polymers have been made through the above approaches and have been demonstrated for many electronic applications such as light-emitting diodes,26−28 solar cells,29,30 and fuel cells,31 these PPP-based polymers generally have low molecular weights, and they are often produced as powders, dilute solutions, or thin films on substrates (e.g., indium tin oxide (ITO) and metal). Nowadays, research endeavors have been devoted to synthesize soluble PPP-based polymers with reasonably high molecular weights. Poly(2,5-benzophenone) (PBP, a PPPbased polymer with benzoyl substituents in the repeating unit) is a new class of rigid-rod promising one because it not only has relatively high molecular weight but also is soluble in some common organic solvents (e.g., DMF, DMAc, and THF); hence, PBP can be easily processed into films.2,32−34 Phillips and co-workers reported that their synthesized PBP had the glass transition temperature of 206 °C, and the polymer had the Received: September 8, 2017 Revised: November 23, 2017

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Figure 1. Schematic illustration depicting the preparation of PBPY/PI blend nanofiber belts via the combination of electrospinning and molecular coupling assembly techniques followed by heat treatment.

treatment. Hence, the use of PAA as carrier/glue polymer will effectively facilitate the formation of electrospun nanofibers, and the performance of final PPP-based nanofiber belts will be high. Within these PBPA-DAB/PAA nanofibers, the PBPA and DAB molecules were coupling-assembled through the neutralization reaction between the end-capped carboxyl groups in PBPA and the amino groups in DAB to form higher molecular weight supramolecules (PBDS), in which the assembly joints were ammonium bonds (i.e., electrostatic interactions between deprotonated carboxylate ions and protonated ammonium ions).51 Upon being heated to 300 °C or higher, the ammonium bonds would be converted into chemical bonds, leading to the formation of pyrrolone molecular structure. Consequently, high-molecular-weight polymer of poly(2,5benzophenone)−pyrrolone (PBPY) with rigid/extended macromolecular backbone was made. Meantime, PAA would be converted into PI. Therefore, the finally obtained materials were PBPY/PI blend nanofiber belts, and the overall preparation process is schematically depicted in Figure 1. During the above preparation process, the heat-treatment temperature was important because it was not only crucial for chemically converting weak ammonium bonds into strong pyrrolone molecular structures but also correlated to mechanical/thermal properties of final PBPY/PI blend nanofiber belts. Therefore, different heat-treatment temperatures (i.e., 300, 330, 350, 370, and 400 °C) were investigated to understand the impacts on chemical structure conversion and on mechanical/thermal properties. Furthermore, due to polydispersity, the amount of end-capped carboxyl groups in PBPA was impossible to be determined accurately; as a result, the amount of DAB coupling agent had to be optimized. Finally, electrospun PBPY/PI nanofiber belts were prepared from different molecular weight PBPAs, and their mechanical properties, thermal stabilities, and chemical resistances were studied. Additionally, these PBPY/PI nanofiber belts exhibited superior chemical resistances toward various harsh environments such as boiling water (100 °C), 7.14 M H2SO4 solution (100 °C), and 6 M KOH solutions (30, 60, and 100 °C), and the retention ratios of mechanical strength upon harsh treatments were also investigated.

5% weight loss temperatures in air and nitrogen of 496 and 495 °C, respectively.32 Wang and Quirk reported that a cast film of PBP exhibited the tensile modulus of >6 GPa and the tensile breaking strength of ∼900 MPa.33 Furthermore, the company of Maxdem, Inc., has successfully commercialized a PBP selfreinforced engineering resin (PX-1000),34 and Friedrich et al. reported that the resin could be readily processed by extrusion and compression molding; the resulting film exhibited the tensile strength, Young’s modulus, and compressive strength of 207, 8.3, and 620 MPa, respectively.8 Nevertheless, there has been no report on the preparation of any PPP-based polymer fibers. The electrospinning technique, which was reinvented by Reneker in the 1990s,35,36 provides a straightforward approach for making continuous polymer fibers with diameters typically being hundreds of nanometers (commonly known as electrospun polymer nanofibers); in recent years, electrospun polymer nanofibers have been investigated for a variety of applications.37−40 Some polymer nanofibers (e.g., the nanofibers of polyacrylonitrile,41 polyamide,42 polyurethane,43 poly(ethylene oxide),44 and poly(vinyl alcohol)45) can be simply made by directly electrospinning their solutions, whereas some other polymer nanofibers can be made by electrospinning the solutions of their precursors followed by postspinning treatments. For example, polyimide (PI) nanofibers can be prepared by electrospinning the solution of its soluble precursor (i.e., poly(amic acid)) followed by the post-treatment of thermal imidization.46−50 Unfortunately, neither approach is applicable to prepare electrospun nanofibers of PPP-based polymers because these polymers are either insoluble or easy to form nonprocessable gels due to rigid macromolecular structures, while their precursors (i.e., PPP-based oligomers) typically have low molecular weights; thus, the continuous nanofibers cannot be generated from electrospinning. Herein, we report an innovative approach for preparing high molecular weight PPP-based polymer nanofiber belts by the combination of electrospinning and molecular coupling assembly techniques followed by heat treatment. In specific, a series of four phthalate-capped poly(2,5-benzophenone) (PBPA) oligomers with varied molecular weights were first synthesized by Ni(II) complex-catalyzed cross-coupling reaction.33 Thereafter, by using these PBPAs as assembling units, 3,3′-diaminobenzidine (DAB) as coupling agent, and poly(amic acid) (PAA) as carrier/glue polymer, four types of PBPA-DAB/ PAA nanofiber belts were prepared by electrospinning the mixture solution of PBPA/DAB/PAA in DMAc. It is necessary to note that PAA is a high-molecular-weight polymer that is easy to be electrospun into morphologically uniform nanofibers; moreover, PAA can be readily converted into the highperformance polymer of PI upon simple high-temperature

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EXPERIMENTAL SECTION

Materials. Triphenylphosphine (PPh3, AR), nickel(II) chloride (NiCl2, anhydrous), and 2,2′-dipyridyl (BPY, AR) were purchased from Bide Pharmatech Co., Ltd. (Shanghai, China). 2,5-Dichlorobenzophenone (98%) and dimethyl 4-bromophthalate (98%) were purchased from Zhengzhou Alfachem Co., Ltd. (Zhengzhou, China). 3,3′,4,4′-Biphenyltetracarboxylic dianhydride (BPDA, 99%), 3,3′diaminobenzidine (DAB, 99%), and 4,4′-diaminobiphenyl (BPA, 99%) were purchased from Changzhou Sunlight Pharmaceutical Co., Ltd. (Changzhou, China). Sodium hydroxide (NaOH, AR), potassium B

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Scheme 1. Synthesis of PBPAs Consisting of Two Steps Including (1) Ni(II) Complex-Catalyzed Cross-Coupling of 2,5Dichlorobenzophenone and Dimethyl 4-Bromophthalate and (2) Hydrolysis of Dimethyl Phthalate-Capped Poly(2,5benzophenone)

Scheme 2. Synthesis Scheme and Chemical Structure of the PBPY Polymer

hydroxide (KOH, AR), tetrahydrofuran (THF, AR), hydrochloric acid (HCl, AR), methanol (AR), sulfuric acid (H2SO4, AR), N,Ndimethylacetamide (DMAc), and zinc powder (Zn) were purchased from Shanghai Chemical Reagents Co. Ltd. (Shanghai, China). Prior to being used, DMAc was dried with excess amount of calcium hydride (CaH2) for 48 h under the stirring condition, and this was followed by distilling under the reduced pressure and then stored with 4 Å molecular sieves under dry nitrogen. Zinc powder was pretreated with 10 wt % hydrochloric acid, filtered, rinsed with anhydrous ether (Et2O), dried in a vacuum oven at 150 °C, and then stored under dry nitrogen. BPDA, BPA, and DAB were prepurified by vacuum sublimation. Catalyst (NiCl2BPy) Preparation. Anhydrous NiCl2 (3.25 g, 25.10 mmol) was mixed with 50 mL of ethanol in a 250 mL threenecked round-bottom flask equipped with a reflux condenser, and the system was then heated to 50 °C. After NiCl2 being dissolved, 25 mL of 2,2′-dipyridyl (3.15 g, 20.10 mmol) ethanol solution was added dropwise using a constant pressure titration funnel. After 12 h, a grassgreen precipitate was generated, and the precipitate was filtered, rinsed with ethanol, dried in a vacuum oven at 150 °C, and then stored under dry argon. The yield of Ni(II) complex catalyst (NiCl2BPy) was ∼84%. Synthesis of Phthalate-Capped Poly(2,5-benzophenone) Oligomers (PBPAs). Molecular weight is one of the key parameters on the properties of polymers; thus, the molecular weight of PBPAs was optimized first. Four soluble PBPAs with different molecular weights were prepared via the Ni(II) complex-catalyzed cross-coupling reaction, wherein the molecular weight was controlled by varying

addition time of dimethyl 4-bromophthalate, which was used as the end-capper.52 The synthesis procedures are shown in Scheme 1. In specific, 2,5-dichlorobenzophenone (2.5 g, 10 mmol), NiCl2BPy (0.29 g, 1 mmol), Zn powder (2 g, 30.6 mmol), and PPh3 (1.04 g, 4 mmol) were added into a 100 mL three-necked round-bottom flask equipped with a reflux condenser and an overhead stirrer under an argon atmosphere; the end-capper dimethyl 4-bromophthalate (0.27 g, 1 mmol) was then added, and the time for adding the end-capper was controlled to adjust the molecular weight of the resulting PBPA. Upon the addition of end-capper at the beginning of reaction (0 h), 2 h later, 4 h later, and 6 h later, four PBPAs with different molecular weights were prepared, which were denoted as P1, P2, P3, and P4, respectively. Subsequently, 30 mL of dried DMAc was added into the flask by using a syringe, and the mixture was then stirred at 70 °C for 12 h to produce a gel/solution. Thereafter, the obtained gel/solution was poured into a coagulation solution of methanol and 10 wt % hydrochloric acid, and the acquired precipitate was filtered and then rinsed with the coagulation solution for three times. Finally, the light yellow dimethyl phthalate-capped poly(2,5-benzophenone) powder (I) was obtained upon drying the precipitate in a vacuum oven at 80 °C, and the yield was ∼75%. The obtained polymer (I) was then dissolved in 30 mL of solution of THF/water (with volume ratio of 1/1) with NaOH (2.4 g, 60 mmol) in a 100 mL three-neck flask equipped with a reflux condenser followed by being magnetically stirred at 60 °C. After 2 h, the color of this solution started to turn into orange; after that, the solution was kept being stirred for an additional 6 h. Thereafter, the solution was cooled down to room temperature, and the pH value of this solution C

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in various harsh conditions including boiling water (100 °C), 7.14 M H2SO4 solution (100 °C), and 6 M KOH solutions (30, 60, and 100 °C). The P4BPY/PI nanofiber belts were placed in the above conditions for different time periods (e.g., 3, 6, 12, 18, 24, 36, 48, and 72 h); subsequently, they were thoroughly rinsed with distilled water and then dried at 30 °C for 12 h in a vacuum oven. Thereafter, the mechanical properties of these nanofibers belts were measured, and the morphology changes were examined by SEM. Characterization. Chemical structures of PBPAs were characterized by nuclear magnetic resonance (NMR) spectroscopy, and 1H NMR spectra were acquired at 30 °C from a Bruker Avance-400 instrument (Germany) with the solvent being deuterated dimethyl sulfoxide (DMSO-d6). Fourier transform infrared spectroscopy (FTIR, Tensor 27, Bruker, Germany) was also employed to study the chemical structures of PBPA, PBDS/PAA precursor nanofiber belts, and PBPY/PI nanofiber belts in the attenuated total reflection (ATR) mode. The molecular weights and polydispersities of PBPA samples were analyzed at room temperature by gel permeation chromatography (GPC, Waters Breeze 2 HPLC, 80 Hz, USA) with trichloromethane (CHCl3) being the solvent. Scanning electron microscopy (SEM, FEI Quanta 200 FEG, USA) was employed to examine the morphologies of different nanofibers. Mechanical properties of the nanofibers belts were measured by using a computer-controlled electromechanical testing machine (Shenzhen Sans Technology Stock Co., China). The tensile tests were carried out at room temperature with the tensile speed of 5 mm/min. The sample thickness (d) was calculated from its density: d = m/(ρ × a × b), according to our previously reported method,48 where m, a, and b are the mass, length, and width of the tested nanofiber belt, while the density (ρ) of a PBPY/PI nanofiber belt was determined from the corresponding cast film and calculated by ρ = m/(d × a × b), where m, d, a, and b are the mass, thickness, length, and width of the corresponding cast film. The mechanical (tensile) properties of each nanofiber belt were tested by using five specimens, and the average value and standard deviation were calculated and reported. Thermogravimetric analysis (TGA, Beijing Henven Scientific Instrument Factory, China) was adopted to study the thermal stability of PBPY/PI nanofiber belts under nitrogen and air atmospheres, and the TGA curves were acquired at the heating rate of 10 °C/min. Dynamic mechanical properties of PBPY/PI nanofiber belts were evaluated by a PerkinElmer dynamic mechanical analyzer (DMA) in tensile mode under the nitrogen atmosphere with the heating rate of 5 °C/min in the temperature range 30−450 °C. Transmission electron microscopy (TEM, JEM-2100) was employed to study the structures of PBPY/PI nanofibers before and after treatments. The orientation of PBPY/PI macromolecules in a nanofiber belt was investigated by small-angle X-ray scattering (SAXS) at Shanghai Synchrotron Radiation Facility (in China).

was adjusted to be lower than 3 (i.e., pH = 1−3) using 0.1 M hydrochloric acid to acquire a precipitate. Subsequently, the precipitate was filtered, boiled in 0.1 M hydrochloric acid for 30 min, and then rinsed with distilled water for several times. Finally, the solid was dried in a vacuum oven at 60 °C to obtain the final product (PBPA) (II), and the yield was ∼90%. Synthesis of PAA from BPDA and BPA. PAA was synthesized according to our previously reported method.49 In brief, equimolar monomers of BPDA and BPA were added in DMAc for being polymerized at a low temperature (−5 to 0 °C) for 24 h, and the solid content was maintained at 10 wt %. The intrinsic viscosity of assynthesized 10 wt % PAA solution was 4.0 dL/g (measured in DMAc at 25 °C). Preparation of PBPY/PI Nanofibers. As shown in Scheme 2, PBPA and DAB were mixed in DMAc (with solid content of ∼30 wt %) at −5 °C and stirred for 2 h to self-assemble into PBPA/DAB supramolecules (PBDS) via the formation of ammonium bonds between these two monomers. Subsequently, the carrier/glue polymer of PAA (10 wt % solution) was added (with the solid mass ratio of (PBPA + DAB)/PAA being 9/1), yielding a 15−18 wt % PBDS/PAA solution for the subsequent electrospinning. Electrospun PBDS/PAA precursor nanofiber belts were prepared from the above solutions. The electrospinning was carried out by applying the positive voltage of 20 kV to the spinneret and the negative voltage of −5 kV to the collector across the gap/distance of 25 cm. A rotating disc with the diameter of 30 cm and edge width of 2 cm was used as the collector, and the rotating speed was set at 1000 rpm; the solution feeding rate was 0.8 mL/h controlled by a digital syringe pump. The obtained belts with aligned PBDS/PAA precursor nanofibers were dried in a vacuum oven at 80 °C for 6 h to completely remove residual solvents. The conversion of PBDS/PAA precursor nanofiber belts to final PBPY/PI nanofiber belts was conducted by using the following heattreatment procedure: (1) room temperature to 250 °C at the heating rate of 10 °C/min, (2) 250 °C for 30 min, (3) 250 °C to final temperature (300, 330, 350, 370, or 400 °C) at the heating rate of 5 °C/min, and (4) the final temperature for 60 min. After the heat treatment, the assembly joints of ammonium bonds were chemically converted into pyrrolone molecular structures (as shown in Scheme 2), while PAA was also chemically converted into PI. Thereafter, the PBPY/PI nanofiber belts (with morphological structures shown in Figure 1) were obtained. Optimization of Heat-Treatment Temperature. Oligomer P2 (Mn ∼ 3284 Da, polydispersity ∼ 1.57) was selected to study the effect of heat-treatment temperature on the properties of PBPY/PI nanofiber belts. P2 (5 g, 1.5 mmol) and DAB (0.33 g, 1.5 mmol) were dissolved in 13 mL of dried DMAc at a low temperature (−5 °C) and then stirred for 2 h; subsequently, 10 wt % BPDA-BPA PAA (5.33 g) and 10 mL of DMAc were added to prepare 18 wt % P2BDS/PAA precursor solution. The precursor solution was then electrospun into nanofibers belts. For the optimization of heat treatment, different final temperatures (i.e., 300, 330, 350, 370, and 400 °C) were studied. Consequently, a series of P2BPY/PI nanofibers belts were obtained, and they were denoted as P2BPY/PI-300, P2BPY/PI-330, P2BPY/PI350, P2BPY/PI-370, and P2BPY/PI-400, respectively. Optimization of Molar Ratio of PBPA/DAB. The molecular weight of PBPA, as well as the amount of −COOH functional groups, could not be accurately measured; such a situation would seriously affect the coupling degree of PBPA and DAB during the assembly process, further leading to the property variation of nanofibers belts. To investigate the influence, a series of P3BPY/PI nanofibers belts were prepared by using P3 (Mn ∼ 4621 Da, polydispersity ∼ 1.62) and DAB with varied molar ratios of 1:0.5, 1:0.6, 1:0.7, 1:0.75, 1:0.8, 1:0.9, and 1:1; and the obtained nanofiber belts were denoted as P3BPY/PI1, P3BPY/PI-2, P3BPY/PI-3, P3BPY/PI-4, P3BPY/PI-5, P3BPY/PI-6, and P3BPY/PI-7, respectively. Thereafter, the structures and properties of these P3BPY/PI nanofibers belts were examined to determine the best molar ratio of PBPA/DAB for subsequent studies. Investigation of Chemical Resistance. Chemical resistance was investigated upon immersing the prepared P4BPY/PI nanofibers belts

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RESULTS AND DISCUSSION

Synthesis of PBPAs. PBPAs were synthesized by Ni(II) complex-catalyzed cross-coupling of 2,5-dichlorobenzophenone,33,53 during which an end-capper of dimethyl 4-bromophthalate was used to control the molecular weight of PBPAs and to simultaneously provide the end groups of orthopositioned dicarboxylic acid for the subsequent coupling assembly. The NiCl2BPy catalyst was prepared ex situ based on the previously reported procedure.54,55 Through varying the time when the end-capper was added, four oligomers of PBPAs (i.e., P1, P2, P3, and P4) with different molecular weights were synthesized. Figure S1 (Supporting Information) shows the GPC results, while the calculated molecular weights (i.e., Mn, Mw, Mp, Mz, and Mz+1) are summarized in Table S1 (Supporting Information). In general, the molecular weights of PBPAs increased with the delay of adding the end-capper; for example, the number-average molecular weight (Mn) increased from 2421 Da (P1) to 5566 Da (P4); the weightaverage molecular weight (Mw) also increased from 4903 to D

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ring; (4) 820, 752, and 698 cm−1, attributed to the out-of-plane C−H wagging of the benzene ring; (5) 3062 and 3026 cm−1, assigned to the aromatic C−H vibration; and (6) 2927 and 2581 cm−1, attributed to the −OH stretching in −COOH.57 The 1H NMR spectra acquired from these PBPA oligomers are shown in Figures S2−S5. The 1H NMR (400 MHz, DMSOd6), d(ppm)-P1: 8.27 (s, 2H), 8.21 (d, 2H), 8.08−7.96 (m, 24H), 7.9−7.82 (m, 12H), 7.77−7.76 (dd, 12H), 7.66−7.62 (m, 24H), 7.62−7.56 (m, 12H), 7.42−7.4 (m, 12H, 7.27−7.23 (m, 2H); d(ppm)-P2: 8.28−8.26 (s, 2H), 8.2 (d, 2H), 8.08− 7.96 (m, 32H), 7.9−8.89 (d, 16H), 7.83−7.73 (m, 16H), 7.69− 7.61 (m, 32H), 7.57−7.53 (m, 16H), 7.41−7.4 (m, 16H), 7.27−7.23 (m, 2H); d(ppm)-P3: 8.28 (s, 2H), 8.2 (d, 2H), 7.9−7.76 (dd, 48H), 7.65−7.6 (dd, 24H), 7.58−7.55 (m, 24H), 7.41−7.36 (m, 48H), 7.26−7.25 (m, 24H), 7.08 (m, 24H), 6.95 (m, 2H); d(ppm)-P4:8.13 (s, 2H), 7.93−7.88 (d, 2H), 7.8− 7.75 (d, 60H), 7.64−7.6 (dd, 30H), 7.6−7.49 (dd, 30H), 7.41− 7.39 (m, 60H), 7.26−7.25 (d, 30H), 7.07 (m, 30H), 7.93−7.87 (d, 2H). The FTIR and 1H NMR results indicated that the synthesized PBPA oligomers contained benzophenone repeating unit and end-capped phthalate groups. Preparation of PBPY/PI Nanofibers. Molecular weights of the above PBPA oligomers were too low to be electrospun into continuous nanofibers; hence, the chain-extension strategy of using a coupling agent (DAB) was adopted to convert PBPA oligomers into higher molecular weight polymers. In specific, the coupling was accomplished through the reaction between the end groups of −COOH in PBPAs and −NH2 in DAB, and the rigid ladder pyrrolone structures would be formed upon neutralization followed by high-temperature treatment. However, the mixture solution containing PBPA and DAB alone could not be electrospun as well because the solution viscosity would not be high enough for the formation of continuous nanofibers. As a result, PAA was used as carrier/glue polymer during electrospinning; note that the PAA/DMAc solution could be easily electrospun into continuous and morphologically uniform nanofibers, while PAA nanofibers could be thermally converted into high-performance PI nanofibers. During electrospinning, only a small amount (10 wt %, solid content ratio) of PAA was used, whereas the PBDS/PAA nanofibers with good morphological structures could be

8193 Da. Therefore, the polymerization degrees of PBPAs were in the range 10−30, indicating that they were oligomers (instead of polymers). The polydispersity index (PDI = Mw/ Mn) values of these PBPA oligomers were in the range from 1.47 to 2.02, which were reasonably in agreement with the theoretical PDI values of polymers/oligomers synthesized by step/condensation polymerization. The peak molecular weight (Mp) of P4 showed the highest value of 6771 Da, while the Mp values of P1, P2, and P3 were lower (i.e., 5458, 3985, and 4986 Da), as shown in Figure S1. It is important to note that the prepared PBPA oligomers were soluble in common organic solvents such as THF, CHCl3, DMF, and DMAc. Chemical structures of PBPA oligomers were characterized by FTIR and 1H NMR. As shown in Figure 2, the four PBPA

Figure 2. FTIR spectra acquired from four PBPA oligomers of P1, P2, P3, and P4.

oligomers had similar absorption bands: (1) 1780−1730 cm−1, assigned to the CO (in −COOH) asymmetric and symmetric stretching; (2) 1662 cm−1, assigned to the CO stretching of carbonyl group in benzophenone unit;56 (3) 1598 cm−1 (strong), 1495 cm−1 (weak), and 1470 and 1443 cm−1 (splitting peaks), attributed to the CC stretching in benzene

Figure 3. SEM images acquired from the nanofiber belts of P1BDS/PAA (a), P2BDS/PAA (b), P3BDS/PAA (c), and P4BDS/PAA (d). SEM images acquired from the resulting nanofiber belts of P1BPY/PI (a′), P2BPY/PI (b′), P3BPY/PI (c′), and P4BPY/PI (d′). E

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and 1280 cm−1 were respectively attributed to the N−H bending vibration (amide II) and the C−N stretching vibration (amide III). These characteristic absorptions indicated that PAA had been converted to PI through thermal imidization. Meantime, PBDS had been converted to PBPY; in other words, the ammonium bonds had been turned into pyrrolone structures (containing CO, CN, and C−N bonds).60,61 Additionally, Figure S6 shows the FTIR spectra acquired from the four types of PBPY/PI nanofiber belts, and the four FTIR spectra had no distinguishable difference. Figure S7 shows that the 13C NMR spectrum acquired from P2BPY/PI nanofiber belts (solid, δ ppm: 220, 197, 151, 138, 128, and 111), which also confirmed the formation of PBPY polymer, and Figure S8 shows representative stress−strain curves acquired from the precursor nanofiber belts (PBDS/PAA); the tensile stress values of the precursor nanofiber belts were in the range from ∼22 MPa for P1BDS/PAA to ∼29 MPa for P4BDS/PAA partially due to the weak ammonium bonds between PBPA and DAB. Influence of Heat-Treatment Process. High-temperature treatment process of electrospun PBDS/PAA nanofiber belts was investigated. The process would not only affect chemical conversions of PBDS into PBPY and PAA into PI but also affect the morphologies and properties (e.g., crystallinity, mechanical strength, and thermal stability) of the final PBPY/ PI blend nanofibers. It is known that the temperature required for the formation of pyrrolone structure is above 200 °C,62,63 while the required temperature for the conversion of PAA into PI is above 300 °C.46 Therefore, the heat-treatment process was studied at the temperatures of 300, 330, 350, 370, and 400 °C, and the P2BDS/PAA precursor nanofiber belt was selected for this study. The SEM images in Figure 5 were acquired from as-

obtained. As shown in Figure 3, the four SEM images (in the top row) were acquired from the electrospun nanofiber belts of P1BDS/PAA (a), P2BDS/PAA (b), P3BDS/PAA (c), and P4BDS/PAA (d). All of the nanofibers were continuous and uniform without microscopically identifiable beads and/or beaded nanofibers. The nanofiber diameters were in the range 700−900 nm, and there were no distinguishable discrepancies among the four kinds of nanofibers. It was also evident that the degrees of nanofiber alignment were reasonably high. Upon the heat treatment at 330 °C, the resulting PBPY/PI nanofibers retained good morphological structures; moreover, there were no appreciable reductions on diameters, as evidenced by the SEM images (in the bottom row of Figure 3). Chemical changes resulted from the coupling and heat treatments were studied by FTIR. The FTIR spectra in Figure 4

Figure 4. FTIR spectra acquired from PBPA oligomer, PBDS/PAA precursor nanofiber belt, and PBPY/PI nanofiber belt. Note that the FTIR spectra acquired from four PBPA oligomers had no appreciable difference; hence, the spectrum of P2 oligomer was included for comparison.

were acquired from the PBPA oligomer, PBDS/PAA nanofiber precursor belt (dried at 60 °C), and PBPY/PI nanofiber belt (treated at 330 °C). Compared to the PBPA spectrum, the PBDS/PAA spectrum had a significant absorption at 3345 cm−1, which could be attributed to the stretching vibration of −NH2 (or −NH3+) in DAB and/or the stretching vibration of −NH (amide A) in PAA. Furthermore, the absorptions at 2927 and 2581 cm−1 (stretching vibration of −OH) in PBDS/PAA spectrum became weak, while new absorptions emerged at 1496 cm−1 (amide II, N−H bending vibration) and 1275 cm−1 (amide III, C−N stretching vibration).58 Additionally, the absorptions of CO at 1780, 1730, and 1662 cm−1 were merged into a wide splitting band around 1600−1662 cm−1 (amide I, CO and CC stretching). The above results indicated that the PBPA and DAB were coupling reacted, leading to the formation of ammonium bonds as the joints. After high-temperature treatment at 330 °C, the absorptions at 3345, 2927, and 2581 cm−1 disappeared. The absorptions in PBPY/PI spectrum at 1774 and 1719 cm−1 were respectively attributed to the asymmetrical and symmetrical stretching of CO in imide structure,59 the absorption at 1668 cm−1 was attributed to the CO stretching in benzophenone structure,56 the absorption at 1600 cm−1 was attributed to the CC stretching in benzene structure, the absorption at 1578 cm−1 (with a shoulder at 1600 cm−1) was attributed to the CN stretching,60 the absorption at 1370 cm−1 was attributed to the C−N stretching in imide structure, and the absorptions at 1496

Figure 5. SEM images acquired from as-electrospun P2BDS/PAA nanofiber belt (a) and the resulting P2BPY/PI nanofiber belts prepared at different heat-treatment temperatures of 300 (b), 330 (c), 350 (d), 370 (e), and 400 °C (f).

electrospun P2BDS/PAA nanofibers (a) as well as from P2BPY/ PI nanofibers (b−f) prepared at different heat-treatment temperatures. It is evident that all of the nanofibers remained continuous and aligned after heat treatment, and the fiber diameters did not have distinguishable changes. The diameter distributions acquired from as-electrospun P2BDS/PAA nanofiber belt and the resulting P2BPY/PI nanofiber belts (prepared at different heat-treatment temperatures) are shown in Figure S9. The diameters were in the range 700−900 nm, and the average diameter was ∼785 nm. As shown in Figures 5e and 5f, the P2BPY/PI nanofibers prepared at high temperatures of 370 F

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Macromolecules and 400 °C were partially conglutinated/fused together, and this situation was caused by melting of nanofibers at such high temperatures. The corresponding SEM images of the cross sections of the P2BPY/PI nanofiber belts (Figure S10) indicate that the nanofiber morphology was retained after heat treatment, while some P2BPY/PI nanofibers were partially conglutinated/fused together when the treatment temperatures were 370 and 400 °C. The FTIR spectra of P2BPY/PI nanofibers (Figure S11) were almost the same when the treatment temperature was 300 °C or higher, revealing that the PBPY/PI structure could be formed when the temperature reached 300 °C yet the chemical structures would not vary appreciably when the temperature was as high as 400 °C. Figure 6 shows stress−strain curves acquired from different P2BPY/PI nanofiber belts. The tensile stress value of the

Figure 6. Representative stress−strain curves acquired from P2BPY/PI nanofiber belts prepared at different heat-treatment temperatures.

Figure 7. (a) 2D SAXS images of P2BPY/PI nanofiber belts prepared at different heat-treatment temperatures of 300 (A), 330 (B), 350 (C), 370 (D), and 400 °C (E). (b) Azimuthally integrated intensities of the 2D SAXS patterns acquired from different P2BPY/PI nanofiber belts.

P2BPY/PI-300 nanofiber belt was ∼163 MPa, while the P2BPY/PI-330 nanofiber belt exhibited the highest value at ∼197 MPa. Mechanical properties of the nanofiber belts would be influenced by several factors such as crystallinity, macromolecular orientation, and degree of fiber alignment. It is noteworthy that both PBPY and PI polymers have rigid-rod backbones, and their macromolecules can be thermally induced to crystallize; as a result, the mechanical strength will be considerably increased. However, if the treatment temperature is too high, the PBPY and PI macromolecules can be decomposed and the nanofibers may be partially melted. In consequence, the mechanical strength will be substantially decreased. Macromolecular orientations in the P2BPY/PI nanofiber belts prepared at different heat-treatment temperatures were investigated by SAXS, and the 2D SAXS patterns are shown in Figure 7; additionally, the corresponding curves of azimuthally integrated intensity are also shown in the figure. In general, the macromolecules in electrospun polymer nanofibers are not perfectly aligned along the fiber axes; instead, the macromolecules are typically oriented to have an average angle (φ). The Herman’s orientation function (f) can be estimated based on the angular spread (φ) of the SAXS peaks using the equation f = 1/2(3 cos2 φ − 1).64 Upon the calculation, the f values of the P2BPY/PI nanofiber belts increased from 0.32 to 0.37 with the increase of heat-treatment temperature from 300 to 400 °C. Although the P2BPY/PI nanofiber belts prepared at 400 °C exhibited higher molecular orientation, the macromolecules might start to degrade at such a high temperature. The wide-angle X-ray diffraction patterns of P2BPY/PI

nanofiber belts prepared at different heat-treatment temperatures are shown in Figure S12. The results showed that the P2BPY/PI-300 and P2BPY/PI-330 had two weak reflection peaks at 2θ angles of 13.5° and 19.8°, which could be attributed to the PBPA oligomer,9 indicating that the crystallinity of PBPY/PI might be very low. After the heat treatment at 350 °C or higher, the diffraction peaks were almost the same (with broader peaks); therefore, the PBPY/PI nanofibers were structurally amorphous as evidenced by no diffraction peaks could be identified. Upon the consideration of high mechanical strength of P2BPY/PI-330 nanofiber belts, the optimal heattreatment temperature was determined at 330 °C for the following studies. Dynamic mechanical analysis (DMA) is an effective tool to understand the thermal and thermal mechanical properties of polymeric materials. Figure S13 shows the DMA results of tan δ (a), storage modulus (b), and loss modulus (c) acquired from different P2BPY/PI nanofiber belts. On the basis of the tan δ and loss modulus results, it is evident that the values of glass transition temperature (Tg) for different P2BPY/PI nanofiber belts were in the range from 299 to 348 °C, which were substantially higher than the previously reported value for PBPbased polymer (206 °C).33 Such high Tg values should be attributed to the high molecular weight of PBPY, wherein both the PBP component and the coupling joints had conjugated rigid-rod structures. The P2BPY/PI nanofiber belts also G

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Figure 8. SEM images of P3BPY/PI nanofiber belts with varied molar ratios of PBPA/DAB being 1:0.5 (a), 1:0.6 (b), 1:0.7 (c), 1:0.75 (d), 1:0.8 (e), 1:0.9 (f), and 1:1 (g).

demonstrated excellent thermal mechanical performance, and the storage modulus (E′) was retained at ∼4 GPa at the testing temperature up to 280 °C (Figure S9b), indicating the PBPY/ PI nanofiber belts could be used in the high-temperature environment. As shown in Figure S14, TGA analysis results indicated that the 5% weight loss temperatures of the P2BPY/PI nanofiber belts were higher than 500 °C. Effect of PBPA/DAB Molar Ratio. In step polymerization, when two functional groups of monomers are in stoichiometric balance (i.e., with molar ratio being 1:1), the molecule weight could become infinitely high in theory. However, the amount of −COOH groups in as-synthesized PBPA was impossible to accurately determine due to the polydispersity. Therefore, the molar ratio of PBPA to DAB (the coupling agent) needed to be optimized, and the P3 oligomer was selected for this study. Figure 8 shows the SEM images of P3BPY/PI nanofiber belts prepared from varied molar ratios of PBPA/DAB (1:0.5, 1:0.6, 1:0.7, 1:0.75, 1:0.8, 1:0.9, and 1:1), and the same heattreatment temperature of 330 °C was adopted. In general, all of the nanofibers were uniform, whereas the P3BPY/PI-3 (1:0.7) nanofibers appeared to possess more desired morphological structure (Figure 8c) that the nanofibers were more uniform with smoother surface morphology and had the higher degree of nanofiber orientation. For the P3BPY/PI nanofibers prepared from lower molar ratios, beaded nanofibers could be occasionally identified, while for the P3BPY/PI nanofibers prepared from higher molar ratios, the nanofibers exhibited some degrees of adhesion/conglutination presumably caused by the melting of excess and/or unreacted DAB monomers. FTIR spectra (Figure S15) verified the chemical structure of PBPY/PI and indicated there was merely no difference among the seven P3BPY/PI nanofibers prepared with varied molar ratios of PBPA/DAB. The tensile test results of seven P3BPY/PI nanofiber belts prepared from different molar ratios of PBPA/DAB are displayed in Figure 9. The values of tensile stress and tensile strain were in the ranges of 200−290 MPa and 8−15%, respectively. As shown in Figure 9b, the tensile stress increased initially and then decreased with the increase of PBPA/DAB molar ratio. The highest tensile stress occurred at the molar ratio of 1:0.7, and the value was ∼285 MPa. The higher tensile stress was attributed to the higher molecular weight of PBPY,

Figure 9. (a) Stress−strain curves of seven P3BPY/PI nanofiber belts prepared with different molar ratios of PBPA/DAB. (b) Variation of tensile stress with PBPA/DAB molar ratio.

suggesting that the PBPA/DAB molar ratio of 1:0.7 might lead to stoichiometric balance. Hence, this molar ratio was used in the subsequent preparations. The DMA and TGA results acquired from different P3BPY/ PI nanofiber belts are shown in Figures S16 and S17, and the corresponding data are summarized in Table 1. The Tg values H

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PBPY/PI nanofiber belts could be potentially used for some engineering applications such as the fabrication of fiberreinforced composites. Figure 11 depicts the DMA tan δ curves and TGA curves of the four PBPY/PI nanofiber belts. The Tg values of the four

Table 1. Thermal Properties of Electrospun P3BPY/PI Nanofibers Belts molar ratio (PBPA/DAB)

Tg (°C)

d5 wt % in N2 (°C)

d5 wt % in air (°C)

char yield (at 800 °C) (wt %)

1:0.5 1:0.6 1:0.7 1:0.75 1:0.8 1:0.9 1:1

335 335 344 343 338 336 336

491 491 523 519 514 506 504

480 490 503 499 485 482 482

75 75 82 78 78 77 78

of P3BPY/PI nanofibers were higher than 330 °C, and the highest value of 344 °C was obtained at the PBPA/DAB molar ratio of 1:0.7, which was consistent with the tensile test results. Meanwhile, TGA curves showed that the 5% weight loss temperatures (d5 wt %) for all kinds of P3BPY/PI nanofibers under nitrogen and air atmospheres were higher than 490 and 480 °C, respectively, whereas the P3BPY/PI-3 (1:0.7) nanofiber belt had the highest values of 523 and 503 °C. The excellent thermal properties were attributed the rigid-rod aromatic structures of PBP, PI, and the heterocyclic coupling joints. Additionally, the char yield values (at 800 °C) of seven P3BPY/ PI nanofiber belts were higher than 75 wt %; hence, the PBPY/ PI nanofiber belts would be good precursors for making carbon nanofiber belts. Effect of Molecular Weight. Upon the optimization of heat-treatment temperature (330 °C) and molar ratio of PBPA/DAB (1:0.7), PBPY/PI nanofibers belts were prepared from the four PBPA oligomers (P1 to P4). Figure 10 shows

Figure 11. Variation of tan δ with temperature (a) and TGA curves (b) of electrospun P1BPY/PI, P2BPY/PI, P3BPY/PI, and P4BPY/PI nanofiber belts prepared under the optimized conditions.

PBPY/PI nanofiber belts were 315, 318, 344, and 348 °C, respectively. It is evident that the Tg values increased with the increase of the molecular weight of PBPA oligomers. The d5 wt % values were also increased from 495 °C for the P1BPY/PI nanofiber belts to 525 °C for the P4BPY/PI nanofiber belts. The excellent thermal properties (particular the high glass transition temperature) are advantageous for the applications in high temperature environments; therefore, the PBPY/PI nanofiber belts/membranes could be used for high-temperature filtration applications such as filtration of industrial dust. Furthermore, the PBPY/PI nanofiber belts were hydrophobic, and this was simply because both PPP-based polymer and PI polymer were hydrophobic. Note that the belt hydrophobicity increased with increasing the content of PPP-based repeating units; as shown in Figure 12, the water contact angles were measured in the range from 100 ± 0.9° to 137 ± 1.0°. As a result, the PBPY/PI nanofiber belts could also be used as waterproof filtration media. Chemical Resistance of PBPY/PI Nanofiber Belts. Good chemical resistance is essential for practical applications in various harsh environments such as high temperature and

Figure 10. Typical stress−strain curves of P1BPY/PI, P2BPY/PI, P3BPY/PI, and P4BPY/PI nanofiber belts prepared under the optimized conditions (i.e., PBPA/DAB molar ratio of 1:0.7, and heat-treatment temperature of 330 °C). The inset showing the photographs of the P4BPY/PI nanofiber belts.

stress−strain curves of the four PBPY/PI nanofiber belts, and the inset shows the photographs of these yellow-colored P4BPY/PI nanofiber belts. The tensile stress values of the P1BPY/PI, P2BPY/PI, P3BPY/PI, and P4BPY/PI nanofiber belts was 183, 247, 285, and 357 MPa, respectively, and the corresponding tensile strain values was 4.8%, 8.1%, 8.6%, and 12.1%. Both the stress and strain values increased with the increase of molecular weight of PBPA oligomers. The high tensile stress values of these PBPY/PI nanofiber belts are similar to the values of some high-performance aromatic polyimide electrospun nanofibers;65,66 hence, the prepared I

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and/or deformations (e.g., shrinkage and expansion). As shown in Figure 14, the P4BPY/PI nanofiber belts could well retain their morphological structures and yellow color upon being soaked in 6 M KOH or 7.14 M H2SO4 solutions at 100 °C for varied times.

Figure 12. Water contact angles of P1BPY/PI (a), P2BPY/PI (b), P3BPY/PI (c), and P4BPY/PI (d) nanofiber belts prepared under the optimized conditions.

corrosive medium. In general, PPP-based polymers possess excellent chemical resistance owing to their aromatic backbone structures. Preliminary test of soaking the four different PBPY/ PI nanofiber belts in strong alkali solution (6 M KOH) at 60 °C for 3 days (72 h) suggested their excellent chemical resistance; for example, the retention ratios of mechanical strength of the four PBPY/PI nanofiber belts were ∼90%, as shown in Figure S18. Thereafter, systematical investigations were carried out by soaking the P4BPY/PI nanofiber belts in different harsh environments including boiling water, 6 M KOH at different temperatures (30, 60, and 100 °C), and 7.14 M H2SO4 at 100 °C for varied soaking time (3−72 h). Figure 13 depicts the tensile stress comparisons of P4BPY/PI nanofiber belts before and after being soaked in the above solutions, and the representative stress−strain curves are shown in Figure S19. The tensile stress of the nanofiber belts after being soaked in 6 M KOH solution at 30 °C had negligible reduction (i.e., the decrease was merely ∼4% upon being soaked for 72 h) (Figure 13a). After being soaked in 6 M KOH solution at 60 °C for 72 h, the tensile stress reduction was ∼9% (Figure 13b). After being soaked in the 6 M KOH solution at 100 °C for 24 and 72 h, the tensile strength of the P4BPY/PI nanofiber belt decreased ∼7% and ∼16%, respectively (Figure 13c). Figure S20 shows the stress−strain curves and stress comparisons of P4BPY/PI nanofiber belts before and after being soaked in 3, 6, and 7.14 M H2SO4 at 100 °C for 72 h. Upon being soaked in 3, 6, and 7.14 M H2SO4 solution at 100 °C for 72 h, the tensile stress reductions were ∼13.2%, ∼ 15.7%, and ∼17%, respectively. Figure 13d shows the P4BPY/PI nanofiber belts being soaked in 7.14 M H2SO4 at 100 °C for varied time periods (3−72 h). In addition, upon being soaked in boiling water at 100 °C for 72 h, the tensile stress reduction was ∼12% (Figure 13e). Despite the decrease of tensile stress, the soaking of the nanofiber belts in both alkali and acid solutions did not cause any color changes

Figure 14. Photographs showing the P4BPY/PI nanofiber belts upon being soaked in 6 M KOH (a) and 7.14 M H2SO4 (b) solutions at 100 °C for different soaking times of 0, 3, 6, 12, 18, 24, 36, 48, and 72 h.

Morphological structures of the PBPY/PI nanofiber belts before and after the above treatments were examined by SEM and TEM. As shown in Figure 15, upon being soaked in different solutions for 72 h, no appreciable variations on morphological structures of the P4BPY/PI nanofiber belts could be identified. More SEM images of the P4BPY/PI nanofiber belts upon the above treatments for different time periods are shown in Figures S21−S25. TEM images (Figure 16) also demonstrated that these P4BPY/PI nanofiber belts had no distinguishable variations. Therefore, the prepared PBPY/PI nanofiber belts possessed excellent chemical stability, which would be sufficient for the applications under various harsh environments. For example, the PBPY/PI nanofiber belts/ membranes might be promising as separators in lithium ion batteries, alkaline batteries, and supercapacitors.

Figure 13. Stress comparisons of the P4BPY/PI nanofiber belts before and after being soaked in 6 M KOH solution at 30 (a), 60 (b), and 100 °C (c) as well as in 7.14 M H2SO4 solution (d) and boil water (e) at 100 °C, for varied soaking times. J

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Figure 15. SEM images of the P4BPY/PI nanofiber belts before (a) and after being soaked in 6 M KOH solution at 30 (b), 60 (c), and 100 °C (d) as well as in 3 (e), 6 (f), and 7.14 M (g) H2SO4 solution and boiling water (h) at 100 °C for 72 h.

Figure 16. TEM images of the P4BPY/PI nanofiber belts before (a) and after being soaked in 6 M KOH solution at 30 (b), 60 (c), and 100 °C (d) as well as in 3 (e), 6 (f), and 7.14 M (g) H2SO4 solution and boiling water (h) at 100 °C for 72 h.

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could be used as filtration media under high-temperature and/ or corrosive conditions, and they could also be used as separators in lithium ion batteries, alkaline batteries, and supercapacitors. It is important to note that this is the first reported study on the preparation of PPP-based polymer nanofibers; additionally, this study also provides an innovative approach for making nanofibers from the polymers that cannot be electrospun directly.

CONCLUSION Four soluble PPP-based oligomers of PBPA with varied molecular weights were synthesized via Ni(II) complexcatalyzed cross-coupling reaction; subsequently, the PBPY/PI blend nanofiber belts were prepared by the combination of electrospinning and molecular coupling assembly techniques followed by heat treatment. In specific, by using PBPA as assembling unit, DAB as coupling agent, and PAA as carrier/ glue polymer, PBPA-DAB/PAA precursor nanofiber belts were electrospun from the mixture solution of PBPA/DAB/PAA in DMAc; within these nanofibers, the PBPA and DAB molecules were coupling-assembled through ammonium bonds. Upon heat treatment, the PBPY/PI nanofiber belts were obtained; since the heat-treatment temperature and the DAB amount had significant influences on the mechanical and thermal properties of final PBPY/PI nanofiber belts, they were investigated and then optimized. The PBPY/PI nanofibers belts exhibited superior thermal and mechanical properties; for example, the Tg values were up to ∼348 °C, the 5% weight loss temperatures were higher than 500 °C in both air and nitrogen atmospheres, the mechanical strengths were as high as 360 MPa, and the storage moduli could be retained above 4 GPa at the temperature up to 280 °C. Furthermore, the PBPY/PI nanofiber belts also demonstrated excellent chemical resistance under various harsh environments such as being soaked in boiling water as well as in 7.14 M H2SO4 or 6 M KOH solutions at elevated temperatures. Therefore, the PBPY/PI blend nanofiber belts could have a variety of applications (particularly those in harsh environments); for example, they

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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.macromol.7b01950. GPC results acquired from four PBPA oligomers of P1, P2, P3, and P4; 1H NMR spectra of the PBPA oligomers; FTIR spectra of four PBPA oligomers and the resulting P1BPY/PI, P2BPY/PI, P3BPY/PI, and P4BPY/PI nanofiber belts; 13C NMR of P2BPY/PI; representative stress−strain curves acquired from the nanofiber belts of P1BDS/PAA, P2BDS/PAA, P3BDS/PAA, and P4BDS/ PAA; diameter distributions acquired from as-electrospun P2BDS/PAA nanofiber belt and the resulting P2BPY/PI nanofiber belts prepared at different heattreatment temperatures; SEM images showing the cross sections of P2BPY/PI nanofiber belts prepared at different heat-treatment temperatures; FTIR spectra acquired from P2BPY/PI nanofiber belts with different heat-treatment temperatures; wide-angle X-ray diffracK

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(6) Grem, G.; Leising, G. Electroluminescence of “wide-bandgap” chemically tunable cyclic conjugated polymers. Synth. Met. 1993, 57, 4105−4110. (7) Shacklette, L.; Chance, R.; Ivory, D.; Miller, G.; Baughman, R. Electrical and optical properties of highly conducting charge-transfer complexes of poly(p-phenylene). Synth. Met. 1980, 1, 307−320. (8) Friedrich, K.; Burkhart, T.; Almajid, A.; Haupert, F. Poly-paraphenylene-copolymer (PPP): A high-strength polymer with interesting mechanical and tribological properties. Int. J. Polym. Mater. 2010, 59, 680−692. (9) Kovacic, P.; Jones, M. B. Dehydro coupling of aromatic nuclei by catalyst-oxidant systems: poly(p-phenylene). Chem. Rev. 1987, 87, 357−379. (10) Speight, J. G.; Kovacic, P.; Koch, F. W. Synthesis and properties of polyphenyls and polyphenylenes. J. Macromol. Sci., Polym. Rev. 1971, 5, 295−386. (11) Fauvarque, J. F.; Petit, M. A.; Digua, A.; Froyer, G. Electrochemical synthesis of poly(1,4-phenylene) films. Makromol. Chem. 1987, 188, 1833−1839. (12) Morita, M.; Komaguchi, K.; Tsutsumi, H.; Matsuda, Y. Electrosynthesis of poly(p-phenylene) films and their application to the electrodes of rechargeable batteries. Electrochim. Acta 1992, 37, 1093−1099. (13) Cai, Z.; She, L.; Wu, L.; Zhong, D. On-surface synthesis of linear polyphenyl wires guided by surface steric effect. J. Phys. Chem. C 2016, 120, 6619−6624. (14) Zhou, X.; Bebensee, F.; Shen, Q.; Bebensee, R.; Cheng, F.; He, Y.; Su, H.; Chen, W.; Xu, G. Q.; Besenbacher, F. On-surface synthesis approach to preparing one-dimensional organometallic and poly-pphenylene chains. Mater. Chem. Front. 2017, 1, 119−127. (15) Talirz, L.; Ruffieux, P.; Fasel, R. On-surface synthesis of atomically precise graphene nanoribbons. Adv. Mater. 2016, 28, 6222− 6231. (16) Chaturvedi, V.; Tanaka, S.; Kaeriyama, K. Preparation of poly(pphenylene) via a new precursor route. Macromolecules 1993, 26, 2607−2611. (17) Gin, D. L.; Conticello, V. P.; Grubbs, R. H. Stereoregular precursors to poly(p-phenylene) via transition-metal-catalyzed polymerization. 1. Precursor design and synthesis. J. Am. Chem. Soc. 1994, 116, 10507−10519. (18) François, B.; Widawski, G.; Rawiso, M.; Cesar, B. Blockcopolymers with conjugated segments: Synthesis and structural characterization. Synth. Met. 1995, 69, 463−466. (19) Poppe, D.; Frey, H.; Kreuer, K.; Heinzel, A.; Mülhaupt, R. Carboxylated and sulfonated poly(arylene-co-arylene sulfone)s: Thermostable polyelectrolytes for fuel cell applications. Macromolecules 2002, 35, 7936−7941. (20) Pasquale, A.; Sheares, V. V. Alkyl-substituted poly(2,5benzophenone)s synthesized via Ni(0)-catalyzed coupling of aromatic dichlorides and their miscible blends. J. Polym. Sci., Part A: Polym. Chem. 1998, 36, 2611−2618. (21) Rehahn, M.; Schlüter, A. D.; Wegner, G. Soluble poly(paraphenylene)s, 3. Variation of the length and the density of the solubilizing side chains. Makromol. Chem. 1990, 191, 1991−2003. (22) Kallitsis, J.; Naarmann, H. Synthesis of some disubstituted poly(p-terphenylenes). Synth. Met. 1991, 44, 247−257. (23) Wallow, T. I.; Novak, B. M. In aqua synthesis of water-soluble poly(p-phenylene) derivatives. J. Am. Chem. Soc. 1991, 113, 7411− 7412. (24) Wright, M. E.; Lott, K. M.; McHugh, M. A.; Shen, Z. Synthesis of fluorinated and hydrocarbon ester functionalized poly(p-phenylenes) and their solubility in supercritical fluids. Macromolecules 2003, 36, 2242−2247. (25) Chaturvedi, V.; Tanaka, S.; Kaeriyama, K. Preparation of poly(pphenylene) via a new precursor route. Macromolecules 1993, 26, 2607−2611. (26) Grem, G.; Leditzky, G.; Ullrich, B.; Leising, G. Blue electroluminescent device based on a conjugated polymer. Synth. Met. 1992, 51, 383−389.

tion patterns acquired from P2BPY/PI nanofiber belts prepared at different heat-treatment temperatures; DMA results of P2BPY/PI nanofiber belts prepared at different heat-treatment temperatures; TGA curves acquired from different P2BPY/PI nanofiber belts; FTIR spectra of P3BPY/PI nanofiber belts prepared with varied molar ratios of PBPA/DAB; DMA results of P3BPY/PI nanofiber belts prepared with varied PBPA/DAB molar ratios; TGA curves acquired from different P3BPY/PI nanofiber belts under a nitrogen atmosphere and air atmosphere with the heating rate of 10 °C/min; stress comparisons of P1BPY/PI, P2BPY/PI, P3BPY/PI, and P4BPY/PI nanofiber belts before and after being soaked in 6 M KOH solution at 60 °C for 72 h; representative stress−strain curves of P4BPY/PI nanofiber belts before and after being soaked in 6 M KOH solution at 30, 60, and 100 °C as well as in 7.14 M H2SO4 solution and boiling water at 100 °C, for varied time periods; representative stress−strain curves and stress comparison of P4BPY/PI nanofiber belts before and after being soaked in 3, 6, and 7.14 M H2SO4 solutions at 100 °C for 72 h; SEM images of the P4BPY/PI nanofiber belts after being soaked under various harsh conditions (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(H.F.) E-mail [email protected]; Ph +1 605-394-1229; Fax 605-394-1232. *(H.H.) E-mail [email protected]; Ph +86 791-88120740; Fax 86-791-88120536. ORCID

Yichun Ding: 0000-0002-7441-800X Zhengtao Zhu: 0000-0002-9311-2110 Hao Fong: 0000-0002-1497-5162 Author Contributions

W.X. and Y.D. made equal contributions. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China through Grants 21574060, 21374044, and 21174058 and by the Graduate Innovation Foundation of Jiangxi Province through Grant YC2016-B033.

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REFERENCES

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DOI: 10.1021/acs.macromol.7b01950 Macromolecules XXXX, XXX, XXX−XXX