Stabilization of Atactic-Polyacrylonitrile under Nitrogen and Air As

Jul 27, 2015 - Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1, Ookay...
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Stabilization of Atactic-Polyacrylonitrile under Nitrogen and Air As Studied by Solid-State NMR Xiaoran Liu,† Wei Chen,† You-lee Hong,† Shichen Yuan,† Shigeki Kuroki,‡ and Toshikazu Miyoshi*,† †

Department of Polymer Science, The University of Akron, Goodyear Polymer Center, Akron, Ohio 44325-3909, United States Department of Organic and Polymeric Materials, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, Japan

‡

S Supporting Information *

ABSTRACT: Solid-state (ss) NMR spectroscopy was applied to study the stabilization process of 30 wt % 13C-labeled atacticpolyacrylonitrile (a-PAN) heat-treated at various temperatures (Ts) under nitrogen and air. Direct polarization magic-angle spinning (DP/MAS) 13C NMR spectra provided quantitative information about the functional groups of stabilized a-PAN. Two dimensional (2D) refocused 13C−13C INADEQUATE and 1 H−13C HETCOR NMR spectra gave through-bond and through-space correlations, respectively, of the complex intermediates and final structures of a-PAN stabilized at different Ts values. By comparing 1D and 2D NMR spectra, it was revealed that the stabilization process of a-PAN under nitrogen is initiated via cyclization, while the stabilization under air proceeds via dehydrogenation. Different initial processes lead to the isolated aromatic ring and ladder formation of the aromatic rings under nitrogen and air, respectively. Side reactions and intermediate structures are also discussed in detail. Through this work, the stabilization index (SI) was defined on the basis of the quantified C-1 and C-3 DP/MAS spectra. The former reached 0.87 at Ts = 370 °C, and further higher Ts values did not affect SI; however, the latter continuously increased up to 0.66 at Ts = 450 °C. All of the experimental results indicated that oxygen plays a vital role on the whole reaction process as well as the final products of stabilized a-PAN. (DTA),3,17 and so on, have been used to characterize the stabilization process of a-PAN. TGA provides weight loss, while DTA detects an exothermal peak. These approaches are useful for determination of the starting temperature of the reaction.17,19 XRD has been applied to detect decrystallization of a-PAN with proceeding of the stabilization process.18 FTIR5,8,9,11,13−15 and 13C high-resolution ss-NMR5,15−17 have been extensively used to identify the molecular level structures of stabilized a-PAN. However, peak separations of individual peaks were insufficient and thus made it difficult to properly identify different functional groups. In the past several decades, ss-NMR techniques including fast magic-angle spinning,20,21 powerful decoupling,22 high magnetic field,23 various recoupling and correlation techniques,24−29 as well as design of samples by selectively isotopic labeling have been well developed. As a result, combination of advanced NMR techniques with isotopic labeling has been successfully applied to elucidate detailed molecular structures including conformation,30−32 packing,33,34 chain-folding structures35,36 of synthetic polymers, as well as 3D structures of peptide37 and large proteins.38 Very recently, Wang et al. investigated the reaction scheme of a-PAN stabilized under an

1. INTRODUCTION Carbon fiber is a revolutionary material that has been of great interest for decades due to its outstanding physical properties, low weight, and high chemical resistance.1 Atactic-polyacrylonitrile (a-PAN) is used as a major precursor to produce carbon fiber.1−5 From a-PAN to carbon fiber, several heat treatment steps of stabilization, carbonization, and graphitization are involved. Among them, stabilization process sets the structure of carbon fiber and significantly influences the mechanical properties.1,2,6 Chemical reaction pathways and final structures of a-PAN during stabilization at 250−400 °C have been theoretically4,7−12 and experimentally13−19 studied over the past six decades. By heat treatment under air, linear polymeric structure is believed to convert to aromatic ladder structure, which makes the residue more heat-resistant and thus could undergo further heat treatment at higher temperatures.1 Houtz first proposed conjugated aromatic ladder structure due to the obvious color change.4 Later, researchers proposed several modified structures including cyclized six-membered ring,7 conjugated polyene backbone,8 and so on. In addition to the intramolecular chemical reactions, intermolecular cross-linking was also proposed by several studies.10−12 Various characterization techniques such as Fourier transform infrared spectroscopy (FT-IR),5,8,9,11,13−15 solid-state nuclear magnetic resonance (ss-NMR),5,15−17 X-ray diffraction (XRD),18 thermal gravity analysis (TGA), 17 differential thermal analysis © XXXX American Chemical Society

Received: May 13, 2015 Revised: July 13, 2015

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DOI: 10.1021/acs.macromol.5b01030 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules argon atmosphere by applying 13C direct polarization magicangle spinning (DP/MAS) assisted by selectively single-site 13C labeling and 13C−13C radio frequency-driven recoupling (RFDR) technique.17 Comparisons between 13C chemical shift for three single-site labeled samples and literature results suggested nine chemical structures involved in stabilization of a-PAN and found that the aromatic ring adopts no ladder formation but an isolated one at the stabilization temperature (Ts) up to 350 °C. They suggested that atactic conformation leads to the isolated ring formation. This suggestion is different from traditionally accepted view on the stabilized structure of aPAN.1−3,5 Because isolated aromatic ring structure is not thermally stable, which may not be favored in the further heat treatment steps, we naturally hold the following questions: Does atactic sequence play an important role for the isolated ring formation? Does a-PAN stabilized under air also form the isolated aromatic ring? These fundamental questions are key issues in the understanding of stabilization mechanism of aPAN under air and inert gas as well as further development of carbon fibers. In this study, we investigate the intermediate, final structures as well as the reaction pathways of 13C-labeled a-PAN stabilized at stabilization temperature up to 450 °C under nitrogen and air by using several ss-NMR techniques. To highlight and quantitatively analyze the intramolecular reactions, we applied 13 C DP/MAS, 13C−13C Incredible Natural Abundance DoublE QUAntum Transfer Experiment (INADEQUATE), and 1 H−13C HETeronuclear CORrelation (HETCOR) techniques. As a result, it was revealed that a-PAN stabilized under nitrogen and air experiences different reaction pathways and forms largely different intermediate structures: The former was initiated by cyclization process, while the latter formed polyene structure through dehydrogenation process. The different initial structures resulted in largely different aromatic ring formation during stabilization process: Stabilization under nitrogen induced the isolated aromatic ring, while that under air lead to the ladder formation including six adjacent aromatic rings as an average. Besides, stabilization under nitrogen induced a large number of chain scissions, while that under air did not. Through the experimentally determined structures, it was concluded that oxygen is of vital importance in inducing the highly stabilized aromatic ladder structure from a-PAN, which works as a precursor of carbon fibers.

NaHSO3 was quickly injected into the system one by one through the septum. The reaction finished after 24 h at 60 °C. The polymerization product was filtered and washed with an excess amount of water, followed by drying under vacuum for 24 h. a-PAN product was white powder with the productivity of ca. 60%. 2.2. Material Characterization. The weight-average molecular weight () and polydispersity (PDI) of the sample were characterized by gel permeation chromatography (GPC) at room temperature, which was performed in dimethylformamide (DMF) by TOSOH HLC-8230 GPC-RI using PMMA as standard. A Varian Mercury 300 NMR was used to determine the stereo regularity of three 13C labeled samples in dimethyl sulfoxide-d6 at room temperature. Stereo regularity was calculated through the integration of split CH carbon peaks. All of the samples were concluded to be a-PAN. Detailed chemical characteristics are summarized in Table 1. Table 1. Chemical Characteristics of Single Site and All Labeled a-PAN sample 13

PAN-1- C PAN-3-13C PAN-13C3

(g·mol−1)

PDI

stereo regularity (mm:mr:rr)

477 996 436 391 437 898

2.947 3.163 2.560

34:37:29 34:40:26 33:39:28

2.3. Heat Treatment of Material. The samples were heated by a programmable Thermolyne benchtop muffle furnace. Starting from room temperature, the heating rate was set as 5 °C/min. After remaining at the desired stabilization temperatures for 15 min, cooling was performed at the rate of 3.3 °C/min. The samples stabilized under air were heated in an open system, while the nitrogen samples were heated under a nitrogen atmosphere. 2.4. 13C Solid-State NMR Measurement. Solid-state NMR experiments were carried out by a Bruker Avance Ultrashield 300 apparatus equipped with a 4 mm double resonance VT CP/MAS probe. The 1H and 13C carrier frequencies were 300.1 and 75.6 MHz, respectively. Samples were packed in a 4 mm Zirconia MAS rotor with a Kel-F drive cap and were measured with a MAS speed of 13 kHz at 25 °C. 1 H spin−lattice relaxation time in the laboratory frame (T1) for unreacted a-PAN as well as a-PAN stabilized under nitrogen and air was measured by inversion−recovery method, and corresponding data are listed in the Supporting Information as Table S3. 13C T1 relaxation time was measured by CP Torchia pulse sequence, and among all of the samples, carbon signal at 177.1 ppm for a-PAN stabilized under air gave the longest T1 values of ca. 25−28 s at various Ts values. Recycle delay was thus set as 150 s in 13C DP/MAS measurements. Typically, 90 accumulations with 13C 90° pulse of 4.57 μs were used to get a good signal-to-noise ratio. The CH signal of adamantine was set as 29.46 ppm as an external reference. In 13C CP/MAS experiments, 13C 90° pulse length, CP contact time, and recycle delay were set as 4.30 μs, 2 ms, and 2 s, respectively. Two-pulse phase-modulated (TPPM) decoupling frequency was set to be 71.43 kHz (180° pulse, 7.0 μs). 2D correlations were measured under the same condition without noting NMR parameters. In refocused 13C−13C 2D INADEQUATE experiment, a total of 128 t1 points with 256 scans were acquired to obtain a good spectral quality. τ is polarization transfer time.26,27 Theoretically, τ = 3.5−5.5 ms gives a maximum polarization transfer for directly bonded 13C−13C spins. The stabilized a-

2. EXPERIMENTAL SECTION 2.1. Material Preparation. Two kinds of single-site 13Clabeled and all 13C-labeled a-PAN with the labeling ratio of 30 wt % were synthesized through REDOX-initiated free radical polymerization. The 13C-enriched monomers acrylonitrile1-13C, acrylonitrile-3-13C, and acrylonitrile-13C3 were used directly without further purification. A two-necked flask with 53 mL of degassed deionized water, 2 g of 13C enriched monomer, and 4.67 g of nonlabeled monomer together with a magnetic stir bar was connected to a vacuum line through one neck by a condenser and an adapter. Teflon septum was attached to the other neck and thus can be used for reagent injection. After nitrogen exchange of the inside environment, 3 mL of water solution containing 0.54 mL of 0.05 mol/L H2SO4 and 0.9 mg (NH4)Fe(SO4)2·12H2O was injected into the system. The system was then kept in the oil bath at a consistent temperature of 60 °C for 30 min. 5 mL of water containing 13.6 mg of Na2S2O8 and 5 mL of water containing of 67.3 mg of B

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Macromolecules PAN sample showed a maximum intensity for the aliphatic signals at τ = 1 ms due to short spin−spin relaxation time. Thereby, τ was set to 1 ms in our experiments. 1H−13C 2D HETCOR spectra were recorded with 90° 1H pulse length of 3.10 μs, CP contact time of 0.5 ms, TPPM decoupling, and FSLG field strength with 80.65 kHz (180° pulse of 6.20 μs) and 98.63 kHz (360° pulse of 10.14 μs), respectively. A total of 55 t1 points with 300 scans were executed. Glycine was used to calibrate 1H chemical shift values.

3. RESULTS 3.1. 13C DP/MAS NMR Spectra. Figure 1 shows the 13C DP/MAS NMR spectra for unreacted 30 wt % C-1-labeled Figure 2. 13C DP/MAS NMR spectra for 30 wt % C-1, C-3, and all 13 C-labeled a-PAN stabilized at (a) 310, (b) 370, and (c) 420 °C under nitrogen with a recycle delay of 150 s. Numbers marked as 1, 2, and 3 above the top spectrum denote the resonances originated from labeled C-1, C-2, and C-3 sites, respectively. Numbers down each peak represent the normalized integration area of corresponding peak.

for the unreacted a-PAN at 121 ppm appeared at Ts = 310 °C. At Ts = 370 °C, however, the shoulder peak at around 170−175 ppm and the two small ones at 65−75 ppm almost disappeared (bottom of Figure 2b). Thereby, these peaks were reasonably attributed to the intermediate/side reaction structures. With further increasing Ts to 420 °C, the signal intensity at 150−160 ppm increased while the -CN signal at 121 ppm gave only 5.3% at Ts = 420 °C (bottom of Figure 2c). For the C-3 spectra shown in the middle of Figure 2, at Ts = 310 °C, the aliphatic C-3 signal moved to 30 ppm, concomitant with appearance of new peaks at 135 (8.0%) and 17 ppm (2.8%). The intensity of the peak at 135 ppm increased to 13.1 and 14.7% at Ts = 370 and 420 °C, respectively. Thereby, this peak should be the main product signal of a-PAN stabilized under nitrogen and could be assigned as the aromatic C-3 on the basis of chemical shift value reported in the literature.39 The peak at 17 ppm was reasonably assigned to the methyl group. Comparisons among C-1, C-3, and all 13C-labeled spectra allowed us to further assign several C-2 signals at different Ts values, as labeled in Figure 2. Figure 3a−c shows the 13C DP/MAS NMR spectra for 30 wt % all, C-3, and C-1 13C- labeled a-PAN stabilized at Ts = 310, 370, and 420 °C under air, respectively. Elevated Ts values also resulted in the decrease in the aliphatic peak intensities as well as the peak intensity increase in the aromatic region. Notably, the aliphatic signals almost disappeared at Ts = 370 °C. Besides, there was no methyl group detected. These spectral changes are largely different from those under nitrogen. For the C-1 spectrum (bottom of Figure 3), besides the unreacted −CN group at 121 ppm, the signal at 150 ppm was observed at Ts = 310 °C. At Ts = 370 °C, this peak was a major component for a-PAN stabilized under air. Another interesting feature is that the remaining −CN signal moved to 116 ppm at 370 °C. (See the detailed Discussion later.) Further increase in Ts did not induce much C-1 spectral change compared with that at Ts = 370 °C. In the C-3 spectrum (middle of Figure 3), the aliphatic C-3 signal appeared at 34 ppm as a major component at Ts = 310 °C. In addition, two clear peaks at 139 and 175 ppm as well as two broad and relatively small peaks at around 157 and 117 ppm were observed. Chemical shift of the C-3 signal (139 ppm) was close to the aromatic C-3 signal at 135 ppm under

Figure 1. 13C DP/MAS NMR spectra for 30 wt % C-1, C-3, and all 13 C-labeled a-PAN with a recycle delay time of 150 s. Numbers marked as 1, 2, and 3 represent the contribution of C-1, C-2, and C-3, respectively. Carbons colored by red in the chemical structures represent labeled 13C sites.

(PAN-1-13C), C-3-labeled (PAN-3-13C), and all 13C-labeled (PAN−13C3) a-PAN measured at 25 °C. Single-site labeling gave chemical shift values for the C-1 and C-3 carbons at 121.3 and 33.5 ppm, respectively. Together with the C-3 and alllabeled (the aliphatic signal) spectra, chemical shift value of the C-2 carbon could be derived as 28.8 ppm. Identification of 13C chemical shift values by using single-site labeled samples was further applied to the understanding of the complex chemical reactions of a-PAN stabilized at various Ts values under nitrogen and air. Figure 2 shows 13C DP/MAS NMR spectra for 30 wt % C-1(bottom), C-3- (middle), and all 13C-labeled a-PAN (top) stabilized at Ts values of 310, 370, and 420 °C under nitrogen. The integration of individual peaks were also depicted in Figure 2, where total integration was normalized to 100% for the allsite-labeled spectrum and 33.3% for the single-site-labeled ones. The normalized values, therefore, are independent of the sample volume and NMR setting parameters. 13C DP/MAS NMR spectra for all 13C-labeled a-PAN clearly indicated that higher Ts values lead to lower intensities of the saturated aliphatic peaks at 10−40 ppm, concomitant with increasing of the unsaturated signals at 90−180 ppm. The integration results for individual peaks were also depicted in Figure 2. For the C-1 spectrum (bottom of Figure 2a), two small signals at 65−75 ppm, low-field signals at 150−175 ppm, in addition to −CN C

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C-labeled a-PAN stabilized at Ts = 310 and 420 °C under nitrogen, as illustrated in Figure 4a,b, respectively. 13C−13C INADEQUATE technique can yield correlation signals between the nuclear spins connected by a covalent bond. For such spins, the correlation signals appear at summation of chemical shifts of the carbon pairs due to double-quantum (DQ) coherence in ω1 dimension, while the signals appear at each single-quantum (SQ) chemical shift in the ω2 dimension. Stabilized a-PAN covered a very wide chemical shift range of 17−180 ppm; thereby, DQ and SQ correlations would drastically enhance the spectral resolution.26−29 At Ts = 310 °C, five peaks labeled by c′, e′, f′, o′, and q′ at 110−125 ppm were clearly identified and separated from each other, as shown in Figure 4a. Peak c′ at 121.5 ppm was well consistent with the −CN group of unreacted a-PAN (121.3 ppm through unreacted a-PAN-1-13C spectrum in Figure 1) and was connected to b′ (C-2) at 23.6 ppm; meanwhile, b′ was connected to a′ (C-3) at 29.8 ppm. These correlations were assigned as the unreacted a-PAN signals. Chemical shifts of a′ and b′ were different from those for C-3 (33.5 ppm) and C-2 (28.8 ppm) of original a-PAN prior to stabilization, as shown in Figure 1. This result implied that neighboring units connected to the unreacted a-PAN component might change due to the stabilization process. Peaks e′ at 106.7 ppm and f′ at 114.0 ppm had strong correlations with peaks g′ at 135.4 ppm (aromatic C-3) and h′ at 149.2 ppm and i′ at 153.7 ppm, respectively, as shown in pink lines in Figure 4a. Thereby, peaks e′ and f′ were assigned to the aromatic C-2, while h′ and i′ arose from the aromatic C1. Elmkaddem et al. synthesized 2-amino-3-methylpyridine and reported corresponding five peaks at 114.4, 116.8, 137.9, 145.8, and 157.7 ppm in solution-state NMR.39 Very similar chemical shifts also supported our assignments. Besides, 1H−13C HETCOR spectrum of a-PAN stabilized at 310 °C under nitrogen with CP time of 500 μs is shown in Figure 5a. The aliphatic, aromatic, and amine protons appeared at 1−2, 5−8, 13

Figure 3. 13C DP/MAS NMR spectra for 30 wt % C-1, C-3, and all 13 C-labeled a-PAN stabilized at (a) 310, (b) 370, and (c) 420 °C under air with a recycle delay of 150s. Numbers marked as 1, 2, and 3 above the top spectrum denote the resonances originated from labeled C-1, C-2, and C-3 sites, respectively. Numbers down each peak indicate the normalized integration area of corresponding peak.

nitrogen and thus was reasonably assigned to the aromatic C-3. The peak at 175 ppm was not observed in the a-PAN C-3 spectra stabilized under nitrogen. This was a characteristic signal for a-PAN stabilized under oxygen. Besides, the two broad shoulder peaks in the C-3 spectra at around 157 and 117 ppm were very similar in chemical shift values with the aromatic C-1 and aromatic C-2 signals, respectively. 3.2. Through-Bond and Space Correlations of a-PAN Stabilized under Nitrogen. Because of limited spectral resolutions of the broadened signals and multiple components having slightly different chemical shifts, it was difficult to capture all chemical structures involved in the stabilization process of a-PAN, even with chemical shift values based on single-site 13C labeling. Detailed chemical structures were investigated by 13C−13C 2D INADEQUATE spectra for all

Figure 4. Refocused 13C−13C 2D INADEQUATE spectra for 30 wt % all 13C-labeled a-PAN stabilized at (a) 310 and (b) 420 °C under nitrogen. 13 C CP/MAS spectra with a contact time of 2 ms were also attached at the top of INADEQUATE spectra. D

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which showed up initially at 300 °C and gradually decreased with the increase in stabilization temperature until it disappeared at Ts = 370 °C (see Figure 2 and Figure S1 in the SI). Besides, a correlation between m′ (at 68.7 ppm) and l′ was assigned as piperidine ring on the basis of the recent literature result.17 For the chain-end C-3, besides peak p′, another chain-end C-3 signal was detected as d′ at 16.4 ppm, which was correlated with the aliphatic C-2 (b′) and thus was reasonably assigned as the chain scission product of the unreacted a-PAN chain. At Ts = 420 °C, as shown in the 1D DP/MAS spectra (Figure 2c), intensities of the aliphatic signals drastically decreased and the dominant signals were the aromatic ones. Besides, two other structures were detected as marked by green and brown lines in Figure 4b. Similar to Ts = 310 °C, correlation between p′ and q′ was also clearly detected. According to the assignment at Ts = 310 °C, this structure was attributed to the chain scission product of the isolated aromatic ring. In fact, weak correlations between q′ and g′, i′ were also detected, which could support this assignment. Correlations between t′ at 17.9 ppm and r′ at 127.3 ppm, g′ at 135.4 ppm (very weak), and s′ at 146.1 ppm illustrated another chain-end structure. Through the solution-state NMR spectrum of compound 3-methylpyridine, chemical shift values for the corresponding peaks were 18.4, 133.1, 136.4, and 150.3 ppm, respectively.45 The experimental results were similar to the reported ones, and these signals were thus assigned to the pyridine ring with the chain-end methyl group attached to the aromatic C-2 as the structure colored by brown inserted in Figure 4b. These two chain-end structures could not be differentiated from each other due to the overlapped signals in the recent report.17 13C−13C INADEQUATE experiments clearly showed the presence of three kinds of the chain-end structures of a-PAN stabilized under nitrogen. The assigned structures together with chemical shift references were summarized in Table S1 in the SI. 3.3. Through-Bond and Space Correlations of a-PAN Stabilized under Air. Figure 6 shows 13 C− 13 C 2D INADEQUATE spectrum for 30 wt % all 13C-labeled a-PAN stabilized at 310 and 420 °C under air. Several signals at 100− 125 ppm were overlapped with each other as similarly observed in a-PAN stabilized under nitrogen. Five peaks (c, d, h, i, and j (only at Ts = 420 °C)) were detected. By comparing 1D spectra of unreacted a-PAN (Figure 1) and INADEQUATE spectra of a-PAN stabilized under nitrogen (Figure 4), correlations between peaks c at 120.5 ppm, b at 27.6 ppm, and a at 34.1 ppm could be assigned as the unreacted a-PAN structure. Peak d (at 114.3 ppm) had a strong correlation with e (aromatic C-3 at 138.8 ppm), f (at 153.7 ppm), and g (at 175.4 ppm). Through this correlation and 1D spectra in Figure 3, d, f, and g were thus assigned as the aromatic C-2, aromatic C-1, and oxidized C-3 (since only observed for a-PAN stabilized under air and its high chemical shift value) in the aromatic ring, respectively. The chemical shift values of the aromatic ring signals of a-PAN stabilized under nitrogen also supported these signal assignments. Notably, the aromatic C-1 and C-2 signals of a-PAN stabilized under air showed singlet peaks, while two different signals were observed for those under nitrogen. In addition, there is no correlation between the aromatic C-2 carbon, d, and aliphatic signals, as illustrated in Figure 6. These two points were striking contrasts with the results under nitrogen. Also, 1H−13C HETCOR of a-PAN stabilized at Ts = 320 °C under air was illustrated in Figure 5b. There was no

Figure 5. 2D 1H−13C HETCOR spectra for 30 wt % all 13C-labeled aPAN stabilized at (a) 310 °C under nitrogen and (b) 320 °C under air, respectively. Lowest contour level is around 10%. (c) Proton slice data at 175.4 ppm with CP contact time of 2.0 ms for 30 wt % all 13C labeled a-PAN stabilized at 320 °C under air.

and 8−11 ppm, respectively.40−43 There are strong correlations between the aromatic carbons and protons. Among three aromatic carbons, the C-3 carbon showed a strong correlation with the directly bonded aromatic protons. In addition, the aromatic C-1 peak gave a correlation with the amine protons at 8−11 ppm. These correlations allowed us to assign the main product as the isolated pyridine ring with −NH2 attached to i′ as the pink structure shown in the bottom of Figure 4a. Also, 13C−13C bond correlations between e′, f′, and the aliphatic carbon (a′) at 29.8 ppm indicated the direct bond connection between the aromatic ring and aliphatic C-3 (a′). 1 H−13C spatial correlations between the aromatic carbons and aliphatic protons were also clearly detected in Figure 5a. Namely, the aromatic ring was isolated from the others and directly bonded to the aliphatic carbon. These spatial and bond correlation results are well consistent with the recent result by Wang et al.17 In addition to the major peaks, minor peaks q′ and o′ were also detected at 119.1 and 119.3 ppm, respectively. Peak q′ had a correlation with the chain-end C-3 (peak p′) at 15.5 ppm. In addition, peak q′ had a weak correlation with the aromatic C-3 g′ at 135.4 ppm and the aromatic C-1 i′ at 153.7 ppm. These correlations indicated the pyridine-based aromatic ring has a bond connection with the methyl group. This structure was attributed to the chain scission product of the isolated aromatic ring. These correlations were observed even at Ts = 420 °C, as illustrated in Figure 4b. Thereby, it was considered that this chain-end structure is the major chain scission reaction of aPAN stabilized under nitrogen. A clear correlation could be found between o′ at 119.3 ppm and n′ at 92.4 ppm (C-2, see Figure 2). Tanner et al. measured the solution-state 13C NMR spectrum of 1,4-dihydropyridine, and chemical shifts for the corresponding carbons on this ring were 128.6 and 96.1 ppm, respectively.44 Thereby, the correlation between peaks o′ and n′ was reasonably assigned to the 1,4-dihydropyridine ring, shown as the light-blue structure in Figure 4a. Peak n′ was also connected to the aliphatic C-3 (peak a′) at 29.8 ppm, which proved that the 1,4-dihydropyridine rings are also isolated. As similarly observed at 110−125 ppm, multiple aliphatic peaks coexisted at 20−35 ppm. At least, three distinguishable peaks labeled by a′ (29.8 ppm), l′ (32.1 ppm), and j′ (31.9 ppm) appeared in the 2D spectrum. The signal j′ was correlated with k′ at 161.2 ppm, which supported previously proposed cyclization intermediate structure by Wang et al.17 Peak k′ corresponds to the strong shoulder C-1 mentioned in Figure 2, E

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adjacent oxidized C-3 due to its lower chemical shift value compared with the aromatic C-2 (peak d). Chemical shift of peak j agreed well with the chemical shift prediction result of compound 1,8-naphthyridine-4,5(1H,8H)-dione.48,49 With the increase in Ts, the fraction of two adjacent oxidized C-3 signals also increased. The structures with corresponding chemical shift references also could be found in Table S2 in the SI. HETCOR spectrum offered additional information about the oxidized carbon at 175 ppm. At a CP time of 500 μs, there was no correlation between the oxidized carbon and any protons, as shown in Figure 5b. With increasing CP time up to 2 ms, a weak correlation with the aromatic protons was detected, and the corresponding slice data for the oxidized carbon are shown as a red curve in Figure 5c. With no correlation detected with C−OH proton (10 to 11 ppm through literature43,50), it was proved that the oxidized carbon forms no C−OH but CO group.

4. DISCUSSION 4.1. Reaction Pathways for a-PAN Stabilized under Nitrogen. Here we consider the chemical reaction scheme of a-PAN stabilized under nitrogen by using 1D and 2D correlation data. At Ts = 310 °C, the cyclized six-membered ring (blue correlation in Figure 4a), 1,4-dihydropyridine (light blue correlation in Figure 4a), piperidine (purple correlation in Figure 4a) as well as the isolated aromatic ring structure (pink correlation in Figure 4a, main residue at high Ts values) were detected. Among three minor components, the cyclized sixmember ring product had obviously higher intensity (7.5%) compared with piperidine (3.6%) and 1,4-dihydropyridine (1.1%) at Ts = 310 °C (see Figure S1 in the SI and integration area in Figure 2). Additionally, cyclization process is believed to be one of the initial processes of stabilized a-PAN.1 Thereby, cyclization process could be assigned as the initial reaction. Minor piperidine and 1,4-dihydropyridine components are reasonably considered to be hydrogenation and isomerization products of the cyclized six-member ring intermediate. At Ts = 370 °C, these products completely disappeared, remaining isolated aromatic ring as the main product. Recently, Wang et al. paid attention to the ratio of the aromatic C-3:C-2:C-1 signals corresponding to the isolated aromatic ring and ladder structure.17 In the former, the ratio of the aromatic C-3:C-2:C-1 is 1:2:2. In the latter, this ratio is 1:1:1 under the assumption of infinite domain structure. On the basis of the integration area of the corresponding peaks, they concluded that the aromatic ring is isolated from each other. We also calculated the ratio. Up to Ts = 370 °C, the ratio was 1:2:2 (see integration values in Figure 2). This result also supported the isolated aromatic ring structure and was well consistent with the result by Wang et al.17 At further high Ts values, the ratio started to decrease. For example, this ratio turned to be 1:1.4:1.4 at Ts = 420 °C. In addition, through C-1 and all labeled spectra of a-PAN stabilized at Ts = 310 °C under nitrogen, the aromatic C-1 and C-2 peaks clearly adopted doublet line shape with similar intensities, as shown in Figures 2a and 4a. As stabilization temperature increased to 420 °C, f′ and i′ in the two peak pairs showed higher intensities compared with e′ and h′, respectively. The chemical shift values of peaks f′ (114.0 ppm) and i′ (153.7 ppm) were close to the aromatic C-2 (d at 114.3 ppm) and C-1 (f at 153.7 ppm) signals for aPAN stabilized under air. On the basis of these experimental results, it was concluded that higher Ts values under nitrogen

Figure 6. 2D refocused 13C−13C INADEQUATE spectrum for 30 wt % all 13C labeled a-PAN stabilized at 310 and 420 °C under air. 13C CP/MAS spectrum was also attached at the top of INADEQUATE spectra.

polarization transfer between the aliphatic protons at 1 to 2 ppm and the aromatic carbons; only polarization transfer from the aliphatic protons to the -CN carbon at 121 ppm for unreacted a-PAN was detected. These experimental results concluded that a-PAN stabilized under air adopts successive aromatic ring formation, namely, ladder formation. Ladder formation did not include −NH2 side group at the C-1 site of the rings. Thereby, the C-1 and C-2 sites adopted chemically equivalent environment and resulted in the singlet peaks. This assignment also agrees well with solution-state NMR results for model compounds of 1, 8-naphthyridine45 and 4-pyridone.46 Peaks labeled by i at 117.0 ppm and h at 108.8 ppm were observed at Ts = 310 °C, and these two showed a clear bond correlation. These two peaks were not observed in 2D INADEQUATE spectra for a-PAN stabilized under nitrogen. Thus, these peaks should be attributed to a distinct structure of a-PAN stabilized under the presence of oxygen. Hsieh et al. synthesized polycyanoacetylene, which consists of polyene backbone and −CN side group and found that the C-1, C-2, and C-3 signals appear at 116, 110, and 158 ppm, respectively.47 Chemical shifts of the former two peaks agreed well with our chemical shift values and their correlations. Thereby, these two peaks were reasonably assigned to the C-1 and C-2 signals in the polyene structure. Because of the signal overlapping in the 150−160 ppm region, it was hard to clearly distinguish the polyene C-3 signal from the aromatic C-1 signal. Besides, a correlation between two weak peaks m (at 173.3 ppm, oxidized C-3) and l (at 39.8 ppm, aliphatic carbon) at Ts = 310 °C proved the existence of oxidation intermediate, which is shown as green structure inserted in Figure 6. As shown in Figure 3c, the 13C signals appeared in the chemical shift range of 90−180 ppm at Ts = 420 °C. Correlation signal patterns were very similar to those at Ts = 310 °C. This means that stabilization at high Ts basically gave the same structures as at low Ts except for one extra peak j at 102.6 ppm, which had correlations with both the aromatic C-1 and oxidized C-3 in Figure 6. This signal could be assigned as the C-2 signal, which was suggested as C-2 between two F

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Macromolecules Scheme 1. Reaction Scheme for a-PAN Stabilized at 290−450 °C under Nitrogen

Scheme 2. Reaction Scheme for a-PAN Stabilized at 290−450 °C under Air

25% through the C-3 spectra at various Ts values, as shown in Figure 3. In addition to dehydrogenation, it was also detected that the small amount of polymer chains experiences oxidization as the intermediate reaction prior to aromatization. The reaction pathways of a-PAN stabilized under air are summarized in Scheme 2. 4.3. Comparison between Stabilization under Nitrogen and Air. By comparing stabilized structures of a-PAN under nitrogen and air, several points are worth stressing. First, two different atmospheres offered completely different reaction pathways in the initial stage. Through different initial steps, stabilization under nitrogen preferred the isolated aromatic structure at Ts up to 370 °C, while stabilization under air lead to the aromatic ladder formation even at a low Ts (e.g., 310 °C). Recently, Wang et al. indicated that a-PAN stabilized under argon prefers the isolated aromatic ring up to 350 °C and suggested that the isolated ring formation during the stabilization process arises from atactic sequence.17 Our results clearly rejected the tacticity effect and concluded that the critical factors to determine the ladder formation are oxygen and the initial reaction processes (cyclization vs dehydrogenation). The different reaction pathways and products were explained as follows: Cyclization process does not require oxygen and is thermally initiated. This process randomly occurs in the a-PAN chain, which subsequently lead to the isolated aromatic rings at low Ts values. At higher temperature, further reactions preferred partial adjacent aromatic ring formation. On the contrary, the dehydrogenation process is originated from oxygen, and this reaction process might induce successive reactions along the backbone and subsequently lead to the ladder formations at even low Ts values. Another interesting feature is the chain-scission process under nitrogen and air. Three kinds of the chain-end structures were clearly detected in a-PAN stabilized under nitrogen. On the contrary, stabilization under air did not induce such structures at higher than 1%. The formation of the chain-end

lead to the partial formation of the adjacent aromatic rings structures. Besides, three different chain scission products were observed on the basis of 2D INADEQUATE. At relatively low Ts values, a small amount of the unreacted a-PAN chain cleaved to form the chain-end structure. The second chain scission gave the isolated aromatic ring with −NH2 side chain in a wide Ts range. Namely, chain scission of the aliphatic chains bonded to the aromatic ring occurred after the formation of isolated aromatic ring. Additionally, chain scission also took place after isomerization, as there was no −NH2 side chain connected to this structure. The reaction pathways of a-PAN stabilized under nitrogen are summarized in Scheme 1. 4.2. Reaction Pathways for a-PAN Stabilized under Air. Here we pay attention to the reaction scheme of a-PAN stabilized under air. According to literature results, two initial reaction pathways were proposed for stabilization of a-PAN under air. One is cyclization and another is dehydrogenation.1,3,16 2D INADEQUATE clearly detected aromatic ring domain formation and polyene backbone for a-PAN stabilized at a low Ts = 310 °C, while no cyclization intermediate was formed at the same or higher Ts values. This observation indicated that dehydrogenation process proceeds in the initial stage of a-PAN stabilization and is followed by aromatization. Ts dependence of 1D DP/MAS proved the increase in the aromatic structure with elevating Ts. Thus, the aromatic domain structures are assigned as the main residue for a-PAN stabilized under air. Also, INADEQUATE and C-1 DP/MAS spectra confirmed polyene structure as a minor component in wide Ts ranges. This indicates that polyene structure is the initial one as well as a part of the final products for a-PAN stabilized under air. Oxygen take-up resulted in the (C-3)O group in the aromatic domain. Partially, two successive (C-3)O groups were also found in the two adjacent aromatic rings at high Ts values. Total fraction of oxygen take-up is calculated as 20− G

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Macromolecules structure originates from the instability of the polymer chains upon heating. Chain-scission process is thus highly related to thermal stability of stabilized a-PAN. As previously described, a-PAN stabilized under nitrogen included isolated aromatic rings that were connected by the aliphatic backbone. The aliphatic parts composed of single bonds were easily cleaved and thus make CH3 terminals. On the contrary, the aromatic ladder domain and polyene structures including double bonds formed during stabilization under air were thermally stable. 4.4. Stabilization Index. To date, experimental results of FT-IR,13 XRD,18 DSC,19 and so on have been used to describe the stabilization index. Nguyen-Thai et al. defined the stabilization index as peak area of aromatic ring structure versus peak area of aromatic ring structure plus unreacted nitrile group (−CN).13 Yu et al. applied X-ray diffraction to study the a-PAN stabilized under air and observed the decrystallization phenomenon. Stabilization can be thus defined and calculated based on the signature peak of 2θ = 17°.18 Through this work, we could precisely evaluate chemical structures of a-PAN stabilized under nitrogen and air. On the basis of the stabilized a-PAN structures, stabilization of a-PAN could be thus defined as the untreated a-PAN converted to the aromatic structure, and the stabilization index (SI) was precisely calculated. In the case of a-PAN stabilized under air, mainly three different structures consisting of ladder aromatic rings (include oxidized part), polyene, and unreacted a-PAN were involved in the whole process. The conjugated aromatic ladder structure was the dominant residue for a-PAN stabilized under air. The C-1 signals representing aromatic, polyene, and unreacted a-PAN showed distinct chemical shift values of 152, 116, and 120 ppm, respectively. Stabilization index for a-PAN stabilized under air (SIair) was thus calculated through the following equation SIair =

C‐1aromatic

Figure 7. SI values for a-PAN stabilized under nitrogen and air as a function of Ts.

Notably, SIair reached 0.87 at Ts = 370 °C. The remaining component was attributed to the polyene structure (see Figures 3 and 6). SIair of 0.87 allowed us to quantitatively calculate approximately six adjacent aromatic rings as a mean structure under the assumption that one polyene segment was connected by one ladder domain on each side. Such an average structure is particularly important in the understanding of the stabilized aPAN structure. To form the aromatic ladder structure, it is necessary to have every adjacent aromatic ring on the same plane. From this result, we can imagine that conformation of original a-PAN would limit long-range order of the ladder structure. In fact, Kaji et al. applied 13C−13C double quantum chemical shift anisotropy correlation to investigate conformations of a-PAN and reported that trans:gauche conformations adopt 9:1 ratio in the a-PAN backbone.30 This ratio is slightly higher than the mean ladder structure determined in this work. Conformational constrains might limit the development of long-range ladder formation under air. In industrial processing, stabilization is performed under stretched states.1 This fact and our experimental evidence implied that conformation of a-PAN is a key factor to lead to more ordered stabilized structure and the further processing of carbon fibers. 4.5. Intramolecular and Intermolecular Reactions. Our strategy mainly highlighted intramolecular reactions of 13Clabeled a-PAN; however, some signals of a-PAN stabilized under air could not be explained in terms of the intramolecular interactions. For example, the C-3 signals at 155.5 and 117.7 ppm (see Figure 3) overlapped with the C-1 f and C-2 d signals in 2D INADEQUATE spectrum (see Figure 6) and were relatively weak comparing with those of the C-1 and C-2 signals. Thereby, it was hard to identify bond correlation of these minor peaks with the C-2 signals in 2D correlations. The chemical shift values of these C-3 signals were largely different from those for the aromatic and oxidized C-3 signals. This suggested that different chemical reactions are additionally involved in the stabilization of a-PAN under air. Similarly, two broad shoulder peaks at 150 and 115 ppm were observed for aPAN stabilized at Ts = 370 °C or higher under nitrogen. (see Figure 2 and Figure S1b in the SI) These two signals might also originate from intermolecular cross-linking. According to literature, several intermolecular cross-linking structures (intermolecular reaction) were proposed.10−12 To highlight intermolecular reactions, different sample design will be necessary. This subject will be one of the future subjects in characterization of stabilized a-PAN.

C‐1aromatic + C‐1polyene + C‐1unreacted

where C-1aromatic, C-1polyene, and C-1unreacted are signal integration area for the DP/MAS NMR spectra at corresponding Ts values (see Figure 3 and Figure S2a in the SI). Under nitrogen, among all signals on the aromatic ring including the aromatic C-1, C-2, and C-3, chemical shift for the former two were easily affected by the group bonded to them, while the aromatic C-3 signal was well-separated from the others. The aromatic C-3 signal could thus be used as characteristic signal. The C-3 spectra for a-PAN stabilized under nitrogen showed three signals corresponding to the aromatic, aliphatic, and chain-end carbons. Then, SInitrogen was calculated using the following equation: SI nitrogen =

C‐3aromatic

C‐3aromatic + C‐3aliphatic + C‐3chain‐end

where C-3 aromatic, C-3aliphatic, and C-3 chain‑end are signal integration area for the DP/MAS NMR spectra at corresponding Ts values (see Figures 2 and Figure S1b in the SI). Using the two equations, the calculated SIair and SInitrogen were plotted as red and blue filled circles, respectively, in Figure 7. It was obvious to see that stabilization under air is definitely more efficient compared with stabilization under nitrogen through both final SI value and speed (slope of the plot). The stabilization under air was almost saturated at Ts = 370 °C, while stabilization process under nitrogen needed much higher Ts. H

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(7) Grassie, N.; Hay, J. N.; McNeill, I. C. Coloration in Acrylonitrile and Methacrylonitrile Polymers. J. Polym. Sci. 1958, 31, 205−206. (8) Berlin, A. A.; Dubinskaya, A. M.; Moshkovskii, Y. S. Heat Treatment of Polyacrylonitrile in Solution in Dimethylformamide. Polym. Sci. U.S.S.R. 1964, 6, 2145−2151. (9) Coleman, M. M.; Petcavich, R. J. Fourier Transform Infrared Studies on the Thermal Degradation of Polyacrylonitrile. J. Polym. Sci., Polym. Phys. Ed. 1978, 16, 821−832. (10) Schurz, J. Discoloration Effects in Acrylonitrile Polymers. J. Polym. Sci. 1958, 28, 438−439. (11) Grassie, N.; Hay, J. N. Thermal Coloration and Insolubilization in Polyacrylonitrile. J. Polym. Sci. 1962, 56, 189−202. (12) Henrici-Olivé, G.; Olivé, S. Inter-versus Intramolecular Oligomerization of Nitrile Groups in Polyacrylonitrile. Polym. Bull. 1981, 5, 457−461. (13) Nguyen-Thai, N. U.; Hong, S. C. Structural Evolution of Poly(acrylonitrile-co-itaconic acid) during Thermal Oxidative Stabilization for Carbon Materials. Macromolecules 2013, 46, 5882−5889. (14) Fu, Z.; Gui, Y.; Cao, C.; Liu, B.; Zhou, C.; Zhang, H. Structure evolution and mechanism of polyacrylonitrile and related copolymers during the stabilization. J. Mater. Sci. 2014, 49, 2864−2874. (15) Fochler, H. S.; Mooney, J. R.; Ball, L. E.; Boyer, R. D.; Grasselli, J. G. Infrared and NMR spectroscopic studies of the thermal degradation of polyacrylonitrile. Spectrochim. Acta 1985, 41A, 271− 278. (16) Usami, T.; Itoh, T.; Ohtani, H.; Tsuge, S. Structural Study of Polyacrylonitrile Fibers during Oxidative Thermal Degradation by Pyrolysis-Gas Chromatography, Solid-State 13C Nuclear Magnetic Resonance, and Fourier Transform Infrared Spectroscopy. Macromolecules 1990, 23, 2460−2465. (17) Wang, Y.; Xu, L.; Wang, M.; Pang, W.; Ge, X. Structural Identification of Polyacrylonitrile during Thermal Treatment by Selective 13C Labeling and Solid-State 13C NMR Spectroscopy. Macromolecules 2014, 47, 3901−3908. (18) Yu, M.-J.; Bai, Y.-J.; Wang, C.-G.; Xu, Y.; Guo, P.-Z. A New Method for the Evaluation of Stabilization Index of Polyacrylonitrile Fibers. Mater. Lett. 2007, 61, 2292−2294. (19) Wu, S.-H.; Qin, X.-H. Effects of the Stabilization Temperature on the Structure and Properties of Polyacrylonitrile-based Stabilized Electrospun Nanofiber Microyarns. J. Therm. Anal. Calorim. 2014, 116, 303−308. (20) Andrew, E. R.; Bradbury, A.; Eades, R. G. Nuclear Magnetic Resonance Spectra from a Crystal Rotated at High Speed. Nature 1958, 182, 1659. (21) Lowe, I. J. Free Induction Decays of Rotating Solids. Phys. Rev. Lett. 1959, 2, 285−287. (22) Bennett, A. E.; Rienstra, C. M.; Auger, M.; Lakshmi, K. V.; Griffin, R. G. Heteronuclear Decoupling in Rotating Solids. J. Chem. Phys. 1995, 103, 6951−6958. (23) Hashi, K.; Shimizu, T.; Goto, A.; Kiyoshi, T.; Matsumoto, S.; Wada, H.; Fujito, T.; Hasegawa, K.; Yoshikawa, M.; Miki, T.; Ito, S.; Hamada, M.; Hayashi, S. Achievement of a 920-MHz High Resolution NMR. J. Magn. Reson. 2002, 156, 318−321. (24) Schmidt-Rohr, K.; Spiess, H. Multidimensional Solid-State NMR and Polymers; Academic Press: San Diego, 1994. (25) Bax, A.; Freeman, R.; Kempsell, S. P. Natural Abundance 13 13 C- C Coupling Observed via Double-Quantum Coherence. J. Am. Chem. Soc. 1980, 102, 4849−4851. (26) Lesage, A.; Auger, C.; Caldarelli, S.; Emsley, L. Determination of Through-Bond Carbon-Carbon Connectivities in Solid-State NMR Using the INADEQUATE Experiment. J. Am. Chem. Soc. 1997, 119, 7867−7868. (27) Lesage, A.; Bardet, M.; Emsley, L. Through-Bond CarbonCarbon Connectivities in Disordered Solids by NMR. J. Am. Chem. Soc. 1999, 121, 10987−10993. (28) Hong, M. Solid-State Dipolar INADEQUATE NMR Spectroscopy with a Large Double-Quantum Spectral Width. J. Magn. Reson. 1999, 136, 86−91.

5. CONCLUSIONS We synthesized 13C single-site C-1,C-3-labeled and all-labeled a-PAN, and demonstrated clear 13C−13C bond correlations of a-PAN stabilized under nitrogen and air. As a result, we have successfully derived the intramolecular reaction pathways and chemical structures of a-PAN stabilized under nitrogen and air as follows: The former was initiated by cyclization process, while the dehydrogenation process was the first step in the latter. Two different reaction conditions involving oxygen or not lead to (i) drastically different aromatization structures, (ii) presence or absence of the chain ends, and (iii) different side reactions. The stabilization behaviors for a-PAN under air were almost saturated at Ts = 370 °C (SIair = 0.87), while SInitrogen built up very slowly with increasing Ts (SInitrogen = 0.66 at Ts = 450 °C). The largely different stabilized structures were reasonably explained in terms of oxygen involved or not in the stabilization process. Among the obtained results, a particularly interesting result was that final structures for aPAN stabilized under air consist of six aromatic rings in addition to one polyene structure as a mean structure. This result might be related to high ordered structures of a-PAN at the molecular level. The ss-NMR approach combined with isotopic labeling has opened possibilities for detailed structural analysis of stabilized a-PAN. This strategy will shed light on further understanding of intermolecular reactions as well as high-order structural effect on the stabilization of a-PAN.

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ASSOCIATED CONTENT

S Supporting Information *

Chemical shift references for 13C spectra, 1H T1 values, and temperature dependence of C-1 and C-3 spectra. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.5b01030.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Nader Hedayat and Dr. Steven S. C. Chuang for the use of ovens, Kaushik Mishra, Chao Peng, and Dr. Abraham Joy for the help in GPC measurement. Financial support of this work from UA start-up fund is gratefully appreciated.

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REFERENCES

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