Chemical Reactions and Their Kinetics of atactic-Polyacrylonitrile As

Dec 23, 2016 - The authors thank Mr. Jiawei Liu and Dr. Steven S. C. Chuang for the use of ovens. This research was financially supported by the Ameri...
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Chemical Reactions and Their Kinetics of atactic-Polyacrylonitrile As Revealed by Solid-State 13C NMR Xiaoran Liu,† Yuta Makita,† You-lee Hong,‡ Yusuke Nishiyama,‡,§ and Toshikazu Miyoshi*,† †

Department of Polymer Science, The University of Akron, Akron, Ohio 44325-3909, United States RIKEN CLST-JEOL Collaboration Center, RIKEN, Yokohama, Kanagawa 230-0045, Japan § JEOL RESONANCE Inc., Akishima, Tokyo 196-8558, Japan ‡

S Supporting Information *

ABSTRACT: Inter- and intramolecular chemical reactions and their kinetics for 13C-labeled atactic-polyacrylonitrile (aPAN) powder heat-treated at 220−290 °C under air and vacuum were investigated by various solid-state nuclear magnetic resonance (ssNMR) techniques. By applying 13C direct polarization magic angle spinning (DPMAS) as well as through-bond and throughspace double quantum/single quantum ssNMR techniques, it was concluded that aPAN heat-treated under air at 290 °C for 300 min adopted the ladder formation, namely, conjugated six-membered aromatic rings with partially cross-linked and oxidized rings and polyene components. In contrast, aPAN heat-treated under vacuum at the same condition thermally decomposed into oligomeric chains that were mainly composed of isolated aromatic rings connected by alkyl segments. Furthermore, early stages of the chemical reactions were investigated by 13C cross-polarization (CP) and DPMAS spectra. The latter provided quantitative information regarding the kinetics of the chemical reactions. As a result, it was shown that chemical reactions under oxygen occurred homogeneously with a higher activation energy (Ea) of 122 ± 3 kJ/mol compared to that of vacuum at 47 ± 2 kJ/mol. By comparing both chemical structures and kinetics under two different conditions, the chemical reaction mechanisms of aPAN will be discussed in detail. 20%), and other structures (10%).10,11 However, this study could not distinguish the cyclized rings from the aromatic ones. Furthermore, other studies using conventional Fourier transformed infrared spectroscopy (FT-IR) faced the same problem.12,13 Thus, it is critical to develop a novel characterization approach with higher spectral and spatial resolutions along with enhanced sensitivity to solve these problems. On the other hand, the physical state (i.e., powder, film, or fiber) of the aPAN precursor needs to be addressed to fundamentally understand the chemical reaction. A previous study using optical microscopy showed that heat-treated aPAN fiber had a core−shell morphology.17 Such observation indicates the difference in the degree of reaction between the outer surface and the inner core of the fiber. The study highlighted that sample size, based on the physical state of aPAN, also has an

1. INTRODUCTION atactic-Polyacrylonitrile (aPAN) is an industrially significant polymer that has been used as a precursor to produce carbon fibers.1−7 Highly stretched aPAN fibers heat-treated under different temperatures and environments lead to lightweight carbon fibers with outstanding mechanical properties. Among various heat treatment steps, the stabilization process that is conducted under air at 200−400 °C converts aPAN into thermally stable chemical structures.1−5 Different stabilization conditions significantly influence the final mechanical properties of the carbon fibers.1,7 Thus, various characterization techniques have been applied to understand the chemical structures of heat-treated aPAN and the reaction kinetics involved.8−16 For example, through X-ray photoelectron spectroscopy and solid-state nuclear magnetic resonance (ssNMR), Morita et al. proposed that chemical structures of reacted aPAN fibers consist of six-membered aromatic rings (pyridine, 30%), oxidized six-membered rings (central ring in acridone, 40%), cyclized rings (2,3,4,5-tetrahydropyridine, © XXXX American Chemical Society

Received: October 14, 2016 Revised: December 14, 2016

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2.2. Heat Treatment of Material. Heat treatment was performed by a programmable Thermolyne benchtop muffle furnace. Reaction temperature was defined as the target temperature in the heating program while reaction time was the time kept at the desired temperature. The heating rate was set at 5 °C/min. After maintaining the reaction temperature for a certain time, cooling was introduced at the rate of 3.3 °C/min. The samples heat-treated under air were heated in an open system while the vacuum samples were heated under sealed glass tubes. 2.3. Differential Scanning Calorimetry (DSC) Measurements. DSC measurement was carried out using a Q10 DSC manufactured by TA Instruments equipped with a refrigerated cooling system. The sample measured under air was heated under an open system without gas flow while the measurement under nitrogen was carried out under flow of dried nitrogen at the flow rate of 30 mL/min. Sample mass for each measurement was around 3 mg. 2.4. 13C Solid-State NMR Measurement. The ssNMR experiments were conducted in a BRUKER Avance Ultrashield 300 NMR spectrometer equipped with a 4 mm double resonance VT CPMAS probe. Carrier frequencies for 1H and 13C were 300.1 and 75.6 MHz, respectively. The MAS speed was set as 13 kHz to eliminate the spinning sideband. The CH carbon of adamantane was used as an external reference as 29.46 ppm. The 13C T1 relaxation time was obtained by Torchia pulse sequence,43 and the longest T1 values were 25−28 s; thus, the recycle delay time was set as 150 s in 13C DPMAS experiments. The 13C 90° pulse length was set as 4.6 μs. In 13C CPMAS measurements, 1H 90° pulse length, CP contact time, and recycle delay time were set as 4.30 μs, 2 ms, and 2 s, respectively. Twopulse phase-modulated (TPPM) decoupling17 was applied with the frequency of 71.4 kHz (180° pulse, 7.0 μs). The same condition was applied to the 2D 13C−13C INADEQUATE experiment, where 128 t1 points with 256 scans were obtained for good spectral quality. The experimental condition for 1H−13C 2D HETCOR spectra was set to be 3.1 μs for the 90° 1H pulse length, 2 ms for the CP contact time, 6.2 μs for the 180° pulse in TPPM decoupling, and 10.1 μs for the 360° in FSLG field strength. A total of 40 t1 points with 128 scans were executed. The 1H chemical shift value was calibrated with glycine. 13 C−13C dipolar based DQ spectra aPAN-3-13C sample were obtained by applying POST C7 pulse sequence26 at a MAS frequency of 10 204 Hz. The sample volume was limited to 12 μL by a rotor with spacer to maintain radio-frequency pulse homogeneity. The 13C 90° pulse length was set as 3.9 μs. 1H continuous wave (CW) decoupling with a decoupling field strength of 125 kHz was applied during excitation and reconversion time. In the 2D experiment, a total of 40 t1 points with 10 240 scans were recorded for good spectral quality.

effect on chemical reactions involved in the heat treatment process. Samples used in this current study were aPAN powders that could effectively avoid size effect observed in aPAN fibers and films. During the past two decades, ssNMR spectroscopy has been extensively developed.18−27 Modern ssNMR techniques are capable of probing conformation,28−31 packing,32−35 intermolecular interactions,36,37 and three-dimensional structures of biomacromolecules38,39 and synthetic polymers.40−42 Recently, this approach has been applied to understand chemical reactions of aPAN powder.15,16 Wang et al. selectively labeled each of the three carbons in aPAN to trace the chemical reactions during heat treatment under argon using 13C DPMAS ssNMR spectra.15 For the first time, this study showed that cyclized six-membered rings (partially saturated six-membered heterocycles) and other intermediate structures can be distinguished by spectroscopy. It was concluded that under argon the major component of the partially reacted aPAN contained alternative arrangement of six-membered aromatic rings with unreacted aPAN segments. Liu et al. applied twodimensional (2D) J-based 13C−13C incredible natural abundance double quantum transfer experiment (INADEQUATE) on all three sites 30% 13C-labeled aPAN (aPAN-13C3) heattreated under air and nitrogen at temperatures up to 450 °C.16 As a result, it was demonstrated that the presence of oxygen leads to the formation of polyene and aromatic ladder structures that consist of consecutive aromatic rings such as pyridine and oxidized aromatic rings. These two works highlighted the effect of oxygen during the late stage of the chemical reactions of aPAN. In this work, we investigate chemical reactions of aPAN powder heat-treated under air and vacuum at temperatures of 220−290 °C to mimic the temperature range used in industry for stabilization.1 Two goals have been achieved through this work: understanding the intra- and intermolecular reactions as well as determining the kinetics involved in the heat treatment processes under different atmospheres. The first objective is achieved by applying 13C−13C INADEQUATE20−23 and 1 H− 13 C hetero-nuclear correlation (HETCOR) 24,25 on aPAN-13C3 samples to understand intramolecular chemical reactions and 13C−13C dipolar-based double quantum/single quantum (DQ/SQ) experiment26 on C-3 single-site-labeled sample (aPAN-3-13C) to access intermolecular reactions. In addition, one-dimensional DP and cross-polarization (CP) MAS ssNMR spectra were recorded to understand the initial stage of chemical reactions. As a result, it was demonstrated that chemical reactions in the presence of oxygen are slower and proceed more homogeneously compared with those under vacuum. From the obtained results, reaction mechanisms of aPAN heat-treated under two different atmospheres will be discussed in detail.

3. EXPERIMENTAL RESULTS 3.1. DSC Measurements. Figure 1 shows the DSC plot for aPAN measured under air (without flow) and nitrogen flow at a heating rate of 5 °C/min. The measurement under air exhibited a broad exothermic peak with an onset temperature of 240 °C while the measurement under nitrogen flow showed a relatively

2. EXPERIMENTAL SECTION 2.1. Material Preparation. Three kinds of 13C-enriched aPAN including two single-site labeled (C-1 and C-3 sites) and all labeled were synthesized through REDOX initiated free radical polymerization with labeling ratio of 30% as aPAN-1-13C, aPAN-3-13C, and aPAN-13C3, respectively. The 13C-enriched monomers acrylonitrile1-13C, acrylonitrile-3-13C, and acrylonitrile-13C3 were obtained from Sigma-Aldrich and used directly without further purification. Detailed synthesis procedure as well as chemical characteristics of the corresponding samples could be found in our previous work.16 aPAN product was white powder with productivity of ca. 60%.

Figure 1. DSC plot for aPAN heated at the heating rate of 5 °C/min under air (red, without flow) and nitrogen flow (blue). B

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Macromolecules intense, sharp exothermal signal starting at 270 °C. Such observations were reported previously.6,44 The different thermal behaviors arose from substantial differences between the chemical reactions of aPAN under air and nitrogen. From the differences in DSC peaks, it is suggested that oxygen dominantly influenced chemical reactions of aPAN. 3.2. Chemical Structures of aPAN Heat-Treated under Air. Figure 2a shows the 13C DPMAS NMR spectra for

carbon sites. Using the same Voigt model, the relative ratio of the peak areas for peaks at 116, 137, 157, and 175 ppm in the C-3 spectrum were determined as 20.0 ± 0.9%, 45.8 ± 1.8%, 22.4 ± 1.2%, and 11.8 ± 1.1%, respectively (Figure S2). 13C signals from C-1 and C-2 natural abundance also contributed to the peaks at 157 and 116 ppm, respectively. Figure 2b shows the 2D 13C−13C INADEQUATE spectrum for aPAN-13C3 heat-treated under air at 290 °C for 300 min. Correlations between signals f, d, e and f, d, g corresponded to C-1, C-2, and C-3 correlations in the six-membered aromatic and oxidized aromatic rings, respectively.16 Additionally, correlation between signals i and j indicated a C-1 and C-2 correlation in the polyene structure.16 According to literature, it is expected that signal h in the backbone of the polyene segment appear at 157 ppm in the aPAN-3-13C spectrum.45 However, the expected correlation between signals j and h was not observed due to the overlapping of the intense d and f signals. A similar trend was seen for the signal at 116 ppm in the C-3 spectrum. The sp2 carbons in graphene can be observed at 124 ppm.46 The chemical shift for the C-3 signal at 116 ppm was close to that of the sp2 carbons in graphene, which initiates the idea that this signal may originate from intermolecular cross-linking as schematically illustrated in Figure 3 (signal k). To further investigate the cross-linking site, 13C−13C dipolar based DQ spectra for aPAN-3-13C were recorded by applying a POST C7 pulse sequence at the MAS frequency of 10 204 Hz.25 The DQ spectrum with τex = 0.45 ms and SQ spectrum are shown in Figure 3a. The DQ efficiencies were obtained by

Figure 2. (a) 13C DPMAS NMR spectra for aPAN-13C3 (red), aPAN3-13C (green), and aPAN-1-13C (blue) heat-treated under air at 290 °C for 300 min and aPAN-13C3 prior to heat treatment (black). Signals a, b, and c correspond to the C-3, C-2, and C-1 sites in aPAN, respectively. (b) 2D 13C−13C INADEQUATE ssNMR spectrum for aPAN-13C3 heat-treated under air at 290 °C for 300 min. Chemical structures and signal assignments were inserted.

aPAN-13C3 (red), aPAN-3-13C (green), and aPAN-1-13C (blue) heat-treated under air at 290 °C for 300 min and unreacted aPAN-13C3 (black) for comparison. The unreacted aPAN showed aliphatic carbon signals at 33 ppm and a CN signal at 121 ppm. The spectrum for the reacted aPAN-3-13C showed broad signals in the chemical shift range of 100−175 ppm. Moreover, the spectrum of aPAN-1-13C included two signals at 151 and 116 ppm that corresponded to the C-1 site in the aromatic pyridine ring (f) and the CN of the polyene structure (i), respectively.16 By adopting a Voigt peak shape, the relative signal intensities of the peaks at 151 and 116 ppm were determined to be 82.1 ± 1.6% and 17.9 ± 1.2%, respectively. The spectrum of aPAN-3-13C showed four peaks at 116, 138, 157, and 175 ppm that covered the entire spectral range of the aPAN-13C3 spectrum. The C-2 signal was generated through subtraction of the aPAN-1-13C and aPAN3-13C spectra from the aPAN-13C3 spectrum, shown as Figure S1 in the Supporting Information, in which one broad signal was observed at around 116 ppm. Thus, only the C-3 site experienced different chemical environments among the three

Figure 3. (a) 13C CPMAS (red) and DQ (blue) ssNMR spectra for aPAN-3-13C heat-treated under air at 290 °C for 300 min at MAS frequency of 10 204 Hz. Excitation time (τex) for the DQ spectrum was set as 0.45 ms. (b) 13C−13C DQ efficiencies as a function of DQ excitation time for the signals at 116 (red), 138 (blue), 157 (green), and 175 ppm (purple) and (c) 13C−13C DQ/SQ correlation spectrum for aPAN-3-13C heat-treated under air at 290 °C for 300 min with τex = 0.45 ms. Asterisk (∗) indicates the contribution from natural abundance. C

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membered aromatic rings, which take 70% of the final residue, 29% participate in the intermolecular cross-linking reaction. 3.3. Chemical Structures of Partially Reacted and Decomposed aPAN under Vacuum. To understand the effect of oxygen in intra- and intermolecular chemical reactions, aPAN heat-treated under vacuum was also studied. Figure 4a

the ratio of signal intensities between the DQ and SQ signals at a given excitation time. Corresponding DQ build-up curves for the k/d*, e, h, and g signals are plotted in red, blue, green, and purple in Figure 3b, respectively. The 13C−13C DQ build-up curves are determined by net dipolar interactions of various spin pairs in addition to chemical shift anisotropy, radiofrequency imperfection, 1H decoupling, and spin−spin relaxation.40−42 Through the DQ build-up curves, it was demonstrated that maximum DQ efficiency of 0.18 was obtained for signals k/d*, g, and h while only 0.11 was achieved for signal e. Considering similar spin environments for all C-3 sites, insufficient 1H decoupling was the source that induced a lower maximum DQ efficiency for signal e. This indicated that all other carbons were not directly bonded to protons except for those at signal e. Proton carbon connectivity was also confirmed by the CPMAS spectrum with a sufficiently short CP contact time of 50 μs. The spectrum showed that signal e has reasonable polarization transfer between the directly bonded 1H and 13C (refer to Figure S3). Such observations also indicated that signal h was not directly bonded with protons, which suggested that further study may be necessary, on the additional reactions that may occur at site h, for proper signal assignment. The DQ correlations included two spin correlations that were mainly contributed by two 13C-labeled spins in addition to a minor contribution between 13C-labeled spins and 13C from natural abundance (signal d*). Among the four signals (k/d*, e, h, and g), the k/d* site was the only one that showed a rapid build-up in DQ efficiency at a short excitation time, ca. 0.1 at τex = 0.45 ms, while other signals only reached 0.03−0.04 at the same τex. Such short τex could highlight the one-bond-separated 13 C pairs. Figure 3c shows 2D 13C−13C dipolar based DQ/SQ correlation spectrum for aPAN-3-13C heat-treated under air at 290 °C for 300 min with τex = 0.45 ms. 2D DQ/SQ techniques could yield signals between two 13C nuclear spins that are spatially correlated. The DQ signals in the ω1 dimension appeared at chemical shift of the sum of chemical shifts of the interacting spin pairs in the ω2 dimension. Signal e showed e−e on-diagonal and e−k/d* off-diagonal correlations. The former corresponded to the correlation between two adjacent 13Clabeled sites that were separated by two bonds while the latter showed correlations that were contributed by both one bond and two bonds separated 13C spin pairs. To clarify, for the e−k/ d* off-diagonal correlation, the one-bond-separated correlation arose from the naturally abundant 13C at the d* site while the two-bond-separated correlation was contributed by the 13Clabeled k site. The e−e and e−g correlations showed spatial connectivity of the aromatic rings, mostly arisen from a ladder formation. Meanwhile, the absence of the g−g correlation indicated that oxidized aromatic C-3 carbons were randomly distributed among the rings. For signal k/d*, an on-diagonal correlation k−k/d* as well as several off-diagonal correlations k/d*−e, h, and g have been clearly detected. The on-diagonal correlation k−k/d* provided direct evidence for the intermolecular cross-linking at the C-3 site. On the basis of 2D INADEQUATE through-bond correlation, quantitative DPMAS spectra, and 2D DQ/SQ through-space correlation, the chemical structure for aPAN heat-treated under air is schematically illustrated in Figure 3c. The residue structure contained six-membered aromatic ring (69.5 ± 2.7%), oxidized six-membered aromatic ring (12.6 ± 1.1%), and polyene units (17.9 ± 1.2%). Among the six-

Figure 4. (a) 13C DPMAS NMR spectra for aPAN-13C3 (red), aPAN3-13C (green), and aPAN-1-13C (blue) heat-treated under vacuum at 290 °C for 300 min. (b) 2D 13C−13C INADEQUATE ssNMR spectrum for aPAN-13C3 heat-treated under vacuum at 290 °C for 300 min. Chemical structures and signal assignments were inserted.

shows 13C DPMAS ssNMR spectra for aPAN-13C3 (red), aPAN-3-13C (green), and aPAN-1-13C (blue) heat-treated under vacuum at 290 °C for 300 min. The DPMAS signals for the samples treated under vacuum appeared at different chemical shift regions compared to samples heat-treated under air at 20−50, 60−70, and 100−170 ppm, respectively. From the appearance, the remaining alkyl chain signals at 20−50 ppm indicated the insufficient reaction in the absence of oxygen. Such observation indicated that the heat-treated sample under vacuum was partially reacted aPAN. Major signals at 122 and 150−155 ppm were observed in the C-1 spectrum and at 137 and 28 ppm for the C-3 spectrum. These peaks were assigned to the reacted aromatic rings (137 and 150−155 ppm) and unreacted aPAN structure (28 and 122 ppm) based on comparison with samples heat-treated under air and with previous work.15,16 To further explore the detailed chemical structures, the 2D 13 C−13C INADEQUATE spectrum for aPAN-13C3 heat-treated under vacuum at 290 °C for 300 min is shown in Figure 4b. Correlations of signals c′, b′, and a′ corresponded to the C-1, C-2, and C-3 site in the unreacted aPAN, respectively. D

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Macromolecules Correlations between signals h′−e′−g′ and i′−f′−g′ represented the formation of six-membered aromatic rings.15,16 Doublet signals h′, i′ and e′, f′ were induced by amine group connected to the i′ site as demonstrated by the pink structure in Figure 4b.15,16 On the contrary, the aromatic C-1 and C-2 signals under air showed singlet peaks such as signals f and d shown in Figure 2. Such differences originated from chemical environment inequivalence (doublet) or equivalence (singlet) of corresponding carbons. To be specific, aPAN heat-treated under air formed ladder structure which resulted in equivalent chemical environments for the C-1 and C-2 carbons in one aromatic ring. Under vacuum, isolated aromatic rings with amine groups connected to one side of the rings induced inequivalent chemical environments.16 The formation of isolated aromatic rings was also proved through another correlation between signals e′ and f′ and signal a′ at 28 ppm in the alkyl chain region. Such a correlation demonstrated that the aromatic rings were covalently bonded to the alkyl carbons. The broad signal k′ at 160−170 ppm that partially overlapped with signals i′ and h′ in the DPMAS spectra can be well separated through 2D INADEQUATE where a correlation with signal j′ at the alkyl region was shown. Such a unique correlation was direct evidence for the cyclized six-membered rings shown as the blue structure in Figure 4b.15,16 Based on current experimental resolution, it was difficult to conclude whether the cyclized six-membered ring would adopt a successive or an isolated sequence since the CH2 carbon appeared at the same chemical shift (signal a′) when it was located in the cyclized rings or at the bridge between the rings. Correlation between signals n′−o′ was attributed to 1,4dihydropyridine rings through dehydrogenation of the cyclized form while the correlation between signals l′−m′ was attributed to isolated piperidine rings through hydrogenation of the cyclized form.15,16 These two structures were assigned as intermediate structures. The 13C DPMAS spectra for aPAN-3-13C showed a signal at 17 ppm that was contributed by a methyl group.16 Such a methyl group was generated by thermal decomposition during the heat treatment process. 2D correlation indicated that two kinds of methyl groups can be detected in the sample. Signal d′ was correlated with signal b′ that represented the C-2 carbon in the unreacted aPAN. The second methyl signal, p′, correlated with signals q′ at 120 ppm that represented the C-2 carbon in the aromatic rings. Such observation indicated that decomposition took place at both unreacted and partially reacted regions. All detected signals can be reasonably interpreted in terms of through-bond correlations arisen by intramolecular reactions. Thus, it was concluded that intermolecular crosslinking does not occur in aPAN heat-treated under vacuum at the conditions studied in this work. 3.4. Chemical Reactions at the Early Stage. Chemical structures that arose from both intermolecular and intramolecular reactions at 290 °C for 300 min under air and vacuum were assigned through the combination of 2D throughbond and through-space ssNMR correlation results. Here, chemical reactions at the early stage (low reaction temperature and/or short time) were further explored. Figures 5a and 5b show 13C CPMAS ssNMR spectra for aPAN-13C3 heat-treated under air and vacuum, respectively. The spectra colored by black, red, blue, and green were samples heat-treated at 250 °C for 300 min and at 220 °C for 300, 180, and 120 min, respectively. Under air, chemical shift values and the line shapes of the aromatic signals (100−180 ppm) remained irrelevant to

Figure 5. 13C CPMAS ssNMR spectra for aPAN-13C3 heat-treated at 250 °C for 300 min (black) and at 220 °C for 300 (red), 180 (blue), and 120 min (green) under air (a) and vacuum (b). Vertical amplification factors are inserted. Spectra with original scale are provided in Figure S4.

the reaction temperature and time. CPMAS ssNMR spectra for single-site labeled aPAN-1-13C as well as aPAN-3-13C heattreated under air at 220 °C for 300 min are also shown in Figure 6a. The C-1 signal at around 170 ppm (signal l) and the

Figure 6. (a) 13C CPMAS ssNMR spectra for aPAN-3-13C (blue) and aPAN-1-13C (red) together with 2D 13C−13C INADEQUATE for aPAN-13C3 heat-treated under air at 220 °C for 300 min. The asterisk (∗) represents signals contributed by 13C in natural abundance. CPMAS spectra with original scale is shown as Figure S6. (b) 1H−13C HETCOR ssNMR spectrum for aPAN-13C3 heat-treated under air at 220 °C for 300 min. (c) 1H slice data through the 13C signals at 138 (signal e) and 137 ppm (signal g′) for aPAN-13C3 heat-treated under air and vacuum, respectively.

C-3 signal at g were observed. 2D 13C−13C INADEQUATE was applied, and the corresponding spectrum is shown in Figure 6a. In addition to g′−d′ correlation, it was found that signal l was correlated with signal m at 40 ppm, which was quite similar to the correlation between signals k′−j′ for aPAN heattreated under vacuum shown in Figure 4. Such an observation indicated that a tiny fraction of cyclized six-membered rings (ca. E

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min (blue), 250 °C for 120 min (red), and 290 °C for 60 min (black) under air are shown in Figure 7a. Thus, the degree of

3% for aPAN heat-treated under air at 220 °C for 300 min through aPAN-1-13C DPMAS spectrum (Figure 6a) also existed as the initial product. Additionally, 2D 13C−13C INADEQUATE for aPAN heattreated under air at 220 °C for 300 min showed a weak diagonal signal at ω2 = 121 ppm. This diagonal signal moved upfield to 118 ppm after heat treatment at 250 °C for 180 min and finally split into two signals at 112 and 118 ppm after treatment at 250 °C and 300 min (refer to Figure S5). The split signals corresponded to signals i and j in the polyene structure. Such a series of 2D 13C−13C INADEQUATE spectra indicated that structural and/or chemical environments of the polyene structure (conformation, packing, and possible further reactions at h site) have changed with increased reaction temperature and time. The obtained data sufficiently proved the presence of two kinds of initial products in addition to the final products at the early stage of the reaction. For chemical reactions under vacuum, peak positions were independent of reaction temperature and time as shown through the CPMAS spectra in Figure 5b. However, as reaction temperature and time decreased, some signals could be enhanced. For example, relative intensities of signals k′ and m′ were much higher at 220 and 250 °C than at 290 °C. The enhancement of signal k′ at the beginning of the reaction provided direct evidence to support cyclization as the initial process of aPAN heat-treated under vacuum. The increase of signal m′ indicated that hydrogenation of the cyclized sixmembered rings served as an intermediate structure during the heat treatment process. Signals e′, f′ and h′, i′ representing the isolated aromatic rings, adopted a doublet line shape while the aromatic signals d and f for aPAN heat-treated under air remained singlet even during the initial stage of the reactions. To confirm spatial connectivity of the aromatic rings during the initial stage of the reaction, 1H−13C HETCOR spectra were obtained for aPAN-13C3 heat-treated at 220 °C for 300 min under both air and vacuum, in which signals from unreacted aPAN were dominant. The 1H−13C HETCOR spectrum for aPAN-13C3 heat-treated under air is depicted in Figure 6b. To highlight 1H−13C long-range correlations over a few angstroms (between aliphatic protons and aromatic carbons), CP contact time was set at 2 ms. The 1H slice data through the carbon signals at 137 ppm (signal g′) for aPAN heat-treated under vacuum and 138 ppm (signal e) for aPAN under air are shown in Figure 6c as blue and red curves, respectively. The peak area ratio for the aromatic and aliphatic proton signals was determined as 3:7 under vacuum and 8:2 under air. On the basis of the 2D HETCOR data and 13C line shape for the aromatic signals, it was demonstrated that isolated aromatic rings are formed under vacuum while the aromatic ladder structure was formed under air even at the early stage of the reactions. 3.5. Quantitative Evaluation of Kinetics of Chemical Reactions under Air and Vacuum. The degree of aromatization (DA) was defined as the relative fraction of the aromatic signals with respect to the overall signals.16 As shown above, the chemical reactions under air and vacuum induced largely different initial, intermediate, and final products. Thus, two different formulas were necessary to calculate the DAs under air and vacuum. For heat treatment under air, the aromatic ladder, polyene, cyclized ring, and unreacted structure were detected in the spectra of aPAN-1-13C. Quantitative 13C DPMAS ssNMR spectra for the aPAN-1-13C heat treatment at 220 °C for 120

Figure 7. 13C DPMAS ssNMR spectra for (a) aPAN-1-13C and (b) aPAN-3-13C sites labeled aPAN heat-treated under air and vacuum, respectively, at 220 °C for 120 min (blue), 250 °C for 120 min (red), and 290 °C for 60 min (black). The asterisk (∗) represents signals contributed by 13C in natural abundance. (c) DAs for aPAN heattreated under air (filled circle) and vacuum (open circle) at 220 (blue), 250 (red), and 290 °C (black) as a function of reaction time. The best fit curves were obtained using eq 3. (d) Arrhenius plot of aromatization reaction rate (k) for aPAN heat-treated under air (filled black) and vacuum (red open circle). The best fit curves provide Ea values of 122 ± 3 and 47 ± 2 kJ/mol under air and vacuum, respectively.

aromatization under air (DAAir) was defined through the equation DAAir = C‐1Aromatic

C‐1Aromatic + C‐1Polyene + C‐1Unreacted + C‐1Cyclized (1)

where C-1 refers to peak areas of corresponding structures indicated by the subscripts. In the case of chemical reactions under vacuum, heat treatment at 290 °C for 300 min (longest reaction time conducted at the highest temperature in the current data set) resulted in a mixture of structures at various stages including the unreacted, initial, intermediate, and final products. As schematically depicted in Figure 4, aPAN heat-treated under vacuum showed alternative sandwich-like arrangement of the aromatic ring bridged by the unreacted alkyl segments. If eq 1 F

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Macromolecules Table 1. Fitting Parameters of the Kinetic Analysis for aPAN Aromatization Reactions air

a

vacuum

temp (°C)

a

tind (s)

k × 105 (s−1)

β

a

tind (s)

k × 105 (s−1)

β

220 250 290

0.82 ± 0.02 0.82 ± 0.02 0.82 ± 0.02

5000 1800 0

1.6 ± 0.3 9.0 ± 0.2 65 ± 4

0.86 ± 0.08 0.99 ± 0.03 0.64 ± 0.04

0.10 ± 0.01 0.24 ± 0.01 0.34 ± 0.01

7200 3200 0

29 ± 1 59 ± 3 121 ± 5

0.99 ± 0.09 0.81 ± 0.05 0.79 ± 0.05

tind was independently determined in terms of 13C DPMAS signals at various reaction periods.

Scheme 1. Chemical Reaction Scheme for aPAN Heat-Treated under Air and Vacuum

atures of 220 and 250 °C was longer than those under air at the same temperatures. The SI build-up curves were analyzed and approximated by the following first-order equation:

was applied to analyze the kinetics, the obtained DA value would be overestimated since it will miscount the alkyl segments between the aromatic rings. This miscounting was shown in previous FT-IR works where DA was evaluated by using the CN and CN bands at 2230 and 1680 cm−1, respectively.13,47 In the DPMAS ssNMR spectra of aPAN-3-13C heat-treated under vacuum shown in Figure 7b, the alkyl and aromatic signals were well separated and assigned. Thus, the DAVacuum could be well determined with such spectra using the equation DA Vacuum =

C‐3Aromatic

C‐3Aromatic + C‐3Aliphatic + C‐3Chain‐End

DA = a(1 − exp( −(k(t − t ind))β ))

(t > t ind)

(3)

where a is saturation factor, k is reaction rate (s−1), t is reaction time (in seconds), and β is the distribution factor (0 < β ≤ 1).48,49 A larger β value indicated more homogeneous kinetics, and β = 1 represented a single reaction rate.49 DAVacuum reached distinct plateau values at all three temperatures. Thus, a was used as a free parameter, Meanwhile, DAAir build-up curves at 220 and 250 °C did not reach a plateau value within 300 min. However, further heat treatment for up to 50 h led to DAAir of 0.82 at both 250 and 220 °C. Thereby, the a value was fixed at 0.82 for the analysis at the two lower temperatures. The best fitted curves are depicted as solid curves in Figure 7c, and the corresponding fitting parameters are listed in Table 1. The fitting results provided different features of chemical reactions under air and vacuum. First, the k value under vacuum was larger by almost 1 order of magnitude under air at the same reaction temperatures. Second, the a value under air was larger than that of vacuum. Lastly, an Ea,Vacuum value of 47 ± 2 kJ/mol was lower than that of air at 122 ± 3 kJ/ mol. Such observations provided different kinetics information for aPAN reactions under air and vacuum. Detailed discussions will be provided later. Similarly, 13C DPMAS ssNMR spectra were used to determine kinetics of chain scission under vacuum and of oxygen uptake and intermolecular cross-linking under air. The rate of chain scission under nitrogen was on the same order of magnitude as the aromatization under vacuum. The chain scission fractions reached 0.03, 0.07, and 0.11 at 220, 250, and

(2)

Each of the peak areas was obtained by peak fitting the experimental data with the Voigt model. The obtained DAAir and DAVacuum values at three different temperatures are plotted in Figure 7c as a function of heat treatment time in filled circles and open circles, respectively. DAAir immediately grew and reached 0.82 at 290 °C, where the remaining fraction of 0.18 corresponded to polyene structure with a dehydrogenated backbone and an unreacted nitrile side-chain. This value was quite similar to that at 420 °C obtained in our previous work.16 With decreasing temperature to 250 and 220 °C, chemical reactions began after inductions time (tind) of 30 and 83.3 min (1800 and 5000 s) at 250 and 220 °C, respectively. At these temperatures DAAir reached 0.64 and 0.18 at 300 min, respectively Under vacuum, overall behaviors of chemical reactions were largely different from those under air. At 290 °C, DAVacuum exhibited a rapid build-up and saturated at 60 min for a value of 0.34 while the DAVacuum only reached a value of 0.24 and 0.10 at 250 and 220 °C, respectively. Moreover, tind at low temperG

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Macromolecules 290 °C, respectively. Thus, mean values of monomer unit number per oligomer can be roughly calculated through the chain-end fractions as 65, 26, and 16 at 220, 250, and 290 °C, respectively, for 300 min. Thus, it was concluded that the original polymer chains decomposed into oligomers after heat treatment under vacuum at high temperatures. Similar to the vacuum case, the rate of oxygen uptake was also on the same order of magnitude as the aromatization reactions under air. Oxygen uptake fraction at the C-3 site was determined as 0.03, 0.10, and 0.14 for aPAN heat-treated under air at 220, 250, and 290 °C, respectively, for 300 min.

Oxygen driven dehydrogenation on the cyclized form instantly induced the aromatic ring structure. Thus, even though two initial routes including dehydrogenation and cyclization existed under air, thermal dehydrogenation driven by oxygen dominated the reaction and determined the chemical structures. This was a crucial difference between chemical reactions under air and vacuum. Meanwhile, well-recognized oxidization on the C-3 site and intermolecular cross-linking also took place, and their reactions proceeded at the same order of magnitude with the aromatization process. Considering structural differences for reactions under air and vacuum, it was concluded that both ladder formation and intermolecular cross-linking formed under air were well reacted chemical structures. Different reaction mechanisms under air and vacuum resulted in different reaction kinetics (refer to Figure 7 and Table 1). DSC onset temperatures and tind under different atmospheres suggested that the initial process (dehydrogenation) under air was relatively easier to be initiated than the cyclization process under vacuum. However, the apparent k values obtained under air were lower by up to 1 order of magnitude than under vacuum at the same temperatures. Furthermore, the DSC results showed a broad peak for reaction under oxygen and a sharp peak for reaction under nitrogen which indicated the differences in distribution width of the overall kinetics as composed by the individual elementary processes. For chemical reactions in solid phase, the chain relaxations of polymers may have also contributed to the kinetics of chemical reactions, which require conformational and packing adjustments induced by chain relaxations. According to previous ssNMR works, it was concluded that unreacted aPAN adopted trans:gauche conformations of 9:1.28−30 Such extended conformation would be preferred for chemical reactions over helical conformation of isotacticPAN.29,30,50 Moreover, ssNMR indicated that aPAN crystal was dynamically conformational disordered crystals above 150−180 °C.51,52 At reaction temperatures above 220 °C studied in this work, chain flexibility was sufficient to induce intramolecular as well as intermolecular reactions. During chemical reactions, chain flexibility changed depending on the degree of the reaction as well as the reacted structures (e.g., aromatic ladder versus isolated aromatic rings). Isolated aromatic ring formations would be related to relative conformations and local dynamics of two successive monomer units. However, the ladder structure with intermolecular crosslinking would require larger scale rearrangement of the chains under air. In addition, the whole chains were involved in the reactions under air while only parts of the chains were involved in the reaction under vacuum. Thus, it can be reasonably explained that the overall chemical reactions under air adopted slower kinetics and higher Ea values than those under vacuum. The same reason can be used to note the differences in the exothermal peak widths determined from DSC for aPAN heattreated under air and nitrogen. Finally, it is interesting to discuss the heat treatment of aPAN in different states (powder, film, and fiber). According to morphological observations, aPAN fibers with diameters of several micrometers showed core−shell morphology after heat treatment.17 Previous 13C high-resolution ssNMR works on the heat-treated fibers detected 13C signals at 60−70 ppm and at 17 ppm.12,14 As demonstrated throughout this work, such signals were intermediate structures and chain ends formed under the absence of oxygen. Thus, ssNMR spectroscopy has the

4. DISCUSSION: ROLE OF OXYGEN IN THE HEAT TREATMENT OF aPAN On the basis of detailed chemical structures arisen from intramolecular as well as intermolecular reactions under different atmospheres, possible chemical reaction schemes of aPAN heat-treated under air and vacuum are illustrated in Scheme 1. Under vacuum, cyclization between nitrile groups in two adjacent aPAN monomer units occurred as the initial process, where cyclized six-membered rings were formed. Through the observation that final residue adopted isolated aromatic rings connected by alkyl segments, the majority of the reactions on the nitrile site would be between two adjacent sites. Such reactions could result in the formation of isolated cyclized sixmembered rings. Thermally induced dehydrogenation dominated the following reactions and thus led to the formation of the isolated aromatic rings under vacuum. Two intermediate structures also formed from the cyclized form, namely, dehydrogenated and hydrogenated six-membered rings from the cyclized form.15 The remaining flexible alkyl chains and the reacted isolated aromatic rings suffered from thermal decomposition which led to oligomeric structures. Moreover, intermolecular cross-linking was not observed under vacuum. The chemical structures of aPAN heat-treated under vacuum were very similar to those under argon15 and nitrogen.16 Thus, it was concluded that the observed chemical structures and discussed reaction mechanisms occurred under the absence of oxygen. Under air, several structures, including aromatic ladder structures with partial oxidation and cross-linking sites, polyene structures, and a tiny amount of cyclized six-membered rings, were detected through 1D and 2D ssNMR spectra. Spectroscopic evidence supporting the presence of polyene structures at 220 and 290 °C for 300 min (refer to 2D 13C−13C INADEQUATE in Figures 6a and 2b) and cyclized sixmembered rings at 220 °C for 300 min (refer to Figure 6a) implied that both dehydrogenation and cyclization processes occurred at the initial stage of the chemical reaction process under air (refer to Scheme 1). The DSC data indicated a lower onset temperature for aPAN heat-treated under air than under nitrogen flow. Similarly, tind for aPAN heat-treated under air was lower than under vacuum at 220 °C (5000 s under air versus 7200 s under vacuum). These differences implied that “oxygen” dominantly induced dehydrogenization along the backbone as the initial process. Moreover, the cyclized form was only 3% under air at 220 °C for 300 min, which was much lower than that under vacuum (15%). No intermediate structure from the cyclized form was detected under air. These observations indicated that the formation probability and lifetime for the cyclized six-membered rings under air were much lower and shorter, respectively, than those under vacuum. H

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potential of quantitatively analyzing heterogeneous reactions of aPAN films and fibers with different thickness and diameters. Such analysis is useful to optimize the film thickness and the fiber diameter on chemical reactions of aPAN.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02239. Illustration of Voigt peak fitting as well as CP time dependence for aPAN-3-13C heat-treated under 290 °C for 300 min; a series of 2D 13C−13C INADEQUATE spectra for aPAN heat-treated under air at various stages (PDF)

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REFERENCES

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5. CONCLUSION Intramolecular as well as intermolecular reactions of aPAN in powder states under air and vacuum at 220−290 °C and their kinetics were successfully analyzed through various ssNMR techniques and 13C isotopic labeling. It was demonstrated that aPAN can be fully reacted by heat treatment under air at 290 °C for 300 min. Fully reacted aPAN consisted of both aromatic ladder structure (82%) with cross-linking and oxidation sites as well as polyene segments (18%). On the contrary, aPAN heattreated under vacuum at the same conditions decomposed into oligomeric structures containing isolated aromatic rings bridged by alkyl segments. No intermolecular cross-linking was observed under vacuum. By comparing the chemical structures formed under the two different conditions at various reaction stages, it was concluded that thermal dehydrogenation driven by oxygen determined the final reacted chemical structures. The importance of oxygen was also highlighted through kinetics analysis of the chemical reactions. In the presence of oxygen, the reactions showed shorter induction time, slower reaction rate, more homogeneous reaction, and higher activation energy than those under vacuum. The kinetic differences between air and vacuum were explained in terms of chemical reactions dominated by chain relaxations of aPAN during the reactions. The fundamental study of aPAN powder presented in current work could provide further insight into chemical reactions of aPAN fibers used in industry.

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AUTHOR INFORMATION

Corresponding Author

*(T.M.) E-mail: [email protected]. ORCID

Toshikazu Miyoshi: 0000-0001-8344-9687 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Mr. Jiawei Liu and Dr. Steven S. C. Chuang for the use of ovens. This research was financially supported by the American Chemical Society Petroleum Research Fund 56168-ND7, the UA start-up fund, and Japan Society for the Promotion of Science P16047. I

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