Article pubs.acs.org/Macromolecules
Structural Identification of Polyacrylonitrile during Thermal Treatment by Selective 13C Labeling and Solid-State 13C NMR Spectroscopy Yusong Wang,†,§ Lianghua Xu,‡ Mozhen Wang,*,† Wenmin Pang,§ and Xuewu Ge*,† †
CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China ‡ National Carbon Fiber Engineering Research Center, Beijing University of Chemical Technology, Beijing 100029, China § Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China ABSTRACT: Four kinds of 13C-labeled polyacrylonitrile (PAN) samples were prepared respectively by solution polymerization of acrylonitrile (AN) with selective 13C labeling of different molecular sites. The composition and structure of the residues from the thermal treatment of PAN in argon at 250 and 350 °C were quantitatively analyzed in detail by oneand two-dimensional solid-state 13C nuclear magnetic resonance (ssNMR) experiments. Compared with the NMR spectrum of each labeled carbon in AN monomer unit, nine chemical structures created during the heat treatment process have been identified accurately. On this basis, four reaction routes were proposed. It is noted that the main chemical change for PAN started from a cyclization reaction at a relatively low temperature, then experienced an aromazation reaction to form a molecular chain basically composed of isolated pyridine units, instead of the commonly reported ladder structure. This work also shows that the combination of selectively 13C-labeled technique and a high spinning speed of 20 kHz in magic-angle spinning (MAS) NMR experiment could improve the detection sensitivity to nearly 2 orders of magnitude, and provide a clear ssNMR spectra with little peak overlaps, which will be helpful to discover the complex reaction mechanism in the manufacture of carbon fibers with high performance.
1. INTRODUCTION The study of the thermal treatment of polyacrylonitrile (PAN) has been a subject of interest for decades owing to its commercial value for the production of carbon fibers.1−3 In order to increase the industrial productivity and improve the quality of the carbon fibers, the identification of the microstructures of the thermal degradation residue and the complex chemical reaction pathways is especially important. A large number of researches have shown that complicated competitive thermal reactions, such as cyclization, dehydrogenation, isomerization, chain scission, aromatization, and oxidative degradation, will take place during the stabilization process of PAN at high temperatures.3−6 Therefore, the products created during the thermal treatment of PAN are diverse and complex7 and strongly depend on the conditions of the thermal treatment, such as temperature and times,2,3,6,8,9 atmosphere,9−11 and even tensile stress.8 The diversity and complexity of the products bring many difficulties in their accurate recognition, which in turn makes the study of the reaction mechanism during thermal treatment process of PAN become more challenging. Up to now, various spectroscopic methods have been used to identify the chemical structures of © 2014 American Chemical Society
the residues after the thermal treatment of PAN, including Fourier transform infrared spectroscopy (FT-IR),2,3,6,8,11−13 solid-state nuclear magnetic resonance (ssNMR),2,6,8,9,12 elemental analysis,2,3,12 X-ray diffraction,12,14 thermal analysis,3,11 pyrolysis−gas chromatography/mass spectrometry (PyGC/MS),2,6 etc. However, the precise chemical reactions occurring during PAN stabilization under either inert or oxidative conditions are still not well understood.7,10,15 NMR spectroscopy is always considered as a powerful tool in the study of polymer chain structure and dynamics.16 As the residue obtained from the thermal treatment of PAN is usually an insoluble resin, 13C ssNMR is suitable to be employed for probing the chemical structural changes.2 They found that linear polymerization of nitrile group was the principal reaction in the decomposition process, and cyclization followed by extended conjugation is the notable exothermic process. Usami (1990)6 mentioned that the main stabilization mechanism of PAN fibers under oxidative thermal degradation was the Received: April 7, 2014 Revised: May 22, 2014 Published: June 3, 2014 3901
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labeled AN monomers using DMSO as solvent and AIBN as an initiator at 67 °C for 48 h. Then, the reaction system was precipitated in excess of distilled water. The obtained solid products were dried at 40 °C for 48 h in a vacuum oven. Sample C(all) was polymerized from mixed monomers composed of uniformly 13C-labeled AN and unlabeled AN (1:5.25 in weight). Samples C(1), C(2), and C(3) were polymerized respectively from C(1)-, C(2)-, and C(3)-labeled acrylonitrile. The chemical structures of the above four samples are illustrated in Scheme 1. The material properties for each 13 C-labeled PAN samples are summarized in Table 1.
formation of cyclic ladder structures, which was indirectly and directly confirmed by Py-GC, ssNMR, and FT-IR, respectively. Martin (2001)9 indicated that degradation has also been shown to be both temperature and time dependent, irrespective of reaction environment, by solid-state 13C NMR and FT-IR. Soulis (2005)8 investigated the structural transformation of the PAN fibers at various treatment conditions (i.e., temperatures, time, stress applied). The pristine and treated fibers were characterized by FTIR, NMR, and thermogravimetric analysis (TGA). The results were used for representing the different regions according to chemical aspects in a plot of temperature versus time. However, due to the spinning sidebands from the low MAS spinning speed (i.e., 5 kHz), the resonances of 13C ssNMR spectra overlap greatly so as to influence the completeness and accuracy of the chemical structural analysis. Recently, Mowery et al.17−19 used 13C enriched polypropylene (PP) to analyze oxidation products formed by thermal aging. By selectively labeling PP with the 13C nucleus, dramatic differences were found in the type and distribution of oxidation products originating from the three carbon atom sites within the PP macromolecule. Time-dependent concentration plots have been obtained, which show the amounts of the various oxidation products originating at the different PP sites, as a function of the extent of material oxidation. In order to make the reaction mechanisms occurring in the heat treatment process of PAN more clearly, we first prepared four kinds of 13C-labeled PAN samples respectively in this work by solution polymerization of acrylonitrile with selective 13C labeling of different molecular sites. The prepared PAN samples were thermally treated at 250 and 350 °C respectively under inert argon atmosphere. Then, the composition and structure of the residues after the heat treatment were quantitatively analyzed by two kinds of 13C ssNMR technique, i.e., onedimensional (1D) 13C direct polarization/magic-angle spinning (DP/MAS) spectroscopy and two-dimensional (2D) 13C−13C radio frequency-driven recoupling (RFDR) spectroscopy. The use of 13C enriched PAN provided an increase in detection sensitivity of nearly 2 orders of magnitude. Meanwhile, the spinning sidebands can be removed from 0 to 200 ppm on a 400 MHz NMR spectrometer since we increased the MAS spinning speed from 5 to 20 kHz, when the 2.5 mm probe is used. The utilization of selectively 13C-labeled PAN helps to weaken the peak overlaps on the NMR spectra and identify the chemical structures of the residues from the thermal treatment more accurately. Furthermore, a main reaction route to form an isolated pyridine units on PAN chains, rather than the generally supposed ladder heteroaromatic structure, was proposed and discussed in detail.
Scheme 1. Chemical Structures of the Prepared Selectively 13 C-labeled PAN samples
Table 1. Material Properties of Selectively 13C-Labeled PAN relative 13C abundance of main-chain carbonsa (%) PAN sample
CN
CH
CH2
Mnb (g·mol−1) (NMR)
C(all) C(1) C(2) C(3)
16 99 1 1
16 1 99 1
16 1 1 99
11500 13600 8600 9700
a
For sample C(all), the relative 13C abundance of main-chain carbons is calculated according to the weight ratio of uniformly 13C-labeled AN to unlabeled AN monomer. bMolecular weight determined by 1H NMR spectroscopy with the PAN powder dissolved in DMSO-d6 at 60 °C.
2.2. Thermal Treatment of PAN. The PAN sample was sealed in a quartz tube and put in a horizontal tube furnace. The sample was heated to 250 or 350 °C at a rate of 5 °C· min−1 under a gas flow of argon (0.10−0.15 dm3·min−1), and kept for 15 min. Then, the quartz tube was taken out from the body of the furnace and rapidly cooled to the room temperature in argon. 2.3. Thermogravimetric Analysis (TGA). TGA was performed with a DTG-60H instrument under argon at a heating rate of 10 °C·min−1. The sample weight was approximately 3 mg. 2.4. 13C ssNMR Spectroscopy. All 13C ssNMR spectra were performed on a Bruker AV III 400WB spectrometer operating at a 13C resonance frequency of 100.6 MHz. A 2.5 mm MAS H/F/X triple resonance probe head was employed. Samples of 5−10 mg were packed inside Zirconia MAS rotor with a diameter of 2.5 mm and a vespel cap. 13 C DP/MAS spectra were acquired with a 200-s recycle delay, a 13C excitation (90°) pulse length of 3.9 μs and 20.0 kHz MAS. Typically 256 scans were operated to obtain a good signal-to-noise ratio. 13C isotropic chemical shifts were
2. EXPERIMENTAL SECTION 2.1. Materials. Uniformly 13C-labeled acrylonitrile (13C3, 99%) and C1-labeled acrylonitrile (acrylonitrile-1-13C) (13C, 99%) were obtained from Cambridge Isotope Laboratories, Inc. C2-labeled acrylonitrile (acrylonitrile-2-13C) (13C, 99%) was obtained from Creative Dynamics, Inc. C3-labeled acrylonitrile (acrylonitrile-3-13C) (13C, 99%) was obtained from International Laboratory USA. Acrylonitrile (AN, analytical grade), dimethyl sulfoxide (DMSO), and azobis(isobutyronitrile) (AIBN) were obtained from Sinopharm Chemical Reagent Co., Ltd. China, and used without further purification. Four 13C-labeled PAN samples, with different 13C labeling schemes referred to as C(all), C(1), C(2), and C(3), were synthesized individually by solution polymerization of 13C3902
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produced during the heat treatment of 13C-labeled PAN samples could be analyzed more clearly and accurately because each labeled carbon in AN units could be accurately recognized on 13C ssNMR spectra. 3.2. The Chemical Changes of PAN during the Thermal Treatment Process Analyzed by Quantitative 13 C ssNMR Spectroscopy. Despite decades of research, the exact chemical changes in PAN molecular chains during the thermal treatment process are still puzzling because the process is very complex, and found to depend strongly on external conditions including atmosphere and temperature.2,3,6,8−11 Oxygen will induce and participate many chemical reactions at high temperatures, which would cause a rather complex product spectrum of the heat treatment of PAN, and not be conducive to the analysis of reaction mechanism of PAN molecular chains. Therefore, the heat treatment of PAN was conducted at an inert atmosphere of argon in this work. A TGA test had been performed under argon before the heat treatment of PAN. The TG-DTA diagram of PAN shown in Figure 2
calibrated on the carbonyl resonance of an external glycine standard, 176.0 ppm with respect to TMS (0.0 ppm). 2D 13C−13C RFDR spectra20−22 of the sample C(all) were recorded by acquiring 128 points in the t1 dimension with 128 scans per FID recorded. A spectral width of 20 000 Hz was used in the ω1 dimension. The recycle delay was set to 2 s. The spinning frequency was 20 kHz, and the mixing time was set to 1.2 or 6.4 ms.
3. RESULTS AND DISCUSSION 3.1. Quantitative 13C ssNMR analysis of 13C-Labeled PAN. The 13C DP/MAS spectrum can give quantitative composition information if the recycle delay is long enough for sufficient relaxation of the 13C magnetization.23 Figure 1
Figure 2. TG-DTA curves of the unlabeled PAN measured under argon. Figure 1. 200-s 13C DP/MAS spectra of selectively 13C-labeled PAN samples C(all), C(1), C(2), and C(3).
exhibited a slight mass loss from 100 to 250 °C, and a large one from 250 to 500 °C. However, a shape exothermal peak appeared at about 270 °C at DTA curve, coincident with the differential scanning calorimetry (DSC) results reported in literature.24 The peak in DTG curve starts from ca. 247 °C, and ends at ca. 350 °C, indicating that the creation of new chemical bonds mainly take place within this temperature period. Therefore, it is necessary to analyze the structures of the residues after the PAN samples were heat treated at 250 and 350 °C respectively, in order to discover the thermal reaction mechanism of PAN. The 13C DP/MAS spectra of the samples C(all), C(1), C(2), and C(3) thermally aged in argon at 250 and 350 °C respectively for 15 min are shown in Figure 3. It can be seen clearly that spinning sidebands of the resonances, which is often difficult to be avoided in a low spinning rate, are not found from 0 to 200 ppm in our work due to the fast sample spinning (20 kHz). Even so, the spectra of PAN heat-treated at high temperatures (Figure 3, part C(all)) still become much more complicated. However, they still can be considered as the summary of the spectra of samples C(1)−C(3) if we compare Figure 3, part C(all), with Figure 3, parts C(1), C(2), and C(3),
shows the 13C DP/MAS spectra of samples C(all), C(1), C(2), and C(3). The selective 13C isotopic labeling makes the resonances of each carbon on the main chain be able to clearly identified in the 13C ssNMR spectra, as displayed in Figure 1, parts C(1), C(2), and C(3). The resonance (peak 1) in Figure 1, part C(1), at 120.5 ppm should be contributed by the 13C atom on nitrile group (−CN). Similarly, the peaks at 28.3 ppm in Figure 1, part C(2) and at 33.3 ppm in Figure 1, part C(3), should be assigned to the resonances of the 13C atom in the methine (−CHCC< unit, as shown in Table 2, since the chemical shift of carbon on isolated >CC< groups locates around 120 ppm. Considering the existence of structure I, it can be explained that a small amount of structure I would proceed an isomerization reaction to form structure IV, as shown in route 2 (see Scheme 2). The chemical shift of C1 and C2 in structure IV just corresponds to Peak d in Figure 3A− C(1) and Peak j in Figure 3A−C(2) respectively according to the empirical calculated value listed in Table 2. Moreover, peaks d and j have the same integral value. However, it is known that the isolated >CC< groups are quite unstable at high temperature evidently, which are commonly dimerized or hydrogenized. Therefore, structure IV would change into structure V and VI if PAN was in a condition of high temperature. This process could be verified by two facts. One is that the peak d and peak j disappeared at the same time in Figure 3B−C(2). The other is the existence of peaks f and g in Figure 3, parts A and B, which just corresponds to the chemical shift of the C atom in aliphatic secondary amines. 3.2.2. The Chemical Reactions on PAN Main Chain. Besides the above reactions occurring on −CN groups, a small amount of reactions had also taken place on the hydrocarbon main chain of PAN because weak peaks m (15 ppm) and q (17 ppm) appear in Figure 3, part C(2), and Figure 3, part C(3), respectively, which is similar to the chemical shift of the C atom at the end of alkanes. The result means a chain scission between C2 and C3 had taken place, as shown in route 3 in Scheme 2, which leads to the formation of structure VII and the appearance of peak q. If the −CN
just as the case of untreated PAN sample (see Figure 1). Therefore, we can identify the chemical status of all carbon atoms in heat-treated PAN through the assignment of each peaks in the relatively simple spectra of samples C(1)−C(3). A summary of these observed resonances (peaks a−q) in Figure 3 and the corresponding possible chemical structure are listed in Table 2. 3.2.1. The Chemical Reactions of −CN Side Groups. First, it can be confirmed by a marked reduction in the relative intensity of peak e (121 ppm, corresponding to the unreacted −CN groups), comparing Figure 3A−C(1) with Figure 3B− C(1), that most of the −CN groups on PAN main chain have reacted within the temperature ranging from 250 to 350 °C. At the same time, a series of new peaks appear in Figure 3, part C(1). A broad peak located at ca. 150 ppm in Figure 3A− C(1) can be clearly differentiated as three resonances under the experiment conditions in this work. It should be related to the formation of imine group (−NCCC< groups can be observed only in Figure 3B−C(2). This could be related with the dissociation of the side −CN groups from the main chain and the formation of an isolated CC on PAN main chain (structure IX), shown as route 4 in Scheme 2. 3.3. 13C−13C RFDR NMR Analysis on the 13C-Labeled Residues. The time-dependence of the cross-peak amplitudes in the 2D homonuclear 13C−13C RFDR spectrum can be employed to determine internuclear distances. Only spins in a close spatial proximity could lead to cross-peaks in a short mixing time so as to facilitate the assignment of 13C resonance. The cross-peaks created from the remote-connected carbon atoms could be detected when the mixing time prolongs.20,21 Therefore, we can use the 2D 13C−13C RFDR spectrum to give the direct evidence of the chemical structure of the main residues from the thermal treatment of PAN. Figure 4A1 shows a contour plot of a 2D RFDR spectrum of uniformly 13C-labeled PAN thermally aged at 250 °C for 15 min recorded at a mixing time of 1.2 ms. The corresponding 1D 13C spectrum is shown at the top of this figure. The crosspeak Cl/Ce (at 24/121 ppm in Figure 4A1) was caused by the correlation of the neighboring C1 and C2 on the unreacted PAN unit (structure PAN in Scheme 2). When the mixing time increased to 6.4 ms, the cross-peak Cp/Ce could be observed in Figure 4A2. This cross-peak become much weaker in Figure 4B because the amount of unreacted PAN become more and more less when the temperature increased from 250 to 350 °C. Another strong cross-peak Ca/Ck (at 33/161 ppm) in Figure 4A1 could be assigned to the heterocyclic unit, i.e., structure I in Scheme 2. The structure could be verified in Figure 4A2 because the cross-peak Ca/Cp could be detected at a long mixing time of 6.4 ms. This peak also nearly disappeared in Figure 4B, implying that the structure I was a transition structure and finally changed into other structures when heated at a higher temperature. The cross-peaks Ci/Co (at 109/136 ppm), Ch/Co (at 115/ 136 ppm), Ci/Cc (at 109/149 ppm), and Ch/Cb (at 115/154 ppm) in Figure 4, parts A1 and B1, were just in accord with the
or a ladder structure reported in many literature reports3,4,6,11 needs further analysis on the quantitative 13C DP/MAS spectra of Figure 3. After all, the chemical shifts of C atoms in the ladder structure are similar to those in the proposed structure, as calculated by a reference model listed in the last line of Table 2. But the most important is, the mole ratio of the corresponding C atoms that have formed the heteroaromatic structure, will be different according to the chemical structures. When PAN was aged at 250 °C for 15 min (Figure 3A), the intensity of peaks a-c (C1) is 23.3, which corresponds to the amount of the C1 atoms forming the heterocyclic structures (I−III) in Scheme 2. The intensity of peaks h and i (C2) in the 3906
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Figure 4. 2D RFDR spectra of 13C full-labeled PAN sample thermally aged in argon at 250 °C (A) and 350 °C (B) for 15 min, detected at a RFDR mixing time of 1.2 (A1, B1) and 6.4 ms (A2, B2).
Scheme 3. Main Chemical Change of Atactic PAN during the Thermal Treatment in Argon at 250−350 °C
heterocyclic structures has an integral of 13.6, while the corresponding peak o (C3) only has an integral of a little bit less than 6.4. It can be seen that the mole ratio of C2:C3 in the heterocyclic structures is close to 2:1, which is in accord with that for the single pyridine rings. When the aging temperature increases to 350 °C (Figure 3B), the corresponding intensity of C1, C2, and C3 in the heterocyclic structures changed as 28.4,
26.4, and 14.8, respectively since most of structure II has changed into structure III. Evidently, the mole ratio of C1:C2:C3 is close to 2:2:1, which implies that the product of the aromatization reaction of PAN at 350 °C for 15 min is basically composed of isolated pyridine units, rather than the ladder structure, in which the mole ratio of C1:C2:C3 should be close to 1:1:1. The formation of this structure may be related 3907
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with the configuration of the original PAN. In our previous work,28 the PAN prepared by solution polymerization of AN monomer has an atactic configuration. The neighboring nitrile group of the isotactic unit will easily take place the cyclization reaction. However, the isotactic units and syndiotatic units nearly have the same proportion in atactic PAN chains. Therefore, it is difficult to grow an extended conjugation, i.e., a ladder structure, in an atactic PAN,29 as illustrated in Scheme 3. Therefore, we can illustrate the main chemical change during the heat treatment of atactic PAN under the conditions of this work in Scheme 3.
4. CONCLUSIONS In this work, we prepared four kinds of 13C-labeled PAN samples respectively by solution polymerization of acrylonitrile monomer with selective 13C labeling of different molecular sites. The composition and structure of the residues from the heat treatment of PAN were quantitatively analyzed in detail by 1D 13C DP/MAS and 2D 13C−13C RFDR ssNMR techniques. Combined with the use of selectively 13C-labeled PAN and a MAS spinning speed of 20 kHz, the detection sensitivity of ssNMR increased nearly 2 orders of magnitude and avoided the peak overlaps on the NMR spectra so that the structures of residues from the thermal treatment of PAN in argon at 250− 350 °C has been identified accurately. Nine chemical structures had been detected, and four reaction routes had been proposed during the heat treatement process in Scheme 2. The main chemical change for PAN prepared from solution polymerization starts from a cyclization reaction at a relatively low temperature, then experience an aromatization reaction to form a molecular chain basically composed of isolated pyridine units, as shown in Scheme 3. It can be expected from Scheme 3 that a ladder structure would likely form when PAN is aged for a long time at 350 °C or directly aged at a temperature higher than 350 °C. This interesting phenomenon needs further studies to be verified.
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AUTHOR INFORMATION
Corresponding Authors
*Telephone: +86-551-6360-0843. Fax: +86-551-6360-1592. Email:
[email protected] (X.G.). *E-mail:
[email protected] (M.W.). Notes
The authors declare no competing financial interest
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ACKNOWLEDGMENTS The authors gratefully acknowledge the National Basic Research Program of China (2011CB605602), the National Natural Science Foundation of China (Nos. 51073146, 51103143, 51173175), and the Fundamental Research Funds for the Central Universities (WK2060200012, 2014).
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REFERENCES
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