Article pubs.acs.org/Macromolecules
Structural Evolution of Poly(acrylonitrile-co-itaconic acid) during Thermal Oxidative Stabilization for Carbon Materials Ngoc Uyen Nguyen-Thai and Sung Chul Hong* Faculty of Nanotechnology and Advanced Materials Engineering, Sejong Polymer Research Center, Sejong University, Seoul 143-747, Republic of Korea S Supporting Information *
ABSTRACT: Fourier transform infrared spectroscopy (FT-IR) is employed for the quantitative tracking of the structural evolution of poly(acrylonitrile-coitaconic acid) (PAI) with different itaconic acid (IA) contents during its thermal oxidative stabilization (TOS). The TOS process includes cyclization, oxidation and tautomerization, as characterized by the evolution of the overlapping peaks of cyclic CC, CN, N−H and CO vibrations in FTIR. The second derivatives of the spectra facilitate the identification of the position of each contributing structure. The following peak-fitting operations and the determination of the molar absorption coefficients using model compounds allow the quantitative tracking of the extent of the TOS process. PAI containing approximately 3 mol % IA exhibits the most efficient TOS process in terms of cyclization, oxygen uptake and dehydrogenation reactions. A quantitative investigation of the evolution of PAI using FT-IR is an efficient way to determine the optimal structural characteristics of the precursors for carbon materials.
1. INTRODUCTION
Vibrational spectroscopy is a highly sensitive analytical tool for monitoring the structural changes of polymers. Fourier transform infrared spectroscopy (FT-IR) is a representative type of vibrational spectroscopy that also allows the evaluation of reaction kinetics. FT-IR has thus been widely used to determine the chemical structures of stabilized PAN precursors and the effect of comonomer content.9,17−25 However, the FTIR spectrum of the stabilized PAN generally exhibits broad and overlapping vibration peaks of oxygen- and nitrogen-containing functional groups in the heteroaromatic ring, which makes the identification and quantitative evaluation of the stabilized PAN quite difficult. Consequently, the quantitative investigation of the evolution of oxygen-containing groups during the TOS process has not been reported in detail. Second derivative and curve-fitting procedures have been applied to the FT-IR spectra of coal to understand its chemical properties to overcome such problems.26−28 The minima of the second derivatives provide convenient criteria for determining the number and positions of the overlapping peaks, which is useful for adequate curve-fitting/deconvolution operations. We attempt to quantitatively investigate the evolution of the PAN copolymer structures, poly(acrylonitrile-co-itaconic acid) (PAI), during the TOS processes for carbon materials using FT-IR. Complex and overlapping spectra of the critical oxygencontaining groups are interpreted through the second-derivative and curve-fitting operations. The technique is used to
Thermal oxidative stabilization (TOS) is indispensible for the formation of carbon materials from polyacrylonitrile (PAN)based precursors.1−7 During this process, cyclization, oxidation, and tautomerization reactions occur, converting linear PAN copolymer chains into cyclic structures. The incorporation of oxygen into the cyclic structure during the oxidation reactions enhances the thermal stability of the precursor,8 which prevents the precursor from decomposing but also leads to successful subsequent high-temperature carbonization processes. The presence of CO and N−H groups in the heterocyclic ring leads to extensive interchain hydrogen bonding between the polymer chains, which results in a more stabilized polymer.9−11 In addition, the oxygen functional groups act as a cross-linking bridge to facilitate the subsequent carbonization processes.12 Because TOS process is exothermic, thermal analysis methods, such as differential scanning calorimetry, differential thermal analysis, and thermogravimetry analysis, have generally been employed to study the kinetic and mechanism of the TOS. Studies on the TOS through the thermal analysis methods report low initiation temperature of TOS in the presence of an acid comonomer, suggesting the initiating effect of the acid comonomer.13 Among the acid comonomers of the PAN copolymers, the most effective comonomer is itaconic acid (IA), probably because of two carboxylic acid groups in its structure.13,14 However, an additional analysis method is needed for better understanding of the structural evolution of PAN precursors because thermal behavior represents both chemical and physical changes.13,15,16 © XXXX American Chemical Society
Received: May 14, 2013 Revised: July 10, 2013
A
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B
a
15 20 30 30 30 60 PAN PAI-1 PAI-2 PAI-3 PAI-4 PAI-7
Polymerization conditions: [AN] = 7.9 M, [AIBN] = 0.011 M in DMSO, temperature = 78 °C. bConversion of each monomer determined by 1H NMR analyses. cf IA = ([IA] × 100)/([AN] + [IA]) in feed. dFIA,NMR = ([IA] × 100)/([AN] + [IA]) in PAI determined from the conversion of each monomer through 1H NMR analyses. eFIA,IR = ([IA] × 100)/([AN] + [IA]) in PAI determined by FT-IR analyses. fMn = Mn,gpc/2.5,30 Mn,gpc = number-average molecular weight determined by GPC with PS standards. gMw = Mw,gpc/2.5,30 Mw,gpc = weight-average molecular weight determined by GPC with PS standards. hMv = viscosity-average molecular weight.31 iNumber-average sequence lengths of AN calculated by the following equation: nAN = (rAN × [AN] + [IA])/[IA].32
63 000 67 100 66 400 69 900 60 400 70 600 1.23 1.29 1.28 1.33 1.19 1.34 1.88 1.85 1.65 1.65 1.61 1.56 96 300 69 500 76 400 79 200 77 700 79 700 51 300 37 600 46 300 47 900 48 200 51 000 0.84 1.87 3.30 3.99 7.12 1.79 1.98 3.33 4.35 6.15 99.9 66.7 77.3 61.9 61.1
FIA,NMR (mol %)d IA AN f IA (mol %)c polymerization time (min) experiment number
conversion (%)b
Table 1. Preparation and Characteristics of PAI Employed in This Studya
FIA,IR (mol %)e
Mn (g mol−1)f
Mw (g mol−1)g
PDI
[η]
2.1. Materials. Acrylonitrile (AN, 99%, Aldrich, St. Louis, MO) was dried over CaH2, distilled under reduced pressure, and stored in a freezer under nitrogen before use. IA (99%, Aldrich), 2,2′azobisisobutyronitrile (AIBN, 98%, Aldrich), nicotinic acid (NC, 98%, Aldrich), and nalidixic acid (NL, 98%, Aldrich) were used as received. Dimethyl sulfoxide (DMSO, 99.8%, Samchun Chemicals, Seoul, Korea) was distilled and stored in the presence of molecular sieves. Other solvents were used without further purification. 2.2. Preparation of PAI. PAI was prepared by free radical polymerization in DMSO at 78 °C, as previously described.29 The preparation condition and the characteristics of the PAIs are summarized in Table 1. Copolymers with IA content in the range of 1−7 mol % were synthesized. The contents of IA and AN in the copolymer were pre-estimated by using reactivity ratios of 0.4682 and 0.3754, respectively. The copolymers were purified by precipitation using a large excess of deionized water from DMSO solution. The copolymers were further purified by Soxhlet extraction using boiling methanol for 8 h. After drying under vacuum at 50 °C, the copolymers were stored in a refrigerator. The PAIs contained 0.84, 1.87, 3.30, 3.99, and 7.12 mol % of IA and were named PAI-1, PAI-2, PAI-3, PAI-4, and PAI-7, respectively. More detailed information on the polymerization procedures and the characterizations of the resulting polymers is provided as Supporting Information. 2.3. Characterizations. An FT-IR instrument (Thermo Nicolet 380) was used to collect the spectra. Each sample was scanned 64 times with a 4 cm−1 resolution. Known amounts of the compounds were mixed and ground in KBr powder using agate mortar to determine the molar absorption coefficients of model compounds. The mixture was then pressed to form pellets for the FT-IR analysis. Thickness values of each pellet were obtained by using digital caliper (Stolz PDIC-151) and used for the normalization procedure of the quantitative experiments. A density value of 2.753 g cm−3 was used for KBr.33 For the FT-IR analyses of polymers, polymer thin films were prepared by casting a 3% (co)polymer solution in DMSO on slide glass. The solvent was evaporated through heat treatment at 80 °C for 5 min. The films were peeled off under water and washed with methanol to remove the remaining DMSO. The films were then transferred onto alumina frames and further dried at 60 °C under vacuum. TOS experiments were performed on the copolymers using the films under air in a high-temperature furnace. The films were removed from the furnace at timed intervals for FT-IR analysis. The copolymer compositions were determined using a calibration curve obtained from the FT-IR spectra. The calibration curve was prepared based on the ratio of the height of the carbonyl peak at 1730 cm−1 and that of the nitrile peak at 2240 cm−1. Standards were prepared by mixing known amounts of the IA comonomer and PAN homopolymer. The molecular weights and molecular weight distributions of the copolymers were determined using gel permeation chromatography (GPC, HLC-8320, Tosoh Ecosec) equipped with a TSK-gel column and refractive index detector. N,N′-Dimethylformamide was used as an eluent at a flow rate of 0.6 mL min−1 at 40 °C. Linear polystyrene standards (PS standard, 1000 to 9.49 × 105 g mol−1) were used for calibration. As previously suggested,30 the molecular-weight values determined from the linear PS standard were divided by 2.5 to obtain the actual molecular weights. The intrinsic viscosity ([η]) values of the PAIs were determined using an Ubbelohde viscometer (capillary diameter of 0.63 mm) at 50 ± 0.5 °C. DMSO was used as the solvent for the PAIs. The viscosity-averaged molecular weight was calculated based on the following equation: [η] = 2.83 × 10−4 × Mv0.758.31 The monomer-to-polymer conversion was determined using proton nuclear magnetic resonance spectroscopy (1H NMR, 500 MHz Bruker Avance), as described in previous studies.29,34
26.0 38.6 40.1 39.3 27.8 34.8
2. EXPERIMENTAL SECTION
0.00 0.70 1.20 1.72 2.00 3.60
Mv (g mol−1)h
nANi
determine the effect of IA content on the TOS process. The analytical procedures provide an efficient methodology for an in-depth understanding of the stabilized structures, which provide guidelines for selecting the optimum PAN copolymer precursors for high-performance carbon materials.
577 57 58 30 18
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Figure 1. FT-IR spectra of PAI containing 3.99 mol % of IA (PAI-4) with different thermal treatment times at 220 °C (a) and a plausible structural transformation scheme for PAI-4 (b) during the thermal treatment process. Downward arrows indicate decreasing peak intensities with thermal treatment time, whereas upward arrows indicate increasing peak intensities: ν, stretching vibration; δ, in-plane bending vibration; τ, out-of-plane bending vibration; ω, wagging vibration. 2.4. Second Derivatives and Curve-Fitting Procedures in FTIR Analyses. The FT-IR absorbance spectra in range of 1000−1800 cm−1 consisted of various overlapping vibration modes of functional groups. Second-derivative spectra were obtained using the equation An″ = (An−1 − 2An + An+1)/(ΔW)2, where An, An″, and ΔW are the absorbance at data point n, the intensity of the second derivative at data point n, and the frequency interval between data points n and n + 1, respectively.35 The positions of the negative peaks in the second derivatives were used to determine the position of each peak in the original spectra.35 PeakFit v4.0 software (SPSS/Jandel, Scientific Software) was used to deconvolute the spectra within a selected range. The curve-fitting parameters of the program were set up in the AutoFit Peaks I Residuals option, which includes a linear two-point baseline, the Savitsky−Golay smoothing function, and the Pearson VII area function for the curve fitting. The Pearson VII function for peak shape was adopted to adjust the shape parameter a3 from Gaussian (a3 = ∞) to Lorentzian (a3 = 1) for the best fit.26,36 The goodness of fit was set to six significant digits, meaning that signal convergence was obtained when chi-square remained unchanged in the sixth decimal place for five consecutive iterations. A typical curve-fitting operation for the copolymer films included baseline correction using EZ OMNIC software.
2870 (νasC−H in CH2), 1450 (δsC−H in CH2), 1360 (mixed mode; δC−H in CH + τC−H, ωC−H in CH2) and 1070 cm−1 (νC−CN).9,37,38 The structural evolution of PAI-4 during the TOS processes is proposed in Figure 1b. The cyclization reaction converted −CN groups to cyclic −CN− and −C−N− groups, as revealed by the gradual disappearance of the νCN peaks at 2240 cm−1 (Figure 1a). The appearance of broad peaks at 1580− 1620 cm−1 represented the formation of cyclic −CN− group and indicated successful cyclization. The broadness of the peaks at 1580−1620 cm−1 stemmed from the overlap of the vibrations of −CN− groups with those of CC and N− H groups, where the CC and N−H groups originated from dehydrogenation reaction as part of the oxidation and tautomerization reactions. The dehydrogenation reaction also reduced the concentration of CH2 groups, resulting in νsC−H (2940 cm−1), νasC−H (2870 cm−1), and δsC−H (1450 cm−1) peaks with reduced intensities, as indicated by the downward arrows in Figure 1a. The vibration peaks at approximately 3350, 2200, and 810 cm−1 gradually appeared with thermal treatment time, representing N−H stretching vibration, α,β-unsaturated nitrile vibration, and the out-of-plane bending of C−H in saturated rings, respectively. These findings clearly demonstrated the success of the TOS reaction. For quantitative evaluation, the peaks needed to be resolved to identify each individual contribution. The second derivatives of the FT-IR spectra in the range of 1000−1800 cm−1 were thus employed to determine the position of each contribution. Representative second-derivative spectra of PAI-4 during TOS are presented in Figure 2. Negative second-derivative peaks indicated the position of each peak in the original spectra.26,27,35 Downward arrows in the Figure indicate decreased peak intensities with thermal treatment time, whereas upward arrows indicate increased peak intensities. The designations and positions of each contributing peak are summarized in Table 2. Although complicated overlapping peaks from mixed vibrational modes were observed in the original FT-IR spectra
3. RESULTS AND DISCUSSION Five PAIs with different IA contents were prepared as model precursors through solution-free radical polymerization. The preparation conditions were carefully designed so that the molecular weights of the PAIs were similar to one another. The similar molecular weight values obtained for each experiment, as determined by GPC (Mn and Mw) and viscometry (Mv) also confirmed the validity of the experiments (Table 1). The PAIs were subjected to the TOS processes at 220 °C. FT-IR spectra of the samples were taken at timed intervals (0, 10, 20, 30, 60, 90, and 150 min). Representative FT-IR spectra of PAI-4 during the TOS process are presented in Figure 1a. The FT-IR spectrum of the initial PAI-4 (Figure 1a, 0 min) exhibited characteristic peaks of the vibrations of nitrile groups (νCN) at 2240 cm−1; carboxylic acid groups at 1730 (νCO), 1200 (mixed modes of νC−O and δO−H),1 and 3100−3600 cm−1 (νOH); and a hydrocarbon backbone at 2940 (νsC−H in CH2), C
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be observed in Figure 1. These behaviors clearly demonstrate the progress of the TOS reaction. One of the most useful characteristic peaks for evaluating the extent of the TOS reaction is νCO peak. The initial PAI-4 gave a νCO peak from IA at 1730 cm−1. Heterocyclic structures from the TOS process afforded νCO peaks at 1715 and 1660 cm−1. As shown in Figure 1b, the cyclization reaction initiated by anionic mechanism afforded an ester structure. However, the carbonyl absorption of the ester group appeared at a higher wavenumber than that of original acid, suggesting the formation of a imide structure.13 The plausible structures of the cyclized PAI are presented in Figure 1b, referring to the literatures from Grassie et al. and Coleman et al.9,13 The νCO peak at 1715 cm−1 represented the carbonyl group in hydronaphthyridine rings, which evolved mainly from IA, whereas that at 1660 cm−1 represented the carbonyl group in acridone rings. (Please refer to the numbers indicated for each functional group in Figure 1b.)39,44 The carbonyl group in acridone rings originated from oxygen-uptake reaction from air. The carbonyl groups promoted interchain hydrogen bonding between CO and N−H of the PAI chains,9−11,24 providing improved thermal stability of the stabilized PAIs. Actually, the behaviors were important for the efficient successive carbonization reaction because the improved stability diminished the evolution of thermally degraded carbon-containing fragments. The involvement of carboxylic acid groups of IA in the TOS reaction to initiate the cyclization reaction was obvious in Figure 2, as confirmed by the disappearance of the νCO of the carboxylic acid group (1730 cm−1, Figure 2, 0 min) and the simultaneous appearance of νCO in hydronaphthyridine rings (peak 1 in Table 2, 1715 cm−1; Figure 2, 10−150 min). The progressive decrease in peaks 12 and 17 along with the increase in peaks 15 and 16 also confirmed the role of the carboxylic acid groups as initiators for the cyclization reaction. The vibration of the carboxylic acid dimer observed at 1700 cm−1 (peak 2 in Table 2; Figure 2; 0, 10, 30, 60 min)37,43 also
Figure 2. Second-derivative spectra of PAI-4 during TOS. Downward arrows indicate decreasing peak intensity with thermal treatment time, whereas upward arrows indicate increased peak intensity.
(Figure 1a), the second derivatives of the spectra were useful for identifying the contributing peaks, especially for oxygenand nitrogen-containing functional groups. The vibrations of amide groups (peaks 6−8 in Table 2),38 ether groups (peaks 16 and 19 in Table 2),37 ketone groups (peak 15 in Table 2),37 hydroxyl groups (peaks 12 and 17 in Table 2),37 coupled O−H in-plane bending with C−H wagging (peaks 11 and 14 in Table 2), and the mixed mode of aromatic C−H, N−H,40 and O− H37 (peak 13 in Table 2) were easily identified in the second derivatives of the spectra. The intensity of several peaks increased significantly during the TOS process, such as νCO (carbonyl group from oxygenuptake reaction, peak 3 in Table 2, 1660 cm−1), νCC + νCN (peaks 4, 5, and 9 in Table 2, 1610, 1580, and 1490 cm−1), δC−H + δN−H + δOH (peak 13 in Table 2, 1380 cm−1), and δC−C(O)−C (peak 15 in Table 2, 1280 cm−1). A simultaneous decrease in the intensity of δC−H (peak 10 in Table 2) can also
Table 2. Interpretation of the Absorbance Peaks of PAI during TOS in FT-IR Analysisa designation of peaks
position of the peaks (cm−1)b
1 2 3
1715 1700 1660
νCO in hydronaphthiridine (carbonyl group mainly originated from IA) νCO in dimer COOH νCO in acridone (carbonyl group mainly originated from oxygen uptake reaction) νCC + νCN
4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
1610 1580 1560 1540 1520 1490 1450 1430 1410 1380 1330 1280 1260 1230 1150 1070
νCC + νCN + δN−H (cyclic structure originated from cyclization and dehydrogenation reaction)
13, 24, 38, 40
νC−N + δN−H of amide
38
νCC of ring δC−H in CH2 δOH + ωCH δC−OH in COOH δC−H + δN−H + δOH in ring δOH + ωCH δC−C(O)‑C in ring νasC−O‑C in ether νC−C + νC−N + νC−O + δO−H of COOH νC−O νsC−O‑C in ether
37 37, 38, 40, 45 37 37 37, 40 37 26, 37 26, 37 1, 26, 46 44, 47 37
corresponding vibration modesc
teferences 1, 24, 39−42 37, 43 24, 38, 39, 42, 44
Thermal treatment temperature = 220 °C. bPosition of the peaks determined from second derivatives of FT-IR spectra. cSymbols: ν, stretching; δ, in plan bending; τ, twisting or out of plane bending; ρ, rocking; ω, wagging. a
D
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Figure 3. Peak fitting by Pearson VII area (mix shape, Gaussian characteristic ∼2%) on the FT-IR spectra of PAI-4 in the range of 1000−1800 cm−1 obtained during the thermal treatment procedures at 220 °C in air for 10 (a), 60 (b), and 150 min (c). The curve at the bottom of each graph is the different spectrum between the experimental spectrum and simulated spectrum.
Table 3. Peak Parameters Corresponding for the Deconvoluted FT-IR Peaks of PAI-4 during TOS as Presented in Figure 3a area under the peaks
a
designation of peaks
position of the peaks (cm−1)b
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19
1715 1700 1660 1610 1580 1560 1540 1520 1490 1450 1430 1410 1380 1330 1280 1260 1230 1150 1070
fwhm of the peaks
area under the peaks
38.58 32.61 52.59 52.40 49.03 76.32 36.93 47.08 19.40 23.31 21.68 26.92 71.51 28.21 69.32 34.72 52.03 74.70 77.36
13.40 11.95 26.57 54.55 71.92 8.93 12.71 12.31 24.62 10.07 15.49 4.52 109.93 9.61 70.92 5.43 25.26 23.30 10.62
10 min 13.28 11.29 8.71 17.51 21.11 4.05 5.23 9.56 1.86 10.00 5.30 2.35 43.11 3.47 24.68 3.42 21.93 11.05 10.46
fwhm of the peaks
area under the peaks
37.82 40.58 57.84 54.80 52.23 77.79 47.60 54.04 47.68 20.86 29.47 25.77 67.00 31.38 74.79 30.74 55.35 61.27 84.02
16.67 11.49 44.35 77.59 100.25 9.06 20.01 13.17 40.30 11.10 18.94 5.84 146.41 13.83 88.38 6.62 28.13 26.99 7.06
60 min
fwhm of the peaks
150 min 38.72 37.13 54.67 54.02 55.43 59.58 53.43 50.62 52.06 22.83 29.77 25.90 68.49 36.28 80.47 33.12 57.26 58.36 107.34
Thermal treatment temperature = 220 °C. bPosition of the peaks determined from second derivatives of FT-IR spectra.
previous attempts, our study combined second-derivative and peak-fitting procedures to study the evolution of cyclized and oxygen-uptaken structures separately, which allowed us to follow each contributing reaction. The extent of the cyclization reaction was calculated from the following equations:22
disappeared gradually with increasing TOS time (90 and 150 min) due to its involvement in the initiation reaction. On the basis of the number and position of the contributing peaks determined from the second-derivative FT-IR spectra, deconvolution of the spectra was attempted by peak-fitting procedures to follow the progress of the TOS reaction (Figure 3 and Table 3). It should be noted that the results of the peakfitting procedures were reproducible. To the best of our knowledge, no previous studies have successfully determined the extent of each contributing reaction due to the overlapping νCO contributions in hydronaphthiridine and acridone groups with cyclized ring vibrations. Previous studies generally followed the overall TOS process by studying the behaviors of the overall shape of the overlapping peaks,17,24 which limited the depth of the understanding of the progress of each contributing reactions. For example, Collins et al. ignored the oxygen uptake reaction in their interpretations; however, this practice may additionally increase the intensity of peaks at ∼1590 cm−1.22 Unlike
extent of cyclization reaction (%) = AC
× 100(%) = AC + =
(A C)∞ . (AN )0
AC (A C)∞ AC (A C)∞
+
AN (AN )0
× 100(%) AN
AC × 100(%) A C + f ·AN
where AC indicated the areas of peaks 4 + 5 (cyclized ring structure). (Please refer to Table 2 and Figure 1b.) The areas of the nitrile peak at 2240 cm−1 (AN) were used as an internal E
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standard for normalization. The subscripts 0 and ∞ indicate time during the TOS reaction, where a sufficiently long TOS process (10 h) was adopted as infinite (∞). The value of f was calculated as 3.355, which was consistent with the value reported by Collins et al.22 This equation provides a quantitative tool to estimate the evolution of the cyclized ring structure. The extent of the cyclization reaction of PAN and various PAIs containing different amounts of IA comonomer is presented in Figure 4. After the TOS at 220 °C for 150 min,
structure, such as the cyclized ring structure (peaks 4 + 5 in Table 2) and the carbonyl groups (peaks 1 and 3 in Table 2), based on the Beer−Lambert equation. For quantitative study, first, the molar absorption coefficients of each contributing groups were investigated using model compounds. In this work, NC and NL were chosen as the model compounds (chemical structures in Figure 5a,b, respectively). NC and NL have structural similarities with the six-membered ring structures of PAN and PAIs evolving during the TOS reaction (Figure 1b). FT-IR has been acknowledged as a useful method to determine the molar absorptivity of certain functional groups at certain wavenumber.27,33 In FT-IR, the absorbance of the nitrogencontaining ring vibrations of NC and NL appeared at 1595 and 1617 cm−1, respectively, whereas that of CO stretching appeared at 1710 cm−1. The absorbance at 1595 and 1617 cm−1 of NC and NL, respectively, corresponded to peaks 4 + 5 of the stabilized structure in Table 2, while that at 1710 cm−1 corresponded to peaks 1 and 3 of the stabilized structure in Table 2. The plots of the absorbance values against the concentration of each contributing structure are presented in Figure 5. Good linear absorption behaviors with the concentration of the contributing structures were observed (R2 > 0.88). The calculated and averaged values of the molar absorption coefficient were 65.8 and 55.2 (cm2 mmol−1) for the carbonyl group (peaks 1 and 3 in Table 2) and the nitrogen-containing ring group (peaks 4 + 5 in Table 2), respectively. The values were approximately the same for each model compound and were used for the quantitative investigation. Figure 6 presents the evolution of the concentration of the cyclized ring group (Figure 6a, peaks 4 + 5 in Table 2), carbonyl group in acridone ring (Figure 6b, peak 3 in Table 2), and carbonyl group in hydronaphthiridine (Figure 6c, peak 1 in Table 2, originating from IA) during TOS. The values were determined based on the molar absorption coefficient obtained above and the area of each peak in the FT-IR analysis (Figure 3). The concentration of the carbonyl group in hydronaphthiridine increased with the IA content (Figure 6c) because the carbonyl group mainly originated from IA. The concentrations of free carbonyl groups were almost constant with TOS time, also supporting this interpretation. Slight increases in the
Figure 4. Extent of the cyclization reaction of PAN and PAI with different IA comonomer contents during TOS at 220 °C as determined by the evolution of peaks 4 + 5.
the extents of cyclization reaction for the PAIs were all above 90%, whereas that of PAN was only 80%, indicating the successful TOS reaction of PAIs and validating the efficiency of IA as an initiation site for the cyclization reaction.24,25,48 The complete cyclization reaction of PAN was obtained after heating for 10 h. In addition, the cyclization reaction of PAIs began immediately after the heating, whereas an induction time (∼20 min) was observed for the cyclization reaction of PAN. The involvement of the carboxylic acid group in the stabilized structure (Figure 2) and the fast stabilization rate of the PAIs indicated the importance of IA as an initiation site (Figure 4). Among the PAIs, PAIs containing 3 to 4 mol % of IA (PAI-3 and PAI-4) exhibited the fastest and most efficient cyclization reactions, suggesting the optimum content of IA in PAIs for the TOS reaction. To investigate the effect of IA content on the TOS reaction more closely, we investigated the evolution of each contributing
Figure 5. Calibration curves to determine the molar absorption coefficients of cyclized ring structures and carbonyl groups, employing NC (a) and NL (b) as model compounds. F
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Figure 6. Evolution of stabilized structures: cyclic ring structures (a, peak 4 + 5 in Table 2), conjugated carbonyl group in an acridone ring (b, peak 3 in Table 2), and free carbonyl group in hydronaphthiridine (c, peak 1 in Table 2) during TOS.
However, the use of the most efficient TOS processes for a certain PAI does not guarantee the highest carbon yield after the following carbonization and graphitization processes. Additional oxygen uptake during TOS may lead to the emission of oxygen during the anaerobic carbonization and graphitization reactions, which would be harmful for the carbon yield.51 A quantitative investigation of the effect of IA content on PAI for the ultimate graphitization reaction will be the next research goal of our group.
concentrations of free carbonyl groups at extended TOS time suggest a minor oxygen uptake reaction to form peak 1. As shown in Figure 6a,b, the synchronized evolutions of the pyridine rings and the carbonyl in the acridone ring (conjugated carbonyl group) with TOS time were observed, suggesting simultaneous cyclization and oxygen uptake reaction from air to form carbonyl groups. The IA content in the PAI copolymer significantly affected the evolution of the stabilized structures. PAIs with 2 or 3 mol % IA exhibited the most efficient cyclization and oxygen uptake reactions (Figure 6a,b), which suggests optimum number-average AN sequence lengths of ∼60 (Table 1). IA in PAI (PAI-2 and PAI-3) enhanced the oxygen uptake reaction to provide a more stabilized structure.8 The improved oxygen uptake reaction probably stemmed from the enhanced oxophilicity of PAI in the presence of IA.22,24 In addition to general radical process of TOS,49 incorporated IAs in PAI also allowed additional ionic initiation process of TOS, which accelerated the overall TOS reaction.13 However, excessive amount of IA (PAI-4 and PAI-7) resulted in lower amounts of cyclization and oxygen uptake reactions (Figure 6a,b).50,51 Probably, excessive amount of IA in PAI inhibited intra/intermolecular ionic propagation reaction of the cyclization through blocking the H-transfer reaction (Figure 7).
4. CONCLUSIONS The structural evolution of PAI with different amounts of IA during TOS processes was investigated through FT-IR analysis. The application of second-derivative and peak-fitting procedures for the FT-IR spectra allowed the identification and quantitative tracking of each contributing structure (free carbonyl group, peak 1 in Table 2; conjugated carbonyl group, peak 3 in Table 2; cyclic ring structure, peaks 4 + 5 in Table 2). The molar absorption coefficients of the cyclized ring groups (55.2 cm2 mmol−1) and carbonyl groups (65.8 cm2 mmol−1) determined from the model compounds were also successfully employed to determine the concentration of the contributing structures. The results suggest that the free carbonyl group originated mainly from IA, whereas the conjugated carbonyl group mainly originated from oxygen uptake from air. The simultaneous evolution of the cyclic structure originating from the cyclization and dehydrogenation reaction suggests the existence of an optimum IA content in terms of TOS efficiency, which was determined to be ∼3 mol % in this study.
■
ASSOCIATED CONTENT
S Supporting Information *
Detailed information on the polymerization procedures and the characterizations of the resulting polymers are provided as a separate file. This material is available free of charge via the Internet at http://pubs.acs.org.
Figure 7. Plausible inhibition effect of excessive IA in PAI on TOS.
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The results were consistent with the previously discussed observations. The extent of the cyclization reaction determined by the evolution of peak 4 + 5 (overall cyclized ring structure, Table 2) with the nitrile peak (2240 cm−1) as an internal standard suggested that PAI with IA content of 3 to 4 mol % was most efficient for the TOS reaction (Figure 4). An investigation of the evolution of peak 3 along with peaks 4 + 5 also suggested that PAI with IA contents of 2 to 3 mol % was the most efficient (Figure 6). Overall, PAI with ∼3 mol % IA most likely affords the most efficient TOS process.
AUTHOR INFORMATION
Corresponding Author
*Tel: +82-2-3408-3750. Fax: +82-2-3408-4342. E-mail:
[email protected] (S. C. Hong). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This research was supported by the Basic Science Research Program through the National Research Foundation of Korea G
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(NRF) funded by the Ministry of Education, Science and Technology (2012R1A1A2005345). This research was also supported by the Fusion Research Program for Green Technologies through the NRF funded by the Ministry of Education, Science and Technology (2012M3C1A1054503).
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dx.doi.org/10.1021/ma401003g | Macromolecules XXXX, XXX, XXX−XXX
