Synergistic effect of comonomers on the thermal oxidative stabilization of polyacrylonitrile copolymers for carbon materials

Accepted Manuscript Synergistic effect of comonomers on the thermal oxidative stabilization of polyacrylonitrile copolymers for carbon materials Dong Won Cho, Sung Chul Hong PII:

S0141-3910(19)30035-7

DOI:

https://doi.org/10.1016/j.polymdegradstab.2019.01.027

Reference:

PDST 8766

To appear in:

Polymer Degradation and Stability

Received Date: 25 October 2018 Revised Date:

23 January 2019

Accepted Date: 24 January 2019

Please cite this article as: Cho DW, Hong SC, Synergistic effect of comonomers on the thermal oxidative stabilization of polyacrylonitrile copolymers for carbon materials, Polymer Degradation and Stability (2019), doi: https://doi.org/10.1016/j.polymdegradstab.2019.01.027. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Synergistic effect of comonomers on the thermal oxidative stabilization of

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Dong Won Cho, Sung Chul Hong*

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polyacrylonitrile copolymers for carbon materials

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Department of Nanotechnology and Advanced Materials Engineering, Sejong University,

Corresponding author. Tel.: +82-2-3408-3750. Fax: +82-2-3408-4342. E-mail address:

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*

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Seoul 143-747, Republic of Korea

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[email protected]

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Abstract

Thermal oxidative stabilization (TOS) behaviors of polyacrylonitrile (PAN) (co)polymers

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with different comonomers, such as itaconic acid (IA), methacrylic acid (MA), methyl methacrylate (MMA), vinyl acetate (VA), and VA/IA mixture, were investigated through Fourier transform infrared spectroscopy analysis to quantitatively determine the role of the

general

radical

mediated

TOS

processes.

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comonomers. Poly(acrylonitrile-co-MMA) exhibited slow progress of TOS, indicating Facilitated

stabilization

behaviors

of

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poly(acrylonitrile-co-IA) (PAIA) and poly(acrylonitrile-co-MA) (PAMA) were observed, indicating additional ionic initiating activity of the comonomer carboxylic units. Poly(acrylonitrile-co-VA) exhibited a certain induction period, followed by an accelerated TOS in the later stage, indicating effective in situ generation of acetic acid through the

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degradation of vinyl acetate units during the TOS. Interestingly, the PAN-based terpolymer containing both VA and IA exhibited not only the most efficient but also the fastest progress of cyclization among the (co)polymers. The results demonstrated a synergistic combination

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of VA and IA in terms of TOS characteristics, likely stemming from the cascade initiation activity of the carboxylic acid groups from IA and VA. The results suggested the importance

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of the type and combination of comonomers as a crucial factor to control the TOS processes, providing technical information to design an optimum precursor for better carbon materials.

Keywords: Thermal oxidative stabilization; polyacrylonitrile; copolymer; terpolymer; acidic unit; precursor; carbon material

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1. Introduction

Carbon materials have attracted great research interest because of their outstanding

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performance [1-5]. As one of the most important carbon precursors for manufacturing carbon materials, polyacrylonitrile (PAN) has attracted significant attention due to its relatively high carbonization yield, highly tunable properties, and cost-effective production [3,6-8]. The PAN

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precursor is converted into carbon materials through subsequent thermal treatments, such as stabilization, carbonization and graphitization processes. Among them, the thermal oxidative

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stabilization (TOS) process is a crucial step in providing high-quality carbon materials. The TOS process includes cyclization, oxidation and tautomerization reactions, converting linear precursors into sturdy cyclic structures that may withstand the subsequent high-temperature thermal treatments [9-12].

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Numerous studies have explored the introduction of comonomers to PAN to improve its TOS characteristics such as precursor processability, degree of cyclization, and controllability on the harsh and sudden radical-mediated exothermic TOS reaction [13-18].

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Reports have suggested that acidic comonomers facilitate the TOS of PAN, which causes an additional low-temperature ionic cyclization reaction during the TOS. Among the acidic

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comonomers, itaconic acid (IA) has been acknowledged as one of the most efficient comonomers due to the presence of two carboxyl groups in its chemical structure [13,17]. Acrylate and methacrylate have also been incorporated into PAN precursors to improve its processability including solubility, drawability and spinnability. Although the neutral comonomers did not contain acidic units, the oxygen uptake reaction of the PAN copolymers was facilitated during TOS due to its bulkiness [19,20]. The resulting carbonyl groups in the 3

ACCEPTED MANUSCRIPT cyclized structure enhanced the thermal stability of the precursor, which kept the precursor from decomposition during the high-temperature carbonization and graphitization processes. Therefore, it is critical to select proper copolymer composition by identifying the comonomer

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contributions to cyclized PAN structures [19]. However, the comprehensive and comparative understanding of the role of comonomers has not been well explored.

In our previous reports, we have successfully demonstrated a simple but useful

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technical tool to track the structural evolutions of poly(acrylonitrile-co-IA) (PAIA) during the TOS process by using Fourier transform infrared spectroscopy (FT-IR) and peak-

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deconvolution/curve-fitting procedures on the spectra (see Section 3 ~ 5 of the Supporting Information) [21-24]. The analytical procedures provided an efficient methodology for an indepth and quantitative understanding of the stabilization process, offering hands-on guidelines for selecting better PAN-based precursors for carbon materials.

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In this study, the analytical tools were further applied to PAN copolymers containing different comonomers. Understanding the role of individual comonomers on the characteristics of TOS processes was attempted. Different comonomers, such as IA,

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methacrylic acid (MA), methyl methacrylate (MMA) and vinyl acetate (VA), were employed as model comonomers. Ultimately, a combination of comonomers was attempted to provide

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terpolymers with better performances in terms of the TOS. The aim of the present work is to provide information on the optimum compositional characteristics of PAN copolymer precursors for better TOS.

2. Experimental 4

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2.1. Materials Acrylonitrile (AN, 99%, Aldrich, St. Louis, MO, USA) was dried through refluxing over

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CaH2, followed by distillation under reduced pressure. The purified AN was then stored at 20 °C under nitrogen. MA (99%, Tokyo Chemical Industry Co., Ltd., Japan), MMA (99%, Aldrich, St. Louis, MO, USA) and VA (99%, Tokyo Chemical Industry Co., Ltd., Tokyo,

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Japan) were also purified through distillation under reduced pressure. All other chemicals, including IA (99%, Aldrich, St. Louis, MO, USA), 2,2-azobis-isobutyronitrile (AIBN, 98%,

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Samchun Chemicals, Seoul, Korea) and dimethyl sulfoxide (DMSO, 99%, Samchun Chemicals, Seoul, Korea) were used without specific purification.

2.2. Preparation of PAN and PAN copolymers

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PAN (co)polymers were prepared by typical free radical polymerization (FRP) technique. Representative procedure to prepare PAIA follows: In a batch-type round-bottom flask, IA (0.55 g, 4.2 mmol) was charged along with a stir bar and a condenser. AN (40ml, 0.6 mol)

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and solvent (DMSO, 32ml) were injected into the reactor by using syringe, followed by bubbling with N2 gas for 30 minutes for degassing. Oil bath was then adopted to maintain the

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temperature of the flask at 78 °C. By injecting a degassed solution of AIBN (0.137 g, 0.83 mmol) in anisole (4 ml), FRP of AN was initiated. After certain polymerization time, the FRP was terminated by removing the reactor from the oil bath. Additional amount of DMSO was added to the resulting viscous solution for enough dilution. The solution was then poured in a large excess (typically 10 times) of deionized water to recover the polymer. Soxhlet extraction procedure on the resulting polymer with methanol was performed for purification for 8 hours. The purified polymer was then dried under vacuum at 60 °C and stored in a 5

ACCEPTED MANUSCRIPT refrigerator. Other copolymers with different comonomers were prepared through the same procedure, keeping the contents of the carbonyl containing functional units in the copolymer to be similar with each other (~3 mol%, indicated with red color in Figure 1 and GCM in Table

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1). The preparation of PAVA and PAVIA were performed at 70 °C due to the boiling point of

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VA (72.7 °C).

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Figure 1. Chemical structures of PAN (co)polymers.

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Table 1. Preparation and characteristics of PAN (co)polymersa

e

g

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1

FCM,NMR = ([Comonomer] × 100)/([AN] + [Comonomer]) in PAN copolymers determined from the conversion of each monomer through H-NMR analyses. GCM = content of carbonyl functional units determined from FCM,NMR and the chemical structure of the comonomers (IA contains two carboxylic acid groups

in its chemical structure). f

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1

Conversion of each monomer determined by H-NMR analyses.

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d

fCM = ([Comonomer] × 100)/([AN] + [Comonomer]) in feed.

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c

fCMb (mol%) 0.70 2.59 2.31 3.98 2.36 (VA) + 0.62 (IA)

Polymerization time (minutes) 15 15 15 15 20

Number-average molecular weight determined by GPC. Weight-average molecular weight determined by GPC.

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b

Comonomer (CM) IA MA MMA VA

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Conversionc(%) FCM,NMRd GCMe Mn f Mwg PDI (mol%) (mol%) (g/mol) (g/mol) AN CM PAN 27.5 237,000 511,000 2.15 PAIA 43.63 99.9 1.59 3.18 92,200 180,000 1.96 PAMA 79.70 90.0 2.89 2.89 66,800 131,800 1.97 PAMMA 66.45 90.91 3.13 3.13 76,300 161,700 2.12 PAVA 75.12 60.0 3.21 3.21 90,100 257,900 2.86 75.73 (VA) + 2.08 (VA)+ PAVIA VA + IA 35 81.40 3.24 81,300 333,000 4.10 71.51 (IA) 0.58 (IA) a Polymerization conditions: [AN] = 8.0 M, [AIBN] = 0.011 M in DMSO, temperature = 78 °C (70 °C for PAVA and PAVIA, due to the boiling point of VA, 72.7 °C) Designation

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2.3. Stabilization procedures By casting (co)polymer solutions (5% in DMSO) on slide glass, film samples for TOS studies were obtained. After the casting, the films were subjected to heating (80 °C, 30 minutes) to

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remove any remaining solvent. The dried polymer films were peeled off in deionized water, followed by further washing with methanol and drying at 60 °C under vacuum for 12 hours on alumina frames. At least three samples (2.0 cm × 2.0 cm × 30 µm) were collected from

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each PAN (co)polymer film for each TOS procedure. TOSs of the polymer samples were

intervals to track their TOS behaviors.

2.4. Characterization

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conducted at 220 °C in air. Each film samples were removed from the furnace at timed

Proton nuclear magnetic resonance spectroscopy (1H-NMR, 500 MHz Bruker Avance) was

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employed to determine the conversion of monomers (see Section 1 of the Supporting Information, Figure S1 and Table S1). Gel permeation chromatography (GPC, 1260 infinity, Agilent, PSS column, refractive index detector; solvent, N,N-dimethylformamide; flow rate,

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0.8 mL/minute) was used to determine molecular weight and molecular weight distribution

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values of the polymers. Poly(methyl methacrylate) standards (4,900 ~ 6.95 × 105 g/mol) were used for calibration (see Section 2 of Supporting Information and Figure S2). FT-IR (Thermo Nicolet 380) was performed in transmission mode at 64 scans at a resolution of 4 cm-1. At least three different samples were repeatedly characterized to afford averaged data and standard deviation numbers. The deconvolutions of the spectra (1,000 ~ 1,800 cm-1) were performed by using PeakFit v4.0 software (SPSS/Jandel, Scientific Software), affording areas of the contributing peaks to determine the extent of cyclization and the concentration of the 8

ACCEPTED MANUSCRIPT chemical structures (see Section 3 ~ 5 of Supporting Information) [21-23]. Thermal characteristics of the polymers were investigated by using differential scanning calorimeter (DSC-4000, PerkinElmer instrument) in the range of 30 °C ~ 430 °C at a heating rate of

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5 °C/minute.

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3. Results and Discussion

Preparation conditions and characteristics of the PAN (co)polymers are summarized in Table

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1. The comonomer contents in the copolymers (FCM,NMR) were carefully controlled by adjusting the fCM values and the conversion of each monomer. GCM for each copolymer, indicating the content of carbonyl-containing functional units in the copolymers, was thus controlled to be similar with each other (~3 mol%) for rational comparison between the

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samples. The resulting PAN copolymers containing IA, MA, MMA, VA, and VA/IA comonomers were named PAIA, PAMA, PAMMA, PAVA and PAVIA, respectively (Figure 1). PAN and the copolymers were subjected to the TOS process at 220 °C in air. To track

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the structural evolution during the TOS process, FT-IR spectra of each samples were taken at

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timed intervals (0, 10, 20, 30, 60, 90, and 120 minutes) [21-23]. Representative FT-IR spectra of PAIA, PAVA and PAVIA during the TOS processes are presented in Figure 2. The FT-IR spectrum of the initial PAIA (Figure 2a, 0 minutes) exhibited characteristic peaks of the vibrations of nitrile groups (νCN) at 2,240 cm-1, carboxylic acid groups at 1,730 cm-1 (νC=O; for PAVA, acetate group at 1,735 cm-1 as shown in Figure 2b), mixed modes of νC-O and δO-H at 1,200 cm-1, and νOH at 3,100-3,600 cm-1. A hydrocarbon backbone exhibited characteristic

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ACCEPTED MANUSCRIPT peaks at 2,940 cm-1 (νsC-H in CH2), 2,870 cm-1 (νasC-H in CH2), 1,450 cm-1 (δsC-H in CH2),

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1,360 cm-1 (mixed mode, δC-H in CH + τC-H, ωC-H in CH2), and 1,070 cm-1 (νC-CN) [25,26].

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Figure 2. FT-IR spectra of PAIA (a), PAVA (b), and PAVIA (c) with different thermal treatment time at 220 °C (Downward arrows indicate

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decreasing peak intensities with thermal treatment time, whereas upward arrows indicate increasing peak intensities).

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The cyclization reaction of the TOS converted the nitrile groups (-C≡N) to cyclic C=N- and -C-N- groups [21], as revealed by the gradual disappearance of the nitrile peaks at 2,240 cm-1 with increasing TOS time (represented as downward arrows in Figure 2). The

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appearance of broad peaks at 1,580 ~ 1,620 cm-1 represented the formation of cyclic -C=N-, C=C and N-H groups with TOS time, indicating a successful cyclization reaction (Figure S3 and S4, see Supporting Information). The C=C and N-H groups stemmed from a

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dehydrogenation reaction as part of the oxidation and tautomerization reactions of the TOS [21]. The dehydrogenation reaction also reduced the concentration of CH2 groups, resulting

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in decreased νsC-H (2,940 cm-1), νasC-H (2,870 cm-1) and δsC-H (1,450 cm-1) peaks during the TOS process, as indicated by the downward arrows in Figure 2. The vibration peaks at approximately 3,350 and 810 cm-1 gradually appeared with thermal treatment time, representing N-H stretching vibration and the out-of-plane bending of =C-H in saturated

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rings, respectively. These findings clearly demonstrated the successful progress of the TOS reaction of the PAN (co)polymers.

The overall extent of the cyclization reaction was calculated from the relative

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intensities of the nitrile peaks at 2,240 cm-1 and the cyclized ring peaks at 1,580 ~ 1,620 cm-1

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(see the Section 5 of the Supporting Information). The progress of cyclization reactions of the PAN (co)polymers is presented in Figure 3. Please be noted that the thickness values of the films were similar with each other (~ 30 µm) and the stabilization data were normalized with film thickness values following the Beer-Lambert equation. Although the stabilized structures do depend on the physical structures of PAN precursors in general, the effect of crystallinity of PAN (co)polymer on their stabilization behavior was not a major factor in this study [24].

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Figure 3. Extent of the cyclization reaction of PAN (co)polymers with different comonomers during TOS at 220 °C.

After the TOS at 220 °C for 120 minutes, the extent of cyclization of PAN reached

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only ~72%, indicating slow progress of cyclization reaction. On the other hand, PAIA and PAMA underwent a fast cyclization reaction. The cyclization reactions of the copolymers started immediately after the heating and reached ~ 85% after 120 minutes. In addition to

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typical radical mediated cyclization reaction of PAN, this finding clearly demonstrated that the carboxylic group of PAIA and PAMA initiated an additional ionic cyclization reaction,

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which facilitated the TOS of the PAN copolymers [13]. Since the GCM values of PAIA and PAMA were similar with each other (3.18 mol% for PAIA and 2.89 mol% for PAMA), relatively identical TOS behaviors were observed (Figure 3). The results suggested that the concentration of the acidic functional groups is the key parameter to initiate the TOS, not the type of acidic groups. The cyclization behavior of PAMMA was quite similar to that of PAN, suggesting the neutral characteristic of MMA. Different from IA and MA, MMA did not afford an anionic 13

ACCEPTED MANUSCRIPT initiation effect on the cyclization reaction of the PAN copolymer due to the absence of acidic units. Instead, acrylate ester comonomers such as MMA and 2-ethylhexyl acrylate retarded the cyclization propagation of nitrile groups [10,27], which resulted in a slightly lower degree of cyclization of PAMMA (69%) than that of PAN (72%) at 120 minutes (Figure 3). The main

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contribution of MMA in the precursor should be the interruption of the AN-AN interaction to increase their processability including solubility, drawability and spinnability [14].

Interestingly, PAVA exhibited stepwise progress of the cyclization reaction. At the

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initial stage (~ 10 minutes), the extent of cyclization of PAVA was similar to that of PAN and PAMMA, indicating slow initiation of the cyclization reaction and the absence of an

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additional ionic cyclization reaction. However, thereafter, PAVA exhibited an accelerated cyclization reaction with a cyclization value of 82% at 120 minutes, which was similar to those of PAIA (86%) and PAMA (85%, Figure 3). The results strongly suggested an in situ generation of acidic units from PAVA during TOS [26]. As shown in Figure 4, the thermal

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degradation of PAVA occurred during the TOS, generating acetic acid from the PAVA backbone [28-31]. The resulting acetate anion then attacked an adjacent nitrile group to initiate an ionic cyclization reaction via rearrangement, oxidation and tautomerization

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reactions, similar to PAIA and PAMA (Figure 4) [10]. The generation of acetic acid through

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thermal degradation of vinyl acetate may also occur under inert atmosphere, which may undergo a chain scission reaction [28]. Since the elimination reaction of acetic acid from PAVA is slow [29], the initial TOS behavior of PAVA resembled PAN and the copolymer with a neutral comonomer such as PAMMA, which exhibited an induction period. During the TOS, due to the generation of acetic acid, the TOS characteristics of PAVA became similar to copolymers with acidic comonomers such as PAIA and PAMA, accelerating the cyclization reaction to ~ 82%. 14

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Figure 4. Plausible structural transformation of PAVA during TOS at 220 °C [10].

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Although PAMMA also contained ester side-groups, no acceleration effect was observed during the TOS (Figure 3). The thermal degradation and scission temperature of the ester side-group of MMA are known to be higher than 300 °C [32-34], suggesting that the

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MMA side-group is quite stable at the TOS temperature (220 °C). In addition, the plausible products from the degradation of the MMA side-group, such as methyl formate, are neutral

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and may not cause an ionic initiation reaction to TOS [35,36]. For the independent and quantitative investigation on the evolution of contributing

chemical structures during TOS, the FT-IR spectra in the range of 1,000 ~ 1,800 cm-1 were investigated through peak-deconvolution/curve-fitting procedures (Figure S3, also see Section 3 ~ 5 of the Supporting Information). As demonstrated in our previous study, identifying the contributing peaks from the overlapped spectra was performed through a 15

ACCEPTED MANUSCRIPT second derivatization of the spectra (see Table S2 and Section 4 of Supporting Information) [21]. Cyclized ring groups, conjugated carbonyl groups in the acridone ring and unconjugated free carbonyl groups in hydronaphthiridine are the three most representative contributing chemical structures of the stabilized PAN (co)polymers (Figure S3 and S4) [21]. Cyclized

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ring groups were attributed to the peaks at 1,580 ~ 1,620 cm-1 (Peaks 4 + 5 in Figure S3). The peaks of conjugated and unconjugated C=O, originating from the oxygen uptake (oxidation) reaction and from comonomers containing C=O, appeared at 1,660 cm-1 (Peak 3) and 1,700 ~

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1,725 cm-1 (Peaks 1 + 2), respectively (Figure S3). The concentration of the cyclized ring and oxygen uptake in the form of the conjugated C=O were determined based on the Beer-

of the Supporting Information) [21].

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Lambert law and using molar absorption coefficients of the model compounds (see Section 5

As shown in Figure 5a, the concentration behaviors of the cyclized rings were similar to those of the cyclization reaction in Figure 3. Both PAIA and PAMA showed the relatively

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identical progress of cyclized ring concentrations with TOS time. PAN and PAMMA exhibited slow accumulation of the cyclized rings due to the absence of anionic TOS process. With an initial induction period, PAVA exhibited accelerated cyclized ring generation in the

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middle of the TOS, which was similar to its behavior of the extent of cyclization.

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(b)

Figure 5. Concentration of cyclized ring (a) and conjugated carbonyl group in acridone ring (b) of PAN (co)polymers with different comonomers during TOS at 220 °C.

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The concentration of C=O in acridone represents the amount of oxidation reaction through the oxygen uptake reaction by (co)polymers (Figure 5b). Copolymers with accelearted TOS by the ionic initiation reaction also facilitated an oxygen uptake reaction.

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The progress rates of the concentration of C=O in acridone of PAIA and PAMA were obviously higher than those of PAN and PAMMA. Similar to the behavior of cyclized ring

during the TOS (Figure 5b).

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concentration, the concentration of C=O in PAVA acridone exhibited stepwise acceleration

The enhanced oxidation reactions of PAIA, PAMA, and PAVA stemmed from their high concentrations of ring structures formed by the accelerated cyclization during the TOS.

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Due to the cyclization reaction, a 1,4-dihydropyridine-like structure was formed, which is readily oxidized to form a 4-pyridone species (Figure S5) [23,37,38]. In addition, the high concentration of the cyclized rings likely decreased the crystallinity and increased the free

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volume of the polymer sample, which facilitated the diffusion of oxygen [39,40]. The quicker oxygen uptake reaction of PAMMA than that of PAN also represented easier oxygen diffusion

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in the presence of methyl ester side groups of PAMMA [41]. The DSC thermograms of PAN (co)polymers are shown in Figure 6 along with

thermal parameters in Table 2 (please also refer Figure S6 and Table S3 for DSC thermograms under inert atmosphere, see Supporting Information). The DSC thermogram of PAN exhibited a relatively sharp exothermic peak with an initiation temperature (Ti) of ~ 217 °C, representing a sudden radical TOS reaction of PAN, which is not desirable due to safety issues of the process. Unlike the DSC thermogram of PAN under inert atmosphere (see 17

ACCEPTED MANUSCRIPT Figure S6 and Table S3 in Supporting information), the two maximums of PAN at 266 °C and 308 °C in air represented the combined cyclization and oxidation reactions of PAN,

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respectively [42].

Figure 6. DSC thermograms of PAN (co)polymers in air.

b c d e

∆He (J/g) 6,441.2 7,686.6 7,934.9 7,045.2 8,334.7 9,113.9

Initiation (onset) temperature of TOS reaction. Final temperature of TOS reaction.

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a

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Table 2. Parameters of DSC thermograms of PAN (co)polymers in air Designation Tia (°C) Tfb (°C) ∆Tc (°C) Tpkd (°C) PAN 216.6 393.5 176.9 307.8 PAIA 168.0 400.0 232.0 307.1 PAMA 162.9 398.0 235.1 314.6 PAMMA 212.8 395.7 182.9 295.6 PAVA 211.5 388.3 176.8 305.7 PAVIA 174.7 382.7 208.0 308.8

Temperature range of the exothermic peak. Peak temperature of TOS reaction. Heat released during the thermal treatment.

The DSC curve of PAMMA exhibited similar thermal behavior with that of PAN, exhibiting relatively sharp exothermic peaks and a similar Ti value of ~ 213 °C. As observed

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ACCEPTED MANUSCRIPT in Figure 3 and Figure 5a, the results indicated that the TOS of PAMMA also followed a similar radical pathway of PAN due to the neutral nature of the comonomer. The incorporation of IA and MA (PAIA and PAMA) dramatically lowered the Ti values (~ 168 °C and ~ 163 °C for PAIA and PAMA, respectively), supporting additional

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anionic initiation of the TOS procedures at a relatively low temperature in the presence of acidic comonomers under oxidative atmosphere [42]. The anionic initiation of the TOS increased ∆T values of the copolymers, suggesting potentially safer TOS procedures.

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Interestingly, the ∆H values of the copolymers (~ 7,687 J/g and ~ 7,935 J/g for PAIA and PAMA, respectively) were higher than those of PAN (~ 6,441 J/g) and PAMMA (~ 7,045 J/g),

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demonstrating increased TOS efficiencies. The ∆H value represented the degree of TOS reactions including cyclization and oxygen uptake reaction [23,43]. PAVA also exhibited similar Ti (~ 212 °C) and ∆T (~ 177 °C) values to those of PAN and PAMMA, suggesting the absence of anionic initiation procedures at the beginning stage of DSC scanning. However,

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interestingly, PAVA exhibited a much higher ∆H value (~ 8,335 J/g) than those of PAN (~ 6,441 J/g) and PAMMA (~ 7,045 J/g). The ∆H value of PAVA was even higher than those of PAIA (~ 7,687 J/g) and PAMA (~ 7,935 J/g). In addition to the typical radical cyclization

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reactions, the result strongly supported the facilitated TOS procedure of PAVA by in situ

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generated acetic acid moieties during the DSC scanning, affording a high degree of cyclization [28,29] (Figure 4). Similar to PAN, the shape of DSC curve of PAVA was also relatively sharp, still suggesting concentrated heat release during stabilization process. More interestingly, the terpolymer, PAVIA, exhibited the highest ∆H value (~ 9,114

J/g) among the (co)polymers in this study. At the same time, the efficient initiation of TOS was confirmed by the relatively low Ti (~ 175 °C) and high ∆T (~ 208 °C) values of PAVIA (Figure 6). The broad exothermic peak and high ∆H value of PAVIA inferred a high degree of 19

ACCEPTED MANUSCRIPT TOS and a safe TOS process. The high degree of the TOS of PAVIA was also confirmed by FT-IR (Figure 2c and Figure 3). With a similar GCM value (3.24 mol% for PAVIA, Table 1), the highest degree of cyclization reaction (~88% after 120 minutes) was obtained for PAVIA among the (co)polymers. The extent of cyclization reaction exhibited efficient initial anionic

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initiation of the TOS reaction in the presence of acidic IA in PAVIA (Figure 3). The cyclization reaction was further accelerated by the additional generation of acetic acid due to the presence of VA in the copolymer during the TOS process (Figure 4), as also evidenced by

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the accelerated and extraordinarily high concentration of the cyclized ring of PAVIA (1.95 mol/cm3 for PAVIA, Figure 5a). The value was obviously the highest among the PAN

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(co)polymers, demonstrating a very efficient TOS reaction. The concentration of C=O in acridone of PAVIA was also the highest among the PAN (co)polymers (Figure 5b), which may assure a stable inter-chain structure after TOS. However, the use of the most efficient TOS processes for a certain PAN (co)polymers does not necessarily guarantee the best

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performances of resulting carbon materials after carbonization and graphitization processes. A investigation of the effect of TOS on ultimate graphitization reaction is still needed as a separate study.

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Overall, by introducing IA and VA together as comonomers in PAN, an ideal

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combination of the desired precursor characteristics, such as efficient initiation of the TOS at low temperature, safe/controlled heat release, and high ultimate degree of the TOS, was attained. The study afforded useful information to design better precursor to obtain carbon materials with improved quality and yield after subsequent carbonization processes.

4. Conclusion 20

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The main goal of this study was to investigate the effect of different comonomers containing different types of carbonyl functional units to the TOS of PAN (co)polymers. Model PAN copolymers with similar contents of carbonyl groups (~3 mol%) from different comonomers

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were successfully prepared for rational comparison between the samples. The PAN copolymer with a neutral comonomer, PAMMA, exhibited quite similar TOS behavior to that of PAN, indicating typical radical TOS pathways. Efficient initiation of the TOS process was

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observed for PAIA and PAMA due to anionic initiation contribution from the acid comonomers (IA and MA), resulting in a high ultimate degree of the TOS. PAVA exhibited

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stepwise progress of the TOS reaction. The TOS of PAVA exhibited initially slow but accelerated TOS, suggesting in situ generation of acetic acid moieties for additional ionic pathways during the TOS. The terpolymer, PAVIA, with VA and IA, exhibited both accelerated and anionically initiated TOS. DSC thermograms of PAVIA exhibited relatively

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low Ti (~ 175 °C) and high ∆T (~ 208 °C) values, which inferred a safe TOS process. PAVIA also exhibited the highest degree of TOS among the (co)polymers, as evidenced by the high ∆H (~ 9,114 J/g), high extent of cyclization reaction (~88% after 120 minutes), and high

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concentration of cyclized ring (1.95 mol/cm3) values. A combination of the desired precursor

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characteristics, such as efficient initiation of the TOS at low temperature, safe/controlled heat release, and high ultimate degree of the TOS, can thus be attained by understanding on the role of comonomers and rational design of optimum precursor.

Supplementary material Supplementary data associated with this article can be found as a separate file. 21

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Acknowledgments This research was supported by the Basic Science Research Program through the National Foundation

of

Korea

(NRF)

by

the

Ministry

of

Education

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(2015R1D1A1A09061172).

funded

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Research

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Table and Figure captions

Table 1. Preparation and characteristics of PAN (co)polymersa

Figure 1. Chemical structures of PAN (co)polymers.

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Table 2. Parameters of DSC thermograms of PAN (co)polymers in air

Figure 2. FT-IR spectra of PAIA (a), PAVA (b), and PAVIA (c) with different thermal

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treatment time at 220 °C (Downward arrows indicate decreasing peak intensities with thermal treatment time, whereas upward arrows indicate increasing peak intensities).

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Figure 3. Extent of the cyclization reaction of PAN (co)polymers with different comonomers during TOS at 220 °C.

Figure 4. Plausible structural transformation of PAVA during TOS at 220 °C [10]. Figure 5. Concentration of cyclized ring (a) and conjugated carbonyl group in acridone ring

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(b) of PAN (co)polymers with different comonomers during TOS at 220 °C.

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Figure 6. DSC thermograms of PAN (co)polymers in air.

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ACCEPTED MANUSCRIPT Highlights

TOS behaviors of PAN (co)polymers for carbon materials are investigated.

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FT-IR is employed to quantitatively track the structural evolution of the precursors. Itaconic acid (IA) and methacrylic acid facilitate stabilization by ionic initiation. Vinyl acetate (VA) accelerates TOS, indicating in situ generation of acetic acid.

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VA and IA comonomer mixture exhibits most efficient cascade TOS activity.