Detailed Cyclization Pathways Identification of Polyacrylonitrile and

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Detailed Cyclization Pathways Identification of Polyacrylonitrile and poly(acrylonitrile-co-itaconic acid) by in situ FTIR and Two-dimensional Correlation Analysis Zhongyu Fu, Baijun Liu, Yuyao Liu, Bing Li, and Huixuan Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01162 • Publication Date (Web): 28 May 2018 Downloaded from http://pubs.acs.org on May 28, 2018

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Industrial & Engineering Chemistry Research

Detailed

Cyclization

Pathways

Identification

of

Polyacrylonitrile

and

poly(acrylonitrile-co-itaconic acid) by in situ FTIR and Two-dimensional Correlation Analysis Zhongyu Fua, Baijun Liua, Yuyao Liua, Bing Lia, Huixuan Zhanga,b∗ a

Engineering Research Center of Synthetic Resin and Special Fiber, Ministry of

Education, School of Chemical Engineering, Changchun University of Technology, Changchun 130012, China b

Key Laboratory of Polymer Ecomaterials, Changchun Institute of Applied Chemistry,

Chinese Academy of Science, Changchun 130022, China ∗

corresponding author: Huixuan Zhang: [email protected], Tel.: +86 (0) 431

85716465, Fax: +86 (0) 431 85716465 Abstract In situ fourier transform infrared spectroscopy (in situ FTIR) and two-dimensional (2D) correlation analysis were applied to investigate the cyclization pathways of polyacrylonitrile homopolymer (PAN) and poly(acrylonitrile-co-itaconic acid) (PAI) during thermal treatment under argon atmosphere. The cyclization pathways for PAN and PAI at different temperatures were summarized based on the 2D correlation analysis. According to the cyclization pathways, the cyclization of PAN was divided into three stages. The vital role of the ends of original PAN chains, especially the conjugated nitrile, in initiating cyclization by the free radical mechanism was verified. A part of IA in PAI chains first converted into anhydride, and then the formed anhydride rather than the carboxyl group initiated cyclization first. The ionic mechanism mainly initiated cyclization at 220 ºC and below. Both for PAN and PAI, 1

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

the length of cyclic structures from the free radical mechanism was ca. three repeated six-membered rings, but that from the ionic mechanism was ca. eight repeated six-membered rings. Keywords: cyclization pathways, in situ FTIR, two-dimensional correlation analysis, cyclic structures length. 1. Introduction Polyacrylonitrile-based carbon fiber (PAN-based CF) is a magical material that has been interesting both for industry and academia for decades.1-6 This is not only because of its excellent mechanical and chemical properties, high strength and modulus, low density, very good creep resistance and chemistry resistance, but also because of its abundant raw material, relatively low manufacture cost and high industrial productivity.7-13 During the fabrication of PAN-based CF, a series of thermal treatments are applied to convert the linear PAN chains into the turbostratic graphite-like structures.5, 14-20 Among these thermal treatments, the thermal oxidative stabilization (TOS) process is commonly conducted under air at 200-300 ºC. The linear PAN chains are converted into thermally stable and oxygen containing six-membered rings during the TOS process so that the high temperature carbonization can be carried out fluently.1,

3, 6, 21-23

It has been accepted by most

researchers that the formed six-membered rings are diverse and complex and strongly depend on the composition of PAN polymers and the thermal treatment conditions, such as temperature and atmosphere.23-29 In the past decades, numerous efforts were made to ascertain the mechanism of cyclization and the formed structures of thermal

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treated PAN polymers by various characterization techniques such as fourier transform infrared spectroscopy (FTIR),24-26,

28, 30

nuclear magnetic resonance

spectrum (NMR),31-34 pyrolysis-gas chromatography (PyGC),35 differential scanning calorimetry (DSC),24, 25, 27, 30, 36, 37 electron paramagnetic resonance (EPR),38 elemental analysis (EA)39. The free radial mechanism for the cyclization of PAN homopolymer and the ionic mechanism for the cyclization of vinyl acid modified PAN polymers are accepted for most researchers today. Recently, Wang et al.34 and Miyoshi et al.31 investigated the scheme of atactic PAN (a-PAN) stabilized under an argon condition and both nitrogen and air conditions by several solid-state NMR (ss-NMR) techniques, respectively. Wang et al.34 suggested that the atactic conformation leads to the formation of isolated ring during thermal treatment under argon atmosphere. And Miyoshi et al.32 indicated that the presence of oxygen leads to the formation of polyene structure, namely conjugated nitrile group, through dehydrogenation reaction at first, and subsequently this polyene structure promotes the formation of successive aromatic rings rather than the isolated ring. These two works highlight the role of oxidative dehydrogenation of linear PAN chains and the resulted unsaturated nitrile groups in the formation of successive six-membered rings.31, 34 However, to overcome the shortages of PAN homopolymer, such as poor solubility and sudden heat release, during the fabrication of PAN precursor fibers and the TOS processes, vinyl acids, such as itaconic acid (IA), are commonly used to copolymerize with AN.25-28, 30, 36, 37 The presence of IA makes the cyclization pathways more complicated because a part of cyclization is initiated by the ionic mechanism. Moreover, most current

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investigations were non-online, which results in that the accuracy strongly depends on the experiment skills, and some important information may be inevitably lost, especially the detailed information of the initiation of cyclization in the early stage. So, the proposed reaction pathways of cyclization are still not satisfied.21, 31, 34 Moreover, it should be pointed out that some saturated and unsaturated ends of PAN chains are generated during the polymerization of AN. However, the roles of these ends of PAN chains in cyclization have been rarely investigated. Generalized two-dimensional (2D) correlation spectroscopy was first proposed by Noda40 in 1993 and has been proven to be a powerful and convenient analytical technique in investigating the glass transition, the reaction mechanism, the crystallization behaviors and the other phase-transition of polymers.41-47 By applying the 2D correlation spectroscopy, some important information, such as the vibration change sequence of special groups, which cannot be readily obtained in conventional 1D FTIR spectra, can be easily captured due to the significantly enhanced spectral resolution. However, the attempts are rarely made to apply 2D correlation spectroscopy in studying cyclization mechanism of PAN and related polymers. Only Zhou et al.48 investigated the TOS reactions of poly(acrylonitrile-co-methyl acrylate) under a dynamic heating process in air condition by using 2D correlation spectroscopy. And they indicated that methyl acrylate introduced cyclization at first, and then a series of TOS reactions occurred subsequently during the dynamic heating process.48 However, the presence of oxygen makes the TOS reactions too complicated to analyze comprehensively. So, the further investigations should be carried out to investigate the

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cyclization reaction pathways more comprehensively by the in situ FTIR and 2D correlation spectroscopy. Moreover, the FTIR spectrum of the thermal treated PAN polymers generally possesses overlapping and broad peaks due to the complicated structures, which makes the quantitative analysis of the formed structures in the thermal treated PAN polymers very difficult. By applying the combination of second-derivative and curve-fitting techniques to FTIR, these problems are overcome.24, 26, 37, 49 In the present work, two representative PAN polymers, PAN homopolymer and p(acrylonitrile-co-itaconic acid) (PAI) containing ca. 0.67 mol% IA, were prepared via traditional free radical polymerization. To obtain fine information of structural evolution of these two PAN polymers during thermal treatment, in situ FTIR and DSC experiments were conducted under inert atmosphere with a scheduled temperature procedure. 2D correlation analysis was adopted to investigate the cyclization pathways of PAN and PAI at different temperatures in detail. Moreover, the length of the formed cyclic structures was calculated by combination of the DSC and the FTIR data. 2 Experimental 2.1 Materials Acrylonitrile (AN) was supplied by Jilin Petrochemical Company, Jilin, China, distilled under reduced pressure and restored in a freezer at -18 ºC prior to its use. Itaconic acid (IA >99%) and ammonium persulfate (APS, >98%) were purchased

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from Sinopharm Chemical Reagent Co., Ltd., Shanghai, China and used as received. Deionized water was prepared by our own lab and used throughout the experiments. 2.2 Preparation of PAN and PAI PAN homopolymer and PAI copolymer containing about 0.67 mol% IA were prepared by a free radical polymerization in water using APS as initiator at 60 ºC for 6 h. Then the reaction mixture was added to excessive deionized water with vigorous agitation, and then the mixture was washed again with deionized water for several times to remove the unreacted monomers. At last, the polymer was dried at 60 ºC under vacuum to a constant weight. 2.3 Characterizations of PAN and PAI 1

H NMR and

13

C NMR sample was dissolved in deuterated dimethyl sulfoxide

(DMSO-d6) (5%, w/v polymer solution), and the measurement was performed at room temperature in a 5 mm o.d. NMR tube using Bruker AVANCE Ⅲ 400 Spectrometer operating at 400 MHz and 100 MHz, respectively (See NMR spectra in supporting information, Figure S1). X-ray diffraction (XRD) pattern of PAN and PAI polymers were obtained using an X-ray diffractometer (Rikagu, SmartLab, CuKα, 0.154056 nm, 9 kV, 200mA) through a 2θ range of 5−90° at a scan speed of 2°/min (See XRD spectra in supporting information, Figure S2). The molecular weights and molecular weight distributions of the PAN and PAI polymers were determined using gel permeation chromatography (GPC, Viscotek GPCmax, Malvern) equipped with refractive

index

detector,

light-scattering

detector

and

viscometer.

N,

N′-dimethylformamide was used as an eluent at a flow rate of 0.7 mL/min at 45 °C

6


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filtered using 0.22 µm syringe filter prior to analysis. Linear polystyrene standards were used for calibration. The glass transition of PAN and PAI polymers was determined by using a Mettler DSC-1 thermal analyzer under nitrogen condition at (50 mL/min) 40 ºC/min in the temperature range from 25 ºC to 150 ºC. For all DSC experiments, 3~5 mg fine powder was used. The characteristics of PAN and PAI were summarized in Table S1. Before isothermal DSC and in situ diffuse reflection FTIR measurements, PAN and PAI polymer powders were grinded in an agate mortar, and the powders were sieved by using a 200 mesh sieve. TGA experiments were carried out by using a PerkinElmer TGA Pyris-1 thermal analyzer in the temperature range from 50 to 350 ºC under nitrogen (40 mL/min) at 10 ºC/min. A temperature procedure was used for isothermal DSC and in situ FTIR measurements and was shown in Figure 1. Isothermal DSC measurements were also carried out by using a Mettler DSC-1 thermal analyzer under nitrogen condition (50 mL/min). For a typical isothermal DSC experiment, the following procedure was adopted. At first, the 3~5 mg fine powder was placed in an aluminum crucible with a perforated cover. Then, this aluminum crucible was placed in DSC at room temperature for at least 10 minutes to remove the air in the chamber. The start temperature was set at 180 °C, and then the sample was heated to this set temperature within several seconds. For each measurement temperature, the sample was kept for 31 minutes. The Origin 8.0 software was used to analyze the DSC data. The DSC experiments were carried out under nitrogen condition, and no oxidation reactions occurred, so the DSC exotherms of PAN and PAI were attributed to the

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cyclization reactions only. The extent of cyclization of these two samples was calculated based on the heat release of the exothermic peak using the equation50,

extent of cyclizatio n = ∆H ∆H total

(1)

Where ∆H is the total heat release of the exothermic peaks between 200 °C to a certain temperature from 220 °C to 260 °C, and ∆Htotal is the total heat release of the exothermic peaks from 200 °C to 260 °C. In situ diffuse reflection FTIR measurements were carried out in the range of 4000-1000 cm-1 by using Nicolet iS-50 FTIR spectrometer equipped with calcium fluoride window, mercury-cadmium-telluride detector and temperature controller. At first, the fine powder sample was placed in an alumina crucible, and then the fine powder was pressed. Weights of in situ FTIR samples were almost the same. Before each in situ FTIR measurements, argon flow was given at least for 15 minutes to remove air. All the in situ FTIR experiments were carried out under a constant argon flow. All FTIR spectra were gathered by 16 scans with a resolution of 4 cm-1 to obtain a good signal-to-noise ratio. For each temperature, the FTIR spectra were collected at the time intervals such as 0, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23 25, 27, 29, 31 minutes (See the FTIR spectra of PAN and PAI in Figure S3).

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Figure 1 The temperature procedure for DSC and FTIR measurements. 2.4 2D correlation analysis 2D correlation analysis in the FTIR range of 2300-2100 cm-1 and 1900-1500 cm-1 were performed with the software 2DCS( Tao Zhou, Sichuan University, China). In the 2D correlation maps, the red-colored regions are defined as the positive correlation intensities, whereas the blue-colored ones are regarded as the negative correlation intensities. To acquire a credible result, the baseline corrections were performed in the regions of 2300-2100 cm-1 and 1900-1500 cm-1 by OMNIC software before 2D correlation analysis. 3. Results and discussion 3.1 DSC measurement of PAN and PAI Figure 2 presents the DSC curves at different temperatures of PAN (Figure 2a) and PAI (Figure 2b). For PAN, the exothermic peaks below 220 °C could be neglected, which demonstrated that there was almost no cyclization occurred. But cyclization initiated by the free radical mechanism occurred sharply and was exhausted mostly as temperature once reached 240 °C. This behavior resulted in the uncontrollable and violent heat release. For PAI, the exothermic behaviors became gentler for all 9



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temperatures. This gentle exothermic behavior of PAI demonstrated that the presence of IA made the occurrence of cyclization more uniform for each temperature. As shown in Figure 3, about 80 % of cyclization for PAN occurred at 240 °C, while cyclization for PAI occurred relatively equably at each temperature.

(a)

(b)

Figure 2 DSC curves of PAN and PAI under nitrogen, (a): PAN, (b): PAI.

Figure 3 Extent of cyclization calculated from DSC curves. 3.2 In situ FTIR measurement of PAN and PAI

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(a)

(b)

Figure 4 In situ FTIR spectra in the range of 2280-2150 cm-1 of thermal treated PAN and PAI, (a): PAN, (b): PAI. (the upward red arrows indicate the increased peak intensity as the thermal treatment temperature and time increased, and the downward blue arrows indicate the decreased peak intensity as the thermal treatment temperature and time increased). Figure 4 presents the in situ FTIR of PAN (Figure 4a) and PAI (Figure 4b) in the range of 2280-2150 cm-1. There are mainly four visible characteristic vibration peaks in the FTIR spectra, which correspond to four kinds of nitrile groups. The peak at ca. 2240 cm-1 corresponds to the vibration of nitrile group in the linear PAN chains.25-27, 48, 51

The peaks at ca. 2211 cm-1 and 2188 cm-1 correspond to the vibration of conjugated

nitrile group and β-amino nitrile group which originated from the disproportionation termination reaction of the free radical polymerization.52 As for the new peak at ca. 2198 cm-1, it is attributed to the β-amino nitrile group due to the termination of cyclization initiated by the free radical mechanism.53 As shown in Figure 4, the intensities of these four peaks at 2240 cm-1, 2211 cm-1, 2198 cm-1 and 2188 cm-1 had very slight change for PAN at 200 °C. While for PAI, the intensities of peaks at 2240

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cm-1, 2211 cm-1 and 2188 cm-1 began to decrease observably at temperature of 200 °C. This difference of initiation temperature of cyclization reaction is attributed to that the presence of IA initiated cyclization through the ionic mechanism at a lower temperature. Moreover, it was found that the nitrile groups at the ends of the PAN chains (2210 cm-1 and 2188 cm-1) also participated into cyclization during the thermal treatment. It should be noticed that the intensity of 2198 cm-1, both for PAN and PAI, just increased significantly as the temperature reached 250 °C and above. Taking the extent of cyclization into account, it was inferred that the termination of cyclization became dominant only in the later stage of thermal treatment.

(a)

(b)

Figure 5 Second derivative spectra in the range of 2280-2150 cm-1 of thermal treated PAN and PAI. (a): PAN, (b): PAI. Because of nitrile groups of PAN possess a high dipole moment (3.9 D), the nitrile groups tend to interact with each other or to form hydrogen bond with the nitrogen atom in PAN chains.54, 55 Second derivative of FTIR spectrum was used to find the positions and the number of nitrile groups, and the corresponding results were shown in Figure 5. The main nitrile band is composed of three peaks, namely ca. 2243 cm-1,

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ca. 2240 cm-1 and ca. 2236 cm-1, which correspond to the stretching vibration of free nitrile, hydrogen-bonding associated nitrile and anti-parallel associated nitrile groups, respectively.54, 55 With the increase of thermal treatment temperature and time, the peak at ca. 2243 cm-1 first became weakened even disappeared. This finding indicated that the nitrile groups in different associations may possess inequable cyclization ability, especially at low temperature.

Figure 6 In situ FTIR spectra in the range of 1900-1500 cm-1 of thermal treated PAI. (the up red arrow indicates the increased peak intensity as the thermal treatment temperature and time increased, and the down blue arrow indicates the decreased peak intensity as the thermal treatment temperature and time increased). Figure 6 presents the in situ FTIR spectra of PAI in the range of 1900-1500 cm-1 at different temperatures. It was found that the IA converted into anhydride (1860 cm-1 and 1780 cm-1)56 rather than initiated cyclization directly at 180 °C. It should be pointed out that there may be extremely small amount of cyclization occurred at 180 °C because the peaks of the C=N, C=C (1584 cm-1 and 1620 cm-1)

23-27, 48, 51

seems increased. But by careful analysis of FTIR in the range of 1640-1560 cm-1, the absolute intensity increase of the peaks of the C=N, C=C is very small. Moreover, the

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intensity of nitrile group at 2240 cm-1 possessed undetected change. So the cyclization occurred at 180 °C is too slight to be observed. As the temperature up to 200 °C and above, the intensities of peaks corresponding to anhydride and carboxylic acid (1735 cm-1)

23-27, 48, 51

decreased observably. At the same time, the carbonyl group in dimer

(1705 cm-1) and the C=N, C=C (1584 cm-1 and 1620 cm-1)

23-27, 48, 51

occurred

observably and increased gradually with the increase of thermal treatment temperature and time. This finding verified that IA can initiate cyclization at a relative low temperature through the ionic mechanism. The intensities of peaks corresponding to anhydride (1860 cm-1 and 1780 cm-1) and carboxylic acid (1735 cm-1) hardly distinguished after the thermal treatment in the range of 200-220 °C. This finding indicated that the cyclization was mainly initiated by the ionic mechanism in the range of 200-220 °C. At 230 °C and above, the cyclization may be initiated mainly even fully by the free radical mechanism, and the gradually occurred peak at 2198 cm-1 also verified this assumption. Based on the discussions above, the positions and corresponding vibration modes of main FTIR peaks of PAN and PAI were summarized and listed in Table S2. 3.3 2D correlation FTIR analyses of thermal treated PAN and PAI

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Synchronous

Asynchronous

Figure 7 2D correlation FTIR spectra calculated from the in situ FTIR spectra of the PAN thermal treated at 240 °C. Figure 7 presents the typical synchronous and asynchronous correlation FTIR spectra calculated from the in situ FTIR spectra of the thermal treated PAN, respectively (See the other spectra in supporting information, Figure S4). During the analysis of 2D correlation FTIR spectra, the Noda’s rule is often used to capture the sequence order of the spectral intensity change of functional groups. The summarization of Noda’s rule is as follows:40 1. If Φ(v1, v2)>0, Ψ(v1, v2)>0 or Φ(v1, v2)

Detailed Cyclization Pathways Identification of Polyacrylonitrile and

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