Pyrolysis and Thermal Stability of Carbon Fiber Polymer Precursors

Chapter 8

Downloaded by MONASH UNIV on November 25, 2014 | http://pubs.acs.org Publication Date (Web): October 14, 2014 | doi: 10.1021/bk-2014-1173.ch008

Pyrolysis and Thermal Stability of Carbon Fiber Polymer Precursors with Different Microstructures G. Santhana Krishnan* Materials Science Division, CSIR-National Aerospace Laboratories, Bangalore-560017, India *E-mail:[email protected]

Acrylonitrile based polymers have emerged as one of the preferred precursors for the production of high tensile carbon fibers. The salient feature of the poly(acrylonitrile) is that it is not stable upto its melting point and tends to decompose prior to melting with sudden release of heat and gas. Therefore it is important to control this thermal degradation and exotherm which is determinant in tensile characteristics of resulting carbon fibers. In this account, the influence of triad tacticity of poly(acrylonitrile) microstructure on the structural changes during the thermal oxidative degradation reactions was investigated applying hyphenated thermal techniques namely Pyrolysis-Gas Chromatography-Mass spectrometry, Evolved gas analysis-Mass Spectrometry, and thermal gravimetric analyzer-FT infrared spectrometry(TGA-FTIR) in the temperature range 200-600°C. The thermal analysis results revealed that nitrile polymerization of pendant cyano functionalities precedes cyclization reaction in case of isotactic rich poly(acrylonitrile) leading to a steady and stable thermal oxidative degradation reaction as compared to atactic rich poly(acrylonitrile). The mass loss accompanying the thermal reactions was accounted for the evolution of hydrogen cyanide, ammonia, and homologs of alkyl nitrile having molar masses between 47 and 147 m/z.The simultaneous TGA-FTIR results of evolved gas analysis also confirm the pyrolysis experimental data.

© 2014 American Chemical Society In Polymer Precursor-Derived Carbon; Hoffman, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Introduction Poly(acrylonitrile) (PAN) based carbon fibers are recognized as a critical part of modern aviation and aerospace applications. They constitute primary, secondary structures of commercial aircrafts, the international space station, satellites, rocket motor casings and expendable launch vehicles. Because of their high specific tensile properties, compressive strength, and increased resistance to corrosion under saline conditions, their use is being extended to other civil applications such as compressed natural gas tanks, anti-seismic reinforcing members of buildings and tethers in oil field rigs apart from traditional domains (1–5). These high tensile carbon fibers (Tensile strength higher than 4.0 GPa) enable the fabrication of light weight, durable, and cryogenic high pressure vessels for storage of compressed hydrogen gases in space launch vehicles (6–8). This kind of trend necessitate further understanding and continuous improvements to the physical characteristics of carbon fiber polymer precursors. The industrial production of high tensile grade carbon fiber includes polymer synthesis, fiber spinning, thermal oxidative stabilization, and carbonization (9). The tensile properties of carbon fiber are highly sensitive to the internal and surface flaws such as voids and defects (10). Hiramatsu Touru studied the dependence of tensile strength of carbon fiber on the sizes of defects and voids formed during their production and reported that the surface defect sizes vary from 2.5µ to less than 0.5µ as tensile strength of carbon fiber filament increases to a value higher than 4.0 GPa (11). Therefore the control of formation of defects and voids becomes significant during the manufacture of high tensile grade carbon fibers. One of the critical stages in the carbon fiber fabrication process is the thermal oxidative stabilization, which involves controlled heat treatment of carbon fiber precursor fiber made of PAN in air at temperatures in the range 200 to 300 °C. Further in the carbonization process, the stabilized fibers are further heated to the temperature up to 3000°C. There exists a trade-off relationship among the density of the oxidized PAN fibers and the subsequent heat treatment temperature and tensile characteristics (12, 13). Figure 1a and b shows the variation of density of PAN fiber precursor during the stabilization process and its influence on the change of limiting oxygen index (14). The high density oxidized PAN fiber leads to a defect free aromatic structure which facilitates the improvement of tensile characteristics in the carbon fibers. The physical characteristics of high tensile carbon fibers with tensile strength above 6000 MPa, is known to depend critically on the surface and internal defects of fiber precursors (14, 15). The carbon fiber failure follows a brittle failure mechanism as predicted by the Griffith model. This model relates the surface free energy of the fracture surface formed on the surface of carbon fiber filament to its breaking strength (16). The Griffith equation is represented as in eq 1.

Here, “σ” is the breaking strength; “ε”, the ultrasonic elastic modulus of the carbon fiber filament; and the ‘C’ is the size of the flaw or defect. 170 In Polymer Precursor-Derived Carbon; Hoffman, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 1. (a) Variation of PAN fiber density with thermal stabilization time, (b) Limiting Oxygen index of oxidized PAN fibers plotted against oxidized PAN fiber density. Source: Drawn from unpublished experimental data. In general, every single filament of a carbon fiber strand has uneven surface structure with a length greater than 0.7 μm. These kind of surface flaws have an assembly of several fibrils extending along the longitudinal axis of the fiber tow known as wrinkle structures. One of the several strategies of the strength improvement mechanisms in the carbon fiber manufacture is to prevent the formation of surface flaws, both internal as well as external. If the sizes of these flaws or defects are on the order of 0.6 μm or less, the tensile strength of the carbon fiber tow is more likely to be improved. The other commonly adopted method is to develop a highly dense structure of the oxidized PAN fiber with a density to a level in excess of 1.35 gcm-3, wherein the defect point formation is suppressed. The thermal oxidative stabilization of PAN fiber precursor before precarbonization is carried out to endure the high temperature treatment. The oxidative stabilization process offers many scopes for the optimization of the quality of resulting carbon fibers in terms of tensile characteristics. This process is usually accompanied by the release of gaseous molecules such as ammonia, hydrogen cyanide, and oxides of carbon in substantial quantities, leading to the formation of large sized surface defects, thereby reducing the tensile strength of the subsequently obtained carbon fibers. Erich Fitzer observed that tensile properties of carbon fibers are heavily influenced by the release of gases during the heat treatment processes up to 1800°C (17). Various techniques of thermal analysis, such as differential thermal analysis (DTA), thermogravimetry (TG) and differential scanning calorimetry (DSC) have become more and more conventionally used for thermal characterization studies of fiber forming high polymers. However, these techniques do not provide direct information on the qualitative and quantitative changes accompanying thermal degradation or cyclization reactions as the method lacks the capability of structural identification. Many research articles discussed the thermal degradation behavior and stability of acrylonitrile based copolymers with different monomers 171 In Polymer Precursor-Derived Carbon; Hoffman, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


(18–25). Grassie and McGuachan investigated the thermal degradation of PAN giving particular attention to the influence of environment, additives and comonomers on the overall process using conventional thermal techniques (26). This chapter describes the beneficial influence of microstructure, in terms of the triad tacticity content of cyano functionalities of poly(acrylonitrile) on their thermal degradation behavior.The detailed thermal investigations were performed applying hyphenated thermal techniques such as Pyrolysis-Gas chromatography-Mass spectrometry (Py-GC-MS), Evolved gas analysis-Mass spectrometry (EGA-MS), and TGA-FTIR, in addition to the confirmation of stereochemical configuration of PAN polymers.

Synthesis of Poly(acrylonitrile) with Different Microstructures Materials Acrylonitrile, (AN) (Sigma Aldrich), was used as monomer in the polymer synthesis. Acrylonitrile was washed with a sodium hydroxide solution of concentration 0.5 wt. % and distilled at its boiling range just before polymerization to remove the inhibitor content if any. The hexagonal crystalline metal salts (Nickel chloride or Magnesium chloride) (Sigma Aldrich) was used as a template compound in the solid state polymerization techniques. Commercially available α, α azoisobutryonitrile (AIBN) (Wako Co.) was used as initiator after crystallization in methanol. Analytical grade dimethylformamide, dimethyl sulphoxide and dimethyl acetamide (Sigma Aldrich) were used for the measurements of polymer solution properties.

Synthesis of Isotactic Poly(acrylonitrile) (I-PAN) and Atactic Poly(acrylonitrile) (A-PAN) Polymerization experiments were carried out in a 500 mL three-necked round bottom flask, equipped with mechanical stirrer and a reflux condenser. The flask was bubbled with ultra-pure N2 gas for 30 min at a flow rate of 30 mL/min. to ensure an oxygen-free atmosphere. The polymerization procedures as described in references were followed for the synthesis of I-PAN and A-PAN (27, 28). The resulting polymer was filtered and washed with methanol and dried at 60°C under vacuum to a constant mass.

Molecular Characterizations Average Molecular Weight Determinations Solution Capillary Viscometry Solution viscometry is one of the simplest and elegant methods of molecular weight determination techniques. It also provides considerable insight on physical 172 In Polymer Precursor-Derived Carbon; Hoffman, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


characteristics of polymer molecules. Sufficient amount of PAN sample was dried in an oven at 75°C for 3h and dissolved in dimethyl sulphoxide at 50 °C. The flow time of the obtained solution between gauge marks is measured at an accuracy of 1/100 seconds using Ubbelohde viscometer at a temperature of 25°C. The measured dropping time is expressed as t (sec). Similarly the dropping time of dimethyl sulphoxide (DMSO) is expressed as t0. Intrinsic viscosity (η) was obtained according to the following eq (2) using the data in the concentration range from 0.1 to 0.5g/dl (29).

Where ηsp = t/t0 −1. The viscosity average molecular weight of PAC was calculated by eq (3) (30).

Membrane Osmometry Membrane osmometry is one of the established primary methods for the determination of number average molecular weight of polymer molecules. Osmotic pressure (Π) measurements of polymer solutions were carried out in the range of polymer concentration (C) from 0.3 to 0.9 g/dl in DMSO at 60°C with Osmomat 090, Gonotec GmbH. Asymmetric two-layer cellulose triacetate membranes, with a diameter of 40 mm and a molecular weight cut off (MWCO) of 5,000 or 20,000 g/mol (type SM 14549, Sartorius GmbH, Gottingen, Germany), were used. Number average molecular weight (Mn) was obtained as Π/C extrapolated to C = 0 according to eq (4).

Where R is gas constant.

Size Exclusion Chromatography (SEC) The chromatography parameters (average molecular weights, molecular weight distribution (Mw/Mn)) were determined by SEC-TDA max instrument of Malvern Viscotek TDA 305 (Triple Detector Array) equipped with I-MBHMW-3078 column, a differential refractometer, a precision low angle light scattering detector, and a viscometer. The molecular weight parameters were computed using SEC data processing system (OmniSEC software). The carrier solvent was DMF containing 0.05 M lithium bromide with a flow a rate of 1 mL/min and injection volume of 100μL at 50 °C. A concentration of ~4mg/ml was 173 In Polymer Precursor-Derived Carbon; Hoffman, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

maintained in all samples. Before the injection, samples were filtered through a PTFE membrane with 0.2µm pore. A set of polymethyl methacrylate standards of narrow molecular weight distribution (PolyCALTM, Viscotek, US) of molecular weight 2.0 x 104 - 4.51 x 105 g/mol was used to used generate conventional calibration curve.


13C

Nuclear Magnetic Resonance Spectroscopy (13C NMR)

The 13C NMR (Bruker AMX-400) spectra of the polymers were recorded in deuterated DMSO using tetramethylsilane (TMS) as an internal standard. Samples were concentrated in dimethyl sulphoxide about 5% (w/w) for 13C NMR by using a 5 mm NMR tube at room temperature. 13C NMR spectra were acquired using 24996 data points, spectral width 22 kHz, broadening 3 Hz, pulse delay 2s, pulse width 90°, and 1024 scans. Nuclear overhauser effect (NOE) was suppressed by gating the decouple sequence. Heteronuclear multiple quantum coherence (HMQC) was performed by using the standard Bruker pulse sequence with a pulse program. The spectrum was obtained with 256 increments in the F1 dimension and 1024 data points in the F2 dimension, with 200 scans and relaxation delay 1.5.

Thermal Characterizations Pyrolysis-Gas Chromatography-Mass Spectrometry The configuration of Py–GC/MS as developed by Chuichi Watanabe et.al. (31), was employed for the evaluation of thermal -oxidative degradation of the polymer samples under investigation. A small amount of powdery sample was taken in a deactivated stainless steel sample cup and then mounted in to the pyrolyzer. The multifunctional pyrolyzer EGA/PY-3030D model (Frontier Laboratories, Japan) which is capable of heating a sample from near room temperature to 800 °C. In single shot pyrolysis, the column was first hold at 40 °C for 2 min. and heated at 20 °C /min. to 320 °C and hold there for 14 min. The pyrolysis temperature was 700 °C at a heating rate 20°C /min. Evolved gases released during the heat-up was directly separated and analyzed by the GC/MS (GC-MS-QP2010, GC-MS ITF temp.280°C, m/z: 27-600 IS temp.: 200). The remaining residue in the sample cup was analyzed on the same system by EGA-MS single shot analysis. In the GC-MS system, the evolved gas mixture was continuously introduced to the sample loops and separated in to the constituents by the chromatographic column. The carrier gas is removed by the separator and only the components to be determined are introduced directly in to the mass spectrometer to obtain mass spectrum. The pattern of thermal degradation and the structural changes accompanying degradation were estimated from the obtained thermogram and pyrograms.

174 In Polymer Precursor-Derived Carbon; Hoffman, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.

TGA-Fourier Transform Infrared Spectrometry The analysis of evolved gases from polymer samples was carried out using a hyphenated system of the TGA 4000/ Pyris 6 (Perkin Elmer) coupled to a frontier FT IR spectrometer. The typical analysis conditions are as follows: wave number, 400-4000 cm-1, resolution, 2 cm-1; purging gas, N2 (flow rate, 20 mL/min). Heating rate, 20 °C, Temperature range, 40 to 650 °C. Specimen mass 12.00 mg.


Molecular Characteristics and Microstructural Elucidation The reaction scheme as in Figure 2 shows the synthetic routes followed for the preparation of PAN polymers with predominantly isotactic and atactic stereo-structures. Different types of molecular weight averages such as viscosity average, number average and weight average molecular weights were estimated with solution viscometry, membrane osmometry and SEC-TDAmax system.

Figure 2. Synthesis pathways for poly(acrylonitrile) with different microstructures.

Figure 3 shows the elution curves generated using SEC triple detector system. The retention volumes observed for the PAN polymer samples ranges from 7.6 to 8.1 mL, the different molecular weight averages of the I-PAN and A-PAN under study is given in Table I. The number and weight average molecular weights of the PAN samples are 10.2-14.3 x104 and 1.61-2.25x105, respectively. 175 In Polymer Precursor-Derived Carbon; Hoffman, et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2014.


Figure 3. The SEC triple detector elution curves for PAN samples.

Table I. Different molecular weight averages of I-PAN and A-PAN under study Mva x 10-5 (gmol-1)

Mnb x 10-4 (gmol-1)

Mnc x 10-4 (gmol-1)

Mwc x 10-5 (gmol-1)

I-PAN/01

2.21

10.2

9.8

2.25

I-PAN/02

2.06

14.3

12.3

2.13

I-PAN/03

1.76

11.2

10.7

1.83

A-PAN/01

1.86

12.7

11.1

1.92

A-PAN/02

1.69

13.6

12.8

1.72

A-PAN/03

1.57

13.9

12.4

1.61

Sample ID

a = Solution viscometry, b = membrane Osmometry, c = SEC I-PAN = Isotactic Poly(acrylonitrile), A-PAN = Atcatic poly(acrylonitrile) samples.


The stereoregularities in terms of triad tacticity contents were estimated using spectroscopic techniques. 13C-NMR spectra of I-PAN and A-PAN are shown in Figure 4a and b. Triad tacticity contents were estimated from the intensities of methine carbon functionality observed at a chemical shift value of 26.50-28.00 ppm. Only methine carbon regions are expanded in Figure 5a and b (bottom). Stereochemical features of vinyl polymer systems are often calculated interms of traid tacticity (32, 33). The measurement of tacticity in polymers is an area characterization. The areas corresponding to three bands,viz., syndiotactic A(rr), atactic A(mr), and isotactic A(mm) are estimated. The number fraction of each type of triad (mm),(mr), and(rr) is then obtained by dividing the area of each individual band by total area A(T). The triad tacticity contents of I-PAN and A-PAN used in this study are presented in Table II. The I-PAN synthesized was found to have a isotactic content of maximum 51.4 wt. % whereas atactic triad tacticity of A-PAN was 48.5 wt.%.


13C-NMR

Table II. Triad tacticity contents of I-PAN and A-PAN Sample Code

Chemical Shift (ppm) mm

mr

Triad tacticity (%) rr

I

A

S

4IS/A2

I-PAN/01

26.77

27.39

27.86 51.4

33.9

14.7

2.629

I-PAN/02

26.76

27.39

27.85 50.1

35.6

14.3

2.261

I-PAN/03

26.75

27.37

27.90 48.7

36.2

15.1

2.214

A-PAN/01

26.76

27.36

27.86 26.4

48.5

25.1

1.121

A-PAN/02

26.76

27.37

27.84 28.1

47.0

24.9

1.245

A-PAN/03

26.74

27.39

27.88 29.7

46.2

24.1

1.232



Figure 4. C-NMR spectra of (a) I-PAN (above) and (b) A-PAN (below).



Pyrolysis and Thermal Stability The EGA-MS curves of total ions of the A-PAN (bottom), and I-PAN (top) are shown in Figure 5. The A-PAN (bottom) curve displays two peaks with maxima 280 and 440 °C, respectively. There exists two humps at 140 and 360 °C, thereby clearly forming four distinct regions of thermal degradation. From the results of EGA-MS, the pyrolysis temperatures of A-PAN in Pyrolysis–GC/MS can be set at 150,180,260,280,300,320,380,400,450,and 500 °C. However, the EGA thermogram of the I-PAN clearly shows a peak at 320 °C and a shoulders at 380 and 420 °C. Table III summarizes the results of identification of the peaks corresponding to the specific mass to charge ratios in the mass spectrum. Most gaseous species evolved are identified as ammonia, hydrogen cyanide and alkyl nitrile compounds.

Table III. Identification of pyrolysates from Py-GC-MS m/z value

Molecular formula

Ionic abundance (%) A-PAN

I-PAN

28

HCN

70

40

41

CH3CN,C3H5,C2H3N

75

50

54

(C2H3CN)H

70

45

80

(C2H4CN)C2H3

10

30

107

(C2H3CN)2H

45

25

121

(C2H3CN)2CH3

25

20

Figures 6 and 7 show the averaged mass spectra of the evolved gases corresponding to the four different regions i.e., α, β, δ, θ, in the A-PAN and I-PAN. In general, the molecular ion peaks of straight-chain alkyl nitriles are either weak or absent except acetonitrile. The base peak of linear nitriles between C4 and C9 is m/z 41. This peak arises as a result of hydrogen rearrangement in a six membered transition state. The thermal degradation mechanism as in reaction scheme (Scheme 1) accounts for formation of some of the diagnosable ionic fractions in the mass spectrum. The relative ionic abundance of evolved gases generated during thermal degradation are higher (>65%) for A-PAN as compared to that of I-PAN (

Pyrolysis and Thermal Stability of Carbon Fiber Polymer Precursors

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