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Stabilization study of polyacrylonitrile/cellulose nanocrystals composite fibers Huibin Chang, Jeffrey Luo, H. Clive Liu, Songlin Zhang, Jin Gyu Park, Zhiyong Liang, and Satish Kumar ACS Appl. Polym. Mater., Just Accepted Manuscript • DOI: 10.1021/acsapm.9b00057 • Publication Date (Web): 12 Apr 2019 Downloaded from http://pubs.acs.org on April 13, 2019

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ACS Applied Polymer Materials

Stabilization study of polyacrylonitrile/cellulose nanocrystals composite fibers Huibin Chang a, b, Jeffrey Luo a, b, H. Clive Liu a, b, Songlin Zhang c, Jin Gyu Park c, Zhiyong Liang c, and Satish Kumar a, b, * a

b

School of Materials Science and Engineering, Georgia Institute of Technology, 801 Ferst Dr. NW, Atlanta, GA, 30332, USA

Renewable Bioproducts Institute, Georgia Institute of Technology, 500 10th Street NW, Atlanta, GA, 30332, USA c High-Performance

* Corresponding

Materials Institute, Florida State University, 2005 Levy Ave., Tallahassee, FL 32310, USA

author:

Email: [email protected]

Abstract The stabilization process is an important step in converting polyacrylonitrile (PAN) to good mechanical property carbon fiber. Currently, sustainable materials such as cellulose nanocrystals (CNCs) are gaining attention for conversion to carbon fibers. In this work, effect of CNCs on the structure, mechanical and thermal properties of stabilized PAN is systematically studied. In the stabilized PAN/CNC fibers, individual stabilized CNCs were found distributed in stabilized PAN, as observed by elemental composition differences in the two regions. When both the precursor fibers were processed at a draw ratio of 10x, then the stabilized PAN/CNC fibers exhibited relatively higher orientation of ladder structure as compared to the stabilized PAN fibers. It was observed that the addition of 40 wt% CNCs into PAN fibers lowers the activation energy of cyclization and crosslinking but does not change the activation energy of oxidation.

Keywords: polyacrylonitrile fiber, cellulose nanocrystals, stabilization

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1.

Introduction Currently, high performance carbon fibers are mainly produced from polyacrylonitrile

(PAN). PAN fibers are converted to carbon fibers via oxidative stabilization followed by carbonization. Stabilization of PAN is typically carried out in air in the temperature range of 180–320 C under tension1. However, stabilization chemistry of PAN is complicated and the mechanism has not been fully understood2-4. The PAN stabilization mainly consists of the following reactions: cyclization, oxidation and crosslinking5. While the stabilization process effects the performance of the resulting based carbon fibers6-7, the stabilization kinetics can change when incorporating other materials such as carbon nanotubes (CNTs), lignin, or CNCs into PAN fibers8-10. It is reported that the presence of CNTs affects the activation energy of stabilization reactions, the orientation of the ladder polymer, and the mechanical properties of the stabilized fiber6, 9. Due to environmental concerns associated with petroleum derived products, renewable materials such as cellulose and lignin are considered potential candidates as carbon fiber precursors. Rayon (a regenerated cellulose fiber) was initially used to make carbon fibers in late 1950s11-12. However, carbon fibers produced from either lignin or cellulose show considerably lower tensile strength and modulus than the PAN based carbon fibers13-18. Thus, PAN containing cellulose or lignin composite fibers have been investigated to make new class of carbon fibers1922.

In our previous studies, PAN containing up to 40 wt% CNC precursor fibers, and the resulting

carbon fibers have been reported. 10, 23-24. At a relatively low draw ratios, the tensile modulus of the composite fiber increased with increase in CNC loading24. High resolution TEM imaging showed that carbonized PAN/CNC fibers exhibited two distinct carbonized regions, one corresponding to the carbonized PAN, and the second region corresponding to carbonized CNC10.


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In addition, based on the batch processing, PAN and PAN/CNC based carbon fibers exhibited comparable mechanical properties10. Since stabilization affects the properties of the carbon fibers, PAN/CNC fiber with 40 wt% CNC was chosen for the study of its stabilization kinetics. Structure, thermal, and mechanical properties of the stabilized PAN/CNC fibers have also been studied. 2.

Experimental Polyacrylonitrile (PAN, Mw: 247,000 g/mol, containing 4 wt% methacrylic acid)

copolymer and cellulose nanocrystals (CNCs) were purchased from Exlan Company (Japan) and University of Maine (USA), respectively. All the CNCS are used as-received and the surface of CNCs was not functionalized in any manner. The detailed solution preparation and fiber spinning process can be found in our previous work24. The CNC content in the composite precursor fiber is 40 wt% and 60 wt% is PAN copolymer. This fiber is referred to PAN/CNC-40. In this study, a final draw ratio of 10x was used. A tube furnace (Micropyretics Heaters International, Cincinnati) was used for stabilization. For stabilization in different gas environment (only air, only N2, or N2 then air), PAN and PAN/CNC-40 fibers were preloaded with 20 MPa stress and then heated to 265°C at a rate of 5 °C/min. The holding time at 265 °C was varied from 1 to 6 hours. Microstructure and elemental mapping of stabilized PAN/CNC fiber was observed under TEM (JEOL, JEM-ARM200cF) based on the previously reported procedure25. For mechanical properties, single precursor and stabilized fibers were tested on a FAVIMAT tensile tester at 1%/s strain rate and at a gauge length of 25.4 mm. PAN fiber has a density of ~ 1.18 g/cm3. Density of 1.34 g/cm3 is calculated for the PAN/CNC-40 precursor fiber using the rule of mixtures, where a density of 1.59 g/cm3 is used for CNC23. A density of 1.4 g/cm3 is assumed for the stabilized fibers26. Fourier transform infrared spectroscopy (FTIR, Spectrum one) was



conducted on an infrared microscope (Perkin Elmer). Wide-angle X-ray diffraction data was collected and analyzed following our previously reported procedure24. Differential scanning calorimetry (DSC) was used to analyze the thermal properties of the fibers (TA Instrument Q200). 3.

Results and discussion At a draw ratio of 10x, PAN/CNC-40 precursor fiber exhibits higher tensile properties

than the control PAN fiber (Table S1). This is attributed to the well-dispersed and highly oriented CNCs, improved PAN chain orientation in the presence of CNC, and potentially effective load transfer between CNC and PAN24,

27-28.

When pyrolized separately, CNCs

pyrolysis occurs at a lower temperature than for the PAN copolymer. However, as shown in our previous study29, the degradation temperature of CNCs in PAN matrix is shifted to relatively higher temperature. As compared to the precursor fibers, the tensile properties of the stabilized fibers in air are lower (Table 1). When stabilized in air for 2 hours, stabilized PAN/CNC-40 fibers show higher tensile modulus than the stabilized PAN fibers. After stabilization in air for more than 4 hours, both stabilized fibers show comparable tensile properties. SEM images of stabilized fibers in air are shown in Figure S1. EF-TEM images (Figure 1) show individual stabilized CNC embedded in stabilized PAN, and differences in the nitrogen and oxygen contents in the two regions of the stabilized fiber. Nitrogen is observed in the stabilized PAN, and oxygen content is higher in stabilized CNC than in stabilized PAN.


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Table 1. Mechanical properties of the stabilized fibers (in air at 265°C under a tension of 20 MPa for different times). PAN fiber

PAN/CNC-40 fiber

Holding time (h)

2

4

6

2

4

6

Tensile modulus (GPa)

12.2 ± 0.3

12.5 ± 0.3

12.2 ± 0.3

15.2 ± 0.3

12.5 ± 0.2

11.3 ± 0.2

Tensile strength (MPa)

318 ± 32

232 ± 12

244 ± 25

299 ± 14

220 ± 18

230 ± 9

Figure 1. Energy-filtered TEM images of stabilized PAN/CNC fiber under a tension of 20 MPa in air at 265°C for 2 hours. (a) zero-loss, (b) nitrogen map, (c) oxygen map, and (d) composite image of carbon, nitrogen, and oxygen. Stabilized PAN and stabilized CNC regions are marked in Figure (d). WAXD of PAN and PAN/CNC-40 precursors and the stabilized fibers in air are shown in Figure S2. PAN peak is at t 2θ = 16.7 ° corresponding to (200),(110) planes. With the increasing stabilization time, the intensity of this peak decreases. This indicates that the PAN structure 5 ACS Paragon Plus Environment


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changes during the stabilization. For PAN/CNC-40 composite fibers, CNC peaks occur at 2θ = 20.5 ° and 22.6 °, corresponding to the CNC (102) and (200) planes, respectively. After stabilization at 265 °C for 6h, the PAN and CNC peaks disappeared completely, and only peak at 2θ = 25.7 ° exists. This indicates the formation of the ladder structure from PAN

6, 30.

The

orientation of the ladder structure in stabilized fibers was used to study the effect of nanofiller on the structure of PAN9. As shown in Figures 2 and S3, the azimuthal scans of stabilized PAN fibers can be fitted by one Gaussian peak. However, the stabilized PAN/CNC-40 fiber, the azimuthal scan cannot be fitted with one Gaussian peak, but is de-convoluted into two peaks. The calculated Herman’s orientation factors based on the azimuthal scans are listed in Table 2. According to the curve fitting, some ladder structure with a high orientation factor of ~0.98 is observed in the composite fibers. Since the stabilized cellulose does not exhibit any peak around 2θ = 25.7° 30, this highly oriented ladder structure should be from stabilized PAN in the vicinity of CNC. Similar result has been reported in stabilized carbon nanotubes (CNTs) reinforced PAN fiber6. In addition, the overall orientation of ladder structure is higher in the stabilized composite fiber than in the stabilized PAN fiber. However, as shown in Table 1, the tensile properties of stabilized PAN and PAN/CNC-40 fibers are comparable after stabilization in air for more than 4 hours. This suggests that, in addition to the orientation of ladder structure, other factors such as the stabilized CNCs would affect the tensile properties of the stabilized PAN/CNC fibers. The chemical structure of the precursor and the stabilized fibers was characterized by FTIR (Figure 3). Both the stabilized PAN and PAN/CNC fibers show similar structural changes. After stabilization in air, the nitrile peak at ~ 2242 cm-1 diminishes and broadens. New peaks occur in the range of 1575 – 1725 cm-1 due to the formation of C＝C, C＝N and C＝O bonds, suggesting the formation of the ladder structure31. 6 ACS Paragon Plus Environment

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Experimental Fitted

90

120

150

180

210

240

270

90

Azimuthal angle ()

Figure 2.

Experiemntal Fitted Fitting curve 1 Fitting curve 2

(b) Intensity (a.u.)

(a) Intensity (a.u.)

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


120

150

180

210

240

270


WAXD azimuthal scan at 2θ = 25.7 ° for stabilized fibers: (a) PAN, and (b)

PAN/CNC-40 fiber (stabilization in air at 265°C for 6 hours).

Table 2. Herman’s orientation factors of the ladder structure in the stabilized fibers. Herman’s orientation factor Stabilization time (hr)

PAN

PAN/CNC-40 Overall

Curve 1

Curve 2

2

0.64

0.76

0.98

0.75

4

0.66

0.77

0.98

0.76

6

0.69

0.79

0.99

0.78



C=O C= C=C

(a) PAN fiber 6hr

Transmittance (%)

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

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(b) PAN/CNC-40 fiber C=O C= 6hr C=C

4hr

4hr

2hr

2hr

Precursor CN Precursor CN

3000

2500

2000

1500

1000

3000

2500

2000

1500

1000

-1

-1

Wavenumber (cm )

Wavenumber (cm )

Figure 3. FTIR spectra of the precursor and stabilized fibers, with stabilization in air for different times as indicated. (a) PAN, and (b) PAN/CNC-40.

Thermal stabilization process of PAN in air is complex where cyclization, oxidation, and crosslinking occur concurrently and transform PAN chain into a thermally stable ladder polymer structure5. Since cyclization reaction can be initiated in air or in inert environment while oxidative environment is required for oxidation and crosslinking26, stabilization reactions have been individually studied by: (i) heating PAN fiber in N2, where mainly cyclization occurs, and (ii) by re-running the samples from (i) under oxidative heat-treatment to initiate oxidation and crosslinking reactions

8-9.

As shown in Figure 4, a single exothermic peak is observed when

PAN is stabilized in N2. While for PAN/CNC-40 fibers, two exothermic peaks are observed from the heat treatment in N2. One peak is related to PAN cyclization and the second peak may relate to CNC reactions including the dehydration and depolymerization of cellulose, or from reactions due to the interaction between PAN and CNC 12, 32-33. Heating the fibers in air after heating in N2, 8 ACS Paragon Plus Environment

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two peaks assigned to oxidation and crosslinking reactions are observed8-9. Due to the presence of CNC, PAN/CNC-40 fiber may go through additional reactions that are not present during PAN copolymer stabilization in air. These may include dehydration and depolymerization of CNC, as well as potential reactions between PAN and CNC, and reactions catalyzed by the presence of CNC or its stabilization products.

4

2.0

1C/min 5C/min 10C/min 15C/min

(a)

1.5

2

1

0

250

1.0

0.5

0.0

Cyclization 200


(b) Heat flow (mW/mg) Exo. 

Heat flow (mW/mg) Exo. 

3

300

350

400

Cyclization

200

250

CNC 300

Temperature (C)

350

400

Temperature (C)

2.0


1.5

2.0

Heat flow (mW/mg) Exo. 

(c) Heat flow (mW/mg) Exo. 

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


1.0

0.5

0.0

1.5

1.0

0.5

0.0

Oxidation 100


(d)

150

200

Oxidation

Crosslink 250

300

350

400

100

150

Temperature (C)

200

Crosslink 250

300

350

400

Temperature (C)

Figure 4. PAN and PAN/CNC-40 fibers at various heating rates in DSC. (a) PAN fiber in N2, (b) PAN/CNC-40 fiber in N2, (c) re-running PAN fiber in air after run in N2, and (d) re-running PAN/CNC-40 fiber in air after run in N2.



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The peak temperatures obtained from DSC at different heating rates are listed in Table S2. The activation energy, pre-exponential factor in Arrhenius equation, and reaction rate constant is calculated following our previous work using Kissinger method25. The plots using this method are shown in Figure S4. As compared to the stabilized PAN/CNT fibers, where CNTs can lower the activation energies of oxidation and the cross-linking reactions 6, 9, addition of CNCs in PAN decreased activation energies of cyclization and crosslinking by 17.5 and 19 %, respectively (Table 3). This is consistent with the previous results on PAN/CNC films33. For oxidation activation energy, the values are comparable for both PAN and PAN/CNC-40 fibers. The preexponential factors for these stabilization reactions are lower by more than an order of magnitude in PAN/CNC than in PAN fiber. For reaction rate constants calculated at 265°C, the values of cyclization and oxidation reactions are slightly lower in PAN/CNC-40 than in PAN fibers. However, the composite fiber shows slightly higher crosslinking reaction rate constant than PAN. Table 3. Kinetic parameters and activation energies of PAN and PAN/CNC-40 stabilization reactions. PAN fiber Reaction activation Pre-exponential energy (KJ/mol) factor (s-1)

PAN/CNC-40 fiber Reaction rate constant (265°C, s-1)

Reaction activation Pre-exponential energy (KJ/mol) factor (s-1)

Reaction rate constant (265°C, s-1)

CNCs in N2

-

-

-

174.4

9.6 x 1013

0.001

Cyclization

198.5

1.6 x 1018

0.085

163.8

4.2 x 1014

0.053

Oxidation

83.5

2.2 x 108

1.76

83.1

6.7 x 107

0.573

Crosslinking

157.7

7.1 x 1012

0.003

127.8

2.0 x 1010

0.008

To further investigate the effect of the presence of CNCs on the structure and properties of PAN, PAN and PAN/CNC-40 fibers were first stabilized in nitrogen for 6 hours and then further stabilized in air for various times, in a tube furnace under tension. Though there are 10 ACS Paragon Plus Environment

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potential reactions between PAN and CNC in nitrogen due to the presence of oxygen in CNC33, WAXD results suggest that oxygen from 40 wt% CNC is still not enough to stabilize PAN and additional oxygen is required. WAXD patterns and integrated scans of fibers stabilized in N2 then in air are shown in Figure S5. The peak in integrated scans at 2θ = 25.7 ° is from the ladder structure. When stabilized in N2 , 2θ = 16.7 ° peak in PAN becomes broader and no peak appears around 2θ = 25.7 °. PAN under N2 results in cyclization and mostly forms isolated aromatic rings, instead of continuous ladder structure that forms under air34. Based on this literature study, it is reasonable to expect that cyclized structure was formed in our PAN and PAN/CNC fibers when stabilized in N2. However, the possibility of additional reactions in the presence of CNC cannot be ruled out. After PAN fibers were subsequently stabilized in air, the peak at 2θ = 16.7 ° disappears and a new peak at 2θ = 25.7 ° appears. For PAN/CNC-40 fiber, peak at 2θ = 16.7 ° of PAN become weak and characteristic peaks of CNCs at 2θ = 20.5 ° and 22.6 ° still exist even after fibers were stabilized in N2 at 265 °C for 6 hours. With increasing stabilization time in air, CNC peak intensity decreases and a new peak appear at 2θ = 25.7 °. The orientation of the ladder structure determined from the azimuthal WAXD scans is listed in Table 4. Compared to the fiber stabilized in air, the azimuthal scans of all the stabilized fibers (stabilized in N2, then in air) are fitted by one curve (Figures 5 and S6). The orientation of ladder structure in stabilized PAN/CNC-40 fibers (N2 then in air) is about 0.69 - 0.7, which is lower than 0.76 - 0.79 for stabilized PAN/CNC-40 fibers in air alone. But when fibers are stabilized in N2 for 6 hours and then in air for 1 and 2 hours, overall orientation of the ladder structure is still slightly higher in stabilized PAN/CNC-40 fibers than that of the correspondingly stabilized PAN fibers. When fibers are stabilized in N2 for 6 hours and then in air for 4 hours, both stabilized fibers show similar value of orientation for the ladder structure.



The chemical structure of stabilized fibers in N2, and fiber stabilized in N2 and then in air was characterized by FTIR (Figure S7). The nitrile peak intensity at ~ 2242 cm-1 in stabilized fibers in N2 weakens and new peaks occur in the range of 1575 – 1725 cm-1 due to the formation of C=O, C＝C or C＝N bonds 31. This indicates the cyclized structure formation from PAN in N2 34. After fibers were stabilized in air, following stabilization in N2, all the stabilized fiber exhibit similar FTIR spectra to that of the fibers stabilized only in air. Combination of FTIR data and integrated WAXD scans suggest that cyclized structure forms when fibers stabilized in N2, and crosslinking and ladder structures form when fibers are further stabilized in air. Mechanical properties of the fibers after stabilization in different gas environments are listed in Table 5. Interestingly, stabilized PAN/CNC-40 fiber in N2 for 6h exhibits comparable tensile modulus to the PAN/CNC-40 precursor fiber. Possible reasons include relatively stable CNCs in PAN matrix or chemical reactions between PAN and CNC to form crosslinked structure. When stabilized in N2 for 6h and then in air for 1h, the tensile strength of both fibers increased as compare to that of the fibers stabilized in N2 for 6h. This is possibly attributed to the formation of ladder structures when fibers are subsequently stabilized in air after N2. After these fibers are subsequently stabilized in air for more than 2 hours, the all fibers show lower mechanical properties. SEM images of stabilized PAN and PAN/CNC-40 fibers in N2 followed by air are shown in Figure S8.


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Experimental Fitted

(b)

Experimental Fitted

Intensity (a.u.)

(a) Intensity (a.u.)

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


90

120

150

180

210

240

90

270

120

150

180

210

240

270



Figure 5. WAXD azimuthal scans at 2θ = 25.7 ° for stabilized fibers. (a) PAN,

and (b)

PAN/CNC-40 (stabilization at 265°C in N2 for 6 hours, then in air for 1 hour). Table 4. Herman’s orientation factor of ladder structure in stabilized fiber (265°C in N2 for 6 hours then in air for different times). Herman’s orientation factor

Stabilization time (hr)

PAN

PAN/CNC-40

1

0.65

0.69

2

0.66

0.69

4

0.70

0.70

Table 5. Mechanical properties of stabilized fibers ( 265°C for 6 hours in N2, and then in air for different times). PAN fiber Holding time (hours in N2, then in air) Tensile modulus (GPa) Tensile strength (MPa)

PAN/CNC-40 fiber

(6, 0)

(6, 1)

(6, 2)

(6, 4)

(6, 0)

(6, 1)

(6, 2)

(6, 4)

11.4 ± 0.3

11.8 ±0.3

11.8 ± 0.3

11.7 ± 0.3

20.8 ±0.3

17.9 ± 0.2

16.3 ± 0.2

11.0 ± 0.4

155 ± 45

258 ± 22

251 ±12

213 ± 5

294 ± 33

324 ± 9

306 ± 7

194 ± 29



4.

Conclusions Stabilization of PAN and PAN/CNC fibers has been systematically studied in different

gas environments. Air environment is required to fully stabilize PAN/CNC fiber. For stabilized PAN/CNC fiber, individually distributed CNCs in PAN was confirmed by their elemental composition differences. As compared to the PAN fibers, the addition of CNCs improves the orientation of the ladder polymer when stabilized PAN/CNC-40 fibers either in air or in N2 followed by air. Addition of 40 wt% CNCs in PAN fibers decreased the activation energies of cyclization and crosslinking reactions by 17.5 and 19 %, respectively. When stabilized in air for 2 hours or in N2 for 6 hours, stabilized PAN/CNC fibers show a higher tensile modulus than that of the stabilized PAN fibers under the same stabilization conditions. Supporting Information Mechanical properties of precursor fibers, and characterization of the stabilized fibers including SEM, FTIR and WAXD. Acknowledgment This work was financially supported by the Air Force Office of Scientific Research (FA 9550-14-1-0194) and Renewable Bioproducts Institute at the Georgia Institute of Technology.


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cellulose nanocrystals

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