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Lightweight MWCNTs-g-PAN Carbon Fiber Precursors; Sensitive High Absorptivity and Novel Wide-Bandgap Conjugated Polymers Mohammad M. Fares, Fahmi A. Abu Al-Rub, and Khansa'a H. Masadeh Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.5b01992 • Publication Date (Web): 07 Sep 2015 Downloaded from http://pubs.acs.org on September 11, 2015

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Industrial & Engineering Chemistry Research

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Lightweight MWCNTs-g-PAN Carbon Fiber Precursors; Sensitive High Absorptivity and Novel Wide-Bandgap Conjugated Polymers Mohammad M. Fares 1,*, Fahmi A. Abu Al-Rub 2, Khansa’a H. Massadeh 1 1

Department of Chemical Sciences, Faculty of Science & Arts, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, JORDAN

2

Department of Chemical Engineering, Faculty of Engineering, Jordan University of Science and Technology, P.O. Box 3030, Irbid 22110, JORDAN

*Correspondence: [email protected]

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ABSTRACT

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We report the fabrication of lightweight MWCNTs-g-PAN carbon fiber precursors as high

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absorptivity and wide-bandgap novel materials. The synthesis of carbon fiber precursors

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performed via grafting process of polyacrylonitrile on the surface of acid functionalized

23

multi walled carbon nanotubes. 1H-NMR and FTIR spectroscopic techniques revealed

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successful grafting process and the formation of MWCNTs-g-PAN fibers with extent of

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stabilization (i.e. degree of conjugation) equals 3.0. Such carbon fiber precursors with 200–

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600 µm thickness demonstrate excellent thermal stability with three degradation steps at

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276, 413 and 850 °C, respectively. Carbon fiber precursors display selective high absorption

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UV radiation and sharp discrete energy difference at 284 nm. They demonstrate linear

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dependence of absorptivity (ε) on weight fraction of multi walled carbon nanotubes in the

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carbon fiber precursors (χCNTs). Large absorptivity of UV radiation occur using χCNTs (%)=

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0.50 specimen due to long chain aligned nano-fibril structures, whereas small absorptivity of

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UV radiation occur using χCNTs (%)= 8.26 specimen due to wrap-up of short length PAN

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chains around carbon nanotubes forming flattered and thicker nanotubes, as depicted by

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ESEM images. Sensitive cutoff UVB absorptions correspond to 4.36–4.38 eV, which

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classifies carbon fiber precursors as promising wide-bandgap materials, and probably adapt

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them to play focal role in many future optoelectronic and/or aeronautic implementations.

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Keywords: Acid functionalized carbon nanotubes, polyacrylonitrile, carbon fiber precursors,

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wide-bandgaps, conjugated polymers, UVB absorption.

1

ACS Paragon Plus Environment


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INTRODUCTION

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Wide-bandgap materials gain spreading and increasing ballistic interest in today’s market due to

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huge number of applications, resulting in billions of dollars of savings for businesses and

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consumers [1]. The numerous applications of such materials include but not limited to; ultraviolet

43

sensors [2, 3], organic light emitting diodes (OLEDs) [4], pathogenic biological UV photodetector

44

for detection of biological materials [5, 6], etc. However, Organic semiconductors have large

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number of applications in electronic devices. Their application mainly depends on their bandgap

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energy difference as being low-, medium- or wide-bandgaps. The properties of lightweight and low

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cost of organic semiconductors overtop conventional inorganic semiconductors that suffer from

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fabrication complexity and high cost [7, 10]. Besides that, thermal stability of conjugated polymers

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that are used, as organic semiconductors, in solar cells is extremely significant. Thermal stability

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considered as cornerstone for the endurance of the semiconductors at high operating temperatures.

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Only few examples of organic semiconductor devices operate at temperatures above 100 °C are

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known [11]. Organic field-effect transistor (OFET) operated well at ambient temperatures but began

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degradation at 120 °C [12]. Similar device began degrading at 180 °C in the presence of a

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protective polymer package [13].

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The use of carbon nanotubes (CNTs) in composites, hybrids, and advanced materials has gain

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remarkable interest due to its remarkable properties such as superior mechanical strength, high

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electrical and thermal conductivity [14-16], energy storage [17], semiconductor devices [18], field

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emission displays [19], catalysis [20], and sensors [21]. Along with that, carbon nanotubes used to

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develop mechanical properties of carbon fiber reinforced plastics (CFRPs) to meet specific high

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performance applications especially in aeronautic and aerospace industry [22, 23], and in the

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development of mechanical properties such as shear strength, interlaminar shear strength,

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interlaminar fracture toughness [24-26].

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On the other hand, surface modification of polymers found outstanding applications including but

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not limited to; smart and stimuli sensitive materials [27-33], oxidative resistance coatings [34-38],

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and wide-bandgap (WBG) materials used in tandem solar cells [39, 40]. WBG materials are also

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strongly incorporated in power saving electronics [41] and in solid-state lighting (SSL) [42].

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Herein, we report the fabrication of novel MWCNTs-g-PAN carbon fiber precursors via grafting of

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polyacrylonitrile on the surface of acid functionalized multi walled carbon nanotubes.

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precursor thin films verified for thermal stability and for UV radiation absorptivity as future

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promising wide-bandgap materials. The chemical structure of carbon fiber precursors characterized

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via 1H-NMR and FTIR spectroscopic techniques, and extent of stabilization (i.e. degree of

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conjugation). The morphologic carbon fiber nanostructures pursued via ESEM technique.

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Such

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EXPERIMENTAL

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Materials & techniques

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Multiwall carbon nanotubes (MWCNTs) purchased from NanocylTM NC7000, Belgium.

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Polyacrylonitrile, PAN, with average molecular weight of 150,000 g/mol, purchased from Sigma-

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Aldrich. The other reagents and solvents were of analytical grade and used as received. 1H-NMR:

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The 1H-NMR spectra of the fibers were recorded on a Bruker Biospin Spectrometer of 400 MHz in

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deuterated DMSO solvent. FTIR: KBr pellets of specimen recorded the vibrational spectra in the

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4000−400 cm−1 range using Shimadzu IRAffinity-1 FTIR spectrophotometer. Thermogravimetric

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Analysis (TGA); samples scanned under flow of nitrogen gas at a heating rate of 10 °C/min and

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25−900 ºC temperature range using Netzsch Proteus thermogravimetric analyzer, Germany.

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Environmental Scanning Electron Microscopy (ESEM); the specimen coated with gold ion by

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sputtering method and scanned using Quanta 450 FEG Environmental Scanning Electron

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Microscope (ESEM). UV-Vis spectrophotometer (UV-Vis); samples scanned in the range of

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200−800 nm using Shimadzu UV-2401 spectrophotometer supplied with a heating control device.

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Compression molding instrument of Carver Inc. Wabash model (USA); Samples exposed to 100 °C

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and 22 MPa pressure for 30 minutes.

90

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Acid Functionalization of MWCNTs

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2.0 g multi walled carbon nanotubes (MWCNTs) located in 50 mL dried round bottom flask

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connected to a condenser, and step-wise addition of 30 ml of 9.0 M nitric acid performed. The

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dispersed solution refluxed at 75 °C for three periods, each 8 hours. The resultant dispersion

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solution was filtered and oxidizable MWCNTs collected. The filtrate solution was subjected to

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centrifugation process to precipitate fine MWCNTs sample suspended in solution. The collected

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functional MWCNTs sample was continuously washed with deionized water to purify MWCNTs

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sample from trapped nitric acid, until pH of the filtrate solution reaches neutral conditions (pH= 6–7

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values). The collected acid functionalized MWCNTs precipitate dried at ambient temperature and

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stored until used elsewhere.

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Fabrication of transparent thin film carbon fiber precursors

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3.0 g of polyacrylonitrile (PAN) slowly dissolved in 50 mL dimethylformamide (DMF) solvent at

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ambient temperature, and 15 mg MWCNTs added step-wise under continuous stirring until

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homogenous dispersed solution occurs. The black dispersed solution transferred to temperature-

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controlled oven equipped at 120 °C, and temperature gradually increased using temperature-control 3



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program until 250 °C reached, and kept there for three hours. After time completion, the resultant

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solid black carbon fiber specimen transferred to compression molding machine and annealed at 100

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°C and 100 MPa for 30 minutes to obtain transparent thin films of MWCNTs-g-PAN carbon fiber.

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The thickness of transparent thin films was measured by digital micrometer. The same process

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repeated to obtain different weight fraction acid functionalized CNTs samples, using 30, 90, and

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270 mg of MWCNTs, respectively as follows (Table 1);

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Table 1. Mass and weight fraction of MWCNTs in MWCNTs-g-PAN carbon fiber precursors. Sample Mass of MWCNT a χCNT b (%) (mg) I 15 0.50 II 30 0.99 III 90 2.91 IV 270 8.26 a Mass of CNT used in presence of 3.0 g polyacrylonitrile (PAN). b Weight fraction of MWCNT in (%).

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RESULTS & DISCUSSION

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Structural identification of MWCNTs-g-PAN carbon fiber precursors

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The oxidation of MWCNTs by nitric acid yields many carboxylic acid functional groups distributed

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all over the backbone structure of the carbon nanotubes. Such carboxylic acid groups play crucial

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role in the cyclization and hence the stabilization of nitrile groups of polyacrylonitrile (PAN) to

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form conjugated chains at lower temperature through an ionic mechanism [43, 44]. Nucleophilic

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attack of carboxylate groups on the carbon nanotubes on the nitrile groups of PAN induced

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cyclization of adjacent nitrile groups and the formation of π-conjugated system at elevated

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temperatures as follows (scheme 1);

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Scheme 1. Schematic illustration for the formation of MWCNTs-g-PAN carbon fiber precursors

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Such cyclization cross-links PAN chains, form π-conjugated system and converts PAN fiber to

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carbon fiber precursor. The yield product is extremely stable. It prevents melting or fusion of the

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fiber, avoid excessive volatilization of elemental carbon at high temperate and hence maximize the

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ultimate carbon yield from the PAN fiber precursor, and adapt the structure to withstand the

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difficulties of high temperature processing [45–49].

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Fig.1 illustrates the 1H-NMR spectrum of MWCNTs-g-PAN carbon fiber precursors. Apparently,

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polyacrylonitrile (PAN) and acid functionalized MWCNTs moieties were available in the spectrum.

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For polyacrylonitrile moiety, nitrile groups converted into π-conjugated groups is not complete.

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Thus, for conjugated polyacrylonitrile moiety; CH peaks of the backbone located at δ = 1.206 ppm

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(peak 2), CH2 peaks of the backbone located at δ= 1.238 and 1.444 ppm (peaks 3 and 4). However,

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unconjugated PAN groups appeared as follows; CH2 peaks of the backbone located at δ = 1.977

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ppm (peak 5), CH peaks of the backbone located at δ= 2.300 and 2.642 ppm (peaks 6 and 7).

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Furthermore, different chemical shifts demonstrated the presence of acid functionalized MWCNTs;

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CH2 peaks with different environments located at δ = 3.312 and 4.545 ppm (peaks 8 and 10). CH

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peaks of the 5.076 and 5.366 ppm corresponding to peaks 11 and 12, respectively. OH peak at δ=

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3.633 ppm (peak 9), and peaks 13-16 located at δ = 6.919, 7.046, 7.175, and 7.309 ppm correspond

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to non-oxidized CH groups in carbon nanotubes moieties.

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Fig. 1 1H-NMR spectrum of MWCNTs-g-PAN carbon fiber precursors

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Furthermore, characteristic peaks of acid functionalized MWCNTs, PAN, and MWCNTs-g-PAN

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carbon fiber precursors are available in Table 2. The grafting of MWCNTs by nucleophilic attack

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into PAN chains is confirmed through the appearance of intensed conjugated (–C=N–) peak at 1606

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cm–1, and decay in intensity for nitrile (–C≡N–) peak at 2244 cm–1. Furthermore, the extent of

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stabilization measures how far the unconjugated nitrile groups (–C≡N–) are converted into

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conjugated (–C=N–) groups. This can be checked by measuring the FTIR absorbance ratio of

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conjugated peak (–C=N–) peak to unconjugated peak (–C≡N–) as follows [44];

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Extent of stabilization (Es) =

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(1)

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Where and are the absorbance of conjugated peak at 1606 cm–1 and

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unconjugated peak at 2244 cm–1, respectively. The extent of stabilization for MWCNTs-g-PAN

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carbon fiber precursors found to be 3.0 as determined from Fig.2 and Table 1 values. Accordingly,

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similar value obtained by dividing the integrated peak area of peak 2 to peak 6 in the NMR

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spectrum (Fig. 1), which demonstrate accurate determination of extent of stabilization for carbon

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fiber precursors.

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172 173 174 175 176 177 178


Fig. 2 FTIR spectra of polyacrylonitrile (PAN), acid functionalized MWCNTs and MWCNTs-gPAN carbon fiber precursors

Table 2. FTIR Characteristic peaks of PAN, acid functionalized MWCNTs and MWCNTs-g-PAN carbon fiber precursors Composite A Functional Group cm−1 Acid Functionalized MWCNTs

PAN

MWCNTs-g-PAN carbon fiber precursors

3558 3477 3417 2928 2855 1638 1617 1385 2941 2872 2244 1631 1456 1360 3421 2930 2868 2244 1660 1606 1403

0.156 0.193 0.226 0.019 0.015 0.119 0.146 0.074 0.465 0.166 0.995 0.181 0.921 0.351 0.043 0.014 0.010 0.009 0.021 0.027 0.013

–OH stretching bounded to aliphatic groups –OH stretching bounded to aromatic groups –OH stretching in carboxyl groups Asymmetric –CH– stretching Symmetric –CH– stretching Conjugated –C C– stretching Asymmetric –COO stretching Symmetric –COO stretching Asymmetric –CH– stretching Symmetric –CH– stretching –C≡N stretching –C=C stretching CH bending of –CH2– groups CH bending of –CH– groups OH stretching of Functional MWCNTs Asymmetric CH stretching Symmetric CH stretching –C≡N stretching of PAN Conjugated –C C– stretching of MWCNTs –C=N– stretching of PAN Symmetric COO stretching of MWCNTs

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For further investigations, surface morphology of carbon fiber precursors was studies via

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environmental scanning electron microscope technique (Fig. 3). Obviously, nucleophilic attack of

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carboxylate groups of MWCNTs at the nitrile groups of PAN established aligned conjugated PAN

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tubular chains that intersect with aligned carbon nanotubes as seen in Fig. 3A. Such aligned 7



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intersections play dominant role in the formation of nano-fibrils structures aligned near each. The

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nano-fibril structures have impact influence on the enhancement of carbon fiber properties such as

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electrical and thermal conductivity, and mechanical strength.

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Fig. 3 ESEM images of different side views for MWCNTs-g-PAN carbon fiber precursors (χCNTs (%) = 0.99)

On the other hand, as weight fraction of MWCNTs increases, the aligned nano-fibril structures seen

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in Figs. 3A, 3B and 3C loosened, and eventually disappear as seen in Figure 4. The loosening of

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nano-fibril structures attributed to large number of carboxylate groups on the surface of CNTs.

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Competitive nucleophilic attack occur on polyacrylonitrile chains result with short chain length of

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polyacrylonitrile per each carboxylate ion. Hence, non-aligned chains and flaky structures observed

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as seen in Figs. 4A and 4B.

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Furthermore, the average width of CNTs threads in acid functionalized CNTs and MWCNTs-g-

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PAN was measured in Figures 4D and 4C, respectively. The images scale was 600 nm in both

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images, and thus the images were comparable to each other. Clearly, the width of CNTs threads in 8


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MWCNTs-g-PAN specimen (Fig. 4C) is larger than CNTs threads in acid functionalized CNTs

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(Fig. 4D). Therefore, careful measurements of the width of the CNTs threads performed several

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times for several images (10 measurements/4 cm2 of the image). This process repeated several times

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for Figs. 4C and 4D, and other related images until stable measurements obtained. Interestingly, the

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average width of CNTs threads in acid functionalized carbon nanotubes was 50 nm, whereas the

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average width of CNTs threads in MWCNTs-g-PAN was 100 nm. This suggests that the CNTs

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were coaxially wrapped by PAN polymer due to strong π–π interactions between CNTs and

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conjugated PAN repeating units [50, 51], which reinforce the strength carbon fiber precursor and

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increase the width of CNTs threads as described in Scheme 2.

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213 214 215 216 217 218

Scheme 2. Wrap-up of conjugated PAN repeating units at high weight fraction of MWCNTs (χCNTs (%) = 8.26

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probably the un-reacted and un-conjugated polyacrylonitrile chains that strongly interact with each

220

other due to high dipole moment of nitrile groups (3.9 D) leading to agglomerated spheroids.

On the other hand, in Fig. 4C the semi-spheroids that appear below the MWCNTs threads are

221 222 223 224 225 226 227 228 229 230 231 232

9



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233

234 235

236 237 238 239

Fig. 4 ESEM images for MWCNTs-g-PAN carbon fiber precursors using (A) and (B) χCNTs (%) = 2.91, (C) χCNTs (%) = 8.26, (D) and (E) Acid functionalized MWCNTs

240 241 10


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Thermal Stability

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Thermogravimetric analysis technique is a tool to investigate thermal stability of compounds and

244

composites against temperature increase.

245

constituents, and therefore indicates how chemical changes improved or disproved the thermal

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stability and properties. Degradation temperature is also an indication of inter- and intra-molecular

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forces between polymeric chains. It is determined, as sharp peak, by the first derivative TGA

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thermogram (DTG). Polyacrylonitrile, PAN has two-step degradation temperatures at 291 and 420

249

°C, respectively (Fig. 5A). The first degradation step corresponds to nitrile oligomerization, which

250

produces volatile products, e.g. NH3, HCN, CH3CN etc and the second degradation step is related to

251

the thermal degradation reaction of the backbone polymer [52]. On the other hand, acid

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functionalized MWCNTs sample has one degradation temperature at 690 °C (Fig. 5B), and

253

MWCNTs-g-PAN thermogram contains Three-step degradation temperatures (Fig. 5C and Table

254

3).

It measures degradation temperature of composite

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Table 3. Thermal properties of PAN, MWCNTs, and MWCNTs-g-PAN carbon fiber precursors Compound Degradation temperature Step I (°°C) Step II (°°C) Step III (°°C) Polyacrylonitrile (PAN) 291 420 -MWCNTs --690 270 413 850 MWCNTs-g-PAN (χCNTs (%) = 0.50) 276 413 850 MWCNTs-g-PAN (χCNTs (%) = 0.99) 285 413 850 MWCNTs-g-PAN (χCNTs (%) = 2.91)

257 258 259

Notably, in step I degradation temperature of PAN incorporated in MWCNTs-g-PAN carbon fiber

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precursors is lower than sole PAN. This indicates that nitrile groups of different chains were aligned

261

during compression molding process at 100 °C and 22 MPa pressure in such a way that allows

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easier nitrile oligomerization, and hence volatile products (NH3, HCN, and CH3CN) emerge at

263

slightly lower degradation temperatures. On the contrary, degradation temperature of CNTs (step

264

III) in MWCNTs-g-PAN carbon fiber precursors is much higher than sole MWCNTs. This indicates

265

that after volatile products of polyacrylonitrile volatilized to the gaseous state, the backbone

266

structures of polyacrylonitrile interact with carbon nanotubes and form a char. This char has larger

267

inter- and intra-molecular forces that allow higher thermal stability against degradation up to 850

268

°C.

269 270 271 11



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272 273 274 275 276 277

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279 280 281 282

Fig. 5 TGA and DTG thermograms for (A) Polyacrylonitrile (PAN), (B) Acid functionalized MWCNTs, and (C) Different weight fractions of MWCNTs-g-PAN carbon fiber precursors

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Sensitive high absorptivity and wide-Bandgap films

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π–conjugated polymers are used extensively as electron-rich donor layer in tandem and in many

285

types solar cell devices due to their consonant optical, electrical, and redox properties. Their

286

properties depend mostly on the energy of their bandgaps (Eg), defined as the difference in energy

287

between the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital

288

(LUMO) [53]. The UV-Vis absorbance spectra of neat thin films for MWCNTs-g-PAN carbon fiber

289

precursors (Fig. 6) demonstrate highly transparency in the ultraviolet region (200–282 nm and 287– 12


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290

399 nm) and in the visible region (400 to 800 nm), whereas, sensitive very high UV absorption

291

occurs in UVB region (280–320 nm) with transmittances ≈ 0%.

292

293 294

Fig. 6 UV-Vis absorbance spectra of thin films for MWCNTs-g-PAN carbon fiber precursors

295 296

On the other hand, absorptivity values (ε) of carbon fiber precursors at different weight fraction of

297

MWCNTs (χCNTs) are shown in Table 3. High absorptivity occur using smaller thickness thin layer

298

and lower weight fraction MWCNTs as seen in Fig. 7a. Such behavior is due to long chain nano-

299

fibril structures and minimum free volume between chains that allow easy movement of electrons in

300

the π-conjugated crystalline regions and hence retain energy. However, low absorptivity occur at

301

higher weight fraction MWCNTs and thicker thin layers due to short chain wrap-up conjugated

302

structures, as in Fig. 4, and large free volume between chains, which hinder absorptivity and

303

dissipate energy.

304

Furthermore, the nonlinear behavior of absorptivity (ε) versus weight fraction of MWCNTs (χ)

305

seen in Fig. 7a can be mathematically described as power law relation as follows;

306

ε ∝ χα

(2)

307

ε = k χα

(3)

308

where k and α are constants related to configurational and geometrical nanostructures of the carbon

309

fiber precursors. By taking power law (ln-ln) for both sides of relation 3;

310

ln (ε) = ln k + α ln (χ)

13


(4)


311

The plot of ln (ε) versus ln (χ) should demonstrate linear behavior with ln k and α as intercept and

312

slope, respectively. Fig. 7b demonstrate linear dependence of absorptivity (ε) on weight fraction of

313

MWCNTs (χ) with linear regression coefficient (R2) of 0.9822. The constants k and α determined

314

from intercept and slope values were 1.3x104 and –1.3 cm–1(wt%)–1, respectively. Such linear

315

dependence allows us to calculate the absorptivity value upon the use of χ= 0.025%. In such case,

316

the absorptivity calculated theoretically is forty four-fold increase than at χ= 0.5%.

317

Table 4. Optical properties of MWCNTs-g-PAN carbon fiber precursors Thickness a π–Conjugated compound ε (µm) (cm–1(wt%)–1) 236 42373 MWCNTs-g-PAN (χCNTs (%) = 0.50) 481 10395 MWCNTs-g-PAN (χCNTs (%) = 0.99) 602 2769 MWCNTs-g-PAN (χCNTs (%) = 2.91) 623 891.7 MWCNTs-g-PAN (χCNTs (%) = 8.26) a Thickness of thin film measured by digital micrometer.

318 319

Eg (eV) 4.37 4.38 4.37 4.36

λmax (nm) 284.5 284.0 284.5 285.0

12.0 40000

10.0

(a)

30000

(b)

8.0

ln (εε)

ε (cm–1 (wt%)–1)

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20000

6.0 ln(ε) = –1.3424 ln(χ) + 9.4867 R² = 0.9814

4.0 10000

2.0 0.0

0 0.0

2.0

4.0

6.0

8.0

-1.0

10.0

χCNTs (%)

0.0

1.0 ln (χ χCNTs)

2.0

320 321 322 323

Fig. 7 (a) Change of absorptivity (ε) versus weight fraction of MWCNTs in the carbon fiber (χCNTs), (b) Power law of ln (ε) versus ln (χCNTs)

324

On the other hand, midpoint λmax for carbon fiber samples were in 284–285 nm range. Such

325

sensitive discrete energy cutoff peaks lie in the UVB region and correspond to optical bandgaps of

326

4.36–4.38 eV (Table 4). Such range of bandgaps situates carbon fiber precursors as wide-bandgap

327

materials with promising properties. The promising properties include; easy fabrication, excellent

328

thermal stability, sharp cutoff absorptivity, wide-bandgap materials, and carbon fiber properties.

329

Such outstanding specifics adapt MWCNTs-g-PAN carbon fiber precursors to play central focal

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role in the fabrication of optoelectronic devices, organic electronics, and tandem solar cells, which

331

can have novel applications such as; control of plants growth, production of Vitamin D, and as

332

dermatologic therapy devices. 14


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333

CONCLUSION

334

Fabrication of MWCNTs-g-PAN carbon fiber precursors via grafting of polyacrylonitrile on the

335

surface of acid functionalized multi walled carbon nanotubes performed. The chemical structure of

336

the carbon fiber precursors verified via 1H-NMR and FTIR means implied successful grafting and

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consequent stabilization process with extent of stabilization equals 3.0. Such carbon fiber precursor

338

form lightweight thin films with micro-scale thickness and demonstrate excellent thermal stability

339

against degradation up to 260 °C. The lightweight transparent thin films display selective high

340

absorptivity of UVB radiation that corresponds to 4.36–4.38 eV wide-bandgap materials. The

341

absorptivity (ε) of the thin films found to depend linearly on weight fraction of multi walled

342

carbon nanotubes (χCNTs); at χCNTs (%) = 0.50 large absorptivity of UVB radiation occurred due to

343

long chain aligned nano-fibril structures. On the contrary, at χCNTs (%) = 8.26 small absorptivity of

344

UVB radiation occurred due to the coaxial wrapping of short length PAN chains around carbon

345

nanotubes forming flattered and thicker nanotubes.

346 347

ACKNOWLEDGMENT

348

Authors wish to acknowledge Jordan University of Science & Technology, Irbid, JORDAN for

349

financial support and facilities.

350 351

352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372

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