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Polymer 54 (2013) 1813e1819

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Polymer

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High flame resistant and strong electrospun polyacrylonitrileecarbonnanotubeseochre nanofibers

SungCheal Moon*, Todd Emrick**

Polymer Science and Engineering Department, Silvio O. Conte National Center for Polymer Research, University of Massachusetts Amherst, Amherst, MA 01003, USA

a r t i c l e i n f o

Article history:Received 14 November 2012Received in revised form5 January 2013Accepted 19 January 2013Available online 9 February 2013

Keywords:Flame resistanceOchreNanofibers

* Corresponding author. Tel./fax: þ82 62 655 7252.** Corresponding author. Tel.: þ1 413 577 1613; fax:

E-mail addresses: scmooon@gmail.com (S.Moon), t(T. Emrick).

0032-3861/$ e see front matter � 2013 Elsevier Ltd.http://dx.doi.org/10.1016/j.polymer.2013.01.053

a b s t r a c t

Environmentally friendly, colorful, and flexible electrospun polyacrylonitrile (PAN)ecarbon nanotubes(CNTs)eochre (Oc) nanofibers (as-spun, stabilized) with very high flame resistance (heat release capacityof 24e143 J g�1 K�1, total heat release of 2.1e8.7 kJ g�1, char yield of 55e74%, limiting oxygen index of22.5e34.5%) and mechanical properties (ultimate tensile strength of 80e177 MPa) were produced usinga mineral, non-toxic, economic Oc and CNT in 1% CNTs and 10% Oc based on polymer concentration froma predominant synergistic effect between CNTs and Oc. This approach using a mineral material is anenvironmentally friendly method which may possibly solve a number of problems related to materialsscience and economics such as improving a flame resistance and lowering the cost for various appli-cations in automobile, protective textile, and other areas.

� 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Thedamage tohuman lifebyfire and the toxicgas it generateshasbeen increasing, but the dilemmas between flame retardation andtoxicity still remain, although smoking and fire deaths are rapidlydecreasing [1e3]. Proposed new flammability regulations could addtens of millions of additional pounds of potentially toxic flame-retardant chemicals such as bromine- and chlorine-containingflame retardants to bed clothing and foam used in upholstered fur-niture [1e3]. In California, Assemblyman Mark Leno introduced AB706, a bill that authorizes the state to consider human health andenvironmentally impacts, as well as fire safety, when regulatingflammability. This bill would prohibit the most toxic classes ofchemicals in furniture, mattresses, and bed clothing and stop thecycle of replacing one toxic flame retardant with another. New Eu-ropean regulations for the Registration, Evaluation, and Author-ization of Chemicals (REACH) require industry to provide data toestablish the safety of new and existing chemicals [2]. Flameretardant chemicals in ourhomes shouldnotposeagreaterhazard toour health and environment than the risk of the fires they are sup-posed to prevent. Equivalent or greater fire safety can be achievedwith new technologies and materials, furniture design, and green

þ1 413 545 0082.semrick@mail.pse.umass.edu

All rights reserved.

chemistry [2]. One strategy, to reduce a smoke densitywithout toxichalogen gases is to induce char formationwhich can act as a barrierand prevent flame spread during a combustion process.

The influence of nanoscale materials such as clays, carbonnanotubes has shown to improve the flame resistance, thermal,mechanical, and electrical properties of polymers [4e10]. However,there are no reports in the literature investigating the flameresistance properties of ochre (Oc, a mineral) in electrospun poly-mer nanofibers system even though they have been widely used inflame resistant and biomedical applications, and other areas.

Electrospinning enables production of continuous polymernanofibers that can be used in protective clothing, biomedicine,composites, and other areas, these nanofibers are expected topossess high axial strength combined with extreme flexibility andvery high open porosity coupled with remarkable specific surfacearea [11].

Introduction of mineral fillers into a polymer makes it possibleto solve a number of problems related to materials science(extending the raw materials base, improving the flame resistanceproperty, etc.), technology (controlling the viscosity and thermalstability), and economics (lowering the production cost of poly-meric composites materials) [12].

Oc, economic material, is a natural composite composed ofkaolin (Al2O3$2SiO2$nH2O), montmorillonite (Al2O3$4SiO2$6H2O),pyrophyllite (Al2O3$4SiO2$H2O), illite (KAl2(OH)2[AlSi3(O,OH)10]),talc (3MgO$4SiO2$H2O), and iron oxides. Oc, which has a honey-combed/duplex structure and a high specific area, is among theearliest pigments used by mankind, derived from naturally tinted

Table 1Composition of the PANeCNTseOc solutions and average diameter of the electro-spun nanofibersmade at applied voltage of 16 kV, flow rate of 0.004mLm�1, take-upvelocity of 9.8 m s�1, and distance to target of 14 cm.

Samples Composition (%) Averagediameter (nm)

PAN CNT Oc DMF

10Pa 10.0 e e 90.0 155 � 2510Pe0.1SWCNTb 10.0 0.1 e 89.9 160 � 3410Pe0.2SWCNT 10.0 0.2 e 89.8 166 � 4010Pe0.2MWCNTc 10.0 0.2 e 89.8 175 � 4510Pe1.0Ocd 10.0 e 1.0 89.0 163 � 3510Pe2.0Oc 10.0 e 2.0 88.0 174 � 2810Pe0.1SWCNTe0.5Oc 10.0 0.1 0.5 89.4 161 � 3710Pe0.1SWCNTe1.0Oc 10.0 0.1 1.0 88.9 162 � 2710Pe0.1SWCNTe2.0Oc 10.0 0.1 2.0 87.9 170 � 3010Pe0.2SWCNTe2.0Oc 10.0 0.2 2.0 87.8 176 � 42St220ee10P-brown 10.0 e e 90.0 144 � 22St220e10Pe0.1

SWCNTe1.0Oc-brown10.0 0.1 1.0 88.9 153 � 25

St240fe10Pe0.1SWCNTe1.0Oc-dark brown

10.0 0.1 1.0 88.9 150 � 20

Commercial-StePANmicrofiber-black

e e e e 1051 � 89

a P: polyacrylonitrile (PAN) copolymer.b SWCNT: single-walled carbon nanotube.c MWCNT: multi-walled carbon nanotube.d Oc: ochre.e St220: stabilized at 200 �C/30 min / 220 �C/30 min.f St240: stabilized at 200 �C/60 min / 230 �C/60 min / 240 �C/60 min.

S. Moon, T. Emrick / Polymer 54 (2013) 1813e18191814

clay containing mineral oxides [12e14]. It can be efficiently used asa compatibilizer [14] and flame retardant with a smoke-suppressing effect from their instinct high absorption and viscousproperties, and variable inorganic components.

Here, we report on an environmentally friendly method toimprove the flame resistance using carbon nanotubes (CNTs) andOc with various inorganic components and high specific surfacearea in an electrospun polymer nanofibers system. The poly-acrylonitrile (PAN)eCNTseOc nanofibers without toxic flameretardant chemicals were prepared by convenient electrospinningtechnique considering various potential applications such asautomobile, protective textile, and other areas. The morphology,flame resistance, and mechanical properties of as-spun and stabi-lized electrospun nanofibers are reported herein.

2. Experimental

2.1. Materials

PAN copolymer with a molecular weight of 112,000 g mol�1 wasreceived from an industrial source. Dimethylformamide (DMF) wasobtained from Sigma Aldrich Co. Single-walled carbon nanotube(SWCNT) with diameter of 1.2e1.5 nm was obtained from SigmaAldrich Co., and multi-walled carbon nanotube (MWCNT) withdiameter of 3e20 nmwas purchased fromWako Pure Chemical Ind.Oc, dried and sterilized, was purchased from Chamtowon Co. (Re-public of Korea) [14]. The energy dispersive X-ray spectroscopy(EDS) data of aggregated Oc with nano-to-micron size were shownin Fig. 1.

2.2. Electrospinning set-up

The compositions of the PANeCNTseOc solutions are presentedin Table 1. All solutions of PANeCNTseOc in DMF were prepared atroom temperature under constant mixing. The electrospinning

Fig. 1. A) SEM images and B) EDS data of the

apparatus consists of a high voltage power supply, a syringe infusionpump, and a grounded stationary (a rectangular, 20 cm � 15 cm,aluminum foil) and rotating (10.2 cm diameter � 2.4 cm width)target. Polymer solution is loaded into a 10 mL syringe capped witha 18-gauge blunt needle and an electrode is clipped onto the needle.The needle, electrode, and grounded target are all enclosed in orderto reduce the effect of air currents on the trajectory of the electro-spun jet. The flow rate of the solution to the needle tip is maintained

aggregated Oc with nano-to-micron size.

S. Moon, T. Emrick / Polymer 54 (2013) 1813e1819 1815

so that a pendent drop remains during electrospinning. All airbubbles are purged prior to electrospinning and the solution iselectrospun at applied voltage of 9e14 kV and flow rate of 0.003e0.004 mL min�1 horizontally onto the target. The 12 cme14 cmdiameter grounded wheel is rotated from 0 to 1878 rpm produc-ing surface velocities ranging from 0 to 9.8 m s�1.

2.3. Structural characterization

The diameter and its distribution, surface appearance, andalignment of the electrospun PANeCNTseOc nanofibers (as-spunand stabilized) were observed by field emission scanning electronmicroscopy (FE-SEM, JOEL JSM 6320FXV). Electrospun nanofiberswere mounted onto SEM plates, sputter coated with gold, andexamined. The average diameter was determined bymeasuring thediameters of the electrospun nanofibers at 100 different points ona SEM image (20,000�) using the annotation and measurementtool on the JEOL program. The diameters are presented as theaverage � standard deviation.

The dispersion of CNTs and Oc in PANeCNTseOc nanofibers wasinvestigated by using transmission electron microscopy (TEM, JEOL1200EX). TEM analysis was operated at an accelerating voltage of100 kV. The PANeCNTseOc nanofibers were placed on a carboncoated copper grid for TEM investigation.

Fourier transform infrared analysis for structural character-ization of the pure and stabilized PANeCNTseOc nanofibers wascarried out on a Fourier transform infrared spectrometer (Nicolet6700, ThermoFisher Scientific, U.S.A).

2.4. Stabilization

Untreated, unidirectional, aligned, electrospun, PANeCNTseOcnanofibers were held at constant length and stabilized (oxidized)in a Thermolyne 4800 furnace. A length of yarns was held ongraphite sheets (3.5 cm � 12 cm). The oven was purged with pre-heated air and then heated to a specific temperature. The sample onthe sheet was placed into the oven, and heated according to a pre-set program. Stabilization was completed in a batch process toavoid unnecessary complexities involving heating rates. The sta-bilized nanofibers, which can be used for protective cloths andinterior materials with variable colors and ultra high flame resist-ance, were prepared from the control of exposure temperature andtime, and flow rate of oxygen [15].

2.5. Flame resistance

2.5.1. Pyrolysis combustion flow calorimetry (PCFC) analysisThe heat-release capacity (HRC), total heat release (THR), and

char yield (CY) of as-spun and stabilized electrospun PANeCNTseOcnanofibers were measured in order to evaluate the flammability forconversion to a highly condensed, thermally stable structure afterstabilization using pyrolysis combustion flow calorimetry (PCFC),micro-scale combustion calorimeter, according to ASTM D 7309[16,17]. Samples of 2e5 mg were pyrolyzed in a commercial device(CDS Pyroprobe 2000) to 900 �C at 1 �C s�1 under N2. The volatileswere swept out continuously by a N2 flow, mixed with a meteredflow of O2, and completely combusted at 900 �C. The consumptionrate of O2 was measured continuously and was used to determinethe heat release using standard methods of oxygen consumptioncalorimetry. The heat release results were taken as the average offive measurements for each sample [15,18,19].

2.5.2. Limiting oxygen index (LOI) analysisThe LOI is defined as the minimum oxygen concentration

required to maintain the downward flame combustion of the

materials. The LOI measurements were carried out in accordancewith ASTM D 2863 [20].

2.6. Mechanical testing

The mechanical behavior of electrospun PANeCNTseOc nano-fibers (as-spun and stabilized) was examined using an Instron 5564with a sample gauge length of 20 mm and a crosshead speed of2 mm min�1 (10% strain rate) in tension at room temperature. Theelectrospun nanofibers were mounted onto Manila paper tabssimilar to that used for single-fiber properties evaluation ofnormal-sized filaments. The original cross-sectional area of theelectrospun PAN nanofibers was used in these calculations and wasdetermined from the measured electrospun nanofiber denier andthe density (1.18 g mL�1) of PAN. The density of the stabilizedelectrospun nanocofiber is assumed to be 1.4 gmL�1, as determinedby ASTM D3800-9 of tow stabilized PAN fiber from an industrialsource. The ultimate tensile strength, initial modulus, and elonga-tion were measured [14,15,19].

3. Results and discussion

3.1. The morphologies of the electrospun PANeCNTseOc nanofibers

The electrospun PANeCNTseOc nanofibers with CNTs and Ocshow good alignment and average diameter of 155 � 25e176 � 42 nm in the range of 1% SW/MWCNT and 10% Oc based onPAN concentration, 16 kV applied voltage, 0.004 mL min�1

flowrate, 9.8 m s�1 take-up velocity, and 14 cm distance to target aspresented in Table 1 and Fig. 2.

As presented in Table 1, the average diameters (160 � 34e175 � 45 nm) of the electrospun 10P-0.1/0.2SW(MW)CNT nano-fibers with only CNTs increase with increasing the amount and sizeof CNTs compared to that (155 � 25 nm) of the electrospun 10Pbecause of the embedment and dispersion of agglomerated CNTsinto the nanofibers although a little CNTs stuck out as shown inFig. 3B and C. The amount of agglomerated and unembeddedMWCNTs increases due to large diameter and content (Fig. 3C).

The average diameters of the electrospun 10P-1.0/2.0Oc nano-fibers with only Oc and the electrospun 10P-0.1/0.2SW(MW)CNT-0.5/1.0/2.0Oc nanofibers with both CNTs and Oc show similar ten-dency with the electrospun 10P-0.1/0.2SW(MW)CNT nanofibersbecause of the embedment of Oc and CNTs into the nanofibers andcoating of Oc on the surface of nanofibers. The main elements of Ocsuch as Al, Si, and Fe are observed on the surface of the electrospun10Pe0.1SWCNTe1.0Oc nanofiber (Fig. 4).

As shown in Fig. 3E, the dispersion and embedment of CNTsimprove by the incorporation of Oc in the PANeCNTeOc systemcompared to the 10Pe0.1SWCNT nanofiber with only SWCNT. It in-dicates that the Oc should be used as a compatibilizer from its in-stinct properties such as high absorption andviscous properties [14].

3.2. Flame resistance of the electrospun PANeCNTseOc nanofibers

The electrospun PANeCNTseOc nanofibers were prepared(Figs. 2 and 3) to improve the flame resistance without toxic flameretardant chemicals.

We proved that Oc, mineral, economic and non-toxic material, isan effective flame retardant. And the synergistic effect from thecombination of CNTs and Oc for improvement of flame resistance isillustrated by the use of PCFC and LOI tester. The PCFC results showsimilar tendency with the LOI results as presented in Table 2.

The improvement of flame resistance (þ43% HRC, þ13% LOI) ofthe electrospun PANeCNTseOc nanofibers is much better thanthose (þ23e28% HRC,þ6% LOI) of the PANeCNTs (or Oc) nanofibers

Fig. 2. SEMmicrophotographs of the electrospun PANeCNTseOc nanofibers made at take-up velocity of 9.8 m s�1. A) 10P, B) 10Pe0.1SWCNT, C) 10Pe1.0Oc, and D) 10Pe0.1SWCNTe1.0Oc.

S. Moon, T. Emrick / Polymer 54 (2013) 1813e18191816

due to the increase of compatibility in the range of 0.1% CNTs and10% Oc based on PAN content.

As presented in Table 2, the HRC, THR, CY, and LOI of the elec-trospun 10P without CNTs and Oc are 253 � 8 J g�1 K�1,15.7 � 0.2 kJ g�1, 40 � 0.5%, and 19.5%, respectively.

The flame resistance of the electrospun 10Pe0.1SWCNT, withonly 1% SWCNT based on PAN content, increases (195 � 4 J g�1 K�1

HRC, 9.9� 0.2 kJ g�1 THR, 50� 0.3% CY, 20.7% LOI) by the dispersionand embedment of SWCNTs. Meanwhile, the flame resistance of theelectrospun 10Pe0.2SWCNT with large amount of SWCNT slightly

Fig. 3. TEM microphotographs of the electrospun PANeCNTseOc nanofibers; A) 10P, B)

increases compared to the 10Pe0.1SWCNT, thus the THR(12.2� 0.1 kJ g�1) increases and the CY (44� 0.5%) decreases in spiteof the low decrement of HRC (172 � 1 J g�1 K�1) and the lowincrement of LOI (21.0%). This suggests that low char formation isinduced by an inefficient embedment of agglomerated CNTs intonano-sized fibers from the decrease of compatibility and theincrease of CNTs content.

Furthermore, in case of the electrospun 10Pe0.2MWCNTnanofiber which contained MWCNT with larger diameter, theHRC increases to 201 � 1 J g�1 K�1, but the THR more increases to

10Pe0.1SWCNT, C) 10Pe0.2MWCNT, D) 10Pe1.0Oc, and E) 10Pe0.1SWCNTe1.0Oc.

Fig. 4. EDS data of the electrospun PANeCNTseOc nanofibers; A) 10P, B) 10Pe0.1SWCNTe1.0Oc.

S. Moon, T. Emrick / Polymer 54 (2013) 1813e1819 1817

12.8 � 0.2 kJ g�1 and the CY further decreases to 42 � 1.0%, also theLOI further decreases to 20.3% because of low compatibility fromlarge amount (2%) of MWCNT (Table 2, Fig. 3C). Therefore, weconfirmed that the content of SWCNTs of 1% based on PAN contentwas effective for improvement of the flame resistance of wellaligned electrospun nanofibers due to the increment of char for-mation from the orientation and introduction of CNTs.

Meanwhile, the 10P-1.0/2.0Oc nanofibers with only Oc showbetter flame resistance than the 10P-0.1/0.2SW(MW)CNT nano-fibers. The HRC of those decreases (181�5/171�1 J g�1 K�1), butthe CY increases (46 � 0.4 / 49 � 0.2%), even though the THR ofthose shows similar values (10.2� 0.1,10.5� 0.1 kJ g�1), also the LOIslightly increases (20.7 / 21.1%) with increasing Oc content due tothe increase of a char formation from an efficient embedment andcoating into/on the nanofibers as presented in Table 2 and Fig. 3D. Itindicates that the Oc, with viscous property, high specific surfacearea, and various inorganic components, is used as a flame retard-ant. Also, we confirmed that 10% Oc based on PAN content waseffective for improvement of the flame resistance of nanofibers.

Table 2Flame resistance and mechanical properties of electrospun PANeCNTseOc nanofibers.

Samples Flame resistance

Limiting oxygenindex (%)

Heat-releasecapacity (J g�1 K�1)

Total heatrelease (kJ g

10Pa 19.5 253 � 8 15.7 � 0.210Pe0.1SWCNTb 20.7 195 � 4 9.9 � 0.210Pe0.2SWCNT 21.0 172 � 1 12.2 � 0.110Pe0.2MWCNTc 20.3 201 � 1 12.8 � 0.210Pe1.0Ocd 20.7 181 � 5 10.2 � 0.110Pe2.0Oc 21.3 171 � 1 10.5 � 0.110Pe0.1SWCNTe0.5Oc e 184 � 4 10.1 � 0.310Pe0.1SWCNTe1.0Oc 22.5 143 � 3 8.7 � 0.210Pe0.1SWCNTe2.0Oc 22.1 150 � 1 9.6 � 0.210Pe0.2SWCNTe2.0Oc e 175 � 2 8.5 � 0.3St190ee10P-yellowish e 95 � 3 8.0 � 0.2St220fe10P-brown e 69 � 1 5.7 � 0.2St240ge10P-dark brown e 28 � 1 2.5 � 0.2St190e10Pe0.1

SWCNTe1.0Oc-yellowish25.1 89 � 3 7.8 � 0.2

St220e10Pe0.1SWCNTe1.0Oc-brown

31.0 59 � 1 5.3 � 0.3

St240e10Pe0.1SWCNTe1.0Oc-dark brown

34.5 24 � 1 2.1 � 0.2

Commercial-St-PAN-blackmicrofiber

34.0e31.1 23 � 1e51 � 4 2.9 � 0.2e

a P: polyacrylonitrile (PAN) copolymer.b SWCNT: single-walled carbon nanotube.c MWCNT: multi-walled carbon nanotube.d Oc: ochre.e St190: stabilized at 150 �C/120 min / 190 �C/120 min.f St220: stabilized at 200 �C/30 min / 220 �C/30 min.g St240: stabilized at 200 �C/60 min / 230 �C/60 min / 240 �C/60 min.

In particular, the electrospun 10Pe0.1SWCNTe1.0Oc nanofiberwith both 1% SWCNT and 10% Oc based on PAN content show thebest values (143 � 3 J g�1 K�1 HRC, 8.7 � 0.2 kJ g�1 THR, 55 � 0.4%CY, and 22.5% LOI). Thus, it shows that the HRC and THR of elec-trospun 10Pe0.1SWCNTe1.0Oc decrease significantly (over 40%) aswell as the hard CY increase predominately (approximately 40%)compared to those of the 10P by the addition both CNTs and Oc ofthose concentrations (Table 2). It indicates that a predominantsynergistic effect for improving the flame resistance induced by thestrong combination of SWCNT and Oc.

3.3. Flame resistance of the stabilized electrospun PANeCNTeOcnanofibers

The electrospun 10Pe0.1SWCNTe1.0Oc nanofiber with thehighest flame resistance was stabilized according to programmedconditions in order to induce the ultra-high flame resistance andthe various colors as well as the maintenance of flexibility for morevariable and special applications.

The ultra-high flame resistant, colorful, and flexible electrospunSt190w240e10Pe0.1SWCNTe1.0Oceyellowishwdark brown nano-fibers are produced by different stabilization conditions (190e240 �Cfor 60e240min) as shown inTable 2 and Figs. 5 and6. It suggests thatthe colors of those are induced by control of stabilization condition aswell as co-monomer for the enhancement of structural changes andquality of the carbon fibers [15,22,24]. They exhibit the very lowHRCof 24�1e89� 3 J g�1 K�1 and THR of 2.1�0.2e7.8� 0.2 kJ g�1, highCY of 55 � 1.0e74 � 1.0%, and high LOI of 25.1e34.5% due to theformation of a thermally stable aromatic ladder structure for thisstabilization condition [15,22,25,26].

The representative Raman spectra for the electrospun PANeCNTseOc nanofibers are shown in Fig. 6. Structural changes areinduced by the incorporation of CNTs [10,22,23]. The cyclizationand dehydrogenation reactions are observed during stabilization. Amajor decrease in the nitrile stretching at 2240 cm�1 and

Mechanical property

�1)Char yield (%) Ultimate tensile

strength (MPa)Initialmodulus (GPa)

Elongation(%)

40 � 0.5 78.5 � 2.2 4.3 � 0.4 25.4 � 2.550 � 0.3 94.2 � 1.0 3.6 � 0.2 30.0 � 1.544 � 0.5 99.1 � 0.6 2.3 � 0.2 47.6 � 2.642 � 1.0 109.2 � 1.2 1.8 � 0.2 44.5 � 3.246 � 0.4 72.6 � 0.3 2.7 � 0.1 28.5 � 2.049 � 0.2 65.5 � 0.2 2.6 � 0.1 36.8 � 2.746 � 0.1 81.5 � 1.9 3.4 � 0.1 29.1 � 1.955 � 0.4 80.1 � 1.5 3.2 � 0.2 28.8 � 2.349 � 1.0 76.2 � 1.3 3.2 � 0.1 34.3 � 2.051 � 1.0 78.6 � 2.0 2.5 � 0.4 39.7 � 1.850 � 1.0 145.5 � 3.3 3.4 � 0.2 20.6 � 1.657 � 1.0 150.5 � 5.3 3.9 � 0.3 11.0 � 2.068 � 1.0 152.5 � 2.5 4.3 � 0.2 7.3 � 0.755 � 1.0 147.9 � 4.6 3.7 � 0.2 17.0 � 0.5

64 � 1.0 155.2 � 3.1 4.3 � 0.1 11.0 � 0.2

74 � 1.0 177.2 � 3.0 4.8 � 0.4 7.4 � 0.2

8.5 � 0.5 64 � 1.0e63 � 0.5 176.6 � 2.9 3.0 � 0.2 20.4 � 0.4

Fig. 5. Photographs of very high flame resistant, colorful, and flexible electrospun10Pe0.1SWCNTe1.0Oc nanofiber with different stabilization conditions; A) St190e10Pe0.1SWCNTe1.0Oc-yellowish, B) St220e10Pe0.1SWCNTe1.0Oc-brown, C) St240e10Pe0.1SWCNTe1.0Oc-dark brown, and D) commercial-St-PAN-black microfiber. (Forinterpretation of the references to color in this figure legend, the reader is referred tothe web version of this article.)

S. Moon, T. Emrick / Polymer 54 (2013) 1813e18191818

methylene band at 2940 cm�1, and a growth of new band at1595 cm�1 in carbonyl-stretch frequency region is observed, alsoa band at 810 cm�1 stemming from the C]CeH group in the aro-matic ring is observed after stabilization of the precursor nano-fibers. As the temperature is increased, an increase indehydrogenation and cyclization reactions is observed [22,27]. Itindicates that the stabilization of the PANeCNTseOc nanofibers hasprogressed effectively considering the similar tendency with that ofthe commercial stabilized PAN microfiber at above 220 �C.

The HRC and THR of those decrease significantly by increasingthe exposure temperature and time, but the CY and LOI increasepredominately compared to those of as-spun. Thus, the flameresistance of the stabilized PANeSWCNTeOc nanofibers is betterthan those of the stabilized PAN nanofiber as well as thecommercial-StePAN-black microfiber.

Fig. 6. Raman spectra for the as-spun and stabilized PANeCNTseOc nanofibers; A) 10Pe0.11.0Oc-brown, D) St240e10Pe0.1SWCNTe1.0Oc-dark brown, and E) commercial-StePAN-bla

Fig. 7. SEM microphotographs of the electrospun St240e10Pe0.1S

The St240e10Pe0.1SWCNTe1.0Oc-dark brown nanofiber(Fig. 5C) stabilized at higher temperature (200 �C/60min/ 230 �C/60 min / 240 �C/60 min) shows the lowest values of HRC(24 � 1 J g�1 K�1) and THR (2.1 � 0.2 kJ g�1), and the highest CY of74 � 1.0% and LOI of 34.5% (Table 2). It exhibits extremely highflame resistance, and the HRC value is superior to those of manyknown polymers already in the low flammable category, such aspoly(vinylidene fluoride) (311 J g�1 K�1), poly(p-phenylene sulfide)(165 J g�1 K�1), poly(ether ether ketone) (155 J g�1 K�1), poly(etherimide) (121 J g�1 K�1), and poly(ether sulfone) (115 J g�1 K�1), aswell as polymers in the ultralow flammability category, such asbisphenol C polyether (16 J g�1 K�1), poly(ether ether ketone)(96 J g�1 K�1), and polybenzimidazole (36 J g�1 K�1), ether arehalogenated or possess the characteristically low solubility of rigidaromatic polymers [18,21].

3.4. Mechanical properties of the as-spun and stabilizedelectrospun PANeCNTeOc nanofibers

The ultimate tensile strength of the electrospun PANeCNTsnanofibers exhibits typical tendency because of the embedmentof CNTs into the nanofibers. Thus, the ultimate tensile strength ofthose increases by increasing the amount and size of CNTs. It isknown that despite a poor dispersion, loading of small filler such asCNTs improves substantially the mechanical properties [22,28].

The ultimate tensile strength of the electrospun 10P-0.1/0.2SW(MW)CNT nanofibers increases to 94.2 � 1.0 MPa,99.1 � 0.6 MPa, and 109.2 � 1.2 MPa, respectively (Table 2).

The ultimate tensile strength of 10Pe0.1SWCNTe1.0Oc nano-fiber with the highest flame resistance is 80.1 �1.5 MPa. Although,this value increases slightly compared with that (78.5� 2.2 MPa) of

SWCNTe1.0Oc, B) St190e10Pe0.1SWCNTe1.0Oc-yellowish, C) St220e10Pe0.1SWCNTeck microfiber.

WCNTe1.0Oc-dark brown nanofibers stabilized up to 240 �C.

S. Moon, T. Emrick / Polymer 54 (2013) 1813e1819 1819

the 10P, but it is quite a good value compared to that (55e75 MPa)of the commercial PC with various engineering applications such aselectric meter housings and covers, computer parts, exteriorautomotive components, and etc. [19].

In particular, the ultimate tensile strength (177.2 � 3.0 MPa) ofstabilized St240e10Pe0.1SWCNTe1.0Oc-dark brown nanofiberwith ultra-high flame resistance (24 � 1 J g�1 K�1 HRC), is muchhigher (approximately 121%) than that of as-spun (80.1 � 1.5 MPa)because of the increase of molecular and nanofibers orientation(Fig. 7) from the shrinkage (average diameter �7.4%, Table 1), andpartial cross-linking from the cyclization (Fig. 6), Also this is goodvalue compared to that (176.6 � 2.9 MPa) of commercial-StePAN-black microfiber. The elongation (7.4 � 0.2%) of that is lower thanthe value (20.4� 0.4%) of commercial one because of the differenceof diameter (Table 2) [15,22].

4. Conclusions

We introduced an effective method to produce the environ-mentally friendly, colorful electrospun PANeCNTseOc nanofibers(as-spun, stabilized) with very high flame resistance and mechani-cal properties using CNTs and economicOc, amineral, with aviscousand high absorptionproperties, and variable inorganic components.

Environmentally friendly, colorful, and flexible electrospunPANeSWCNTeOc nanofibers with average diameter of 144 � 22e162 � 27 nm and very high flame resistance (24e143 J g�1 K�1

HRC, 2.1e8.7 kJ g�1 THR, 55e74% CY, 22.5e34.5% LOI) were pro-duced using Oc in 10% and CNTs in 1% based on polymer concen-tration from a predominant synergistic effect between CNTs and Oc.

The ultimate tensile strength of electrospun 10Pe0.1SWCNTe1.0Oc nanofiber with the highest flame resistance was80.1 �1.5 MPa, and that was significantly increased (approximately121%) to 177.2 � 3.0 MPa after stabilization process because of theincrease of orientation and partial cross-linking caused bycyclization.

We believe these materials have a huge potential in variousapplications such as the interior of airplane, automobile, and pro-tective textiles, and other areas as an environmentally friendly highflame resistant and strong materials.

Acknowledgments

This paper is dedicated to the memory of Dr. Richard J. Farris.

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