Journal of Materials Processing Technology 209 (2009) 4617–4620

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Journal of Materials Processing Technology

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Hot-pressed electrospun PAN nano fibers: An idea for flexible carbon mat

Ashraf A. Ali a,∗, G.C. Rutledgeb

a Mechanical Engineering Department, College of Engineering in AlKharj, KSU, Saudi Arabiab Chemical Engineering Department, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

a r t i c l e i n f o

Article history:Received 22 July 2008Received in revised form 20 October 2008Accepted 17 November 2008

Keywords:ElectrospinningNano fiber matCarbonMechanical propertiesRaman spectroscopy

a b s t r a c t

Fibers on the nano scale are characterized by its high surface area per unit mass which is associated withhigh surface free energy. It seems to be an interesting idea to take advantage of this high free surface energyin electrospun in-plane randomly oriented (quasi-isotropic) multi-layered PAN fibrous mat by stabilizingthe as electrospun structure at 220 ◦C under suitable pressure in oxygen environment; such treatmentnot only activates its high surface energy but also allows the contribution of larger number of moleculeson the nano fiber surfaces as well as enhances the bonding between fibers. Mechanical examinationof the hot pressed electrospun PAN Nano fibers mat showed higher flexibility than commercial carbonfiber as well as 2-D structure that can be advantageous in applications where the forces are equal in alldirections and no specific orientation are required. Stress–strain curves of the hot pressed electro-spunPAN mats showed ductile behavior. The absolute values for tensile strength ranged from 55 to 63 MPasimilar to some ductile pure metals such as aluminum, with much larger strain (6.5–8.25%). The modulusvalue for the fabric was found to be a measure for the enhanced surface free energy (nano size bond)and not the measure for single nano fiber properties, which was proved by preliminary examination forthe fracture pattern of the fabric mat. This revealed de-lamination between the mat’s layers breakingthe bond in between within a value approximately equal or slightly larger than the modulus for bulk

PAN polymer (2.80–3.04 GPa) with measured Poisson’s ratio of 0.33. Raman analysis for the hot pressedsamples showed a formation of disordered carbon structure at 1360 cm−1 and ordered carbon structure

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. Introduction

Fitzer and Heine (1998) stated, based on literature survey as aart of Chapter 3 in Book titled fiber reinforcements for compositeaterials, that; “fibers of polyacrylonitrile (PAN) are the precursor

f 90% of commercial carbon fibers”. Also, it is generally thoughthat the better the degree of molecular orientation in the originalAN-based carbon fibers, the better the mechanical properties, inarticular the modulus, of the resultant fibers. Based on the previ-us two statements research interest in the area of electrospun PANano fibers can be proudly explained.

Warner et al. (1998) explained the threshold of the jet forma-ion in the electrospinning process as a high electric field is appliedo a hanging droplet of polymer solution contained in a capillary

ube, when the applied electric field overcomes surface tension aharged jet of the solution is ejected. Ali (2006) utilized electrospin-ing process to produce different ultra fine PAN structures such asoil like shape, ultrafine monofilament and nano fibroses mat. In

∗ Corresponding author. Tel.: +966 5455860x130; fax: +966 5455728.E-mail addresses: afattah@ksu.edu.sa, hassanien65@yahoo.com (A.A. Ali).

924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.jmatprotec.2008.11.027

© 2008 Elsevier B.V. All rights reserved.

his paper it has been explained that; nano fibroses mat producedupon removal of the solvent by either of two ways: the evapora-tion of the solvent through the distance between the spinneret andthe collector or by dissolving the solvent inside a coagulating bath,the former one will be used in the present work as it produces theminimum fiber diameters.

Interest in the electrospinning process has increased in recentyears; Doshi and Reneker (1995) published a series of articles in theelectrospinning of polymer solutions which opened the door formany other researcher in this area. Also, Reneker and Chun (1996)studied the idea of making nano fiber membranes out of electro-spun polymers and studied the formation of beaded structuresduring the processes which should be avoided in this present work.Ali et al. (2002) discovered the formation of PAN nano fiber yarnunder certain conditions during the electrospinning of PAN/DMFpolymer solution. Also, Ali and El-Hamid (2006) optimized the elec-trospinning process for precursor carbon nano fibers from PAN/DMF polymer solution which has to be considered as a process-

ing condition reference for the present work. Baumgarten (1971)has also correlated spinning atmosphere with the occurrence ofthe jet-splaying phenomenon in PAN system. One of the mostimportant applications of nano PAN fibers after heat treatmentis in the nano composite structure with CNTs to produce nano

4618 A.A. Ali, G.C. Rutledge / Journal of Materials Processing Technology 209 (2009) 4617–4620

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lected fiber mat has been trimmed then the mat placed in betweentwo aluminum plates of 110 mm × 105 mm dimensions and 10 mmthickness after covered with aluminum foil. The mold with the matin between has been placed in a hot press set, CARVER model USAof maximum 10 metric tons and 500 ◦C, to reach 220 ◦C with no

Fig. 1. Electro-spun PAN nano fibers m

arbon–carbon composite fibrils. Ko et al. (2002) showed the possi-ility of CNT alignment in electrospun PAN/DMF polymer solution.lso, Ko et al. (2003) measured the mechanical properties of singleAN nano fiber before and after carbonization by using AFM andound its modulus values in the range of commercial carbon fibers.lso, Mack et al. (2005) used platelet shape of nano Graphite withAN/DMF system to produce nano fibril carbon composite afterarbonizing the electrospun nano fibroses structure.

Electrospun nano fibers are characterized by its high surfacerea; this characterization showed a different behavior for the nanoized fibers like super absorbent characteristics for electrospunAM nano fibers studied by Ali (2008). Also, nano reinforcementize effect has been measured by Ko et al. (2003) for carbon nanoube and by Mack et al. (2005) for Graphite nano platelet which haseen correlated to the unusual behavior of molecular chains on theurface of nano scale fibers. It is well known that, atoms at a freeurface experience a different local environment than do atoms inhe bulk of a material; as a result, the energy associated with thesetoms will, in general, be different from that of the atoms in theulk. The excess energy associated with surface atoms is called sur-ace free energy. In traditional continuum mechanics, such surfaceree energy is typically neglected because it is associated with onlyfew layers of atoms near the surface and the ratio of the volumeccupied by the surface atoms and the total volume of material ofnterest is extremely small. However, for nano size particles, wiresnd films, the surface to volume ratio becomes significant, and sooes the effect of surface free energy. The effect of surface energyn the elastic behavior at the nano level has been studied in the lit-rature. For example Dingreville et al. (2005) showed 20% increasen the axial Young’s modulus for 4 nm diameter copper wire. Also,anda et al. (2003) showed 5–6 times increase in material surfacenergy for Ag nano particles relative to bulk.

He et al. (2007) found a size effect in the elastic property oflectrospun PAN nano fibers below 150 nm.

In the other hand, nano diamond nucleation has been reportedor carbon nano materials below the required pressure and tem-erature for similar bulk materials. Sun et al. (2005) demonstratedhe formation of chain like diamond nano wires from MWCNTs at011 cm−2 nucleation density with diameters ranged from 4 to 8 nmnd 200 nm long. Also, Guillou et al. (2007) showed nano diamonducleation and growth below 2273 K temperature and 15 GPa pres-ure, they also mentioned that; graphitization of disordered carbons not a requisite to the formation of nano diamonds.

So, it can be summarized from the previous literature that,aterials in nano level behave physically, mechanically and ther-odynamically different than bulk materials. Also, electrospun

ber mats are characterized by their random pattern and nano sizebers.

) As electro-spun and (b) hot-pressed.

The present study is an attempt to use the high surface energy ofthe electrospun PAN nano fibers as a cohesive bond to build a firmstrong precursor carbon nano fibers fabric. Also, it uses the highdegree of entanglement inside the nano fibrous structure to gain ahigher degree of flexibility in the produced fabric.

2. Experimental work

2.1. Electrospinning and hot-press processes

Polyacrylonitrile (PAN) of 86,200 g/mol molecular weight fromAldrich has been used with10% weight concentration withDimethylformamide (DMF) to form a polymer solution after hotstirring for 3 h at 60 ◦C to ensure a complete solubility. The PAN/DMFpolymer solution has been poured to fill a clean syringe of 10 ml vol-ume, the syringe placed in a controlled pump set to give flow rateof 0.025 ml/min. the syringe connected to a rubber tube of 30 cmlength and 2 mm inner diameter, the other side of the rubber tubehas been connected to a metal tube of 1.018 mm inner diameter and35 cm length, the metal tube has been placed in a hole drilled in acircular aluminum disc of 15 cm diameter and 1 cm thickness, thealuminum disc has been connected to the power supply at 38 kV ofpositive charge. A metal screen collector of 15 cm × 15 cm dimen-sions has been centered vertically with 30 cm distance away fromthe orifice of the metal tube and covered with aluminum foil. Thecharge has been applied first then the pump flow rate to ensure nodroplets on the collected aluminum foil, the electro-spun fibers hasbeen collected for about 8 h. About 2 cm from each side of the col-

Fig. 2. Machine stress–strain of hot-pressed electro-spun PAN nano fibers mat.

A.A. Ali, G.C. Rutledge / Journal of Materials Processing Technology 209 (2009) 4617–4620 4619

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pplying pressure for 1 h till the plates’ temperature reaches theaximum set temperature. Then 1 metric ton has been applied for

nother 1 h. The hot press shut off to cool down for another 1 h keep-ng the pressure till it reaches 100 ◦C then the pressure has beeneleased completely from the fabric except the weight of the upperluminum plate till it cooled down to room temperature. Also, Mor-hological and chemical structural behavior of the nano fiber matsefore and after hot pressing have been investigated using SEM andaman spectroscopy respectively.

.2. Tensile test

Five tensile samples of approximately 60 mm length and 5 mmidth have been cut from the hot-pressed nano fibroses mat. Each

ample has been weighted by 3 digits balance. Zwick machine with00 N maximum load cell and 1 �N accuracy has been used withn optical extensometer set-up to investigate the tensile proper-ies (strength, modulus and Poisson’s ratio) of the hot pressed nanobroses fabric. Also, Preliminary fracture patterns of the tensileamples have been investigated using an optical microscope.

. Results and discussion

.1. Morphological and mechanical properties results

SEM pictures Fig. 1 show the fiber diameters and pattern for thes electrospun PAN and the hot-pressed one. It has been found thathe fiber kept its morphological behavior after the hot pressing.

lso, the measured average diameter for the as electrospun nanobroses mat found to be 139 nm with 12 nm standard deviation and34 nm with 33 nm standard deviation for the hot pressed one.

The stress–strain behavior of five samples has been tested andhown in Fig. 2, the elastic–plastic behavior of the samples was

Fig. 4. Fracture patterns for tensile sample. (a) De-lamina

Fig. 5. Raman analysis for as electrospun and hot pressed mats.

similar to the behavior of many other materials such as metalsand polymers and was not similar to the behavior of commer-cial in-plane random fiber mats that affected by the orientationof their fibers in the applied force direction. However, such abehavior indicates the absence of the single nano fiber role in themeasured stress–strain. The measured maximum and minimumtensile strength for the hot pressed samples found to be 63.17 MPaand 55.58 MPa simultaneously while the measured average ten-sile strength for the as electrospun mat samples found to be equal8.73 MPa with 3 MPa standard deviation.

The optical extensometer has been used to measure the actualstrain of the samples and to eliminate the machine compliance andsample to grip slip. Each sample has been marked with four linestwo horizontal and two vertical to measure the strain in both direc-tions, the optical extensometer has been attached by laptop runningwith software program to detect the difference between the twomarked lines during the test and as function of the test time whichis already correlate to the tensile (Zwick) machine strain collectedby its attached computer.

The measured Poisson’s ratio for the fabric found to be 0.33.Also, Fig. 3 shows the difference between machine stress–strain andthe one calculated based on the measured strain from the opticalextensometer, the later has been used to calculate the modulus ofelasticity of the fabric which has been found within the range of thePAN bulk values, that may be explained as the indication for bondstrength between the fibers and not the modulus of single nano

fiber that has been measured in previous works (He et al., 2007;Ko et al., 2003; Mack et al., 2005) and found to be much larger.The fabric modulus value for the hot pressed samples found to be2.89 GPa with 0.1 GPa standard deviation.

tion plan profile and (b) De-lamination side profile.

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620 A.A. Ali, G.C. Rutledge / Journal of Materia

Preliminary study for the fracture patterns as shown in Fig. 4as been used to show the de-lamination between the fabric layers.his de-lamination indicates the bases of the mechanical propertiesesults, which has been used to interpret the low modulus value ofhe tested mat. More investigation for the fracture patterns will beiscussed deeply in another paper.

.2. Raman analysis

Raman analysis for the hot pressed and as electrospun samplesan be shown in Fig. 5. Two peaks at 1360 cm−1 (D-Breathing Band)nd 1580 cm−1 (G-Breathing Band) have been detected for the hotressed sample indicates the formation of disordered and orderedraphite structures respectively.

. Conclusions

The conclusions can be summarized in the following points:

1. The advantage of high surface free energy in electrospun PANnano fibroses structure of 139 ± 12 nm fiber diameter has beenused to build a new non-woven precursor carbon nano fibrosesfabric.

. The non-woven nano fibroses fabric behaves mechanically sim-ilar to ductile materials with 0.33 Poisson’s ratio. It is alsocharacterized by its high flexibility (8.5% strain to fracture).

. Preliminary study for the fracture patterns showed a de-lamination between the mat’s layers showed minimumthickness in the range of 900 nm. More investigation for the frac-ture patterns of hot pressed samples with and without CNT willbe discussed in another paper.

. The measured mat strength (59.55 ± 2.5MPa) and modulus(2.89 ± 0.1 GPa) found to be corresponding to the bond betweennano fibers and not the single nano fiber property.

. Raman analysis showed formation of ordered and disorderedgraphite structure for the hot pressed mats.

The previous conclusions demonstrate the ability to use the sur-ace energy as a cohesive bond to build a flexible carbon nanobroses fabric. High resolution morphological analysis (AFM or/andRTEM) to examine the level of graphite order and to check theossibility of forming nano diamond like structures as well as elec-

rical conductivity measurements are essentially required to judgehe innovation of the proposed technique to build new flexibletructural nano fibroses mats. Potential applications of these mats thermal resistant or wear coated layers or as a reinforcementaterial with CNT for ballistic, armor, space shuttles and similar

essing Technology 209 (2009) 4617–4620

applications that require mechanical and thermal stabilities areproposed.

Acknowledgment

The first author would like to thank Fulbright Commission(Egypt Office) for sponsoring his visit to institute for solder nan-otechnology, MIT, USA where the core of this research work hasbeen achieved.

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