Modification of polyacrylonitrile (PAN) carbon fiber precursor via post-spinning plasticization and...

Carbon 40 (2002) 25–45

Modification of polyacrylonitrile (PAN) carbon fiber precursorvia post-spinning plasticization and stretching in dimethyl

formamide (DMF)*J.C. Chen, I.R. Harrison

Polymer Science Program, Department of Materials Science and Engineering, The Pennsylvania State University, University Park,PA 16802, USA

Received 29 January 1999; accepted 15 January 2001

Abstract

This study investigates the possibility of using a post-spinning plasticization and stretching process to eliminate suspectedproperty-limiting factors in polyacrylonitrile-based carbon fibers. This process was performed with the intention of removingsurface defects (to improve tensile strength), attenuating fiber diameter (to promote more uniform heat treatment), andreducing molecular dipole interactions (to facilitate further molecular orientation). Among the various organic and inorganicsolutions tested, treatment using aqueous dimethyl formamide (DMF) offered far and away the best properties and wastherefore selected for further testing. Tested individually (as single filaments), fibers exposed to 80% DMF for 10 s gave thehighest precursor values of elastic modulus (9.07 GPa) and tensile strength (675 MPa). While fibers treated in 80% DMFgave a 73% improvement in elastic modulus and a 53% improvement in tensile strength over as-received PAN, limitations insample preparation and carbonization necessitated a reduction in DMF concentration (to 30%) to allow extraction ofindividual carbon fibers for tensile testing. Despite this compromise, results for fibers carbonized at 10008C ultimatelyshowed a 32% improvement in carbon fiber elastic modulus and a 14% improvement in carbon fiber tensile strength overregularly prepared carbon fibers. These results show that, to a certain extent, improvements in PAN precursor properties cantranslate to corresponding improvements in subsequently produced carbon fibers. Additional characterization using wideangle X-ray scattering (WAXS) and scanning electron microscopy (SEM) suggests that these improvements are due in partto improved lateral order as well as the successful elimination of surface defects and prevention of skin-core formation. 2002 Published by Elsevier Science Ltd.

Keywords: A. Carbon precursor; B. Carbonization; C. Scanning electron microscopy (SEM); X-ray diffraction; D. Mechanical properties

1. Introduction they are uniquely qualified for use in primary and sec-ondary structural components in the aerospace industry.

The act of drawing polyacrylonitrile (PAN) precursor Specifically, the primary structures of fighter aircraft todayfibers is essentially an attempt to improve mechanical require the use of ‘extremely-high-performance carbonproperties (particularly elastic modulus and tensile fibers,’ providing the driving force to produce carbonstrength) through molecular orientation. In this case, fibers with superior mechanical properties. It has also beenoptimization of PAN precursor fibers would ideally result deemed desirable to obtain carbon fibers with high strain toin enhanced performance of the resulting carbon fibers for failure (.2%) for use in the primary load-bearing struc-use in aerospace applications specified by the Air Force tures of next generation fighter aircraft [1,2]. Such prop-[1]. Since carbon fibers generally have the inherent combi- erties have in fact been realized in PAN-based carbonnation of high strength, high stiffness, and light weight, fibers produced by Toray Industries, Inc. Their T1000

grade fibers have a tensile strength of 7 GPa and a Young’s(elastic) modulus of 490 GPa [2]. Structurally speaking*Corresponding author. Tel.: 11-814-865-3130; fax: 11-814-(based on C–C bond strengths), it should be theoretically863-8675.

E-mail address: [email protected] (I.R. Harrison). possible to achieve an elastic modulus of 1000 GPa and a

0008-6223/02/$ – see front matter 2002 Published by Elsevier Science Ltd.PI I : S0008-6223( 01 )00050-1

26 J.C. Chen, I.R. Harrison / Carbon 40 (2002) 25 –45

Table 1tensile strength of 100 GPa, although these values haveClassification of carbon fibers based on HTT [7]been approached with little or no success (especially for

tensile strength) when adjusting PAN spinning and heating Carbon fiber category HTT Tensile Young’sconditions alone [1,3]. Post-spinning (pre-stabilization) (8C) strength modulusmodification of these fibers, on the other hand, may go a (GPa) (GPa)long way towards bridging the gap. Low Modulus Type 1000 1.72–2.41 138–172

While no unanimously accepted conclusions have been Intermediate Modulus (IM) Type 1500 3.45–4.13 241–276reached concerning the structure of PAN, some general High Modulus (HM) Type 2500 2.41–2.76 345–482theories have been widely agreed upon. The strong dipolarinteractions between the nitrile groups of PAN, along withthe close proximity of the groups in space, are largely a carbon fiber with about 50% the original precursor mass,believed to give rise to very large intramolecular dipolar at least 90% carbon by weight, and good mechanicaland steric repulsions which force the molecular chains into properties [10,12]. In contrast to the stabilization step, noan irregular helical conformation [4]. The twisted, kinked tension is applied to the fiber during carbonization, since atmolecule may be thought of as a more or less rigid this stage no amount of tension can further improve thestructure, fitting within a cylinder or rod. Parallel rods molecular orientation. The overall result is that the linearconstitute the major part of the ordered phase in a fibril, chain structure of the original PAN precursor has beeneach rod containing PAN in the helical shape [5]. The rods transformed to a planar structure in the final carbon fiber.are not in perfect alignment with respect to the rod ends, After stabilization, the resulting ladder polymer struc-but rather exhibit some misalignment [4,6]. The disordered tures undergo dehydrogenation and link up in the lateralregions connecting the rods are suggested to consist of direction, producing a graphite-like layer or ribbon struc-loops, folds, entangled chains, chain ends, defects, ture. Arranging themselves approximately parallel to thecomonomer sequences, tie chains, etc. Wide angle X-ray fiber axis, the graphite ribbons are said to pack togetherscattering (WAXS) data has revealed the lateral dimen- with two-dimensional order only and rotational disordersions of the ordered domains to be in the range of 5–20 between layers (i.e., a turbostratic structure) to formnm, depending on the thermal history of the precursor microfibrils [14–16]. The carbon fiber structure tends tofibers. The length of these domains is on the order of 8–10 become more graphitic (with improved molecular orienta-nm, approximately twice that of the disordered regions. tion) as the HTT is raised. Consequently, carbon fibers areWAXS and molecular model studies have estimated the generally divided into three broad categories depending onrod diameter to be |0.6 nm [5]. the HTT and the subsequent effects on mechanical prop-

In the heat treatment process which converts PAN fiber erties (as shown in Table 1) [7,17]. Although HM Typeto carbon fiber, the stabilization or oxidation step is fibers have high modulus, they also have a low strain tointended to prevent melting or fusion of the fiber, avoid failure. Since a high strain to failure is essential (par-excessive volatization of elemental carbon in the sub- ticularly in fiber-reinforced composites), IM Type and Lowsequent carbonization step [7,8], and thereby maximize the Modulus Type fibers are commercially predominant [18].ultimate carbon yield from the precursor. During stabiliza- Not surprisingly, the final mechanical properties oftion, the PAN fiber precursor is heated between 200 and carbon fibers are significantly influenced by the corre-3008C in air or in an oxygen-containing atmosphere under sponding properties of their precursors. In fact, a study hastension (to prevent shrinkage and maintain molecular revealed what appears to be a direct correlation betweenorientation), resulting in cyclization and the formation of a the primary Young’s moduli of various PAN-based pre-thermally stable aromatic ladder polymer [9–11]. Upon cursors and the resulting carbon fibers, as shown in Tablecyclization, the fiber changes in appearance from white to 2. A plot of carbon fiber modulus (E ) versus precursorc

dark brown or black and should not burn even if subjectedto an ordinary match flame (commonly referred to as the

Table 2‘flame test’) [8]. Stabilized fibers commonly contain fromElastic moduli for a variety of precursor fibers and correspondingabout 50 to 60% carbon by weight [12]. Oxidized PANcarbon fibers [19]

fibers result in greater yield of high-performance carbonFiber E E E /Efibers because they have preferentially an aromatic charac- p c c p

type (GPa) (GPa)ter preventing the backbone carbon chain from extensivesplitting. Toray (T ) 4.34 89.7 20.72

In proceeding to eliminate the noncarbon elements from Hercules 5.38 110.3 20.5Textile 5.86 133.1 22.7the stabilized PAN fiber as volatiles (i.e., H O, HCN, NH ,2 3CLRI, Madras 6.34 131.0 20.7CO, CO , N , etc.) [10], the fiber is further heated (at a2 2Toray (T ) 7.45 158.6 21.3lslow rate to retain molecular orientation) to a heat treat-Beslon 8.48 172.4 20.3ment temperature (HTT) of 800–30008C in an inertCourtella 9.31 193.1 20.7atmosphere during a carbonization step [13], giving rise to

J.C. Chen, I.R. Harrison / Carbon 40 (2002) 25 –45 27

modulus (E ) depicts a straight line passing through the minimal defects or irregularities and improved orientation.p

origin [19]. The average ratio of the two (E /E ) was Subsequent heat treatment of this modified precursorc p

found to be about 20 [6,19,20]. With this result in mind, it would ideally result in a uniformly stabilized fiber (withoutis then feasible to further improve as-received precursors a skin-core morphology) and eventually a carbon fiberby drawing them prior to stabilization in an attempt to with a high degree of orientation of graphitic planes, fewerobtain a corresponding improvement in the resulting defects per unit volume, and therefore dramatically im-carbon fibers. PAN fibers, unlike the majority of man-made proved mechanical properties (e.g., elastic modulus, tensilefibers, can be further stretched at temperatures above their strength, strain to failure). This study investigates thesecond-order transition temperature of |808C (but below possibility of improving precursor PAN fibers (and ulti-their decomposition temperature of |1808C) without relax- mately their carbonized counterparts) through this methodation on cooling and without removal of tension (shrinkage of modification.occurs when the temperature is raised above 808C withouttension) [8,21], allowing the PAN fibers to undergo a‘self-ordering’ process resulting in improved lateral and 2. Experimentalorientational order [20,22]. Prior studies have shown thatstretching PAN precursor fibers in the prestabilization stage 2.1. Materialsdoes indeed result in an increase in the Young’s modulusof the final carbon fiber. However, the stretching process 2.1.1. PAN precursor fiberapparently produced no significant effect on the tensile The PAN fibers used in this study were type SAFstrength of the carbon fiber [20]. (Special Acrylic Fiber), supplied by Courtaulds Fibers,

It is generally accepted that a major limitation to the U.K. [6]. This particular precursor was originally specifiedtensile strength of PAN-based carbon fibers is the presence as an atactic random copolymer of acrylonitrile, meth-of surface defects and large fiber diameters. Recently, there acrylate, and itaconic acid that is wet-spun from a sodiumhas been a trend towards thinning precursor fibers to obtain thiocyanate solution. The presence of these comonomerscarbon fibers with small diameters (4.5–5.0 mm compared initiates the stabilization or cyclization reaction and ulti-to the 7–8 mm diameters usually obtained), providing mately produces a carbon fiber of higher quality than thatuniformity of thermal stabilization in very short times and produced from PAN homopolymer alone [6,26,27]. As-carbon fibers with fewer defects per unit volume [2,23,24]. received fibers were obtained as tows of 6000 fibers withThe smaller resulting diameters reduce any temperature an average denier of 1.09 and a mean diameter of 11.3 mm.gradients across the fiber resulting in faster and moreuniform stabilization across the cross-section (thereby 2.1.2. Plasticizeravoiding formation of a skin-core morphology) [24]. As a post-spinning treatment, PAN precursor was plasti-

Elastic modulus, on the other hand, is controlled by the cized using aqueous organic solutions of N,N-dimethyl-degree of orientation of molecular chains (and ultimately formamide (DMF) (from Fluka Chemical) or dimethylsul-graphitic planes) in the fiber axis direction as well as the foxide (DMSO) (from Fisher Scientific) as well as aqueoussize of crystallites [15,23,25]. Since the dipole–dipole inorganic solutions of zinc chloride (ZnCl ) or copper (I)2

interactions among the nitrile groups obstruct the molecu- chloride (CuCl) (both from Fisher Scientific). Solutionslar chains from becoming oriented during stretching, contained concentrations ranging from 10 to 80% DMF,reduction of these interactions can make the drawing from 20 to 80% DMSO, from 10 to 30% ZnCl , or from 52

process more effective. In particular, the introduction of to 20% CuCl, in distilled water.solvent molecules and/or heat has been cited as aneffective means of decreasing these interactions [6]. 2.2. Infrared spectroscopy

Hence, an ideal post-spinning modification schemewould allow the removal of surface defects, attenuation of Infrared spectroscopy (in DRIFT mode) was performedfiber diameter, and molecular orientation prior to heat on SAF PAN precursor fibers to verify their compositiontreatment in order to obtain fibers with a good balance of (particularly comonomer types) and observe the effects ofstiffness and strength. It is conceivable that drawing a fiber stabilization and carbonization. IR DRIFT measurementsplasticized by aqueous organic or inorganic solvents at were made on a Digilab Model FTS-45 Fourier transform

21high temperatures (|1008C) could simultaneously produce infrared spectrometer using a resolution of 2 cm and 64these effects. Upon heating, the liquid film surrounding the scans per sample.wet fiber should become progressively richer in thesolvent, thereby partially dissolving or swelling the pre- 2.3. Heat treatmentcursor fiber. As the fiber is stretched, the solvent would beexpected to evaporate, precipitating the polymer in a Fibers were heat treated using a Lindberg (Modelhighly oriented form. Ideally, this process would produce a 55346) three-zone laboratory tube furnace. Fiber towsprecursor fiber with decreased diameter, a surface with were placed in a quartz glass tube extending through the


furnace. This setup involved first adding epoxy putty to the (50.8 mm). Upon loading the mounted sample in the gripsends of the drawn tow followed by tying a Kevlar fiber and of the Instron tester, the center portion of the tab was cuta stainless steel wire (0.0060 diameter) directly into the with scissors or burned with a hot wire (it was found to becuring epoxy at one end. The steel wire was left hanging preferable to use scissors for samples that were not heatfreely outside the tube, while the Kevlar fiber was tied to a treated to avoid heat damage from the hot wire). Thepost at one end. At the other end, a Kevlar fiber was tied sample was then stressed at a constant crosshead speed ofaround the tow and also tied to the opposite post. The 5.00 mm/min until failure. For each sample type, 10Kevlar fibers were intended to maintain tension as the tow samples were selected at random and tested.shrank during stabilization, while the steel wire allowedeasy sample removal after carbonization. 2.6. Drawing individual fibers

Stabilization was carried out under restraint at constantlength (i.e., the tow was tied at each end) in continuously Most fibers in this study were drawn prior to beingflowing air (4 l /min total) at a heating rate of 18C/min to tensile tested as usual. To facilitate drawing and eventual2308C. The furnace temperature was programmed to remounting of fibers, modified tensile tabs with a moreremain at 2308C for a period of 5 h. All three zones were accommodating 2 in. gauge length (as opposed to 1 in.)given the same heating program. Fiber tows that were were used for initial fiber mounting. As before, cementstabilized but not carbonized were then immediately was placed at the top and bottom ends of the slot to fix thequenched in air. fiber at the proper length. However, for the drawing

Fiber tows that were carbonized were done so immedi- procedure, tape was also applied over these cemented areasately after stabilization (i.e., after 5 h at 2308C) without (on both sides of the tab) to protect the cement fromdiscontinuity. Carbonization was performed without re- exposure to the plasticizer and thereby prevent fiberstraint (i.e., the tow was cut at one end) in continuously slippage. Those fibers that were plasticized were done soflowing inert nitrogen gas (4 l /min total) at a heating rate by floating the tensile tab (face up) in the plasticizerof 18C/min to 10008C with no hold time at 10008C. Upon solution prior to drawing. Those fibers that were drawn atcompletion of carbonization, fiber tows were then immedi- higher temperatures were done so using an environmentalately quenched in air. Again, all three zones were given the chamber designed for use with the Instron testing ap-same heating program. Carbon fibers produced by this paratus. Preliminary drawing was performed on fibers (fivemethod were of Low Modulus Type (as specified in Table samples for each type) at a crosshead speed of 5.001). Higher heat treatment temperatures (required for mm/min until sample failure in order to determine theproducing HM Type and IM Type carbon fibers) were not maximum possible extension. Subsequently, a high exten-possible using the given Lindberg furnace (maximum sion limit was set so that fibers could be drawn to a pointrecommended temperature for short duration use was just prior to failure and remain intact for tensile testing.specified at 10008C). Hence, new fiber samples were made and drawn this finite

amount (again at a crosshead speed of 5.00 mm/min).2.4. Fiber diameter determination These fibers were then carefully removed from the used

drawing tab and remounted onto the usual tensile tab (withSingle slit laser diffraction was performed on fiber the 1 in. gauge length). Fiber diameter measurement and

samples using a Melles Griot 10 mW Helium–Neon laser subsequent tensile testing of these drawn fibers was thento determine their corresponding diameters. Accurate performed in the usual manner.diameter measurements were essential to calculating thenecessary tensile properties. To ensure the reliability of 2.7. Drawing fiber towsthis technique, diameters calculated via laser diffractionwere first compared to those obtained via scanning electron In the case where previously drawn fibers were to bemicroscopy (SEM). subsequently stabilized and carbonized, entire fiber tows

were needed to accommodate the furnace and provide an2.5. Tensile testing individual fibers adequate quantity of fibers for testing. As a result, entire

tows were drawn on the Instron testing apparatus (asFiber mechanical properties (particularly tensile strength opposed to one fiber at a time) to the desired draw ratio

and elastic modulus values) were obtained using ASTM D prior to heat treatment and subsequent tensile testing.3379-75 guidelines and an Instron Static Mechanical In preparing fiber tows for drawing, 15.5 in. (393.7 mm)Tester (Model 4201) equipped with a 10 Newton load cell lengths of tow were first cut and then secured approximate-[28]. Randomly selected fibers were centered and mounted ly 1.5 in. (38.1 mm) from each end using epoxy putty.onto slotted paper tabs with a 1 in. (25.4 mm) gauge Tows that were to be solution treated were then saturatedlength. Cement was placed at the top and bottom ends of in the appropriate DMF solutions (30% DMF and 80%the slot to fix the fiber at the appropriate gauge length. For DMF) for specific times (1 min and 10 s, respectively).the actual testing setup, the grip distance was set at 2 in. Tows were then mounted into the Instron testing apparatus


and environmental chamber at a gauge length of 9.5 in. 2.9. Scanning electron microscopy (SEM)(241.7 mm). Through preliminary drawing, maximumpossible tow extension (just prior to failure) was pre- Fiber diameter and tow structure were observed using andetermined for solution-treated tows. Drawing (to the SX-40A scanning electron microscope from Internationalappropriate extension) was then performed on tows using a Scientific Instruments. Individual fiber cross-sections were1 kN load cell and a crosshead speed of 5.00 mm/min observed using a Philips XL 20 scanning electron micro-until sample failure. scope. Fiber and tow samples were gold coated prior to

analysis. Fiber tows used for cross-sectional views werefractured in liquid nitrogen prior to gold coating. In-

2.8. Tensile testing fiber tow samples dividual fibers used for cross-sectional views were subject-ed to actual tensile failure prior to gold coating.

Due to their extremely fragile nature, the carbonizedtows were again mounted onto modified paper tabs (cut 2.10. X-ray analysisthis time from heavy-stock paper) to provide a level ofstability while mounting samples into the Instron ap- Wide angle X-ray scattering (WAXS) was performed onparatus. The tensile testing setup used was nearly identical undrawn and drawn precursor tows in an attempt toto that mentioned previously for individual fibers mounted observe any improvements in orientation and/or crys-on paper tabs, except that this time a 1 kN load cell and a 1 tallinity. Flat-plate pinhole patterns were obtained usingin. grip distance (25.4 mm) was used. Upon loading the unfiltered Fe K X-ray radiation, a sample-to-film distancetensile tab in the grips of the Instron tester, the center of |8.0 cm, and an exposure time of |2 h. In addition, anportion of the tab was burned with a hot wire, and the azimuthal scan was performed with an X-ray diffractome-sample was then subjected to a crosshead speed of 5.00 ter using monochromatic Cu Ka radiation.mm/min until failure. For each sample type, three sampleswere selected at random and tested.

Precursor tows were found to be inappropriate for use 3. Results and discussionwith paper tabs due to the observed tendency of the towsto slip during testing. As a result, precursor tow samples 3.1. Verification of precursor fiber compositionwere prepared by cutting 3 in. lengths of tow and wrappingthe ends with masking tape, leaving a 1 in. gauge length. Infrared spectroscopy was used to verify the originalSamples were then tested, again using a 1 kN load cell, a specifications of comonomer types contained in the SAFgrip distance of 1 in. (25.4 mm), and a 5.00 mm/min precursor fibers. The spectrum obtained (shown in Fig. 1)

21crosshead speed. For each sample type, three samples were shows a characteristic peak at 2243 cm (due to nitrileselected at random and tested. stretch), indicating the likely and expected presence of

Fig. 1. IR spectrum of SAF precursor fiber.


21acrylonitrile. The peak at 1733 cm is assigned to Since the far-field approximation only applies at small um

carbonyl stretching. In conjunction with the peaks at 1073 values, calculation of fiber diameters using this equation is21and 1071 cm (most likely due to C–O–C bend), the most appropriate for first-order minima (m 5 1).

carbonyl peak seems to confirm the presence of meth-acrylate (H C=C(CH )CO R) as a comonomer. In con- 3.2.1. Laser diffraction vs. SEM2 3 2

junction with the broad absorption around 3600–3300 As a comparative study, fiber diameters were measured21cm (due to OH stretch) and the peaks in the 1300–1000 by both laser diffraction and scanning electron microscopy21cm range (due in part to C–O stretch), the carbonyl peak methods for various fiber types spanning a wide range of

also confirms the likely presence of an acid comonomer magnitudes. Specifically, five samples of carbonized SAF(probably itaconic acid (HO CCH C(=CH )CO H)). PAN fiber, five samples of as-received SAF PAN fiber, a2 2 2 2

Overall, there appears to be nothing in the spectrum sample of human hair, and a sample of polyethylenedirectly contradicting the claim that the SAF precursor terephthalate (PET) fiber were examined altogether. Afibers contain comonomers in the form of acrylonitrile, fiber-to-screen distance (D) of 40 cm was used in all cases.methacrylate, and itaconic acid. If anything, the original In calculating the fiber diameters via laser diffraction, thespecifications appear to be confirmed. distances of both first-order minima (m 5 1) from the beam

center (D9) were measured and averaged together. Thefibers measured via laser diffraction are the exact same3.2. Fiber diameter measurementsones observed using SEM. Fig. 2 shows the direct com-parison of fiber diameters measured by the two methods.For the purposes of tensile testing, it was necessary toBased on this comparison, fiber diameter measurement viaobtain a quick but reliable measure of fiber diameter tolaser diffraction appears to be in good agreement withensure reliable calculations of tensile strength and elasticSEM. As a result, laser diffraction was used to obtain fibermodulus. While scanning electron microscopy (SEM) candiameter information needed for the tensile testing pro-provide accurate representations of fiber diameter, it tendscedure.to be a time-consuming and tedious process since the

diameter measurement must be taken from the exact same3.3. Effect of stabilization and carbonizationfiber that is to be tensile tested. As a result, laser

diffraction was attempted as an alternate means of obtain-To get a first-hand look at the effects of stabilization anding fiber diameter. With the aid of a Helium–Neon (He–

carbonization on fiber properties, as-received untreated,Ne) laser, a screen composed of ground glass (ground sidestabilized, and carbonized PAN fibers were prepared andfacing the laser), and an optical bench, it was possible toheat treated as fiber tows and tensile tested upon extractiontake a fiber (already mounted on the tensile tab) andof individual fibers (10 fiber samples were tested for eachdiffract laser light from the fiber as it would from a singlespecimen type). A summary of the tensile results is shownslit [29]. This process causes the formation of alternatingin Fig. 3. Representative stress–strain behavior of theselight and dark bands (maxima and minima) to be projectedfibers is provided in Fig. 4. The effects of stabilizationonto the screen. The positions of the minima are given bywere generally unremarkable compared to the effects ofthe following equation:carbonization. Stabilization produced little change in elas-

a sin u 5 ml tic modulus, strain to failure, and fiber diameter comparedm

to untreated PAN but resulted in decreased tensile strengthwhere m is the order of diffraction, u the angle ofm and toughness. These decreased values were perhaps duediffraction (for order m), l the wavelength of light (He– to conversion of nitrile to conjugated C=N and theNe)50.6328 mm, and a the fiber diameter. formation of ladder polymers, causing a drop in cohesive

By the far-field approximation (i.e., Fraunhofer diffrac- energy between the relative chains [30]. Carbonization, ontion): the other hand, produced dramatic improvements in elastic

modulus and tensile strength and dramatic drops in straintan u | sin u .m mto failure, toughness, and fiber diameter, results that are to

Therefore, the following substitutions can be made: be expected from carbon fibers based on the formation of acyclized, planar structure. For these particular SAF PAN

a tan u 5 mlm fibers, the ratio of carbon fiber to precursor modulus(E /E ) is calculated to be about 16, which is relativelyc p

a(D9 /D) 5 ml close to the average value of 20 provided in the literaturefor selected PAN-based precursors and correspondingwhere D is the distance between fiber and screen, and D9carbon fibers [19].the distance between beam center and minima of order m.

Rearranging gives3.3.1. IR analysis

a 5 ml(D/D9). Infrared spectroscopy was performed on as-received


Fig. 2. Comparison of fiber diameters measured by laser diffraction and SEM.

precursor, stabilized, and carbonized fibers as shown in which stabilized fibers are subjected to an ordinary matchFig. 5. In this case, special focus was made on changes flame to see whether or not they burn [8]. When actual

21occurring in the nitrile group, in the 2260–2200 cm stabilized fiber samples were subjected to this flame testrange, since during stabilization, the nitrile group should using a Bunsen burner, they indeed did not burn andexhibit a tendency to be converted into C=N bonds to simply continued to glow in the flame. This result clearlyproduce ladder polymers [31]. As can be seen from the indicated an adequate stabilization heat treatment.spectra, the nitrile group shows a characteristically strong

21peak at 2243 cm for the as-received precursor (in 3.4. Effect of drawing plasticized PAN fibers21addition, there is a small peak at 2349 cm that appears

to be an overtone). Upon stabilization, the peak at 2243 PAN fibers were first tested for extensibility, then for21cm decreases in intensity, most likely due to reaction of elastic modulus, tensile strength, strain to failure, and

the nitrile group during cyclization to form conjugated or toughness. Preliminary drawing of these fibers at |1008Caromatic C=N [9,32]. In addition, there is the formation of until failure (five samples for each type) indicated an

21a new, small peak at 2337 cm that has been previously extension at maximum load. With a fair amount of surveyattributed to conjugated C=N [8,33,34]. Furthermore, the work, optimized solution concentrations and exposure

21formation of a large shoulder at 2216 cm is evident, times were determined (based on these extensibility mea-again probably due to nitrile conjugation with double surements) for each plasticizer solution used. Hence, fibersbonds or aromatic rings, which typically shifts the nitrile were then drawn a finite amount approaching this exten-

21absorption (22.43 cm ) to the right [9]. Taking into sion (enough to try to orient the fiber but not enough toaccount all these observations for the spectrum of stabi- break it). Upon remounting of these fibers onto thelized PAN, it seems more than likely that a cyclization appropriate paper tabs, fibers were tensile tested as usual.reaction did indeed take place. Finally, upon carbonization, Final tensile properties of PAN fibers saturated in various

21the peak at 2243 cm virtually disappears, indicating that organic and inorganic plasticizers and drawn at |1008Cmost of the remaining nitrile groups were lost in the form are given in Fig. 6. These results reflect the optimizedof HCN gas [35]. solution concentrations and exposure times for each plasti-

cizer solution used. As can be seen from the plot, the3.3.2. Flame test treatment using organic dimethyl formamide (DMF) clear-

As mentioned previously, the adequacy of the stabiliza- ly offered far and away the best properties among thosetion reaction can be evaluated by a simple flame test in tested and was therefore selected for further study. Spe-


Fig. 3. Tensile properties of as-received untreated, stabilized, and carbonized PAN precursor fibers: (a) elastic modulus; (b) tensile strength;(c) strain to failure; (d) toughness; (e) fiber diameter.

cifically, this post-spinning treatment provided a 73% 3.5. Effect of carbonization on DMF-treated fibersimprovement in elastic modulus and a 53% improvementin tensile strength over undrawn, untreated fibers. As demonstrated in other studies [36], even unremark-


Fig. 4. Stress–strain behavior of SAF PAN fibers: (a) as-received untreated; (b) stabilized; (c) carbonized.

able improvements in precursor properties sometimes modulus, E /E for SAF PAN fibers was already calcu-c p

translate into remarkable improvements in carbon fiber lated to be about 16. Since DMF-treated PAN precursorsproperties. Given the modest improvement in properties of gave modulus values as high as 9.07 GPa, it was thoughtDMF-treated PAN precursor, this possibility could not be that the corresponding carbonized fibers could provideruled out. Carbonized SAF PAN fibers have been found to modulus values as high as 145 GPa (a 74% improvementhave an elastic modulus of 83.5 GPa (compared to 5.24 over regularly prepared carbon fibers) assuming that theGPa for the as-received precursor). Keeping in mind the correlation held. With this prospect in mind, the effect ofreported linear correlation of carbon fiber and precursor carbonization on these modified fibers was investigated.

Fig. 5. IR spectra of SAF PAN fibers: (a) as-received untreated; (b) stabilized; (c) carbonized.


Fig. 6. Tensile results for PAN fibers plasticized in various aqueous organic and inorganic solutions and drawn at |1008C: (a) elasticmodulus; (b) tensile strength; (c) strain to failure; (d) toughness.


Fig. 6. (continued)

However, it cannot be overemphasized that obtaining such provided a maximum improvement of only about 40% inlarge improvements in carbon fiber properties is con- carbon fiber modulus [26].siderably more difficult than it might appear. In theliterature, post-spinning modification methods incorporat- 3.5.1. Tensile test results of towsing either catalysts, coatings, or plasticizers have generally The fact remains that when tested individually, pre-


cursor PAN fibers show the best properties when treated in data set. When considering E as a function of E for thesec p

80% DMF for 10 s at 1008C. However, obtaining test tows, the existence of linearity might be argued (butresults from the corresponding individual carbon fibers is perhaps not to the extent that the values are in line withextremely difficult (if not impossible) due to both the scale those reported for individual fibers). Additional data wouldof the carbonization process and the tendency of tow fibers be needed to verify such a claim.to fuse together into a single strand upon solution treat-ment in high DMF concentration. It was believed thatindividual fiber properties should be able to translate to 3.5.1.1. Morphological observations. Scanning electroncorresponding tow properties. As a result, an attempt was microscopy was conducted on various tow samples in anmade to actually tensile test entire tows of fibers with the effort to illustrate how solution modification and carboni-underlying objective of assessing the properties of carbon- zation can affect the observed properties of fiber tows.ized fibers that were initially treated in 80% DMF. Taking an untreated, undrawn PAN tow, the micrograph in

In preparing drawn tows, a maximum draw ratio of 1.06 Fig. 9 clearly shows that individual precursor fibers remain(maximum extension of 14 mm) could be given to tows separated while carbonized fibers show a greater tendencytreated in an intermediate concentration of 30% DMF (for to adhere to one another. More intriguing structural1 min at 808C), while a maximum draw ratio of 1.07 features manifest themselves with modification in 80%(maximum extension of 18 mm) could be given to tows DMF solution.treated in 80% DMF (for 10 s at 1008C). In comparison, As shown in Fig. 10, solution modification in 80% DMFthe maximum draw ratios for individually drawn fibers (saturated 10 s, drawn at 1008C) clearly causes thetended to be considerably larger (e.g., as described previ- individual PAN fibers to partially dissolve and fuse to-ously, individual fibers treated in 80% DMF could be gether. At higher magnifications, some interesting ‘rippled’given a draw ratio of 1.20). Upon extension, tows were textures are evident, probably a remnant of precursor fibersmaintained at the appropriate draw ratio for 30 min before that were twisted in the plane. These twisted fibers may inrelease. fact have contributed to the observed decreases in pre-

Results of the tensile tests are given in Fig. 7. Tensile cursor strain and toughness with solution modification (asstrength values for untreated, undrawn PAN precursor seen in Fig. 7), acting as entanglements and thereforeappear to translate well from individual fiber to tow, but limiting factors for extensibility (e.g., L /L 5 1.07 for this0

the similarities end there. General trends for the drawn tow compared to 1.20 for individual fibers).tows appear to be reversed from those observed previously Carbonization of these tows greatly facilitates the intro-for individually drawn fibers (treated in DMF). Here, duction of surface flaws (as shown in Fig. 11a) that wouldhigher draw ratios in higher solution concentrations result severely limit properties like tensile strength, strain toin overall decreases in elastic modulus, tensile strength, failure, and toughness. Other carbonized tows (that werestrain to failure, and toughness for both precursor and preferentially tensile tested) do not show such obviouscarbon fibers. surface defects as shown in Fig. 11b, but the tensile

These observed trends may be due to the introduction of properties were limited nonetheless (as previously ob-flaws in the surface of the drawn tow fibers or the served in Fig. 7). It is worth noting that, in some cases, theformation of internal voids, which may be much more twisted fibers observed for the precursor tows remaineddifficult to control on this macroscopic scale. Surface flaws apparent in the carbonized tows. With the fibers fusedand internal voids would clearly act as stress concentrators, together the way they were, it is understandable whysignificantly limiting the corresponding tensile properties. individual fibers could not be extracted from the tow.The resulting stress concentrations would be magnified in Focusing on the cross-sectional area of the solution-carbonized tows due to their extremely brittle and fragile treated tows revealed a number of property-limiting fea-nature. This possibility seems to be validated by the tensile tures (as shown in Fig. 12). The precursor tow samplestrength results for the tows. Here, tensile strength values shows the expected presence of internal voids, which limitare dramatically lower for carbonized tows than for the properties such as tensile strength. Likewise, the carbon-corresponding precursor tows (contrary to previous results ized tow sample shows two very prominent voids accom-for fibers tested individually). These results are at least panied by numerous smaller voids which can again ac-consistent with the hypothesis that tensile strength is count for the decrease in tensile strength compared toseverely limited by the presence of both internal and untreated, undrawn fibers (Fig. 7). Furthermore, closeexternal defects. Likewise, while precursor values for observation of the carbon tow cross-section verifies thestrain and toughness are still higher than the carbon values presence of a ‘skin-core’ type morphology. This morpholo-(consistent with the results for individually tested fibers), gy is most likely an indirect result of densification of thehere the carbon values are much lower in comparison tow during the drawing stage, preventing uniform heat(barely showing up on the given plots). transfer and oxygen diffusion from taking place across the

As shown in Fig. 8, data points for the tows do not even tow cross-section during stabilization. The end result isapproach the magnitudes of the original linear correlation that the tow was not uniformly stabilized from the inside


Fig. 7. Tensile results for precursor and carbonized tows initially drawn in various DMF concentrations: (a) elastic modulus; (b) tensilestrength; (c) strain to failure; (d) toughness.

out. The stress concentrations associated with this ‘skin- anticipated improvements in tow tensile properties. Suchcore’ morphology in combination with those arising from improvements would therefore seem limited to individuallybulk and surface flaws would be enough to deny any treated, individually drawn fibers.


Fig. 7. (continued)

3.5.2. Individual fibers extracted from drawn, carbonized entire fiber tows did not yield favorable results, the nexttows best method is to first draw and carbonize entire tows

Clearly, the most ideal method of accurately determin- followed by extraction of individual fibers for tensileing the effects of drawing and carbonizing fibers is to do testing (10 samples for each specimen type). In thisso using individual fibers (consistent with the previous context, a maximum solution concentration of only 30%precursor fiber testing using drawing tabs). However, since DMF could be used because higher DMF concentrationsthis method is not practical for carbonizing fibers (due to caused the fibers in the tow to fuse together, making thethe size requirements of the furnace), and since testing removal of individual fibers impossible. Using 30% DMF,


Fig. 8. Properties of drawn tows compared to original E vs. Ec p Fig. 10. SEM micrograph of PAN precursor tow modified in 80%data. DMF (saturated 10 s, drawn at 1008C).

tows could be extended 14 mm, providing a maximum towdraw ratio of 1.06. Upon receiving this extension, towswere either immediately released from tension or main-tained at this draw ratio for 30 min for comparisonpurposes. Individual fibers from these precursor or sub-sequently carbonized tows were then mounted onto tensiletabs (1 in. slot) for testing. Results of these tests are givenin Fig. 13. As shown in the plots, the benefit of maintain-ing extension for 30 min most clearly manifests itself inthe modulus results. While maintaining extension providesessentially no improvement in precursor modulus com-pared to immediate release, it does result in a 15%improvement in carbon fiber modulus (an example ofunremarkable improvements in precursor fibers giving wayto more notable improvements in their carbonized counter-parts). In addition, the benefit of saturating fibers in 30%DMF compared to undrawn, untreated fibers is also readilyapparent, providing an 81% improvement in precursormodulus and a 32% improvement in carbon fiber modulusover undrawn, untreated fibers (68 and 14% improvementsin tensile strength, respectively). It is also worth notingthat the carbon fiber to precursor modulus ratio dropsslightly with treatment in 30% DMF, implying that anycorrelation between E and E is not likely to be linear forc p

DMF-treated fibers, as shown in Fig. 14.

3.5.2.1. Morphological observations. In this case, scan-ning electron microscopy was used to observe the cross-sectional fracture surfaces of individual carbon fibers(those that were previously untreated and undrawn andthose that were previously treated in 30% DMF for 1 minat 808C). In fact, these fibers were the very same ones usedfor the tow tensile test with the fracture surfaces directlyresulting from tensile failure during the test. MicrographsFig. 9. SEM micrographs of untreated, undrawn PAN fiber tows

(2003 magnification): (a) precursor; (b) carbonized. of the individual fibers are provided in Figs. 15 and 16 and


Fig. 12. Cross-sectional view of PAN tows initially modified inFig. 11. SEM micrographs of carbonized PAN tow initially 80% DMF (saturated 10 s, drawn at 1008C): (a) precursor; (b)modified in 80% DMF (saturated 10 s, drawn at 1008C): (a) carbonized.surface flaws evident; (b) surface flaws not as obvious.

provide a revealing look at the effects of skin-core treated with 30% DMF show a strikingly different surfacemorphologies and outer surface defects. morphology, as shown in Fig. 16. Here, the fracture

Carbon fibers that were previously untreated and un- surfaces appear extremely uniform, smooth, and flat,drawn appear to exhibit a skin-core structure as shown in without any readily apparent evidence of skin-core mor-Fig. 15. While many fibers in the sample exhibit a rough, phologies or concave surfaces. Furthermore, the outerconcave fracture surface (as shown in Fig. 15a) which may surfaces of the fibers are generally smoother, and surfaceindicate skin-core behavior (an oriented ‘skin’ can be flaws are also not as readily apparent as the more obviousinferred from the outer edge of the fracture surface), others defects shown in the undrawn, untreated carbon fibers.show unmistakable evidence of core pull-out (as seen in More uniform stabilization heat treatment (with less volati-Fig. 15b). In addition, the outer surfaces of the fibers zation) seems to be indicated for carbon fibers previouslythemselves are rather rough and moreover show a large treated with 30% DMF, resulting in fewer stress con-number of surface defects. This combination of skin-core centrations and therefore significantly improved tensilemorphologies and surface defects provides a significant properties over the undrawn, untreated carbon fibers.number of stress concentrations, which probably accountfor the lower overall tensile properties of previously 3.6. X-ray analysisundrawn, untreated carbon fibers.

On the other hand, carbon fibers that were previously In an effort to qualitatively observe the effects of


Fig. 13. Tensile results for individual precursor and carbonized fibers extracted from drawn tows: (a) elastic modulus; (b) tensile strength;(c) strain to failure; (d) toughness.

plasticization and drawing on actual fiber orientation, flat- structure [4,8,30,37–39]. These arc reflections are slightlyplate pinhole patterns were taken from untreated, undrawn shorter and sharper for the precursor tow treated in 80%tows and tows treated with 80% DMF (saturated 10 s, DMF, implying better overall lateral order (i.e., betterdrawn at 1008C) using wide angle X-ray scattering orientation of molecular chains with respect to the fiber(WAXS). Patterns from the precursor tows (given in Fig. axis) and larger crystallite size, respectively, than un-17) show prominent equatorial reflections corresponding to treated, undrawn precursor PAN [19,40,41].the (100) plane (2u 5 178) of the pseudo-hexagonal PAN Off-equatorial peaks (resulting from longitudinal order)


Fig. 13. (continued)

are not apparent, implying that only lateral order is present. orientation in the longitudinal and perhaps the lateralThis result is generally preferred, since during carboniza- direction.tion, longitudinal ordering (due to (101) planes) has been A semi-quantitative analysis of these precursor tows wasreported to hinder the process of molecular chain orienta- obtained by performing an azimuthal scan using an X-raytion with respect to the fiber axis [4,19,41]. The presence diffractometer. In doing so, the tows were initially scannedof an outer ring is evident for the precursor treated in 80% to detect the precise locations of their characteristic 2u

DMF; this ring may indicate the presence of some random values (these were found to be 16.858 for the untreated,


Fig. 14. Properties of individual fibers extracted from drawn towscompared to original E vs. E data.c p

Fig. 16. SEM micrographs of individual carbon fiber previouslytreated in 30% DMF (saturated 1 min, drawn at 808C) (80003

magnification).

undrawn tow and 16.888 for the tow treated in 80% DMF).With the detector fixed, the respective fiber tow sampleswere each rotated 908, the equivalent of scanning along theDebye ring from the equator (x 5 08) to the top or bottomof the ring (x 5 908). The resulting intensity data isprovided in the plots in Fig. 18. These results seem toconfirm that the arc reflections in the pinhole patterns(corresponding to x 5 08) have a slightly sharper drop inintensity with increasing x, implying slightly better lateralorder. In conjunction with the pinhole patterns, theseresults appear to be consistent with the structural changesin PAN (upon post-spinning modification) proposed byother researchers [36]. Post-spinning stretching (this timein 80% DMF) indeed appears to slightly improve orienta-Fig. 15. SEM micrographs of individual carbon fiber previously

untreated, undrawn (80003 magnification). tion of molecular chains along the fiber axis.


Fig. 17. WAXS flat-plate pinhole patterns of precursor fiber tows: (a) untreated, undrawn; (b) treated in 80% DMF (saturated 10 s, drawn at1008C).

4. Summary and conclusions 30% DMF solution was used. Specifically, there was an81% increase in precursor modulus and a 32% increase in

With the intention of removing surface defects and carbon fiber (HTT 10008C) modulus over undrawn, un-attenuating fiber diameter while simultaneously inducing treated fibers (68 and 14% increases in tensile strength,molecular orientation, post-spinning modification of in- respectively).dividual polyacrylonitrile fibers was performed using Plasticization in 30% DMF was indeed shown to havesolution plasticization methods. Among the various or- positive influences on carbon fiber mechanical propertiesganic and inorganic solutions tested, treatment using due in no small part to its ability to minimize stressorganic DMF offered far and away the best properties, concentrations in individual carbon fibers. In contrast togiving an 18% improvement in elastic modulus and a 12% previously undrawn, untreated carbon fibers, fibers treatedimprovement in tensile strength over drawn, untreated in 30% DMF were shown by SEM analysis to havePAN (73 and 53%, respectively, over as-received PAN). remarkably uniform, smooth, and flat fracture surfaces

In an effort to observe how individual precursor fiber without a readily apparent skin-core morphology. Outerproperties translate into carbon fiber properties, entire fiber surfaces of the fibers were also smoother, with fewertows were drawn and tested to allow the possibility of surface defects than in previously untreated, undrawncarbonization in the tube furnace. In this context, sample carbon fibers. These observed morphologies are a likelypreparation and testing was approached in two ways: first result of a more uniform stabilization heat treatmentby sequentially solution-treating, drawing, and tensile without excess volatization.testing entire fiber tows, and second by solution-treating In addition, through qualitative and semi-quantitativeand drawing followed by extraction and tensile testing of X-ray analysis, some improved orientational order wasindividual fibers. Results indicated that the former method observed for fibers that were treated in 80% DMF andis less preferable than the latter, giving the counterintuitive drawn. Flat-plate pinhole patterns and intensity data forresult of decreasing modulus and tensile strength with precursor fibers seemed to confirm a morphological modelincreasing solution concentration. Scanning electron mi- showing slightly improved lateral order upon treatment incroscopy (SEM) analysis suggested that this outcome may 80% DMF and drawing.have been the direct result of entanglements, surface flaws,internal voids, and a skin-core morphology, all of whichwere observed in the fiber tows. While the argument ofremoving defects via plasticization was eventually shown Acknowledgementsto have merit when considering fibers individually (asdemonstrated by SEM studies), the same argument cannot Special thanks go to the Air Force Office of Scientificbe made when considering entire fiber tows. In contrast, Research for funding and support of this work. The authorsthe latter sample preparation and testing method provided would also like to thank Dr. Earle Ryba of the Penn Statethe anticipated increase in properties like elastic modulus Materials Science and Engineering Department’s X-rayand tensile strength (for precursor and carbon fibers) when Diffraction Laboratory for all his help in showing how to


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