Structure development and physical properties achieved in the drawing and/or annealing of PEN fibers

Structure Development and Physical Properties Achieved inthe Drawing and/or Annealing of PEN Fibers

GANG WU,1 MING LIU,1 XIAONING LI,2 JOHN A. CUCULO3

1 College of Material Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China

2 Department of Fibrous Material Engineering, Beijing Institute of Clothing Technology, Beijing 100029, China

3 Fiber & Polymer Science Program, College of Textiles, North Carolina State University, Raleigh, North Carolina 27695

Received 22 March 1999; revised 23 February 2000; accepted 28 February 2000

ABSTRACT: As-spun poly(ethylene-2,6-naphthalate) (PEN) fibers (i.e., precursors) pre-pared from high molecular weight polymer were drawn and/or annealed under variousconditions. Structure and property variations taking place during the treatment pro-cess were followed via wide-angle X-ray scattering (WAXS), small-angle X-ray scatter-ing, differential scanning calorimetry (DSC), and mechanical testing. Both the WAXSand DSC measurements of the cold-drawn samples stretched from a low-speed-spunamorphous fiber indicate that strain-induced crystallization can occur at a temperaturebelow the glass-transition temperature and that the resultant crystal is in the a-formmodification. In contrast, when the same precursor was subjected to constrained an-nealing, its amorphous characteristics remained unchanged even though the annealingwas performed at 200 °C. These results may imply that the application of stretchingstress is more important than elevated temperatures in producing a-form crystalliza-tion. The crystalline structure of the hot-drawn samples depends significantly on themorphology of the precursor fibers. When the precursor was wound at a very low speedand in a predominantly amorphous state, hot drawing induced the formation of crystalsthat were apparently pure a-form modification. For the b-form crystallized precursorswound at higher speeds, a partial crystalline transition from the b form to the a formwas observed during the hot drawing. In contrast with the mechanical properties of theas-spun fibers, those of the hot-drawn products are not improved remarkably becausethe draw ratio is extremely limited for most as-spun fibers in which an orientedcrystalline structure has already formed. © 2000 John Wiley & Sons, Inc. J Polym Sci B:Polym Phys 38: 1424–1435, 2000Keywords: poly(ethylene-2,6-naphthalate); fiber; drawing; annealing; structuralchange; crystalline transition

INTRODUCTION

Drawing and annealing operations are importantsteps in the processing of an oriented polymericmaterial. An investigation of the structuralchanges accompanying these operations provides

considerable useful information about the mor-phological characteristics, including molecularorientation and crystalline behavior.1–3

Poly(ethylene-2,6-naphthalate) (PEN) is a rel-atively new polyester with rigid naphthalenerings in its backbone. Accordingly, PEN exhibits ahigher glass-transition temperature, a highermelting point, and a higher modulus than poly-(ethylene terephthalate) (PET). These character-istics make this polymer an excellent candidate

Correspondence to: G. Wu (E-mail: [email protected])Journal of Polymer Science: Part B: Polymer Physics, Vol. 38, 1424–1435 (2000)© 2000 John Wiley & Sons, Inc.

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for a variety of industrial applications, such ashigh-performance tire-cord yarn.4,5

There have been several studies on the influ-ence of processing conditions on the structure andproperties of melt-spun PEN fiber.6–11 This poly-mer is reported to possess two crystalline modifi-cations, and a crystalline transition phenomenonmay occur during the formation processing, in-cluding extrusion, elongation, and heat treat-ment. In our previous study,12 the melt spinningof PEN fibers was performed at different take-upvelocities ranging from 0.5 to 10 km/min. Theeffect of take-up velocity on the structure andproperties of as-spun fibers was also investigatedthrough general characterization and measure-ments. An obvious difference between our resultsand the results of others is that our as-spun fiberspossess a well-developed molecular orientationand high crystallinity even though they are pre-pared at a quite low take-up velocity. Because themolecular weights or intrinsic viscosities (IVs) ofthe PEN chip used in our study and in the studiesof others are significantly different (the measuredIVs are 0.89 dL/g in our work and 0.6 dL/g inothers’ works), this discrepancy has been attrib-uted to the enhanced rheological drag experi-enced along the spin line induced by the use ofhigh molecular weight PEN in our laboratory. Ofthe previously cited articles, however, only a fewof them were concerned with the investigation ofthe structural changes in drawn and/or annealedPEN filaments.7–10

In this study, as-spun PEN fibers (i.e., precur-sor samples) prepared at various take-up veloci-ties were drawn and/or annealed. The effects ofthe as-spun structure, which was affected signif-icantly by the spinning, drawing, and annealingconditions, on the structure and properties of theresultant fibers were studied by X-ray scattering,differential scanning calorimetry (DSC), and ten-sile measurements.

EXPERIMENTAL

Sample Preparation

The original samples used in this study were PENas-spun fibers. The details of such precursors pro-duced by the melt-spinning process have beendescribed previously.12 In brief, a high molecularweight PEN chip obtained by solid -state polymer-ization, with an IV of 0.89 dL/g as measured in a60/40 wt % phenol/tetrachloroethane solvent at

25 °C, was used. This IV, however, is believed tobe slightly lower than the true value of the rawmaterial because the chips required for dissolu-tion a melting and quenching treatment prior tothe IV solution preparation due to their high crys-tallinity. PEN chips were dried and extruded witha screw 25 mm in diameter at a temperature of310 °C. A round single-hole spinneret with a0.6-mm diameter was used together with athroughput wind-up speed arrangement to pro-duce approximately 4.5-denier as-spun fibers. Nocrossflow or radial quench chamber was used inthe spinning process. The as-spun fiber was col-lected at various take-up velocities varying from0.5 to 10 km/min. From these precursors, threekinds of posttreated samples were preparedthrough cold drawing, constrained annealing, andhot-drawing techniques.

Only low-speed-spun fiber collected at 0.5 km/min was used in the cold-drawing process becauseof its high capacity for stretching. In this case, theas-spun fiber was stretched in two different ways:one was drawn with different draw ratios (DRs)between 1.0 and 3.07 in the air at a constant roomtemperature, and the other was drawn with aconstant DR of 3.5, in an oil bath at differenttemperatures ranging from 60 to 140 °C. Drawingwas performed through a hand-operated stretcher.The drawing rate could not be monitored becauseof the limitation of the device but is believed to beabout 1022 1/s. After drawing, samples weretaken out from the oil bath immediately andquenched at room temperature. The actual DRwas determined from the displacement of inkmarks made on the samples.

To obtain the annealed samples, the as-spunfibers collected at 0.5, 5, and 10 km/min wereheat-set under a constant length condition in anoil bath maintained at the desired temperaturesranging from 25 to 240 °C for 30 min.

All as-spun fibers wound at the various speedsvarying from 0.5 to 10 km/min were hot-drawn togenerate the third type of sample. Drawing wasperformed in three stages. In the first stage, as-spun fiber was stretched as taut as possible butwith a minimum of filament breakage at the de-formation rate mentioned previously in an oilbath maintained at the low temperature of 145°C. These intermediate filaments were then im-mediately subjected to the second-stage drawing,in which the oil temperature was kept at 180 °C,and the DR was slightly lower than the maximumto obtain break-free continuous operation. In thelast stage, these filaments were heat-set under

DRAWING AND ANNEALING OF PEN FIBERS 1425

constant length conditions in a bath at 220 °C for30 min. The total DR achieved for as-spun PENfibers collected at various take-up speeds is pre-sented in the Results and Discussion section (seeFig. 10).

Characterization Techniques

Wide-angle X-ray scattering (WAXS) intensitywas measured with a Rigaku Denki ModelD/max-rB X-ray diffractometer system. Nickel-filtered CuKa radiation (l 5 0.1542 nm) gener-ated at 40 kV and 50 mA was used throughout allthe WAXS measurements. The scattering inten-sities were recorded every 0.1° from 2u scans inthe range 5–40° at a scanning speed of 4°/min.When the crystallinity of fiber samples needed tobe determined, fibers were finely cut and madeinto powderlike isotropic samples. The small-an-gle X-ray scattering (SAXS) intensity curves formeridional (parallel to fiber direction) and equa-torial scans were measured with the same systemoperated at 40 kV and 100 mA. The scans weredone at 0.02° intervals in the 2u range 0.105–1.465°. The measurement of two-dimensionalSAXS patterns was carried out with the diffrac-tometer operated at 44 kV and 200 mA. The datawere obtained on a Fuji imaging plate system in a2048 3 2500 matrix. A sample-plate distance of510 mm and an exposure time of 60 min wereused.

Thermal analysis were performed on aPerkinElmer DSC-7 differential scanning calo-rimeter. The calibration of the apparatus for tem-perature and energy was made with an indiumreference. The weight of the PEN samples wasabout 8–10 mg, and the heating rate was 20 °C/min.

The mechanical properties of the fiber sampleswere measured on an Instron Model 1122 tensiletester at room temperature. All tests were per-formed with a gauge length of 25 mm and a con-stant crosshead speed of 20 mm/min. An averageof 10 individual tensile determinations was re-corded for each sample.

RESULTS AND DISCUSSION

Cold Drawing

First, the PEN as-spun fiber taken up at 0.5 km/min was stretched to different DRs at room tem-perature. Figure 1 shows the change in the equa-

torial WAXS traces for fiber samples with variousDRs. Among these curves, as-spun fiber withoutany stretching (DR 5 1) shows a broad and nearlysymmetric pattern. Its peak centers at about 22°,which almost coincides with the peak position ofthe amorphous scattering,6,10 indicating that thesample is essentially totally amorphous. In com-parison with the as-spun fiber, the diffuse scat-tering curves of cold-drawn fibers appear to be-come slightly narrower and gradually more asym-metric in their shapes with increasing DR. Thepeak position of the drawn fiber with the highestDR shifts to nearly 25°. These features may indi-cate the existence of a slightly developed crystal-line structure in the predominantly amorphousfiber samples, even though the reflections fromthese small and imperfect crystals are not yetstrong enough to form separate diffraction peaks.This observation may also imply that stretchedPEN amorphous molecules are able to crystallize

Figure 1. Change in the WAXS curves for the cold-drawn samples with the indicated DR from 0.5 km/minas-spun PEN fiber.

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below the glass-transition temperature, which isreported to be around 120 °C,13,14 as in the case ofPET.15,16 To test this conclusion, these sampleswere also examined via the thermal analysis tech-nique.

Normalized DSC thermograms of the five PENfiber samples are presented in Figure 2. All fibersexhibit the typical glass transition, namely, ashoulder appearing at about 120 °C and a meltingendotherm with the peak temperature at 260–262 °C. The cold-crystallization exothermic peakrelated to thermally induced crystallization dur-ing the heating process is at about 200 °C in theas-spun fiber (DR 5 1) and varies when the fiberis cold-drawn with different DRs. The position ofthe exothermic peak shifts to lower temperatureswith increasing DR. This phenomenon is verysimilar to the well-known fact observed in thePET and PEN melt-spinning process where thecold-crystallization temperature in as-spun fibers

decreases with increasing take-up velocity.10,11,17

Both of these phenomena may be understood withthe same explanation. As the DR or take-up ve-locity increases, the noncrystalline molecules areincreasingly oriented before the initial crystalli-zation, allowing cold crystallization to occur eas-ier and shifting the peak to lower temperatures.However, the area of the cold-crystallization peakdecreases significantly with increasing DR,whereas the area under the crystalline meltingpeak does not vary drastically. The difference ob-tained by the subtraction of the area under thecold-crystallization peak from the melting peak isdirectly proportional to the crystallinity in a sam-ple before the thermal scan, and the crystallinity,Xc, could be calculated by the application of thefollowing relation:10,14

Xc 5DHm 2 DHC

DH o

where DHo is the fusion heat of 100% crystallinePEN and DHc and DHm are the exotherm of coldcrystallization and the crystalline heat of fusion,respectively.

As shown in Figure 2, a straight baseline wasdrawn to separate the cold-crystallization exo-thermic peak and the crystalline melting peak.The heats of cold crystallization and fusion weresimply determined by comparison of the corre-sponding area, as measured with a planimeter.An actual calculation of the crystallinity, how-ever, is a difficult task because of the possibility ofthe coexistence of two crystalline modifications inPEN, as described later, and the lack of the heatof fusion for either form. To simplify the problem,the crystalline form was not distinguished, andthe sole value that appeared in the literature,DHo 5 25 kJ/mol, was used in the calculation.18

The crystallinity thus obtained is then called ap-parent crystallinity in this study. Figure 3 showsthe effect of DR on apparent crystallinity in cold-drawn fibers. The crystallinity seems to be in-crease with increasing DR. This result provesthat the stretched PEN molecules can be crystal-lized during the cold-drawing process eventhough the drawing is performed at room temper-ature.

In comparison with sample series described inFigures 1 and 2 that were stretched at room tem-perature to various DRs from the 0.5 km/minas-spun fiber, the same precursor was cold-drawnat different temperatures to the same DR. The

Figure 2. Change in the DSC scans for the cold-drawn samples with the indicated DR from 0.5 km/minas-spun PEN fiber. The baseline is drawn as a straightdotted line.


fiber samples in this series have a common DR of3.5 but were stretched at 60, 80, 100, and 120 °C,respectively. Another sample stretched at 140 °C,which is a little higher than the glass-transitiontemperature of PEN, was also prepared for fur-ther study of the temperature effect on the struc-ture of the drawn product. The WAXS traces offive such fiber samples are shown in Figure 4. Atlower temperatures, the WAXS curves also ex-hibit a broad unresolved profile. Such peaks be-come resolvable as the drawing temperature in-creases to 100 °C, which is still below the glass-transition temperature. As seen in the lowerthree curves, where fiber samples were respec-tively drawn at 100, 120, and 140 °C, three dif-fraction peaks appear gradually. It is well-knownthat PEN exhibits two crystal modifications, thatis, a- and b-form crystals.13 The characteristicreflection peaks are located at 2u 5 15.7, 23.3, and27.0° for the (010), (100), and (2110) planes of thea form and at 2u 5 18.6 and 26.9° for the (020)and (200) planes of the b form. As shown in Fig-ure 4, three reflections appearing in the bottomtrace may be attributed to the (010), (100) and(2110) reflections from the a modification. Be-cause the WAXS profiles of all five samples seemto change systematically in their shape with in-creasing drawing temperature, it may be con-cluded that the originally nucleated crystals inthe other four samples prepared at 60, 80, 100,and 120 °C also consist of the a modification,although the resolved pattern has not yet beenobserved. The crystal sizes Lhkl, therefore, couldbe calculated with a curve-fitting program andthe Scherrer equation.19 As shown in Figure 5,

crystalline sizes (L010, L100, L2110) increase grad-ually with increasing drawing temperature, evenbelow the glass-transition temperature, provingagain that strain-induced crystallization can oc-cur during the cold-drawing process of PEN. Ac-tually, Murakami et al.20 performed the colddrawing of an unoriented amorphous PEN film at65 °C and examined the structural change bymeasuring X-ray diffraction patterns. Thechanges in WAXS intensity observed in this studyare similar to those reported by the aforemen-tioned authors. From their experiments, they con-cluded that the cold-drawn film with a relativelyhigh DR is in the oriented amorphous state butwithout any crystalline structure. In contrast, theresults derived from our WAXS and DSC mea-surements led to a slightly different explanation.Besides the development of an oriented amor-phous phase, strain-induced crystallization in-deed occurs during cold drawing, even though the

Figure 4. Change in the WAXS curves for drawnsamples stretched at the indicated drawing tempera-ture from 0.5 km/min as-spun PEN fiber.

Figure 3. Plots of the apparent crystallinity versusthe cold-draw ratio for samples stretched from 0.5 km/min as-spun PEN fiber.

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drawing temperature is below the glass-transi-tion temperature.

Constrained Annealing

During the annealing process, the as-spun fiberswound at 0.5, 5, and 10 km/min were subjected toheat setting under a constant length condition atcertain temperatures chosen from 140 to 240 °Cfor 30 min. In addition, three as-spun fibers with-out heat treatment (25 °C samples) were alsoused for comparison. This generated three newsample series, that is, Series 05, 5, and 10, whichcorrespond to the take-up velocities 0.5, 5, and 10km/min, respectively, in as-spun fiber spinning.Figure 6(a) presents the equatorial WAXS scansfor Series 05. As seen in this figure, even whenthe annealing temperature was elevated to 200°C, the amorphous characteristic of the intensitycurves remains unchanged, indicating the diffi-culty of crystallization under this condition. Incomparison to the situation revealed in Figure 4,where strain-induced crystallization occurs at aquite low temperature, a qualitative conclusionmay be reached that the application of tensile oraxial stress is more important than the tempera-ture in forming a crystalline structure in a post-treated PEN fiber sample, at least in the stretch-ing and temperature ranges we used. At higherannealing temperatures of 220 and 240 °C, theintensity curves show resolved diffraction peaksat about 2u 5 15.7, 23.3, and 27.0°. These peaksare attributed to the (010), (100) and (2110) re-flections of the a modification. The appearance ofthese peaks indicates that a-form crystals were

developed in the 0.5 km/min fibers at high anneal-ing temperatures. This is consistent with the re-sults of Cakmak and Kim,10 who found that at anannealing temperature of 220 °C, fibers spun at0.5 and 1 km/min exhibit exclusively a-form crys-tals.

The equatorial WAXS scans of Series 5 and 10are shown in Figure 6(b,c). The as-spun fiberstaken up at 5 and 10 km/min, as presented at thetop of the figures, exhibit well-developed crystal-line structures. Two strong peaks appearing atabout 2u 5 18.6 and 26.9° are (020) and (200)reflections from the b modification, whereas twoweak ones located at 2u 5 15.7 and 23.3° provethe existence of the a modification in the 10 km/min as-spun fiber. These features are somewhatdifferent from those in other reports, where theas-spun fibers produced at similar speeds revealbroad or poorly resolved scattering peaks.6,9,11

Through an examination of the relationship be-tween the thread-line tension monitored by onlinemeasurement and the IV and crystallinity datafor individual as-spun fibers, the discrepancy be-tween our results and others has been attributedto the high spinning stress generated by the highmolecular weight polymer used in our study.12

When the 5 km/min as-spun fiber was subjectedto heat setting at various temperatures, as shownin Figure 6(b), crystalline reflections of a010 anda100 at 2u 5 15.7 and 23.3° start to appear atrelatively high temperatures, indicating that thecontent of the a-form crystal is promoted by theannealing temperature being increased. Theb-form reflections seem not to vary drastically intheir shape in the experimental range chosen,

Figure 6. Change in the WAXS curves for heat-setsamples annealed at the indicated temperatures from0.5, 5, and 10 km/min as-spun PEN fibers.

Figure 5. Plots of the apparent crystallite sizes ver-sus the drawing temperature for the samples stretchedfrom 0.5 km/min as-spun PEN fiber.


although its content and crystallite size may alsoincrease. However, as shown in Figure 6(c), thereis no substantial variation in the peak shape andrelative intensity of both the a- and b-form reflec-tions among seven curves, and neither a trace ofthe crystalline transition from the b form to the aform nor a reversion was found. This indicatesthat the crystalline structure containing the bmodification together with the a modification isquite stable in the 10 km/min as-spun fiber in awide temperature range.

In an early study, Cakmak and Kim10 reportedthat the SAXS patterns of heat-set PEN fibersthat were spun at speeds of 1.5–4 km/min exhibittwo streaklike meridional reflections. In thiswork, SAXS measurements were performed to ex-amine the structural changes in annealed fibersthat were taken up at much higher speeds, from 4to 10 km/min, and annealed at 240 °C for 30 min.The scattering intensity recorded with the imag-ing plate, with the direct beam at the center of thepattern, is shown in Figure 7. The strong streakalong the meridian is a common feature of most ofthese patterns, except for samples spun at 9 and10 km/min. The lengths of the meridional streaksseem to decrease when the take-up speed is in-

creased from 4 to 8 km/min, indicating that thetransverse dimension of the crystallites increaseswith the increase of the take-up speed.19 For thetwo patterns shown in Figures7(f,g), the streakintensity is a little weaker. The nature of thisphenomenon was not studied, but a small electrondensity difference between the crystalline andhighly oriented noncrystalline phases at this su-perhigh take-up speed is a possible cause.

The SAXS scans were carried out on Series 05,5, and 10. Scattering intensity curves along boththe equator (E) and the meridian (M) for the as-spun fibers, as well as the meridional scans (M)for the heat-set samples, are presented in Figure8(a–c) for the three series. In the case of theas-spun fiber taken up at 0.5 km/min, as shown inthe top two curves in Figure 8(a), both the equa-torial and meridional scans exhibit only a weakpeak, and the peak intensity in the meridionaldirection seems to be slightly higher than that ofthe equator. This may indicate slight crystalliza-tion and molecular orientation within the as-spunfiber. As the annealing temperature increases,the meridional SAXS curves seem to be un-changed. From what we discussed previously[Fig. 6(a)], the Series 05 samples have a very low

Figure 7. SAXS patterns of heat-set samples annealed at 240 °C for 30 min: (a) 4km/min, (b) 5 km/min, (c) 6 km/min, (d) 7 km/min, (e) 8 km/min, (f) 9 km/min, and (g)10 km/min.

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crystallinity even up to an annealing temperatureof 200 °C. Accordingly, their meridional SAXSintensities are low, and the respective shapes ofthe maxima are obscure. Furthermore, when theannealing temperature is elevated to 220 or 240°C, the curve still seems to be almost unaltered inboth intensity and shape, although the corre-sponding WAXS patterns presented in Figure 6(a)indicate the existence of a developed crystallinephase. This may be attributed to the low take-upspeed at which the precursor fiber was spun. Thecorresponding superstructure in the high-temper-ature annealed fibers, therefore, may resemble atexture consisting of crystalline and amorphousdomains with a negligible molecular orientationrather than microfibrils oriented parallel to thefiber direction, resulting in quite weak maxima inthe meridional scans. As shown in Figure 8(b),SAXS curves for Series 5 reveal an ordinary sit-uation, where every intensity curve exhibits apeak in the scan range. An obvious intensity dis-crepancy at the peak maximum between the me-ridional and equatorial scans for the as-spun fi-bers indicates that a well-developed crystallinephase and a preferred molecular orientation existin this precursor. From the meridional scans ofthe annealed samples, it can be also observed thatthe position of the peak shifts toward small scat-tering angles and the maximum intensity be-comes stronger with increasing annealing tem-perature. This observation agrees well with nu-merous studies in which the annealing ofsynthetic fibers leads to both a reflection sharp-ening and a long period increase.19 As shown inFigure 7 where the SAXS patterns for heat-set

PEN fibers exhibit two-streak meridional reflec-tion, such variation in both the peak intensity andposition indicates an increasing crystallinity andlong period. The corresponding long period in-creases from about 19 to 27 nm when the anneal-ing temperature is elevated from 25 to 240 °C. Asshown in Figure 8(c), the variation tendency inSeries 10 is similar to that of Series 5, except thatthe peak intensity in all the scan curves is a littleweaker. A possible cause for the observation of aweaker intensity has already been discussed.

The DSC thermograms of Series 05 annealedat various temperatures are given in Figure 9(a).A shoulder corresponding to the glass transitionis found at a constant temperature of about 120°C up to an annealing temperature of 200 °C. Forthe samples heat-set below 200 °C, the glass tran-sition is followed by the cold-crystallization exo-therm with a peak temperature of about 200 °C,which is followed by a melting endotherm atabout 260 °C. As shown in this figure, the area ofthe cold-crystallization exotherm peak is aboutthe same as that of the melting endotherm peak,indicating that these samples contained very lit-tle crystallinity before the DSC scan. As the an-nealing temperature rises to 220 °C or 240 °C, thecold-crystallization exotherm peak disappears,and only the melting endotherm can be observedin the DSC curve. These features are in accordwith the structure analyses that we present inFigure 6(a).

Figure 9(b,c) shows the DSC curves for Series 5and 10, respectively. In both figures, only themelting endothermic peak is observed, and thelocations of the melting peaks in each figure are

Figure 9. Change in the DSC scans for heat-set sam-ples annealed at the indicated temperatures from 0.5,5, and 10 km/min as-spun PEN fibers.

Figure 8. Change in the SAXS curves for heat-setsamples annealed at the indicated temperatures from0.5, 5, and 10 km/min as-spun PEN fibers.


rather constant, from about 280 °C for the Series5 fibers to about 296 °C for the Series 10 fibers.The melting peaks of both series samples aremuch narrower than those of the 0.5 km/min sam-ples, suggesting that the fibers spun at the higherspeeds of 5 km/min and 10 km/min have a well-developed and more perfect crystalline structure.

Hot Drawing

As mentioned in the Experimental section, allPEN as-spun fibers were hot-drawn in threesteps. Figure 10 shows the total DR as a functionof the take-up velocity at which various precursorfibers were prepared, as derived from the litera-ture7,9 and this work. It is obvious that the totalDR decreases as take-up velocity increases andapproaches unity over 4 km/min. The total DRsobtained in this work are generally lower thanthose from other works because of the high mo-lecular weight PEN used in our as-spun fibers.12

This causes a higher spin-line stress and inducesboth higher molecular orientation and highercrystallinity in the fibers, resulting in a re-stricted, lower capacity for stretching than thatobserved in low-IV fibers.

The as-spun PEN fibers taken up at variousvelocities were characterized through WAXSmeasurements in our previous study.12 Afterthese fibers were subjected to a hot-drawing pro-cess, the WAXS scans of the resultant samplesshowed obvious changes. Figure 11 shows theWAXS traces for all the hot-drawn fiber samples,which were finely cut and reduced to a powderlikestate. In the low-speed range of 0.5–1.3 km/min,the reflections from the (010), (100) and (2110)

planes of the a-form crystal are clearly observedat 2u 5 15.7, 23.3, and 27.0°, respectively, wherethe precursor fibers reveal only broad unresolvedcurves.12 This indicates that well-developeda-form crystals are formed preferentially in theoriginally amorphous fibers during the hot draw-ing. This is consistent with results reported byothers.8,10 At and above the 1.5 km/min take-upspeed, it is clear that the predominant b-formcrystals in the as-spun fibers were transformed toa large extent into a-form crystals, as indicated bythe manifest a-form peaks located at the afore-mentioned angles, whereas the b-form crystalswere partially preserved, as indicated by the de-generative b200 peak located at 2u 5 18.6°. As theprecursor fiber wind-up speed increases, theb-form crystals in the corresponding hot-drawnsample seem to become more resistant to change.This may be confirmed by the apparent changesin the diffraction intensities at each characteristic

Figure 11. WAXS scans of hot-drawn samples pre-pared from various as-spun PEN fibers that were takenup at the indicated speeds.

Figure 10. Plots of the total DR versus the take-upspeed at which the as-spun PEN fibers were prepared.

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angle. For example, the intensity of the a-formdiffraction peaks located at 15.7 and 23.3° is rel-atively low for the precursor spun at a higherspeed, whereas the b-form diffraction located at18.6° and a diffraction peak appeared at 26–27°,which was attributed to the reflection overlapfrom the a2110 and b200 planes, seem to be rela-tively strong. This observation disagrees some-what with the work reported by Nagai et al.7, whofound that a complete crystalline transition fromthe b- to the a-form crystals occurs during hotdrawing. For a hot-drawn yarn that was origi-nally spun at a speed of 5 km/min, their resultshows a trace of the a-form modification.

To discuss the crystalline transition phenome-non quantitatively, the WAXS scans of all thehot-drawn samples shown in Figure 11 as well asthe WAXS scan of all the precursor fibers pre-sented in ref. 12 were resolved into one amor-phous peak and several crystalline peaks re-flected respectively from both the a and b forms.By the peak-separation procedure, the relativearea of amorphous scattering and reflection peaksin both the as-spun and corresponding hot-drawnfibers could be calculated as a function of thetake-up velocity, and the results are presented inFigure 12(a) for the as-spun fibers and in Figure12(b) for the hot-drawn samples. As shown inFigure 12, two sets of curves for both as-spun andcorresponding hot-drawn fibers are different inthe percentage value of the peak area. In the caseof the as-spun fiber, the crystalline peak areassignificantly increased when the take-up speed iselevated to 1.5 km/min. Although the content ra-tio of the two crystal modification could not beevaluated because the a2110 and b200 reflectionsoverlap in the vicinity of 2u 5 27°, it may beconcluded that the predominant crystal in theas-spun fibers is the b-form rather than thea-form modification because the a100 and a010peak areas were quite small, even though theywere slightly increased in the superhigh-speedrange.

After any precursor fiber spun at differentspeeds undergoes hot drawing, as shown in Fig-ure 12(b), the area of the diffraction peaks belong-ing to the a form increase significantly. This im-plies that hot drawing induces a preferred devel-opment of the a-form crystal for low-speed-spunamorphous precursors or impels a b 3 a-formtransition for high-speed-spun precursors thatcontain a number of b-form crystals. The level ofthe b3 a-form transition seems to depend on theperfection of the b-form crystals in the as-spun

fibers. Both the crystallite size of the b modifica-tion (b020, 2u 5 18.6°) and the melting tempera-ture of the as-spun fibers increase gradually withincreasing take-up speed, suggesting that theb-form crystal becomes more perfect at higherspeeds.12 In combination with the results dis-cussed previously, it may be concluded that thehigher the take-up speed chosen in the melt-spin-ning process was, the more perfect the b-formcrystals in the as-spun precursor were, the lowerthe level of the b 3 a transition was, and thehigher the concentration of the b-form crystals inthe hot-drawn samples was.

However, in contrast to the hot-drawing-in-duced crystalline phase transition just de-scribed, this phenomenon was of little or noimportance when the as-spun fibers were heat-set at a high temperature but under a constantlength condition [Fig. 6(b,c)]. Although the totalDR for most as-spun fibers in the hot-drawingprocess, as shown in Figure 10, is very low andclose to unity, the drawing procedure was per-formed near the stress limit. This comparison,therefore, indicates again that stretching ten-sion plays a major role in promoting a-form

Figure 12. Take-up speed dependence of the percent-age of the peak area for (a) as-spun PEN fibers and (b)hot-drawn fibers.


crystallization. Furthermore, the crystallinitydata for the hot-drawn PEN fibers can be simplycalculated from the summation of all the crys-talline peak areas. As shown in Figure 12(b), itvaries from 50 – 60%, depending slightly on thetake-up velocity at which the as-spun fiber wasproduced. The crystallinity index reported byIizuka and Yabuki6 for 3–5 km/min as-spunPEN fiber is about 80%. As they pointed out,this extremely high value was determined by anequatorial WAXS scan on the fibrous sample.Because the crystallinity reported in this studywas measured on a powderlike sample, the crys-tallinity data are thought to be more reliable.

Tensile tests were interpreted in terms of sev-eral mechanical characteristics, including initialYoung’s modulus, tenacity, and elongation atbreak. These properties of the hot-drawn PENfibers are plotted in Figure 13 against the take-upvelocity at which the precursor fibers were pre-pared. The tenacity and modulus of the hot-drawn fibers increase with increasing velocity,whereas the elongation to break decreases. Incomparison with the mechanical properties of theas-spun fiber taken up at the same speed reportedin our previous work,12 the modulus of the hot-drawn fibers is higher by about 10–15 %, presum-ably a result of elevated crystallinity, whereas thetenacity of the hot-drawn fibers is almost thesame as that of the as-spun fibers. Such unalteredtenacity obtained in the hot-drawn fiber samplesmay be attributed to the low hot-draw ratio (DR' 1) achieved in most precursors in which a well-developed crystalline structure has alreadyformed.

CONCLUSIONS

From as-spun PEN fibers prepared by melt spin-ning, three types of posttreated samples were pro-duced through cold drawing, constrained anneal-ing, and hot-drawing processes. The effects ofdrawing and/or annealing conditions as well asthe morphology of the precursor fibers on thestructure and properties of the resultant sampleshave been characterized via measurements of X-ray scattering (WAXS and SAXS), DSC, and me-chanical testing. For cold-drawn samplesstretched from low-speed-spun amorphous fiber,the equatorial WAXS scans show asymmetric orresolved patterns, and the area of the cold-crys-tallization peak appearing in the DSC scans de-creases significantly with increasing DR. This in-dicates that stretched amorphous molecules areable to crystallize below the glass-transition tem-perature and generate the a-form modification. Incontrast, when the same as-spun fiber was sub-jected to constrained annealing, the amorphouscharacteristic observed in the precursor remainedunchanged even though the annealing was per-formed at a quite high temperature. This discrep-ancy between the cold-drawn and annealed sam-ples may imply that the application of elongationstress is more important in forming the a-formmodification than elevated temperature is. Thecrystalline structure of the hot-drawn samplesdepends strongly on the morphology of the pre-cursor fiber. When the precursor was spun at avery low speed and was in a predominantly amor-phous state, hot drawing induced what seemed tobe the pure a-form crystal. For the b-form crys-tallized precursors wound at higher speeds, a par-tial crystalline transition from the b form to the aform occurs during the hot drawing. The level ofthis transition depends on the perfection of theb-form crystals within the precursors. For thesuperhigh-speed-spun fibers where the b-formcrystal may be perfectly developed, the b-formcrystals remain after the hot drawing. In compar-ison to the mechanical properties of the as-spunfibers, those of the hot-drawn products are notremarkably improved because the DR is ex-tremely limited for most as-spun fibers in whichan oriented crystalline structure has alreadyformed.

The authors gratefully acknowledge the support of theNational Natural Science Foundation of China (GrantNo. 59873003).

Figure 13. Plots of the mechanical properties of thehot-drawn fibers versus the take-up speed at which theas-spun PEN fibers were prepared.

1434 WU ET AL.

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Documents

Structure development and physical properties achieved in the drawing and/or annealing of PEN fibers