transcrystallization of ptfe fiber/pp composites—iii. effect of fiber pulling on the...

Transcrystallization of PTFE Fiber/PP Composites—III. Effect of Fiber Pulling on the Crystallization Kinetics

CHI WANG, C.-R. LIU

Department of Chemical Engineering, Yuan-Ze University, Neili, Taoyuan Taiwan 320, Republic of China

Received 11 July 1997; revised 6 November 1997; accepted 11 December 1997

ABSTRACT: The effect of shear rates on the transcrystallization of polypropylene (PP)on the polytetrafluoroethylene (PTFE) fibers has been quantitatively investigated us-ing a polarized optical microscope equipped with a hot stage and a tensile testingmachine. The PTFE fibers were pulled at different rates, from 0.17 to 8.33 mm/s, toinduce a range of shear rates, about 0.02 to 1.16 1/s, in the PP melt adjacent to thefiber. The induction time, nucleation rate, and saturated nucleation density at the fibersurface were determined at various crystallization temperatures. It was found thatboth the nucleation rate and the saturated nucleation density increase with increasingshear rates. However, the induction time is significantly reduced. Based on the theoryof heterogeneous nucleation, the interfacial free energy difference functions DsTCL ofPP on PTFE fibers at different levels of shear rates were determined and comparedwith that obtained from crystallization under quiescent conditions. Results showedthat the magnitude of DsTCL decreased to be about one-third of that for the quiescentcrystallization, when a shear rate of 1.16 1/s was applied. The application of a shearstress to the supercooled PP melt by fiber pulling leads to enhance the development oftranscrystallinity. Moreover, both the thickness and the crystal growth rate of trans-crystalline layers were found to increase with the increasing rate of fiber pulling,especially at low crystallization temperatures where regime III prevails (see text).Surface morphology of PTFE fibers was revealed using a scanning electron microscopeand an atomic force microscope. It is argued that the presence of fibrillar-type featuresat the fiber surface is the main factor responsible for the development of transcrystallin-ity. q 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1361–1370, 1998Keywords: transcrystallinity; PTFE fiber/PP composites; heterogeneous nucleation;crystal growth rate; orientation

INTRODUCTION found in the bulk. It has been concluded thatYoung’s modulus of TCL is superior to those of

It is well established that when fibers are incorpo- the bulk materials crystallized by itself.1,2 Severalrated in semicrystalline polymers, under appro- suggestions have been proposed to account for thepriate conditions, a highly oriented layer is devel- formation of TCL.3–5 The most interesting subjectoped at the fiber/matrix interface. This distinct is the flow-induced (stress-induced or strain-in-morphology is called transcrystalline layer (TCL), duced) crystallization to result in the morphologyand is the consequence of a high nucleating ability of TCL.6,7

of the fibers, compared to that of the bulk matrix. In the quiescent crystallization conditions,In contrast, isotropic spherulites are normally transcrystallization of PP (or Nylon) does not

take place when glass fibers are used. However,the presence of TCL has been reported after the

Correspondence to: C. Wangapplication of a stress to the glass fibers.6–9 Flow-

Journal of Polymer Science: Part B: Polymer Physics, Vol. 36, 1361–1370 (1998)q 1998 John Wiley & Sons, Inc. CCC 0887-6266/98/081361-10 induced transcrystallization has also been found

1361

8Q5A 7013/ 8Q5A$$7013 04-02-98 15:52:20 polpa W: Poly Physics

1362 WANG AND LIU

in the composites of high modulus carbon fiber/PP.10 This flow-induced crystallization is of prac-tical importance, especially in polymer processingwhere a relatively large shear rate is usuallyfound. Chien and Weiss11 had used a parallelplate rheometer to investigate the effect of shearrate on the crystallization of poly(ether ether ke-tone). Results showed that shear flow has a pro-nounced effect on the reduction of induction time, Figure 1. Sketch of the fiber pulling to study thedefined as the time elapsed from the start of strain-induced transcrystallization of PP on the PTFEshearing to the onset of crystallization.11,12

fiber (the diameter of the fiber is 30 mm, and the dimen-Buerger et al.13 found that there is an inverse sions of the mold are 10 1 6 1 1 mm3).

relation between the molecular weight and theminimum shear rate necessary to produce stress- mal effect. Then, the system was programmed toinduced morphology. Recently, a continuum model cool to the crystallization temperature, Tc , with awas developed to account for the flow-induced crys- rate of 207C/min. After the temperature reachedtallization.14 Tc , crystallization time was recorded with a stop-

In spite of the fact that PTFE possesses a watch and simultaneously the fiber was pulledrather low surface energy, it has been demon- with a tensile testing machine. Different pullingstrated that transcrystallinity of PP forms easily rates, U , were applied and the time of pullingon the PTFE fibers under quiescent conditions.15 was fixed to be 60 s, unless specified elsewhere.It is attributed to a lower interfacial free energy Crystallization of polypropylene on the PTFE fi-difference Ds of TCL, compared to that of the PP bers was observed with a polarized optical micro-bulk. Effects of shear rates, induced by pulling scope (POM, Nikon MICROPHOT-FXA). The sys-the fiber, at the fiber surface on the transcrystalli- tem was purged with dry nitrogen to eliminatezation are current interest. The objective of this any possible degradation in all cases.work is to reveal the effect of shear rate on the The number of nuclei was counted directlynucleation process of transcrystallinity. Quantita- through the eyepiece of a microscope. The nucle-

ation density on the fiber was expressed by numbertive measurements of Ds at different shear rates,of nuclei per unit length of fiber. The nucleationbased on the theory of heterogeneous nucleation,rate, I (number of nuclei/mm s), was calculatedare essential. Magnitudes of Ds are used to char-from the linear portion in the plot of nucleationacterize the ability of nucleation of transcrys-density versus crystallization time. The inductiontallinity.time, ti , for nucleation was determined from theintercept of the linear nucleation density to thetime axis. The nuclei density on the fiber at satura-EXPERIMENTALtion, N` , was determined as well. The definitionsof induction time, nucleation rate, and saturatedThe isotactic PP powder without any additivesnucleation density are given in Figure 2.was generously supplied by Taiwan Polypropyl-

To reveal the fiber-pulling effect on the rate ofene Co. The melting temperature of the PP iscrystal growth, measurement of TCL thickness164.17C using DuPont 910 DSC at a heating ratewas carried out as well. The thickness of the well-of 107C/min. The viscosity average moleculardeveloped TCL was measured as a function ofweight was about 2.8 1 105. PTFE fibers with atime while the fiber was pulled continuously. Indiameter about 30 mm were provided by Du Pontthis study, nucleation study was carried out atCo. Acetone was used to clean the surface of fibers147–1507C to obtain reliable numbers of nuclei.before use. A scanning electron microscope (SEM)The study of crystal growth rate, however, waswas used to exam the uniformity of the fiber ra-conducted in a lower temperature range, 136–dius. A small rectangular mold, 10 1 6 1 1 mm3,1427C, to obtain accurate measurement of TCLmade of silicone rubber was used to contain thethickness.PP powder, as shown in Figure 1. A single PTFE

fiber was embedded in the powder. Prior to theRESULTS AND DISCUSSIONinitiation of crystallization, the specimen was

held in a well-controlled hot stage (Mettler, FP- It has been pointed out that the hydrodynamicorigin of the flow-induced orientation in polymer82) at 2007C for 10 min to erase the previous ther-


TRANSCRYSTALLIZATION OF PTFE FIBER/PP COMPOSITION 1363

Figure 4. Formation of crystals nucleated from row-Figure 2. Definitions of induction time, ti , nucleationnucleation process at the fiber end. The arrow pointingrate, I , and nucleation density at saturation, N` .the direction of pulling; U Å 8.33 mm/s, Tc Å 1387C;crystallization time: (a) 28 min (b) 28.5 min.

melt is due to the presence of entanglements,which causes the elastic extension of the chainsegments between them.13 The chain segments on the PTFE fibers pulled at a small rate is similarbetween entanglement points are stretched from to that takes place under quiescent conditions.15

a state of random coil to become extended chains Separated nuclei were easily observed, and crys-in the field of shear flow. These extended chains tal grew in the radial direction until the impinge-may serve as the seeds of nucleation if they reach ment of the neighboring crystals [Fig. 3(a) andthe dimensions of the critical nucleus. Meanwhile, (b)] . TCL formed thereafter and grew perpendic-relaxation of the extended chains has to be taken ularly to the fiber axis [Fig. 3(c) and (d)] . A dif-into consideration to develop a stable nucleus. As ferent mechanism, i.e., row-nucleation process,shown in Figure 3, the nucleation process of PP was observed, however, when the rate of fiber

pulling was relatively large and a low crystalliza-tion temperature was used. Figure 4 clearly illus-trates the formation of row nuclei at the fiber end.A similar morphology, termed cylindrites,16 hasbeen observed in molten PP when a slight shearstress was applied to the cover plate. This is ex-pected because the development of stable PP nu-clei is dependent on both orientation and relax-ation of the polymer chains in the field of shear.Row nucleation is favored in the conditions of highorientation and slow relaxation of polymer chains.From the observation of TCL using POM, we con-cluded that separated nuclei are always observedat the early stage of nucleation provided the ap-propriate conditions are met, i.e., high tempera-tures and low fiber-pulling rates. The potentialnuclei are not activated at the same time in thenucleation process of transcrystallization becauseof the difference in the relaxation times of PP mol-ecules, which results from the distribution of mo-lecular weight. Thus, nuclei were able to becounted directly through the eyepiece of the POM.

Figure 5 shows the effect of pulling time onthe nucleation process at Tc Å 1507C with a fiber-Figure 3. Processes of nucleation and crystal growthpulling rate of 1.67 mm/s. It is interesting to noteof PP transcrystallinity on PTFE fibers when the fiberthat the nucleation rates, determined from theis pulled continuously. The arrow pointing the directioninitial slopes, were approximately constant. Aof pulling; U Å 0.17 mm/s and Tc Å 1477C; crystalliza-

tion time: (a) 20 (b) 50 (c) 130 (d) 150 min. value of 0.085 #/mm/s was obtained in spite of a


1364 WANG AND LIU

site effect on the induction time. Similar resultswere obtained for specimens crystallized at differ-ent crystallization temperatures used in thisstudy (147–1507C).

According to the theory of nucleation, the rateof heterogeneous nucleation I is given by17

log I Å log Io 0U*

2.303R (Tc 0 T` )

0 16sseDsTo2

m

2.303kTc (DTDhf f )2 (1)

where Io is a constant, U* is the activation energyFigure 5. Nucleation density of PP on the PTFE fiber related to molecules to transport across the phasesurface vs. crystallization time for fibers being pulled boundary, R is a gas constant, Tc is the crystalliza-at different times (Tc Å 1507C). tion temperature, T` is the temperature below

which crystallization ceases, Tom is the equilib-

rium melting temperature of the polymer, DT iswide range of pulling times, from 30 to 300 s. The the degree of supercooling (ÅTo

m 0 Tc ) , Dhf is theinduction time was greatly reduced when a longer heat of fusion per unit volume of the polymer, andpulling time was applied, which reduced the level f is a correcting factor, being equal to 2Tc / (Tcof relaxation of extended PP chains. The satu- / To

m ) . Further details regarding these defini-rated nucleation density, however, increased with tions are given in the ref. 18. Values of U*, T` ,increasing time of pulling. It is attributed to the and Dhf are taken from literature18 to be 6.28 kJ/fact that more effective nuclei (extended chains) mol, 232 K, and 1.96 1 109 erg/cm3, respectively.are activated when more work (longer pulling

s and se are the lateral and fold surface energies,time) is done to the system. The shear rate in- respectively. To take account of the energy changeduced at the fiber surface is about 0.23 1/s [calcu- caused by the creation of new surface on the toplated from eq. (3) and discussed below]. Thus, the of foreign substrates, a quantity termed interfa-shear strain of PP melt at the fiber surface is cial free energy difference, Ds, is incorporated.about 7 for a pulling time of 30 s. The strain seems This quantity is important in describing the nu-sufficiently large to make the segments between cleating ability of the substrates. Consequently,entanglements oriented and serve as nucleation the surface energy parameter sseDs is deter-seeds. However, it should be noted that relaxation mined from the slope by plotting of log I / U*/of these extended chains takes place after the 2.303R (Tc 0 T` ) vs. 1/Tc (DT f )2 . In this study,pulling stops. Indeed, the strain-induced crystalli-zation is associated with the competition betweenthe orientation and the relaxation of polymer mol-ecules. In the following experiments, a constanttime of fiber pulling, 60 s, was used, but differentpulling rates were applied, from 0.17 to 8.33 mm/s. The corresponding shear strains of PP chainsadjacent to the fiber were estimated to be in therange of 1.2 to 70.

Effect of Fiber Pulling on the Nucleation Rate

Variations of nucleation density vs. crystalliza-tion time at 1487C under the influence of fiberpulling are shown in Figure 6. It is evident thatthe nucleation rate and the saturated nucleation Figure 6. Nucleation density of PP on the PTFE fiberdensity increased with increasing rate of fiber surface as a function of crystallization time with differ-

ent fiber pulling rates (Tc Å 1487C).pulling, U . But, the pulling rate showed an oppo-



Table I. Values of sseDsTCL and DsTCL Determinedfrom Nucleation Study of PP on PTFE Fibersat Different Rates of Fiber Pulling

Pulling Rate Shear Rate sseDsTCL DsTCL

(mm/s) (1/s) (erg3/cm6) (erg/cm2)

0 0 466 { 54 0.64 { 0.070.17 0.02 461 { 78 0.63 { 0.110.83 0.12 354 { 28 0.48 { 0.041.67 0.23 347 { 58 0.47 { 0.088.33 1.16 152 { 29 0.21 { 0.04

rates. Discussion will be provided in the next sec-tion.

Figure 7. Variation of nucleation rate with super- As expected and shown in Figure 8, a largercooling, according to eq. (1), to determine the sseDsTCL induction time is necessary to develop TCL at aparameters at different fiber pulling rates. higher crystallization temperature. Moreover, the

larger the fiber pulling rate, the smaller the in-duction time. In other words, the shear rates in-duced at the fiber surface promote the nucleationDsTCL is used to denote the interfacial free energyof PP to take place at an early time. The nucle-difference for nucleation taking place in the TCLation density at saturation, N` , decreases slightlyon PTFE fibers. The value of DsTCL is used towith the increase of the crystallization tempera-characterize the nucleating ability of TCL to taketure, as shown in Figure 9. The effect is not pro-place. From a thermodynamic point of view, thenounced. Nevertheless, fiber pulling shows a moresmaller the DsTCL magnitude, the easier for thesignificant effect on the increase of N` than thetranscrystallinity to develop.3,15,19

crystallization temperature does. The value of N`Figure 7 shows the variation of nucleationis about 1.5 times larger than that under quies-rates with crystallization temperatures for PP tocent conditions, when a rate of 8.33 mm/s is usedcrystallize on PTFE fibers at different fiber pull-to pull the fibers. It is attributed to a largering rates, according to eq. (1). Owing to the lowamount of nucleation sites to activate due to theshear rate, the effect of the fiber pulling on thefact that more polymers chains are aligned whenactivation energy, U*, was assumed to be negligi-a larger fiber-pulling rate (or shear rate) is used.ble. The supercooling DT was calculated using the

equilibrium melting temperature of 458 K, sug- Effect of Fiber Pulling on the Crystal Growth Rategested by Clark and Hoffman.18 It is evident from

Figure 10 shows the variation of TCL thicknessFigure 7 that nucleation rate decreased with in-with crystallization time at 1367C for fibers beingcreasing Tc for nucleation taking place with or

without the application of fiber pulling. As men-tioned previously, the nucleation rate increasedwith increasing rate of fiber pulling in all Tc inves-tigated. Values of sseDsTCL were determined fromthe slopes, according to eq. (1) by a linear regres-sion, and were tabulated in Table I.

After deducing the sseDsTCL values from thenucleation study, one has to estimate the valueof sse to determine DsTCL. However, the surfaceenergy parameter sse can be solely determinedfrom the study of crystal growth rate. It shouldbe noted that not only the nucleation rate butalso the crystal growth rate will be affected in theprocess of stress-induced crystallization. We havealso investigated the change in the crystal growth Figure 8. Variation of induction time vs. crystalliza-

tion temperature at different fiber pulling rates.rate of TCL when the fibers are pulled to different


1366 WANG AND LIU

Figure 9. Variation of nucleation density at satura- Figure 11. Variation of TCL thickness with crystalli-tion vs. crystallization temperature at different fiber zation time at different fiber pulling rates. (The fiberspulling rates. are pulled continuously, Tc Å 1427C.)

ature is used, as shown in Figure 11 for crystalli-pulled continuously. Because of the continuous zation at 1427C. When the fiber was pulled at 3.33movement of the fiber in the field of observation, mm/s, only a relatively small increase, about 6%,direct measurement of the TCL thickness through in the crystal growth rate was found, comparedthe eyepieces at a fixed location was unfeasible. to that in the quiescent condition. Accurate mea-Instead, optical photographs at different times surements of TCL thickness at Tc higher thanwere taken after a layer of transcrystallinity was 1427C were unfeasible due to the limited fiberwell developed [Fig. 3(c) and (d)] . Measurements length (10 mm, Fig. 1) to observe under the POMwere carried out by taking the average of the TCL as well as a prolonged time for the layer of trans-thickness along the fiber axis at an interval of 50 crystallinity to develop. Difficulties were encoun-mm over a distance of 300 mm for each photograph. tered in TCL thickness measurement at Tc lowerIt is apparent from Figure 10 that a thicker TCL than 1367C due to the fact that rapid nucleationdevelops when the fiber is pulled at a higher of PP spherulites occurs in the bulk, which resultsspeed. Moreover, the growth rate of TCL, deter- in the impingement of TCL with spherulites tak-mined from the slope, increases with increasing ing place in a short time. Thus, the thickness ofrate of fiber pulling. Nevertheless, the effect is TCL is too thin to be measured accurately.diminished when a higher crystallization temper- Hoffman et al. have derived the regime the-

ory,18,20 on the basis of secondary nucleation the-ory, for the relation between crystal growth rateand crystallization temperature. The crystalgrowth rate, G , is described by the equation

log G Å log Go 0U*

2.303R (Tc 0 T` )

0 nbsseTom

2.303kDhf TcDT f(2)

where b is the layer thickness and n is 4 for re-gimes I and III and 2 for regime II. The value ofsse can be determined from the slope of log G/ U*/2.303R (Tc 0 T` ) vs. 1/TcDT f . It has beendemonstrated that the regime theory can be suc-cessfully employed in the crystal growth of TCLFigure 10. Variation of TCL thickness with crystalli-under quiescent conditions.15 Attempting waszation time at different fiber pulling rates. (The fibers

are pulled continuously, Tc Å 1367C.) made to assess eq. (2) under the influence of shear



Hoffman’s theory, regimes are envisioned as rela-tive levels of i and g . In regime II, multiple second-ary nuclei form on the existing crystal surface be-fore the previous layer is complete. The growthrate G is proportional to ( ig )1/2 . Regime III com-mences at a lower temperature when much moresecondary nuclei develop and a rough surface ofgrowing front is observed. In regime III, thegrowth rate G is proportional to i and independentof g . It has been pointed out that21 g is much moredependent on the mobility of polymer chains thani . The effect of fiber pulling is to enhance the chainmobility of PP adjacent to the crystal growingfront and promote the level of g more effectively.

Figure 12. Variation of TCL thickness with crystalli- Indeed, a more pronounced effect was observed atzation time at different crystallization temperatures. low temperatures, as seen in Figure 10. At high(The fiber are pulled continuously at 3.33 mm/s.) temperatures, however, the crystallization be-

comes more nucleation controlled, which makesthe effect of fiber pulling less significant on crystalrate on the simplifying assumption that the acti-

vation energy of chain motion, U*, in the melt is growth, as seen in Figure 11.approximately constant despite the application ofthe fiber pulling. The effect of crystallization tem-

Effect of Fiber Pulling on DsTCLperature on the crystal growth rate of TCL isgiven in Figure 12 for specimens with the fiber Because the nucleation rate study was carried outbeing pulled at 3.33 mm/s. The growth rate de- at high temperatures, 147 to 1527C, the value ofcreases as the crystallization temperature is in- sse determined from the slope in regime II wascreased. The growth rate data are further ana- reasonably assumed to be independent of the fiberlyzed, according to eq. (2), and results are given pulling rates in this temperature range, accordingin Figure 13 for TCL crystallized at different tem- Figure 13. Moreover, because the time of fiberperatures and various rates of fiber pulling. pulling (60 s) is much smaller than the induction

Apparently, a transition between regime II and time of nucleation, as shown in Figure 6, the fiber-regime III was found at a supercooling DT Å 477C pulling effect on the crystal growth and thus onwhen TCL grew in the quiescent crystallization the sse value is though to be negligible. Thus,condition, i.e., U Å 0 mm/s. The ratio of the slopein regime III to that in regime II is found to beabout 2.05 in accordance with the prediction bythe theory.20 This is consistent with previous re-sults15 for a thinner specimen. Moreover, the sur-face energy parameter, sse , determined from theslope according to eq. (2) is about 730 erg2/cm4.

When the fiber is pulled during the process oftranscrystallization, on the other hand, a devia-tion from the solid line is evident from Figure13, especially at low Tc (regime III) . It has beenrecognized18 that crystal growth rate, G , is depen-dent on two factors, i.e., the surface (secondary)nucleation rate, i , and the substrate completionrate, g , at the growing front. The former is favoredat high supercooling (low crystallization tempera-tures), where polymer chains are characterizedby low energy levels. On the other hand, low su-percooling (high crystallization temperatures) fa- Figure 13. Plot of log G / U*/2.303R (Tc 0 T` )vors the latter because of a lower melt viscosity against 1/TcDTf for PP in TCL at different fiber

pulling rates. (G : in the unit of cm/s.)and thus increasing chain mobility. According to


1368 WANG AND LIU

the sse value of 730 erg2/cm4, derived from thequiescent conditions, was used. Values of DsTCL

calculated by dividing sseDsTCL with sse are tab-ulated in the Table I as well. For transcrystalliza-tion taking place under the quiescent condition,the present value of DsTCL is close to the one re-ported previously,15 about 0.75 { 0.12 erg/cm2

for specimens of 30 mm thin. As the pulling rateincreased, the DsTCL value decreases, which indi-cates that the energy barrier for PP to nucleateon the PTFE fibers is reduced. Thus, TCL is easierto develop when the fiber is pulled.

Because the dimensions of the silicone rubbermold are much larger than the diameter of thePTFE fibers (Fig. 1), the fiber is regarded as beingdragged in an infinite PP melt medium. When thePTFE fiber is pulled with a rate of U , a field ofshear stress is induced in the PP melt adjacent tothe fiber. The shear rate at the fiber surface, g

h,

can be derived from the modification of axial an-nual couette flow22 and is expressed as follows,

ghÅ qU

R(3)

where R is the fiber radius and q is a parameterequal to 1 0 1/n , where n is the power law indexof the PP melt. Thus, the shear rate at the fibersurface is proportional to the rate of fiber pulling.Table I also lists the shear rates calculated fromeq. (3) with a value of n equal to 0.323 for PPmelt.23 It’s evident that the value of DsTCL de-creases with the increasing of the shear rate. Theshear stress at the fiber surface was determinedfrom the product of the shear rate and the viscos-ity. The estimated viscosity of PP melt23 is about4.4 1 104 Pars in the temperature range investi-gated. From Table I, the level of PP nucleation onthe fiber is promoted when a larger stress is used.Moreover, the maximum shear stress at the fiber

Figure 14. Surface topography of PTFE fibers (a),surface in this study is about 0.05 MPa when aSEM micrograph (b), and (c) AFM surface image. (Thepulling rate of 8.33 mm/s is applied. It is of interestarrows showing the directions of fiber axis.)

to note that such a small level of stress can sig-nificantly increase the nucleation rate and pro-mote the development of TCL. This finding is con- cles plays an important role in the formation ofsistent with results reported by Hass and Max- transcrystallinity.9,25,26 Figure 14 illustrates thewell,24 who demonstrated that crystallization surface topography of PTFE fibers revealed bytime was several orders of magnitude less, after both SEM and atomic force microscope (AFM).the application of a small shear stress, than re- AFM images were obtained using a Nanoscopequired in the quiescent condition. III microscope (Digital Instruments, Inc.) . It is

evident that the presence of irregular distur-Possible Nucleation Sites for TCL bances, i.e., ‘‘valleys,’’ ‘‘ridges,’’ and relative

smooth regions along the fiber axis, was found atIt has been shown that the surface morphologyrather than the surface chemistry of foreign parti- the fiber surface [Fig. 14(a)] . The detailed



roughness of the representative area, 5 1 5 mm2 fiber, and thus enhance the ability of nucleation.Shear rates, predicted by eq. (3), are valid onlyin the SEM micrograph was resolved by AFM andfor fibers with smooth surface. Owing to theshown in Figure 14(b) and (c). The fibrillar-typeroughness variation of the fiber surface, as shownmorphology, as revealed in Figure 14(b), couldin Figure 14, localized disturbance in the magni-be the consequence of stretching during the fibertudes of the shear rate are expected. Shear ratesmanufacturing. In addition to numerous stria-are different from place to place from a viewpointtions along the main fiber direction, grains of dif-of microscopic scale. The deeper the ‘‘valley,’’ theferent sizes, corresponding to the smooth regionlarger the shear rate and the shear stresses atin the SEM micrograph, were resolved in AFMthat location. Moreover, the dimensions of theimage, Figure 14(c) . In the region of fibrillar-typecritical nuclei are much smaller than those of themorphology, the typical distance between ridgesfiber fissures in all cases. Thus, initial nucleiranged from 100 to 800 nm, and the height ofmight locate in these ‘‘valleys’’ (grooves or steps)individual ridges ranged from 20 to 50 nm. Theand are treated as anchored molecules that en-roughness in the grain-type region, however, washance the adhesion of fiber/matrix through me-relatively small, about 6 and 150 nm for the ridgechanical interlocking.27 When considering theheight and distance between ridges, respectively.poor wettability between PTFE and PP due toIn any case, small-scale steps or grooves aretheir low surface energies, it is unlikely for theclearly observed along the fiber axis at the fiberadsorption between PP molecules and the fiber tosurface.take place. We propose that nucleation of TCLIt is of interest to estimate the size of the criti-is associated with the localized alignment of PPcal nuclei to compare with the depth and widthchains in the ‘‘valleys,’’ resulting from high shearof these grooves. The dimensions of the criticalrates. Indeed, at the early stage of nucleationnuclei are:17 a* Å 4sT o

m /DhfDT , b* Å 2DsTom /

[Fig. 3(a)] , individual nuclei attached to the fiberDhfDT , and l*Å 4seTo

m /DhfDT , where a* is crit-move at the same rate with the dragged fiber. Thisical width of the nucleus, b* is critical thicknessalso supports that the nucleation sites for TCLof the crystal strand, and l* is critical length ofare located at the fiber surface ( ‘‘valleys’’ ) , ratherthe folded chains. Assuming the crystal growththan in the PP melt near the fiber, which might

taking place on the (110) plane, the corresponding result in the row nucleation. Thus, the surfacevalue18 of s is 11.5 erg/cm2. Thus, the surface roughness (morphology) of the fiber plays a domi-energy of the folding plane, se , was derived to be nant role in determining the development of TCL.63.5 erg/cm2. After substituting the appropriatevalues, the deduced values of a* and l* are about2.7 and 15.0 nm, respectively. These two dimen- CONCLUSIONSsions are independent of shear rate and aresmaller than the depth and width of the ‘‘valleys’’ In spite of the low surface energy of PTFE fibers,at PTFE fiber surface. On the other hand, the it has been demonstrated in a previous article15

dimension of b* decreases with increasing shear that transcrystallinity of PP forms easily on therates. Thus, it is easier for a critical nucleus to fibers under quiescent conditions. This is ex-deposit at the fiber surface when the fiber is plained in terms of a lower interfacial free energypulled. It should be noted, however, that the di- difference of TCL, compared to that in the bulk.mension of b* deduced in this manner is smaller An experimental study is reported here of the de-than that of a mono-molecular layer, about 0.626 pendence of the transcrystallization on the shearnm for the (110) growing plane.18 Thus, the depo- rates. Different levels of shear rates were inducedsition of the critical nucleus at the fiber surface at the fiber surface by pulling the fibers with vari-seems questionable. This discrepancy might be at- ous speeds. Particular attention is given to thetributed to the overestimated value of Dhf for the influence of shear rate on the rates of nucleationcritical nucleus, as pointed out by Ishida and and crystal growth of PP. Results primarily showBussi.3 By any means, the magnitude of b* should that both the nucleation rate and the saturatedbe smaller than both a* and l*. nucleation density increase with the applied

When the fiber is pulled, the PP molecules adja- shear rates. However, the induction time is re-cent to the fiber are stretched due to the shear duced as the shear rate is increased. The magni-flow. The effect of shear is to reduce the thickness tude of Ds is reduced to be one-third of that for

crystallization under quiescent conditions when aof the first crystal layer, which deposits on the


1370 WANG AND LIU

8. E. Devaux and B. Chabert, Polym. Commun., 32,shear rate of 1.16 1/s is applied. Thus, the applica-464 (1991).tion of a shear stress to the supercooled PP melt

9. D. G. Gray, J. Polym. Sci., Polym. Lett. Ed., 12,by fiber pulling leads to enhance the development645 (1974).of transcrystallinity. Moreover, both the thickness

10. R. H. Burton, T. M. Day, and M. J. Folkes, Polym.and the crystal growth rate of transcrystalline Commun., 25, 361 (1984).layers were found to increase with increasing rate 11. M. C. Chien and R. A. Weiss, Polym. Eng. Sci., 28,of fiber pulling, especially at low crystallization 6 (1988).temperatures where regime III prevails. From the 12. R. R. Lagasse and B. Maxwell, Polym. Eng. Sci.,study of surface morphology of PTFE fibers, it is 16, 189 (1976).

13. D. E. Buerger, K. Engberg, J.-F. Jansson, andargued that the presence of fibrillar-type featuresU. W. Gedde, Polym. Bull., 22, 593 (1989).at the fiber surface is the main factor responsible

14. A. C. Bushman and A. J. McHugh, J. Polym. Sci.,for the development of transcrystallinity.Part B: Polym. Phys., 34, 2393 (1996).

15. C. Wang and L. M. Hwang, J. Polym. Sci., Part B:Polym. Phys., 34, 47 (1996).This research has been supported by National Science

16. J. Varga, J. Mater. Sci., 27, 2557 (1992).Council under contract NSC87-2216-E-155-004.17. B. Wunderlich, Macromolecular Physics, Vol. 2, Ac-

ademic Press, New York, 1976.18. E. J. Clark and J. D. Hoffman, Macromolecules, 17,

878 (1984).REFERENCES AND NOTES 19. C. Wang and C.-R. Liu, Polymer, 38, 4715 (1997).

20. J. D. Hoffman, Polymer, 24, 3 (1983).21. W. S. Lamber and P. J. Phillips, Macromolecules,

1. T. K. Kwei, H. Schonhorn, and H. L. Frish, J. Appl. 27, 3537 (1994).Phys., 38, 2512 (1967). 22. S. Middleman, Fundamentals of Polymer Pro-

2. N. Klein, G. Marom, A. Pegoretti, and C. Migliar- cessing, McGraw-Hill, New York, 1977, p. 98.esi, Composites, 26, 707 (1995). 23. W.-Y. Chiu, L.-W. Chen, C. Wang, and D.-C. Wang,

3. H. Ishida and P. Bussi, Macromolecules, 24, 3569 J. Appl. Polym. Sci., 43, 39 (1991).(1991). 24. T. W. Haas and B. Maxwell, Polym. Eng. Sci., 9,

4. T. He and R. S. Porter, J. Appl. Polym. Sci., 35, 225 (1969).1945 (1988). 25. T. Hata, K. Ohsaka, T. Yamada, K. Nakamae, N.

5. A. Kellar and S. Sawada, Makromol. Chem., 74, Shibata, and T. Matsumoto, Proc. 16th Annual190 (1964). Symposium, Adhesion Society, Williamsburg, VA,

6. J. L. Thomason and A. A. Van Rooyen, J. Mater. February, 1993, p. 180.Sci., 27, 897 (1992). 26. C. Wang and C.-R. Liu, Polymer, to appear.

7. J. Varga and J. Karger–Kocsis, J. Polym. Sci., Part 27. C. Wang and L. M. Hwang, J. Polym. Sci., Part B:Polym. Phys., 34, 1435 (1996).B: Polym. Phys., 34, 657 (1996).


transcrystallization of ptfe fiber/pp composites—iii. effect of fiber pulling on the...

Documents