Structure Development of Polyamide-66Fibers during Drawing and Their Microstructure Characterization
NADARAJAH VASANTHAN,SIGRID B.RUETSCH,DAVID R.SALEM
TRI/Princeton,601Prospect Avenue,Princeton,New Jersey08542
Received15February2002;revised3June2002;accepted10June2002
ABSTRACT: Structure development during drawing was studied for three sets of poly-
amide-66(PA66)?bers with density,optical microscopy,wide-angle X-ray diffraction,
and Fourier transform infrared spectroscopy.The crystallinity,estimated by density
measurements,remained virtually constant with increasing draw ratios,indicating
that stress-induced crystallization did not occur for the PA66?bers drawn at room
temperature,but there was a rapid transformation from a hedrite morphology to a
?brillar one.The absence of stress-induced crystallization differed from the behavior of
polyamide-6,and this was attributed to the stronger hydrogen bonding between poly-
amide chains and the higher glass-transition temperature of PA66.Polarized infrared
spectroscopy was used to measure the transition-moment angles of the vibrations at
936and906cm?1,which were found to be48and60°,respectively.The crystalline
orientation was estimated from the band at936cm?1,and the increase with an
increasing draw ratio was in close quantitative agreement with X-ray diffraction data;
this showed that infrared spectroscopy could be used reliably to measure the crystalline
orientation of PA66?bers.Because we were unable to obtain the transition-moment
angle of the amorphous bands,the amorphous orientation was obtained with Stein’s
equation.The amorphous orientation developed more slowly than the crystalline ori-
entation,which is typical behavior for?exible-chain polymers.?2002Wiley Periodicals,
Inc.J Polym Sci Part B:Polym Phys40:1940–1948,2002
Keywords:PA66?bers;infrared spectroscopy;transition moment angle;hydrogen
bonding
INTRODUCTION
Polyamide-66(PA66)is a?ber-forming semicrys-talline polymer usually synthesized by condensa-tion polymerization.1Polyamide?bers are gener-ally produced by melt spinning.Its structure de-velopment during spinning and heat setting has received much attention and is well documented in the literature.Fibers produced by these pro-cesses may not have the required crystallinity or molecular orientation to provide speci?c mechan-ical properties or good dyeing properties.Sub-stantial improvements in the mechanical proper-ties of PA66?bers have been achieved through various?ber-forming routes involve spinning fol-lowed by drawing.2–5
The physicomechanical properties of these?-bers depend on processing parameters such as the spinning velocity and drawing conditions.Struc-tural parameters such as the crystallinity,crystal perfection,crystallite size,crystalline orientation, and amorphous orientation are altered during these processes.PA66?bers have been character-ized by wide-angle X-ray diffraction(WAXD),6,7 small-angle X-ray scattering,8differential scan-ning calorimetry,9and Fourier transform infra-
Correspondence to: N. Vasanthan (E-mail: vnada@ https://www.doczj.com/doc/2e2534021.html,)
Journal of Polymer Science:Part B:Polymer Physics,Vol.40,1940–1948(2002)?2002Wiley Periodicals,Inc.
1940
red(FTIR)spectroscopy.10The drawing of poly-meric?bers in?uences molecular orientation and, often,the degree of crystallinity.Molecular orien-tation can be measured directly or indirectly with various physical techniques.Crystalline orienta-tion is usually measured with WAXD measure-ments,but infrared spectroscopy has not been applied extensively to the determination of crys-talline and amorphous orientation because of sampling dif?culties.A method has been devel-oped to measure the amorphous orientation of poly(ethylene terephthalate)directly with an in-trinsic?uorescence technique,11,12but this method is not suitable for polyamides.WAXD can also be used as a direct method of characterizing amorphous orientation,13but the predominance of overlapping crystalline re?ections that often overwhelm the amorphous scattering makes this a dif?cult procedure to apply to most semicrystal-line?bers.In addition,it has been known for some time that the chain orientation is not homo-geneous in the amorphous phase,and tie mole-cules connecting neighboring crystallites are more oriented than other polymer chains within the amorphous phase.Not many direct physical techniques are available to monitor the amor-phous orientation of semicrystalline polyamide?-bers,and most studies have resorted to indirect techniques that can be unreliable.
In the work presented here,as-spun PA66?-bers were drawn to different draw ratios,and the structure development of these PA66?bers was studied with WAXD,optical microscopy,and FTIR spectroscopy.The lack of a suitable method to measure the amorphous orientation of poly-amides directly led us to further explore the use of polarized infrared spectroscopy as a method to measure crystalline and amorphous orientation. Structural parameters obtained by X-ray diffrac-tion and FTIR spectroscopy are also compared in our discussion.
EXPERIMENTAL
Materials
One set of melt-spun PA66yarn(set S1)obtained from Solutia was drawn to different draw ratios (2–7)in our laboratory with a tensile tester at room temperature(23°C).Two more sets of PA66?bers were melt-spun and drawn at room temper-ature to different draw ratios(1.5–3.5)at DuPont (set D)and Solutia(set S2).
Density
Density values(?)were determined on small pieces of the?ber samples with a density gradient
column at23°C.Each value was the average of
three determinations.The column liquids used
were carbon tetrachloride(CCl4)and n-heptane.
Samples were allowed approximately24h for
equilibration in the gradient column before the
reading was taken.The volume fraction crystal-
linity(X c)was determined from
X c?(???a)/(?c??a)
The densities of the crystalline phase(?c)and amorphous phase(?a)of PA66were taken to be
1.24and1.09g/cm3,respectively.14
Birefringence Measurements
The birefringence of all the samples investigated
was obtained by the measurement of the parallel
and perpendicular refractive indices.These re-
fractive indices were measured with a transmit-
ted light interference microscope(Jenoptic Jena,
Inc.,Peravel Intephako)with a grating-plate disk
and a Mach–Zehender interferometer.A?lter
was used to produce monochromatic light(??551nm).Two adjacent sections of a?ber were mounted in different liquids of a suitable refrac-
tive index to obtain parallel and perpendicular
refractive indices,showing fringe de?ections on
the longitudinally viewed?ber well below one
order,that is,less than the distance between two
equidistant background lines.Fringe de?ections
or displacement measurements on the optically
sheared,duplicated image of the?ber were ob-
tained in the center of the?ber with parallel and
perpendicularly vibrating light.The procedure
was repeated several times,and the average val-
ues were used for calculations.
WAXD
WAXD measurements were carried out with a
Phillips X-ray diffractometer in the transmission
mode with curved crystal monochromatized radi-
ation of40kV and30mA.Fiber samples of a
constant mass were wound parallel to each other
on the sample holder.Equatorial scans were ob-
tained in a2?range of5–60°,with intensity data collected every0.05°for a period of10s.The equatorial scans were scanned from5to60°,and meridional scans were scanned from5to20°.The STRUCTURE DEVELOPMENT OF POLYAMIDE-661941
equatorial scans were resolved into three re?ec-tions.The apparent crystallite sizes of the crys-talline re?ections were determined with the Scherrer equation:15
L hkl??/b cos?
where L is the crystallite size,?is the Bragg angle,and b is the full width at half-maximum. The degree of crystalline orientation was obtained with Herman’s equation,based on the azimuthal half-width of the equatorial re?ection.16
FTIR Spectroscopy
Infrared spectra were collected on a Nicolet560 FTIR spectrometer equipped with an Advantage microscope with a liquid-nitrogen-cooled mercury–cadmium–telluride detector.All the spectra were
obtained on single?bers with the transmission mode.At least256scans were made to achieve an adequate signal-to-noise ratio.All subtractions were carried out with standard Ominic software. The micro-FTIR technique was used for polariza-tion studies.Two different transmission spectra were obtained for each sample with the incident beam parallel and perpendicular to the?ber axis. The infrared dichroism was calculated as(D?A?/ A?).Curve?tting was carried out with the Peak-solve program.The peaks were assumed to be Lorentzian with a linear baseline.
Scanning Electron Microscopy
The scanning electron microscopic observations of ?ber were performed with a Hitachi S4500?eld emission scanning electron microscope at differ-ent magni?cations.
RESULTS AND DISCUSSION
Density,Birefringence,and Scanning Electron Microscopy
Two sets of PA66?bers were received from Solu-tia(sets S1and S2).S1was drawn at TRI/Prince-ton to different draw ratios to obtain samples with different microstructures.S2was drawn to different draw ratios at Solutia.We also obtained PA66?bers drawn to different draw ratios from DuPont(set D).The densities of all three sets of PA66?bers drawn to different draw ratios at room temperature were measured with a density gradient column.The crystallinity of these three sets of PA66?bers,estimated from the density measurements,is plotted in Figure1.It is clear from Figure1that the crystallinity of PA66?bers drawn at room temperature did not change with an increasing draw ratio.Because increasing crystallinity is not generally observed when poly-mers are drawn below the glass-transition tem-perature(T g)because of insuf?cient molecular mobility,the observation we made for PA66?bers is not surprising.However,this is entirely differ-ent from the behavior of polyamide-6(PA6)?bers, for which the density values increase with an increasing draw ratio.13,17This results from?–?transformation as well as strain-induced crystal-lization from the oriented amorphous phase.17 Because increasing crystallinity is not gener-ally observed when polymers are drawn below T g because of insuf?cient molecular mobility,the ob-servation we made for PA66?bers would be ex-pected.The?relaxation in polyamides is gener-ally associated with T g,and the fact that,at a given humidity,PA6displays a lower?-relax-ation temperature than PA66may explain its ten-dency to crystallize during room-temperature drawing.The lower T g for PA6may arise from weaker intermolecular hydrogen bonding.Nor-mal coordinate calculations18that do not incorpo-rate the hydrogen-bonding interaction have been made for polyamides,and these put the amide-1 band at1644cm?1.The fact that this band ap-pears at1640cm?1in PA6and at1630cm?1in PA66provides additional support for the notion that hydrogen bonding is stronger in
PA66. Figure 1.Volume fraction crystallinity versus the draw ratio for three sets of PA66?bers received from DuPont(D)and Solutia(S1and S2)and drawn at room temperature.
1942VASANTHAN,RUETSCH,AND SALEM
The birefringence (?n )values for drawn PA66?bers (set S1)are plotted against the draw ratios in Figure 2.The birefringence values were ob-served only for the Solutia ?bers drawn in our laboratory.PA66?bers obtained from Solutia ap-peared to have trilobal cross sections,so obtaining reliable birefringence data is dif ?cult.The Mon-santo samples are primarily considered for our discussion.The birefringence at ?rst increased rapidly at lower draw ratios and then slowly in-creased at higher draw ratios.The birefringence provides information about overall orientation;that is,it includes both crystalline and amor-phous orientation.Stein and Norris 19suggested that the total birefringence is the sum of the crystalline and amorphous birefringence:
?n ?X c f c ?n c 0?(1?X c )f a ?n a
(1)
where ?n c 0and ?n a 0
are the intrinsic birefringence of the crystalline and amorphous phases,respec-tively,and f c and f a are the crystalline and amor-phous orientation functions,respectively.Al-though there is no signi ?cant change in crystal-linity,birefringence increases with an increasing draw ratio,and this suggests that the crystalline and/or amorphous orientation changes with drawing.This is discussed later.
The surface morphology of PA66?bers ob-tained from Solutia (set S1)drawn to different draw ratios was studied with scanning electron microscopy.Figure 3shows the scanning electron micrographs of drawn PA66?bers.It can be clearly seen that undrawn PA66?bers show hedrite or axialite morphology.Hedrites are
par-
Figure 2.Birefringence of PA66?bers (set S1)versus the draw
ratio.
Figure 3.Scanning electron micrographs of drawn PA66?bers (set S1)with the following draw ratios:(a)1,(b)1.5,and (c)2.
STRUCTURE DEVELOPMENT OF POLYAMIDE-661943
tially developed spherulites.Hedrite structures are formed during the spinning because of insuf-?cient time for the formation of complete spheri-cal structures.As we increase the draw ratio,these hedrites become smaller and disappear completely;this results in a predominantly ?bril-type morphology at higher draw ratios.Similar behavior has been observed during the drawing of other semicrystalline polymers such as polyethyl-ene and polypropylene.20WAXD
Equatorial X-ray diffraction patterns were ob-tained for various drawn PA66?bers (not shown).
The X-ray diffraction pattern of the undrawn PA66?bers consisted of two major re ?ections with an amorphous halo,con ?rming that the un-drawn ?bers were partially crystalline.Curve ?t-ting was carried out to obtain the half-width of each re ?ection.The apparent crystallite size was estimated from the half-width of these re ?ections and was found to increase rapidly when the ?ber was drawn from the undrawn state to a draw ratio of 2(Fig.4).There was no signi ?cant change in the crystallite size at higher draw ratios.The large increase in the apparent crystallite size at low draw ratios may be attributed to the rapid change from hedrite morphology to ?brillar mor-phology,as mentioned earlier.Our similar stud-ies on PA6?bers showed a continuous increase in the crystallite size up to higher draw ratios.The crystalline orientation of the PA66?bers obtained from Solutia was estimated from azimuthal scans of the (020)re ?ection and showed that crystalline orientation increased continuously,as shown later.
FTIR Spectroscopy
FTIR spectra of drawn PA66?bers are shown in Figure 5.The band at 1200cm ?1and the band at 936cm ?1were attributed previously to the crys-talline phase,whereas the bands at 1180and 1630cm ?1were shown to be reference bands.We previously reported 10a method of determining the crystallinity of PA66with the bands at
936
Figure 4.Crystal width of ?200re ?ections of Solutia PA66?bers versus the draw
ratios.
Figure 5.FTIR spectra of drawn PA66?bers.
1944VASANTHAN,RUETSCH,AND SALEM
and 924cm ?1.We found that the crystallinity of the PA66?bers drawn in our laboratory to differ-ent draw ratios was approximately 35%by the independent infrared method,which is in close agreement with our density data.
Polarized infrared studies could be performed in the mid-infrared range to obtain information about the molecular orientation.One of the prac-tical dif ?culties is identifying infrared bands that have an absorbance lower than approximately 1to satisfy the Beer –Lambert law.Polyamides are strongly absorbing,and only a few bands satisfy these requirements.On the basis of previous as-signments,we identi ?ed the bands at 936,906,and 924cm ?1as suitable for investigating crys-talline and amorphous orientation.The transi-tion-moment angles for these vibrations are not known.Once the value of the transition-moment angle for a given band is determined,the molec-ular orientation functions can be determined.Figure 6shows infrared spectra taken for PA66?bers drawn to different draw ratios under par-allel and perpendicular polarization.Parallel and perpendicular bands were identi ?ed from the lit-erature.The absorption of infrared radiation de-pends on the transition-moment vector and ap-plied electric vector.For solids and crystals,each molecule has a ?xed direction.Therefore,the ab-sorbance of a vibrational mode depends on the transition-moment vector of the normal vibration (M )and the applied electric vector (E ).The quan-tity ME determines the molecular transition mo-ment.A maximum absorption will occur when M
and E are parallel,and minimum absorption will occur when they are perpendicular.The dichroic ratio (D )is given by 21
D ?
A ?A ?
(2)
The infrared dichroic ratio of a uniaxially ori-ented polymer depends on two characteristic an-gles,?and ?.?is the angle between the polymer chain axis and reference direction (?ber axis),and ?is the angle between the polymer chain and the transition moment,as shown in Figure 7.Gener-ally,overall chain orientation is given by follow-ing
equation:
Figure 6.FTIR spectra of PA6?bers taken under parallel and perpendicular polar-ization along with an unoriented calculated
spectrum.
Figure 7.Schematic representation of orientation.
STRUCTURE DEVELOPMENT OF POLYAMIDE-661945
?P 2?cos ????f ???D ?1?/?D ?2??
?1/2?3cos 2??1)
(3)
Optical measurement is particularly useful for the determination of the orientation https://www.doczj.com/doc/2e2534021.html,bining and rearranging eqs 2and 3give eq 4,from which the transition-moment angle (?)can be obtained:
?n /X c ?n c 0
?2?D ?1?/?D ?2?/?3cos 2??1)
??1?X c )f a ?n a 0/X c ?n c 0
(4)
The dichroic ratios of the bands at 936and 906
cm ?1are plotted as a function of the draw ratio,as shown in Figure 8.To obtain the orientation function,we obtained the direction of the transi-tion moment for the band at 936cm ?1by plotting ?n /X c ?n c 0against 2(D ?1)/(D ?2)(eq 3)in Figure
9(a)(r 2
?0.97).The transition-moment angle was established as 48°.The transition-moment angle was obtained for the band at 906cm ?1from a similar plot shown in Figure 9(b)(r 2?0.90).The transition-moment angle for the band was estab-lished to be 60°.It has been known for some time that bands being determined as parallel or per-pendicular in direction on the basis of the transi-tion moment make an angle smaller or greater than 55°with respect to the chain axis.Therefore,the bands at 936and 906cm ?1can be attributed to parallel and perpendicular dichroism,respec-tively.
Crystalline orientation functions of drawn PA66?bers were calculated with the angle of 48°,and the values are given as a function of the draw
ratio in Figure 10.Samanta et al.22obtained the transition-moment angle for the band at 936cm ?1with attenuated total re ?ection (ATR)po-larized infrared spectroscopy and X-ray diffrac-tion as 42°.However,ATR polarized spectroscopy is not an accurate and established technique for polarized infrared investigation.Furthermore,any quantitative work with ATR spectroscopy makes the assumption that there is no signi ?cant difference in the microstructure properties of the surface and bulk.It is necessary to compare the ATR and transmission polarized infrared data,and a detailed investigation is in progress in our laboratory.
Similar experiments were carried out to obtain the transition-moment angle for the amorphous bands at 924and 1136cm ?1,but because of ex-cessive data scattering,we were not able to obtain reliable data.It is important to note that the dichroic ratios of these bands do not show a sys-tematic increase or decrease with increasing
draw
Figure 9.Plots of (a)936-cm ?1and (b)906-cm ?1bands for the determination of the transition-moment
angle.
Figure 8.Dichroic ratios of 936and 906cm ?1versus the draw ratios.
1946VASANTHAN,RUETSCH,AND SALEM
ratios.We do not have any explanation for this behavior.The amorphous phase consists of sev-eral possible conformations,and the transition-moment angle we observe by this method is the angle for the average conformation.If we assume that the populations of different conformations change with the draw ratio,the average transi-tion-moment angle would be expected to change as well.We suspect this might be the reason for this kind of behavior.Because we have so far been unable to directly obtain the orientation function for the amorphous phase from infrared spectros-copy,we resorted to eq 1,for which ?n was ob-tained from the birefringence method and X c and ?P 2(cos ?)?c were obtained from infrared measure-ments.The calculated amorphous orientation functions are also shown in Figure 10.The rapid development of the crystalline orientation,?P 2(cos ?)?c ,to almost full ?ber-axis orientation compared with the slower development of orientation in the noncrystalline regions,?P 2(cos ?)?a ,is common be-havior for ?exible-chain polymers.20The crystal-line orientation functions obtained by X-ray dif-fraction are plotted along with the crystalline ori-entation functions obtained by FTIR spectroscopy in Figure 11.It is apparent from the plot that the techniques agree very well.
CONCLUSIONS
We have characterized the microstructure of drawn PA66?bers by polarized FTIR spectros-copy.Structural information was obtained by density,X-ray diffraction,and optical microscopy
measurements.The transition-moment angles of the infrared vibrations in PA66at 936and 906cm ?1were found to be 48and 60°,respectively.The crystalline orientation was estimated from the band at 936cm ?1,and the increase with an increasing draw ratio was in close quantitative agreement with X-ray diffraction data;this showed that infrared spectroscopy could be used reliably to measure the crystalline orientation of PA66?bers.A reliable determination of the tran-sition-moment angles of the amorphous infrared bands could not be obtained,but an application of Stein ’s equation showed that the amorphous ori-entation developed more slowly with the draw ratio than the crystalline orientation,with ulti-mate values of approximately 0.6and 0.98,re-spectively.We have also found that the drawing of semicrystalline PA66?bers from the unori-ented or poorly oriented state involves a rapid transformation from a hedrite morphology to a ?brillar morphology but does not involve an in-crease in crystallinity.The absence of stress-in-duced crystallization differs from the behavior of PA6,and this was attributed to the stronger hy-drogen bonding between polyamide chains and the higher T g of PA66.
These studies were undertaken in connection with the TRI project “Fiber Structure and Heat Setting Proper-ties ”,which was supported by a group of TRI partici-pants,including DuPont,DSM,BASF,Honeywell,and Milliken Research Corp.The authors are also indebted to Dennis W.Briant for experimental
work.
Figure 10.Crystalline and amorphous orientation versus the draw
ratio.
Figure https://www.doczj.com/doc/2e2534021.html,parison of the crystalline orientation of PA66?bers obtained by (E )X-ray and (F )FTIR methods.
STRUCTURE DEVELOPMENT OF POLYAMIDE-661947
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