Crystal transformation from the a -to the b -form upon tensile
drawing of poly(L -lactic acid)
Kazuyo Takahashi a ,Daisuke Sawai a ,Takafumi Yokoyama a ,Tetsuo Kanamoto a,*,
Suong-Hyu Hyon b
a
Department of Applied Chemistry,Tokyo University of Science,Kagurazaka,Shinjuku-ku,Tokyo 162-8601,Japan b
Institute for Frontier Medical Sciences,Kyoto University,53Kawara-cho,Shogoin,Sakyo-ku,Kyoto 606-8507,Japan
Received 22December 2003;received in revised form 12March 2004;accepted 24March 2004
Abstract
A ?lm of poly(L -lactic acid)(PLLA)consisting of highly oriented a crystals was uniaxially drawn by tensile force.The effects of the draw ratio (DR),draw temperature eT d T;and draw stress on the crystal/crystal transformation from the a -to the b -form crystals were studied.At the initial stage of drawing,the highly oriented a crystals of the starting ?lm transformed into a broader orientation distribution,and signi?cant crystal disorder was introduced.Upon further drawing,the a crystals steadily transformed into b crystals with increasing the DR.For the drawing at a constant T d ;the crystal transformation proceeded more ef?ciently at a higher draw rate and,hence,at a higher draw stress.Furthermore,for the drawing at a constant draw rate,the transformation proceeded with DR most ef?ciently for the tensile draw at a T d around 1408C,although the draw stress increased with decreasing the T d :The present result combined with the previous ?nding in the drawing of PLLA by solid-state extrusion [Macromolecules 36(2003)3601]suggests that there is a T d of around 1408C at which the crystal transformation proceeds most ef?ciently with DR,suggesting that there are two factors that have opposite effects on the ef?ciency of the crystal transformation with increasing the T d :However,as a result of the combined effects of the T d and DR on the crystal transformation and the ductility increase with the T d ;an oriented ?lm consisting predominantly of b crystals was obtained by tensile drawing at a T d in the range of 140–1708C to the highest DR achieved at each T d :q 2004Elsevier Ltd.All rights reserved.
Keywords:Tensile drawing of poly(L -lactic acid);Crystal transformation from a to b crystals;Ef?ciency of crystal transformation
1.Introduction
Much attention has recently been given to poly(L -lactic acid)(PLLA)because it is a polymer produced from sustainable plant products,such as corn and potato,and is biocompatible and biodegradable,producing nontoxic lactic acid in the human body [1,2].Therefore,PLLA is being used for biomedical applications,including bone-?xation devices and sutures for microsurgery [3–5].Since this polymer has good mechanical properties [6]with a high melting temperature (,1808C),its application for more general purposes,for example,as ?bers [7–11],?lms and structural materials [12,13],has been evaluated.
PLLA takes three crystal forms depending on the conditions for crystallization and drawing.The most
common a -form (orthorhombic)has a 10/7helical chain conformation and is obtained by crystallization from melt [7,14–16],solutions [7,16,17],or glass [18].The b -form (orthorhombic or trigonal)takes a 3/2helical conformation and is produced by drawing of a semicrystalline ?lm with a crystals [15,19,20].Eling et al.[7]reported that b crystals were generated upon tensile drawing at a high temperature to a higher draw ratio (DR),whereas the drawing at a low temperature and/or a low DR produced a crystals.Thus,the drawn products of PLLA commonly consist of a crystals or a mixture of a and b crystals.Recently,a new g -form was obtained by epitaxial crystallization of PLLA on hexa-methylbenzene [21].This polymorph also has an ortho-rhombic unit cell containing two regularly packed antiparallel 3/2helices.Since the physical and mechanical properties as well as the chemical stability of a polymer,in general,strongly depend on the morphology and the crystal
0032-3861/$-see front matter q 2004Elsevier Ltd.All rights reserved.
doi:10.1016/j.polymer.2004.03.108
Polymer 45(2004)4969–4976
https://www.doczj.com/doc/179962378.html,/locate/polymer
*Corresponding author.
structure,the conditions for the formation of the b-form crystal in PLLA should be clari?ed to control the properties of this polymer.
Previously,we have shown that,in the solid-state extrusion of semicrystalline polymers,the deformation in an extrusion die proceeded in complex shear and exten-sional?elds under high pressure[19,22].The crystal transformation in PLLA from a crystals into oriented b crystals was promoted slightly more by shear deformation at the sheath of an extrudate than by extensional deformation at the core of the extrudate[19].Furthermore,it was found that the crystal transformation with extrusion DR proceeded most ef?ciently for the extrusion at1308C among the extrusion conducted at100,130,and1708C[20].In contrast to the complex deformation mode in solid-state extrusion,a simple extensional deformation may be expected for tensile drawing.
In this article,we discuss the effects of the draw temperatureeT dT;DR,and draw stress on the a to b crystal transformation upon tensile drawing of a PLLA?lm consisting of highly oriented a crystals.Tensile drawing was made over wide ranges of temperatures and draw rates. The drawn products were characterized by wide-angle X-ray diffraction(WAXD)and polarized FT-IR spectra as a function of the DR and T d:
2.Experimental section
2.1.Drawing
Dried PLLA with a viscosity-average molecular weight eM vTof8.3￡104was compression-molded at2008C into a ?lm of0.1mm thickness followed by rapid cooling to ,08C.The melt-quenched amorphous?lm thus obtained had an M v of6.6￡104.A ribbon with0.5cm width and 5cm length cut from the amorphous?lm was drawn by tensile force at808C to a DR of6.The drawn?lm was annealed while keeping the sample length constant at 1708C for1h,producing a?lm consisting of highly oriented a crystals with high crystallinity.This highly oriented a?lm was further drawn by tensile force at a T d between25and1808C and a constant crosshead speed corresponding to an initial strain rate of0.5–100/min.
2.2.Characterization
WAXD patterns were recorded by a?at-plate camera and by diffractometer scans.WAXD photographs were obtained with Cu K a radiation generated at40kV and25mA on a Rigaku Geiger?ex RAD-3A and monochromatized with a graphite crystal.WAXD pro?les were recorded with Ni-?ltered Cu K a radiation generated at40kV and150mA on a Rigaku Rota?ex RU-200rotating anode X-ray generator equipped with a diffractometer and a pulse height discriminator.Thee00‘Tpro?les on the meridian were measured by a symmetrical transmission mode.
The crystal transformation from a to b crystals was also followed by infrared(IR)spectroscopy.Polarized spectra were recorded using a JEOL Fourier transform infrared spectrometer(JRS-FT7000W)at a resolution of1cm21and room temperature.An oriented?lm consisting of a crystals (10/7helix)exhibits an absorption band at923cm21that is assigned to the CH3rocking mode,showing the transition moment perpendicular to the chain axis[23–25].In contrast,a highly oriented b?lm showed a band at 912cm21that was assigned to the CH3rocking mode of b crystals(3/2helix),which shows a transition moment perpendicular to the chain axis[20,25].
The total absorbance A0for an oriented sample is given by the average of the three absorbances of A x;A y;and A z which were measured with the incident beams polarized along the three orthogonal axes of x;y;and z;respectively. For a sample uniaxially oriented along the?ber axis z;the absorbances of A x and A y may be equal[26].Thus,
A0?eA zt2A xT=3e1TDensities were measured by a pycnometer method at 30.0^0.18C using pure water as a?lling?uid to avoid any signi?cant absorption of a liquid.The crystallinity of a sample was calculated assuming a crystal–amorphous two-phase structure.The densities of an amorphous phase,r a; an a crystal,r ceaT;and a b crystal,r cebTwere assumed to be 1.245^0.001g/cm3[19], 1.285g/cm3[15],and 1.301g/cm3[19],respectively.
3.Results and discussion
3.1.Drawing behavior
Fig.1shows the WAXD patterns of an a?lm prepared by drawing of an amorphous?lm at808C to a DR of6and a highly oriented sample prepared by annealing of
the
Fig.1.WAXD photographs of a PLLA?lm consisting of a crystals prepared by tensile drawing of an amorphous?lm at808C to a DR of6(a) and a highly oriented?lm consisting of a crystals obtained by annealing of the cold-drawn?lm at1708C for1h while keeping the sample length constant.
K.Takahashi et al./Polymer45(2004)4969–4976 4970
as-drawn ?lm at 1708C for 1h while keeping the sample length constant.The WAXD of the as-drawn ?lm shows a poor crystallinity and not well-aligned chain orientation,as revealed by the appearance of only several broad re?ections overlapped with diffuse scattering.These re?ections could be indexed by assuming either an orthorhombic a -form crystal [15],with unit cell constants of a ?1:06;b ?0:61;and c ?2:88nm (?ber axis),or an orthorhombic b -form crystal [15,19],with cell constants of a ?1:04;b ?1:77;and c ?0:90nm (?ber axis).However,the meridional scan of this sample clearly showed the existence of the (0010)a re?ection from a crystals at 2u ?31:08but no re?ection at 2u ?29:98;where the (003)b re?ection from b crystals might have been observed if b crystals existed.These observations indicate that the as-drawn sample consisted of poorly oriented a crystals with signi?cant disorder.Upon annealing of this ?lm at 1708C,the resultant ?lm exhibited an increased number of sharp and circular re?ections from a crystals with no streak along the layer lines,showing high chain orientation and high crystal perfection as seen in Fig.1(b).
Fig.2shows the nominal stress/strain curves for the tensile drawing of a highly oriented a ?lm recorded at a constant draw temperature eT d Tof 1708C near the melting temperature eT m ?1808C Tand at different crosshead speeds corresponding to the initial strain rates of 0.5–100/min.When the draw was made at a lower rate,thin areas formed at several portions,and the deformation proceeded heterogeneously producing no uniformly drawn products.When the draw rate was increased,however,the draw stress increased,and a more uniform draw was achieved.Thus,uniformly drawn ?lms were obtained at higher crosshead speeds corresponding to initial strain rates of 50–100/min.
Furthermore,the ductility varied complexly with the draw rate.The maximum achieved DR increased with increasing the draw rate,reaching a maximum of ,4(corresponding to a strain of ,3in Fig.2)at a crosshead speed corresponding to an initial strain rate of 50/min.At yet higher draw rates,the ductility decreased again.Thus,the tensile drawing was made at a constant crosshead speed corresponding to an initial strain rate of 50/min.
3.2.Crystal transformation during tensile drawing Fig.3shows WAXD photographs for a DR series prepared by tensile drawing of a highly oriented a ?lm at a T d of 1708C and a constant crosshead speed corresponding to an initial strain rate of 50/min.The WAXD pattern of the initial sample showed many sharp re?ections from highly oriented a crystals.However,upon drawing to a DR of 1.6,most of the sharp but weak re?ections merged into the background and the remaining re?ections showed a broad intensity distribution along the azimuthal direction.These observations reveal that the initially highly oriented a crystals were deformed and exhibited a broad orientation distribution around the ?ber axis.Upon further drawing,the number of re?ections from a crystals further decreased,and the remaining re?ections became broad spots.Concurrently with these changes,the intensities of streaks along the equator and layer lines became stronger as the DR was increased,showing that signi?cant paracrystalline disorder was introduced both along and perpendicularly to the chain axis [15,19,20].Although most of the broad re?ections could be indexed on the basis of either the a -or b -form crystal,the appearance of such streaks suggested the formation of b crystals because such streaks are character-istic of b -form crystals [15,19].
The crystal transformation can be followed by the relative intensity of the (003)b re?ection from b crystals and the (0010)a re?ection from a crystals recorded by WAXD meridional scans,as shown in Fig.4.The intensity of the (003)b re?ection gradually increased with the DR at the expense of the intensity of the (0010)a re?ection,showing a steady transformation of a crystals to b crystals.Since the degree of chain orientation for a crystals was slightly different from that for b crystals within a given sample,the relative amount of b crystals could not be directly correlated to the intensity ratio of the (0010)a and the (003)b re?ections as previously done in the solid-state coextrusion of a PLLA ?lm consisted of randomly oriented a crystals [20].
The formation of b crystals was also evaluated from the FT-IR spectra that were corrected for the chain orientation according to Eq.(1).Fig.5shows the IR spectra for a draw ratio series prepared by drawing of a highly oriented a ?lm at a T d of 1708C.To determine the relative amount of crystals formed within a drawn sample,the ratio of the absorption coef?cients K e912Tb =K e923Ta for IR bands at 912and 923cm 21assigned to the CH 3rocking
modes
Fig.2.Stress/strain curves recorded at 1708C and different crosshead speeds corresponding to initial strain rates e_g Tin the range of 0.5–100/min.
K.Takahashi et al./Polymer 45(2004)4969–49764971
[20,25]of b and a crystals,respectively,should be known.According to the Lambert–Beer law,absorbance A is given by
A ?log eI 0=I T?KdX c
e2T
where I 0and I are the intensities of the incident beam and absorbed beam,respectively,K ;the absorption coef?cient,d ;the sample thickness,and X c ;the crystallinity of a sample consisting of only a or b crystals.Therefore,the ratio of K e912Tb =K e923Ta is given by K e912Tb =K e923Ta
?A 0e912Tb d ea TX c ea T=A 0e923Ta d eb TX c eb T
e3T
where A 0e923Ta and A 0e912Tb are the total absorbances of the bands at 923and 912cm 21,respectively,d ea Tand d eb T;the thicknesses of the samples consisting of only a and b crystals,respectively,and X c ea Tand X c eb T;the crystal-linities of the a and b samples,respectively.
The absorbances A 0e923T0a s for a samples were measured
on two ?lms;one consisting of highly oriented a crystals and one consisting of randomly oriented a crystals.The absorbance A 0e912Tb for a b sample was measured on an extrudate prepared by solid-state coextrusion [20].By using the measured values of A 0;d ;and X c for the a and b samples,as shown in Table 1,the K e912Tb =K e923Ta was determined to be 1.1.Thus,the ratio of b crystals to the sum of a and b crystals within a drawn ?lm can be calculated by A 0e912Tb =?1:1A 0e923Ta tA 0e912Tb
e4
T
Fig.3.WAXD photographs for a draw ratio series prepared by the drawing of a highly oriented a ?lm at a T d of 1708C and a constant crosshead speed corresponding to an initial strain rate of 50/min.
Table 1
IR absorbances for unoriented and oriented a samples at 923cm 21and an oriented b sample at 912cm 21
Crystallinity eX c T
Thickness ed ￡106m TAbsorbance (A 0)
Oriented a sample 0.7529 3.12(at 923cm 21)Unoriented a sample 0.4648 3.24(at 923cm 21)b sample
0.51
38
3.04(at 912cm 21)
K.Takahashi et al./Polymer 45(2004)4969–4976
4972
As the pro?le for each of the IR bands at 912and 923cm 21was well approximated by a Gaussian function,the observed IR spectra were resolved into the appropriate peaks at 912and 923cm 21assuming Gaussian functions that were determined from the samples consisting of only a crystals or b crystals,as shown in Fig.5.The relative amounts of b crystals estimated from the IR spectra are shown in Fig.6as a function of the DR.They increased almost linearly with the DR and approached ,0.9(90%)at a DR $3.5.
3.3.Effect of draw temperature
Fig.7shows the effect of the draw temperature,T d ;on the nominal stress/strain curves for the tensile drawing of a highly oriented a ?lm recorded at a constant crosshead speed giving an initial strain rate of 50/min in the temperature range of 25–1808C.With increasing the T d ;the draw stress decreased and the ductility increased.More speci?cally,for the drawing at a lower T d #508C (glass transition temperature,T g ?608C),the draw
stress
Fig.4.WAXD meridional pro?les for the draw ratio series shown in Fig.3
.
Fig.5.Polarized FT-IR spectra for the draw ratio series shown in Figs.3and 4.The spectra were corrected for the chain orientation.Observed (—)and decomposed spectra assuming Gaussian functions (--
-).Fig.6.Fractions of b crystals as a function of the draw ratio estimated from the IR spectra in Fig.5
.
Fig.7.Stress/strain curves recorded at a constant crosshead speed
corresponding to an initial strain rate e_g Tof 50/min at different T 0
d
s in the range of room temperature to 1808C.
K.Takahashi et al./Polymer 45(2004)4969–49764973
increased rapidly with the strain until sample failure occurred.However,for the drawing at a higher T d of 100–1808C,the draw stress initially increased rapidly and then stayed almost constant at higher strains until failure occurred.The maximum DR achieved increased rapidly with a T d above the T g(608C)and reached a maximum of a DR of,4(corresponding to a strain of,3in Fig.7)at a T d of1708C,followed by a rapid decrease at yet higher T0d s: These results showed that the tensile drawing of a highly oriented a?lm at a T d of1708C and a constant crosshead speed corresponding to an initial strain rate of50/min gave a uniform product with the highest DR of,4.
To examine the effect of the T d on the crystal transformation,a highly oriented a?lm was tensile-drawn at T0d s in the range of100–1808C to a constant DR of3, which was the highest DR commonly achievable at these T0d s:Fig.8shows WAXD photographs for a series of such samples.Upon drawing at a T d of1008C to a DR of3,most of the sharp and spotty re?ections from the highly oriented a crystals of the starting sample(see Fig.1(b)(disappeared, and a few broad re?ections continued to overlap with streaks on the equator and layer lines.These observations suggest that the crystal orientation was disturbed and that signi?cant crystal disorders were induced both along and perpendicularly to the chain axis.With increasing the T d up to1408C,the broad re?ections became slightly more circular whereas the streaks on the layer lines were signi?cantly stronger.Most of the broad re?ections could be indexed on the basis of either the a-or the b-form.For the drawing at the highest possible T d of1808C,the WAXD pattern consisted of a mixture of a number of sharp re?ections from a crystals and a few broad re?ections observed for the samples prepared at lower T0d s:This indicates that the ef?ciency of the crystal transformation from a to b crystals,de?ned by the ratio of b crystals to the sum of a and b crystals at a given DR,is lower at a high T d near the T m:
In order to determine the effect of the T d on the formation of b crystals more quantitatively,the relative amounts of b crystals were determined from the FT-IR spectra for a series of samples drawn to a constant DR of3at different T0d s of 100–1808C.As shown in Fig.9,the fraction of b crystals increased with increasing the T d from a T d of1008C to a maximum at a T d of,1408C.At yet higher T0d s;it decreased rapidly with the T d:
Our previous study[20]on the effect of drawing variables on the ef?ciency of the crystal transformation in solid-state extrusion of PLLA revealed that the transform-ation proceeded most ef?ciently for the extrusion around 1308C.Therefore,the combination of the present?nding in tensile drawing and that in the previous solid-state extrusion suggests that the ef?ciency of the crystal transformation from a to b crystals upon drawing is primarily determined by the deformation temperature.
3.4.Effect of draw stress
In a previous study[20]on the drawing of a PLLA?lm consisting of randomly oriented a crystals by solid-state coextrusion at1708C,it was found that the crystal transformation from a to b crystals proceeded with extrusion DR more rapidly for the extrusion at a higher extrusion pressure.As shown in Fig.2,the nominal draw stress increased with an increasing draw rate.Therefore,the effect of the draw stress on the ef?ciency of the crystal transformation was studied by drawing a highly oriented
a Fig.8.WAXD photographs for a T d series drawn to a constant DR of3at
different T0d s in the range of100–1808
C.
Fig.9.Fractions of b crystals at a constant draw ratio of3as a function of
the drawing temperature estimated from the IR spectra(A).
K.Takahashi et al./Polymer45(2004)4969–4976
4974
?lm at different draw rates and a T d of 1708C to a constant DR of 3.The DR of 3corresponds to the highest DR commonly achievable for the drawing at crosshead speeds,giving initial strain rates in the range of 20–100/min.When the crosshead speed was increased from 20,50,to 100/min,the true draw stress at a DR of 3(strain of 2),estimated by multiplying the nominal stress by the DR,increased from approximately 55,65,to 80MPa,respectively.The WAXD meridional pro?les in Fig.10show that the intensity of the (003)b re?ection increased with increasing the draw stress at the expense of the intensity of the (0010)a re?ection.These results show that,when the tensile draw was made at a constant T d but at different draw rates,the crystal transformation proceeded with DR more ef?ciently for the draw at a higher draw rate and,hence,a higher draw stress.As have been discussed,there is an optimum T d of ,1408C at which the ef?ciency of crystal transformation is the highest for both tensile drawing and solid-state extrusion.As seen in Fig.10,for the drawing at a given T d ;the ef?ciency was higher for the drawing at a higher draw rate and,hence,at a higher draw stress suggesting,that the crystal transformation is enhanced by a higher draw stress.For the draw at a given draw rate,indeed,the true draw stress at a given strain increased steadily with decreasing the T d (Fig.7),and the ef?ciency of the crystal transformation increased rapidly with decreasing the T d
from 180to 1408C.However,at yet lower T 0
d s ;th
e ef?ciency o
f the crystal transformation decreased with
decreasing the T d even though the draw stress increased steadily with decreasing the T d :These facts suggest that there is another factor,which has an opposite effect to that of the draw stress on the crystal transformation when the T d is changed.When the T d is lowered,the crystals likely become harder,and the crystal transformation may be suppressed,as suggested by the fact that the ductility decreased with lowering the T d (Fig.7).As a result of the combined effects of the draw stress,which increases with decreasing the T d and enhances the crystal transformation,and the crystal hardening,which proceeds with decreasing the T d and suppresses the transformation,the ef?ciency of the crystal transformation became the highest at a T d of ,1408C.
4.Conclusion
A PLLA ?lm consisting of highly oriented a crystals was tensile-drawn at a T d of 25–1808C and constant crosshead speeds corresponding to the initial strain rates of 0.5–100/min.To determine the effects of the DR,T d ;and draw stress on the crystal transformation from the a -to the b -form,the drawn products were characterized by WAXD and polarized FT-IR spectroscopy.At the initial stage of drawing,the highly oriented a crystals within the starting sample were inclined to the draw direction,and,upon further drawing,they progressively transformed into oriented b crystals.These observations suggest that macroscopically extensional deformation for tensile draw-ing proceeded by microscopically shear deformation.The crystal transformation,evaluated by the ratio of b crystals to the sum of a and b crystals at a given DR that was determined from the IR absorbances at a 912cm 21band for b crystals and a 923cm 21band for a crystals,was higher for the drawing at a higher draw stress when compared at a given T d :However,there was a T d of ,1408C,at which the crystal transformation proceeded most ef?ciently with the DR even though the draw stress steadily increased with decreasing the T d :This ?nding suggests that the ef?ciency of the crystal transformation is controlled by two factors of the draw stress and crystal hardening,which had opposite effects when the T d was changed.However,an oriented sample consisting predominantly of b crystals was obtained
by tensile draw in a wide range of T 0
d
s as a result of the combined effects of T d and DR on the crystal transformation and the ductility increase with the T d :
Acknowledgements
This work was partly supported by the Grant-in-Aid from Of?ce of Technology Transfer,Japan Science and Tech-nology
Corporation.
Fig.10.WAXD meridional pro?les for samples with a DR of 3prepared at a T d of 1708C but at different crosshead speeds corresponding to the initial strain rates of 20,50,and 100/min.The true draw stresses at a DR of 3for different draw rates are shown below the strain rates.The decomposition of the observed pro?les into three peaks was made by assuming a symmetrical function consisting of Gaussian and Cauchy pro?les for each peak.
K.Takahashi et al./Polymer 45(2004)4969–49764975
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