Effect of annealing on the microstructure
Research Article
Received:11May2011Revised:8July2011Accepted:10July2011Published online in Wiley Online Library:14October2011 (https://www.sodocs.net/doc/3e15701641.html,)DOI10.1002/pi.3180
Effect of annealing on the microstructure and mechanical properties of polypropylene with oriented shish-kebab structure
Hongwei Bai,a Hua Deng,a?Qin Zhang,a Ke Wang,a Qiang Fu,a?
Zhijie Zhang b and Yongfeng Men b
Abstract
Annealing is thought to be an effective method to promote chain rearrangement in semicrystalline polymers and improve their physical properties.However,little attention has been paid to the annealing of?ow-oriented semicrystalline polymers despite its importance in polymer processing.In this work,the microstructural evolution of injection-moulded polypropylene with an oriented shish-kebab structure upon annealing has been explored with differential scanning calorimetry,small-angle X-ray scattering and scanning electron microscopy,Fourier transform infrared spectroscopy and dynamic mechanical analysis.The results show that annealing gives rise to a chain rearrangement in both the crystalline and amorphous phases.Accompanied by the growth and perfecting of the kebabs,relaxation of the initially oriented chains in the amorphous phase is observed.Then, the relationship between the structure and the resulting mechanical properties is established.
c 2011Society of Chemical Industry
Keywords:polypropylene;shish-kebab;annealing;molecular orientation;toughness
INTRODUCTION
It is well known that isotactic polypropylene,one of the most im-portant semicrystalline polymers with fast expansion in market share,can crystallize into two distinct crystalline morpholo-gies from the melt:spherulites and shish-kebab superstructure. Isotropic spherulites are often observed under quiescent crystal-lization conditions,while anisotropic shish-kebabs can only be obtained during elongational or shear-?ow-induced crystalliza-tion,where the?brillar entity originating from stretched chains in the melt under?ow acts as shish to induce epitaxial growth of kebabs(parallel stacks of folded-chain crystalline lamellae)orthog-onally to the?ow direction.1–9Compared with spherulites,the exclusive formation of such a highly oriented structure has been demonstrated to be a good route for the preparation of polymer articles with dramatically improved mechanical properties.10–12 Unfortunately,although injection moulding is the most common and convenient processing operation to manufacture polymer articles with regular geometric pro?les,only a small amount of oriented crystalline structures can be formed in relatively thick moulded parts due to the complicated temperature gradient and shear gradient.13–15In the thin outer(shell)zone,includ-ing the skin layer and the shear layer,highly oriented structures are formed due to the rapid cooling rate induced by the cold walls of the mould and the presence of?ow-induced crystal-lization,whereas in the inner(core)zone,a large amount of oriented structures are transformed into isotropic spherulites be-cause the relatively low cooling rate allows for a strong relaxation of initially stretched molecules during subsequent crystallization at quiescent conditions.For this reason,shear-controlled orienta-tioninjectionmoulding(SCORIM),16–18vibration-assistedinjection moulding19–21and push–pull processing22,23have been devel-oped in recent years to suppress the relaxation of the oriented melt during cooling solidi?cation.In our group,dynamic pack-ing injection moulding(DPIM),a type of SCORIM technique,has been successfully used to prepare injection-moulded parts with a highly oriented shish-kebab structure.24–30In such a process,an oscillatory shear?eld is imposed on the cooling polymer melts during packing stage with two pistons moving reversibly at the same rate.It has been observed that the relative proportions of ori-ented structures and isotropic spherulites in PP injection-moulded bars can be as high as80%at high shear intensity,thus causing signi?cant improvements in both impact toughness and tensile strength.27
Due to their importance in polymer processing,the formation mechanism1–3and threshold?ow conditions required for the onset4–8of an oriented shish-kebab structure has drawn consid-erable interest over the past decades.In contrast,little attention has been paid to the annealing of?ow-oriented polymers,31–36al-though annealing below the melting temperature is thought to be effective in inducing chain rearrangement in semicrystalline poly-mers and then improving their physical properties.37–43Moreover,?Correspondence to:Qiang Fu and Hua Deng,College of Polymer Science and Engineering,State Key Laboratory of Polymer Materials Engineering, Sichuan University,Chengdu610065,PR China.E-mail:qiangfu@https://www.sodocs.net/doc/3e15701641.html,;
Huadeng@https://www.sodocs.net/doc/3e15701641.html,
a College of Polymer Science and Engineering,State Key Laboratory of Polymer
Materials Engineering,Sichuan University,Chengdu610065,PR China
b State Key Laboratory of Polymer Physics and Chemistry,Changchun Institute
of Applied Chemistry,Chinese Academy of Sciences,Changchun130022,PR China
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to our best knowledge,up to now most studies have been limited to thin?lms and?bres,31,32,34–36and no effort has been made on the injection-moulded parts probably because it is dif?cult to ob-tain highly oriented structures percolating through the moulded parts.With regard to PP with an isotropic structure,annealing only permits a precise adjustment of the?ne crystalline structures, such as chain rearrangement in the interphase of crystalline and amorphous phases,without apparently affecting the superstruc-ture consisting of crystalline lamellae and even the crystallinity in some cases.37,43–46However,such structural reorganization upon annealing is expected to be markedly different from that in PP with an oriented structure(e.g.shish-kebabs),where the oriented shish may provide nucleating sites for the lateral growth of new (secondary)kebabs.32,34,47Therefore,it is very important to give a description of the nature of annealing induced microstructural changes and the subsequent structure–property relationships in injection-moulded PP with a highly oriented structure. Inthiswork,thepreparationofPPinjection-mouldedpartswitha high content of highly oriented shish-kebab structure was realized with the DPIM technique.The microstructural development upon annealing has been analysed with DSC,small-angle X-ray scattering(SAXS)and SEM.More importantly,in order to gain more insight into the chain reorganization in the amorphous phase during annealing and its role in determining the mechanical responses of?ow-oriented PP,Fourier transform infrared(FTIR) spectroscopy and dynamic mechanical analysis(DMA)has also been employed.Molecular mechanisms for the annealing induced changes in mechanical properties are proposed. EXPERIMENTAL
Materials
A commercial isotactic PP(trademarked as T30S)with a melt?ow rate of3g per10min(230?C,2.16kg)was supplied by Dushanzi Petrochemical Co.Ltd,Kelamayi,China.It had a weight-average molecular weight of3.99×105g mol?1and a dispersity of4.6.
Sample preparation
The injection-moulded bars of highly oriented PP were prepared using the DPIM technique,before which PP melt was?rst injected into a mould with the aid of an injection moulding machine (SZ100g,Ningbo,China)at a barrel temperature of200?C.The frequency of the two moving pistons was0.3Hz,and the shear rate of the melt was around10s?1.A more detailed description of the DPIM equipment can be found elsewhere.28
The as-prepared bars with a thickness of around3.6mm were annealed in a vacuum oven at different temperatures between 105to155?C,for various time intervals,i.e.0.5,1.5,3,6,9,12,18h. After annealing,the specimens were cooled in the atmosphere and then conditioned at room temperature for48h before testing.For comparison,the unannealed specimen was used as a reference. Mechanical testing
The notched impact strength was measured using an Izod impact tester(VJ-40,Chengde,China).A SANS universal tensile tester (Shenzhen,China)with a crosshead speed of50mm min?1was used to investigate the in?uence of annealing on tensile properties of the oriented PP.All the measurements were carried out at room temperature(23?C)and the average value reported was derived from at least six specimens.
DMA testing was measured with a TA Q800equipment(USA). The testing was carried out in three-point bending mode with a support span of about20mm from?40to150?C at a heating rate of3?C min?1.A sinusoidally oscillating strain of0.1%at a frequency of1Hz was imposed on the rectangular bar of each specimen.
DSC
The melting behaviours of the specimens were characterized with a Perkin-Elmer Pyris-1DSC(USA)under a dry nitrogen atmosphere. For each measurement,the specimen of around5mg cut from the highly oriented region of the moulded bars(400–800μm below the surface parallel to the thickness direction)was heated from30 to200?C at a rate of10?C min?1.
The degree of crystallinity(X c)for each specimen was evaluated with following expression:
X c=
H m
H m
(1)
where H m is the measured fusion enthalpy and H m0is the fusion enthalpy of100%crystalline PP.The value of H m0was selected as177.0J g?1.48
Two-dimensional SAXS
Shear-induced oriented structures in the injection-moulded bars were analysed with a NanoStar X-ray diffractometer(Bruker AXS Inc.,USA)with a wavelength of0.154nm.The sample-to-detector distance was1076mm.Specimens with a thickness of 1mm were cut from the highly oriented region of the moulded bars(200–1200μm below the surface parallel to the thickness direction)along the?ow direction,and SAXS patterns were recorded with a HI-STAR detector at room temperature.
SEM
Crystalline morphology was characterized with an FEI Inspect F?eld emission scanning electron microscope(FE-SEM,USA). The specimens were cut from the highly oriented region of the moulded bars(400–800μm below the surface parallel to the thickness direction)in liquid nitrogen along the?ow direction,and were then etched chemically with a permanganic etchant as de-scribed by Olley and Bassett.49Prior to the SEM characterizations, all the specimens were sputter coated with a gold layer.
FTIR spectroscopy
To detect the chain orientation in both amorphous and crystalline phases,FTIR measurements were performed on a Nicolet6700 spectrometer equipped with a Nicolet Continuum IR microscope (Thermo Scienti?c,USA).Specimens about60μm in thickness were cut from the middle part of the bars along a plane parallel to the thickness direction,and the measurement was carried out with steps of100μm over half the thickness of the injection-moulded bars.At each step,two polarized FTIR spectra(by rotating a wire grid ZnSe polarizer),namely parallel and perpendicular to the?ow direction,were recorded with32scans at a4cm?1resolution in the wavenumber range400–4000cm?1.In this way,the extent of chain orientation at each step can be described by an orientation
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https://www.sodocs.net/doc/3e15701641.html, H Bai et al. parameter f:
f=R?1
R+1
g
3cos2α?1
2
(2)
R=A//
A⊥
(3)
where A//and A⊥are the absorbance from beams polarized parallel and perpendicular to the?ow direction,respectively,andαis the angle between the local chain axis and the transition moment associated with the absorption band used for the measurement.To determine the orientation parameter of PP,the998and973cm?1 absorption bands were selected as adopted by Huy et al.50The 998cm?1band is attributed to the crystalline phase,whereas the973cm?1band is associated with both the amorphous and crystalline phases.In this case,chain orientation in the crystalline phase(f c)and average chain orientation(f av)can be calculated using the998and973cm?1bands,respectively,for which the value ofαwas considered as18?.51Then,chain orientation in the amorphous phase(f am)can be estimated according to the
equation
f am=f av?X c f c
1?X c
(4)
where X c is the degree of crystallinity calculated from DSC measurements.
RESULTS AND DISCUSSION
Mechanical properties
Figure1(a)presents the notched Izod impact strength of?ow-oriented PP as a function of annealing temperature.Obviously,the impact strength improves gradually until it reaches a maximum value at around145?C with increasing annealing temperature. Furthermore,during annealing at145?C,the impact toughness of the?ow-oriented PP is found to increase predominantly within the?rst0.5h,followed by a slight increase and?nally a plateau(see Fig.1(b)).This indicates that the annealing effect on impact toughness is largely dependent on both annealing temperature and annealing time.Annealing at145?C seems to be an optimum temperature condition for obtaining the best toughening effect.The representative stress–strain curves of?ow-oriented PP annealed in different conditions are shown in Fig.2. It is evident that annealing depresses the yield stress and shifts the onset of strain hardening toward lower strains(as indicated by the arrows).More importantly,similar to the effect observed on impact strength,no extra changes in tensile properties can be obtained for annealing temperatures above145?C and annealing time beyond0.5h.
Considering that changes in mechanical properties upon annealing are indicative of structural evolution which can only be triggered if the annealing temperature is above a critical value,37,40it is believed that annealing at145?C signi?cantly promotes the chain rearrangement in?ow-oriented PP.Therefore, we mainly focus on the structural development and subsequent structure–property relationships during annealing at145?C to clarify the molecular mechanisms for annealing induced changes in mechanical properties.
Structural development upon annealing
It has been reported in our previous work that PP injection-moulded parts prepared with the DPIM technique exhibit
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Figure1.The notched Izod impact strength of?ow-oriented PP before and after being annealed at(a)different temperatures for6h and(b)145?C for different periods.
a typical‘core–shell’structure.24,27,28In the shell zone the shish-keba
b superstructure is dominant,whereas in the core zone isotropi
c spherulites prevail.The area ratio of shell/core can be effectively controlle
d by adjusting th
e shear intensity imposed on the PP melt.27Here,in order to distinguish the structural hierarchy o
f as-prepared PP injection-moulded parts, the orientation distribution alon
g the thickness direction is determined wit
h FTIR measurements(see Fig.3).As expected,the relative content of highly oriented shish-kebab structure(located 0–1500μm below the surface)is estimated to be as high as 83%,suggesting that the shish-kebab structure is the dominant crystalline structure responsible for the mechanical properties of the injection-moulded parts.The exclusive formation of the shish-kebab structure in a highly oriented zone is clearly demonstrated by SEM of the crystalline morphologies as shown in Fig.4(a). However,no apparent changes in the superstructure can be observed after annealing(see Fig.4(b)).Therefore,more detailed investigations on the evolution of the shish-kebab structure upon annealing were carried out.
Figure5presents the DSC heating traces of?ow-oriented PP annealed at145?C for different periods.All the thermograms exhibit multiple melting endotherms.For the unannealed sample, a typical?ngerprint of the melting of PP with a shish-kebab structure is observed.52The melting peak at lower temperature
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1020304050
60S t r e s s (M P a )
Strain (%)
(a)
10
20
3040
50
60
1020304050
60(b)
S t r e s s (M P a )
Strain (%)Figure 2.The representative stress–strain curves of ?ow-oriented PP before and after being annealed at (a)different temperatures for 6h and (b)145?C for different periods.
(near 164?
C)is related to the fusion of kebabs,whereas the small shoulder peak at higher temperature (around 170?C)is attributed to the fusion of shish.This result is in agreement with the earlier in situ SAXS and DSC studies on the shish-kebab structure,where the melting temperature of the shish is about 5–10?C higher than that of the kebabs.3,52However,it should be noted that the length of the shish (which can be regarded as lamellar thickness)in the as-prepared ?ow-oriented PP may be relatively small because there is only a shoulder peak overlapping with the melting peak of the kebabs rather than the usually observed single melting peak exhibited in the DSC thermogram.After annealing,another shoulder peak (the so-called ‘annealing peak’)is present in the thermograms just a few degrees above the annealing temperature (i.e.145?C).The appearance of the annealing peak can be tentatively ascribed to the melting of thin lamellae (secondary kebabs)induced by the initial shish during annealing.With increasing annealing time,both the magnitude and onset temperature of the annealing peak increase at ?rst and then remain constant as the annealing time exceeds 0.5h,indicating that the most signi?cant development of the secondary kebabs occurs during the ?rst 0.5h.Moreover,the primary kebabs become more perfect upon annealing as evidenced by a slight increase in their melting temperatures.For quantitative comparison,the data on crystallinity are also
0.05
0.100.150.200.250.300.350.40
0.45(a)
O r i e n t a t i o n p a r a m e t e r
Distance from skin to core (μm)
(b)
O r i e n t a t i o n p a r a m e t e r
Distance from skin to core (μm)
Figure 3.Orientation parameters f an and f c over half the thickness of ?ow-oriented PP before and after being annealed at 145?C for 6h.
presented in the pro?le as an inset.Clearly,annealing gives rise to signi?cant increases in the values of crystallinity,from 38.9%for the unannealed sample to 52.3%for the sample annealed at 145?C for 6h.
The formation of the shish-kebab structure in PP injection-moulded parts and its evolution during subsequent annealing are demonstrated more clearly with two-dimensional SAXS.Figure 6showsrepresentativeSAXSpatternsof?ow-orientedPPbeforeand after being annealed at 145?C.As expected,for the unannealed sample,the pattern exhibits both strong meridianal scattering lobes arising from lamellar stakes (kebabs)with the lamella normal parallel to the ?ow direction and a relatively weak equatorial streak originating from ?brillar structures (shishes)in the ?ow direction,suggesting the formation of a highly oriented shish-kebab structure in the as-prepared PP injection-moulded parts.As annealing progresses,the scattering intensity along the meridian increases signi?cantly,as a result of the development of secondary kebabs and the perfection of primary kebabs.The decreased scattering intensity of the equatorial streak may be associated with the relaxation of some unstable shish.Furthermore,similar to the DSC result discussed above,the length of ?brillar shish is relatively short as evidenced by the broad breadth of the equatorial streak in the meridianal direction.34,53More importantly,in order to gain more insight on the structural changes induced by annealing,some important structural parameters,e.g.the crystalline long
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Figure4.SEM micrographs of?ow induced shish-kebab superstructure for(a)unannealed and(b)annealed(at145?C for6h)PP samples.The arrow indicates the?ow direction.
130140150160170180
Temperature (°C)
Figure5.DSC heating curves of?ow-oriented PP before and after being annealed at145?C for the indicated times.The data on crystallinity are given in the pro?le as an inset.
period(L),the average lamellar thickness(L c)and the amorphous thickness(L a),are obtained using the one-dimensional electron density correlation function method.54,55As shown in Fig.7,both L and L c are reduced upon annealing,indicating the formation of thin secondary kebabs between the primary kebabs during annealing.
As demonstrated by numerous investigations,the appearance of secondary crystallization during annealing must involve structure reorganization in the amorphous phase,which is believed to play a crucial role in determining the mechanical properties of semicrystalline polymers.32,43,56To trace the chain rearrangement in the amorphous phase during annealing,FTIR was conducted and the results are shown in Fig.3.Interestingly,the chain orientation in the amorphous phase(f am)decreases during annealing,which indicates the relaxation of stretched chains along the?ow direction(Fig.3(a)).Generally,it is expected that
(a)(b)
Figure6.Two-dimensional SAXS patterns of?ow-oriented PP(a)before and(b)after being annealed at145?C for6h.The?ow direction is vertical.
K
(
z
)
Z / nm
Figure7.The curves of the one-dimensional electron density correlation function along the meridian for?ow-oriented PP annealed at145?C for the indicated times.L,crystalline long period;L c,average lamellar thickness; L a,amorphous thickness.
both the development of secondary kebabs and the perfection of oriented primary kebabs during annealing can cause segmental reorientation in the amorphous phase;however,the relaxation of the oriented chains is dominant.Thus,the average chain orientation in the amorphous phase is depressed.Furthermore, the increased chain orientation in the crystalline phase(f c)is obtained due to ordering perfecting of primary kebabs and the perfect packing of secondary kebabs(Fig.3(b)).A similar result has been observed during annealing of solution-electrospun PP ?bres.35
It is clear from the above discussion that annealing induces not only the development of secondary kebabs and the perfection of primary kebabs,but also the relaxation of some oriented chains in the amorphous phase.Such an annealing effect on the structural development can be further con?rmed with DMA measurements as shown in Fig.8.In plots of mechanical loss factor(tanδ)versus temperature,the maximum at lower temperature is ascribed to the glass transition of unconstrained chains in the amorphous phase(β-relaxation),while the other at higher temperature is ascribed to the relaxation of constrained amorphous chains associated with crystals,such as rigid amorphous chains(α-relaxation).57–59As expected,the relaxation of oriented chains in the amorphous phase gives rise to an obvious decrease in glass transition temperature(T g),while the development and perfection of the crystalline structure causes a signi?cant enhancement in
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20
40
60
80
100120140
T a n δ (a . u .)
Temperature (°C)
Figure 8.Time evolution of the mechanical loss factor (tan δ)for ?ow-oriented PP during annealing at 145?C for the indicated times.
Figure 9.Schematic illustrations showing the structural changes of ?ow-oriented PP (a)before and (b)after annealing at 145?C.The ?ow direction is horizontal.
α-relaxation.In addition,some of the chains in the amorphous phase are rearranged into the crystalline phase during annealing,thus leading to a signi?cant decrease in the quantity (or density)of the remaining amorphous chains.This also contributes to the decreased T g .
A schematic diagram of the annealing induced structural development is illustrated in Fig.9.
Structure–property relationship
AccordingtothemorphologicalmodelproposedbySamon et al .34,the ?ow-oriented PP can be regarded as a short ?bre reinforced composite,where the relatively short shish act as ?bres and the
amorphous phase acts as the polymer matrix.In this case,the mechanical properties are dominated by the amorphous phase.Therefore,the depressed yield stress upon annealing should be associated with the decreased orientation and quality of the chains in the amorphous phase which decrease the stress transfer between crystalline and amorphous phase.60After yielding,the amorphous chains become almost fully stretched and further shear of the amorphous component is impossible;then strain hardening https://www.sodocs.net/doc/3e15701641.html,pared with the unannealed sample,the extendability of the amorphous chains is greatly reduced due to the increased content of secondary kebabs upon annealing which can act as physical crosslink points of the molecular network,thus causing decreased strain for the onset of strain hardening.
The toughness of PP is generally believed to be mainly dependent on the energy dissipated as a result of the plastic deformation in the crystalline phase.However,the increase in crystallinity does not guarantee enhancement in toughness.There is a competition between the plastic deformation in the crystalline phase and the cavitation in the amorphous phase during deformation.61,62While the stress needed to initiate cavitation is higher than that to trigger plastic deformation,crystallite shearing occurs prior to the cavitation.Here,the development and perfecting of crystalline structures can enhance the resistance of crystallite shearing,whereas the depressed quantity (or density)of the amorphous chains can decrease that of cavitation.In other words,annealing makes it easier to initiate cavitation before crystallite shearing.This phenomenon hasbeencon?rmedinannealedpolymerswithisotropicstructures,such as high density polyethylene.63Similar to the role of cavitated elastomer particles in toughening polymers,it has been demonstrated that the presence of submicrometre voids can trigger plastic deformation of the surrounding crystalline phase by altering the stress state.64Therefore,cavitation may be the dominant toughening mechanism for annealing induced excellent toughness.During deformation,cavitation occurs ?rst in the amorphous phase with decreased density and then induces plastic large-scale deformation of the newly formed and perfected kebabs,thus dissipating more impact energy compared with unannealed samples.
CONCLUSIONS
In summary,PP injection-moulded parts with a highly oriented shish-kebab structure have been successfully prepared with the DPIM technique.During subsequent annealing,some initially oriented chains in the amorphous phase are relaxed and a portion of them is involved in the lateral growth of secondary kebabs on the surface of the ?brillar shish.In addition,the perfecting of primary kebabs is observed.The structural changes in the amorphous phase play a crucial role in determining the mechanical properties,i.e.the depressed yield strength,decreased strain for the onset of strain hardening,and improved impact toughness.
ACKNOWLEDGEMENT
We express our sincere thanks to the National Natural Science Foundation of China for ?nancial support (50903048,21034005).
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