Reversible Lamellar Thickening Induced by Crystal Transition in Poly

The structural evolution of poly(butylene succinate) (PBS) during tensile deformation was investigated by in situ synchrotron X-ray scattering. Crysta...
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Reversible Lamellar Thickening Induced by Crystal Transition in Poly(butylene succinate) Guoming Liu,† Liuchun Zheng,† Xiuqin Zhang,‡ Chuncheng Li,† Shichun Jiang,§ and Dujin Wang†,* †

Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing, 100190, China ‡ Beijing Key Laboratory of Clothing Materials R & D and Assessment, Department of Materials Science & Engineering, Beijing Institute of Fashion Technology, Beijing, 100029, China § School of Materials Science and Engineering, Tianjin University, Tianjin, 300072, China ABSTRACT: The structural evolution of poly(butylene succinate) (PBS) during tensile deformation was investigated by in situ synchrotron X-ray scattering. Crystal transition during stretching was identified at 30−90 °C. An increase of long period was observed during the α−β crystal transition, which was attributed to the increase of both amorphous layer thickness and crystalline layer thickness (lamellar thickness). The reversibility of crystal transition and correlation of lamellar thickening with crystal transition were confirmed by a “stepcycle” deformation measurement. The variation of the amorphous layer was partially recoverable, while the variation of lamellar thickness was nearly fully recoverable. The different repeating length in unit cell along the chain axis in different crystal forms resulted in the variation of the lamellar thickness. The different recoverability of structural parameters was interpreted by the different dynamics of the amorphous and crystalline phase.

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INTRODUCTION Semicrystalline polymers, with typical lamellar structure built from alternatively packed crystalline layers and amorphous layers, exhibit a complex behavior during plastic deformation1 In general, an isotropic polycrystalline structure gradually transforms to a highly oriented fibrillar structure with the chain axis preferentially aligned along the drawn direction during stretching. The structure evolution was first suggested to be achieved by inter- and inner-lamellar slips.2−6 Second, others believed that there was a “stress-induced melting and recrystallization” during stretching.7,8 On the basis of true stress−strain measurements, Strobl et al.9−13 proposed that both mechanisms work during tensile deformation. Upon stretching, block slippage within the crystalline lamellae takes place first, followed by the stress-induced fragmentation and recrystallization of the polymeric chains at larger deformation. However, Galeski et al. claimed that this process could be explained on the ground of crystallographic mechanism alone.6,14 More efforts are devoted to explore the details and the nature of lamellar−fibillar transition of semicrystalline polymers currently. The complexity of plastic deformation of semicrystalline polymers lies also in other phenomena, such as cavitation and crystal transition. Cavitation is reported in a variety of semicrystalline polymers, such as nylon-6 (PA6),15 high density polyethylene (HDPE)16 and polypropylene (PP).17 The advantage of cavitation is that the cavities induce the appearance of crazing and shear yielding, the most energy dissipative processes, at a reduced stress level.14 On the other © 2012 American Chemical Society

hand, stress induced crystal transition has been discovered in many polymers. Most of them show irreversible transitions, such as PA6,18,19 poly(vinylidene fluoride) (PVDF),20 poly(ethylene oxybenzoate) (PEB).21 Polymers with reversible crystal transitions include poly(butylene terephthalate) [PBT],22,23 poly(ethylene oxide) (PEO),24 etc. Poly(butylene succinate) (PBS) is one of the semicrystalline biodegradable polyesters with reversible crystal transition. Normally, PBS exhibits a monoclinic α crystal form.25 Ichikawa et al.26,27 reported a β crystal form at stretched PBS fibers. The transition occurred reversibly under application and removal of stress. The conformation is T7GTG̅ for the α form, and T10 for the β form, where T, G, and G̅ denoted trans, gauche, and minus gauche, respectively. The transition was recognized as thermodynamically first order transition with the free energy difference ΔG = 1.6 kJ/mol of monomer unit.27 Ichikawa et al.28 have refined the unit cell parameters for α and β crystals. For the α form, the cell dimensions are a = 0.523 nm, b = 0.912 nm, c (chain axis) = 1.090 nm, and β = 123.98°; for the β form, a = 0.584 nm, b = 0:832 nm, c (chain axis) = 1.186 nm, and β = 131.68°. Although the crystal transition of PBS has been investigated on atomic or subnano scale (unit cell parameters and chain conformations), the structural transformation on submicro scale during tensile deformation has not been reported yet. Received: March 15, 2012 Revised: June 16, 2012 Published: June 27, 2012 5487

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the melting endothermic peak, a small exothermic peak at around 93.6 °C can be observed. This peak can be attributed to recrystallization of the partially melted imperfect lamellae of PBS, according to previous reports.31−34 Because of the melting-recrystallization phenomenon, the baseline of the melting curve was difficult to identify for calculating the melting enthalpy. Thus, the crystallinity of PBS was estimated by the crystallization enthalpy. The result is 29%, taking the melting enthalpy of 100% crystalline PBS 200 J/g.35 Uniaxial Deformation Behavior. Figure 2 shows the tensile behavior of PBS at varied temperatures. The stress−

Questions such as on which stage of plastic deformation does the α−β transition take place, and more importantly, what happens on the lamellar structure during crystal transition, are much unknown. In the present study, we investigated the structural evolution of PBS by in situ wide-angle and smallangle X-ray scattering (WAXS/SAXS) during tensile deformation and step-cycle deformation, aiming to provide a deeper understanding of the deformation mechanism of semicrystalline polymers with reversible crystal transitions.

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EXPERIMENTAL SECTION

Materials and Sample Preparation. PBS was synthesized by a two-step method. A low molecular weight dihydroxytelechelic− polyester prepolymer was first synthesized, and the PBS polymer was obtained by a chain extension reaction using hexamethylene diisocyanate as the chain extender. The details of the polymer synthesis can be found elsewhere.29,30 The number-average molecular weight (Mn) of prepolymer was 4.4 × 103 g/mol as determined by 1H NMR. The molecular weight of PBS was Mn = 1.08 × 105 g/mol, Mw = 2.24 × 105 g/mol (weight-average), respectively, as determined by gel permeation chromatography (GPC, equipped with a refractive index detector), using chloroform as the solvent and monodisperse polystyrene samples as the calibration standard. PBS plaques with thickness of 1 mm were compressed at 160 °C and then rapidly quenched in water. Mini tensile bars with 28 mm long and 2 mm wide were cut from those plaques. Characterization Methods. In situ X-ray scattering measurements were carried out at the beamline 1W2A in the Beijing Synchrotron Radiation Facility (BSRF). The wavelength of the radiation source was λ = 1.54 Å. The mini tensile bars were stretched at a crosshead speed of 1.20 mm/min on a Linkam TST350 hotstage and scattering patterns were collected in situ. Scattering patterns were collected by a MAR CCD (MAR-USA) detector with a resolution of 2048 × 2048 pixels (pixel size: 79 × 79 μm2). Image acquisition time was 10 s. The sample to detector distance was 1535 mm for SAXS and 168 mm for WAXS. All the X-ray patterns were corrected for background scattering, air scattering and beam fluctuations. The melting and crystallization behavior of PBS was examined with a Perkin-Elmer differential scanning calorimeter (DSC 7). The instrument was calibrated with indium before measurements. Temperature scans were performed in the temperature range from 20 to 160 °C at a heating/cooling rate of 10 °C/min under nitrogen atmosphere.

Figure 2. Engineering stress−strain curves of PBS at different temperatures.

strain curve of PBS stretched at 30 °C displays a sharp yield with the yield stress of 30 MPa. After a postyield flat region (100−200%), the stress increases with increasing strains until fracture, indicating typical strain hardening feature. The stress− strain curve at 60 °C has similar shape as that at 30 °C. The stress at 60 °C is lower than that at 30 °C under the same strain. At 90 °C, no obvious yield point can be seen and the stress at low and intermediate strains is much lower than that stretched at 60 °C. This is because of the increased molecular mobility at higher stretching temperature. Table 1 summarizes the mechanical data.

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RESULTS AND DISCUSSION Crystallization and Melting Behavior. Figure 1 illustrates the thermographs of PBS during melting and crystallization. The PBS in the present study displays a melting temperature of 109.0 °C, and a crystallization temperature of 67.9 °C. Before

Table 1. Summary of the Mechanical Data temperature (°C)

yield stress (MPa)

fracture strain (%)

fracture stress (MPa)

30 60 90

29.9 22.6 −

758 707 735

57.3 52.8 48.9

Formation of β Crystals under Strain. To explore the structural evolution of PBS during stretching, WAXS and SAXS patterns were collected in situ. Figure 3 shows selected 2D WAXS and SAXS patterns under different strains at 30 °C. For unstretched PBS, four diffraction rings in the WAXS pattern can be indexed as 020/11̅ 1 with 2θ = 19.6°, 021 with 2θ = 21.8°, 110 with 2θ = 22.5°, 1̅21 with 2θ = 26.0° and 111 with 2θ = 28.9° reflections from the α crystal. The magnified scattering pattern and 1D intensity profiles of unstretched PBS are illustrated in Figure 4. All reflections are isotropic, corresponding to isotropic crystalline phase in the initial PBS. Upon deformation (strain = 50%), the WAXS pattern becomes anisotropic, with the 020/1̅11 and 021/110 reflections

Figure 1. DSC thermographs of PBS at 10 °C/min. 5488

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Figure 3. Selected WAXS and SAXS patterns of PBS stretched at 30 °C. The stretching direction is horizontal and is defined as meridian hereafter. Black arrow indicates the emergence of the 021β reflection.

reflections in unstretched PBS such as 020/1̅11α and 021/110α, which compose of more than one reflections with similar Bragg angle, are spatially separated because of the fiber symmetry, as seen in Figure 5(B). We observed that the crystal transition during stretching also exists at elevated temperatures at 60 and 90 °C. Those scattering patterns were not shown here. Superstructure Evolution during Stretching. Information about changes in the lamellar structure during stretching can be extracted directly from the 2D SAXS patterns. The diffuse ring in SAXS for the unstreched PBS (Figure 4) arises from the periodic lamellar structure. The position of the peak qmax is related to the long period L by the Bragg’s law.

Figure 4. Indexed WAXS pattern of unstretched PBS (A) and the corresponding intensity profile obtained from equatorial slice (B).

intensified on the equator. At strain =100%, the WAXS reflections turn into arcs. With the strain further increased, the arcs in WAXS patterns become shorter and gradually become spot-like, indicating that the orientation of crystal increases with strain. At strain = 500%, an additional off-equatorial reflection emergences (arrow in Figure 3) at a Bragg angle (2θ = 23.6°) slightly larger than that of 021α reflection (2θ = 21.8°) (represents reflection from α crystal, hereinafter), can be indexed as the 021β reflection. Meanwhile, the two strongest reflections earlier indexed as 020/1̅11α and 021/110α move closer to each other. This is another sign of the α−β crystal transition, because the Bragg angles (2θ) of 020β and 110β are closer than those of 020α and 110α. Figure 5 shows the magnified WAXS patterns of stretched and released PBS samples. All the reflections in Figure 5A can

L=

2π qmax

where q, the scattering vector, is defined as q = 4π(sin θ)/λ, where λ is the X-ray wavelength, θ is one-half the scattering angle (2θ). At this point of data treatment, no geometrical considerations are taken into account, which will be addressed later. As seen in Figure 3, the isotropic long period ring indicates that the lamellar stacks are randomly distributed in the initial PBS. Upon deformation (strain = 50%), the SAXS pattern becomes arc-like with the intensity on the meridian stronger than the intensity on the equator. At strain = 100%, the long period arcs turns into two blob-like reflections on the meridian. Meanwhile, a streak appears at the equator. This pattern is consistent with the fibillar structure, as first described by Bonart and Hosemann in 1962.36 As the strain further increased, the SAXS patterns keep the blob-like feature. Theoretically, the calculation of the long period should take symmetry considerations into account. Multiplication of the intensity value with q2 (Lorentz correction) is necessary for ideally isotropic systems, while no Lorentz correction should be applied for perfectly aligned lamellar systems. Ruland37 have shown that in any partially oriented system a 1D intensity along stretching direction can be analyzed in a similar fashion as that in an isotropic system. Men et al.38,39 have pointed out that for a semiquantitative discussion of the dependence of the long period on the strain the type of data treatment (Lorentz correction versus no correction) only plays a minor role. In the present study, we also found that despite the exact values were different, the characteristic variations of the long period derived from either Lorentz corrected or uncorrected data were nearly the same. For this reason, only the Lorentz-corrected data was presented. The process of data treatment in the present experiment is as follows. A 1D meridional slice was taken through the SAXS pattern and a plot of the intensity I(q) versus the scattering vector was constructed. The intensity data then was Lorentz-corrected by multiplying the observed intensity by

Figure 5. Indexed WAXS patterns of PBS. (A) strain = 700% under load; (B) stretched at strain = 700% and releasing the load. The stretch was carried out at 30 °C and the stretching direction is horizontal.

be indexed as the reflections of the β crystal. This clearly confirms the existence of the crystal transition in PBS under stretching. After releasing the stretching force, as seen in Figure 5B, the reflections can be indexed as that of the α crystal, indicating that the α−β crystal transition is reversible upon loading and unloading. It should be mentioned that convoluted 5489

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q2 and plotting the result as a function of q. The value of long period was calculated by the Bragg’s law as shown above. As seen in Figure 6, at small strain region (50−100%), there is an

Figure 7. 1-D electron density correlation functions for analyzing the lamellar structure. The data is from the SAXS of the unstretched PBS at 30 °C.

shows an example illustrating the calculation process of structural parameters. It must be mentioned that the L obtained by the z value of the first maxima of K(z) is very close to that calculated by the Bragg’s law as seen in Figure 6. Figure 8 presents the change of structural parameters (la and lc) during stretching. It can be observed that la varies similar as

Figure 6. Variation of the long period of PBS stretched at different temperatures. Data were calculated from the peak maxima in Lorentz corrected intensity profiles by the Bragg’s law.

increase in the long period due to stretching of the amorphous regions and possibly an accompanying slight rearrangement of lamellae. Subsequently there is a drop in the long period. This behavior seems to be in line with the assumption of a fragmentation and recrystallization process occurring during deformation. At strains = 100−200%, there is a plateau region in which L is nearly constant. The plateau corresponds to the postyield plateau in the stress−strain curves (Figure 2). At strains higher than 200% (corresponding to the strain hardening region), L starts to increase with increasing strain. In other polymers such as high density polyethylene (HDPE),40 ethylene−propylene copolymer,41 poly(ether ester) (PBT−PTMO),42 poly(ε-caprolactone) (PCL) and its blends with poly(vinyl methyl ether) (PVME),43 the long period kept nearly constant at high strains. Samon et al.44 observed that the long period increases with strain in cold drawn nylon-6 fibers, and the increase of long period was attributed to the increase of amorphous layer thickness. To demonstrate whether the increase of the long period (L) comes from the amorphous layer (la) or the crystalline layer (lc), we need to separate the contribution of each component. The average thickness of the amorphous and crystalline layers measured along the stretching direction can be evaluated from the one-dimensional electron density correlation function K(z)1,45 ∞

K (z ) =

∫0 I(q)q2 cos(qz) dq ∞

∫0 I(q)q2 dq

where z is parallel to the drawing direction, I(q) is the 1D intensity profile as depicted above. Multiplication of q2 to I(q) is always carried out to account for the partially orientation. It must be noted that it is impossible to decide whether it is the amorphous or the crystalline thickness that is read out from the correlation function without prior knowledge of crystallinity. Because the crystallinity of the sample used in the present study is 29%, much lower than 50%, the smaller value is assigned as the average thickness of the crystalline layer, which is in accordance with previous reports.46,47 The thickness of amorphous layer can be calculated by la = L - lc. Figure 7

Figure 8. Crystalline layer thickness and amorphous layer thickness of PBS along the stretching direction during tensile at different temperatures.

L in both 30 and 60 °C. Initially there is an increase in la, followed by a drop at small strains (0−100%). Subsequently, la reaches a plateau region and then gradually increases with increasing strain. The variation of lc at small and intermediate strain (0−300%) is not remarkable. Until strains higher than 5490

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Figure 9. Stress−strain curves together with the scattering patterns during step-cycle deformation at 30 °C.

workers.28 The result, 9%, is lower than the variation of the crystalline layer thickness (12.5% at 30 °C, 14% at 60 °C), the reason for which will be discussed later. Structural Changes during Step-Cycle Deformation. Apparently, the lc variation and the crystal transition take place simultaneously. To clarify whether they are correlated with each other, a “step-cycle” deformation measurement was performed as follows: Normal tensile deformation was first conducted at 30 °C until strain =370% (to yield a fibrillar structure), then conducted stepwise to progressively higher tensile strains. Figure 9 is the stress−strain curve together with the scattering patterns at each step. For WAXS patterns under load, reflections for the β crystal appear and gradually become stronger with increasing strain. For samples removing the load, all reflections in WAXS can be identified as that for the α crystal. This observation again unambiguously confirms that the α−β transition is fully reversible. The SAXS patterns are further analyzed by 1-D correlation function. As seen in Figure 10, L, la and lc exhibit reversible thickening and thinning during the step-cycle deformation. Upon loading, L increases with increasing strain, similar to that in Figure 6. After unloading, L becomes smaller compared to

300%, lc increases gradually with increasing strain. It is obvious that the extent of variation of la during deformation is much higher than that of lc. On the basis of the above discussion, the structure has transformed to an oriented fibrillar structure at strains 200− 300% (Figure 3). The stretching force produces a normal stress perpendicular to the lamellar plane within microfibrils upon further deformation. From 300% to 700%, la increases from 4.8 to 6.2 nmaround 30%while lc increases from 3.2 to 3.6 nmaround 12.5%, stretched at 30 °C. Similarly, la increases from 5.5 to 6.7 nmaround 22%while lc increases from 3.6 to 4.1 nmaround 14%, stretched at 60 °C. The increase of la results form the stretching of the entangled network. It is interesting to compare the change of lc before and after crystal transition with the change of repeating length in the chain axies between different crystal forms. The repeating length change in chain axies in the α and β crystal unit cell can be calculated by cβ − cα × 100% = 9% cα where cα is the c value of the α crystal and cβ is the c value of the β crystal, respectively, as reported by Ichikawa and co5491

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Figure 12. Schematics showing the unit cell change and corresponding lamellar thickness change during crystal transition of PBS. Straight red lines represent polymer chains in the crystalline lamellae, while coillike curves represent polymer chains in the amorphous phase.

Figure 10. Variation of the structural parameters of PBS during the step-cycle deformation at 30 °C.

Peterlin,3 the microfibrills keep largely intact, and only longitudinal sliding between adjacent microfibrills can proceed during deformation. Therefore, it is reasonable to suppose that the number of the monomer units within a crystalline layer keeps constant during crystal transition. Subsequently, the lamellar thickness lc becomes larger. Inversely, after releasing the stress, the crystal transforms back to the α form. The lamellar thickness becomes lower accordingly. It should be noted that the difference of the c value between the two crystal modifications (9%) is lower than the measured lamellar thickening (12.5% at 30 °C, 14% at 60 °C). One reason should be the possible calculation error. As the lamellar thickness of PBS is rather small (around 3−4 nm), a 0.1 nm error in lamellar thickness would lead to a ∼3% error in relative lamellar thickening. Another possible reason might be the variation of chain tilt in different crystal modifications. In the released or low stress state (the α form), the chains in a portion of the lamellae might be slightly tilted with respect to the lamellar normal. Under high tensile stress, the tilting angle is expected to become smaller or even be vanished accompanied by the α−β crystal transition. The possible slight change of tilting angle could also contribute to the lamellar thickening. Nevertheless, the reversible lamellar thickening mainly stems from the reversible crystal transition. Another important feature is the different recoverability of la and lc. The fully recoverability of lc indicate that the crystal transition is fully reversible in our experimental time scale. In the treatment of the dynamic mechanical properties of semicrystalline polymers, the crystalline phase has been treated as an elastic “spring” to describe its extremely fast strain−stress response.48−50 In our case, it seems that the structural relaxation of crystalline phase is also relatively fast despite the existence of crystal transition, because we did not mean to wait for the structural relaxation before scattering measurements. The amorphous phase is believed to be viscoelastic, typically with a characteristic relaxation time. A possible explanation for the partial recoverability of the amorphous phase is that the amorphous phase needs longer time than that experimentally provided to relax to the same “metastable state” as that before deformation. Additionally, the amorphous phase may undergo viscous flow upon large strain, which can be another possible reason for the lower recoverability of la.

the corresponding L with load. It is noticed that L does not decrease to a common value for all strains. Instead, the L free of load increases with increasing strain. Similar results can be observed in la. Meanwhile, compared with la, lc varies in a lesser extent. Although lc increases with increasing strain under load similar to L and la, it recovers nearly to an identical value upon releasing the force. Figure 11 is a magnified plot focusing on lc.

Figure 11. Magnified plot of the variation of the lamellar thickness of PBS during step-cycle deformation at 30 °C.

It can be observed that lc oscillating during loading and unloading. This observation strengthens the correlation between crystal transition and variation in lamellar thickness. The upper line goes upward with increasing strain, while the lower line is roughly horizontal. Interplay Between Crystal Transition and Lamellar Thickening. As above-mentioned, L, la and lc increase under uniaxial stress applied on the fibrillar structure, and decrease upon stress relaxation. The change is highly reversible upon loading and unloading. Figure 12 is a schematic illustrating the structural parameter variation during crystal transition. The variation of la results from the stretching of the entangled amorphous network. lc also has thickening and thinning process during step-cycle deformation. And it can almost fully recover after unloading. We believe that the lc variation is correlated with the crystal transition. Upon deformation, the α crystal within microfibrils gradually transforms into the β form with chain conformation changes from T7GTG̅ to T10. This yields a higher c value in the β crystal. As has been pointed out by

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CONCLUSIONS Crystal transition was observed during tensile deformation of poly(butylene succinate) (PBS) at 30−90 °C. An increase of long period was identified during stretching. The step-cycle 5492

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deformation measurement showed that the long period L reversibly increased under stress and decreased upon removing stress. The contribution of crystalline layer thickness (lamellar thickness) and amorphous layer thickness to the long spacing was discriminated by correlation function analysis. The variation of the amorphous layer thickness was partially recoverable while the variation of lamellar thickness was nearly fully recoverable. The reversible lamellar thickening was shown to mainly stem from by the reversible crystal transition: the repeating length in chain direction in the β crystal is 9% longer than that in α form due to the conformation change. The recoverability can be interpreted by considering the different dynamics of amorphous and crystalline phase. The crystal transition of PBS is expected to be a relatively fast process. Reversible lamellar thickening and thinning is expected to be a general character in semicrystalline polymers with a reversible crystal transition.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China is gratefully acknowledged (50925313, 11179031). The BSRF is acknowledged for kindly providing the beam time.

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REFERENCES

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