Article Cite This: Macromolecules XXXX, XXX, XXX−XXX
Structural Evolution of Hard-Elastic Isotactic Polypropylene Film during Uniaxial Tensile Deformation: The Effect of Temperature Yuanfei Lin, Xueyu Li, Lingpu Meng, Xiaowei Chen, Fei Lv, Qianlei Zhang, Rui Zhang, and Liangbin Li* National Synchrotron Radiation Lab, CAS Key Laboratory of Soft Matter Chemistry, Anhui Provincial Engineering Laboratory of Advanced Functional Polymer Film, University of Science and Technology of China, Hefei, China S Supporting Information *
ABSTRACT: The effects of temperature on the nonlinear mechanical behaviors of hard-elastic isotactic polypropylene films are systematically studied with in-situ ultrafast synchrotron radiation small- and wide-angle X-ray scattering techniques (SAXS/ WAXS) during uniaxial tensile deformation at temperatures from 30 to 160 °C. Based on the mechanical behaviors and structural evolutions in strain−temperature two-dimensional space, three temperature regions (I, II, and III) are clearly defined with the α relaxation temperature (Tα ≈ 80 °C) and the onset of melting temperature (Tonset ≈ 135 °C) as boundaries, where different mechanisms dominate the nonlinear deformations after yield. In region I, microstrain in lamellar stacks εm obtains an accelerated increase after yield and reaches a value significantly larger than corresponding macrostrain ε, during which neither slipping, melting, nor cavitation occurs. We propose stress-induced microphase separation of interlamellar amorphous to be responsible to the hyperelastic behavior in region I. Above Tα in region II, due to reduced cohesive strength and enhanced chain mobility, the irreversible reduction of crystallinity and the formation of slender cavities suggest that crystal slipping overwhelms microphase separation and plays the major role in nonlinear deformation, during which chains in lamellar crystals are pulled out and recrystallize into nanofibrillar bridges. In region III above Tonset, melting−recrystallization dictates the nonlinear deformation. A schematic roadmap for structural evolution is constructed in strain−temperature space, which may guide the processing of microporous membranes for lithium battery separators as well as other high performance polymer fibers and films. extended-chain crystals, while biaxial-oriented polymer films are produced by stretching at temperatures around either Tm or glass transition temperature (Tg).20−22 The production of isotactic polypropylene (iPP) microporous separators for lithium battery involves two steps of poststretch, namely cold-stretch around room temperature and hot-stretch close to Tm,23−25 where the former is thought to result in lamellar separation while the latter leads to enlargement of pores via lamella−fiber transition.26,27 In these industrial processing, tensile deformation with high strain rate drives deformation and reconstruction of initial structure in the precursor films at different temperature regions, where the underlying nonequilibrium physics and the corresponding nonlinear mechanical behaviors are not yet well understood.
1. INTRODUCTION The relationship between structural evolution and nonlinear mechanical behaviors during tensile deformation, such as yield, strain softening, and hardening, is a long-standing nonequilibrium challenge of semicrystalline polymeric materials, which is the basic knowledge not only for predicting service behaviors of materials but also for guiding the processing of high performance products.1−3 For predicting service behavior, the study on structure−property relation is literally self-evident, which enjoys the same importance for processing though not obvious. Industrial processing of high performance products, such as fibers,4−7 biaxially oriented films,8−10 microporous membranes,11−15 etc., not only involves the solidification from liquid to crystal but also undergoes crystal deformations and phase transitions in poststretch process.16 The high strength of ultrahigh molecular weight polyethylene (UHMWPE) fiber precisely comes from poststretch at temperatures near melting point (Tm),17−19 during which lamellar crystals transform into © XXXX American Chemical Society
Received: February 2, 2018 Revised: March 22, 2018
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DOI: 10.1021/acs.macromol.8b00255 Macromolecules XXXX, XXX, XXX−XXX
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Figure 1. Schematic diagram of lamellar stack in series in iPP film sample. One of the lamellar stacks is zoomed in for a better description. Cry (green) denotes for lamellar crystal, while Am (origin) represent amorphous inside lamellar stacks.
tensile stress on isotropic aligned lamellar stacks results in heterogeneous stress distribution involving all components of stress tensor.65,66 Evidently, correlating a simple stress−strain curve to multiscale heterogeneous structures is a formidable challenge. Another issue in plastic deformation of semicrystalline polymer is the effects of temperature.6,15,18,34,52,59,64 In addition to Tm and Tg, α relaxation temperature Tα differentiates the chain mobility in crystal lattice at higher and lower temperature sides, which is evidenced not only by spectroscopic methods67−74 but also from drawing ability.73,75−78 Instead of pulling chains through the crystallites directly, thermally activated helical jump motions in the crystalline region are observed through nuclear magnetic resonance (NMR) during the mechanical α relaxation.71,73 For ordered structure formed with low supercooling temperature (ΔT), local helical jumps of stems effectively result in chain diffusions, while they do not lead to cooperative chain diffusions between the crystalline and amorphous regions due to the irregular chain folding in structure with disordered trajectory at high ΔT.71,72,74,79 With α relaxation activated by temperature effects, random diffusion of chains into and out of crystallites occurs through local helical jump, which will reorganize amorphous regions, thus redistributing the local force on tie molecules and preventing the premature breaking of taut tie chains without crystal melting. Combining in-situ structural measurements like SR SAXS/WAXS with tensile deformation study on samples with simplified structural level in wide temperature range is an effective approach to assign specific deformation mechanisms in strain−temperature space, which may aid to establish the relationship between structural evolution and nonlinear mechanical behaviors. In this work, with ultrafast in-situ synchrotron radiation SAXS and WAXS, the structural evolution of iPP precursor films with highly oriented lamellar stacks during uniaxial tensile deformation is studied at wide temperature range (30−160 °C), during which a stretching rate around industry processing conditions is employed. Corresponding to Tα and the onset melting temperature Tonset, three temperature regions are defined with 80 °C (Tα) and 135 °C (Tonset) as the boundaries, i.e., region I (Tβ < Td < Tα), region II (Tα < Td < Tonset), and region III (Tonset < Td < Tm). Different deformation behaviors, including linear and hyperelastic deformations, plastic deformation through crystal slipping, and melting−recrystallization, in three strain zones during stretching are determined by the coupled effects of strain and temperature.
Several structural deformation mechanisms have been proposed to account for the nonlinear mechanical behaviors of semicrystalline polymeric materials, i.e., crystal slipping assisted by dislocation,28−34 melt−recrystallization,35,36 void formation,37−39 microphase separation,40,41 etc. In Young’s theory, the plastic yield of semicrystalline polymers, such as iPP and PE, is attributed to the nucleation of dislocation in crystals at temperatures above Tg,32,34,42−45 while shearing of lamellae followed by fragmentation in crystal blocks is believed to determine the onset of nonlinear mechanical behavior in fibrous materials.46,47 Besides, stress-induced melting−recrystallization seems more probable to take place at temperature near Tm.48−50 In our previous works, crystal slip via melting is also proposed to account for yielding and strain softening zone for all temperature regions, while crystal slipping without melting occurs via splitting planes for strain hardening in hightemperature regions for UHMWPE.6,51 In iPP with random packed lamellae, yield at low temperature region ( εh). In region I (40 and 60 °C), stress first increases linearly in zone A and then gradually goes through a plateau or weak softening (zone B), which is followed by a strain hardening (zone C). Tensile deformations in region II (90 and 120 °C) first behave obvious strain softening with stress decreasing after reaching a maximum at εy and then enter into a narrow stress plateau in zone B. Strain hardening of region II is less obvious with lower transient modulus in zone C as compared with that of region I. In region III, different from the phenomena observed in regions I and II, zone C follows closely at the end of yielding without a stress plateau or strain softening in zone B.
(1)
where Ac and Aa are the fitted areas of crystalline and amorphous regions in WAXS patterns, respectively. In addition, orientation parameter of crystal ( f 040) was estimated from the full width at halfmaximum (fwhm) of the azimuthal distribution of scattering intensity of one picked crystallographic planes (040 plane here). According to Herman’s equation,90 f 040 is defined as f040 =
3⟨cos φ2⟩ − 1 2
(2)
where φ is the angle between the reference direction (tensile direction) and the normal direction of the 040 crystallographic plane. Thus, f 040 equals to 1 when all lamellae are perpendicular to the tensile direction since the normal direction of (040) plane is perpendicular to tensile direction, while 0 for no preferred orientation situation. 2.4. Scanning Electron Microscope (SEM) Measurement. The images of surface morphology of samples used in this work were taken using a field emission scanning electron microscope (Nova NanoSEM 450) in Sichuan University. The surface morphologies of samples were observed at 3 kV without etching process. Samples were sputter-coated with a gold ion beam for 20 s before tests to enhance electrical conductibility.
3. RESULTS 3.1. Mechanical Behaviors. We first extract nonlinear mechanical information from the stress−strain curves and then D
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Figure 4. Representative 2D-SAXS patterns of iPP hard-elastic films stretched at different strains at 40, 60, 90, 120, and 150 °C. The upper right numbers refer to detailed macrostrain (%). Tensile direction is horizontal.
Figure 5. (a) Representative 1D SAXS intensity distribution profiles with Lorentz correction of hard elastic iPP stretched at 60 °C along meridian direction. The inset is the mask protocol used for the integration. (b) Evolution of long period along meridian (Lm, red square) and the corresponding strain difference (Δε, blue circle) with increasing strain at 60 °C. The engineering stress−strain curve is also replotted for reference.
3.2. SAXS Results. Figure 4 depicts typical 2D SAXS patterns collected in situ during tensile test at the five representative Td (40, 60, 90, 120, and 150 °C), where tensile direction (TD) is in horizontal. The two-point scattering signal concentrating in meridian of the original patterns (the first column in Figure 4) stands for the oriented and parallel aligned periodic lamellae. The emergence of cavitation or large domains with density contrast gives the lozenge-shaped scattering close to beamstop, while fibril structure shows symmetric streaks in equator. Coincided with the different mechanical properties in different temperature regions, the structural features demonstrated in the 2D SAXS patterns also present different evolutions in the three regions. In region I, taking 40 and 60 °C as examples, the two-point scattering moves toward beamstop, indicating an increase of the long period, while no obvious equatorial streak signal appears in the whole deformation process at 40 °C though weak and diffusive streak scattering starts to emerge at strain close to fracture at temperature (like 60 °C) close to the boundary of regions I and II. In regions II and III, the equatorial streak scattering clearly appears in zone B and is gradually
strengthened in zone C, while the lozenge-shaped scattering appears once strain going into zone B (εn) in regions I and II but never occurs in region III. These results suggest that in the nonlinear deformation zones B and C cavities or large-scale domains with density contrast form in regions I and II while microfibrils are generated in regions II and III, which demonstrate that different structural mechanisms dominate the nonlinear deformation behaviors in different temperature regions. To elucidate the structural mechanisms of the nonlinear deformations in different regions, we focus on the evolutions of long period and equatorial scattering signal. As the example of region I, Figure 5a presents several representative onedimensional scattering intensity curves with Lorentz correction during tensile deformation, which are integrated along meridian direction and contain mainly the scattering from periodic lamellar stacks. The peak position moves to smaller q value during the whole stretching, especially in zone B. With the peak position qm, we calculate the long period of lamellar stacks Lm with Bragg’s law Lm(ε) = 2π/qm(ε),3 which is plotted vs macrostrain ε in Figure 5b together with the engineering E
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Figure 6. (a) Representative 1D SAXS intensity distribution profiles with Lorentz correction of iPP stretched at 120 °C along meridian direction. (b) Evolution of long period along meridian (Lm, red square) and the corresponding strain difference (Δε, blue circle) with increasing strain at 120 °C. The engineering stress−strain curve is also replotted for reference.
Figure 7. (a) Representative 1D SAXS intensity distribution profiles with Lorentz correction of iPP stretched at 150 °C along meridian direction. (b) Evolution of long period along meridian (Lm, red square) and the corresponding strain difference (Δε, blue circle) with increasing strain at 150 °C. The engineering stress−strain curve is also replotted for reference.
stress−strain curve for comparison. Lm increases slightly from 18.8 to 19.4 nm in the linear deformation zone A, while the increase trend is drastically accelerated as strain exceeding εn into zone B, where long period of around 26.8 nm is obtained around εh. From εn to εh a macrostrain ε increase of 10.4% corresponds to an increase of microstrain εm of about 39.4%, which is nearly 3 times larger than the macrostrain ε and is in strong conflict with the mechanical model in series. As the layer normal of lamellar stacks lays in the tensile axis, one would expect that microstrain εm should be smaller than macrostrain ε as the surrounding amorphous matrix of lamellar stacks should also sustain portion of macrostrain ε, which however is not in line with the results shown in Figure 5b. The unexpectedly accelerated increase of Lm in zone B is more strikingly demonstrated by Δε(ε) = ε − εm(ε) as depicted in Figure 5b. Δε first increases in zone A to a maximum of about 3.5% at εn and then drops down sharply to negative values in zone B. Δε reaches a minimum value of about −26.9% around εh and then turns to increase and finally reaches positive values in zone C. The overlarge value of εm or negative value of Δε suggests that the increase of long period of lamellar stacks is not only driven by external strain but also promoted by a spontaneous selfacceleration mechanism in zone B. As cavitation is a passive and destructive process, which can only lead a maximum microstrain εm equal to macrostrain ε, the observation that the increase of εm reaches about 3 times larger than that of ε cannot be attributed to cavitation, although lozenge scattering close to beamstop indicates the formation of larger scale domains with density contrast in zone B. We propose stress-induced microphase separation of interlamellar amorphous phase as
the spontaneous self-acceleration mechanism to account for the overlarge εm or negative Δε in zone B, which will be discussed later. For region II, the structural evolution at 120 °C is chosen as the representative. Figure 6a illustrates selective 1D integrated SAXS profiles in meridian. With the same approach employed in Figure 5, we obtain the evolutions of long period Lm and Δε, which are plotted vs macrostrain ε in Figure 6b. Although Lm follows a monotonic increase trend with different slopes in different strain zones, the overall increase keeps in low level. In zone B from εn to εh, Lm increases from 19.1 nm to around 21.4 nm, which is quite small as compared with that in lowtemperature region I. The most worthy noted point is that Δε maintains positive value in the whole deformation process, indicating that microstrain εm is always smaller than macrostrain ε. In fact, εm only occupies small portion of ε, especially in strain hardening zone C where Lm and εm reach plateaus without obvious changes. Note that with the interference of strong scattering close to beamstop, long period in meridian (Lm) is difficult to be recognized at 80 and 90 °C with increase of strain. We cannot differentiate a peak from strong background scattering in 1D integration profiles even with Lorentz correction at the two temperatures. Figure 7 presents the 1D SAXS curves in meridian (a) and the evolutions of long period and Δε (b) during deformation at 150 °C, which is taken to represent region III. Similar to that in region II, Lm also follows a monotonic increase trend with macrostrain ε, which shows an accelerated increase from around 22.0 to 26.8 nm in zone B and initial zone C until strain of 56.9%, after which a slowdown increase to nearly 30 nm is F
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Figure 8. (a) Evolution of microstrain along meridian direction at selected deformation temperatures. (b) Difference value (Δε) between microstrain of amorphous phase along meridian direction (εm) and macrostrain (ε) at different Td. (c) Enlarged plotting in small strain zone of Δε vs ε. (d) Evolution of the variation slope of Δε in zone C (kΔε) with increasing deformation temperature.
Figure 9. Evolutions of typical 1D SAXS integration profiles within equatorial-mask region during stretching at three temperatures: (a) 60, (b) 120, and (c) 150 °C. The numbers in the legend represent the strains of each curve.
better view. In the linear elastic deformation zone A, both εm and Δε follow linear increase with macrostrain ε in the three temperature regions, which is in correspondence with the mechanical model in series. In the nonlinear deformation zones B and C, the deformations of lamellar stacks are obviously different among the three temperature regions. In region I, microstrain εm presents an accelerated increase in zone B, resulting in microstrain εm significantly larger than the corresponding macrostrain ε (namely εm ≫ ε) and negative Δε (Figures 8b and 8c). In contrast, the phenomenon of εm ≫ ε and negative Δε observed in region I does not occur in high temperature regions II and III. For regions II and III, εm shows similar evolution trend in zone B, while an obvious difference of εm or Δε occurs in strain hardening zone C, where the increment of εm is stopped for region II and a continuous weak increase proceeds for region III. Figure 8d presents the slope kΔε of Δε−ε curves in zone C, which represents the relative strain contributions from those factors outside lamellar stacks.
followed until strain of 180%. The increment is obviously larger than that at 120 °C, which however does not lead to a negative value of Δε, indicating that macrostrain ε is always larger than the corresponding microstrain εm in the whole stretching process. Note that due to the temperature effects of lamellar thickening and the melting of some unstable thin crystals, the original long period is much larger than that observed at 60 and 120 °C. Figure 8 summarizes the evolutions of microstrain εm and strain difference Δε obtained at different Td, which are calculated based on the evolutions of long periods (Lm) during tensile deformation. For the convenience of reading and comparison, the microstrains εm at the five representative temperatures are chosen as references and plotted versus macrostrain ε in Figure 8a. To highlight the different evolutions in the three regions, the evolutions of Δε with macrostrain at Td ranging from 30 to 160 °C are described in Figure 8b, and an enlarged plot in small strain zone is plotted in Figure 8c for a G
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Figure 10. (a) Evolution of WAXS patterns of iPP cast films taken at different strains at 120 °C. Stretching direction is in horizontal direction. (b) Azimuthal integration profiles of (040) crystal plane when stretched at 120 °C. (c) 1D WAXS intensity distribution profiles of iPP cast films stretched at 120 °C. (d) Evolution of crystallinity (Xc, red square) and orientation parameter of (040) crystal plane ( f 040, blue circle) during stretching at 120 °C. (e) Evolutions of correlation length (L040, red square) and lattice spacing (d040, blue circle) related to (040) crystal plane as a function of strain at 120 °C. Engineering stress−strain curves are replotted in (d) and (e) as references.
kΔε equals 1 means that long period Lm keeps constant, and macrostrain ε is contributed by other factors like plastic deformation via slipping or melting−recrystallization. In region I, kΔε decreases from 0.71 to 0.49 with Td increasing. kΔε maintains a value of nearly 1 in region II while decreases to values below 1 (about 0.96) in region III. These results reveal that long period Lm keeps a continuous increase trend in zone C of region I, while macrostrain is mainly contributed by Δε via crystal slipping or melting−recrystallization in regions II and III. Figure 9 presents 1D SAXS scattering intensity profiles integrated in equator area during stretching at the three temperatures: 60 °C (a), 120 °C (b), and 150 °C (c). The vertical coordinates are set as logarithmic in order to check whether periodical scattering actually occurs in equator, which might come from the formation of domains with density contrast perpendicular to the stretching direction. In region I (60 °C), combined with the increase of the overall intensity, a weak and broad scattering peak with qmax of around 0.3 nm−1 emerges with strain going into zone B, which reveals that density-contrast domains with an average periodicity in the order of 20 nm form. The broad peak width and the strong scattering at high q tail suggest that these density-contrast domains have diffusive interface, which is closely similar to that from phase-separated polymer blends and is attributed to microphase separation of interlamellar amorphous (Am) as discussed later. In region II, similar scattering peak with qmax of about 0.26 nm−1 is also observed at 120 °C in Figure 9b, which
shows stronger intensity with narrower width than that in region I. The narrow peak width and low scattering tail at high q suggest that domains with sharp scattering interface form in region II, which probably comes from the scattering density difference between polymer and air or the formation of cavities. Note that the periodicity revealed by SAXS peak in Figure 9b corresponds well with the period of fibrillar bridges shown in SEM pictures as shown later. In region III (150 °C, Figure 9c), the intensity Ie follows a monotonic attenuation with q increasing without any hint of the occurrence of scattering peak, suggesting that neither phase separation nor cavitation occurs in high-temperature region III. 3.3. WAXS Results. Since WAXS patterns do not present significant difference during stretching at all experimental temperatures, we only present several patterns and the corresponding analysis of 120 °C as an example in Figure 10. (The patterns of other temperatures are presented in Figure S3 of the Supporting Information.) As shown in Figure 10a, WAXS patterns only contain the diffractions of oriented iPP αcrystal. Figures 10b and 10c plot the azimuthal integration of scattering from (040) crystal plane and 1D integration profiles of the whole patterns, respectively. On the basis of the fwhm of azimuthal integration curves, we calculate the orientation parameter of crystal ( f 040) quantitatively according to eq 2, while through peak fitting on 1D integration profiles the evolutions of bulk crystallinities (Xc), correlation length (L040), and crystal spacing (d040) of 040 plane are obtained. L040 and d040 are estimated with Scherrer’s equation and Bragg’s law,3 H
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Figure 11. Evolutions of some structural parameters calculated from WAXS patterns at the five representative temperatures: (a) crystallinity (Xc); (b) correlation length of 040 crystal plane (L040); (c) orientation parameter of 040 crystal plane (f 040).
Figure 12. Evolution of crystallinity (Xc) and correlation length (εL040) as a function of strain during cyclic tensile deformation at different deformation temperatures: (a1) Xc at 60 °C; (a2) εL040 at 60 °C; (b1) Xc at 120 °C; (b2) εL040 at 120 °C; (c1) Xc at 150 °C; (c2) εL040 at 150 °C. The corresponding stress−strain curves are plotted together for the convenience of correlating. The open and solid symbols are for stretching and returning, respectively.
spacing d040 obtains linear decrease at first and then continues to decrease slowly but still linearly after going through a small increment in the strain softening process. The evolution of d040 is perfectly inversely correlated with the stress−strain curve, suggesting the dominant role of stress here. In other words, the decrement of crystal spacing d040 mainly comes from the results of stress-induced lattice distortion. With the same method mentioned above for 120 °C, we calculate the three important crystal parameters (Xc, L040, and f 040) at other deformation temperatures Td. The results of five representative temperatures are compared in Figure 11. In Figure 11a, the initial crystallinity decreases from 63.9% to 53.7% with Td increasing due to the partial melting of crystals especially at Td > 90 °C. In region I, Xc almost remains constant in zone A, followed by a reduction until fracture. Xc only decreases from 63.9% to 60.9% before fracture at 40 °C and from 62.2% to 54.9% at 60 °C, respectively. In region II, Xc decreases monotonously first in zone B but slows down in zone C at 90 °C, while Xc obtains some increment in zone C and reaches a value still lower than the initial one at 120 °C. In region III, melting−recrystallization occurs obviously at 150 °C in zones B and C, where Xc increases to a value even higher
respectively. Note that the correlation length L040 (calculated from the width of diffraction peak) contains the contributions from (i) internal lattice distortion and (ii) actual crystal size. These quantitative data of crystal are plotted in Figures 10d and 10e together with corresponding stress−strain curve. As shown in Figure 10d, f 040 remains constant in zone A and then decreases to a minimum value (from 0.95 to 0.72) in zone B, followed by an increase back to around 0.93 in zone C. Xc first remains constant in zone A and then presents a decrement from 57.1% to 53.0% in zone B, probably resulting from either crystal broken or melting. After that there is a small plateau followed by a slight increase to around 56% for Xc in zone C, which evidences the occurrence of melting−recrystallization at such condition. Note that the turning points of f 040 and Xc coincide well with the boundaries of different strain zones. As illustrated in Figure 10e, the correlation length L040 first decreases linearly from 22.1 to 17.6 nm until εy and then remains almost constant in zone B, followed by a further decreasing with smaller slope in zone C. The main decrement before yielding during stretching is probably originated from stress induced lattice distortion, of which the turning point coincides well with the stress evolution. Similarly, crystal I
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Figure 13. SEM images of samples stretched at different conditions: (a1) 40% at 40 °C; (b1) 40% at 120 °C; (c1) 40% at 150 °C; (a2) 120% at 40 °C; (b2) 120% at 120 °C; (c2) 120% at 150 °C. The deformation direction is horizontal.
process, Xc recovers nearly completely to the initial value along the same path as that during stretching at 60 °C in region I (Figure 12(a1)), suggesting that no crystal destructions like slipping or melting occurs here. On the other hand, as shown in Figure 12(b1) Xc at 120 °C in region II only recovers partially but always possesses higher values compared with the values at the same strain during stretching, supporting the occurrence of recrystallization with temperature effects. Interestingly, the variation of Xc at 150 °C presents obvious increasing trend during the returning as shown in Figure 12(c1), which means that recrystallization even occurs in the returning stretching. In region II (120 °C) stress-induced melting−recrystallization is not massive and seems to occur at stress concentration points, while it is more obvious and massive in region III (150 °C). As shown in Figure 12(a2) for region I, εL040 and σ match with each other perfectly in the cyclic test, which unambiguously confirms that the variation of L040 indeed stems from the imposed stress instead of crystal slip or other plastic mechanisms. And εL040 does not vary proportionally with σ at 120 °C as shown in Figure 12(b2), which cannot fully recover to its initial value. This irreversibility is probably originated from stress-induced destruction of crystal via slipping or melting, which is also supported by the partial recovery of Xc. As illustrated in Figure 12(c2), εL040 turns to partially matching with σ again with Td increasing in region III to 150 °C, which, however, is still unable to recover to its initial value. As recrystallization massively occurs here, L040 is also contributed by newly formed crystals. 3.4. Surface Morphology after Stretching in the Three Regions. For direct visualization of the different morphological evolutions at the three temperature regions, Figure 13 presents several SEM images, which are taken without etching process to check the different surface morphologies of samples stretched to strains of 40% and 120% at the three typical temperatures in different regions (40, 120, and 150 °C). Here strain of 40% is located in the initial stage of zone C, of which the morphology change is mainly due to the deformation in zone B, and the morphology of strain of 120% is provided here to check the morphological evolution in zone C. At 40 °C (on behalf of region I), the surface morphology is almost unchanged when stretched to strain of 40% as presented in Figure 13(a1), and most lamellae still perform oriented state with layer normal along stretching direction after stretched to 120% (near fracture
than the initial one. For correlation length L040 in Figure 11b, significant reduction mainly occurs during stretching in the linear elastic deformation zone A, while after yielding the reduction trend sharply slows down, suggesting that L040 is dominated by stress-induced lattice distortion rather than irreversible deformation like crystal slipping or melting− crystallization. Note that the reduction of L040 at 150 °C (region III) is not so obvious when compared with that in regions I and II, which probably results from combined effects of melting of original crystal and the formation of new fibrillar crystal. In Figure 11c, f 040 also presents obviously different characteristics among the three temperature regions. The reduction of f 040 is motivated at εn for all Td, which proceeds until broken at 40 and 60 °C in region I but turns to increase in zone C after going through a minimum value in the end of zone B in regions II and III. Nevertheless, f 040 keeps at high level above 0.9 during tensile deformation at all Td except at 120 °C, which reaches a minimum value of 0.69. This lowest orientation at 120 °C may be ascribed to the combined effects of stressinduced crystal melting and broken, which overwhelms crystallization of highly oriented fibrillar crystals. In order to verify elastic or plastic deformation of crystals during stretching in zone B, the evolutions of crystallinity (Xc) and the relative variation of correlation length L040 (εL040) during cyclic deformations are provided in Figure 12. εL040 is defined as εL040 = (L040(0) − L040(ε))/L040(0) × 100%, where L040(0) and L040(ε) are the correlation lengths at macrostrain of 0 and ε, respectively. The preset return strain is set as 40% (initial stage in zone C) for the three temperatures: (a) 60, (b) 120, and (c) 150 °C. The first row (Figure 12(a1/b1/c1)) is the evolution of crystallinity Xc, while the second row (Figure 12(a2/b2/c2)) is for εL040, where stress−strain curves are replotted in double-Y form for reference. In order to compare the elastic recovery property among the three temperatures, here we define residual strains as the strain where stress reaches zero value during returning deformation process. The residual strain at 60 °C reaches down to about 2.8%, indicating that the reversible elastic deformation dominates in region I although stress−strain curves already enter the nonlinear zones B and C. At 120 and 150 °C residual strains are about 19.9% and 25.3%, respectively, implying the occurrence of irreversible plastic deformation, which is further evidenced by the variation of Xc as shown in the first row in Figure 12. During the returning J
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Figure 14. Contour maps of the evolutions of tensile and structural parameters as a function of strain within experimental temperature range (30− 160 °C): (a) engineering stress (σ); (b) microstrain (εm); (c) strain difference (Δε); (d) crystallinity (Xc); (e) strain of correlation length of 040 crystal plane (εL040); (f) orientation parameter of 040 crystal plane ( f 040). The black squares are on behalf of εn, while the red circles represent for εh at different deformation temperatures.
line and red circle-line in the six contour maps, respectively. Note that mechanical and structural data at the five representative temperatures have been discussed and analyzed above, so we will not focus on the details of the contour maps but concentrate more on the direct and visualized characters among the three temperature regions, which are much easier to be defined through the color evolution and contour lines. Spectra from blue to red color as shown in the scale bar represents the mechanical and structural parameters increase from low to high values, while the denser contour lines corresponds with the larger variation rate. Combined with the εn and εh lines, the structural evolutions do behave differently in the three strain zones, mainly by the density of contour lines, especially between zone A and zone B. The boundary between regions I and II is mainly decided by the existence of minus value (navy area) of Δεm at Td not larger than 70 °C in Figure 14c. The area colors of Xc and εm in Figures 14b and 14d changing from navy to green along strain axis provide evidence for recrystallization, which clearly builds the boundary between regions II and III. Combining in-situ ultrafast SAXS/WAXS and ex-situ SEM measurements, several interesting results and conclusions can be drawn. (i) The evolutions of morphological and mechanical data all consistently lead to the division of three temperature regions, namely regions I, II, and III. The temperature boundaries are in great accordance with α relaxation temperature (Tα ≈ 80 °C) and the onset temperature of initial melting (Tonset ≈ 135 °C), implying that different chain mobility of crystals corresponds to different mechanical behaviors with characteristic structural evolution. (ii) The strain space can also be roughly divided into three zones: linear elastic deformation zone A (0 < ε < εn), initial nonlinear mechanical zone B (εn < ε < εh, including yielding and stress plateau or strain softening), and strain hardening zone C (ε > εh). (iii) Based on the evolution of detailed structural parameters (Lm, εm, Ie, Xc, L040,
strain in Figure 13(b1)). Comparing Figures 13(a1) and 13(a2), lamellar crystals are seldom destroyed or going through plastic deformation like slipping even near fracture, which coincides well with the perfect reversibility of mechanical behavior and the microscopic structural parameters measured by SAXS and WAXS as mentioned above (Lm, Ie, Xc, L040, etc.). With Td higher than Tα (region II) as illustrated in Figure 13(b1), massive slender cavities are observed when stretched to strain of 40% at 120 °C, which are arranged in rows with intervals of lamellar stacks skeleton. Further stretched to 120% (Figure 13(b2)), larger but still slender cavities are observed combined with disordered lamellae, which resemble the trace of crystal slipping. Here the bridges between cavities probably result from stretch-induced crystallization, of which the period (around 20−40 nm) corresponds well with the periodicity observed in equatorial SAXS peak (qmax of about 0.26 nm−1) in Figure 9b. As Figures 13(c1) and 13(c2) present, the surface morphologies are rather different when stretched at temperatures near melting temperature (region III, 150 °C). Most lamellae are still presenting oriented status while some local regions (darker areas in Figure 13(c1)) seem more disordered when stretched to 40%. The disordered level is strengthened in the process of strain hardening (120% in Figure 13(c2)), where some fibrillar crystals are formed along tensile direction with random distribution. Corresponding with the absence of lozenge scattering near beamstop in SAXS patterns (Figure 4), cavities are not observed in Figures 13(c1) and 13(c2).
4. DISCUSSION Before discussion, we summarize nonlinear mechanical behaviors and structural evolutions during tensile deformation at temperatures from 30 to 160 °C. In strain−temperature space, σ, εm, Δεm, Xc, εL040, and f 040 are plotted in contour maps in Figures 14a−f, respectively. Extracted from stress−strain curves at different Td, εn and εh are plotted with black squareK
DOI: 10.1021/acs.macromol.8b00255 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules
Figure 15. Schematic images of the deformation mechanism in hard elastic iPP films in the wide temperatures range (30−160 °C). Deformation direction is horizontal. The black square-line and red circle-line are representative for εn and εh at different Td as the boundaries for zones A, B, and C, respectively. (a−h) present the deformation behavior at typical strain zones in different temperature regions, respectively, and (b′), (d′), (e′), and (g′) are the zoom-in images of (b), (d), (e), and (g), respectively.
d040, and f 040) and different surface morphologies, different deformation mechanisms trigger the initial nonlinear mechanical behaviors (zone B) and contribute to further strain hardening (zone C). (a) In low temperature (region I), hyperelastic deformation in zone B is proposed to be triggered by microphase separation of interlamellar amorphous. (b) In region II crystal slipping through pulling out chains from lamellar crystals and the simultaneous recrystallization of nanofibrillar bridges competes with microphase separation, resulting in plastic deformation with the formation of cavities in zone B, while microfibers form through further slipping in zone C. (c) Melting−recrystallization dominates the nonlinear deformation in zones B and C and results in the formation of fibrous crystals in region III. These different nonlinear deformation mechanisms in different temperature regions essentially stem from the completion between cohesive strengths of amorphous and crystal phases, which will be discussed in the following. For the convenience of discussion, schematic images of deformation mechanisms of lamellar stacks embedded in residual amorphous matrix in the two-dimensional temperature−strain space are depicted in Figure 15. 4.1. Hyperelastic Deformation in Region I (Tβ < T < Tα). We propose that microphase separation of interlamellar amorphous is mainly responsible for the hyperelastic deformation behavior in the nonlinear zone B, as schematically illustrated be Figures 15b and 15b′. Before discussing the microphase separation mechanism, we first exclude the other two possibilities, namely (i) crystal slipping or melting− recrystallization and (ii) cavitation. (i) Early mechanical models of semicrystalline polymers attribute the nonlinear deformation to Martensitic transition, crystal slipping, or melting−recrystallization, which are all
related to irreversible plastic deformation of crystals rather than amorphous phase. As shown in Figures 12(a1) and 12(a2), crystallinity Xc and correlation length L040 all follow nearly 100% reversible process during the cyclic deformation with strain even exceeding εh (zone C). Correspondingly, the surface morphologies after stretching to zone C as presented in Figures 13(a1) and 13(a2) almost have nothing different with initial morphology, where only oriented lamellar stacks exist without the occurrence of cavities. Moreover, the evolution of εL040 matches perfectly with stress−strain curve, indicating that the variation of correlation length L040 is simply due to stressinduced lattice distortion rather than crystal slipping. Thus, we can conclude that no plastic deformation via crystal slipping or melting−recrystallization occurs in zone B at low temperature region I, which is in line with nearly 100% elastic deformation of hard-elastic iPP film. (ii) As strong lozenge scattering emerges close to beamstop when strain entering the nonlinear deformation zone B, cavitation of amorphous area might be one possibility to trigger yield. With in-situ atomic force measurements, early studies indeed reported the occurrence of cavitation.91 However, as cavitation is a passive energy consuming process rather than a spontaneous phase transition, if cavitation occurred in interlamellar amorphous it could only lead to a maximum microstrain εm equal to macrostrain ε, which is contradictory to the experimental results. As shown in Figures 5, 8, and 14, the increment of εm in zone B can reach nearly 3 times larger than that of the corresponding ε. Thus, we can also conclude that cavitation is not the main mechanism to drive the nonlinear deformation behavior in zone B at region I. Note that the early observation of cavitation with AFM may be due to low strain or creep effect, which is indeed also observed in our creep L
DOI: 10.1021/acs.macromol.8b00255 Macromolecules XXXX, XXX, XXX−XXX
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Macromolecules experiments in PE.92 Nevertheless, those cavities are not responsible for the hyperelastic behavior in zone B, which is more possibly due to further structural evolution based on microphase separation which will be discussed in region II with a high temperature. After excluding the possibilities of crystal destruction and cavitation, we come to the microphase separation model as illustrated in Figures 15b and 15b′. As tie-chain (bridge and trapped entanglement) and cilia (loop) heterogeneously distribute in interlamellar amorphous, we propose stressinduced microphase separation between domains with rich tie-chains and cilia, which is closely analogue to stress-induced phase separation in dynamic asymmetric blends. Here tiechains are locked by the hard lamellar crystal framework, while cilia possessing freedom can diffuse in the scale of radius of gyration of polymer chain. During the tensile deformation, tiechains sustain higher stress and squeeze out cilia to form high density domains, while domains with rich cilia segments possess a low density. As stress-induced microphase separation is a nonequilibrium reversible phase transition, it is a spontaneous process as strain entering zone B, which can account for the accelerated increase of ε m to values larger than the corresponding ε. Note stress-induced microphase separation is a stochastic process, which occurs in portion of lamellar stacks. If we define the whole initial sample length on one deformation unit as L(0) composed of total n periods Lm(0) and residual amorphous phase with thickness along stretching direction of LAm′(0), the initial sample length is L(0) = nLm(0) + LAm′(0), provided that microphase separation occurs in a portion (m periods) of the total lamellar stacks (n periods), where n > m. Thus, there exist residual lamellar stacks with (n − m) periods without microphase separation, which have a periodicity of Lx(ε) smaller than those with microphase separation of Lm(ε). Then the expression of Δε is written as Δε = ε − εm ≈ [(n − m)(Lx(ε) − Lm(ε)) + LAm′(ε)]/nLm(0). Considering n > m and Lx(ε) < Lm(ε), (n − m)(Lx(ε) − Lm(ε)) is a negative value in zone B. When its absolute value is larger than LAm′(ε), Δε obtains negative value in zone B, corresponding well with what we observed in region I. The broad scattering peak in equator (Ie) indicates the average periodicity of the phase-separated domains, which is in the order of radius of gyration of polymer chains. The density contrast between lamellar stacks with and without microphase separation contributes to the lozenge scattering close to beamstop. As shown in Figure 15c, a microphase separation is developed in zone C with further lamellar separation, inducing the sharper scattering interface between two phases. Thus, the lozenge scattering is generally shrunk into beamstop with strain increasing in zone C. The samples are going through fracture before the occurrence and growth of cavities in crystals or amorphous phase with microphase separation. The essential physics for stress-induced microphase separation of interlamellar amorphous phase is coming from the Poisson contraction effect. Because of large aspect ratio of lamellar crystal and amorphous layers and 2−3 orders of modulus difference of the two layers, tensile deformation along their layer normal will induce Poisson contract of amorphous layers in lateral direction, which, however, is confined by crystal layers via chemical connected tie chains. In low temperature region I, the cohesive strength of crystals is sufficient to prevent it from being destroyed through either slipping or melting, while the cohesive strength of amorphous phase cannot hold its initial structure in zone B, and eventually microphase separation
is induced by stress. Microphase separation disperses the macroscopic Poisson contract into nanometer domains through local diffusion of chain segments. As will be discussed later, if the cohesive strength of crystals is weaker than that in amorphous phase, crystals may be destroyed before the occurrence of microphase separation of amorphous phase. 4.2. Crystal Slipping and Microfibers Formation in Region II (Tα
