Stretching-Induced Uniform Micropores Formation: An in Situ SAXS

Macromolecules , Article ASAP. DOI: 10.1021/acs.macromol.7b02335. Publication Date (Web): April 26, 2018. Copyright © 2018 American Chemical Society...
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Stretching-Induced Uniform Micropores Formation: An in Situ SAXS/ WAXS Study Caihong Lei,*,† Ruijie Xu,*,† Ziqin Tian,† Henghui Huang,† Jiayi Xie,† and Xingqi Zhu‡ †

Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, P. R. China ‡ Beijing Application Lab, Bruker China, Beijing 100192, P. R. China S Supporting Information *

ABSTRACT: Although the microporous membrane prepared based on the melt stretching mechanism has been commercialized for more than 20 years, the formation process from the initial lamellae structure to final fiber connecting bridges and pores is still unclear. In this work, to clarify the transformation mechanism, in situ SAXS and WAXS were carried out during hot stretching at 130 °C to 100%. The scattering patterns from the annealed film, cold stretched film to 20% (stretched at room temperature), and heating to 130 °C were also collected. The preparation technology was similar to that during the commercial fabrication. It was found that during cold stretching to 20% many long and narrow crazes are formed between separated lamellae clusters, and a part of destroyed crystals appeared. After heating to 130 °C, oriented structure and needlelike voids appeared, which was related to the shrinkage of oriented amorphous chains along the transverse direction, due to the tension stress effect. Also some oriented crystal structure was formed. During hot stretching to 20%, the lamellae which are close to the craze wall are rotated as the fibril crystal as the axle and the connecting bridges were formed among the separated lamellae cluster. Further stretching to 100%, these connecting bridges transformed to fiber bridges, contributed by strain-induced crystallization. During the whole hot stretching, the amorphous chains oriented along the machine direction and also crystallized into fiber bridges. This is the first time to clearly describe the lamellae to fiber bridges transformation during the preparation of microporous membrane.

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INTRODUCTION The polypropylene (PP) microporous membrane could be prepared based on the melt stretching mechanism. The stretched pores are beneficial for the transmitting of Li ions from negative to positive anode; hence, this kind of membrane has been the most used separator for Li-ion batteries due to its suitable pore size, excellent mechanical strength, and chemical stability. During the production line of this kind of microporous membrane, the fabrication process mainly covers three stages: (1) production of the precursor film with prerequisite rownucleated lamellar crystalline structure, (2) annealing to thicken the lamellae and improve lamellae orientation and uniformity, and (3) continuous four steps: cold stretching (stretching at room temperature), heating to hot stretching temperature, hot stretching within 130−145 °C, and heat-setting within 145− 155 °C. The heat-setting is carried out to improve the dimensional stability of stretched microporous membrane.1,2 In stages 1 and 2, there have been many works to build the materials−processing−structure−properties relationship. A detailed figure of material and process parameters effect on lamellae structure and film properties has been drawn out.3−7 As for the stage 3, the fundamental scientific problem is where the pores are initiated and how they are grown during the deformation process. Cavitation is a consequent result during drawing of semicrystalline polymers, but the location of appeared voids is still at the center of the debate. There have been many studies focusing on drawing deformation of the © XXXX American Chemical Society

spherulites system, and basically there are two different arguments in the literature: (1) Galeski8−10 pointed out that cavitation started in the amorphous phase. They believed that during deformation of the material cavities could be generated in the interspherulitic regions and the interlamellar regions, inside the spherulites. (2) Men11−13 considered that cavitation started in the crystalline phase, where cavities were initiated first in the crystalline phase as a result of breakage of crystalline skeleton during tensile deformation. In the preoriented system, there are still many controversies as to the stretching induced pore formation. For the pore formation process, stage 3, it is well accepted that cold stretching within the stretching ratio of 20−30% induces the formation of initial pores and hot stretching to 100% enlarges the pores and induces the transition of lamellar structure to fiber connecting bridges.1 In early works, Sadeghi et al.14 proposed that during cold stretching void was formed as a result of short tie chain scission. Later works from Lei, Liu, and Ajji believed that the unstable secondary crystals formed during annealing were transformed to the initial pores.15−17 In our recent work, the contribution of typical daughter crystal in PP could not be neglected.18 Contrary to these, Li et al.19 mentioned that the amorphous region could be stretched and Received: November 4, 2017 Revised: April 14, 2018

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DOI: 10.1021/acs.macromol.7b02335 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Surface morphology of stretched and heat-set microporous membranes prepared at different hot stretching ratios, cold stretched to 20% and hot stretched to 20%, 40%, 60%, 80%, and 100% (a−f). The inset schematic figure in (f) means the connecting bridge crystal formation. the microporous membranes were prepared by stretching using an Instron 5500R machine equipped with a heating chamber. A drawing speed of 833 μm/s was applied during cold and hot stretching steps. The annealed film was first stretched to 20% in the heating chamber at room temperature. After cold stretching, we stopped the machine and increased the heating chamber temperature to 130 °C, and then the cold stretched film was stretched to different ratios under 130 °C. The initial film thickness is 20 μm, and the final microporous membrane thickness is about 18−19 μm. The thickness shows nearly no change during the stretching process. Finally, the stretched membranes were heat-set under 145 °C for 10 min, and their surface morphology was characterized by scanning electron microscopy (SEM, S3400N, Hitachi, Japan). All the samples were sputtered with a platinum ion beam for 300 s before testing. In Situ SAXS and WAXS. Two-dimensional SAXS and WAXS measurements were performed using synchrotron radiation with λ = 0.154 nm at Beamline 1W2A of Beijing Synchrotron Radiation Facility (Beijing, China). In the in situ SAXS and WAXS experiment, we set the Linkam TST350 equipment between the X-ray source and the detector.22 Mar165-CCD was set at 5000 mm Linkam TST350 detector distance in the direction of the beam for SAXS data collections and 300 mm for WAXS data collections.22 The stretching velocity was set at 833 μm/s. The SAXS and WAXS signals were collected when the sample was stretched to set position, and the exposure time was 15 s; then the stretching was continued. The whole process was the same with the above microporous membrane preparation for SEM observation. We must point out that this stop stretching process has no effect on the final pore structure. The stretching sample is 50 mm long and 10 mm wide. The distance between two holders is 30 mm.

broken, and lateral shrinkage of amorphous region induced pore formation. As for hot stretching, Sadeghi et al.20 believed that stretching strain induced long tie chains that orient and crystallize along the machine direction. In fact, at the hot stretching temperature, the disentanglement, orientation, and crystallization of amorphous chains could not be neglected. Compared with the SEM morphology of only cold-stretched film, it is directly seen that the lamellae are further separated into lamellae cluster; the cluster size is decreased during hot stretching, and more connecting fiber bridges are formed, indicating the occurrence of lamellae transformation into fiber bridges. To summarize these works, the researchers only described the phenomenon of pore growth process during the cold and hot stretching, but there is still many disputes in where and how the pores appear and grow. Recently, Xiong et al.21 give a new recognition about stretching-induced pores formation in the preoriented film. They believed that elastic deformation occurs at the beginning of stretching followed by the crystal shear deformation and finally by the initiation of cavitation. But it is well-known that the modulus of the amorphous region is much lower than the crystalline region when the temperature is lower than the αc relaxation temperature; the amorphous region is much easier be broken than the crystalline region. It is seems that the initial structure deformation appeared in the amorphous region. Although the microporous membrane based on the melt stretching mechanism has been commercially fabricated by Celgard from USA for more than 20 years and Senior from China for more than 10 years, however, up to now it is still unclear where the initial pores appear and how the lamellae transform to fiber connecting bridges during the above continuous cold stretching, heating, and hot stretching steps. To clarify the formation process, the in situ small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) with a temperature-controlled experimental stretching setup (Linkam TST350) were used to follow the cold stretching, heating, and hot stretching process. The transformation mechanism was proposed.

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RESULTS AND DISCUSSION To directly exhibit the microstructure change during cold and hot stretching, the surface morphology of stretched and heatset microporous membranes which have been cold stretched to 20% and hot stretched to different ratios is shown in Figure 1. Here, all the samples have to be heat-set at 145 °C to escape the shrinkage and stabilize the structure of stretched membrane after the stretching stress is relieved.1 The cold stretched film after heat-setting shows long and narrow crazes between the separated lamellae clusters and only ten sized nanosized pores in the craze. The pore size is much less than that after hot stretching. It is apparent that a lot of connecting fiber-like bridges and pores among the separated lamellae clusters are formed after hot stretching. With the increase of hot stretching ratio, more separated lamellae cluster is initiated and their thickness is decreased. At the same time, the connecting bridges

EXPERIMENTAL SECTION

Preparation of Stretched Microporous Membranes and SEM Characterization. The annealed polypropylene precursor films were supplied by Shenzhen Senior Technology Material Co., Ltd., China. Their 100% elastic recovery is high to 96%. For SEM characterization, B

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Figure 2. In situ WAXS scattering patterns (a), the scattering intensity of main lamellae (b), the relative content of daughter crystal (c), the crystallinity (d), and the relative content of connecting bridge crystal and fitted peak fwhm (e) changes during hot stretching to 100%.

crystal structure for the PP system is also listed in the pattern.23 The scattering intensity of the (110) plane along the vertical direction, the relative content of daughter crystals based on the integral area ratio between the daughter crystal’s peak, the total diffraction peak in (110) plane azimuthal angle curve,23 and the crystallinity are calculated. Within region I (from cold stretching to heating), compared with that of annealed film, the cold stretching and increasing temperature to 130 °C result in the decrease of scattering intensity, relative content of daughter crystal, and crystallinity by 51.7%, 29.3%, and 15.6%, respectively. The deterioration of daughter crystal and recrystallized part (formed during annealing6 and included in the main lamellae intensity) results in the decrease of crystallinity. During our former work, it has been proposed that during cold stretching to 20%, the deterioration of daughter crystal and recrystallized part contributed to the formation of initial pores.18 And we notice that after cold stretched to 20%, a pair of dispersed signals very close to the

length and slit pore size are increased. The SEM results directly show that hot stretching induces further lamellae cluster separation and lamellae conversion to connecting bridges; at the same time, pores are formed. However, only surface SEM images could not clarify how lamellae transforms to connecting bridges. In addition, the final heat-setting process during the preparation of microporous membrane will induce the shrinkage in amorphous region and the melt-recrystallization behavior, leading to the deformation of the membrane surface structure. To clarify the transformation process from lamellae to fiber during stretching, the in situ WAXS and SAXS patterns were collected during cold stretching, heating to hot stretching temperature, and hot stretching to 100%. Figure 2 gives the WAXS patterns of annealed film, cold-stretched film, increasing temperature to 130 °C, and hot-stretched films to different ratios. The typical (110), (040), and (130) planes of the α crystal are exhibited. In addition, the special mother−daughter C

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Figure 3. In situ SAXS patterns (a), the calculated lamellae lateral size (b), the connecting bridges length (c), and bridges gap during hot stretching (d).

accompanying the increase of pores length. It is strange that after hot stretching to more than 40% (region III) the scattering intensity and the crystallinity are improved by 60.1% and 111.5%, respectively. The relative content of daughter crystals does not change much after hot stretching to 60%, where further separation of lamellae cluster to thin cluster is limited. The force support element during the whole deformation process is the lamellae cluster, about 5−7 layers of lamella. The residual daughter crystals are located among those unseparated lamellae cluster. They will not change during the follow stretching. Combined with the above SEM results, it can be deduced that during hot stretching to 40% the deterioration of main and daughter crystal is higher than the formation of connecting fiber bridge crystals, whereas further stretching to 100% induces more connecting fiber bridges, resulting in the apparent improvement of crystallinity. The residual daughter crystal among the lamellae cluster will not continue to be reduced during the following hot stretching. The signals along equatorial direction totally disappear, since the pore size is larger than the detection range of WAXS. The spiculate signals turn stronger, originating from the amount of connecting bridge crystals formation and the crystallinity increase. On the basis of the WAXS result, it is possible that the cold stretching to 20% makes the lamellae cluster parallel separation, and the crazes are formed between the neighborhood separated lamellae clusters. The observation of initial pores in the SEM of cold stretched film may be from the heat-setting process. To characterize the surface morphology of cold stretched film,

beam stop appear in the equatorial direction. This means that the lamellae clusters are separated, and some crazes are formed between the neighborhood lamellae cluster during cold stretching. Similar signals have also be recognized by Rozanski24 during PP tensile drawing in the spherulite system. Also for the preoriented film, there tens of layers of lamellae crystals are vertical accumulation along the thickness direction. The 20% strain will cause the crazes randomly appear in each layer lamellae crystal, all crazes are located in amorphous region between adjacent deformed lamellae clusters after stretching. These crazes are not the small through holes. The orientated amorphous molecules are located in the crazes. During heating, the mobility of amorphous molecular chains is obviously increased, and the rapid relaxation of extended amorphous chains induces the irregular arrangement of the unit cell, and the diffraction intensity turns weak. At the same time, the signal from the craze turns stronger. The shrinkage in amorphous region results in the appearance of initial small pores, and the density contrast between the craze and lamellae cluster region increases. The spiculate signals in meridianal direction appear after increasing temperature to 130 °C. Some striplike pores appear. Within region II, hot stretching to 40% further induces the decrease of scattering intensity, relative content of daughter crystal, and crystallinity by 28.6%, 58.6%, and 31.6%, respectively. This means that during the separation of lamellae cluster induced by hot stretching more daughter crystals are destroyed. The signals in equatorial direction turn weak due to the craze increase and transform to pores and connecting bridge crystal. The spiculate signals turn stronger gradually D

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Figure 4. Schematic diagram of azimuthal scans of the lamellar peaks along qy.

regularly. During the following heating and hot stretching, the scattering pattern along the equatorial direction continuously moves to near the beam stop. The lamellae cluster structure is continuously separated during the following hot stretching; at the same time, the secondary periodical signals disappear. Along the meridional direction, after cold stretching, the scattering signal is not apparent, but heating and hot stretching to 20% results in apparent pattern. During heating, as mentioned above, the appearance of voids among the amorphous region leads to the density difference between the voids and amorphous chains, resulting in the scattering pattern along the meridional direction. Upon further stretching to 40%, the void signals become narrow and sharp. Some new scattering patterns, similar to that from the shish structure, overlay the void signals. Here, some weak fiber bridges begin to appear. The formation of fiber connecting bridges improves the density difference between the fiber and void. These signals are improved by further stretching to 100%. The corresponding lamellae lateral size based on the width of the scattering patterns along the azimuthal direction,27 connecting bridge length based on the Ruland method,28 and bridge gap are calculated. During the stretching process, the shape of SAXS patterns are apparently changed. This change is mainly due to the lamellae structure deformation, such as long period and the lamellae lateral size. We integrate the SAXS signal along the radial direction and discover that the long period of lamellae (Figure S3) shows nearly no change. This means that the single lamellae structure does not change during the stretching. Then the shape of SAXS pattern deformation could be attributed to the lamellae lateral dimension change.29 The lateral size can be derived from the width Δqy of the peaks at half-height in the equatorial direction according to

heat-setting has to be carried out after cold stretching due to the hard elastic characteristics of annealed film.18 During the heating process to heat-setting temperature, the cold stretching destroyed crystals that may melt and orient along the machine direction due to the effect of heat-setting tension. At the same time, some amorphous chains also orient along the tension. Then, they crystallize, and initial connecting bridges are induced; here this crystallization process will induce the volume shrinkage. Jia et al.25 pointed out that the crystallization in the amorphous region will appear when temperature is higher than 120 °C. Meanwhile, the other part of amorphous chains will shrink along the transversal direction, leading to the appearance of initial pores. It is the heat-setting that leads to the appearance of initial connecting bridges and pores for only the cold stretched film. We also notice that with hot stretching ratio increasing, the shape of diffraction arc shows overt change. A new diffraction signal along mother crystal direction appears accompanying the connecting bridge crystal formation step by step. Mao et al.26 mentioned that this overlapped signal could be separated into two parts based on the peak separation software. The azimuthal integral curve of (110) plane during stretching is shown in Figure S1, and two Lorenz peaks are used to fit the diffraction peak (Figure S2). The area ratio of the sharp peak increase could be used to describe the connecting bridge crystal growth process. As shown in Figure 2e, the content of connecting bridge crystal is continually increased with the lamellae cluster separation. The fwhm of lamellae skeleton is decreased until the hot stretching ratio is larger than 40%. Within range I and II, the arrangement of lamellae cluster is influenced by the stretching force, and a part of lamellae crystal may be stretched out from the initial cluster to transform to the fiber-like crystal. Within region III, it is mainly the stable growth of connecting bridge crystals. Figure 3 gives the SAXS patterns, the calculated lamellae lateral size (Llateral), the connecting bridges length, and bridges gap (Lm) during hot stretching. The SAXS pattern of annealed film is also included. For the annealed film, two scattering spots appear along the equatorial direction, indicating the existence of oriented crystalline structure.1 After cold stretching, the scattering signals move near to the beam stop. From the above SEM, it can be deduced that the separation of lamellae cluster during cold stretching induces the increase of long period, and the signals move to the beam stop. At the same time, it is strange that secondary scattering signals appear, indicating high periodicity structure induced by cold stretching. The stretching force compels lamellae crystals arranged more

L lateral =

2π Δq

(1)

Figure 4 shows the schematic diagram of azimuthal scans of the lamellar peaks along qy. The average lateral dimensions of the lamellae are obtained from the profiles of the intensity distribution along the straight line, I(qy). The curves of I(qy) are fitted with two Lorentz functions as shown in Figure 4. The width of the resulting Lorentz function (Δqy) is related to the lateral size of the crystalline lamellae.30 It must be mentioned that the use of this method is only valid when the orientation is perfect. Here, for sake of simplicity, we assume a perfect orientation of crystalline lamellae. E

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Figure 5. Azimuthal angle integral curve of SAXS streak and the Lorentz fitted line (a) and the relationship between the Bobs and 2π/q (b).

In former work, the Ruland streak method has been well used to calculate the shish length for the system with shishkebab crystalline structure.31,32 Based on this method, the apparent azimuthal width (Bobs) is a function of the length of shish (Lshish) and the azimuthal width (Bϕ) due to the misorientation of shish. As the Lorentzian profile is a proper model for the oriented distribution, the relation becomes Bobs =

2π ⟨Lshish⟩q

+ Bϕ

(2)

Here Bobs represents the integral width of the azimuthal profile from the streak at q. Based on the above equation, Lshish can be obtained from the Bobs versus q curves. In this study, the shish length is approximate to the stretched pore length, and all the azimuthal distributions are fit with Lorentz functions well. Figure 5 gives a schematic diagram of integrate curves and give a typical example of calculating Lshish and Bϕ. Then, based on eq 2, the average connecting bridge crystal length was calculated. Since the pore and connecting bridge crystals are parallel to each other, the signals along meridional direction reflect their superimposed information. The length from Ruland streak method is the weight-averaged length of the sample.33 This result has considered the effect of wide distribution of connecting bridge crystal length and density contrast between the pore and the crystals. From the SEM images in Figure 1, we notice that the stretching-induced connecting bridge crystal is arranged neatly, and the distance of neighborhood bridge crystals is similar; thus, a pair of symmetrical scattering signals from the adjacent fibrils crystals formed pore’s walls will appear along the meridional direction.34 We integrate the SAXS streak signal along the radial direction; the Iq2−q curves are given in Figure 6. Before integration, the instrument background was subtracted considering sample absorption.35 In the onedimensional scattering intensity distribution, generally, no multiplication q2 to I(q) is performed because of the anisotropic orientation of the lamellae in the samples, where q is the scattering vector, q = 4π sin θ/λ, λ is the X-ray wavelength, and 2θ is the scattering angle. Here, to amplify the scattering curve difference between different stretching stage, q2I(q) curves are given. The average bridge gap is calculated by Bragg equation, Im = 2π/q; here q is the peak of Iq2−q curve.

Figure 6. Iq2−q curves along the meridional direction.

Within region I, the calculated lamellae lateral size is decreased from 135 to 120 nm. The increasing temperature process makes the mobility of lamellae crystal increase, which decreases the lamellae lateral projected dimensions. Here, since no signals from connecting fiber bridges could be observed; their length and gap could not be given. After increasing the temperature to 130 °C, the pore length and width are about 20 and 17.5 nm, respectively. Within regions II and III, the lamellae lateral size is continuously decreased to 70 nm, whereas the length and gap of connecting fiber bridges are increased to 220 and 27.5 nm, respectively. During hot stretching, the main lamellae are continuously destroyed along their both ends. At the same time, the connecting bridges length are increased, which is in agreement with the above SEM results. We must declare that the dimension from SAXS is much smaller than that of SEM images in Figure 1. The main reason is that the microporous membrane has more than ten layers of pores. The SAXS result is given from the whole sample, whereas the SEM image only gives the size of surface pore. The similar result also given by Li et al.19 The above WAXS results give the information that during cold stretching and hot stretching to 10%, the daughter crystal and some main lamellae are destroyed, while the crystallinity is increased apparently after hot stretching to 40%, as shown in Figure 2b,d. During cold stretching and initial hot stretching, the destroyed F

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Figure 7. Obtained SAXS scattering patterns from the subtraction between near two patterns, such as the pattern (b) is obtained by subtracting cold stretched pattern from increasing temperature to 130 °C pattern.

Figure 8. Schematic of connecting fiber bridges formation process during hot stretching.

subtracting adjacent two scattering signals in Figure 3. The differential SAXS patterns can give the deformation structure information in the stretching process. The obtained new patterns are shown in Figure 7. For example, the new pattern (a) is obtained by subtracting the pattern of annealed film from the scattering pattern of the cold stretched pattern. In pattern (a), it is apparent that secondary periodical structure exists in the cold stretched film. In pattern (b), obtained from the subtraction between the heating and the cold stretching, along

lamellae structure has not been rearranged and crystallized. Although the fiber signal appears after hot stretching to 20%, the damage of lamellae structure is higher than the formation of fiber crystals; hence, the crystallinity is still decreased compared with that of cold-stretched sample. After hot stretching to 100%, the lamellae lateral size is about 70 nm, whereas the fiber crystal gap is about 27.5 nm. To further characterize the structure change during stretching, the new scattering patterns were obtained by G

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the transverse direction further increases the pore size. During hot stretching to more than 40%, the continuous decrease of lamellae lateral size indicates that the lamellae ends are continuously pulled out and connect with the above bridges, leading to the increase of connecting bridges length. The amorphous chains also further orient and the bridges gap are increased. At this time, the strain-induced crystallization phenomenon becomes strong, and these connecting bridges transform to fiber crystals. The formation process of fiber connecting bridges explains the existence of bridges bundle in Figure 1f. In addition, as seen from Figure 1, during hot stretching, the separated lamellae cluster is further separated to thin clusters, and new fiber connecting bridges are induced. During hot stretching until 60%, the content of daughter crystal is continuously decreased, meaning that during hot stretching, among the separated cluster, the daughter crystals are continuously destroyed. At high hot stretching temperature, the destroyed daughter crystals melt and orient along the machine direction. The lamellae closing to the craze wall will rotate, and new connecting bridges are formed. High stretching ratio induces their transformation to fiber connecting bridges, and new fiber connecting bridges are continuously initialized; at the same time, more pores are formed. It can be seen that during stretching voids are first initialized due to the orientation of amorphous chains, and then connecting fiber bridges are formed to support the stretched pore structure.

the equatorial direction, there are three scattering spots. The outside black spot represents the secondary periodical structure, meaning that after heating, high structure periodicity is remained. The inner black spot represents the lamellae structure within the separated lamellae cluster, indicating that during cold stretching and heating, the lamellae structure is kept. The white spot represents the long and narrow craze, and the intensive scattering comes from the difference in electron density between the material inside the crazes oriented perpendicularly to the tensile direction and the polymer matrix.36 Along the meridional direction, the weak and diffused blue signal is from the heating-induced initial voids. After hot stretching to 20% (in patterns c and d), the scattering pattern representing the craze turns to four leaf, which moves to near the beam stop and disappears during the following hot stretching. The appearance of four leaf indicates the occurrence of zigzag deformation of crazes wall. This zigzag change is mainly since the lamellae, near the craze wall, rotates with stretched orientation structure as the center. The hot stretching temperature is higher than Tαc.37 The lamellae rotation or molecular chain stretched out of the lamellae is more easily occurring. In Figure S3, the lamellae long period shows nearly no change during the whole stretching process; hence, the hot stretching mainly induce the lamellae rotation deformation.38,39 The disappearance of secondary signal means that during hot stretching the lamellae structure periodicity is destroyed. The void scattering signal becomes narrow and strong. At the same time, the lamellae signal is still remained. After hot stretching to higher than 40% (in patterns e, f, g, and h), the fiber crystal signals, similar to that of shish structure, are superimposing with the void signals. The separated lamellae cluster further moves to the beam stop, indicating further separation. However, even though the stretching ratio is high to 100%, the lamellae within the separated lamellae cluster are still remained. Based on the above SAXS and WAXS results, the fiber connecting bridges formation process during the preparation of microporous membrane is shown in Figure 8. In the annealed film, there are mother main lamellae, daughter crystal, and unstable recrystallized part formed during annealing between the lamellae cluster and tie chains. After cold stretching to 20%, the damage of daughter crystal and unstable recrystallized part among the separated lamellae cluster happens, and some long and narrow crazes are formed, but no voids appear. During heating to hot stretching temperature, the destroyed crystal parts melt and orient along the machine direction due to the existence of tension stress. Depending on the oriented structure, some amorphous chains disentangle and orient along the machine direction. Their orientation along the machine direction induces the shrinkage along the transverse direction, initializing the appearance of initial pores. Some oriented structure may crystallize and the initial long and narrow pores may appear, explaining the appearance of signal along the meridional direction in WAXS patterns during heating. During hot stretching to 20%, the lamellae, near the craze wall, rotates with stretched orientation structure as the center and the craze further increase. At the same time, the lamellae lateral size is continuously decreased. These stretched parts connect with the oriented part formed during heating. Some bridges appear connecting the separated lamellae cluster. Some bridges are crystallized during stretching, but they could not be sensitized by SAXS. The amorphous chains further orient along the machine direction, and their shrinkage along

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CONCLUSIONS The whole transformation process from initial lamellae structure to final pore and fiber connecting bridges during the preparation of microporous membrane based on melt stretching mechanism are in situ followed using SAXS and WAXS. Cold stretching to 20% induces the separation of lamellae cluster, but no voids can be identified. The tension stress along the machine direction during heating to hot stretching temperature induces the orientation of melted crystal part and amorphous chains. The initial voids are observed. During hot stretching, the lamellae close to the craze wall will rotate by the stretching with the above oriented structure, and strain-induced crystallization results in the transformation of connecting bridges to fiber bridges, which support the stretched pore structure. This is the first time to clarify the fiber connecting bridges formation during hot stretching into microporous membrane.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02335. Azimuthal angle integral curve of the (110) plane in mother crystal range during stretching; the peak separation schematic graphic of each sample; the integral curve of the lamellae patterns and the calculated long period change during stretching (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*(C.H.L.) E-mail [email protected]; Tel 86-20-39322570. *(R.J.X.) E-mail [email protected]; Tel 86-20-39322570. ORCID

Caihong Lei: 0000-0002-3660-7715 H

DOI: 10.1021/acs.macromol.7b02335 Macromolecules XXXX, XXX, XXX−XXX

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Project of National Science Foundation of China under Grant (51603047 and 51773044), Guangzhou Science and Technology Plan Project (201510010037), High Level Talents in Higher School, Guangdong Province Major Key Projects of Applied Research and Development of Science and Technology (2015B090925021), Guangdong Province Science and Technology Plan Project (2016A010103030), Guangdong Province Ordinary University Innovation Project, and the PhD Start-up Fund of Natural Science Foundation of Guangdong Province, China (2016A030310344), for financial support.

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DOI: 10.1021/acs.macromol.7b02335 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules (37) Chang, B.; Schneider, K.; Vogel, R.; Heinrich, G. Influence of Annealing on Mechanical αc-Relaxation of Isotactic Polypropylene: A Study from the Intermediate Phase Perspective. Macromol. Mater. Eng. 2017, 302, 1700291. (38) Thünemann, A. F.; Ruland, W. Microvoids in polyacrylonitrile fibers: a small-angle X-ray scattering study. Macromolecules 2000, 33, 1848−1852. (39) Gerrits, N. S. J. A.; Tervoort, Y. Deformation mechanisms during uniaxial drawing of melt-crystallized ultra-high molecular weight polyethylene. J. Mater. Sci. 1992, 27, 1385−1390.

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DOI: 10.1021/acs.macromol.7b02335 Macromolecules XXXX, XXX, XXX−XXX