As more and more electric devices are entering into our life, especially the demand of electric vehicles is increasing. So rapid development of EV or HEV requests high power Li-ion batteries (LIBs) to have higher energy, longer life and better safety than before. The process of producing microporous membrane is melt-extrusion/annealing/uniaxial-stretching (MEAUS), which is a low-cost, simple, green and broad method. It is key to form the oriented lamellar by stress-induced crystallization then lamellar thickening and crystal defect elimination are also important. The extruding-casting machine was shown in Fig.1. In this process, processing conditions would affect the crystal morphologies, such as extruder die temperature[1], melt draw ratio[2, 3] and the air knife[4],which all have been studied and that are important for influence of crystal morphologies.
Poly(4-methyl-1-pentene) is a type of polyolefin, which has good comprehensive performance, such as mechanical property, heat resistance, environmental resistance and chemical resistance[5]. So far, many researchers has reported methods of poly(4-methyl-1-pentene) membrane, which include phase separation, thermally induced phase separation and the principle, the process, the effects and the properties of each method are all introduced[5]. However, the most serious drawback is pollution caused by solvent. Therefore, the best way is MEAUS.As reported that it is possible for polymers with αc relaxation to ultradrawing, in contrast polymers without αc relaxation can undergo ultradrawing or solid-state processing[6-8]. M.B. Johnson[9]has shown that the αc relaxation TPX is relatively large in magnitude and breadth and the entire temperature range is between Tg and Tm. Then it is possible for TPX to be made into microporous membrane.
Many people have researched the processing condition effect on properties of cast film and membrane. Sang-Young Lee et al.[10] analysis the processing-structure–property relationship of HDPE precursor films which is the relationship of melt extension and annealing temperature – lamellar crystalline structure – hard elasticity; and it is found that the cast films with high stress levels could form well-developed membrane, which have thicker lamellar, superior air permeability and hard elasticity of cast films. Caihong Lei et al.[11]reported that the orientation, elastic recovery, and lamellar lateral dimension were improved with melt draw ratio increasing to 120; and the porosity of corresponding membrane was increased from 37.8-45.5%; If the MDR was increased further to 200, the long period, crystalline phase thickness, crystallinity, and lamellae cluster size were constant value; however, the orientation was improved in MDR range, which was the key factor to affect membrane properties. Pierre J. Carreau et al.[12] indicated that the higher draw ratio, the better crystal orientation. A higher crystallinity of sample was, a better the crystallites orientation were reported by H. Shanak et al.[13].
In this work, we prepared different draw ratios cast film. Small-Angle X-ray Scattering (SAXS) and Wide-Angle X-ray Scattering (WAXD) were used to characterize the lamellae structure and orientation. The morphology and average pore size of stretched membrane were measured. Our aim is to figure out the relationship among the orientation, lamellar structure, elastic recovery, and stretched membrane property.
Experimental
Material. The Poly(4-methyl-1-pentene) ,TPX, was purchased from Mitsui Chemicals America, Inc. Its melt flow rate (MFR) value is 26 g/min (P=5kg, 260 oC). The molecular weights are Mn=84318 g/mol, Mw=167806 g/mol, the dispersity index (PDI) is 1.99 with measured by a GPC. The melting peak point (Tm) is 232 °C which is measured by differential scanning calorimetry (DSC;TA-Q20, United States) at a rate of 10 °C/min,.
Preparation of TPX films
The cast film was prepared by cast extrusion under controlled processing conditions, and the apparatus was equipped with a slot die (Fig.1). In the extrusion process, to obtain the oriented crystalline structures, the effect of uniaxial (machine direction, MD) stretching to TPX melt was obvious. The cast extrusion was equipped with a 0.2 mm thick and 32 cm width slot die and purchased from Shanghai Kechuang Rubber Plastic Mechanical Equipment Co., Ltd. The die temperature was set at 250 oC and roll rate were carried out at 7, 8, 9 and 10 rpm respectively. During extrusion, the extrude velocity of the die exit was constant, so the take-up speed was important for the calculation of draw ratios. Since the melt will retract before cooling, the draw ratio was calculated by the die thickness and the film thickness. Then we mark the sample as R-5.42, R-8.33, R-9.09 and R-10.00.
Fig.1 Schematic diagram of extruding-casting machine.
Analysis
Calorimetric properties of cast films were characterized by differential scanning calorimeter (DSC TA Q20). Operating temperatures of each sample was from 40 oC to 270 oC at a rate of 20 oC min-1 with dry nitrogen gas during the measurements. The fusion enthalpy of a perfectly crystalline TPX was 61.89 J/g[14].
X-ray diffraction patterns were measured on a DX-1000 automatic diffractometer operating at a step size of 0.02 with nickel-filtered Cu Ka radiation.
The elastic recovery can be used as an index to represent the structural arrangement and the hard elasticity of lamellar crystalline structure, which was measured by Instron 5500R machine with a environmental chamber at a deformation rate of 50 mm/min at 55 oC and the sample was strained to 50%. Finaly, the elastic recovery (ER 50) was calculated by the following equaton[7, 10, 15]：
ER(%)=(L-L^’)/(L-L_0 )×100%
where L0 was the initial length of the film without extension, L was the length when strained to 50%, and L’ was the length at the end of extension.
Scanning electron microscopy (SEM; JEOLJSM-5900LV, Japan) was used to observe lamellar crystalline morphology. To remove the amorphous part, the cast films were soaked in a solution at 80 °C, which was a mixture with 80 mL water, 20 mL concentrated sulfuric acid mixed and 50 g chromium trioxide. The etching time was 4h, and the samples were rinsed off and washed with distilled water then ultrasonic 5 min.
Small- and Wide-Angle X-ray Scattering
SAXS and WAXD were carried out at the beamline BL16B1 of the Shanghai Synchrotron Radiation Facility (SSRF, Shanghai, China). The energy of the X-ray radiation was 3.5 GeV and its wavelength was 0.124 nm. The sample-to-detector distances were about 1820 mm and 136 mm for SAXS and WAXD, respectively. The distances were calibrated with the silver behanate scattering peaks. The data of sample acquisition time for each pattern was 40s and 10 s for SAXS and WAXD, respectively. Data processing was performed with the computer program “Fit2D”[16].
Results and discussion
Influence of DR on the Crystallization of Cast Films. In this work, the different draw ratios cast films were obtained by melt extrusion process. Fig.1 shows the DSC melting curves of cast films with different draw ratios. All samples have a main melting peak and a shoulder peak, which demonstrates that there are two sizes lamellar in the structural arrangement. With DR increasing, the shoulder peak become weak and melting points have no significant change. However, compared with the film at DR of 5.41, the crystallinity (Table 1) of cast films is decreased. In the previous report, stretching can induce crystallization of PP, with the DR increasing, more molecular chains are arranged to lattice, so the crystallinity of PP are improved. However, the DR influence on TPX is opponent. As we all known that the cooling time of melt from die to the roll becomes shorter with increasing DR. and compare to PP, the non-isothermal crystallization of TPX is relatively fast. The larger DR applying to melt, the greater stress field to molecular chains, which have more opportunities to arrange along the direction of stress. However, the cooling time become short, then the number of molecular arranging into lattice is limited. Therefore crystallization properties of TPX itself play a dominant role in the non-isothermal crystallization process, then the crystallinity of cast films go down with increasing DR.
Fig.1 The DSC curves of RT18XB cast films with different draw ratio.
Table1 Melting parameters of cast films with different draw ratios
DR Tm(oC) Crystallinity
5.41 232.38 0.5893
8.33 233.02 0.5749
9.09 233.78 0.5692
10.00 233.32 0.5675
Fig.2 demonstrates that the WAXD curves of RT18XB cast films with different draw ratios. The WAXD is a useful method to analysis the crystal structure. The WAXD curve of R-0 is the sample that is obtained by compression-molded. It is apparent that each characteristic diffraction peaks are obvious. Compared with R-0, the pronounced change is that the intensity of 200 with the cast film at different DR become weak, and some characteristic diffraction peaks, (131)(122)(321), even disappear. It is probable that the stress field is not conductive to the growth of the crystal plane.
Fig.2 The WAXD curves of RT18XB cast films with different draw ratios.
Fig.3 The work recovery and recovery ratio of cast films with different draw ratio.
Hard elastic has a good elastic recoverability. The work recovery was the ratio between the work of unloading elastic deformation and the total work done during the elastic extension in the loading-unloading cycle, which is related with the energy consumption during recovery process. And if the ratio is bigger, the energy consumption is not obvious. The work recovery and recovery ratio of cast films at different draw ratio were shown in Fig.3. It can be seen that the elastic recovery and work recovery increases apparently.
Deformation mechanism of cast films with parallel lamellar crystalline structure is mainly the lamellar deformation; When the lamellar network is destroyed slightly and recovers well, improving DR is benefit to the orientation of lamellar. At the same time, increasing the number of tie chains in the amorphous and reducing the density of chain entanglement contribute to hard elastic of materials. In the previous report, lamellar is distortion in the low stress field and entanglement density become big, which will reduce hard elastic of materials [15, 17, 18]. Fig.4 shows the lamellar morphology of cast films at different DRs with etching the amorphous. At DR of 5.41 and 8.33, the irregular lamellar structure is obvious and most is distortion. When the DR becomes higher, most lamellar structures are parallel to each other, which is perpendicular to the MD. This is consistent with the above report.
Fig.4 The SEM of TPX cast films with draw ratios.
Fig.5 The difference in two cycles stretching experiment of RT18XB cast films at different draw ratios.
In the loading-unloading cycle, when the extension is stopped at strain 50%, the stress value drops immediately, which is related with Tie chains and lamellar network according to Elyashevich’s research[15]. The difference in two cycles stretching experiment of cast films at different draw ratios were shown in Fig.5. In the process, the number of tie chains is small in this work, which is negligible in comparison with the lamellar network contribution to stress drops. The difference value obviously increases as draw ratios, which is dependent on perfect network of lamellae.
2D-SAXS measurement was an effective characterization method to investigate the lamellar structure of cast films at different draw ratios. Fig.6 shows SAXS patterns of precursor films with different draw ratios. The stress induces meridional signal along the qy direction at polar zone, which is the parallel lamellar structure.
One-dimensional electron density correlation function (EDCF) can be denoted as K(z), which is a powerful tool in revealing the average thickness of the amorphous and crystalline regions of the two-phase systems along the meridianal direction, where K(z) as follows[19-23]:
K(z)=(∫_0^∞▒〖I(q_y)cos(q_y z)dq_y 〗)/(∫_0^∞▒〖I(q_y)dq_y 〗)
where z denotes the drawing direction. And q_y^2 to q_y is not multiplicated because of the anisotropic orientation of the lamellar in the samples[19, 24, 25]. SAXS correlation functions for cast films with different draw ratios were shown in Fig.7. The inset in Fig.7 demonstrates that how the average thickness of crystalline phase layer (dc) and long period (dac) are derived. Because the crystallinity of the sample in this work is lower than 60%, we regard the small value as crystalline phase layer. The average thickness of amorphous layer is calculated by the equation d_a=d_ac-d_c. A linear crystallinity (φ_(c,l)=d_c⁄d_ac ) can be defined by the crystalline layer and long period[23]. The value of dc, dac, da and φc,l are shown in table 2. With the draw ratio increasing, the value of dc, dac and da decreased, but the linear crystallinity is improved.
In the previous report, the narrowing SAXS pattern along the equatorial direction is derived from the increase of lamellae lateral dimension[26]. The lateral size can be derived from the width Δqx of peaks at half height in equatorial direction according to:
L_lateral=2π/〖∆q〗_x
The curves of I(qx) are fitted with two Lorentz functions as shown in Fig.8b. The width of the resulting Lorentz function (Δqx) is related to the lateral size of lamellae[22, 27]. 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 the crystalline lamellae[22]. The lateral size with increasing draw ratio is shown in table 2. It is apparent that the data is not changed.
Fig.6 SAXS patterns of precursor films with different draw ratios.
Fig.7 SAXS correlation functions for cast films with different draw ratios. The long spacing (dac) and the lamellar thickness (dc) can be obtained from the correlation function as shown in this figure.
Fig.8 (a)SAXS pattern integration area and (b) fit procedure used for the evaluation of the lamellae lateral size. (c) SAXS: azimuthal scan of the lamellar peaks along qx at different MDR values
Table 2 Lamellae Structure Parameters of samples with different draw ratios.
DR dc (nm) dac(nm) da(nm) φc,l Llateral(nm) f200
5.41 10.40 25.33 14.93 0.4106 42.11 0.094
8.33 10.47 24.76 14.29 0.4230 42.81 0.126
9.09 10.42 24.85 14.43 0.4352 41.84 0.130
10.00 9.79 23.12 13.33 0.4228 42.12 0.147
Fig.9 illustrates the 2D-WAXD pattern s of cast films with different draw ratios. The sharp diffraction arcs instead of rings are seen for crystallographic planes (200) cast films, indicating the presence of oriented lamellar structure[28]. The intensity of the (200) crystalline plane is plotted as a function of the azimuthal angle. For uniaxially oriented films, the set-of-plane typically followed is the (200)[9, 29]. The orientation value, f200 is calculated by the Hermans’ orientation function :
f_200=(3∙〖cos〗^2 θ_200∙(〖sin〗^2 φ_200 ) ̅-1)/2
Where the angle φ200 is the azimuthal dependence of the scattered intensity for the (200) reflection. The quantity (〖sin〗^2 φ_200 ) ̅ can be determined according to the following equation:
(〖sin〗^2 φ_200 ) ̅=(∫_0^(π⁄2)▒〖I(φ)〖sin〗^2 φ_200 〖cosθ〗_200 dφ〗)/(∫_0^(π⁄2)▒〖I(φ)〖cosθ〗_200 dφ〗)
where I(φ) is the relative intensity at the angle ψ200 for the (200) reflection. And (〖sin〗^2 φ_200 ) ̅ may be approximated by measuring the half width of the (200) reflection[9, 30, 31]. The 200 orientation values (f200) of cast films are shown in table 2. It is found that the f200 increase with draw ratio apparently, implying that higher draw ratio induces better orientation.
Fig.9 The 2D-WAXD paterns of cast films with different draw ratios.(a) DR=5.41,(b)DR=8.33,(c)DR=9.09,(d)DR=10.00.
In previous report of study on draw ratio, with DR increasing the crystallinity of cast films is improved or it is a constant value[11, 32] and the orientation value is enhanced. However, for TPX this material the higher draw ratio, the lower crystallinity. Conversely, the cast film with high draw ration has a high orientation value. In my work, the crystallization rate of TPX is relatively fast, which is faster than that of polypropylene (PP). The TPX melt flows out from the die, it will cool down after roll. Melt solidification time become shorter with draw ratio increasing. Molecular chains did not have enough time to arrange into crystal lattice, resulting that the crystallinity of cast film decreased during this process. However, in the greater stress field, the parallel lamellae will arranged more orderly perpendicular to the direction of stress.
In order to compare the characteristics of the membrane with different draw ratios, Fig.10 shows the surface morphologies of the microporous membranes obtained by stretching cast films with different draw ratios. It is obvious that the pore number becomes more as draw ratio increases. But the pores morphology is not perfect, which is due to the lamellae orientation at low stress field. The pore size distributions and average pore size of membranes prepared by cast films with different draw ratios are shown in Fig.11 and Fig.12. Obviously, the stress levels are an important factor, which affect the pore number and morphology by stretching. The average pore size increased with increasing draw ratio excluding DR 5.41.
Fig.10 Surface morphologies of the membranes obtained by stretching cast films with different draw ratios.
Fig.11Pore size distribution of membranes prepared by cast films with different draw ratios.
Fig.12 The average pore size of membranes prepared by cast films with different draw ratios.【透气性与孔径一起作图】
Conclusion
In this work, the lamellae structure of TPX cast films with different draw ratios and corresponding membrane properties were discussed. With draw ratios increased, the crystallinity of cast films is reduced, however, the linear crystallinity is improved. And all samples have the same crystalline form. The elastic recovery and stress difference are increased, which result from the improved orientation. Crystalline phase layer and long period are reduced and the lamellae lateral dimension does not change. The pore number and average pore size are increased. This work investigates the effect of stress levels on the crystal structure and corresponding membrane.
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