Influence of Melt-Draw Ratio on the Crystalline Structure and

Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, School of Materials and Energy, ... Publication Date (Web): March 4, 2015 ...
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Influence of Melt-Draw Ratio on the Crystalline Structure and Properties of Polypropylene Cast Film and Stretched Microporous Membrane Ruijie Xu, Xiande Chen, Jiayi Xie, Qi Cai, and Caihong Lei* Guangdong Provincial Key Laboratory of Functional Soft Condensed Matter, School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, PR China ABSTRACT: The orientation and crystallization during melt stretching were characterized, and their influence on the lamellar morphology and stretched polypropylene pore structure was clarified. During melt stretching, the MDR range from 40−200 could be divided into two regions. In region I, MDR below 120, the crystalline morphology transformed from ellipsoid spherulites to lamellae structure, and the orientation, elastic recovery, and lamellar lateral dimension were enhanced. The porosity of corresponding stretched microporous membrane was increased from 37.8−45.5%. In region II, with the MDR increasing to 200, the orientation and lamellae lateral dimension were increased, but the elastic recovery did not change much. The porosity of corresponding microporous membrane was improved further to 60.3%. The long period, crystalline phase thickness, crystallinity, and lamellae cluster size were kept constant within the whole MDR range, but the orientation was improved from 0.23 ± 0.02 to 0.41 ± 0.03. Apparently, the orientation induced the increase of lamellar lateral dimension, and it was the main factor deciding the properties of stretched microporous membrane. materials,12−14,18−21 such as molecular weight and molecular weight distribution, and applied processing conditions22−25 including temperature of die, chill roll and thermal-setting rolls, the velocity difference between the extruder, and take-up rolls and air cooling. Sadeghi12 et al. found that the resin with high molecular weight had a tendency to form a planar crystalline morphology as the draw ratio increased. Johnson21 reported that the long melt-relaxation time determined the lamellae structure in the cast film. Compared with the material parameters, the applied process condition, especially the meltdraw ratio (MDR), is more important for preparing the initial cast film with row-nucleated crystalline structure. Many studies have focused on the effect of MDR on the properties of initial film and stretched microporous membrane. Tabatabaei et al.26,27 gave the microporous membrane structure and corresponding properties change in a narrow MDR ranges from 60−90. The orientation and the tensile properties along the machine direction were increased with the MDR improving. Sadeghi16 indicated that the lamellae stack crystals appeared with increasing MDR. Also during melt extrusion process, with increasing MDR from 0 to 77, the poly(vinylidiene fluoride) crystals transformed from spherulites to lamellae structure.17 Accompanying with the crystalline morphology transformation, the orientation and crystallization are also affected. In early years, the linear relationship between orientation and draw ratio during spinning process was reported.28 Zhou et al.29 reported that the overall orientation of PP α-phase crystal induced by elongational flow was significantly enhanced with increasing MDR. The crystallinity and crystal size were both

1. INTRODUCTION In 1960, the polyoxymethylene springy fiber and polypropylene (PP) filament with hard elastic behavior were first fabricated by the inventors at Celanese and DuPont company.1,2 Later, many new materials were used to fabricate hard elastic materials such as polypivalolactone, nylon, polyethylene (PE), poly(iso-butene oxide), etc.3−6 It was found that by stretching hard elastic materials, some micropores could be formed, and these micropores would be enlarged to 200 nm after stretching 100%.7,8 In 1969, Zimmerman9 proposed a melt-stretching method to fabricate microporous membrane. In 1981, the first commercial PP microporous membrane was used in the Li-ion battery field as a separator by Celgard. Later, this method was also used to fabricate PE microporous membrane. Compared with PE, PP shows higher melting point, and its microporous membrane exhibits better dimensional stability at higher temperature when it is used in the Li battery. But because of the technical security, some related work is limited and owned by the above companies. Detailed work is needed to clarify the relationship among the materials, processing, structure, and properties. On the basis of melt-stretching method, the fabrication process covers three stages:10,11 (1) production of the precursor film with prerequisite row-nucleated lamellar crystalline structure, contributed by stress-induced crystallization, (2) annealing to thicken the lamellae and improve lamellae orientation and uniformity, and (3) stretching of the film at low temperature for pore creation and then at high temperature to enlarge the pores. During the whole process, the first stage is most important for the control of stretched pore distribution and permeability. The initial lamellae structure might originate from shish-kebab structure12−15 or the spherulitic deformation.16,17 There have been many detailed works about the influence of physical properties of raw © XXXX American Chemical Society

Received: January 16, 2015 Revised: March 3, 2015 Accepted: March 4, 2015

A

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speed of 50 mm/min was applied. After stretching, the films were heat-set at 145 °C for 5 min to improve dimensional stability. Here, the heat-setting temperature was set at 145 °C since it has been found that the corresponding microporous membrane showed best air permeability and dimensional stability.36 For FTIR spectroscopy, spectra were recorded on a Nicolet Magna 860 FTIR instrument from Thermo Electron Corp. (DTGS detector, resolution 4/cm, accumulation of 128 scans). The beam was polarized by means of a Spectra-Tech zinc selenide wire grid polarizer from Thermo Electron Corp. The dichroic ratio, R, was calculated by the ratio of the absorbance from beams polarized parallel (A//) and perpendicular (A⊥) to the melt extrusion direction. Then the data were evaluated in degree of orientation, f:

increased.30,31 The mechanical properties, such as elastic modulus, yield strength, and stress-hardening, of the oriented polymer increased synchronizedly with increasing MDR.17,32 For the stretched microporous membrane, Lee33 et al. reported that the higher the MDR was, the higher the elastic recovery of cast film, and the better the air permeability of stretched microporous membrane. Olifirenko34 indicated that with increasing MDR, the pore number and sizes of HDPE/oxidized PE microporous membrane were increased. It can be seen that MDR induces the crystal transformation and the increase of orientation. In addition, the lamellae thickness, lamellae lateral dimension, lamellae cluster size, and tie chain fraction might also be influenced by melt-stretching field. However, up to now, no such work has been carried out to build the relationship between MDR and the corresponding lamellae parameters. We want to know, during the stressinduced crystallization process, how the orientation and lamellar parameters influence the property of initial film such as elastic recovery and whether the orientation or lamellae parameters determine the elastic recovery and the property of corresponding stretched microporous membrane. In this article, the PP cast films were prepared at different melt-draw ratios. The orientation and lamellae structure parameter were characterized using Fourier transform infrared (FTIR) and two-dimensional small-angle X-ray scattering (2D SAXS). The structure and properties of stretched microporous membrane were characterized. The relationship among the orientation, lamellar structure, elastic recovery, and stretched pore property was built.

R=

f=

A// A⊥

(1)

R +2 R−1 × 0 R+2 R0 − 1

(2)

R0 was given by R 0 = 2 cot2 Ψ

(3)

where Ψ was the angle between the polymer chain axis and the transition moment of the investigated absorption band. Here, the 998 cm−1 band corresponded to the crystalline phase; the angle Ψ was 18°.37 A PerkinElmer DSC 7 was used to measure the melting curves of cast films from 80−190 °C at a heating rate of 10 °C/ min. The crystallinity was calculated as

2. EXPERIMENTAL SECTION Material. A homopolypropylene resin with a melt flow rate value of 2.0 g/10 min (under ASTM D 1238 conditions of 230 °C and 2.16 kg) from Yangzi petrochemical company, China, was used. The melting peak point (Tm) and crystallinity, obtained from differential scanning calorimetry (DSC; PerkinElmer DSC 7, MA, United States) at a rate of 10 °C/ min, were 164.2 °C and 39.0%, respectively. The reported crystallinity results were obtained using a heat of fusion of 209 J/g for fully crystalline PP.35 The weight-average molecular weight and polydispersity index, measured using a GPC (Viscotek model 350) at 140 °C and 1,2,4-trichlorobenzene (TCB) as a solvent, were about 754 kg/mol and 6.29, respectively. The xylene soluble content at room temperature was less than 4%. Cast Film and Stretched Microporous Membrane Preparation. The cast film was prepared by cast extrusion through a T-slot die followed by stretching and thermal-setting. During extrusion, the uniaxial (machine direction, MD) stretching was applied to PP melt, which resulted in the oriented crystalline structures. The die temperature was set at 210 °C. The MDRs were set at 40, 80, 120,160, and 200, respectively. The draw ratio was determined by the take-up speed since the extrude velocity at the exit of the die was constant. The films were produced at chill roll temperature of 80 °C. An air knife with dimensions of 1.5 mm opening and 200 cm width was mounted close to the die to provide cooling to the film surface right at the exit of the die. The prepared cast film was annealed for 30 min at 145 °C in a hot oven. The stretching of the annealed PP film was taken using an Instron 5500R machine equipped with a heating chamber. The annealed film was first stretched to 25% at room temperature and then stretched to 100% at 130 °C. A drawing

Xc (%) =

ΔHm ΔHm 0

× 100 (4)

where ΔHm was the measured value of fusion enthalpy and ΔHm0 was the fusion enthalpy of a perfectly crystalline PP, that was, 209 J/g.35 The elastic recovery, as a practical and simple method to characterize the property of cast films, was tested using an Instron 5500R machine at a deformation rate of 50 mm/min. It was determined along stretching direction of the film. The percent elastic recovery (ER %) was calculated by the following equation: ER (%) =

L − L′ × 100 L − L0

(5)

where L0 was the initial length of the film before extension, L was the length when strained to 100%, and L′ was the length at the end of extension. 2D SAXS measurements were performed using synchrotron radiation with λ = 0.154 nm at Beamline 1W2A of Beijing Synchrotron Radiation Facility (Beijing, China). Mar165-CCD was set at 5000 mm sample−detector distance in the direction of the beam for SAXS data collections.38 The surface morphology of cast films and stretched microporous membranes was characterized by scanning electron microscopy (SEM; S3400−N, Hitachi, Japan). The cast films were etched with a solution of 800 mL of water and 200 mL of concentrated sulfuric acid mixed with 125 g of chromium oxide at 70 °C. The etching time was 25 min, and the samples were rinsed off and washed with distilled water. All B

DOI: 10.1021/acs.iecr.5b00215 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research the samples were sputter coated with platinum for 300 s before examination. The air permeability of stretched microporous membranes was characterized by Gurley Densometer model No.4150 (Gurley Precision Instruments, New York, USA) according to ASTM D726. The Gurley value was defined as the time required for a specific amount of air (100 cc) to pass through a specific area of the membrane under a specific pressure (here, 20 kgf/cm2) where low Gurley value corresponded to high air permeability. The porosity was measured using liquid absorption methods according to American Society for Testing and Materials (ASTM) D-2873. The Gurley value and porosity were given based on testing of three samples.

Table 1. Air Permeability and Porosity of Stretched Microporous Membranes melt-draw ratio

porosity (%)

Gurley value (s/100 cm3)

40 80 120 160 200

37.8 39.6 45.5 54.1 60.3

2345 469 236 192 188

stretched membrane depends on the porosity, membrane thickness, and pore tortuosity. Here, the improvement of air permeability may come from the increase of porosity and the decrease of membrane thickness. The obtained membrane thickness at MDR of 40 is about 40 μm, whereas that at MDR of 200 is about 12 μm. The improvement of pore tortuosity induced by higher MDR can also not be neglected. The results in Table 1 indicate the apparent influence of MDR on membrane porosity. Influence of MDR on the Crystallization of Cast Films. During melt-stretching process, crystallization and orientation are two important factors affecting the obtained initial crystalline morphology and the stretched pore structure. The above results have shown that different pore structure can be obtained at different MDR values. Then, we want to know whether the crystallization or the orientation is the main reason inducing the different crystalline and pore structure. Figure 2 shows the surface morphology of cast films prepared at different MDRs after the amorphous regions are etched off.

3. RESULTS AND DISCUSSION Influence of MDR on the Structure and Properties of Stretched Microporous Membrane. It is well-known that the initial structure and property of cast film determine the pore structure of stretched microporous membrane. The pore structures of stretched microporous membrane, which was prepared at different MDRs and first stretched at room temperature to 25% and then stretched at 130 °C to 100%, are shown in Figure 1. It is apparent that all the cast films can be

Figure 1. SEM of stretched microporous membrane prepared at different MDRs and stretched at room temperature to 25% and then at 130 °C to 100%.

stretched to form pore structure, but apparent difference exists. At MDRs of 40 and 80, the twisted lamellae arrangement, nonuniform pore size, and some closed pore areas can be observed on the surface, whereas at other MDR values, apparent uniform connecting bridges and lamellae structure are seen. On the basis of the surface morphology of stretched microporous membrane, the MDR range can be divided into two regions. Within region I (MDR from 40−120), pore structure can be formed by stretching, but there are still amount of defects on the surface. These defects are decreased within region II (MDR from 120−200). The properties of stretched microporous membranes are listed in Table 1. With increasing MDR, the porosity is increased from 37.8% to 60.3%, and the Gurley value is decreased from 2345 to 188 s/100 cm3. The air permeability of

Figure 2. Crystalline morphology of cast films prepared at different MDR values.

At MDR value of 40, the ellipsoid spherulites with different size and irregular lamellae cluster crystals can be observed. At MDR value of 80, a few small ellipsoid spherulite crystals exist among some lamellae, as shown in the areas marked by arrows in Figure 2. When the MDR value is higher than 120, only the row-nucleated morphology can be observed, which is perpendicular to the melt-stretching direction. Sadeghi23 et al. C

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Industrial & Engineering Chemistry Research mentioned that when the crystallization rate induced by stretching was shorter than the mean relaxation time, the crystalline morphology showed tendency to form the lamellae structure. Similar crystallization transition process was also reported by Hu17 and Lee33 during PVDF and PE meltstretching extrusion processes. Figure 3 gives the DSC curves of cast films prepared at different MDR values. Compared with the film prepared at

Figure 3. DSC curves of cast films prepared at different MDR values.

MDR of 40, the pronounced change is the main melting peak moving to low temperature within region I. The crystalline morphology transition shown in Figure 2 is the main reason for this phenomenon. Within region II, the main melting point does not shift, but a weak right shoulder appears, and the end point shifts to higher temperature. It is probable due to the formation of more perfect crystals or some shish-like crystals. Although the crystalline morphology shows large difference, the crystallinity at different MDRs is nearly same, about 48.6%. This means that MDR has little effect on crystallinity. To further investigate the lamellar structure of cast films at different MDR values, 2D SAXS testing was used. Figure 4 gives the SAXS patterns of cast films prepared at different MDR values. It is apparent that the melt stretching induces pronounced meridional pattern along the qy direction, which indicates the formation of row-nucleated crystalline structure. Contrary to the SEM results, the SAXS pattern shows no pronounced differences within regions I and II. Although at MDR value of 40, there are many large spherulites, the lamellae within the spherulites still keep highly oriented. With increasing MDR from 40 to 120, the scattering intensity increases and then decreases with further increasing MDR to 200. The increase is due to the higher periodicity of the formed structure. The crystalline morphology transition within region I explains this behavior. According to this, the intensity should increase within region II, although the structure change is small. Here, the intensity decrease is due to the smaller quantity of crystalline structure in these films, since with increasing MDR from 120 to 200, the cast film thickness is decreased from 30 to 18 μm. Along the meridional direction, the scattering curves of cast films prepared at different MDR values are shown in Figure 5. On the basis of the two phase model, two parameters to describe the lamellar structure are derived, namely, the long period L

Figure 4. 2D SAXS patterns of cast films prepared at different MDR values.

L = La + Lc

(6)

where La and Lc are the average thickness of amorphous and crystalline layers, respectively. The long period of lamellae stacks along the melt stretching direction can be obtained by considering the scattering intensity distribution along the qy direction I(qy), which is obtained by meridional scans along qy for certain intervals of qx. The profiles of I(qy) at different MDR values are shown in Figure 5, panel a. The long period is obtained from the peak position of I(qy) according to Bragg’s law: L=

2π qy,max

(7)

It can be seen that the scattering peak position does not change much for different MDR values. The average thickness of amorphous and crystalline regions along the meridional direction can be derived from the one-dimensional correlation function of the electron density distribution in the lamellar stacks K(r) as follows:39−42 ∞

K (r ) =

D

∫0 I(qy) cos(qyr ) dqy ∞

∫0 I(qy) dqy

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Figure 5. SAXS: one-dimensional scattering intensity distribution along the melt-stretching direction at different (a) MDR values, (b) correlation functions, and (c) plots of Porod’s law. The long period (L) and the average crystalline phase thickness (Lc) can be obtained from the correlation function as shown in the inset of panel b.

where r is the distance perpendicular to the lamellar surface. The Lorentz correction where I(qy) is multiplied by qy2 is not applied because of the anisotropic orientation of the lamellae in the samples.39,41,42 Figure 5, panel b shows the one-dimensional correlation function along qy. The inset in Figure 5, panel b shows how the average thicknesses of crystalline phase layer (Lc) and long period (L) are derived. The crystallinity of cast films is lower than 50%. Hence, the small value is assigned to be the average crystalline phase layer thickness. The values of L and Lc at different MDR values are listed in Table 2. Within

increase long period but shows little effect on the lamellae thickness. In fact, in the semicrystalline polymer system, the diffuse transition layer exists between the crystalline and amorphous phase. There are two general approaches to estimate diffuse transition layer thickness (Ld) from SAXS data. One is using the one-dimensional correlation function (1DCF) based on a linear profile of electron density,41,43−45 and the other is using a modified Porod’s law.46,47 Compared with the Porod’s method, the 1DCF result is affected by the intensity profiles, extrapolation, and the statistical stability of the intensities in the high-q region.48 For the ideal two-phase model with sharp boundaries at the crystal−amorphous interface, the Porod’s law can be used to describe the asymptotic behavior of the background-subtracted SAXS curves at the high-q region:46

Table 2. Lamellae Structure Parameters of Cast Films Prepared at Different MDR Values MDR

L (nm)

Lc (nm)

La (nm)

Ld (nm)

40 80 120 160 200

11.7 11.4 12.1 12.2 12.2

4.74 4.75 4.73 4.75 4.74

7.01 6.65 7.37 7.45 7.46

2.39 2.65 2.36 2.05 2.02

lim q 4I(q) = Kq

(9)

q →∞

Here, Kq is the Porod constant. The Porod’s law can be written as46 lim I(q) = Kq

q →∞

region I, the long period is decreased slightly due to the crystalline morphology transformation, and then the long period is increased and stabilized at about 12 nm. The crystalline phase thickness shows nearly no change within regions I and II. Thus, the melt extrusion process can slightly

exp( −σ 2q2) q2

+ Ifl

(10)

where σ is the thickness of the sigmoidal-gradient electron density transition layer of crystal−amorphous interface, and Ifl is the background intensity resulting from thermal density fluctuations. After deducting the background intensity, the plot

Figure 6. (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. E

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pronounced change of lamellae lateral size but does not lead apparent change to long period and lamellar thickness. Influence of MDR on the Orientation and Elastic Recovery of Cast Films. During melt-stretching, the stretching flow induces the formation of row-nucleated crystalline structure. Elastic recovery is an important technical parameter to characterize the properties of cast films with rownucleated crystalline structure. Figure 8 gives the crystalline

of Porod’s law appears negative deviations. So, the diffuse transition layer thickness can be approximately calculated as49 Ld = σ ≈

2π ( −k)

(11) 4

4

where k is the slope of Iq versus q at high-q region after deducting the thermal density fluctuations. When calculating the slope, the boundaries of the high-q region should be q > 1/ L, where L is the minimal thickness of the Lc or the La.50 The plots of Porod’s law are shown in Figure 5, panel c, and the boundary layer thicknesses are also listed in Table 2. Contrary to the crystalline phase thickness, the Ld is increased within region I and deceased within region II. The crystalline morphology transformation induces the transition layer to be slightly increased. Within region II, higher MDR induces more perfect crystal, which results in thin transition layer. Along the equatorial direction, with increasing MDR, the SAXS patterns exhibit a pronounced narrowing. Such a narrowing is due to the increase of lamellae lateral dimension.51 The lateral size can be derived from the width Δqx of the peaks at half height in the equatorial direction according to 2π L lateral = Δqx (12) Figure 6 shows azimuthal scans of the lamellar peaks along qx at different MDR values. The average lateral dimension of the lamellae is obtained from the profiles of the intensity distribution along the straight line, I(qx). The curves of I(qx) are fitted with two Lorentz functions as shown in Figure 6, panel b. The width of the resulting Lorentz function (Δqx) is related to the lateral size of the crystalline lamellae.39,51−53 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. The data in Figure 7 indicate an obvious enlargement of lamellae lateral size with increasing MDR. It is surprising to find

Figure 8. Crystalline orientation degree and elastic recovery of cast films prepared at different MDR values.

orientation degree and elastic recovery value of cast films prepared at different MDR values. It can be seen that within region I, the crystalline orientation degree increases apparently from 0.23 ± 0.02 to 0.37 ± 0.02, and the elastic recovery increases from 43.4 ± 3.1% to 83.7 ± 7.1%. Within region II, the crystalline orientation degree increases from 0.37 ± 0.02 to 0.41 ± 0.03, and the elastic recovery does not change much. It is apparent that higher MDR induces better orientation. Influence of MDR on the Lamellae Cluster Size and Tie Chains Fraction of Cast Films. In the semicrystalline polymer, the extended tie molecules connect each mosaic block to assemble the lamellae cluster. The lamellae clusters as the structure unit bears the external force. The probability that a chain penetrates a stack of layers of thickness L composed of n amorphous layers with the thickness nLa and (n + 1) crystalline lamellae with thickness (n + 1)Lc can be calculated according to the Huang−Brown scheme54−56 by ∞

P(L) =

Figure 7. Lateral lamellar size of cast films prepared at different MDR values.

∫L r 2 exp[−3r 2/2⟨r 2⟩] dr ∞

∫0 r 2 exp[−3r 2/2⟨r 2⟩] dr

(13)

where ⟨r2⟩ is the mean square end-to-end distance of the chain. Because the probability density function, ρ(L) is obtained by the derivative of P(L) with L:

that the lateral size is increased apparently from 19.1 ± 1.2 nm at MDR of 40 to 39.3 ± 3.8 nm at MDR of 120. Within region II, the lateral size is further increased to 54.6 ± 4.3 nm. The ordered arrangement of molecular chains is perpendicular to the melt-stretching direction, and these chains are gradually extended. Murthy51 et al. mentioned that during Nylon fiber spinning process, with increasing draw ratio, the lamellae length increased. It can be seen from SAXS results that MDR induces

3/2 4 ⎛ 3 ⎞ ρ (L ) = ⎜ ⎟ L2 exp[−3r 2/2⟨r 2⟩] π ⎝ 2⟨r 2⟩ ⎠

(14)

The weight-average lamellar cluster length, Lw, can be obtained as F

DOI: 10.1021/acs.iecr.5b00215 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Lw =

∫L L2ρ(L) dL ∞

∫x Lρ(L) dL

=

3 4

Relationship among MDR and Crystalline or Orientation Properties. During melt extrusion, the crystalline structure changes from spherulites to lamellae cluster. Table 4 lists the related crystalline and orientation parameters within regions I and II. Some studies considered12−15 that the shishkebab structure was formed during melt extrusion, and the others16,17 indicated that the lamellae cluster structure was derived via spherulitic deformation. In Figure 4, no equatorial scattering signal can be observed. This means no shish structure formed during extrusion process. The spherulitic deformation mechanism fits our SEM results in Figure 2. Within region I, the crystalline morphology changes with increasing MDR. The elastic recovery is improved from 43.4 ± 3.1% to 83.7 ± 7.1%, the orientation of crystalline phase is increased from 0.23 ± 0.02 to 0.37 ± 0.02, and the lateral dimension is extended to 39.3 ± 3.8 nm. But the crystallinity and crystalline phase thickness show no apparent change. The tie-chain fraction is decreased a little within the structure transformation area. Within region II, the elastic recovery is only increased from 83.7 ± 7.1 to 83.9 ± 7.7%. Obviously, the MDR shows no effect on the increase of elastic recovery within region II. Simultaneously, the crystallinity, crystalline phase thickness, and long period do not change. The lamellae lateral dimension is enlarged to 54.2 ± 4.8 nm at MDR of 160 but stops increasing even though the MDR is further increased. The lamellae structure change could not obviously induce the increase of elastic recovery. The crystalline phase orientation is directly increased to 0.41 ± 0.03. The tie-chain fraction is constant. It is apparent that the elastic recovery is determined by the orientation. The increase of orientation results in the improvement of lamellae lateral dimension in cast films. Since the secondary annealing and stretching technology during the third stage mentioned in the introduction to fabricate microporous membrane is same, the increase of orientation and lamellae lateral dimension in the cast film leads to the improvement of pore arrangement and the increase of porosity in stretched microporous membrane. It is the orientation, not crystallinity or lamellae thickness, that decides the structure and properties of cast films and stretched microporous membranes.

2π 2 3 2πC M w1/2 ⟨r ⟩ = b 3 4 3M 0 (15)

where M0 is the molar mass of the chain, C is the characteristic ratio, and b is the bond length. It is found that the values of Lw are proportional to the square root of the weight-average molecular weight and almost equal to ⟨r2⟩1/2. In the case of PP, the value of Lw can be estimated using the values of b = 0.154 nm and C = 5.4. The lamellae cluster length formed during melt extrusion, only decided by the weight-average molecular weight but not MDR values, is about 51.4 nm. The long tie chain penetrates about 4−5 lamellae layers to form a single lamellae cluster. The probability to form a tie molecular Ft can be determined by the following equation:56,57 ∞

Ft ≡

∫L r 2 exp(−βr 2) dr ∞ 2 r 0

3∫

2

=

exp( −βr ) dr

⎞ 2 ⎛⎜ 3 Γ , βL2⎟ ⎠ 3 π ⎝2

(16)

where Γ is the incomplete gamma function, and r is the end-toend distance of the random coil in the melt, β = (3/2r2̅ )1/2, where r ̅ is the root−mean−square of the end-to-end distance, and L = 2Lc+ La is the critical distance required to form a tie molecule. Then, the value of Ft can be considered to be the weight fraction of tie molecules. This is based on the experimental results of small-angle neutron scattering of PE58,59 and PP60 that the radius of gyration in the melt state does not change on rapid crystallization. The value of r ̅ is calculated from r ̅ = (Cnb2)1/2

(17)

where C is the characteristic ratio, 5.4; b is the bond length, 0.154 nm; and n is the number of bonds. In this work, according to Huang and Brown,56 the weight-average molecular weight Mw is used to determine the value of n in eq 17. The critical distance and tie molecular fraction are listed in Table 3. Similar to that of long period, the tie molecular fraction is slightly increased from 0.17 to 0.18 with MDR improving.

4. CONCLUSIONS The lamellae structure of PP cast films prepared at different MDR values and the corresponding microporous membrane properties were studied. On the basis of the stretched microporous membrane properties, the melt-draw ratio from 40−200 was divided into two regions. The crystallinity, long period, and crystalline phase thickness did not change much from region I to region II. Within region I (MDR from 40− 120), the spherulite and lamellae crystals coexisted, the elastic recovery was increased obviously, the crystalline phase

Table 3. Lamellae Parameter and Tie-Molecule Fraction of Cast Films Prepared at Different MDR Values MDR

long period (nm)

crystalline phase thickness (nm)

critical distance (nm)

tie-molecule fraction

40 80 120 160 200

11.7 11.4 12.1 12.2 12.2

4.74 4.75 4.73 4.75 4.74

16.49 16.15 16.83 16.95 16.94

0.172 0.165 0.179 0.180 0.181

Table 4. MDR, Elastic Recovery, Crystalline Parameters, and Orientation region I melt-draw ratio elastic recovery ER100 (%) long period (nm) crystalline phase thickness (nm) lamellae lateral dimension (nm) crystallinity (%) crystalline phase orientation tie-molecule fraction

40 43.4 ± 3.1 11.7 4.74 19.1 48.6 0.23 ± 0.02 0.172

80 76.8 ± 6.5 11.4 4.75 31.4 48.6 0.33 ± 0.02 0.165 G

region II 120 83.7 ± 7.1 12.1 4.73 39.3 48.6 0.37 ± 0.02 0.179

160 83.7 ± 7.6 12.2 4.75 54.2 48.7 0.38 ± 0.03 0.180

200 83.9 ± 7.7 12.2 4.74 54.6 48.6 0.41 ± 0.03 0.181

DOI: 10.1021/acs.iecr.5b00215 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Article

Industrial & Engineering Chemistry Research orientation was increased from 0.23 ± 0.02 to 0.37 ± 0.03, and the lamellae lateral dimension was enlarged from 19.1 ± 1.2 nm to 39.3 ± 3.8 nm. Within region II (MDR from 120−200), the elastic recovery showed no apparent change. The orientation was increased to 0.41 ± 0.03, and the lamellae lateral dimension was increased to 54.6 ± 4.3 nm. By comparing the change of crystalline phase thickness, orientation, and lamellae lateral length, it could be deduced that the crystalline phase orientation was the main factor influencing the structure and properties of cast films and stretched microporous membrane. Within region I, the increase of orientation and lateral length lead to the increase of porosity and elastic recovery. Within region II, the increase of orientation and lateral length resulted in the improvement of lamellar structure and further increase of porosity, although the elastic recovery was not further increased. This work clarifies the importance of orientation during the preparation of initial cast film on the structure and properties of stretched microporous membrane.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-20-39322570. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank National Science Foundation of China under Grant No. 51003017 for financial support. They also want to thank Shenzhen Senior Materials Company, Ltd. for generously supplying raw materials.

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DOI: 10.1021/acs.iecr.5b00215 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX