Property Development for Biaxial Drawing of Ethylene

Article pubs.acs.org/JPCB

Property Development for Biaxial Drawing of EthyleneTetrafluoroehtylene Copolymer Films and Resultant Fractural Behavior Analyzed by in Situ X‑ray Measurements Hiroki Uehara,*,† Yasunori Ono,† Masaki Kakiage,†,⊥ Takumi Sakamura,† Hiroyasu Masunaga,‡ Yasumasa Yukawa,§ Yoshiaki Higuchi,§ Hiroki Kamiya,∥ and Takeshi Yamanobe† †

Division of Molecular Science, Faculty of Science and Technology, Gunma University, Kiryu, Gunma 376-8515, Japan Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan § R & D Division, Asahi Glass Co., Ltd., Ichihara, Chiba 290-8566, Japan ∥ AGC Chemicals Americas, Inc. Exton, Pennsylvania 19341, United States ‡

S Supporting Information *

ABSTRACT: The property development of the ethylene-tetrafluoroethylene copolymer (ETFE) membrane induced by simultaneous biaxial drawing was investigated. Commonly, tensile strength can be increased by drawing; conversely, tear resistance decreases. In this study, the balance between tensile strength and tear resistance for the resultant ETFE membrane was optimized achieved by a combination of lamination of low molecular weight (LMW) and high molecular weight (HMW) layers and subsequent biaxial drawing. The structural factor determining tear resistance of these biaxially drawn membranes was determined based on in situ small-angle X-ray scattering (SAXS) measurement during tensile deformation simulating tearing tests. Lozenge shaped scattering, which indicated inclined lamellae, was observed during such tensile deformation of the resultant membranes. Remarkably, this inclined lamellar structure was observed for the pure LMW membrane; however, it also appeared at the interface between LMW and HMW layers within biaxially drawn membranes. For the membrane exhibiting the highest tearing strength, the fraction of such inclined lamella increased up to the critical strain corresponding to the actual sample breaking. These results confirm that the inclined lamellar structure absorbed strain during membrane tearing.

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structure. We13 have reported that uniaxial drawing can develop the tensile strength and yielding stress of ETFE films, due to the formation of extended chain crystals (ECCs) during drawing. Another important mechanical property of the membrane is tearing strength, which usually decreases along the drawing direction while exhibiting a remarkably higher value perpendicular to the drawing direction. Biaxial drawing effectively cancels these contrasting effects of molecular orientation on the tensile strength and tearing strength. The resultant biaxial orientation of molecules can often balance them. Therefore, biaxial drawing, including simultaneous or sequential techniques, is widely adopted for manufacturing polymer thin membranes of PE, 14−17 PTFE,18−20 poly(propylene),21−25 poly(ethylene terephtalate) (PET),22,26,27 poly(lactic acid),28−31 polyamide,32 polystyrene,33 and poly(oxymethylene).34 These biaxially drawn membranes are industrially utilized as solar cells or display

INTRODUCTION

Ethylene-tetrafluoroethylene copolymer (ETFE) has both excellent processability, attributed to polyethylene (PE), and superior heat-resistance, weather resistance, and chemical stability, similar to poly(tetrafluoroethylene) (PTFE). Therefore, ETFE sheets are used as stadia roofs and agricultural housing.1−7 For these applications, creep relaxes the initial tension with settling, causing undesirable membrane flapping in wind or rain. Therefore, thick ETFE sheets are still commercially used to prevent such creep.4 However, for stadia roofs or agricultural housing, a thinner membrane is preferred for high transparency and weight reduction, which reduce the construction and illumination costs. Therefore, both of creep restriction and membrane thinning are in demand for manufacturing of ETFE films. Also, ETFE is now widely used as a precursor of the proton-exchange membrane for fuel cells,8−12 where membrane thinning is advantageous for stacking membrane electrode assemblies, resulting in higher output power. A possible approach for this membrane thinning is introducing molecular orientation within the membrane © 2015 American Chemical Society

Received: September 9, 2014 Revised: February 12, 2015 Published: February 19, 2015 4284

DOI: 10.1021/jp509093g J. Phys. Chem. B 2015, 119, 4284−4293

Article

The Journal of Physical Chemistry B

80 to 150 °C and a cross-head speed of 1.0 mm/s along vertical and horizontal directions up to a draw ratio (DR) of 3.5 × 3.5. DR and Td were maintained for 1 min, and the drawn membrane was then heated to 220 °C for annealing but shrunk to a final DR of 3.0 × 3.0 in order to prevent membrane breaking during annealing. After the above annealing at 220 °C for 5 min, samples were cooled to room temperature. Pure LMW film was annealed at 215 °C, due to its lower Tm. Mechanical Tests. The mechanical properties of the biaxially drawn membranes were measured by a Tensilon RTC-1325A tensile tester (A&D, Japan) at room temperature. Strips 5 mm wide and 50 mm long were cut from the prepared membranes for tensile tests. Tensile tests were performed at a CHS of 100 mm/min for 30 mm gauge length. Tensile strength was calculated from the maximum recorded stress at the breaking point. Tearing tests were conducted at a CHS of 200 mm/min when an initial notch 10 mm long was made at the center of strips that were 25 mm wide and 30 mm long. Two ends of the strips at the top side were turned in the vertical direction and clamped. The stress profile was recorded up to 20 mm tearing. The area below the recorded stress profile indicates tearing energy, which was divided by tested membrane thickness. In Situ SAXS Measurements. In order to simulate the above tearing deformation by in situ X-ray measurement for the tensile test, a specially shaped specimen was cut from the tested biaxially drawn membrane. A dumbbell specimen with sharp narrow zone 4 mm wide was cut from the prepared membranes. An in situ SAXS measurement during tensile testing was performed using a synchrotron radiation source at SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan). An extension device13,40−45 was installed in beamline BL40B2, and SAXS images were continuously recorded during tensile testing on a cooling CCD camera (Hamamatsu Photonics K.K., C4880). A 0.3-mm-diameter beam was radiated perpendicular to the membrane surface (through-viewed) at the center of the narrower zone of the sample. The wavelength of the synchrotron beam was 1.00 Å. The camera length was 2.0 m. The exposure time for each pattern was 5.0 s, with a time interval of 6.0 s for data storage. Tensile deformation at room temperature was performed at 5 mm/min up to a strain of 0.6. The returning stress for the hysteresis profile was also recorded with corresponding in situ SAXS patterns. Structural Analyses for the Prepared Membranes. The structures of the prepared membranes were analyzed by ex situ SAXS and wide-angle X-ray diffraction (WAXD) measurements under the static condition at room temperature. These X-ray measurements were also performed at SPring-8 beamline BL40B2. SAXS measurement condition is same as the in situ measurement described above, but WAXD patterns were recorded on the flat-panel (Hamamatsu Photonics, C9732DK). The wavelength of the synchrotron beam was 1.00 Å. The exposure time for each pattern was 3.0 s for WAXD image recording. The camera length was 70 mm. In the case of the ex situ WAXD measurements, the edge-viewed patterns were also recorded with parallel X-ray radiation along the membrane surface, as well as usual through-viewed recording for the above in situ and ex situ SAXS measurements. DSC measurements were also carried out for the prepared membranes in order to evaluate the crystallinity. The heating condition is the same as described above.

panels.35−37 Sequential drawing facilitate a continuous production line, resulting in superior cost performance; however, simultaneous drawing provides better homogeneity of the mechanical properties due to the radial molecular orientation within the resultant membranes. This study adopts the latter technique in order to achieve the most desirable membrane structures, balancing tensile strength and tearing strength. Our previous study13 indicated both the oriented molecules in crystalline phase and the tie molecules in amorphous phase improve the mechanical strength for uniaxially drawn ETFE films. Therefore, a combination of these different phases should be optimized within biaxially drawn ETFE membranes. This study achieves this by laminating two layers having different deformation characteristics (i.e., plastic and elastic components having different molecular weights (MWs)). These layers also play corresponding roles in membrane-breaking during tensile and tearing tests. Recently, Carr et al.38,39 reported that laminating fluoropolymer and PET layers provides an effective gas barrier. In order to detect the fracturing mechanism of the prepared ETFE membrane biaxially drawn in this study, we performed in situ measurement during breaking tests. We13,40−45 reported earlier that in situ X-ray measurement is a powerful tool to analyze the deformation mechanism of a polymeric film. In particular, small-angle X-ray scattering (SAXS) can detect the phase arrangement of crystalline and amorphous components, which will destruct during tensile breaking or tearing.

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EXPERIMENTAL SECTION Samples. Three hundred micron thick ETFE films were supplied by the Asahi Glass Company. Different grades were prepared by melt-extrusion from ETFE pellets with higher MW (HMW) and lower MW (LMW). Table 1 compares the Table 1. Rheological and Thermal Properties of Original Films grade

MFI (g/10 min)

Tg (°C)

Tm (°C)

ΔHm (cal/g)

HMW LMW

4.6−5.7 10−20

99 76

255 225

27.2 11.1

rheological and melting properties of these initial materials. The melt-flow index (MFI) represents the weight in grams extruded at 297 °C under an applied pressure of 300 MPa. Lower MFI indicates higher MW, exhibiting higher glass transition temperature (Tg), melting temperature (Tm) and fusion heat (ΔHf). These thermal properties were determined by differential scanning calorimetry (DSC) using a PerkinElmer Pyris 1 DSC. DSC heating scans were performed from 30 to 275 °C at a rate of 10 °C/min in a nitrogen gas flow. The sample weight was 5 mg. Sample Tm was evaluated as the peak temperature of the melting endotherm. Temperature and fusion heat were calibrated using indium and lead standards. These LMW and HMW films were laminated by rolling at 290 °C, resulting in a three-layered structure, with a HMW film sandwiched between two LMW films. Three thickness ratios of LMW/HMW/LMW (1/1/1, 1/2/1, and 1/4/1) were applied. The total film thickness was adjusted to 300 μm, which is the same as that of pure HMW and LMW films. Biaxial Drawing. The above ETFE films were cut into 45 × 45 mm2, where the drawn area excluding chucking regions was 25 × 25 mm2. These square films were biaxially drawn at Td = 4285


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Figure 1. Photographs taken before (a) and after (b) biaxial drawing up to 3.0 × 3.0 for the LMW film. The dotted lines emphasize the crossing marks preinked on the initial film before drawing. Eight chucks move simultaneously in the vertical and horizontal directions during biaxial drawing.

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film could not be biaxially drawn in a Td range of 80 to 150 °C. The second approach involves annealing the biaxially drawn membrane. It has been reported that annealing polyethylene membrane after biaxial drawing increases both tensile strength and tearing strength,16 due to effective reduction of the molecular orientation within the resultant membrane. To prepare a laminated film, a core layer of HMW was sandwiched between two LMW layers. This LMW/HMW/ LMW film was biaxially drawn in the vertical and horizontal directions at 120 °C. Surprisingly, these films could be drawn up to a biaxial DR of 3.5 × 3.5 without any pin-holes, which exceeds the maximum achievable DR of 3.0 × 3.0 for the pure LMW film, independent of lamination ratio. These laminated membranes also exhibit the necking during biaxial drawing process. Such necking fully spreads on the film surface at a DR of 3.0 × 3.0 for all the laminated membranes, which is similar to the pure LMW film although the pure HMW film cannot be biaxially drawn. In other words, the homogeneously oriented membranes with HMW could be prepared by biaxial drawing in this study, which are beneficial for the later tensile and tearing testing. Further annealing was performed at 220 °C, which is just below the Tm of pure LMW film (Table 1), but such a highly drawn membrane often broke during cooling to room temperature, due to the increase of the shrinking stress. Therefore, such residual stress was relaxed by returning DR to 3.0 × 3.0 without sag, and the film was annealed at 220 °C for 5 min. The resultant membrane properties are compared in Figure 2. Both yielding stress and tensile strength are lower than for the unshrunk LMW membrane with a biaxial DR of 3.0 × 3.0 (Table 2). These reductions are ascribed to molecular relaxation induced by annealing near Tm. However, these properties still increased 1.7 to 2.4 times those of the undrawn HMW film. In contrast, tearing energy was higher than for the 3.0 × 3.0 film of pure LMW, independent of the lamination ratio, meaning that molecular relaxation by annealing effectively improves tearing resistance. In particular, the highest energy of 3.3 N/mm for 20 mm tearing was obtained for LMW/HMW/ LMW = 1/1/1 with a DR of 3.0 × 3.0, which is 2.2 times that for a pure LMW membrane with the same DR. In order to confirm the structural change induced by such relaxation and annealing processes, ex situ WAXD and SAXS patterns recorded under static condition at room temperature are compared between the unannealed 3.0 × 3.0 membrane and

RESULTS AND DISCUSSION First, the HMW film was biaxially drawn at the Td range of 80 to 150 °C, which exceeds Tg of ETFE.46 Above Td of 160 °C, the film immediately broke when biaxial drawing was started. The maximum biaxial DR of 3.0 × 3.0 could be obtained at Td of 120 °C, where necking was observable just after the draw began. However, the membrane often tears before such necking spreads to its ends. For biaxial drawing of LMW, full spreading of necking was achieved up to a maximum DR of 3.0 × 3.0 at Td of 120 °C. A typical affine deformation was confirmed from the similarity of crossing ink marks before and after drawing (Figure 1). The resultant membrane thickness was about 20 μm. Table 2 compares the mechanical properties of the initial Table 2. Comparison of Mechanical Properties of Initial and a Biaxially Drawn Film grade

DR

yield stressa (MPa)

HMW LMW LMW

1.0 × 1.0 1.0 × 1.0 3.0 × 3.0

25 20 40

tensile strengtha (MPa)

tear energyb (N·20 mm/mm)

65 62 161

50.7 34.4 1.47

a

Estimated from usual tensile tests at room temperature. bEstimated from tearing tests at room temperature.

undrawn films of HMW and LMW and the biaxially drawn LMW membrane. The mechanical properties of HMW film are superior to those of LMW film, but they could not be biaxially drawn, as described above. For the LMW film, yielding stress increases 2.0 times and tensile strength increases 2.6 times by biaxial drawing up to 3.0 × 3.0 compared to those of the initial undrawn film. In contrast, biaxial drawing significantly decreases tearing stress. This implies that higher molecular orientation is disadvantageous for developing tearing strength but advantageous for tensile strength. This study applied two different approaches to develop the mechanical properties. One approach is laminating the HMW film with the LMW film. A poorly ductile film is often drawable when laminated with a highly ductile film. For example, fluoropolymer laminated with PET can be successfully drawn biaxially.38 In this study, similar lamination is applied for biaxially drawing poorly ductile HMW film. Melt-blending of LMW and HMW pellets was attempted, but the prepared blend 4286


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Figure 2. Lamination ratio dependence of resultant yield stress, tensile stress (a) and tear energy (b) for the membranes biaxially drawn and subsequently annealed at 220 °C with biaxial DR of 3.0 × 3.0.

the annealed 3.0 × 3.0 membrane shrunk from initial 3.5 × 3.5 for pure LMW in Figure 3. Independent of annealing, the through-viewed WAXD for the biaxially drawn membrane are isotropic, due to the random chain orientation on the membrane surface. However, the annealing gives the remarkable long-period scattering on the SAXS pattern. Figure 4 compares the scattering profiles extracted from these SAXS patterns. Here, the air scattering pattern for the empty cell under the same SAXS measurement condition indicates that the central scattering around the beam stop does not affect the SAXS data with the larger q region than 0.005 Å−1 (see the Supporting Information, Figure S1). Apparent growth of the scattering peak intensity with annealing indicates the crystallinity increment. Corresponding DSC results in Figure 5 also exhibit the higher crystallinity after annealing. In contrast, the edge-viewed WAXD patterns suggest that the anisotropic orientation of the crystalline molecules, independent of annealing. Over ten pieces of membranes were stacked with maintaining the biaxial draw directions parallel and perpendicular to the incident beam which is radiated along the membrane surface. Therefore, such an edge-viewed pattern is also regarded as the end-viewed one. In edge-viewed WAXD patterns in Figure 3, the above stacked membranes are set horizontal. Independent of annealing, the edge-viewed WAXD patterns for both 3.0 × 3.0 membranes contain three oriented (100) reflections, one is on the equator (perpendicular to the membrane surface), and the others are along the directions tilted 30° and 60° from the equator. The intensities of the equatorial arcs are always stronger than those of diagonal arcs. A comparison of both 3.0 × 3.0 membranes suggests that the annealing increase the membrane crystallinity with keeping the molecular orientation, which agrees with the results from the

Figure 3. Comparison of ex situ WAXD and SAXS patterns for the biaxially drawn membranes of unannealed (a) and annealed pure LMW (b). WAXD patterns were both through-viewed (top) and edgeviewed at room temperature (bottom). SAXS patterns (middle) were through-viewed, as same as the later in situ measurements. The resultant biaxial DR was both 3.0 × 3.0.

Figure 4. SAXS profile extracted from the patterns depicted in Figure 3. (a) Unannealed pure LMW and (b) annealed pure LMW.

corresponding through-viewed patterns and DSC profiles. Here, the WAXD patterns are recorded on the flat plate camera set at the half diagonal side from the incident beam. If the whole pattern was recorded, there are totally 6 arcs on an 4287


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Figure 5. DSC heating profiles for the membranes prepared in this study. (a) Unannealed pure LMW, (b) annealed pure LMW, (c) 1/1/ 1, (d) 1/2/1, and (e) 1/4/1. Although the membrane (a) was just biaxially drawn up to a DR of 3.0 × 3.0, the latter four membranes (b− e) were biaxially drawn up to an initial DR of 3.5 × 3.5, and subsequently annealed with biaxial DR of 3.0 × 3.0. Figure 6. Stress-strain curve recorded during fracturing test for the biaxially drawn LMW membrane with the corresponding change of in situ SAXS patterns. This fracturing test was conducted at room temperature along the vertical direction in SAXS patterns. Strain was denoted in each pattern. After a strain of 0.6 was reached, the reverse measurement was performed.

equator and two inclined directions. Such 6-point arcs are attributed to the hexagonal chain packing for the most stable crystalline modification of ETFE molecules,47,48 where the caxis orients along the incident beam as end-viewed pattern. Here, the stronger intensity for equatorial reflections suggests the other set of c-axis orientation perpendicular to the incident beam. Such double orientations of the c-axis agree with biaxial drawing directions set along and perpendicular the incident beam. In the case of the edge-viewed SAXS patterns, the interfaces between the stacked membranes gives the strong streaks along the meridional direction, thus the effective analyses are difficult. The other laminated membranes exhibit similar structures to the annealed pure LMW membrane, but the higher crystallinity is confirmable with increasing HMW content. Especially, their DSC profiles contain two different endotherms, as depicted in Figure 5. These correspond to the melting of lower-Tm LMW and that of the higher-Tm HMW, as previously described in Table 1. The ratio of these endotherms agrees well with the lamination ratio of the membrane, indicating the desirable structural homogeneities for the prepared membranes. In order to characterize the fractural mechanism of such laminated membranes, in situ SAXS measurements were performed during breaking tests in this study. For in situ analysis of fractural behavior, the applied deformation should be confined to the limited space within the sample. Therefore, two semicircular notches are cut from specimen sides at the longitudinal center. This specimen preparation can concentrate the fracture at the area targeted by in situ X-ray measurement when it was tensile-deformed along the specimen length. A similar notched specimen has been adopted for analysis of the fractural behavior on tensile tests for multilayer polymer films.49 Figure 6 depicts the series of in situ SAXS patterns and the corresponding stress−strain profile recorded during fractural deformation for the biaxially drawn membrane of pure LMW. It should be noted that the isotropic pattern was confirmed by ex situ SAXS measurements performed before the test, as depicted in Figure 3. A lozenge-shaped scattering pattern was obtained at a strain of 0.02. This result indicates that the lamellae within the prepared membrane are slightly inclined toward the tensile direction even in the early stage of testing. This SAXS pattern is horizontally narrowed at a strain of 0.1 just beyond the yielding stress in the stress profile. Scattering intensity gradually decreases with increasing strain, leading to the disappearance

of the lozenge pattern at a higher strain of 0.58. The fact that that such a lozenge pattern does not reappear even when the applied stress is released from 0.6 strain indicates that the initial ETFE lamellae are collapsed during this tensile breaking in the notched area. A similar tensile test was performed for the biaxially drawn 1/ 1/1 membrane (Figure 7). As well as the pure LMW

Figure 7. Stress-strain curve recorded during fracturing test for the biaxially drawn 1/1/1 membrane with the corresponding change of in situ SAXS patterns.

membrane, the isotropic pattern was obtained before tensile testing (Supporting Information, Figure S2). Independent of applied strain, the scattering intensity is higher than that for the pure LMW membrane even with the same DR of 3.0 × 3.0. Therefore, the characteristic lozenge pattern in the early stage of testing was clearer than that for the pure LMW layer. In contrast, the testing result for the biaxially drawn 1/2/1 membrane indicates lower scattering intensity (Figure 8). 4288


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The profiles recorded in the azimuthal angle range of 90° to 270° were plotted as a function of applied strain during testing (Figure 10). Here, the color gradation from red to blue corresponds to observed scattering intensity changes from lower to higher. In the scattering intensity change corresponding to a 20 nmlong period for the biaxially drawn membrane of pure LMW (top row in Figure 10a), peaks are recognizable at azimuthal angles of 120° and 240° in the early stage of testing. These peak positions gradually shift toward the central 180°; they gradually become less intense with increasing applied strain and disappear beyond 0.4 strain. In contrast, scattering corresponding to a 10 nm-long period (bottom row in Figure 10a) exhibits a strong peak at 180° even at the start of the tensile test. The intensity of this central peak increases with increasing strain, but decreases in the latter stage of testing, which is similar to the results for the above change for a 20 nm-long period. Comparing these changes in azimuthal scans extracted at different q positions for pure LMW film indicates that the orientation of the initial thicker lamellae with a 20 nm-long period are inclined ±30° toward the tensile fracturing direction in the early stage of testing, but thin lamellae with a 10 nm-long period gradually stack parallel to the tensile direction in the latter stage. Similar results were obtained for the biaxially drawn 1/1/1 membrane, independent of period length (Figure 10b). However, scattering intensities for both the inclined lamellae with a 20 nm-long period and the parallel lamellae with a 10 nm-long period exceed that for the pure LMW membrane. These results confirm that the HMW layer has inclined and parallel lamellae, as well as the LMW layer. The biaxially drawn 1/2/1 membrane also exhibits a similar position shift for the 20 nm-long period (top row in Figure 10c); however, the scattering intensity is remarkably lower than that of the 1/1/1 membrane. Also, there is a peak at the central 180° at the beginning of testing. Another difference is observed for scattering changes in the parallel lamellae (bottom row in Figure 10c). The peak intensity at the central 180° increases up to the end of testing, when it decreases for pure LMW and 1/ 1/1 membranes (Figure 10a,b). For the 1/4/1 membrane, the scattering intensity of the inclined lamellae is lower than that for the 1/2/1 membrane, as depicted in the bottom column in Figure 10d. Correspondingly, the intensity of the central peak at the beginning of testing is higher. In contrast, the trend of a 10 nm-long period is similar to that for the 1/2/1 membrane (Figure 10c), but the scattering intensity is highest among all membranes tested in this study, indicating that the parallel lamellae remarkably increases with increasing testing strain. Basically, the lozenge SAXS pattern corresponds to a fourpoint pattern, but the lower lateral continuity of the tilted lamellae gives the less separation of the four-point scattering, like ellipsoid shape. This is similar to the ellipsoid SAXS pattern obtained for chevron arrangements for deformed block copolymer.50,51 In contrast, the two-point scatterings perpendicular to the deformation axis are significant with increasing HMW content in Figure 9, but not so in Figure 6 for pure LMW membrane. Therefore, these two-point scatterings are attributed to the crystalline lamellae contained in HMW layers with the higher crystallinity. These lamellae are not tilted but orient parallel along the deformation direction where the crystalline molecules highly orient along deformation axis for the membranes with the higher HMW content, as confirmed by


However, the anisotropy of the scattering pattern is remarkable, resulting in a clear transition from a lozenge to a spot pattern oriented on the equator. Such a trend is more significant in Figure 9, where the other 1/4/1 membrane with the same DR


of 3.0 × 3.0 but the lowest LMW content is tested, resulting in a spot pattern even at the early stage of testing. These results suggest that lamellar deformation during pseudo tearing tests is dominated by the composition of LMW and HMW layers. In order to analyze differences between lamellar deformation mechanisms in detail, the profiles along the azimuthal scan are extracted at the given q. The targeted q values are q = 0.025− 0.032 Å−1, corresponding to a 20 nm-long period, and q = 0.051−0.058 Å−1, corresponding to a 10 nm-long period. The series of the intensity profiles for the initial membranes indicate the long period peak at the former q ≈ 0.03 Å−1 still exists before in situ SAXS measurements, as depicted in Figures 3 and S3 (Supporting Information). This corresponds to the initial lamellar arrangements within all the DR 3.0 × 3.0 membrane. 4289


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Figure 10. Stacked line profiles of azimuthal scans at q = 0.03 A−1 (top) and 0.055 A−1 (bottom) for the series of in situ SAXS patterns for biaxially drawn membranes composed of different component ratios. (a) Pure LMW, (b) 1/1/1, (c) 1/2/1, and (d) 1/4/1. Intensity is represented as color gradation from red (higher intensity) to blue (lower intensity). Extraction of the azimuthal profile is schematically denoted as a dotted circle in the inset SAXS patterns in (a).

ex- situ WAXD pattern recorded after in situ SAXS measurements, as depicted in Figure 11. Preferential molecular orientation along the membrane surface for the biaxially drawn membranes, as depicted in Figure 3, is effective for narrowing the stacking long-period of the lamellae into 10 nm during the in situ SAXS measurements. An ease of the later uniaxial orientation induced by tensile deformation during in situ SAXS measurements enhances the transformation from

initial thicker lamellae into the deformed thinner ones, like pantographs. This is the most characteristic aspect of the deformation mechanism of the biaxial drawn membranes prepared in this study. The above lamellar deformation indicates that inclined lamellae with a 20 nm-long period are attributed to LMW components. The highest scattering intensity was obtained for the 1/1/1 membrane, suggesting that the former inclined lamellae are formed even in the HMW layer. Such inclined lamellae are characteristic of LMW; thus, inclined lamellae within the HMW layer are also induced by deformation of the LMW layer. Considering that this 1/1/1 membrane exhibits the highest tearing strength (Figure 2b), synchronization of deformation within LMW and HMW layers is evaluated from the amount of inclined lamellae. The azimuthal angle profiles extracted at q = 0.025−0.032 Å−1, corresponding to a 20 nmlong period, were fitted by a combination of inclined (120° and 240°) and parallel (180°) components in the fracturing direction. The area percentages of the former components are plotted as a function of testing strain in Figure 12; the values for typical strains at every 0.1 are plotted. Here, these intensity values in Figure 12 were normalized by the membrane thickness calculated for the given strain. After the in situ SAXS measurements depicted in Figures 6−9, they became all 17 μm. Therefore, the thickness change during in situ SAXS measurements are almost same, independent of the lamination ratio. In the early stage of fracturing for the 1/2/1 membrane, no value is plotted because its counter component of parallel lamellae is very low. The obtained results demonstrate that the inclined lamellar component decreases monotonically with increasing applied strain for the pure LMW membrane. In contrast, that of the 1/1/1 membrane remains constant up to 0.3 strain. Such resistance of lamellar deformation results in higher tearing strength for the 1/1/1 membrane. Assuming that the amount of inclined lamellar component with the LMW layer is the same as that for the pure LMW

Figure 11. Comparison of ex situ WAXD patterns through-viewed for the resultant samples obtained after in situ SAXS measurements depicted in Figures 6−9. (a) Pure LMW, (b) 1/1/1, (c) 1/2/1, and (d) 1/4/1. Each tested direction for in situ measurement was horizontal in the corresponding WAXD pattern. 4290


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inclined lamellae are transformed into thinner lamellae arranged parallel to the in situ testing direction with the uniaxial molecular orientation. Considering that the origin of such inclined lamellae is ascribed to the LMW component, the inclined lamellae are first formed within the LMW layer in the early stage of testing, but are followed by those formed within the HMW layer by the former lamellar deformation. This phenomenon is concentrated at the lamination boundaries between the LMW and HMW layers. A possible fractural mechanism for the 1/1/1 membrane with biaxial DR of 3.0 × 3.0 is summarized here. The HMW component exhibits two types of lamellar deformation during biaxial drawing, depending on its laminated position within the initial film. The internal position within the HMW layer exhibits parallel lamellae, reflecting the deformation trend of the HMW component. In contrast, inclined lamellae are obtained for the interface of the HMW component to the LMW component. It should be noted that the chain orientation in the testing direction is a key for determining breaking strength. The former parallel lamellae have higher tensile strength, due to the agreement of chain orientation along the membrane surface and tensile testing direction. In contrast, the initial inclined lamellae absorb stress absorption in tear testing, resulting in higher strength. These results indicate that an optimum balance between inclined and parallel lamellae within the HMW components increases tearing strength and tensile strength for the biaxially drawn 1/1/1 membrane.

Figure 12. Changes in intensities attributed to inclined lamellar component during fracturing tests for the biaxially drawn membranes prepared at different lamination ratios. The intensities estimated from the azimuthal profiles extracted at q = 0.025−0.032 A−1 for the series of in situ SAXS patterns. Green denotes pure LMW, red denotes a lamination ratio of 1/1/1, and purple denotes a lamination ratio 1/2/ 1. These intensity values were normalized by the membrane thickness calculated for the given strain.

membrane, the inclined lamellar component within the HMW layer (IHMW) may be estimated using eq 1. IHMW = I − ILMW ×

LMW content (%) 100(%)

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CONCLUSIONS Biaxial drawing of ETFE films produces homogeneously transparent membranes without any pin holes. Lamination with ETFE films having different MWs successfully balances tensile strength and tearing strength of biaxially drawn membranes. In particular, annealing just below the Tm of the LMW component increases tearing strength. Such tearing deformation of biaxially drawn membranes was simulated by in situ SAXS measurement, indicating the effect of change in lamellar structure within biaxially drawn membranes. The obtained results indicate that tearing strength was dominated by the amount of inclined lamellae formed during tearing simulation in in situ SAXS measurements. These characteristic inclined lamellae were clearly detectable for deformation of LMW film. It should be noted that the HMW layer within the laminated film also contains these inclined lamellae, which are induced by deformation of the LMW layer. The fraction of inclined lamellae increases with increasing strain up to the critical value corresponding to sample breaking for the biaxially drawn 1/1/1 membrane, which exhibits the highest tearing strength. These results suggest that inclined lamellae absorb strain on tearing. This membrane also contains parallel lamellae, resulting in the highest tensile strength. A combination of biaxial drawing and initial film lamination effectively balances the tensile strength and tearing strength of the ETFE membranes prepared in this study.

(1)

Here, I is the scattering peak area of the inclined lamellar component obtained for each DR = 3.0 × 3.0 membrane, and ILMW is that for the pure LMW membrane obtained at the corresponding strain. The calculated IHMW is plotted as a function of applied strain in Figure 13. The IHMW value increases with increasing strain for the 1/1/1 membrane up to 0.3 strain but decreases at higher strain. A corresponding increase of scattering intensity of the parallel lamellae with a 10nm-long period (Figure 10b) suggests that the thicker initial

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ASSOCIATED CONTENT

S Supporting Information *

The SAXS air scattering pattern for the empty cell, throughviewed ex situ SAXS patterns for the laminated membranes, and extracted SAXS profiles. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 13. Estimated intensities attributed to inclined lamellar component in the HMW layer. These intensities were calculated using eq 1. Red denotes a lamination ratio 1/1/1, and purple denotes a lamination ratio of 1/2/1. 4291


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Present Address ⊥

Department of Applied Chemistry, Saitama University, 255 Shimo-Okubo, Sakura-ku, Saitama 338−8570, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS In situ WAXD measurement using synchrotron radiation was performed at SPring-8, Japan Synchrotron Radiation Research Institute (Proposal Nos. 2009B1780 and 2012B1117). We appreciate the cooperation of Dr. Sono Sasaki (JASRI and Kyoto Institute of Technology). This work was partly supported by Industrial Technology Research Grant Program from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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Property Development for Biaxial Drawing of Ethylene

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