Phase Transition during Heating of Nanostructured Ultrahigh

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Phase Transition during Heating of Nanostructured Ultra-High Molecular Weight Polyethylene Membranes Hiroki Uehara, Takuya Tamura, Hideyuki Yamashita, Takeshi Yamanobe, and Hiroyasu Masunaga J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/acs.jpcb.5b07086 • Publication Date (Web): 25 Nov 2015 Downloaded from http://pubs.acs.org on December 13, 2015

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The Journal of Physical Chemistry

Phase Transition during Heating of Nanostructured Ultra-High Molecular Weight Polyethylene Membranes

Hiroki Uehara,1* Takuya Tamura,1 Hideyuki Yamashita,1 Takeshi Yamanobe,1 and Hiroyasu Masunaga2

1. Division of Molecular Science, Faculty of Science and Technology, Gunma University, Kiryu, Gunma 376-8515, Japan 2. Japan Synchrotron Radiation Research Institute (JASRI/SPring-8), Kouto 1-1-1, Sayo-cho, Sayo-gun, Hyogo 679-5148, Japan

[email protected]

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Abstract Ultra-high molecular weight polyethylene (UHMW-PE) membranes were prepared using biaxial melt-drawing and subsequent melt-shrinking. Electron microscopy observations indicate that the former membrane has more extended-chain crystals (ECCs), whereas the latter is composed of folded-chain crystals (FCCs). Corresponding double-melting endotherms are recorded on differential scanning calorimetry (DSC) measurements. Detailed assignments of such double-melting components are performed using in-situ X-ray measurements during heating. Wide- and small-angle X-ray diffraction and scattering (WAXD/SAXS) images were simultaneously recorded at SPring-8. Changes in WAXD images indicate that the orthorhombic reflection peak begins to decrease at 130oC, followed by the appearance of the hexagonal reflection peak beyond 145oC for both membranes, but the latter meltshrunk membrane exhibits weaker hexagonal reflection intensity. Simultaneous SAXS results indicate that FCCs rapidly disappear at 135oC for the melt-shrunk membrane, resulting in a sharper endotherm. In contrast, residual ECCs restrict the melting of FCCs for the melt-drawn membrane, resulting in a broader endotherm of FCC melting at a slightly higher temperature position.


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Introduction

We have demonstrated that biaxial melt-drawing of ultra-high molecular weight polyethylene (UHMW-PE) increases tensile strength but decreases tearing strength, due to extended-chain crystals (ECCs).1,2 Both strengths can be balanced by subsequent biaxial melt-shrinking immediately after meltdrawing, resulting in homogeneous membrane morphology composed of folded-chain crystals (FCCs). It should be noted that such biaxial melt-drawing and subsequent melt-shrinking are not allowed for usual high-density PE (HDPE) with conventional MW. The low melt-viscosity of the latter HDPE can not transmit the applied stress when draw is started above melting temperature (Tm), thus gives immediate breaking. Differential scanning calorimetry (DSC) analysis for these UHMW-PE membranes indicates double-melting endotherms at 132 and 152oC; however, the latter peak for the melt-drawn membrane is larger than that for the melt-shrunk membrane.1 Such double-melting endotherms are observed for the DSC heating profile of uniaxially melt-drawn films of UHMW-PE.3-6 With increasing biaxial draw ratio (DR), the ECC content increases while the latter endotherm develops, indicating that the higher-temperature-side endotherm at 150oC is attributed to ECCs. Considering that this temperature exceeds the thermodynamic equilibrium Tm of the orthorhombic PE crystals, phase transition during DSC heating is expected. Indeed, crystalline transition from stable orthorhombic to mobile hexagonal forms is observed above 150oC for DSC heating of uniaxially melt-drawn UHMW-PE film.7 In contrast, the lower-temperature-side melting endotherm is larger for the melt-shrunk membrane than for the melt-drawn membrane. If the counter higher-temperature-side endotherm is attributed to ECCs as described above, the lower one is necessarily ascribed to FCCs, because the membranes contain ECCs and FCCs. Another interesting factor is the shape of these lower-temperature-side endotherms. A sharper endotherm is obtained for the melt-shrunk membrane than for the melt-drawn membrane. Considering the difference between the areas of the higher-temperature-side endotherms of these membranes, such features of the lower-temperature-side endotherm are also affected by residual ECCs even during melting of FCCs. However, there is no direct evidence of such melting assignment ACS Paragon Plus Environment

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for these biaxially melt-drawn and melt-shrunk membranes of UHMW-PE. For example, the formation of a crystallite population of different size is still possible explanation of such double endotherm of UHMW-PE membrane.8 In this study, such characteristic melting behavior for biaxially melt-drawn and subsequently meltshrunk UHMW-PE membranes is analyzed. Here, in-situ X-ray measurements effectively analyze overlapping different melting mechanisms for ECCs and FCCs. Our previous studies9 indicate that simultaneous wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) can be used to analyze both dimensional changes in molecular and phase separation levels for uniaxially meltdrawn film. Such melting proceeds within a short time of several minutes; thus, high time-resolution is desirable. A synchrotron radiation source with high luminescence is useful for in-situ analyses of melting behaviors complicated with crystal transition. Simultaneous in-situ WAXD/SAXS measurements have been applied during heating,10 uniaxial drawing11,12 or tensile testing13,14 of UHMW-PE materials. In this study, this methodology is performed at SPring-8 during heating of biaxially melt-drawn and subsequently melt-shrunk membranes of UHMW-PE. The obtained in-situ measurement results give us direct evidences for assignment of double endotherms for biaxially-drawn UHMW-PE membrane.

Experimental Film preparation. The UHMW-PE used was Hizex 340M supplied by Mitsui Chemical. The viscosity average MWs were 3.5 x 106 g/mol. UHMW-PE powder was sandwiched between commercial polyimide films (UPILEX-125S, Ube) and compression-molded into the film at 180oC and 5MPa for 10min in vacuum, followed by slow cooling to room temperature. A press machine equipped with a vacuum chamber (Boldwin, Japan) was used. The resultant film was 300µm thick. Biaxial drawing. The prepared UHMW-PE films were biaxially drawn at 150oC and a crosshead speed (CHS) of 5mm/min along vertical and horizontal directions up to a maximum DR of 8x8. A biaxial


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drawing machine (Island Industries, Japan) was used. Measurements. Morphologies of the resultant membranes were observed using a Hitachi field-emission SEM S-4800 operated at 1.0kV. The sample membranes were uncoated for SEM observations; thus, artifacts were negligible. A Perkin-Elmer Diamond DSC was used for DSC measurements. Heating scans were performed from 50 to 180oC at 2oC/min under a nitrogen gas flow. The sample Tm was evaluated as the peak temperature of the melting endotherm. Temperature and fusion heat were calibrated using indium and tin standards. To avoid the effects of membrane shrinkage and heat-transfer delay during heating scan, a small amount of silicone oil was placed between the sample and the bottom of the DSC sample pan. In-situ X-ray measurements were carried out during heating of the prepared membranes using synchrotron radiation at the BL40B2 beamline of SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan). Both through-viewed and edge-viewed patterns were recorded (Supporting Information, Figure S1). Because the resultant melt-drawn or melt-shrunk membranes were very thin (a few tens of micrometers), the diffraction or scattering intensity from one piece of the membrane was less effective for quantitative analysis of the melting behavior of the targeted membrane. Therefore, several pieces of the membranes were stacked with adjusting the vertical and horizontal biaxial drawing directions and sandwiched within the window of the lead holder (Supporting Information, Figure S1b). A heating chamber7,8 was installed in the beamline, and WAXD and SAXS images were continuously recorded on flat-panel (Hamamatsu Photonics, C9732DK) and cooling-type CCD cameras (Hamamatsu Photonics, C4880) with an intensifier. The wavelength (λ) of the synchrotron beam was 1.00 angstrom. Corresponding beam energy was 15keV. Camera length was 50mm (WAXD) and 2000mm (SAXS). The exposure time was 3.5sec for WAXD and 2.0sec for SAXS image recording, with a time interval of 4.0sec. The heating temperature ranged from 50 to 180oC at 2oC/min. Corresponding WAXD/SAXS image capture was every 0.25oC. Concerning the SAXS profile analyses, the Lorentz correction was performed.


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Result Membrane Morphologies. The as-prepared morphologies of the biaxially melt-drawn and subsequently melt-shrunk membranes targeted in this study are characterized in Figure 1. The meltdrawn membrane with biaxial DR of 8x8 exhibits a few tens of nanometer wide oriented fibrillar ECCs closely covering the membrane surface. Also, typical “shish-kebab”15 morphology is observed, where stacked FCCs grow epitaxially perpendicular to the ECC orientation axis. The orientations of these crystalline structures are random on the membrane surface. In contrast, the membrane with a final biaxial DR of 4x4, which was melt-shrunk from the initial 7x7 membrane in the molten state, exhibits a network of FCC lamellae with fewer ECC fibrils. One of the most characteristic features of these FCCs is the constant thickness of 30nm. This value is similar to the MW between the adjacent entanglements of PE (2400-3400Da).16,17 This agreement indicates that molecular entanglements are trapped on the amorphous surfaces of FCCs.18 The UHMWPE chain contains many entanglements, which are disentangled by melt-drawing but reformed by meltshrinking. Rearranging the entanglements of UHMW-PE molecules connects the amorphous region, resulting in the network morphology of FCCs.1 Molecular and Phase Orientations. To characterize the arrangement and orientation of these ECCs and FCCs, WAXD and SAXS measurements were performed for biaxially melt-drawn and subsequently melt-shrunk UHMW-PE membranes prepared in this study. Figure 2 compares the patterns obtained when the incident X-ray beam was radiated perpendicular (through-viewed) or parallel (edge-viewed) to the membrane surface. The vertical direction on the biaxial drawing is arranged vertically in these patterns. The through-viewed WAXD and SAXS patterns exhibit non-oriented rings for both membranes. The two reflection rings in WAXD patterns are assigned to the orthorhombic (110) and (200) reflections. These results suggest the random orientation of molecular chains and phase arrangements on the membrane surface, which agrees with the morphologies depicted in Figure 1.


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In contrast, the edge-viewed patterns are quite different for biaxially melt-drawn and subsequent melt-shrunk membranes. The former membrane exhibits a six-point pattern on WAXD, indicating duplicated molecular orientation. This pattern could be resolved into two equatorial spots and two pairs of diagonal spots tilted 60o toward the equator. The edge-viewed SAXS pattern of this membrane exhibits strong streaks spread on the equator, indicating that the ECCs oriented parallel to the drawing axis. Other weak spots are recognized on the meridian. These spots correspond to the lamellar stacking perpendicular to the molecular axis, which has been observed for solution-crystallized UHMW-PE films.19,20 These results suggest that ECCs are formed and oriented along the membrane surface during biaxial melt-drawing. The other FCCs are epitaxially crystallized on the ECC surface;1,2 thus, FCCs and ECCs are perpendicular to each other, whereas their molecular orientations are parallel. This combined crystalline structure is coincident with the shish-kebab morphology15 depicted in Figure 1a. Here, the diagonal pair of (110) reflection spots indicate the tilting of both ECC orientation and epitaxial crystallization direction of FCCs, which is evidenced later. Indeed, the tilting angle of such diagonal reflections depends on the biaxial DR.1,2 Figure 3 compares the azimuthal angle scans of the (110) and (200) reflections for the edge-viewed WAXD patterns at various DRs. There is a combination of the central equatorial peak and an asymmetrical set of tilted reflections on both azimuthal sides. The intensity of the central equatorial peak increases with biaxial DR, indicating the growth of ECCs parallel oriented along the drawing axis. Correspondingly, the tilting angle of side reflection peaks gradually increases with biaxial DR and reach 60o at 8x8. The origin of such tilting reflection is discussed later. Further stress relaxation is induced by melt-shrinking after biaxial melt-drawing, as depicted in Figure 2 (right side). The edge-viewed WAXD pattern of the melt-shrunk membrane exhibits arcshaped (110) reflections located on the equator, which is quite different from the characteristic six-point pattern of the former melt-drawn membrane. Subsequent melt-shrinking relaxes the molecular orientation induced by initial biaxial melt-drawing. This agrees well with the wider distribution of lamellar orientation for the melt-shrunk membrane (Figure 1b). However, the SAXS pattern includes a


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pair of clearer meridional spots than those of the melt-drawn membrane, because the FCCs become major for the melt-shrunk membrane. Correspondingly, homogeneous lamellar FCCs are observed on the SEM image of the melt-shrunk membrane presented in Figure 1b. In contrast, equatorial streaks are much weaker, indicating fewer ECCs for the melt-shrunk membrane. This feature also agrees with the SEM image in Figure 1b. In-situ WAXD measurement of melt-drawn membrane. First, the membrane biaxially melt-drawn at 150oC up to a DR of 8x8 is targeted. As depicted in Figure 4, there are double endotherms at 135oC and at 152oC, indicating the coexistence of two types of PE crystals having different thermal properties. Similar double-melting endotherms have been obtained for the uniaxially melt-drawn film of UHMWPE,3-7 where lower- and higher-temperature-side endotherms are attributed to melting of lamellar FCCs formed on cooling after melt-drawing and fibrillar ECCs formed during melt-drawing. However, the previous uniaxially and present biaxially melt-drawn UHMW-PEs differ in the peak position of the higher-temperature-side endotherm; the latter one is 7oC higher than the former one. It should be noted that the equilibrium Tm of the orthorhombic crystals of PE is 145oC,21 thus, the endotherm exceeding this critical Tm is attributed to the melting of the other crystalline form. Indeed, our previous in-situ WAXD measurement during heating of the uniaxially melt-drawn UHMW-PE film indicated that transformation from orthorhombic into hexagonal form occurs during heating with ends fixed above 150oC.7,22 This hexagonal phase is known as a mesophase that crystallizes at high temperature and pressure.23-28 However, this transient hexagonal phase completely disappears when cooled to room temperature. This result confirms that in-situ measurement is a powerful tool to prove the existence of such a mesophase. Although the other unstable crystalline form of monoclinic phase is obtained by cold drawing29,30 and polymerization31,32 of normal MW or UHMW-PE at room temperature, it immediately transforms into the orthorhombic form at 60oC during heating,32,33 thus is not issued in the present study. In this study, in-situ WAXD measurements were simultaneously recorded during heating of biaxially melt-drawn and melt-shrunk membranes, and the obtained results were compared with the


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corresponding DSC melting endotherm. Combination with in-situ SAXS measurements enables the structural detection of FCCs or ECCs on a larger scale than with WAXD measurements alone. Here, the static WAXD/SAXS results in Figure 1 suggest that the edge-viewed patterns emphasized the difference between melt-drawn and melt-shrunk membranes rather than through-viewed ones; thus, in-situ WAXD/SAXS patterns were also edge-viewed during heating. The heating rate was fixed at 2oC/min. for both heating scans to determine the relationship between the melting behaviors detected by DSC and in-situ WAXD/SAXS measurements. Figure 4 also depicts changes in in-situ WAXD/SAXS patterns recorded during heating of the biaxially melt-drawn membrane with DR 8x8 with the corresponding DSC melting endotherm. The series of in-situ WAXD patterns indicates that orthorhombic (110) and (200) reflections weaken above 130oC, which corresponds to the Tm of lamellar FCCs, resulting in an increase of the amorphous scattering halo observed just inside the (110) reflection. Beyond 150oC, these orthorhombic reflection intensities further decrease, and hexagonal (100) reflections appear both on the equator and tilted ±60o from the equator. Finally, only hexagonal reflections remain with the amorphous halo at 155oC. Correspondingly, the meridional spots on the in-situ SAXS pattern, indicating the lamellar FCC stack, disappear at 135oC. In contrast, equatorial streaks weaken at 150oC, due to the melting of ECCs, but finally disappear at 155oC, where hexagonal reflections remain on the in-situ WAXD pattern recorded simultaneously. This time lag between WAXD and SAXS intensity reductions attributed to ECC melting suggest that the hexagonal form observed here is similar to a nematic feature of the liquid crystalline phase, where the molecules orientate but slide toward each other. This hexagonal form contains gauche conformation that travels along the molecular chain,34 which expands the molecular cross-section perpendicular to the chain axis. Here, no exotherm is observed on the corresponding DSC profile, indicating that this hexagonal form does not crystallize during heating. We conclude that some of the orthorhombic form transforms into hexagonal form with slight melting of the lamellar FCCs keeping the orthorhombic form. Specifically, there are two types of orthorhombic crystals; one melts at


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135oC, and the other transforms into hexagonal form at 152oC. The difference between these orthorhombic crystals may be attributed to their structural features (e.g., the thermodynamic stability of the crystals). Lamellar FCCs have a wider interface contacting the amorphous layer, which increases their melting free energy, resulting in lower Tm. In contrast, ECCs have longer crystalline stems, maintaining hexagonal chain packing even at an elevated temperature. Line profiles were extracted from the series of in-situ WAXD patterns in order to characterize these phase transitions during heating of the biaxially melt-drawn membrane with DR 8x8. Figure 5a depicts changes in the 2θ profiles extracted along the equator of the in-situ WAXD patterns in Figure 4. Here, the profiles are duplicated as a function of measurement temperature, and the recorded intensity is represented by color gradation from low (blue) to high (red). Intensities of both orthorhombic (110) and (200) reflections decrease with increasing temperature above 135oC. In contrast, the intensity of the amorphous halo increases in the corresponding temperature region. Considering that the Tm of the lamellar FCCs is 132oC in the DSC melting thermogram of undrawn initial film (not shown here), these simultaneous intensity changes indicate lamellar FCC melting. When further heating approaches 150oC, these orthorhombic reflections completely disappear, and the hexagonal (100) reflection appears. Since this temperature exceeds the equilibrium Tm of the orthorhombic lamellar crystals (145oC15), such hexagonal crystals are attributed to oriented ECCs. Here, the equatorial profiles obtained during heating are deconvoluted into orthorhombic or hexagonal reflection peaks and amorphous halo. The resultant integral intensities of these resolved reflection peaks or amorphous halo are plotted as a function of temperature in Figure 6a. Peak deconvolution was performed in the 2θ range of 8 to 18o. Even before the lower-temperature-side endotherm, slight intensity increases are observed for orthorhombic (110) and (200) reflections and the amorphous halo, suggesting lamellar thickening of FCCs during heating.19 At 135oC, the intensities of (110) and (200) reflections rapidly decrease, due to melting of lamellar FCCs. This temperature position coincides with the lower-temperature-side Tm on the corresponding DSC profile (Figure 4). A hexagonal (100)


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reflection is recognized beyond 145oC, indicating that phase transition from orthorhombic to hexagonal form occurs with partial melting of the orthorhombic crystals. These results suggest that melting of lamellar FCCs induces phase transition of fibrillar ECCs into hexagonal form. Maximum intensity of the hexagonal (100) reflection peak was obtained at 150oC, which coincides with the position of the higher-temperature-side endotherm on the corresponding DSC profile. This means that the highertemperature-side endotherm corresponds to such phase transition for ECCs. As described above, the hexagonal form has gauche conformation,34 similar to the amorphous state; thus, there is no endotherm within such a higher temperature region on DSC profiles, although the hexagonal (100) reflection remains up to 160oC. Also of interest is the characteristic higher intensity for the orthorhombic (200) reflection. In the case of non-oriented orthorhombic crystals such as the undrawn initial film, the (200) reflection intensity is a third of the (110) reflection intensity. Similar higher intensity of the equatorial (200) reflection has been reported for uniaxially melt-drawn UHMW-PE film,35 exhibiting typical shish-kebab morphology, composed of ECCs and FCCs, with both molecular orientations parallel to drawing direction. Such a structural coincidence in terms of higher (200) reflection intensity indicates that the biaxially meltdrawn membrane in the present study also contains similar shish-kebab morphology oriented along the vertical direction of WAXD images. It should be noted that the (200) reflection intensity exceeds that of the (110) reflection when FCCs begin to melt beyond 135oC, meaning that such a predominant (200) reflection is attributed to the ECCs that still survive at 145oC. In turn, the early decrease of orthorhombic reflection intensities just beyond 135oC is attributed to the melting of FCCs, whereas the later decrease beyond 145oC is ascribed to phase transition into hexagonal form for ECCs. These continuous decreases of orthorhombic reflection imply that melting of FCCs and transition of ECCs are synchronizing during heating. These results suggest that both lamellar FCCs and fibrillar ECCs contribute to two equatorial reflection peaks; however, the other two pairs of diagonal points tilting 60o toward the equator have not


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been analyzed. Therefore, such diagonal profiles were also extracted from the series of in-situ WAXD along the line 60o tilted toward the equator, and these profiles are stacked in Figure 5b. The hexagonal (100) reflection is observable even in such diagonal profiles above 145oC. This result means that tilted ECCs coexist with those parallel to the drawing axis. Furthermore, two-step decreases of the orthorhombic (110) reflection are clearly recognizable. These diagonal profile changes were evaluated by peak resolution, similar to that for the melt-drawn membrane in Figure 6a. Figure 6b plots the integral intensities of the amorphous halo, orthorhombic (110), (200), and hexagonal (100) reflection peaks. These tilted profiles indicate the usual combination of higher (110) and lower (200) reflection intensities, which is quite different from the equatorial profiles in Figure 5b; therefore, the predominant a-axis orientation is lost at an azimuthal angle of 60o. Regarding temperature dependence, rapid decreases of the orthorhombic (110) reflection were recognized at both 135oC and 145oC in Figure 6b, corresponding to melting of lamellar FCCs and phase transition of fibrillar ECCs. Such stepwise decreases of orthorhombic reflection intensities are quite different from their continuous decreases for the series of equatorial profiles in Figure 6a. This means that these two sets of FCCs and ECCs, with parallel molecules and molecules tilted toward the drawing axis, exhibit different connections between them. However, the intensity of the hexagonal (100) reflection peak is much lower than that along the equator, suggesting fewer tilted ECCs than parallel ECCs. These in-situ measurement results give the clear assignments of double DSC endotherms for biaxially melt-drawn membrane, i.e. melting of FCCs (lower Tm) and ECCs (higher Tm). In other words, the formation of a crystallite population of different size is not the case for the origin of double endotherms observed in this study. In-situ WAXD measurement of melt-shrunk membrane. DSC heating scan was combined with insitu WAXD and SAXS patterns recorded during heating of the melt-shrunk membrane in Figure 7. There are two endothermic peaks, similar to the DSC profile for the melt-drawn membrane in Figure 4;


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however, the higher-temperature peak is significantly depressed. In contrast, the sharper endotherm is located at 132oC, which is slightly lower than the lower-temperature-side Tm for the melt-drawn membrane (135oC). These melting characteristics are also considered with the in-situ WAXD/SAXS measurement results, and compared to those for the melt-drawn membrane. Beyond 130oC, the orthorhombic (110) and (200) reflections begin to decrease with a corresponding decrease of the meridional spot on the SAXS patterns. These coincident decreases of WAXD and SAXS intensities are attributed to the melting of lamellar FCCs. However, the hexagonal (100) reflection is less recognizable even beyond this lower-temperature endotherm, which is quite different from the trend of the melt-drawn membrane in Figure 4. Correspondingly, the equatorial SAXS streak attributed to ECCs is weaker than that for the melt-drawn membrane for the entire temperature range. To quantitatively analyze the melting behavior of the melt-shrunk membrane, 2θ line profiles were extracted along the equator for the series of in-situ WAXD patterns (Figure 8a). The orthorhombic (110) and (200) reflections rapidly decrease beyond 130oC, whereas the amorphous halo increases, due to the melting of lamellar FCCs. Beyond 150oC, the hexagonal reflection appears but is much weaker than that for the melt-drawn membrane in Figure 5a. This indicates that the ECC amount for the melt-shrunk membrane is very small, which coincides with the smaller endotherm at 150oC for the DSC profile in Figure 7. Here, these equatorial profile changes for the melt-shrunk membrane are quite similar to the 60o-tilted ones for the melt-drawn membrane depicted in Figure 6b. Corresponding profile deconvolution results in Figure 8b depict that the integral intensity of the orthorhombic (110) reflection is higher than that of the (200) reflection, which is usual for the unoriented UHMW-PE films before drawing. Stepwise decreases of these orthorhombic reflection intensities at 130oC and 150oC also coincide with the 60otilted profile change for the melt-drawn membrane. Such a coincidence of the intensity changes in the orthorhombic reflections is attributed to the characteristic shish-kebab structure with the lower amount


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of ECCs both for the melt-shrunk membrane and the tilted region for the melt-drawn membrane. In-situ SAXS measurement during heating. Changes in in-situ SAXS patterns recorded during heating were also analyzed using line profile extraction. For equatorial streaks attributed to the ECC arrangement parallel to the drawing direction, scattering intensities are too strong to analyze for the melt-drawn membrane because of the larger amount of ECCs, as indicated by the corresponding DSC melting profile in Figure 4. Although the comparison of the equatorial to the meridional intensity gives the lamellar alignment ratio,36 but this analysis is difficult in the present study. Therefore, changes in meridional spots during heating are compared for melt-drawn and melt-shrunk membranes (Figure 9). These meridional scatterings include information on FCCs oriented along the drawing direction, although the 60o-tilted FCCs are not considered here. The conventional 2θ value is converted into the scattering vector q = 4πsinθ/λ, where λ is the wavelength of the radiated X-ray beam (1.00A). Plots beyond q = 0.005 (A-1) were analyzed considering the scattering region of the beam stopper (Supporting Information, Figure S2). Scattering intensity was multiplied by the Lorentz factor, q2, and represented by color gradation, the same as the WAXD line profile changes in Figures 5a and 8a. Before heating at room temperature, the long periods D (= 2π/q) calculated from the peak position of the scattering line profiles were 56 and 54nm for the melt-drawn and melt-shrunk membranes. Here, the thicknesses of the brighter (darker) regions in Figure 1, corresponding to crystalline (amorphous) layers, are 30nm (20nm). The total value of 50nm agrees with the above-mentioned long periods estimated from SAXS profiles. Comparison of crystalline thickness of FCCs stacked along the meridian in meltdrawn membranes and that in melt-shrunk membranes indicates that a combination of higher Tm and larger D for the former membrane is reasonable, assuming similar crystallinity. During heating of both membranes, the scattering peak disappears at 135oC, corresponding to the melting of FCCs, which coincides with the results of in-situ WAXD analyses. With heating of these membranes, the peak position shifts slightly to the lower q side, and intensity increases before FCC melting, due to lamellar thickening during heating, which is indicated for the in-situ WAXD patterns in


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Figure 6. These outline features in the meridional SAXS profiles are common for both membranes; however, quantitative analysis of scattering peak intensities distinguishes detailed differences for melting FCCs. Figure 10 plots the temperature dependence of the integral intensities and long period values of SAXS scattering peaks corresponding to FCC stacking. Integral intensity decreases beyond the Tm of the lower-temperature endotherms, which is slightly higher for the melt-drawn membrane. Another difference between the melt-drawn and melt-shrunk membranes is the temperature dependence of intensity decrease, which is more rapid for the melt-shrunk membrane. These characteristics of SAXS profile changes agree with those of the DSC profiles (i.e., lower Tm and narrower lower-temperature endotherm), which are ascribed to lamellar morphologies with more homogeneous and thicker meltshrunk membranes.

Discussion Structural models for melt-drawn and melt-shrunk membranes.

Characteristic features of ECC

and FCC structures of biaxially melt-drawn and melt-shrunk membranes prepared in this study are compared in Figure 11. The melt-shrunk membrane contains two types of shish-kebabs; one oriented along the drawing direction, and the other tilted 60o from the drawing direction. The former sets of ECCs and FCCs are interconnected, as indicated by the overlapping phenomenon of FCC melting and FCC transition (Figure 6a). During cooling after melt-drawing, FCCs epitaxially spread out from the surface of ECCs, playing the role of connector-tying ECCs. This means that melting of such connecting FCCs is restricted by ECC backbones that remain beyond the Tm of FCCs. Therefore, FCC melting and ECC transition become coordinated for the shish-kebab oriented along the drawing direction, resulting in a broader DSC endotherm of FCC melting at 135oC for the melt-drawn membrane. In contrast, the other set of 60o-tilted shish-kebab exhibits stepwise decreases of orthorhombic reflection intensities during heating, similar to the melt-shrunk membrane in Figure 7b. There are fewer ECCs within 60o-tilted shish-kebab although they still transition into hexagonal form at elevated


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temperature. Therefore, less interconnection of ECCs through FCCs is produced, while the melting behavior of the FCCs dominates on DSC, due to their larger content rather than that of ECCs, within the DSC profile for the melt-drawn membrane. Similar two-step reductions of orthorhombic reflection intensities are observable for the melt-shrunk membrane, meaning that the FCCs within each shishkebab melt individually, resulting in a narrower endotherm at 132oC. Formation mechanism of tilted shish-kebab. The difference between uniaxially and biaxially meltdrawing allows us assign the formation mechanism of tilted shish-kebab. As depicted in Figure 12, the edge-viewed WAXD pattern for uniaxially melt-drawn film contains exhibits none of the 60o-tilted reflection, independent of sample DR. Namely, the uniaxial melt-drawing gives the superior molecular orientation, compared to the biaxial melt-drawing. Here, we focus the dimensional change during both melt-drawings. The initial film thickness is 300µm, but the resultant value is 5µm for biaxial meltdrawing at 8x8, which is remarkably lower than 55µm for uniaxial melt-drawing even at a half DR of 32. Instead, the width of the uniaxially melt-drawn film is much narrowed from initial 3mm to final 500µm. These comparisons suggest that the sample thinning is much enhanced for the former biaxially meltdrawn membrane. Such membrane thinning causes the compression force applied perpendicular to the membrane surface during biaxial melt-drawing. This effect contributes the tilting of the ECC formed during biaxial melt-drawing, giving the molecular tilting of some of the resultant shish-kebabs. Namely, there are two types of ECCs, i.e. one is remained straight toward the drawing direction, but the other is tilted after biaxial melt-drawing. We37-39 have revealed that the stress transmission thorough entanglements is the driving force for ECC formation. There are two types of entanglements, i.e., “shallow” entanglements are disentangled during melt-drawing but the other “deep” entanglements are semi-permanent.3-5 Namely, melt-drawing segregates the amorphous region into still entangled and disentangled portions. This entanglement concept is experimentally supported by in-situ WAXD and NMR measurements during uniaxial meltdrawing of UHMW-PE.35,40 The deep entanglements restricts the growth of ECC length, giving the


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shorter ECCs, but the shallow entanglements are disentangled into the longer ECCs. The above compression force caused by membrane thinning is effective for the former shorter ECCs, which are connected by deeply entangled amorphous component each other. Such ECC connection through deeply entangled amorphous region is similar to the soft joint between crystalline bones. Therefore, the membrane thinning gives the symmetrical tilting toward drawing direction, like pantographs, as depicted in Figure 11. Such membrane thinning is enhanced with increasing biaxial DR, thus the tilting angle of the orthorhombic reflections for the melt-drawn membranes finally reach 60o at the highest DR examined in this study, as depicted in Figure 3.

Conclusions In-situ WAXD/SAXS measurement during heating of a biaxially melt-drawn UHMW-PE membrane indicates that the higher-temperature endotherm is attributable to the phase transition of ECCs, which first transform from the usual orthorhombic form into hexagonal form during heating at 150oC, and finally disappear at 160oC. In contrast, the lower-temperature endotherm is ascribed to the melting of FCCs, which are less oriented than ECCs. For the membrane melt-shrunk immediately after biaxial melt-drawing, less hexagonal reflection was observed on heating, indicating fewer ECCs. Therefore, the area of the higher-temperature endotherm is smaller than that of the biaxially melt-drawn membrane. In contrast, the lower-temperature endotherm becomes narrower, which coincides with the resultant homogeneous FCC networks. Correspondingly, the in-situ SAXS profile change exhibits a rapid decrease of long-period intensity at 130oC, corresponding to the lower-temperature endotherm.

Acknowledgments In-situ WAXD and SAXS measurements using synchrotron radiation were performed at the BL40B2 beamline of SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (2012B1117 and 2015A1161). This work was partly supported by the Mukai Science and Technology


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Foundation. Supporting Information Available: Schematic representations for X-ray images and SAXS line profiles extracted along the meridional of prepared membranes. This material is available free of charge via the Internet at http://pubs.acs.org.


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FIGURE CAPTIONS

Figure 1. SEM images of the membranes prepared in this study. (a) Biaxially melt-drawn at 150oC and up to 8x8 and (b) melt-shrunk from 7x7 to 4x4. Horizontal and vertical drawing directions in the biaxial membrane preparation were maintained for both images. Figure 2. WAXD (top) and SAXS (bottom) patterns through-viewed (//) with an incident X-ray beam perpendicular to the flat membrane surface and edge-viewed (┴) to the membrane edge. Membranes were biaxially melt-drawn up to 8x8 (left) and resultant 4x4 shrank from 7x7 (right). All drawing and shrinking were performed at 150oC. Vertical drawing directions in the biaxial membrane preparation were maintained for all images. Figure 3．．Comparison of the azimuthal angle scans extracted for the edge-viewed WAXD pattern for biaxial drawn UHMW-PE membranes with the different biaxial DRs. (a) (110) reflection; (b) (200) reflection. The corresponding azimuthal scan is indicated by the dashed arrow within the inset WAXD pattern for the 8x8 membrane, which is same in Fig. 2. The equatorial position in the pattern is set at 0o in the azimuthal scan. For comparison, the intensity of every profile is vertically shift with the constant height interval. Figure 4. DSC thermograms for a biaxial DR = 8×8 sample and corresponding in-situ WAXD (top) and SAXS (bottom) images recorded with edge viewed. Draw direction is vertical. Figure 5. Duplicated WAXD line profiles recorded during heating for an edge-viewed biaxial DR = 8x8 membrane. A series of profiles was extracted (a) along the equator and (b) with 60o tilting from the equator. Intensity is represented by color gradation from lowest (blue) to highest (red). The dotted arrows in the inset images represent the extracting directions. Draw direction in the inset images is vertical. The dotted lines indicate peak positions of double-melting endotherms in Figure 4. Figure 6. Changes in integral intensities estimated from the series of in-situ WAXD line profiles ACS Paragon Plus Environment

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depicted in Fig. 5 for a biaxial DR = 8x8 membrane. The line profiles extracted (a) along the equator and (b) with 60o tilting from the equator were deconvoluted into amorphous, hexagonal (100)h, and orthorhombic (110)o and (200)o reflection peaks. Figure 7. DSC thermograms for a resultant DR = 4×4 membrane melt-shrunk from 8x8 and corresponding in-situ edge-viewed WAXD (top) and SAXS (bottom) images. Draw direction in the inset images is vertical. Figure 8. (a) Duplicated WAXD line profiles recorded edge-viewed during heating for a resultant DR = 4x4 membrane melt-shrunk from 7x7. A series of line profiles was extracted along the equator. Draw direction in the inset image is vertical. (b) Changes in integral intensities estimated from the series of insitu WAXD line profiles depicted in (a). The dotted lines indicate peak positions of double-melting endotherms in Figure 7. Figure 9. Duplicated SAXS line profiles extracted along the meridian during heating for the prepared membranes. (a) DR = 8×8 membrane and (b) resultant DR = 4x4 membrane melt-shrunk from 7x7. The meanings of the dotted lines and the arrows in the inset figures are the same as in Figs. 5 and 8. Draw direction in the inset images is vertical. Figure 10. Changes in integral intensities and long-period values estimated from the series of meridional SAXS line profiles depicted in Figure 9. (a) DR = 8×8 membrane and (b) resultant DR = 4x4 membrane melt-shrunk from 7x7. Figure 11. Schematic representations of structural differences between (a) melt-drawn and (b) meltshrunk membranes prepared in this study. Green molecules indicate FCCs, and blue molecules indicate ECCs. (a) The melt-drawn membrane includes two FCC/ECC arrangements, where molecules orient parallel to or 60o-tilted toward the drawing direction. In contrast, (b) the melt-shrunk membrane exhibits random orientation.


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Figure 12．．WAXD (top) and SAXS (bottom) patterns through-viewed and edge-viewed for the films uniaxially melt-drawn at 150oC up to DRs of 8 (left) and 32 (right). Drawing direction is vertical.


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Figure 1

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500nm

(b)

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Figure 2

7x7 → 4x4

8x8 ┴

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Figure 3

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Figure 4

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Figure 5

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Figure 7

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(100)h

160 140 120

1700 1712 1725 1737 1750 1763 1776 1789 1802 1815 1828 1841 1855 1868 1882 1896 1909 1923 1937 1951 1966 1980 1994 2009 2023 2038 2053 2068 2083 2098 2114 2129 2144 2160 2176 2192 2208 2224 2240 2256 2273 2289 2306 2323 2340 2357 2374 2391 2409 2426 2444 2462 2479 2498 2516 2534 2553 2571 2590 2609 2628 2647 2666 2686 2705 2725 2745 2765 2785 2805 2826 2846 2867 2888 2909 2930 2951 2973 2994 3016 3038 3060 3083 3105 3128 3151 3173 3197 3220 3243 3267 3291 3315 3339 3363 3388 3412 3437 3462 3487 3513 3538 3564 3590 3616 3643 3669 3696 3723 3750 3777 3805 3833 3860 3889 3917 3945 3974 4003 4032 4062 4091 4121 4151 4181 4212 4242 4273 4305 4336 4367 4399 4431 4464 4496 4529 4562 4595 4629 4662 4696 4730 4765 4800 4835 4870 4905 4941 4977 5013 5050 5087 5124 5161 5199 5236 5275 5313 5352 5391 5430 5469 5509 5549 5590 5631 5672 5713 5755 5796 5839 5881 5924 5967 6011 6054 6099 6143 6188 6233 6278 6324 6370 6416 6463 6510 6558 6605 6654 6702 6751 6800 6850 6899 6950 7000 7051 7103 7154 7207 7259 7312 7365 7419 7473 7527 7582 7637 7693 7749 7805 7862 7920 7977 8035 8094 8153 8212 8272 8332 8393 8454 8516 8578 8640 8703 8767 8830 8895 8960 9025 9091 9157 9224 9291 9358 9427 9495 9564 9634 9704 9775 9846 9918 9990 1.006E4 1.014E4 1.021E4 1.028E4 1.036E4 1.043E4 1.051E4 1.059E4 1.066E4 1.074E4 1.082E4 1.09E4 1.098E4 1.106E4 1.114E4 1.122E4 1.13E4 1.138E4 1.147E4 1.155E4 1.163E4 1.172E4 1.181E4 1.189E4 1.198E4 1.206E4 1.215E4 1.224E4 1.233E4 1.242E4 1.251E4 1.26E4 1.269E4 1.279E4 1.288E4 1.297E4 1.307E4 1.316E4 1.326E4 1.336E4 1.345E4 1.355E4 1.365E4 1.375E4 1.385E4 1.395E4 1.405E4 1.415E4 1.426E4 1.436E4 1.447E4 1.457E4 1.468E4 1.478E4 1.489E4 1.5E4

100 80 60

(110)o

10

12

(200)o

14

16

18

20

2 (deg.) 20000

(b)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

15000

10000

(110)o

Amorphous

5000

(200)o

(100)h

0 60

80

100

120

140

160

o

Temperature ( C)


8

Page 33 of 37

Figure 9

180 160

o

Temperature ( C)

(a) 140 120 100 80 60 0.01

0.02

0.03 -1

0.04

0.05

0.04

0.05

q (A )

180

(b) 160

o

Temperature ( C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


140 120 100 80 60 0.01

0.02

0.03 -1

q (A )


9


Figure 10

8000

100

6000

80

4000

60

2000

Long Period (nm)

Integral Intensity (arb. unit)

(a)

40

Integral Intensity Long Period 0

20 60

80

100

120

140

160

o

Temperature ( C)

8000

100

(b) 6000

80

4000

60

2000

Long Period (nm)

Integral Intensity (arb. unit)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 37

40

Integral Intensity Long Period 0

20 60

80

100

120

140

160

o

Temperature ( C)


10

Page 35 of 37

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 11

(a)

(b)


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 37

Figure 12

DR=32

DR=8 //

┴

┴

//

(110)o

WAXD (200)o

SAXS


12

Page 37 of 37

TOC

180

Melt-Drawn UHMW-PE

Melt-Shrunk UHMW-PE Amorphous

Amorphous

o

Temperature ( C)

o

(100)h

140 120 100 80 60 10

1700 1712 1725 1737 1750 1763 1776 1789 1802 1815 1828 1841 1855 1868 1882 1896 1909 1923 1937 1951 1966 1980 1994 2009 2023 2038 2053 2068 2083 2098 2114 2129 2144 2160 2176 2192 2208 2224 2240 2256 2273 2289 2306 2323 2340 2357 2374 2391 2409 2426 2444 2462 2479 2498 2516 2534 2553 2571 2590 2609 2628 2647 2666 2686 2705 2725 2745 2765 2785 2805 2826 2846 2867 2888 2909 2930 2951 2973 2994 3016 3038 3060 3083 3105 3128 3151 3173 3197 3220 3243 3267 3291 3315 3339 3363 3388 3412 3437 3462 3487 3513 3538 3564 3590 3616 3643 3669 3696 3723 3750 3777 3805 3833 3860 3889 3917 3945 3974 4003 4032 4062 4091 4121 4151 4181 4212 4242 4273 4305 4336 4367 4399 4431 4464 4496 4529 4562 4595 4629 4662 4696 4730 4765 4800 4835 4870 4905 4941 4977 5013 5050 5087 5124 5161 5199 5236 5275 5313 5352 5391 5430 5469 5509 5549 5590 5631 5672 5713 5755 5796 5839 5881 5924 5967 6011 6054 6099 6143 6188 6233 6278 6324 6370 6416 6463 6510 6558 6605 6654 6702 6751 6800 6850 6899 6950 7000 7051 7103 7154 7207 7259 7312 7365 7419 7473 7527 7582 7637 7693 7749 7805 7862 7920 7977 8035 8094 8153 8212 8272 8332 8393 8454 8516 8578 8640 8703 8767 8830 8895 8960 9025 9091 9157 9224 9291 9358 9427 9495 9564 9634 9704 9775 9846 9918 9990 1.006E4 1.014E4 1.021E4 1.028E4 1.036E4 1.043E4 1.051E4 1.059E4 1.066E4 1.074E4 1.082E4 1.09E4 1.098E4 1.106E4 1.114E4 1.122E4 1.13E4 1.138E4 1.147E4 1.155E4 1.163E4 1.172E4 1.181E4 1.189E4 1.198E4 1.206E4 1.215E4 1.224E4 1.233E4 1.242E4 1.251E4 1.26E4 1.269E4 1.279E4 1.288E4 1.297E4 1.307E4 1.316E4 1.326E4 1.336E4 1.345E4 1.355E4 1.365E4 1.375E4 1.385E4 1.395E4 1.405E4 1.415E4 1.426E4 1.436E4 1.447E4 1.457E4 1.468E4 1.478E4 1.489E4 1.5E4

180

160

Temperature ( C)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(110)o

12

14

16

2 (deg.)

140 120 100 80 60

(200)o

18

20

(100)h

160

10

(110)o

12

(200)o

14

16

2 (deg.)

18

20


13

Phase Transition during Heating of Nanostructured Ultrahigh

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