Precursor of Shish-Kebab above the Melting Temperature by

Apr 2, 2013 - ABSTRACT: Preparation of strong fiber with ultrahigh strength and modus is a dream of polymer scientists and engineers. It is believed t...
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Precursor of Shish-Kebab above the Melting Temperature by Microbeam X‑ray Scattering Toshiji Kanaya,†,* Inga A. Polec,† Tetsuaki Fujiwara,† Rintaro Inoue,† Koji Nishida,† Tomohiko Matsuura,† Hiroki Ogawa,‡ and Noboru Ohta‡ †

Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611-0011, Japan Japan Synchrotron Radiation Research Institute, Sayo-gun, Hyogo-ken 679-5198, Japan

‡

ABSTRACT: Preparation of strong fiber with ultrahigh strength and modus is a dream of polymer scientists and engineers. It is believed that a shish-kebab is the structure origin of the ultrahigh strength and modulus fiber. This article presents our investigation on the inner structure of shish-kebab precursor of isotactic polystyrene in μm scale formed by shear flow above the nominal melting temperature Tm using microbeam wide- and small-angle X-ray scattering (WAXS and SAXS). The microbeam WAXS experiments indicated that the precursor included crystallites with degree of crystallinity of ∼0.15%, which had higher melting temperature than normal lamellar crystals, meaning that its size (the length) was longer in the c-axis of crystals than the normal lamellar crystals. The microbeam SAXS experiments confirmed that approximately 1% of the crystallites in the precursor were rather long in the direction of c-axis. It is expected that such crystallites work as an initiator of a shish when it is below Tm.

1. INTRODUCTION Oriented polymer morphology formed during and after shear, elongational and/or mixed flows has caused enormous interest of both scientific and industrial societies.1−5 This is due to a significant improvement in physical and mechanical properties of the materials that experience such treatment. The understanding of polymer crystallization mechanism under flows is therefore essential not only to control processing methods and technologies to achieve desirable properties but also can potentially enable us to design new, more profitable processing techniques complying with polymer dynamics and crystallization mechanisms. Application of shear and/or elongational flows to polymer melts results in creation of so-called shish-kebab structure. Generally it is considered as extended chain crystal (central core, i.e., shish) with periodically grown folded chain lamella crystals (kebabs).1,3−5 In reality, however, it exhibits complex and hierarchical structure.6−8 Extensive studies on shish-kebab formation show that a precursor is created before the shishkebab,8−24 suggesting that final morphology of the shish-kebab is dominated by the precursor. The precursor must be then the key to understand the complexity of the shish-kebab structure and its formation. The precursor has been observed in both nanometer to tens of nanometers scale8−13 when SAXS and WAXS were applied and micrometer scales14−24 when polarized optical microscope (POM), in situ birefringence, and light scattering were used. A small-angle neutron scattering (SANS) study on shish-kebab in a wide spatial scale7 elucidated that a large oriented structure in μm scale included some extended chain crystals with approximately an 10 nm diameter. © XXXX American Chemical Society

This confirms existence of oriented structures in both naonmeter and micrometer scales. This allows us to assume that the micrometer scale structure works as a precursor or a scaffold for the shish-kebab formation in nanometer scale. Another unsolved problem in this field is whether a precursor contains crystal or not. Noncrystallized shish-like structure observed in isotactic polypropylene (iPP) above the nominal melting temperature Tm by SAXS was assigned to the precursor.9 Other SAXS and WAXS measurements on polyethylene (PE) blend of low and high molecular weight components showed that formed, in tens nm scale, needlelike precursors contained limited crystallinity even above the equilibrium melting temperature Tm0.11,12 Nonetheless, due to the complexity of the structure and the technique limitations, the experiments above the nominal melting temperature Tm are still under debate. In the following article we are presenting the results of the microbeam SAXS and WAXS measurements on the isotactic polystyrene (iPS) precursor above nominal Tm. The WAXS data exhibited weak Bragg diffractions from the precursor in μm scale, indicating that the precursor included very small amount of crystals. The SAXS measurements showed that approximately 1% of the existing crystallites in the precursor are ∼140 nm long in the c-axis direction. These nm scale crystals must be precursors of shish in the oriented structure in μm scale. Received: January 10, 2013 Revised: March 11, 2013

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direction run was repeated 10 and 8 times for WAXS and SAXS, respectively. For each given temperature, the mapping area of 200 by 200 μm for the WAXS experiments and 160 by 135 μm for the SAXS experiments were analyzed. When the first run finished, the temperature was increased to 255, 260, and, subsequently, 265 °C with a heating rate of 30 °C/min to perform the next three runs. After the increase in temperature, the sample was stabilized for 30 min before mapping at each temperature. The acquisition time of 2 s was applied for each position. The data were corrected for the scattering from the empty cell with the kapton windows and air.

2. EXPERIMENTAL SECTION 2.1. Materials and Samples. Isotactic polystyrene (iPS) with weight-average molecular weight Mw of 400 000 g/mol and molecular weight distribution Mw/Mn of 2.0 was used in this work. The polymer was purchased from Scientific Polymer Products. The nominal melting temperature (Tm), determined by differential scanning calorimeter (DSC) measurement with a heating rate of 5 °C/min, was 223 ± 1 °C. The samples used for the experiments were compression molded at 290 °C for 10 min and subsequently cooled in air. Disks of approximately 13 mm diameter and 0.25 mm thickness were cut from such prepared film. 2.2. Microbeam WAXS and SAXS Measurements. The microbeam WAXS and SAXS experiments were performed at the BL40XU beamline in a synchrotron radiation facility, SPring-8 in Nishiharima, Japan. The beam size evaluated by a full-width at halfmaximum (fwhm) was 5 μm. Wavelength of incident X-ray was 0.83 Å, and the X-ray detector was a beryllium-windowed image intensifier (V5445P, Hamamatsu Photonics, Hamamatsu, Japan) coupled to a cooled CCD camera (ORCA-II-ER, Hamamatsu Photonics). The SAXS detector (with a pixel size of 90.5 μm) and the WAXS detector (with a pixel size of 160.6 μm) were placed at 1820 and 130 mm from the sample, respectively. A Linkam CSS-450 shear cell was used to impose both temperature and shear. The shear cell was fixed vertically to a precisely moving in x and z direction platform. The iPS samples were placed on the stainless plates with windows covered with a 25 μm thick kapton film located in the shear flow cell. The temperature protocol utilized during both the microbeam WAXS and SAXS experiments is shown in Figure 1.

3. RESULTS AND DISCUSSION As mentioned briefly in the Introduction, it is considered that a precursor (or scaffold) has hierarchic structure in nanometer and micrometer scales and is created before formation of a shish-kebab. Our SANS studies on a drawn blend of medium molecular weight polyethylene (PE) with a small amount of ultrahigh molecular weight component7 revealed existence of the large oriented objects in μm scale containing several extended chain crystals (shish) in nm scale. To observe and further understand the mechanism of this shish-kebab, polarized optical microscope (POM) measurements were performed. The structure formation experiment was conducted on iPS under shear flow above the nominal melting temperature Tm. The resultant morphology with a several tens of mixcrometers in diameter and several hundred micrometers in length was observed (Figure 2a). The long oriented objects relaxed in length and width in the early stage of annealing.26 In addition, they flow slightly during the annealing due to the convection. The small changes of the precursor observed in Figure 2a must be due to these reasons. Except for the changes noted, these long oriented objects neither dissolved nor grew for more than 24 h while kept at this temperature. Interestingly, this structure was very similar in size and shape to that one observed in SANS measurements on drawn PE.7 The experiment on the iPS was now modified. Namely, the iPS prepared at 250 °C was cooled to 210 °C below its Tm of 223 °C. As illustrated in Figure 2b, because crystallization had started around the precursor, the ordinary SAXS measurements showed two-spot pattern along the flow direction. It indicated that oriented lamellar crystals (or kebab) had grown on/in the long objects, showing that it worked as an incubator of shishkebab. Hence, this long entity is called precursor. After 1 h annealing at 210 °C, the iPS sample was heated to 290 °C with a heating rate of 10 °C/min. The crystals (kebabs) which had grown on/in the precursor melted around Tm as revealed by the SAXS pattern in Figure 2b. However, the precursor originally formed at 250 °C was still detectable even above Tm and melted at around 270 °C, close to the equilibrium melting temperature Tm0. The experimental outcome described above provides information to formulate two most possible hypotheses for the inner structures of the shear-induced precursor: (1) the precursor includes some crystals with higher melting temperature than that of normal lamella crystals and work as cross-links for noncrystallized polymer chain to form a gel-like structure, and (2) the precursor has liquid crystal-like structure because iPS chains can easily form a 3/1 helix structure which is rather rigid and could work as mesogen in liquid crystals.27 The fact that the melting of the precursor was close to the equilibrium melting temperature Tm0 suggested that the former possibility is more likely. In order to confirm this hypothesis, the regular SAXS and WAXS measurements on the precursor induced at 250 °C by pulse shear with a shear

Figure 1. Temperature and time protocol for both the microbeam WAXS and SAXS experiments. The samples were heated to 290 °C with a heating rate of 30 °C/ min, above the equilibrium melting temperature of the iPS crystals (=289 °C,25), and held at this temperature for 5 min to melt to remove any previous thermal history. During this time, the gap between the stainless plates was reduced from 2500 to 30 μm. After the time period of 5 min, the sample was cooled to shearing temperature of 250 °C with the same cooling rate. After reaching the shearing temperature, shear flow with rate of 720 s−1 was applied for shear time ts of 8 s giving shear strain of 576 000%. Note that the shear rate of 720 s−1 for the kapton windows effectively corresponds to ∼70 s−1 for the quartz windows, resulting in the shear strain of 56 000%. This must be due to the softness of the 25 μm thick kapton film. The strong shear condition was selected to produce many precursors in the microbeam experiments. Just after applying the shear flow, we observed that the small precursor disappeared as shown in a previous paper,26 and the effects of cessation of the flow were serious, and then the sample was kept at the shearing temperature for 150 min to stabilize the oriented structure. The first 20 min microbeam mapping was performed after that time. First, the platform moved in z direction making in total 40 and 27 steps for WAXS and SAXS, respectively. Each step was equal to 5 μm and it took 4 s to change the platform position between each individual step. Then in 4 s, the platform came back to the starting position moving simultaneously by 20 μm in x-direction. The xB

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Figure 2. (a) Polarized optical microscope (POM) image of the precursor existing for 24 h at 250 °C and (b) structure evolution of the shish-kebab precursor recorded by POM and SAXS when cooled to 210 °C below Tm and then heated to 290 °C above Tm0.

rate of 30 s−1 and strain of 12 000%21 were carried out. However, there were no signs of crystals, suggesting that there are no crystals within the detection limit. After the regular SAXS/WAXS experiments and before the current experiments we re-examined the experimental conditions to reach a conclusion that the degree of crystallinity was too low to detect by the regular SAXS/WAXS. Therefore, we decided to use a microbeam synchrotron (SR) X-ray to confirm whether or not a precursor includes crystals. The microbeam X-ray is an excellent tool to overcome this difficulty. Figure 3a shows the POM picture of the precursor formed by applying a pulse shear at 250 °C and the microbeam WAXS experiments were performed on the precursor.

patterns on the precursor are shown in Figure 3b. Very clear, although weak, Bragg diffractions were observed almost everywhere in the precursor area (Figure 3b, upper). When the microbeam was out of the precursor, only the amorphous halo was detected, no Bragg peaks (Figure 3b, lower). These findings give the direct evidence that the precursor includes crystallites. The Miller indexes assigned to the Bragg peaks observed in our experiment (Figure 3b, upper) are based on the reported iPS crystal data.28 The isolated Bragg peaks clearly shows that the c-axis (chain axis) is aligned along the flow direction. Figure 4a shows the 1D diffraction curve obtained by averaging the WAXS intensity along the direction of the (220) plane. The dashed line is a curve fitted to amorphous halo. Subtraction of the amorphous contribution allows us to extract

Figure 3. (a) Polarized optical microscope (POM) image of the selected oriented entity together with the mapping area scanned by microbeam X-ray and (b) 2D microbeam WAXS images on shishkebab precursor (upper) and out of it (lower).

The pink color of the precursor is due to the oriented kapton film used as a cell window not to the retardation of the precursor. As described in the Experimental Section, the step intervals during the mapping were 5 and 20 μm in the z- and xdirections, respectively. The microbeam WAXS mapping was performed by scanning the sample area indicated by both the white and red circles in Figure 3a. The red dots indicated places where the Bragg peaks were observed. The white circles pointed the positions where the Bragg reflections were not detected. The observed 2D WAXS

Figure 4. (a) 1D intensity curve calculated from the WAXS microbeam mapping outcome. Inset shows the results of fit to the Bragg peak from (220) plane. (b) Crystallinity distribution along the mapping area. C

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the Bragg peak and fit the peak with Lorentz function (Inset of Figure 4a). Consequently, taking into account the average thickness of the precursor, it was possible to evaluate an apparent crystallinity in the precursor, which was 0.15% in average in the precursor and 0.004% in the mapping sample. Note that the crystallinity evaluated is apparent because it was evaluated from only the (220) diffraction. These numbers are so low that it became clear why the crystals in the precursor could have not been observed by regular WAXS measurement in our previous work.21 On the basis of the mapping results, the distribution of the crystallinity in the precursor was also calculated. As illustrated in Figure 4b, distribution of the crystallites in the precursor within the beam size (5 μm) resolution seems to be homogeneous. However, is the homogeneous distribution of crystallites in the precursor acceptable or not? To conduct this discussion, we need information on the size of crystallites. Inset of Figure 4a shows the Bragg peak from the (220) plane after subtracting the amorphous contribution. The peak width ΔQ/Q is about 9% which is much higher than the Q resolution of the WAXS machine (about 1%). Generally, it is assumed that broadening of a Bragg peak is mainly caused by the limited size of and the defects in crystallites. The size of a polymer crystallite is generally in nm scale, implying the peak width is caused by the limited size of the crystallites. Hence, we have evaluated the size crystallites D along the (220) and (211) planes at 250, 255, and 260 °C using the Sherrer equation given by D = (Kλ)/ (BcosΘ); where B, λ, θ, and K are a width of Bragg peak due to the limited size, a wavelength of the X-ray, half of a scattering angle, and Sherrer’s constant near unity, respectively. The width of Bragg peak was evaluated by fitting the Lorentz function to the observed peak (see inset of Figure 4a) and Scherrer’s constant K of 1 was taken. The evaluated sizes along (211) and (220) planes are as follows: (14 ± 2.5 and 12 ± 2.1 nm), (16 ± 1.7 and 12 ± 1.7 nm), and (16 ± 2.1 and 13 ± 1.9 nm) at 250, 255, and 260 °C, respectively. The widths are rather fluctuating indicating the size fluctuation of the crystallites, however, it can be said that the average crystallite size is around 12−16 nm (14 nm in average). The average size along the (211) plane is larger than that along (220) plane, suggesting that the size along the c-axis is larger than those long other axes. The size along the caxis was also evaluated from the melting temperature of the precursor. It is well-known that a value of the melting temperature for polymer crystallites is dominated by the size of the crystallites. The surface energy of crystallites is responsible for the phenomenon. The surface energy of the plane normal to the c-axis is especially large and hence the melting temperature Tm(D) considerably depends on a crystallite size in the c-axis direction. In other words, Tm(D) is determined by the lamellae thickness. In our experiments, the melting temperature of the precursor was between 265 and 270 °C. It must then correspond to Tm(D) of crystallites in the precursor. According to the data published by Strobl et al.,25 who measured Tm(D) of iPS as a function of a lamella crystal thickness as well, Tm(D) of 265 and 270 °C correspond to the lamella crystal thickness (or the size along the c-axis) of 16 and 20 nm, respectively. Comparing these values to the size evaluated from the Sherrer’s equation presented above, it seems that the size along the c-axis is slightly larger than the size along the a- and b-axes. Note that we observed the distribution of the disappearance temperatures of the precursors (see Figure 2a). This must be due to the distribution of the crystallite sizes in the c-axis direction (or the distribution of the melting

temperatures) in the precursor. These results naturally raise a question if these crystallites sizes can explain the homogeneous distribution of crystallites in the precursors. We found the homogeneous distribution of the crystallites in the precursor within the microbeam resolution (∼5 mm in fwhm). The homogeneous distribution in two dimension means that there is at least one crystallite in an irradiated volume in the precursor. Knowing a precursor thickness along the depth direction (10.4 μm in average measured by a microscope) and simply assuming that the beam cross section is a circle with a 5 μm in diameter, we evaluated the irradiated volume of 20.4 × 109 nm3. The degree of crystallinity in the precursor is 0.15% as shown above. Hence, the total volume of crystallites in the precursor in the irradiated volume is 0.306 × 109 nm3. Assuming the size of the crystallites is 12, 12, and 16 for a-, b-, and c-axes, the volume is 2304 nm3 for a crystallite. If the total volume of crystallites in the irradiated volume is divided by the volume of a single crystallite, the number of the crystallites in the irradiated volume is 1.33 × 105. This number is large enough for the homogeneous distribution of crystallites in the precursor. The microbeam SAXS measurements on the precursor were performed as well. The POM picture of the precursor taken before the SAXS measurements at 250 °C is shown in Figure 5a.

Figure 5. (a) Polarized optical microscope (POM) image of the selected oriented entity together with the mapping area scanned by microbeam X-ray and (b) 2D microbeam SAXS images on the precursor in micrometer scale. Strong and weak streak scattering (lower and middle) normal to the orientation direction was detected at the red and yellow positions, but not at the white positions (upper).

The microbeam SAXS experiment was performed under the same shear and temperature conditions as the WAXS measurements. After applying the shear flow, the sample was kept for 150 min to stabilize. During this time, some precursors moved and rotated, and the orientation direction of the precursor did not always follow the flow direction as seen in Figure 5a. The red, yellow and white dotes in Figure 5a indicates the area where the microbeam SAXS mapping experiments were performed. The red and yellow circles depict the places where the strong and weak streak scatterings normal to the orientation direction were detected. The white circles show their absence. The streak scatterings were observed only in very few positions. Its fraction is about 1%, slightly depending on the temperature within the experimental fluctuations. The streak scattering normal to the orientation D

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In order to evaluate the length 2H of the shish, the scattering intensity along Qz was analyzed according to eq 2. For this analysis the scattering intensity I(Qz) along Qz at four given Qy values was plotted in Figure 6b; note that Qy = 0 in the measurement. According to eq 2, the scattering intensity I(Qz) must be scaled by a factor of 1/[2J1(QxR)/QxR]2. Hence we first fitted the sum of the theoretical function and the background to the observed I(Qz) at Qx = 0.277 nm−1 and evaluated the length of 2H to be 140 nm. For the fitting process, the background was the observed scattering intensity where no streak scattering was observed. The result of the fit is shown by a thick solid curve in Figure 6b for Qx = 0.277 nm−1 while the background is plotted with a dashed line. Then, using the same length (=140 nm) we fitted the theoretical curve to other observed curves at Qx = 0.262, 0.292, and 0.307 nm−1 only by changing the scaling factor. The results are plotted with thin solid curves in Figure 6b. The scaling worked well, suggesting that the assumption of cylindrical shape is appropriate for the analysis. The evaluated length 2H of 140 nm is rather short compared with that observed by TEM for PE crystallized in extruder (several hundred micrometers)4,5 but long compared with the normal lamellar thickness of iPS (∼10 nm).25 The contour length of iPS chain used in the experiment is about 1000 nm and hence the 140 nm does not correspond to the fully extended chain crystal. As indicated by the mapping result, the formation of such long crystallites is very few (∼1% among the crystallites in the precursor). These results imply that the fully extension of polymer chains by a shear flow is not easy, and hence elongational flow and/or slow drawing near the melting temperature is necessary to extend chain crystals in total. The results described above clearly indicate the presence of many small crystallites formed by shear flow in the precursor even above the nominal melting temperature. These crystallites are larger in size along the c-axis and have higher melting temperature than the normal lamellae crystals. Additionally, ∼1% of the existing crystallites in the precursor have rather large length of ∼140 nm in the c-axis direction. These crystallites must act as incubators for shish-kebab, especially for shish crystal (extended chain crystal) at temperatures below the nominal melting temperature. Finally we discuss a growth possibility of the long crystallites above the nominal melting temperature Tm although we did not observe the growth in the experiment. According to a paper by Strobl et al.25 the equilibrium melting temperature Tm0 of iPS is 289 °C, and hence in principle there is a possibility of the growth of the long crystals at 250 °C. In fact, Petermann et al.30 have reported the growth of shish from the molten iPS on a cleaved TEM grid at a high temperature (250 °C) above Tm. However, we did not observe the growth at 250 °C. We are expecting two possibilities for that: one is that the growth rate at 250 °C is so slow that the growth is invisible within the experimental time and the other is that some external stimulations such as flow or stress is necessary for the growth. At the moment, however, we have no conclusions.

direction suggests that a long object in nm scale was aligned along the orientation direction in the precursor. This must be shish crystal (or extended chain crystal). As proved in the microbeam WAXS experiments crystals exist everywhere in the precursor. Therefore, it is considered that fraction of the shish crystals (or the extended chain crystals) is about 1% of the existing crystals in the precursor. It should be mentioned that short crystals cannot be observed in the SAXS experiments because of the scattering intensity from those is spread over a broad angle range. Assuming for simplicity that the shape of a shish is a long cylinder, we evaluated a size of the shish in the precursor. Because the number of shish is small, it can be considered that the shish is isolated in the precursor, and hence, we assume that we observed the form factor of a cylinder in this measurement. The form factor I(Q) has been calculated for a cylinder with radius R and length 2H as29 I(Q ) = [A(Q , R , H )]2

(1)

where A is the amplitude of scattered wave given below. A (Q , R , H ) =

2 2 sin(Q zH ) 2J1( Q x + Q y R )

Q zH

Q x2 + Q y2 R

(2)

Here, J1 is the first order cylindrical Bessel function, the components of scattering vector Qx and Qz are defined in Figure 5b, and Qy is parallel to the beam direction and negligibly small in the SAXS geometry. According to the eqs 1 and 2, the radius of the cylinder can be evaluated from the scattering intensity along Qx at any Qz. It is clear, however, the scattering intensity at Qz = 0 is the strongest, and hence we plotted I(Qx) at Qz = 0 in Figure 6a with a theoretical function calculated from eq 1 at Qz = 0.

Figure 6. (a) SAXS intensity I(Qx) at Qz = 0 and the theoretical function of a cylinder (form factor) along Qx (see eq 2). (b) SAXS intensity I(Qz) at Qy = 0 and the theoretical function of cylinder (form factor) along Qz (see eq 2) at Qx = 0.262, 0.277 0.292, and 0.307 nm−1.

The best agreement between the observed and calculated intensities gives the radius of 9.1 nm. This value agrees with the shish radius reported7 rather well. The observed scattering intensity does not show oscillation with Qx, suggesting large thermal fluctuations of the radius of the shish and/or the distribution of the radii. It is noted that the intensity I(Qx) in the high Qx region above ∼0.3 nm−1 gradually merges to I(Qy) ∼ Qx−3 curve which is an asymptotic form of a form factor of cylinder, supporting this cylinder model again. In any case, the radius of the shish evaluated here was acceptable as that of the extended chain crystal.

4. CONCLUSIONS In the present article, we present our investigation on the inner structure of the large oriented objects in μm scale formed by shear flow above the nominal melting temperature Tm. These large oriented entities worked as a precursor of shish-kebab when they were cooled to a temperature below Tm. The microbeam WAXS experiments on the precursor clearly E

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(20) Ogino, Y.; Fukushima, H.; Takahashi, N.; Matsuba, G.; Nishida, K.; Kanaya, T. Macromolecules 2006, 39, 7617−7625. (21) Hayashi, Y.; Matsuba, G.; Zhao, Y.; Nishida, K.; Kanaya, T. Polymer 2009, 50, 2095−2103. (22) Gutierrez, M. C. G.; Alfonso, G.; Riekel, C.; Azzurri, F. Macromolecules 2004, 37, 478−485. (23) Azzurri, F.; Alfonso, G. C. Macromolecules 2005, 38, 1723−1728. (24) Azzurri, F.; Alfonso, G. C. Macromolecules 2008, 41, 1377−1383. (25) Al-Hussein, M.; Strobl, G. Macromolecules 2002, 35, 1672− 1676. (26) Zhao, Y.; Matsuba, G.; Nishida, K.; Fujiwara, T.; Inoue, R.; Polec, I.; Deng, C.; Kanaya, T. J. Polym. Sci., Part B: Polym. Phys. 2010, 49, 214−221. (27) Kaji, K.; Nishida, K.; Kanaya, T.; Matsuba, G.; Konishi, T.; Imai, M., In Interphases and Mesophases in Polymer Crystallization III; Advances in Polymer Science 191; Allegra, A., Ed.; Springer: Berlin and Heidelberg, Germany, 2005; pp 187−240. (28) Natta, G.; Corradini, P.; Bassi, I. W. Nuovo Cimento (Suppl. 1) 1960, 15, 68−82. (29) Shibayama, M.; Nomura, S.; Hashimoto, T.; Thomas, E. L. J. Appl. Phys. 1989, 66, 4188−4197. (30) Lieberwirth, I.; Loos, J.; Petermann, J.; Keller, A. J. Polym. Sci., Part B: Polym. Phys. 2000, 38, 1183−1187.

showed that there are small crystallites with higher melting temperature than normal lamellar crystals. Therefore, it is suggested that the precursor can survive even at temperatures above Tm and has an oriented gel-like structure with cross-links of crystallites. The microbeam SAXS experiments revealed that the ∼1% of the crystallites in the precursor are approximately 140 nm long in the c-axis direction, which is similar to the shish crystal (extended chain crystal) in nm scale. It is expected that the crystallites act as an incubator of the shish crystals when it is below the melting temperature.

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AUTHOR INFORMATION

Corresponding Author

*(T.K.) Telephone: +81-774-38-3140. Fax: +81-77-4383-146. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The microbeam WAXS and SAXS experiments were performed in the research subjects (2010A1259, 2011A1076) in Spring-8 and we are indebted to the machine time. We are grateful to Professor G. Matsuba for valuable discussion.

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REFERENCES

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