J. Phys. Chem. B 2008, 112, 5311-5316
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Phase Transitions during Heating of Melt-Drawn Ultrahigh Molecular Weight Polyethylenes Having Different Molecular Characteristics Masaki Kakiage,†,# Miho Sekiya,† Takeshi Yamanobe,† Tadashi Komoto,† Sono Sasaki,‡ Syozo Murakami,§ and Hiroki Uehara*,† Department of Chemistry, Gunma UniVersity, Kiryu, Gunma 376-8515, Japan, Japan Synchrotron Radiation Research Institute, Sayo, Hyogo 679-5198, Japan, and Heian Jogakuin UniVersity, Takatsuki, Osaka 569-1092, Japan ReceiVed: October 6, 2007
Changes in the crystalline structure during heating of melt-drawn ultrahigh molecular weight polyethylenes (UHMW-PEs) having different molecular characteristics were analyzed by in situ wide-angle X-ray diffraction measurements. A phase transition from the orthorhombic into the hexagonal phase was observed for all samples, but the perfection was enhanced and the possible temperature window for the hexagonal phase was greater for the sample containing only a higher molecular weight component. In contrast, an increase in retractive stress during heating was confirmed for the sample containing a lower molecular weight component, reflecting melting of the folded-chain crystal (FCC). Differential scanning calorimetry and transmission electron microscopy revealed the dependency of the molecular characteristics of the sample on the resultant morphologies. These results demonstrate that the existence of FCC determines both the quality and the width of the temperature window for the hexagonal phase during heating of melt-drawn UHMW-PEs.
Introduction When an oriented polyethylene (PE) sample was heated under tension, an initial orthorhombic phase transformed, through an intermediate hexagonal phase, into melt,1-9 as indicated by the solid arrow in Figure 1. Considering that the crystal modification of the hexagonal phase is the same as that of the extendedchain crystal (ECC) that is usually formed under high temperature and pressure,10-14 it can be reasonably assumed that this highly oriented chain arrangement significantly affects the phase transition conditions, including temperature and stress, during heating of preoriented PE. As reported previously, ultrahigh molecular weight (UHMW) PE can even be drawn from the molten state using its higher melt viscosity.15-18 On the basis of in situ wide-angle X-ray diffraction (WAXD) measurements using synchrotron radiation, we demonstrated that transient crystallization into a metastable hexagonal phase occurred during the oriented crystallization into a final orthorhombic phase that occurs during such melt-drawing of UHMW-PE (Figure 1, broken arrow).19,20 Furthermore, we recently reported that this transient crystallization into the hexagonal phase during melt-drawing was dominated by the entanglement characteristics, i.e., the average number of entanglements per chain of the sample films.21 In addition, we demonstrated that these molecular entanglement characteristics correlated with the phase development into ECC based on in situ small-angle X-ray scattering (SAXS) measurements.22 Thus, the molecular entanglement characteristics also determine the mechanism of phase development into ECC during meltdrawing. In contrast, a folded-chain crystal (FCC) develops during cooling after the melt-drawing process.15-18 Consistent * Corresponding author. E-mail:
[email protected]. † Gunma University. ‡ Japan Synchrotron Radiation Research Institute. § Heian Jogakuin University. # Research Fellow of the Japan Society for the Promotion of Science.
Figure 1. Stress-temperature phase diagram for UHMW-PE film. The solid arrow represents the heating process for an oriented sample, and the broken arrow represents the melt-drawing process at 155 °C.
with such morphologies, differential scanning calorimetry (DSC) of this melt-drawn sample exhibited double endotherms, corresponding to the melting of the ECC and FCC.15-18 The combination of ECC and FCC in the melt-drawn sample also affects the phase transition behavior during heating. In this study, we discuss the relationship between phase transitions and molecular characteristics during heating of meltdrawn UHMW-PE samples, using in situ WAXD measurements. In order to analyze the structural changes the heating process of oriented UHMW-PE, numerous in situ X-ray measurements have been conducted using a synchrotron radiation beam,7,8,23,24 but this involves a reciprocal space analysis. In contrast, morphological analysis in a real-space has hardly been examined. In this study, the origins of the different structural arrangements were comprehensively investigated by analyzing the morphologies of melt-drawn samples using transmission electron microscopy (TEM) observations. Another aspect of this study is the adoption of in situ retractive stress measurements taken during heating. The crystalline transition behavior of an oriented PE sample is
10.1021/jp709782g CCC: $40.75 © 2008 American Chemical Society Published on Web 04/08/2008
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TABLE 1: Component Percentages of the Different UHMW-PE Materials in the Prepared Films metallocene-catalyzed material film M-100/0 M-50/50 Ziegler
higher Mv lower Mv Ziegler-catalyzed material () 1.07 × 107) () 1.73 × 106) (Mv ) 1.00 × 107) 100 50
0 50 100
synchronized with its stress relaxation.1-3 Thus, the combination of these in situ measurements of WAXD and retractive stress reveals the relationship between phase transition and chain relaxation during heating. Experimental Section Initial Materials. Three kinds of UHMW-PE materials were supplied by Asahi Kasei Chemicals Co. Of these, two kinds with different MWs were prepared using a metallocene catalyst system with viscosity-average MWs (Mv) of 1.07 × 107 and 1.73 × 106 g/mol, and a third was prepared using a Ziegler catalyst system with Mv of 1.00 × 107 g/mol. These materials featured a narrower MW distribution (MWD) for the metallocene-catalyzed materials and a broader MWD for the Zieglercatalyzed material. Gel permeation chromatography (GPC) measurements were carried out on these materials, but components with MWs exceeding 107 g/mol cannot be detected by the GPC method. Thus, an accurate analysis of the MWDs of these materials was difficult. Film Preparation. For the metallocene-catalyzed materials with the narrower MWDs, two kinds of films were prepared with component ratios (wt %) of 100/0 (M-100/0) and 50/50 (M-50/50) for both the higher and lower Mv materials, as indicated in Table 1. Appropriate amounts of these UHMWPEs, 0.2 wt % in total, were dissolved in p-xylene at the boiling point under a nitrogen gas flow. Antioxidants, namely, 0.5 wt % (based on polymer) of both octadecyl 3-(3,5-di-tert-butyl4-hydroxyphenyl)propanoate and bis(2,4-di-tert-butylphenyl)pentaerythritol diphosphite, were added. A gel-like aggregation was precipitated by slow cooling to room temperature (RT), filtered into a mat, and then soaked in an excess amount of acetone for p-xylene exchange. The mats were dried at RT in a vacuum. The dried mats were compression molded into films at 180 °C and 30 MPa for 10 min, followed by slow cooling to RT. Before this molding procedure, a 0.5 wt % acetone solution of the above-mentioned antioxidants was applied to the surface of the dried mats. The resultant film thickness was 0.7 mm. A Ziegler-catalyzed film with higher Mv and broader MWD was also prepared as described above. This dissolving process of the initial powder material for the film preparation allows us to eliminate the effect of initial powder boundary on retractive stress. Melt-Drawn Sample Preparation. A dumbbell-shaped drawing specimen was cut from each of the three kinds of compression-molded films, with the straight region 4 mm wide and 12.5 mm long. All melt-drawings were made at 155 °C, well above the sample melting temperature (Tm), using a previously designed high-temperature extension device.25,26 A chamber covered the straight region of the dumbbell specimen. Before drawing, the sample specimen was held at 155 °C for 5 min for temperature equilibration. The cross-head speed of drawing was always 24 mm/min. The three kinds of samples were melt-drawn through the end of each strain-hardening region, where phase transitions from the hexagonal into orthorhombic phase were almost completed for all films,19-21 followed
Figure 2. Temperature dependence of the retractive stress along the oriented direction recorded during heating with the corresponding changes in the in situ WAXD patterns for the melt-drawn M-100/0 sample. The oriented direction for the WAXD patterns is horizontal. The crystalline reflection region on the equator is enlarged. The temperature in Celsius is indicated for each pattern.
Figure 3. Temperature dependence of the retractive stress along the oriented direction recorded during heating with the corresponding changes in the in situ WAXD patterns for the melt-drawn M-50/50 sample.
by draw stopping and slow cooling to RT. The prepared sample draw ratios (DRs) were 11.0 for the melt-drawn M-100/0, 26.5 for the melt-drawn M-50/50, and 13.0 for the melt-drawn Ziegler samples. Measurements. A Perkin-Elmer Pyris 1 DSC was used for DSC measurements. Heating scans were performed up to 180 °C at a rate of 10 °C/min under a nitrogen gas flow. The sample Tm was evaluated as the peak temperature of the melting endotherm. The temperature was calibrated using indium and tin standards. To avoid melting the melt-drawn sample under a constrained state and delay in heat transfer during the heating scan, a small amount of silicone oil was placed between the sample and the bottom of a DSC sample pan. In situ WAXD measurements were carried out during the heating process using a synchrotron radiation source at the BL40B2 beamline of SPring-8 (Japan Synchrotron Radiation Research Institute, Hyogo, Japan). The melt-drawn samples were placed in a sample holder in the high-temperature furnace installed in the beamline, where the measurement temperature was detected by thermocouples placed less than 1 mm away from the sample surface. All heatings were performed up to 180 °C at a rate of 5 °C/min under tension. Due to the results of pilot tests using unoriented samples with the thickness less than 0.7 mm, the effect of sample thickness was not recognized in this heating condition. The retractive stress during the heating process was recorded using
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Figure 4. Temperature dependence of the retractive stress along the oriented direction recorded during heating with the corresponding changes in the in situ WAXD patterns for the melt-drawn Ziegler sample.
Figure 6. Three-dimensional plot of WAXD line profiles extracted along the equators of a series of in situ WAXD patterns recorded during heating of the M-50/50 sample.
Figure 5. Three-dimensional plot of WAXD line profiles extracted along the equators of a series of in situ WAXD patterns recorded during heating of the M-100/0 sample. Difference in intensity is represented by a color gradation.
a load cell (Kyowa Electronic Instruments Co., Ltd., LUR-A50NSA1) installed in the sample holder. WAXD images were continuously recorded during the heating process on a coolingtype CCD camera (Hamamatsu Photonics K.K., C4880). The wavelength of the synchrotron beam was 1.00 Å. The exposure time for each pattern was 0.05 s with a time interval of 7 s. TEM observations of the melt-drawn samples were made with a JEOL JEM-1200EX electron microscope operated at 80 kV. The samples were stained by RuO4 vapor and embedded in an epoxy resin, causing the amorphous phase to appear as a darker region in the image. The assembly was cut into thin sections 50 nm thick with a Reichert Ultracut S microtome for TEM observation. Results and Discussion First, we recorded the retractive stress along the oriented direction during heating and the corresponding changes in the
Figure 7. Three-dimensional plot of WAXD line profiles extracted along the equators of a series of in situ WAXD patterns recorded during heating of the Ziegler sample.
in situ WAXD patterns. A comparison of the obtained results enables us to discuss the phase diagram (Figure 1) for three kinds of melt-drawn UHMW-PE samples with different molecular characteristics. These samples were melt-drawn through the end of each strain-hardening region, where the phase transitions from the hexagonal into the orthorhombic phase were almost completed for all films19-21 because crystal perfection, especially ECC, was almost achieved through the melt-drawing process in this region.22
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Figure 9. DSC melting thermograms for M-100/0, M-50/50, and Ziegler samples.
Figure 8. Changes in the integral intensities of the hexagonal (100) and orthorhombic (110) reflection peaks evaluated from the equatorial line profiles in Figures 5-7: (top) M-100/0, (middle) M-50/50, and (bottom) Ziegler samples. The broken arrows indicate the incomplete transition temperature window, and the solid arrows indicate the complete transition window.
Figure 2 depicts the temperature dependence of the retractive stress recorded during heating, along with the corresponding changes in the in situ WAXD patterns for the melt-drawn M-100/0 sample (DR ) 11.0) containing only the higher Mv component with the narrower MWD. The retractive stress exhibits a gradual decrease with heating, up to the final breaking point. The WAXD pattern before heating exhibits only oriented orthorhombic (110) and (200) reflections. Upon heating, a hexagonal (100) reflection appears at 160 °C with the initial orthorhombic reflections. Only hexagonal (100) reflection is observed at 170 °C, followed by the sample breaking at 173 °C. Similar measurements of the retractive stress and in situ WAXD patterns were made for the melt-drawn M-50/50 sample (DR ) 26.5), which contains the lower Mv component with the narrower MWD. Figure 3 depicts the temperature dependence of the retractive stress recorded during heating with the corresponding changes in the in situ WAXD patterns for the melt-drawn M-50/50 sample. The retractive stress initially decreases but begins to increase beyond 120 °C upon heating, which is quite different from the continuous decrease observed for the M-100/0 sample. The hexagonal (100) reflection appears at 160 °C in the WAXD patterns. However, the initial orthorhombic (110) reflection did not disappear completely on further heating, which is also different from the result for the M-100/0 sample in Figure 2. The M-50/50 sample finally broke at 161 °C, at which point both hexagonal (100) and orthorhombic (110) reflections were observed. In contrast, the melt-drawn Ziegler sample (DR ) 13.0), composed of the higher Mv component but with the broader MWD, exhibits intermediate behaviors for the above transition. Figure 4 depicts the temperature dependence of the retractive stress recorded during heating with the corresponding changes in the in situ WAXD patterns for the Ziegler sample. The retractive stress begins to increase beyond 120 °C, similar to the M-50/50 sample containing the lower Mv component. However, the change in the WAXD patterns is similar to that for the M-100/0 sample, i.e., the hexagonal (100) reflection appears at 160 °C and becomes dominant at 166 °C. Cor-
respondingly, the sample breaks at 167 °C, between the corresponding temperatures for the M-100/0 and M-50/50 samples. These transitions were quantitatively analyzed by comparing equatorial line profiles extracted from the series of in situ WAXD patterns depicted in Figures 2-4. The obtained 2θ profiles were plotted as a function of temperature. Changes in the equatorial line profiles for the M-100/0, M-50/50, and Ziegler samples during heating are indicated in Figures 5-7. For the M-100/0 sample (Figure 5), the transition into the hexagonal phase begins beyond 160 °C and is completed at approximately 165 °C. This hexagonal phase survives until the sample breaks at 173 °C. In contrast, for the M-50/50 sample (Figure 6), the transition into the hexagonal phase begins around 155 °C. However, the transition behavior is less strict than for the M-100/0 sample, and the sample breaks with coexistence of the hexagonal and orthorhombic phases, i.e., before the transition is complete. For the Ziegler sample (Figure 7), the transition into the hexagonal phase begins beyond 160 °C. The sample breaks at 167 °C immediately after the transition is completed around 165 °C. Such sample breaking with complete transition into the hexagonal phase is similar to that for the M-100/0 sample. The differences in phase transitions during the heating process of these melt-drawn UHMW-PE samples were characterized using the integral intensities of the amorphous scattering and crystalline reflections. The extracted equatorial line profiles (Figures 5-7) were resolved into amorphous scattering, hexagonal (100), and orthorhombic (110) and (200) reflection peaks using the Voigt function, which combines the Lorentzian and Gaussian functions. Among these resolved peaks, the hexagonal (100) and orthorhombic (110) ones were chosen for the evaluation of crystalline phase transition. Changes in the integral intensities of these peaks during heating of the M-100/0, M-50/ 50, and Ziegler samples are summarized in Figure 8. For the M-100/0 sample (top column), it can be clearly recognized that the orthorhombic phase rapidly transforms into the hexagonal phase around 161 °C. The temperature window with only hexagonal phase reaches approximately 6 °C from 167 °C. Waddon and Keller27,28 also represent the temperature window of minimum extrusion pressure in melt flow of PE, with flowinduced hexagonal phase. In contrast, for the M-50/50 sample (middle column), the transition into the hexagonal phase is recognized from approximately 155 °C, but remains incomplete, with the orthorhombic phase surviving, even just before the sample breaks at 161 °C. Thus, the resultant integral intensity
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Figure 10. Series of TEM images of the (a) M-100/0, (b) M-50/50, and (c) Ziegler samples. The oriented direction is always horizontal. All scale bars are 500 nm.
of the hexagonal (100) reflection peak is depressed. The M-50/ 50 sample was expected to transform completely at the higher temperature due to the highly oriented state with a DR of 26.5, but the transition behavior was quite contrary to the expectation. This means that the molecular characteristics of the sample dominate rather than the sample orientation, for the phase transition of the melt-drawn sample during heating. For the Ziegler sample (bottom column), the orthorhombic phase rapidly transforms into the hexagonal phase, and only the hexagonal phase exists from 165 °C. However, the temperature window with only hexagonal phase of this sample is approximately 2 °C, narrower than that of the M-100/0 sample. DSC measurements and TEM observations were carried out in order to characterize the morphologies of these melt-drawn UHMW-PE samples with their different molecular characteristics. DSC melting thermograms for the M-100/0, M-50/50, and Ziegler samples are summarized in Figure 9. The meltdrawn sample exhibited double endotherms, corresponding to melting of the ECC and FCC.15-18 These double endotherms were also induced by the difference in the molecular characteristics.29,30 The endotherms corresponding to the melting of the ECC are observed at approximately 140 °C for all samples. In contrast, the endotherm corresponding to the melting of the FCC is observed at approximately 134 °C, lower than that of the ECC. This lower temperature endotherm was observed only for the M-50/50 sample. Figure 10 presents a series of TEM images of the M-100/0, M-50/50, and Ziegler samples. The oriented structures of the ECCs are observed for the M-100/0 sample (Figure 10a). The Ziegler sample (Figure 10c) exhibits slight FCCs in addition to the oriented ECCs. Here, the ECC structure for the M-100/0 and Ziegler samples exhibited the characteristic lozenge pattern. Considering no four-point pattern attributed to the lozenge crystalline block was recognized in the in situ SAXS pattern recorded during melt-drawing,22 this lozenge crystalline block must be formed by the sample reduction during cooling. In contrast, a typical shish-kebab structure consisting of longitudinally oriented ECC and paralleloriented FCC15-18 was observed for the M-50/50 sample (Figure 10b). Indeed, the existence of FCC is clearly recognized in only the M-50/50 sample, due to the substantial amount of the lower Mv component contains. As described above, the FCC exhibits a lower Tm. Consequently, this component will appear as nonoriented amorphous in the higher temperature region during heating, where the hexagonal phase appears. Therefore, the effect of these characteristic FCCs on the structural changes during heating was evaluated by meridional analyses of in situ WAXD patterns. Line profiles along the meridians, which are useful for evaluating amorphous scattering in the nonoriented state,19,20 were extracted from the series of in situ WAXD patterns depicted in Figures 2-4. Changes in the integral intensity of the meridional
Figure 11. Changes in the integral intensity of the amorphous scattering evaluated from the meridional line profiles: (top) M-100/0, (middle) M-50/50, and (bottom) Ziegler samples.
amorphous scattering during heating of the M-100/0, M-50/50, and Ziegler samples are summarized in Figure 11. For the M-100/0 sample (top column), a constant value is obtained even during heating. In contrast, for the M-50/50 (middle column) and Ziegler (bottom column) samples, the integral intensity increases beyond 100 °C. Considering that the morphologies of the M-50/50 and Ziegler samples, both of which contain the lower MW component, exhibit the FCC in Figure 10, the increase in meridional amorphous scattering corresponds to melting of the FCC formed by the lower MW component with less entanglement. Furthermore, a comparison of the changes in retractive stress and the integral intensity of the meridional amorphous scattering reveals the correlation of their temperature tendencies for each sample, i.e., a constant value for the M-100/0 sample and an increase for the M-50/50 and Ziegler samples upon heating. Thus, the increase in retractive stress upon heating can be attributed to the melting of the FCC. We now consider the perfection of the phase transition from the orthorhombic into hexagonal phases for these melt-drawn UHMW-PE samples. The M-100/0 sample did not exhibit an increase in retractive stress upon heating but did exhibit a complete transition into the hexagonal phase. Furthermore, this sample had a wider temperature window in which only the hexagonal phase was recognized. FCC was hardly formed for the M-100/0 sample containing only the higher Mv component with the narrower MWD, which predominantly forms ECC during melt-drawing. Therefore, it is reasonably concluded that
5316 J. Phys. Chem. B, Vol. 112, No. 17, 2008 a lesser amount of FCC induces perfection of the phase transition and extension of the temperature window of the hexagonal phase. The restricted chain relaxation of the ECC leads to the complete transition into the hexagonal phase for the M-100/0 sample. Also, sample breaking is prevented due to the thermal stability of the ECC with less FCC present. In contrast, FCC was formed in the M-50/50 and Ziegler samples containing the lower MW component. The melting of this FCC accelerated the chain relaxation of the ECC, resulting in an incomplete transition into the hexagonal phase with the sample breaking at a lower temperature. Thus, the transition into the hexagonal phase for the M-50/50 and Ziegler samples is less remarkable than that for the M-100/0 sample. The Ziegler sample with the broader MWD was expected to include FCC like the M-50/50 sample, corresponding to the increase in retractive stress upon heating. However, the lower temperature endotherm assigned to the melting of the FCC was not clearly detected by the DSC measurement. TEM observation of this Ziegler sample also revealed a morphology consisting mainly of ECC. It should be noted that the Ziegler sample includes the lower MW component, but its effect upon the meltdrawing behavior was less.20 This is due to the continuous existence of various MW components, as represented by the broader MWD. The middle range of MW can serve as a bridge between the higher and lower MW components to cover the whole MW region. Therefore, the lower MW component is pulled into the higher MW component during melt-drawing, resulting in a homogeneous ECC structure. In contrast, the meltdrawing of M-50/50 film produces a heterogeneous structure due to phase separation into more and less entangled components.21 The middle MW component of the Ziegler film helps prevent such phase separation of molecular entanglement during melt-drawing. The ECC-dominant structure of the Ziegler sample results in a complete transition into the hexagonal phase. The temperature window with only hexagonal phase also reflects this structural characteristic, although it is narrower than that of the M-100/0 sample because the ECC structure contains the lower MW component pulled into the higher MW one. This crossover structure of more and less entangled components produces the complex melting behavior of the Ziegler sample. Conclusions Phase transitions during heating of a series of melt-drawn UHMW-PE samples having different molecular characteristics were analyzed using in situ WAXD measurements. For a metallocene-catalyzed sample containing only a higher Mv component with a narrower MWD, the transition into the hexagonal phase proceeded completely, with only the hexagonal phase appearing over a wide temperature window of 6 °C. In contrast, for another metallocene-catalyzed sample containing a lower Mv component, an increase in retractive stress and an incomplete transition into the hexagonal phase were observed. The Ziegler-catalyzed sample with a broader MWD exhibited both phenomena, i.e., an increase in retractive stress was recognized and the transition into the hexagonal phase proceeded completely. However, the temperature window with only hexagonal phase was 2 °C, narrower than that of the metallocene-catalyzed sample containing a component with similar Mv. The increase in retractive stress upon heating corresponded to the melting of the FCC. Considering that the transition into the hexagonal phase proceeded sufficiently for this sample with no increase in retractive stress during heating, it could be concluded that the existence of the FCC dominated both the perfection of the phase transition and the width of the temper-
Kakiage et al. ature window of the hexagonal phase. Morphological analyses using DSC and TEM also support the above phase transition mechanisms for each sample. These results demonstrate that the hexagonal transition behavior of melt-drawn UHMW-PE samples is significantly affected by molecular characteristics resulting from the initial sample preparation. Acknowledgment. In situ measurements using synchrotron radiation were performed at the BL40B2 in the SPring-8 with the approval of the Japan Synchrotron Radiation Research Institute (JASRI) (Proposal No. 2005A0700-NI-np). We appreciate the cooperation of Drs. Hiroyasu Masunaga and Katsuaki Inoue (JASRI). This work was partly supported by The Sumitomo Foundation and the Industrial Technology Research Grant Program in ’04 from the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References and Notes (1) Pennings, A. J.; Zwijnenburg, A. J. Polym. Sci., Polym. Phys. Ed. 1979, 17, 1011. (2) Rastogi, S.; Odell, J. A. Polymer 1993, 34, 1523. (3) Uehara, H.; Kanamoto, T.; Kawaguchi, A.; Murakami, S. Macromolecules 1996, 29, 1540. (4) Tashiro, K.; Sasaki, S.; Kobayashi, M. Macromolecules 1996, 29, 7460. (5) Kuwabara, K.; Horii, F. Macromolecules 1999, 32, 5600. (6) Kwon, Y. K.; Boller, A.; Pyda, M.; Wunderlich, B. Polymer 2000, 41, 6237. (7) Rein, D. M.; Shavit, L.; Khalfin, R. L.; Cohen, Y.; Terry, A.; Rastogi, S. J. Polym. Sci., Part B: Polym. Phys. 2004, 42, 53. (8) Ratner, S.; Weinberg, A.; Wachtel, E.; Moret, P. M.; Marom, G. Macromol. Rapid Commun. 2004, 25, 1150. (9) Watanabe, S.; Dybal, J.; Tashiro, K.; Ozaki, Y. Polymer 2006, 47, 2010. (10) Wunderlich, B.; Arakawa, T. J. Polym. Sci., Part A: Polym. Chem. 1964, 2, 3697. (11) Bassett, D. C.; Turner, B. Nature (London), Phys. Sci. 1972, 240, 146. (12) Asahi, T. J. Polym. Sci., Polym. Phys. Ed. 1984, 22, 175. (13) Rastogi, S.; Kurelec, L.; Lemstra, P. J. Macromolecules 1998, 31, 5022. (14) Kurelec, L.; Rastogi, S.; Meier, R. J.; Lemstra, P. J. Macromolecules 2000, 33, 5593. (15) Bashir, Z.; Keller, A. Colloid Polym. Sci. 1989, 267, 116. (16) Uehara, H.; Nakae, M.; Kanamoto, T.; Zachariades, A. E.; Porter, R. S. Macromolecules 1999, 32, 2761. (17) Nakae, M.; Uehara, H.; Kanamoto, T.; Ohama, T.; Porter, R. S. J. Polym. Sci., Part B: Polym. Phys. 1999, 37, 1921. (18) Nakae, M.; Uehara, H.; Kanamoto, T.; Zachariades, A. E.; Porter, R. S. Macromolecules 2000, 33, 2632. (19) Uehara, H.; Kakiage, M.; Yamanobe, T.; Komoto, T.; Murakami, S. Macromol. Rapid Commun. 2006, 27, 966. (20) Kakiage, M.; Yamanobe, T.; Komoto, T.; Murakami, S.; Uehara, H. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 2455. (21) Kakiage, M.; Yamanobe, T.; Komoto, T.; Murakami, S.; Uehara, H. Polymer 2006, 47, 8053. (22) Kakiage, M.; Sekiya, M.; Yamanobe, T.; Komoto, T.; Sasaki, S.; Murakami, S.; Uehara, H. Polymer 2007, 48, 7385. (23) Yang, L.; Somani, R. H.; Sics, I.; Hsiao, B. S.; Kolb, R.; Fruitwala, H.; Ong, C. Macromolecules 2004, 37, 4845. (24) Zuo, F.; Keum, J. K.; Yang, L.; Somani, R. H.; Hsiao, B. S. Macromolecules 2006, 39, 2209. (25) Murakami, S.; Tanno, K.; Tsuji, M.; Kohjiya, S. Bull. Inst. Chem. Res., Kyoto UniV. 1995, 72, 418. (26) Murakami, S. Nippon Kagaku Kaishi 2000, 2, 141. (27) Waddon, A. J.; Keller, A. J. Polym. Sci., Part B: Polym. Phys. 1990, 28, 1063. (28) Waddon, A. J.; Keller, A. J. Polym. Sci., Part B: Polym. Phys. 1992, 30, 923. (29) Mirabella, F. M. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 2800. (30) Mirabella, F. M. J. Polym. Sci., Part B: Polym. Phys. 2006, 44, 2369.
