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Polymorphic Transformation of Isotactic Poly(1-butene) in Form III upon Heating: In Situ Synchrotron Small- and Wide-Angle X-ray Scattering Studies Zhiyong Jiang,† Yingying Sun,† Yujing Tang,† Yuqing Lai,† Se´rgio S. Funari,‡ Rainer Gehrke,‡ and Yongfeng Men*,† State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Renmin Street 5625, 130022 Changchun, P.R. China, and HASYLAB am DESY, Notkestr. 85, 22607 Hamburg, Germany ReceiVed: February 2, 2010; ReVised Manuscript ReceiVed: March 31, 2010
The phase transformation of form III isotactic poly(1-butene) was investigated as a function of temperature. The polymer was isothermally precipitated from a dilute solution in iso-amyl acetate and observed with realtime synchrotron small- and wide-angle X-ray scattering techniques. The results confirmed that the polymorphic transition of form III was strongly dependent on temperature. The phase transformation from form III to form I′ proceeded at a temperature of ca. 80 °C. This was accompanied by the presence of two distinctly different lamellar periodicities arising from form III and I′ crystals, respectively. The coexistence of form III and I′ crystals can persist up to 103 °C, followed by melting and recrystallizing into form II crystals. Finally, the reflections resulting from form II crystals disappeared at 118 °C. 1. Introduction Isotactic poly(1-butene) (P1B), renowned for its superior creep resistance and high temperature resistance, is a technically important semicrystalline polymer. The material has been applied in many fields, e.g., pressurized tanks, tubes, and hot water pipes.1 Aside from its useful physical properties, P1B is characterized by interesting polymorphous behavior. It is now experimentally supported and widely recognized that the mechanical properties and deformation behavior of polymer materials depend on molecular orientation and crystalline form.2-6 As a result, a detailed understanding of the crystalline transformations arising from heating and annealing is essential to comprehend the macroscopic performance of the material, thereby providing possible routes for improvement. P1B exhibits four crystalline modifications. Depending on the formation conditions, P1B crystal polymorphism is characterized by different helix conformations and crystal unit-cell dimensions: form I is a twinned, hexagonal crystal lattice with a 31 helix;7 form I′ is untwinned and hexagonal, with a 31 helix;8 form II is tetragonal, with an 113 helix;9,10 and form III is orthorhombic, with a 41 helix.8,11 The routes for the formation and transformation of the four crystalline forms have been examined in considerable detail. P1B generally crystallizes into the metastable tetragonal form II modification during cooling down from the molten state at atmospheric pressure.7,12 Subject to time, the unstable form II crystals transform spontaneously into the stable hexagonal form I crystals via a solid state transformation.13,14 This phase transformation can be greatly accelerated by the application of mechanical deformation15-18 or by the addition of specific additives, such as sodium salicylate.19 Additionally, form I′ crystals can be obtained upon crystallization of the melt under high pressure (1500-2000 atm).20,21 Bulk samples of P1B forms III and I′ can be pre* Corresponding author. Phone: +86 431 85262907. Fax: +86 431 85262954. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ HASYLAB am DESY.
cipitated from the polymer solution depending on the nature of the solvent, the solution concentration, and the crystallization temperature.8,22,23 In particular, the three crystalline modificationssI′, II, and IIIscan be generated by bulk crystallization on the appropriate organic substrates through epitaxy.11,24 Although the orthorhombic form III and the untwinned, hexagonal form I′ crystals are stable at room temperature,25,26 they transform into tetragonal polymorphs upon heating.27 Their further transformation into the twinned, hexagonal form I occurs when standing at room temperature. The structure and morphology of the four crystalline forms of P1B have been extensively studied. Samples of forms I and II are composed of spherulitic formations built up of lamellar crystals.22,28 The morphology of form III in bulk samples also consists of lamellar formations but differs from that of forms I and II because of radial twisting in the spherulites.22,28,29 Additionally, P1B has a three-phase structure composed of crystalline and mobile amorphous microphases (MAF) together with additional rigid amorphous nanophases (RAF).30,31 It was recently reported that the relative amounts of the three phases depend on thermal history, and the coupling between the crystal and amorphous fractions was shown to be largely affected by crystal polymorphism.32,33 The RAF is coupled more strongly to the crystals and relaxes at higher temperatures when the form II crystals are transformed into form I. Many studies have dealt with the crystallographic, morphological, and conformational changes accompanying the phase transition of form II to form I.34-41 The polymorphic behavior of form III crystals, however, has been addressed relatively less than that of form I and II, presumably due to the complicated phase transformation involved in the course of its heating and deformation.23,26-28 Studies on the polymorphic transformation of P1B have been performed employing X-ray diffraction,20,36 differential scanning calorimetry (DSC),28,42 transmission electron microscopy (TEM),43-45 and nuclear magnetic resonance (NMR) spectroscopy.46-48 Obviously, none of the abovementioned methods are able to elucidate the relations between the phase transformations on the length scale of crystalline
10.1021/jp101017n 2010 American Chemical Society Published on Web 04/20/2010
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lamellae as a function of temperature. Combined small- (SAXS) and wide-angle X-ray scattering (WAXS) measurements, using synchrotron radiation, can nevertheless be carried out online while heating samples with morphologies similar to the bulk material. This technique enables large-scale evaluations of the structural evolution of the length scale of molecules (WAXS) and lamellae (SAXS). The purpose of the present work was thus to follow the phase transformation of form III P1B into other forms (I′ and II) during heating by means of time-resolved SAXS and WAXS measurements. As will be reported in the following sections, the crystalline transformations of form III crystals were highly temperature-dependent. It must be stressed that, although the interconversions between the form III crystals and other crystalline modifications may be accompanied by a change in the three phase structure of P1B, we do not take the characteristics of both crystal fractions and amorphous phase into account in the present study because SAXS is sensitive to the electron density contrast between the crystalline lamellae and amorphous phase. Therefore, a one-dimensional correlation function based on a two-phase system, which gives the long spacing corresponding to the sum of the average thickness of the amorphous layers and the crystalline lamellae, can be employed to describe the structural evolution of P1B with modification III. 2. Experimental Methods Sample Preparation. The P1B sample with a weightaveraged molecular weight of 185 000 was purchased from Aldrich Chemical Co. The virgin sample consisted of form I crystals produced by the slow transformation of form II crystals aged at room temperature. It was dissolved in a 0.2 wt % solution of iso-amyl acetate at a temperature of 120 °C, and quickly transferred to a controlled, 55 °C water bath for isothermal crystallization. When precipitated completely, the specimen was obtained by slowly filtering the crystal suspension at room temperature, followed by solvent evaporation in a vacuum oven. After all of the solvent had been flashed off, the specimen was compressed, yielding a polymer plate with a thickness of about 0.5 mm. Simultaneous SAXS/WAXS Measurement. Real-time SAXS and WAXS measurements were performed at the synchrotron beamline A2 at HASYLAB, DESY, Hamburg, Germany. The wavelength of the X-ray radiation was 0.15 nm. A piece of specimen sheet was tightly wrapped with a thin aluminum foil in order to promote thermal conductivity. It was then mounted onto a sample holder in the hot stage installed at the beamline at a sample-to-SAXS detector distance of 2290 mm. At this distance, the effective scattering vector q (q ) (4π/λ) sin θ, where 2θ is the scattering angle and λ the wavelength) range was 0.15-1.18 nm-1. Meanwhile, the accessible scattering angle for WAXS was positioned at 8.76 nm-1 < q < 25.89 nm-1. Scattering patterns were registered by means of a twodimensional detector for SAXS and a one-dimensional detector for WAXS. In situ SAXS and WAXS measurements were carried out simultaneously during the heating scan from 30 to 160 °C at a rate of 1 K/min, and each pattern was collected within 60 s. The two-dimensional SAXS patterns were calibrated for background scattering and normalized with respect to the primary beam intensity. The scattering patterns after calibration were averaged over all directions at a constant q, resulting in one-dimensional scattering intensity curves. Lorentz correction (multiplication of I by q2) was performed in order to calculate the long spacing of the lamellar stacks.49 The one-dimensional
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Figure 1. DSC thermogram of P1B with form III crystalline modification measured at a heating rate of 1 K/min.
WAXS data were also calibrated, using poly(ethylene terephthalate) as the reference standard, and background corrected. DSC Characterization. To characterize the melting behavior of the as-received specimen, differential scanning calorimetry (DSC) measurements were conducted on a DSC Q100 instrument (TA Instruments). The thermal behavior of the specimen was derived from the melting thermogram, measured by a DSC heating scan over a temperature range of 30-160 °C at a rate of 1 K/min under nitrogen gas flow. The melting point Tm denoted the minimum of the DSC trace during heating. 3. Results and Discussion Before structural changes during crystal transformation upon heating were considered, the as-received specimen was measured by DSC. The DSC trace of the P1B form III specimen, recorded at a heating rate of 1 K/min, is shown in Figure 1. At first sight, triple endothermic peaks can be observed at 89, 98, and 114 °C. These are the characteristic features of form III of P1B. Additionally, the presence of a sharp exothermic peak at 100 °C, subsequent to the major endothermic peak at 98 °C, indicates the occurrence of melting, followed by a recrystallization process. Apparently, the endothermic peak at 114 °C marks the melting of the recrystallized form II crystals. Since the position of an endothermic peak corresponding to the melting of form III and I′ crystals is at the same temperature range of 90-100 °C,50 we cannot distinguish one form from the other based on the DSC melting curve alone. Therefore, another technique should be utilized in order to explore the origins of the two endothermic peaks at the lower temperatures. However, the possibility of the existence of form I crystals can be eliminated across the entire heating run, since no fusion heat signal was detected around 130 °C at which the melting of form I occurs. For the purpose of identification of microscopic events associated with phase transition, WAXS was employed to characterize the crystal form upon heating. Figure 2 illustrates the variation of WAXS profiles of the as-received P1B specimen as a function of temperature. The WAXS scan, recorded at room temperature, was typical of the form III crystals. It gave a sequence of diffraction reflections of 9.3, 10.5, 12.6, 13.5, 15.1, and 17.6 nm-1. These values can be assigned, in that order to the (110), (200), (111), (201), (120), and (301) lattice planes of form III crystals. In addition, the
Polymorphic Transformation of Form III Isotactic P1B
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Figure 2. Selected WAXS profiles of the as-prepared P1B sample as a function of temperature taken at a heating rate of 1 K/min.
diffractogram taken at 30 °C exhibited a weak shoulder at the low q side of the (120) reflection of the form III crystals, reflecting a relatively small amount of modification I′ crystals present in the as-prepared sample. It is evident that the specimen underwent a comprehensive crystalline transition during the heating run, as revealed by the WAXS patterns. When the temperature increased, the reflections of form III crystals gradually decreased in intensity and slightly moved toward lower scattering angles. On the other hand, the shoulder developed gradually as the temperature was raised to 80 °C and then was superimposed on the original reflection, forming a new broad diffraction peak at around 14.7 nm-1. It should be noted that, as a consequence of the crystalline phase transformation, the peaks resulting from form III crystals decreased in magnitude, while the newly established reflection worked in the opposite direction and intensified until 100 °C was reached during heating. At a temperature of 102 °C, only a small quantity of form III and form I′ crystals still existed. Simultaneously, the reflections typical of the tetragonal form II, following the melting of form III and I′ crystals, could be observed. The mechanism involved in this transformation was the melting and recrystallization process, as evidenced by the DSC results. It is worth mentioning that the transition of forms III and I′ to II was rapid, and they were able to coexist in the same system over a narrow temperature range during the heating run. When the temperature was further increased, the form II crystals started to melt at 110 °C and finally vanished at 118 °C, at which point a completely amorphous pattern was recorded. The SAXS results during heating are shown in Figures 3, 4, and 5. The one-dimensional scattering intensity distributions, as illustrated in Figure 3, can be used to qualitatively evaluate, by noting the shape and peak position, the changed organization of the lamellar stacks that occurred in the process of heating. Furthermore, the technique of one-dimensional electron density correlation function analysis has often been used to give detailed structural information of the system. The autocorrelation function K(z) can be derived from the inverse Fourier transformation of the experimentally obtained intensity distribution I(q) as follows:49,51,52
K(z) )
∫0∞ I(q)q2 cos(qz) dq ∫0∞ I(q)q2 dq
(1)
Figure 3. SAXS: selected one-dimensional scattering intensity distributions as a function of temperature for P1B in form III measured at a heating rate of 1 K/min.
Figure 4. SAXS: selected correlation functions for the as-prepared P1B sample at different temperatures during heating scan. The correlation function K(z) was determined by the cosine transformation of the scattering intensity I(q). The long spacing of the lamellar stacks can be derived from the peak position of the correlation function, as indicated by the filled symbols.
where z denotes the location measured along a trajectory normal to the lamellar surfaces. In the above consideration, the multiplication of I(q) with q2 was performed because of the isotropically distributed stacks of parallel lamellar crystallites in the sample.49 For systems with a structure of stacks of lamellae, the correlation function shows characteristic features that allow the long spacing defined as the average thickness of a lamella together with one interlamellar amorphous layer measured along the lamellar normal to be determined.51,52 The resultant correlation functions are presented as a function of temperature in Figure 4. The long spacing dac can be obtained from the maximum position of the correlation function as marked by the filled symbols. It
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Figure 5. SAXS: evolution of the long spacing of the lamellar stacks as a function of temperature for the as-prepared P1B sample measured at a heating rate of 1 K/min.
needs to be emphasized that the inverse Fourier transform is linear, which can be expressed as
F[I1(q) + I2(q)] ) F[I1(q)] + F[I2(q)]
(2)
where the symbol F represents the inverse Fourier transform. As a consequence, the linearity of the correlation function makes it possible to distinguish respective long spacings of the two crystalline structures present in the system. In addition, the value of the long spacing for form II crystals was calculated from the SAXS scans using the Bragg equation because of the broad plateau occurring in the correlation functions. The temperature dependence of the long spacing, as shown in Figure 5, demonstrates distinct characteristic behavior in different temperature domains. At room temperature, two periodicities were evaluated from the correlation function. The possibility of the presence of a long spacing value assigned to the modification I′ crystals at the onset of heating scan can be excluded in terms of the evolution of long spacing as a function of temperature, because the magnitude of the reflections arising from form I′ crystals continuously increased up to ∼100 °C as identified by the WAXS data and thus no substantial melting occurred at 86 °C. As a result, these two long spacings at low temperatures were produced by the form III lamellar crystals. The absence of long spacing for form I′ crystals might be due to the lack of stacks of crystalline lamellae at the early stages of heating scan in spite of the existence of a small quantity of form I′ crystals at the beginning. These two populations of periodic lamellar stacks stemmed from the primary structure built up during the first isothermal solution crystallization and the additional crystallites generated during slowly filtering the crystal suspension at room temperature, respectively. Obviously, the long spacing of thinner form III lamellar stacks increased continuously when the temperature was elevated, accompanied by a gradual decrease of the long spacing of thicker ones. This behavior was indicative of a melting of defective lamellae within thinner lamellar stacks and a crystallization of the free polymeric segments in the thicker stack established during the initial isothermal crystallization. These two processes gave an opposite thermal effect and therefore cannot be observed clearly in the DSC scan. When the temperature was raised to 75 °C, the long spacing for both lamellar stacks started to increase due to partial melting. As the temperature was further increased, the long spacing values for form III and I′ crystals emerged within the
temperature range 86-103 °C. It should be noted that the presence of long spacing for form I′ crystalline lamellae was postponed to higher temperature with respect to the WAXS results. On one hand, the long spacing of form III crystals experienced an even more pronounced increase, caused by the selective melting of thermally less stable crystals in the lamellar stacks. On the other hand, the long spacing for form I′ crystals gave a larger value, compared to form III crystals; e.g., it was about 8 nm at the beginning. With increasing temperature, the long spacing value for form I′ declined at the early stages of phase transformation and then remained essentially constant until 100 °C, which could be attributed to crystalline transformation subsequent to the selective melting of form III crystals. Thereafter, it tends to increase because of substantial melting, which is in line with the reduction in magnitude of WAXS diffraction reflections. It must be stressed that the absence of long spacing for form II at the end of this interval was ascribed to the smaller electron density difference between the form II crystalline lamellae and the amorphous layers in between as compared to form III and I′ crystals. When the form III and I′ crystals were completely molten, and only form II crystals were left, the long spacing was almost constant (about 22 nm) starting from 104 °C. At still higher temperatures, because the scattering intensity distributions exhibited humps or plateaus, the long spacing was not calculated. Let us now discuss the transformation mechanism underlying the phase transition during the heating of P1B form III crystals. It has been generally accepted that a solid state crystal-to-crystal process is responsible for the phase transformation from form III to form I′, as investigated principally by NMR spectroscopy.23,48 Although this transformation is proposed to be hindered by a conformational constraint during the transformation of form III into form I′ which arises from the fact that form III is an isochiral crystal phase, whereas forms I′ and II can be regarded as “conformational stereocomplexes” in which right-handed helices are surrounded by left-handed helices, and vice versa,29,53 there exists no direct evidence to suppose this view. In addition, the known loss packing of polymeric chain segments in form III P1B crystals due to its lowered density may provide enough freedom for the solid crystal-to-crystal transformation of form III to form I′. In the present experiments, the evolution of long spacing for form III and I′ crystals showed a quite different tendency as a function of temperature. Moreover, the discrepancy between the long spacing values derived from form III and I′ crystals was observed distinctly. In order to understand the observed much enlarged long spacing values of form I′ crystalline lamellae compared to the original form III lamellae, we need to recall the DSC and WAXS results, as shown in Figures 1 and 2. Obviously, accompanying the appearance of form I′ crystals, two endothermic peaks showed up representing a neat melting of form III crystalline lamellae. At the same temperature interval, phase transformation of form III to I′ occurred within lamellar stacks. Melting of lamellar stacks can proceed via different routes, namely, surface melting which occurs only at the surface of crystalline lamellae, stack melting where lamellae in the same stacks melt simultaneously, and sequential melting which is characterized by selective melting of individual lamellae within a stack in light of the thermal stability.54 The last melting mechanism results in an increase of lamellar long spacing during melting. Thus, the observed larger long spacing of form I′ crystalline lamellae than original form III ones can be understood as a consequence of phase transformation of part of the form III lamellar stacks after
Polymorphic Transformation of Form III Isotactic P1B sequential melting. There are, in addition, two points favoring this assignment. First, sequential melting of the form III lamellar stacks partially released constraints of the chain segments in the crystalline phase and thus facilitated the crystal-to-crystal transformation of form III to I′; second, the similarity of melting temperature between the resultant form I′ crystals and form III crystallites indicated that the average thickness of the crystalline lamellae for form I′ approximated the value measured for form III crystals. Together with the enhanced long spacing of this form I′ crystalline lamellae, it is safe to conclude that the transformation of form III to form I′ proceeded via a crystalto-crystal transformation because otherwise a melting and recrystallization route will produce either thicker lamella or smaller long spacing than the current one. 4. Conclusions The phase transformation upon heating of the P1B sample with form III crystals was characterized by simultaneous synchrotron SAXS and WAXS techniques. On the basis of the temperature-dependent long spacing analysis, combined with the WAXS results, it was shown that three temperature intervals were revealed in terms of the types of crystalline modification. In the first case (T ) 30-85 °C), two populations of form III lamellar crystals were observed in addition to the presence of a small quantity of form I′ crystals. The variation of long spacing as a function of temperature for these two populations of lamellar stacks showed different tendencies at lower temperatures. When heating was continued, the long spacing for both lamellar stacks increased gradually due to selective melting. In the second interval (T ) 86-103 °C), the form I′ crystals developed via crystalline phase transformation of form III crystals accompanied by selective melting of thinner form III crystalline lamellae. This resulted in the coexistence of form III and I′ crystals. Finally, in the third interval (T ) 104-118 °C), only form II crystals with unchanged long spacing from the early part of this interval were seen, and then they vanished when approaching the temperature of final melting. Acknowledgment. Y.M. thanks the National Natural Science Foundation of China (20734006, 50603024 and 50921062), the National Basic Research Program of China (2005CB623800), and HASYLAB project II-20080190. References and Notes (1) Gedde, U. W.; Wiebke, J.; Leijstrom, H.; Ifwarson, M. Polym. Eng. Sci. 1994, 34, 1773. (2) Nakamura, K.; Aoike, T.; Usaka, K.; Kanamoto, T. Macromolecules 1999, 32, 4975. (3) Yang, Y. C.; Geil, P. H. Makromol. Chem. 1985, 186, 1961. (4) Kalay, G.; Kalay, C. R. J. Polym. Sci., Polym. Phys. 2002, 40, 1828. (5) Kalay, G.; Kalay, C. R. J. Appl. Polym. Sci. 2003, 88, 814. (6) Weynant, E.; Haudin, J. M.; G’sell, C. J. Mater. Sci. 1982, 17, 1017. (7) Natta, G.; Corradini, P.; Bassi, I. W. NuoVo Cimento Suppl. 1960, 15, 52. (8) Miller, R. L.; Holland, V. F. Polym. Lett. 1964, 2, 519. (9) Turner-Jones, A. J. Polym. Sci. 1963, B1, 455. (10) Petraccone, V.; Pirozzi, B.; Frasci, A.; Corradini, P. Eur. Polym. J. 1976, 12, 323.
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