Initial lamellar thickness dependency of recrystallization behavior of poly(4‐methyl‐1‐pentene)

The isothermal crystallization and subsequent melting process in semicrystalline poly(4‐methyl‐1‐pentene) were investigated via temperature‐dependent small‐ and wide‐angle X‐ray scattering and Flash DSC techniques. In a phase diagram of inversed crystalline lamellar thickness and temperature, the crystallization and melting lines can be described by two linear dependencies of different slopes and different limiting temperatures at infinite lamellar thickness. Upon subsequent heating, recrystallization lines with different slopes were observed for samples with different lamellar thickness, indicating changes in surface free energy difference between stabilized crystallites and mesomorphic phase. The surface free energy of native crystallites with extended‐chain conformation decreased with increasing lamellar thickness due to a more ordered surface region and less chain ends which changes cooperatively with mesomorphic phase. The surface free energy of stabilized crystallites remained unchanged for all lamellar thickness. Therefore, the recrystallization lines with different slopes are consequences of changes in surface free energy of mesomorphic phase. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 219–224     

New insight into melting and crystallization behavior in semicrystalline poly(ethylene terephthalate)

After isothermal crystallization, poly(ethylene terephthalate) (PET) showed double endothermic behavior in the differential scanning calorimetry (DSC) heating scan. During the heating scans of semicrystalline PET, a metastable melt which comes from melting thinner lamellar crystal populations formed between the low and the upper endothermic temperatures. The metastable melt can recrystallize immediately just above the low melting temperature and form thicker lamellae than the original ones. The thickness and perfection depends on the crystallization time and crystallization temperature. The crystallization kinetics of this metastable melt can be determined by means of DSC. The kinetics analysis showed that the isothermal crystallization of the metastable PET melt proceeds with an Avrami exponent of n = 1.0 ∼ 1.2, probably reflecting one‐dimensional or irregular line growth of the crystal occurring between the existing main lamellae with heterogeneous nucleation. This is in agreement with the hypothesis that the melting peaks are associated with two distinct crystal populations with different thicknesses. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 53–60, 2000     

Crystallization and melting of polyethylene copolymers: Differential scanning calorimetry and atomic force microscopy studies

The Hoffman–Lauritzen theory of secondary, surface nucleation and growth was primarily relied upon for about 40 years after its introduction in about 1960 to rationalize the crystallization of flexible chain polymers into lamellar crystals. However, in about 1998, Strobl and coworkers introduced a different model for crystallization, based on the stage‐wise formation of lamellae. Two major components of this model were as follows: (1) the concept of the formation of a mesomorphic melt as a precursor to crystallization and (2) the control of the melting temperature range of lamellar crystals of homogeneous polyolefin copolymers by an inner degree of order or perfection rather than on the crystal thickness. The first concept is in disagreement with the HL theory and the second with the Gibbs‐Thomson theory, which associates melting temperature with lamella thickness. In the present study, differential scanning calorimetry and atomic force microscopy were successfully employed to monitor the in situ quiescent crystallization of polyethylene homopolymer and copolymer. In the present study, evidence was not found to support the concept of lamellae with equal thickness melting over a broad temperature range. Some evidence was found that might be interpreted to support the concept of a mesomorphic melt as a precursor to crystallization. At present, the model promoted by Strobl and coworkers appears to be at an uncertain stage at which strong proof or disproof are not available. However, this alternative model has injected a new vitality into the study of crystallization of flexible chain polymers. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2369–2388, 2006     

New insights into the multiple melting behaviors of the semicrystalline ethylene‐hexene copolymer: Origins of quintuple melting peaks

A semicrystalline ethylene‐hexene copolymer (PEH) was subjected to a simple thermal treatment procedure as follows: the sample was isothermally crystallized at a certain isothermal crystallization temperature from melt, and then was quenched in liquid nitrogen. Quintuple melting peaks could be observed in heating scan of the sample by using differential scanning calorimeter (DSC). Particularly, an intriguing endothermic peak (termed as Peak 0) was found to locate at about 45 °C. The multiple melting behaviors for this semicrystalline ethylene‐hexene copolymer were investigated in details by using DSC. Wide‐angle X‐ray diffraction (WAXD) technique was applied to examine the crystal forms to provide complementary information for interpreting the multiple melting behaviors. Convincing results indicated that Peak 0 was due to the melting of crystals formed at room temperature from the much highly branched ethylene sequences. Direct heating scans from isothermal crystallization temperature (T_c, 104–118 °C) were examined for comparison, which indicated that the multiple melting behaviors depended on isothermal crystallization temperature and time. A triple melting behavior could be observed after a relatively short isothermal crystallization time at a low T_c (104–112 °C), which could be attributed to a combination of melting of two coexistent lamellar stack populations with different lamellar thicknesses and the melting‐recrystallization‐remelting (mrr) event. A dual melting behavior could be observed for isothermal crystallization with both a long enough time at a low T_c and a short or long time at an intermediate T_c (114 °C), which was ascribed to two different crystal populations. At a high T_c (116–118 °C), crystallizable ethylene sequences were so few that only one single broad melting peak could be observed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2100–2115, 2008     

Isothermal melt and cold crystallization kinetics of poly(aryl ether ketone ether ketone ketone)

The isothermal melt and cold crystallization kinetics of poly(aryl ether ketone ether ketone ketone) are investigated by differential scanning calorimetry over two temperature regions. The Avrami equation describes the primary stage of isothermal crystallization kinetics with the exponent n ≈ 2 for both melt and cold crystallization. With the Hoffman–Weeks method, the equilibrium melting point is estimated to be 406 °C. From the spherulitic growth equation proposed by Hoffman and Lauritzen, the nucleation parameter (K_g) of the isothermal melt and cold crystallization is estimated. In addition, the K_g value of the isothermal melt crystallization is compared to those of the other poly(aryl ether ketone)s. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1992–1997, 2000     

Morphology of homogeneous copolymers of ethylene and 1‐octene. III. Structural changes during heating as revealed by time‐resolved SAXS and WAXD

The structural changes of two linear polyethylenes, LPEs, with different molar mass and of two homogeneous copolymers of ethylene and 1‐octene with comparable comonomer content but different molar mass were monitored during heating at 10 °C per minute using synchrotron radiation SAXS. Two sets of samples, cooled at 0.1 °C per minute and quenched in liquid nitrogen, respectively, were studied. All LPEs display surface melting between room temperature and the end melting temperature, whereas complete melting, according to lamellar thickness, only occurs at the highest temperatures where DSC displays a pronounced melting peak. There is recrystallization followed by isothermal lamellar thickening if annealing steps are inserted. The lamellar crystals of slowly cooled homogeneous copolymers melt in the reverse order of their formation, that is, crystals melt according to their thickness. Quenching creates unstable crystals through the cocrystallization of ethylene sequences with different length. These crystals repeatedly melt and co‐recrystallize during heating. The exothermic heat due to recrystallization partially compensates the endothermic heat due to melting resulting in a narrow overall DSC melting peak with its maximum at a higher temperature than the melting peak of slowly cooled copolymers. With increasing temperature, the crystallinity of quenched copolymers overtakes the one of slowly cooled samples due to co‐recrystallization by which an overcrowding of leaving chains at the crystal surfaces is avoided. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1975–1991, 2000     

Melting behavior of a poly(ether‐block‐amide) copolymer melt‐crystallized under quiescent,isothermal conditions

Segmented poly(ether‐block‐amide) copolymers are typically known as polyamide‐based thermoplastic elastomers consisting of hard, crystallizable polyamide block and flexible, amorphous polyether block. The melting characteristics of a poly(ether‐block‐amide) copolymer melt‐crystallized under various quiescent, isothermal conditions were calorimetrically investigated using differential scanning calorimetry (DSC). For such crystallized copolymer samples, their crystalline structures under ambient condition and the structural evolutions upon heating from ambient to complete melting were characterized using ambient and variable‐temperature wide‐angle X‐ray diffractometry (WAXD), respectively. It was observed that dependent of specific crystallization conditions, the copolymer samples exhibited one, two, or three melting endotherms. The ambient WAXD results indicated that all melt‐crystallized copolymer samples only exhibited γ‐form crystals associated with the hexagonal habits of the polyamide homopolymer, whereas variable‐temperature WAXD data suggested that upon heating from ambient, a melt‐crystallized copolymer might exhibit so‐called Brill transition before complete melting. Based on various DSC and variable‐temperature WAXD experimental results obtained in this study, the applicability of different melting mechanisms that might be responsible for multiple melting characteristics of various crystallized PEBA copolymer samples were discussed. It was postulated that the low (T _m1) endotherm was primarily because of the disruption of less thermally stable, short‐range ordered structure of amorphous polyamide segments of the copolymer, which was only formed after the completion of primary crystallization via so‐called annealing effects. The intermediate (T_m2) and high (T_m3) endotherms were attributed to the melting of primary crystals within polyamide crystalline microdomains of the copolymer. The appearance of these two melting endotherms might be somehow complicated by thermally induced Brill transition. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2035–2046, 2008     

Isothermal crystallization kinetics and melting behavior of poly(ethylene terephthalate)/barite nanocomposites

Poly(ethylene terephthalate) (PET)/Barite nanocomposites were prepared by direct melt compounding. The effects of PET‐Barite interfacial interaction on the dynamic mechanical properties and crystallization were investigated by DMA and DSC. The results showed that Barite can act as a nucleating agent and the nucleation activity can be increased when the Barite was surface‐modified (SABarite). SABarite nanoparticles induced preferential lamellae orientation because of the strong interfacial interaction between PET chains and SABarite nanoparticles, which was not the case in Barite filled PET as determined by WAXD. For PET/Barite nanocomposites, the Avrami exponent n increased with increasing crystallization temperature. Although at the same crystallization temperature, the n value will decrease with increasing SABarite content, indicating of the enhancement of the nucleation activity. Avrami analyses suggest that the nucleation mechanism is different. The activation energy determined from Arrhenius equation reduced dramatically for PET/SABarite nanocomposite, confirming the strong interfacial interaction between PET chains and SABarite nanoparticles can reduce the crystallization free energy barrier for nucleus formation. In the DSC scan after isothermal crystallization process, double melting behavior was found. And the double endotherms could be attributed to the melting of recrystallized less perfect crystallites or the secondary lamellae produced during different crystallization processes. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 655–668, 2009     

Effects of carbon nanotubes on crystallization and melting behavior of poly(L‐lactide) via DSC and TMDSC studies

In this work, multiwalled carbon nanotubes (MWNTs) were surface‐modified and grafted with poly(L ‐lactide) to obtain poly(L ‐lactide)‐grafted MWNTs (i.e. MWNTs‐g‐PLLA). Films of the PLLA/MWNTs‐g‐PLLA nanocomposites were then prepared by a solution casting method to investigate the effects of the MWNTs‐g‐PLLA on nonisothermal and isothermal melt‐crystallizations of the PLLA matrix using DSC and TMDSC. DSC data found that MWNTs significantly enhanced the nonisothermal melt‐crystallization from the melt and the cold‐crystallization rates of PLLA on the subsequent heating. Temperature‐modulated differential scanning calorimetry (TMDSC) analysis on the quenched PLLA nanocomposites found that, in addition to an exothermic cold‐crystallization peak in the range of 80–120 °C, an exothermic peak in the range of 150–165 °C, attributed to recrystallization, appeared before the main melting peak in the total and nonreversing heat flow curves. The presence of the recrystallization peak signified the ongoing process of crystal perfection and, if any, the formation of secondary crystals during the heating scan. Double melting endotherms appeared for the isothermally melt‐crystallized PLLA samples at 110 °C. TMDSC analysis found that the double lamellar thickness model, other than the melting‐recrystallization model, was responsible for the double melting peaks in PLLA nanocomposites. Polarized optical microscopy images found that the nucleation rate of PLLA was enhanced by MWNTs. TMDSC analysis found that the incorporation of MWNTs caused PLLA to decrease the heat‐capacity increase (namely, ΔC_p) and the C_p at glass transition temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1870–1881, 2007     

Analysis of the complex thermal behavior of poly(L‐lactic acid) film. II. Samples crystallized from the melt

The complex thermal behavior of poly(l ‐lactic acid) films crystallized from the melt, either isothermally or nonisothermally, was studied by differential scanning calorimetry (DSC), wide angle X‐ray diffraction, and small angle X‐ray scattering. The variation of the thermal behavior with crystallization temperature, time, and cooling rate was documented and analyzed. After nonisothermal crystallization at low cooling rates that develop high crystallinity, an obvious double melting peak appears at modest heating rates (e.g., 10 °C/min). At higher heating rates, these samples exhibit only single melting. However, an unusual form of double melting occurs under the majority of the conditions studied under either isothermal or nonisothermal conditions. In this case, double melting is marked by the appearance of a recrystallization exotherm just prior to the final melting that obscures the observation of the melting of the crystals formed during the initial crystallization process. The occurrence of double melting in melt‐crystallized samples was concluded to be the result of a melt‐recrystallization process occurring during the subsequent DSC heating scan; it is a function of crystalline perfection, not the initial crystallinity, nor whether or not the crystallization reached completion at the crystallization temperature. Many other very interesting observations are also discussed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3378–3391, 2006     

Confirmation of a nanohybrid shish‐kebab (NHSK) structure in composites of PET and MWCNTs

Here, the confirmation of an oriented nanohybrid shish‐kebab (NHSK) crystalline structure in a series of composites of poly(ethylene terephthalate) (PET) and multiwall carbon nanotubes (MWCNTs) is reported. The combined use of small‐ and wide‐angle X‐ray scattering (SAXS/WAXS) and thermal analysis has been used to investigate the morphology development in PET‐MWCNT nanocomposites under hot isothermal crystallization conditions. The MWCNTs act as both heterogeneous nucleating agents and surfaces (oriented shish structures) for the epitaxial growth of PET crystallites (kebabs) giving an oriented crystalline morphology. In contrast, the PET homopolymer does not show any residual oriented crystalline morphology during isothermal crystallization but gave a sporadic nucleation of a classic unoriented lamellar structure with slower crystallization kinetics. The results provide a valuable insight into the role of MWCNTs as nanoparticulate fillers in the morphology development and subsequent modification of physical properties in engineering polymers. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 132–137     

SAXS and DSC studies of the crystallization and melting phenomena of poly(ϵ‐caprolactone)

The relationships between the crystallization temperature, T_c, the crystal thickness, d_c and the melting peak temperature, T_m of poly(ϵ‐caprolactone) have been determined by carrying out time‐ and temperature‐dependent small angle x‐ray scattering experiments. A two‐step melting has been suggested, resulting in the occurrence of two well defined independent boundary lines, indicating the transformation from melt into the partially crystalline state. For crystallization temperatures lower than 40°C, during heating, more pronounced peak shifts are observed with the final melting of the crystallites having the same thickness of d_c ≈︁ 7 nm. In this region, it is evident that crystals have relatively good stabilities, since they have fairly uniform thickness. At higher temperatures, above 40°C, T_m increases with T_c, together with the thickness. The transformation of the melt into the partially crystalline state depicts a two‐step process, beginning with the formation of a well defined initial structure with lower order, which is subsequently stabilized.     

Structure development during melt spinning and subsequent annealing of polybutene‐1 fibers

The structural development during the melt spinning and subsequent annealing of polybutene‐1 fibers was studied with in situ wide‐angle X‐ray scattering techniques. The online spinning apparatus consisted of a vertically translating extruder that allowed different distances from the spinneret to the stationary X‐ray beam to be sampled. For all take‐up speeds examined, phase II crystals mainly were formed, with only a small population of phase I crystals existing. As the take‐up speed was increased, the crystallinity also increased, indicating that strain‐induced crystallization prevailed. The crystalline orientations observed online were very close to perfect alignment with the fiber axis. In addition, annealing studies were performed to study aspects of the gradual phase II to phase I transformation as functions of time and prior processing take‐up speed. This transformation was strongly dependent on the take‐up speed. The dependence appears to be connected to local stress enhancement via chains connecting crystallites. The results also seem to indicate that at low take‐up speeds (17 mpm) there is a series connectivity of amorphous and crystalline components in the fiber, whereas at greater take‐up speeds (100 and 250 mpm), the morphology grows into some type of three‐dimensional network, possibly a shish–kebob‐type morphology. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1872–1882, 2000     

Morphology and crystallization behavior of poly(l‐lactide)/poly(butylene oxalate) blends

The isothermal crystallization of poly(l ‐lactide) (PLLA) in blends with poly(butylene oxalate) (PBOX) is investigated by time‐resolved small‐angle X‐ray scattering, differential scanning calorimetry, and optical microscopy. We focus on the temperatures at which only PLLA crystallizes while PBOX is amorphous. It is obtained that the addition of PBOX causes a reduction of the melting temperature of PLLA. The lamellar thickness of PLLA crystals decreases whereas the amorphous layer thickness increases with blend composition, suggesting the occurrence of the interlamellar incorporation upon the addition of PBOX. The crystal growth rate and morphology of PLLA/PBOX blends are analyzed by polarized optical microscopy. The spherulite growth rate of PLLA is found to increase with the addition of PBOX. Analysis of the isothermal crystallization in terms of the Lauritzen and Hoffman equation give the reduction of the fold surface free energy upon the addition of PBOX in PLLA, indicating that the mobility of the PLLA chains is significantly improved due to the presence of PBOX. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2015 , 53, 192–202     

间规1,2-聚丁二烯结晶结构研究

以间规1,2-聚丁二烯(s-PB)为研究对象,通过原位同步辐射小角X射线散射(SR-SAXS)和广角X射线衍射(WAXD)研究其结晶结构的变化过程.SR-SAXS曲线中存在明显的散射峰,表明在等温结晶过程中形成有序结晶结构;在等温结晶后间规1,2-聚丁二烯的片晶厚度、微晶尺寸均正比于1/Tc∞-T,根据高分子结晶中介相机理可以做出合理的解释.     

Characterization,crystallization kinetics,and melting behavior of poly(ethylene succinate) copolyester containing 10 mol % butylene succinate

Copolyester was synthesized and characterized as having 89.9 mol % ethylene succinate units and 10.1 mol % butylene succinate units in a random sequence, as revealed by NMR. Isothermal crystallization kinetics was studied in the temperature range (T_c) from 30 to 73 °C using differential scanning calorimetry (DSC). The melting behavior after isothermal crystallization was investigated using DSC by varying the T_c, the heating rate and the crystallization time. DSC curves showed triple melting peaks. The melting behavior indicates that the upper melting peaks are associated primarily with the melting of lamellar crystals with various stabilities. As the T_c increases, the contribution of recrystallization slowly decreases and finally disappears. A Hoffman‐Weeks linear plot gives an equilibrium melting temperature of 107.0 °C. The spherulite growth of this copolyester from 80 to 20 °C at a cooling rate of 2 or 4 °C/min was monitored and recorded using an optical microscope equipped with a CCD camera. Continuous growth rates between melting and glass transition temperatures can be obtained after curve‐fitting procedures. These data fit well with those data points measured in the isothermal experiments. These data were analyzed with the Hoffman and Lauritzen theory. A regime II → III transition was detected at around 52 °C. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2431–2442, 2008     

Structure and deformation recovery of the thermoplastic polyurethane spherulite

We investigated the structure and deformation behavior of the thermoplastic polyurethane (TPU) spherulite by optical microscopy, tensile testing, Hv light scattering, and small angle X‐ray scattering. The TPU spherulite structure obtained by melt crystallization was coarse consisting of bundle‐like structure containing hard segment (HS) lamellar domain in which the HS domains were stacked and the HS chain direction was perpendicular to the longitudinal axis of the HS domain. By stretching, the spherulite was deformed to ellipsoidal one and the stacked HS lamellar domains were tilted in the stretching direction. The deformed spherulite and the tilted HS domain in the spherulite were recovered to the unstretched state by retraction. The recovery of the structure is ascribed to the characteristic spherulite structure consisting of rubbery soft segment matrix physically cross‐linked with the stacked HS domain. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1585–1594     

Crystallization studies of isotactic polypropylene containing nanostructured polyhedral oligomeric silsesquioxane molecules under quiescent and shear conditions

Crystallization studies at quiescent and shear states in isotactic polypropylene (iPP) containing nanostructured polyhedral oligomeric silsesquioxane (POSS) molecules were performed with in situ small‐angle X‐ray scattering (SAXS) and differential scanning calorimetry (DSC). DSC was used to characterize the quiescent crystallization behavior. It was observed that the addition of POSS molecules increased the crystallization rate of iPP under both isothermal and nonisothermal conditions, which suggests that POSS crystals act as nucleating agents. Furthermore, the crystallization rate was significantly reduced at a POSS concentration of 30 wt %, which suggests a retarded growth mechanism due to the molecular dispersion of POSS in the matrix. In situ SAXS was used to study the behavior of shear‐induced crystallization at temperatures of 140, 145, and 150 °C in samples with POSS concentrations of 10, 20, and 30 wt %. The SAXS patterns showed scattering maxima along the shear direction, which corresponded to a lamellar structure developed perpendicularly to the flow direction. The crystallization half‐time was calculated from the total scattered intensity of the SAXS image. The oriented fraction, defined as the fraction of scattered intensity from the oriented component to the total scattered intensity, was also calculated. The addition of POSS significantly increased the crystallization rate during shear compared with the rate for the neat polymer without POSS. We postulate that although POSS crystals have a limited role in shear‐induced crystallization, molecularly dispersed POSS molecules behave as weak crosslinkers in polymer melts and increase the relaxation time of iPP chains after shear. Therefore, the overall orientation of the polymer chains is improved and a faster crystallization rate is obtained with the addition of POSS. Moreover, higher POSS concentrations resulted in faster crystallization rates during shear. The addition of POSS decreased the average long‐period value of crystallized iPP after shear, which indicates that iPP nuclei are probably initiated in large numbers near molecularly dispersed POSS molecules. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2727–2739, 2001     

Morphology of polyanhydride copolymers: Time‐resolved small‐angle X‐ray scattering studies of crystallization

Synchrotron small‐angle X‐ray scattering (SAXS) was used to study the isothermal crystallization kinetics of a family of polyanhydride copolymers consisting of 1,6‐bis(p‐carboxyphenoxy)hexane and sebacic acid monomers. In situ SAXS experiments permitted the direct observation of the crystallization kinetics. The structural parameters (the long period, lamellar thickness, and degree of crystallinity) were obtained from Lorentz‐corrected intensity profiles, one‐dimensional correlation functions, and interface distribution functions to form a comprehensive picture of the crystal morphology. The combination of these three analyses provided information not only on the lamellar dimensions but also on the polydispersity (nonuniformity) of these dimensions. Where possible, the crystallization kinetics were interpreted with a modified version of the Avrami equation. The results can be used to perform the rational design of controlled‐drug‐release formulations because crystallinity affects drug‐release kinetics. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 463–477, 2005     

Crystallization kinetics and multiple melting phenomena of a flame‐retardant phosphorus containing copolyester

Melt, cold isothermal crystallization kinetics, and multiple melting phenomena are investigated by differential scanning calorimetry (DSC) for a flame‐retardant phosphorus containing copolyester. The crystallization kinetics was investigated by the Avrami equation. The Avrami exponent is about 2.6 for melt crystallization and about 2 for cold crystallization. The crystallization activation energy for melt crystallization and for cold crystallization is −64.7 and 145.5, respectively. Three melting endotherms are found in the DSC scan, and they are explained in terms of secondary crystallization, primary crystallization, and recrystallization during the scan. A strong evidence of a two‐stage crystallization mechanism was also observed in the DSC isothermal experiment and X‐ray diffraction. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2269–2277, 1999