Study on Melting and Recrystallization of Poly(butylene succinate) Lamellar Crystals via Step Heating Differential Scanning Calorimetry

Differential scanning calorimetry (DSC) has been widely applied to study crystallization and melting of materials.However,for polymeric lamellar crystals,the melting thermogram during heating process usually exhibits a broad endothermic peak or even multiple endotherms,which may result from changes of metastability via recrystallization process.Sometimes,the recrystallization exotherm cannot be observed due to its overlapping with the melting endotherm.In this work,we employed a step heating procedure consisting of successive heating and temperature holding stages to measure the metastability of isothermally crystallized poly(butylene succinate) (PBS) crystals.With this approach we could gain the fraction of crystals melted at different temperature ranges and quantitatively detect the melting-recrystallization behavior.The melting-recrystallization behavior depends on the polymer chain structure and the crystallization temperature.For instance,PBS block copolymer hardly shows recrystallization behavior while PBS oligomer and high molecular weight PBS homopolymer demonstrate remarkable melting-recrystallization phenomenon.High molecular weight PBS isothermally crystallized in the low temperature range shows multiple melting-recrystallization while those isothermally crystallized at elevated temperatures do not exhibit observable recrystallization behavior.Furthermore,the melting endotherms were fitted via the melting kinetics equations.The original isothermally crystallized lamellae demonstrate quite different melting kinetics from the recrystallized lamellar crystals that melt at the highest temperature range,which is attributed to the different degrees of stabilization.Finally,the mechanism of melting-recrystallization is briefly discussed.We propose that apparent meltrecrystallization phenomenon be observed when melting of preformed lamellar crystals and recrystallization of thicker lamellae have similar free energy barrier.     

Correlation between melting behavior and ringed spherulites in poly(trimethylene terephthalate)

The lamellar types as revealed by the multiple melting peaks and possible mechanisms of ringed spherulites in poly(trimethylene terephthalate) (PTT) were analyzed with differential scanning calorimetry (DSC), optical microscopy, and scanning electron microscopy. Several interesting correlations were found. If PTT is melt‐crystallized in a certain temperature range, it shows multiple melting peaks and rings in PTT. Once rings are formed in the original melt‐crystallized PTT, they do not disappear but persist and become even more apparent upon postcrystallization annealing at higher temperatures. Furthermore, for PTT that is capable of exhibiting ringed spherulites, a temperature range exists where rings do not form. This behavior can be interpreted in relation with the demonstrated thermal behavior in PTT. Reorganization took place upon postcrystallization scanning or annealing to or at higher temperatures. A postulation was proposed and rigorously tested with evidence to correlate the ringed spherulites and melting behavior. Rings in PTT may be related to multiple lamellae in the spherulites. Consequently, if a temperature of crystallization is selected so that there is only one type of lamella in the spherulites, then there should be no rings. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 80–93, 2002     

Time-resolved synchrotron X-ray study of crystalline phase transition in poly(aryl ether ketone ketone) containing alternated terephthalic/isophthalic moieties

Unique crystallization and melting behavior in poly(aryl ether ketone ketone) containing alternated terephthalic and isophthalic moieties were studied by time-resolved synchrotron x-ray methods. Recently, this material has been shown to exhibit three polymorphs (forms I, II, and III). In this work, we further investigated their distinctive thermal properties and found that form I is the dominating and the most thermally stable phase while form II is favored by fast nucleation conditions and is the least stable phase. On the other hand, form III represents a minor intermediate phase that usually coexists with form I and can be transferred from form II and to form I. Structural and morphological changes in form I have been followed by simultaneous wide-angle x-ray diffraction (WAXD)/small-angle x-ray scattering (SAXS) measurements during cold- or melt-crystallization and subsequent melting. In all cases, a larger dimensional change was found in the crystallographic a-axis than the b-axis during heating and cooling. This may be due to the greater lateral stress variation with respect to temperature along the a direction of the primary lamellae which is induced by either the formation of secondary lamellae or the preferential chain-folding direction in poly(aryl ether ketone ketone)s. During the phase transitions of form II ← III in the cold-crystallized specimen and form III ← I in the melt-crystallized samples, lamellar variables (long period, lamellar thickness, and invariant) obtained from SAXS remain almost constant. This indicates that the density distribution in the long spacing is independent of the melting in form II or III. For melt-crystallization, the corresponding changes in unit-cell dimensions and lamellar morphology during the annealing-induced low endotherm are most consistent with the argument that these changes are due to the melting of thin lamellar population. © 1995 John Wiley & Sons, Inc.     

Study on melting and recrystallization of poly(butylene succinate) lamellar crystals <Emphasis Type="Italic">via</Emphasis> step heating differential scanning calorimetry

Differential scanning calorimetry (DSC) has been widely applied to study crystallization and melting of materials. However, for polymeric lamellar crystals, the melting thermogram during heating process usually exhibits a broad endothermic peak or even multiple endotherms, which may result from changes of metastability via recrystallization process. Sometimes, the recrystallization exotherm cannot be observed due to its overlapping with the melting endotherm. In this work, we employed a step heating procedure consisting of successive heating and temperature holding stages to measure the metastability of isothermally crystallized poly(butylene succinate) (PBS) crystals. With this approach we could gain the fraction of crystals melted at different temperature ranges and quantitatively detect the melting-recrystallization behavior. The melting-recrystallization behavior depends on the polymer chain structure and the crystallization temperature. For instance, PBS block copolymer hardly shows recrystallization behavior while PBS oligomer and high molecular weight PBS homopolymer demonstrate remarkable melting-recrystallization phenomenon. High molecular weight PBS isothermally crystallized in the low temperature range shows multiple melting-recrystallization while those isothermally crystallized at elevated temperatures do not exhibit observable recrystallization behavior. Furthermore, the melting endotherms were fitted via the melting kinetics equations. The original isothermally crystallized lamellae demonstrate quite different melting kinetics from the recrystallized lamellar crystals that melt at the highest temperature range, which is attributed to the different degrees of stabilization. Finally, the mechanism of melting-recrystallization is briefly discussed. We propose that apparent melt-recrystallization phenomenon be observed when melting of preformed lamellar crystals and recrystallization of thicker lamellae have similar free energy barrier.     

Morphological Changes of Linear,Branched Polyethylenes and their Blends during Crystallization and Subsequent Melting by Synchrotron SAXS and DSC

Summary: The crystalline structure and phase morphology of linear, branched polyethylenes and their blends during crystallization and subsequent melting were investigated, using a combination of differential scanning calorimetry (DSC), and synchrotron small angle X-ray scattering (SAXS). A linear polyethylene (PE1) with weight-average molecular weight (M_w) of 114 000 g/mol, and two branched polyethylene copolymers, containing 4.8 mol% (PE4) and 15.3 mol% (PE10) hexane, with molecular weights of 93 000 g/mol and 46 000 g/mol were used as pure samples. Two blends, PE1-4 and PE1-10, each with a weight ratio of 50/50, were prepared by solution blending. Our results indicate that in PE4 a phase separation within the branched component itself occurred, forming a broad distribution of lamellar thicknesses during the crystallization process. PE10 on the other hand did hardly crystallize because of the high degree of branching. Co-crystallization of both components took place in blend PE1-4 and liquid-liquid phase separation occurred in the melt of PE1-10. Morphological parameters were determined by using Bragg's law and the correlation function, respectively. The detected semicrystalline morphology can be well described by the lamellar insertion mode where thin lamellae develop between thicker primary lamellae. During subsequent heating, lamellae melted in the reversed sequence of their formation. The evolution of the structural parameters as a function of temperature revealed that surface melting began at first, and then the complete melting of stacks occurred until the final melting temperature was reached.     

基于同步辐射超小角X射线散射的高密度聚乙烯空洞化行为研究

以一系列高温结晶后自然冷却的高密度聚乙烯(HDPE)为研究对象,利用同步辐射超小角X射线散射(USAXS)和示差扫描量热技术(DSC)对样品的微观结构进行了分析,并在线研究了单轴拉伸过程中的空洞化行为.结果表明,结晶温度高于110℃后自然冷却到室温的样品中存在热稳定性不同的两组片晶,等温过程形成结构完善的厚片晶,而在冷却过程会形成有缺陷的薄片晶,两组片晶的熔点分别在133和110℃附近.在30℃拉伸时,所有样品都可观察到空洞化并伴随发白现象.并且,等温结晶中形成片晶厚度越大的样品,相应的空洞化现象越明显.在拉伸过程中,空洞出现在屈服点附近,其法向方向平行于拉伸方向,后随应变的增加发生转向,法向方向与拉伸方向垂直.样品中空穴的长度为900~1200 nm.另一方面,随着冷却过程生成薄片晶比例的增加,空洞化趋势下降.此外,提高拉伸温度,样品更倾向发生塑性形变,空洞化程度减弱.     

Morphology of cold-crystallized polyamide 6

The structure of cold-crystallized polyamide 6 (PA 6) has been analyzed by wide-angle X-ray scattering, atomic force microscopy, and polarizing optical microscopy. It has been found that ordering of initially fully amorphous and glassy PA 6 on slow heating to temperatures higher than the glass transition temperature results in formation of spatially non-organized short lamellae/nodules with a size depending on the maximum annealing temperature. In contrast, melt crystallization at low supercooling is connected with formation of spherulites and laterally extended lamellae. The observed experimental results demonstrate that crystals of qualitatively different morphology and higher-order organization can be generated by variation of the pathway of crystallization. It is assumed that the different structures obtained in melt- and cold-crystallized samples are related to heterogeneous and homogeneous nucleation, respectively.     

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     

Morphology development and crystallization behavior of poly(ethylene terephthalate)/poly(trimethylene terephthalate) blends

The spherulite morphology and crystallization behavior of poly(ethylene terephthalate) (PET)/poly(trimethylene terephthalate) (PTT) blends were investigated with optical microscopy (OM), small-angle light scattering (SALS), and small-angle X-ray scattering (SAXS). The thermal analysis showed that PET and PTT were miscible in the melt over the entire composition range. The rejected distance of non-crystallizable species, which was represented in terms of the parameter δ, played an important role in determining the morphological patterns of the blends at a specific crystallization temperature regime. The parameter δ could be controlled by variation of the composition, the crystallization temperature, and the level of transesterification. In the case of two-step crystallization, the crystallization of PTT commenced in the interspherulitic region between the grown PET crystals and proceeded until the interspherulitic space was filled with PTT crystals. The spherulitic surface of the PET crystals acted as nucleation sites where PTT preferentially crystallized, leading to the formation of non-spherulitic crystalline texture. The SALS results suggested that the growth pattern of the PET crystals was significantly changed by the presence of the PTT molecules. The lamellar morphology parameters were evaluated by a one-dimensional correlation function analysis. The blends that crystallized above the melting point of PTT showed a larger amorphous layer thickness than the pure PET, indicating that the non-crystallizable PTT component might be incorporated into the interlamellar region of the PET crystals. With an increased level of transesterification, the exclusion of non-crystallizable species from the lamellar stacks was favorable due to the lower crystal growth rates. As a result, the amorphous layer thickness of the PET crystals decreased as the annealing time in the melt state was increased.     

Confined lamella formation in crystalline-crystalline poly(ethylene oxide)-b-poly(ɛ-caprolactone) diblock copolymers

The real time and in situ investigation of the crystallization process and structure transitions of asymmetric crystalline-crystalline diblock copolymers from the melt was performed with synchrotron simultaneous SAXS/WAXS. The asymmetric poly(ethylene oxide)-b-poly(ε-caprolactone) diblock copolymers were chosen for the present study. It was shown that the short blocks crystallized later than the long blocks and final lamellar structure was formed in all of the asymmetric diblock copolymers. The final lamellar structure was confirmed by AFM observation. The SAXS data were analyzed with different methods for the early stage of the crystallization. The Guinier plots indicated that there were no isolated domains formed before the formation of lamellae in the asymmetric diblock copolymers during the crystallization process. Debye-Bueche plots implied the formation of correlated domains during crystallization.     

Effects of miscible diluent of poly(ether imide) on ring-banded morphology of poly(trimethylene terephthalate)

The melting, crystallization, and self-packed ring patterns in the spherulites of miscible blends comprising poly(trimethylene terephthalate) (PTT) and poly(ether imide) (PEI) were revealed by optical, scanning electron microscopies (PLM and SEM) and differential scanning calorimetry (DSC). Morphology and melting behavior of the miscible PTT/PEI blends were compared with the neat PTT. Ringed spherulites appeared in the miscible PTT/PEI blends at all crystallization temperatures up to 220 °C, whereas at this high temperature no rings were seen in the neat PTT. A postulation was proposed, and interrelations between rings in spherulites and the multiple lamellae distributions were investigated. The specific interactions and the segregation of amorphous PEI were discussed for interpreting the morphological changes of 220 °C-melt-crystallized PTT/PEI samples. Interlamellar segregation of PEI might be associated with multiple lamellae in the spherulites of PTT/PEI blends; therefore, rings were more easily formed in the PTT/PEI blends at all crystallization temperatures. A postulated model of uneven lamellar growth, coupled with periodical spiraling, more properly describes the possible origin of ring bands from combined effects of both interactions and segregation between the amorphous PEI and PTT in blends.     

Time-resolved SAXS studies of morphological changes in cold crystallized poly(ethylene terephthalate) during annealing and heating

Structural changes occurring during crystallization of quenched amorphous poly(ethylene terephthalate) (PET) and subsequent cooling/heating cycles have been studied by real-time small-angle x-ray scattering (SAXS), using synchrotron radiation. Initial crystallization is found to occur by insertion of new lamellae between the existing ones, while rapid continuous melting/recrystallization happens when the cold-crystallized PET samples are heated above the previous highest annealing temperature. Such melting/recrystalization results in irreversible increases in the lamellar long period, the crystal thickness and the density difference between the crystalline and amorphous regions; in contrast, at temperatures below the prior highest crystallization temperature, the structural changes are dominated by reversible effects such as thermal expansion. However, throughout the entire temperature range up to the melting point around 250 °C, the crystal core thickness remains quite small, less than ca. 50 Å, and the linear crystallinity of lamellar stacks remains nearly constant around 0.3. Such a low crystallinity indicates the presence of thick order-disorder interfacial layers on the lamellar surface, whose thickness increases with temperature.Dedicated to Prof. E. W. Fischer on the occasion of his 65th birthday.     

Characterization of polyethylene crystallization from an oriented melt by molecular dynamics simulation

Molecular dynamics is used to characterize the process of crystallization for a united atom model of polyethylene. An oriented melt is produced by uniaxial deformation under constant load, followed by quenching below the melting temperature at zero load. The development of crystallinity is monitored simultaneously using molecular-based order parameters for density, energy, and orientation. For crystallization temperatures ranging from 325 to 375 K, these simulations clearly show the hallmarks of crystal nucleation and growth. We can identify multiple nucleation events, lamellar growth up to the limit imposed by periodic boundaries of the simulation cell, and lamellar thickening. We observe a competition between the rate of nucleation, which results in multiple crystallites, the rate of chain extension, which results in thicker lamellae, and the rate of chain conformational relaxation, which is manifested in lower degrees of residual order in the noncrystalline portion of the simulation. The temperature dependence of lamellar thickness is in accord with experimental data. At the higher temperatures, tilted chain lamellae are observed to form with lamellar interfaces corresponding approximately to the [201] facet, indicative of the influence of interfacial energy.     

Equilibrium Crystallization Temperature of Syndiotactic Polystyrene <Emphasis Type="Italic">γ</Emphasis> Form

The crystallization behavior of syndiotactic polystyrene (sPS) γ form undergoing annealing at various temperatures was investigated using the thermodynamic phase diagram based on Strobl's crystallization theory.On the basis of the differential scanning calorimetric results,it was observed that γ form melt-recrystallization occurred at a higher temperature with the increasing lamellar thickness,which resulted from the pre-annealing at the elevating temperature after acetone induced crystallization.Further temperature dependent small-angle X-ray scattering (SAXS) measurement revealed the evolution of the γ form lamellae upon heating until phase transition,involving three different regimes:lamellae stable region (25-90 ℃),melt-recrystallization region (90-185 ℃) and pre-phase transition region (185-195 ℃).As a result,recrystallization line,equilibrium recrystallization line and melting line were developed for the sPS γform crystallization process.Since the melt of γform involved a γto-α/β form phase transition,the melting line was also denoted as the phase transition line in this special case.Therefore,the equilibrium crystallization temperature and melting (phase transition)temperatures were determined at around 390 and 220 ℃ on the basis of the thermodynamic phase diagram of the sPS γform.     

X-ray scattering and thermal analysis study of the effects of molecular weight on phase structure in blends of poly(butylene terephthalate) with polycarbonate

Blends of Poly(butylene terephthalate), PBT, with Polycarbonate, PC, were studied for a range of molecular weights and blend compositions. Blends were available in PBT/PC compositions 80/20 and 40/60, and with M_w designated by H (high) or L (low). Samples were prepared by melt crystallization, or by cold crystallization following a rapid quench from the melt. Addition of PC reduces the crystallization kinetics of PBT so that the resulting crystals are more perfect than those which form in the homopolymer. Degree of crystallinity of the blends followed the rank ordering: L/L > L/H > H/L = H/H. The glass transition behavior was investigated using dynamic mechanical analysis (DMA) and modulated differential scanning calorimetry (MDSC). All blends exhibited two glass transitions at intermediate temperatures between the T_gs of the homopolymers, indicating existence of a PBT-rich phase and a PC-rich phase. Blends L/L were most, and H/H the least, miscible. Small-angle X-ray scattering was performed at room temperature on cold crystallized blends, or at elevated temperature during melt crystallization. The long period was consistently larger, and the linear stack crystallinity was consistently smaller, in blends L/L or H/L. These results indicate that in blends containing low M_w PC, there is more PC located within the PBT-rich phase. The long period was consistently smaller in cold crystallized samples, while the linear stack crystallinity was nearly the same, regardless of melt or cold crystallization treatment. Reduction of the average long period in cold crystallized samples could result from crystallization of PBT within the PC-rich phase. This is consistent with thermal analysis results, which indicate that cold crystallized samples have greater overall crystallinity than melt crystallized samples. A hypothetical liquid phase diagram is presented to explain the differences between melt and cold crystallized blends. © 1996 John Wiley & Sons, Inc.     

Crystallization of polyethylene at elevated pressures

The crystallization from the melt of three sharp polyethylene fractions has been studied at 5 kbar. It has been shown that the thickness of so-called extended-chain lamellae is a function of time, temperature, and molecular weight. There is by no means just the fully extended molecular configuration present. Crystallization is qualitatively similar to that of chain-folded crystals at 1 bar, giving an optimum lamellar thickness which increases with time and decreasing supercooling. Fractional crystallization is widespread and is a major cause of disparate lamellar thickness. Isothermal thickening of lamellae during crystallization has been established directly. Morphological detail suggests further that layers can increase their thickness tenfold over their initial size.     

Lamellar morphology of polyethylene: Electron microscopy of a melt-crystallized sharp fraction

The morphology of linear polyethylene of M_w/M_n 1.19 crystallized isothermally from the melt has been investigated. Electron microscopy (EM) of stained thin sections gave exceptionally well-defined images. There were no qualitative differences between Regime I and Regime II crystallization, but the lamellae grown at lower supercooling were thicker, larger, and had fewer spiral growths per unit area of fold surface. Small-angle x-ray scattering indicated a much sharper distribution of lamellar thicknesses than EM and Raman LAM did. This is evidence of the complementary nature of the techniques and not a contradiction. EM also revealed nonrepresentative features in detail. There were extremely thin lamellae formed on cooling, indicating molecular weight segregation even in this fractionated material, and there were extremely thick lamellae, attributed to isothermal thickening. As the representative lamellae are also supposed to have thickened isothermally, this raises the issue of when and how the lamellae which are the primary products of crystallization can be identified.     

Analysis of dual melting behavior in cold-crystallized poly(ether diphenyl ether metaketone)

By focusing on cold-crystallized poly(ether diphenyl ether metaketone) (PEKm), a more in-depth understanding of the nature of the crystalline morphology has been gained, which may lead to thorough mechanisms for interpreting the observed thermal behavior in PEKm. Apparently, cold-crystallized PEKm containing initially only a single P1 crystal can exhibit dual melting peaks (300 and 320 °C), with the second high-melting peak corresponding to the P2 crystal that was subsequently formed via P1 melting/repacking during the scan. However, dual morphism (preexisting P1 and P2 crystals) could be intentionally introduced into PEKm if it was cold-crystallized at temperature schemes of decreasing order. The P1 and P2 crystals possess the same unit cells (orthorhombic) and thus they differ only in the lamellae populations. The dual lamellar morphism in this PEKm sample also exhibited similar dual melting peaks during scanning, which correspond to melting of the individual P1 and P2 in a sequential order. This study has thus provided important clues in and shed new light on the interpretation of multiple melting with respect to polymorphism in polymers. Relationships between the low-melting and high-melting lamellae in cold-crystallized polyketone polymer have been thoroughly explored. Received: 9 January 2001/Accepted: 3 February 2001     

Influence of the order of polymer melt on the crystallization behavior: I. Double melting endotherms of isotactic polypropylene

The influence of the order of polymer melt on the subsequent crystallization and melting has been carefully studied. The experimental data show that the order of isotactic polypropylene (iPP) melt decreases with increases in the fusion temperature. For an iPP sample isothermally crystallized at 130 °C for half an hour, the degree of order of melt is higher when the fusion temperature is lower than about 170.5 °C, hence the lamellae formed in a rapid cooling process are perfect. If the fusion temperature is not higher than 167 °C, some thicker lamellae can exist in the melt. The melting of these unmelted lamellae and those lamellae recrystallized in the cooling process result in double endotherms. On the other hand, when the fusion temperature is higher than 170.5 °C, the order of the iPP melt decreases greatly; thus, the lamellae formed in the following cooling process are imperfect. At a lower heating rate, the recrystallization or reorganization of these imperfect lamellae also leads to double melting endotherms. Received: June 16, 2000 Accepted: October 16, 2000     

Crystallization and melting of cold-crystallized poly(phenylene sulfide)

In this study, we report the melting behavior of poly(phenylene sulfide), PPS, which has been cold-crystallized from the rubbery amorphous state. We find that the crystallization kinetics are faster for cold-crystallized PPS than for melt-crystallized material, due to formation during quenching of a short-range ordered, but noncrystalline, structure. We observe that the endothermic response of cold-crystallized PPS at a large undercooling consists of a low temperature endotherm, followed by an exothermic region, and by the main higher melting endotherm. The lower melting peak temperature of cold-crystallized PPS increases as the crystallization temperature increases, but the main upper melting peak temperature remains almost the same. The size of the exothermic region is strongly related to the degree of undercooling, and must be taken into account in order properly to determine the degree of crystallinity of the material prior to the scan. When the crystallization time is varied, we see a systematic decrease in the size of the main endotherm, and an increase in size of the lower melting endotherm. This suggests that a portion of the main endothermic response is due to reorganization during the scan. Annealing will not only increase the degree of crystallinity but also improve the crystal perfection; therefore the ability of an annealed sample to reorganize decreases as the annealing time increases. However, an additional third melting peak is seen when a cold-crystallized sample is annealed at high temperature for a sufficiently long residence time. The existence of the third melting peak suggests that more than one kind of distribution of crystal perfection may occur when PPS has been cold-crystallized and subsequently annealed.