THE DOUBLE MELTING PEAKS OF POLY(ETHYLENE TEREPHTHALATE)

Three sets of PET samples, comprising original (undrawn), uniaxially drawn and biaxially ones, after annealed at 230°, 240°and 250℃respectively, were subjected to DSC thermal analysis, X-ray diffraction analysis and IR analysis. The results indicate that the phenomenon of double melting peaks during DSC analysis is due to the partial melting and recrystallization of the crystallite at the moment of thermal scanning. The lower temperature peak, which varies slightly according to annealing condition, corresponds to the melting of imperfect crystallite, and the higher temperature peak corresponds to the melting of better organized crystallite. In the course of temperature scanning, the unit cell parameters of PET remains unchanged while the crystals turn to better crystal lattice, greater crystal size and more regular folding.We also found that there is a slight reduction in crystal size between the two melting peaks, and an explanation is suggested for this phenomenon.     

聚对苯二甲酸乙二酯的熔融双峰

未拉伸聚对苯二甲酸乙二酯(PET)、单轴拉伸PET、双轴拉伸PET在不同条件下热处理,并进行了DSC热分析、X-射线衍射分析、红外分析。结果表明:熔融双峰现象起因于DSC等速升温过程中PET的部分熔融和重结晶;低温峰是等温热处理过程中形成的不完善微晶的熔融峰,高温峰是重结晶后较完善的微晶的熔融峰;在等速升温过程中,PET晶胞参数不变、晶格完善、晶粒增大,并且规则折迭链增加,但晶粒随温度增大的过程中出现低谷。     

Chain-folding measurements in annealed poly(ethylene terephthalate)

Chain folding in unoriented poly(ethylene terephthalate) (PET) films has been investigated as a function of annealing time and temperature. To meet this objective dynamic mechanical, infrared, and molecular weight measurements were used, together with selective chemical degradation to remove chain folds and amorphous regions. The β dispersion in the dynamic mechanical spectrum of PET is here tentatively associated with motions of methylene and/or carboxyl groups in irregular chain folds; the β dispersion is not found in quenched amorphous polymer, in polymer where amorphous regions and chain folds have been removed, or in highly annealed PET where the irregular folds have regularized. Upon mild crystallization and annealing (30 min at 110°C) of initially amorphous film a large β dispersion appears and then diminishes upon further annealing at 220°C. As the β dispersion diminishes, the infrared regular fold band increases more than the crystallinity band, indicating regularization of folds. The molecular weight of the degraded residue corresponds approximately to typical fold-period dimensions (～130 Å), and increases on annealing as expected from lamellar thickening. The degradation process has, by fold removal, reduced the chains in the crystals to a very short, uniform length.     

Relation between melting behavior and physical structure in polymers

The phenomenon of double melting, as manifested by two characteristic endotherms in the melting region on a differential thermal analysis (DTA) scan, has been studied in nylon 66 and polystyrene as a function of sample treatment by annealing or drawing. A variety of techniques were used in these studies including DTA, x-ray diffraction, electron microscopy, and mechanical testing. It is shown that the two endotherms are not caused by a bimodal crystal size distribution, by recrystallization, by orientation changes, or by phase changes. It is proposed that one endotherm is caused by the melting of foldedchain crystals, while the other is due to the melting of less perfect bundle crystals. This view is well supported by the results, especially by the DTA measurements made at different heating rates. Published data on the thermal behavior of annealed and drawn poly(ethylene terephthalate) and on polyethylene crystallized at various pressures may also be explained on this basis if it is allowed that in polyethylene the chains may be more extended.     

Melting behavior of poly(butylene succinate) during heating scan by DSC

The melting behavior of isothermally crystallized poly(butylene succinate) (PBS) has been investigated using differential scanning calorimetry (DSC) and wide‐angle X‐ray analysis. The samples crystallized between 80°C to 100°C show middle endotherm at the position just before the high exotherm, while the others under 80°C show two endotherms (low and high). From the results of the melting peak vs. crystallization temperature plot, it was suggested that the middle endotherm corresponds to the melting process of the original crystallites and the high endotherms to the melting process of the recrystallized ones. As the DSC heating rate was increased, the peak temperature of the low and middle endotherms increased and that of the high endotherm decreased, indicating that the low endotherm was due to the original crystallites as well as the middle endotherm. Consequently, in the heating scan of PBS, the existence of two kinds of morphologically different crystallites as well as the process of melting and recrystallization becomes evident. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1357–1366, 1999     

CRYSTALLIZATION AND MELTING OF NYLON 610

Differential scanning calorimetry was used to study the crystallization andmelting of nylon 610. For nylon 610 crystallized from the melt state (260℃), the overall rateof bulk crystallization can be described by a simple Avrami equation with Avrami exponentn ≈ 2, independent of crystallization temperature. With the experimentally obtainedT_m~0 (235℃ ～ 255℃) of nylon 610, the fold surface free energy σ_e was determined to be35 ～38 erg/cm~2. The effects of annealing temperature and time on the melting of quenchednylon 610 were also investigated. For nylon 610 quenched at room temperature there isonly one DSC endotherm peak DSC scans on annealed samples exhibited an endothermpeak at approximately 10℃ above the annealing temperature. The size and position of theendothermic peak is strongly related to annealing temperature and time. An additionalthird melting was observed when quenched nylon 610 was annealed at high temperaturefor a sufficiently long residence time. The existence of the third melting peak suggests thatmore than one kind of distribution of lamella thickness may occur when quenched nylon610 is annealed. The implications of these results in terms of crystal thickening mechanismwere discussed.     

The double melting behavior of a liquid crystalline polyimide derived from PMDA and 1,3‐bis[4‐(4′‐aminophenoxyl) cumyl] benzene

The double melting behavior of a thermotropic liquid crystalline polyimide was studied by means of differential scanning calorimetry (DSC), polarized light microscopy (PLM), transmission electron microscopy (TEM), wide‐angle X‐ray diffraction (WAXD), and small‐angle X‐ray scattering (SAXS). This liquid crystalline polyimide exhibited a normal melting peak around 278 °C and transformed into a smectic A phase. The smectic A phase changed to nematic phase upon heating to 298 °C, then became isotropic melt around 345 °C. The samples annealed or isothermally crystallized at lower temperature showed double melting endotherms during heating scan. The annealing‐induced melting endotherm was highly dependent on annealing conditions, whereas the normal melting endotherm was almost not influenced by annealing when the annealing temperature was low. Various possibilities for the lower melting endotherm are discussed. The equilibrium melting points of both melting peaks were extrapolated to be 283.2 °C. Combined analytical results showed that the double melting peaks were from the melting of the two types of crystallites generated from two crystallization processes: a slow and a fast one. Fast crystallization may start from the well‐aligned liquid crystal domains, whereas the slow one may be from the fringed or amorphous regions. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3018–3031, 2000     

The annealing and thermal analysis of poly(butylene terephthalate)

When poly(butylene terephthalate) (PBT) is annealed, a second endotherm is often displayed in a subsequent scanning thermal analysis at a temperature below that of the original endotherm, and this new endotherm appears to grow with annealing at the expense of the original. This growth is not due to chemical changes, because the thermogram obtained before annealing is recovered after complete melting. But a physical change would also seem unlikely because the transformation of higher-melting into lower-melting crystals is generally prohibited by thermodynamics. Two hypotheses to explain the result were tested. The first is that higher-melting crystals are not transformed into lower-melting crystals. Instead, because of recrystallization during thermal analysis, the single endotherm that results without annealing overestimates the population of high-melting crystals present before the analysis. This hypothesis was tested by extending to annealing a mathematical analysis previously used to describe the thermal scanning behavior of specimens crystallized at different cooling rates. Though most features of the thermograms obtained after annealing were able to be described, the decrease in the higher-temperature endotherm concomitant with growth of the lower endotherm was not. The second hypothesis is that the transformation of higher-melting to lower-melting crystals during annealing is allowed because it is coupled to the crystallization of formerly amorphous material. The amount of such crystallization observed for PBT was found to be sufficient to satisfy thermodynamic requirements, suggesting that this hypothesis is correct. © 1994 John Wiley & Sons, Inc.     

Influence of annealing and chain defects on the melting behaviour of poly(vinylidene fluoride)

The melting behaviour of poly(vinylidene fluoride) (PVDF) was investigated by differential scanning calorimetry (DSC) and small- and wide-angle X-ray scattering in order to study the influence of the chain defects content and of the temperature of annealing on the crystallization and melting behaviour.All the DSC scans show a double endotherm and the analysis of the data suggests that the low temperature endotherm is due to the melting of a population of thin lamellae, whose thickness increases during the annealing, but a high content of chain defects prevents the lamellar thickening and the main effect in this case is the crystallization of thin lamellae from a portion of polymer which did not crystallize during the quenching from the melt. Furthermore, the two melting endotherms, which are observed, can be partially ascribed to a melting-recrystallization process.Furthermore, stepwise isothermal cooling was performed in a differential scanning calorimeter followed by melting scans of fractionated PVDF samples to point out the possible presence of a series of endothermic peaks.     

Annealing effects in poly(vinylidene fluoride) as revealed by specific volume measurements,differential scanning calorimetry,and electron microscopy

Specimens of poly(vinylidene fluoride), crystal form II, annealed at different temperatures between 130 and 180°C were characterized by specific volume measurements, differential scanning calorimetry (DSC), and electron micróscopy. The degree of crystallinity calculated from the specific volume changed only by 15% i.e., from 50% to 65%. On the other hand, the melting behavior changed with annealing conditions. When a specimen was annealed above 170°C, two endothermic peaks appeared on either side of the annealing temperature. Results from DSC measurements made at different heating rates and electron microscopy showed that the two endotherms were caused by a bimodal distribution of lamellar thicknesses. The equilibrium melting point was found to be 210°C from the linear relation of the melting point and the annealing temperature. The equilibrium enthalpy and entropy of fusion were found to be 1.6 keal/mole and 3.3 eu/mole of repeat units by measurement on polymer–diluent mixtures. The surface free energy was found to be 5.1 kcal/mole of lamellar sequences from the plot of melting point versus reciprocal lamellar thickness obtained by electron microscopy. From a plot of enthalpy of fusion versus reciprocal lamellar thickness the surface enthalpy was found to be 20 keal/mole of lamellar sequences. These data lead to the estimate that a chain fold consists of about 30 repeat units.     

热处理条件对R-PET/LLDPE-g-MA共混物中PET玻璃化转变行为的影响

对回收聚对苯二甲酸乙二酯(R-PET)/LLDPE-g-MA马来酸酐改性的线性低密度聚乙烯共混物进行不同条件的热处理, 采用差示扫描量热仪(DSC)研究共混物基体PET的玻璃化转变行为. 结果表明, 当热处理温度低于PET的玻璃化转变温度(Tg)时, PET的玻璃化转变区域出现热焓松弛现象. 随着热处理温度的增加, PET的Tg逐渐升高; 在50~70 ℃下热处理48 h后, PET的Tg逐渐稳定. 当热处理温度高于PET的Tg而低于100 ℃时, PET的玻璃化转变区域出现2个热流转变, FTIR分析表明, PET分子构象开始发生变化. 当热处理温度为100 ℃时, DSC曲线上PET的玻璃化转变消失, PET的结晶度明显增加, 说明PET开始冷结晶的温度在90~100 ℃之间.     

A dynamical study of the melting behavior of polypivalolactone single crystals by the X-ray diffraction method

Wide-angle and small-angle X-ray diffraction patterns of polypivalolactone single crystals were recorded as a function of temperature while heating at a constant rate. Long periods were found to increase abruptly at a certain temperature, while the degree of crystallinity decreased up to that temperature and then recovered on further heating, to an extent depending on the rate of heating. Multiple endotherms were observed in DSC measurements. The temperature at which the first endotherm peak was observed corresponded exactly to the temperature of the long period jump mentioned above. From these data, it is concluded that the original lamellar crystals melt at each peak temperature in the DSC thermogram and new lamellae are produced by subsequent crystallization. Multiple endotherms in the DSC thermogram are thus considered to represent repeated melt-recrystallization processes.     

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.     

Etude du comportement thermique de la poly(pivalolactone) par diffraction des rayons-x aux petits angles et par calorimetrie dynamique

Poly(α,α-dimethyl-β-propiolactone) (PPL), known as poly(pivalolactone), has been studied by differential scanning calorimetry (DSC) and small-angle X-ray diffraction (SAXR). DSC measurements indicate the presence of two melting endotherms. Peak 1 and Peak 2, the latter at lower temperatures. Peak 1 is relatively unaffected by the crystallization temperature and its relative intensity decreases with heating rate. Peak 2 is greatly influenced by the crystallization temperature of the sample and its relative intensity increases with heating rate. Peak 2 is associated with the true melting of the PPL samples and Peak 1 with a recrystallization process during the heating cycle. SAXR long periods increase with crystallization and annealing temperatures. Similar increases in density, in melting temperature, in lamella thickness, and in degree of crystallinity have been observed. These results lead to a thermodynamic melting temperature of 268 ± 3 for PPL, and to interfacial free energies of, respectively. 13 × 10^?7 J cm^?2 and (43 ± 4) × 10^?7 J cm^?2 for the lateral surface and the fold surface of the PPL crystal.     

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.     

Synthesis and crystallization behavior of rigid copolyesters with biphenyl‐4,4′‐dicarboxylate and 2,6‐naphthalenedicarboxylate in the main chain

Structurally rigid copolyester thermoplastics were synthesized from 1,4‐cyclohexanedimethanol and the diesters dimethyl biphenyl‐4,4′‐dicarboxylate and dimethyl 2,6‐naphthalenedicarboxylate (DMN) via conventional melt transesterification. Conventional differential scanning calorimetry (CDSC) showed all compositions to exhibit multiple endotherms upon heating. Wide‐angle X‐ray diffraction analysis showed copolyester compositions to exhibit the crystalline structure of either the homopolyester Poly(1,4‐cyclohexylenedimethylene 2,6‐naphthalate) (PCN) or the homopolyester Poly(1,4‐cyclohexylenedimethylene 4,4′‐bibenzoate) (PCB), but not both simultaneously. Further thermal analysis using CDSC and fast DSC investigated the origin of the multiple endotherm behavior. While three endotherms are observed for low heating rates, the upper two endotherms appear to merge at heating rates about 1–5 °C s^?1 and a single endotherm remains above heating rates about 10–50 °C s^?1. While the behavior of the upper two endotherms is undeniably consistent with the mechanism of melting–recrystallization–remelting (MRR), we suggest that the low endotherm is likely associated with the melting of constrained secondary crystals, although MRR effects cannot be ruled out. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 973–980     

Estimation of the degree of fusion of rigid PVC from thermal analysis measurements

PVC that has been processed exhibits two endothermic peaks in DSC heating curves. These endotherms are separated by a temperature. Tc, which is characteristic of the maximum temperature reached in prior processing. The relative value of the lower temperature endotherm has been proposed as a measure of degree of gelation of unplasticized PVC. This measure is not exact, however, because it does not reflect the extent of breakdown of primary PVC particles. An alternative DSC measurement technique is proposed that reflects the degree of fusion more accurately. The nature of the various endotherms that are observed is a subject of speculation in this article.     

聚对苯二甲酸乙二酯的熔融双峰与形态

非晶PET等温结晶后,用DSC、WAXD、SAXS、密度和透射电镜等方法,考察了结晶PET在升温过程中的结构变化,进一步证实了过程中发生部分熔融再结晶;同时形态也起了明显变化:片晶增厚,片晶侧向尺寸增大,由节瘤状晶粒堆砌部分地转变为典型的片晶堆砌,构成片晶的微晶尺寸增大,晶体趋于完善,折迭表面的规则折迭增加。这样,在等温结晶时生成的结构状态转变为更稳定的形态,因而相应地在DSC曲线上出现两个熔融峰。     

Effect of chain defects on the morphology and thermal behavior of solution-grown single crystals of syndiotactic polypropylene

The morphology of solution-grown single crystals of syndiotactic polypropylene with different degree of stereoregularity is compared. A sector formation phenomenon, found in some monolayer single crystals, is discussed in terms of possible crystallographic fold planes, growth planes, and gemination planes. A correlation between thermodynamic and morphological properties such as apparent enthalpy of fusion, critical long spacing, critical annealing temperature, and the number of configurational chain defects along the macromolecule has been found. Two endothermic peaks are observed in the DSC thermograms of single-crystal aggregates of syndiotactic polypropylene. The low-temperature peak is attributed to melting of crystals or parts of crystals with incorporated chain defects. The high-temperature peak corresponds to the melting endotherm of more regular crystalline zones. The peak-area ratio seems to depend on the degree of stereoregularity.