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     

Melting behaviors,crystallization kinetics,and spherulitic morphologies of poly(butylene succinate) and its copolyester modified with rosin maleopimaric acid anhydride

This article investigated the melting behaviors, crystallization kinetics, and spherulitic morphologies of poly(butylene succinate) (PBS) and its copolyester (PBSR) modified with rosin maleopimaric acid anhydride, using wide‐angle X‐ray diffraction, differential scanning calorimeter (DSC), and polarized optical microscope. Subsequent DSC scans of isothermally crystallized PBS and PBSR exhibited two melting endotherms, respectively, which was due to the melt‐recrystallization process occurring during the DSC scans. The equilibrium melting point of PBSR (125.9 °C) was lower than that of PBS (139 °C). The commonly used Avrami equation was used to describe the isothermal crystallization kinetics. For nonisothermal crystallization studies, the model combining Avrami equation and Ozawa equation was employed. The result showed a consistent trend in the crystallization process. The crystallization rate was decreased, the perfection of crystals was decreased, the recrystallization was reduced, and the spherulitic morphologies were changed when the huge hydrogenated phenanthrene ring was added into the chain of PBS. The activation energy (ΔE) for the isothermal crystallization process determined by Arrhenius method was 255.9 kJ/mol for PBS and 345.7 kJ/mol for PBSR. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 900–913, 2006     

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     

Nonisothermal cold‐crystallization kinetics of poly(trimethylene terephthalate)

The nonisothermal cold‐crystallization kinetics and subsequent melting behavior of poly(trimethylene terephthalate) (PTT) were investigated with differential scanning calorimetry. The Avrami, Tobin, and Ozawa equations were applied to describe the kinetics of the crystallization process. Both the Avrami and Tobin crystallization rate parameters increased with the heating rate. The Ozawa crystallization rate increased with the temperature. The ability of PTT to crystallize from the glassy state at a unit heating rate was determined with Ziabicki's kinetic crystallizability index, which was found to be about 0.89. The effective energy barrier describing the nonisothermal cold‐crystallization process of PTT was estimated by the differential isoconversional method of Friedman and was found to range between about 114.5 and 158.8 kJ mol^?1. In its subsequent melting, PTT exhibited double‐melting behavior for heating rates lower than or equal to 10 °C min^?1 and single‐melting behavior for heating rates greater than or equal to 12.5 °C min^?1. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4151–4163, 2004     

Reflection–absorption infrared spectroscopy investigation of the crystallization kinetics of poly(ethylene terephthalate) ultrathin films

Reflection–absorption infrared spectroscopy was used to study the crystallization behavior of poly(ethylene terephthalate) (PET) ultrathin films. The crystallinity of ultrathin films was estimated by the fraction of trans conformers of PET. The isothermal and nonisothermal crystallization kinetics of ultrathin films with different thicknesses were investigated. The thinner PET film showed slower kinetics during isothermal crystallization than the thicker film. Moreover, the final crystallinity of films with various thicknesses were reduced with decreasing thickness. An Avrami equation was used to fit the acquired results. The Avrami exponents decreased with the film thickness. As for the nonisothermal crystallization, the cold‐crystallization starting temperature shifted to a lower temperature as the film thickness increased. The influence of the substrate on the crystallization kinetics of the films was also studied. The half‐crystallization times and final crystallinities of ultrathin films adsorbed onto a self‐assembled‐monolayer‐treated surface and an untreated substrate were clearly different, although their thickness dependence was similar. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4440–4447, 2004     

Analysis of crystallization behavior and crystal modifications of poly(butylene‐2,6‐naphthalene dicarboxylate)

Information on the crystalline structure and the properties of poly(butylene‐2,6‐naphthalene dicarboxylate) (PBN) has not been well reported until now, but it is known that there are two different crystal modifications in PBN, as follows: one is formed in isotropic samples by annealing (α form); another appears by annealing with tension (β form). The relation between the crystal modifications and the kinetics of isothermal crystallization for PBN was investigated using in‐situ Fourier transform infrared spectroscopy (FTIR) and wide‐angle X‐ray diffraction (WAXD). The melting behavior of each crystalline form was also studied by means of FTIR and differential scanning calorimetry (DSC) measurements. From the analysis of the melt‐crystallized PBN specimens, the two crystalline forms coexisted in the isotropic samples melt‐crystallized at 230°C, but only the α crystal modification was observed in the films annealed at lower temperatures. In addition, it was revealed that, at 230°C, the β modification was formed only in the primary crystallization process. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 561–574, 1999     

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     

Nonisothermal crystallization and melting behavior of poly(β‐hydroxybutyrate)–Poly(vinyl‐acetate) blends

Nonisothermal crystallization and melting behavior of poly(β‐hydroxybutyrate) (PHB)–poly(vinyl acetate) (PVAc) blends from the melt were investigated by differential scanning calorimetry using various cooling rates. The results show that crystallization of PHB from the melt in the PHB–PVAc blends depends greatly upon cooling rates and blend compositions. For a given composition, the crystallization process begins at higher temperatures when slower scanning rates are used. At a given cooling rate, the presence of PVAc reduces the overall PHB crystallization rate. The Avrami analysis modified by Jeziorny and a new method were used to describe the nonisothermal crystallization process of PHB–PVAc blends very well. The double‐melting phenomenon is found to be caused by crystallization during heating in DSC. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 443–450, 1999     

Isothermal and nonisothermal crystallization kinetics of nylon‐46

Isothermal and nonisothermal crystallization kinetics of nylon‐46 were investigated with differential scanning calorimetry. The equilibrium melting enthalpy and the equilibrium melting temperature of nylon‐46 were determined to be 155.58 J/g and 307.10 °C, respectively. The isothermal crystallization process was described by the Avrami equation. The lateral surface free energy and the end surface free energy of nylon‐46 were calculated to be 8.28 and 138.54 erg/cm², respectively. The work of chain folding was determined to be 7.12 kcal/mol. The activation energies were determined to be 568.25 and 337.80 kJ/mol for isothermal and nonisothermal crystallization, respectively. A convenient method was applied to describe the nonisothermal crystallization kinetics of nylon‐46 by a combination of the Avrami and Ozawa equations. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1784–1793, 2002     

Crystallization kinetics and morphology of biodegradable poly(butylene succinate‐co‐propylene succinate)s

The crystallization behavior of biodegradable poly(butylene succinate) and copolyesters poly(butylene succinate‐co‐propylene succinate)s (PBSPS) was investigated by using ¹H NMR, DSC and POM, respectively. Isothermal crystallization kinetics of the polyesters has been analyzed by the Avrami equation. The 2.2‐2.8 range of Avrami exponential n indicated that the crystallization mechanism was a heterogeneous nucleation with spherical growth geometry in the crystallization process of polyesters. Multiple melting peaks were observed during heating process after isothermal crystallization, and it could be explained by the melting and recrystallization model. PBSPS was identified to have the same crystal structure with that of PBS by using wide‐angle X‐ray diffraction (WAXD), suggesting that only BS unit crystallized while the PS unit was in an amorphous state. The crystal structure of polyesters was not affected by the crystallization temperatures, too. Besides the normal extinction crosses under the POM, the double‐banded extinction patterns with periodic distance along the radial direction were also observed in the spherulites of PBS and PBSPS. The morphology of spherulites strongly depended on the crystallization temperature. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 420–428, 2007     

Crystallization kinetics and crystallization behavior of syndiotactic polystyrene/clay nanocomposites

We investigated the effects of montmorillonite (clay) on the crystallization kinetics of syndiotactic polystyrene (sPS) with isothermal differential scanning calorimetry analyses. The clay was dispersed into the sPS matrix via melt blending on a scale of 1–2 nm or up to about 100 nm, depending on the surfactant treatment. For a crystallization temperature of 240 °C, the isothermal crystallization data were fitted well with the Avrami crystallization equation. Crystallization data on the kinetic parameters (i.e., the crystallization rate constant, Avrami exponent, clay content, and clay/surfactant cation‐exchange ratio) were also investigated. Experimental results indicated that the crystallization rate constant of the sPS nanocomposite increased with increasing clay content. The clay played a vital role in facilitating the formation on the thermodynamically more favorable all‐β‐form crystal when the sPS was melt‐crystallized. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2097–2107, 2001     

Nonisothermal crystallization studies on poly(4‐hydroxybutyric acid‐alt‐glycolic acid)

The crystallization behavior of a new sequential polyester constituted by glycolic acid and 4‐hydroxybutyric acid has been studied under nonisothermal conditions. Nonisothermal melt crystallization has been followed by means of hot‐stage optical microscopy (HSOM), with experiments performed at different cooling rates. Two crystallization regimes have been found, which is in good agreement with previous isothermal studies and with the different spherulitic morphologies that were observed. The kinetics of both glass and melt crystallizations has also been studied by differential scanning calorimetry (DSC) and considering the typical Avrami, Ozawa, and Cazé analyses. Only the last gave Avrami exponents, which were in good agreement with those measured under isothermal conditions, suggesting a spherulitic growth with a predetermined nucleation. Isoconversional data of melt and glass nonisothermal crystallizations have been combined to obtain the Hoffman and Lauritzen parameters. Results again indicate the existence of two crystallization regimes with nucleation constants close to those deduced from isothermal DSC experiments. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 121–133, 2008     

Effect of partial melting on the crystallization kinetics of nylon‐1212

The isothermal and nonisothermal crystallization kinetics of partially melted nylon‐1212 was investigated with differential scanning calorimetry. Because of partial melting, the pre‐existing crystals changed the crystallization mechanism and had a strong effect on the crystallization process. The Avrami exponent and interfacial free energy of the chain‐folded surface of partially melted nylon‐1212 were higher than those of completely melted nylon‐1212. The work of chain folding was determined to be 5.9 kcal/mol. The activation energy of the isothermal crystallization process was determined to be 399.1 kJ/mol, far higher than that of complete melting. The crystallization rate coefficient and Jeziorny analysis indicated that the ability of nonisothermal crystallization for partially melted nylon‐1212 was enhanced. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3222–3230, 2005     

Nonisothermal crystallization and melting behavior of syndiotactic polypropylenes of different microstructure

Syndiotactic polypropylenes and their copolymers with 1‐olefins were synthesized using two metallocene/MAO catalytic systems, and the effect of the different microstructures on nonisothermal crystallization and subsequent melting was studied. Using differential scanning calorimetry (DSC) it was observed that samples with lower content of defects showed crystallization on cooling from the melt, and a double melting peak in the subsequent heating scan, the latter associated with melt, recrystallization and remelt processes that it was confirmed by its nonreversing exothermic process found by means of temperature modulated DSC (MDSC). However, polymers with high amount of defects showed cold crystallization on heating followed by a melting process, that it was observed by MDSC. Wide angle X‐ray diffraction was used for characterizing the changes of crystalline forms in relationship with crystallization process. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 798–806, 2008     

Crystallization kinetics and morphology of poly(butylene succinate‐co‐adipate)

The crystallization kinetics of biodegradable poly(butylene succinate‐co‐adipate) (PBS/A) copolyester was investigated by using differential scanning calorimetry (DSC) and polarized optical microscopy (POM), respectively. The Avrami and Ozawa equations were used to analyze the isothermal and nonisothermal crystallization kinetics, respectively. By using wide‐angle X‐ray diffraction (WAXD), PBS/A was identified to have the same crystal structure with that of PBS. The spherulitic growth rates of PBS/A measured in isothermal conditions are very well comparable with those measured by nonisothermal procedures (cooling rates ranged from 0.5 to 15 °C/min). The kinetic data were examined with the Hoffman–Lauritzen nucleation theory. The observed spherulites of PBS/A with different shapes and textures strongly depend on the crystallization temperatures. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3231–3241, 2005     

Nonisothermal crystallization and melting behavior of a luminescent conjugated polymer,poly(9,9‐dihexylfluorene‐alt‐co‐2,5‐didecyloxy‐1,4‐phenylene)

The nonisothermal crystallization kinetics of a luminescent conjugated polymer, poly(9,9‐dihexylfluorene‐alt‐co‐2,5‐didecyloxy‐1,4‐phenylene) (PF6OC10) with three different molecular weights was investigated by differential scanning calorimetry under different cooling rates from the melt. With increasing molecular weight of PF6OC10, the temperature range of crystallization peak steadily became narrower and shifted to higher temperature region and the crystallization rate increased. It was found that the Ozawa method failed to describe the nonisothermal crystallization behavior of PF6OC10. Although the Avrami method did not effectively describe the nonisothermal crystallization kinetics of PF6OC10 for overall process, it was valid for describing the early stage of crystallization with an Avrami exponent n of about 3. The combined method proposed in our previous report was able to satisfactorily describe the nonisothermal crystallization behavior of PF6OC10. The crystallization activation energies determined by Kissinger, Takhor, and Augis‐Bennett models were comparable. The melting temperature of PF6OC10 increased with increasing molecular weight. For low‐molecular‐weight sample, PF6OC10 showed the characteristic of double melting phenomenon. The interval between the two melting peaks decreased with increasing molecular weight, and only one melting peak was observed for the high‐molecular‐weight sample. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 976–987, 2007     

Analysis of the complex thermal behavior of poly(L‐lactic acid) film. I. Samples crystallized from the glassy state

The melting behavior of poly(L ‐lactic acid) film crystallized from the glassy state, either isothermally or nonisothermally, was studied by wide angle X‐ray diffraction (WAXD), small angle X‐ray scattering (SAXS), differential scanning calorimetry (DSC), and temperature‐modulated differential scanning calorimetry (TMDSC). Up to three crystallization and two melting peaks were observed. It was concluded that these effects could largely be accounted for on the basis of a “melt‐recrystallization” mechanism. When molecular weight is low, two melting endotherms are readily observed. But, without TMDSC, the double melting phenomena of high molecular weight PLLA is often masked by an exotherm just prior to the final melting, as metastable crystals undergo melt‐recrystallization during heating in the DSC. The appearance of a double cold‐crystallization peak during the DSC heating scan of amorphous PLLA film is the net effect of cold crystallization and melt‐recrystallization of metastable crystals formed during the initial cold crystallization. Samples cold‐crystallized at 80 and 90 °C did not exhibit a long period, although substantial crystallinity developed. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 3200–3214, 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     

Crystallization kinetics and melting behavior of syndiotactic 1,2‐polybutadiene

Differential scanning calorimetry was used to investigate the isothermal crystallization, subsequent melting behavior, and nonisothermal crystallization of syndiotactic 1,2‐polybutadiene (st‐1,2‐PB) produced with an iron‐based catalyst system. The isothermal crystallization of two fractions was analyzed according to the Avrami equation. The morphology of the crystallite was observed with polarized optical microscopy. Double melting peaks were observed for the samples isothermally crystallized at 125–155 °C. The low‐temperature melting peak, which appeared approximately 5 °C above the crystallization temperature, was attributed to the melting of imperfect crystals formed by the less stereoregular fraction. The high‐temperature melting peak was associated with the melting of perfect crystals formed by the stereoregular fraction. With the Hoffman–Weeks approach, the value of the equilibrium melting temperature was derived. During the nonisothermal crystallization, the Ozawa method was limited in obtaining the kinetic parameters of st‐1,2‐PB. A new method that combined the Ozawa method and the Avrami method was employed to analyze the nonisothermal crystallization of st‐1,2‐PB. The activation energies of crystallization under nonisothermal conditions were calculated. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 553–561, 2005     

Crystallization behavior of polypropylene with or without sodium benzoate as a nucleating agent

The crystallization kinetics of polypropylene (PP) with or without sodium benzoate as a nucleating agent were investigated by means of DSC and polarized optical microscopy in isothermal and nonisothermal modes. A modified Avrami equation was applied to the kinetic analysis of isothermal crystallization. The addition of the nucleating agent up to its saturation concentration increased the crystallization temperature by 15 °C and shortened both the isothermal and nonisothermal crystallization half‐times. It was concluded that the sodium benzoate acted as a good nucleating agent for α‐form PP. By adding the nuclefier to PP, adequately controlled spherulites increased the mechanical properties including especially the Izod impact strength and shortened cycle time of PP. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1001–1016, 2001