Hoffman-Lauritzen parameters for non-isothermal crystallization of poly(ethylene terephthalate) and poly(ethylene oxide) melts

Summary By applying an advanced isoconversional method to DSC data one can evaluate a dependence of the effective activation energy (the temperature coefficient of the growth rate) on the relative extent of melt crystallization. The conversion dependence can further be converted into a temperature dependence and parameterized in terms of the Hoffman-Lauritzen equation. For poly(ethylene terephthalate) (PET) we observe a transition from regime I to II. Poly(ethylene oxide) (PEO) crystallization appears to begin in regime II and then undergoes 2 consecutive changes that however cannot be clearly interpreted as regime III. The K_g and _e parameters obtained for regime I and II (PET) and regime II (PEO) are consistent with the respective parameters reported for isothermal crystallization.     

Critical Considerations on the Isoconversional Methods. III. On the evaluation of the activation energy from non-isothermal data

The differential and integral isoconversional methods for evaluation the activation energy, described in the first note of this series, were applied for: a) simulated data for two successive reactions; b) dehydration of calcium oxalate monohydrate. It was shown that for these systems the activation energy depends on the conversion degree as well as on the method of evaluation. This revised version was published online in August 2006 with corrections to the Cover Date.     

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     

Isothermal crystallization kinetics of poly(trimethylene terephthalate) under the influence of self-seeding nucleation

The effect of self-seeding nucleation on the crystallization behavior of poly(trimethylene terephthalate) (PTT) was studied. Differential scanning calorimetry (DSC) indicated that the crystallization temperature of PTT notably increased after self-seeding nucleation. Avrami equation was applied in the analysis of the isothermal crystallization process of PTT. The resulting average value of the Avrami exponent at n = 3.34 suggests that primary crystallization may correspond to a three-dimensional spherulitic growth. Self-seeding nucleation, leading to a decrease in active energy for crystallization and chain folding work, promotes the overall crystallization process of PTT. Translated from Acta Polymerica Sinica, 2006, (3): 414–417 (in Chinese)     

Isothermal and nonisothermal crystallization kinetics of poly(propylene terephthalate)

The kinetics of crystallization of poly(propylene terephthalate) (PPT) samples of different molecular weights were studied under both isothermal and nonisothermal conditions. The Avrami and Lauritzen–Hoffmann treatments were applied to evaluate kinetic parameters of PPT isothermal crystallization. It was found that crystallization is faster for low‐molecular‐weight samples. The modified Avrami equation, and the combined Avrami–Ozawa method were found to successfully describe the nonisothermal crystallization process. Also, the analysis of Lauritzen–Hoffmmann was tested and it resulted in values close to those obtained with isothermal crystallization data. The nonisothermal kinetic data were corrected for the effect of the temperature lag and shifted alone with the isothermal kinetic data to obtain a single master curve, according to the method of Chan and Isayev, testifying to the consistency between the isothermal and corrected nonisothermal data. A new method for ranking of polymers, referring to the crystallization rates, was also introduced. This involved a new index that combines the maximum crystallization rate observed during cooling with the average crystallization rates over the temperature range of the crystallization peak. Furthermore, the effective energy barrier of the dynamic process was evaluated with the isoconversional methods of Flynn and Friedmann. It was found that the energy barrier is lower for the low‐molecular‐weight PPT. The effect of the catalyst remnants on the crystallization kinetics was also investigated and it was found that this is significant only for low‐molecular‐weight samples. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3775–3796, 2004     

Heterogeneous nucleation on the crystallization poly(ethylene terephthalate)

The crystallization behavior of poly(ethylene terephthalate) (PET) with disodium terephthalate (DST) as nucleating agent was investigated. A detailed analysis of the crystallization course from the melt was made with the Avrami expression. The results demonstrated that DST additive can promote the PET crystallization rate in its entire crystallizable temperature range, and the acceleration degree of DST decreases with increasing temperature after a temperature higher than 180 °C. The values of the Avrami exponent indicated that the crystallization mode in Avrami theory is not suitable for the crystallization of these polymers, and the mechanism of the heterogeneous nucleation on PET crystallization is discussed. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2135–2144, 2003     

A new iterative linear integral isoconversional method for the determination of the activation energy varying with the conversion degree

The conventional linear integral isoconversional methods may lead to important errors in the determination of the activation energy when the significant variation of the activation energy with the conversion degree occurs. Vyazovkin proposed an advanced nonlinear isoconversional method, which allows the activation energy to be accurately determined [Vyazovkin, J Comput Chem 2001, 22, 178]. However, the use of the Vyazovkin method raises the problem of the time‐consuming minimization without derivatives. A new iterative linear integral isoconversional method for the determination of the activation energy as a function of the conversion degree has been proposed, which is capable of providing valid values of the activation energy even if the latter strongly varies with the conversion degree. Also, the new method leads to the correct values of the activation energy in much less time than the Vyazovkin method. The application of the new method is illustrated by processing of theoretically simulated data of a strongly varying activation energy process. © 2009 Wiley Periodicals, Inc. J Comput Chem 2009     

Non-isothermal crystallization kinetics of poly(trimethylene terephthalate)/poly(ethylene 2,6-naphthalate) blends

The glass-transition temperature and non-isothermal crystallization of poly(trimethylene terephthalate)/poly(ethylene 2,6-naphthalate) (PTT/PEN) blends were investigated by using differential scanning calorimeter (DSC). The results suggested that the binary blends showed different crystallization and melting behaviors due to their different component of PTT and PEN. All of the samples exhibited a single glass-transition temperature, indicating that the component PTT and PEN were miscible in amorphous phase. The value of T_g predicted well by Gordon-Taylor equation decreased gradually with increasing of PTT content. The commonly used Avrami equation modified by Jeziorny, Ozawa theory and the method developed by Mo were used, respectively, to fit the primary stage of non-isothermal crystallization. The kinetic parameters suggested that the PTT content improved the crystallization of PEN in the binary blend. The crystallization growth dimension, crystallization rate and the degree of crystallinity of the blends were increased with the increasing content of PTT. The effective activation energy calculated by the advanced iso-conversional method developed by Vyazovkin also concluded that the value of E_a depended not only on the system but also on temperature, that is, the binary blend with more PTT component had higher crystallization ability and the crystallization ability is increased with increasing temperature. The kinetic parameters U^* and K_g were also determined, respectively, by the Hoffman-Lauritzen theory.     

The effect of annealing on the crystallization of poly(ethylene terephthalate)/polycarbonate blends

The effect of annealing on the morphology and subsequent crystallization kinetics of poly (ethylene terephthalate)/polycarbonate blends have been investigated using differential scanning calorimetry (DSC), polarized light microscopy, and scanning electron microscopy (SEM). During annealing transesterification and phase coarsening occurred, and the final properties were compromizes between these two competing effects. Initially, the effect of phase separation dominated and the rate of cold crystallization of PET increased. Transesterification, however, became increasingly important and the rate of crystallization decreased progressively until finally the blend completely lost the ability to crystallize. At this stage in the reaction a single glass transition was observed and uniform glassy material observed in the SEM. The maximum crystallinity of the blend achieved on heating showed the same trend in first increasing and then decreasing with annealing time. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 2129–2136, 2004     

Non-isothermal crystallization kinetics of hot drawn polyester films

The non-isothermal crystallization kinetics of hot drawn poly(ethylene terephthalate) films were studied using the Kissinger and Ozawa equations. The influence of the initial drawing on the crystallization kinetics was investigated. The values of the apparent activation energy and of the Avrami exponent indicates that the nucleation and growth of crystallites depend greatly on the stress submitted to the samples.     

Synthesis and non‐isothermal crystallization kinetics of poly(ethylene terephthalate)‐co‐poly(propylene glycol) copolymers

Poly(ethylene terephthalate)‐co‐poly(propylene glycol) (PET‐co‐PPG) copolymers with PPG ratio ranging from 0 to 0.90 mol% were synthesized by the melt copolycondensation. The intrinsic viscosity, structure, non‐isothermal crystallization behavior, nucleation and spherulitic growth of the copolymers were investigated by Ubbelohde viscometer, Proton Nuclear Magnetic Resonance (¹H‐NMR), differential scanning calorimetry, and polarized optical microscopy, respectively. The non‐isothermal crystallization process of the copolymers was analyzed by Avrami, Ozawa, Mo's, Kissinger, and Dobreva methods, respectively. The results showed that the crystallizability of PET was apparently enhanced with incorporating a small amount of PPG, which first rose and then reduced with increasing amount of PPG in the copolymers at a given cooling rate. The crystallization mechanism was a three‐dimensional growth with both instantaneous and sporadic nucleation. Particularly, PET‐co‐PPG containing 0.60 mol% PPG exhibited the highest crystallizability among all the copolymers. Copyright © 2015 John Wiley & Sons, Ltd.     

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     

PET分子量对PET/PC共混体系高压结晶行为的影响

以往高压合成的聚合物伸直链晶体尺寸较小且合成时间过长,不能满足单晶应用的要求。加入10wt%PC于不同分子量的PET中,采用自制高压装置,制备共混体系,用WAXD,SEM,DSC等表征手段,研究了PET/PC共混体系在高压下的结晶行为。研究结果表明:通过PET/PC体系在高压下化学反应诱导的自组装,不同分子量PET的PET/PC共混体系均在较短的时间内生成了尺寸较大的聚酯伸直链晶体,对应于三斜晶系,其中PET/PC体系最适于大尺寸伸直链晶体的合成,仅需6hr时间即生成PE需200hr方可合成的40μm厚度的伸直链晶体。同时提出了PET1/PC体系中伸直链晶体高温下缺陷自我修复模型。     

Morphology development and crystallization behavior of poly(ethylene terephthalate)/phenoxy blend

The morphological development and crystallization behavior of a poly(ethylene terephthalate)/poly(hydroxyl ether of bisphenol A) (phenoxy) blend were studied with time‐resolved light scattering, optical microscopy, differential scanning calorimetry, and small‐angle X‐ray scattering (SAXS). During annealing at 280 °C, liquid–liquid phase separation via spinodal decomposition proceeded in the melt‐extruded specimen. After the formation of a domain structure, the blend slowly underwent phase homogenization by the interchange reactions between the two polymers. Specimens annealed for various times (t_s) at 280 °C were subjected to a temperature drop and the effects of liquid‐phase changes on crystallization were then investigated. The shifts in the position of the cold‐crystallization peaks indicated that the crystallization rate is associated with the composition change of the separated phases as well as the change of the sequence distribution in polymer chains during annealing. The morphological parameters at the lamellar level were determined by a correlation function analysis on the SAXS data. The crystal thickness (l_c) increased with t_s, whereas the amorphous layer thickness (l_a) showed little dependence on t_s. Observation of a constant l_a value revealed that a large number of noncrystallizable species formed by the interchange reactions between the two polymers were excluded from the lamellar stacks and resided in the interfibrillar regions, interspherulitic regions, or both. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 223–232, 2008     

Melt‐crystallization mechanism of poly(ethylene terephthalate)/multi‐walled carbon nanotubes prepared by in situ polymerization

A series of poly(ethylene terephthalate)/multi‐walled carbon nanotubes (PET/MWCNTs) nanocomposites were prepared by in situ polymerization using different amounts of multi‐walled carbon nanotubes (MWCNTs). The polymerization of poly(ethylene terephthalate) (PET) was carried out by the two‐stage melt polycondensation method. The intrinsic viscosity (IV) of the composites is ranged between 0.31 and 0.63 dL/g depending on the concentration of the MWCNTs. A decrease of IV was found by increasing MWCNTs content. This is due to the reactions taking place between the two components leading to branched and crosslinked macromolecules. These reactions are, mainly, responsible for thermal behavior of nanocomposites. The melting point of the nanocomposites was shifted to slightly higher temperatures by the addition till 0.55 wt % of MWCNTs while for higher concentration was reduced. The degree of crystallinity in all nanocomposites was, also, reduced by increasing MWCNTs amount. However, from crystallization temperature, it was found that MWCNTs till 1 wt % can enhance the crystallization rate of PET, whereas at higher content (2 wt %), the trend is the opposite due to the formation of crosslinked macromolecules. From the extended crystallization analysis, it was proved that MWCNTs act as nucleating agents for PET crystallization. Additionally, the crystallization mechanism due to the existence of MWCNT becomes more complicated because two mechanisms with different activation energies are taking place in the different degrees of crystallization, depending on the percentage of MWCNT. The effect of molecular weight also plays an important role. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 1452–1466, 2009     

New polymer syntheses. CIX. Biodegradable,alternating copolyesters of terephthalic acid,aliphatic dicarboxylic acids,and alkane diols

Copolyesters with an alternating sequence of terephthalic acid and aliphatic dicarboxylic acids were prepared with three different methods. First, dicarboxylic acid dichlorides were reacted with bis(2‐hydroxyethyl)terephthalate (BHET) in refluxing 1,2‐dichlorobenzene. Second, the same monomers were polycondensed at 0–20 °C in the presence of pyridine. Third, dicarboxylic acid dichlorides and silylated BHET were polycondensed in bulk. Only this third method gave satisfactory molecular weights. Matrix‐assisted laser desorption/ionization time‐of‐flight mass spectrometry revealed that the copolyesters prepared by the pyridine and silyl methods might have contained considerable fractions of cyclic oligoesters and polyesters despite the absence of transesterification and backbiting processes. The alternating sequences and thermal properties were characterized with ¹H NMR spectroscopy and differential scanning calorimetry measurements, respectively. In agreement with the alternating sequence, all copolyesters proved to be crystalline, but the crystallization was extremely slow [slower than that of poly(ethylene terephthalate)]. A second series of alternating copolyesters was prepared by the polycondensation of silylated bis(4‐hydroxybut‐ yl)terephthalate with various aliphatic dicarboxylic acid dichlorides. The resulting copolyesters showed significantly higher rates of crystallization, and the melting temperatures were higher than those of the BHET‐based copolyesters. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3371–3382, 2001     

Transient and athermal effects in the crystallization of polymers. I. Isothermal crystallization

The isothermal crystallization of poly(propylene) and poly(ethylene terephthalate) was investigated with differential scanning calorimetry and optical microscopy. It was found that the induction time depends on the cooling rate to a constant temperature. The isothermal crystallization of the investigated polymers is a complex process and cannot be adequately described by the simple Avrami equation with time‐independent parameters. The results indicate that crystallization is composed of several nucleation mechanisms. The homogeneous nucleation occurring from thermal fluctuations is preceded by the nucleation on not completely melted crystalline residues that can become stable by an athermal mechanism as well as nucleation on heterogeneities. The nucleation rate depends on time, with the maximum shortly after the start of crystallization attributed to nucleation on crystalline residues (possible athermal nucleation) and on heterogeneities. However, the spherulitic growth rate and the exponent n do not change with the time of crystallization. The time dependence of the crystallization rate corresponds to the changes in the nucleation rate with time. The steady‐state crystallization rate in thermal nucleation is lower than the rate determined in a classical way from the half‐time of crystallization. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1835–1849, 2002     

New insight into the crystallization behavior of poly(ethylene terephthalate)/clay nanocomposites

Crystallization behavior of poly(ethylene terephthalate) (PET)/clay nanocomposites has been investigated in terms of differential scanning calorimeter (DSC) analysis, polarizing optical microscopy (POM), and scanning electron microscopy (SEM) observation. The nanocomposites for investigation were prepared via in situ polycondensation. Crystalline morphologies were observed through POM and SEM. The nonisothermal and isothermal crystallization rates of different samples were determined for comparison based on DSC data. Secondary nucleation analysis was also performed based on bulk crystallization data derived from DSC analysis. The results revealed that nucleating abilities of montmorillonites (MMT) depended on the dispersion state of clay in matrix, the surface modification status, and the metallic derivatives released from MMT during in situ synthesis. The quantities of metallic elements released were measured by inductively coupled plasma (ICP) analysis. The results showed that the release of these metallic derivatives was also affected by surfactant molecules anchored on the surface of MMT. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2380–2394, 2008     

Rapid crystallization and thermoreversible gelation of poly(ethylene terephthalate) in polymer/oligomer binary system

Poly(ethylene terephthalate) (PET) was rapidly crystallized through thermoreversible gelation in a liquid ethylene glycol oligomer or in epoxy resin. The solutions formed gel rapidly on cooling. Polarized light microscopy and small-angle light scattering showed that these gels contain large, regular PET spherulites. The gels may be formed by two consecutive processes: the phase separation and crystallization, and gelation by formation of a three-dimensional PET network in the oligomer solvents, where the nodes of the network are PET spherulites. The crystallinity of PET recovered from polymer/oligomer gels is near 72% measured by wide-angle X-ray diffraction method, which is about 20% higher than PET samples crystallized by solution crystallization in small molecule solvent, high temperature annealing, and stretching techniques. It takes only a few minutes to form the highly crystalline phase PET in the PET/oligomer system, and the crystallinity of the dried gel is independent of the concentration of the original solution. Excimer-fluoresence and Raman spectroscopic studies indicated that PET recovered from the gels are in an ordered state with few chain entanglements. The entanglement density of the recovered PET recovered from a 20 wt % solution in ethylene glycol oligomer is as low as that of freeze-extracted PET from a 0.5 wt % solution in phenol. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 1219–1225, 1998     

Thermal behavior and crystallization kinetics of random poly(ethylene‐Co‐2,2‐Bis[4‐(ethylenoxy)‐1,4‐phenylene]propane terephthalate) copolyesters

The thermal behavior of poly(ethylene‐co‐2,2‐bis[4‐(ethylenoxy)‐1,4‐phenylene]propane terephthalate) (PET/BHEEBT) copolymers was investigated by thermogravimetric analysis and differential scanning calorimetry. A good thermal stability was found for all the samples. The thermal analysis carried out using DSC technique showed that the T_m of the copolymers decreased with increasing BHEEBT unit content, differently from T_g, which on the contrary increased. Wide‐angle X‐ray diffraction measurements permitted identifying the kind of crystalline structure of PET in all the semicrystalline samples. The multiple endotherms similar to PET were also evidenced in the PET/BHEEBT samples, due to melting and recrystallization processes. By applying the Hoffman–Weeks' method, the T_m° of PET and its copolymers was derived. The isothermal crystallization kinetics was analyzed according to Avrami's treatment and values of the exponent n close to 3 were obtained, independently of T_c and composition. Moreover, the introduction of BHEEBT units was found to decrease PET crystallization rate. Lastly, the presence of a crystal‐amorphous interphase was evidenced. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1441–1454, 2005