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     

Non-isothermal crystallization and melting behavior of compatibilized polypropylene/recycled poly(ethylene terephthalate) blends

Binary blends of polypropylene (PP)/recycled poly(ethylene terephthalate) (r-PET), r-PET/maleic anhydride grafted PP (PP-g-MA), r-PET/glycidyl methacrylate grafted PP (PP-g-GMA), and ternary blends of PP/r-PET (80/20 w/w) compatibilized with various amounts (2-10 wt%) of PP-g-MA or PP-g-GMA were prepared on a twin-screw extruder. The non-isothermal crystallization and melting behavior, and the crystallization morphology were investigated by DSC and POM. The chemical reactions of r-PET with PP-g-MA and PP-g-GMA were characterized by FT-IR. DSC results show that the crystallization peak temperatures of r-PET and PP increased when blending them together, due to the heterogeneous nucleation effect on each other. The of r-PET increased with increasing the content of PP-g-MA while slightly influenced by the content of PP-g-GMA in the binary blends of r-PET with grafted PP, implying different reactivity of r-PET with PP-g-MA and PP-g-GMA. The of PP in the ternary blends retained or slightly decreased, dependent on the compatibilizers and their contents. The melting peak temperature of r-PET in PP/r-PET blends compatibilized by PP-g-MA was lower than that of compatibilized by PP-g-GMA, indicating that PP-g-MA had stronger reactivity towards r-PET compared to PP-g-GMA. The crystallization and melting behavior of blends was influenced by the pre-melting temperature, especially the melting behavior of r-PET in the blends. The crystallization behavior of PP in the blends was also evaluated by Mo’s method. POM confirmed the heterogeneous nucleation effect of r-PET on PP.     

Solvent‐induced crystallization in poly(ethylene terephthalate) during mass transport

Solvent transport in poly(ethylene terephthalate) (PET) and related phase transformation were investigated. The data of mass sorption were analyzed according to Harmon's model for Case I (Fickian), Case II (swelling), and anomalous transport. This transport process in PET is accompanied by the induced crystallization of the original amorphous state. The transformation was examined by wide‐angle X‐ray scattering, small‐angle X‐ray scattering, differential scanning calorimetry, and Fourier transform infrared spectroscopy. During this process, the matrix is under a strain state that causes different kinetic paths of crystallization as compared with that by thermal annealing. This state of strain assists the development of the solvent‐induced crystallization. The model regarding crystallization was proposed in terms of the study of long period L, the crystal thickness l_c, and the thickness of amorphous layer l_a obtained from the one‐dimensional correlation function and interface distribution function. Different kinetic paths were discovered for different crystallization processes. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1444–1453, 2002     

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     

The triple melting behavior of poly(ethylene terephthalate): Molecular weight effects

The melting behavior of isothermally crystallized PET has been studied using linear heating in a differential scanning calorimeter (DSC). Variables such as crystallization temperature, crystallization time, heating rate, and average molecular weight are the main focus of the study. On the basis of several experimental techniques, a correlation of the melting behavior of PET with the amount of secondary crystallization was found to exist. It was observed that the triple melting of PET is a function of programmable DSC variables such as crystallization temperature, crystallization time, and heating rate. However, in testing the hypothesis that there was a correlation between melting endotherms and secondary crystallization inside spherulites, it was found necessary to use a DSC-independent variable in order to enhance the observed effects. Therefore, on the basis of a crystallization model that involves secondary branching along the edges of parent lamellar structures, it was speculated that an increase in the average molecular weight could affect the triple melting of PET due to an increase of rejected portions of the macromolecules. It was found that the second melting endotherm increased, apparently, at the expense of the third one as the average molecular weight was increased. The second melting endotherm was also found to correlate proportionally with the amount of secondary crystallization inside spherulites. The results support a model of crystallization which basically consists of parent crystals and at least one population of secondary, probably metastable, crystals. This latter structural component must involve excluded portions of the macromolecules that did not crystallize during the isothermal crystallization period of the parent crystals. An increase of molecular weight gives rise to a higher entanglement density which in turn increases the fraction of initially rejected chain sections and therefore the amount of secondary crystallization. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1757–1774, 1997     

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.     

Structural features of crystallized poly(ethylene terephthalate) polymers

Polyethylene terephthalate (PET) is a widely used polymeric material. In this work, the microstructural features before and after the solid‐state polymerization (SSP) of several DuPont PET products were investigated by low‐voltage scanning electron microscopy (LV‐SEM) and atomic force microscopy (AFM). The microstructural features on the cross section of various PET samples included crystallites, voids, boundaries, defects, and amorphous phases. The SEM images revealed layered and stepped structural features at the micron and 10‐micron scales that are highly crystallized at the near‐edge region of the cross section for both linear and branched PET samples after the SSP process. The AFM images demonstrate that the degree of crystallization for the linear and branched PET samples increases gradually from the central area to the edge on the cross section. The linear crystallized PET has a higher degree of orientation than the branched crystallized PET in the 10‐micron to micron scales, but their crystalline structures have no significant differences in the submicron to nanometer scales. The PET crystallization process occurs when the molecular chains in the amorphous phase are aligned and folded to form straight molecular chains at the nanometer scale, and small crystallites are formed. The crystallites aggregate and align together into a polygon rod‐like‐shaped crystallites at the submicron scale. Finally, large crystallites at the micron size are formed that appear on the edge area of the cross section. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 40: 245–254, 2002     

Microkinetics of crystallization in poly(ethylene terephthalate)

The lamellar growth kinetics and lamellar thickness of poly(ethylene terephthalate) crystallized from the glassy state have been determined as a function of crystallization temperature. Values of end and side surface free energies have been estimated as well as the residual lamellar thickness. Analyses carried out using secondary nucleation approaches indicate that the width of a critical nucleus is comparable to the effective substrate length for multiple nucleation in this and other slowly crystallizing polymers at high supercoolings. A “universal” critical value of T/T²ΔT below which the strip completion process ceases was found to exist. All crystallization must, therefore, occur through the deposition of critical nuclei. Models are proposed for this process which appear to be consistent with both neutron scattering and infrared experiments on quenched polyethylenes. Comparison of crystallization rates, expressed as “jump” rates, with relaxation frequencies suggest that in order for crystallization to occur at any given temperature the relaxation frequency must be at least two decades faster than the crystal “jump” rate.     

Partially renewable copolyesters prepared from acetalized d‐glucitol by solid‐state modification of poly(butylene terephthalate)

The backbone of poly(butylene terephthalate) (PBT) was modified with 2,4:3,5‐di‐O‐methylene‐D ‐glucitol (Glux) using solid‐state modification (SSM). The obtained copolyesters proved to have a non‐random overall chemical microstructure. The thermal properties of these semicrystalline, block‐like, Glux‐based materials were extraordinary, showing higher melting points, and glass transition temperatures compared with other sugar‐based copolyesters prepared by SSM. These remarkable thermal properties were a direct result of the inherently rigid structure of Glux and the relatively slow randomization of the block‐like chemical microstructure of the Glux‐based copolyesters in the melt. SSM proved to be a versatile tool for preparing partially biobased copolyesters with superior thermal properties. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2014 , 52, 164–177     

Crystallization and melting behavior of liquid crystalline copolyesters based on poly(ethylene terephthalate)

A series of copolyesters were prepared by the incorporation of p‐hydroxybenzoic acid (HBA), hydroquinone (HQ), and terephthalic acid (TA) into poly(ethylene terephthalate) (PET). On the basis of viscosity measurements, high molar mass copolyesters were obtained in the syntheses, and ¹H‐NMR analyses indicated the total insertion of comonomers. They exhibit nematic phase above melting temperature, as observed by polarized light microscope (PLM). Their crystallization and melting behaviors were also studied by differential scanning calorimetry (DSC) and wide angle X‐ray diffraction (WAXD). It was found that these copolyesters are more crystalline than copolyesters prepared from PET and HBA. Introduction of HQ/TA disrupts longer rigid‐rod sequences formed by HBA, and thus enhances molecular motion and increases crystallization rate and crystallinity. Isothermal crystallization at solid phase polymerization conditions (up to 24 h at 200°C) resulted in increased copolymer randomness (by NMR) and higher melting point, the latter attributed to structural annealing. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 369–377, 1999     

Melt-crystallization behavior and isothermal crystallization kinetics of crystalline/crystalline blends of poly(ethylene terephthalate)/poly(trimethylene terephthalate)

The melt-crystallization and isothermal melt-crystallization kinetics of poly(ethylene terephthalate)/poly(trimethylene terephthalate) blends (PET/PTT) were investigated by differential scanning calorimetry (DSC) and polarized optical microscopy. Although PET and PTT in the binary blends are miscible at amorphous state, they will crystallize individually when cooled from the melt. In the DSC measurements, PET component with higher supercooling degree will crystallize first, and then the crystallite of PET will be the nucleating agent for PTT, which induce the crystallization of PTT at higher temperature. On the other hand, in both blends of PET80/PTT20 and PET60/PTT40, the PET component will crystallize at higher temperature with faster crystallization rate due to the dilute effect of PTT. So the commingled minor addition of one component to another helps to improve the crystallization of the blends. For blends of PET20/PTT80 and PET40/PTT60, isothermal crystallization kinetics evaluated in terms of the Avrami equation suggest different crystallization mechanisms occurred. The more PET content in blends, the fast crystallization rate is. The Avrami exponent, n = 3, suggests a three-dimensional growth of the crystals in both blends, which is further demonstrated by the spherulites formed in all blends. The crystalline blends show multiple-melting peaks during heating process.     

Physical transitions and crystallization phenomena in poly(ethylene terephthalate) studied by microhardness

The crystallization behavior of poly(ethylene terephthalate) both with and without sodium montanate, a crystal nucleating agent, has been studied using the microhardness technique. The kinetics of crystallization from the glassy state were investigated in real time by measuring the microhardness H at different crystallization temperatures. Results are discussed in terms of the Avrami equation. Values of the Avrami exponent n of about 3 are observed for samples irrespective of nucleating agent. For samples with nucleant two crystallization ranges are observed: a first range which corresponds to a fast crystallization from nucleating agent particles and a second range which is associated with a slow self-crystallization mode. New transitions evidenced by the presence of a small maximum in H as a function of annealing time and temperature are detected at temperatures above T_g for physically aged samples. The kinetics of this transition have also been examined. It is further shown that the presence of nucleating agent induces a hardening at room temperature which is similar to the effect produced by the physical aging of the samples below T_g. Finally, it is found that aging reduces the rate of creep of the material under the indenter. © 1993 John Wiley & Sons, Inc.     

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     

Phase behavior and structure development in extruded poly(ethylene terephthalate)/poly(ethylene‐2,6‐naphthalate) blend

Liquid–liquid phase separation and subsequent homogenization during annealing in an extruded poly(ethylene terephthalate) (PET)/poly(ethylene‐2,6‐naphthalate) (PEN) blend were investigated with time‐resolved light scattering and optical microscopy. In the initial stage, the domain structure was developed by demixing via spinodal decomposition. In the later stage, the blend was homogenized by transesterification between the two polyesters. The crystallization rate depended on the sequence distribution of polymer chains, which was determined by the level of transesterification rather than the composition change of separated phases. When the crystallization of PEN preceded that of PET, PEN showed a higher melting point. However, when the crystallization rate of PEN was slower than that of PET, the previously formed PET crystals suppressed the crystallization of PEN, causing the coarse crystalline structure of PEN to have a lower melting point. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2625–2633, 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.     

Systematic study on chemical oxidative and solid‐state polymerization of poly(3,4‐ethylenedithiathiophene)

Systematic research on the synthesis, chemical oxidative polymerization of 3,4‐ethylenedithiathiophene (EDTT) in the presence of surfactants or not, and solid‐state polymerization of 2,5‐dibromo‐3,4‐ethylenedithiathiophene (DBEDTT) and 2,5‐diiodo‐3,4‐ethylenedithiathiophene (DIEDTT) under solventless and oxidant‐free conditions has been investigated. Effects of oxidants (Fe³⁺ salts, persulfate salts, peroxides, and Ce⁴⁺ salts), solvents (H₂O, CH₃CN/H₂O, and CH₃CN), surfactants, and so forth on polymerization reactions and properties of poly(3,4‐ethylenedithiathiophene) (PEDTT) were discussed. Characterizations indicated that FeCl₃ was more suitable oxidant for oxidative polymerization of EDTT, while CH₃CN was a better solvent to form PEDTT powders with higher yields and electrical conductivities. Dispersing these powders in aqueous polystyrene sulfonic acid (PSSH) solution showed better stability and film‐forming property than sodium dodecylsulfate and sodium dodecyl benzene sulfonate. Oxidative polymerization of EDTT in aqueous PSSH solutions formed the solution processable PEDTT dispersions with good storing stability and film‐forming performance. Solvent treatment showed indistinctive effect on electrical conductivity of free‐standing PEDTT films. As‐formed PEDTT synthesized from solid‐state polymerization showed similar electrical conductivity, poorer stability, but better thermoelectric property than oxidative polymerization. Contrastingly, PEDTT synthesized from DIEDTT showed higher electrical conductivity (0.18 S cm^?1) than DBEDTT which showed better thermoelectric property with higher power factor value (6.7 × 10^?9 W m^?1 K^?2). © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Solid state blending of poly(ethylene terephthalate) with polystyrene: Extent of PET amorphization and compositional effects on crystallizability

Polystyrene (PS) and poly(ethylene terephthalate) (PET) were blended together in the solid state via cryogenic mechanical attrition (CMA) and in the melt through conventional twin‐screw extrusion. CMA PS/PET blend morphologies were characterized both qualitatively and quantitatively through microscopy and thermal analysis. Specifically, CMA reduced the dispersed‐phase domain size and its distribution relative to simple melt extrusion, although not to the extent attained with added chemical compatibilizers. CMA also amorphized the PET phase and depressed the PET cold crystallization rate, which was measured by post‐CMA nonisothermal MDSC analysis. The PET amorphization efficiency and crystallizability for CMA PS/PET blends were the highest and lowest, respectively, at the PS/PET phase inversion. These concomitant phenomena are known to be caused by CMA‐induced PET crystal defect formation and subsequent entropic stabilization. Such behaviors are linked to the enhanced presence of an uncrystallizable rigid amorphous PET phase, and the weight fraction of this rigid amorphous fraction (RAF PET) was quantified and also maximized near the PS/PET phase inversion. Moreover, the increased compatibilization and amorphization efficiencies and reduced PET crystallizability were determined to be interdependent. These studies have verified that CMA of PET with PS is more efficient than extrusion due to the formation of nonequilibrium, metastable morphologies that can be more precisely controlled and better stabilized with an interesting, composition‐dependent interplay between PET crystallizability and the extent of PS/PET compatibilization. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1348–1359, 2008