Multiple Melting Endotherms Research Articles

Synthesis, structure, and thermal property of poly(trimethylene terephthalate-co-trimethylene 2,6-naphthalate) copolymers

Poly(trimethylene terephthalate-co-trimethylene 2,6-naphthalate)s (P(TT-co-TN)s) with various copolymer composition were synthesized, and their chain structure, thermal property and crystalline structure were investigated by using1H-NMR spectroscopy, differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD), respectively. It was found from sequence analysis that all the P(TT-co-TN) copolymers synthesized have a statistical random distribution of TT and TN units. It was also observed from DSC thermograms that the glass transition temperature increases linearly with increasing the TN comonomer content, whereas the melting temperature of copolymer decreases with increasing the corresponding comonomer content in respective PTT- and PTN-based copolymer, showing pseudo-eutectic melting behavior. All the samples melt-crystallized isothermally except for P(TT-co-66 mol % TN) exhibit multiple melting endotherms and clear X-ray diffraction patterns. The multiple melting behavior originates from the dual lamellar population and/or the melting-recrystallization-remelting. The X-ray diffraction patterns are largely divided into two classes depending on the copolymer composition, i.e., PTT and PTNβ-form diffraction patterns, without exhibiting cocrystallization.

Secondary Crystallization Behavior of Poly(ethylene isophthalate-co-terephthalate): Time-Resolved Small-Angle X-ray Scattering and Calorimetry Studies

Time-resolved small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) analyses were used to study the isothermal crystallization and remelting of poly(ethylene isophthalate-co-terephthalate)s containing 0−10 mol % isophthalate unit. For each of the polymers considered, evidence of the formation of both primary and secondary crystals was found in the SAXS pattern and its invariant Q, as well as in the DSC thermogram, which showed multiple melting endotherms. The melting of secondary crystals was found to shift significantly toward the high-temperature region with increasing crystallization time, indicating that the secondary crystals become thick and perfect over time. The thickness of the lamellar crystals was found to be very sensitive to the crystallization temperature, but no thickening was observed throughout the entire crystallization run, regardless of composition. The formation of secondary crystals, which favorably occurs during the later stages of crystallization, was found to cause a peak shift and an intensity increase in the SAXS pattern, a decrease in the SAXS invariant Q, and a decrease in the thickness of the amorphous layers in the lamellar stacks formed during primary crystallization. However, formation of secondary crystals was found not to be properly accounted for in the determination of the lamellar thickness from the SAXS patterns. The present results indicate that the secondary crystallization causes densification and shrinkage of the amorphous layers and that the resulting secondary crystals have lower electron density than the primary crystals. On the basis of the present results, we propose that secondary crystallization involves the formation of short-range molecular order in the amorphous layers of the lamellar stacks as well as in the amorphous regions between the lamellar stacks. This short-range ordered structure, which is likely a type of single thin lamella, thin lamellae, or fringed micelle-like order, has a lower electron density than the lamellar crystal formed by primary crystallization.

Miscibility, melting, and crystallization of poly(butylene‐2,6‐naphthalate)/poly(ether imide) blends

AbstractWe prepared blends of poly(butylene‐2,6‐naphthalate) (PBN) and poly(ether imide) (PEI) by solution‐casting from dichloroacetic acid solutions. The miscibility, crystallization, and melting behavior of the blends were investigated with differential scanning calorimetry (DSC) and dynamic mechanical analysis. PBN was miscible with PEI over the entire range of compositions, as shown by the existence of single composition‐dependent glass‐transition temperatures. In addition, a negative polymer–polymer interaction parameter was calculated, with the Nishi–Wang equation, based on the melting depression of PBN. In nonisothermal crystallization investigations, the depression of the crystallization temperature of PBN depended on the composition of the blend and the cooling rate; the presence of PEI reduced the number of PBN segments migrating to the crystallite/melt interface. Melting, recrystallization, and remelting processes occurring during the DSC heating scan caused the occurrence of multiple melting endotherms for PBN. We explored the effects of various experimental conditions on the melting behavior of PBN/PEI blends. The extent of recrystallization of the PBN component during DSC heating scans decreased as the PEI content, the heating rate, the crystallization temperature, and the crystallization time increased. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1694–1704, 2004

Crystallization and melting of isotactic rolypropylene in response to temperature modulation

Isotactic polypropylene (iPP) was crystallized using temperature modulation in a differential scanning calorimeter (DSC) to thicken the crystals formed on cooling from the melt. A cool-heat modulation method was adopted for the preparation of the samples under a series of conditions. The effect of modulation parameters, such as temperature amplitude and period was monitored with the heating rate that followed. Thickening of the lamellae as a result of the crystallization treatment enabled by the cool-heat method lead to an increase in the peak melting temperature and the final traces of melting. For instance, iPP melting peak shifted by up to 3.5°C with temperature amplitude of 1.0°C while the crystallinity was increased from 0.45 (linearly cooled) to 0.53. Multiple melting endotherms were also observed in some cases, but this was sensitive to the temperature changes experienced on cooling. Even with a slower underlying cooling rate and small temperature amplitudes, some recrystallization and reorganization occurred during the subsequent heating scan. The crystallinity was increased significantly and this was attributed to the crystal perfection that occurred at the crystal growth surface. In addition, temperature modulated differential scanning calorimetry (TMDSC) has been used to study the melting of iPP for various crystallization treatments. The reversing and non-reversing contribution under the experimental time scale was modified by the relative crystal stability formed during crystallization. Much of the melting of iPP was found to be irreversible.

Influence of thermal treatments, molecular weight, and molecular weight distribution on the crystallization of β‐isotactic polypropylene

AbstractSix samples of isotactic polypropylene were examined to study the influence of the thermal treatments and the molecular weights and their distribution on the β‐crystallization of the polymer. The highest amount of the β‐phase was obtained by isothermal crystallization and in correspondence of high average molecular weights and wide molecular weight distributions. Small‐angle X‐ray scattering pointed out that a partial β‐crystallization seems not to influence the lamellar morphology parameters. Differential scanning calorimetry measurements, at different heating rates, allowed us to confirm that the multiple melting endotherms behavior of the β‐phase is to be correlated to a melting–recrystallization–melting mechanism. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 91: 1008–1012, 2004

Effect of polydimethylsiloxane macrodiol molecular weight on properties and morphology of polyurethane and polyurethaneurea

AbstractTwo series of polyurethanes and polyurethaneureas were prepared using a two‐step bulk‐solution polymerization procedure. Each series consisted of three polymers based on three molecular weights of α,ω‐bis(6‐hydroxyethoxypropyl) polydimethylsiloxane (PDMS): 940, 1913, and 2955. The soft segment in all cases was an 80:20 mixture of PDMS and poly(hexamethylene oxide) (MW 700), and the hard segment was based on 4,4′‐methylenediphenyl diisocyanate and mixed chain extenders (40 wt %). In the polyurethane (PU) series the chain extender was a 60:40 (mol) mixture of BDO and 1,3‐bis(4‐hydroxybutyl)1,1,3,3‐tetramethyldisiloxane (BHTD), whereas in the polyurethaneurea (PUU) series it was a 60:40 (mol) mixture of BHTD and 1,2‐ethylenediamine. The polymerization was carried out by preparing a prepolymer using a bulk polymerization procedure followed by chain extension in a solution of N,N‐dimethylacetamide. Polymers were characterized by size‐exclusion chromatography, tensile testing, differential scanning calorimetry (DSC), and dynamic mechanical thermal analysis (DMTA). The number‐average molecular weights of the polymers in the PU series were in the range of 114,300–124,500, whereas they were in the 78,400–103,300 range for the PUU series. Polymers with good clarity and mechanical properties were obtained with PDMS‐940 and PDMS‐1913, but those obtained from PDMS‐2955, despite having good tensile strength, had a low percentage of elongation, high modulus, and poor clarity. DSC and DMTA results indicated that regardless of the PDMS molecular weight, the siloxane segments existed as a highly phase‐separated state. This poor compatibility was consistent with the low solubility of PDMS compared to that of the hard‐segment‐forming components. The polymers in the polyurethane series exhibited multiple melting endotherms, attributed to the melting of ordered domains from different hard segments. The combined heats of fusion were similar for the materials in the PU series. In contrast, the polymers in the PUU series showed a hard‐segment order that was significantly less defined, with the heat of fusion approximately a third to a half that of the materials in the polyurethane series. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 1565–1573, 2003

Effects of several lanthanide trichloride hydrates on the melting behavior and spherulitic superstructure of poly(ethylene oxide)

AbstractBinary mixtures of poly(ethylene oxide) (PEO) with the trichloride hydrates of lanthanum, cerium, europium, terbium, and ytterbium have been studied with calorimetry, polarized optical microscopy, and infrared spectroscopy. Melting‐point depression of the PEO‐rich phase occurs in all cases. At sufficiently high concentrations of the low molecular weight lanthanide complex, crystallization of the polymer is absent. The lighter lanthanides with larger ionic radii, such as lanthanum and cerium, are more effective in suppressing PEO crystallization from solution or the molten state because they are more oxophilic. The spherulitic superstructure of PEO disappears at rather low concentrations of the lanthanide salts, between 2 and 8 mol % Ln3+. Lanthanum and terbium are most efficient at disrupting the formation of PEO spherulites, and europium is least efficient. Infrared spectroscopy identifies twisting and wagging vibrational absorptions of CH2 groups in the polymer that are sensitive to the morphologies of these mixtures. Modifications of the PEO infrared absorbances in the presence of these five lanthanide salts correlate more closely with the presence or absence of major PEO melting, not the formation of a spherulitic superstructure. The phase behavior is rather simple, with no evidence of eutectic solidification upon cooling from the molten state. Multiple melting endotherms are observed in the differential scanning calorimetry heating traces of binary mixtures containing 8 mol % Yb3+ and between 10 and 20 mol % Eu3+, but the concentration dependence of these first‐order endothermic transitions is not characteristic of eutectic phase behavior. The presence of trivalent cations, such as Eu3+ or Yb3+, in these complexes perturbs the crystallization kinetics of PEO upon cooling from the molten state, as well as the melting behavior upon heating. Ion–dipole or electrostatic interactions between the lanthanide cation and the ether oxygen of PEO might alter the surface free energy at the periphery of the crystalline lamellae and perturb the chain‐folding characteristics of PEO. Consequently, coupling between the amorphous matrix and the PEO crystallites is strengthened, and this provides stability for the existence of multiple‐chain‐folded crystals composed of rather thin lamellae that could be responsible for multiple melting behavior. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 2200–2213, 2003

Multiple melting behaviour of poly(ethylene terephthalate)

Differential scanning calorimetry (DSC) and temperature modulated DSC (MTDSC) have been used to investigate the melting behaviour of poly(ethylene terephthalate) (PET). Multiple melting endotherms were observed even at high heating rates, e.g. 160 K min −1 and these have been attributed to the presence of two different distributions of lamella thickness and re-crystallisation (reorganisation) on heating. This has been confirmed by MTDSC—the presence of endotherms and an exotherm in the reversing component of the heat flow during heating. Examination of the endotherms of samples heating stepwise indicated that further crystallisation took place above the isothermal crystallisation temperature ( T c). Some part of this was associated with lamella thickening and crystal perfecting. The multiple melting endotherms observed are a consequence of the balance between the melting and re-crystallisation and the lamella thickness distribution existing within the sample, prior to heating. The triple melting endotherms observed are attributed to the melting of secondary and primary lamellae produced on crystallisation and to thickened lamellae produced during heating to the melting point.

Structure–property relationships in metallocene polyethylenes

The development of new polyolefins based on metallocene technologies represent a considerable advance in the performance of polyethylenes available for a wide range of applications. The ability to obtain a homogeneous short chain branching distribution and control molecular weight is remarkable. The added ability to introduce long chain branching (LCB) into what would otherwise be linear polyethylenes has opened up a broad range of processing possibilities. Through a review of this technology, it becomes apparent that significant elements of the fundamentals underlying the materials science of these new polyolefins are in need of clarification. The morphology of metallocene polyethylenes, particularly in the low-crystallinity range, has yet to be conclusively resolved. The occurrence of multiple melting endotherms for ethylene/α-olefin copolymers with, ostensibly, a homogeneous branching distribution is in need of explanation. LCB incorporated into the linear copolymers enhances shear thinning and melt elasticity. Yet to be fully determined, however, are the types of long chain branch architectures that most effectively promote these desirable rheological attributes, and to what degree these types are present in metallocene polyolefins.

Thermal behavior of gum arabic in comparison with cashew gum

Thermal behavior of gum arabic and cashew gum, the exudate polysaccharides from Acacia and Anacardium occidentale L., containing different gum/water concentrations: 100/0, 80/20, 60/40, 50/50, 40/60, 20/80 and 5/95% w/w, were studied by differential scanning calorimetry (DSC) and thermogravimetry (TG). DSC thermal profiles for gum arabic with low water content (0–40%) showed an endothermic event at about 90°C (Tonset) and with increasing water content (50–80%) multiple melting endotherms and associated enthalpies were observed. Similar behavior was found in the DSC curves of cashew gum with Tonset of about 97°C. TG curves show two decomposition stages, the major decomposition occurred at 252°C (Tonset) to arabic and cashew gums pure.

Calorimetric and Infrared Spectroscopic Analysis of Multiple Melting Endotherms of Poly(ethylene terephthalate)

Multiple melting endotherms of poly(ethylene terephthalate) (PET) were investigated with differential scanning calorimetry (DSC), Fourier transition infrared spectroscopy (FT-IR) and temperature modulated DSC (MDSC) by examining PET samples subjected to special schemes of crystallization and annealing treatment at different temperatures. Upon one-step and two-step annealing, a series of multiple minor peaks in the PET were demonstrated by DSC. FT-IR showed that the multiple endothermic minor peaks were due to melting of imperfect crystals during crystallization. From MDSC curves direct evidence can be obtained for explanation the multiple melting mechanisms in these cold crystallized polymers. The morphology and melting mechanism of semicrystalline polymers depend on the thermal history of crystallization or annealing. When the sample is crystallized at an isothermal temperature or at multiple ascending temperatures, the hypothesis of melting of original low-temperature crystals and reorganization (recrystallization) into high-melt crystals during DSC scanning has been found responsible for the observed multiple melting behavior; when the sample is isothermally crystallized and annealed at multiple descending temperatures, the proposal of multiple morphologies is more responsible for the multiple melting peaks.

MELTING BEHAVIOR OF POLYETHYLENE TEREPHTHALATE COPOLYESTER AND POLYSTYRENE BLENDS

The melting behavior of polyethylene terephthalate (PET) copolyester/polystyrene (PS) blends was studied using differential scanning calorimetry. Multiple melting endotherms were observed for the PET copolyester as well as the PET copolyester/PS blend at Tc = 483 and 493 K, respectively. A higher crystallization temperature resulted in the reorganization of the crystalline species and an increase in area under the reorganization peak. Higher melting temperatures for PS-rich blends may be attributed to the formation of a stiffer matrix around the crystallites and the possible inhibition of crystal thickening. Melting behavior shows that the PET copolyester and PS form miscible blends up to 30 wt.% of PS, whereas with the lowering of the isothermal crystallization temperature to 483 K, miscibility was found only up to 10 wt.% of PS.

Crystalline Morphology and Melting Behavior of 100% Syndiotactic Polystyrene and 70/30 Blend of Syndiotactic Polystyrene and Poly(2, 6-dimethyl-1, 4-phenylenoxide)

The thermal behavior of isothermally crystallized syndiotactic polystyrene (SPS) and 70/30 SPS/poly(2,6-dimethyl-1,4-diphenylene oxide) (PPO) blend was investigated by differential scanning calorimetry. Pure SPS, which was crystallized at Tc=220 and 240°C had single melting endotherm and multiple endotherms, respectively, whereas the 70/30 SPS/PPO blends showed the opposite behavior with respect to Tc. Investigation by polarizing optical microscopy and small angle X-ray scattering revealed that the rejected impurity (PPO in blend and noncrystallizable chains with low molecular weight in pure SPS) during SPS crystallization resides between primary lamellae of SPS and thus the recrystallization strongly suppressed for the pure SPS crystallized at Tc=220°C and the 70/30 SPS/PPO blend crystallized at Tc=240°C. This may induce single melting endotherm. In the pure SPS crystallized at Tc=240°C and the 70/30 SPS/PPO blend crystallized at Tc=220°C, the rejected impurity resided between SPS crystalline fibrils, which consist of lamellae. This may allow recrystallization between the primary lamellae which would exhibit sharp multiple melting endotherms.

Temperature modulated DSC studies of melting and recrystallization in polymers exhibiting multiple endotherms

Temperature-modulated DSC (TMDSC) is used to characterize melting and recrystallization in polymers exhibiting multiple melting endotherms. Poly(ethylene-2,6-naphthalenedicarboxylate)(PEN) and poly(oxy-1,4-phenyleneoxy-1,4-phenylenecarbonyl-1,4-phenylene)(PEEK) are chosen, and the data show the detailed contributions of thermal and processing histories to properties. The results are supplemented by standard DSC at different heating rates. By independent very rapid heating rate methods, the temperature at which the polymer first completely flows is used as a measure of the end of melting of crystals originally present in the sample, and is shown to be well below the final DSC melting point because of recrystallization during the DSC heating scan. This is true even for long annealing times at moderately high temperatures. The TMDSC signal detects endothermic peaks or shoulders corresponding to the melting of crystals originally present in the sample, and such information are not available from standard DSC because of offsetting exothermic and endothermic signals. The TMDSC data prove that the “low endotherm”—routinely detected by standard DSC a few degrees above isothermal annealing temperatures—is not a true “low endotherm”, but is a superposition of early melting of secondary crystals with almost simultaneous exothermic recrystallization. It is not a distinct endotherm because the degree of recrystallization measured in the non-reversing signal of TMDSC increases continuously up to and sometimes through the final melting region. This description of the thermal scan considers both primary crystals, secondary crystals, and recrystallization during the heating scan.

Melting behavior and identification of polymorphic crystals in syndiotactic polystyrene

Melting peaks and resulted crystal forms in syndiotactic polystyrene (s-PS) subjected to various thermal treatments were thoroughly investigated using DSC and X-ray diffraction. This study concluded that the four major crystal forms were individually generated and no transformation occurred between α′, α″, β′ and β″ forms during thermal treatments. Thickening of these crystals, however, was likely during either slower heating scan (i.e. dynamic annealing) or extended isothermal annealing at relatively high temperatures. Different polymorphs were found to dominate the observed multiple melting endotherms of a thermally treated s-PS sample, which was erased of the thermal history in advance. Through cross-reference between the thermograms and X-ray crystallographs, each of the multiple melting peaks in DSC thermograms of s-PS was individually assigned and associated with the identified crystal forms. The melting peak of β-form is always located at lower temperature than those of α-form. Meanwhile, β-form is less thermally stable than α-form.

Properties and Film Applications of Metallocene-Based Isotactic Polypropylenes

When using a stepwise isothermal segregation technique (SIST) by DSC, multiple melting endotherms are observed for metallocene-based isotactic polypropylenes (miPPs), whereas only a single melting peak is obtained for standard iPPs made by the Ziegler-Natta (Z-N) catalyst system. The multiple endotherms suggest discrete morphological populations of segregation effects due to heterogeneity in stereo defect distribution among chains. A distinct peak at about 150TC may be attributable to the presence of 2-1 insertion defect, which is detected only in the miPPs using 13C NMR. This lower melting family of polymer species may result in the unique stress-strain profile observed when using a laboratory scale biaxial film stretcher. The miPPs require lower draw temperatures and show a similar yielding behavior to that of high crystallinity iPPs at an equivalent yield stress; however, they show an earlier onset and sharp increase of stress due to strain hardening. The insertion defects inherent to the miPPs as well as the high isotacticity may thus control the final film properties which result in superior optical, mechanical, barrier and COF (coefficient of friction) properties in the cast and BOPP (biaxially oriented) film applications.

Isothermal crystallization kinetics and melting behavior of poly(β-hydroxybutyrate)/poly(vinyl acetate) blends

The overall isothermal crystallization kinetics and melting behavior of poly(beta-hydroxybutyrate) (PHB)/poly(vinyl acetate) (PVAc) blends were studied by using differential scanning calorimetry(DSC). The Avrami analysis indicates that the addition of PVAc into PHB results in the decrease in the overall crystallization rate of the PHB phase, but does not affect PHB's nucleation mechanism and geometry of crystal growth. The activation energy of the overall process of crystallization increases with the increasing PVAc content in the blends. The phenomenon of multiple melting endotherms is observed, which is caused by melting and recrystallization during the DSC heating run. (C) 1998 Elsevier Science Ltd. All rights reserved.

Thermal behavior of poly(hexamethylene terephthalate) oligomers. I. Melting behavior and morphology of the crystalline phase

The thermal behavior of poly(hexamethylene terephthalate) (PHT) oligomers containing variable amounts of isophthalic (IPA) monomer has been investigated. Poly(hexamethylene terephthalate-co-isophthalate) 3400 was first studied. This polymer has a Mn of 3400, is constituted by heterodisperse PHT units (crystallizable units) with a Mn of 2000, contains an average number of 2.6 IPA per chain, and has carboxylic chain ends. DSC results have shown that this polymer exhibits multiple melting endotherms. These are described in terms of (i) the existence of two melting zones located on both sides of Tc and associated with two polymer fractions differentiated by their crystallizable unit length, (ii) different crystalline populations growing at Tc, (iii) a reorganization process. A simultaneous small (SAXS) and wide (WAXS) angle time resolved X-ray scattering study using synchrotron radiation has revealed that the different crystalline populations are characterized by different lamellar structures rather than by different crystalline lattices. Other PHTcoIs and PHTs differing from PHTcoI 3400 by their molecular weight and IPA content were also studied using DSC. IPA acts as a defect for crystallinity and causes Tm and ΔHm depression in the PHTcoIs. When the molecular weight of PHT increases, Tm increases and ΔHm decreases as usually reported in the literature. Finally, two crosslinked polymers (PHTcoI 3400 and PHTcoI 4700) were investigated. The insoluble fraction of crosslinked PHTcoI 3400 does not crystallize anymore while the one of PHTcoI 4700 does. This results from shorter crystallizable units and from the distance between the crosslinks of the former. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1–18, 1999

Crystallization and Melting Behavior of Poly(ferrocenyldimethylsilanes) Obtained by Anionic Polymerization

A series of poly(ferrocenyldimethylsilanes) (PFS) with narrow molar mass distribution were prepared via anionic ring-opening polymerization. The thermal behavior and morphology of these materials were investigated. Differential scanning calorimetry (DSC) experiments showed multiple melting endotherms upon heating after isothermal crystallization. Melting of thin lamellae and recrystallization into thicker lamellae accounted for these multiple transitions. Wide-angle X-ray scattering (WAXS) showed that the crystal structure remained unchanged during heating. An increase in the long-period observed with temperature-dependent small-angle X-ray scattering (SAXS) supported the assumption that lamellar thickening occurs. The semicrystalline polymers showed a melting behavior as described by the Hoffman−Weeks equation, leading to an equilibrium melting temperature of 143 °C.

Multiple melting and crystal annealing of poly(ethylene terephthalate) in its blends with poly(ether imide)

The melting behaviour and crystal annealing of poly(ethylene terephthalate) (PET) in its blends with poly(ether imide) (PEI) have been investigated by differential scanning calorimetry (d.s.c.). The multiple melting endotherms of PET were considered to arise from the occurrences of initial melting, recrystallization, and remelting processes in the melting region. The effects of blend composition, heating rate and crystallization duration on the multiple melting behaviour of PET/PEI blends were examined. It was found that the extent of recrystallization during d.s.c. heating decreased with increasing PEI composition and crystallization duration. The extent of recrystallization was suggested to be controlled by the degree of undercooling, T° m − T m and the remixing after initial melting. The reduction in extent of recrystallization of PET upon blending with PEI was attributed to the remixing between PEI and PET after the initial melting. The effect of blend composition on the annealing rate of PET crystals was also evaluated from the change of the onset of melting with annealing time. The results showed that the PET crystals exhibited a slower annealing in the blends than in the pure state. This observation was correlated with the molecular morphology in the crystalline/amorphous blends.