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The complex melting behavior of isotactic polypropylene, after isothermal crystallization, was studied within the context of step-like melting mechanisms which were previously proposed for high temperature polymers. The morphological characteristics of the melting process were also studied as a function of molecular weight, and close similarities were observed with respect to high temperature polymers. Positive birefringence crystals of low molecular weight samples developed double melting behavior in three steps. The first melting step was assigned to continuous melting of secondary crosshatch reversing lamellae, together with recrystallization of the remaining isothermal crystals. In the second melting step (first melting endotherm), crystals tended to lose their original coarse negative birefringence due to melting of secondary reversing branching. This effect rendered new, finer texture, but still negative birefringence crystals. In the third melting step (second melting endotherm), there was a combination of melting of two crystal populations, one consisting of the remaining fraction of reversing primary crystals, and the other consisting of nonreversing primary crystals. A crosshatch secondary branching model was therefore proposed to explain the overall results. Mixed birefringence spherulites of high molecular weight samples displayed similar, although proportional, behavior under identical crystallization and melting conditions corroborating the proposed melting mechanism.

The blending and barrier properties of the MPAEVOH blends of modified polyamide (MPA) and ethylene vinyl alcohol copolymer (EVOH) were systematically in- vestigated in this study. After blending MPA in EVOH resin, a noticeable "negative deviation" was found on the plot of the oxygen permeation rate versus MPA content when the MPA contents present in MPAEVOH resins reach about 80 wt %. The peak tempera- tures associated with the main melting endotherm of MPA and EVOH reduce significantly with increasing the EVOH and MPA contents present in MPAEVOH resins, respectively. The melting endotherm and X-ray diffraction peak associated with EVOH crystal phases disappear gradually as the MPA contents present in MPAEVOH increase. In fact, the melting endotherm and X-ray scattering peak corresponding to EVOH crystals almost disappear as the EVOH contents present in MPAEVOH specimens are less than 20 wt %. Further Fourier-Transform Infrared (FT-IR) investigations indicate that the strengths of intermolecular hydrogen bonds of MPAEVOH specimens reduce significantly as the MPA contents increase, wherein the self-associated hydroxyl-hydroxyl bonds within EVOH resins almost disappear as the EVOH contents reduce to be less than about 20 wt %. As expected, the average sizes of the free volume holes of MPAEVOH specimens increase significantly as the MPA contents increase. However, somewhat surprisingly, a clear negative deviation was found on the plot of the numbers and fractional free volumes of free volume holes against the MPA contents as the EVOH contents are close to about 20 wt %. The interesting barrier properties of the MPA, EVOH, and MPAEVOH specimens were investigated in terms of the free volume and intermolecular interaction properties in the amorphous phases of MPAEVOH specimens described above. © 2005 Wiley Periodicals, Inc. J

The melting behavior of poly(butylene succinate-co-adipate) (PBSA) isothermally crystallized from the melt was investigated by differential scanning calorimetry. Triple, double, or single melting endotherms were observed in subsequent heating scan for the samples isothermally crystallized at different temperatures. These endothermic peaks were labeled as I, II, and III for low-, middle-, and high-temperature melting endotherms, respectively. The independence of endotherm III to the crystallization temperature, the existence of an exothermic crystallization peak just below the endotherm III, and the heating rate dependence of endotherm III indicated that endotherm III was due to the remelting of recrystallized lamellar during a heating scan. The influence of crystallization time on the melting behavior of PBSA showed that endotherms II and III developed prior to endotherm I; endotherm III developed rather simultaneously with endotherm II. Further investigation showed that the peak temperature of endotherm I increased linearly with the logarithm of the crystallization time. It suggested that endotherm II was attributed to the melting of the primary lamellae, while endotherm I was due to the melting of secondary lamellae. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3077–3082, 2005

In this study, we examine the effects of heating, nucleation, cooling, and reheating on the thermal properties and structure of metallocene isotactic polypropylene (m-iPP) that had been prepared initially in a standard state containing nearly equal amounts of the crystallographic α and γ phases. Heat treatment was achieved through partial melting and annealing by the heating of samples to self-nucleation temperatures (Tn's) that spanned and exceeded the entire range of melting of the standard state, from 122 to 160 °C. The relative amounts of α and γ crystals are determined from the area under the unique wide-angle X-ray reflections. The lower and upper endotherms are caused by the melting of γ and α crystals, respectively. Four distinct regions of Tn were identified on the basis of the thermal and structural parameters of m-iPP. In region I, Tn is below the peak melting temperature of the γ phase. Here, γ crystals are annealed and α crystals are barely affected by Tn. In region II, Tn is above the peak of the lower endotherm but below the peak of the upper endotherm. γ crystals melt, and α crystals anneal. In both regions I and II, the portion of the sample melted at Tn recrystallizes epitaxially with existing parent α lamellae as the substrates, and the amount of α always exceeds the amount of γ. In region III, Tn is above the peak of the upper endotherm, and all γ crystals and some or all α crystals are melted at Tn. The number of α-crystal nuclei steadily decreases as Tn increases, causing systematic depression of the crystallization and melting temperatures seen during cooling. Finally, in region IV, Tn exceeds the upper endotherm, and only small self-nuclei or heterogeneous nuclei remain. Recrystallization is now suppressed to lower temperatures. For regions III and IV, a crossover behavior in the relative amounts of α and γ is observed during cooling from Tn. Because of the effective nucleating ability of α toward γ, as the temperature drops, the amount of γ increases and then exceeds the amount of α. With subsequent reheating, the reverse crossover occurs because of the lower melting point of γ. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 40: 1644–1660, 2002

A simultaneous wide-angle and small-angle X-ray scattering study of two poly(ethylene naphthalene 2,6-dicarboxylate) samples crystallized from the glassy state at different annealing temperatures for different annealing times was carried out with synchrotron radiation. Either single or dual melting was induced in the samples, as confirmed by differential scanning calorimetry (DSC). The correlation function and interface distribution function were calculated to evaluate microstructural parameters such as the long spacing, the thickness of the amorphous and crystalline phases, and the width of the size distributions. The sample with dual melting behavior exhibited an abrupt increase of all microstructural parameters at temperatures above the melting of the lowest endotherm, whereas the sample revealing a single melting endotherm did not show such a sudden change. This finding agrees with the concept that the appearance of two melting peaks in DSC traces can be explained by the dual lamellar stacking model. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 881–894, 2001

The microkinetics of crystallization and the melting behavior of solid-state polymerized poly(ethylene terephthalate) (PET) have been studied. After correlating crystallization data with secondary nucleation theory and a single regime of growth, a positive correlation was found between molecular weight, free energy products, and transport activation energy. The results indicate that the crystallization behavior of PET might be correlated with regime III if the secondary nucleation theory is assumed to apply. They also support a model of crystallization involving primary and secondary crystallization with increased branching when the molecular weight increases. The melting behavior of PET, previously studied at commercial levels of molecular weight, and proposed before as morphologically the reverse of the crystallization process (Medellin-Rodriguez et al. J Poym Sci B Polym Phys 1997, 35, 1757), continues with solid-state polymerized PET. In particular, it was observed that the second melting endotherm continues to increase with the molecular weight, the process involving a proportional decrease of the third endotherm. This characteristic continues even up to levels where only the first and second melting endotherms are present. The first melting endotherm was also studied, and the results indicated that it is probably related to low molecular weight segregated fractions.

Nylon 46 fibers produced by the high-temperature zone-drawing treatment were treated by repeating high-tension annealing treatments, that is, a high-tension multiannealing (HTMA) treatment to improve their tensile properties. The HTMA treatment was carried out at a repetition time of 10 times and treating temperature of 110°C under high tension (538.2 MPa) close to the tensile strength at break. Although the HTMA treatment was carried out at 110°C, which is much lower than the crystallization temperature of 265°C for nylon 46, the degree of crystallinity increased up to 59%. The orientation factor of crystallites increased dramatically up to 0.949 by the first high-temperature zone-drawing treatment and slightly during the subsequent treatments. This observation indicated that the orientation of crystallites due to slippage among molecular chains did not occur during the HTMA treatment. The treatments shifted the melting peak to slightly higher temperatures, and the HTMA fiber has a melting endotherm peaking at 285°C. The fiber obtained finally had a storage modulus of 12.5 GPa at 25°C.

The melting behavior of poly(butylene terephthalate) crystallized isothermally for various times was examined using differential scanning calorimetry. After short crystallization times, the DSC analysis gave two melting peaks, but after longer times, the analysis gave three peaks. The latter triplet of DSC peaks can be denoted as low, middle, and high, starting with the lowest temperature endotherm. The DSC peaks were simulated using a measured recrystallization rate and behavior for PBT and an assumed initial melting point distribution. The low and middle peaks represent the original melting peaks arising from isothermal crystallization. The high melting peak arises from recrystallization during the DSC heating scan.

The melting behavior of poly(ethylene terephthalate co-1,4-cyclohexylene dimethylene terephthalate) [PET/CT] random copolyesters has been studied. The basis of this analysis was the triple melting behavior of PET homopolymers, which is commonly observed after a period of isothermal crystallization followed by linear heating in a differential scanning calorimeter. Both ET and CT homopolymers are able to crystallize, and as a consequence, the copolymer morphology depends on the ET/CT ratio. It has been reported that at low CT concentrations, the ET units can crystallize with complete rejection of the CT units and that at high CT concentrations, the CT units can cocrystallize with the ET units. In the present work, low CT concentrations were selected, as they are completely rejected from the ET crystals. The purpose was to further test the hypothesis that in the triple melting behavior of PET homopolymers, the second DSC melting endotherm is related to secondary species crystallized by material rejected from the primary crystals. This concept arose from our previous work, where it was speculated that increasing the average molecular-weight of PET would enhance molecular entanglement and increase secondary crystallization. This process would give rise to a higher amount of species being rejected from the main crystals, i.e., an increase of secondary crystallization would occur, and as a consequence the second melting endotherm would be enhanced. Similar to the effect of molecular weight, such behavior has been observed as a function of rejected copolymer content. This gives support to our previously proposed step-like crystallization and melting mechanism for PET homopolymers, and has the potential to be extended to other high temperature semicrystalline polymeric systems. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 763–781, 1998

Differential scanning calorimetry (DSC) is one of the most widely used technique for measuring crystallinity in the polymer industry. The major source of error in the crystalline index (CIDSC) of low crystallinity polymeric articles, is the development of further crystallinity during the DSC scan. Although, this type of cold crystallization is obvious, and thus accounted for in polymers like polyethylene terephthalate, nylons are a difficult class of materials in that respect. The major contributing factors to the failure of DSC in measuring low levels of crystallinity in nylons are identified to be (1) silent crystallization between the glass (Tg) and melting (Tm) transitions, (2) extreme difficulties in packing a moisture-free nylon in the sample pan (the response due to traces of moisture being a broad endotherm competing with a broad exothermic crystallization), and (3) a sub-Tm exotherm, especially in low crystallinity nylons, due to relaxation of the processing-induced stresses. These factors, specific to nylons, mask the observation of cold crystallization and lead to substantially higher than real crystallinities. This manuscript deals with such complications and corrective actions using commercial nylon 6 films of CIDSC = 0−40%. X-ray diffraction measurements have been included to support the validity of our improved DSC methodology. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 2219–2231, 1997

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

The double melting endotherm of spunbonded isotactic polypropylene (iPP) fabrics was investigated by monitoring changes in the solid-state NMR spectrum that result from thermal annealing. The DSC melting thermogram was found to change from a double to a single endotherm at anneal temperatures ≥156°C, with a concomitant increase in percent crystallinity. All of the carbon resonances in the CP/MAS NMR spectrum of the purely crystalline phase of iPP were found to be composed of multiple peaks with relative intensities that depend on anneal temperature. By monitoring the changes in the distribution of intensity among the various peaks of a given resonance, a transition temperature of 156°C was identified. Arguments are presented that this redistribution of intensity within a given carbon resonance characterizes the transformation from the α1 to the α2 monoclinic crystal form. The exothermicity associated with this transformation is responsible for the observation of a double melting endotherm by DSC. The splitting patterns observed in the NMR spectrum are discussed in terms of interlayer distances between layers of isochiral helices and the density of exposed methyls at the contact faces of these interlayers. © 1996 John Wiley & Sons, Inc.

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

DSC of an interpenetrating network polymer of composition 25% polyurethane-75% poly(methyl methacrylate) shows a slowly increasing heat capacity, instead of the usual glass transition endotherm, whose onset temperature is not clearly discernible. On aging of the polymer at several temperatures between 193 and 333 K, an endothermic peak is observed whose onset is in the vicinity of the respective temperature of aging. The area under these peaks increases with increasing aging time at a fixed temperature. The effects are attributed to a very broad disribution of relaxation times, which may be represented by either a sum of discrete structural relaxation times of local network arrangement or by a nonexponential relaxation function which is equivalent to a distribution of relaxation times

A high temperature endotherm has been reported recently for PE samples during a slow T ramp which has been associated with a phase change in strained chains. From comparison of NMR and calorimetric data on nascent and melt/recrystallized samples, the following origin of the strain is now proposed. Nascent PE is a homogeneous material which contains a sizeable proportion of short-range ordered chains interrupted by loose knots. Due to expansion during melting, the knots become tighter and a physical network which encompasses a fraction of the chains is produced. The network so formed interrupts the melting process in the ordered chains. In PE and other polyolefins, melting resumes at higher T. The high T endotherms of physical and chemical networks are compared

The effects of fiber-forming processes on the microstructure and macrostructure and overall orientation in sulfonated poly (ethylene terephthalate) (SPET) fibers are reported. The processing parameters examined include drawing, crimping, relaxing, and annealing. Drawing and annealing cause changes in both the crystalline structure and molecular packing in the noncrystalline regions, while crimping and relaxing appear to affect only the noncrystalline regions. A bimodal melting endotherm was observed for the SPET fibers. Experimental data suggest the low-temperature endotherm of the SPET fibers originates from melting of the crystalline structure formed on drawing, and that the high-temperature endotherm results from melting the heat-induced crystals formed during fiber processing and/or thermal analysis

In this study, we report the melting behavior of poly(phenylene sulfide), PPS, which has been cold-crystallized from the rubbery amorphous state. We find that the crystallization kinetics are faster for cold-crystallized PPS than for melt-crystallized material, due to formation during quenching of a short-range ordered, but noncrystalline, structure. We observe that the endothermic response of cold-crystallized PPS at a large undercooling consists of a low temperature endotherm, followed by an exothermic region, and by the main higher melting endotherm. The lower melting peak temperature of cold-crystallized PPS increases as the crystallization temperature increases, but the main upper melting peak temperature remains almost the same. The size of the exothermic region is strongly related to the degree of undercooling, and must be taken into account in order properly to determine the degree of crystallinity of the material prior to the scan. When the crystallization time is varied, we see a systematic decrease in the size of the main endotherm, and an increase in size of the lower melting endotherm. This suggests that a portion of the main endothermic response is due to reorganization during the scan. Annealing will not only increase the degree of crystallinity but also improve the crystal perfection; therefore the ability of an annealed sample to reorganize decreases as the annealing time increases. However, an additional third melting peak is seen when a cold-crystallized sample is annealed at high temperature for a sufficiently long residence time. The existence of the third melting peak suggests that more than one kind of distribution of crystal perfection may occur when PPS has been cold-crystallized and subsequently annealed.

The effect of the heat treatment on as-spun fibers made of a commercial liquid crystalline polymer, Vectra®, was examined through tensile properties and differential scanning calorimetry (DSC) measurements. The heat treatment increased the fiber strength considerably, and it also increased the elongation at break. On the other hand, the treatment had a weaker effect on the modulus. In general, the effects of treatment depended on the temperature and on the treatment time used. DSC measurements showed the effect of the treatment on the behavior of the melting endotherm. In light of the development of this endotherm, it seems possible that the structure formed during treatment depends essentially on the treatment temperature. On the whole, an unambiguous correlation between tensile properties and calorimetric quantities could not be concluded.

Detailed thermal characterization of sulfo-EPDM ionomers was carried out. The endotherms observed via differential scanning calorimetry (DSC) and multiple inflections observed via thermal mechanical analysis (TMA) are speculated to be due to associated ionic species in the polymers. A large time dependence of the DSC endotherm was observed, suggesting substantial variations in the nature of the ionic species over this period. The influence of ionomer composition on thermal characteristics was probed with whole polymers and polymer fractions. In general, trends in the DSC and TMA data paralleled those expected on the basis of the anticipated effect of polymer sulfonation level, molecular weight, and counterion type on the strength of the physical crosslinking of these polymers. Both the DSC endotherms and the TMA inflection regions are rationalized in terms of a previously developed ionic-bond exchange model.

Thermal and electron diffraction studies of single crystals derived from the tetramer of p-acetoxybenzoic acid have revealed the presence of two consecutive crystallographic transformations, i.e., topotactic transitions. Thus, differential scanning calorimetry of the oligomer indicated two irreversible phase transitions, an endotherm followed by an exotherm, between 150 and 180°C, in addition to the crystal-to-nematic transition at 260°C. Electron diffraction analysis further elucidated the transformation occurring during thermal treatment of the original oligomer. Hence it could be shown that the solution-crystallized material (α form) underwent a solid–solid phase transition to the β modification at a temperature corresponding to the observed endotherm. This β form then converted to a more stable modification (β′ form) which represented a superlattice structure derived from the β modification.