Influence of melting conditions on the thermal behaviour of poly(l-lactic acid)

The influence of melting temperature and time on the thermal behaviour of poly(l-lactic acid) (PLLA) was studied with differential scanning calorimetry (DSC). Different melting conditions were investigated at temperature ranging from 200 to 210 °C, and for time from 2 to 20 min. For lower-molecular-weight PLLA, a single exothermic peak could be observed at cooling rate of 2 °C/min, after melted at different conditions. The obtained peak temperature and degrees of crystallinity dramatically increased with an increase of melting temperature or time. During subsequent heating scans, double melting peaks could be observed, which were significantly affected by prior melting conditions. The degradation of this material in the melt and the melt/re-crystallization mechanism might be responsible for the observations above. Apart from double melting, double cold crystallization peaks were observed during heating traces for this material after fast cooling (20 °C/min) from the melt. Prior melting conditions could significantly influence the cold crystallization behaviour. The competition between the crystallization from the nuclei remained after cooling, and that from spontaneous nucleation might be responsible for the appearance of double peaks. Additionally, the influence of melting conditions on the thermal behaviour of PLLA was dependent on the initial molecular weight.     

Melting behavior of poly(butylene succinate) during heating scan by DSC

The melting behavior of isothermally crystallized poly(butylene succinate) (PBS) has been investigated using differential scanning calorimetry (DSC) and wide‐angle X‐ray analysis. The samples crystallized between 80°C to 100°C show middle endotherm at the position just before the high exotherm, while the others under 80°C show two endotherms (low and high). From the results of the melting peak vs. crystallization temperature plot, it was suggested that the middle endotherm corresponds to the melting process of the original crystallites and the high endotherms to the melting process of the recrystallized ones. As the DSC heating rate was increased, the peak temperature of the low and middle endotherms increased and that of the high endotherm decreased, indicating that the low endotherm was due to the original crystallites as well as the middle endotherm. Consequently, in the heating scan of PBS, the existence of two kinds of morphologically different crystallites as well as the process of melting and recrystallization becomes evident. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1357–1366, 1999     

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     

Thermal analysis of the multiple melting behavior of poly(butylene succinate‐co‐adipate)

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     

Isothermal melt-crystallization and melting behavior for three linear aromatic polyesters

Isothermal crystallization and subsequent melting behavior for three different types of linear aromatic polyester, namely poly(ethylene terephthalate) (PET), poly(trimethylene terephthalate) (PTT), and poly(butylene terephthalate) (PBT), were investigated (with an emphasis on PTT in comparison with PET and PBT). These polyesters were different in the number of methylene groups (i.e. 2, 3, and 4 for PET, PTT, and PBT, respectively). Isothermal crystallization studies were carried out in a differential scanning calorimeter (DSC) over the crystallization temperature range of 182-208 °C. The wide-angle X-ray diffraction (WAXD) technique was used to obtain information about crystal modification and apparent degree of crystallinity. The kinetics of the crystallization process was assessed by a direct fitting of the experimental data to the Avrami, Tobin, and Malkin macrokinetic models. It was found that the crystallization rates of these polyesters were in the following order: PBT>PTT>PET, and the melting of these polyesters exhibited multiple-melting phenomenon. Lastly, the equilibrium melting temperature for these polyesters was estimated based on the linear and non-linear Hoffman-Weeks (LHW and NLHW) extrapolative methods.     

DSC analysis of isothermal melt-crystallization, glass transition and melting behavior of poly(l-lactide) with different molecular weights

Isothermal melt-crystallization, glass transition and melting behavior of poly(l-lactide) (PLLA) with different molecular weights were investigated by using differential scanning calorimetry. Analysis by Avrami equation showed that crystallization was initiated by heterogeneous nucleation, followed by 3-dimensional growth. The maximum reciprocal half-time of crystallization (1/t_1/2) was detected at 105 °C. Double endothermic peaks were observed around the glass transition for PLLA with intermediate crystallinities, indicating the coexistence of bulk-like and confined amorphous regions. Double-melting behavior was analyzed and combined with the equilibrium melting temperature evaluation by non-linear Hoffman-Weeks extrapolation, from which a value of 207.6 °C was deduced for PLLA of infinite molecular weight. Lauritzen-Hoffman theory was employed to analyze the crystallization kinetics. Regime II-III transition was found to occur at 120 °C for PLLA of lower molecular weight. The crystal morphology was also examined by scanning electron microscopy through chemical etching method.     

The double melting behavior of a liquid crystalline polyimide derived from PMDA and 1,3‐bis[4‐(4′‐aminophenoxyl) cumyl] benzene

The double melting behavior of a thermotropic liquid crystalline polyimide was studied by means of differential scanning calorimetry (DSC), polarized light microscopy (PLM), transmission electron microscopy (TEM), wide‐angle X‐ray diffraction (WAXD), and small‐angle X‐ray scattering (SAXS). This liquid crystalline polyimide exhibited a normal melting peak around 278 °C and transformed into a smectic A phase. The smectic A phase changed to nematic phase upon heating to 298 °C, then became isotropic melt around 345 °C. The samples annealed or isothermally crystallized at lower temperature showed double melting endotherms during heating scan. The annealing‐induced melting endotherm was highly dependent on annealing conditions, whereas the normal melting endotherm was almost not influenced by annealing when the annealing temperature was low. Various possibilities for the lower melting endotherm are discussed. The equilibrium melting points of both melting peaks were extrapolated to be 283.2 °C. Combined analytical results showed that the double melting peaks were from the melting of the two types of crystallites generated from two crystallization processes: a slow and a fast one. Fast crystallization may start from the well‐aligned liquid crystal domains, whereas the slow one may be from the fringed or amorphous regions. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 3018–3031, 2000     

Confined crystallization,melting behavior and morphology in PEG-b-PLA diblock copolymers: Amorphous versus crystalline PLA

In diblock copolymers, the constraining effects of different stereochemical structure of high-T_m block on crystallization and melting behaviors of other constituent are supposed to be different. In this work, PEG-b-PDLLA and PEG-b-PLLA were synthesized, and crystallization kinetics, crystalline structure, melting behaviors of PEG blocks and morphology development in these systems were evaluated. Compared to those connected to PLLA, PEG-b-PDLLA exhibited lower crystallization rates, implying that connectivity of amorphous chain exerted more pronounced effect on crystallization rate of PEG than that of steric hindrance of PLLA crystallites. While all PEG-b-PDLLA samples showed a single endothermic peak during heating process, multiple melting peaks were observed in PEG-b-PLLA associated with composition, crystallization temperature and cooling rate of PLLA. A lamellar structure was formed by the crystallization of PEG in all PEG-b-PDLLA, however, when PEG-b-PLLA crystallized at room temperature directly, unexpected results occurred: lamellar for diblock copolymers with 31.5 and 48.0 wt% PLA or cylindrical structure for the diblock copolymers with 56.1 and 63.8 wt% PLA. Depending on composition, PEG-b-PLLA created one or two types of lamellar stacks after sequential crystallization of PLLA and PEG. © 2020 Wiley Periodicals, Inc. J. Polym. Sci. 2020 , 58, 455–465     

Characterization,crystallization kinetics,and melting behavior of poly(ethylene succinate) copolyester containing 10 mol % butylene succinate

Copolyester was synthesized and characterized as having 89.9 mol % ethylene succinate units and 10.1 mol % butylene succinate units in a random sequence, as revealed by NMR. Isothermal crystallization kinetics was studied in the temperature range (T_c) from 30 to 73 °C using differential scanning calorimetry (DSC). The melting behavior after isothermal crystallization was investigated using DSC by varying the T_c, the heating rate and the crystallization time. DSC curves showed triple melting peaks. The melting behavior indicates that the upper melting peaks are associated primarily with the melting of lamellar crystals with various stabilities. As the T_c increases, the contribution of recrystallization slowly decreases and finally disappears. A Hoffman‐Weeks linear plot gives an equilibrium melting temperature of 107.0 °C. The spherulite growth of this copolyester from 80 to 20 °C at a cooling rate of 2 or 4 °C/min was monitored and recorded using an optical microscope equipped with a CCD camera. Continuous growth rates between melting and glass transition temperatures can be obtained after curve‐fitting procedures. These data fit well with those data points measured in the isothermal experiments. These data were analyzed with the Hoffman and Lauritzen theory. A regime II → III transition was detected at around 52 °C. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2431–2442, 2008     

The multiple melting endotherms from poly(butylene terephthalate)

The melting behavior of poly(butylene terephthalate) (PBT) has been investigated, and a simulation has been performed to determine whether the multiple melting endotherms observed during the thermal analysis of PBT can be explained by the simultaneous melting and recrystallization of an initial distribution of crystal melting temperatures that contains only one maximum and two inflection points. Specimens that were cooled at constant rates from the melt showed between one and three melting endotherms upon heating in a differential scanning calorimeter (DSC). The position and breadth of the crystallization exotherms upon cooling from the melt and small-angle x-ray scattering showed that as the cooling rate is increased, the distribution of melting temperatures broadens and shifts to lower temperatures. By combining temperature-dependent recrystallization with an initial distribution of melting temperatures, simulated DSC curves were produced that agreed well with experimental DSC curves. In instances of triple peaked curves, the high temperature peak was due to crystals formed during the scanning process, and the middle and low temperature peaks were due to crystals originally present in the material. Satisfactory agreement between the experimental and simulated curves was found without considering additional crystallization from the amorphous regions during the scanning process.     

Correlation between melting behavior and ringed spherulites in poly(trimethylene terephthalate)

The lamellar types as revealed by the multiple melting peaks and possible mechanisms of ringed spherulites in poly(trimethylene terephthalate) (PTT) were analyzed with differential scanning calorimetry (DSC), optical microscopy, and scanning electron microscopy. Several interesting correlations were found. If PTT is melt‐crystallized in a certain temperature range, it shows multiple melting peaks and rings in PTT. Once rings are formed in the original melt‐crystallized PTT, they do not disappear but persist and become even more apparent upon postcrystallization annealing at higher temperatures. Furthermore, for PTT that is capable of exhibiting ringed spherulites, a temperature range exists where rings do not form. This behavior can be interpreted in relation with the demonstrated thermal behavior in PTT. Reorganization took place upon postcrystallization scanning or annealing to or at higher temperatures. A postulation was proposed and rigorously tested with evidence to correlate the ringed spherulites and melting behavior. Rings in PTT may be related to multiple lamellae in the spherulites. Consequently, if a temperature of crystallization is selected so that there is only one type of lamella in the spherulites, then there should be no rings. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 80–93, 2002     

Nonisothermal crystallization kinetics and spherulite morphology of poly(trimethylene terephthalate)

The nonisothermal melt crystallization behavior of poly(trimethylene terephthalate) (PTT) was investigated using the DSC technique. PTT peak exothermic crystallization temperature was found to move to lower temperatures as the cooling rate was increased. The modified Avrami equation exponent, n, was 4 when the cooling rates were between 5 and 15 °C/min, indicating a thermal nucleation and a three-dimensional spherical growth mechanism. When the cooling rate was increased to 25 °C/min, n gradually decreased to near 3, implying the nucleation mechanism changed to an athermal mode. PTT nonisothermal crystallization behavior could also be analyzed using the Ozawa equation and the combined equations of Ozawa and Avrami with very good fit of the data.PTT spherulite morphologies and the sign of the birefringence depended strongly on the spherulite's growth temperature. When the growth temperature was decreased from 222 to 170 °C, the spherulite changed from a saturation-type dendritic morphology to one with a colorful banded texture; the sign of the birefringence also changed in the following order: from a weakly positive spherulite → mixed spherulite → weakly negative spherulite → negative spherulite → positive spherulite → negative spherulite → positive spherulite.     

Effect of dyeing on melting behavior of poly(lactic acid) fabric

The effect of the dyeing on the melting behavior of poly(lactic acid) fabrics was investigated by differential scanning calorimeter. The DSC melting peaks at 10°C min^-1 of the untreated poly(lactic acid) fabric were observed at a temperature higher than those of the dyed fabrics. The restricting force from the extended tie molecules along the fiber axis seems to decrease in the dyeing process. When the sample was rapidly heated, the crystallites melted at lower temperatures since recrystallization was restricted. It was estimated, based on the heating-rate dependency of melting behavior, that the original crystallites of the untreated sample melted at 146.1°C and those of the dyed samples melted at higher temperatures, suggesting that their crystallites are grown to be more perfect in the dyeing process. This revised version was published online in July 2006 with corrections to the Cover Date.     

Crystallization and melting of isotactic polypropylene 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.This revised version was published online in November 2005 with corrections to the Cover Date.     

Evaluation of the physical stability and local crystallization of amorphous terfenadine using XRD-DSC and micro-TA

It is very difficult to follow rapid changes in polymorphic transformation and crystallization and to estimate the species recrystallized from the amorphous form. The aim of this study was to clarify the structural changes of amorphous terfenadine and to evaluate the polymorphs crystallized from amorphous samples using XRD-DSC and an atomic force microscope with a thermal probe (micro-TA). Amorphous samples were prepared by grinding or rapid cooling of the melt. The rapid structural transitions of samples were followed by the XRD-DSC system. On the DSC trace of the quenched terfenadine, two exotherms were observed, while only one exothermic peak was observed in the DSC scan of a ground sample. From the in situ data obtained by the XRD-DSC system, the stable form of terfenadine was recrystallized during heating of the ground amorphous sample, whereas the metastable form was recrystallized from the quenched amorphous sample and the crystallized polymorph changed to the stable form. Obtained data suggested that recrystallized species could be related to the homogeneity of samples. When the stored sample surface was scanned by atomic force microscopy (AFM), heterogeneous crystallization was observed. By using micro-TA, melting temperatures at various points were measured, and polymorph forms I and II were crystallized in each region. The percentages of the crystallized form I stored at 120 and 135 °C were 47 and 79%, respectively. This result suggested that increasing the storage temperature increased the crystallization of form I, the stable form, confirming the temperature dependency of the crystallized form. The crystallization behavior of amorphous drug was affected by the annealing temperature. Micro-TA would be useful for detecting the inhomogeneities in polymorphs crystallized from amorphous drug.     

Melting behavior of isotactic polystyrene revealed by differential scanning calorimetry and transmission electron microscopy

The triple melting behavior and lamellar morphologies of isotactic polystyrene isothermally crystallized from the glassy state have been investigated by differential scanning calorimetry (DSC), temperature-modulated DSC and transmission electron microscopy (TEM). The combination of thermal analysis measurements and morphological observations indicates that: (1) The lowest endothermic peak, the so-called “annealing peak” (T_a), is not associated with the melting of the subsidiary crystals formed by secondary crystallization as often suggested in the literature, but probably with a constrained interphase between the amorphous and crystalline regions; (2) Within spherulites two lamellar populations with different degrees of perfection (or thermal stability) are confirmed by direct TEM observations following partial melting experiments, which are responsible for the so-called double melting peaks (T_m,1 and T_m,2) at higher temperatures observed in DSC curves; (3) The highest endothermic peak (T_m,2) is partially originated from the melting of the recrystallized lamellae formed during heating process in DSC.     

Reversible and irreversible heat capacity of poly(trimethylene terephthalate) analyzed by temperature‐modulated differential scanning calorimetry

The heat capacity of poly(trimethylene terephthalate) (PTT) has been analyzed using temperature‐modulated differential scanning calorimetry (TMDSC) and compared with results obtained earlier from adiabatic calorimetry and standard differential scanning calorimetry (DSC). Using quasi‐isothermal TMDSC, the apparent reversing and nonreversing heat capacities were determined from 220 to 540 K, including glass and melting transitions. Truly reversible and time‐dependent irreversible heat effects were separated. The extrapolated vibrational heat capacity of the solid and the total heat capacity of the liquid served as baselines for the analysis. As one approaches the melting region from lower temperature, semicrystalline PTT shows a reversing heat capacity, which is larger than that of the liquid, an observation that is common also for other polymers. This higher heat capacity is interpreted as a reversible surface or bulk melting and crystallization, which does not need to undergo molecular nucleation. Additional time‐dependent, reversing contributions, dominating at temperatures even closer to the melting peak, are linked to reorganization and recrystallization (annealing), while the major melting is fully irreversible (nonreversing contribution). © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 622–631, 2000     

Thermal analysis of the melting process of poly(trimethylene terephthalate) using FTIR micro-spectroscopy

Thermal spectra of poly(trimethylene terephthalate) (PTT) were collected over a temperature range of 40-250 °C by FTIR micro-spectroscopy. Based on the changes of absorbance ratio corresponding to characteristic groups in low and high vibration energy states, the apparent enthalpy differences of vibration energy states transformation (ΔH_v) in the melting process have been calculated by van’t Hoff equation at constant pressure. In comparison with the values of ΔH_v, the status of participation for the vibration mode of various characteristic groups in PTT macromolecular chain segments was analyzed. It was found that the vibration modes related to the trimethylene glycol unit (O-CH₂-CH₂-CH₂-O) of PTT behaved significant sensitivity and made prominent contribution in the melting process. By the summarization of corresponding data, it has shown that the melting course concerned amorphous phase began at as early as 218 °C, accompanied by the occurrence of crystallization to certain extent, and the ending point was at approximately 238 °C; whereas the melting course concerned crystalline phase began till 228 °C, with the top value of 238 °C, and ended at 242 °C. Besides, for the particular ordered arrangement of chain segments of aromatic polyesters in the melting course, FTIR analysis has provided a reasonable explanation on a molecular level.     

Thermal and X‐ray analysis of polymorphic crystals,melting, and crystalline transformation in poly(butylene adipate)

Polymorphic crystals and complex multiple melting behavior in an aliphatic biodegradable polyester, poly(butylene adipate) (PBA), were thoroughly examined by wide‐angle X‐ray diffraction (WAXD) and differential scanning calorimetry (DSC). Further clarification on mechanisms of multiple melting peaks related to polymorphic crystal forms in PBA was attempted. More stable α‐form crystal is normally favored for crystallization from melt at higher temperatures (31–35 °C), or upon slow cooling from the melt; while the β‐form is the favored species for crystallization at low temperatures (25–28 °C). We further proved that PBA crystallization could also result in all α‐form even at low temperatures (25–28 °C) if it crystallized with the presence of prior α‐form nuclei. PBA packed with both crystal forms could display as many as four melting peaks (P1 ? P4, in ascending temperature order). However, PBA initially containing only the α‐crystal exhibited dual melting peaks of P1 and P3, which are attributed to dual lamellar distributions of the α‐crystal. By contrast, PBA initially containing only the β‐crystal could also exhibit dual melting peaks (P2 and P4) upon scanning. While P2 is clearly associated with melting of the initial β‐crystal, the fourth melting peak (P4), appearing rather broad, was determined to be associated with superimposed thermal events of crystal transformation from β‐ to α‐crystal and final re‐melting of the new re‐organized α‐crystal. Crystal transformation from one to the other or vice versa, lamellae thickening, annealing at molten state, and influence on crystal polymorphism in PBA were analyzed. Relationships and mechanisms of dual peaks for isolate α‐ or β‐crystals in PBA are discussed. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 1662–1672, 2005     

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

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