Low molecular weight poly(ethylene terephthalate) samples were crystallized isothermally at 120–245°C from both the amorphous state and the melt. Isothermal annealing of these polymers at 215°C provided polymers which exhibited multiple melting peaks in thermal analysis, referred to as form I and form II, as assigned by Bell and Dumbleton. In these samples the peak temperature of the form II melting endotherm and the average crystallite size are dependent on the temperature of initial crystallization. This result requires a mechanism for retaining some structural feature during the conversion from morphological form I to form II. DSC thermograms obtained at varying heating rates on samples showing only form II endotherms support the assignment of superheating as the cause of the shift to higher peak temperatures with increasing heating rate. 相似文献
The multiple melting behavior of poly(ethylene terephthalate) (PET) was investigated with differential scanning calorimetry (DSC) by examining PET samples having been subjected to special schemes of crystallization and annealing treatment at multiple descending temperatures. Upon such step-wise annealing in decreasing temperatures, the existence of doublet melting peaks in addition to a series of multiple minor peaks in the PET has been demonstrated using carefully designed thermal schemes. Using the Hoffman theory, multiple lamellae populations, might be suggested to be simultaneously present in the PET subjected to such thermal treatments. However, direct experimental evidence has yet to be provided. The low-temperature minor crystals simply melt during normal scanning without having time enough to reorganize into higher-melt crystals. Nevertheless, the effect of scanning on non-isothermal crystallization does exist but is primarily confined to the temperature range much below the main melting region where the crystallization of polymer chains can progress at a reasonable rate. At higher temperatures near the main melting region, annealing for extended times is required in order to result in relative changes of the melting endotherms of the upper and lower peaks in the main melting doublet. In all we have shown that interpretations of the multiple melting phenomenon in semicrystalline polymers can be better refined. 相似文献
This study investigated the capacity of fast-scan (400 °C min?1) against conventional (10 °C min?1) differential scanning calorimetry (DSC) techniques to track crystallization phenomenon in tolbutamide–polyethylene glycol 3000 composites prepared by hot melt method (mass ratios 1:1, 1:5, and 1:9) and stored at 25 and 75 % relative humidities. Drug crystallization in composites was indicated by X-ray diffractometry (XRD) and scanning electron microscopy characterization over 40 days storage. With reference to XRD as gold measurement standard, fast-scan DSC could not map the crystallization events of composites (Pearson correlation: fast-scan DSC peak temperature and enthalpy versus XRD peak intensity and area, p > 0.05). Conventional DSC was able to indicate marked drug crystallization through an increase in endothermic enthalpy value of peaks at high temperature regimes between 250 and 360 °C due to formation of high melting point crystal form. 相似文献
Flaxseed oil is rich in the alpha-linolenic acid. The effect of heating on the thermal properties of flaxseed oil extracted from flax seeds has been investigated. The flaxseed oils were heated at a certain temperature (75, 105, and 135 °C, respectively) for 48 h. The melting curve (from ?75 to 100 °C) of flaxseed oil was determined by differential scanning calorimetry (DSC) at intervals of 4 h. Three DSC parameters of exothermic event and endothermic event, namely, peak temperature (Tpeak), enthalpy, and temperature range were determined. The initial flaxseed oil exhibited an exothermic peak, two endothermic peaks, and two endothermic shoulders between ?68 and ?5 °C in the melting profile. Heating temperature had a significant influence on the oxidative deterioration of flaxseed oil. The melting curve and parameters of flaxseed oil were almost not changed when flaxseed oil was heated at 75 °C. However, the endothermic peaks of melting curve decreased dramatically with the increasing of heating time when heating temperature was above 105 °C. There is almost no change of melting heat flow of flaxseed oil when heating time exceeded 32 h at 135 °C. The preliminary results suggest that the DSC melting profile can be used as a fast and direct way to assess the deterioration degree of flaxseed oil. 相似文献
This work is concerned with the melting behaviour and accuracy of differential scanning calorimeter (DSC) analyses of poly(3-hydroxybutyrate) (PHB), a semi-crystalline thermoplastic polymer completely biodegradable and biocompatible, and obtained from renewable resources. Melting parameters of PHB were determined for the first fusion event applying standard experimental procedures for thermal analysis, using heating rates ranging between 1 °C/min and 20 °C/min. The analyses of DSC energy flow scans showed a complex melting peak that may be resolved into three elementary peaks having different intensities at different melting temperatures. Peak temperatures depend on heating rate, while the total crystallinity detected was independent of the rate. A study of 24 DSC runs showed good temperature reproducibility (±0.5 °C), but poor reproducibility of mass crystallinity (±10%). 相似文献
The impact of the physical aging process on the electrical and thermal properties of semicrystalline polybutylene terephthalate (PBT) was investigated by means of thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC). The TSDC technique was used to study the relaxation modes of PBT in the temperature range ?50° to + 120°C. The obtained spectra revealed two peaks located at temperature maxima of 45° and 93°C. The peak appearing at 45°C corresponds to the dielectric manifestation of the glass transition phenomenon (α-relaxation). The aim of this work is to study the effect of physical aging on this relaxation. The recording of TSDC peaks of aged PBT at different temperatures for different times revealed a reduction in their intensities and their shift towards higher temperatures when the aging becomes significant. This result can be explained by the diminution of molecular chain mobility, which is directly related to the area under the peak representing the polarization of the sample. This result was confirmed by DSC measurements, which revealed the growth and the shift of the peak, which is superimposed on the jump of the heat capacity, located around 38°C and characteristic of the glass transition, towards higher temperatures where aging becomes significant. 相似文献
Summary: The polymorphisms in poly(hexamethylene terephthalate) (PHT), along with their associated melting and spherulite morphologies, were examined by differential scanning calorimetry (DSC), wide‐angle X‐ray diffraction (WAXD), and polarized‐light microscopy (PLM). The morphology and crystal cells were dependent on the temperature of crystallization. When melt‐crystallized at low temperatures (90–135 °C), PHT showed at least five melting peaks and two re‐crystallization peaks upon DSC scanning, and the samples displayed various fractions of both α and β crystals. However, only a single melting peak was obtained in PHT melt‐crystallized at 140 °C or above, which displayed a single type of β crystal. In addition, two different forms of spherulites were identified in melt‐crystallized PHT, with one being a typical Maltese‐cross spherulite containing the α crystal, and the other being a dendrite‐type packed mainly with the β crystal. This study provides timely evidence for a critical interpretation of the relationship between multiple melting and polymorphisms (unit cells and spherulites) in polymers, including semi‐crystalline polyesters.
WAXD diffractograms for PHT melt‐crystallized at 140 °C, revealing a single type of β‐crystal cell. 相似文献
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. 相似文献