Melting endotherm

The SCB distribution (SCBD) has been extensively studied by fractionation based on compositional difference as well as molecular size. The analysis by cross fractionation, which involves stepwise separation of the molecules on the basis of composition and molecular size, has provided information of inter- and intramolecular SCBD in much detail. The temperature-rising elution fractionation (TREE) method, which separates polymer molecules according to their composition, has been used for HP LDPE it has been found that SCB composition is more or less uniform [24,25]. It can be observed from the appearance of only one melt endotherm peak in the analysis by differential scanning calorimetry (DSC) (Fig. 1) [26]. Wild et al. [27] reported that HP LDPE prepared by tubular reactor exhibits broader SCBD than that prepared by an autoclave reactor. The SCBD can also be varied by changing the polymerization conditions. From the cross fractionation of commercial HP LDPE samples, it has been found that low-MW species generally have more SCBs [13,24]. 

Figure 4 DSC melting endotherms of P7MB after isothermal crystallization at 135°C, starting from the isotropic melt [10]. The curves correspond to 0, 3,6, and 35 min of crystallization time, from bottom to top. Scanning rate 5°C/min.

The DSC technique enables crystalline materials to be characterised by their melting point. Additives that come into this category include hydrocarbon waxes and poly(tetrafluoroethylene) (PFTE) lubricant. The presence of PTFE in low friction acetal mouldings can be established from the detection of the PTFE melting endotherm at 320°C. 

The only thermal event in the differential thermal analysis curve of (/))-penicillamine is the melting endotherm at 185 °C. Either polymorph of (z>)-penicillamine gives the same endotherm [2]. 

Thermal methods of analysis were used to study the polymorphism associated with pindolol [29]. The overlapping melting endotherms of the different forms were separated by peak fitting analysis and grouping of materials by the temperature of... 

Fig. 4 DTA thermogram of chloroquine diphosphate, form II, illustrating the melting endotherm of form II (196°C), recrystallization to form I, and final melting endotherm attributable to the converted form I (216°C). (Data adapted from Ref. 18.)...

The melting endotherm is followed immediately by a strong exothermic degradation. Since bromocriptine mesilate decomposes under melting, the transition temperature is strongly dependent on the heating rate. A broad but weak endotherm between 40 and 100 °C indicates the volatilization of sorbed recrystallization solvent (usually butanone-2, see section 3). 

Figure 8 shows the DSC thermogram of griseofulvin obtained with a DuPont Model 900 Thermal Analyzer. A single sharp melting endotherm occurs for this substance with onset temperature at 216 C. 

Figure 4.8 shows the comparison of three lots of loperamide hydrochloride, each obtained from a different supplier. The displayed thermograms represent normal behavior for this material, and while the figure shows the uniqueness of each source, the variations were within acceptable limits. Owing to the decomposition that followed on the end of the melting endotherm, specific heats of fusion were not calculated in this case. 

DSC analysis of a mixture of PBT cyclic oligomers containing the stannox-ane 3 showed only the melting endotherm (AH = 68 J/g), with no exotherm evident. Apparently, polymerization starts as the cyclics melt. Cooling showed the crystallization of the polymer, while the second heating stage displayed only the melting point of the PBT polymer at 213 °C (A H = 54 J/g) the polymer had an Mw of 117 000, and was 97 % polymerized (from GPC analysis). 

Smith et al. [64] prepared a series of PET/PTT copolyesters, and found that addition of the other component suppressed the melting point of the respective homopolymer. Between 37 and 60 % PTT content, the copolymers became amorphous and did not show any melting endotherms in the differential thermal analyzer scans. A similar behavior was observed by Balakrishnan and coworkers [102] in PET/PTT copolyesters prepared by the transesterification of PET with PDO, and by the copolymerization of EG and PDO with DMT [103, 104], The non-crystallizing behavior of copolymers with intermediate contents of the respective component is similar to that of a eutectic mixture, indicating formation of random copolyesters. The 7 g and solubility temperature of the copolyesters were, however, continuous and went through minima with increasing PTT content [64],... 

These observations complement results of DSC experiments performed on macroscopic gels which show no melting endotherm for the primary crosslinks found in the initial stage of the gelation. DSC peaks appear only when significant time has elapsed after quenching. 

Endotherm - sulfur melting. Endotherm - sulfur boiling. 

A melt endotherm was observed for brinzolamide with an onset of 121 °C and a maximum of 131 °C. The DSC thermogram is shown in Figure 3. 

The strong exothermic peak has its maximum at 197.41°C C corresponds to the melting transition, and is characterized by a heat of fusion equal to 86.36 J/g. The equivalence of the baseline prior to and subsequent to the melting endotherm suggests that the compound melts without decomposition. 

A single melting endotherm was observed having an onset temperature of 113.6°C, and a peak maximum of 114.6°C. Integration of this thermal feature indicated an enthalpy of fusion equal to 105.6 J/g. 

The DSC thermogram for Form I was obtained at a heating rate of 20 C/minute under nitrogen, and is shown in Figure 3. The sole thermal event noted was the melting endotherm which began with an onset temperature of 259°C, and exhibited a peak maximum at 275"C. The endothermic heat of fusion was determined to be 248 J/g. 

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