Exothermal peaks polymers

When the glass transition temperature of the polymer sample is reached in the DSC experiment, the plot will show an incline. It is obvious that the heat capacity increases at T, and therefore DSC can monitor the of a polymer. Usually the middle of the incline is taken to be the Tg. Above Tg, the polymer chains are much more mobile and thus might move into a more ordered arrangement they may assume crystalline or liquid-crystalline order. When polymers self-or-ganize in that way, they give off heat which can be seen as an exothermal peak in... 

The DSC of primary acetylenic monomer 68 showed the presence of two distinct exotherm peaks. The first peak of 202 °C was attributed to the acetylene reacting separately with a second acetylene while the exothermic peak of 270 °C was proposed to be a benzocyclobutene reacting with a second benzocyclobutene. The resulting polymer was believed to be a highly crosslinked network and to some extent the Tg of 380 °C would support this contention. Interestingly, this homopolymer had poor thermal stability at 343 °C in air, with a retention of 41 % of its weight after 200 h. 

Monomer 95 was shown to have a melting point of 93 °C by DSC (10°C/min) and an exothermic peak at 261 °C. A rescan of the DSC sample showed that the Tg was 249 °C. The TGA indicated that the major weight loss occured at 430 °C with 10% weight loss occuring at 470 °C. Interestingly, the only reference to 17 was shown in both a table and TGA trace with no indication as to how the monomer/polymer behaved in a DSC polymerization... 

Ozonization of the polypropylene powder creates the peroxidic species in the polymer, as well. The activation energy [41] of the thermal decomposition of these peroxides is 100 kJ/mol. In the decomposition of peroxides more than one type of radicals was trapped. Moreover, the three exotherms (peak at 40,90, and 130 °C) were observed on DSC thermograms of ozonized sample which also indicates the presence of several types of peroxides. Besides the peroxidic bonds in polymer, selective thermal decomposition may occur also with such bonds in the polymer as, e.g., with end groups containing the initiator moieties [42], This, however, takes place at higher temperatures than it corresponds to usual temperatures at which the thermo-oxidation starts. 

Increasing the concentration of catalyst brings about a reduction in the time to the exothermic peak. The exothermic peak temperature increases as the percentage of catalyst is increased. This increase in temperature is due to the autoacceleration effect that occurs when the viscosity of the monomer-polymer solution increases very rapidly with polymer formation. The percentage of conversion is approximately constant, except for a drop at the 1.2% and 1.5% vazo... 

MMA monomer, which causes cross-linking to the point where the polymer will decompose before it melts. On the average, the exothermic peak temperature remains about the same as the concentration of the TMPTMA is increased from 1.0 to 20% by weight of MMA. 

The stability of the polyyne-type polymers can be examined by thermogravimetry and differential thermal analysis (TG/DTA). From the DTA curves in Figure 10.5, an exothermic peak is observed in all cases. The very broad peak for PpPE is observed around at 350°C, while the sharp peak is observed at 207°C for PpPB and at 138 C for PpPO, respectively. The exothermic peak temperature should be related with thermal stability of the polyyne-type polymers, thus it is concluded that the most stable is PpPE... 

A differential scanning calorimeter (DSC 1-B, Perkin-Elmer Corporation) was used to determine the extent of cure 10-mg to 20-mg specimens were tested at a scanning rate of 10°C/min. An exothermic peak on the thermograph indicates the heat of reaction whereas an endothermic peak in the amorphous polymer indicates the presence of residual stresses or the occurrence of a transition such as the glass transition. The presence of an exothermic peak in the DSC-scan of a pre-cured sample is an indication of incomplete curing. 

The nature of the two endothermal processes has been revealed by study with POM that at 434 °K is the melting of the crystals and the formation of a nematic phase, while at 467 °K is the isotropization of the mesophase. If the polymer is cooled down from its isotropic liquid state the curve (B) is obtained. In the cooling process the first exothermal peak occurs at 463 °K which is the formation of the nematic phase as revealed by POM. The second exothermal peak is centered at 387 °K corresponding to the crystallization of nematic polymer. The jump in (B) of the glass transition is not as clear as in the heating curve. This is understandable because the glass transition is not a genuine thermodynamic transition. 

The data is recorded and can be plotted at the computer. From these DSC plots, thermal events such as melting points, phase change temperatures, chemical reaction temperatures and glass transition temperature of polymers can be determined. An endothermic peak is plotted in the upward direction and an exothermic peak in the downward direction... 

At about 100°C, a weak endothermic peak due to the loss of sorbed water was observed in the nylon 66 curve. In air there was an exothermic reaction initiating at about 1855C and forming a small endothermic peak at a A7. n of about 250°C, the latter being caused by the fusion of the polymer (mp about 255°C). In nitrogen, the exothermic peaks were not present, suggesting thal the air reactions were due to an oxidation reaction. The two endothermic peaks in the nitrogen curve were due to the fusion of the polymer and to the... 

The DSC diagrams of AITPApva-peg. CuTPApva-peg, AITSApva-peg, and CuTSApvA-PEG samples display the endothermic peaks assigned to the evolution of water and volatile compounds such as aldehydes, ketones and ethers, but they do not explicitly show the exothermic peak due to Keggin anion decomposition. For example, below 250 X3 the DSC of AITPApva-peg sample (Fig. 3b) presented the endothermic peaks associated with the release of physisorbed water and the dehydration of both the AITPA salt and the polymer. The other... 

Thermogravimetric curves for the polyacetylene, there are two exothermic peaks at 145 and 325°C [16]. The first of these corresponds to an irreversible cis-trans isomerization. Migration of hydrogen occurs at 325°C, open chain and cross-linking without the formation of polyacetylene volatile products. The color of the polymer becomes brown. A large number of defects appears. In the infrared spectrum there are absorption bands characteristic of the CH, CH3, -C=C- and -C2H5- groups [16]. 

In this equation, the standard heat of polymerization is for full conversion, and AH is the area under the exothermic peak (Figure 2(a)). The percent conversion can be calculated from the ratio of AH to AHq. As mentioned earlier, DSC thermal scans can be used to measure the unreacted components in the cured polymers or networks. Figure 2(b) shows the DSC heating curves for a photo-cross-linked PHEMA-EGDMA network. An exothermal peak immediately associated with thermal polymerization of uncured compounds appeared in the first thermal scan. This peak disappeared when the sample is scanned after being kept at elevated temperature for 20 min. The uncured compounds may influence the structural, mechanical, and physical... 

Figure 2 (a) Schematic illustration of a photo-DSC device (left) and a typical heat flow curve obtained during light exposure (right). In this plot, fp indicates the time to reach the maximum polymerization, (b) DSC thermal scan for photo-cross-linked HEMA-EGDMA polymer. Solid and dashed lines represent first and second runs, respectively. The exothermic peak indicates the thermal polymerization of the unreacted monomers. 

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