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And of course the current thermal analysis instruments have been undergoing continuous development. TA TIP. The art of interpreting curves has yet to be integrated into commercially available com- puter programs. The interpretation of a DSC measurement curve is therefore still some- thing you have to do yourself. It requires a considerable amount of experience in thermal analysis as well as a knowledge of the possible reactions that your particular sample can undergo.
This article presents tips and information that should help you with the systematic inter- pretation of DSC curves.
Recognizing artifacts. The first thing to do is to examine the curve for any obvious artifacts that could lead to a possible misinterpretation of the results. Artifacts are effects that are not caused by the sample under investigation. Figure 1 shows examples of a number of such artifacts. They include:. Afterward, on melting, they make good contact with the pan Fig.
Part 1: Dynamic measurements. NEW in our sales program. The entry of cool air into the measuring cell due to a poorly adjusted measuring. Electrical effects:. The lid of the pan bursts as a result of increasing vapor pressure of the sample. This produces an endothermic peak with. Intermittent often periodic closing of the hole in the lid of the pan due to droplets that condense or to samples that foam. Contamination of the sensors caused by residues of a sample from previous experiments.
The thermal effects characteristic for this substance always occur at the same temperature. This problem can often be overcome by heating the system in air or oxygen.
This type of artifact is very dependent on the contaminant. Artifacts caused by pans that are not inert also look very similar. Figure 3 shows an example of this. Artifacts can also interfere with automatic evaluations with EvalMacro , especially those using automatic limits. Measurement conditions You define the temperature range and the heating rate for the measurement based on your knowledge of the physical and chemi- cal properties of the sample.
Further information on this can be found in UserCom 3. Make a note of the total weight of the sample and pan so that you can detect a loss of weight by. DSC artifacts details are given in the text : An artifact can very often be identified by repeat- ing the measurement with a new sample of the same substance and observing whether the effect oc- curs again either at the same place or at a different place on the curve.
Exceptions to this are f and h, which can be very reproducible. Above: Artifact due to a PE film that was not pressed down firmly in the pan dotted line. The sample of film that was pressed down on the base of the pan with the lid of a light Al pan gave the "correct" melting curve. Below: DSC heating curve of 1. The first measurement is often performed using a pan with a pierced lid and nitrogen as a purge gas.
The cooling rate that can be used depends on the cooling option installed in your system. Differences between the first and the second heating curves can be very informative. This freezes any possible meta- stable states. The sample is then measured a second time. A very conve- nient way to shock cool the sample to room temperature is to use the auto- matic sample robot. It deposits the hot sample on the cold aluminum turn- table, which cools it down to room temperature within a few seconds.
If you do not have a sample robot, you can wait until the sample has reached its final temperature and then remove the pan with tweezers and place it on a cold aluminum surface with a 2 mm diameter hole for the pin or immerse it for about 10 seconds in liquid nitrogen. Below: In an open pan, water evaporates before the boiling point is reached. Above: In a hermetically sealed pan at constant volume , there is no boiling point.
If the ordinate scale is expanded 20 times, an exothermic peak can be observed that is due to the reaction of aluminum with water see the expanded section of the curve.
If no thermal effects occur In this case your sample is inert in the tem- perature range used for the measurement and you have only measured the tempera- ture dependent heat capacity. An inert sample does not undergo any loss. After opening the pan, it looks exactly the same as before the measurement.
This can be confirmed with the aid of a micro- scope for reflected light. If you are interested in c p values, you need. Check the plausibil-. To make absolutely sure that no effects oc- cur, extend the temperature range of the measurement and measure larger samples.
If thermal effects are visible. Thermal effects are distinct deviations from the more or less straight line DSC curve. They are caused by the sample undergoing physical transitions or chemical reactions. If two effects overlap, try to separate them.
Here, one should take into account that faster heating rates cause a marked shift in the peak maxima of chemical reactions to higher tempera- tures. To a lesser extent, this also applies to. The onset temperatures of the melting pro- cesses of nonpolymeric substances are, how- ever, independent of the heating rate. The shape of the DSC curve is usually very characteristic and helps to identify the na- ture of the effect.
In the following sections, examples of the most important effects and their typical curve shapes will be discussed. Physical transitions Physical transitions can in principle be measured as many times as desired if.
This, however, is not always the case and. Many substances in fact solidify from the melt at fast cooling rates to a glassy amorphous state. This is the reason why no melting peak occurs on heating the same sample a second time. Some metastable crystal modifications crystallize only in the presence of certain solvents. Any sample lost by evaporation cannot of course condense in the sample pan on cooling because the purge gas has already removed it from the measuring cell.
Melting, crystallization and mesophase transitions The heat of fusion and the melting point can be determined from the melting curve. With pure substances, where the low tem- perature side of the melting peak is almost a straight line Fig. Impure and poly- meric samples, whose melting curves are concave in shape, are characterized by the temperatures of their peak maxima Fig. Partially crystalline polymers give rise to very broad melting peaks be-.
Many organic compounds melt with de- composition exothermic or endothermic, Figs. An endothermic peak in a DSC heating curve is a melting peak if. A number of substances exhibit a marked degree of sublimation around the melting temperature. If hermetically sealed pans are used, the DSC curve is not affected by sublimation and evapo- ration.
Powdery organic substances, in particular, form a melt that on cooling either solidifies to a glass with no exothermic crystallization peak or crystallizes with an exothermic peak. Comment: Many metals have a high melting point oxide layer on their surface. After melting, the oxide layer remains behind as a rigid envelope. Precious metals have no oxide layer and form spherical droplets on melting. The heat of fusion on nonpolymeric organic substances is almost always between Jg - 1 and Jg - 1.
The melting peak is increasingly sharper, the purer the substance and the smaller the size of the sample. Very small quantities of pure substances give peaks with half-widths of less than 1 K. Impure samples and mixtures often show several peaks. Substances with eutectic im- purities exhibit two peaks Fig. Sometimes the eu- tectic is amorphous so the first peak is missing.
Liquid crystals remain anisotropic even after the melting peak. The melt does not become isotropic until one or more small sharp peaks of mesophase transitions have occurred Fig. An exothermic peak on a cooling curve is a crystallization peak if. Substances that crystallize rapidly show an almost vertical line after nucleation until if the sample is large enough the melting temperature is reached Figs. If the liquid phase consists of a number of individual droplets, the degree of super- cooling of each droplet is different so that several peaks are observed Fig.
Organic and other "poorly crystallizing" compounds form a solid glass on cooling. Melting processes: a: a nonpolymeric pure substance; b: a sample wit a eutectic impurity; c:. Such amorphous samples can then crystallize on heating to temperatures above the glass transition temperature de- vitrification, cold crystallization. Cold crystallization can often occur in two steps. On further heating, polymorphic transi- tions can occur before the solid phase fi- nally melts Fig.
When the melt of a sample containing eu- tectic impurities is cooled, the main com- ponent often crystallizes out Fig. It can, however, solidify to a glass Fig.
Interpretation of DSC Curves in Polymer Analysis 2000 - Toledo
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